WO2019234393A1 - Aqueous electrolytes - Google Patents

Abstract

An electrolyte for an electrochemical device comprises an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated; and a water- absorbent polymer. The water-absorbent polymer may be dissolved in the aqueous solution. The solution may be an over-saturated aqueous salt solution. The electrolyte may be incorporated into an electrochemical device comprising electrodes. The electrolyte may be made by heating a water-containing liquid to a temperature near and below its boiling point; maintaining the temperature of the liquid whilst adding a water-absorbent polymer to the liquid and dissolving a salt into the liquid until a saturated solution is formed; and cooling the solution to room temperature.

Description

AQUEOUS ELECTROLYTES

The invention relates to an electrolyte for an electrochemical device such as a supercapacitor, battery or electrochemical synthesis device, and more specifically to aqueous electrolytes with improved electrochemical windows, and also to a method of making such an electrolyte and electrochemical devices using such an electrolyte.

As would be understood by the skilled person, electrochemical devices generally have two electrodes and an electrolyte, and which electrode is the anode and which the cathode reverses depending on usage (e.g. during (re-)charging of a battery as compared to discharging). The terms “negative electrode” and“positive electrode” may be used to avoid changing terminology when the direction of operation of the system is reversed. The term“cathode” is used herein to denote the electrode which gains electrons. The term “anode” is used herein to denote the electrode which donates/loses electrons. In the discharge mode of a battery or supercapacitor, the cathode is the positive electrode, and the anode is the negative electrode. In the charging mode of battery or supercapacitor, the cathode is the negative electrode, and the anode is the positive electrode. The“working electrode” is the electrode on which a reaction of interest is occurring; the anode or the cathode may be the working electrode depending on whether the reaction on the electrode is an oxidation or a reduction, respectively.

Herein, the invention is primarily described in relation to batteries; the skilled person would appreciate that the same principles could be applied to other electrochemical devices, such as supercapacitors, cells for electrochemical synthesis.

Whilst the field of batteries and supercapacitors has been developed for many years, there are still some challenges, for example for large scale applications in automobiles and energy storage, particularly renewable energy storage. Although various types of batteries have been commercialised for different applications, the barriers for these batteries for automobile and large scale energy storage applications have not yet been overcome. Among current commercial batteries, lithium ion batteries (Li-ion batteries) generally have the highest energy density, and thus are widely used for energy storage and electric vehicles. However, conventional Li-ion batteries have many drawbacks. The major challenges include: (a) low energy density; (b) safety issues associated with the electrolyte; (c) toxicity associated with the electrolyte; (d) high cost associated with the electrolyte and electrode materials; (e) difficulty associated with fast-charging; and (f) limited life-time/cyclability.

In addition to batteries based on Li⁺ ion conducting electrolytes, batteries based on other types of ion conductor are also being developed and some of them have been commercialised. Typical ions used as charge carriers in batteries are Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Fe²⁺, Fe³⁺, Cu⁺, Cu²⁺, Zn²⁺, Ag⁺, NH₄ ⁺, F ,OH and the likes. Stable and conductive electrolyte materials may facilitate commercialisation.

According to a first aspect of the invention, there is provided an electrolyte for an electrochemical device. The electrolyte comprises an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated (the salt solution may therefore be referred to as a saturated aqueous salt solution); and a water-absorbent polymer. The skilled person would understand that a solution comprises a solvent (in this case an aqueous solvent, i.e. a solvent which either is or comprises water) and a solute (the solute in this case being or comprising the salt). The solute is dissolved in the solvent, forming the solution.

The skilled person would understand that a“saturated” solution of a salt is a solution of a high enough concentration that no more of that salt can be dissolved into it. The skilled person would also appreciate that the solubility limit of a salt - i.e. how much of the salt can dissolve in a set volume of a solvent - may vary with temperature. In particular, the solubility limit often increases as temperature increases - i.e. more salt can be dissolved in the same amount of solvent at higher temperatures. Solubility is generally defined at room temperature (20-25°C) - a sufficient quantity of the salt is therefore dissolved in the solution of the electrolyte of the first aspect for the solution to be saturated at room temperature (20-25°C). A sufficient quantity of the salt may be present in the electrolyte (dissolved or otherwise) for the solution to be/remain saturated at a higher temperature, e.g. 40°C, 50°C 60°C, 70°C, 80°C or 90°C. A sufficient quantity of the salt may be present in the electrolyte (dissolved or otherwise) for the solution to be/remain saturated at all operating temperatures expected in use of the electrolyte in an electrochemical device. If the solubility limit of the salt increases with temperature, an electrolyte which is saturated at a first temperature is likely to be supersaturated at a second, lower, temperature (i.e. to comprise a saturated solution and additional, undissolved, salt).

The skilled person would understand that a“water-absorbent” polymer is a polymer capable of absorbing water; i.e. molecules (or ions) of water are taken in by the polymer. A water absorbent polymer may, for example, be capable of taking up between 50% and 100000% of its own weight (i.e. from 0.5 times its own weight to 1000 times its own weight) in water, and optionally may be capable of taking up at least its own weight in water.

The water-absorbent polymer may be dissolved in the aqueous solution. Alternatively, the water-absorbent polymer may absorb water, optionally swelling in the process, but may not itself dissolve. The water-absorbent polymer may be suspended in the aqueous solution. Additionally or alternatively, some of the water-absorbent polymer may be dissolved, and some suspended. Optionally, more than one water-absorbent polymer may be present.

The saturated aqueous salt solution may be an over-saturated (i.e. supersaturated) aqueous salt solution. The electrolyte may be a gel (e.g. at room temperature). The electrolyte may comprise some of the salt in a crystallised form, distributed throughout the gel. The amount of salt present in a crystallised form may be small compared to the amount of electrolyte - the skilled person would appreciate that the amount of salt present in a crystallised form may depend on temperature and solubility limit of the salt; it may depend on the level of super-saturation of the electrolyte.

The weight of the water-absorbent polymer may be up to 30 %, or optionally up to 10 % of the weight of the solvent of the electrolyte (the solvent may be a single solvent or a mixture of different solvents). This may be described as the electrolyte comprising up to 10 wt.%, or optionally up to 30 wt.%, water-absorbent polymer (the weight percentage being calculated based on solvent weight). The amount of water-absorbent polymer added may be a minimum of 0.01 wt.% of the solvent, optionally a minimum of 0.1 wt.%, optionally a minimum of 1 wt.%, and further optionally a minimum of 5 wt.%. The water-absorbent polymer may be PVA. In various embodiments, a range of 1 wt.% to 30 wt.% PVA, and optionally more than 5 wt.% PVA may be used. The water-absorbent polymer may be or comprise one or more of the following: (i) polyvinyl alcohol (PVA); (ii) polyvinyl pyrrolidone (PVP); (iii) polypropylene alcohol (PPA); or (iv) polyethylene glycol (PEG).

The water-absorbent polymer may be or comprise a superabsorbent polymer (SAP). The amount of SAP added may be 0.1 wt.% of the solvent, for example around 0.3 wt.% of the solvent. Two or more water-absorbent polymers may be present; for example both a (optionally) non-water- soluble SAP and a water-soluble water-absorbent polymer which is not a SAP. The water-soluble water-absorbent polymer may be up to 10 wt.%, or optionally up to 40 wt.%, of the solvent. In embodiments including a SAP, the superabsorbent polymer may be or comprise one or more of the following: (i) a polyacrylate salt, optionally sodium polyacrylate; (ii) a polyacrylamide; (iii) a polysaccharide; (iv) a polypeptide; (v) polyacrylonitrile (PAN); or (vi) a polyvinyl alcohol copolymer.

In embodiments including a SAP, the superabsorbent polymer may form between 0.01 wt.% and 5 wt.% of the electrolyte, optionally between 0.1 wt.% and 5 wt.% or between 0.5 wt.% and 5 wt.% of the electrolyte, and further optionally between 0.1 wt.% and 2 wt.% of the electrolyte.

The aqueous solution may be an aqueous metal acetate solution. The metal of the metal acetate may be selected from a group of metals having a negative standard electrode potential, relative to the standard hydrogen electrode, when in their elemental forms. The metal of the metal acetate may be, for example, potassium, caesium, magnesium, lithium, sodium, or zinc. The electrolyte may further comprise an additional metal salt dissolved in the solution. The metal of the metal cation of the additional metal salt may be different from the metal of the metal acetate but selected from the same group of metals. The anion of the additional metal salt may be a nitrate, a sulfate, a phosphate, a halide, or an organic salt with a chain length smaller than or equal to 6. The molar ratio of the metal acetate to the additional metal salt may be between 3 : 1 and 100: 1, inclusive.

The aqueous salt solution may be an aqueous metal chlorate solution such as a lithium perchlorate solution. The aqueous salt solution may be an aqueous metal nitrate solution. The aqueous salt solution may be an aqueous metal chloride solution.

The aqueous solution may be a mixed salt solution comprising a plurality of different salts (a multi-salt solution), and optionally may be a bi-salt solution comprising two different salts.

The skilled person would appreciate that the“electrochemical window” of a substance is the voltage range over which the substance is neither oxidised nor reduced, and that electrochemical window width is an important characteristic for electrolytes. The electrochemical window is calculated by subtracting the reduction potential (cathodic limit) from the oxidation potential (anodic limit), and is a term commonly used to indicate the potential range and the potential difference of an electrochemical system. The skilled person would appreciate that the electrochemical window is important for the efficiency of an electrode, because, outside of the range of the electrochemical window, the solvent (e.g. water, for an aqueous electrolyte) is electrolysed, wasting energy and reducing efficiency.

Electrolytes and methods for extending the electrochemical windows of aqueous electrolytes are disclosed herein. The electrolytes of this aspect of the invention include a polymeric component as well as one or more salts and may therefore be described as composite electrolytes. The solvent may be water, or may comprise water and one or more additional solvents. The salt(s) of the aqueous salt solution may be referred to as the“main” salt(s) of the electrolyte - other additives, such as the water- absorbent polymer, may also be salts but are present in smaller quantities than the main salt(s). The aqueous salt solution may be super-saturated (also referred to as over-saturated). In a super-saturated solution, an amount of undissolved salt is present with the saturated aqueous solution - a super-saturated solution may therefore be described as a saturated solution of the salt(s) with some of the same salt(s) in an undissolved form therein - e.g. as crystallites suspended within the solution. Use of a super-saturated solution as an electrolyte may provide one or more of the following advantages:

1. reduce or minimise the activity of water and increase or maximise the electrochemical window of the electrolyte, leading to a high battery voltage, thus high power and energy density;

2. decrease the freezing point of the solution, so facilitating use of the electrolyte at temperatures below 0 °C, the freezing point of water. This may be particularly useful for outdoor applications, or for electric vehicles in winter or in a relatively cold country, e.g. Norway;

3. maintain electrolyte saturation during operation. The skilled person would appreciate that many electrochemical devices release heat during operation, so warming the electrolyte. Therefore, the working temperature of a battery or the likes is often higher than the surrounding temperature. For most salts, the solubility of the salt increases with increasing temperature. As such, if a saturated (but not super-saturated) solution is used, as temperature increases the solution may become unsaturated, and the electrochemical window may decrease. The battery developed at room temperature (normally classed as round 20-25 °C) may then not be stable - for example, electrolysis of water may happen at the higher working temperature due to the decreased electrochemical window (affected by the increased solubility limit at higher temperature). Electrolysis of water releases H₂ and 0₂, thus potentially causing safety problems. However, this can be overcome by the use of a super-saturated solution. At elevated temperature, the over-saturated solution may become less over-saturated, or saturated, thus the decrease in electrochemical window may be zero, or very small. The desired electrochemical window can therefore be retained, and the battery or other electrochemical device may be more safely, and/or more efficiently, operated at the higher temperature;

4. increase the boiling point of the solution, which may allow the electrolyte to be used at a higher temperature.

A disadvantage of using a super-saturated solution may be that the usage of salt may be higher, so potentially increasing cost. The skilled person would appreciate that a relatively low cost inorganic salt, or combination of salts, may be selected whilst maintaining electrochemical window width. The water absorption of the polymer may also at least partially counter the increase in salt use by reducing the amount of the salt dissolvable in a set volume of solvent.

As the aqueous electrolyte is a saturated solution, and optionally super-saturated, it may be described as a“ water-in-salt” electrolyte to reflect the relatively large amount of salt present as compared to the solvent. In particular, a water-in-salt electrolyte may be defined as an electrolyte for which the salt exceeds the solvent by both weight and volume (see L. M. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. L. Fan, C. Luo, C. S.Wang, K. Xu, Science 350, 938-943 (2015)). The electrolyte may also be referred to as a salt-polymer-H₂0 composite electrolyte, or as an aqueous salt-polymer composite electrolyte. The more standard situation of the solvent being the major component of the solution and the solute/salt the minor component may therefore be reversed in such embodiments. An over-saturated (supersaturated) water-in-salt gel electrolyte is provided in various embodiments. Such an electrolyte may offer a wider electrochemical window for electrochemical devices, such as batteries and supercapacitors.

The electrolyte may be a sol (an inorganic colloidal suspension) or a gel, and may therefore be referred to as a sol electrolyte or a gel electrolyte. In a supersaturated gel electrolyte, the undissolved salt may be distributed, preferably evenly, throughout the gel. The undissolved salt may be held in place by the structure of the gel. The electrolyte may be a hydrogel.

The aqueous salt solution comprises a solvent - the salt is dissolved in the solvent. The water- absorbent polymer may be dissolved in the solvent (a soluble polymer) or may absorb the water and expand (e.g. a super-absorbent polymer which may or may not be soluble in the solvent).

The solvent may be or comprise water - the solvent may consist of water, or, in alternative embodiments, the solvent may comprise a mixture of water and one or more organic solvents. Solvents comprising a mixture of water and one or more organic solvents may be azeotropes - i.e. a constant boiling point mixture of two or more liquids. The salt solution may comprise one or more different salts. The salts may be organic and/or inorganic salts.

According to a second aspect of the invention, there is provided the use of a supersaturated solution as a sol or gel electrolyte in an electrochemical device, such as a battery, supercapacitor or the likes, the supersaturated solution including a salt and a water-absorbent polymer dissolved therein.

The electrolyte may be as described for the first aspect. The electrolyte, and therefore the electrochemical device, may have a wider electrochemical window than an electrochemical device using the same salt in an un-saturated solution and/or without the absorbent polymer. The electrolyte and/or the electrochemical device may be as described in any other aspect.

According to a third aspect of the invention, there is provided an electrochemical device comprising an electrolyte comprising an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated, and a water-absorbent polymer; a positive electrode in contact with the electrolyte; and a negative electrode in contact with the electrolyte.

The electrochemical device may be a battery. The electrochemical device may be a supercapacitor. The electrochemical device may be an electrochemical synthesis device. The electrolyte may be the electrolyte as described with respect to the first aspect.

According to a fourth aspect of the invention, there is provided a method of making an electrolyte for an electrochemical device, the method comprising heating a liquid to a first temperature, wherein the liquid is or comprises water; maintaining the temperature of the liquid whilst: adding a water- absorbent polymer to the liquid, and dissolving a salt into the liquid until the liquid is saturated; and cooling the mixture to a second temperature.

Adding the water-absorbent polymer to the liquid may form a sol, and the salt may then be dissolved into the sol until the sol is saturated. The sol may transform into a gel on cooling to the second temperature. If the water-absorbent polymer dissolves completely into the liquid, a solution which is not a sol may be formed instead of a sol - the solution may become a gel on cooling to the second temperature.

The first temperature may be a temperature near and below the liquid’s boiling point. The method may comprise heating the liquid to a temperature above 90°C, and optionally to the temperature of 95°C. The first temperature may be above an expected maximum temperature of the electrolyte in use in an electrochemical device and below the liquid’s boiling point. The second temperature may be room temperature (e.g. 20-25°C).

The liquid may be water. The liquid may be an azeotrope comprising water and one or more organic solvents, and wherein optionally the water forms at least 50% of the liquid by weight.

The water-absorbent polymer may be or comprise one or more of the following: (i) polyvinyl alcohol (PVA); (ii) polyvinyl pyrrolidone (PVP); (iii) polypropylene alcohol (PPA); or (iv) polyethylene glycol (PEG).

The amount of the water-absorbent polymer added may be between 0.1 wt.% and 40 wt.%, and optionally between 0.1 wt.% and 30 wt.% (based on solvent weight). The amount may be between 5 wt.% and 30 wt.%, optionally between 5 wt.% and 20 wt.%, and optionally around 10 wt.%, if the water-absorbent polymer is not a SAP. The amount may be between 0.1 wt.% and 5 wt.%, and optionally around 1 wt.%, if the water-absorbent polymer is a SAP.

The salt may be a metal chlorate, a metal acetate, a metal nitrate or a metal chloride.

The salt and the water-absorbent polymer may be selected such that the electrolyte formed is as described with respect to the first aspect.

The skilled person will appreciate that prior art electrolytes are generally prepared at room temperature, so not forming a super-saturated solution. The skilled person will appreciate that the method described, involving heating the mixture to a higher temperature (referred to as a“high” temperature, or the“first” temperature), usually below but close to the boiling point of the solvent, may allow a larger amount of the salt(s) to be dissolved than would be possible in the same volume (or mass) of solvent at room temperature. A saturated solution may be formed at the high temperature; when the solution is then cooled to room temperature, the solubility of the salt(s) in the solvent may decrease, so forming an over-saturated (supersaturated) solution.

