WO2018011576A1

WO2018011576A1 - Electrode material

Info

Publication number: WO2018011576A1
Application number: PCT/GB2017/052049
Authority: WO
Inventors: Jonas SOTTMANN; Matthias Herrmann; Helmer FJELLVÅG
Original assignee: Universitetet I Oslo; Golding, Louise
Priority date: 2016-07-13
Filing date: 2017-07-12
Publication date: 2018-01-18
Also published as: GB201612174D0

Abstract

The invention provides sodium-ion and lithium-ion cells comprising: (i) a positive electrode; (ii) a negative electrode comprising: an active material of formula (I): X_pM_m(AO₄)_n (I); or a composite of active materials of formulae (II) and (III): X_jM (II) + X_k(AO₄) (III); and (iii) an electrolyte; wherein in formula (I): X is Na or Li; each M is independently selected from the group consisting of the group IV metals having an oxidation state of from -4 to +2, and the group V metals having an oxidation state of from -3 to +3; each A is independently selected from the group consisting of the transition metals, and the main group elements from groups III to VII; p is 0 or any positive rational number which is less than 36, preferably p is 0; m and n are integers independently selected from 1, 2 and 3; and p, m and n are selected such that the compound of formula (I) is electrically neutral; wherein in formula (II) and (III): X and A are as defined in formula (I); M is selected from the group consisting of the group IV metals having an oxidation state of from -4 to 0, and the group V metals having an oxidation state of from -3 to 0; j is 0 or any positive rational number up to 4; k is any positive rational number from 1 to 6, preferably from 1 to 4, e.g. 1, 2 or 3; and j and k are selected such that each compound of formula (II) and formula (III) is electrically neutral. Also provided are batteries, in particular rechargeable batteries, which comprise such cells or a plurality of such cells.

Description

Electrode material

Technical field

The present invention relates generally to the field of cells and batteries and, in particular, to rechargeable cells and batteries. More particularly, it relates to negative electrode materials (also referred to herein as "anode" materials) for use in both sodium-ion and lithium-ion cells and batteries.

Background of the invention

Lithium-ion batteries (LIBs) are the power source of choice in portable devices and electric drive vehicles due to their lightweight construction, relatively low cost and high energy density. However, mature as the technology may seem, improvements in terms of safety, life, and particularly, energy density are still required to fulfil the demand of these applications.

The development of low-cost and efficient storage of off-peak electric energy generated by intermittent renewable energy sources is becoming a crucial issue. The ambient rechargeable battery technology offers good opportunities but, for efficient large-scale stationary storage, Li-ion batteries are not considered as the best option due to the high-costs and shortage of lithium resources (see e.g. Table 1 in Slater et al., Adv. Funct. Mater. 23: 947-958, 2013).

Unlike lithium, sodium is relatively cheap and readily available worldwide. Contrary to LIBs, in sodium-ion batteries (SIBs) the lighter and cheaper aluminium foil instead of copper foil can be used as current collector for the anode which saves weight and costs. Similarities in the chemistries of LIB and SIB technologies are likely to facilitate a fast and cost-effective scale up of SIB production. It follows that there is a huge incentive to develop rechargeable, low-cost SI Bs of reasonable energy density with high charge and discharge rates.

The analogy between SIB technology and LIB systems does not, however, extend to the choice of anode material. Graphite is the most commonly used anode material in LIBs, but cannot be used in SIBs due to the inability of sodium to intercalate reversibly with graphite. It is therefore fundamental to develop suitable alternative anode materials for SIBs. Lithium-ion batteries comprise one or more positive electrodes, one or more negative electrodes and an electrolyte provided within a housing or casing. Both the positive and negative electrodes include active materials which are capable of accepting and releasing lithium ions. During charging and discharging of the battery, lithium ions move between the positive electrode and the negative electrode. For example, when the battery is discharged the lithium ions flow from the negative electrode to the positive electrode. When the battery is charged, the lithium ions move back to the negative electrode. The electrochemical roles of the electrodes reverse between anode (negative electrode) and cathode (positive electrode) depending on the direction of current flow through the cell. In a sodium-ion battery it is sodium ions which are the charge carriers and which are responsible for the generation of a current in an external circuit.

Graphite is most commonly used as the negative electrode material in LIBs and can intercalate one lithium ion per six carbon atoms with a theoretical capacity of 370 mAh/g (750 mAh/cm³). However, the sodium-binding capacity of graphite is very low due to a weak binding between sodium and carbon. In contrast to LIBs, hard carbons are presently the most promising carbonaceous materials for use in the production of SIB negative electrodes (anodes). They exhibit reversible capacities of up to 300 mAh/g (450 mAh/cm³) in SIBs and are thereby comparable to graphite in LIBs. However, high-rate capability is problematic due to sodium plating (< 0.1 V) during discharge and removal of sodium ions from enclosed pores in the structure during charge (see, for example, Bommier et al., Isr. J. Chem. 55: 486-507, 2015 and Luo et al., Acc. Chem. Res. 49: 231-24, 2016).

Higher gravimetric and volumetric capacities as well as better rate-capabilities can be achieved with so-called alloying anode materials which contain group IV and V metals. In contrast to carbon materials, these can combine with several lithium or sodium atoms. Prominent examples are the use of silicon for LIBs (3579 mAh/g or 8303 mAh/cm³) (Wang et al., Nano Energy 8: 71-77, 2014) and red phosphorus for SI Bs (2596 mAh/g or 5893 mAh/cm³) (Kim et al., Adv. Mater. 25: 3045-3049, 2013 and Qian et al., Angew. Chem. Int. Ed. 52: 4633-4636, 2013). However, a significant drawback when using these materials is that large associated volume changes (about 300% for the afore-mentioned examples) can occur during sodiation and desodiation which can deteriorate the cycling stability (see Larcher et al., J. Mater. Chem. 17: 3759-3772, 2007 and Obrovac et al., Chem. Rev. 1 14: 1 1444-11502, 2014). In many cases, crystallite size engineering and careful selection of the binder and carbon matrix plays a key role in compensating for the large volume changes and the low electrical conductivity of all or some phases present during

electrochemical cycling (see Bommier et al., above).

There are therefore a number of challenges associated with the design and production of lower cost SIBs. It would be advantageous to provide a SIB (especially a rechargeable SIB) which addresses some or all of these challenges, in particular a SI B which provides superior specific capacities compared to existing carbon-based anodes, which shows good capacity stability, and which has high discharge and a high rate of performance.

The present invention addresses these needs by way of a new family of active materials for use in the preparation of anodes (negative electrodes) of SIBs, in particular rechargeable SIBs. The properties of such materials also make these suitable for use in LIBs.

Summary of invention

We have now discovered that a particular family of active materials and related 'composite' materials may be used in the production of negative electrodes (anodes) for use in both primary (non-rechargeable) and secondary (rechargeable) cells and batteries, especially in SIBs. These have high specific capacities and low charge- discharge potentials vs sodium, low environmental load, and low manufacturing costs. These electrode materials can be used in both SIB and LIB technology.

