Electrode precursor composition

Abstract

An electrode precursor composition for an alkali secondary metal ion secondary cell is described. The electrode precursor composition comprises a polymer electrolyte gel matrix phase, a dispersed phas

Description

B ACKGROUND Lithium-ion secondary batteries are the leading battery technology currently used in applications from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. They contain a plurality of lithium-ion secondary cells, which is 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. One approach to avoiding the use of sacrificial solvent is preparing gel electrodes. These electrodes can be formed from a composition prepared by mixing the necessary components such as electrochemically active material, conductive additive, polymer, and a liquid electrolyte, and subsequently subjecting the composition to a thermal treatment. The cell manufacturing costs are reduced because the gel components can be produced by simpler processing steps without the need for slow and energy intensive drying of solvent needed for solvent cast electrodes. SUMMARY Despite the advantages of gel-electrode based solid-state cells, it has been found that gelelectrode based solid-state cells may have reduced cell performance in comparison to some conventional cells. The present inventors have realised that the replacement of some or all of the conventional conductive additives used in gel-electrode based solid-state calls with a mixture of tubular and sheet carbon materials could offer the potential for improved performance of gelelectrode based solid-state cells. Accordingly, in a first aspect, the present invention provides an electrode precursor composition for an alkali metal ion secondary cell. The electrode precursor composition comprises: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and a conductive additive. The conductive additive comprises a mixture as a majority component, the mixture consisting of tubular carbon material and sheet carbon material. The mixture may consist of tubular carbon material and sheet carbon material in a ratio in the range of from 9:1 to 1:1 (i.e. 9:1 tubularsheet to 1:1 tubular:sheet). For example, the mixture may consist of tubular carbon material and sheet carbon material in a ratio in the range of from 8:2 to 1:1, or in the range of from 7:3 to 1:1, or in the range of from 6:4 to 1:1, or in the range of from 9:1 to 6:4, or in the range of from 8:2 to 6:4, or in the range of from 7:3 to 6:4, or in the range of from 9:1 to 7:3, or in the range of from 8:2 to 7:3, or even in the range of from 9:1 to 8:2. The mixture may consist of tubular carbon material and sheet carbon material in a ratio of about 9:1, or in a ratio of about 8:2, or in a ratio of about 7:3, or in a ratio of about 6:4 or in a ratio of about 1:1. The term ‘majority component’ is used herein to define that the mixture of tubular carbon material and sheet carbon material constitutes at least 50 wt% of the conductive additive. For example, the conductive additive may comprise 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, 95 wt% or more, or even 99 wt% or more of the mixture of tubular carbon material and sheet carbon material. The mixture may constitute at least 50 vol% of the conductive additive, for example the mixture may constitute at least 60 vol% or more, 70 vol % or more, 80 vol % or more, 90 vol % or more, 95 vol % or more, or even 99 vol % or more of the conductive additive. The conductive additive may consist essentially of, or consist of, the mixture of tubular carbon material and sheet carbon material. In this way, the electrode precursor composition may not comprise alternative non-tubular and non-sheet carbon conductive additive components. That is, the electrode precursor composition may contain substantially no conductive carbonaceous material other than the mixture of tubular and sheet carbon materials - e.g. the electrode precursor composition may not comprise amorphous carbon (such as carbon black). In some embodiments, the electrode precursor composition may contain less than 0.5 wt% of conductive carbonaceous materials other than the mixture of tubular and sheet carbon material, e.g. 0.1 wt% or less, 0.05 wt% or less, or 0.01 wt% or less. The term ‘tubular carbon’ is used herein to define carbonaceous materials having a generally tubular form. Tubular carbon materials may include, but are not limited to, carbon nanotubes (CNTs) (including SWCNTs and MWCNTs) as well as carbon fibres such as carbon nanofibers (CNFs) and vapor-grown carbon fibres (VGCFs). The term ‘sheet carbon’ is used herein to define carbonaceous materials having a generally sheet-like or planar form. Sheet carbon materials may include, but are not limited to, graphite, graphene, graphene oxide (GO), graphene nanoplatelets (GNPs), and reduced graphene oxide (RGO). The present inventors have found that by providing a conductive additive which comprises a mixture of tubular carbon material and sheet carbon material as a majority component, electrodes having improved electrochemical performance may result. In particular, the present inventors have found that by providing a conductive additive which comprises a mixture of tubular carbon material and sheet carbon material as a majority component, the tortuosity values of the