Energy storage devices and components thereof

Abstract

A method of preparing an electrode precursor composition for an alkali metal ion secondary cell is described. The electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a dispersed phase comprising electrochemically active material and carbon nanotubes. The method comprises steps of mixing an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to produce a premix, drying the premix to remove at least some of, or substantially all of, the dispersion medium to produce a dried premix, and mixing the dried premix with an electrolyte salt solution to produce the electrode precursor composition. An electrode precursor composition, an electrode, an electrochemical secondary cell, and an electrochemical energy storage device are also described.

Description

[0001] ENERGY STORAGE DEVICES AND COMPONENTS THEREOF

[0002] BACKGROUND

[0003] Lithium-ion secondary batteries are the leading battery technology currently powering devices ranging from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. Lithium-ion batteries contain a plurality of lithium-ion secondary cells, one example of an alkali metal ion secondary cell.

[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. One known 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, a gelling polymer binder, and a liquid electrolyte to form the gelled electrode material comprising the electrochemically active material. Such gel electrodes are described in WO 2017 / 017023 AL

[0005] Gel electrodes may be made from a precursor composition by thermal processing, for example hot rolling or extrusion. The choice of the components used during the preparation of the precursor composition may be limited to components which are chemically and physically compatible with one another. This places practical limitations on the range of available components, potentially increasing the overall cost of the manufacturing process and reducing the versatility of the electrode product. Furthermore the choice of ingredients may be limited to those which do not necessarily have the best affinity to the electrochemically active materials present, which reduces the strength of binding within the gel electrode and may destabilise the electrode structure and / or cause poor electrode performance.

[0006] SUMMARY

[0007] There is a need for more versatile methods of preparing gel electrode precursor compositions and gel electrodes, and in particular a need for such methods that allow for satisfactory performance of the electrodes produced from said gel electrode precursor compositions.

[0008] The present inventors have realised that the addition of carbon nanotubes to a gel electrode could offer the potential for improved performance of gel-electrode based solid-state cells.

[0009] However, carbon nanotubes are typically supplied in a carbon nanotube dispersion and not handled as dry powders due to health concerns, and it has been found that various carbon nanotube dispersions have physical and / or chemical incompatibilities with some of the components typically used in the preparation of gel electrodes, thereby making them difficult or impossible to incorporate using conventional mixing methods.

[0010] Accordingly, the invention relates generally to a method of preparing an electrode precursor composition for an alkali metal ion secondary cell, wherein the method comprises an initial step of preparing a dried premix from a mixture of an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium.

[0011] In a first aspect, the present invention provides a method of preparing an electrode precursor composition for an alkali metal ion secondary cell, the electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a dispersed phase comprising electrochemically active material and carbon nanotubes, the method comprising steps of: mixing an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to produce a premix; drying the premix to remove at least some of, or substantially all of, the dispersion medium to produce a dried premix; and mixing the dried premix with an electrolyte salt solution to produce the electrode precursor composition.

[0012] To form the gel matrix phase, a gelling polymer binder may be introduced during the method, so that the gelling polymer binder is present in the resulting electrode precursor composition. In some methods, a gelling polymer binder may be introduced in the step of mixing an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to produce a premix. Alternative or additionally, a gelling polymer binder may be introduced in the step of mixing the dried premix with an electrolyte salt solution to produce the electrode precursor composition.

[0013] By providing a method in which the carbon nanotubes are first incorporated as part of a premix, a convenient method is provided which enables a wider range of carbon nanotube dispersions to be used in the manufacture of gel electrode precursor compositions, and / or allows ingredients to be incorporated into gel electrode precursor compositions which were previously difficult or impossible to incorporate due to the other incompatible ingredients present - as one example, aqueous carbon nanotube dispersions were not previously possible to incorporate into a gel electrode via conventional methods, due to material incompatibilities, in particular between organic components of the gel electrode precursor, such as the electrolyte salt solution and / or the gelling polymer binder with the water in the aqueous dispersion medium. However, in the present method, by introducing the carbon nanotubes in a premix formation stage, it becomes more straightforward to manufacture an electrode precursor composition containing otherwise incompatible ingredients. It has been found that the premix of electrochemically active material and carbon nanotubes can be relatively easily combined with other components of the electrode precursor composition - e.g. with an electrolyte salt solution and / or a gelling polymer binder, to prepare the electrode precursor composition without issues of physical incompatibility.

[0014] Furthermore, it has been found that by incorporating the carbon nanotubes in a premix stage as described above, it may be easier to achieve suitably good dispersion of the carbon nanotubes in the resultant electrode precursor composition as compared with methods where carbon nanotubes are introduced at a different stage of the manufacturing process. This is because good dispersion can be achieved in the pre-mix stage due to the relatively higher solids content of the premix, and this good dispersion ay then be maintained when the remix is mixed with further components to for the electrode precursor composition.

