Electrode precursor composition

Abstract

An electrode precursor composition for an alkali metal ion secondary cell is described. The electrode precursor composition comprises: a polymer-electrolyte gel matrix phase, a dispersed phase comprising an electrochemically active material, and a conductive additive. The conductive additive comprises a tubular carbon material as a majority component. An electrode of the same composition, a method of producing said electrode, an electrochemical secondary cell incorporating said electrode, and electrochemical energy storage device incorporating said cell are also described.

Description

[0001] ELECTRODE PRECURSOR COMPOSITION

[0002] BACKGROUND

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] SUMMARY

[0008] 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.

[0009] 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 tubular carbon materials could offer the potential for improved performance of gel-electrode based solid- state cells. Accordingly, in a first aspect, the present invention provides an electrode precursor composition for an alkali metal ion secondary cell, comprising: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and a conductive additive; wherein the conductive additive comprises a tubular carbon material as a majority component.

[0010] The term ‘majority component’ is used herein to define that the component constitutes at least 50 wt% of the conductive additive. Preferably, the conductive additive is 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, 95 wt% or more, 99 wt% or more tubular carbon. The component may also constitute at least 50 vol% of the conductive additive, for example 60 vol% or more, 70 vol % or more, 80 vol % or more, 90 vol % or more, 95 vol % or more, 99 vol % or more tubular carbon.

[0011] In some embodiments, the conductive additive consists essentially of, or consist of a tubular carbon material. In such embodiments, the electrode precursor composition may not comprise alternative non-tubular conductive additive components. That is, the electrode precursor composition may contain substantially no conductive carbonaceous material other than the tubular carbon materials - e.g. the electrode precursor composition may not comprise graphite, graphene and / or 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 tubular carbon material, e.g. 0.1 wt% or less, 0.05 wt% or less, or 0.01 wt% or less.

[0012] 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 single-walled carbon nanotubes (SWCNTs) and multi -walled carbon nanotubes (MWCNTs)) as well as carbon fibres such as carbon nanofibers (CNFs) and vapor-grown carbon fibres (VGCFs). The present inventors have found that by providing a conductive additive which comprises tubular carbon 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 tubular carbon 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.

[0013] The electrode precursor compositions of the invention, when formed into electrodes, may therefore deliver good rate capability, i.e. the ability to discharge at higher rates than known electrodes, and retain a greater amount of initial capacity after cycling at higher rates.

[0014] The electrode precursor compositions of the invention, when formed into electrodes, also deliver greater discharge capacity than comparative compositions, for a given active material loading and discharge rate.

[0015] In addition to this improvement in rate capability, the electrode precursor compositions of the invention offer a way to deliver higher energy density without compromising rate capability. The lower tortuosity of the present compositions means that, for a given active material loading, an electrode of the invention will deliver better rate capability than a comparative electrode. It is therefore possible to increase the active material loading in electrodes, and therefore the total electrode mass, made from the present composition (e.g. by increasing the electrode thickness), which will have the effect of increasing the energy density of the electrode and therefore increasing the discharge capacity. Nevertheless, due to the reduced tortuosity, the electrode of the present invention with higher loading will still provide good rate capability, for example equivalent to or greater than the comparative electrode, despite the greater loading.

[0016] Put otherwise, it is possible to increase the thickness and therefore active material loading in the present electrodes beyond that which would be possible for comparative electrodes (thereby increasing energy density), because the detrimental impact on rate capability caused by a thicker electrode (i.e. increased loading) is lessened in the present electrodes due to the reduced tortuosity.

[0017] Therefore, the electrodes made from the present precursor composition offer two advantages: (a) an improved rate capability, when compared with a comparative electrode of similar active material loading (e.g. similar thickness), or (b) an improved energy density while maintaining comparable rate capability, by increasing the active material loading (e.g. thickness) of the present electrode relative to a comparative electrode.

[0018] 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.

[0019] 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. Preferably, the tubular carbon material comprises, consists essentially of, or consists 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.

[0020] In some embodiments, the electrochemically active 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 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. 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.

[0021] 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.

[0022] The electrochemically active material may be a particulate material, i.e. materials 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.

[0023] In some embodiments, the electrochemically active material makes 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, in some embodiments, the electrochemically active material may make up about 64 vol% of the electrode precursor composition. As noted above, the dispersed phase further comprises a conductive additive which comprises a tubular carbon material as a majority component. In some embodiments, the conductive additive is 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. In some embodiments, 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, in some embodiments, the conductive additive may be present in amounts of about 1.29 vol%, 1.89 vol%, or 2.48 vol%.

