Energy storage devices and components thereof

Abstract

A method of preparing an electrode precursor composition for use in an alkali metal ion secondary cell comprises forming a first mixture from one or more electrolyte components, a gelling polymer, and carbon nanotubes, and mixing the first mixture with an electrochemically active material to thereby form an electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a dispersed phase comprising electrochemically active material and carbon nanotubes. The electrolyte component preferably comprises an alkali metal salt such as LiBF4, LiTFSI, LiFSI or LiTDI and an organic solvent. The carbon nanotubes may comprise single wall (SWCNT) and / or multi-wall carbon nanotubes (MWCNT). The carbon nanotubes may be dispersed in the polymer-electrolyte gel matrix using sonication, ultrasonication and / or rotor-stator homogenisation. The gelling polymer may comprise one or more of PVDF, HFP, PVDF-HFP, PMMA, PAN and / or PEO. Also disclosed are an electrode precursor composition and an electrode for an alkali metal ion secondary cell obtained or obtainable by the method, an electrochemical secondary cell comprising the electrode, and an electrochemical energy storage device comprising the electrochemical secondary cell.

Description

Field of the Invention The present invention relates to an electrode precursor composition for an alkali metal ion secondary cell, and a method for making said electrode precursor composition. The invention also relates to electrodes, cells and energy storage devices made from such precursor compositions, along with methods of preparing electrodes for alkali metal ion secondary cells. Background Lithium-ion secondary batteries are the leading battery technology currently powering devices ranging from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. Lithium-ion batteries contain a plurality of lithium-ion secondary cells, one example of an alkali metal ion secondary cell. Traditional lithium ion battery components such as electrodes are made from a solvent cast process that uses sacrificial solvent. This is an energetically expensive step, and a process that avoids using sacrificial solvent is therefore desirable. A further major drawback of lithium-ion technology and other alkali-metal ion secondary cell technology is that a liquid electrolyte is often 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. This gives rise to safety issues since the liquid electrolytes are often highly flammable. This is a particular problem for electric vehicles, where a collision with another vehicle is relatively likely and the resulting impact may cause damage to the battery and ignition of the electrolyte. It is also a problem for devices used in the home, where a lithium-ion battery fire could cause damage to property, serious injury or even death. One approach to avoiding the use of sacrificial solvent, and the need for liquid electrolyte within the cell, is preparing gel electrodes. These electrodes can be formed from a composition prepared by mixing the necessary components such as electrochemically active material, a gelling polymer, and a liquid electrolyte to form the gelled electrode material comprising the electrochemically active material. Such gel electrodes are described in WO 2017 / 017023 A1, which attempts to manufacture electrochemical devices free of liquid electrolytes. Gel electrodes are assembled together with other gel or solid-state components to form a cell having no free liquid components, thereby reducing the risk of fire due to the removal of free liquid from the cell. The cell manufacturing costs are also reduced because the gel components can be produced by simpler processing steps without the need for slow drying of solvent needed for solvent cast electrodes. However, despite these advantages, gel-electrode based solid-state cells may have reduced cell performance in comparison to some conventional cells. Accordingly, there is a need for electrochemical cells which not only offer improved safety benefit but also demonstrate suitable performance. Summary of the Invention 5 The present inventors have realised that the addition of carbon nanotubes to a gel electrode could offer the potential for improved performance of gel-electrode based solid-state cells. For conventional, solvent cast electrodes, carbon nanotubes are typically supplied as dispersions in sacrificial solvents and mixed with the other components of the electrode precursor slurry during the solvent casting process. The solvent is then removed during the 10 casting / drying step. However, in these methods, it is necessary to predisperse the carbon nanotubes at low solid contents if good dispersion of the carbon nanotubes in the slurry for solvent casting is to be achieved, due to the aspect ratio of SWCNTs and their resulting impact on dispersion viscosities. Furthermore, the use of a dispersing agent (commonly polymers used as binders for conventional electrode design) is typically necessary to maintain good 15 dispersion. For gel electrodes, and in particular gel electrodes which are produced by a thermal processing method, it would be preferred if a solvent evaporation step was not included as part of the production method, to limit the overall complexity and cost of the method (as well as the 1 requirement for solvent capture, depending on the solvent in question). 