Electrode precursor composition, electrode, cell, device, and methods

Abstract

An electrode precursor composition comprising: a polymer-electrolyte gel matrix phase comprising a blend of at least a first polymer, a second polymer a liquid electrolyte; and a dispersed phase compr

Description

BACKGROUND Lithium-ion secondary batteries are used in a variety of applications from small personal devices to electric vehicles. Lithium-ion batteries can have high energy density and long cycle life, among other properties. They can contain a plurality of lithium-ion secondary cells, which is one example of an alkali metal ion secondary cell. Lithium-ion battery components such as electrodes can be made from a solvent cast, process that uses sacrificial solvent. A liquid electrolyte may be used within the lithium-ion cells of the battery, to provide conductivity of lithium ions within the cell between the solid, solvent cast anode and cathode. A different approach involves use of gel electrodes. These electrodes can be formed from a composition prepared by mixing the necessary' components such as electrochemically active material, polymer, and a liquid electrolyte, and subsequently subjecting the composition to a treatment, such as thermal treatment. Some gel electrode compositions contain a small amount ofliquid electrolyte to aid gelation of the polymer. SUMMARY Provided herein is an electrode precursor composition for an alkali metal ion secondary cell. The composition comprises a polymer-electrolyte gel matrix phase comprising a polymer blend and a liquid electrolyte. The polymer blend is made up of a blend of polymers, the polymer blend comprising a first polymer which is polymethylmethacrylate (PMMA) having a weight average molecular weight of at least. IxlO5 Da. The polymer blend also comprises a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer. The liquid electrolyte comprises an organic solvent and an alkali metal salt. The polymer-electrolyte gel matrix phase includes the first polymer in an amount of between 0.1 and 10 volume% (vol%) of the total volume of the polymer blend i.e. the total volume of the first polymer, the second polymer, and any optional further polymer(s). In addition to the polymer-electrolyte gel matrix phase, the composition comprises a dispersed phase comprising an electrochemically active material. In solvent cast processes for producing component such as electrodes, the use of a sacrificial solvent such as N-methylpyrrolidone (NMP) is an energetically expensive step. Gel electrodes of the kind that can be produced using the compositions described herein can avoid such additional cost, thereby significantly reducing the cost of cell production. The use of a small amount of liquid electrolyte means a gel may more easily form and the processability of the gel electrodes is improved, reducing the cost of manufacture. However, in prior art compositions, this in turn can make the finished gel electrode more prone to dissolution in the electrolyte which is present between the electrodes within the finished cell, reducing the operable lifetime of the cell. By contrast, poly mers having poor solubility in electrolyte may ensure that, once assembled, the gel electrodes are not degraded by the electrolyte present in the cell. However such electrodes can be difficult to manufacture due to the poor gelation between the polymer and the electrolyte. To balance factors such as electrode processability, cell performance, and longevity, the present gel precursor compositions employ a first polymer that acts as a processing aid. The first polymer typically has a relatively high solubility in the electrolyte chosen and can assist in the formation of a more easily processible gel during manufacture of the gel electrode, without compromising the cell performance or cell lifetime. In some embodiments, the first polymer may not form a gel. In some embodiments, the first polymer may form a liquid in the electrolyte chosen at the amount chosen, or in some embodiments it may dissolve from the gel in situ. Further, the present inventors find that use of an additive amount of a very highly soluble (first) polymer can reduce or eliminate the risk of introducing cell instability that could occur for example if a larger fraction of the first polymer was used. Even so, the addition of such small amounts of first polymer to an electrode composition can improve the processability, without detrimental impact on electrochemistry. That is, the present-electrode precursor compositions contain only a relatively small amount of the first polymer, which means that there can be little change in structural stability and cell performance which are imparted by the second polymer and any further polymetys), while still manifesting large improvements in processability. A further benefit of the present electrode precursor compositions is that the concentration of the alkali metal salt in the overall composition is higher than would otherwise be possible without the presence of the first polymer in the polymer blend. A. higher salt concentration increases the conductivity of the gel electrode, improving electrochemical performance of the cell. Due to the presence of the first (more soluble) polymer in the blend, a greater quantity of the liquid electrolyte can be incorporated into the electrode precursor composition. The greater quantity of liquid electrolyte means a higher salt concentration and consequently improved electrode performance, without any noticeable effect on electrode structural stability. The first polymer is polymethylmethacrylate (PMMA). Thus, the first polymer is a homopolymer having only methyl methacrylate (MMA) as constituent monomers. The first polymer has a weight average molecular weight of at least IxlO5 Da. Such polymers are considered to have particularly high solubility in typical liquid electrolytes used in the preparation of gel electrodes, thereby readily achieving the effects described above of improved processability, cell performance, and longevity. in some embodiments, the first polymer makes up at least 0.5 vol%, at least 1.0 vol% or at least 2.0 vol% of the total volume of the polymer blend i.e. the total volume of the first, second and optional further polymers. In some embodiments, the first polymer is present in an amount of up to 9.8 vol%, up to 9.5 vol% or up to 9.0 vol% of the total volume of the polymer blend. These are considered particularly useful amounts of the first polymer for which the effects described above can be observed. In some embodiments, the first polymer makes up from 0.5 vol% to 9.8 vol%, for example from 1.0 vol% to 9.5 vol% or from 2.0 vol% to 9.0 vol% of the total volume of the polymer blend i.e. the total volume of the first, second and optional further polymers. In some embodiments, the first polymer has a weight average molecular weight of at least IxlO6 Da. Such polymers are considered to have particularly high solubility in typical liquid electrolytes used in the preparation of gel electrodes, thereby readily achieving the effects described above of improved processability, cell performance, and longevity. The second polymer comprises at least 75 mol% VDF as constituent monomer. In some embodiments, the second polymer has a weight average molecular weight of at least IxlO5 Da. Such polymers are considered to have lower solubility in typical liquid electrolytes used in the preparation of gel electrodes, thereby imparting good structural performance to the electrodes. In some embodiments, the polymer blend comprises at least one further polymer i.e. a third polymer. Any third or further polymer which is present is different, to the first and second polymers. In some embodiments, the third polymer comprises at least 75 mol% VDF as constituent monomer. The judicious choice of second and third polymers, in combination with the first polymer, can provide exemplary balance between processability and durability of the cell during use. This is particularly notable in embodiments where the second polymer has a particularly low solubility in the liquid electrolyte chosen. For example, in some embodiments the second polymer may have a higher weight average molecular weight than the third polymer. In some embodiments, the second polymer is present in an amount of at least 10 vol% of the total volume of the polymer blend. This is considered a particularly useful range amount of the second polymer for which noticeable gelling effects can be observed. In some embodiments, the second polymer is a polyvinylidene fluoride (PVDF) homopolymer, or a PVDF copolymer. In some embodiments, a further polymer is a poly vinylidene fluoride (PVDF) homopolymer, a polytetrafluoroethylene (PTFE) homopolymer, a polychlorotrifluoroethylene (PCTFE) homopolymer, a polymethylmethacrylate (PMMA) homopolymer, a PVDF copolymer, a PTFE copolymer, a PCTFE copolymer, or a PMMA copolymer. In some embodiments, a