A flexible lithium-ion battery

Abstract

A flexible lithium-ion battery comprising a cathode 110, an anode 112 and an electrolyte 120 comprising lithium ions contained within a flexible casing 118, wherein each of the cathode and the anode i

Description

The invention relates to a flexible lithium-ion battery comprising a cathode, and anode and an electrolyte contained within a flexible casing. The cathode and anode each include a carbon-based fabric current collector comprising a porous network of graphitic carbon fibre and an electroactive composition at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre and thus supported on the carbon-based fabric current collector. The invention also relates to a method of producing such a flexible lithium-ion battery, and a flexible article of manufacture comprising the flexible lithium-ion battery. Background of Invention Lithium-ion batteries are produced in a wide range of configurations depending on the intended application. The components of such devices, including the cathode, anode and casing, are usually rigid, with a hard casing in particular defining a fixed configuration for the battery as a whole. For some applications, however, a lithium-ion battery should be flexible, for example to allow the battery to fit into differently shaped cavities or to flexibly adapt its configuration to accommodate movement. One such application is in garments or other wearable devices with integrated electronics or power systems designed to operate portable equipment. A flexible, low weight and relatively flat (planar) configuration is preferred because the battery can then conveniently be integrated into the fabric of a garment without causing discomfort or restricting the movement of the wearer. Other applications of interest include portable electronic devices such as medical monitoring devices. For such battery configurations, the anode and the cathode are contained within a flexible casing, which is typically a pouch constructed of a flexible barrier film. The electrodes must therefore also have a planar configuration and be suitably flexible to accommodate movement of the battery. Furthermore, the electrodes should ideally have a high conductivity, good areal capacity, and high retention of the initial capacity over many cycles and despite flexing of the battery in use. Flexible batteries for garment applications have previously been reported. For example, US patent 8,192,863 discloses a lithium-ion battery with an ionic liquid electrolyte and flexible electrodes comprising cathode and anode materials supported on fabric current collectors. The fabric current collectors were formed by integrating an intersecting network of conductive threads into an otherwise conventional fabric composed of non-conductive fibres. While this approach provides good flexibility, the current collectors are insufficiently electrically conductive for many practical applications. Electrodes for flexible batteries have also been fabricated on more conductive current collectors. For example, US2017 / 0373284 describes an electrode produced by doctor-blading an electrode precursor slurry onto a nonwoven mat of multiwall carbon nanotubes (MWNT’s). However, such current collectors are not sufficiently flexible for many applications, such as in garments, and we have found that intimate integration of the electroactive material through the internal porosity of the non-woven mat is challenging. As a result, the electroactive material tends to form as a layer on the outside of the mat. This layer is susceptible to cracking when the electrode is flexed and adversely affects the electrical conductivity of the electrode. A further difficulty with many flexible batteries is the vulnerability to air and moisture ingress or electrolyte leakage through the flexible casing, particularly at the location where the electrical connections (e.g. metal wires or tabs) between the electrodes and the external battery terminals pass through the flexible casing. Even if a hermetic seal is created initially between the barrier film and the electrical connections, there is a risk of breaking the seal as the battery is flexed, leading to rapid degradation of the battery performance in use. There is therefore an ongoing need for flexible lithium-ion batteries, and methods of producing such batteries, which at least partially address one or more of the above-mentioned short-comings, or provide a useful alternative. A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. Summary of Invention The inventors have now discovered that flexible lithium-ion batteries with an attractive balance of properties can be fabricated by using a carbon-based fabric, comprising a porous network of graphitic carbon fibre with a high carbon content, as the flexible current collector. Despite the high graphitic carbon content needed to provide good electrical conductivity, the carbon-based fabric is sufficiently flexible for applications where the battery must accommodate significant movement, for example in garment applications. Moreover, the porosity of carbon-based fabric comprising a network of graphitic carbon fibre has been found particularly well suited to receive an electrode precursor slurry such that the resultant electroactive composition is intimately integrated with the fabric. The electroactive composition can thus be infiltrated through the thickness of the fabric and on both sides. For the case of a woven carbon-based fabric comprising a weave of carbon fibre yarns, the inventors have observed that the electroactive composition can be infiltrated through the weave, in the pores between the interwoven