1 20 Accordingly, the invention relates generally to a method of preparing an electrode precursor composition for use in an alkali metal ion secondary cell, and in particular to a method in which carbon nanotubes are incorporated in the electrode precursor composition by incorporation in electrolyte components of the gel electrode. In a first aspect, the present invention provides a method of preparing an electrode precursor 25 composition for use in an alkali metal ion secondary cell, the method comprising: forming a first mixture from one or more electrolyte components, a gelling polymer, and carbon nanotubes; and mixing the first mixture with an electrochemically active material to thereby form an electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a 30 dispersed phase comprising electrochemically active material and carbon nanotubes. By providing a method in which carbon nanotubes are incorporated with electrolyte components of the electrode precursor composition, the need for a sacrificial solvent evaporation step is removed (because the carbon nanotubes can be directly incorporated into the polymerelectrolyte gel matrix phase). 35 Furthermore, such methods may enable processing and effective dispersion of the carbon nanotubes even at low solid content amounts, and even in the absence of a dedicated dispersing agent for the carbon nanotubes, because the gelling polymer can act as a dispersing agent. The electrode precursor composition made by the above method may find use as a precursor material for the preparation of a gel electrode, as discussed below. 5 Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise. 10 The one or more electrolyte components may include a solvent suitable for use as an electrolyte solvent in a gel electrode, for example an organic solvent. The one or more electrolyte components may include a salt. In some embodiments, the one or more electrolyte components may constitute an electrolyte salt solution. However, in alternative embodiments, the first mixture may be formed by mixing a solvent, a gelling polymer and carbon nanotubes 15 alone. An electrolyte salt may be subsequently added to the first mixture. In some embodiments, the one or more electrolyte components comprises a solvent comprising 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 20 carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. Preferably, the electrolyte component(s) comprises a solvent with low vapor pressure and high flash point to enable safe processing. An example of a solvent fulfilling these criteria is propylene carbonate. Accordingly, the one or more electrolyte components may comprise or consist of propylene 25 carbonate, or a blend of propylene carbon with one or more of the above listed solvents. In some embodiments, the one or more electrolyte components comprises an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). Typically, it refers to lithium, sodium and potassium. The anion of the alkali metal salt may be any suitable anion. Typical anions are known to the 30 skilled person and may be chosen based on the kind of alkali metal. In some embodiments, when the alkali metal is lithium, the anion of the salt comprises a halogen such as fluorine. Examples include BF4-, PF6-, TFSI-, FSI- and TDI-. In some embodiments, the one or more electrolyte components comprises a lithium salt. Examples of suitable lithium salts include LiPFs, LiBF4, LiTFSI, LiFSI and LiTDI. Preferably the 35 salt is a thermally stable salt. It has been found that LiPFs has relatively low thermal stability relative to other available lithium salts, and accordingly use of LiPFs may be avoided - that is, in some embodiments, the electrolyte component(s) do not include LiPF6. One or more kinds of alkali metal salt may be used in accordance with the present invention. Typically, but not exclusively, when more than one kind of alkali metal salt is used, they share a common alkali metal. The gelling polymer may be selected from one or more of Polyvinylidene fluoride (PVDF) 5 (optionally functionalised PVDF), hexafluoropropylene (HFP), Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP),), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), poly(ethylene oxide) (PEO) or mixtures or co-polymers thereof. However, this list is not exhaustive, and other commonly used polymers for gel electrolytes may also be suitable. Accordingly, in some embodiments, the gelling polymer may be selected from one or more of: 10 poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(ethylene oxide) (PEO), poly(ethyleneglycol dimethylether), poly(ethyleneglycol diethylether), poly[bis(methoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, 15 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, CXI      polyfuran, polycarbazole, tetrathiafulvalene-substituted polystyrene, ferrocene-substituted polyethylene, carbazole-substituted polyethylene, polyoxyphenazine, poly(heteroacene), 20   poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide-co-methoxy-polyethyleneglycolacrylate] ___     (Li[PSTFSI-co-MPEGA]), sulfonated poly(phenylene oxide) (PPO), N,N-dimethylacryl amide (DMAAm), lithium 2-acrylamido-2-methyl-1-propane sulfonate (LiAMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co-Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium 25 sorbate)+BF3], PEO copolymer, and PEO terpolymer, or mixtures or co-polymers thereof. In some preferred arrangements, the gelling polymer may comprise a PVDF-HFP copolymer, or a mixture of polymers having high HFP content (over 50% HFP), or combination of these. The order of addition of components in the step of forming the first mixture from one or more electrolyte components, a gelling polymer, and carbon nanotubes is not of critical importance. 