In one aspect the invention provides a sodium-ion or lithium-ion cell (e.g. a rechargeable sodium-ion or lithium-ion cell) comprising:

a positive electrode;

a negative electrode comprising an active material of formula (I):

(wherein:

X is Na or Li; each M is independently selected from the group consisting of the group IV metals having an oxidation state of from -4 to +2, and the group V metals having an oxidation state of from -3 to +3;

each A is independently selected from the group consisting of the transition metals, and the main group elements from groups III to VII;

p is 0 or any positive rational number which is less than 36;

m and n are integers independently selected from 1 , 2 and 3; and

p, m and n are selected such that the compound of formula (I) is electrically neutral); and

an electrolyte.

In another aspect the invention provides a sodium-ion or lithium-ion cell (e.g. a rechargeable sodium-ion or lithium-ion cell) comprising:

a positive electrode;

a negative electrode comprising a composite of active materials of formulae (II) and (III):

X_jM (II) + X_k(A0₄) (III)

(wherein:

X is Na or Li;

M is selected from the group consisting of the group IV metals having an oxidation state of from -4 to 0, and the group V metals having an oxidation state of from -3 to 0;

A is selected from the group consisting of the transition metals, and the main group elements from groups III to VII;

j is 0 or any positive rational number up to 4;

k is any positive rational number from 1 to 6, preferably from 1 to 4, e.g. 1 , 2 or 3; and

j and k are selected such that each compound of formula (II) and formula (III) is electrically neutral);

and

an electrolyte.

In another aspect the invention provides a sodium-ion or lithium-ion battery (e.g. a rechargeable sodium-ion or lithium-ion battery) which comprises at least one cell as herein described within a suitable housing. In a further aspect the invention provides an electrode comprising: (i) an active material of formula (I) as herein described, or a composite of active materials of formulae (II) and (III) as herein described; (ii) optionally a conductive carbon material; and (iii) optionally a binder.

In a yet further aspect the invention provides the use of a compound of formula (I) as herein described, or a composite of active materials of formulae (II) and (III) as herein described, as an active electrode material, e.g. as a negative electrode material (anode).

Detailed description of invention

As used herein a "cell" is a basic electrochemical unit comprising a positive electrode (also referred to herein as the "cathode"), a negative electrode (also referred to herein as the "anode"), and an electrolyte. When the electrolyte is a liquid electrolyte the cell will normally also comprise a separator situated between the electrodes. The separator is usually a semi-permeable membrane (often a polymeric membrane) which permits ionic charge carriers to travel through the electrolyte from one electrode to the other whilst separating the electrodes in order to prevent short circuits.

A "lithium-ion cell" (or "lithium cell") is an example of an electrochemical cell in which the ionic charge carriers are lithium ions. A "sodium-ion cell" (or "sodium cell") is an example of an electrochemical cell in which the ionic charge carriers are sodium ions.

As used herein, a "battery" is a cell or plurality of cells which are ready for use.

Typically a battery comprises a plurality of cells, a housing, a current collector at each of the positive and negative electrodes which enable connection to an external circuit, and optionally a protection circuit. Preferably a battery will comprise a plurality of cells with electrical interconnections between the cells.

The term "lithium-ion battery" (or, equivalently, "lithium battery") is a battery in which the ionic charge carriers are lithium ions (and therefore a lithium battery comprises at least one lithium-ion cell, typically a plurality of lithium-ion cells). The term "sodium- ion battery" (or "sodium battery") should be construed accordingly. The function of the electrolyte in a lithium- or sodium-ion cell is to facilitate the movement of lithium or sodium ions from the negative electrode to the positive electrode during discharge of the cell and their movement in the reverse direction during charging (which is driven by an external power source). During discharge of the cell, electrochemical reduction takes place at the positive electrode as electrons flow through an external circuit (i.e. a circuit external to the cell) towards the positive electrode while cations move within the cell from the electrolyte to the positive electrode. These processes are reversed, at least partially, during cell charging: connection of the cell to an external electrical power source (a charging circuit) which applies an over-voltage (a higher voltage than that produced by the cell, having the same polarity) causes the ion flow and electron flow to occur in the reverse directions compared to discharging. The positive electrode is commonly referred to during discharging as the "cathode" and the negative electrode as the "anode". This terminology is conventional in the field of batteries and therefore this is adopted herein.

The active materials of both the negative and positive electrodes are such that these allow the partly reversible movement of sodium or lithium ions into or out of their structures as appropriate. Although not wishing to be bound by theory, it is understood that movement of sodium or lithium ions into and out of their structures will typically occur via "insertion" and "extraction" mechanisms (also known as "intercalation" and "de-intercalation", respectively). Alternatively, the ions may bind and release from the structures (i.e. bind reversibly) by "conversion" or "alloying" reactions which are described, for example, in Tarascon et al., Nature 414 (6861): 359-367, 2001 , and in Cabana et al., Adv. Mater. 22: E170-E192, 2010). It is not intended that the invention should in any way be limited by the precise mechanism by which the active materials enable the movement of sodium or lithium ions.

In the cells according to the invention the negative electrode (anode) comprises an active material of general formula (I) as herein defined, or it may comprise a combination or mixture (also referred to herein as a "composite") of active materials of formulae (II) and (III) as herein defined. The relationship between these structures is described in more detail below and in the accompanying example.

In formula (I), each M is either a group IV metal having an oxidation state of from -4 to +2, or a group V metal having an oxidation state of from -3 to +3. The term "metal" as used herein is to be construed broadly and is considered to encompass "metalloids". M may be selected from any of the following metals: silicon, germanium, tin, lead, arsenic, antimony and bismuth. Preferred for use in the invention are active materials of formula (I) in which M is bismuth, antimony, lead, tin or silicon, more preferably bismuth, antimony, lead or tin.

Where m is 2 or 3, each M may be the same or different. Where these are different, formula (I) encompasses mixed metal compounds. Suitable mixed metal compounds include those containing bismuth and antimony, or tin and lead.

In formula (I) each A is either a transition metal, or a main group element from any of groups III to VII. As will be understood, A should be capable of combining with oxygen to form an oxyanion of formula A0₄. First and second row transition metals are generally preferred, in particular titanium, vanadium, chromium, manganese, copper, niobium, molybdenum and cadmium. The main group elements may be metals (including metalloids) or non-metals and will typically be selected from periods 3, 4 or 5 of the periodic table. Suitable main group elements, A, include silicon, phosphorus, sulphur, germanium, arsenic, selenium, antimony and bismuth.

Where n is 2 or 3, each A may be the same or different. Where these are different, formula (I) encompasses compounds containing different oxyanions. For example, (A0₄)_n where n is either 2 or 3 may be selected from the following groups:

(ΑΌ₄ΧΑΌ₄), (ΑΌ₄)(ΑΌ₄)(Α"Ό₄), (ΑΌ₄)₂(ΑΌ₄), or (ΑΌ₄)(ΑΌ₄)₂ in which A', A" and A'" are used to denote different groups A as herein defined. Suitable mixed oxyanion compounds include those containing V0₄ ^3" and Mo0₄ ^2", for example.