yarns and also between the carbon fibre filaments bundled together in each yarn. This ensures that the flexible electrodes have excellent mechanical durability despite flexing in use and facilitates good current transfer from the electrode material to the current collector during charging and discharging. The conductive and porous properties of the carbon-based fabric may be further exploited by using a terminal portion (or tab) of the carbon-based fabric current collectors as the electrical connection between the electrodes and the external battery terminals. An effective seal may be created where the tabs pass through the flexible casing of the battery by infiltrating a hydrophobic polymeric sealant through the porosity of the carbon-based fabric. The inventors have found that such a seal mitigates electrolyte loss via wicking through the tabs and facilitates excellent capacity retention of the battery over many charge-discharge cycles. In accordance with a first aspect the invention provides a flexible lithium-ion battery comprising a cathode, an anode and an electrolyte comprising lithium ions contained within a flexible casing, wherein each of the cathode and the anode independently comprises: a flexible current collector comprising a carbon-based fabric comprising a porous network of graphitic carbon fibre, wherein the graphitic carbon fibre comprises at least 85 wt.% carbon; and an electroactive composition supported on the flexible current collector, the electroactive composition comprising an electrode material and a polymeric binder, wherein the electrode material of the cathode is a lithium-intercalating cathode material and the electrode material of the anode is a lithium-intercalating anode material, wherein the electroactive composition is at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre. In some embodiments, the carbon-based fabric is a woven fabric comprising a weave of graphitic carbon fibre. The electroactive composition is thus infiltrated through the weave of graphitic carbon fibre. The weave of graphitic carbon fibre may comprise a weave of carbon fibre yarns, each yarn comprising a bundle of carbon fibre filaments, and the electroactive composition is interspersed between the carbon fibre filaments of the carbon fiber yarns. In some embodiments, the graphitic carbon fibre comprises at least 90 wt.% carbon, or at least 95 wt.% carbon, such as at least 99 wt.% carbon. In some embodiments, the graphitic carbon fibre is not functionalised with carbon nanostructures. In some embodiments, the carbon-based fabric comprises the graphitic carbon fibre in an amount of at least 90 wt.% based on the total weight of the carbon-based fabric. In some embodiments, the carbon-based fabric consists of the graphitic carbon fibre. In some embodiments, the carbon-based fabric has an electric resistivity of less than 0.1 ohm.cm, or less than 0.01 ohm.cm, such as less than 0.002 ohm.cm. In some embodiments, the polymeric binder has a tensile modulus of less than 1 GPa, or less than 0.5 GPa, such as less than 0.1 GPa, for example less than 0.05 GPa. In some embodiments, the polymeric binder is a polyurethane elastomer, such as a polyester polycaprolactone polyurethane elastomer. In some embodiments, the polymeric binder is present in an amount in the range of 1 wt.% to 15 wt.%, such as in the range of 2 wt.% to 12 wt.%, based on the total weight of the electroactive composition. In some embodiments, the electroactive composition comprises a conductive additive, such as a conductive carbon additive. In some embodiments, the electroactive composition comprises the electrode material in an amount in the range of 50 wt.% to 94 wt.%, the polymeric binder in an amount in the range of 1 wt.% to 15 wt.% and the conductive additive in an amount in the range of 5 wt.% to 20 wt.%, each based on the total weight of the electroactive composition. In some embodiments, the lithium-intercalating cathode material is selected from the group consisting of graphite, graphite composites with silicon, lithium metal, alloys of lithium metal, lithiated carbonaceous materials, and lithium titanate. In some embodiments, the lithium-intercalating cathode material is lithium titanate (LTO). In some embodiments, the lithium-intercalating anode material is a lithium metal oxide or lithium metal phosphate. In some embodiments, the lithium-intercalating anode material is lithium iron phosphate (LFP). In some embodiments, each of the cathode and the anode has an electrical resistivity of less than 10 ohm / cm, such as less than 5 ohm / cm. In some embodiments, the flexible current collectors of the cathode and anode each comprise a terminal portion of the carbon-based fabric which passes through the flexible casing to provide an electrical terminal for the flexible lithium-ion battery, wherein the flexible casing is sealed against ingress of air or egress of the electrolyte where the terminal portion passes through the flexible casing by a polymeric sealant infiltrated through the porosity of the porous network of graphitic carbon fibre in the carbon-based fabric. The polymeric sealant may be a heat-activated thermoplastic, for example a polyolefin, such as a polyethylene, e.g. linear low density polyethylene. The carbon-based fabric may be a woven fabric comprising a weave of graphitic carbon fibre, and the polymeric sealant is infiltrated through the weave of the carbon-based woven fabric. The weave of graphitic carbon fibre may comprise a weave of carbon fibre yarns, each yarn comprising a bundle of carbon fibre filaments, and the polymeric sealant is infiltrated between the carbon fibre filaments of the carbon fiber yarns. In some embodiments, the flexible lithium-ion battery has an initial areal capacity of at least 70% of theoretical capacity, or at least 80% of theoretical capacity, when cycled at C / 10 between 1.0 and 2.6 V, based on the loading of electrode material on the cathode and the anode. In some embodiments, the flexible lithium-ion battery exhibits a capacity retention of at least 90% after 36 cycles when cycled at C / 10 between 1.0 and 2.6 V. In accordance with a second aspect the invention provides a method of producing a flexible lithium-ion battery, the method comprising: producing a cathode and an anode by: (i) providing a flexible current collector comprising a carbon-based fabric comprising a porous network of graphitic carbon fibre, wherein the graphitic carbon fibre comprises at least 85 wt.