Electrode precursor composition

Abstract

An electrode precursor composition suitable for preparing a gel electrode comprises an organic solvent, an alkali metal salt, an electronically insulating polymer and an organic redox-active component

Description

15 04 25 Field of the Invention The present invention relates to an electrode precursor composition for an alkali metal 5 ion secondary cell. The invention also relates to electrodes, cells and energy storage devices made from such precursor compositions, along with methods of preparing electrodes for alkali metal ion secondary cells. Background of the Invention 10 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. 15 Traditional lithium ion battery components such as electrodes are made from a solvent cast process that uses sacrificial solvent. This is an energetically expensive step, and a process that avoids using sacrificial solvent is therefore desirable. One approach to avoiding the use of sacrificial solvent is preparing gel electrodes. 20 These electrodes can be formed from a composition prepared by mixing the necessary components such as electrochemically active material, polymer (typically an electronically insulating polymer), and a solvent or liquid electrolyte, and subsequently subjecting the composition to a processing treatment such as extrusion, to form the electrode. 25 The use of a gel in the preparation of an electrode using such processing treatments must balance at least two opposing requirements. On the one hand, the electrode must have sufficiently high energy density to be of practical use. For this, a higher solids content (higher content of electrochemically active material) is desirable. On the other 30 hand, the gel must be sufficiently soft to be processed to form the electrode. For this, a lower solids content (lower content of electrochemically active material) is desirable, so that the material behaves sufficiently fluidly to be processable. There is a need for electrode precursor compositions which satisfy the above criteria. 35 15 04 25 Summary of the Invention The invention is defined in accordance with the appended claims. In general, the present inventors propose to address the above problems by making use 5 of redox active organic molecules as a fraction of the electrode precursor composition. In particular, the inventors generally propose to replace a portion of the usual electronically insulating polymer with redox active organic molecules. Since the electronically insulating polymer is a structural component, which is typically swellable, the volume fraction of the redox active organic component should be smaller than the 10 volume fraction of the electronically insulating polymer. In this way, the inventors believe it is possible to increase the energy density of a gel electrode compared to a corresponding gel electrode without the redox active organic component. For example, if the electrode precursor composition requires a reduced 15 amount of electrochemically active material for processability, then the presence of a redox active organic component can at least partially compensate for the expected corresponding reduction in energy density. Similarly, if the electrode precursor composition does not require a reduced amount of electrochemically active material for processability, then the use of a redox active organic component can add capacity and 20 result in an electrode with higher energy density than expected for a gel electrode without the redox active organic component. The present disclosure is based on this finding. Use of redox active polymers in electrodes are known, as discussed by e.g. Poizot et al. 25 (Chern. Rev. 2020, 120, 14, 6490-6557). For example, Ducetal. (ACS Appl. Polym. Mater. 2020, 2,6,2366-2379) discloses a long-range electronic conductor based on PEO and PEDOT:PSS. The authors report that extrusion of these blends introduced phase segregation phenomena requiring an acidic post-treatment to restore electronic conductivities. US 9,722,249 B2 and DE 102012022976 A1 disclose compositions 30 comprising complexes of polythiophene and polyanions, at least one lithium-containing compound, and at least one solvent, wherein the composition comprises less than 1 g of a material comprising elemental carbon, based on 1 g of the polythiophenes, or comprises no material at all comprising elemental carbon. JP 2004-158286 A discloses electrodes containing PEDOT and PVDF as binder produced by molding. JP 2011- 35 100594 A discloses a composition comprising a conductive polymer, formed by polymerizing ethylenedioxy thiophene or a derivative in the presence of at least one kind 15 04 25 of polymeric sulfonic acid selected from the group consisting of polystyrene sulfonic acid, sulfonated polyester, and phenolsulfonic acid novolak resin, containing the polymeric sulfonic acid as a dopant, and an organic solvent and a carbonaceous conductive material, as well as an electrode mixture composition which also includes a binder an 5 electrode active material. CN 110429279 A discloses an organic positive electrode material formed by copolymerization of a ruthenium or osmium derivative as a comonomer and which contains a conjugated aromatic fused ring as an electrochemical redox site. 10 However, these documents