The high temperature may be between 80 °C and 100 °C, optionally between 90 °C and 98 °C, and optionally equal to or around 95°C. The high temperature may be chosen based on the boiling point of the solvent, and/or on a known intended maximum working temperature of the electrolyte. For example, a typical working temperature of a lithium ion battery is currently in the range of -25 to 45 °C, and is expected to expand to the range of - 40 to 70 °C in future. For batteries based on aqueous electrolytes, the freezing point of the electrolyte is generally well below 0 °C due to the presence of salts in the solution. The lowest operating temperature generally cannot be lower than the freezing point of the aqueous electrolyte. The highest operating temperature generally cannot be above the boiling point of the solution or the eutectic point of the solvents in the case of mixed solvents. The operating temperature range may therefore vary depending on the salt and the solvent used (e.g. pure water or mixed water and organic solvent(s)), but may generally be in the range of -20 to 60 °C. The high temperature may therefore be selected to be higher than 60 to 70 °C, optionally with a safety margin of at least 5-10°C. As the solution is cooled, becoming supersaturated, some of the salt(s) may come out of solution, e.g. by crystallisation. At room temperature, the super-saturated solution may therefore comprise some undissolved salt(s) in the solution. Crystallites of the undissolved salt(s) may be suspended within the solution. The addition of a water-absorbent polymer, such as PVA, whilst the mixture is at the high temperature may cause a gel to form, which may be uniform. In some embodiments, a sol or gel is formed at the high temperature. In other embodiments, the mixture may remain as a liquid (solution, with the polymer fully dissolved) or sol at the high temperature, but solidify into a gel (or sol depending on the temperature) as the mixture is cooled. The skilled person would appreciate that a saturated solution prepared at room temperature with addition of a polymer such as PVA may generally take the form of a sol. By contrast, the saturated solution prepared at high temperature, e.g. 95 °C, with addition of a polymer, such as PVA, is generally a sol at the high temperature and after cooling down to lower temperature or room temperature, gradually changes to super-saturated gel state. After cooling down to room temperature, there may be inclusions of undissolved salts suspended within the gel. This may be referred to as a supersaturated gel electrolyte. The inclusions may be evenly distributed throughout the gel, which may facilitate dissolving of the inclusions into the gel on subsequent heating.

The skilled person would appreciate that electrochemical devices generally require current collectors to connect the device into a circuit and allow electron flow. Suitable, stable, current collectors suitable for use in batteries, supercapacitors and other electrochemical devices are discussed herein, with a focus on the current collector for a positive electrode. The skilled person would appreciate that, if the electrochemical window of the system is broadened, the current collector should be stable over the wider voltage range - different materials may therefore be used as compared to prior art current collectors. The current collector may be used as the substrate or holder for electrode materials in electrochemical devices of various embodiments.

According to a fifth aspect of the invention, there is provided an electrolyte for an electrochemical device, the electrolyte comprising an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated.

The saturated aqueous salt solution may be an over-saturated aqueous salt solution. The saturated solution may be as described for the electrolyte of the first aspect; the skilled person would appreciate that the absence of the polymer, as compared to the first aspect, may reduce electrochemical window width and/or increase the amount of salt needed for the same level of (super-)saturation, but may still out-perform unsaturated electrolyte solutions.

According to a sixth aspect of the invention, there is provided an electrochemical device comprising: an electrolyte comprising an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated, and optionally over-saturated; a positive electrode in contact with the electrolyte; and a negative electrode in contact with the electrolyte.

According to another aspect of the invention, there is provided supercapacitor, battery or electrochemical synthesis device comprising: a salt-water-polymer composite electrolyte; a positive electrode, wherein the positive electrode is the cathode when a battery or supercapacitor is discharging whilst it is the anode when a battery or supercapacitor is charging; and a negative electrode, wherein the negative electrode is the anode when a battery or supercapacitor is discharging whilst it is the cathode when a battery or supercapacitor is charging.

The electrolyte may contain salts and water. The electrolyte may contain either inorganic salts or organic salts, or a mixture of organic and inorganic salts. The electrolyte may be a saturated or over-saturated solution of salts dissolved in water, or water mixed with one or more other solvents, optionally including both inorganic and organic solvents. The electrolyte may or may not contain at least one polymer; ideally the polymer can absorb water.

The supercapacitor, battery or electrochemical synthesis device may include one or more current collectors - for example one for each electrode. The or each current collector may be selected to be chemically compatible with the electrolyte and with one or both of the electrodes. The or each current collector may be selected to sustain high and low applied voltage without being oxidised or reduced.

The salt(s) may be typical inorganic salts, optionally containing one or more of:

(i) cations such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Fe²⁺, Fe³⁺, Cu⁺, Cu²⁺, Zn²⁺, Ag⁺, NH₄ ⁺, etc. (e.g. Bi³⁺, Ni²⁺) ; and/or

(ii) anions such as S0₄ ² , C10₄ , CT, N0₃ , P0₄ ³ , OH , Br , F etc.

The salt(s) may be or comprise one or more organic salts, optionally comprising one or more of the cations listed above for inorganic salts. The anions may be or comprise one or more of: acetate (Ac), trifluoromethane sulfonate (Tf), bis(trifluoromethane sulfonyl)imide (TfSI), bis(fluorosulfonyl)imide (FSI), tetrafluorophosphate (BF₄), hexafluorophosphate (PF₆), bis(pentafluoroethane sulfonyl) imide (BETI), 4,5-dicyano-2-trifluoromethanoimidazole (DCMI), [fluoro(nonafluorobutane) sulfonyl] imide (FNF), and fluorinated acetate anions, such as trifluoroacetate (AcF).

The polymer may be polyvinyl alcohol (PVA), and/or polyethylene glycol (PEG). The polymer may be or comprise polyvinyl pyrrolidone (PVP) and/or polypropylene alcohol (PPA).

A relatively small amount (e.g. up to 5 wt.%) of super water absorbent may be added to the salt-polymer-H₂0 composite electrolyte. This may serve to reduce the solubility of inorganic and organic salts in the solution without significantly changing the electrochemical window and conductivity. In various embodiments, the amount of the super water absorbent added may be between 0.1 wt.% and 5 wt.%, and optionally between 0.5 wt.% and 5 wt.%, and further optionally around 1 wt.%. In one embodiment, 1 wt.% of sodium polyacrylate was used as the super water absorbent. The skilled person would appreciate that the amount added may be adjusted for different SAPs dependent on the absorption properties of the SAP.

The super water absorbent polymer (SAP) may be selected from the salts of polyacrylate, for example containing one or more cations such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Fe²⁺, Fe³⁺, Cu⁺, Cu²⁺, Zn²⁺, Ag⁺, NH⁴⁺ ions, and/or Bi³⁺, Ni²⁺. In alternative or additional embodiments, the super water absorbent may be acrylonitrile / copolymers or other materials which can strongly absorb water.

In embodiments with one or more current collector, the current collector(s) may be or comprise a sheet/foil/mesh (or foam) of metal or metal oxide or metal nitride or metal carbide or a composite of metal/polymer/ceramic. In some embodiments, the metal, polymer or ceramic or their composite may be used as a substrate; for example a thin layer of conductive metal, polymer or ceramic materials may be coated on the surface. The thin layer may be formed using a composite of a polymer binder and conductive metal or ceramic materials such as oxides, nitrides, carbides, nitride-oxide, carbide-oxide etc. In embodiments with a current collector containing a metal, the metal of the current collector may be or comprise one or more of: aluminium, stainless steel, titanium, silver, gold, platinum, vanadium, molybdenum, zirconium, chromium, zinc, niobium (or nickel). For example, titanium foam, may be used as current collector for both positive and negative electrodes.

In embodiments with a current collector containing a metal oxide, the metal oxide may be sub- stoichiometric titanium oxides with formula Ti_n0_2n-i (n=4-9), conductive oxides with spinel or perovskite or other crystal structure, for example, A- or/and B-site doped chromates, vanadates, molybdenates or the likes.

In embodiments with a current collector containing a metal nitride or carbide or the likes, the conductive metal nitrides/nitride oxides, metal carbides/carbide oxides may be or comprise one or more of CrN, VN, TiN, TiC, WC, or the likes (see the following paper for various options: Y. Zhong, X. H. Xia, F. Shi, J. Y. Zhan, J. P. Tu, H. J. Fan, Advanced Science 3, (2016) 1500286).

In embodiments with a current collector containing a metal oxide, nitride or carbide or the likes, methods for preparation of the conductive oxides/nitrides/carbides and coating a layer thereof onto a substrate may be or comprise one or more of the below, or any suitable method known in the art: (i) a sol-gel process, (ii) chemical vapour deposition (CVD), (iii) physical vapour deposition (PVD) such as plasma or thermal PVD, (iv)precipitation, (v) solid state reaction, (vi) electro-deposition, (vii) solvent thermal synthesis, or (viii) microwave synthesis. Additionally or alternatively, combustion synthesis may be performed.

The skilled person would understand that features described with respect to one aspect of the invention may be applied, mutatis mutandis , to any other aspect of the invention. There now follows by way of example only a detailed description of embodiments of the present invention with reference to the accompanying drawings in which:

Figure 1 illustrates a standard electrochemical setup for testing of an electrolyte of various embodiments;

Figure 2 shows linear sweep voltametry (LSV) curves (-0.4 to 1.6 V vs. Ag/AgCl) of 1 m (i.e. 1 molal = 1 mole of salt per kg of solvent) LiC10₄ aqueous electrolyte with various scanning rates;

Figure 3 shows LSV curves (-2.0 to 2.0 V vs. Ag/AgCl) of a saturated (10 m) LiC10₄ aqueous electrolyte with various scanning rates;

Figure 4 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of LiC10₄-PVA aqueous electrolyte with various scanning rates;

Figure 5 shows an electrochemical impedance spectroscopy (EIS) plot of carbon cloth|LiC10₄- PVA|carbon cloth cell at room temperature;

Figure 6 shows LSV curves (-2 to 2.5 V vs. Ag/AgCl) for a NaC10₄-PVA aqueous electrolyte with various scanning rates;

Figure 7 shows an EIS plot of carbon cloth|NaC10₄-PVA|carbon cloth cell at room temperature;

Figure 8 shows LSV curves (-2 to 2.5 V vs. Ag/AgCl) of a Mg(C10₄)₂-PVA aqueous electrolyte with various scanning rates;

Figure 9 shows an EIS plot of carbon cloth|Mg(C10₄)₂-PVA|carbon cloth cell at room temperature; Figure 10 shows LSV curves (-0.5 to 1.5 V vs. Ag/AgCl) of a Zn(C10₄)₂-PVA aqueous electrolyte with various scanning rates;

Figure 11 shows an EIS plot of carbon cloth|Zn(C10₄)₂-PVA|carbon cloth cell at room temperature;

Figure 12 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of LiTfSI-PVA aqueous electrolyte with various scanning rates;

Figure 13 shows an EIS plot of carbon cloth|LiTfSI-PVA|carbon cloth cell at room temperature;

Figure 14 shows LSV curves (-2.5 to 3 V vs. Ag/AgCl) of LiOTf-PVA aqueous electrolyte with various scanning rates;

Figure 15 shows an EIS plot of carbon cloth|LiOTf-PVA|carbon cloth cell at room temperature;

Figure 16 shows LSV curves (-2.5 to 2.5 V vs. Ag/AgCl) of a LiC0₂CF₃ -PVA aqueous electrolyte with various scanning rates;

Figure 17 shows an EIS plot of carbon cloth|LiC0₂CF₃-PVA|carbon cloth cell at room temperature;

Figure 18 shows LSV curves (-2.0 to 2.5 V vs. Ag/AgCl) of Zn(OTf)₂-PVA aqueous electrolyte with various scanning rates;

Figure 19 shows an EIS plot of carbon cloth|Zn(OTf)₂-PVA|carbon cloth cell at room temperature;

Figure 20 shows LSV curves (-3 to 3 V vs. Ag/AgCl) of LiC10₄+LiOTf-PVA aqueous electrolyte with various scanning rates;

Figure 21 shows EIS plot of carbon cloth|LiC10₄+LiOTf-PVA|carbon cloth cell at room temperature;

Figure 22 shows LSV curves (-2.0 to 2.0 V vs. Ag/AgCl) of LiC10₄-PEG aqueous electrolyte with various scanning rates;

Figure 23 shows an EIS plot of Pt mesh|LiC10₄-PEG|Pt mesh cell at room temperature;

Figure 24 shows LSV curves (-2.0 to 2.5 V vs. Ag/AgCl) of LiC10₄+ poly(acrylic acid sodium salt)-PVA aqueous electrolyte with various scanning rates;

Figure 25 shows an EIS plot of carbon cloth|LiC10₄+ poly(acrylic acid sodium salt)-PVA|carbon cloth cell at room temperature;

Figure 26 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of ZnS0₄-PVA electrolyte with various scanning rates;

Figure 27 shows an EIS plot of carbon cloth|ZnS0₄-PVA|carbon cloth cell at room temperature;

Figure 28 shows cyclic voltametry (CV) curves (-0.5 to 2.0 V vs. Ag/AgCl) within 30 loops for a A1 foil|LiC10₄-PVA| A1 foil three-electrode cell with 1 mV/s scanning rate;

Figure 29 shows CV curves (-0.5 to 2.0 V vs. Ag/AgCl) within 30 loops for a carbon cloth|LiC10₄- PVA|carbon cloth three-electrode cell with 1 mV/s scanning rate;

Figure 30 shows CV curves (-0.5 to 1.7 V vs. Ag/AgCl) within 30 loops for a steel mesh|LiC10₄- PVA|steel mesh three-electrode cell with 1 mV/s scanning rate; Figure 31 shows CV curves (-0.5 to 2.0 V vs. Ag/AgCl) within 30 loops of Ti foil|LiC10₄-PVA|Ti foil three-electrode cell with 1 mV/s scanning rate;

Figure 32 shows CV curves (-0.1 to 2.4 V vs. Ag/AgCl) within 100 loops of a Ti foil|LiC10₄-PVA|Ti foil coin cell with 1 mV/s scanning rate;

Figure 33 shows an EIS plot of a Pt mesh|various acetate salt-based electrolytes|Pt mesh cell at room temperature;

Figure 34 shows LSV curves (-1.5 to 2 V vs. Ag/AgCl) of a 31 m KAc aqueous gel with various scanning rates;

Figure 35 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 47 m CsAc aqueous gel with various scanning rates;

Figure 36 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 7 m Mg(Ac)₂ aqueous gel with various scanning rates;

Figure 37 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 10 m LiAcF+31 m KAc aqueous gel with various scanning rates (AcF = trifluoroacetate);

Figure 38 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 1 m LiAc+31 m KAc aqueous gel with various scanning rates;

Figure 39 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 1 m LiN0₃+31 m KAc aqueous gel with various scanning rates;

Figure 40 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 1 m Li₂S0₄+31 m KAc aqueous gel with various scanning rates;

Figure 41 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 1 m Mg(Ac)₂+31 m KAc aqueous gel with various scanning rates;

Figure 42 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 1 m Zn(Ac)₂+31 m KAc aqueous gel with various scanning rates;

Figure 43 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of a 1 m Zn(Ac)₂+31 m KAc+ poly(acrylic acid sodium salt) aqueous gel with various scanning rates;

Figure 44 shows a saturated 10 m LiC10₄-PVA-H₂0 sol at 95°C (left) and the supersaturated 10 m LiC10₄-PVA-H₂0 gel electrolyte formed by cooling the solution to 25°C (right);

Figure 45 shows LSV curves demonstrating the electrochemical window of LiC10₄-PVA-H₂0 gel electrolytes at room temperature for solutions of three different concentrations;

Figure 46 illustrates a method of an embodiment;

Figure 47A shows results of electrochemical window tests on a 10 m LiC10₄-PVA electrolyte under different scanning rates;

Figure 47B shows pH values for various aqueous solutions;

Figure 47C shows results of electrochemical window test on three different electrolytes (4.5 m Zn(C10₄)₂- PVA, 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA and 1 m Zn(C10₄)₂+10 m LiC10₄-PVA);

Figure 47D shows CV curves for a Zn symmetric cell using the 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA electrolyte under 1 mV/s scanning rate;

Figure 48 shows (a) Fourier transform infrared (FTIR) and (b) thermal analysis results of electrolyte material testing, and (c) optical image of electrolyte tape cast on glasee fiber and (d) SEM image of the electrolyte;;

Figure 49 shows SEM images and EDS mappings of the self-supported LiMn₂0₄-carbon cloth cathode; Figure 50 shows electrochemical data obtained for an electrode of an embodiment;

Figure 51 shows XRD and SEM image data for a Zn anode before and after cycling;

Figure 52 shows results of an electrochemical window test on a 10 m LiC10₄ aqueous solution (without PVA) under scanning rates ranging from 50, 20, 10, or 5 to 1 mV/s;

Figure 53 shows EIS plots for various electrolyte discs;

Figure 54 illustrates an electrochemical setup for testing of an electrolyte of various embodiments;

Figure 55 shows CV data for symmetric cells of two embodiments;

Figure 56 shows galvanostatic Zn stripping/plating in a Zn symmetric cell under 0.2 mA cm ² current density within 50 cycles;

Figure 57 shows CV and chronocoulometry data for an asymmetric Zn coin cell, using Pt for the counter electrode;

Figure 58 shows TGA test data for a 10 m LiC10₄-PVA electrolyte from room temperature to 600°C;

Figure 59 shows (a) an SEM image of electrolyte with 25 pm resolution, and (b, c, d, e) corresponding elemental mappings for O, Zn, C and Cl;

Figure 60 shows an XRD pattern of a LiMn₂0₄-carbon cloth cathode before testing;

Figure 61 shows SEM images of a Zn anode with various magnifications before cycling;

Figure 62 shows (a) EIS spectra of a 1 m ZnC104-PVA gel and a 0.1 m KC1 aqueous solution with Zn foil electrodes, and (b) the relationship between Z’-R and w ^{1 2} for a Zn| 1 m Zn(C10₄)₂-PVA|Zn coin cell;

Figure 63A shows LSV curves for 10 m LiC10₄-PVA and 6 m LiC10₄-PVA electrolytes at various temperatures at a 1 mV/s scanning rate;

Figure 63B shows EIS plots for the 6 m LiC10₄-PVA electrolyte of Figure 63A at four different temperatures, alongside an EIS plot for 0.1m KC1 at room temperature for reference, focusing on the x-intercept;

Figure 63C shows EIS plots for the 10 m LiC10₄-PVA electrolyte of Figure 63A at four different temperatures, alongside the EIS plot for 0.1m KC1 at room temperature for reference, focusing on the x-intercept;

Figure 64 shows an XRD pattern of LiMn₂0₄-carbon cloth cathode after charge-discharge cycling;

Figure 65 shows the cycling stability and coulombic efficiencies of a Zn| l m Zn(C10₄)₂+10 m LiC10₄- PVA|LiMn₂0₄-titanium foil full cell with 200 mA g ¹ current between 0 and 2.0 vs. Zn/Zn²⁺ within 1000 cycles;

Figure 66 shows EIS spectra of a Zn/LiMn₂0₄ full cell before and after charge-discharge cycling;

Figure 67 shows optical images of a full cell and various cell components after charge-discharge cycling;

Figure 68 shows FTIR spectra of 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA electrolyte collected before and after the charge-discharge cycling;

Figure 69 shows (a, b) SEM images of a self-supported LiMn₂0₄-carbon cloth cathode with various magnifications, and (c, d, e) SEM images of the self-supported LiMn₂0₄-carbon cloth cathode and corresponding elemental mappings of O and Mn;

Figure 70A shows EIS plots for a 31 m KAc electrolyte at four different temperatures, alongside the EIS plot for 0.1 m KC1 at room temperature for reference;

Figure 70B shows EIS plots for a 45 m KAc electrolyte at four different temperatures, alongside an EIS plot for 0.1 m KC1 at room temperature for reference;

Figure 70C shows LSV curves for the 31 m KAc electrolyte of Figure 70A and the 45 m KAc electrolyte electrolyte of Figure 70B at various temperatures at a 1 mV/s scanning rate; Figure 71 shows the results of stable window tests for 33 m LiN0₃-PVA and 15 m LiN0₃-PVA electrolytes at various temperatures at a 1 mV/s scanning rate;

Figure 72 shows EIS plots of (a) 33 m LiN0₃-PVA and (b) 15 m LiN0₃-PVA at various temperatures;

Figure 73 shows the results of stable window tests for 13 m LiCl-PVA and 10 m LiCl-PVA electrolytes at various temperatures at a 1 mV/s scanning rate; and

Figure 74 shows EIS plots of (a) 13 m LiCl-PVA and (b) 10 m LiCl-PVA at various temperatures.