[0015] The electrode precursor composition made by the above method may find use as a precursor material for the preparation of a gel electrode, as discussed below.

[0016] Further preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

[0017] The carbon nanotubes in the carbon nanotube dispersion may include single-walled carbon nanotubes (SWCNTs). The carbon nanotubes may include multi -walled carbon nanotubes (MWNTs). In some arrangements, the carbon nanotubes may include substantially only SWCNTs. In other arrangements, the carbon nanotubes may include substantially only MWCNTs. In yet other arrangements, the carbon nanotube may include a mixture of SWCNTs and MWCNTs. The use of SWCNTs may be particularly preferred in view of the improved electrochemical performance which may be provided by use of SWCNTs in comparison to use of MWCNTs.

[0018] The carbon nanotubes may be present in the carbon nanotube dispersion in an amount of from 0.05 to 2 wt%, or 0.05 to 1 wt%, based on the total weight of components in the carbon nanotube dispersion. In some embodiments, the carbon nanotubes may be present in the carbon nanotube dispersion in an amount of 0.05 wt% or more, 0.1 wt% or more, 0.5 wt% or more or 1 wt% or more. In some embodiments, the carbon nanotubes may be present in the carbon nanotube dispersion in an amount of 2 wt% or less, 1.5 wt% or less, or 1 wt% or less. The carbon nanotubes may have an aspect ratio in a range of greater than 1000, preferably greater than 5000.

[0019] The dispersion medium for the carbon nanotube dispersion may be any suitable dispersion medium. For example, it may be an aqueous dispersion medium, or a non-aqueous dispersion medium. Aqueous dispersion mediums may be preferred over non-aqueous dispersion mediums, as they are generally lower in cost, and may have a lower environmental impact than many nonaqueous dispersion mediums, or limit the need for solvent recovery during the step of drying the premix. However, it is also contemplated that in some methods, a non-aqueous dispersion medium (e.g. N-methyl-2-pyrrolidone, NMP) may be used. Accordingly, in some embodiments, the dispersion medium comprises water. In such embodiments, water may be the primary component of the dispersion medium. It may be preferable to use a dispersion medium having water as the primary component in view of environmental impact and safety. However, any compatible solvent that evaporates at a relatively low temperature (200 °C or less, preferably 150 °C or less, preferably 120 °C or less) and which does not substantially impact the thermal stability of other components of the electrode precursor mixture may be used. In other embodiments, the dispersion medium comprises NMP, IPA or mixtures thereof.

[0020] In some embodiments, the carbon nanotube dispersion may further comprise a dispersion agent. The dispersion agent may be a polymer. The polymer may be e.g. CMC, although other suitable polymers are contemplated.

[0021] The viscosity of the carbon nanotube dispersion may be selected to be in a range of 200 to 10,000 cP, as measured at a shear rate of 20 reciprocal seconds. Selection of an appropriate viscosity can help to ensure suitable mixing of the carbon nanotube dispersion as part of the premix. The viscosity may depend on the concentration of carbon nanotubes in the dispersion, as well as the composition of the dispersion medium.

[0022] The identity of the electrochemically active material in the method of the invention is not of particular importance. The benefits of the invention based on preparation of the premix may be achieved for any active material which could be present in an electrode or electrode precursor composition. The skilled person will be aware of a large number of possible cathode active materials (also called positive active materials) and anode active materials (also called negative active materials) which may be used in the present invention.

[0023] The electrochemically 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. In some embodiments, the electrochemically material is a positive active material and the electrode precursor composition is a cathode precursor composition. In these embodiments, the positive active material may be a lithium transition metal oxide material. In some 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.

[0024] In some embodiments, the electrochemically active material is a negative active material and the electrode precursor composition is an anode precursor composition. In these embodiments, the electrochemically active material may comprise carbon (suitably graphite), graphene, silicon, silicon oxide, or mixtures thereof, including mixtures of composites thereof. Preferably, the negative active material is selected from one or more of graphite, silicon and silicon oxide. As above, such active materials are commercially available or may be manufactured by methods known to the skilled person

[0025] In some embodiments the electrochemically active material is a material which undergoes significant volume change during a charge / discharge cycle when used in an alkali metal ion secondary cell, e.g. as a result of incorporation of the alkali metal ions into the active material layer. For example, the active material layer may undergo volume changes of 100% or more, 200% or more, or 300% or more when the active material alkali metal ions are incorporated into the active material layer to saturation, e.g. when fully lithiated, for a lithium-ion cell.

[0026] The volumetric median particle size (Dso) of the electrochemically active material may be from 0.5 to 50 pm, for example from 1 to 40 pm, from 2 to 30 pm, from 5 to 25 pm or from 5 to 20 pm. Dso is the volumetric median particle size. In other words, it represents the particle size in microns which splits the volume distribution of a population of particles in half, with 50 vol% of the particles having a particle size below that value and 50 vol% having a particle size above that value.