do not describe the concept of increasing the energy density of gel electrodes by using redox active organic molecules in a lower volume fraction than electronically insulating polymer. Accordingly, a general proposal of the present disclosure is to provide a processable gel 15 electrode precursor composition containing a redox active organic component, an electronically insulating polymer, an alkali metal salt, and an organic solvent. This composition may be subsequently combined with electrochemically active material and a conductive additive, before being further processed to form an electrode. Using this general procedure, the inventors find that the electrode precursor composition is highly 20 processable while also forming an electrode with increased energy density. A first aspect of the disclosure concerns an electrode precursor composition suitable for preparing a gel electrode, the composition comprising an organic solvent, an alkali metal salt, an electronically insulating polymer, and a redox active organic component, wherein 25 the redox active organic component is present in a smaller volume fraction than the electronically insulating polymer. The resulting electrode precursor composition typically contains a polymeric gel matrix phase. In this way, the electrode precursor composition typically has a gel-like quality 30 and can be processed into a thin-film electrode. In general, it is difficult to obtain dense electrodes having more than 70% by volume solids content and this results in limitation of the energy density of the electrode. Advantageously, the use of the organic fraction as a processing aid and active material 35 as a way of solving this problem, as is disclosed in the present application, has not been described previously. 15 04 25 According to a second aspect, the present disclosure concerns a method of preparing an electrode for an alkali metal ion secondary cell, the method comprising a providing step, comprising providing an electrode precursor composition 5 according to the first aspect; and a processing step, comprising processing the electrode precursor composition to form an electrode. Further aspects concern an electrode for an alkali metal ion secondary cell prepared 10 from an electrode precursor composition or method according to the present disclosure, an electrochemical secondary cell, such as an alkali metal ion secondary cell, comprising an electrode according to the present disclosure, an electrochemical energy storage device comprising an electrochemical cell according to the present disclosure, and use of a redox active organic component in a method of preparing an electrode for 15 an alkali metal ion secondary cell using a high temperature processing step. Brief Description of the Drawings Figure 1 is a schematic diagram of theoretical electrodes having (a),(b) ideal packing of electrochemically active material and (c),(d) non-ideal packing of electrochemically 20 active material. Detailed Description and Preferred Embodiments Preferred and / or optional features of the disclosure will now be set out. Any aspect of the disclosure may be combined with any other aspect of the disclosure unless the 25 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 disclosure unless the context demands otherwise. Unless otherwise defined, all technical and scientific terms used herein have the same 30 meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Methods and materials are described herein for use in the present disclosure; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, and other references mentioned 35 herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. 15 04 25 Methods described herein usually employ ambient temperature of a typical laboratory, which is typically between 20 and 30°C and preferably around 25°C, at atmospheric pressure, unless a different condition is defined herein or is more usually employed e.g. 5 for a particular apparatus. The present disclosure generally describes the use of a redox active organic component in electrode precursor compositions. The redox active organic component comprises molecules which can undergo reversible oxidation and reduction processes. The redox 10 active organic component may comprise one or more of redox active small organic molecules, and / or redox active organic polymers. Depending on the nature of the redox active organic component, particularly polymer, a dopant may be used to achieve practically useful effects. 15 It will be understood that the redox active organic component should be present in the electrode precursor composition in an amount which is sufficient to achieve the desired amount of increase in energy density. Accordingly, in some embodiments, the redox active organic component is present in the electrode precursor composition in an amount of at least 1 vol%, based on the total volume of the electrode precursor composition, for 20 example at least 2 vol%, at least 3 vol%, or at least 5 vol%. Furthermore, the redox active organic component is present in the electrode precursor composition in a smaller volume fraction than the electronically insulating polymer. Accordingly, in some embodiments, the redox active organic component is present in the 25 electrode precursor composition in an amount of up to 20 vol%, based on the total volume of the electrode precursor composition, for example up to 18 vol%, up to 15 vol%, up to 10 vol%, or up to 8 vol%. Accordingly, in some embodiments, the redox active organic component is present in the 30 electrode precursor composition in an amount of between 1 to 20 vol%, based on the total volume of the electrode precursor composition. Other combinations of the above values may be combined to form a suitable range, such as between 2 to 18 vol%, between 2 to 10 vol%, or between 5 to 8 vol%. 