The oxyanion (A0₄) will typically be tetrahedral in geometry, although it may also be square planar. Suitable oxyanions may be selected by those skilled in the art for any given metal, or combinations of metal, M. A may, for example, be chosen from any of the following elements: titanium, vanadium, chromium, manganese, copper, niobium, molybdenum, cadmium, silicon, phosphorus, sulphur, germanium, arsenic, selenium, antimony and bismuth. Preferably, A will be silicon, phosphorus, sulphur, titanium, vanadium, arsenic, niobium, molybdenum, antimony or bismuth. More preferably A will be vanadium, molybdenum, phosphorus or sulphur. Examples of the oxyanion include the following: V0₄ ^3", P0₄ ^3", Mo0₄ ^2", S0₄ ^2", Si0₄ ^4", Ti0₄ ^4", Nb0₄ ^3", As0₄ ^3", Sb0₄ ^3", and Bi0₄ ^3". Preferred for use in the invention are active materials of formula (I) in which at least one of the oxyanions is selected from V0₄ ^3", P0₄ ^3", Mo0₄ ^2", S0₄ ^2", Si0₄ ^4", Ti0₄ ^4", Nb0₄ ^3", preferably from V0₄ ^3", P0₄ ^3", Mo0₄ ^2", and S0₄ ^2". Preferably, all oxyanions which are present in the compound of formula (I) will be identical and will be selected from such groups.

In formula (I), p may be 0 or it may be a positive rational number such that the active material is partially sodiated or lithiated. The value of p will be dependent on the degree of pre-sodiation or pre-lithiation of the material but typically this will be a positive rational number less than 36. p will generally range from 0 to 24, preferably from 0 to 6, e.g. 0 or 1.

Where p is other than zero, the compounds of formula (I) contain a proportion of sodium or lithium. As the compounds take up a greater proportion of sodium or lithium, these may be expected to dissociate into composite materials comprising more than one compound, for example compounds of formula (II) and (III) as defined herein.

As will be understood, where p in formula (I) is zero, no sodium or lithium will be present. In this case, the active materials may be represented by formula (la):

M_m(A0₄)_n (la)

(wherein:

each M is independently selected from the group consisting of the group IV metals having an oxidation state of +2, and the group V metals having an oxidation state of +3;

m and n are integers independently selected from 1 , 2 and 3 and are selected such that the compound of formula (la) is electrically neutral).

In formula (la), metal M and element A may be chosen from any of the metals and elements listed above in respect of formula (I). As would be understood, in the compounds of formula (la) the oxyanion will either be A0₄ ^{3m n"} or A0₄ ^{2m n"} depending on the charge on the metal, M, and so may carry a charge of -1 , -2 or -3. In one embodiment the active material for use in the invention is a compound of formula (lb):

Bi_m(A0₄), (lb) in which A is as hereinbefore defined, preferably vanadium, molybdenum or phosphorus;

m is an integer selected from 1 or 2; and

n is an integer selected from 1 or 3.

Examples of compounds of formula (lb) include Bi₄(Si0₄)₃, BiP0₄, Bi₂(S0₄)₃, Bi₄(Ti0₄)₃, BiV0₄, Bi₂Cu0₄, Bi₄(Ge0₄)₃, BiAs0₄, BiNb0₄, Bi₂(Mo0₄)₃, Bi₂Cd0₄, BiSb0₄, and BiBi0₄.

In another embodiment the active material for use in the invention is a compound of formula (lc):

Pb_m(A0₄), (lc) in which A is as hereinbefore defined, preferably vanadium, molybdenum or phosphorus;

m is an integer selected from 1 or 3; and

n is an integer selected from 1 or 2.

Examples of compounds of formula (lc) include Pb₂Si0₄, Pb₃(P0₄)₂, PbS0₄, Pb₃(V0₄)₂, PbCr0₄, Pb₂Mn0₄, PbSe0₄, and PbMo0₄.

In the case where M is either antimony, arsenic or tin, examples of active materials for use in the invention include: SbP0₄, Sb₂(S0₄)₃, SbV0₄, Sb₄(Ge0₄)₃, SbNb0₄, AsP0₄, As₂(S0₄)₃, Sn₃(P0₄)₂, and SnS0₄.

Preferred for use in the invention are the following active materials: Bi(V0₄), Bi(P0₄), Bi₂(Mo0₄)₃, Pb(Mo0₄), Sn(S0₄), and Sb₂(S0₄)₃.

In a further aspect the active electrode material for use in the invention may comprise a combination of compounds of formula (II) and (III) (also referred to herein as a "composite"). As discussed in the accompanying example, such compounds are produced during the conversion reaction of a compound of formula (I) during the first discharge of the cell. As such, these can be expected to function, when used in combination, as active negative electrode (anode) materials.

In formulae (II) and (III), metal M and element A may be chosen from any of the metals and elements listed above in respect of formula (I), (la), (lb) or (lc). A suitable molar ratio of compounds (II) and (III) in the composite can readily be determined by those skilled in the art, however, typically these may be used in a molar ratio which is less than or equal to n:m where m and n are as defined in formula (I).

As will be understood, the value of j in the compound of formula (II) will be dependent on the oxidation state of the metal, M. Where M is a group IV metal, j may be 0 or any positive rational number up to and including 4. Where M is a group V metal, j may be 0 or any positive rational number up to and including 3. In one embodiment, j may be zero.

The materials proposed herein for use as the active material of the negative electrode are known in the art and many are commercially available, e.g. from Sigma Aldrich, Goodfellow, Alfa Aesar, American Elements, Heubach Color, Heucotech, DCC, BASF and Cappelle.

Composite materials of formula (II) and (III) may be produced, for example, by ball milling a suitable metal, M, with a compound of formula (III). For example, a metal such as bismuth, silicon or tin might be ball milled with Na₂Mo0₄ or Na₃V0₄ to form a suitable composite material. One example of a suitable composite material is that formed from Bi and Na₂Mo0₄. In an embodiment, these materials may be combined in a weight ratio of about 1 : 1.5 (Bi : Na₂Mo0₄).

Other components of the negative electrode will typically include a conductive additive such as carbon, e.g. carbon black, graphene, graphene oxide, graphite, hard carbons, and nanostructured carbons (including single and multi-walled carbon nanotubes). These may be present in an amount from 0 to 80 wt.%, preferably 5 to 40 wt.%, more preferably 20 to 40 wt.%, e.g. about 30 wt.% (based on the weight of the electrode).

A suitable binder material may also be used to hold together the various components of the negative electrode. Suitable binder materials include polymeric materials such as polyvinylidine fluoride (PVDF), elastomeric polymers such as polyvinylalcohol (PVA), poly(acrylic acid) (PAA), PAN, CMC, and PTFE. Conductive binders may also be used, such as poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic eser (PFM). Any binder may be present in an amount from 0 to 50 wt.%, preferably 5 to 25 wt.%, more preferably 15 to 25 wt.%, e.g. about 20 wt.% (based on the weight of the electrode).

The amount of active material present in the electrode may range from 1 to 100 wt.%, preferably from 45 to 90 wt.%, more preferably 40 to 60 wt.% (based on the weight of the electrode).

The negative electrode may be prepared by methods known in the art and will generally involve mixing of the active material, the conductive additive and the binder in a suitable solvent (e.g. ethanol). In order to increase the surface area of the active material this may be milled or ground into fine particulate form prior to mixing using known techniques such as ball-milling. Suitable particle sizes may range from 1 to 1 ,000 nm, preferably 1 to 400 nm, e.g. from 3 to 50 nm. Depending on the choice of conductive additive, this may also be milled prior to use, or alternatively milled together with the active material. The resulting slurry may then be coated onto a suitable current collector, for example a metal disc or metal foil. The resulting electrodes may be flat or planar or may be wrapped or wound in a spiral

configuration. Alternatively, these may be folded.