% carbon; (ii) applying an electrode precursor slurry to the flexible current collector so that the electrode precursor slurry is at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre, the electrode precursor slurry comprising an electrode material and a polymeric binder dispersed in a solvent, wherein the electrode material of the cathode is a lithium-intercalating cathode material and the electrode material of the anode is a lithium-intercalating anode material; and (iii) drying the electrode precursor slurry to provide an electroactive composition comprising the electrode material and the polymeric binder supported on the flexible current collector; and functionally arranging the cathode, the anode and an electrolyte comprising lithium ions within a flexible casing. In some embodiments, the carbon-based fabric is a woven fabric comprising a weave of graphitic carbon fibre, and wherein the electrode precursor slurry is interspersed through the weave of graphitic carbon fibre. The weave of graphitic carbon fibre may comprise a weave of carbon fibre yams, each yarn comprising a bundle of carbon fibre filaments, and the electrode precursor slurry is interspersed between the carbon fibre filaments of the carbon fiber yarns. In some embodiments, the electrode precursor slurry is applied to the flexible current collector by spraying. The flexible current collector may be mounted vertically and the electrode precursor slurry is sprayed horizontally onto the flexible current collector via two or more passes applied to each side of the flexible current collector. In some embodiments, the solvent is dimethylformamide. In some embodiments, the graphitic carbon fibre comprises at least 90 wt.% carbon, or at least 95 wt.% carbon, such as at least 99 wt.% carbon. In some embodiments, the polymeric binder has a tensile modulus of less than 1 GPa, or less than 0.5 GPa, such as less than 0.1 GPa, for example less than 0.05 GPa. In accordance with a third aspect the invention provides a flexible article of manufacture comprising a flexible lithium-ion battery according to any embodiment of the first aspect. In some embodiments, the flexible lithium-ion battery is integrated into a flexible fabric portion of the flexible article of manufacture. In some embodiments, the flexible article of manufacture is a garment. Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof. Further aspects of the invention appear below in the detailed description of the invention. Brief Description of Drawings Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which: Figure 1 schematically depicts, in side cross-sectional view, a flexible lithium-ion battery according to some embodiments of the invention. Figure 2 schematically depicts, in plan view, the flexible lithium-ion battery of Figure 1. Figure 3 schematically depicts a sheet of carbon cloth with polyethylene tab seals, vertically mounted for spraying with electrode precursor slurry, as done in Example 3 Figure 4 schematically depicts apparatus for spraying a vertically mounted sheet of carbon cloth with electrode precursor slurry, as done in Example 3. Figure 5 is a SEM image showing an electroactive composition intimately interspersed through the weave of graphitic carbon fibre in a carbon cloth current collector, as produced in Example 2. Figure 6 is a graph depicting the battery capacity as a function of cycle number for a flexible lithium-ion battery as produced in Example 5. Figure 7 is a low magnification SEM image showing the infiltration of molten polyethylene from a heat activated polyethylene tab seal through the weave of a woven carbon cloth comprising graphitic carbon fibre, as produced in Example 12. [1] Figure 8 is a high magnification SEM image showing the infiltration of molten polyethylene from a heat activated polyethylene tab seal into into the internal porosity of a multifilamentous yam in a woven carbon cloth comprising graphitic carbon fibre, as produced in Example 12. Detailed Description The present invention relates to a flexible lithium-ion battery comprising a cathode, an anode and an electrolyte comprising lithium ions contained within a flexible casing. Each of the cathode and the anode independently comprises a flexible current collector comprising, and typically formed of, a carbon-based fabric and an electroactive composition supported on the flexible current collector. The carbon-based fabric comprises a porous network of graphitic carbon fibre with a carbon content of at least 85 wt.