35 It is noted that the total vol% of all ingredients in the electrode precursor composition is 100 vol%. 15 04 25 Redox active organic components can undergo reversible oxidation and reduction processes. Accordingly, they represent a particularly useful class of component because they have the ability to store charge. Thus, the use of a redox active organic 5 component achieves compensation or improvement of energy density of an electrode. Figure 1 is used to show schematically the way the energy density of an electrode can be increased compared to a corresponding electrode without redox active organic component. In Figure 1, (a) shows a close to ideal electrode with near perfect packing of 10 electrochemically active particles. Theoretically, this has a >73% solids packing (close to ideal) with -68% volume electrochemically active material (black circles) and the intervening space filled with gel (white). This can result in an active material capacity of around 210 mAhg-1, and an 15 electrode capacity of around 183 mAhg-1. The same electrode as (a) is shown in (b) but with a redox active conductive component (grey) included in the gel in the pores, which may be represented e.g. by PhenQ (1,10-phenanthroline-5,6-dione, a cationic type of molecule which is discussed further herein). While the other values remain the same, the electrode capacity may be increased to 196 mAhg-1. 20 A similar trend can be seen for a lower packing efficiency scenario. Figure 1 (c) shows a non-ideal electrode, such as of the kind which may more typically be achieved through an extrusion process. This electrode has an imperfect packing of <73% solids packing and -63% volume of electrochemically active particles (black circles) and the intervening 25 space filled with gel (white). This can result in an active material capacity of around 210 mAhg-1, and electrode capacity of 177 mAhg-1. The same electrode as (c) is shown in (d) but with a redox active conductive component such as 1,8 PhenQ (grey) included in the gel in the pores. While the other values remain the same, the electrode capacity may be increased to 196 mAhg-1. Thus, it can be seen that replacing some of the gel 30 polymer with a redox active organic component can compensate somewhat for loss of volumetric capacities and increase the electrode capacity. Thus, higher energies may be achieved. This is particularly important for the non-ideal example, since this already has lower energy density. 35 In each of (a) to (d), the other components such as conductive additive and electronically insulating polymer are not shown. 15 04 25 In cathodes, the charge is stored through anionic insertion. That is, the compensation of energy density is achieved by an anionic doping reaction. The skilled person will be familiar with electrochemical storage mechanisms using redox-active organics; in this 5 regard we further refer to Figure 9 and Table 1 of Poizot et al. (Chern. Rev. 2020, 120, 14, 6490-6557). Figure 9a of Poizot et al. shows an example of anionic compensation of r a p-type system in which a polymer (shown as &) and comprising a conjugated double bond can be reversibly doped with anion A’. This process is shown for a two-fold reversible doping step (n=2). Each doping step involves loss of one electron from the 10 polymer. Accordingly, in embodiments using a redox active component, such as a redox active polymer, which undergoes an anionic doping reaction, the gel electrode can comprise up to 8 vol% of the redox active polymer, such as up to 7 vol%, up to 6 vol% or up to 5 vol% 15 of the redox active polymer, to avoid effective depletion of anions and thus maintain cell performance. Many redox active organic molecules, both small molecules and polymers, will be known to the skilled person. They may be used in the present disclosure alone or in 20 combination. Purely by way of example and not limitation, redox active organic molecules suitable for use in the disclosure include one or more of poly(3,4-etheylenedioxythiophene) (PEDOT), polyaniline (PANI), polypyrrole (PPy), as well as derivatives and doped derivatives thereof, such as poly(3,4-etheylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS). Further examples include polyacetylene, 25 coronene, polydopamine, 4,5-PhenQ, and others set out in Table 2 of Poizot et al. (Chern. Rev. 2020, 120, 14, 6490-6557) which is herein incorporated by reference in its entirety. In some embodiments, the redox active organic molecules for use in the disclosure have 30 a minimum redox activity of about 100 mAhg'1 with an average voltage about 2.5V. In some embodiments, the redox activity is higher, such as about 150 mAhg'1 or about 200 mAhg'1. No particular upper limit is envisaged, as higher redox activity