[0024] In some embodiments, the dispersed phase comprises 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.

[0025] In some embodiments, the dispersed phase comprises 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.

[0026] In some embodiments, the polymer-electrolyte gel matrix phase is 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. In some embodiments, the one or more electrolyte components may constitute an electrolyte salt solution or liquid electrolyte.

[0027] 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, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluor opropylene carbonate and y- butyrolactone.

[0028] 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.

[0029] 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 carbonate, or a blend of propylene carbonate with one or more of the above listed solvents.

[0030] 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). The alkali metal salt may be a lithium, sodium, or potassium salt.

[0031] 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. In some embodiments, when the alkali metal is lithium, the anion of the salt comprises a halogen such as fluorine. Examples include BFE, PFe", TFST, FSI", OTf, DFOB" and TDI". In some embodiments, the one or more electrolyte components comprises a lithium salt. In some embodiments, the electrolyte comprises a mixture of at least two different lithium salts. Examples of suitable lithium salts include LiPFe, LiBF4, LiTFSI, LiFSI, LiOTf, LiDFOB and LiTDI. Preferably the salt is a thermally stable salt. It has been found that LiPFe has relatively low thermal stability relative to other available lithium salts, and accordingly use of LiPFe may be avoided - that is, in some embodiments, the electrolyte component(s) do not include LiPFe.

[0032] 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.

[0033] 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 therefore comprises a gel comprising the polymer(s) and absorbed liquid electrolyte.

[0034] The gelling polymer may comprise one or more gelling polymers independently selected from carboxymethyl cellulose (CMC), 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), polyethylene oxide) (PEO), poly-L-lactic acid (PLA), polystyrene (PS), poly(ethyleneglycol dimethylether), poly(ethyleneglycol diethylether), poly [bi s(m ethoxy 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-l -propane sulfonate (LiAMPS), Poly(lithium 2-Acryl ami do-2 -Methylpropanesulfonic Acid-Co- Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbatej+BFs], PEO copolymer, PEO terpolymer, and NIPPON SHOKUB Al® polymer, or mixtures or co-polymers thereof.

[0035] In some preferred examples, the gelling polymer comprises one or more gelling polymers independently selected from carboxymethyl cellulose (CMC), 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)

[0036] In some embodiments, the polymer-electrolyte gel matrix phase makes 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, in some embodiments, the polymer-electrolyte gel matrix phase may make up about 33.52 vol%, about 34.11 vol%, or about 34.72 vol% of the electrode precursor composition.

[0037] 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.

[0038] A second aspect of the invention is an electrode for use in an alkali metal ion secondary cell comprising: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and a conductive additive; wherein the conductive additive comprises a tubular carbon material as a majority component. In some embodiments, the electrode demonstrates 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. As also described above, the electrodes may deliver increased energy density when active material loading is increased, without compromising rate capability. In some embodiments, the electrode may demonstrate a tortuosity in a range of from 1-2. The tortuosity may be, for example, 1.6 or less, 1.5 or less, 1.4 or less, or 1.3 or less.

[0039] In some embodiments, the electrode has 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.

[0040] In some embodiments, 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 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 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.

[0041] In some embodiments, the capacity retention of the electrode is 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 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. In some embodiments, the electrode is produced by processing an electrode precursor composition according to the first aspect to form a film or coating.

[0042] In some embodiments, the processing comprises thermal processing and / or extrusion. The electrode may be an extruded electrode. In other embodiments, the electrode may be a hot- rolled electrode. In other embodiments, the electrode is prepared by extruding an electrode precursor composition according to the first aspect through a die to form a film.

[0043] In some embodiments, the electrode is both extruded and hot-rolled, for example by a process which comprises first extruding the electrode through a die followed by hot-rolling the electrode down to a desired thickness.

[0044] In some embodiments, the electrode is a cathode.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] Such thicknesses may provide an electrode with a good balance of energy density and rate capability, but which is particularly effective for use in high-power applications which require high charge / discharge rate, since these thicknesses provide a modest active material loading while allowing high rate capability due to the low tortuosity.

[0051] In some embodiments, the total electrode mass loading is at least 250 gsm, for example greater than 250 gsm, greater than 300 gsm, greater than 350 gsm, greater than 400 gsm, greater than 450 gsm, greater than 500 gsm, greater than 550 gsm, greater than 600 gsm, greater than 650 gsm, greater than 700 gsm, greater than 750 gsm, greater than 800 gsm, greater than 850 gsm, greater than 900 gsm, greater than 950 gsm, greater than 1000 gsm, or greater than 1040 gsm. Such high mass loadings, which would not traditionally be possible due to detrimental effects on rate capability, have been observed to be achievable in the present electrode while still delivering acceptable rate capability. This provides an electrode of very high energy density and good rate capability.

