WO2023105202A1

WO2023105202A1 - Electrochemical cell

Info

Publication number: WO2023105202A1
Application number: PCT/GB2022/053092
Authority: WO
Inventors: Adrien Marie AMIGUES
Original assignee: Oxlid Ltd
Priority date: 2021-12-07
Filing date: 2022-12-06
Publication date: 2023-06-15
Also published as: GB2615511A; GB202117662D0

Abstract

An electrochemical cell comprising: an anode comprising an alkali metal, an alkali metal alloy, silicon, carbon, or a silicon-carbon composite material; a cathode comprising a metal sulphide; and a liquid electrolyte, wherein the polysulfide solubility of the electrolyte is less than 500mM; a method of producing the electrochemical cell; and an electrochemical cell assembly comprising at least one electrochemical cell.

Description

Electrochemical Cell

The invention relates to an electrochemical cell, methods of manufacture of the electrochemical cell, and an electrochemical cell assembly comprising at least one of the electrochemical cells. In particular, the invention relates to electrochemical cells including a cathode comprising a metal sulphide, and an electrolyte having a polysulfide solubility less than 500 mM.

In recent years there has been an increased demand for "green energy" due to the detrimental impact that fossil fuels have on the environment. One energy source that has received a great deal of interest is battery technology, in particular rechargeable batteries.

Of the rechargeable battery technologies, lithium-ion (Li-ion) battery technology dominates the commercial market, as a result of its high energy density compared to competing technologies, such as nickel-cadmium batteries (Ni-Cd). However, Li-ion batteries are expensive to produce, highly flammable, and typically require the use of cobalt and/or nickel in the production of the cathodes. Both cobalt and nickel are costly materials, and there are concerns over the security of the supply chain. In addition, cobalt can be toxic if not handled correctly, increasing the operational complexity of both the manufacture and end of life recycling processes.

In recent years, lithium-sulphur (Li-S) cells have received widespread attention because of their advantages over Li-ion batteries. For instance, they have a higher gravimetric energy (i.e., the measure of how much energy a battery contains in proportion to its weight, which is typically measured in 'watt-hours per kilogram (Wh/kg)', wherein a watt-hour is a measure of electrical energy that is equivalent to the consumption of one watt for one hour), a lower raw material cost, and are more environmentally friendly. Moreover, they do not require the use of nickel or cobalt in their manufacture. Furthermore, there are safety benefits associated with use of Li-S batteries over Li-ion batteries, as there is no longer a need for free metal ions in the materials. Instead, Li-S batteries proceed via a "conversion mechanism", whereby sulphur and lithium react to form polysulphides.

However, there is aann important disadvantage surrounding the generation of polysulphides in conventional Li-S batteries. Polysulfides generated at the electrodes dissolve in a liquid electrolyte and undergo a "shuttling effect" between the anode and cathode, which results in an irreversible loss of sulphur. This can result in capacity loss and be detrimental to cyclability of the battery (i.e., the measure of times they can be recharged before they start to break down).

One way that the issue of polysulphide shuffle has been partly addressed is through the use of metal sulphide cathodes in conventional Li-S cells. As noted in the literature article "Applications of MoS₂ in the cathode of lithium sulfur batteries" (RSC Adv. 2020, 10, 7384), the metal-sulphur bonds of the metal sulphide cathode can bind the polysulphides through electrostatic or chemical bonds, reducing polysulfide shuttling within the electrolyte. However, the use of metal sulphides is not sufficient to inhibit all of the polysulfide shuttle. In addition, metal sulphides tend to have low sulphur utilisation (i.e., the amount of sulphur that is de-lithiated/lithiated during cycling) in conventional Li-S batteries, which can impact the gravimetric energy of the cell.

An additional way that the effect of polysulphide shuffle has been limited is through the addition of certain additives in the electrolyte, such as additives that include N-0 bonds (e.g., lithium nitrate (LiNO₃)). However, there are numerous disadvantages associated with the use of these additives, such as a narrow operating and storage temperature window, cell swelling due to formation of gases during cycling, not to mention safety implications. Therefore, there is a need to eradicate the need for such additives.

In addition, conventional Li-S batteries often use flammable liquids as the electrolyte, which has resulted in concern over their safety. As a result, there has been a great deal of interest in Li-S solid state batteries (SSBs), which use an inorganic solid-state electrolyte that do not dissolve polysulfides during battery cycling.

An example of a solid-state battery can be found in literature article "Exfoliated M0S2 as Electrode for All-Solid-State Rechargeable Lithium-Ion Batteries" Q. Phys. Chem. C 2019, 123, 19, 12126 -12134), which describes the use of metal sulphide cathodes and solid-state electrolytes for lithium-ion batteries.

Whilst safety would be improved using a solid-state battery, the manufacture of solid- state batteries on a large scale is difficult, and there remains the issue of low sulphur utilisation from use of metal sulphide as the cathode in Li-S solid-state batteries. Moreover, there would be poor interfacial contact between the electrolyte and electrode in a solid-state battery, which may result in high impedance within the cell. Moreover, in order to retain as good an interfacial contact as possible between the solid-state electrolyte and cathode, a high amount of pressure is required. An alternative way of tackling the polysulphide shuffle effect is shown in WO 2020/053604. Instead of using electrolytes salts such as LiNOs or solid-state electrolytes, there is described the combination of the use of a low porosity cathode having an electrochemically active sulphur component with a liquid electrolyte having no or a low polysulphide solubility (which ultimately reduces polysulphide shuffle).

Building upon this, WO 2021/074634 demonstrates that a combination of a highly concentrated electrolyte with aa low porosity cathode comprising both an electrochemically active sulphur and an electronically conductive carbon material can result in high gravimetric energy and volumetric energy (i.e., the measure of the energy content of a battery in relation to its volume, which is typically measured in 'watt-hours per litre (Wh/L)') densities. Whilst this technology was shown to have high gravimetric energy, it still suffers from low cyclability. Without being bound by theory, this could be a result of the poor mechanical strength of the carbon-sulphur composite, which is not able to withstand the large volume of expansion of sulphur during delithiation and lithiation. As such, the interfacial contact between carbon and sulphur would be lost during cycling.

Notwithstanding the above-mentioned advances in the field, there remains a need for an electrochemical cell having not only a high gravimetric energy, volumetric energy density, and broad operating/storage temperature window, but also having improved cyclability.

The invention is intended to overcome or ameliorate at least some aspects of the above- mentioned problems.

Accordingly, in a first aspect of the invention there is provided an electrochemical cell comprising: an anode comprising an alkali metal, alkali metal alloy, silicon, carbon, or a silicon-carbon composite material; a cathode comprising a metal sulphide; and a liquid electrolyte, wherein the polysulfide solubility of the electrolyte is less than 500mM.