[0027] The skilled person will appreciate that the volume median particle size Dso can be measured using a Malvern Mastersizer 3000 using the light scattering method set out in ASTM B 822-20, applying the Mie scattering theory.

[0028] As discussed above, the method comprises mixing an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to produce a premix. Optionally, this step may also include mixing a gelling polymer binder as part of the premix.

[0029] In some embodiments, the premix contains from 50 wt% to 90 wt% (e.g. 55 wt% or more, 60 wt% or more, 65 wt% or more or 70 wt% or more, 85 wt% or less, 80 wt% or less, 75 wt% or less) electrochemically active material, based on the total weight of premix. In some embodiments, the premix may contain about 72 wt% (e.g. about 71.6 wt%) electrochemically active material, based on the total weight of premix. This wt% may be defined based on the premix composition before the step of drying the premix to remove at least some of, or substantially all of, the dispersion medium.

[0030] The electrochemically active material and carbon nanotube dispersion may be present in the premix at a ratio of 1 : 1 to 5: l, for example from 2: 1 to 4:l, e.g. about 3: l the ratio being calculated based on the relative weights of these components in the premix (prior to drying).

[0031] In this way, the electrochemically active material and carbon nanotubes may be present in the premix at a ratio of 500: 1 to 1000: 1, for example from 600: 1 to 800: 1, e.g. about 750: 1 the ratio being calculated based on the relative weight of these components in the premix (prior to drying).

[0032] Where a gelling polymer binder is also introduced in the premix, this may be introduced in amounts of from 2 wt% to 10 wt%, for example 3 wt% or more, 4 wt % or more, 9 wt% or less, 8 wt% or less, 7 wt% or less, 6 wt % or less or 5 wt% or less, e.g. about 4.3 wt%, based on the total weight of premix (prior to drying).

[0033] The premix may be prepared by any suitable mixing method. In some methods, the mixing may be performed via acoustic mixing. In some methods, the mixing may be performed by planetary mixing. Acoustic mixing may be particularly suitable for methods in which the gelling polymer binder is introduced in the premix stage, especially where such binder materials do not have high compatibility with the dispersion medium for the carbon nanotube dispersion, and the premix is therefore performed on a low-liquid content mixture as a ‘ semi-dry’ mixing step. Planetary mixing may be particularly appropriate where the premix has a higher liquid content or is in the form of a slurry (e.g. where the carbon nanotube dispersion is an aqueous dispersion, and an aqueous gelling polymer binder such as PEO is also added).

[0034] In some embodiments, a further conductive additive (i.e. a conductive additive in addition to the carbon nanotubes) is added during the method, so that the further conductive additive is present in the resulting electrode precursor composition. The conductive additive may be incorporated into the electrode precursor composition at any suitable time and using any suitable method. For example, the conductive additive may suitably be included when producing the premix, or when mixing the dried premix with the electrolyte salt solution.

[0035] In some embodiments the conductive additive comprises one or more of carbon black and graphite. In some embodiments, the conductive additive comprises or consists of carbon black. Examples of commercially available carbon black include Super P ®, Ketjen Black, Super C and Denka Black

[0036] The conductive additive may be added to the premix to provide 0.1 wt% to 5 wt% conductive additive, for example 0.2 wt% or more, 0.5 wt% or more, 0.7 wt% or more, 4 wt% or less, 3 wt% or less, 2 wt% or less, or 1 wt% or less, e.g. about 0.8 wt% based on the total weight of the premix.

[0037] The method comprises drying the premix by removing at least some of, or substantially all of, the dispersion medium to produce a dried premix. In some embodiments the slurry is dried to remove at least 90 wt% of the dispersion medium, based on the total amount of dispersion medium in the original slurry, for example at least 95 wt%, or at least 99 wt%.

[0038] In some embodiments, the step of drying the slurry is performed to reduce the amount of dispersion medium to an amount of 150 ppm or less, e.g. 100 ppm or less, or 50 ppm or less. Where the dispersion medium is water, it may be particularly important for the remaining amount of water in the slurry to be reduced to the above values, as excess moisture remaining in the dried premix may reduce the compatibility of the dried premix with other components of the electrode precursor composition.

[0039] In some embodiments, the step of drying the premix is performed at a temperature of 150 °C or less, e.g. 120 °C or less.

[0040] The drying step may be performed in any suitable manner, however in one preferred arrangement, the drying step is performed under vacuum, for example in a vacuum oven. The drying step may be performed for a time period in a range of from 1 hour to 48 hours, e.g. about 12 hours. Preferably drying is performed for a time of at least 1 hour, at least 2 hours, at least 3 hours, at least 4 hours or at least 5 hours.

[0041] In one suitable arrangement, drying is achieved by placing the slurry in a vacuum oven at a temperature of under 50 °C overnight (i.e. for around 12 hours).