is generally desirable. Redox activity can be measured using suitable means known in the art. 15 04 25 The electrode precursor composition of the present disclosure also comprises an electronically insulating polymer. These polymers are not intrinsically conducting 5 polymers, and do not conduct electricity or conduct electricity to a negligible degree. Typically, these polymers are substantially free of conjugated double bonds which allow the flow of electrons across the polymer backbone. The electronically insulating polymer(s) for use in the present electrode precursor 10 compositions are polymers typically used in the preparation of gel electrodes. They are generally swellable. They may be used in the present disclosure alone or in combination. Many will be known to the skilled person. Purely by way of example and not limitation, electronically insulating polymers suitable for use in the present disclosure include one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), 15 polymethylmethacrylate (PMMA) and polyacrylonitrile. In some embodiments, the electronically insulating polymer is present in the electrode precursor composition in an amount of at least 5 vol%, based on the total volume of the electrode precursor composition, such as at least 7 vol%, at least 10 vol% or at least 15 20 vol%. In some embodiments, the electronically insulating polymer is present in the electrode precursor composition in an amount of up to 30 vol%, based on the total volume of the electrode precursor composition, such as up to 28 vol%, up to 25 vol%, or up to 20 vol%. 25 Accordingly, in some embodiments, the electronically insulating polymer is present in the electrode precursor composition in an amount of between 5 to 30 vol%, based on the total volume of the electrode precursor composition. Other combinations of the above values may be combined to form a suitable range, such as between 5 to 28 vol%, 30 between 7 to 25 vol%, or between 10 to 20 vol%. The electrode precursor composition of the present disclosure comprises at least one alkali metal salt. 35 The alkali metal of the at least one alkali metal salt may be any suitable alkali metal (Group I of the periodic table). Typically, it may be lithium, sodium and potassium. In 15 04 25 some embodiments, the at least one alkali metal of the alkali metal salt is lithium, and this may be preferred in all embodiments of the disclosure. The anion of the at least one alkali metal salt may be any suitable anion. Typical anions 5 are known to the skilled person and may be chosen based on the nature of the alkali metal. In some embodiments, when the at least one alkali metal is lithium, the anion of the or each alkali metal salt is independently selected from: a lithium borate salt, a lithium imide salt, and a lithium imidazolide salt, or comprises a halogen such as fluorine. Examples include BF4; PFe’, TFSI-, FSI' and TDI'. 10 Accordingly, in some embodiments, the at least one alkali metal salt comprises one or more of lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium difluoro(oxalate)borate (LiDFOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl) imide (LiFSI), lithium 2-trifluoromethyl-4,5-dicyanoimidaxolide 15 (LiTDI) and lithium hexafluorophosphate (LiPFe). In some embodiments, the at least one alkali metal salt comprises one or more of LiFSI, LiDFOB and LiTDI 20 In some particularly useful embodiments, the at least one alkali metal salt comprises one or more of LiBF4, LiPF6, LiTFSI, LiFSI and LiTDI. One or more kinds of alkali metal salt may be used in accordance with the present disclosure. Typically, but not exclusively, when more than one kind of alkali metal salt is 25 used, they share a common alkali metal cation. Typically, but not exclusively, when more than one salt is present, they are each a lithium salt. The organic solvent used in the present disclosure is an organic solvent which is typically used in the manufacture of electrodes for alkali metal secondary cells. The 30 organic solvent should suitably be capable of being blended with, e.g. to form a solution with the polymer and redox active organic component of the electrode precursor composition. In some embodiments, the organic solvent is non-aqueous. In some embodiments, the 35 solvent comprises one or more cyclic or linear carbonate, ether or nitrile compounds. In 15 04 25 some embodiments the solvent comprises one or more cyclic or linear carbonate compounds. In some embodiments the one or more linear or cyclic carbonate compound comprises 5 one or more cyclic carbonate compounds. In some embodiments the solvent comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. 