further polymer is a poly vinylidene fluoride (PVDF) homopolymer, or a PVDF copolymer. In some embodiments, the second and / or a further polymer comprises hexafluoropropylene (HFP) as a constituent monomer. In some embodiments, the second and / or a further polymer comprises up to 2 niol% of one or more constituent monomers other than VDF and HFP. Such polymers are considered to have exemplary solubilities as well as achieving gelling benefits described herein. In some embodiments, the electrochemically active material is present in an amount of between 50 and 75 vol%, such as at least 60 vol%, of the total volume of the electrode precursor composition. Such a volume amount of electrochemically active material can achieve useful performance electrodes. In some embodiments, the dispersed phase further comprises a conductive additive. The presence of a conductive additive can improve the performance of the electrodes. In some embodiments, the liquid electrolyte is present in an amount of at least 25 vol% of the total volume of the electrode precursor composition. This amount can achieve particularly good electrode processability. In some embodiments, the organic solvent comprises one or more cyclic or linear carbonate compounds. In some embodiments, the organic solvent comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. Such organic solvents can be particularly suitable for balancing the solubility profile of the above-mentioned polymer blend, and safety. In some embodiments, the alkali metal salt comprises one or more of LiPFe, LiBF4, lithium bis(fluorosulfonyl) imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium 2-trifluoromethyl-4,5”dicyanoimidazole (LiTDI). Such alkali metal salts are suitable for providing electrodes for lithium-ion secondary batteries. In some embodiments, the electrode precursor composition comprises between 20 and 50 vol% of the polymer-electrolyte gel matrix phase, based on the total composition volume. This proportion of polymer-electrolyte gel matrix phase is considered to provide exemplary performance properties. In some embodiments, the polymer-electrolyte gel matrix phase comprises 3 to 30 vol% of polymer blend, based on the total volume of polymer-electrolyte gel matrix phase. Such range is considered to provide compositions having exemplary processability properties without significantly affecting structural or cell performance and longevity properties. In some embodiments, the electrode precursor composition is for a lithium-ion secondary’ electrochemical cell. It is considered that the present, compositions are particularly suitable for providing electrodes that can be used in a lithium-ion secondary electrochemical cell. Also provided herein is a method of preparing an electrode precursor composition as described herein. The method can comprise mixing the electrochemically active material with the first, second and any optional further polymer(s) and the electrolyte. Also provided herein is the use of between 0.1 and 10 vol% of a first polymer in a polymer-electrolyte gel matrix phase of a composition for an alkali metal ion secondary cell, the composition comprising the polymer-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising the first polymer which is polymethylmethacrylate (PMMA) having a v / eight average molecular weight of at least IxlCP Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent, monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt; and wherein the dispersed phase comprises an electrochemically active material. In some embodiments, the use results in an electrode precursor composition as described herein. The use of an additive amount of the first polymer in a composition for an alkali metal ion secondary' cell, to form a composition as described herein, provides at least the advantages explained above. Also provided herein is an electrode for use in an alkali metal ion secondary cell. The electrode comprises a polymer-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising a first polymer which is polymethylmethacrylate (PMMA) having a weight average molecular weight of at least IxlO5 Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt; wherein the dispersed phase comprises an electrochemically active material; and wherein the first polymer makes up between 0.1 and 10 vol% of the total volume of the polymer blend. In some embodiments, the electrode comprises or is produced from an electrode precursor composition as provided herein. The relevant above-mentioned effects described for the gel electrode precursor composition described herein also apply to the electrode described herein. Also provided herein is a method of producing an electrode. The method comprises processing an electrode precursor composition described herein to form a film or coating. In some embodiments, the processing comprises thermal processing or extrusion. Thermal processing or extrusion is found to provide electrodes, such as electrodes described herein, in a straightforward and relatively low-cost manner. Also provided herein is an electrochemical secondary cell comprising an electrode described herein, or as produced by a method described herein. Also provided herein is an electrochemical energy storage device comprising an electrochemical secondary' cell described herein. Since the electrochemical secondary cell and the electrochemical energy storage device each comprise an electrode as described herein, the relevant above mentioned effects described for the gel electrode precursor composition described herein also apply. BRIEF DESC RIPTION OF THE DRAWINGS Figure 1 shows photographic images of gel electrodes. Figure 1(a) shows an image of a gel electrode prepared from an electrode precursor composition not containing any first polymer (PMMA) as described herein. Figure 1(b) shows an image of a gel electrode prepared from an electrode precursor composition as described herein. The circle of Figure 1(a) indicates an exemplary area of the electrode having electrolyte loss. DETAILED DESCRIPTION Electrode Precursor Composition The electrode precursor composition for an alkali metal ion secondary cell described herein comprises a polymer-electrolyte gel matrix phase, and a dispersed phase. The electrode precursor composition may find use as a precursor material for the preparation of a gel electrode. The polymer-electrolyte gel matrix phase comprises a polymer blend comprising a first polymer, a second polymer and optionally at least one further polymer. The first polymer is PMAIA. The second polymer is different from the first polymer and comprises VDF as constituent monomer. Any optional further polymer is different from the first and second polymers. In some embodiments, an optional further polymer may differ from the first and second polymers in type of constituent nionomer(s), amount of constituent monomer(s), and / or in weight average molecular weight. The polymer-electrolyte gel matrix phase also comprises a liquid electrolyte. In some embodiments, the polymer-electrolyte gel matrix phase consists of the polymer blend and the liquid electrolyte. In general, the first (most soluble) polymer is present primarily to improve the processability of the electrode precursor composition. On the other hand, the second and any optional further (less soluble) polymer(s) are typically chosen for their ability to form a gel capable of use as a gel electrode. That, is, the first polymer is used to improve the processability of a polymer (e.g. in embodiments containing only a second polymer) or polymer combination (e.g. in embodiments containing a second polymer and at least one further polymer, such as a second and third polymer) that can form gel suitable for use as a gel electrode. Thus, the second and any optional further polymers perform a more structural function in the finished gel electrode than the first polymer. The first polymer, which is PMMA having a weight average molecular weight of at least 1xl05 Da, has a solubility in the liquid electrolyte which is higher than the solubility of the second polymer, which comprises at least 75 mol% VDF as constituent monomer, in the liquid electrolyte. Thus, the first polymer has the highest solubility in the liquid electrolyte of any of the polymers of the polymer blend. The first polymer makes up at least 0.] vol%, and at most 10 vol%, of the total volume of the first, second and any optional further polymers i.e. of the total volume of the polymers of the polymer blend. Put differently, the first polymer is present in an additive amount. In some embodiments, the first polymer makes up at least 0.1 vol%, at least 0.2 vol%, at least 0.5 vol%, at least 1.0 vol%, or at least 2.0 vol% of the total volume of the first second and any further polymers. In some embodiments, the first polymer makes up at most 9.8 vol%, at most 9.5 vol%, or at most 9.0 vol% of the total volume of the first second and any further polymers. Any