resulting electrodes (tortuosity values associated with Li+_ diffusion) may be reduced in comparison to electrodes which do not comprise tubular carbon as the majority component of the conductive additive in the electrode. As will be discussed in greater detail below, electrodes produced from electrode precursor compositions according to the first aspect may demonstrate a tortuosity of 2.2 or less, as measured using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode, e.g. as described in Johannes Landesfeind et al 2016 J. Electrochem. Soc. 163 A1373. When the electrodes have a tortuosity of 2.2 or less, the electrodes may demonstrate suitable ionic resistance to allow for improved performance in high-power applications. Moreover, the present inventors have found that, in comparison to a conductive additive which comprises only a tubular carbon material as a majority component, by providing a conductive additive which comprises a mixture of tubular carbon material and sheet carbon material as a majority component, the ionic conductivity of the resulting electrodes may be increased. Additionally, or alternatively, the processability of the resulting electrodes may be increased (i.e. the load required to be applied to the resulting electrode to roll it to the desired thickness may be reduced). The dispersed phase may consist of, or consist essentially of, the electrochemically active material and the conductive additive. That is, the dispersed phase may comprise substantially no other components in addition to the electrochemically active material and the conductive additive. The tubular carbon material may be a material selected from single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs), vapor-grown carbon fibres (VGCFs), or mixtures thereof. Suitably, the tubular carbon material may comprise, consist essentially of, or consist of, multi-walled carbon nanotubes (MWCNTs). It has been found that use of MWCNTs can provide suitable performance with lower cost and manufacturing complexity than use of SWCNTs or carbon-fibre-based tubular carbon materials. The sheet carbon material may be a material selected from graphite, graphene, graphene oxide (GO), graphene nanoplatelets (GNPs), reduced graphene oxide (RGO), or mixtures thereof. Suitably, the sheet carbon material may comprise, consist essentially of, or consist of, graphite. The electrochemically active material may be a positive active material. Thus, the electrode precursor composition may be a cathode precursor composition. The positive active material may be a lithium transition metal oxide material. The positive active material may be 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. The positive active material may be a lithium transition metal oxide material comprising lithium and one or more transition metals selected from nickel, cobalt and manganese. The positive active material may be 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 (LMFP) and lithium nickel vanadate (LNV). Suitably, the positive active material may be 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. The electrochemically active material 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. The electrochemically active material may be a particulate material, i.e. a material made up of a plurality of discrete particles. The particles may comprise primary particles and / or secondary particles formed from the agglomeration of a plurality of primary particles. The electrochemically active material may make up at least 50 vol% of the electrode precursor composition, based on the total volume of 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%. Suitably, the electrochemically active material may make up about 64 vol% of the electrode precursor composition. As noted above, the dispersed phase comprises a conductive additive which comprises a mixture as a majority component, the mixture consisting of tubular carbon material and sheet carbon material. The conductive additive may be present in an amount of from 0.1 vol% to 10 vol% based on the total volume of electrode precursor composition. For example, the conductive additive may make up from 0.3 vol% to 5 vol%, from 0.4 vol% to 4 vol%, from 0.5 vol% to 3 vol% or from 0.55 vol% to 2.5 vol% of the electrode precursor composition. The conductive additive may make up 1 vol% or more of the electrode precursor composition, for example from 1 vol% to 3 vol% of the electrode precursor composition. Suitably, the conductive additive may be present in amounts of about 1.29 vol%, about 1.89 vol%, or about 2.48 vol%. The dispersed phase may comprise from 1.2 vol% to 5 vol% of the conductive additive, based on the total volume of the dispersed phase, for example the dispersed phase may comprise 1.3 vol% or more, 1.4 vol% or more, 1.5 vol% or more, 1.6 vol% or more, 1.7 vol% or more, 1.8 vol% or more, 1.9 vol% or more, or 2 vol% or more of the conductive additive, based on the total volume of the dispersed phase. The dispersed phase may comprise 4.5 vol% or less, 4 vol% or less, 3.9 vol% or less, 3.8 vol% or less, 3.7 vol% or less, 3.6 vol% or less, or 3.5 vol% or less based on the total volume of the dispersed phase. The dispersed phase may comprise from 95 vol% to 98.8 vol% of the electrochemically active material, based on the total volume of the dispersed phase, for example the dispersed phase may comprise 96 vol% or more, 97 vol% or more or 98 vol% or more of the electrochemically active material. The dispersed phase may comprise 98 vol% or less of the electrochemically active material. The dispersed phase may comprise from 95 vol% to 98 vol%, or from 96.5 vol% to 98 vol% of the electrochemically active material. The polymer-electrolyte gel matrix phase may be formed from one or more electrolyte components and at least one gelling polymer. 