30 However, in some embodiments, the step of forming the first mixture may include sub steps of: mixing the one or more electrolyte components and the gelling polymer to form a polymer-electrolyte mixture or a polymer-electrolyte component-based solution; and dispersing the carbon nanotubes in the polymer-electrolyte mixture or polymerelectrolyte component-based solution to form the first mixture. 35 The step of dispersing the carbon nanotubes in the polymer-electrolyte mixture or polymerelectrolyte component-based solution may be performed by any conventional method known to the person skilled in the art. In some embodiments, this step may be performed via one or more of: sonication, ultrasonication, and / or rotor-stator homogenisation. In some embodiments, the volume ratio of electrolyte components (e.g. electrolyte salt solution comprising solvent + salt) to other components (e.g. to gelling polymer and carbon nanotubes) in the first mixture may be in a range of from 5:1 to 100:1. In some embodiments, the volume ratio of gelling polymer to carbon nanotubes in the first 5 mixture may be in a range of from 9:1 to 1:1. In some embodiments, a heating step may be performed during formation of the first mixture. For example, heating may be performed during a step of mixing the one or more electrolyte components and the gelling polymer to form a polymer-electrolyte mixture or a polymerelectrolyte component-based solution. This heating can assist in dissolution and gelation of the 10 gelling polymer. Where a heating step is performed, this may be carried out at any suitable temperature. The suitable temperature may depend on the solubility of the polymer, however generally, any temperature that is below the flashpoint of solvent(s) in the first mixture may be suitable. In one example, for a first mixture which comprises PVDF-HFP polymer in a solvent comprising at least one carbonate compound, a heating step may be performed at a 15 temperature of 80 °C or more. In some other embodiments, no dedicated heating step is required during formation of the first mixture. For example, a dedicated heating step may not be required where the gelling polymer is a polymer which dissolves in the one or more electrolyte components (e.g. in the electrolyte solvent) at room temperature. 20 The carbon nanotubes may include single-walled carbon nanotubes (SWCNTs). The carbon nanotubes may include multi-walled carbon nanotubes (MWNTs). In some arrangements, the carbon nanotubes may include substantially only SWCNTs. In other arrangements, the carbon nanotubes may include substantially only MWCNTs. In yet other arrangements, the carbon nanotube may include a mixture of SWCNTs and MWCNTs. 25 The carbon nanotubes may have any suitable aspect ratio. The dynamic viscosity of the first mixture may be selected to be in a range of 100 to 20000 cP, for example in a range of from 100-10000 cP, as determined by use of a suitable rheometer, such as a cone and plate rheometer. The dynamic viscosity may be determined at a shear rate of 100s1. Selection of an appropriate viscosity can help to ensure suitable dispersion of the 30 carbon nanotubes throughout the first mixture. In some embodiments, the first mixture and / or the electrode precursor composition does not comprise a dedicated dispersing agent (i.e. a dispersing agent which is added in addition to any inherent dispersing capabilities of other components already present in the first mixture). This is because, as discussed above, the gelling polymer can act as a suitable dispersion agent when 35 forming the first mixture. For example, in some embodiments, the electrode precursor composition does not comprise carboxymethyl cellulose (CMC), or contains an immeasurably low amount of CMC, such as less than 1% or substantially 0%. In this way, electrode precursor compositions or gel electrodes produced from said electrode precursor compositions according to the present invention may be distinguishable from electrode precursor compositions / gel electrodes comprising carbon nanotubes, but where the carbon nanotubes have been incorporated in the electrode precursor compositions / gel electrodes by a method not according 5 to the present invention. This is because in other methods, it would typically be necessary to include one or more such further dedicated dispersing agents in order to provide for suitable dispersion of the carbon nanotubes within the electrode precursor composition. As explained above, this is not necessary in the present invention. Similar comments may also apply in respect of PVDF without any functionalisation: in some embodiments the electrode precursor 