Preparation of the negative electrode may be conducted under conditions which minimise oxidation of the active material, e.g. under conditions avoiding the presence of oxygen and water.

Typically, the positive electrode (cathode) will comprise an active material which contains either lithium or sodium which may be intercalated and de-intercalated during charging and discharging of the cell, respectively.

In a lithium-ion cell the positive electrode is commonly a layered oxide such as lithium cobalt oxide (LiCo0₂), a polyanion such as lithium iron phosphate (LiFeP0₄), or a spinel such as lithium manganese oxide (LiMn₂0₄). Other commonly-used positive electrode materials include lithium nickel cobalt aluminium oxide

(LiNio.8Coo.15Alo.05O2 (NCA)), lithium nickel manganese colbalt oxide

(LiN sCo^Mn^Oa (NMC)), and lithium nickel oxide (Li_1-zNi_1+z0₂ where 0<z<0.2). Other layered compounds which may be used include LiTiS₂ and Li₂Mn0₃. Other spinels which may be used include LiCo₂0₄ and LiMn_{1 5}Nio_.50₄. Other suitable materials for the positive electrode in a lithium-ion cell include: LiMnP0₄, LiCoP0₄, LiFeS0₄F and LiVP0₄F.

In a sodium-ion cell the positive electrode may be selected from hexacyanoferrates / Prussian blue analogues such as Na_xMFe(CN)₆ (where M is Ni, Cu, Fe, Mn, Co or Zn and x is from 0 to 2); fluorides such as MF₃ and NaMF₃ (where M is Ni, Fe or Mn); layered transition metal oxides such as NaCo0₂; tunnel-type oxides such as

Na₀₄₄MnO₂ ; olivine phosphates such as NaMP0₄ (where M is Fe or Mn);

pyrophosphates and mixed polyanions such as Na₂FeP₂0₇; and fluorophosphates, e.g. Na₃V₂(P0₄)₂F₃.

Other examples of suitable materials for the positive electrode in a sodium-ion cell include: NaTi0₂(P0₄)₃, Na₃TiP₃0₉N, Na₂Mn[Mn(CN)₆], Na₂MnP₂0₇, Na₂Fe₂(S0₄)₃, Na_1.5VP0_4.8Fo.7, Na₂FeP0₄F, P2-Na_{2 3}Mn_{1 2}Fe_{1 2}0₂, NaNi_{1 2}Mn_{1 2}0₂.

An electrolyte is provided between the positive and negative electrodes to provide a suitable medium through which the lithium or sodium ions can travel. The electrolyte may either be a liquid or a solid, and suitable materials are known in the art.

Liquid electrolytes may take the form of a lithium or sodium salt dissolved in one or more non-aqueous solvents, for example in at least one organic carbonate, DME, THF or triglyme. Examples include one or more organic carbonates (e.g. ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylmethyl carbonate, etc.) containing a complex of lithium ions (e.g. lithium

hexafluorophosphate, lithium hexafluoroarsenate monohydrate, lithium perchlorate, lithium tetrafluoroborate or lithium triflate) or a complex of sodium ions (e.g. sodium bis(tri-fluoromethane) sulfonimide (NaTFSI), sodium triflate (NaOTf), sodium perchlorate (NaCI0₄), or sodium hexa-fluorophosphate (NaPF₆).

Electrolyte additives may also be present to provide a more stable solid-electrolyte (SEI) interface. Examples of suitable electrolyte additives include fluoroethylene carbonate (FEC) and vinylene carbonate (VC). FEC is preferred for use in sodium- ion cells and batteries, whereas VC is more suited for use in lithium-ion cells and batteries. In one embodiment a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) and a salt of LiPF₆ or NaPF₆ may be employed as the electrolyte. EC and DEC may be used in a ratio of about 1 : 1. Optionally, this electrolyte may be used in combination with FEC in a sodium-ion cell or battery.

The electrolyte may be an ionic liquid. Suitable ionic liquids are known in the art and include those described in The Journal of Power Sources: 194 (2009), pages 601- 609 (see, in particular, page 603 in respect of suitable ionic liquids and additives for use with different Li-based electrochemical systems). For use in sodium-ion cells and batteries, the following are particularly suitable: NaTFSI-doped N-methyl-N- propylpyrrolidinium bis(fluorosulfonyl)imide (NaFSI-C1 C3pyrFSI); 1-butyl(propyl)-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4(C3)mpyr]-[TFSI]) doped with NaTFSI. In these liquids, the TFSI anion may be replaced by the FSI (i.e.

(bis(fluorosulfonyl)imide) anion.

Solid state electrolytes such as lithium phosphorus oxynitride (LiPON), Li₃N, Lil, "NASICON" (sodium super ionic conductors of formula Na_1+x r₂Si_xP₃__x0₁₂, 0≤ x≤ 3, and their lithium analogues), γ-Ι_ί₃Ρ0₄ or LiP0₃, may also be employed. Other examples include β-ΑΙ₂0₃ (Ν3₂0-11ΑΙ₂0₃), β"-ΑΙ₂0₃, and Na₃PS₄. Poly(ethylene oxide) (PEO) is effective in solvating a variety of salts and is widely used as a solid electrolyte. PEO may be used with sodium salts including NaCI0₃ or NaCI0₄. Any of these may be used with poly(ethylene glycol) (PEG) which functions as a plasticizer and leads to an increase in the conductivity

A suitable separator may be provided between the positive and negative electrodes in order to prevent their direct contact with one another. The choice of separator is dependent on the choice of electrolyte material, e.g. whether this is a liquid or solid. Suitable separators are known in the art and include porous polymer materials such as polypropylene/polyethylene copolymers. The presence of pores in the separator material allow for the movement of electrolyte and lithium or sodium ions through the separator. Glassfibre may also be used.

Also provided herein is a battery which comprises at least one lithium-ion or sodium- ion cell as described herein. The cell (or plurality of cells) are provided within a suitable battery housing. The housing encases the components of the cell, providing structural integrity and also provides a barrier between the cell and the external environment so that the electrodes and electrolyte are protected from reaction with atmospheric gases, liquids, etc. Any conventional housing may be used. This may be formed of a metal or metal alloy such as aluminium or alloys thereof, titanium or titanium alloys, stainless steel or other suitable materials. Alternatively, the battery housing may be made of a plastic material. The precise size, shape and

configuration of the battery may be selected based on its intended use. The configuration of the cells for use in the battery may be selected accordingly. For example, these may be flat plate electrodes, wound or coiled electrodes, or folded (e.g. Z-folded).

The battery also comprises a current collector at each of the positive and negative electrodes which enable connection to an external circuit. These are made of a conductive material such as a metal, e.g. aluminium or an aluminium alloy, or copper or a copper alloy. Other materials which may be used include silver and silver alloys, titanium and titanium alloys, nickel and nickel alloys. The current collectors may take various different forms and configurations dependent, for example, on the size and shape of the battery and its intended use. For example, these may be a thin foil of the metal or metal alloy material or may take the form of a metal grid. The use of aluminium is advantageous since this is relatively inexpensive, highly conductive and easy to form into a current collector.

In one embodiment, the active electrode material may be provided on the current collector, e.g. in the form of a coating on the current collector. Where this is coated on the current collector, it may be coated on one or both sides.