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. Suitably, the one or more electrolyte components may constitute an electrolyte salt solution or liquid electrolyte. The one or more electrolyte components may comprise a solvent comprising one or more cyclic or linear carbonate compounds. The solvent may comprise one or more cyclic carbonate compounds. For example, the solvent may comprise one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. The solvent may comprise a blend of at least two different compounds, for example at least three or at least four different compounds. The solvent may comprise a blend of at least two different organic carbonate compounds, for example at least three or at least four different organic carbonate compounds. Suitably, the electrolyte component(s) may comprise a solvent with low vapor pressure and / or high flash point. This may beneficially 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 carbonate, or a blend of propylene carbonate with one or more of the above listed solvents. The one or more electrolyte components may comprise an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). The alkali metal salt may be a lithium, sodium, or potassium salt. The anion of the alkali metal salt may be any suitable anion. Typical anions are known to the skilled person and may be chosen based on the kind of alkali metal. When the alkali metal is lithium, the anion of the salt may comprise a halogen such as fluorine. Examples include BF4-, PF6-, TFSI-, FSI-, OTf-, DFOB- and TDI-. The one or more electrolyte components may comprise a lithium salt. The electrolyte may comprise a mixture of at least two different lithium salts. Examples of suitable lithium salts include LiPF6, LiBF4, LiTFSI, LiFSI, LiOTf, LiDFOB and LiTDI. Suitable the salt is a thermally stable salt. It has been found that LiPF6 has relatively low thermal stability relative to other available lithium salts, and accordingly use of LiPF6 may be avoided -that is, the electrolyte component(s) may 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 may share a common alkali metal. The polymer-electrolyte gel matrix phase may comprise a gel matrix formed by the gelling of one or more gelling polymers when the polymer(s) absorb a liquid electrolyte. The polymer-electrolyte gel matrix phase may therefore comprise a gel comprising the polymer(s) and absorbed liquid electrolyte. The gelling polymer may comprise one or more gelling polymers independently selected from poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA), polystyrene (PS), poly(ethyleneglycol dimethylether), poly(ethyleneglycol di ethyl ether), poly[bis(methoxy ethoxy ethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, 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, poly naphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene-substituted polystyrene, ferrocene-substituted polyethylene, carbazolesubstituted polyethylene, polyoxyphenazine, poly(heteroacene), 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-l-propane sulfonate (Li AMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co-Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, PEO terpolymer, and NIPPON SHOKUBAI® polymer, or mixtures or co-polymers thereof. In some examples, the gelling polymer may comprise one or more gelling polymers independently selected from poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA) and polystyrene (PS) The polymer-electrolyte gel matrix phase may make up from 20 vol% to 50 vol% of the electrode precursor composition, for example from 25 vol% to 45 vol%, from 28 vol% to 42 vol%, from 30 vol% to 40 vol%, from 31 vol% to 39 vol% or from 32 vol% to 38 vol%. Suitably, the polymer-electrolyte gel matrix phase may make up about 33 vol%, about 34 vol%, or about 35 vol% of the electrode precursor composition. For example, the polymerelectrolyte gel matrix phase may make up about 33.52 vol%, about 34.11 vol%, or about 34.72 vol% of the electrode precursor composition. The electrode precursor composition may be for a lithium-ion secondary electrochemical cell. The electrode precursor composition may be a cathode precursor composition. A second aspect of the invention provides an electrode for use in an alkali metal ion secondary cell. The electrode comprises: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and a conductive additive. The conductive additive comprises a mixture as a majority component, the mixture consisting of tubular carbon material and sheet carbon material. The electrode may demonstrate a tortuosity of 2.2 or less, as measured using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode, e.g. as described in Johannes Landesfeind et al 2016 J. Electrochem. Soc. 163 A1373. When the electrodes have a tortuosity of 2.2 or less, the electrodes may demonstrate suitable ionic