[0052] Throughout the present disclosure, “total electrode mass loading” refers to the total electrode mass in grams, per square metre of electrode area, including all components of the electrode, such as polymer, electrolyte, active material, conductive additive and any further optional additives which may be present.

[0053] In some embodiments, the total electrode mass loading is from 250 to 1200 gsm, for example from 300 to 1200 gsm, from 350 to 1200 gsm, from 400 to 1100 gsm, from 400 to 1050 gsm, from 400 to 1000 gsm, from 400 to 900 gsm, from 400 to 800 gsm, from 400 to 750 gsm, from 400 to 700 gsm or from 300 to 500 gsm. In some embodiments, the total electrode mass loading is from 500 to 1200 gsm, for example from 550 to 1100 gsm, from 600 to 1100 gsm, from 650 to 1100 gsm, from 700 to 1100 gsm, from 750 to 1100 gsm, from 800 to 1100 gsm or from 900 to 1100 gsm.

[0054] In some embodiments, the electrode has a thickness of greater than 80 pm, for example greater than 90 pm, greater than 100 pm, greater than 110 pm, greater than 120 pm, greater than 130 pm, greater than 140 pm, greater than 150 pm, greater than 160 pm, greater than 170 pm, greater than 180 pm, greater than 190 pm, greater than 200 pm, greater than 210 pm, greater than 220 pm, greater than 230 pm, greater than 240 pm, greater than 250 pm, greater than 260 pm, greater than 270 pm, greater than 280 pm or greater than 290 pm.

[0055] The processes used to make the electrodes of the invention allow greater thicknesses, and therefore greater total electrode mass loadings, to be achieved than traditional solvent-cast preparation methods which are unable to make such thick electrodes. As a result, electrodes having such increased thicknesses are made possible, and as explained above, provide increased energy density without compromising rate capability. In some embodiments, the electrode has a thickness of from 80 pm to 300 pm, for example from 100 pm to 300 pm, from 130 pm to 300 pm, from 150 pm to 300 pm, from 200 pm to 300 pm, from 250 pm to 300 pm or from 280 pm to 300 pm.

[0056] In some embodiments, the total electrode mass loading is at least 250 gsm and the electrode has a thickness of greater than 80 pm. In some embodiments, the total electrode mass loading is at least 350 gsm and the electrode has a thickness of greater than 100 pm. In some embodiments, the total electrode mass loading is at least 350 gsm and the electrode has a thickness of greater than 110 pm. In some embodiments, the total electrode mass loading is at least 400 gsm and the electrode has a thickness of greater than 100 pm. In some embodiments, the total electrode mass loading is at least 450 gsm and the electrode has a thickness of greater than 130 pm. In some embodiments, the total electrode mass loading is at least 500 gsm and the electrode has a thickness of greater than 130 pm. In some embodiments, the total electrode mass loading is at least 600 gsm and the electrode has a thickness of greater than 130 pm. In some embodiments, the total electrode mass loading is at least 1000 gsm and the electrode has a thickness of greater than 250 pm.

[0057] 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.

[0058] 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.

[0059] 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. Preferably the cell is a lithium-ion secondary cell. In some embodiments the electrochemical secondary cell comprises 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. In some embodiments the electrochemical secondary cell comprises an electrode according to the second aspect laminated with a current collector, for example a metallic foil.

[0060] A fourth aspect of the invention provides an electrochemical energy storage device comprising an electrochemical secondary cell according to the third aspect. In some embodiments, the electrochemical energy storage device is a battery. In some embodiments, the electrochemical energy storage device is a lithium-ion battery.

[0061] A fifth aspect of the invention provides a method of preparing an electrode for an alkali metal ion secondary cell, comprising: mixing a polymer, an electrolyte, an electrochemically active material, and a conductive additive comprising tubular carbon as a majority component, to form an electrode precursor composition according to the first aspect; and processing the electrode precursor composition to form an electrode film.

[0062] In some embodiments, the electrode film has a thickness of from 500 to 700 pm.

[0063] In some embodiments, the method further comprises cutting the electrode film to form an electrode of predetermined dimensions.

[0064] 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 pm.