[0042] Where the drying step is performed at an elevated temperature (i.e. above room temperature), the dried premix may be allowed to cool after the drying step. This may be natural cooling e.g. performed by opening a door of the oven in which the drying step is performed and allowing the temperature of the premix to equilibrate to room temperature (e.g. about 25 °C). Alternatively, an active cooling step may be performed, e.g. to reduce the temperature of the dried premix from the drying temperature to room temperature in a shorter time period than would be achieved by natural cooling.

[0043] The ratio of carbon nanotubes to electrochemically active material in the dried premix will typically remain substantially unchanged as compared with the ratio prior to drying. That is, the electrochemically active material and carbon nanotubes may be present in the premix at a ratio of 500: 1 to 1000: 1, for example from 600: 1 to 800: 1, e.g. about 750: 1, the ratio being calculated based on the relative weight of these components.

[0044] The method may comprise a step of processing the dried premix by mechanically agitating the dried premix. This may assist in breaking up any agglomerates within the dried premix. The mechanical agitation may be performed e.g. using an acoustic mixer.

[0045] In some methods, further active material may be added to the premix after the drying step. That is, the method may include a step of mixing the dried premix with further active material. The further active material may comprise or consist of the same active material as present in the premix or may comprise or consist of a different active material to the active material present in the premix. The step of mixing the dried premix with further active material may take place before, or at substantially the same time as mixing the dried premix with the electrolyte salt solution to produce the electrode precursor composition.

[0046] The dried premix is then combined with an electrolyte salt solution and, optionally, a gelling polymer binder (where this is not added an another stage of the method), to produce an electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a dispersed phase comprising electrochemically active material and carbon nanotubes. In some embodiments, the electrolyte salt solution comprises or consists of a solvent comprising one or more cyclic or linear carbonate compounds. In some embodiments the solvent comprises one or more cyclic carbonate compounds. In some embodiments the solvent comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluor opropylene carbonate and y-butyrolactone.

[0047] In some embodiments, the electrolyte salt solution further comprises an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). Typically, it refers to lithium, sodium and potassium.

[0048] 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 BF4-, PF6-, TFSI-, FSI- and TDI-.

[0049] In some embodiments, the electrolyte salt solution comprises a lithium salt. Examples of suitable lithium salts include LiPFe, LiBF4, LiTFSI, LiFSI and LiTDI.

[0050] 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.

[0051] The gelling polymer binder may be selected from one or more of: poly vinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polyacrylic acid (PAA), poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(ethyleneglycol dimethylether), poly(ethyleneglycol di ethyl ether), poly [bi s(m ethoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, poly di sulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, 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- Acrylamido-2-Methylpropanesulfonic Acid-Co- Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, and PEO terpolymer.

[0052] The electrolyte salt solution may be combined with the dried premix in relative amounts of 0.5:1 to 10: 1, based on the total weight of these components. In some embodiments, the ratio of electrolyte salt solution to dried premix may be from 3: 1 to 7: 1, 4: 1 to 6:1, or about 5:1, e.g. about 4.7: 1.

[0053] In some embodiments, heating is performed during the step of mixing the dried premix with the electrolyte salt solution. Application of heat may encourage gelation of any gelling polymer binder that is present, which as noted above, may be introduced either as part of the premix, or additionally or alternatively during the step of mixing the dried premix with an electrolyte salt solution to produce the electrode precursor composition.

[0054] In some embodiments, the method does not include a dedicated solvent extraction step, unlike traditional solvent casting methods, because solvents that are present in the dried premix can be retained in the gel structure of the electrode precursor material.

[0055] In some embodiments, the step of mixing the dried premix with the electrolyte salt solution is performed in a twin screw extruder. It has been found that use of a twin screw extruder can provide for more homogenous mixing that other possible mixing methods. Whilst the use of twin screw extruders has been found to be particularly convenient, it is also contemplated that alternative mixing methods could be used.

[0056] In a second aspect, the present invention provides an electrode precursor composition for an alkali metal ion secondary cell, obtained or obtainable by a method according to the first aspect, comprising: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and carbon nanotubes.

[0057] The electrode precursor composition of the invention contains a polymer-electrolyte gel matrix phase and a dispersed phase of solid particulate material incorporating carbon nanotubes dispersed through the matrix phase. In this way, the electrode precursor composition has a gel-like composition and can be processed into a thin-film electrode with a similar gel-like composition, where the electrode structure contains liquid electrolyte (the electrolyte salt solution) trapped within the matrix phase due to the gelled nature of the polymer. Traditional solid-state electrodes formed by solvent casting a slurry onto a substrate followed by drying require the separate addition of a free liquid electrolyte to the cell when assembled, creating a cell which is a fire risk due to the flammability of the free liquid electrolyte. The replacement of such solid-state electrodes with a gel electrode prepared from the precursor composition of the invention reduces this risk and provides a cell of increased safety.