resistance to allow for improved performance in high-power applications. In some embodiments, the electrode may demonstrate a tortuosity in a range of from 1-2. The tortuosity maybe, for example, 1.7 or less, 1.6 or less, 1.5 or less, 1.4 or less, or 1.3 or less. The electrode may have an ionic conductivity of 1.2 mS / cm or more, as measured at 30 °C using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode, e.g. as described in Johannes Landesfeind et al 2016 J. Electrochem. Soc. 163 A1373. For example, the ionic conductivity may be 1.29 mS / cm or more, 1.3 mS / cm or more, 1.4 mS / cm or more, 1.5 mS / cm or more, 1.6 mS / cm or more, 1.7 mS / cm or more, 1.8 mS / cm or more or 1.9 mS / cm or more as measured at 30 °C using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode. The capacity retention of the electrode may be 20% or more at a C rate of 10C, as assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70 pl of electrolyte at 45 °C. For example, the capacity retention of the electrode may be 25% or more, 30% or more, 35% or more, 38%, or more, or 39% or more, or 60% or more, or 65% or more, or 69% or more at a C rate of 10C as assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70 pl of electrolyte at 45 °C. The capacity retention of the electrode may be 70% or more at a C rate of 5C, as assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70 pl of electrolyte at 45 °C. For example, the capacity retention of the electrode may be 76% or more, 77% or more, 80% or more, 83% or more, or 84% or more, or 86% or more at a C rate of 5C as assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70 pl of electrolyte at 45 °C. The electrode may be produced by processing an electrode precursor composition according to the first aspect to form a film or coating. The processing may comprise thermal processing and / or extrusion. The electrode may be an extruded electrode. The electrode may be a hot-rolled electrode. The electrode may be prepared by extruding an electrode precursor composition according to the first aspect through a die to form a film. Suitably, the electrode may be a cathode. All of the compositional options and preferences set out above for the electrode precursor composition of the first aspect apply equally to the electrode of the second aspect, 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. The thermal processing may comprise 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. The thermal processing may comprise 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. The thermal processing may comprise 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. The thermal processing may comprise extruding the electrode. The thermal processing may comprise extruding the electrode using an extrusion apparatus comprising one or more screw feeding sections and an extrusion die. The temperature of the die may be 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. The temperature of the die may be up to 150 °C, for example up to 140 °C or up to 130 °C. The temperature of the die may be 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 electrode may have 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. The electrode may have 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. The electrode may have 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. The electrode may have a porosity of less than about 5% by volume. In some cases, the porosity of the electrode may be 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 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 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. A third aspect of the invention provides an electrochemical secondary cell comprising an electrode according to the second aspect. The cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell. Suitably, the cell is a lithium-ion secondary cell. The electrochemical secondary cell may comprise a first electrode according to the second aspect, wherein the first electrode is a cathode, and a second electrode, wherein the second electrode is an anode, and an electrolyte between the cathode and the anode. The electrochemical secondary cell may comprise an electrode according to the second aspect laminated with a current collector, for example a metallic foil. A fourth aspect of the invention provides an electrochemical energy storage device comprising an electrochemical secondary cell according to the third aspect. The electrochemical energy storage device may be a battery. The electrochemical energy storage device may be a lithium-ion battery. A fifth aspect of the invention provides a method of preparing an electrode for an alkali metal ion secondary cell. The method comprises: mixing a polymer, an electrolyte, an electrochemically active material, and a conductive additive comprising a mixture as a majority component, the mixture consisting of tubular carbon material and sheet carbon material, to form an electrode precursor composition according to the first aspect; and processing the electrode precursor composition to form an electrode film. The electrode film may have a thickness of from 500 to 700 pm. The method may comprise cutting the electrode film to form an electrode of predetermined dimensions. The method may comprise 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 pm. The temperature during thermal processing may be 90 °C or more, e.g. from 100 to 140 °C. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the results for sweeping loads from 20 - 50 kN in increments of 10 kN to roll electrode precursor compositions into a gel electrode film. EXAMPLES &DETAILED DESCRIPTION For the samples tested, the amount of active material, conductive additive, polymer and electrolyte were determined by calculation of the desired vol % and then conversion of this amount to a desired wt %. The appropriate amounts of each were then weighed and mixed. This mixture was then fed into a twin-screw extruder with three mixing zones at several intervals. The main body of the twin screw extruder was held at 120 degrees over the mixing zones, with a ramp from 40 degrees from the input port and a drop off to 80 degrees at the exit. After this material was fed into the twin-screw extruder it was collected in the form of a granular mixture. This granular mixture was then rolled into a thin film. Precursor material was fed into a hot 5 roller assembly to create a film of target thickness, typically 50-70 pm depending on formulation. Two gel electrode precursor compositions were prepared according to the compositions shown in Table 1: 10 Component Fl (Comparative) F2 Amoum / vol% Active grade 1 NMC 64.00 64.00 Polymer grade 1 PVDF / PVDF copolymer 0.47 0.47 Polymer grade 2 PVDF / PVDF copolymer 3.26 3.26 Tubular carbon grade 1 - MWCNTs 2.48 1.99 Sheet carbon grade 1 - synthetic graphite 0 0.5 Electrolyte formulation 1 29.79 29.79 The electrode precursor compositions in Table 1 were formed into a gel electrode film by the method set out above. 15 The tortuosity, ionic and electronic conductivity values (IC, EC) of these electrode films was then measured, with the results being set out in Table 2, below: Formulation code IC (mS / cm) @ 30 °C EC (mS / cm) @RT Tortuosity (dimensionless) Fl (Comparative) 1.59 132 1.8 F2 1.81 91 1.6 These results were determined using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode, according to methodology described in the following reference: Johannes Landesfeind et al 2016 J. Electrochem. Soc. 163 A1373. Rate performance of the electrode films was then assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70ul of a filling electrolyte, with the results being set in out Table 3, below: % Capacity retention @45 °C Formulation code 5C 10C Fl (Comparative) 86% 69% F2 86% 69% The load required to roll the electrode precursor compositions in Table 1 into a gel electrode film was then investigated. Figure A shows the results for sweeping loads from 20 - 50 kN in increments of 10 kN. In order to determine the sweeping loads, the granular mixture was passed through rollers to which set loads of 20, 30, 40, and 50 kN were applied. The resulting gsm (grams per square metre) of the film produced for each set load was then measured. Once the data was gathered, a logarithmic decay was applied to the results to create the graph shown in Figure A. For a more processible electrode precursor composition, a lower coating weight of the thin film produced was observed at the applied load. The load required to roll the electrode precursor compositions in Table 1 into a gel electrode film having a coating weight of 220 gsm is shown in Table 4 below: Formulation code kN for 220 gsm Fl (Comparative) 46.9 F2 29.6 It can be seen, by comparing comparative example Fl to example F2, that the ionic conductivity significantly increases from 1.59 mS / cm to 1.81 mS / cm when 20% of the MWCNT are replaced with synthetic graphite. By comparing the rate performance of these two examples, it is shown that the high-rate performance of example F2 remains unchanged in comparison to comparative example F2. This is despite the fact that the electronic conductivity of example F2 is considerably lower than example Fl. By comparing comparative example Fl to example F2, it is shown that replacing 20% of the MWCNT with synthetic graphite results in a lower tortuosity of 1.6, thereby showing improvement relative to comparative example Fl which demonstrates a tortuosity of 1.8. Moreover, by comparing comparative example Fl to example F2, it is shown that when 20% of the MWCNT volume was replaced with synthetic graphite, a 36.9% reduction in load was required to roll the electrode to target thickness at a coating weight of 220 gsm. As shown in Figure A, when sweeping loads from 20 - 50 kN in increments of 10 kN, for example F2 the gsm and thickness is lower at every load. It is concluded from these examples that when using tubular carbon materials as a majority component of a conductive additive in an electrode precursor composition, replacing a portion of the tubular carbon material with synthetic graphite can provide for both improved conductivity of electrodes formed from said electrode precursor composition, and improved processability in terms of the load required to roll the electrode to target thickness. Notably, this is achieved without compromising electrochemical performance. In particular, replacing 20% of the tubular carbon material with synthetic graphite has been found to result in a 13.8% increase in ionic conductivity, and a 36.9% reduction in load required to roll the electrode to target thickness at a coating weight of 220 gsm.