In the battery, the cells are electrically interconnected. These may be coupled in series or in parallel.

In an embodiment the battery further comprises a protection circuit. This may be located within the housing or outside it. In some embodiments according the invention, however, this may not be required.

A variety of conventional battery structures are known in the art and any of these are suitable for the batteries herein described. Suitable battery structures include, for example, the cylindrical cell, button cell, prismatic cell and pouch structures. (Note that the use of the term "cell" is conventional in the art when describing battery shape, but in this context it may describe a battery comprising multiple cells within the meaning of the term as herein defined). The batteries according to the invention may be used in a variety of applications and for any use for which conventional lithium-ion batteries are known. Examples of devices which may be powered using a battery (or batteries) as herein described include, in particular, a computer (e.g. a laptop/notebook computer, a tablet computer, a personal digital assistant, a smartwatch, hand-held electronic gaming device, or games console); an e-reader; a mobile phone (e.g. a smartphone); a flashlight (torch); a vehicle (e.g. an electric buggy, electric car or hybrid car, electric vessel or hybrid vessel, electric train or hybrid train, electric wheelchair, model aircraft, drone aircraft, or piloted aircraft); an implantable medical device (e.g. a pacemaker, defibrillator, nerve stimulator, cochlear implant, implantable drug administration device, active monitoring device; a power tool (e.g. a drill, sander, saw, hedge trimmer, strimmer, or lawnmower); and in large scale home energy storage (e.g. the Tesla power wall, Bosch BPT-S 5 Hybrid) or energy storage power plants (e.g. installations in Feldheim and Dresden, Germany).

The invention is illustrated by way of the following non-limiting examples and the accompany figures, in which:

Figure 1 - Voltage profiles of (a) Bi(VO₄)/C-20min, (b) Bi(PO₄)/C-20min, (c)

Bi₂(MoO₄)₃/C-20min and (d) Pb(MoO₄)/C-20min vs Na/Na⁺ for different number of cycles.

Figure 2 - Cycling performance of Bi(V0₄)/C and Bi₂(Mo0₄)₃/C ball milled for 20 min and 24 h. Fig. (a) and (b) show the capacity retention with cycle number for the first 100 cycles Bi(V0₄)/C and Bi₂(Mo0₄)₃/C, respectively. Figs, (c)-(f) show the corresponding voltage profiles vs Na/Na+.

Figure 3 - Long-term cycling performance of Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h. After the 100th cycle the current was increased to 300 mA/g.

Figure 4 - Rate performance of (a) Bi(V0₄)/C-24h and (b) Bi₂(Mo0₄)₃/C-24h.

Figure 5 - Cycling performance of (a) Bi(V0₄)/C-24h and (b) Bi₂(Mo0₄)₃/C-24h cycled in different voltage ranges vs Li/Li+ for the first 50 cycles and corresponding (c/d) voltage profiles measured in the range of 0.01 V to 2.5 V vs Li/Li+. Figure 6 - The voltage profile of Bi(V0₄) and Bi₂(Mo0₄)₃ vs Na/Na+ and Li/Li+ compared to the shift in Bi L3 and Mo K absorption edge position. A shift in the absorption edge position of an element to lower (higher) energies corresponds to a decrease (increase) of its average oxidation state.

Figure 7 - The voltage profile of (a) Pb(MoO₄)/C-20min, (b) Sn(SO₄)/C-20min and (c) Sb₂(SO₄)₃/C-20min vs Na/Na⁺ for the first 5 cycles.

Figure 8 - Capacity retention vs cycle number for the first 250 cycles for

Pb(MoO₄)/C-20min.

Figure 9 - Voltage profiles of Bi/Na₂Mo0₄/C-24h vs Na/Na⁺ for different number of cycles.

Figure 10 - Specific capacity with cycle number for the first 100 cycles of the

Bi/Na₂Mo0₄/C-24h electrode cycled vs Na/Na⁺.

Examples

Example 1 Experimental:

The M_m(A0₄)_n /C composites were prepared by milling of commercial M_m(A0₄)_n powder (Bi(V0₄), Bi(P0₄), Bi₂(Mo0₄)₃, Sb₂(S0₄)₃, Pb(Mo0₄) and Sn(S0₄) purchased from Alfa Aesar or Sigma Aldrich) and conductive carbon black (C, Timcal Super P) in a mass ratio of 7:3 under Argon. Ball-milling was conducted using a Fritsch Mini- Mill Pulverisette 23 at 50 Hz with a ball-to-powder ratio of 10: 1 for 20 min

(M_m(AO₄)_n/C-20min) and a Fritsch Planetary Micro Mill Pulverisette 7 at 720 rpm with a ball-to-powder ratio of 20: 1 for 24 h (M_m(A0₄)_n/C-24h). Grinding balls and bowls were made of steel. The composites were kept under inert conditions to prevent oxidation.

Electrode preparation was performed in a glove box (M. Braun) with 0₂ and H₂0 levels less than 0.1 ppm. The working electrode was prepared by spreading slurry composed of 70 wt % of bismuth carbon composite, 10 wt % of conductive carbon black (Super P, Timcal) and 20 wt % poly(acrylic acid) (PAA, Sigma Aldrich) as binder dissolved in degassed absolute ethanol on Al foil for SIBs and Cu foil for LIBs. PAA binder can accommodate the large expected volume expansions (see Kim et al., Adv. Mater. 25, 3045-3049, 2013; Komaba et al., J. Phys. Chem. C 115, 13487- 13495, 2011 ; and Magasinski et al., ACS Appl. Mater. Interfaces 2, 3004-3010, 2010). Drying of the electrodes was carried out under vacuum at 60°C overnight. The electrodes were thereafter transferred to and stored in the glove box. In the glove box the working electrode was cut into disks with a mass loading of active material of about 1 mg/cm².

The battery was assembled in coin cells (2032) in the glove box. The working electrode was separated from the Na metal disk as counter electrode by electrolyte soaked glass fibres (GF/C, Whatman). As electrolyte a 1 M solution of NaPF₆ in ethylene carbonate/diethyl carbonate (EC/DEC, 1 :1 in wt) solution with the addition of 5 wt % FEC was prepared. All electrolyte constituents were purchased from Sigma Aldrich. The use of FEC (fluoroethylene carbonate) as electrolyte additive was found to lead to more stable (and thus beneficial) formation of the solid- electrolyte interface (SEI) layer (see Qian et al., Chem. Commun. 48, 7070-7072, 2012; Darwiche et al., JACS 134, 20805-2081 1 , 2012; and Bodenes et al., J. Power Sources 273, 14-24, 2015).

Galvanostatic cycling was performed at a current of 150 mA/g (unless otherwise specified) in a voltage range of 0.01 V to 2 V vs Na/Na⁺ and 0.01 V to 2.5 V vs Li/Li⁺ using a Bat-Small battery cycler (Astrol). Specific capacity values of the M_m(A0₄)_n/C composites are expressed on the basis of the mass of M_m(A0₄)_n.