The electrochemical cell has a high gravimetric energy, a high volumetric energy, a broad operating and storage temperature range, a good interfacial stability between the cathode and the electrolyte (resulting in high sulphur utilisation), and a long cycle life. Without being bound by theory, the electrochemical cell according to the first aspect of the invention is believed to operate via a solid-state mechanism, i.e., via the formation of solid (unsolvated) polysulfide species. In such solid-state mechanisms, cathodes according to conventional Li-S batteries and Li-S solid state batteries may have insufficient transport of lithium ion to the active sulphur species present in the cathode, and/or an insufficient sulphur/carbon interface to enable high sulphur utilisations via a solid-state mechanism. However, the combination of a cathode comprising a metal sulphide with a liquid electrolyte with poor polysulfide solubility may mitigate this issue via the formation of solid polysulfide species that remain in the cathode.

Optionally, the metal sulphide has a one-dimensional structure including, but not limited to, nanotubes, wires and rods. These structures allow for a high metal/sulphur interface, which result in fast kinetics. In addition, the use of one-dimensional structures provides a cathode with good structural stability. Moreover, without being bound by theory, use of one-dimensional structures allows for higher sulphur loadings, which would result in a cell having both a high gravimetric energy and volumetric energy density.

Optionally, the metal sulphide has a two-dimensional layered structure. Two-dimensional layered structures, such as two dimensional nanosheets, form networks through overlapping and stacking with one another. Without being bound by theory, these structures have a large metal/sulphur interface able to effectively trap the polysulphides formed, holding these in place through chemical bonds. The high metal/sulphur interface also results in faster kinetics.

In addition, without being bound by theory, a layered structure is able to accommodate the volume expansion of sulphur, and conduct lithium ions (Li⁺). Moreover, a layered structure can be exfoliated to obtain small particles and increase the electrochemical performance of the battery.

Optionally, the metal sulphide has a three-dimensional structure including, but not limited to, metal sulphide nanoparticles such as core-shell structured metal sulphides and flower-like metal sulphide nanomaterials. Three-dimensional structures can withstand volume expansion of sulphur. Without being bound by theory, polysulphides could be held within three-dimensional structures. As such, use of three-dimensional structures provide a cathode with optimised kinetics. In addition, three-dimensional structures can have higher sulphur loadings compared to one or two-dimensional structures, which provides a cell with a higher gravimetric and volumetric energy density.

The metal sulphide may comprise one or more metals, such that mono-metal sulphides may be used or mixed-metal sulphides comprising two, three or more metals (bi-metal, tri-metal or multi-metal systems). The mono-metal sulphide may have the structural formula M_xS_y, whereby M is a metal, The metal may be a transition metal, an alkali earth metal, an alkali

metal, or a post-transition metal (i.e., metals found in groups 13-16 of the periodic table). Optionally, the metal may be selected from molybdenum (Mo), tin (Sn), titanium (Ti), vanadium (V), tungsten (W), tantalum (Ta), hafnium (Hf), or rhenium (Re).

The mono-metal sulphide may be selected from α-manganese sulphide

manganese sulphide

γ-manganese sulphide

iron sulphide (FeS,

ferrous disulphide

cobalt sulphide zinc

sulphide (ZnS), copper sulphide (CuS), bismuth (III) sulphide , germanium

sulphide (GeS), germanium disulphide

lithium sulphide

calcium sulphide (CaS), tin (II) sulphide (SnS), tin (IV) sulphide

antimony trisulfide

indium sulphide

α-indium sulphide

sulphide

, γ- indium sulphide

zirconium sulphide (ZrS), cerium sulphide

molybdenum disulphide

molybdenum trisulphide

silver sulphide

cadmium sulphide (CdS), tungsten disulphide

, nickel sulphide (NiS), vanadium sulphide

titanium sulphide

lead sulphide (PbS), niobium sulphide (NbS), niobium disulphide

tantalum disulphide

tellurium disulphide rhodium(III) sulphide

palladium sulphide (PdS), palladium disulphide rhenium disulphide

osmium sulphide

, platinum sulphide (PtS),

iridium disulphide

iridium(III) sulphide chromium sulphide (CrS),

chromium(III) sulphide barium sulphide (BaS), strontium sulphide (SrS),

caesium sulphide rubidium sulphide thallium(I) sulphide , beryllium

sulfide (BeS), ytterbium sulfide (YbS), hafnium disulphide

or combinations thereof. It is noted that the term "copper sulphide" includes chemical compounds and minerals with the formula

where

For instance, the term copper sulphide may include

or

It is noted that the term "nickel sulphide" includes

Optionally, the metal sulphide may be a mixed metal sulphide, wherein the metals may be selected from a transition metal ion, an alkali earth metal, an alkali metal, a posttransition metal, or combinations thereof. Optionally, the mixed metal sulphide comprises molybdenum (Mo), tin (Sn), titanium (Ti), vanadium (V), tungsten (W), tantalum (Ta), hafnium (Hf), or rhenium (Re), or combinations thereof. Examples of mixed-metal sulphides include, but are not limited to,

or any combination thereof.

Optionally, the metal sulphide is a metal disulphide. The metal disulphide may be selected from molybdenum disulphide

tungsten disulphide

hafnium disulphide

tin sulphide

, titanium sulphide

vanadium sulphide

tantalum disulphide

, rhenium disulphide

or combinations thereof. Without being bound by theory, the transition metal disulphides that fall within the transition metal dichalcogenide family are particularly beneficial, as they can be in the form of a monolayer where the metal atom is located between two sulphur atoms.

Often the metal sulphide comprises molybdenum disulphide. It may be the case that the molybdenum disulphide is in the form of the IT polymorph 2H polymorph

or 3R polymorph

Often, molybdenum disulphide is in the form of

the 1T polymorph. The octahedral or trigonal antiprismatic geometry of the 1T polymorph is able to form strong bonds with polysulphides, reducing the possibility of polysulphide shuffle. Moreover, the octahedral or trigonal antiprismatic geometry of the 1T polymorph ccaann provide higher lithium diffusion, which leads to enhanced electrochemical performance. Optionally, the metal sulphide is in particulate form. The particles may be of a size in the range of from 1 μm to 20 μm, often in the range of from 1 μm to 5 μm. Having metal sulphide particles falling within this size range results in higher sulphur utilisation, which leads to enhanced power performance. Particle size analysis can be determined using any known technique, such as dynamic image analysis (DIA), static laser light scattering, dynamic light scattering (DLS), sieve analysis, or by visual analysis of Transmission Electron Microscopy (TEM) or Scanning Electron Microscopy (SEM) images.

Particles can be obtained via conventional means including, but not limited to, chemical/physical exfoliation, bead milling, jet milling and/or ball milling.