Claims

1. An electrode precursor composition for an alkali metal ion secondary cell, comprising:a polymer-electrolyte gel matrix phase; anda dispersed phase comprising an electrochemically active material and a conductive additive;wherein the conductive additive comprises a mixture as a majority component, the mixture consisting of tubular carbon material and sheet carbon material.

2. The electrode precursor composition according to claim 1, wherein the mixture consists of tubular carbon material and sheet carbon material in a ratio in the range of from 9:1 to 7:3.

3. The electrode precursor composition according to claim 1 or claim 2, wherein the mixture consists of tubular carbon material and sheet carbon material in a ratio of about 8:2.

4. The electrode precursor composition according to any one of the preceding claims, wherein the mixture constitutes 90 wt% or more of the conductive additive, optionally wherein the conductive additive consists essentially of the mixture.

5. The electrode precursor composition according to any one of the preceding claims, wherein the electrode precursor composition contains substantially no conductive carbonaceous material other than the mixture, optionally wherein the electrode precursor composition contains substantially no amorphous carbon such as carbon black.

6. The electrode precursor composition according to any one of the preceding claims, wherein the tubular carbon material comprises or consists of single-walled carbon nanotubes (SWCNTs) and / or multi-walled carbon nanotubes (MWCNTs).

7. The electrode precursor composition according to claim 6, wherein the tubular carbon material consists essentially of multi-walled carbon nanotubes (MWCNTs).

8. The electrode precursor composition according to any one of the preceding claims, wherein the sheet carbon material comprises of consists essentially of graphite and / or graphene.

9. The electrode precursor composition according to claim 8, wherein the sheet carbon material comprises of consists essentially of graphite.

10. The electrode precursor composition according to any one of the preceding claims, preceding claim, wherein the electrochemically active material is a positive active material.

11. The electrode precursor composition according to claim 10, wherein the electrochemically active material is a lithium transition metal oxide material.

12. The electrode precursor composition according to any one of the preceding claims, wherein the electrochemically active material makes up at least 50 vol% of the electrode precursor composition, based on the total composition volume.

13. The 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 volume.

14. The electrode precursor composition according to any one of the preceding claims, wherein the conductive additive is present in an amount of from 0.1 vol% to 10 vol%, based on the total weight / volume of electrode precursor composition.

15. The electrode precursor composition according to any one of the preceding claims, wherein the polymer-electrolyte gel matrix phase is formed from one or more electrolyte components and at least one gelling polymer, and wherein the gelling polymer comprises one or more gelling polymers independently selected from poly(ethyleneglycoldimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly (ethylene oxide) (PEO), poly-L-lactic acid (PLA), polystyrene (PS), poly(ethyleneglycol dimethylether), poly(ethyleneglycol diethylether), poly[bis(methoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, 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, tetrathiafulvalene-substituted polystyrene, ferrocene-substituted polyethylene, carbazole-substituted polyethylene, polyoxyphenazine, poly(heteroacene), 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 sorbate)+BFj], PEO copolymer, and PEO terpolymer.

16. The electrode precursor composition according to claim 15, wherein the one or more electrolyte components include:(i) a solvent suitable for use as an electrolyte solvent in a gel electrode, for example an organic solvent; and(ii) a salt.

17. An electrode for use in an alkali metal ion secondary cell comprising: a polymer-electrolyte gel matrix phase; anda dispersed phase comprising an electrochemically active material and a conductive additive;wherein the conductive additive comprises a mixture as a majority component, the mixture consisting of tubular carbon material and sheet conductive material.

18. The electrode according to claim 17, produced by processing an electrode precursor composition according to any one of claims 1 to 16 to form a film or coating.

19. The electrode according to claim 18, wherein the processing comprises thermal processing or extrusion.

20. The electrode according to any one of claims 17 to 19, wherein the electrode has a tortuosity of 2.2 or less, as measured using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode.

21. The electrode according to any one of claims 17 to 20, wherein the electrode has an ionic conductivity of 1.2 mS / cm or more, as measured at 30 °C.

22. The electrode according to any one of claims 17 to 21, wherein the capacity retention of the electrode is 20% or more at a C rate of 10C, as assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70ul of electrolyte at 45 °C.

23. An electrochemical secondary cell comprising an electrode according to any one of claims 17 to 22.

24. An electrochemical energy storage device comprising an electrochemical secondary cell according to claim 23.

25. A method of producing an electrode comprising:mixing a polymer, an electrolyte, an electrochemically active material and a conductive additive comprising a mixture as a majority component, the mixture consisting of tubular carbon material and sheet carbon material, to form an electrode precursor composition according to any one of claims 1 to 16; andprocessing the electrode precursor composition to form an electrode film.

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