[0065] In some embodiments, the final electrode produced by the method after all processing steps has a total electrode mass loading greater than 250 gsm, for example greater than 250 gsm, greater than 300 gsm, greater than 350 gsm, greater than 400 gsm, greater than 450 gsm, greater than 500 gsm, greater than 550 gsm, greater than 600 gsm, greater than 650 gsm, greater than 700 gsm, greater than 750 gsm, greater than 800 gsm, greater than 850 gsm, greater than 900 gsm, greater than 950 gsm, greater than 1000 gsm, or greater than 1040 gsm. Such high loadings, which would not traditionally be possible due to detrimental effects on rate capability, have been observed to be achievable in the present electrode while still delivering acceptable rate capability. This provides an electrode of very high energy density and good rate capability.

[0066] In some embodiments, the final electrode produced by the method after all processing steps has a total electrode mass loading within the range 250 to 1200 gsm, for example from 300 to 1200 gsm, from 350 to 1200 gsm, from 400 to 1100 gsm, from 400 to 1050 gsm, from 400 to 1000 gsm, from 400 to 900 gsm, from 400 to 800 gsm, from 400 to 750 gsm, from 400 to 700 gsm or from 300 to 500 gsm. In some embodiments, the total electrode mass loading is from 500 to 1200 gsm, for example from 550 to 1100 gsm, from 600 to 1100 gsm, from 650 to 1100 gsm, from 700 to 1100 gsm, from 750 to 1100 gsm, from 800 to 1100 gsm or from 900 to 1100 gsm.

[0067] In some embodiments, the method comprises performing a second thermal processing step on the cut film to reduce the thickness of the film to a value greater than 80 pm, for example greater than 90 pm, greater than 100 pm, greater than 110 pm, greater than 120 pm, greater than 130 pm, greater than 140 pm, greater than 150 pm, greater than 160 pm, greater than 170 pm, greater than 180 pm, greater than 190 pm, greater than 200 pm, greater than 210 pm, greater than 220 pm, greater than 230 pm, greater than 240 pm, greater than 250 pm, greater than 260 pm, greater than 270 pm, greater than 280 pm or greater than 290 pm.

[0068] The processes used to make the electrodes of the invention allow greater thicknesses, and therefore greater active material loadings, to be achieved than traditional solvent-cast preparation methods which are unable to make such thick electrodes. As a result, electrodes having such increased thicknesses are made possible, and as explained above, provide increased energy density without compromising rate capability.

[0069] In some embodiments, the method comprises performing a second thermal processing step on the cut film to reduce the thickness of the film to within the range from 80 pm to 300 pm, for example from 100 pm to 300 pm, from 130 pm to 300 pm, from 150 pm to 300 pm, from 200 pm to 300 pm, from 250 pm to 300 pm or from 280 pm to 300 pm.

[0070] In some embodiments, the final electrode produced by the method after all processing steps has a total electrode mass loading of at least 250 gsm and the electrode has a thickness of greater than 80 pm. In some embodiments, the total electrode mass loading is at least 350 gsm and the electrode has a thickness of greater than 100 pm. In some embodiments, the total electrode mass loading is at least 350 gsm and the electrode has a thickness of greater than 110 pm. In some embodiments, the total electrode mass loading is at least 400 gsm and the electrode has a thickness of greater than 100 pm. In some embodiments, the total electrode mass loading is at least 450 gsm and the electrode has a thickness of greater than 130 pm. In some embodiments, the total electrode mass loading is at least 500 gsm and the electrode has a thickness of greater than 130 pm. In some embodiments, the total electrode mass loading is at least 600 gsm and the electrode has a thickness of greater than 130 pm. In some embodiments, the total electrode mass loading is at least 1000 gsm and the electrode has a thickness of greater than 250 pm.

[0071] In some embodiments, the temperature during thermal processing is 90 °C or more, e.g. from 100 to 140 °C.

[0072] A sixth aspect of the invention is the use of the composition according to the first aspect to do one or more of:

[0073] (a) increase the rate capability of an electrode or electrochemical cell;

[0074] (b) increase the discharge capacity of an electrode or electrochemical cell;

[0075] (c) reduce the tortuosity of an electrode; and

[0076] (d) increase the ionic conductivity of an electrode.

[0077] BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 shows half-cell discharge profiles of a comparative formulation and an inventive cathode formulation, as total electrode mass loading values of 200, 300, 400 and 500 gsm. The discharge current applied for all half cells was -20 mA, which corresponds to about 5C for the 200 gsm examples. Figure 2 shows half-cell charge and discharge profiles for a cathode based on inventive formulation F5, with a total electrode mass loading of 1000 gsm cycled at C / 15.