10 composition does not comprise non-functionalised (PVDF) or contains an immeasurably low amount of non-functionalised PVDF, such as less than 1% or substantially 0%. The identity of the electrochemically active material in the method of the invention is not of particular importance. The benefits of the invention based on the addition of the carbon nanotubes to electrolyte components of the electrode precursor composition may be achieved 15 for any active material which could be present in an electrode or electrode precursor composition. The skilled person will be aware of a large number of possible cathode active materials (also called positive active materials) and anode active materials (also called negative CXI active materials) which may be used in the present invention. The electrochemically active material may be a particulate material, i.e. materials made up of a T”20   plurality of discrete particles. The particles may comprise primary particles and / or secondary particles formed from the agglomeration of a plurality of primary particles. 1 In some embodiments, the electrochemically material is a positive active material and the electrode precursor composition is a cathode precursor composition. In these embodiments, the positive active material may be a lithium transition metal oxide material. In some 25 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 30 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 35 lithium nickel manganese cobalt oxide (NMC), optionally doped with another metal such as aluminium. Such positive 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. In some embodiments, the electrochemically active material is a negative active material and the electrode precursor composition is an anode precursor composition. In these embodiments, 5 the electrochemically active material may comprise carbon (suitably graphite or hard carbon), silicon, silicon-carbon composite, silicon oxide, or mixtures thereof. Preferably, the negative active material is selected from one or more of graphite, silicon and silicon oxide. As above, such active materials are commercially available or may be manufactured by methods known to the skilled person. 10 In some embodiments the electrochemically active material is a material which undergoes significant volume change during a charge / discharge cycle when used in an alkali metal ion secondary cell, e.g. as a result of incorporation of the alkali metal ions into the active material layer. For example, the active material layer may undergo volume changes of 100% or more, 200% or more, or 300% or more when the active material alkali metal ions are incorporated into 15 the active material layer to saturation, e.g. when fully lithiated, for a lithium-ion cell. The volumetric median particle size (D50) of the electrochemically active material may be from 0.5 to 50 pm, for example from 1 to 40 pm, from 2 to 30 pm, from 3 to 25 pm or from 4 to 15 pm. D50 is the volumetric median particle size. In other words, it represents the particle size in microns which splits the volume distribution of a population of particles in half, with 50 vol% of the particles having a particle size below that value and 50 vol% having a particle size above that value. The skilled person will appreciate that the volume median particle size D50 can be measured using a Malvem<R™> MastersizeKR™) 3000 using the light scattering method set out in ASTM 25   B822-20, applying the Mie scattering theory.The first mixture and the electrochemically active material may be combined in relative amounts at a ratio of e.g. 1:4 to 1:10 or more, the ratio being calculated based on the relative weights of these components. Further details relating to the proportions of components in the electrode precursor composition once mixed are set out later. 30 Further details about the relative amounts of components in the resulting electrode precursor composition are discussed below. In some embodiments, heating is performed during the step of mixing the first mixture with an electrochemically active material to form the electrode precursor composition. This may assist in formation of the electrode precursor composition. Where a heating step is performed, this may 35 be carried out at any suitable temperature. The suitable temperature may depend on the solubility of the polymer, however generally, any temperature that is below the flashpoint of solvent(s) in the first mixture may be suitable. In one example, for a first mixture which comprises PVDF-HFP polymer in a solvent comprising at least one carbonate compound, a heating step may be performed at a temperature of 80 °C or more. In some embodiments, gelation of the gelling polymer occurs during the step of forming the first mixture from one or more electrolyte components, a gelling polymer, and carbon nanotubes. In 5 other embodiments, gelation of the gelling polymer occurs during the step of mixing the first mixture with an electrochemically active material to form the electrode precursor composition, for example during a thermal processing step performed during this mixing stage. The precise point at which gelation occurs will depend on the gelling polymer used, and also on the identity of the one or more electrolyte components (e.g. on the identity of the electrolyte solvent). In 10 some embodiments, a gelling polymer which dissolves in the electrolyte component(s) at room temperature may be selected. In some embodiments, the method does not include a dedicated solvent extraction step, unlike traditional solvent casting methods, because solvents that are present in the first mixture can be retained in the gel structure of the electrode precursor material. 