10 In some embodiments, the alkali metal salt and organic solvent together is present in an amount of at least 1 vol%, based on the total volume of the electrode precursor composition, such as at least 5 vol%, at least 10 vol% or at least 15 vol%. In some embodiments, the alkali metal salt and organic solvent is present in an amount 15 of up to 50 vol%, based on the total volume of the electrode precursor composition, such as up to 45 vol%, up to 35 vol% or up to 25 vol%. Accordingly, in some embodiments, the alkali metal salt and organic solvent is present in an amount of between 1 to 50 vol%, based on the total volume of the electrode 20 precursor composition. Other combinations of the above values may be combined to form a suitable range, such as between 5 to 45 vol%, between 5 to 35 vol%, or between 10 to 25 vol%. The skilled person will be aware of a large number of possible electrochemically active 25 materials, including cathode active materials (also called positive active materials) and anode active materials (also called negative active materials), which may be used in the present disclosure. The electrochemically active material is a particulate material, i.e. a material made up of 30 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 active material is a positive active material. That is, the electrode functions as a cathode. 35 15 04 25 In some embodiments, the positive active material is a lithium transition metal oxide material. In some embodiments, the positive active material is a lithium transition metal oxide material, or a lithium transition metal phosphate material, or a lithium transition metal sulfide material. These comprise, respectively, a mixed metal oxide, phosphate or 5 sulfide 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, phosphate or sulfide material comprising lithium and one or more transition metals selected from nickel, cobalt and manganese. In some embodiments, the positive active material is a lithium transition metal oxide 10 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 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 15 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. 20 Such electrochemically active materials are commercially available or may be manufactured by methods known to the skilled person, for example through the precipitation of mixed metal hydroxide intermediates from a reaction mixture containing different precursor metal salts, followed by calcination to form a mixed metal oxide and optionally lithiation to incorporate lithium into the oxide. 25 The electrochemically active materials may be undoped or uncoated or may contain one or more dopants and / or a coating. For example, the electrochemically active material may be doped with small amounts of one or more metal elements. The electrochemically active material may comprise a carbon coating on the surface of the 30 particles of the material. In some embodiments, the electrochemically active material is present in an amount of at least 55 vol%, based on the total volume of the electrode precursor composition, such as at least 56 vol%, at least 57 vol%, at least 58 vol%, at least 59 vol%, at least 60 vol%, 35 at least 61 vol%, or at least 62 vol%. 15 04 25 In some embodiments, the electrochemically active material is present in an amount of up to 75 vol%, based on the total volume of the electrode precursor composition, such as up to 74 vol%, up to 73 vol%, up to 72 vol%, up to 71 vol%, up to 70 vol%, up to 69 vol % or up to 68 vol%. 5 In some embodiments the electrochemically active material is present in an amount of from 55 to 75 vol%, based on the total volume of the electrode precursor composition. Other combinations of the above values may be combined to form a suitable range, such as from 55 to 70 vol%, from 55 to 69 vol%, from 55 to 68 vol%, from 58 to 68 vol%, or 10 from 60 to 68 vol%. In some embodiments the electrochemically active material makes up from 62 to 75 vol% of the electrode precursor composition, for example from 62 to 70 vol%, from 62 to 69 vol%, from 62 to 68 vol% or from 62 to 69 vol%. 15 In some embodiments the electrochemically active material is present in an amount of between 62 to 75 vol% of the electrode precursor composition, such as from 60 to 75 vol%, from 60 to 72 vol%, from 60 to 70 vol% or from 60 to 68 vol%. 20 In some embodiments, the electrode precursor composition further comprises a conductive additive, preferably which includes a conductive carbon. The conductive additive may be particulate. In some embodiments, the conductive additive comprises or consists of one or more of 25 carbon black, carbon nanotubes, graphene and graphite. In some embodiments, the conductive additive comprises or consists of carbon black or graphite, preferably carbon black. Examples of commercially available carbon black include Ketjen Black<R™) and Super C65. 30 In some embodiments, the conductive additive is present in an amount of at least 1 vol%, based on the total volume of the precursor composition, such as at least 2 vol%, at least 3 vol%, or at least 4 vol%. In some embodiments, the conductive additive is present in an amount of up to 10 vol%, 35 based on the total volume of the electrode precursor composition, such as up to 9 vol%, up to 8 vol%, or up to 7 vol%. 15 04 25 Accordingly, in some embodiments, the conductive additive is present in an amount of between 1 to 10 vol%, based on the total volume of the electrode precursor composition. Other combinations of the above values may be combined to form a suitable range, such 5 as between 1 to 9 vol%, between 1 to 8 vol%, between 2 to 8 vol% or between 3 to 7 vol%. In embodiments where a composition contains redox active organic component (in an amount of q vol%), electronically insulating polymer (in an amount of z vol%), alkali 10 metal salt, organic solvent and electrochemically active material (together in an amount totalling x vol%), and conductive additive (in an amount of y vol%), then x + y + z + q = 100 vol%. In some such embodiments, typically (x+y) >(q+z). 