Herein, if PVA is specified to be present and not quantified, 10 wt.% PVA was used (as a percentage of solvent weight). In the following description, like or corresponding reference numerals are used for like or corresponding features.

Figure 1 illustrates an electrochemical set-up 100 used for testing of various electrolytes 103 described in this application, for example to make LSV and CV measurements. The cell 100 comprises a working electrode 105 and a counter electrode 104. In some embodiments, both the working 105 and counter electrodes 104 are made from the same material. In various embodiments, the working 105 and counter electrodes 104 are made from titanium mesh, platinum mesh, stainless steel mesh, aluminium foil, or carbon cloth; the skilled person would appreciate that any suitable electrode material known in the art may be used in additional or alternative embodiments. A silver/silver chloride (Ag/AgCl,) electrode was used as a reference electrode 102 in the embodiments described. The skilled person would appreciate that a different reference electrode, or no reference electrode, may be used in other embodiments. A potentiostat 101 was used to measure the potential across the working and counter electrodes. In the embodiments described below, a Solartron^® 1287 Electrochemical Interface (Solartron Analytical, UK) was used as the potentiostat 101; in alternative or additional embodiments, different apparatus may be used. In the embodiment being described, the electrolyte 103 is contained within a beaker 108, and a lid 106 (in this case, a PTFE plate, although the skilled person would appreciate that any suitable lid, plate or film may be used) is provided to seal the beaker 108 and provide entry points for the electrodes 102, 104, 105. In the embodiment being described, the beaker 108 is placed on a hotplate 107 to facilitate thermal testing, as described below. Insulation (not shown) for the beaker 108 may be provided.

The skilled person would appreciate that the cell arrangement 100 shown in Figure 1 may be most easily used with a liquid electrolyte 103, and that alternative arrangements may be preferable for solid or quasi-solid electrolytes (e.g. gels). In various embodiments discussed below in which the electrolyte 103 takes the form of a solid or quasi-solid gel 103’, the arrangement 100’ shown in Figure 54 was used. To take electrochemical impedance spectroscopy (EIS) measurements, and the likes, the electrolyte gels 103’ were moulded into disks 103’ which were sandwiched between carbon cloths 104’, 105’, which served as current collectors and electrodes. The resultant layered disks 100’ were held immobilised in a jig for the EIS measurements, which were taken using a Solartron 1287 Electrochemical Interface in the embodiments described. A Solartron 1250 Frequency Response Analyser (FRA) was used for conductivity measurements.

The embodiments described herein extend the electrochemical window as compared to known electrolytes by use of a saturated aqueous salt solution as an electrolyte 103. In some embodiments, the solution 103 is an over-saturated aqueous solution, i.e., a small amount of undissolved salt is present with the saturated aqueous solution. The skilled person would appreciate that the amount of undissolved salt present may depend on the extent of oversaturation of the solution and on the salt (e.g. the solubility difference between the selected high temperature at which the solution is created and room temperature), but that amounts may be selected such that the ionic conductivity of the solution remains higher than 10 ⁴ S/cm, and optionally higher than 10 ³ S/cm. When a gel electrolyte 103’ is used, the undissolved salt may be held in place within the gel. When a liquid electrolyte 103 is used, the undissolved salt may be suspended within the liquid, or may float or sink depending on size and density.

It has been found that the electrochemical window for a saturated or over-saturated aqueous salt solution 103 is wider than for an unsaturated aqueous solution of the same salt(s). For example, when the scanning rate was 1 mV/s, it was found that the electrochemical window for a (unsaturated) 1 m LiC10₄ aqueous solution was 1.2 V, but increased to 2.0 V for a (super-saturated) 10 m LiC10₄ aqueous solution (see Table 1, below). Due to the strong solvation of both cations and anions from the dissolved and ionised salts, the over-potential for electrolysis of water for evolution of H₂ and 0₂ may be increased when the solution 103 is saturated or over-saturated, effectively supressing the electrolysis of water, and increasing the electrochemical window. This mechanism was proposed by Fic et al. (K. Fic, G. Lota, M. Meller, E. Frackowiak, Energy & Environmental Science 5, 5842-5850 (2012)).

In the work discussed herein, it was found that the addition of a water-absorbent polymer, such as polyvinyl alcohol (PVA), to the saturated or over-saturated salt solution can further expand the electrochemical window. For example, the electrochemical window for a saturated LiC10₄ aqueous solution is 2.0 V, whist it is 2.6 V for a saturated LiC10₄ solution with addition of 10wt% PVA (see Table 1). In the embodiments described herein, PVA with a molecular weight of between 10,000 and 100,000 a.m.u., and more specifically or around 50,000 a.m.u., was used. The skilled person would appreciate that the amount of PVA may be adjusted accordingly for different molecular weights. In the embodiments described herein, the PVA was 98 to 99% hydrolysed. The skilled person would appreciate that the amount of PVA may be adjusted accordingly for different levels of hydrolysis, as this may affect the solubility and/or water absorption performance of the PVA. In the embodiments described herein, 10 wt.% of PVA was used (i.e. lOg PVA per 100 g water), unless otherwise specified. Here, the weight percentage of the water-absorbent polymer means the mass of dissolved polymer in 100 g of the solvent, which is water in the embodiments being described. The solubility limit of the PVA used in the embodiments being described in water at 95 °C is around 25g in lOOg water and a minimum of 5 wt.% PVA was found to be needed to obtain a homogeneous gel. However, a functional electrolyte showing the benefits of the addition of PVA could be obtained with as little as 1 wt.% PVA. The skilled person would appreciate that a minimum quantity of the water absorbent polymer needed for the benefits described herein would vary, depending on the polymer itself.

In Table 1 and Table 2, electrochemical windows and ionic conductivities for a variety of candidate electrolyte solutions 103 are listed. Wider electrochemical windows were seen for higher concentration solutions of the same salt, and on the addition of a water-absorbent polymer to the solution. It is theorised that the addition of a water-absorbent additive, such as certain polymers including PVA, may effectively decrease the activity of water molecules, as the water molecules are ‘dragged’ by the polymer or other water-absorbent chemicals (e.g. silica gel, CaCl₂, Na₂S0₄, CaS0₄, molecular sieves, montmorillonite etc.); the reactivity of the water molecules may decrease accordingly, so increasing the over-potential for electrolysis of water to release ¾ and 0₂, so expanding the electrochemical window. The water absorbent additive may also compete with the salt - reducing the amount of salt required for saturation of the solution, thus reducing the usage of salts whilst keep the solution saturated or super-saturated. The ionic conductivities of the measured electrolytes listed in Table 1 are generally in the range of 10 ⁴ to 10 ² S/cm - high enough to be used as electrolytes for batteries or supercapacitors or cells for electrochemical synthesis.

An expanded electrochemical window was observed for a saturated aqueous LiC10₄ solution 103 upon the addition of PVA. In the embodiment being described, the salt is lithium perchlorate (LiC10₄). The skilled person would appreciate that other salts may be used, in addition to or instead of lithium perchlorate, in other embodiments. For example, in other embodiments aqueous electrolytes 103 containing any soluble organic or inorganic salts may be used. The skilled person will appreciate that a salt comprises a cation and an anion. The cation is a metal cation in most of the embodiments described herein, but non-metallic cations (such as NH₄ ⁺) may be used in some embodiments. For example, the salts used in various embodiments may include one or more of Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Fe²⁺, Fe³⁺, Zn²⁺, Bi³⁺, Ni²⁺, NH₄ ⁺ cations.

The anions for these salts, in various embodiments, may include one or more of S0₄ ² (sulfates); C10₄ (chlorates); N0₃ (nitrates); P0₄ ³ (phosphates); CT, Br , I (halides); Ac (acetates), trifluoro methane sulfonate (Tf); bis(trifluoromethane sulfonyl)imide (TfSI), bis(fluorosulfonyl)imide (FSI); tetrafluoro-phosphate (BF₄); hexafluorophosphate (PF₆); bis(pentafluoroethane sulfonyl) imide (BETI); 4,5-dicyano-2-trifluoromethanoimidazole (DCMI); [fluoro(nonafluorobutane) sulfonyl] imide (FNF); fluorinated acetate anions, such as trifluoroacetate (AcF); an organic anion with a chain length less than or equal to six, e.g. an acetate anion, or the likes. For example, a metal acetate may be used, for example a Group I or Group II metal acetate (e.g. KAc, CsAc) or an acetate salt of another metal anion (e.g. Zn²⁺, Al³⁺, Fe³⁺). In the embodiments being described, the metal for the metal cation is selected from metals having a negative standard electrode potential (relative to the standard hydrogen electrode) when in their elemental form, e.g. the half-cell reaction electrode potential for K⁺ +e K (for potassium) is negative (i.e. smaller than zero), making K a member of that group of metals, or the equivalent half-cell reaction electrode potential for other metal cations is negative. In one embodiment, the anion of the salt is HO ions. In this embodiment, the electrolyte is an alkaline solution.

In embodiments using metal acetates, the skilled person would appreciate that generally metal acetate with large cations (such as Na⁺, K⁺ or Cs⁺ ions), or more highly charged cations (such as Mg²⁺, Zn²⁺ ions), will have relatively high polarisation (by comparison to smaller or less highly charged cations), and thus may absorb more water molecules around the cations. When water is split to form ¾ and 0₂, these water molecules separate from the polarised cations before splitting. The strong interaction between the polarised cations and the water molecules may therefore increase the amount of energy input needed to split the water. The over-potential for splitting or electrolysis of water therefore increases, thus extending the electrochemical window for the salt solution to an extent dependent on salt concentration. As indicated in Table 2 (below), the electrochemical window width for over saturated alkaline acetate aqueous solution is in the order of: LiAc<NaAc<KAc<CsAc. Over-saturated Mg(Ac)₂ and Zn(Ac)₂ aqueous solutions have wide electrochemical windows due to the cation charge being higher. In various embodiments, salts (acetate or non-acetate salts) with cations such as Li⁺, Na⁺, Mg²⁺, Ca²⁺, Al³⁺, Fe²⁺, Fe³⁺, Zn²⁺, Bi³⁺, Ni²⁺ etc. may be added into super-saturated acetate solutions, such as KAc, CsAc which have a wide electrochemical window. The added salt may introduce sufficient ionic conduction of the added cations, whilst the supersaturated KAc or CsAc may sustain the wide electrochemical window. This kind of bi-salt (two salts), or more generally multi-salt (two or more different salts), super-saturated solution may provide beneficial properties for electrolytes for batteries or supercapacitors or electrochemical cells

The skilled person would appreciate that combinations of different salts may be used in the same salt solution. A mixture of cations and of anions may therefore be present, forming a complex aqueous salt solution. In embodiments with mixed salt electrolytes, the salts are selected to be chemically compatible, and such that no insoluble precipitate is formed as a result of reactions (a precipitate of one or both salts may be present in over-saturated solutions in which the solubility limit is exceeded, but this precipitate is soluble in the solvent).

Table 1 - electrochemical windows and ionic conductivities for various aqueous electrolytes.

In Table 1, 10 wt.% PVA was used for all electrolytes for which PVA is mentioned. Similarly, 10 wt.% of PEG was used in the PEG-containing electrolyte. By contrast, just 0.3 wt.% poly(acrylic acid sodium salt) was used in the PAAS containing electrolyte. In Table 1, with the exception of the 1 m LiC10₄ solution and the solutions of organic salts, all the listed solutions are super-saturated.

In various embodiments, it was found that the electrochemical window of a mixed salt solution 103 may be wider than the electrochemical window for a saturated solution containing only one salt selected from the salts used for the mixed salt solution. For example, the electrochemical window of a saturated lithium trifluoro-methanesulfonate (LiOTf) aqueous solution with PVA is 3.1 V (from -1.1 to 2.0 V at 1 mV/s, as shown in Table 1, above), that of a saturated LiC10₄ aqueous solution with PVA is 2.6 V, whilst the electrochemical window of a saturated or over-saturated aqueous solution of LiOTf blended with LiC10₄ with the molar ratio of LiOTf to LiC10₄ of 1 : 10 with addition of PVA is 3.3 V; higher than for the electrolyte containing only one of LiOTf or LiC10₄. The use of a mixture of different salts with different anions, in this case with the same Li⁺ cations, may therefore effectively expand the electrochemical window. Use of a mixture of different salts with different cations, such as Li⁺ ions and Zn²⁺ ions, may also effectively expand the electrochemical window. Such an embodiment is described in more detail in Example 3, below.

In various embodiments, one or more“super absorbers” - i.e. chemicals with strong water absorption properties, for example having the ability to absorb as much as 100 to 1000, or optionally 500 to 1000, times their mass of water - are added to the electrolyte 103. One or more of the super absorbers may be a super-absorbent polymer (SAP). In embodiments using a SAP, the SAP may be the only polymer in the electrolyte, or may be used in combination with one or more other SAPs, and/or with one or more less-absorbent polymers (such as PVA). The use of a super-absorber may expand the electrochemical window and/or reduce the usage of inorganic salts (by decreasing the amount of salt per unit solvent volume required for a saturated or super-saturated solution).

As shown in Table 1, the addition of 0.3 wt.% (with respect to the solvent; here, water) of poly(acrylic acid sodium salt) to the LiC10₄-PVA increased the electrochemical window by 0.1 V. In various embodiments, one or more of the following SAPs is used: (i) a polyacrylate salt, optionally sodium polyacrylate (poly(acrylic acid sodium salt), also referred to as PAAS); (ii) poly acrylamide; (iii) a polysaccharide; (iv) a polypeptide; (v) polyacrylonitrile (PAN); or (vi) a suitable polyvinyl alcohol copolymer. The skilled person will appreciate that, whilst PVA, PEG and the likes are water-soluble polymers that can be used to form a gel electrolyte, they are not generally classed as SAPs as their water absorption ability is lower. Indeed, many SAPs may not dissolve in water, instead expanding as they absorb water without forming a solution - such SAPs can generally only be“diluted” with water until the maximum extent of their expansion from water absorption is reached, so a true solution of the SAP is not formed. Other SAPs may be soluble in water, forming a highly viscous solution. As the term“soluble” is used herein, if a substance is soluble in a solvent, the substance dissolves in the solvent to form a single phase (provided enough solvent is added, where“enough” may depend on temperature and/or other parameters). If a SAP takes up water and swells but remains identifiably a separate phase from the surrounding solvent, the SAP is not classed as soluble herein - instead of a single-phase solution, a suspension of water-swollen SAP molecules may be formed. Solubility may therefore be assessed by visual inspection, filtering, or any suitable method known in the art. Adding such a non-SAP water-soluble, water-absorbent polymer in addition to a SAP may further expand the electrochemical window of the electrolyte, however; this is thought to be due to the water absorption properties of the water-soluble, water-absorbent polymer (such as PVA).

Use of sodium polyacrylate provides an anionic polyelectrolyte with negatively charged carboxylic groups in the main chain, and sodium cations. The skilled person would appreciate that sodium-neutralised polyacrylic acids are the most common polyacrylate commonly used in industry, but that other polyacrylate salts such as potassium, lithium, zinc and/or ammonium acrylates may also be produced and serve as SAPs. It was found that adding a small amount of poly(acrylic acid sodium salt)

- e.g. less than or equal to 10 wt.% or optionally less than or equal to 5 wt.% - can significantly reduce the solubility of organic or inorganic salts in water whilst retaining the width of the electrochemical window. The use of a super-absorber may reduce the usage of salts in the electrolyte, thus reducing the cost for salts leading to reduced overall cost. In the embodiments being described, the quantity of added SAPs is less than 10 wt.% of the electrolyte 103, and typically between 0.01 - 5 % by weight, or 0.1 - 2 % by weight.