15 In some embodiments, the step of mixing the first mixture with an electrochemically active material is performed by a kneading process. In some embodiments, the step of mixing the first mixture with an electrochemically active material is performed in a twin screw extruder. It has been found that use of a twin screw extruder can provide for more homogenous mixing that other possible mixing methods. 20 In some embodiments, a further conductive additive (i.e. a conductive additive in addition to the carbon nanotubes) is added during the method, for example during or after the step of mixing the first mixture with an electrochemically active material, so that the further conductive additive is present in the resulting electrode precursor composition. The conductive additive may be incorporated into the electrode precursor composition at any suitable time and using any 25 suitable method. For example, the conductive additive may suitably be included either when forming the first mixture, or when forming the electrode precursor composition by mixing the first mixture with an electrochemically active material. Preferably, the further conductive additive(s) are added at a point when the materials for forming the electrode precursor composition are in a flowable state, to allow for effective dispersion of the further conductive additive within the 30 resulting electrode precursor composition. In some embodiments the conductive additive comprises one or more of carbon black and graphite. In some embodiments, the conductive additive comprises or consists of carbon black. Examples of commercially available carbon black include Ketjen Black<R™) and Super C65. Proportions in which the conductive additive may be present in the electrode precursor 35 composition are discussed in further detail below. In some embodiments, further polymeric and / or electrolyte components may be added during or after the step of mixing the first mixture with an electrochemically active material, so that the further polymeric and / or electrolyte components are present in the resulting electrode precursor composition. Where further polymeric components are added, these may suitably be selected from the species identified about as suitable gelling polymers for use in the present invention. In one preferred arrangement, a lower molecular weight PVDF-HFP polymer or high HFP 5 content (over 50%) polymer mixture or combination thereof is used as the initial gelling polymer for the first mixture which acts to disperse the carbon nanotubes, and a higher molecular weight PVDF-HFP, or lower HFP content (less than 50%) polymer mixture or combination thereof is subsequently added during or after the step of mixing the first mixture with an electrochemically active material to provide the bulk of the polymer for forming an electrolyte gel. 10 Also envisaged herein is an electrode precursor composition for an alkali metal ion secondary cell, obtained or obtainable by a method according to the first aspect, comprising: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and carbon nanotubes. 25 The envisaged electrode precursor composition contains a polymer-electrolyte gel matrix phase and a dispersed phase of solid particulate material incorporating carbon nanotubes dispersed through the matrix phase. In this way, the electrode precursor composition has a gel-like composition and can be processed into a thin-film electrode with a similar gel-like composition, where the electrode structure contains liquid electrolyte (the electrolyte salt solution) trapped within the matrix phase due to the gelled nature of the polymer. Traditional solid-state electrodes formed by solvent casting a slurry onto a substrate followed by drying require the separate addition of a free liquid electrolyte to the cell when assembled, creating a cell which is a fire risk due to the flammability of the free liquid electrolyte. The replacement of such solidstate electrodes with a gel electrode prepared from the envisaged precursor composition reduces this risk and provides a cell of increased safety. Furthermore, the presence of carbon nanotubes as part of the dispersed phase of solid particulate may increase the capacity relation of an electrode formed from the electrode precursor composition. Without wishing to be bound by theory, the present inventors suggest that the presence of carbon nanotubes may provide for improved electrical connection across 30 the particulate phase of the electrode precursor composition, as the carbon nanotubes, in particular due to their large aspect ratio and excellent conductivity, may act to bridge gaps between particles of the electrochemically active material. Optional features discussed above relating to the nature of the electrochemically active material, carbon nanotubes, electrolyte salt solution