15 In some such embodiments, q / (q+z) is typically at least 0.25, such as at least 0.27 or at least 0.30. In further examples, q / (q+z) may be at least 0.32, at least 0.35 or at least 0.40. In some such embodiments, q / (q+z) is typically less than 0.5, such as up to 0.49, up to 0.47, up to 0.45 or up to 0.40. Accordingly, in some such embodiments, when x+y 20 >70 vol% then q / (q+z) is typically between 0.25 and less than 0.50. Other combinations of the above values may be combined to form a suitable range, such as between 0.27 and less than 0.50, between 0.27 and 0.49, or between 0.30 and 0.45. Also provided herein is a method of making an electrode precursor composition as 25 described herein. In general, the method comprises blending the redox active organic component, the electronically insulating polymer, the alkali metal salt, and the organic solvent. Typically, this blend will form a gel. Without wishing to be bound by theory, it is considered that cooling provides the gel with sufficient mechanical strength. The ingredients can be blended simultaneously or in any appropriate order, for any suitable 30 time, in any suitable way. For example, the appropriate amounts of each may be simultaneously added and mixed. Subsequently, the blend of organic solvent, alkali metal salt, electronically insulating polymer, and redox active organic component, is combined with the electrochemically 35 active material and the conductive additive, and optionally any further components. The electrochemically active material and the conductive additive can be combined with the 15 04 25 blend simultaneously or in any appropriate order, for any suitable time, in any suitable way. Typically the components will be mixed together using a variety of standard mixing 5 equipment such as planetary mixing to ensure a good dispersion of components. The mixture will then be passed through a twin-screw extruder at elevated temperatures (>110 °C) which will may combine the polymers and liquids into a viscous gel. Alternatively, the components maybe fed individually into the extruder via individual feeders and the mixing occurs inside the extruder. The material can then either be 10 extruded directly through a die head into a thin sheet or can be extruded roughly before being calendared into a film. This film can then be adhered to a metal foil which can be assembled into a battery. Any suitable equipment for preparing electrode precursor compositions may be used to 15 prepare the electrode precursor compositions herein, for example standard mixing equipment. The electrode precursor composition may subsequently be processed into an electrode as described herein. 20 Also provided herein is a method of preparing an electrode for an alkali metal ion secondary cell. In general, the method includes a providing step, comprising providing an electrode precursor composition as described herein, and a processing step, comprising processing the electrode precursor composition to form an electrode. Thus, 25 it will be understood that electrode precursor compositions as described herein are used for preparing electrodes according to the disclosure. Accordingly, steps described elsewhere herein for preparing the electrode precursor compositions apply equally to the providing step of the method of preparing an electrode. 30 In some embodiments, the processing step comprises one or more of extrusion and thermal processing. In some embodiments, the processing step comprises at least thermal processing. That is, the processing step comprises a step of applying high temperature in a fluid state 35 formation process. 15 04 25 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 5 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 10 130 °C. The roller assembly may comprise two rollers separated by a small distance such that the electrode precursor composition is pressed into a thin film when passed through the rollers. 15 In some embodiments the processing step comprises at least extruding the electrode, or extrusion. In some embodiments the processing step comprises extruding the electrode using an extrusion apparatus comprising one or more screw feeding sections and an extrusion die. In some embodiments, extrusion is achieved using a twin screw extruder. 20 In some embodiments in which both extrusion and thermal processing are used in the processing step, 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 25 some embodiments the temperature of the die is from 50 °C to 150 °C, for example from 60 °C to 150 °C, from 70 °C to 150 °C, from 80 °C to 150 °C, from 80 °C to 140 °C, from 90 °C to 140 °C, from 100 °C to 140 °C or from 110 °C to 130 °C. In some embodiments the electrode has a thickness of less than 150 pm, for example 30 less than 100 pm, less than 90 pm, less than 80 pm or less than 70 pm. In some embodiments the electrode has a thickness of from 40 to 150 pm, for example from 40 to 100 pm, from 40 to 90 pm, from 40 to 80 pm, from 40 to 70 pm or from 50 to 70 pm. In particular embodiments, the electrode has a thickness of from 40 to 100 pm. 35 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 15 04 25 70 pm, and comprises the electrochemically active material in an amount of from 60 to 75 vol% of the electrode precursor composition, for example from 60 to 70 vol%, from 60 to 69 vol%, from 60 to 68 vol% or from 62 to 69 vol%. 