combination of the above-listed values may be combined to form a suitable range, such as between 0.1 and 9.5 vol%, between 0.5 and 9.0 vol%, or between 1.0 and 9.0 vol%. It is noted that the sum of the volumes of the first, second and any further polymers is the total volume of the polymer blend, and the sum is 100 vol %. Put differently, the volume ratio of the first polymer to the second polymer and any further polymer is from 0.001 to 0.1. Within this range a good balance of processability and durability can be obtained by the polymer blend In some embodiments, the volume ratio of the first polymer to the second polymer and any further polymer is at least 0.002, at least 0.005, at least 0.01 or at least 0.02. In some embodiments, the volume ratio of the first polymer to the second polymer and any further polymer is at most 0.098, at most 0.095, or at most 0.090. Any combination of the above-listed values may be combined to form a suitable range, such as between 0.001 and 0.095, between 0.005 and 0.090, or between 0.01 and 0.090. In some embodiments, the first polymer is present in an amount of at least 0.01 vol% of the total volume of the electrode precursor composition, such as at least 0.02 vol%, or at least 0.05 vol%. In some embodiments, the first polymer is present in an amount of up to 1.0 vol%, such as up to 0.8 vol% or up to 0.5 vol% based on the total volume of the electrode precursor composition. Any combination of the above-listed values may be combined to form a suitable range, such as between 0.01 and 1.0 vol%, between 0.02 and 1.0 vol%, or between 0.02 and 0.8 vol%, based on the total volume of the electrode precursor composition. The first polymer has a weight average molecular weight of at least lx 105 Da. In some embodiments, the first polymer is a high molecular weight polymer. In some embodiments, the first polymer is an ultra-high molecular weight polymer. In some embodiments, the first polymer has a weight average molecular weight of at least IxlO6 Da, at least 2xl06 Da, or at least 3x106 Da. An upper limit is not particularly specified. Herein, the weight average molecular weight can be measured using a gel permeation chromatography (GPC) method. Universal calibration according to a standard method, e.g. ASTM-D6474-99, may be used. UV-vis spectra and absorbances may be recorded on a spectrophotometer. Calculation of the molecular weight can be carried out by appropriate analytical software, such as CIRRUS® GPC software. GPC measurements can be carried out on a suitable HPLC system. For example, PALMA standards and organic solvent eluents are appropriate for measuring polymers containing MAIA or VDF. By way of further example, the weight average molecular weight of a VDF-containing polymer can be measured according to the method described in “Polyvinylidene Fluoride Analysis on Agilent PLgel 10 gm MIXED-B and GPC / SEC” Application Note (Graham Cleaver, Agilent Technologies, Inc. 2015). Here, PMMA standards are used and DMSO is eluent. The weight average molecular weight of PMMA can be measured according to the method described in “Synthesis of ultra-high molecular weight polyfmethyl methacrylate) with hydrosilane as initiator” (Yuan et al., Materials Today Communications, 2022). Here, PMMA standards are used and THE is eluent. In some embodiments, the first polymer can exhibit spontaneous dissolution in the liquid electrolyte at room temperature. This may, depending on the polymer, be a very slow process, but in some embodiments exposing the liquid electrolyte to the first polymer at room temperature without any specific mixing leads to the formation of a gel. In addition to the above-described first polymer, the polymer blend also comprises a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, such as at least 78 mol% VDF as constituent monomer, at least 80 mol% VDF as constituent monomer, or at least 85 mol% VDF as constituent monomer. In embodiments where the second polymer has 100 mol% VDF as constituent monomer, it is PVDF homopolymer. As used herein, “mol0 / ” refers to the molar proportion of monomers present in the polymer, where the total moles contained in the polymer is 100%. In some embodiments, the second polymer has a weight average molecular weight which is lower than that of the first polymer. In some embodiments, the second polymer has a weight average molecular weight of at least lx 105 Da, such as at least 2x10’ Da or at least 5x10’ Da. An upper limit is not particularly specified. In some embodiments, the second polymer may be poly (vinylidene difluoride) (PVDF) homopolymer or a copolymer thereof. In some embodiments, the second polymer may be a copolymer comprising VDF, such as poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). As used herein, “copolymer” refers to a polymer containing more than one kind of monomer. Thus, for example, a PVDF copolymer contains one or more kinds of monomer in addition to VDF monomer units, and a PMMA-PEO copolymer contains at least MMA and EO monomers. The copolymer may be any suitable kind of copolymer, such as a random copolymer, an alternating copolymer, or a block copolymer etc. The one or more kinds of monomer in addition to the monomer mentioned may be of any suitable kind as long as the effects described herein are achieved. Suitably, a copolymer may contain the mentioned monomers in an amount of at least 20 weight% (wt%) such as at least 30 wt%, at least 50 wt%, at least 70 wt% or at least 90 wt% based on the weight of the copolymer. In some embodiments, the second polymer has the lowest solubility in the liquid electrolyte of the polymers making up the polymer blend e g. the lowest solubility in the liquid electrolyte of the first, second and any optional further polymer. In some embodiments, in which the polymer blend consists of first and second polymer, the proportion (vol%) of second polymer is determined by the amount of first polymer. In some embodiments, in which the polymer blend includes optional further polymer(s), the second polymer is present in an amount of at least 5.0 vol% of the total volume of the polymer blend, such as at least 8.0 vol% or at least 10.0 vol%. The upper limit of the amount of second polymer is determined by the amount(s) of other polyrner(s) in the polymer blend. In some embodiments, the second polymer is present in an amount of at least 0.1 vol% of the total volume of the electrode precursor composition, such as at least 0.2 vol%, or at least 0.3 vol%. In some embodiments, the second polymer is present in an amount of up to 5.0 vol%, such as up to 4.0 vol%, up to 3.0 vol% or up to 1.0 vol%. Any combination of the above-listed values may be combined to form a suitable range, such as between 0.1 and 5.0 vol%, between 0.1 and 4.0 vol%, or between 0.3 and 3.0 vol%. In some embodiments, the polymer blend comprises a further (third) polymer. In some embodiments, the polymer blend comprises more than one further polymer, such as a third polymer and a fourth polymer. Each of the one or more further polymer(s) are different from the first and second polymers. So long as the difference does not impact the effects described herein, the optional one or more further polymers may differ from the first and second polymers in any respect. In some embodiments, optional further polymer(s) may have the same constituent monomers as the first or second polymers but differ in some other property e.g. molecular weight. In some embodiments, optional further polymer(s) may differ from the first or second polymer in their constituent monomer units, for example they may comprise monomers having different functional groups to those of the first or second polymer. In some embodiments, in which there are two or more further polymer(s), the way in which the further polymers differ from the first and second polymers may be the same or different. In some embodiments in which the polymer blend comprises a first, second and third polymer, the third polymer has a solubility in the liquid electrolyte which is intermediate between that of the first and second polymers. In this way, the electrode precursor composition can provide a particularly good balance between processability, through the presence of the more soluble polymers, and durability during use of the cell, through the presence of the less soluble polymer. In such embodiments, the second polymer is less prone to dissolution into the cell electrolyte during use of the cell than the first or third polymers, and therefore contributes to good cell lifetime. In some embodiments, any further polymer, such as a third polymer, may independently be selected from poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propylenegiycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylomtrile) (PAN), polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(ethylene oxide) (PEO), poly(ethyleneglycol dimethylether), poly(ethyleneglycol di ethylether), polychlorotrifluoroethylene (PCTFE), polytetrafluoroethylene (PTFE), poly[bis(methoxy ethoxyethoxide)-phosphazene], poiy(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, poiythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene-substituted polystyrene, ferrocene-substituted polyethylene, carbazole-substituted polyethylene, polyoxyphenazine, poly(heteroacene), poly[(4-styrenesulfonyr)(trifluoromethanesulfonyl)imide-co-methoxy-polyethyleneglycolacrylate] ([PSTFSI-co-MPEGA]), sulfonated poly(phenylene oxide) (PPO), N,N-dimethylacryl amide (DMAAm), lithium 2-aciyIamido-2-methyl~l-propane sulfonate (LiAMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co-Vinyl Tri ethoxy silane), polyethyleneoxi de(PEO) / poly(lithium sorbate), PEO / poly (lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, PEO terpolymer, and NIPPON SHOKUBAI® polymer. In some embodiments, any further polymer, such as a third polymer, is not PMMA. In some embodiments, any further polymer, such as a third polymer, is independently selected from polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polychlorotrifluoroethylene (PCTFE), polytetrafluoroethylene (PTFE), poly(dimethylsiloxane) (PDMS), polyacene, polydi sulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, and [PSTFSI-co-MPEGA], In some embodiments, any further polymer, such as a third polymer, is independently selected from poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polytvinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polychlorotrifluoroethylene (PCTFE), polytetrafluoroethylene (PTFE), polyfluorene, and [PSTFSI-co-MPEGA], In some embodiments, any further polymer, such as a third polymer, is independently selected from poly(vinylidene difluoride) (PVDF) or a copolymer comprising VDF, such as polytvinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). In some embodiments, the polymer blend comprises or consists of a first polymer, a second polymer and a third polymer. In some such embodiments, the second polymer and / or third polymer are each independently selected from a PVDF homopolymer or copolymer containing VDF monomers. In some such embodiments, the second polymer and a third polymer are each a PVDF copolymer or are each a PVDF homopolymer. In some such embodiments, the PVDF copolymer is a copolymer comprising hexafluoropropylene (HFP) monomers as well as VFD monomers. In some such embodiments, the second polymer is selected from PVDF, or a copolymer thereof, and the third polymer is selected from a PVDF homopolymer, or a copolymer thereof In some such embodiments, the second polymer is a PVDF copolymer, and the third polymer is selected from PVDF homopolymer or a PVDF copolymer. In some such embodiments, the second polymer is selected from a PVDF homopolymer or a PVDF copolymer, and the third polymer is a PVDF copolymer. In some such embodiments, the second polymer comprises VDF and HFP monomer as constituent monomers and the third polymer is a PVDF copolymer. In some embodiments, in which the polymer blend comprises a third polymer, the third polymer has a weight average molecular weight which is lower than that of the second polymer. That is, in some embodiments, the third polymer has a lower weight average molecular weight than the second or third polymer. In some embodiments, the third polymer has a weight average molecular weight of at least IxlO4 Da, at least 5xl04 Da or at least 1x10’ Da. An upper limit is not particularly specified. In some embodiments, in which the polymer blend comprises a third polymer, the third polymer is present in an amount of at least 50 vol%, such as at least 60 vol% or at least 70 vol%, of the total volume of the polymer blend The upper limit of the amount of third polymer wdll be determined by the amounts of the first, second and any further polymers in the polymer blend. In some embodiments, in which the polymer blend comprises a third polymer, the third polymer is present in an amount of at least 2.6 vol% of the total volume of the electrode precursor composition, such as at least 2.8 vol%, at least 3.0 vol%, or at least 3.2 vol%. In some embodiments, the third polymer is present in an amount of up to 10 vol%, such as up to 9.5 vol%, up to 9.0 vol% or up to 8.5 vol%. Any combination of the above-listed values may be combined to form a suitable range, such as between 2.6 and 10 vol%, between 2.8 and 9.0 vol%, or between 3.2 and 8.5 vol%. In some embodiments in which the polymer blend comprises first, second and third polymers, and in which the first polymer has the highest solubility in the liquid electrolyte and the second polymer has the lowest solubility in the liquid electrolyte so that the third polymer has a solubility in the liquid electrolyte which is intermediate between the first and third polymers, the volume ratio of the second polymer to the third polymer is from 1:4 to 1:10, such as from 1:5 to 1:9 or from 1:6 to 1:8. Such ratios provide particularly stable and processible compositions. Accordingly, in some embodiments, the electrode precursor composition for an alkali metal ion secondary cell comprises a polymer-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising a first polymer which is PMMA having a weight average molecular weight of at least IxlO6 Da, a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer and having a weight average molecular weight of at least 1x10’ Da, and a third polymer which comprises VDF as constituent monomer and having a weight average molecular weight lower than that of the second polymer, the liquid electrolyte comprising an organic solvent and an alkali metal salt; wherein the dispersed phase comprises an electrochemically active material; and wherein the first polymer makes up between 0.1 and 10 vol% of the total volume of the polymer blend. In some embodiments, an additional energy input can be applied to initiate dissolution of the second and / or optional further polymers. In some embodiments, dissolution of the second and / or third polymer in the liquid electrolyte may be carried out at elevated temperature (for example, at or greater than 30 °C, at or greater than 50 °C or at or greater than 80 °C). Stability of the gel can be evaluated using a number of methods: ® A visual assessment would indicate a single phase of material for a stable gel, with no evident liquid-rich phases on the surface, ® An optical assessment via microscopy or similar would not show any evidence of phase separation; * Further analytical assessment, for example by DSC, spectroscopy methods including FTIR, RAMAN, or EDX, or x-ray diffraction, would indicate homogenous, single phase mixtures. In some embodiments, the second polymer and / or any further polymer exhibits no tendency to swell or dissolve in the liquid electrolyte at elevated temperatures. In some embodiments the second polymer and / or any further polymer shows no obvious change of physical state when exposed to the liquid electrolyte at high temperatures, even when utilising high shear mixing techniques. This can be confirmed by a visual assessment which would indicate unchanged polymer when exposed to the liquid electrolyte. In addition, DSC of this mixture would likely show an endothermic melting peak at the same temperature as the raw7 polymer, indicative that the crystalline polymer phase is unchanged in the material after exposure to the liquid electrolyte. In some embodiments the second polymer and / or any further polymer show's partial tendency to swell or dissolve in the liquid electrolyte at elevated temperatures. Slight swelling of the second polymer in the liquid electrolyte in this way may occur but may be difficult to visually observe. In this case DSC measurements of the mixture may indicate that an endothermic melting peak of the polymer in the mixture is different to that of the raw polymer, indicative of change in crystallinity on exposure to liquid electrolyte. In some embodiments, the second polymer and / or any further polymer may show significant swelling and plasticise within the liquid electrolyte but may be unable to take up all of the liquid electrolyte at the desired quantity. A visual assessment would indicate free liquid around the gel revealing this behaviour. In some embodiments, the second polymer and / or any further polymer may swell and plasticise with the whole quantity of liquid electrolyte at elevated temperatures, but evaluation of this gel in this state at this temperature might suggest a non-homogenous mixture. For example, rheological assessment of this mixture may be difficult to measure, or noisy, suggestive of inhomogeneous composition or phase separation. In some embodiments, the second polymer and / or any further polymer may show the ability to swell or dissolve in the liquid electrolyte at elevated temperatures, but the mixture may not be stable at room temperature. A polymer such as this might swell and plasticise completely with the liquid electrolyte at elevated temperature, but when