and gelling polymer used in the method of 35 the first aspect also apply to the envisaged electrode precursor composition and so will not be repeated. However, the ratios of components in the resulting electrode precursor composition should be discussed. In some embodiments, the electrochemically active material makes up at least 50 vol% of the electrode precursor composition, for example at least 55 vol%, at least 60 vol%, at least 62 vol%, at least 64 vol%, or at least 65 vol%. In some embodiments, the electrochemically active material makes up no more than 68 vol% of the electrode precursor composition. By weight, 5 the electrochemically active material may make up at least 80 wt% of the electrode precursor composition, more preferably at least 85 wt% of the electrode precursor composition. In some embodiments, the carbon nanotubes make up at least 0.01 vol% of the electrode precursor composition, for example at least 0.05 vol%, or at least 0.1 vol%. The amount of carbon nanotubes present may be selected based on the form of the carbon nanotubes. For 10 example, where the carbon nanotubes comprise or consist of MWCNTs, these may be present in amounts of 5 vol% or less. Where the carbon nanotubes comprise or consist of SWCNTs, these may be present in amounts of 1 vol% or less. In some preferred embodiments, carbon nanotubes may make up 0.25 vol% or less of the electrode precursor composition. The mass of carbon nanotubes may by less than 1% by weight, more preferably less than 0.2% by weight, of 15 the electrode precursor composition. It has been found that these amounts can provide suitable performance whilst reducing or minimizing cost. As discussed above in relation to the first aspect, in some embodiments, the dispersed phase further comprises a further conductive additive (i.e. a conductive additive in addition to the carbon nanotubes present in the dispersed phase). This may be a particulate conductive "1”20 additive. Where a conductive additive is present, this may comprise or consist of one or more of carbon black and graphite. In some embodiments, the conductive additive comprises or T— consists of carbon black. Examples of commercially available carbon black include Ketjen Black<R™) and Super C65. In some embodiments, the conductive additive is present in an amount of <5 vol% based on the 25 total volume of the electrode precursor composition, although the precise amount added may be selected based on the choice of active material(s) and the amount of type of carbon nanotube present in the dispersed phase. For example, in some embodiments, the conductive additive is present in an amount of 1 vol% or more, 2 vol% or more, 3 vol% or more or 4 vol% or more based on the total volume of the electrode precursor composition 30 The dispersed phase of the electrode precursor composition may consist of the electrochemically active material, carbon nanotubes, and a further conductive additive. In some embodiments, the polymer-electrolyte gel matrix phase comprises or consists of a mixture of a gelling polymer and an electrolyte salt solution (the electrolyte salt solution may also be referred to as a liquid electrolyte), wherein the vol% of polymer in the gel matrix phase 35 is in a range of from 5 to 30 vol%. In some embodiments, the electrode precursor composition is for a lithium-ion secondary electrochemical cell. In some embodiments, the electrode precursor composition is an anode precursor composition. In other embodiments, the electrode precursor composition is a cathode composition. Also envisaged herein is an electrode for use in an alkali metal ion secondary cell comprising an electrochemically active material and carbon nanotubes dispersed in a polymer-electrolyte gel 5 matrix. The electrode may be obtained or obtainable by processing the envisaged electrode precursor composition into an electrode. In some embodiments, the electrode is produced by processing the envisaged electrode precursor composition to form a film. All of the compositional options and preferences set out above for the electrode precursor 10 composition of the envisaged electrode precursor apply equally to the envisaged electrode, including the identities and the relative amounts of the various components of the composition, which do not change during the processing of the precursor composition into the electrode. In some embodiments, a small amount of solvent evaporation may occur during processing, in particular during thermal processing. However, preferably such solvent loss is minimised by 15 appropriate process control such that the composition of the electrode precursor composition remains substantially unchanged even after thermal processing. C\j In some embodiments, the processing comprises thermal processing and / or extrusion. 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 ■^—20   extruding an electrode precursor composition through a die to form a film. 1        Accordingly, in a second aspect, the present invention provides a method of preparing an electrode for an alkali metal ion secondary cell, comprising: preparing an electrode precursor composition by the method according to the first aspect; and 25          thermally processing the electrode precursor composition to form an electrode film. 