5 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. 10 In some embodiments, the method of the disclosure further comprises shaping and / or cutting the electrode to form an electrode of predetermined dimensions. In some embodiments, the electrode film has a thickness of from 500 to 700 pm. In 15 some embodiments, the method of the disclosure further comprises performing a second thermal processing step on a cut film to reduce the thickness of the film to within a range of 50 to 70 pm. In some embodiments, the temperature during that thermal processing step is from 100 20 to 140 °C. Also provided herein as an aspect of the disclosure is an electrode prepared by a method of the disclosure or from an electrode precursor composition of the disclosure. Typically, the electrode which results from the method of the disclosure is a gel 25 electrode. In some embodiments, the electrode which is prepared by the method of the disclosure has less than 10% porosity, such as less than 8% porosity, less than 5% porosity, less than 3% porosity or less than 1% porosity. In some embodiments, the electrode which is 30 prepared by the method of the disclosure has 0% porosity. The electrode porosity may be determined for example by comparison of theoretical density with observed density. That is, a skilled person can calculate the theoretical density of the electrode, and then measure the actual electrode density, using suitable 35 means. Subsequently, the actual density is subtracted from the theoretical density to provide the porosity. 15 04 25 Without wishing to be bound by theory, the inventors consider that a lower porosity is generally advantageous because it reduces the tortuosity for the ions (e.g. lithium ions) to move through the gel of the electrode. Thus, the initial rate capability of the electrode 5 is expected to be higher compared with an electrode having higher porosity. For similar reasons, it is expected that the electronic conductivity might be impacted in a similar way. Thus, the inventors expect the energy density is related to the porosity. Also provided as an aspect of the disclosure is an electrochemical secondary cell 10 comprising such electrode. The cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell. In some embodiments, the cell is a lithium-ion secondary cell. In some embodiments the electrochemical secondary cell comprises a first electrode 15 according to the disclosure, wherein the first electrode is a cathode, and an anode. In some embodiments, the electrochemical secondary cell comprises a second electrode according to the disclosure, wherein the second electrode is an anode, and a cathode. In some embodiments, the electrochemical secondary cell comprises a first and a second electrode according to the disclosure, wherein the first electrode is a cathode, 20 and the second electrode is an anode. In each of these embodiments, the electrochemical secondary cell also comprises an electrolyte between the cathode and the anode. In some embodiments, the electrolyte is a liquid electrolyte. In some embodiments, the liquid electrolyte comprises or is a solution comprising an alkali metal salt as described herein. 25 In some embodiments the electrochemical secondary cell comprises an electrode according to the disclosure laminated with a current collector, for example a metallic foil. Also provided herein as an aspect of the disclosure is an electrochemical energy storage 30 device comprising an electrochemical secondary cell of the disclosure. In some embodiments, the electrochemical energy storage device is a battery. In some embodiments, the electrochemical energy storage device is a lithium-ion battery. Also provided herein as an aspect of the disclosure is the use of a redox active organic 35 component in a method of preparing an electrode for an alkali metal ion secondary cell using at least one of a thermal processing step or an extrusion step. The redox active organic component of this aspect is preferably a redox active organic component as described herein for use in the first aspect. The method of preparing an electrode of this aspect is preferably a method of the second aspect as described herein. 5 In some embodiments, such use provides the electrode with an increased energy density compared to a corresponding electrode without the redox active organic component. Examples 10 The following is an exemplary electrode precursor composition for use in the present invention: 15 04 25 Ingredient Vol% of Composition Specific Example 1 Specific Example 2 Redox active organic component 1-20 5 10 Electronically insulating polymer (e.g. one or more of PVDF, PEO or PAN) 5-30 5 10 Alkali metal salt (e.g. one or more of LiFSI, LiTFSI, orLiBF4); and organic solvent (e.g. one or more of ethylene carbonate, propylene carbonate, vinylidene carbonate, fluoroethylene carbonate, and y-butyrolactone) 1-50 15 20 Electrochemically active material (e.g. one or more of LiCo02, NMC, or LiMn2O4) upto 75, preferably at least 45 70 55 Conductive carbon 1-10 5 10 15 04 25