[0058] Furthermore, the presence of carbon nanotubes as part of the dispersed phase of solid particulate may increase the capacity relation of an electrode formed from the electrode precursor composition. Without wishing to be bound by theory, the present inventors suggest that the presence of carbon nanotubes may provide for improved electrical connection across the particulate phase of the electrode precursor composition, as the carbon nanotubes, in particular due to their large aspect ratio and excellent conductivity, may act to bridge gaps between particles of the electrochemically active material.

[0059] Optional features discussed above relating to the nature of the chemical components (e.g. electrochemically active material, carbon nanotubes, electrolyte salt solution, gelling polymer binder etc.) used in the method of the first aspect also apply to this aspect and so will not be repeated. However, the ratios of components in the resulting electrode precursor composition should be discussed.

[0060] In some embodiments, the electrochemically active material makes up at least 50 vol% of the electrode precursor composition, for example at least 55 vol%, at least 60 vol%, at least 62 vol%, at least 64 vol%, or at least 65 vol%. In some embodiments, the electrochemically active material makes up no more than 95 vol% of the electrode precursor composition, for example no more than 90 vol%, no more than 85 vol%, no more than 80 vol% or no more than 75 vol%. In some embodiments, the electrochemically active material makes up about 69 vol% of the electrode precursor composition.

[0061] In some embodiments, the carbon nanotubes make up at least 0.01 wt% of the electrode precursor composition, for example at least 0.05 wt%. In some embodiments, the carbon nanotubes make up about 0.1 wt% of the electrode precursor composition.

[0062] In some embodiments, the dispersed phase further comprises a further conductive additive (i.e. a conductive additive in addition to the carbon nanotubes present in the dispersed phase). This may be a particulate conductive additive. Where a conductive additive is present, this may comprise or consist of one or more of carbon black and graphite. In some embodiments, the conductive additive comprises or consists of carbon black. Examples of commercially available carbon black include Super P ®, Ketjen Black, Super C and Denka Black.

[0063] In some embodiments, the conductive additive is present in an amount of from 0.5 wt% to 2 wt%, based on the total weight of the electrode precursor composition, for example at least 0.6 wt%, at least 0.7 wt% or at least 0.8 wt%. In some embodiments, the conductive additive may constitute about 0.9 wt% or about 1.0 vol% of the electrode precursor composition.

[0064] In some embodiments, the dispersed phase of the electrode precursor composition comprises from 95 wt% to 99.99 wt% electrochemically active material, based on the total volume of the dispersed phase of the electrode precursor composition, for example at least 96 wt%, at least 97 wt% or at least 98 wt%. In some embodiments, the electrochemically active material may constitute about 98.7 wt% or about 98.4 vol% of the dispersed phase of the electrode precursor composition.

[0065] In some embodiments, the dispersed phase of the electrode precursor composition comprises from 0.5 wt% to 3 wt% conductive additive, e.g. 0.7 wt% to 2.2. wt%, based on the total volume of the dispersed phase of the electrode precursor composition. In some embodiments, the conductive additive may constitute about 1.2 wt% or about 1.4 vol% of the dispersed phase of the electrode precursor composition.

[0066] In some embodiments, the dispersed phase of the electrode precursor composition comprises from 0.05 vol% to 1 vol% carbon nanotubes, based on the total volume of the dispersed phase of the electrode precursor composition, for example at least 0.1 vol. In some embodiments, the carbon nanotubes may constitute about 0.15 vol% of the dispersed phase of the electrode precursor composition.

[0067] The dispersed phase of the electrode precursor composition may consist of the electrochemically active material, carbon nanotubes, and a further conductive additive.

[0068] In some embodiments, the polymer-electrolyte gel matrix phase comprises a mixture of a gelling polymer binder and an electrolyte salt solution (the electrolyte salt solution may also be referred to as a liquid electrolyte), wherein the weight ratio of electrolyte:polymer is from 1 : 1 to 6: 1, for example 2: 1 to 5: 1, or 3: 1 to 4: 1 e.g. about 3.8: 1. In some embodiments, the polymer electrolyte gel matrix phase consists of the gelling polymer binder and the electrolyte salt solution.

[0069] In some embodiments the electrode precursor composition comprises from 5 to 50 wt% polymer- electrolyte gel matrix phase, based on the total electrode precursor composition weight or volume respectively. For example, in some embodiments, the polymer-electrolyte gel matrix phase makes up at least 10 wt% of the electrode precursor composition, for example at least 15 wt%, or at least 20 wt%. In some embodiments, the polymer-electrolyte gel matrix phase makes up no more than 50 wt% of the electrode precursor composition, for example no more than 40 wt%, no more than 30 wt%, or no more than 25 wt%. In some embodiments, the polymer-electrolyte gel matrix phase makes up about 22.1 wt% or 29.8 vol% of the electrode precursor composition. In some embodiments the polymer-electrolyte gel matrix phase comprises 5 to 50 wt% gelling polymer binder, based on the total weight or volume respectively of the polymer-electrolyte gel matrix phase. For example, in some embodiments, the gelling polymer binder makes up at least 10 wt% of the polymer-electrolyte gel matrix phase, for example at least 15 wt%, or at least 20 wt%. In some embodiments, the gelling polymer binder makes up no more than 50 wt% of the polymer-electrolyte gel matrix phase, for example no more than 40 wt%, no more than 30 wt%, or no more than 25 wt%. In some embodiments, the gelling polymer binder makes up about 20.7 wt% or 16.8 vol% of the polymer-electrolyte gel matrix phase.