returned to room temperature it would exhibit behaviour that suggests it is inhomogeneous or unstable. For example, visual assessment of this material may indicate liquid being rejected from a gel mass, as droplets forming on the surface, or pooling around the bulk material. In some embodiments, the electrode precursor composition comprises the polymer blend in an amount of from 2 to 10 vol%, based on the total volume of electrode precursor composition, for example from 2 to 9 vol%, from 2 to 8 vol%, from 3 to 8 vol% or from 3 to 5 vol%. In some embodiments, the polymer-electrolyte gel matrix phase comprises the polymer blend in an amount of from 3 to 30 vol%, based on the total volume of polymer-electrolyte gel matrix phase, for example from 3 to 25 vol%, from 3 to 24 vol%, from 5 to 24 vol%, from 10 to 23 vol%, from 10 to 22 vol%, or from about 10 to 15 vol%. in some embodiments, the polymer-electrolyte gel matrix phase comprises the liquid electrolyte in an amount of from 70 to 97 vol%, based on the total volume of polymer-electrolyte gel matrix phase, for example from 75 to 90 vol%, or from 85 to 90 vol%. In some embodiments, the electrode precursor composition comprises the polymerelectrolyte gel matrix phase in an amount of from 20 to 50 vol%, based on the total volume of electrode precursor composition, for example from 25 to 45 vol%, from 25 to 40 vol%, or from 30 to 40 vol%. In some embodiments, the electrode precursor composition comprises the liquid electrolyte in an amount of at least 25 vol%, such as from 25 to 45 vol%, based on the total volume of electrode precursor composition, for example from 26 to 44 vol%, from 27 to 43 vol%, from 28 to 42 vol% or from 29 to 42 vol%. As explained above, the present compositions allow the amount of liquid electrolyte present to be increased while maintaining good structural stability of the gel electrode. So, in some embodiments, the liquid electrolyte makes up at least 31 vol% of the electrode precursor composition, for example greater than 31 vol%, for example greater than 31.00 vo1%, for example from greater than 31 vol% to 35 vol%. Compositions containing a single type of polymer to provide structural stability may be unable to incorporate such levels of electrolyte. By contrast, compositions containing a single type of polymer which can incorporate such levels of electrolyte may not provide the necessary structural stability and the electrodes are degraded by the cell electrolyte during use, shortening the lifetime of the cell. Indeed, compositions including only two types of polymers to provide structural stability and high levels of electrolyte may not provide sufficient longevity of the cell performance; that is, cell lifetime can be lower compared to cells containing compositions described herein. The present compositions can allow for a higher level of liquid electrolyte in the gel electrode (and thereby a higher level of alkali metal salt), while maintaining good structural stability and longevity compared to other compositions. The polymer-electrolyte gel matrix phase comprises a gel matrix formed by the swelling of the swellable polymer blend when the polymer blend absorbs a liquid electrolyte. The polymer-electrolyte gel matrix phase therefore comprises a gel comprising the polymer blend and absorbed liquid electrolyte. In some embodiments, the polymer-electrolyte gel matrix phase consists of the polymer blend described herein and the liquid electrolyte described herein. The skilled person will understand that the relative solubilities of the first, second and any further polymers in the liquid electrolyte can depend not only on the nature of the first, second and any further polymers chosen, but also on the choice of liquid electrolyte and / or choice of the components of the liquid electrolyte e.g. solvent. In some embodiments, the liquid electrolyte is present in an amount of at least 25 vol% of the total volume of the electrode precursor composition. In some embodiments, the liquid electrolyte is present in an amount of at least 28 vol%, at least 30 vol%, or at least 32 vol% of the total volume of the electrode precursor composition. In some embodiments the liquid electrolyte is present in an amount of up to 50 vol%, such as up to 48 vol%, up to 45 vol%, up to 42 vol% or up to 40 vol% of the total volume of the electrode precursor composition. Any combination of these values may be combined to form a suitable range, such as between 25 and 50 vol%, between 28 and 48 vol%, or between 32 and 45 vol%. The liquid electrolyte contains an organic solvent and an alkali metal salt. In some embodiments, the liquid electrolyte consists of the organic solvent and alkali metal salt. In some embodiments, the organic solvent comprises or consists of one or more cyclic or linear carbonate compounds. In some embodiments the solvent comprises one or more cyclic carbonate compounds. In some embodiments the solvent, comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. In some embodiments the solvent comprises one or more of ethylene carbonate, propylene carbonate, vinylene carbonate and fluoroethylene carbonate. In some embodiments the solvent comprises or consists of a mixture of ethylene carbonate, propylene carbonate, vinylene carbonate and fluoroethylene carbonate. In some embodiments the solvent comprises a blend of at least two different compounds, for example at least three or at least four different compounds. In some embodiments the solvent comprises a blend of at least two different organic carbonate compounds, for example at least three or at least four different organic carbonate compounds. In some embodiments the solvent is not a blend but consists of one compound. In some embodiments the solvent comprises ethylene carbonate in an amount of at least 50 wt% based on the total weight of solvent, for example at least 55 wt%, at least 60 wt% or at least 65 wt%. In some embodiments the solvent comprises ethylene carbonate in an amount of up to 80 wt% based on the total weight of solvent, for example up to 75 wt% or up to 70 wt%. In some embodiments the solvent comprises ethylene carbonate in an amount of from 50 to 80 wl% based on the total weight of solvent, for example from 60 to 75 wt%. In some embodiments the solvent comprises propylene carbonate in an amount of at least 10 wt% based on the total weight of solvent, for example at least 15 wt%, at least 20 wt% or at least 22 wt%. In some embodiments the solvent comprises propylene carbonate in an amount of up to 45 wt% based on the total weight of solvent, for example up to 42 wi% or up to 40 wt%. In some embodiments the solvent comprises propylene carbonate in an amount of from 10 to 45 wt% based on the total weight of solvent, for example from 20 to 40 wt%. In some embodiments, the liquid electrolyte further comprises one or more lithium salts. Examples of suitable lithium salts include LiPFe, LiBF4, lithium bis(fluorosulfonyl) imide (LiFSI), lithium 24rifluoromethyl-4,5-dicyanoimidazole (LiTDI), and lithium bi s(trifluoromethanesul fonyl)i mide (LiTFSI). In some embodiments, the liquid electrolyte comprises one or more lithium salts selected from LiPFg, LiBF4, lithium bis(fluorosulfonyl) imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI). In some embodiments, the liquid electrolyte comprises a solvent as described above and a lithium salt component comprising or consisting of one or more lithium salts selected from LiPFo, LiBF4, LiFSI, LiTFSI and LiTDI. In some embodiments, the liquid electrolyte comprises a solvent as described above and a lithium salt component comprising or consisting of one or more lithium salts selected from LiFSI and LiTDI. In some embodiments, the liquid electrolyte comprises or consists of an organic solvent and an alkali metal salt; wherein the organic solvent consists of one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyroiactone; and the alkali metal salt consists of one or more of LiPFs, LiBF4, LiFSI, LiTFSI and LiTDI. In some embodiments, the liquid electrolyte comprises or consists of an organic solvent and an alkali metal salt; wherein the organic solvent comprises or consists of one or more of ethylene carbonate, propylene carbonate, vinylene carbonate and fluoroethylene carbonate; and the alkali metal salt comprises or consists of one or more of LiFSI and LiTDI. In some embodiments, the liquid electrolyte comprises or consists of a mixture of at least two different lithium salts. In some embodiments the solvent comprises or consists of a blend of at least two different organic carbonate compounds, for example at least two or at least three or at least four different organic carbonate compounds, and the liquid electrolyte comprises or consists of a mixture of at least two different lithium salts. In some embodiments, the liquid electrolyte consists of one kind of alkali metal salt and one kind of solvent, such as one kind of lithium salt and one kind of