Other water superabsorbent materials may be used in the aqueous electrolyte 103, as well as or instead of the sodium polyacrylate, to reduce the quantity of the salt and therefore the cost. The skilled person would appreciate that, for different salts and different solvents, an appropriate amount of the SAP to add may vary. For example, for a 30 ml saturated LiC10₄ solution, it was found that, if the quantity of added sodium polyacrylate was less than ~0.1g, no LiC10₄ seeded out from the solution 103

- a small amount of sodium polyacrylate could be added to the saturated solution without forcing any of the salt out of solution. When the quantity of added sodium polyacrylate was increased, some LiC10₄ seeded out, showing that the added sodium polyacrylate reduces the solubility limit of LiC10₄ because both are competing for the solvent, which is limited in quantity. When more than 0.5g of sodium polyacrylate was added to a 30 ml solution, more LiC10 came out of solution and the ionic conductivity dropped by around two magnitudes. The low conductivity means that such a mixture of LiC10₄ - sodium polyacrylate may be unsuitable for use as the electrolyte 103 for batteries.

Some SAPs are hydrogels - hydrogels are macromolecular polymer gels comprising a network of crosslinked polymer chains. Hydrogels can be synthesised from hydrophilic monomers by either chain or step growth, using a functional cross-linker to promote network formation. The hydrophilicity of the components can provide strong absorption of water - hydrogels often have a net-like structure, and this structure, along with void imperfections, further enhance the hydrogel's ability to absorb large amounts of water via hydrogen bonding.

In Example 5 below, with addition of O. lg poly(acrylic acid sodium salt), the usage of LiC10₄ salt was reduced from 31.9g to 19.08g, saving 12.82g, or 40%. The electrochemical window expanded from 2.6 V for a saturated LiC10₄-PVA-H₂0 electrolyte to 2.7 V for saturated LiC10₄-PVA- poly(acrylic acid sodium salt) - H₂0 composite electrolyte (i.e. on addition of a suberabsorber). The ionic conductivities of these LiC10₄-PVA-H₂0 electrolytes with or without O. lg poly(acrylic acid sodium salt) were found to be similar. Due to the addition of poly(acrylic acid sodium salt), the solubility of LiC10₄ in water decreased whilst the electrochemical window was at least substantially unaffected (the window expanded by 0.1 V for a saturated aqueous LiC10₄ solution with PVA when sodium polyacrylate was added) therefore the electrolyte with 0.1 g poly(acrylic acid sodium salt) is still a saturated LiC10₄ solution although the usage of LiC10₄ has been reduced by 40% due to the reduction in the amount of solvent available to dissolve the salt when the SAP is present.

As for the use of a water-soluble, water-absorbent polymer such as PVA, the addition of a superabsorbent such as poly(acrylic acid sodium salt) can be extended to other aqueous salt solutions as described above for PVA and is not limited to lithium perchlorate solutions.

In various embodiments, polyacrylic acid salts other than sodium polyacrylate may be used - for example with other cations such as Li⁺, K⁺, NH₄ ⁺, Zn²⁺ etc. Further, the skilled person would appreciate that the use of SAPs is not limited to polyacrylic acid salts. Any other chemicals, e.g. silica gel, CaCl₂, Na₂S0₄, CaS0₄, molecular sieve, montmorillonite, or polymers/resins which can strongly absorb water may be used to reduce the solubility/usage of the main salt(s), thus potentially reducing the overall cost without sacrificing electrochemical window width and/or ionic conductivity.

Double-network (DN) gels have drawn attention as innovative materials having both a high water content (ca. 90 wt.%) and high mechanical strength and toughness (Jian Ping Gong, Soft Matter, 2010, 6, 2583-2590). DN gels are characterised by a network structure comprising two types of polymer components with opposed properties: the minor component is abundantly cross-linked polyelectrolytes (forming a rigid skeleton - the“first network”) and the major component is poorly cross-linked neutral polymers (forming a ductile substance - the“second network”). For DN gels synthesised under suitable conditions (such as polymer choice, atmosphere for the reaction, etc.), the DN gels are found to have strengths and hardnesses comparable to those of rubbers and soft load- bearing bio-tissues. Due to the excellent water absorbing capability and mechanical properties, the combination of double-network gels with salts may form a composite gel electrolyte with excellent mechanical strength - a self-supporting membrane electrolyte may therefore be made and used directly in electrochemical devices such as batteries or supercapacitors.

For all the aqueous electrolyte 103 embodiments described above, one of more inorganic or organic solvents may be added to further expand the electrochemical window, provided that these solvents are miscible with water. In embodiments using one or more organic solvents, the organic solvents are selected such that the salts have high solubility in both water and the selected organic solvents - thus the activity of water may be reduced or minimised in order to provide a wide electrochemical window. It may be desirable that the organic solvents themselves have a wide electrochemical window, to reduce the probability of, or avoid, decomposition when a large voltage is applied to the composite electrolyte.

In embodiments in which perchlorate salts are used as the electrolyte salt, the strong oxidation properties of perchlorate ions may place limitations on current collector materials - the current collectors for both electrodes 104, 105 should be chemically compatible with perchlorate ions, avoiding chemical reaction between current collectors and perchlorate ions under applied positive voltages. As the positive and negative electrodes 104, 105 swap during cyclic voltammetry and linear sweep voltammetry tests (and more generally during charging and discharging of an electrochemical device), the current collectors for both electrodes 104, 105 are selected to be stable over the expected range of applied voltages. It was found that stainless steel, carbon cloth and aluminium foil were not stable at the upper end of the expected ranges of potential, whilst titanium (Ti) foil (purity > 99.9%) or Ti mesh or Ti foam was stable at an applied voltage ranging from -0.5 to 2.0 V vs. Ag/AgCl reference electrode. The use of Ti foil or foam as current collector for a two electrode coin cell, battery, supercapacitor, or other electrochemical device may allow the device to be charged at a higher voltage to the maximum power and energy density without decomposition of the aqueous electrolyte. The skilled person would appreciate that requirements for current collectors of electrochemical devices with aqueous electrolytes may differ from those for non-aqueous electrolytes. For example, the current collector must be stable in contact with the salt, water and other additives (such as PVA) in the range of applied voltage, and in particular at applied high positive voltage. The pH value of the electrolyte may also be an important parameter in choosing suitable materials for electrode current collectors.

Cheap materials such as stainless steel foil, foam, or mesh may be used the current collectors in some embodiments if the surface is coated with a thin, dense layer of conductive protection materials such as metal, metal oxides, nitrides or carbides etc. Provided that these protection layers are stable in the electrochemical device operating environment, cheap stainless steel or the likes can be used as the substrate of the current collector.

The current collector for a high voltage aqueous electrolyte must be stable at high (positive electrode) or low (negative electrode) applied voltages. This is a challenge for high voltage electrochemical devices such as batteries and supercapacitors based on aqueous electrolytes or gel- electrolytes containing water. The skilled person would appreciate that“high” and“low” depend on the width and location of the electrochemical window for the electrolyte, and on the electrode materials used - the current collector should be selected not to necessitate a reduction in the useable voltage range insofar as possible.

Current collector stability at high applied positive voltage (oxidation) is even more important because most known suitable materials are unstable on oxidation (high positive voltage) and relatively more stable on reduction (low negative voltage). In the embodiments discussed herein, a stable metal such as titanium is used for the current collector. The current collectors can be in the form of Ti foil, foam, sheet or mesh. In addition to metals, electronically conductive metal oxides, nitrides, carbides, nitride-oxides, or carbide-oxides can be used for the current collectors. These ceramic materials are often fragile; a thin layer of the conductive ceramic may be coated on top of a substrate with better flexibility, such as a polymer or a composite, to provide improved structural properties. In alternative or additional embodiments, these conductive metal or ceramic materials can be mixed with a binder, typically a polymer, to form a conductive composite material meeting the requirements on stability, conductivity and mechanical strength for use as a current collector for high voltage electrochemical devices such as batteries and supercapacitors. The skilled person would appreciate that these conductive and stable current collectors may be used for any electrochemical devices, and are not limited to use with electrolytes comprising perchlorates, either with or without H₂0.

Example Embodiments:

Example 1 : LiC10₄ is commonly used in conventional organic electrolytes of lithium ion batteries. LiC10₄ is less expensive (£119.10/500g in Alfa Aesar with 98% purity) than various organic salts such as lithium bistrifluoromethanesulfonimidate (LiTfSI) (£121.70/50g in Alfa Aesar with 98% purity) and lithium trifluoro methane sulfonate (LiOTf) (£90.50/50g in Alfa Aesar with 97% purity) which have been used in aqueous electrolytes of lithium ion batteries (see L. M. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. L. Fan, C. Luo, C. S. Wang, K. Xu, Science 350, 938-943 (2015)), and may also provide an expanded electrochemical window for aqueous “water in salt” electrolytes with the formation of an electrode-electrolyte interphase. LiC10₄ was tested as a part of this work in the interests of reducing electrolyte cost; a LiC10₄-polymer-water composite electrolyte was formed.

Firstly, in order to compare the electrochemical window differences between electrolytes using an unsaturated LiC10₄ solution, a saturated LiC10₄ solution, and a saturated LiC10₄ solution with 10 wt.% PVA added, the following solutions were prepared:

• 1 m LiC10₄ aqueous solution (unsaturated);

• 10 m LiC10₄ aqueous solution (super-saturated); and

• 10 m LiC10₄ aqueous solution (super-saturated) with 10 wt.% PVA added.

Data for these solutions are provided in the first three rows of Table 1, above. These solutions were produced by dissolving 3.19g (1 m solution) or 31.9 (10 m solutions) of LiC10₄ (Alfa Aesar) in 30 ml deionised water to form the aqueous solutions. 3 g poly(vinyl alcohol) (PVA) (Alfa Aesar, medium molecular weight of 31,000-50,000, 98-99% hydrolysed) was then dissolved into the third solution. The mixture was stirred at 95 "C for five hours to obtain homogeneous aqueous electrolytes, which are gel electrolytes for those soultions including PVA. To characterise the electrochemical windows of these electrolyte solutions, platinum meshes, which had been ultrasonically treated in 2-propanol, were inserted into the aqueous solution or gel as the working and counter electrodes. An Ag/AgCl reference electrode was also inserted. A Solartron 1287 was then used to record linear sweep voltammetry (LSV) curves for the electrolytes 103; the results are shown in Fig. 2, Fig. 3 and Fig. 4 respectively. In these Figures, the area between the dashed lines is the stable electrochemical window for the scanning rates investigated - it can be seen that the gradient of the curves is very shallow (almost flat) in this region.

In Figure 2, the curve 201 for a 1 mV/s scanning rate shows less variation in current over the tested voltage range than the curves 202-205 for the higher scanning rates, with the curve 205 for the 50 mV/s scanning rate having the highest total variation in current (starting lowest, and ending highest). The intermediate curves 202-204 for scanning rates of 5, 10, and 20 mV/s respectively fall between these extremes. Within the stable electrochemical window, all five curves approximately overlie each other, showing consistent behaviour for the different scan rates. In Figures 3 and 4, similar trends are observed, with the curves 301-305 and 401-405 similarly being numbered consecutively in order of increasing scan rate. With a 1 mV/s scanning rate, it can be seen that the electrochemical windows of 1 m LiC10₄ and 10 m LiC10₄ solutions can reach 1.2 V (+0.3-1.5 V vs. Ag/AgCl) and 2.0 V (-0.3-1.7 V vs.Ag/AgCl), respectively, showing the increased window width when using a saturated solution. For the 10 m LiC10₄-PVA-water composite electrolyte, the electrochemical window was wider still, reaching 2.6 V (-0.9-1.7 V vs.Ag/AgCl).

For testing the ionic conductivity of the LiC10₄-PVA-water composite electrolyte, the aqueous gel was poured into mould to form a disk. Carbon cloth was then introduced around the gel to form sandwich-structure cells, with a layer of gel between two layers of carbon cloth, which were then immobilised in a jig. Electrochemical impedance spectroscopy (EIS) was then performed using a Solartron 1287/1250 with 10 mV amplitude and frequency range of 65535-0.1 Hz (65535 Hz being the highest available frequency for the Solartron 1250 FRA), and the results are shown in Fig.5 - the ionic conductivty of that electrolyte was found to be 7.3 x lO ³ S cm ¹.

Example 2: The data shown in Fig.4 and Fig.5 demonstrate that a LiC10₄-PVA-water composite gel can work as an electrolyte with a relatively high ionic conductivity and wide electrochemical window for aqueous lithium ion batteries. The skilled person would appreciate that other perchlorates such as NaC10₄, Mg(C10₄)₂ and Zn(C10₄)₂ could correspondingly be used in sodium ion batteries, magnesium ion batteries and zinc ion batteries, respectively.

Saturated aqueous solutions of NaC10₄, Mg(C10₄)₂ and Zn(C10₄)₂ were prepared by mixing 62.85 g NaC10₄ H₂0 (Alfa Aesar), 15.01 g Mg(C10₄)₂ (Alfa Aesar) and 13.88 g Zn(C10₄)₂ (Alfa Aesar) respectivly with 30 ml deionised water, into which 3 g of PVA had previously been dissolved to form a gel. The mixtures were stirred vigorously at 95^“C for various time periods (5 to 10 hours) to obtain homogeneous aqueous gel electrolytes on cooling to room temperature. To characterise electrochemical windows, platinum meshes, which had been ultrasonically treated in 2-propanol, were inserted into the gel as the working and counter electrodes. An Ag/AgCl electrode working as a reference electrode was also inserted. A Solartron 1287 was employed to obtain linear sweep voltammetry (LSV) curves for the gel electrolytes; the electrochemical windows of NaC10₄-PVA, Mg(C10₄)₂-PVA and Zn(C10₄)₂-PVA are shown in Fig.6, Fig.8 and Fig.10 respectively with their windows found to be 2.8 (-0.9-1.9 V vs.Ag/AgCl), 2.5 (-0.7-1.8 V vs.Ag/AgCl) and 2.0 V (-0.5-1.5 V vs.Ag/AgCl), respectvely, at a 1 mV/s scanning rate. In Figures 6, 8 and 10, the curves 601-605, 801- 805 and 1001-1005 are numbered consecutively in order of increasing scan rate.

To test the ionic conductivity of the NaC10₄-PVA, Mg(C10₄)₂-PVA and Zn(C10₄)₂-PVA composite electrolytes, the aqueous gels were poured into a mould to form disks. Carbon cloth was then introduced around the gel to form sandwich-structure cells, with a layer of gel between two layers of carbon cloth, which were then immobilised in a jig. EIS was then performed using the Solartron 1287 with a 10 mV amplitude and frequency range of 65535-0.1 Hz. The results are shown in Fig.7, Fig.9 and Fig.11, demonstrating ionic conductivities of 8.0>< 10 ³, 1.2>< 10 ³ and 1.6x l0 ³ S cm ¹ for the NaC10₄-PVA, Mg(C10₄)₂-PVA and Zn(C10₄)₂-PVA composite gel electrolytes, respectively.

Example 3: lithium bistrifluoromethanesulfonimidate (LiTfSI) was chosen as the (main) salt for the “water in salt” electrolyte of this embodiment due to its high solubility in water and desirable stability against hydrolysis. Once its concentration reaches 5 m, the water-in-salt definition applies, as the salt exceeds the solvent in this binary system by both weight and volume. A 5 m solution was used in this example to form the LiTfSI composite gel electrolyte with the addition of 10 wt.% PVA (with respect to the solvent). The electrochemical window and ionic conductivity of this composite gel electrolyte were tested and the results shown in Fig.12 and Fig.13. The electrochemical window is shown to reach 3.0 V (-1.5-1.5 V vs.Ag/AgCl) when the scanning rate is 1 mV/s and the ionic conductivity approached 3.7 x l0 ³ S cm ¹ at room temperature. In Figure 12, the curves 1201 -1205 are numbered consecutively in order of increasing scan rate.

In addition, different organic salts were then added to the LiTfSI composite gel electrolyte. In particular, two mixed salt solutions were prepared; one with the addition of lithium trifluoromethanesulfonate (LiOTf) and the other with the addition of zinc trifluoromethanesulfonate (Zn(OTf)₂). The concentration of both LiOTf and Zn(OTf)₂ was 5 m, and their electrochemical windows and ionic conductivities were characterised using the method described above. The results are shown in Fig.14 and Fig.15 for LiOTf and Fig.18 and Fig.19 for Zn(OTf)₂. In Figures 14 and 18, the curves 1401 -1405 and 1801 -1805 are numbered consecutively in order of increasing scan rate. The electrochemical windows of LiOTf and Zn(OTf)₂ were found to be 3.2 (-1.2-2.0 V vs.Ag/AgCl) and 2.6 V (-0.9-1.7 V vs.Ag/AgCl), respectively. The ionic conductivities of LiOTf and Zn(OTf)₂ were found to be 5.0 10"’ and 2.5 HP' S cm ¹, respectively. For comparison, 10 m lithium trifluoroacetate (LiC0₂CF₃) was used with 10 wt.% PVA to form a gel electrolyte and its electrochemical window and EIS data are shown in Fig.16 and Fig.17. In Figure 16, the curves 1601 -1605 are again numbered consecutively in order of increasing scan rate.

Although the electrochemical windows at 50 to 5 mV/s scanning rates are wide enough for use as an electrolyte - especially the window demonstrated at 5 mV/s, which can approach 2.7 V (-0.7-2.0 V vs. Ag/AgCl) - once the scanning rate is reduced to 1 mV/s, the window shrinks dramatically, demonstrating poor stability. Based on the comparision of different types of organic salts, LiOTf was chosen to be mixed with LiC10₄ in a 1 : 10 molar ratio, to determine whether the window of a pure LiC10₄ electrolyte could be expanded. It was found that using the mixed salts extended the window to 3.3 V (-1.0-2.3 V vs.Ag/AgCl) as shown in Fig.20, with l . l x l 0 ³ S cm ¹ ionic conductivty as shown in Fig.21. In Figure 20, the curves 2001 -2005 are numbered consecutively in order of increasing scan rate - the same applies to corresponding reference numerals for LSV curves in Figures 22, 24 and 26. The mixture of different salts was found to effectively expand the electrochemcial window.

Example 4: As discussed above, PVA can be used as a, or the, polymer component of the salt-polymer- water composite electrolyte, but other polymers may be used instead of, or as well as, PVA. In this example, polyethylene glycol (PEG) was used in the place of PVA. 31.9 g of LiC10₄ was dissolved in 30 ml deionised water to form a 10 m saturated aqueous solutions. After dissolving 3 g of PEG (Alfa Aesar, PEG 6000, average mol weight (based on OH value) 5400 - 6600) into the solution, it was found that the electrolyte still formed an aqueous solution rather than a gel.