Optionally, it may be the case that the metal sulphide cathode further comprises sulphur. For instance, the cathode may comprise metal sulphide-sulphur composites. When the metal sulphide cathode further comprises sulphur, the sulphur may be incorporated into the metal sulphides through conventional methods. For instance, this could be achieved via melt infusion whereby metal sulphides are immersed in sulphur at approximately 150 to 160 °C. Without being bound by theory, when a melt infusion method is used, melted sulphur can diffuse throughout the pores of the metal sulphide.

Optionally, the average pore diameters of the metal sulphide are in the range of from 0.1 nm and 20 nm, more often in the range of from 1 nm and 10 nm. Pore sizes falling within these particular ranges allow for higher sulphur utilisation.

When the metal sulphide cathode further comprises sulphur, it may be the case that the metal sulphide has a porous one-dimensional, two-dimensional, or three-dimensional structure.

Optionally, the metal sulphide particles additionally comprise a coating, the coating comprising a ceramic material, a polymeric material, or a combination thereof. Use of an ionically conducting coating helps contain the polysulphides within the cathode and retain the structure of the metal host which improves cyclability. The provision of an ionically conducting coating on one-dimensional, two-dimensional, or three-dimensional metal sulphide structures is particularly effective at inhibition of polysulphide dissolution. With respect to two-dimensional layered metal sulphides, use of a coating also inhibits exfoliation of the layered structure during cycling.

Optionally, the coating has a low area specific lithium and/or sodium ionic resistance. Typically, the area specific lithium and/or sodium ionic resistance is less than 20 Ω/cm², more typically less than 5 Ω/cm². As used herein, the unit "Ω/cm²" relates to the area of the coating in contact with the electrolyte.

The coating may be applied to the metal sulphide particles using conventional coating techniques. For instance, the coating can be applied via chemical vapour deposition (CVD), plasma-enhanced CVD, sol-gel techniques, hydrothermal or solvothermal precipitation, molecular layer deposition (MLD), or atomic layer deposition (ALD). Where the coating comprises a ceramic material, it is typically the case that the coating is applied via LID. Where the coating comprises a ceramic-polymer composite, it is typically the case that the coating is applied via MLD.

The ceramic material may have a crystalline, polycrystalline, partially crystalline, or amorphous structure. Suitable ceramic materials include, but are not limited to, oxides, carbonates, nitrides, carbides, silicides, sulphides, oxysulphides, and/or oxynitrides of metals and/or metalloids. Examples of ceramic materials that can be used include, but are not limited to, oxides such as titanium oxide, aluminium oxide, zinc oxide, silicon oxide, boron oxide, vanadium oxide, zirconium oxide, magnesium oxide, or combinations thereof; nitrides such as aluminium nitride, boron nitride, silicon nitride, or combinations thereof; carbides such as tungsten carbide (WC), chromium carbide

titanium carbide (TiC), tantalum carbide (TaC), silicon carbide (e.g. sintered silicon carbide (SSiC), liquid phase sintered silicon carbide (LPS-SiC), reaction bonded Silicon Carbide (RBSiC), nitride bonded silicon carbide (NSiC), or recrystalised silicon carbide), or combinations thereof; hydrides such as

(where X = Cl, Br, or I), LiNH,

or combinations thereof; or any combination thereof. Often, the coating comprises a ceramic oxide selected from titanium oxide, aluminium oxide, zinc oxide, silicon oxide, boron oxide, vanadium oxide, zirconium oxide, magnesium oxide, or combinations thereof, more often the coating comprises aluminium oxide.

Examples of the polymeric material include, but are not limited to, poly(p-phenylene vinylene), poly(acetylene)s, polyphenylenes, polyphenylene sulphide, polyanilines, polythiophenes, polycarbazoles, polyfluorenes, polyazulenes, polypyrenes, poly(3,4- ethylenedioxyth iophene) polystyrene sulfonate (PEDOT:PSS), polyindoles, polypyrenes, polynaphthalenes, or polyethylene oxide; or combinations thereof. Often, the polymeric material comprises polyethylene oxide.

It may be the case that the coating comprises a ceramic-polymer composite material. Examples of ceramic-polymer composite materials include, but are not limited to, metalcones (e.g., alucone, zincone, zircone, titacone, or combinations thereof). Optionally, the coating has a thickness in the range of from 1 nm to 150 nm, often less than 100 nm, more often less than 75 nm. A coating in this range allows for fast lithium or sodium ion diffusion, as well as allowing conduction of electrons.

As noted above, the anode comprises an alkali metal, alkali metal alloy, silicon, carbon, or a silicon-carbon composite material.

Optionally, the alkali metal or alkali metal alloy comprises lithium and/or sodium. It may be the case that the anode comprises a foil formed of lithium metal or lithium metal alloy. Examples of lithium alloys include, but are not limited to, lithium indium alloy, lithium aluminium alloy, lithium magnesium alloy and lithium boron alloy. It may be the case that the anode comprises a foil formed of sodium metal or sodium metal alloy. Examples of sodium alloys include, but are not limited to, sodium indium alloy, sodium aluminium alloy, sodium magnesium alloy and sodium boron alloy. Often the anode is a lithium metal foil or a sodium metal foil because of their high specific capacity.

Alternatively, the anode may comprise silicon. Where the anode comprises silicon, this may be lithiated or sodiated. As used herein, the term "lithiated" takes its usual meaning in the art and refers to the combination or impregnation with lithium or a lithium compound. Similarly, the term "sodiated" takes its usual meaning in the art and refers to the combination or impregnation with sodium or a sodium compound.

Alternatively, the anode may comprise carbon, for instance as carbon nanotubes, carbon nanofibers, graphene, graphene oxide, graphite, carbon black, or combinations thereof.

It may be the case that the anode comprises a silicon-carbon composite material. Examples of silicon-carbon composites include, but are not limited to, Silicon-doped graphite.

The electrolyte according to the first aspect of the invention has a polysulphide solubility at room temperature (approximately 20 °C) of less than 500 mM. For example, the liquid electrolyte may have a polysulphide solubility less than 400 mM, optionally less than 200 mM, optionally less than 150 mM, optionally less than 100 mM, optionally less than 10 mM, or optionally less than 1 mM. In some cases, the electrolyte may not dissolve polysulfides. For example, the electrolyte may have a polysulfide solubility in the range of from 0.001 mM to 500 mM, often 0.01 to 400 mM, often 0.1 mM to 200 mM, more often 1 mM to 10 mM. Correspondingly, the electrolyte may have a low solubility for sulphur-containing species (such as polysulfides and sulphur) in general.

The use of an electrolyte having poor or no solubility of polysulfides can prevent polysulfide shuttle within an electrolyte and is therefore beneficial in cells such as lithium-sulphur cells. As noted above, the polysulfide shuttle effect is an undesirable reaction, as it results in loss of coulombic efficiency and can impact cyclability.

It is typically the case that the electrolyte comprises a suitable solvent system, liquid or gel, or mixture of liquids and/or gels; and an alkali metal salt.