organic carbonate compound. In some embodiments, the liquid electrolyte contains only one kind of alkali metal salt, and a blend of at least two different organic carbonate compounds. In some embodiments, the liquid electrolyte comprises a mixture of at least two different alkali metal salts, such as two different lithium salts, and only one kind of organic carbonate compound as solvent. In some embodiments, the liquid electrolyte comprises or consists of a mixture of two different alkali metal salts, such as LiFSI and LiTDI, and at least two kinds of organic carbonate compounds as solvent. In some embodiments, the total concentration of the lithium salt(s) in the organic solvent is at least 10 wt%, based on the total weight of liquid electrolyte, for example at least 11 wd%, at least 12 wt%, at least 13 wt%, at least 14 wt% or at least 15 wt%. In some embodiments, the total concentration of the lithium salt(s) in the organic solvent is up to 50 wt%, based on the total weight of liquid electrolyte, for example up to 40 wt%, up to 30 wt%, up to 25 wt% or up to 24 wt%. In some embodiments, the total concentration of the lithium salt(s) in the organic solvent is from 10 to 30 wt%, based on the total weight of liquid electrolyte, for example from 12 to 25 wt% or from 15 to 25 wt%. The dispersed phase comprises an electrochemically active material. In some embodiments, the electrochemically active material makes up at least 50 vol% of the total volume of the electrode precursor composition, such as at least 52 vol%, at least 55 vol%, or at least 57 vol%. In some embodiments, the electrochemically active material makes up up to 75 vol% of the total volume of the electrode precursor composition, such as up to 73 vol%, up to 70 vol% or up to 68 vol%. Any combination of these values may be combined to form a suitable range, such as between 50 and 75 vol%, between 50 and 73 vol%, or between 52 and 73 vol% of the total volume of the electrode precursor composition. In some embodiments, the dispersed phase consists of an electrochemically active material. The electrochemically active material is a particulate material, i.e a material made up of a plurality of discrete particles. The particles may comprise primary particles and / or secondary panicles formed from the agglomeration of a plurality of primary particles. In some embodiments, the electrochemically active material is a positive active material. Such materials may suitably be used to form an electrode which is a cathode. In some embodiments, the positive active material is a lithium transition metal oxide material. In some embodiments, the positive active material is a lithium transition metal oxide material comprising a mixed metal oxide of lithium and one or more transition metals, optionally further comprising one or more additional non-transition metals. In some embodiments, the positive active material is a lithium transition metal oxide material comprising lithium and one or more transition metals selected from nickel, cobalt and manganese. In some embodiments, the positive active material is selected from one or more of lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt oxide (NCO), aluminium-doped lithium nickel cobalt oxide (NCA), lithium nickel manganese cobalt oxide (NMC), lithium nickel oxide (LNO), lithium nickel manganese oxide (LNMO). lithium iron phosphate (LFP), lithium manganese iron phosphate (LFP) and lithium nickel vanadate (LNV). In some embodiments, the positive active material is lithium nickel manganese cobalt oxide (NMC), optionally doped with another metal such as aluminium. In some cases, the electrochemically active material may comprise carbon, suitably graphite, graphene or a blend of carbon and a silicon oxide. Such electrochemically active materials are commercially available or may be manufactured by methods known to the skilled person, for example through the precipitation of mixed metal hydroxide intermediates from a reaction mixture containing different precursor metal salts, followed by calcination to form a mixed metal oxide and optionally lithiation to incorporate lithium into the oxide. The electrochemically active materials may be undoped or uncoated or may contain one or more dopants and / or a coating. For example, the electrochemically active material may be doped with small amounts of one or more metal elements. The electrochemically active material may comprise a carbon coating on the surface of the particles of the material. In some embodiments, the electrochemically active material is a negative active material. Such compositions may suitably be used to prepare an electrode which is an anode. In some embodiments, the electrochemically active material has a bimodal particle size distribution. In some embodiments, the electrode precursor composition is for a lithium-ion secondary electrochemical cell. In some embodiments, the electrode precursor composition is a cathode precursor composition. Electrode / Mso provided herein is an electrode for use in an alkali metal ion secondary cell. The electrode comprises a polymer-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising a first polymer which is polymethylmethacrylate (PMMA) having a weight average molecular weight of at least IxlO3 Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt; wherein the dispersed phase comprises an electrochemically active material; and wherein the first polymer makes up between 0.1 and 10 vol% of the total volume of the polymer blend. In some embodiments, the electrode comprises or is produced by processing an electrode precursor composition described herein to form a film. In some embodiments, the electrode is an extruded electrode. In other embodiments, the electrode is a hot-rolled electrode. In other embodiments, the electrode is prepared by extruding an electrode precursor composition described herein through a die to form a film. In some embodiments, the electrode is a cathode or an anode. In some embodiments, the electrode is a cathode. All of the compositional options set out for the electrode precursor composition described herein apply equally to the electrode described herein, including the identities and the relative amounts of the various components of the composition which are not typically expected to change during the processing of the precursor composition into the electrode. In some embodiments, the processing comprises thermal processing or extrusion. In some embodiments the thermal processing comprises passing the electrode precursor composition through a roller assembly at a temperature of at least 50 °C, for example at least 60 °C, at least 70 °C, at least. 80 °C, at least 90 °C or at least 100 °C. In some embodiments the thermal processing comprises passing the electrode precursor composition through rollers at a temperature of up to 150 °C, for example up to 140 °C or up to 130 °C. In some embodiments the thermal processing comprises passing the electrode precursor composition through rollers at a temperature of from 50 °C to 150 °C, for example from 60 °C to 150 °C, from 70 °C to 150 °C, from 80 °C to 150 °C, from 80 °C to 140 °C. from 90 °C to 140 °C, from 100 °C to 140 °C or from 110 °C to 130 °C. The roller assembly may comprise two rollers separated by a small distance such that the electrode is pressed into a thin film when passed through the rollers. In some embodiments the thermal processing comprises extruding the electrode. In some embodiments the thermal processing comprises extruding the electrode using an extrusion apparatus comprising one or more screw feeding sections and an extrusion die. In some embodiments, the temperature of the die is at least 50 °C, for example at least 60 °C, at least 70 °C, at least 80 °C, at least 90 °C or at least 100 °C. In some embodiments the temperature of the die is up to 150 °C, for example up to 140 °C or up to 130 °C. In some embodiments the temperature of the die is from 50 °C to 150 °C, for example from 60 °C to 150 °C, from 70 °C to 150 °C, from 80 °C to 150 °C, from 80 °C to 140 °C, from 90 °C to 140 °C, from 100 °C to 140 °C or from 110 °C to 130 °C. in some embodiments the electrode has a thickness of less than 150 pm, for example less than 100 pm, less than 90 pm, less than 80 pm or less than 70 pm. In some embodiments the electrode has a thickness of from 40 to 150 pm, for example from 40 to 100 pm, from 40 to 90 pm, from 40 to 80 pm, from 40 to 70 pm or from 50 to 70 pm. In some embodiments the electrode has a thickness of from 40 to 150 pm, for example from 40 to 100 pm, from 40 to 90 pm, from 40 to 80 pm, from 40 to 70 pm or from 50 to 70 pm. In some embodiments the electrode has a porosity of less than about 5% by volume. In some cases, the porosity of the electrode is less than 5 vol%, less than 3 vol% or less than 2 vol%. To phrase in another manner, the volumetric density of the electrode may be at least 95%, suitably at least about 97% or 98% of