In operando X-ray absorption near egde spectroscopy (XANES) was performed at the Swiss-Norwegian Beam Lines (SNBL), BM01 B, at the ESRF. Bismuth L3 edge and Mo K edge XANES were collected in transmission mode using a Si (1 1 1) channel-cut type monochromator. The second crystal was detuned at about 70 % to reduce higher harmonics. The XANES data were analysed using ATHENA (Ravel et al., Journal of Synchrotron Radiation 12, 537-541 , 2005) for absorption edge determination and spectrum normalization to an edge jump of unity. The absorption edge position was determined as the maximum of the first derivative of the spectrum. The relative shift in absorption energy was calculated with respect to metallic Bi (L3 edge at 13419 eV) and Mo (K edge at 20000 eV). A Mo and Ir foil was used as a reference (Ir L1 edge at 13419 eV). The electrochemical cycling for the in operando characterization was performed in Swagelok type electrochemical cells with Kapton windows which are available at SNBL. The battery assembly was kept identical to the coin cells. The first 1.5 galvanostatic cycles were followed in operando. The applied current was chosen such that each charge/discharge took approximately 10 hours. XANES data collection (2 min scan) was performed in sequence on several cells.

Results:

The voltage profiles of Bi(VO₄)/C-20min, Bi(PO₄)/C-20min and Bi₂(MoO₄)₃/C-20min vs Na/Na⁺ are shown in Fig. 1. After the initial charge two reversible plateaus are observed in the voltage profile of the Bi-based anodes. The voltage drop in the low voltage plateau of Bi(VO₄)/C-20min and Bi₂(MoO₄)₃/C-20min during the first charge steps (Fig. 1a and 1c) is not produced by a change in potential of the working electrode, but by solvent interaction at the Na counter electrode which causes polarization (Rudola et al., Electrochem. Commun. 46, 56-59, 2014). Such a voltage drop is generally observed in flat voltage profiles at high rates of two-phase Na electrode materials. The voltage drop is not discernible for Bi(PO₄)/C-20min.

Fig. 1 d and Fig. 7a show the voltage profile of Pb(MoO₄)/C-20min. This confirms that Bi (representative of a group V metal) can be replaced by a group IV metal. It is clear that detrimental catalytic electrolyte decomposition at the lead metal surface as reported by others (e.g. Ellis et al. J. Electrochem. Soc. 161 , A416-A421 , 2014) is avoided and a reversible cycling regime is established after the initial charge. At this current the reversible capacity for Pb(MoO₄)/C-20min compared to pure lead (485 mAh/g), however, remains low. Further examples of the flexibility in choosing various combinations of M and A are given by Sb₂(SO₄)₃/C-20min and Sn(S0₄) /C-20min which also show reversible cycling behaviour after the initial charge as demonstrated for the first 5 cycles in Fig. 7.

The average operation voltage for Bi₂(MoO₄)₃/C-20min, Bi(VO₄)/C-20min, Bi(P0₄)/C- 20min is 0.57 V < 0.59 V < 0.62 V. For Bi_m(A0₄)_n the average operation voltage lies in the range of voltages which allow for safe operation (meaning low risk for formation of metallic Na) while maintaining a large enough voltage difference between the anode and cathode materials (important criteria for the energy density). The average operation voltage further depends on the group V or IV element M in M_m(A0₄)n as evidenced by comparison of Bi₂(MoO₄)₃/C-20min and Pb(Mo0₄)/C- 20min which have an average operation voltage of 0.57 V and 0.45 V, respectively. The average operation voltage can thus be optimized choosing the appropriate combination of M and A in M_m(A0₄)_n.

Fig. 2 compares the cycling performance of Bi(V0₄) and Bi₂(Mo0₄)₃ ball milled with carbon for 20 min and 24 h. The voltage profiles seem only weakly affected by the milling conditions, while the cycling stability is drastically improved by prolonged milling. It is reasonable to assume that Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h have smaller particle sizes and improved carbon coating which results in better electronic conductivity and thus better cycling performance compared to Bi(VO₄)/C-20min and Bi₂(MoO₄)₃/C-20min. Quite stable capacity retention was found for

Pb(MoO₄)/C-20min over 250 cycles as shown in Fig. 8. Even better capacity retention might be obtained in Pb(Mo0₄)/C-24h in analogy to Bi₂(Mo0₄)₃.

Fig. 3 shows the long term cycling performance of Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C- 24h. For the first 100 cycles a specific current of 150 mA/g was applied. The first discharge capacity is 367 mAh/g and 351 mAh/g of which 93 % and 91 % are retained after 100 cycles for Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h, respectively. After the 100^th cycle the specific current was increased to 300 mA/g. This current corresponds approximately to a charge rate of 1C which means that the battery is fully charged (discharged) in 1 h. A charge rate of 1C further corresponds to the time scales required for large-scale energy storage applications. Also at this rate both materials show excellent capacity retention over 225 cycles (325 in total). The obtained reversible capacity values are thus superior compared to the hard carbon and the capacity retention at rates suitable for large-scale storage applications is excellent for conversion/alloying materials and superior to hard carbons. The specific capacity of a material is a function of the material's ability to exchange Na atoms and its molar weight. If the elements M and A in M_m(A0₄)_n are chosen from lighter elements the specific capacity can be increased with respect to Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h.

Further rate studies were performed as shown in Fig. 4. The applied specific current was varied between 150 mA/g (0.5C) and 3000 mA/g (10C) over 200 cycles. Up to currents of 900 mA/g specific capacities of more than 300 mAh/g and 250 mAh/g are retained for Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h, respectively. Even at the highest applied current of 3000 mA/g still about 100 mAh/g of capacity can be extracted. The rate performance is an important criterion for potential applications. The presented materials show excellent rate performance. Furthermore, almost no capacity losses were found after 200 cycles under varying charge rates and application of large currents which further underlines the excellent cycling stability of this family of materials.

The cycling performance of Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h vs Li/Li⁺ is shown in Fig. 5. For Li higher reversible capacities can be achieved. Higher reversible capacities and improved cycling stability are obtained if a larger current window, i.e. 0.01V to 2.5V, is chosen. The reversibility of the redox reactions in the case of Bi₂(Mo0₄)₃/C-24h is also much improved if the larger voltage range is chosen. The voltage profiles of Bi(V0₄)/C-24h and Bi₂(Mo0₄)₃/C-24h measured in the range of 0.01 V to 2.5 V vs Li/Li⁺ show that the redox reactions are taking place at higher voltages compared to Na. It is also worth noting that the voltage profile of

Bi₂(Mo0₄)₃/C-24h appears more sloped compared to the SIB. Different lithiation and sodiation mechanisms are expected because of the differences in reversible capacities and voltage profiles between the LIB and SIB.

The sodiation and lithiation mechanisms of Bi(V0₄) and Bi₂(Mo0₄)₃ were investigated by in operando X-ray absorption spectroscopy (XANES) as shown in Fig. 6.