Suitable organic solvents for use in the electrolyte are ethers (e.g. linear ethers, diethyl ether (DEE), diglyme (2-methoxyethyl ether), tetraglyme, tetra hydrofuran, 2- methyltetrahydrofuran, dimethoxyethane (DME), dioxolane (DIOX); carbonates (e.g. dimethylcarbonate, diethylcarbonate, ethyl methylcarbonate, methylpropylcarbonate, ethylene carbonate (EC), propylene carbonate (PC); sulfones (e.g. dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), tetramethyl sulfone (TMS); esters (e.g. methyl formate, ethyl formate, methyl propionate, methylpropylpropionate, ethylpropylpropionate, ethyl acetate and methyl butyrate); ketones (e.g. methyl ethyl ketone); nitriles (e.g. acetonitrile, proprionitrile, isobutyronitrile); amides (e.g. dimethylformamide, dimethylacetamide, hexamethyl phosphoamide, N, N, N, N- tetraethyl sulfamide); lactams/lactones (e.g. N- methyl-2-pyrrolidone, butyrolactone); ureas (e.g. tetra methyl urea); sulfoxides (e.g. dimethyl sulfoxide); phosphates (e.g. trimethyl phosphate, triethyl phosphate, tributyl phosphate); phosphoramides (e.g. hexamethylphosphoramide). Further suitable solvents include toluene, benzene, heptane, xylene, dichloromethane, and pyridine.

It may be the case that the ethers, carbonates, sulfones, esters, ketones, nitriles, amides, lactams, ureas, phosphates, phosphoramides are fluorinated. An example of a fluorinated ether is l,l,2,2,-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

The electrolyte may comprise one or more ionic liquids as solvent. Said ionic liquids may comprise salts comprising organic cations such aass imidazolium, ammonium, pyrrolidinium, and/or organic anions such as bis(trifluoromethanesulfonyl)imide

bis(fluorosulfonyl)imide triflate, tetrafluoroborate dicyanamide chloride

The ionic liquid is liquid at room temperature (20 °C). Examples of suitable ionic liquids include (N,N-diethyl-N-methyl- N(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl), N,N-diethyl-N-methyl-N- propylammonium bis(fluorosulfonyl)imide, N,N-diethyl-N-methyl-N-propylammonium bis(fluorosulfonyl)imide, N,N-dimethyl-N-ethyl-N-(3-methoxypropyl)ammonium bis(fluorosulfonyl)imide, N,N-dimethyl-N-ethyl-N-(3-methoxypropyl)ammonium bis(trifluoromethanesulfonyl)imide, N,N-dimethyl-N-ethyl-N-benzyl ammonium bis(trifluoromethanesulfonyl)imide, N,N-dimethyl-N-Ethyl-N-phenylethyl ammonium bis(trifluoromethanesulfonyl)imide, N-Ethyl-N,N-dimethyl-N-(2- methoxyethyl)ammonium bis(fluorosulfonyl)imide, N-Ethyl-N,N-dimethyl-N-(2- methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, N-tributyl-N- methylammonium bis(trifluoromethanesulfonyl)imide, N-tributyl-N-methyl ammonium dicyanamide, N-tributyl- N-methylammonium iodide, N-trimethyl-N-butyl ammonium bis(trifluoromethanesulfonyl)imide, N-trimethyl-N-butylammonium bromide, N-trimethyl- N- hexylammonium bis(trifluoromethanesulfonyl)imide, N-trimethyl-N-propylammonium bis(fluorosulfonyl)imide, N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide, (N,N-diethyl-N-methyl-N(2methoxyethyl)ammonium bis(fluorosulfonyl)imide, 1 -Butyl-1 - methylpyrrolidinium bis(fluorosulfonyl)imide, 1- ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1 - methyl-1 -(2- methoxyethyl)pyrrolidinium bis(fluorosulfonyl)imide, N,N-diethyl-N-methyl-N- propylammonium bis(fluorosulfonyl)imide, N-Ethyl-N,N-dimethyl-N- (2- methoxyethyl)ammonium bis(fluorosulfonyl)imide, N-propyl-N-methylpiperidinium bis(fluorosulfonyl)imide, N-trimethyl-N-butyl ammonium bis(fluorosulfonyl)imide, N- methyl-N- butyl-piperidinium bis(trifluoromethanesulfonyl) imide, N-methyl-N- propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, and combinations thereof.

Alternatively, or additionally, the liquid electrolyte may be a gel electrolyte. The gel electrolyte may comprise polyethylene oxide with a gelling liquid electrolyte, for example an ether such as dimethyl ether. In one example, the electrolyte may comprise polyethylene oxide in combination with LiTFSI in dimethylether.

Any combination of one or more of the above solvents may be included in the electrolyte. For example, the electrolyte may comprise the combination of an ionic liquid with a fluorinated ether, or the combination of an ionic liquid within a gel, or the combination of a fluorinated ether within a gel. The electrolyte may comprise a combination of two or more of any of the liquids and/or gels detailed above.

Optionally, the liquid electrolyte comprises a solvent selected from linear ethers, diethyl ether (DEE), tetra hydrofuran (THF), Dimethoxyethane (DME), Dioxolane (DIOX), Diglyme, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl formate (MF), ethyl formate (EF), methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB), methyl ethyl ketone, acetonitrile (ACN), propionitrile (PN), isobutyronitrile (iBN), Dimethylformamide (DMF), Dimethylacetamide (DMAc), N-Methyl-2-pyrrolidone (NMP), Tetram ethyl urea (TMU), Dimethyl sulfoxide (DMSO), Trimethyl phosphate, Triethyl phosphate, Hexamethylphosphoramide, toluene, benzene, heptane, xylene, dichloromethane, ionic liquids, fluorinated ethers, gels, or a combination thereof; and at least one alkali metal salt.

Optionally, the alkali metal salt comprises lithium when the anode comprises lithium or a lithium alloy; and the alkali metal salt comprises sodium when the anode comprises sodium or a sodium alloy.

Optionally, the alkali metal salt is at least one lithium salt selected from lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium perchlorate, lithium sulfate, , lithium trifluoromethanesulfonate, lithium bis(trifluoromethane)sulfonimide, lithium bis(fluorosufonyl)imide, lithium bis(oxalate) borate, lithium difluoro(oxalate)borate, lithium bis(pentafluoroethanesulfonyl)imide, lithium 2-trifluoromethyl-4,5- dicyanoimidazole, and combinations thereof; or the alkali metal salt is at least one sodium salt selected from sodium hexafluoroarsenate, sodium hexafluorophosphate, sodium perchlorate, sodium sulfate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethane)sulfonimide, sodium bis(fluorosufonyl)imide, sodium bis(oxalate) borate, sodium difluoro(oxalate)borate, sodium bis(pentafluoroethanesulfonyl)imide, sodium 2-trifluoromethyl-4,5-dicyanoimidazole, and combinations thereof.