[0079] EXAMPLES & DETAILED DESCRIPTION

[0080] For all 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.

[0081] This granular mixture was then rolled into a thin film. Precursor material was fed into a hot roller assembly to create a film of target thickness.

[0082] Gel electrode precursor compositions were prepared according to the compositions shown in Table 1 :

[0083] Table 1

[0084] Each of the electrode precursor compositions in Table 1 were formed into a gel electrode film by the method set out above, to produce films with a target thickness in the range 50- 70 pm.

[0085] One further gel electrode precursor composition was prepared according to the composition shown in Table 1 A:

[0086] Table 1A

[0087] The tortuosity, ionic and electronic conductivity values (IC, EC) of the electrode films made from formulations F1-F7 were then measured, with the results being set out in Table 2, below:

[0088] Table 2

[0089] 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

[0090] Rate performance of the electrode films was then assessed under standardised conditions in a lithium metal half-cell with glass fibre separator and 70 pLof electrolyte, with the results being set in out Table 3, below:

[0091] Table 3

[0092] It can be shown, by comparing comparative example Fl to example F2 that the ionic conductivity significantly increases from 1.13 mS / cm to 1.74mS / cm when carbon blacks are replaced with MWCNTS.

[0093] By comparing the rate performance of these two examples it is shown that the high-rate performance of example F2 is significantly increased in comparison to comparative example Fl, from 12.5% at IOC to 20%. This is despite the fact that the electronic conductivity of example F2 is considerably lower than example Fl.

[0094] By comparing examples F3 to F4 it can be shown that it is possible to significantly raise the electronic conductivity of the electrode by increasing the vol% of tubular carbon used as the conductive additive, with only a moderate drop in IC. This results in a considerable improvement in rate performance.

[0095] By comparing examples F5 to F6 and F7, it can be shown that replacing the tubular carbons with carbon blacks results in a considerably lower ionic conductivity, and subsequent lower-rate performance, regardless of the changes to electronic conductivity. However, each of these examples which uses 50 vol% or more of tubular carbon shows improved electrochemical performance relative to comparative example Fl, with decreased tortuosity, and increased ionic and electronic conductivity as compared with the comparative example comprising no tubular carbon.

[0096] All examples according to the invention were seen to demonstrate tortuosity values of 2 or less, specifically of 1.6 or less, thereby showing improvement relative to comparative example Fl which demonstrated a tortuosity of 2.2.

[0097] Furthermore, all examples according to the invention were seen to demonstrate ionic conductivity values of 1.29 mS / cm @ 30 °C or more, thereby showing improvement relative to comparative example Fl which demonstrate ionic conductivity value of 1.13 mS / cm @ 30 °C.

[0098] Formulations F8 (comparative) and F5 were formed into electrode layers of varying thicknesses (thereby having varying total electrode mass loadings), using the method described above. The resultant electrodes had a total electrode mass loadings of 200 gsm, 300 gsm, 400 gsm and 500 gsm, each ± 3 gsm (final electrode thicknesses of 54-56 pm, 82-85 pm, 111-113 pm and 136-138 pm, respectively). These total electrode mass loadings refer to the total mass of finished electrode per square metre.

[0099] Half-cell discharge profiles for these electrodes were then obtained using a discharge current of -20 mA (corresponding to about 5C for the 200 gsm electrodes). The results are shown in Figure 1. Multiple lines of the same type in a single chart represent separate tests on identical cells, to confirm repeatability of the results.

[0100] It is evident that, for any given areal mass loading of active material, the F5 formulation provides higher discharge capacity at a given discharge rate. The corollary of this is that the loading of the inventive electrode could be reduced (i.e. a thinner electrode used) but still deliver a capacity equivalent to a thicker standard electrode. For example, it is evident from Figure 1 that the capacity of the 300 gsm F5 formulation is similar to that of the 400 gsm F8 (comparative) formulation. The inventive formulations therefore allow thinner electrodes to be made without jeopardizing discharge capacity or rate capability, or provide a significantly increased capacity if the same thickness of electrode or a thicker electrode is used.