the density of a perfectly non-porous electrode. In some cases, the extruded electrode may for part of an extruded monolith which includes one or more further layers which are present in an electrochemical battery'. For instance, the monolith may include a separator layer, and / or may include the other electrode (i.e. the extruded monolith may include both a cathode and anode). The different layers may be coextruded and have different compositions from one another. Cell and Device Also provided herein is an electrochemical secondary' cell comprising an electrode described herein. In some embodiments, the cell is an alkali metal ion secondary-’ cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell. In some embodiments, the cell is a lithium-ion secondary cell. In some embodiments, the electrochemical secondary' cell comprises one electrode as described herein. In some embodiments, the electrochemical secondary cell comprises more than one, such as two or more than two, electrodes as described herein. In some embodiments the electrochemical secondary' cell comprises a first electrode described herein, wherein the first electrode is a cathode, and a second electrode described herein, wherein the second electrode is an anode, and an electrolyte between the cathode and the anode. In some embodiments the electrochemical secondary cell comprises an electrode described herein laminated with a current collector, for example a metallic foil Also provided herein is an electrochemical energy storage device comprising an electrochemical secondary cell described herein. In some embodiments, the electrochemical energy storage device is a battery In some embodiments, the electrochemical energy storage device is a lithium-ion battery. Methods / Mso provided herein is a method of preparing an electrode precursor composition set out herein. The method comprises mixing the electrochemically active material with the first, second and optional further polymerfs) and the electrolyte. Other components, such as any conductive additive, may be mixed at the same time or later. Mixing may be carried out by any suitable means, in particular any suitable kind of .mixing apparatus for any suitable time. In some embodiments, the mixing is carried out at room temperature (roughly 18-25 °C) at atmospheric pressure. Also provided herein is the use of between 0.1 and 10 vol% of the first polymer in a polymer-electrolyte gel matrix phase of a composition for an alkali metal ion secondary’ cell, the composition comprising the polymer-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising the first polymer which is polymethylmethacrylate (PMMA) having a. weight average molecular weight of at least lx 105 Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt; and wherein the dispersed phase comprises an electrochemically active material. Accordingly, the use provides the first polymer as a processing additive to gel electrode compositions. Typically, the use results in a composition as described herein. Also provided herein is a method of producing an electrode. The method comprises processing an electrode precursor composition described herein to form a film or a coating. In some embodiments, the processing forms a film. In some embodiments, the method is a method of producing an electrode for an alkali metal ion secondary' cell. In some embodiments, the processing comprises thermal processing or extrusion. In some embodiments, the method comprises: mixing polymers, an electrolyte and an electrochemically active material to form an electrode precursor composition described herein; and thermally processing or extruding the electrode precursor composition to form an electrode film or coating. In some embodiments, the processing forms a film. In some embodiments, the processing comprises thermal processing. In some embodiments, the electrode film has a thickness of from 500 to 700 pm. In some embodiments, the method further comprises cutting the electrode film to form an electrode of predetermined dimensions. In some embodiments, the method further comprises performing a second thermal processing step on the cut film to reduce the thickness of the film to within a range of 50 to 70 um. In some embodiments, the film is provided at the desired thickness without a second thermal processing step i.e. the thermal processing or extrusion step results in a film of desired thickness. In some embodiments, the temperature during thermal processing is from 100 to 140 °C. Examples Electrode precursor compositions were prepared according to the formulations shown in Table 1 below. The electrode precursor compositions were subjected to processing steps and the resulting compounded material subjected to pressure at room temperature (between 20 and 25°C) to reduce target thickness. The same pressure was applied to each example and its respective comparative example (i.e. Example I and Comparative Example 1 had the same pressure applied; Example 2 and Comparative Example 2 had the same pressure applied; and Example 3 and Comparative Example 3 had the same pressure applied). After subjecting the compounded materials to pressure, a visual assessment was carried out. The mass amounts of the components to be included in the composition was calculated based on the target vol% amount to be used, and the determined density of the component, using the known formula mass ::: density x volume. In general, density may be determined by available data (in a textbook or similar) or from first principles using a suitable method known to a skilled person. For solid components -such as particulate solids, polymers, conductive carbon - density can be determined using a method such as helium pycnometry to measure the volume and material weight accurately (this is not powder density or tap density). For liquid components, density measurements can be taken using suitable liquid density apparatus such as a volumetric flask. Density measurements as used herein are at 25°C and atmospheric pressure. The examples used the following components: - the second polymer is an ultra-high molecular weight PVDF copolymer, the polymer comprising VDF as constituent monomers; - the third polymer is a high molecular weight PVDF copolymer, the polymer comprising VDF as constituent monomer units, - the first polymer is PMMA; additive 1 is carbon black; and additive 2 is multi-wall carbon nanotubes; electrolytes 1 and 2 are each present at IM. Electrolyte 1 has the following composition, where the % values provided are a proportion of the total electrolyte, which is 100 wt%: Proportion of component in electrolyte (wt%) Ethylene carbonate 58.19% Fluoroethylene carbonate 2.29% LiFSI 12.47% Propylene carbonate 19.40% LiTDI 3.20% Vinylene carbonate 4.45% Electrolyte 2 is a mixture of two carbonate solvents and two lithium salts. Table 1 First Polymer (vol%) Example 1 0.34 Comparative Example 1 Example 2 0.17 Comparative 1 Example 2 1 Example 3 | 0.08 Comparative Example 3 Second Polymer (vol%) 0.48 0.48 0.47 0.47 0.47 0 47 Third Polymer (vol%) 3.04 3.38 3.15 3.32 1 3.23 3.32 Amount of first polymer based on total volume of poiymer blend (vol%) 8.8 4.5 2.! Electrolyte 1 (vol%) Electrolyte 2 (vol%) 30.86................. ................30.86________________ _________________30.32_________________ 30.32 30.32 30.32 Cathode active material (vol %) 64.00 64.00 64.00 64.00 1 64.00 64.00 Additive 1 (vol%) 1.29 1.29 Additive 2 (vol%) 1.89 1.89 | 1.89 1.89 It was found that, after application of pressure, electrolyte was squeezed out from the compounded material of Comparative Examples 1, 2 and 3. .After application of corresponding pressure to the compounded material of Example L electrolyte was not observed to be squeezed out. After application of pressure to the compounded material of 5 Example 2, it was observed that electrolyte retention was considerably improved compared to the result of Comparative Example 2. Similarly, after application of pressure to the compounded material of Example 3, it was observed that the electrolyte retention was considerably improved compared to the result of Comparative Example 3. 10 Figure 1(a) shows the result of Comparative Example 1 i.e. a composition having no first polymer, and subsequently processed and pressed as described above. The circled portion shows areas of the compounded material in which electrolyte loss can be observed visually. Other areas are also visible, though not indicated. Figure 1 (b) show's the result of Example 1 i.e. a composition as described herein containing a first polymer as an additive 15 in the composition used to form the material of Figure 1(a). The material shown in Figure 1(b) was processed and pressed in the same way as the material of Figure 1(a). Figure 1(b) shows no area of visible electrolyte loss. It is noted that these images are taken with overlying polymer film (used when pressure is applied) carefully removed, for ease of comparison.