In the high voltage region (>0.7 V) during the initial sodiation (charge) the Bi L3 edge shifts irreversibly to the value for metallic Bi (Fig. 6(i) and Fig. 6(iii)). The following conversion reactions were thus assigned for Bi(V0₄) and Bi₂(Mo0₄)₃ to this plateau:

(a) 3 Na + Bi(V0₄) Bi + Na₃(V0₄)

(b) 6 Na + Bi₂(Mo0₄)₃ 2 Bi + 3 Na₂(Mo0₄)

After this reaction the slope associated with the changes in Bi L3 edge is reduced for both samples. This can be seen as a clear indication for reduction of V(lll) and Mo(VI) in Na₃(V0₄) and Na₂(Mo0₄), respectively, taking place in parallel with the reversible Bi alloying. Reversible changes in the Mo K edge were indeed observed for Bi₂(Mo0₄)₃ (Fig. 6(iii)). They are most pronounced below 0.3 V during discharge and below 0.7 V during discharge. Due to the analogous behavior of the Bi L3 edge and the change in Mo oxidation state the V is also expected to be redox active. The following reversible alloying and most likely conversion-type reactions are thus suggested: (c) (3+x) Na + Bi + Na₃(V0₄) Na₃Bi + Na_3+x(V0₄)

(d) (6+3y) Na + 2 Bi + 3 Na₂(Mo0₄) 2 Na₃Bi + 3 Na_2+y(Mo0₄)

During the first lithiation (Fig. 6(H) and Fig. 6(iv)) the Bi L3 edge shifts to about the value for metallic Bi where it is constant over a short compositional range starting at about 240 mAh/g for both materials before it is further decreased. At this capacity value the Mo K edge starts to shift significantly during lithiation of Bi₂(Mo0₄)₃. Due to the similarities in Bi L3 edge shift in Bi(V0₄) and Bi₂(Mo0₄)₃ during

lithiation/delithiation, similar changes in oxidation state as observed for Mo are expected for V. Different sodiation and lithiation mechanisms are further expected. The reactions (a)-(d) should also apply when replacing Na by Li. The x and y values might be larger and/or reactions (a) and (b) might be more reversible for Li.

After the initial conversion of Bi(V0₄) and Bi₂(Mo0₄)₃ the X_3+x(V0₄) and X_2+y(Mo0₄) with X = Na, Li, respectively, are believed to provide a suitable confinement for the Bi nanoparticles allowing for high rate performance and enabling stable capacity retention over hundreds of cycles.

The revealed sodiation and lithiation mechanisms suggest that composites of formulae X M and X_k(A0₄) are formed during cycling. These might indeed provide promising starting materials for SIB, LIB anodes.

Conclusions:

Both Bi(V0₄) and Bi₂(Mo0₄)₃ show improved battery performance with respect to metallic Bi (Sottmann et al. , How Crystallite Size Controls Reaction Path in Non- Aqueous Metal Ion Batteries: The Example of Sodium Bismuth Alloying. Chem. Mater. 2016). They further provide superior specific capacities compared to best carbon based anodes for both SIB and LIB technology, excellent capacity stability and high rate performance. They operate in the optimal voltage range for SIBs, have low environmental load and low manufacturing costs.

Preliminary results on Bi(P0₄) , Sb₂(S0₄)₃, Pb(Mo0₄) and Sn(S0₄) indicate that other group V and IV compounds with anions composed of tetrahedrally arranged oxygens will exhibit similar superior SIB and LIB performance. Within the family of M_m(A0₄)_n specific capacity and average operation voltage can be tuned by choosing A and M. The formation of the X_k(A0₄) matrix during conversion of M_m(A0₄)_n might be seen as one of the major reasons for the high cycling stability. Composites of the formulae X_jM and X_k(A0₄) are formed during cycling. These might indeed provide promising starting materials for SIB and LIB anodes.

Example 2

Experimental:

A negative electrode comprising a composite of active materials of formulae (II) and (III), i.e. Bi and Na₂Mo0₄, was prepared using ball milling.

A Fritsch Planetary Micro Mill Pulverisette 7 at 720 rpm with a ball-to-powder ratio of about 20: 1 was used for 12 hours to form a composite of Bi and Na₂Mo0₄ in a weight ratio of 40 to 60 wt.%. Then Carbon Super P (C) from Timcal was added to the grinding bowls and the ball milling was continued for another 12 hours. The Bi to Na₂Mo0₄ to C ratio was 28:42:30 by weight (Bi/Na₂Mo0₄/C-24h). Grinding balls and bowls were made of steel. The composites were kept under inert conditions to prevent oxidation.

Electrode preparation was performed in a glove box (M. Braun) with 0₂ and H₂0 levels less than 0.1 ppm. The working electrode was prepared by spreading a slurry composed of 70 wt. % of Bi/Na₂Mo0₄/C-24h, 10 wt. % of conductive carbon black (Super P, Timcal) and 20 wt. % Poly-(vinylidene fluoride) (PVdF, Sigma Aldrich) as a binder dissolved in degassed N-methylpyrrolidone (NMP, Sigma Aldrich) on Al foil for SI Bs and Cu foil for LIBs. Drying of the electrodes was carried out under vacuum at 60°C overnight. The electrodes were thereafter transferred to and stored in the glove box. In the glove box the working electrode was cut into disks with a mass loading of active material of about 1 mg/cm².

A battery was assembled in coin cells (2032) in the glove box. The working electrode was separated from the Na metal disk as counter electrode by electrolyte soaked glass fibres (GF/C, Whatman). As electrolyte a 1 M solution of NaPF₆ in ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by weight) solution with the addition of 5 wt. % FEC was prepared. All electrolyte constituents were purchased from Sigma Aldrich. The use of FEC (fluoroethylene carbonate) as electrolyte additive was found to lead to more stable (and thus beneficial) formation of the solid-electrolyte interface (SEI) layer (see Qian et al., Chem. Commun. 48: 7070-7072, 2012; Darwiche et al., JACS 134: 20805-2081 1 , 2012; and Bodenes et al., J. Power Sources 273: 14-24, 2015).

Galvanostatic cycling was performed at a current of 150 mA/g (unless otherwise specified) in a voltage range of 0.01 V to 2.5 V vs Na/Na⁺ and 0.01 V to 2.5 V vs Li/Li⁺ using a Bat-Small battery cycler (Astrol). Specific capacity values of the Bi/Na₂Mo0₄/C composites are expressed on the basis of the mass of Bi/Na₂Mo0₄.

Results:

The negative electrode comprising a composite of active materials of formulae (II) and (III) - Bi/Na₂Mo0₄/C-24h - reveals two voltage plateaus which can be associated with the Na-Bi alloying reaction. The voltage profiles for various cycle numbers are shown in Figure 9. The voltage drop in the low voltage plateau during the first charge steps (Fig. 9a and 9c) is not produced by a change in potential of the working electrode, but by solvent interaction at the Na counter electrode which causes polarization (Rudola et al., Electrochem. Commun. 46: 56-59, 2014). Such a voltage drop is generally observed in flat voltage profiles at high rates of two-phase Na electrode materials. Figure 10 shows its stable cycling behavior and high Coulombic efficiency of 76.3% (low loss of reversible capacity of 23.7%) in the initial cycle. In the 100th cycle the Coulombic efficiency is 98.9%. With a capacity retention of 69% this electrode shows better cycling stability and Coulombic efficiency than its counterpart without Na₂Mo0₄ (Bi/C-24h in Sottmann et al., Chem. Mat. 28: 2750-2756, 2016).

Claims

Claims:

1. A sodium-ion or lithium-ion cell comprising:

a positive electrode;

a negative electrode comprising an active material of formula (I):

(wherein:

X is Na or Li;

each M is independently selected from the group consisting of the group IV metals having an oxidation state of from -4 to +2, and the group V metals having an oxidation state of from -3 to +3;

p is 0 or any positive rational number which is less than 36;

m and n are integers independently selected from 1 , 2 and 3; and p, m and n are selected such that the compound of formula (I) is electrically neutral); and

an electrolyte.