[0070] In some embodiments the polymer-electrolyte gel matrix phase comprises 50 to 95 wt% electrolyte salt solution, based on the total weight or volume respectively of polymer-electrolyte gel matrix phase. For example, in some embodiments, the electrolyte salt solution makes up at least 60 wt% of the polymer-electrolyte gel matrix phase, for example at least 70 wt%, or at least 75 wt%. In some embodiments, the electrolyte salt solution makes up no more than 90 wt% of the polymer- electrolyte gel matrix phase, for example no more than 85 wt%, or no more than 80 wt%. In some embodiments, the electrolyte salt solution makes up about 79.3 wt% or 83.2 vol% of the polymer- electrolyte gel matrix phase.

[0071] In some embodiments, the electrode precursor composition is for a lithium-ion secondary electrochemical cell. In some embodiments, the electrode precursor composition is an anode precursor composition. In other embodiments, the electrode precursor composition is a cathode composition.

[0072] In a third aspect, the present invention provides an electrode for use in an alkali metal ion secondary cell comprising an electrochemically active material and carbon nanotubes dispersed in a polymer-electrolyte gel matrix.

[0073] The electrode may be obtained or obtainable by processing the electrode precursor composition according to the second aspect into an electrode. In some embodiments, the electrode is produced by processing an electrode precursor composition according to the second aspect to form a film.

[0074] In some embodiments, the electrode is an extruded electrode. In other embodiments, the electrode is a hot-rolled electrode. In other embodiments, the electrode is prepared by extruding an electrode precursor composition through a die to form a film.

[0075] In some embodiments the electrode is formed by one or more hot-rolling steps carried out on an electrode precursor composition, followed by one or more extrusion steps. All of the compositional options and preferences set out above for the electrode precursor composition of the second aspect apply equally to the electrode of the third aspect, including the identities and the relative amounts of the various components of the composition, which do not substantially change during the processing of the precursor composition into the electrode.

[0076] In some embodiments, the processing comprises thermal processing or extrusion.

[0077] Accordingly, in a fourth aspect, the present invention provides a method of preparing an electrode for an alkali metal ion secondary cell, comprising: preparing an electrode precursor composition by the method according to the first aspect; and thermally processing the electrode precursor composition to form an electrode film.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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 20 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.

[0082] In some embodiments the electrode has a thickness of from 20 to 150 pm, for example from 40 to 100 pm, from 40 to 90 pm, from 40 to 80 pm, from 40 to 70 pm or from 50 to 70 pm, and comprises the electrochemically active material in an amount of from 50 to 75 vol% of the cathode, for example from 55 to 70 vol%, from 60 to 69 vol%, from 62 to 68 vol% or from 64 to 69 vol%.

[0083] 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.

[0084] In some cases, the extruded electrode may form part of an extruded monolith which includes one or more further layers which are present in an electrochemical battery. For instance, the monolith may include a separator layer, and / or may include another electrode (i.e. the extruded monolith may include both a cathode and anode). The different layers may be coextruded and have different compositions from one another.

[0085] The carbon nanotubes may be well distributed within the electrode, i.e. substantially homogeneously distributed. The distribution of carbon nanotubes can be determined by investigation of the electrode e.g. using SEM, EDX, or RAMAN mapping.

[0086] The electrode may have a capacity retention of 90% or more after 5 or more charge-discharge cycles at a C rate of C / 10. The capacity retention is preferably 95% or more, for example 96% or more, 97% or more, 98% or more or 99% or more. In some embodiments, the capacity retention of the electrode may be substantially equal to 100%.

[0087] A fifth aspect of the invention provides an electrochemical secondary cell comprising an electrode according to the third 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 cathode, an anode, and an electrolyte between the cathode and the anode, wherein one or both of the anode or cathode are an electrode according to the third aspect. In some embodiments the electrochemical secondary cell comprises an electrode according to the third aspect laminated with a current collector, for example a metallic foil.

[0088] A sixth aspect of the invention provides an electrochemical energy storage device comprising an electrochemical secondary cell according to the fourth 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.

[0089] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0090] BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 shows the charge and discharge capacities of two different anodes over five charging cycles. One anode is a first anode according to the invention including SWCNT and formed using a premix method, and the other anode is a comparative anode without SWCNT and formed without using a premix method.