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 30 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. It is noted that the preferred temperature for hot-rolling may 35 be selected based on the identity of the polymer in the electrode precursor composition. 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 5 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 10   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 15 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, and comprises the electrochemically active material in an amount of from 50 to 75 vol% of the cathode, for example from 55 to 70 vol%, from 60 to 69 vol%, from 62 to 68 vol% or from 64 to 1 20   69vol%. 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. 25 In some cases, the extruded electrode may form 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 another 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. 30 The carbon nanotubes may be well distributed within the electrode. The distribution of carbon nanotubes can be determined by investigation of the electrode e.g. using SEM or TEM. Where the carbon nanotubes comprise SWCNT, these may not be easily visible using SEM, however may be observed using TEM. Where the carbon nanotubes comprise MWCNT, these may be observed by either SEM or TEM. 35 The electrode may have a capacity retention of 90% or more after 5 or more charge-discharge cycles at a C rate of C / 10. The capacity retention is preferably 95% or more, for example 96% or more, 97% or more, 98% or more or 99% or more. In some embodiments, the capacity retention of the electrode may be substantially equal to 100%. Also envisaged herein is an electrochemical secondary cell comprising the envisaged electrode. The cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a 5 lithium-ion secondary cell. Preferably the cell is a lithium-ion secondary cell. In some embodiments the electrochemical secondary cell comprises a cathode, an anode, and an electrolyte between the cathode and the anode, wherein one or both of the anode or cathode are envisaged electrodes. In some embodiments the electrochemical secondary cell comprises an envisaged electrode 10 laminated with a current collector, for example a metallic foil. Also envisaged herein is an electrochemical energy storage device comprising the envisaged electrochemical secondary cell. In some embodiments, the electrochemical energy storage device is a battery. In some embodiments, the electrochemical energy storage device is a lithium-ion battery. 15 The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. CXI Summary & Description of the Figures 1— Embodiments illustrating the principles of the invention will now be discussed with reference to ?n the accompanying figure in which: Fig. 1 is a block process-flow diagram showing steps in a method 100 according to the present invention. The method includes methods steps as follows: 1. [Step S1 ] Mix electrolyte solvent and salt with gelling polymer to form polymer solution (may require heat). This step may be performed in multiple sub steps, e.g. a first sub- 25           step of mixing electrolyte solvent and salt to form an electrolyte, followed by dissolving a gelling polymer (e.g. PVDF-HFP polymer) in electrolyte to form a polymer solution) 2. [Step S2] Add carbon nanotubes (e.g. SWCNTs) and disperse with sonication or high shear mixing to form SWCNT dispersion constituting a first mixture (may require heat) 3. [Step S3] Add additional components (actives, conducting additives and any additional 30          polymer / electrolyte components) & Distribute and disperse materials at elevated temperature (using equipment such as twin-screw compounder). This step may be performed in multiple sub steps, e.g. a sub-step of pumping the first mixture dispersion into a twin-screw extruder at elevated temperature, and optionally add additional polymer powder if required to form a gel in the first feeder, followed by further sub step(s) of adding active material components and any additional carbon components via powder feeders, downstream. 4. [Step S3] Process into electrode (through extrusion or hot-rolling, e.g. hot rolling the resultant agglomerates into an electrode film onto foil). 5 The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 10 While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the 15 invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for CXI the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. 1” Any section headings used herein are for organizational purposes only and are not to be 20 construed as limiting the subject matter described. 1 Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 25 It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as 30 approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A method of preparing an electrode precursor composition for use in an alkali metal ion secondary cell, the method comprising:forming a first mixture from one or more electrolyte components, a gelling polymer, and5 carbon nanotubes; andmixing the first mixture with an electrochemically active material to thereby form an electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a dispersed phase comprising electrochemically active material and carbon nanotubes.