Often, the alkali metal salt is lithium trifluoromethanesulphonate (also known as lithium triflate or LiOTf), lithium bis-trifluoromethanesulfonimide (LiTFSI), and/or lithium bis(fluorosufonyl)imide (LiFSI). In conventional Li-S cells, LiFSI lacks stability in the presence of polysulphides, such that this salt would usually be considered unsuitable. However, with the cathode of the invention, which operates without the formation of polysulphides, the electrolyte can include salts and solvents, such as LiFSI, that otherwise would not be stable, resulting in a broader range of materials that can be used in the fabrication of the claimed cells.

Optionally, the concentration of the at least one alkali metal salt is at least 75% of the saturation concentration of the solvent system of the electrolyte. As used herein, the term "saturation concentration" relates to the extent of solubility of a particular solute in a particular solvent. The point of saturation is where the addition of solute does not result in an increase in concentration. Often the concentration of the at least one alkali metal salt is at least 80% of the saturation concentration of the solvent system, often at least 85% of the saturation concentration of the solvent system, often at least 90% of the saturation concentration of the solvent system. It may be the case that the concentration of the at least one alkali metal salt is about 100% of the saturation concentration, i.e., the electrolyte is fully saturated by the alkali metal salt. For example, the concentration of lithium or sodium salt in the electrolyte may be within the range of 0.05 M to 10 M, often 1 M to 5 M, for example, 3 M. It may be the case that the lithium salt is present in the electrolyte at a concentration of 0.1 M to 6 M, often 0.5 M to 3 M, for example, 1 M.

The saturation concentration is determined at room temperature, for example at 20 °C. The saturation concentration of polysulfides within a particular solvent may be determined by known methods, for example by determining the point at which just enough electrolyte is added to dissolve all solid residues.

The electrolyte is liquid across the range of operating temperatures of the cell, which may be from -30 to 120 °C, often from -10 to 90 °C, often from 0 to 60 °C. Operating pressures of the cell may be from 5 mbar to 100 bar, often from 10 mbar to 50 bar, for example 100 mbar to 20 bar. It may be the case that the cell according to the invention is operated at room temperature and pressure. The high concentration of the electrolyte in accordance with the invention means that the electrolyte has a lower vapour pressure than a standard electrolyte. Thus, the cell in accordance with the invention may perform better than a standard lithium-sulphur cell at a low pressure. The liquid electrolyte may be a gel electrolyte.

In conventional Li-S cells, certain additives may be included in the electrolyte to prevent or limit the effect of polysulfide shuttle. For instance, additives including N-0 bonds, such as UNO3. However, there are some disadvantages to the inclusion of additives of this nature, such as depletion during cell operation and resultant cell swelling due to formation of gases during cycling, particularly at higher temperatures. This could not only have safety implications but can also have an adverse effect on cycle life of the cell. Moreover, these additives may also limit the voltage range of the cell. The electrolyte of the invention removes the need for these additives. It may be the case that the electrolyte according to the invention does not comprise additives comprising N-O bonds.

Optionally, the electrolyte loading is in the range of from 0.5 μL/mAh to 3 μL/mAh. Often in the range of from 1 μL/mAh to 1.5 μL/mAh. As used herein, " μL/mAh" relates to the electrolyte/sulphur ratio in the cell (i.e., pl of electrolyte per milliampere-hour of sulphur). Typically, conventional Li-S requires a high amount of electrolyte (i.e., high loading) in order to dissolve polysulfides contained in the cathode. With cells according to the invention this is not necessary. A low electrolyte loading is beneficial as it makes the cell lighter, resulting in higher gravimetric energy. Optionally, the cathode further comprises an electronically conductive current collector. The current collector may comprise aluminium, copper, titanium, platinum, zinc, or stainless steel. Often, the current collector is aluminium foil. Aluminium foil is able to form a passive film, which results in a stable electrolyte/aluminium interface. Aluminium foil is also light weight, low cost, and provides strong adhesion to the metal sulphide in the cathode. Moreover, aluminium foil has good electronic conductivity. It may be the case that the current collector is coated with a protective layer. Typically, the protective layer is a carbon-based layer.

Optionally, the cathode further comprises a binder. Without being bound by theory, the binder may act to bind the cathode components together. Additionally, or alteratively, the binder may also help bind the cathode components to the current collector. In doing so, the binder can provide a cathode with enhanced mechanical robustness and can improve the processability of the cathode.

The binder may be a polymeric binder, for example, a polyether such as poly(ethylene oxide)s, polyethylene glycols, polypropylene glycols, polytetra methylene glycols (PTMGs), polytetra methylene ether glycols (PTMEGs), or mixtures thereof.

The binder may be selected from halogenated polymers, for instance, the binder may be selected from a fluorinated polymer. Examples of suitable binders include, but are not limited to, poly(vinylidene fluoride) (PVDF), often in the a form poly(trifluoroethylene) (PVF3); polytetrafluoroethylene (PTFE); copolymers of vinylidene fluoride with either hexafluoropropylene (HFP) or trifluoroethylene (VF3) or tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE); fluoroethylene/propylene (FEP) copolymers; copolymers of ethylene with either fluoroethylene/propylene (FEP) or tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE); perfluoropropyl vinyl ether (PPVE); perfluoroethyl vinyl ether (PEVE); and copolymers of ethylene with perfluoromethyl vinyl ether (PMVE); or blends or mixtures thereof.

Other examples of suitable binders include polyacrylonitrile, polyurethane, PVDF-acrylic co-polymer; polyacrylic acid, polyimides and polyvinyl alcohol. Further suitable binders include rubber (e.g., styrene butadiene rubber), cellulose (e.g., carboxymethyl cellulose) or gelatine.

Optionally, the binder is selected from polyethylene oxide (PEO), polyvinylidene fluoride

polyacetylene, polyphenylene vinylene, poly(3,4-ethylenedioxythiophene), polyphenylene sulphide, gelatine, or combinations thereof.

Optionally, the cathode may comprise 0.05 to 20 weight % binder based on the total weight of the cathode, often 0.5 to 10 weight %, for example 1 to 5 weight %, for example 2 to 3 wt%.

Optionally, the cathode further comprises an ionically conductive material. The ionically conductive material may have a bulk ionic conductivity of greater than

at 25 °C, for example greater than

Where the cathode contains an electroactive, ionically conductive material such as a further ionically conductive

material may be absent.

Optionally, the ionically conductive material is selected from a conducting ceramic material, an ionically conducting polymer, or a combination thereof.