2. A cell as claimed in claim 1 , wherein in formula (I), p is 0.

3. A sodium-ion or lithium-ion cell comprising:

a positive electrode;

X_jM (II) + X_k(A0₄) (III)

(wherein:

X and A are as defined in claim 1 ;

j is 0 or any positive rational number up to 4;

k is any positive rational number from 1 to 6, preferably from 1 to 4, e.g. 1 , 2 or 3; and j and k are selected such that each compound of formula (II) and formula (III) is electrically neutral);

and

an electrolyte.

4. A cell as claimed in claim 1 , wherein the active material is a compound of formula (la):

M_m(A0₄)_n (la)

(wherein:

5. A cell as claimed in any one of claims 1 to 4, wherein M is selected from any of the following metals: silicon, germanium, tin, lead, arsenic, antimony and bismuth.

6. A cell as claimed in claim 5, wherein M is bismuth, antimony, lead or tin.

7. A cell as claimed in any one of claims 1 to 6, wherein A is a first or second row transition metal.

8. A cell as claimed in claim 7, wherein A is titanium, vanadium, chromium, manganese, copper, niobium, molybdenum or cadmium.

9. A cell as claimed in any one of claims 1 to 6, wherein A is an element selected from periods 3, 4 or 5 of the periodic table.

10. A cell as claimed in claim 9, wherein A is silicon, phosphorus, sulphur, germanium, arsenic, selenium, antimony or bismuth. A cell as claimed in claim 1 , wherein the active material is a compound of

Bi_m(A0₄), (lb) in which A is as defined in any one of claims 7 to 10, preferably vanadium, molybdenum or phosphorus;

m is an integer selected from 1 or 2; and

n is an integer selected from 1 or 3.

12. A cell as claimed in claim 11 , wherein the compound of formula (lb) is Bi₄(Si0₄)3, BiP0₄, Bi₂(S0₄)₃, Bi₄(Ti0₄)₃, BiV0₄, Bi₂Cu0₄, Bi₄(Ge0₄)₃, BiAs0₄, BiNb0₄, Bi₂(Mo0₄)₃, Bi₂Cd0₄, BiSb0₄, or BiBi0₄.

13. A cell as claimed in claim 1 , wherein the active material is a compound of formula (lc):

Pb_m(A0₄), (lc) in which A is as defined in any one of claims 7 to 10, preferably vanadium, molybdenum or phosphorus;

m is an integer selected from 1 or 3; and

n is an integer selected from 1 or 2.

14. A cell as claimed in claim 13, wherein the compound of formula (lc) is Pb₂Si0₄, Pb₃(P0₄)₂, PbS0₄, Pb₃(V0₄)₂, PbCr0₄, Pb₂Mn0₄, PbSe0₄, or PbMo0₄.

15. A cell as claimed in claim 1 , wherein the active material is SbP0₄, Sb₂(S0₄)₃, SbV0₄, Sb₄(Ge0₄)₃, SbNb0₄, AsP0₄, As₂(S0₄)₃, Sn₃(P0₄)₂, or SnS0₄.

16. A cell as claimed in claim 1 , wherein the active material is Bi(V0₄), Bi(P0₄), Bi₂(Mo0₄)₃, Pb(Mo0₄), Sn(S0₄), or Sb₂(S0₄)₃.

17. A cell as claimed in any one of claims 1 to 16 which is a lithium-ion cell in which the positive electrode comprises at least one active material which is a layered oxide such as lithium cobalt oxide (LiCo0₂), a polyanion such as lithium iron phosphate (LiFeP0₄), or a spinel such as lithium manganese oxide (LiMn₂0₄).

18. A cell as claimed in any one of claims 1 to 16 which is a sodium-ion cell in which the positive electrode comprises at least one active material which is a hexacyanoferrate / Prussian blue analogue such as Na_xMFe(CN)₆ (where M is Ni, Cu, Fe, Mn, Co or Zn and x is from 0 to 2); a fluoride such as MF₃ and NaMF₃ (where M is Ni, Fe or Mn); a layered transition metal oxide such as NaCo0₂; a tunnel-type oxide such as Na_{0 44}MnO₂ ; an olivine phosphate such as NaMP0₄ (where M is Fe or Mn); a pyrophosphate or mixed polyanion such as Na₂FeP₂0₇; or a fluorophosphate, e.g. Na₃V₂(P0₄)₂F₃.

19. A cell as claimed in any one of claims 1 to 18, wherein the electrolyte comprises a lithium or sodium salt dissolved in one or more non-aqueous solvents.

20. A cell as claimed in claim 19, wherein the electrolyte comprises a lithium or sodium salt dissolved in a solvent which comprises at least one organic carbonate, DME, THF or triglyme.

21. A cell as claimed in claim 20, wherein the solvent comprises one or more organic carbonates selected from ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, and ethylmethyl carbonate, containing a complex of lithium ions (e.g. lithium hexafluorophosphate, lithium hexafluoroarsenate

monohydrate, lithium perchlorate, lithium tetrafluoroborate or lithium triflate) or a complex of sodium ions (e.g. sodium bis(tri-fluoromethane) sulfonimide (NaTFSI), sodium triflate (NaOTf), sodium perchlorate (NaCI0₄), or sodium hexafluorophosphate (NaPF₆).

22. A cell as claimed in claim 21 , wherein the electrolyte comprises a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) and a salt of LiPF₆ or NaPF₆.

23. A cell as claimed in any one of claims 1 to 22, which further comprises an electrolyte additive selected from fluoroethylene carbonate (FEC) and vinylene carbonate (VC).

24. A cell as claimed in any one of the preceding claims which is a rechargeable sodium-ion or lithium-ion cell.

25. A battery comprising at least one cell, preferably a plurality of cells, as claimed in any one of claims 1 to 24 within a suitable housing.

26. A battery as claimed in claim 25 which is rechargeable.

27. An electrode comprising: (i) an active material of formula (I), or a composite of active materials of formulae (II) and (III) as claimed in any one of claims 1 to 16; (ii) optionally a conductive carbon material; and (iii) optionally a binder.

28. An electrode as claimed in claim 27 which comprises a conductive carbon material selected from carbon black, graphene, graphene oxide, graphite, hard carbons, and nanostructured carbons.

29. An electrode as claimed in claim 28, wherein the conductive carbon material is present in an amount from 5 to 40 wt.% (based on the weight of the electrode).

30. An electrode as claimed in any one of claims 27 to 29 which comprises a binder which is a polymeric material (e.g. polyvinylidine fluoride (PVDF)), an elastomeric polymer (e.g. polyvinylalcohol (PVA), poly(acrylic acid) (PAA), PAN, CMC, or PTFE), or poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic eser (PFM).

31. An electrode as claimed in claim 30, wherein the binder is present in an amount from 5 to 25 wt.% (based on the weight of the electrode).

32. An electrode as claimed in any one of claims 27 to 31 which comprises said active material or composite of active materials in an amount from 45 to 90 wt.% (based on the weight of the electrode).

33. Use of a compound of formula (I), or a composite of active materials of formulae (II) and (III) as claimed in any one of claims 1 to 16 as an active electrode material, e.g. as a negative electrode material (anode).