[0101] Furthermore, as total electrode mass loading increases, the difference in discharge capacity between the F5 electrode and the F8 electrode also increases, i.e. the discharge capacity of the comparative electrode falters at higher loadings, but this is not the case for the inventive electrode. This shows that the inventive compositions provide significant improvements in discharge capacity at a given discharge rate as total electrode mass loading increases. Without wishing to be bound by theory, it is believed that the low tortuosity facilitated by the tubular carbon additive means that the electrode can be made thicker without significant limitation on rate capability or discharge capacity. By contrast, for the comparative formulation F8, discharge capacity does not increase significantly as loading increases, which may be due to the increased tortuosity making the additional active material “inaccessible”.

[0102] To test whether it was possible for the formulations of the invention to provide functional electrodes of very high total electrode mass loadings (i.e. very high thickness), a further electrode was prepared from formulation F5 with an total electrode mass loading of 1048 gsm ± 3 gsm (final thickness of 296 pm). Half-cell charge and discharge curves for the electrode are shown in Figure 2, based on cycling at C / 15. The results show the ability of the formulation to be formed into a thick electrode, and the ability of the electrode to deliver good discharge capacity despite the increased thickness. More generally, it is surprising that it is possible to form electrodes having such high thickness which still demonstrate good performance.

[0103] It is concluded from these examples that the use of tubular carbon materials as a majority component of a conductive additive in an electrode precursor composition can provide for significantly improved electrochemical performance of electrodes formed from said electrode precursor composition. In particular, significant improvements in high-rate performance are seen.

Claims

CLAIMS1. An electrode precursor composition for an alkali metal ion secondary cell, comprising: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and a conductive additive; wherein the conductive additive comprises a tubular carbon material as a majority component.

2. The electrode precursor composition according to claim 1 wherein the tubular carbon material constitutes 90 wt% or more of the conductive additive, optionally wherein the conductive additive consists essentially of the tubular carbon material.

3. The electrode precursor composition according to claim 1 or claim 2 wherein the electrode precursor composition contains substantially no conductive carbonaceous material other than the tubular carbon material, optionally wherein the electrode precursor composition contains substantially no graphite, graphene and / or amorphous carbon such as carbon black.

4. 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).

5. The electrode precursor composition according to claim 4 wherein the tubular carbon material consists essentially of multi-walled carbon nanotubes (MWCNTs).

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

7. The electrode precursor composition according to claim 6, wherein the electrochemically active material is a lithium transition metal oxide material.

8. 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.

9. 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.

10. 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.

11. 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 carboxymethyl cellulose (CMC), 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 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- ethylenedi oxy thiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, 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 (LiAMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co- Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbatej+BFs], PEO copolymer, PEO terpolymer, and NIPPON SHOKUBAI® polymer.

12. The electrode precursor composition according to claim 11 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.

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

14. The electrode according to claim 13, produced by processing an electrode precursor composition according to any one of claims 1 to 12 to form a film or coating.

15. The electrode according to claim 14, wherein the processing comprises thermal processing or extrusion.

16. The electrode according to any one of claims 13 to 15, wherein the electrode has a tortuosity of 2.2 or less, as measured using electrochemical impedance spectroscopy of a symmetric cell incorporating said electrode.

17. The electrode according to any one of claims 13 to 16, wherein the electrode has an ionic conductivity of 1.2 mS / cm or more, as measured at 30 °C.

18. The electrode according to any one of claims 13 to 17, wherein the total electrode mass loading, defined as the total mass of the electrode per unit area, is at least 250 gsm.

19. The electrode according to any one of claims 13 to 18, wherein the electrode has a thickness of greater than 80 pm.

20. The electrode according to any one of claims 13 to 19, wherein the total electrode mass loading is at least 400 gsm and the electrode has a thickness of greater than 100 pm.

21. The electrode according to any one of claims 13 to 20, 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 70 pLof electrolyte at 45 °C.

22. An electrochemical secondary cell comprising an electrode according to any one of claims 13 to 21.

23. An electrochemical energy storage device comprising an electrochemical secondary cell according to claim 22.

24. A method of producing an electrode comprising: mixing a polymer, an electrolyte, an electrochemically active material and a conductive additive comprising tubular carbon as a majority component to form an electrode precursor composition according to any one of claims 1 to 12; and processing the electrode precursor composition to form an electrode film25. The method according to claim 24, wherein the total electrode mass loading in the final electrode film after processing, defined as the total mass of the electrode per unit area, is at least 400 gsm and the electrode has a thickness of greater than 100 pm.

26. The use of the composition according to any one of claims 1-12 to do one or more of:(a) increase the rate capability of an electrode or electrochemical cell; (b) increase the discharge capacity of an electrode or electrochemical cell;(c) reduce the tortuosity of an electrode; and(d) increase the ionic conductivity of an electrode.

Images