For the electrochemical window test, the platinum meshes were inserted into the 10 m LiC10₄- PEG solution and the Solartron 1287 used to determine LSV curves of the electrolyte, as for the tests described above. For the ionic conductivity test, as the liquid electrolyte could not be molded into a disk, the electrolyte was tested with Pt meshes as electrodes 104, 105 inserted directly into the solution 103. The data from the electrochemical window and ionic conductivity testing of 10 m LiC10₄-PEG solution are shown in Fig.22 and Fig.23, respectively, indicating that the window of 10 m LiC10₄-PEG is 2.6 V (-0.9-1.7 V vs.Ag/AgCl) and the ionic conductivity is 5.7>< 10 ² S cm ¹. This experiment demonstrates that other water-absorbent polymers can also be used in the saturated or over-saturated aquoues or gel electrolytes for high voltage electrochemical devices.

Example 5: As discussed above, it was found that poly(acrylic acid sodium salt) can significantly reduce the solubility of organic or inorganic salts in water whilst the electrochemical window is retained, thus the quantity of salt used can be reduced, reducing the cost of preparing salt-polymer- water composite electrolytes. In this example, 19.08 g LiC10₄ and 0.1 g poly(acrylic acid sodium salt) (Alfa Aesar) were dissolved into 30 ml deionised water with 3 g PVA to replace the 10 m (31.9 g) LiC10 aqueous solution used in the previous Example.

Data concerning the electrochemical window and ionic conductivity of LiC10₄-poly(acrylic acid sodium salt)-PVA were collected via LSV and EIS tests using the Solartron 1287/1250, and the results are shown in Fig. 24 and Fig. 25. An electrochemical window of 2.7 V (-1.1-1.6 V vs.Ag/AgCl) was found at 1 mV/s scanning rate, and an ionic conductivity of 4.0>< 10 ⁴ S cm ¹ at room temperature. The cost for 0.1 g poly(acrylic acid sodium salt) is much less that that for the saved 12.82g LiC10₄ salt. The skilled person would appreciate that a similar strategy can be applied to other aqueous electrolytes/gel-electrolytes containing water.

Example 6: Although the electrochemical window of a ZnC10₄-PVA composite electrolyte is 2 V at a 1 mV/s scanning rate, ZnS0₄ is cheaper and so its viability was investigated. The cost for 500 g of each from Alfa Aesar is £91.70 for ZnC10₄-7H₂0 (Reagent Grade), and £12.10 for ZnS0₄ 7H₂0 (98%). ZnS0₄-7H₂0 (Alfa Aesar) was dissolved into 30 ml deionised water at 95 °C with 3 g PVA to obtain a super-saturated ZnS0₄ aqueous solution to test whether or not a wider electrochemical window than that of 4 m ZnC10₄-PVA could be obtained. Electrochemical window and ionic conductivity data of 4 m ZnS0₄-PVA were collected via LSV and EIS tests using the Solartron 1287/1250, and the results are shown in Fig.26 and Fig.27 - a 2.3 V (-0.7-1.6 V vs.Ag/AgCl) electrochemical window was demonstrated at a 1 mV/s scanning rate and 6.9>< 10 ³ S cm ¹ ionic conductivity at room temperature.

Example 7: Due to the strong oxidation properties of perchlorate ions, the current collectors for both electrodes are selected to be chemically compatible with perchlorate ions when perchlorate salts are used as the electrolytes. The current collectors are selected to be able to sustain the applied high voltage at the positive electrode and low voltage at negative electrode. The“high” and“low” voltage are determined based on the electrode materials and the stable electrochemical window of the electrolyte.

In this example, A1 foil, carbon cloth, stainless steel mesh and Ti foil were tested as current collectors, and inserted into a LiC10₄-PVA electrolyte as working and counter electrodes. An Ag/AgCl reference electrode was used to form a three-electrode cell and obtain cyclic voltammetry (CV) curves. In the embodiment being described, 30 loops were performed using the Solartron 1287 with the applied voltage ranging from -0.5 to 2.0 V, except for the cell with stainless steel mesh, for which the applied voltage ranged from -0.5 to 1.7 V. In Fig.28 and Fig.29, peaks with high current (above 1 mA) can be seen when the voltage approaches -0.5 V and 2.0 V for the cells with A1 foil and carbon cloth, while in Fig.30, peaks of oxidation and reduction of steel mesh can be seen around 0.2 and 1.2 V. For Ti foil, in Fig.31 , no distinct peaks are visible within the applied voltage range; the current for that voltage range was below 0.3 mA.

In addition, a Ti foil|LiC10₄-PVA|Ti coin cell was assembled and underwent a 100 loop CV test from -0.1 to 2.4 V, with the results shown in Fig. 32. No distinct peaks are visible, and no current over 0.1 mA was detected; this demonstrates the stability of Ti foil against the LiC10₄-PVA electrolyte. It has been found that the leaking current became smaller upon cycling, indicating that a more stable product (such as titanium oxide) was formed on the surface of Ti foil, making it even more stable on cycling.

Example 8: Three different LiC10₄-PVA-H₂0 electrolytes were then prepared and tested to investigate the effect of saturation:

• An unsaturated 1 m LiC10₄-PVA-H₂0 solution prepared at room temperature;

• A saturated 6 m LiC10₄-PVA-H₂0 solution prepared at room temperature; and

• A supersaturated 10 m LiC10₄-PVA-H₂0 electrolyte prepared at 95 °C and then cooled to room temperature.

Each of these three electrolytes contained 10 wt.% PVA and formed a gel. Figure 44 shows the effect that temperature has on the electrolyte - it shows the saturated 10 m LiC10₄-PVA-H₂0 solution at 95 °C (left) and the supersaturated 10 m LiC10₄-PVA-H₂0 gel electrolyte at 25 °C after cooling to room temperature (right). This gel-forming process is reversible; the electrolyte re-liquefies when heated. In the embodiment being described, the liquid electrolyte is transparent at 95 °C but translucent/opaque as a gel at 25 °C.

Figure 45 illustrates results of electrochemical window characterisation of the three electrolytes, which was carried out at room temperature, showing LSV curves demonstrating the electrochemical window of LiC10₄-PVA-H₂0 gel electrolytes at room temperature for unsaturated lm LiC10₄-PVA-H₂0, saturated 6m LiC10₄-PVA-H₂0 prepared at room temperature, and supersaturated 10m LiC10₄-PVA-H₂0 gel electrolytes prepared at 95 °C and cooled down. The electrochemical window of the LiC10₄-PVA-H₂0 gel electrolytes was measured at room temperature. The electrochemical window for the unsaturated lm LiC10₄-PVA-H₂0 solution was found to be from -0.6 V to +1.7 V, i.e. a window width of 2.3 V. For the saturated 6 m LiC10₄-PVA-H₂0 solution, the electrochemical window was found to be from -1.1 V to 1.3 V, i.e. a window width of 2.4 V, which is an increase of 0.1 V as compared to the unsaturated solution. For the supersaturated 10 m LiC10₄- PVA-H₂0 solution, the electrochemical window was found to be from -1.4 V to +1.9 V, i.e. a window width of 3.3 V, which is an increase of IV as compared to the unsaturated solution, and of 0.9 V as compared to the 6 m saturated solution. The supersaturated 10 m LiC10₄-PVA-H₂0 therefore has a much wider electrochemical window than either of the other solutions tested in this Example.

Example 9: Various acetate salt-based electrolytes were made and characterised. The electrochemical window and conductivity of the acetate-based electrolytes were measured as described above, and the results are provided in Table 2 (below). It can be seen that electrochemical windows with widths of around 3.0 V were recorded for various electrolytes; the skilled person would appreciate that this is relatively broad for aqueous electrolytes, as known aqueous electrolytes generally have a window width of 1.6 V or below. Of the acetate salt electrolytes tested, KAc demonstrated promising properties. CsAc is generally more expensive than KAc, but showed better performance with a wide electrochemical window. Mg(Ac)₂ and a mixed-salt electrolyte comprising KAc+ Li₂S0₄ also demonstrated promising properties. The skilled person would appreciate that the addition of Li₂S0₄ means that the KAc- Li₂S0₄ -H₂0 electrolyte contains and conducts Li⁺ ions, which may be of particular utility for lithium ion batteries, and that the same approach could be taken with salts containing Na⁺, Zn²⁺, Mg²⁺, Al³⁺, K⁺, Fe³⁺, Fe²⁺, Ni²⁺, Bi³⁺ in a saturated metal acetate solution (e.g. a KAc solution) to for corresponding metal ion batteries.

Lithium trifluoroacetate (LiAcF) also showed strong performance; lithium trifluoro acetate mono hydrate was used as the precursor in preparing the supersaturated solution of KAc plus 10 m LiAcF. Adding poly(acrylic acid sodium salt) was found to make the acetate electrolyte unstable, unlike the perchlorate salt based electrolytes, and so is not recommended for acetate electrolytes.

The skilled person would appreciate that a variety of different metal ions may be used in the acetate salts, not limited to Group I or Group II metals. The metal ion type may be selected based on its usability for batteries (or other electrochemical devices). In the embodiments being described, the metal of the metal cation of the metal acetate is selected from the group of metals having a negative standard electrode potential (relative to the standard hydrogen electrode) when in its elemental form (e.g. for the half-cell reaction K⁺ +e K for potassium).

In various embodiments, mixed salt electrolytes are used, for example adding another acetate salt with different cations from the first acetate salt (e.g. adding Zn(Ac)₂ into a supersaturated KAc solution), adding a sulphate, optionally with a different cation from the first acetate salt (e.g. adding Li₂S0₄ into supersaturated KAc solution), or adding organic salts with anions listed above). In various embodiments, KAc or CsAc is chosen as the primary salt of the electrolyte due to the wide electrochemical window provided; a secondary salt with a specific cation suited to the intended use of the electrolyte may then be added.

Table 2 - characterisation of acetate electrolytes

Stable window at Electrochemical Ionic conductivity

1 mV/s (V vs. window width (S cm ¹)

Ag/AgCl) (V)

Where “m” represents the molality (mol-salt in kg-solvent) . The skilled person would appreciate that using molality - based on mass instead of on volume as for more conventional measures of solubility - is that precisely measuring the volume of the solvent, which changes with temperature, would be non-trivial whereas mass can easily be measured, and the mass of the salt treated as constant with temperature

All the materials listed in Table 2, except for those shown in the first and last rows (which had no stable window), may be used as the electrolyte 103 for a battery of an embodiment, as all of them have an electrochemical window (EV) wider than 2.0 V. The higher the EV, the higher the battery voltage, and the higher the power and energy density, can be.

Na, K, Cs, Mg, Zn acetates all showed good performance, with CsAc being best but also the most expensive. Considering both cost and the desire for a wide EV, an over-saturated KAc aqueous solution 103, optionally with the addition of one or more other salts such as LiAcF, Li₂S0₄, and/or Mg(Ac)₂ is recommended for high energy density batteries.

Results on the addition of lithium, zinc and magnesium salts to over-saturated aqueous KAc solutions are shown in Table 2. Other potassium or sodium salts, such as K₂S0₄, and/or Na₂S0₄ could be added, although the skilled person would appreciate that adding a different potassium salt into an over-saturated KAc may not be as beneficial as using a different metal, as the KAc itself will conduct K⁺ ions; whereas adding Na₂S0₄ would form a composite electrolyte with a wide EV which could be used as electrolyte 103 for sodium batteries. Figure 33 shows EIS data for cells 100 using Pt mesh electrodes and each of the electrolytes listed above, at room temperature - the conductivities listed above were calculated using these data.

Figure 34 shows LSV curves (-1.5 to 2 V vs. Ag/AgCl) for the 31 m KAc aqueous gel with various scanning rates (curves numbered consecutively in order of decreasing scan rate for this Figure and all LSV curve figures for electrolytes tabulated above). Figure 35 shows LSV curves (-2 to 2 V vs. Ag/AgCl) for the 47 m CsAc aqueous gel with various scanning rates. Figure 36 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of 7 m Mg(Ac)₂ aqueous gel with various scanning rates. Figure 37 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of 10 m LiAcF+31 m KAc aqueous gel with various scanning rates. Figure 38 shows LSV curves (-2 to 2 V vs. Ag/AgCl) of 1 m LiAc+31 m KAc aqueous gel with various scanning rates. Correspondingly, Figures 39 to 43 show LSV curves (-2 to 2 V vs. Ag/AgCl) with various scanning rates for: 1 m LiN0₃+31 m KAc aqueous gel; 1 m Li₂S0₄+31 m KAc aqueous gel; 1 m Mg(Ac)₂+31 m KAc aqueous gel; 1 m Mg(Ac)₂+31 m KAc aqueous gel; and 1 m Zn(Ac)₂+3 l m KAc aqueous gel. A method 4600 of preparing a supersaturated composite electrolyte as disclosed herein is shown in Figure 46. The method 4600 comprises heating 4602 a solvent to a first temperature. The first temperature is selected to be above an expected maximum working temperature of the electrolyte such that a saturated solution at the first temperature will remain saturated or supersaturated during use of the electrolyte, across the expected temperature range. The first temperature is near, but below, the solvent’s boiling point in the embodiments being described. In the embodiment being described, the solvent is water. In alternative embodiments, the solvent may comprise water mixed with one or more other solvents. In the embodiment being described, the temperature is raised to 95°C. In alternative embodiments, a different temperature, for example between 80°C and 100°C, may be selected.

In embodiments in which the solvent comprises one or more component other than water, the selected temperature may be adjusted in accordance with the boiling point of the azeotrope formed. The addition of one or more organic solvents (for example 5% ethanol, 95% water) to the water may reduce the boiling point as compared to that of pure water; therefore the high temperature selected for preparation of the saturated solution may be selected to be lower than for pure water, to avoid or reduce the loss of the solvent(s) by evaporation. In embodiments in which one or more organic solvents is used, the organic solvent(s) are selected to be miscible with water, and not to react with the salt(s) or polymer (e.g. SAPs) in the composite gel electrolytes. For example, PVA can dissolve in hot water and in the organic solvent dimethyl sulfoxide (DMSO), and PVP can dissolve in tetrahydrofuran (THF) - these solvents may therefore be suitable for mixing with water in electrolytes containing these polymers. The skilled person would appreciate that many polymers dissolve in alcohols, and that alcohols may therefore also be suitable choices for some electrolytes.

The temperature of the solvent is then maintained 4604 whilst further steps are performed. The maintained (first) temperature may be referred to as a high temperature.

A water-absorbent polymer is added 4606 to the solvent. In the embodiment being described, a set amount of the water-absorbent polymer is dissolved into the solvent. In alternative or additional embodiments, a sol may be formed. The set amount of the polymer is selected such that a gel can be formed (either at the high temperature or once cooled, as discussed below). In various embodiments, the amount of the water-absorbent polymer is between 0.1 wt.% and 40 wt.%, and optionally between 0.1 wt.% and 30 wt.%. If the water-absorbent polymer is not a SAP (being e.g. PVA), between 1 wt.% and 30 wt.%, optionally between 5 wt.% and 30 wt.%, and optionally around 10 wt.% of the polymer may be used. If the water-absorbent polymer is a SAP, between 0.01 wt.% and 5 wt.%, optionally between 1 wt.% and 5 wt.%, and optionally around 1 wt.% of the polymer may be used.

In the embodiment being described, the water-absorbent polymer is PVA (which is generally not classed as a SAP unless in a cross-linked, co-polymer form) and the amount added is around 10 wt.%; the skilled person will appreciate that different polymers may be used in alternative or additional embodiments, as described above - for example, polyvinyl pyrrolidone (PVP), polypropylene alcohol (PPA), or polyethylene glycol (PEG) may be used instead of, or as well as, PVA.

A salt is added 4608 to the solvent. The salt is dissolved into the solvent until no more salt will dissolve; i.e. until a saturated solution has been formed. In the embodiment being described, a single salt is added. In alternative embodiments, two or more different salts may be added, so forming a mixed salt solution. In the embodiment being described, the salt is a metal chlorate salt and more specifically is LiC10₄. In alternative or additional embodiments, a different salt, such as a metal acetate salt, may be used. In the embodiments being described, the mixture is stirred whilst it is maintained at the high temperature, to facilitate the dissolving of the polymer and the salt and the formation of a homogeneous solution. In the embodiment being described, salt is added until a saturated LiC10₄ solution is formed; for LiC10₄, this is a ~10 m solution, which is a higher concentration than would be possible at room temperature as the solubility of LiC10₄ in H₂0 is higher at 95°C than at room temperature.

In the embodiment being described, sufficient polymer is added 4606 for the solution to become a sol or gel at the high temperature. In other embodiments, the solution may remain a liquid at the high temperature.

The solution is then cooled 4610. In the embodiment being described, the solution is cooled from the high temperature to room temperature. In embodiments in which the solution remained a liquid or sol at the high temperature, a gel may be formed as the solution cools to room temperature.

At room temperature, there was found to be a small amount of crystallised LiC10₄ homogeneously distributed throughout the LiC10₄-PVA-H₂0 gel electrolyte in the embodiment being described. The skilled person would appreciate that these LiC10₄ inclusions will dissolve in to the mixture to form a homogenous gel on re-heating to 95 °C.

The resultant LiC10₄-PVA-H₂0 gel electrolyte 103’ is an example of an‘over-saturated water- in-salt’ (OS-WiS) electrolyte 103. The electrochemical window of the over-saturated 10 m LiC10₄- PVA-H₂0 (at room temperature, the concentration of the dissolved LiC10₄ is less than 10 m and the remaining LiC10₄ is present as suspended inclusions) prepared by the method 4600 described above was found to be 0.3 - 0.4 V wider that the that for a saturated (at room temperature) 6 m LiC10₄-PVA- H₂0 gel electrolyte. The wider electrochemical window may provide a higher energy density because the energy density of a battery is proportional to the square of the voltage. The skilled person would appreciate that the same method 4600 may be applied to all suitable salts known in the art, including the organic salts mentioned herein (e g. acetates).