The ceramic material may have a crystalline, polycrystalline, partially crystalline, or amorphous structure. Suitable ceramic materials include, but are not limited to, oxides, carbonates, nitrides, carbides, sulfides, oxysulfides, and/or oxynitrides of metals and/or metalloids. Where the anode comprises lithium or a lithium alloy, the ceramic material generally comprises lithium; similarly, where the anode comprises sodium or a sodium alloy, the ceramic material generally comprises sodium. Non-limiting examples of suitable ceramic materials of sufficient ionic conductivity for use in lithium-based systems may be produced by a combination of various lithium compounds, such as ceramic materials including lithium include lithium oxides

where R is scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and/or lutetium), lithium carbonate

lithium nitrides (e.g.,

lithium oxysuifide, lithium oxynitride, lithium garnet-type oxides

lithium phosphorus oxynitride, lithium silicosulfide, lithium germanosulfide,

lithium lanthanum oxides, lithium titanium oxides, lithium borosulfide, lithium aluminosulfide, lithium phosphosulfide, lithium silicate, lithium borate, lithium aluminate, lithium phosphate, lithium halides, and combinations of the above. In certain cases, the ceramic material comprises a lithium oxide, a lithium nitride, or a lithium oxysuifide. In some embodiments, the ceramic includes a carbonate and/or a carbide.

Examples of ceramic materials that can be used as a lithium-ion containing conductive material include: Li-containing oxides such as Nasicon structure such as

other oxides such as

sulphides such as

antiperovskites such as

hydrides such as

(where X = Cl, Br, or I), LiNH,

borates or phosphates such as

carbonates or

hydroxides such as

fluorides such as LiF; nitrides such as

sulphides such as lithium borosulphides; lithium phosphosulfides, lithium aluminosulfides, oxysulfides, praseodymium oxide. At least one of said ceramic materials may be used, or combinations thereof. As noted above, where the anode comprises sodium metal or a sodium alloy, the sodium ion equivalent of any of these conductive materials may be utilised.

In some examples, the ionically conductive material may be formed of a polymeric material which is inherently ionically conductive, such as Nafion. Alternatively, polymers blended with lithium (or sodium) salts, which can achieve bulk conductivities of greater than 10"⁷ S/cm, may also be used. Examples of suitable polymers include, but are not limited to, ethylene oxide (EO) based polymers (for example PEO); acrylate based polymer (for example PMMA); polyamines (polyethyleneimine); siloxanes (poly(dimethylsiloxane)); polyheteroaromatic compounds (e.g., polybenzimidazole); polyamides (e.g. Nylons), polyimides (e.g. Kapton); polyvinyls (e.g. polyacrylamide, poly(2-vinyl pyridine), poly(N-vinylpyrrolidone), poly(methylcyanoacrylate), poly(vinyl acetate), poly (vinyl alcohol), poly(vinyl chloride), poly(vinyl fluoride); inorganic polymers (e.g. polysilane, polysilazane. polyphosphazene, polyphosphonate); polyurethanes; polyolefins (e.g., polypropylene, polytetrafluoroethylene); polyesters (e.g., polycarbonate, polybutylene terephthalate). Optionally, co-block polymers such as Nafion may be used. At least one of said polymeric materials may be used, or combinations thereof. It may be the case that the cathode contains ceramic particles in combination with one or more ionically conductive polymers.

Optionally, the conducting ceramic material is selected from at least one of lithium lanthanum zirconium oxide

(LLZO) , lithium aluminium titanium phosphate (LATP), lithium germanium phosphorus sulphide

(LGPS), or lithium sulphide-phosphorous pentasulphide

and the ionically conductive polymer is selected from at least one of polypyrole, polythiophene, polyaniline, polyacetylene, polyphenylene vinylene and poly(3,4- ethylenedioxythiophene), or combinations thereof.

Optionally, the cathode contains from 1 to 60 % by weight ionically conductive material based on the total weight of the cathode. Optionally, the cathode further comprises an electronically conductive carbon material. Often, the electronically conductive carbon material is selected from carbon nanotubes, carbon nanofibers, graphene, graphene oxide, graphite, carbon black, activated carbon (e.g. Maxorb III®), and combinations thereof. The electronically conductive carbon material may be present in the range of from 0.1 to 30 wt%, optionally 1 to 20 wt%, optionally 5 to 15 wt% of the total weight of the cathode. Inclusion of an electronically conductive carbon material results in an increase of the electronic conductivity within the cathode. In addition, inclusion of an electronically conductive carbon material adds a degree of porosity to the cathode, allowing for a better electrolyte penetration, which contributes to shortening the lithium-ion migration path within the cathode. As a result, power performance of the cell is enhanced.

Optionally, the metal sulphide is pre-lithiated. The pre-lithiation of metal sulphides can provide enhanced structural stability, lithium-ion conductivity, and improve the overall electrochemical performance of the cell.

Optionally, tthhee metal sulphide further comprises selenide forming a metal sulphoselenide, optionally wherein the metal sulphoselenide has the structural formula

where M is a metal and where

The metal may be a transition metal ion, an alkali earth metal, an alkali metal, or a post-transition metal (i.e., metals found in groups 13-16 of the periodic table). Lithium-selenide (Li-Se) batteries are a similar technology to Li-S batteries. Use of a metal sulphoselenide has the advantage of having enhanced power, owing to the high electronic conductivity of Se.

Optionally, the cathode has a thickness in the range of from 20 μm to 200 μm, often in the range of 50 μm to 150 μm. The cathode may be a single or double-sided cathode, although dimensions are quoted without the current collector. Metal sulphides are dense materials and allow for thinner cathodes than cathodes using conventional carbon/sulphur composites. A thinner cathode results in a cell with higher volumetric energy density.

In a second aspect of the invention there is provided a method of producing a cell according to the first aspect of the invention comprising the following steps:

(i) forming a cathode by (a) mixing a metal sulphide with a solvent to produce a slurry, (b) depositing the slurry onto a current collector, (c) removing the solvent to produce a cathode, and (d) cutting the cathode into the desired shape; (ii) placing a separator on the cathode;

(iii) placing an anode on the separator; and

(iv) adding an electrolyte, wherein the polysulfide solubility of the electrolyte is less than 500mM.

It may be the case that the cell is a pouch cell, a prismatic cell, or a cylindrical cell.

Where the cell is a pouch cell, the method may further include the step of placing the cathode, separator, and anode in a pouch prior to addition of the electrolyte.

A plurality of cells according to the first aspect of the invention may be combined to form a cell stack.

The solvent may be selected from water or a suitable organic solvent, such as N-Methyl- 2-pyrrolidone (NMP).

The current collector may comprise aluminium, copper, titanium, or stainless steel. Often, the current collector is aluminium foil.

The separator may be formed from a wide variety of materials. Examples of material used for the separator include, but are not limited to, polyol efin -based materials such as polyethylene, polypropylene, and combinations thereof.

The metal sulphide may be mixed with, for example, an ionically conductive material such as a ceramic material or polymeric material (or combinations thereof) and/or an electronically conductive carbon material and further optional components such as binders prior to forming the slurry. Examples of the ionically conductive material, electronically conductive carbon material, and binder(s) are detailed above in relation to the first aspect of the invention.