[0092] Figure 2 shows the charge and discharge capacities of two different anodes over five charging cycles. One anode is a second anode according to the invention including SWCNT and formed using a premix method and the other anode is a comparative anode without SWCNT and formed without using a premix method.

[0093] DETAILED DESCRIPTION - EXAMPLES

[0094] Example 1 - first anode

[0095] Initially 22.8g of an aqueous carbon nanotube dispersion (0.4 wt % carbon nanotubes dispersed in water) was mixed with 1.61g of PEO binder and 0.82g of C65 as a conductive additive using a planetary mixing method. 7.00g of SiOx and 63.0g of graphite as negative active materials were then mixed using a planetary mixing method to form the premix slurry.

[0096] The resultant slurry contained 70.0g of negative active material (10 wt% SiOx, 90 wt% graphite), 0.091g of single wall carbon nanotubes and 22.5g water.

[0097] The slurry was dried in a vacuum oven overnight at 50 °C. 2.56g of PVDF was then added to the premix with 15.8g of electrolyte. This was then mixed in a twin screw extruder whilst being heated up to a temperature of 120 ° C.

[0098] The resultant gel anode precursor composition was then hot rolled to form a gel anode having a thickness of 47 microns laminated onto 16 micron copper foil.

[0099] Comparative Example 1

[0100] A comparative anode precursor composition was prepared by adding all powders to a mixing vessel. 7.00g of SiOx and 63g of Graphite as negative active material, 0.911g of C65 as a conductive additive, 1.61g of PEO, and 2.56g of PVDF were dispersed by stirring. 16.0g of liquid electrolyte was added to this premix, which was then mixed in a twin screw extruder whilst being heated up to a temperature of 120 ° C.

[0101] The resultant gel anode precursor composition was then hot rolled to form a gel anode having a thickness of 47 microns laminated onto 16 micron copper foil.

[0102] Example 2 - second anode

[0103] Initially most dry materials were added to a mixing vessel. 56.0g of Graphite as negative active material, 0.821g of C65 as a conductive additive, and 4.17g of PVDF granules. 22.8g of aqueous carbon nanotube dispersion (0.4 wt % carbon nanotubes dispersed in water) was added and acoustically mixed to form the wet premix. This was then dried at 120°C under vacuum for 60-70 hours.

[0104] The dried premix was acoustically mixed to break up agglomerates before 14g of SiOx as further negative active material was added (such that the active material constituted 20 wt% SiOx, 80 wt% graphite) and acoustically mixed. Finally 14.6g of liquid electrolyte was added to the premix and acoustically mixed.

[0105] This was then mixed in a twin screw extruder whilst being heated up to a temperature of 120 ° C

[0106] The resultant gel anode precursor composition was then hot rolled to form a gel anode having a thickness of 41 microns laminated onto 16 micron copper foil.

[0107] Comparative Example 2

[0108] Initially all dry materials were added to a mixing vessel. 14.0g of SiOx and 56.0g of Graphite as negative active material, 0.821g of C65 as a conductive additive, 0.091g of multi-wall carbon nanotubes, and 4.17g of PVDF granules were mixed acoustically.

[0109] 14.6g of liquid electrolyte was added to the powders and acoustically mixed .

[0110] This was then mixed in a twin screw extruder whilst being heated up to a temperature of 120 ° C

[0111] The resultant gel anode precursor composition was then hot rolled to form a gel anode having a thickness of 44 microns laminated onto 16 micron copper foil. Electrochemical testing

[0112] The anodes of Examples 1 and 2, and Comparative Examples 1 and 2 were tested by constructing half-cells that contain the anode electrode and a lithium metal counter electrode. The prototype cell consists of glass fibre as a separator and electrolyte (typically the same type of electrolyte that was used in the electrode production). The voltage limits of the prototype cell were from 0.01 V to 1 V with a current of C / 10 for five cycles. The cells were tested at 30 °C.

[0113] The results of the tests are shown in Figure 1 and Figure 2.

[0114] The results of Fig. 1 indicate that the capacity retention after formation for the gel anode with SWCNT was greatly improved compared with the anode of Comparative Example 1, being 100.2 % over 5 cycles compared to 79.3 % for Comparative Example 1. This value is also comparable with the capacity retention for known solvent cast anodes, indicating that electrodes produced according to the method of the present invention demonstrate suitable performance.

[0115] In Fig. 2, the dashed lines indicate the initial lithiation for each sample which in both cases is ~ 620 mAh / g due to the increased wt % of Si containing materials compared to Fig. 1. However, it can also be seen from this data that the anode with SWCNT (Example 2) has a much higher first cycle efficiency (85.0 Vs. 70.9 %), higher delithiation capacity (537 Vs. 438 mAh / g) and higher capacity retention (99.9 Vs. 65.1 %) over those 5 cycles as compared with the same values for Comparative Example 2.