In the embodiment being described, the salt is added 4608 after the water-absorbent polymer has been added 4606. In alternative embodiments, the polymer may be added 4606 after the salt; in such embodiments, some of the salt may come out of the solution when the polymer is added. For example, in one embodiment, the solvent is heated 4602 to a temperature close to its boiling point (e.g. 95 °C) and as much of the salt as will dissolve is dissolved 4608 into the solvent to form a saturated solution at the high temperature. PVA is then added 4606 to the solution whilst at 95°C and dissolves therein. Other soluble polymers may be used instead of, or as well as, PVA in other embodiments. In the embodiment being described, sufficient polymer is added for a gel electrolyte to be formed at 95 °C. In alternative embodiments, such as embodiments in which less PVA is added, the electrolyte may remain liquid or become a sol at the high temperature, but become a gel on cooling. In further alternative embodiments, such as embodiments in which even less PVA is added, the electrolyte may remain liquid even after cooling. In some embodiments, a non-soluble water absorbent polymer, which may be a super-absorber such as various hydrogels, is additionally added to the solution, either whilst it is at 95 °C or during or after cooling. The solution is then cooled 4610 to room temperature (e.g. 25 °C). The prepared electrolyte is a super-saturated solution once cooled, as the solubility of the salt is lower at the lower temperature - the solution contains some undissolved salts. For gel electrolytes, small inclusions of undissolved salts are suspended within the gel. The skilled person would appreciate that the sizes of the inclusions may vary depending on one or more of the salts, the polymer, concentrations, the rate of cooling, the rate of stirring, and the likes.

The skilled person would appreciate that electrolyte performance may vary with temperature - for example due to temperature effects on diffusion and the solubility limit of the salt(s). Figure 63A shows LSV curves for a super-saturated 10 m LiC10₄-PVA electrolyte (saturated at 95°C) and a saturated 6 m LiC10₄-PVA electrolyte (saturated at room temperature) at four different temperatures - 80°C, 60 °C, 40°C, and room temperature - at a 1 mV/s scanning rate. It can be seen that the more concentrated / supersaturated solution has a broader EV and maintains its EV width better at higher temperatures. Figures 63 B and C provide EIS data for the same electrolytes and temperatures, illustrating that resistance (x-axis intercept) is generally lower at all temperatures for the LiC10₄-PVA electrolytes than for the KC1 reference, and decreases with increasing temperature. Figures 70A-C provide equivalent data for super-saturated 45 m and saturated 31 m KAc electrolytes, for reference.

Example 10: Supersaturated gel electrolytes formed from other salts such as nitrates and halides.

Figure 71 shows the electrochemical windows of a supersaturated 33 m LiN0_{3 -} PVA-H₂0 gel electrolyte (10 wt.% PVA with respect to water) and a saturated 15 m LiN0_{3 -} PVA-H₂0 gel electrolyte (10 wt.% PVA with respect to water) at room temperature, 40°C, 60 °C and 80 °C. At all temperatures, the electrochemical window of the super-saturated 33 m LiNi0₃-PVA-H₂0 gel electrolyte is shown to be wider than that of the saturated 15 m LiNi0₃-PVA-H₂0 gel electrolyte. EIS testing was performed using a Solartron 1287/1250 and the results are shown in Figure 72.

Figure 73 shows the electrochemical windows of a supersaturated 13 m LiCl-PVA-H₂0 gel electrolyte (10 wt.% PVA with respect to water) and a saturated 10m LiCl-PVA-H₂0 gel electrolyte (10 wt.% PVA with respect to water) at room temperature, 40°C, 60 °C and 80 °C. At all temperatures, the electrochemical window of the super-saturated 13 m LiCl-PVA-H₂0 gel electrolyte is shown to be wider than that of the saturated 10 m LiCl-PVA-H₂0 gel electrolyte. EIS testing was performed using a Solartron 1287/1250 and the results are shown in Figure 74. Table 6, below, summarises the data obtained for these electrolytes, and their preparation is discussed in more detail below.

Table 6 - Electrolyte electrochemical windows and conductivities

To prepare the super-saturated (at room temperature) L1NO₃-PVA (33 m), 10 wt.% PVA was added to 20 ml deionised water at 95 "C to form a sol. 45.5 g LiN0₃ (Alfa Aesar) was then dissolved in the sol, which was stirred for 5 hours. After cooling to room temperature, the quasi-solid-state electrolyte was formed, with the PVA causing the salt which crystallised out of the solution to be suspended and dispersed within the colloidal electrolyte. These transitions between gels and sols are reversible at 95 "C or at room temperature. Similarly, to prepare the 15 m LiN0₃-PVA“water-in-salt” electrolyte, sufficient salt was dissolved into the solvent for the solution to be saturated at room temperature - after cooling down to room temperature, the electrolyte therefore remained as a transparent gel (with no salt crystals as it is not super-saturated). LSV curves of both the super- saturated and saturated LiN0₃-PVA were measured at 80 "C, 60 "C, 40 "C and room temperature in a three-electrode system. The results are shown in Fig. 71, demonstrating that the super-saturated LiN0₃- PVA provides wider stable windows (from 2.1 V at 80 "C to 2.7 V at room temperature) than saturated LiN0₃-PVA (from 1.6 V at 80 "C to 2.3 V at room temperature). To get the ionic conductivity of the electrolytes at different temperatures, 0.1 m KC1 was tested at room temperature in a three-electrode system 100 as a standard sample, then the same set-up was used to test both electrolytes at different temperatures; the results are shown in Fig. 72. At room temperature or higher temperatures, the ionic conductivity of the super-saturated“water-in-salt” LiN0₃-PVA electrolyte (from 1.28x 10 * S cm ¹ at 80 °C to 2.51 x l0 ² S cm ¹ at room temperature) are lower than the saturated (but not super-saturated) “water-in-salt” LiN0₃-PVA (from 1.94x 10 ' S cm ¹ at 80 °C to 7.63 x lO ² S cm ¹ at room temperature), because the crystallised salts included within the super-saturated gel are usually insulators. However, the ionic conductivity for the super-saturated electrolyte remained above l.Ox lO ² S cm ¹ at every temperature due to the dispersion of the insulating crystallites allowing for many conductive pathways through the electrolyte. To prepare the super-saturated LiCl-PVA (13 m) solution, 10 wt.% PVA was dissolved in 20 ml deionised water at 95 °C to form a sol. 11.0 g LiCl (Alfa Aesar) was then added in the sol, which was stirred for five hours. After cooling to room temperature, the quasi-solid-state electrolyte was again formed, as described above. Similarly, to prepare the 10 m LiCl-PVA“water-in-salt” electrolyte, sufficient salt was dissolved into the solvent for the solution to be saturated at room temperature - after cooling down to room temperature, the electrolyte therefore remained as a transparent gel (with no salt crystals as it is not super-saturated). LSV curves of both super-saturated and saturated LiCl-PVA were recorded at 80 °C, 60 °C, 40 °C and room temperature in a three-electrode system 100. The results are shown in Fig. 73, in which the super-saturated LiCl-PVA is shown to provide wider stable windows (from 1.2 V at 80 °C to 2.2 V at room temperature) than the saturated (but not super-saturated) LiCl- PVA (from 1.1 V at 80 °C to 1.8 V at room temperature). To get the ionic conductivity of the electrolytes at different temperatures, 0.1 m KC1 was tested at room temperature in a three-electrode system 100 as a standard sample, then the same set-up was used to test the individual electrolytes at different temperatures; the results are shown in Fig. 74. Although, at room temperature or higher temperatures, the ionic conductivities of the super-saturated“water-in-salt” LiCl-PVA (from 1.26x l0^-1 S cm ¹ at 80 °C to 2.95 >< 10 ² S cm ¹ at room temperature) are lower than those of the saturated (but not super-saturated)“water-in-salt” LiCl-PVA (from 2.06x l0^-1 S cm ¹ at 80“C to 4.97x l0 ² S cm ¹ at room temperature), its ionic conductivity is still sufficient for use as an electrolyte.

Example 11 : In this example, use of an electrolyte as described above in a battery is described.

In the embodiment being described, relatively inexpensive inorganic perchlorate salts were used in combination with polyvinyl alcohol (PVA) as the electrolyte for the battery. An electrochemical window of 2.6 V was achieved using a super-saturated 10 m LiC10₄-PVA electrolyte (10 wt.% PVA). Zn(C10₄)₂ was then added to the LiC10₄-PVA electrolyte to introduce Zn²⁺ ion conduction. Following experimentation on relative ratios, a combination of 1 m Zn(C10₄)₂+10 m LiC10₄-PVA was found to give good performance when used as the electrolyte in an aqueous rechargeable Zn-Li hybrid battery. The wide electrochemical window was retained on the addition of the Zn(C10₄)₂.

Based on this novel and inexpensive 1 m Zn(C10₄)₂ +10 m LiC10₄-PVA gel electrolyte, a rechargeable hybrid battery was prepared. In the embodiment being described, zinc foil was used as the anode, and binder-free LiMn₂0₄ grown on carbon cloth was used as the cathode. The binders commonly used for integrating the electrodes, such as polyvinylidenefluoride (PVDF) and polytetrafluoroethylene (PTFE), are insulators and thus decrease the electronic conductivity of the electrode and shield the active surface of electrolyte (as discussed in J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X.W. Lou, Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage , Adv. Mater. 24 (2012) 5166-5180). To avoid the presence of binders, electrochemical deposition and hydrothermal synthesis were employed successively in the embodiment being described to obtain the self-supported LiMn₂0₄ cathode on the carbon cloth (as discussed in H. Xia, Q. Xia, B. Lin, J. Zhu, J.K. Seo, Y.S. Meng, Self-standing porous LiMn204 nanowall arrays as promising cathodes for advanced 3D microbatteries and flexible lithium-ion batteries , Nano Energy 22 (2016) 475-482). Therefore a binder-free LiMn₂0₄ cathode was used in the hybrid Zn-Li battery of this embodiment.

The hybrid battery was found to deliver a discharge capacity of 90.7 mAh-g ¹ and an energy density of 120.8 Wh-kg ¹ for >300 cycles, with a maximum discharge capacity and energy density of 120 mAh g-1 (based on active materials at cathode) and 150.1 Wh kg-1 respectively from the 200th to 250th cycle. A current density of 200 mA g ¹ was maintained during the cycling. The columbic efficiency was 86.7% when carbon cloth was used as current collector, but nearly 100% columbic efficiency was achieved when Ti foil was used as the cathode current collector. It was found that Ti foam is an excellent current collector for this type of battery. After cycling, no clear signs of dendrite formation were observed on the Zn anode, demonstrating that a common problem for rechargeable batteries with Zn metal anodes had been avoided.

The skilled person would appreciate that these results demonstrate that a promising hybrid aqueous battery for energy storage has been developed. A wide electrochemcial window can be achieved through the use of low cost inorganic salts, such as perchlorates, to be used for new batteries with high energy density. Further experimental details are provided below.

Experimental

Fabrication of“water-in-salt” electrolytes and self-supported LiMn₂0₄

The electrolyte (1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA) was prepared by dissolving

Zn(C10₄)₂-6H₂0 (Alfa Aesar, Reagent Grade) and LiC10₄ (Alfa Aesar, 98%) in deionised water to achieve the desired molality (moles of dissolved salts in 1 kg solvent) to obtain an over-statured aqueous solution. 10 wt.% poly(vinyl alcohol) (PVA) (Sigma-Aldrich, 99+% hydrolysed) was then added, and vigorous stirring at 95^“C performed for five hours to obtain a homogeneous quasi-solid-state aqueous electrolyte.

To prepare the self-supported LiMn₂0₄-carbon cloth cathode, a three-electrode electrochemical cell was used with a solution of 0.015 m Mn(00CCH₃)₂-4H₂0 (Sigma-Aldrich, Mn 22%) and 0.015 m Na₂S0₄ (Sigma Aldrich, >99.0%) as the electrolyte, a platinum mesh as the counter electrode and Ag/AgCl (in saturated KC1) as the reference electrode, to deposit the intermediate product Mn(OH)₂ on carbon cloth substrate at a constant potential of 1.4 V vs. Ag/AgCl for 900 s. After the deposition, the sample was rinsed with deionised water and dried in the oven at 50^“C overnight to convert the Mn(OH)₂ into Mn₃0₄ (see discussion in H. Xia, Q. Xia, B. Lin, J. Zhu, J.K. Seo, Y.S. Meng, Self-standing porous LiMn204 nanowall arrays as promising cathodes for advanced 3D microbatteries and flexible lithium- ion batteries , Nano Energy 22 (2016) 475-482).

The Mn₃0₄-carbon cloth was then immersed in 15 ml of a 0.02 m LiOH (Sigma-Aldrich, >98.0%) solution, and then transferred into a 50 ml Teflon-lined stainless steel autoclave and heated at 210 °C for 17 hours. After that, the sample was rinsed with deionised water and dried in a vacuum oven at 100 °C. The same process was used to prepare the LiMn₂0₄-Ti foil cathode.

Assembly of Zn \ l m Zn(ClO₄)₂ + 10 m LiCl0₄-PVA \LiMn₂0₄-carbon cloth full cell

Commercial zinc foil was polished using zinc powder (Alfa Aesar, median 6-9 micron, 97.5%) for around 15 minutes, followed by being washed with soap and deionised water, rinsed with 2- proponal and dried at 60 °C in a vacuum oven for three hours (see discussion in T.K.A. Hoang, M. Acton, H.T.H. Chen, Y. Huang, T.N.L. Doan, P. Chen, Sustainable gel electrolyte containing Pb2+ as corrosion inhibitor and dendrite suppressor for the zinc anode in the rechargeable hybrid aqueous battery , Materials Today Energy 4 (2017) 34-40).

Meanwhile, the“water-in-salt” electrolyte was heated on a hot plate at 95 °C so as to melt it sufficiently to flow, and then cast on glass microfiber filters (Whatman) to be converted into a quasi solid state on cooling. Both the Zn anode and the LiMn₂0₄-carbon cloth cathode were cut into round discs with 12 mm diameters using a precision disc cutting machine (Kejing, MSK-T10), while the electrolyte was cut into a 16 mm-diameter disc. Finally, the anode, cathode and electrolyte were assembled together in the CR2016 coin cell using a hydraulic crimping machine (Kejing, MSK-110).

Material Characterisation

A Thermo Scientific STAR A214 pH meter was employed to measure pH values of different aqueous solutions including dissolved salts at various concentrations.

X-ray diffraction (XRD) data was collected on a PANanalyticalX’Pert Pro in the Bragg- Brentano reflection geometry with a Ni-filtered Cu Ka source (1.5405 A), fitted with the X’Celerator detector and an Empyrean CuLFF XRD tube. Absolute scans in the 2Q range of 10-90° with step sizes of 0.0167° were used during data collection.

Scanning electron microscopy (SEM) measurements were carried out on a ZEISS SUPRA 55- VP Field Emission Scanning Electron Microscope equipped with an energy dispersive X-ray (EDX) spectrometer that allows elemental composition analysis. Thermal analysis was conducted using a NETZSCH STA 449 F3-Jupiter Thermal Analyser on heating from room temperature to 600 °C in air, with a heating rate of 10 °C/min and a flow rate of compressed air of 50 mi min ¹. FT-IR measurements were carried out on a Broker Vertex 70V IR spectrometer.

Electrochemical characterisation

To test the electrochemical windows of the electrolytes, two platinum meshes (ultrasonically washed in 2 -propanol) were inserted into the heated electrolyte as the working electrode and counter electrode, and an Ag/AgCl reference electrode was provided. A Solartron 1470E multichannel cell test system was employed to acquire linear sweep voltammetry (LSV) curves of electrolytes.

To test ionic conductivities of different electrolytes, heated electrolytes were poured into moulds to obtain discs. Carbon cloth was then introduced around the gel to form sandwich-structure cells, with a layer of gel between two layers of carbon cloth, which were then immobilised in a jig for electrochemical impedance spectroscopy (EIS) characterisation using an integrated Solartron 1455A frequency response analyser with 10 mV bias and 100 kHz-0.1 Hz frequency range.

To test the mobility of Zn²⁺ ion in the gel electrolyte, both Zn| l m Zn(C10₄)₂-PVA|Zn and Zn|0.1 m KCl|Zn were assembled in CR2016 coin cells and compared via EIS examination. The Zn|0.1 m KCl|Zn cell was prepared using the same techniques as detailed above. The assembled Zn| l m Zn(C10₄)₂+10 m LiC10₄-PVA|LiMn₂0₄-carbon cloth CR2016 coin cells was left for 8 hours following fabrication to allow equilibrium conditions to be attained. After that, both cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (0.6-2.1 V vs. Zn/Zn²⁺) measurements were carried out using a Solartron 1470E multichannel cell test system at room temperature. Results and discussion

Ionic conductivity and electrochemical window of perchlorate-based water in salt electrolytes

Sufficient ionic conductivity and a wide electrochemical window are essential requirements for the use of an electrolyte in batteries with high energy and power densities. For Li-0₂ batteries, the electrolytes containing LiC10₄ usually perform better (greater discharge capacity) than lithium bis(trifluoromethane sulfonyl)imide (LiTfSI)-containing electrolytes, which is generally attributed to the better solvent viscosity, oxygen solubility and stability against oxygen (see R. Younesi, G.M. Veith, P. Johansson, K. Edstrom, T. Vegge, Lithium salts for advanced lithium batteries: Li-metal, Li 0₂, and Li-S, Energy & Environmental Science 8 (2015) 1905-1922, G.M. Veith, J. Nanda, L.H. Delmau, N.J. Dudney, Influence of lithium salts on the discharge chemistry of Li-air cells, The Journal of Physical Chemistry Letters 3 (2012) 1242-1247, and R. Younesi, M. Hahlin, K. Edstrom, Surface characterization of the carbon cathode and the lithium anode of Li-02 batteries using LiCl04 or LiBOB salts, ACS Applied Materials & Interfaces 5 (2013) 1333-1341).