Claims

1, An electrode precursor composition for an alkali metal ion secondary cell, comprising:a polymer-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising a first polymer which is polymethylmethacrylate (PMMA) having a weight average molecular weight of at least 1x103 Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt;wherein the dispersed phase comprises an electrochemically active material;and wherein the first polymer makes up between 0.1 and 10 vol% of the total volume of the polymer blend.

2. An electrode precursor composition according to claim 1, wherein the first polymer is present in an amount of at least 0.5 vol% of the total volume of the polymer blend and / or up to 9 vol% of the total volume of the polymer blend.

3. An electrode precursor composition according to any one of the preceding claims, wherein the first polymer has a weight average molecular weight of at least lxlO6Da.

4. An electrode precursor composition according to any one of the preceding claims, wherein the second polymer has a weight average molecular weight of at least IxlO5 Da.

5. An electrode precursor composition according to any one of the preceding claims, wherein the polymer blend comprises a third polymer which is different from the first and second polymers and which optionally comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer.

6. An electrode precursor composition according to claim 5, wherein the first polymer has a weight average molecular weight which is greater than that of the second polymer,and optionally wherein the third polymer has a weight average molecular weight which is lower than that of the second polymer.

7. An electrode precursor composition according to any one of the preceding claims, wherein at least one of the second polymer or an optional further polymer comprises hexafluoropropylene (HFP) as a constituent monomer, and optionally up to 2 mol% of one or more constituent monomers other than VDF and HFP.

8. An electrode precursor composition according to any one of the preceding claims, wherein the electrochemically active material is present in an amount of 50 to 75 vol% of the total volume of the electrode precursor composition.

9. An electrode precursor composition according to claim 8, wherein the electrochemically active material is present in an amount of at least 60 vol% of the total volume of the electrode precursor composition.

10. An electrode precursor composition according to any one of the preceding claims, wherein the dispersed phase further comprises a conductive additive.

11. .An electrode precursor composition according to any one of the preceding claims, wherein the liquid electrolyte is present in an amount of at least 25 vol% of the total volume of the electrode precursor composition.

12. An electrode precursor composition according to any one of the preceding claims, wherein the organic solvent comprises one or more cyclic or linear carbonate compounds.

13. An electrode precursor composition according to any one of the preceding claims, wherein the organic solvent comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone.

14. An electrode precursor composition according to any one of the preceding claims, wherein the alkali metal salt comprises one or more of LiPFe, LiBFh, lithium bis(fluorosulfonyl) imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium 2-trifluoroniethyl-4,5-dicyanoimidazole (LiTDI).

15. An electrode precursor composition according to any one of the preceding claims, wherein the electrode precursor composition comprises from 20 vol% to 50 vol% of the polymer-electrolyte gel matrix phase, based on the total composition volume16. An electrode precursor composition according to any one of the preceding claims, wherein the polymer-electrolyte gel matrix phase comprises 3 to 30 vol% of the polymer blend, based on the total volume of polymer-electrolyte gel matrix phase.

17. An electrode precursor composition according to any one of the preceding claims, which is for a lithium-ion secondary electrochemical cell.

18. A method of preparing an electrode precursor composition according to any one of the preceding claims, comprising mixing the electrochemically active material with the first and second polymers, and optionally a further polymer, and the electrolyte.

19. Use of between 0.1 and 10 vol% of a first polymer in a polymer-electrolyte gel matrix phase of a composition for an alkali metal ion secondary cell, the composition comprising:the polynier-electrolyte gel matrix phase and a dispersed phase, wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising the first polymer which is polymethylmethacrylate (PMMA) having a weight average molecular weight of at least IxlO5 Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt; andwherein the dispersed phase comprises an electrochemically active material.

20. An electrode for use in an alkali metal ion secondary cell, comprising a polymer-electrolyte gel matrix phase and a dispersed phase,wherein the polymer-electrolyte gel matrix phase comprises a polymer blend and a liquid electrolyte, the polymer blend comprising a first polymer which is polymethylmethacrylate (PMMA) having a weight average molecular weight of at least lx 105 Da, and a second polymer which comprises at least 75 mol% vinylidene fluoride (VDF) as constituent monomer, and wherein the liquid electrolyte comprises an organic solvent and an alkali metal salt;wherein the dispersed phase comprises an electrochemically active material; andwherein the first polymer makes up between 0.1 and 10 vol% of the total volume of the polymer blend.

21. An electrode according to claim 20, comprising or produced from an electrode precursor composition according to any one of claims 1 to 19.

22. A method of producing an electrode comprising processing an electrode precursor composition according to any one of claims 1 to 19 to form a film or coating.

23. A method according to claim 22, wherein the processing comprises thermal processing or exfrusiom24. An electrochemical secondary cell comprising an electrode according to any one of claims 20 or 21. or as produced by a method according to any one of claims 22 or 23.

25. An electrochemical energy storage device comprising an electrochemical secondary cell according to claim 24.

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