Optionally, the method further comprises the step of calendaring or pressing of the cathode prior to cutting. As used herein, the term "calendaring" refers to the compaction process for the cathode. Calendaring can be carried out via conventional methods, such as through the use of calendar rollers. Where calendaring takes place via calendar rollers, the cathode may be passed through rollers up to five times, often one or two times. The rollers may be made of any suitable material, for example steel, glass, or ceramics. A force may be applied on the rollers of 0 kN to 100 kN, often 0 to 80 kN, often example 20 to 80 kN. Calendaring may take place at room temperature (i.e., in the range of from 15 to 25 °C). Heating may optionally be applied to the rollers. The temperature of the rollers may be in the range of from 15 to 80 °C. During calendaring or pressing, the thickness of the cathode decreases. The thickness of the cathode following calendaring or pressing may be from 1 to 50 μm, often 10 to 40 μm, often 15 to 30 μm. Calendaring may result in the reduction of the porosity of the cathode, which allows for a lower electrolyte loading. Moreover, calendaring may also bring about smoothing and levelling of the cathode, which can help extend the life cycle of the cell without affecting the cell utilisation.

In a third aspect of the invention there is provided an electrochemical cell assembly comprising at least one electrochemical cell according to the first aspect of the invention; and a means of applying pressure to the at least one electrochemical cell.

During cycling of the cell, there is an expansion of sulphur, which results in swelling of the cathode. Swelling of the cathode can result in an increase in the overall porosity of the cathode and a decrease in the metal/sulphur interface, which can negatively impact cyclability. Application of pressure to the at least one electrochemical cell can help safeguard the integrity of the cathode and help extend the life cycle of the cell.

Optionally, the means of applying pressure comprises at least one of a band, wrap or tubing positioned on the outside of the cell assembly. A band, wrap or tubing positioned on the outside of the cell assembly allows for a stable constricting force to be applied during cycling. As used herein, the outside of the cell refers to the surface of the anode. Pressure may be applied across the entire surface of the anode. Alternatively, the force may be applied over a portion of the surface of the anode, such as over at least 20% of the surface of the anode. Often, pressure may be applied over at least 40% of the surface of the anode, often over at least 60%, often over at least 80%.

The cell assembly may comprise one or more plates located outside the cell. Where one or more plates are present, pressure may be applied to the one or more plates.

The cell assembly may be located within a housing. When the cell is located within a housing, pressure may be applied to the housing.

The band wrap or tubing can be made of any suitable material, such as elastic materials or shrink-wrap materials. Examples of suitable elastic materials include, but are not limited to, natural or synthetic rubber materials. Examples of shrink-wrap materials include, but are not limited to, polyvinyl chloride (PVC), polyethylene (PE), and polyolefin (POF).

Pressure may be applied to the cell or plurality of cells present in the cell assembly continuously. Alternatively, the pressure may vary over time. Other means of applying pressure may include use of screws or weights.

Unless otherwise stated, each of the integers described may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the invention often "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alterative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about".

In order that the invention may be more readily understood, it will be described further with reference to the figures and to the specific examples hereinafter.

Figure 1 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 1;

Figure 2 illustrates electrochemical performance data (charge-discharge voltage curves) of a cell designed in accordance with Example 2; and

Figure 3 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 3.

Example 1

A lithium sulphur cell with a metal sulphide containing cathode is provided. The cathode can comprise 70 wt.% tungsten disulphide (WS2) nanotubes, impregnated with 10% sulphur, with the metal sulphide-sulphur composite as the active material, 10 wt.% Ketjen Black as a conductive additive and 10 wt.% PEO as a binder.

The cathode powder can be prepared by simple agitation and mixing methods with the use of a three-roll mill. The binder can be added in a second step to form an aqueous slurry which can be coated onto an aluminium based current collector to form a cathode.

The liquid electrolyte of the cell can contain a lithium salt, the salt being at a concentration above 75% of its saturation concentration. In this example, the liquid electrolyte may consist of lithium bis(fluorosufonyl)imide (LiFSI) dissolved within Di methoxyethane (DME) to a molar concentration of 4.5 M. Lithium metal foil 100 micron thick can be utilised as the negative electrode (anode). The liquid electrolyte component is held within an inert separator placed between the electrodes.

Electrochemical performance data characteristic of the cell is provided in Figure 1, based upon cycling of the cell between 1 and 3V under an applied current equivalent to a rate of C/10 based upon the total sulphur content of the cathode, measured by mass, and assuming the theoretical capacity of sulphur to be 1672 mA h

Example 2

A lithium sulphur cell with a metal sulphide containing cathode is provided. The cathode can comprise 90 wt.% molybdenum sulphide (M0S2) nanosheets as the active material and 10 wt.% PEO as a binder.

The liquid electrolyte of the cell can contain a lithium salt, the salt being at a concentration above 75% of its saturation concentration. In this example, the liquid electrolyte may consist of lithium bis(fluorosufonyl)imide (LiFSI) dissolved within Di methoxyethane (DME) to a molar concentration of 4.5 M. Lithium metal foil 100 micron thick can be utilised as the negative electrode (anode). The liquid electrolyte component is held within an inert separator placed between the electrodes. Electrochemical performance data characteristic of the cell is provided in Figure 2, based upon cycling of the cell between 1 and 3V under an applied current equivalent to a

Rate of C/10 based upon the total sulphur content of the cathode, measured by mass, and assuming the theoretical capacity of sulphur to be 1672 mA h

Example 3

A lithium sulphur cell with a metal sulphide containing cathode is provided. The cathode can comprise 70 wt.% flower-like molybdenum sulphide

with metal sulphide particles encapsulated with aluminium oxide, 20 wt.% Ketjen Black as a conductive additive and 10 wt.% PEO as a binder.

Electrochemical performance data characteristic of the cell is provided in Figure 3, based upon cycling of the cell between 1 and 3V under an applied current equivalent to a rate of C/10 based upon the total sulphur content of the cathode, measured by mass, and assuming the theoretical capacity of sulphur to be 1672 mA h

It would be appreciated that the cell, method and cell assembly of the invention is capable of being implemented in a variety of ways, only a few of which have been illustrated and described above.

Claims

1. An electrochemical cell comprising: an anode comprising an alkali metal, an alkali metal alloy, silicon, carbon, or a siliconcarbon composite material; a cathode comprising a metal sulphide; and a liquid electrolyte, wherein the polysulfide solubility of the liquid electrolyte is less than 500 mM.

2. A cell according to claim 1, wherein the metal sulphide has a one-dimensional nanotube structure.

3. A cell according to claim 1, wherein the metal sulphide has a two-dimensional layered structure.

A cell according to claim 1, wherein the metal sulphide has a three-dimensional structure, optionally selected from core-shell structured nanoparticles and/or flower-like nanomaterials.