[0116] Without wishing to be bound by theory it is believed that the addition of the carbon nanotubes to the electrode may provide for improved electrical connection across the particulate phase of the electrode precursor composition, as the carbon nanotubes, in particular due to their large aspect ratio and excellent conductivity, may act to bridge gaps between particles of the electrochemically active material - and in particular may act to bridge gaps cause by expansion and contraction of the electrode active material on cycling. This effect is seen in particular by comparing the results in Fig. 1 and Fig. 2 - where the anode uses an increased amount of SiOx active material, this increased amount of SiOx active induces more stress on the microstructure of the anode expansion / contraction, and a greater benefit of the invention is observed.

Claims

CLAIMS:

1. A method of preparing an electrode precursor composition for an alkali metal ion secondary cell, the electrode precursor composition comprising a polymer-electrolyte gel matrix phase and a dispersed phase comprising electrochemically active material and carbon nanotubes, the method comprising steps of: mixing an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to produce a premix; drying the premix to remove at least some of, or substantially all of, the dispersion medium to produce a dried premix; and mixing the dried premix with an electrolyte salt solution to produce the electrode precursor composition.

2. The method according to claim 1 wherein a gelling polymer binder is introduced in the step of mixing an electrochemically active material with a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to produce a premix.

3. The method according to claim 1 or claim 2 wherein a gelling polymer binder is introduced in the step of mixing the dried premix with an electrolyte salt solution to produce the electrode precursor composition.

4. The method according to any one of the preceding claims wherein the carbon nanotube dispersion medium is an aqueous dispersion medium.

5. The method according to any one of the preceding claims, wherein the carbon nanotubes include single-walled carbon nanotubes (SWCNTs).

6. The method according to any one of the preceding claims, wherein the carbon nanotubes include multi -walled carbon nanotubes (MWNTs).

7. The method according to any one of the preceding claims wherein the carbon nanotubes are present in the carbon nanotube dispersion in an amount of from 0.05 to 1 wt%, optionally in an amount of about 0.4 wt%.

8. The method according to any one of the preceding claims wherein the carbon nanotubes have an aspect ratio of greater than 1000, optionally greater than 5000.

9. The method according to any one of the preceding claims, wherein the electrochemically active material comprises either:(i) a positive active material, optionally a lithium transition metal oxide material; or(ii) a negative active material, optionally carbon, silicon, silicon-carbon composite, silicon oxide, or mixtures thereof.

10. The method according to any one of the preceding claims, further comprising the addition of a further conductive additive during formation of the premix.

11. The method according to claim 10, wherein the further conductive additive comprises one or more of carbon black or graphite.

12. The method according to any one of the preceding claims wherein the carbon nanotube dispersion is an aqueous dispersion, and wherein the step of drying the premix is performed to reduce the amount of water to an amount of 150 ppm or less.

13. The method according to any one of the preceding claims wherein the step of drying the premix is performed at a temperature of 150 degrees or less.

14. The method according to any one of the preceding claims wherein the electrolyte salt solution comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone as a solvent.

15. The method according to any one of the preceding claims wherein the electrolyte salt solution comprises a lithium salt selected from one or more of LiPFe, LiBF4, LiTFSI, LiFSI and LiTDI.

16. The method according to any one of the preceding claims as dependent on claim 2 or 3, wherein the gelling polymer binder comprises one or more polymers selected from polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polyacrylic acid (PAA), poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), 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, ferrocenesubstituted 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-m ethyl- 1 -propane sulfonate (LiAMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co- VinylTriethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, and PEO terpolymer.

17. The method according to any one of the preceding claims, wherein the electrolyte salt solution and the dried premix are combined in relative amounts of 0.5: 1 to 10: 1, optionally about 4.7: 1, based on total weight of these components.

18. The method according to any one of the preceding claims wherein heating is performed during the step of mixing the dried premix with the electrolyte salt solution.

19. The method according to any one of the preceding claims wherein the step of mixing the dried premix with the electrolyte salt solution is performed in a twin screw extruder.

20. A method of preparing an electrode for an alkali metal ion secondary cell, comprising: preparing an electrode precursor composition by the method according to any one of claims 1 to 19; and thermally processing the electrode precursor composition to form an electrode film.

21. An electrode precursor composition or an electrode for an alkali metal ion secondary cell, obtained or obtainable by a method according to any one of claims 1 to 19, comprising: a polymer-electrolyte gel matrix phase; and a dispersed phase comprising an electrochemically active material and carbon nanotubes.

22. The electrode according to claim 21, wherein the electrode has a capacity retention of 90% or more after 5 or more charge-discharge cycles at a C rate of C / 10.

23. An electrochemical secondary cell comprising an electrode according to any one of claim 21 or claim 2224. An electrochemical energy storage device comprising an electrochemical secondary cell according to claim 23.

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