However, for aqueous rechargeable batteries (ARBs), LiC10₄ or any other perchlorates are seldom used as the electrolyte salts due to safety and toxicity concerns (see R. Younesi, G.M. Veith, P. Johansson, K. Edstrom, T. Vegge, Lithium salts for advanced lithium batteries: Li-metal, Li 0₂, and Li-S, Energy & Environmental Science 8 (2015) 1905-1922). In the embodiments being described, the use of PVA serves to retain the perchlorates in the quasi-solid-state electrolyte (see G. Wang, X. Lu, Y. Ling, T. Zhai, H. Wang, Y. Tong, Y. Li, LiCl/PVA gel electrolyte stabilizes vanadium oxide nanowire electrodes for pseudocapacitors, ACS Nano 6 (2012) 10296-10302).

In a similar way to LiTfSI, the higher the concentration of LiC10₄, the higher the viscosity of the solution - the skilled person would appreciate that the higher viscosity may lower the electrochemical activity of water thus expanding the electrochemical window of the aqueous electrolyte (see V. Kolosnitsyn, E. Kuzmina, E. Karaseva, Influence of lithium salts on physicochemical properties of lithium polysulphide solutions in sulfolane, ECS Transactions 19 (2009) 25-30, and C. Yang, L. Suo, O. Borodin, F. Wang, W. Sun, T. Gao, X. Fan, S. Hou, Z. Ma, K. Amine, K. Xu, C. Wang, Unique aqueous Li-ion/sulfur chemistry with high energy density and reversibility, Proceedings of the National Academy of Sciences of the United States of America 114 (2017) 6197-6202). In this example, it was found that, the electrochemical window of the 10 m LiC10₄ aqueous solution was 2.0 V (-0.3 to 1.7 V vs. Ag/AgCl) at a scanning rate of 1 mV/s (Fig. 52, 5201). After adding PVA, the electrochemical window of the 10 m LiC10₄-PVA gel electrolyte further expanded to 2.6 V (see Figure 47A). Figure 47 shows results of electrolyte testing, in particular of:

(A) an electrochemical window test on 10 m LiC10₄-PVA electrolyte under different scanning rates;

(B) the pH value of various aqueous solutions;

(C) an electrochemical window test on three different electrolytes (4.5 m Zn(C10₄)₂-PVA, 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA and 1 m Zn(C10₄)₂+ 10 m LiC10₄-PVA), at a scanning rate of with 1 mV/s; and

(D) CV curves of a Zn symmetric cell using the 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA electrolyte, tested at a 1 mV/s scanning rate.

Results of the electrochemical window measurement on the 10 m LiC10₄-PVA gel electrolyte at different scanning rates are shown in Fig. 47A. As the scanning rate was decreased from 50 to 1 mV/s, the window varied from 2.2 V (-0.7 to 1.5 V vs. Ag/AgCl) to 2.6 V (-0.9 to 1.7 V vs. Ag/AgCl), which is comparable to the performance of saturated LiTfSI-H₂0 electrolytes in terms of stability and rate capability (see L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, “Water- in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries, Science 350 (2015) 938-94), while the price of LiC10₄ is much lower than that of LiTfSI. Due to the rapidly increased viscosity after dissolving saturated LiC10₄ and PVA, the ionic conductivity of the 10 m LiC10₄-PVA electrolyte is 7.3 x lO ³ S cm ¹ at room temperature (obtained from the EIS data shown in Fig. 53a). The ionic conductivity of the 10 m LiC10₄-PVA gel electrolyte is also comparable with those of widely-used ceramic and polymeric solid-state electrolytes (see J.W. Fergus, Ceramic and polymeric solid electrolytes for lithium-ion batteries, Journal of Power Sources 195 (2010) 4554-4569).

In order to use the 10 m LiC10₄-PVA electrolyte in an aqueous rechargeable hybrid zinc battery, Zn²⁺ ion conduction was introduced. Owing to the similar chemical properties to LiC10₄, Zn(C10₄)₂ was added to introduce Zn²⁺ ions and combine with LiC10₄ to form a water in bi-salt (WiBS) electrolyte.

Previous work (see J.-Y. Luo, W.-J. Cui, P. He, Y.-Y. Xia, Raising the cycling stability of aqueous lithium-ion batteries by eliminating oxygen in the electrolyte, Nature Chemistry 2 (2010) 760) indicates that the pH values of aqueous electrolytes can significantly affect their performance. The concentration of the added Zn(C10₄)₂ was therefore adjusted based on the pH values of the aqueous solutions. As shown in Fig. 47B, the pH of saturated Zn(C10₄)₂ (4.5 m) and 1 m Zn(C10₄)₂ aqueous solutions were 1.53 and 2.99 respectively (Table 3, below, provides pH values as obtained using a pH meter). The acidic environment, formed due to hydrolysis of Zn²⁺ ions, will enhance the activity towards the hydrogen evolution reaction (see the paper of Luo et al., cited above). However, with the increase in LiC10₄ concentration, the pH increases accordingly, eventually reaching 4.92 which is deemed a weak acid. It has been reported that, with the addition of PVA, the electrolyte can approach neutrality (see G. Wang, X. Lu, Y. Ling, T. Zhai, H. Wang, Y. Tong, Y. Li, LiCl/PVA gel electrolyte stabilizes vanadium oxide nanowire electrodes for pseudocapacitors, ACS Nano 6 (2012) 10296- 10302). Therefore the activity for the hydrogen evolution reaction may decrease, thus expanding the electrochemical window.

Table 3 - pH measurements of aqueous electrolyte solutions

To further demonstrate the advantages of the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA gel electrolyte, 4.5 m Zn(C10₄)₂-PVA and 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA gel electrolytes were prepared for comparison. As shown in Figure 47C, the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte is stable within the voltage range of -1.0 to 1.45 V vs. Ag/AgCl when tested using linear sweep voltammetry (LSV) with 1 mV/s scanning rate. However, the other two electrolytes showed relatively narrow windows of -0.58 to 0.97 and -0.8 to 1.4 V respectively. The ionic conductivities are compared in Fig. 53, which shows data gathered using EIS tests - the electrochemical windows and ionic conductivities of these electrolytes are summarised in Table 4, below: Table 4 - Electrochemical windows and ionic conductivities of 4.5 m Zn(C10₄)₂-PVA, 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA and 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA electrolytes, each with 10 wt.% PVA.

Electrochemical window (vs. Ag/AgCl) at 1 mV/s Ionic conductivity

[V] [S cm ¹]

10 m LiC10₄-PVA -0.9 to 1.7 7.3 x lO ³ S cm ¹

4.5 m Zn(C10₄)₂-PVA -0.58 to 0.97 9.9x l0 ³

1 m Zn(C10₄)₂+5 m LiC10₄- PVA -0.80 to 1.40 9.1 x lO ³

1 m Zn(C10₄)₂+10 m LiC10₄- PVA -1.0 to 1.45 2.2x l0 ³

Figure 53 shows EIS plots for the four different electrolyte disks tested, which had various sizes:

(a) 10 m LiC10₄-PVA (1.5 cm diameter, 0.32 cm thickness);

(b) 4.5 m Zn(C10₄)₂-PVA (1.3 cm diameter, 0.5 cm thickness);

(c) 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA (1.3 cm diameter, 0.5 cm thickness); and

(d) 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA (1.3 cm diameter, 0.5 cm thickness).

The ionic conductivity of 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte is 2.2 x l0 ³ S cm ¹ which is slightly lower than that of the 4.5 m ZnC10₄-PVA (9.9 >< 10 ³ S cm ¹) and 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA electrolyte(9.1 ^c 10 ³ S cm ¹). However, it is still high enough for use as an electrolyte in batteries.

To prove the feasibility and reversibility of Zn plating/stripping with these electrolytes, Zn symmetric cells were assembled in CR2016 coin cells, and tested using cyclic voltammetry (CV) at a 1 mV/s scanning rate within the voltage range of -0.6 to 0.6 V vs. Zn/Zn²⁺. In Fig. 47D, the symmetric peaks of oxidation and reduction of the Zn symmetric cell with 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte from the first to the fifth CV scan can be observed at around 0.2 and -0.2 V, which demonstrates stable Zn plating/stripping. The other two electrolytes displayed unstable CV curves under the same circumstances, as shown in Fig. 55. In particular, Figure 55 shows: (a) CV curves of the Zn symmetric cell based on the 4.5 m Zn(C10₄)₂ electrolyte, at a 1 mV/s scanning rate; and (b) CV curves of the Zn symmetric cell based on the 1 m Zn(C10₄)₂ + 5 m LiC10₄-PVA electrolyte, at a 1 mV/s scanning rate.

Moreover, to demonstrate reversibility of Zn plating/stripping, the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA based Zn symmetric cell was characterised by galvanostatic cycling with a current density of 0.2 mA cm ² and potential limitation of -0.3 to 0.3 V vs. Zn/Zn²⁺ for 50 cycles. The average coulombic efficiency (CE) of that Zn symmetric cell within 50 cycles is 92% (derived from Fig. 56), while the real CE of Zn plating/stripping was characterised via CV tests from -0.6 V to 0.6 V vs. Zn/Zn²⁺ on the coin cell with Zn foil as the work electrode and Pt mesh as the counter and reference electrodes (the skilled person would appreciate that when Zn was used as counter electrode, it may be oxidised at high voltage thus the potential of Zn counter electrode may change and the measured CE be inaccurate. Using Pt as the counter electrode avoids this problem, so allowing a more accurate“real” CE to be measured). The CV curves and corresponding chronocoulometry curves of that coin cell are shown in Figure 57. In particular Figure 56 shows galvanostatic Zn stripping/plating in a Zn symmetric cell under 0.2 mA cm ² current density within 50 cycles, and Figure 57 shows: (a) the CV plot for Zn plating/stripping in a coin cell using Pt mesh as the working electrode and Zn as the reference and counter electrodes at a 1 mV/s scanning rate; and (b) the corresponding chronocoulometry curves. From Figure 57, it can be seen that the CE of the cell increases from 91.3% in first cycle to 95.1% in the fourth cycle, which is better than that of 80% for the Zn anode in conventional aqueous electrolytes, although slightly lower than the CE of a Pt/Zn cell with Zn(TfSI)₂ and LiTfSI mixed electrolyte (see F. Wang, O. Borodin, T. Gao,

X. Fan, W. Sun, F. Han, A. Faraone, J.A. Dura, K. Xu, C. Wang, Highly reversible zinc metal anode for aqueous batteries , Nature Materials 17 (2018) 543-549, and Y. Li, H. Dai, Recent advances in zinc air batteries , Chemical Society Reviews 43 (2014) 5257-5275).

Characterisation of cell components prior to assembly

Figure 48 shows results of electrolyte testing discussed in more detail below, in particular:

(a) FTIR spectra of the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte and the original chemicals (lines for the starting materials serving as reference);

(b) TGA analysis on the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte from room temperature to

600°C;

(c) an optical image of the electrolyte after tape casting on the glass microfibre filter; and

(d) an SEM image of the electrolyte surface with 37 ^c magnification.

In order to characterise the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte, Fourier transform infrared (FTIR) analysis was employed to identify its composition and stretching bands - i.e. absorption bands due to stretching vibrations (Fig. 48a). For comparison, all the starting materials used to make the electrolyte were also tested. In the curve for PVA powder, a broad band in the range of 3600-3100 cm ¹ is observed and ascribed to O-H stretching vibrations, which can also be seen in the curve of the electrolyte (due to the presence of water) as well as the curve of Zn(C10₄)₂-6H₂0 (due to the presence of water of crystallisation). The band for the PVA powder at 2950-2910 cm ¹ is attributed to the asymmetric and symmetric stretching modes of -CH₂- groups (see H. Liao, Y. Liu, Q. Wang, W. Duan, Structure and properties of porous poly(vinyl alcohol) hydrogel beads prepared through a physical chemical crosslinking method , Journal of Applied Polymer Science 135 (2018) 46402). The peaks at 1431 and 1338 cm ¹, which also exist in the curve for the electrolyte as two weak peaks, are attributed to the bending mode of -CH₂-. The peaks for Zn(C10₄)₂-6H₂0 and the electrolyte on C10₄ symmetric and asymmetric stretching band are present at -1050 cm ¹ and -1610 cm ¹ respectively (see Y. Chen,

Y.-H. Zhang, L.-J. Zhao, A TR-FTIR spectroscopic studies on aqueous LiCl0₄, NaCl0₄, and Mg(Cl0₄)₂ solutions , Physical Chemistry Chemical Physics 6 (2004) 537-542). For LiC10₄, these two peaks shift towards marginally higher wavenumbers. The shift of the peaks of LiC10₄ may be due to the absence of water when compared with Zn(C10₄)₂-6H₂0 and the water-containing electrolyte.

Thermal gravimetric analysis (TGA, 4805) and differential scanning calorimetry (DSC, 4806) analysis were used to characterise the thermal properties of the electrolyte. In Fig. 48b, the first peak of the DSC curve occurs at ~110 °C, which indicates the phase transition of the electrolyte from a quasi solid state to a liquid state; below that temperature the weight loss in the TGA curve is mainly caused by evaporation of water. Above that temperature, it can be seen from the TGA curve that the acceleration of weight loss is due to the decomposition of PVA. A further, faster weight loss and another peak of the DSC curve can be seen at around 220 °C, which confirms the decomposition of perchlorates (see T. Sheela, R.F. Bhajantri, P.M.G. Nambissan, V. Ravindrachary, B. Lobo, J. Naik, S.G. Rathod, Ionic conductivity and free volume related microstructural properties of LiCIO 4/PVA/NaAlg polymer composites: Positron annihilation spectroscopic studies , Journal of Non- Crystalline Solids 454 (2016) 19-30). By contrast, the fastest weight loss from the 10 m LiC10₄-PVA electrolyte (without ZnC10₄) happens at a higher temperature of around 280 °C (shown in Fig. 58, which presents data from a TGA test on the 10 m LiC10₄-PVA electrolyte from room temperature to 600 °C) due to the lower decomposition temperature of ZnC10₄ as compared to that of LiC10₄ (see S. Gordon, C. Campbell, Differential thermal analysis of inorganic compounds, Analytical Chemistry 27 (1955) 1102-1109).

An optical image of the quasi-solid-state electrolyte after tape casting on the glass microfiber filter and cutting into a round disc of 16 mm diameter is shown in Fig. 48c, whilst the scanning electron microscope (SEM) image of that electrolyte with 37 ^c magnification is shown in Fig. 48d. There are some dents on the surface of electrolyte visible in Fig. 48d. For SEM pictures with high magnification, it can be seen that there are some small pores (see Fig. 59a, an SEM image of electrolyte with a 25 pm scale bar marked), which could be attributed to the loss of water in the electrolyte under the high vacuum of the SEM measurement environment. The mappings of O, Zn, C and Cl elements (Fig. 59b, c, d and e) demonstrate the homogeneous distributions of these elements. The morphology and self- supported structure of the LiMn₂0₄-carbon cloth cathode characterised by SEM and energy dispersive X-ray (EDX) are shown in Fig. 49. Figure 49 shows SEM images of the self-supported LiMn₂0₄-carbon cloth cathode at various magnifications: (a) x 266; (b) x 1690; (c) x 3090; (d) x 21.69k; and (e) x 3090 (identical to (c), provided for comparison with (i) and (g)). Figure 49 also includes elemental mappings for oxygen (f) and manganese (g), for the region of the self-supported LiMn₂0₄-carbon cloth cathode shown in SEM image (e).

The low-magnification SEM images of the prepared LiMn₂0₄ grown on the carbon cloth demonstrate a uniform coating on the ordered woven structure (Fig. 49a). LiMn₂0₄ grown on the single carbon fibre is homogeneously distributed (Fig. 49b and c). The further enlarged SEM images with 21.69k X magnification in Fig. 49d shows the closely distributed particles with particle size from 100 to 500 nm. To analyse the composition of this structure, the part of cathode shown in Fig. 49c was selected for EDX testing.

According to the element mapping, both Mn and O are homogeneously distributed on the carbon fibre (Fig. 49e, f and g). The XRD pattern of the LiMn₂0₄-carbon cloth cathode is shown in Fig. 60, in which both phases of spinel LiMn₂0₄ (ICDD: 04-006-9472) and carbon in hexagonal crystal system (ICDD: 04-018-7559) can be observed and identified. This indicates that LiMn₂0₄ has been successfully grown on the carbon cloth. The surface of the polished Zn anode was also characterised by SEM in Fig. 61a, b and c with various magnifications (magnifications of 121, 478 and 600 times for (a), (b) and (c) respectively), and found to be smooth.

Performance of the hybrid Zn-Li battery

As the 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte can conduct both Li⁺ and Zn²⁺ ions, both ionic conductors can be used to make hybrid Zn-Li batteries. Unlike the traditional“rocking-chair” lithium- ion battery that works on the basis of the migration of Li ions between the cathode and anode, a mixed ion-battery involves the migration of one type of ion between the electrolyte and the electrodes. During repeated charging and discharging of mixed ion -batteries, the total concentration of mixed ions should remain constant to ensure the charge neutrality of the electrolytes (see L. Chen, Q. Gu, X. Zhou, S. Lee, Y. Xia, Z. Liu, New-concept batteries based on aqueous Li⁺/Na⁺ mixed-ion electrolytes , Scientific Reports 3 (2013) 1946). In the Zn| l m Zn(C10₄)₂+10 m LiC10₄-PVA|LiMn₂0₄-carbon cloth hybrid battery described herein, the well-established Li⁺ ion insertion/extraction occurs reversibly at the LiMn₂0₄ cathode, whereas Zn strips/plates at the Zn anode. Therefore, the electrochemical reversibility of this hybrid battery is determined by the CE of Zn stripping/plating (see the 2018 paper of Wang et al., cited above).