2. The method according to claim 1 wherein the one or more electrolyte components10 comprises an organic solvent.

3. The method according to claim 2 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.15   4. The method according to any one of the preceding claims wherein the one or moreelectrolyte components comprises an alkali metal salt.CM5. The method according to claim 4 wherein the alkali metal salt is a lithium salt selected from one or more of LiBF4, LiTFSI, LiFSI and LiTDL1—6. The method according to any one of the preceding claims wherein the step of forming1 20 the first mixture includes sub steps of:mixing the one or more electrolyte components and the gelling polymer to form a polymer-electrolyte mixture or a polymer-electrolyte component-based solution; anddispersing the carbon nanotubes in the polymer-electrolyte mixture or polymerelectrolyte component-based solution to form the first mixture.25   7. The method according to claim 6 wherein the step of dispersing the carbon nanotubes inthe polymer-electrolyte mixture is performed via one or more of: sonication, ultrasonication, and / or rotor-stator homogenisation.

8. The method according to any one of the preceding claims, wherein the carbon nanotubes include single-walled carbon nanotubes (SWCNTs).30   9. The method according to any one of the preceding claims, wherein the carbonnanotubes include multi-walled carbon nanotubes (MWNTs).

10. The method according to any one of the preceding claims wherein the carbon nanotubes are present in the resulting electrode precursor composition in an amount of 1 vol% or less.

11. The method according to any one of the preceding claims wherein the gelling polymer35 comprises one or more polymers selected from Polyvinylidene fluoride (PVDF),hexafluoropropylene (HFP), Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP),), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), and / or poly(ethylene oxide) (PEO) or mixtures or co-polymers thereof.

12. The method according to any one of the preceding claims wherein the ratio of the5 volume of electrolyte components to the volume of gelling polymer + carbon nanotubes in the first mixture is in a range of from 5:1 to 100:1.

13. The method according to any one of the preceding claims wherein the ratio of the volume of gelling polymer to the volume of carbon nanotubes in the first mixture is in a range of from 9:1 to 1:1.10   14. The method according to any one of the preceding claims, wherein the electrochemicallyactive material comprises a positive active material.

15. The method according to claim 14, wherein the positive active material comprises a lithium transition metal oxide material.

16. The method according to any one of claims 1 to 13, wherein the electrochemically active 15 material comprises a negative active material.

17. The method according to claim 16, wherein the negative active material comprises carbon, silicon, silicon-carbon composite, silicon oxide, or mixtures thereof.

18. The method according to any one of the preceding claims, further comprising the ____ addition of a conductive additive during the step of forming the first mixture, or during the step of 20 mixing the first mixture with the electrochemically active material.

19. The method according to claim 18, wherein the conductive additive material comprises one or more of carbon black or graphite.

20. The method according to any one of the preceding claims wherein heating is performed during the step of mixing the first mixture with the electrochemically active material.25   21. The method according to any one of the preceding claims wherein the step of mixing thefirst mixture with the electrochemically active material is performed in a twin screw extruder.

22. A method of preparing an electrode for an alkali metal ion secondary cell, comprising: preparing an electrode precursor composition by the method according to any one of claims 1 to 21; and30          thermally processing the electrode precursor composition to form an electrode film.

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