5. A cell according to any preceding claim, wherein the metal sulphide is selected from molybdenum disulphide

tin sulphide

titanium sulphide vanadium sulphide tungsten disulphide

hafnium disulphide (

tantalum disulphide

Rhenium disulphide or combinations thereof.

6. A cell according to any preceding claim, wherein the metal sulphide is a multimetal sulphide.

7. A cell according to any of claims 1 to 4, wherein the metal sulphide is a monometal sulphide with a structural formula wherein and M is selected

from Mo, Sn, Ti, V, W, Ta, Hf, or Re.

8. A cell according to any preceding claim, wherein the metal sulphide comprises molybdenum disulphide.

9. A cell according to any preceding claim, wherein the metal sulphide is in particulate form.

10. A cell according to claim 9, wherein the metal sulphide particles additionally comprise a coating, the coating comprising a ceramic material, a polymeric material, or a combination thereof.

11. A cell according to claim 10, wherein the coating has a thickness in the range of from 1 nm to 150 nm.

12. A cell according to any preceding claim, wherein the metal sulphide further comprise sulphur.

13. A cell according to any preceding claim, wherein the liquid electrolyte has a polysulphide solubility less than 400 mM, optionally less than 200 mM, optionally less than 100 mM.

14. A cell according to any preceding claim, wherein the alkali metal or alkali metal alloy comprises lithium and/or sodium.

15. A cell according to any of claims 1 to 13, wherein the anode comprises silicon.

16. A cell according to any claims 1 to 13, wherein the anode comprises a carbon material.

17. A cell according to any of claims 1 to 13, wherein the anode comprises a siliconcarbon composite material.

18. A cell according to any preceding claim, wherein the liquid electrolyte comprises a solvent selected from linear ethers, diethyl ether (DEE), tetra hydrofuran (THF), Di methoxyethane (DME), Dioxolane (DIOX), Diglyme, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl formate (MF), ethyl formate (EF), methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB), methyl ethyl ketone, acetonitrile (ACN), propionitrile (PN), isobutyronitrile (iBM), Dimethylformamide (DMF), Dimethylacetamide (DMAc), N-Methyl- 2-pyrrolidone (NMP), Tetram ethyl urea (TMU), Dimethyl sulfoxide (DMSO), Trimethyl phosphate, Triethyl phosphate, Hexamethylphosphoramide, toluene, benzene, heptane, xylene, dichloromethane, ionic liquids, fluorinated ethers, gels, or a combination thereof; and at least one alkali metal salt.

19. A cell according to claim 18, wherein the alkali metal salt comprises lithium when the anode comprises lithium or a lithium alloy; and wherein the alkali metal salt comprises sodium when the anode comprises sodium or a sodium alloy.

20. A cell according to claim 19, wherein the alkali metal salt is at least one lithium salt selected from lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium perchlorate, lithium sulfate, lithium nitrate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethane)sulfonimide, lithium bis(fluorosufonyl)imide, lithium bis(oxalate) borate, lithium difluoro(oxalate)borate, lithium bis(pentafluoroethanesulfonyl)imide, lithium 2-trifluoromethyl-4,5-dicyanoimidazole, and combinations thereof; or wherein the alkali metal salt is at least one sodium salt selected from sodium hexafluoroarsenate, sodium hexafluorophosphate, sodium perchlorate, sodium sulfate, sodium nitrate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethane)sulfonimide, sodium bis(fluorosufonyl)imide, sodium bis(oxalate) borate, sodium difluoro(oxalate)borate,

dicyanoimidazole, and combinations thereof.

21. A cell according to any of claims 18 to 20, wherein the concentration of the at least one alkali metal salt is at least 75% of the saturation concentration of the electrolyte.

22. A cell according to any preceding claim, wherein the cathode further comprises a current collector.

23. A cell according to claim 22, wherein the current collector is aluminium foil.

24. A cell according to any preceding claim, wherein the cathode further comprises a binder.

25. A cell according to claim 18, wherein the binder is selected from polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), Nafion, polypyrole, polythiophene, polyaniline, polyvinyl alcohol, poly(ethylene) imine, polyacetylene, polyphenylene vinylene, poly(3,4- ethylenedioxythiophene), polyphenylene sulphide, gelatine, or combinations thereof.

26. A cell according to claim 24 or 25, wherein the cathode comprises 0.05 to 20 weight % binder based on the total weight of the cathode.

27. A cell according to any preceding claim, wherein the cathode further comprises an ionically conductive material.

28. A cell according to claim 27, wherein the ionically conductive material is selected from a conducting ceramic material, an ionically conducting polymer, or a combination thereof.

29. A cell according to claim 28, wherein the alkali metal or alkali metal alloy comprises lithium, and wherein the conducting ceramic material is selected from at least one of lithium lanthanum zirconium oxide (LLZO), lithium aluminium titanium phosphate (LATP), lithium germanium phosphorus sulphide (LGPS), oorr lithium sulphide- phosphorous pentasulphide; and the ionically conductive polymer is selected from at least one of polypyrole, polythiophene, polyaniline, polyacetylene, polyphenylene vinylene and poly(3,4-ethylenedioxythiophene).

30. A cell according to any of claims 27 to 29, wherein the cathode contains from 1 to 60 % by weight ionically conductive material based on the total weight of the cathode.

31. A cell according to any preceding claim, wherein the cathode further comprises an electronically conductive carbon material.

32. A cell according to claim 31, wherein the electronically conductive carbon material is selected from carbon nanotubes, carbon nanofibers, graphene, graphene oxide, graphite, carbon black, activated carbon and combinations thereof.

33. A cell according to any preceding claim, wherein the metal sulphide is pre- lithiated.

34. A cell according to any preceding claim, wherein the metal sulphide further comprises selenide forming a metal sulphoselenide.

35. A cell according to any preceding claim, wherein the electrolyte loading is in the range of from 0.5 μL/mAh to 3 μL/mAh.

36. A cell according to any preceding claim, wherein the cathode has a thickness in the range of from 20 μm to 200 μm.

37. A method of producing a cell according to any of claims 1 to 36 comprising the following steps:

(i) forming a cathode by (a) mixing a metal sulphide with a solvent to produce a slurry, (b) depositing the slurry onto a current collector, (c) removing the solvent to produce a cathode, and (d) cutting the cathode into the desired shape;

(v) placing a separator on the cathode;

(vi) placing an anode on the separator; and

(vii) adding an electrolyte, wherein the polysulfide solubility of the electrolyte is less than 500mM.

38. A method according to claim 37, further comprising calendaring or pressing the cathode prior to cutting.

39. An electrochemical cell assembly comprising at least one electrochemical cell according to any of claims 1 to 36; and a means of applying pressure to the at least one electrochemical cell.

40. An electrochemical cell assembly according to claim 39, wherein the means of applying pressure comprises at least one of a band, wrap or tubing positioned on the outside of the cell assembly.