Due to the nature of the hybrid battery, the standard galvanostatic intermittent titration technique (GITT) cannot be employed for characterising the mobility of Zn²⁺ ions (see N. Zhang, F. Cheng, Y. Liu, Q. Zhao, K. Lei, C. Chen, X. Liu, J. Chen, Cation-Deficient Spinel ZnMn₂0₄ Cathode in Zn(CF₃S0₃)₂ Electrolyte for Rechargeable Aqueous Zn-Ion Battery , Journal of the American Chemical Society 138 (2016) 12894-12901), hence EIS was used to demonstrate the desired mobility of Zn²⁺ ions in the 1 m ZnC10₄-PVA gel electrolyte. Zn foils were used as the work, counter and reference electrodes in coin cells (Fig. 62a). For calibration, the impedance of a 0.1 m KC1 solution was measured using the same set-up with the same volume, and these results are plotted together. In the high frequency region of that EIS plot, the conductivity of the 1 m ZnC10₄-PVA gel electrolyte was determined to be 4.28 * 10 ² S cm ¹ , while in the low frequency region, the line presenting the process of Warburg semi-infinite diffusion provides information regarding the diffusion of Zn²⁺ ions in the interphase between Zn foils and the gel electrolyte.

According to the formula: D=R²T²/2A²n⁴F⁴C²o² - (l)

Zn²⁺ ion diffusion coefficient of the cell depends on the Warburg impedance coefficient, s, with R, T, A, n, F and C being the gas constant, absolute temperature, electrode area, electron number, Faraday constant, and molar concentration of ions, respectively, all of which are known s has a relationship with Z’ as shown in equation (2):

Z’=R+oco ^1/2 ... (2)

In equation (2), R is resistance, not the gas constant - R relates to the resistance of the electrolyte and of charge transfer. Through the interdependence between Z’ and root square angular frequency, w ^{1 2}, exhibited in Fig. 62b, s can be determined as 3136.8. From this, Zn²⁺ ion diffusion coefficient can be determined as 5.5 ^c 10 ¹² cm²-s⁴.

To determine the loading of active matter (in this embodiment, LiMn₂0₄) on the carbon cloth after electrochemical deposition and hydrothermal reaction, ten round disks of carbon cloth of the same size were processed using the same steps. The mass of active matter for the cathode was found to be 1.1 mg, with little variation between disks.

Figure 50 shows results from electrochemical testing, including: (a) CV curves of Zn foil and LiMn₂0₄- carbon cloth with Pt mesh as counter electrode and Ag/AgCl as reference electrode under 1 mV/s scanning rate; (b) charge-discharge curves of Zn/LiMn₂0₄ full cell with 200 mA g ¹ current between 0.6 and 2.1 V vs. Zn/Zn²⁺; and (c) cycling stability and discharge capacity of Zn/LiMn₂0₄ full cell with 200 mA g ¹ current between 0.6 and 2.1 V vs. Zn/Zn²⁺ for 300 cycles.

In Fig. 50, part (a), the CV curves of both the Zn anode and the LiMn₂0₄-carbon cloth were collected separately at a 1 mV/s scanning rate in a three-electrode cell, with 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA as the electrolyte, Pt mesh as counter electrode, and Ag/AgCl as reference electrode. The peaks of oxidation and reduction were found to occur at -0.95 and -0.70 V vs. Ag/AgCl (4.21 and 3.96 V vs. Li/Li⁺), indicating the insertion/extraction reaction of Li⁺ ions in LiMn₂0₄ (see J.-Y. Luo, Y.-Y. Xia, Aqueous lithium-ion battery LiTi2(P04)3/LiMn204 with high power and energy densities as well as superior cycling stability , Advanced Functional Materials 17 (2007) 3877-3884), while the other pair of redox peaks appear at— 0.75 and— 1.1 V vs. Ag/AgCl (-0.23 and— 0.12 V vs. Zn/Zn²⁺), representing Zn plating/striping (see W. Sun, F. Wang, S. Hou, C. Yang, X. Fan, Z. Ma, T. Gao, F. Han, R. Hu, M. Zhu, C. Wang, Zn/Mn02 battery chemistry with H⁺ and Zn²⁺ coinsertion , Journal of the American Chemical Society 139 (2017) 9775-9778).

In Fig. 50b and 50c, the galvanostatic charge-discharge curves (b) and corresponding cycling tests for the first to 300^th cycles (c) are shown, obtained at a current density of 200 mA g ¹ within the voltage range of 0.6 to 2.1 V vs. Zn/Zn²⁺. From the 1^st cycle to the 200^th cycle, the discharge capacity increased continuously from 34.5 to 120 mAh-g ¹ due to the increasing activation of the cathode in the process of charging and discharging. From the 200^th to the 250^th cycle, the discharge capacity and energy density (obtained by integrating the discharge curve) remained stable around 120 mAh-g ¹ and 150.1 Wh-kg ¹, whereas those of the 300^th cycle decreased to 90.7 mAh-g ¹ and 120.8 Wh-kg ¹, with 75.6% capacity retention compared to the max discharge capacity. The shrink of capacity after the 250^th cycle may be caused by the detachment of active matter from carbon cloth in the cathode, or by the broken current collector carbon cloth, which was observed in the SEM images in Fig. 69, with magnification of x 381 and x 1720 for Figure 69 (a) and (b) respectively.

It is noteworthy that a second charge plateau (~ 2 V) can be observed in the 200^th and 300^th charge-discharge cycles in Fig. 50b; this may be generated by the formation of Mn0₂ after repeating charge-discharge reactions and following the insertion/extraction of Zn²⁺. This hypothesis is supported by the XRD test on the cathode after cycling, shown in Fig. 64, in which Mn0₂ (ICDD: 00-050-0866) and Lii ₂₄Mni ₇₆0₄ (ICDD: 04-018-6436) were identified - the generation of Mn0₂ may therefore be the reason why capacity of the full cell increased until the 200^th cycle.

During the cycling test, the average CEs over the first 300 cycles was found to be around 86.7%; the skilled person would appreciate that this is not as high as current commercial Li-ion batteries, but better than traditional aqueous batteries with, for example, 2 m ZnS0₄ (75%), 2 m Zn(CH₃COO)₂ (80%) and 6 m KOH (50%) aqueous solutions as electrolytes (see the 2018 paper of Wang et al. cited above). The possible oxidation of carbon cloth as the substrate and current collector of the cathode under high voltage may account for the correspondingly lower CEs, compared with currently widely used Li-ion batteries. To test this hypothesis, a LiMn₂0₄-titanium foil cathode prepared by the same method as the LiMn₂0₄-carbon cloth cathode was assembled with 1 m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte and Zn metal anode and tested at a current density of 200 mA g ¹ with the voltage range of 0 to 2.0 V vs. Zn/Zn²⁺ for 1000 cycles. As demonstrated by the results shown in Fig. 65, although the capacity of that coin cell is not as high as some known cells, its CEs can reach nearly 100% after 100 cycles, and maintain this level for 1000 cycles. This experiment demonstrates that nearly 100% CE can be achieved when Ti foil is used as the cathode current collector. This is because Ti foil is more stable than carbon cloth at high potential.

EIS data of the hybrid battery before and after cycling is shown in Fig. 66, in which resistances of both the electrolyte and the interphase can be seen to have increased after repeated galvanostatic charge-discharge cycling. After this test, the hybrid battery was separated, and optical images of the different components are shown in Fig. 67. In Fig. 67, optical images of the following after charge- discharge cycling are shown: (a) the Zn/LiMn₂0₄ full cell; (b) the 1 m Zn(C10₄)₂ + 10 m LiC10₄-PVA electrolyte; (c) the Zn anode; and (d) the LiMn₂0₄-carbon cloth cathode. All components were characterised further to compare data with the pre-testing components, to assess the differences induced by cycling. Figure 51 illustrates results from this testing, including: (a) an XRD test of Zn anode before and after charge/discharge cycling; (b) an SEM image of a cross-section of the Zn anode after cycling; (c) an SEM image of the Zn anode after cycling; and (d) an elemental mapping of zinc on the Zn anode after cycling. As shown in Fig. 51a, X-ray diffraction (XRD) spectra of the Zn foil before and after cycling were compared, and no obvious differences observed. Zn is the major constituent (ICDD: 03-065-3358), despite some weak peaks of ZnO (ICDD: 01 -075-1526) due to the oxidation of Zn in air. SEM image of the cross section of the Zn anode after cycling are exhibited in Fig. 51b; considering the SEM image (Fig. 51c) and the elemental mapping of Zn (Fig. 5 Id) and d, it can be seen that the Zn anode is almost dendrite-free.

For rechargeable batteries with metal anodes, dendrites induced by the uneven metal deposition can lead to thermal runaway and explosion hazards, which curb the advancement of metal anodes and induce research in interfacial engineering and electrolyte engineering to address those issues (see D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nature Nanotechnology 12 (2017) 194-206). In the hybrid battery of the embodiment being described, the high salt concentration can increase the threshold critical current density for cations becoming depleted in the electrolyte, thus suppressing the formation of zinc dendrites; this may improve the safety of this battery. Although the pH of the aqueous solution of 1 m Zn(C10₄)₂+10 m LiC10₄ was found to be 4.92, the likelihood of zinc dendrites being dissolved by the electrolyte, affecting the dendrite-free result, is small as the addition of PVA moves the pH further towards neutrality (see G. Wang, X. Lu, Y. Ling, T. Zhai, H. Wang, Y. Tong, Y. Li, LiCl/PVA gel electrolyte stabilizes vanadium oxide nanowire electrodes for pseudocapacitors , ACS Nano 6 (2012) 10296-10302).

In Fig. 68, FTIR spectra of the 1 m Zn(Cl0₄)₂ + 10 m LiCl0₄-PVA electrolyte before 6801 and after 6802 cycling testing are shown to be largely the same, indicating that the electrolyte is stable. Fig. 69a-e illustrates an SEM image (a, b and c) and corresponding elemental mappings of O (d) and Mn (e) for the LiMn₂0₄-carbon cloth cathode after cycling; except some detachment of active matter, the morphology and particle size were maintained. It was observed that the carbon fibre was broken after the cycling, possibly due to the volume change of the LiMn₂0₄ on the surface during cycling of the batter and the oxidation of carbon cloth when charged to the high voltage. A more robust and redox stable cathode substrate such as Ti foil or foam may therefore provide better performance.

Summary of Example 11

Zn ion-based aqueous rechargeable batteries (ARBs) may offer advantages such as high abundance, non-toxicity, low redox potential and stability in water. However, current Zn ion-based ARBs offer restricted energy density because of the narrow electrochemical windows. In an example embodiment, the low-cost inorganic salt LiC10₄ was used to replace the expensive organic LiTfSI salt which has been used in prior work to increase window width. The electrochemical window of the water-in-salt (WiS) electrolyte with statured LiC10₄ (10 m) and PVA has been shown to reach 2.6 V, which is comparable to the LiTfSI-based WiS electrolytes. To improve or optimise the composition of WiBS electrolytes containing LiC10₄ and ZnC10₄ for hybrid batteries, pH, electrochemical window, and ionic conductivity are jointly assessed to evaluate the different electrolytes. A i m Zn(C10₄)₂+10 m LiC10₄-PVA electrolyte was deemed a strong candidate, in part due to the reversibility.

To overcome the disadvantages of the presence of binders in electrodes, self-supported LiMn₂0₄-carbon cloth was prepared by electrochemical deposition and hydrothermal reaction and used for the cathode in the embodiment being described. The skilled person would appreciate that the same technique may be used for other salts.

Finally, the Zn anode and LiMn₂0₄-carbon cloth cathode were assembled with the WiBS electrolyte to form the quasi-solid-state full cell for use in hybrid batteries with a voltage of 2.1 V. During the charge-discharge test for the hybrid battery, the discharge capacity increased continuously over the first 200 cycles, to 120 mAh-g ¹ discharge capacity and 150.1 Wh-kg ¹ energy density. These decreased to 90.7 mAh-g ¹ and 120.8 Wh-kg ¹ respectively by the 300^th cycle. After cycling, the anode was tested by XRD, SEM and EDS, and the Zn anode was found to be almost dendrite-free after 300 cycles with columbic efficiency of 86.7%. A columbic efficiency of nearly 100% was achieved when Ti foil was used as the cathode current collector.

The above results demonstrate a viable route to industrialise Zn ion-based ARBs using low-cost saturated or supersaturated WiS electrolytes, and optionally composite WiS-polymer electrolytes, which may, for example, comprise one or more inorganic perchlorates and PVA. This is a promising hybrid aqueous battery for energy storage. A wide electrochemical window is achieved with the aqueous gel electrolyte using a low cost inorganic salt.

Claims

1. An electrolyte for an electrochemical device, the electrolyte comprising:

an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated; and

a water-absorbent polymer.

2. The electrolyte of claim 1 wherein the water-absorbent polymer is dissolved in the aqueous solution.

3. The electrolyte of claim 1 or claim 2 wherein the saturated aqueous salt solution is an over saturated aqueous salt solution.

4. The electrolyte of any preceding claim, wherein the electrolyte is a gel.

5. The electrolyte of claim 4 as dependent on claim 3, wherein the electrolyte comprises some of the salt in a crystallised form, distributed throughout the gel.

6. The electrolyte of any preceding claim wherein the water-absorbent polymer forms up to 10 wt.% of the electrolyte (based on solvent weight), and optionally wherein the water- absorbent polymer is polyvinyl alcohol (PVA) and the weight of the PVA is between 1 wt.% and 30 wt.%.

7. The electrolyte of any preceding claim wherein the water-absorbent polymer is or comprises one or more of the following:

(i) polyvinyl alcohol (PVA);

(ii) polyvinyl pyrrolidone (PVP);

(iii) polypropylene alcohol (PPA); or

(iv) polyethylene glycol (PEG).

8. The electrolyte of any preceding claim wherein the water-absorbent polymer is or comprises a superabsorbent polymer (SAP), and wherein optionally the electrolyte comprises both a non-water-soluble SAP and a water-soluble water-absorbent polymer which is not a SAP, the water-soluble water-absorbent polymer(s) optionally forming up to 10 wt.% (with respect to the weight of solvent of the aqueous solution) of the electrolyte.

9. The electrolyte of claim 8, wherein the superabsorbent polymer is or comprises one or more of the following:

(i) a polyacrylate salt, optionally sodium polyacrylate; (ii) a polyacrylamide;

(iii) a polysaccharide;

(iv) a polypeptide;

(v) polyacrylonitrile (PAN); or

(v) a polyvinyl alcohol copolymer.

10. The electrolyte of claim 8 or claim 9 wherein the superabsorbent polymer forms between 0.01 wt.% and 5 wt.% of the electrolyte (wt.% based on solvent weight), optionally between 0.1 wt.% and 5 wt.% or between 0.5 wt.% and 5 wt.% of the electrolyte, and further optionally between 0.1 wt.% and 2 wt.% of the electrolyte.

11. The electrolyte of any preceding claim wherein the aqueous solution is an aqueous metal acetate solution, and wherein optionally at least one of the following applies:

(i) the metal of the metal acetate is selected from a group of metals having a negative standard electrode potential, relative to the standard hydrogen electrode, when in their elemental forms; and/or

(ii) the metal of the metal acetate is potassium, caesium, magnesium or zinc.

12. The electrolyte of claim 11, further comprising an additional metal salt dissolved in the solution, wherein the metal of the metal cation of the additional metal salt is different from the metal of the metal acetate but selected from the same group of metals, and the anion of the additional metal salt is a nitrate, a sulfate, a phosphate, a halide, or an organic salt with a chain length smaller than or equal to 6, and wherein the molar ratio of the metal acetate to the additional metal salt is between 3 : 1 and 100: 1, inclusive.

13. The electrolyte of any of claims 1 to 10 wherein the aqueous salt solution is an aqueous metal chlorate solution such as a lithium perchlorate solution.

14. The electrolyte of any preceding claim wherein the aqueous solution is a mixed salt solution comprising a plurality of different salts, and optionally is a bi-salt solution comprising two different salts.

15. An electrochemical device comprising:

an electrolyte comprising an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated, and a water-absorbent polymer;

a positive electrode in contact with the electrolyte; and

a negative electrode in contact with the electrolyte.

16. The electrochemical device of claim 15, wherein at least one of the following applies: (i) the electrochemical device is a supercapacitor, battery or electrochemical synthesis device; and/or

(ii) the electrolyte is the electrolyte of any of claims 1 to 14.

17. A method of making an electrolyte for an electrochemical device, the method comprising: heating a liquid to a temperature near and below its boiling point, wherein the liquid is or comprises water;

maintaining the temperature of the liquid whilst:

adding a water-absorbent polymer to the liquid;

dissolving a salt into the liquid until a saturated solution is formed; and cooling the solution to room temperature.

18. The method of claim 17, wherein the method comprises heating the liquid to a temperature above 90°C, and optionally to the temperature of 95°C.

19. The method of claim 17 or claim 18, wherein the liquid is an azeotrope comprising water and one or more organic solvents, and wherein optionally the water forms at least 50% of the liquid by weight.

20. The method of any of claims 17 to 19, wherein the water-absorbent polymer is or comprises one or more of the following:

(i) polyvinyl alcohol (PVA);

(ii) polyvinyl pyrrolidone (PVP);

(iii) polypropylene alcohol (PPA); or

(iv) polyethylene glycol (PEG).

21. The method of any of claims 17 to 20, wherein the amount of the water-absorbent polymer is between 0.01 wt.% and 30 wt.% (based on liquid weight), and optionally:

(i) between 5 wt.% and 30 wt.%, and optionally around 10 wt.%, if the water- absorbent polymer is not a SAP; or

(ii) between 0.01 wt.% and 5 wt.%, and optionally around 1 wt.%, if the water- absorbent polymer is a SAP.

22. The method of any of claims 17 to 21, wherein the salt is a metal chlorate or metal acetate salt.

23. The method of any of claims 17 to 22, wherein the salt and the water-absorbent polymer are selected such that the electrolyte formed is as claimed in any of claims 1 to 14.

24. An electrolyte for an electrochemical device, the electrolyte comprising:

an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated, and optionally over-saturated.

25. An electrochemical device comprising:

an electrolyte comprising an aqueous salt solution, wherein a sufficient quantity of the salt is dissolved therein for the solution to be saturated, and optionally over-saturated;

a positive electrode in contact with the electrolyte; and

a negative electrode in contact with the electrolyte.