Claims

1. A gel electrode precursor composition, the composition comprising an organic solvent, an alkali metal salt, an electronically insulating polymer, and a redox active polymer, wherein the redox active polymer is present in a smaller volume fraction than 5 the electronically insulating polymer, and wherein the redox active polymer is present in the gel electrode precursor composition in an amount of up to a maximum of 10 vol%, based on the total volume of the gel electrode precursor composition.

2. A gel electrode precursor composition according to claim 1, wherein the redox10 active polymer is present in the gel electrode precursor composition in an amount of up to 8 vol%, based on the total volume of the gel electrode precursor composition.

3. A gel electrode precursor composition according to claim 1 or claim 2, wherein the redox active polymer comprises one or more selected from poly(3,4-15 etheylenedioxythiophene), polyaniline, polypyrrole, and doped derivatives thereof.

4. A gel electrode precursor composition according to any one of the preceding claims, wherein the redox active polymer comprises poly(3,4-etheylenedioxythiophene)-polystyrene sulfonate.

205. A gel electrode precursor composition according to any one of the preceding claims, wherein the electronically insulating polymer comprises one or more selected from polyvinylidene fluoride, polyethylene oxide, polymethylmethacrylate and polyacrylonitrile.

256. A gel electrode precursor composition according to claim 5, wherein the electronically insulating polymer is present in the gel electrode precursor composition in an amount of between 5 to 30 vol%, based on the total volume of the gel electrode precursor composition.

307. A gel electrode precursor composition according to any one of the preceding claims, wherein the alkali metal salt comprises a lithium salt.

8. A gel electrode precursor composition according to claim 7, wherein the lithium 35 salt comprises one or more of LiBF4, LiPF6, LiTFSI, LiFSI or LiTDI.15 04 259. A gel electrode precursor composition according to any one of the preceding claims, wherein the organic solvent comprises one or more of a carbonate, an ether, or a nitrile.5 10. A gel electrode precursor composition according to any one of the precedingclaims, further comprising an electrochemically active material.

11. A gel electrode precursor composition according to claim 10, wherein the electrochemically active material comprises a positive active material.1012. A gel electrode precursor composition according to claim 10 or claim 11, wherein the electrochemically active material is present in the gel electrode precursor composition in an amount of at least 60 vol%, based on the total volume of the gel electrode precursor composition.1513. A gel electrode precursor composition according to any one of the preceding claims, further comprising an additive which includes a conductive carbon.

14. A gel electrode precursor composition according to claim 13, wherein the20 conductive carbon comprises one or more of carbon black, carbon nanotubes, graphene and graphite.

15. A gel electrode precursor composition according to claim 13 or claim 14, wherein the conductive carbon is present in the gel electrode precursor composition in an25 amount of between 1 to 10 vol%, based on the volume of the gel electrode precursor composition.

16. A method of preparing an electrode for an alkali metal ion secondary cell, the method comprising30 a providing step, comprising providing a gel electrode precursor compositionaccording to any one of claims 1 to 15; anda processing step, comprising processing the gel electrode precursor composition to form an electrode.35 17. A method according to claim 16, wherein the processing step comprises one ormore of extrusion and thermal processing.

18. A method according to claim 16 or claim 17, wherein the providing step comprises providing a gel electrode precursor composition according to any one of claims 1 to 10, subsequently combining the gel electrode precursor composition with an 5 electrochemically active material and conductive carbon.

19. An electrode for an alkali metal ion secondary cell prepared from a gel electrode precursor composition according to any one of claims 1 to 15 or prepared by a method according to any one of claims 16 to 18.1020. An electrochemical secondary cell comprising a gel electrode according to claim 19.

21. An electrochemical energy storage device comprising an electrochemical cell 15 according to claim 20.15 04 25

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