% to provide high electrical conductivity. The electroactive composition comprises an electrode material, a polymeric binder and optionally a conductive additive. The electrode material of the cathode is a lithium-intercalating cathode material and the electrode material of the anode is a lithium-intercalating anode material. The electroactive composition is at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre, so that the electroactive composition is intimately integrated into the carbonbased fabric of the flexible current collector. As a result, the electrodes as a whole remain flexible, with the electroactive composition remaining supported on and electronically integrated with the flexible current collector despite flexing of the battery in use. Furthermore, because the bulk of electroactive composition is closely associated with graphitic carbon fibres, and not present in a discrete layer overlying the current collector, the electrodes retain excellent electrical conductivity and charge transfer capability. A flexible lithium-ion battery according to some embodiments disclosed herein is schematically depicted in Figures 1 and 2. Flexible lithium-ion battery 100 comprises flexible cathode 110 and flexible anode 112, separated by a porous flexible separator 114. The sheet-like electrodes and separator are functionally arranged inside flexible casing 118, which is in the form of a planar pouch fabricated from sheet portions 132 and 134 of a multi-layered laminate barrier film. The flexible casing contains electrolyte 120, such as a liquid electrolyte, to provide ionic conductivity between the electrodes. The electrolyte is thus in contact with the electrodes, the separator and typically also the inside surface of the flexible casing. The device components appear spaced apart in Figure 1, for greater clarity, but it will be appreciated that the sheet-like electrodes, separator and laminate walls of the flexible casing are overlaid in close contact so that flexible lithium-ion battery 100 has a generally flat, yet flexible, configuration. Cathode 110 comprises a flexible current collector 122 with an electroactive composition 124 supported thereon. Similarly, anode 112 comprises a flexible current collector 126 with an electroactive composition 128 supported thereon. Cathode 110 and anode 112 comprise current collector tabs 130, 131 which are extensions of the carbon-based woven fabric of flexible current collectors 122, 126 that are not coated with electroactive composition. Both current collectors are formed of an electrically conductive carbonbased woven fabric comprising a weave of graphitic carbon fibre with a carbon content of at least 85 wt.% carbon. The electroactive composition of each electrode is present in a layer which is intimately integrated with the carbon-based woven fabric of the flexible current collector because the electroactive composition is infiltrated through the porosity provided by the weave of graphitic carbon fibre. The electroactive composition infiltrates the pores between the interwoven yarns and may also penetrate between the individual carbon fibre filaments bundled together in the carbon fibre yarns which form the woven fabric. Electroactive composition 124 comprises a particulate cathode material (e.g. LiFePO4; LFP) and electroactive composition 128 comprises a particulate anode material (e.g. Li4Ti50i2; LTO). Electroactive compositions 124, 128 also include a polymeric binder, preferably a low modulus polymeric binder which does not unduly restrict the flexibility of the electrodes, and optionally also a conductive additive such as carbon black. As best seen in Figure 2, flexible casing 118 is a planar pouch of generally rectangular configuration, formed by two sheet portions 132, 134 (optionally folded halves of a single sheet) of multi-layer laminate barrier film which are heat-sealed together in heat sealed regions 136 around the periphery of flexible casing 118. The current collector tabs 130 extend through one sealed edge of the planar pouch to provide electrical terminals of the device. Sheet portions 132, 134 are carefully sealed onto current collector tabs 130, 131 in tab seal regions 138 to avoid or limit air and moisture ingress or electrolyte leakage at the location where the current collector tabs pass through flexible casing 118. The seal is created by infiltrating a polymeric sealant, such as a thermoplastic polyolefin, through the weave of the carbon-based woven fabric. Flexible current collector comprising a carbon-based fabric The cathode and anode both include a current collector to support the electroactive composition. The current collectors must be suitably flexible to ensure that the lithium-ion battery as a whole is flexible enough to accommodate expected movements of the battery in use. Moreover, this flexibility, and the corresponding flexing which actually occurs in use, should not unacceptably compromise other important requirements of the current collector such as high electrical conductivity and the capability to physically support and electronically communicate with a layer of electroactive composition. The inventors have found that a suitable balance of properties is provided by a carbon-based fabric comprising a porous network of graphitic carbon fibre, wherein the graphitic carbon fibre comprises at least 85 wt.% carbon. As used herein, a carbon-based fabric is a woven or non-woven fabric composed primarily of fibres which consist predominantly of elemental carbon. The carbon-based fabric comprises, and typically consists of, graphitic carbon fibre, the individual filaments of which may optionally be arranged in multifilamentous fibres, threads, tows or yarns. The graphitic carbon fibre is arranged in an interconnected network to provide electrical conductivity. Carbonbased fabrics comprising such a network of graphitic carbon fibre are thus porous substrates, with pores commonly being present both between the fibres or yarns, whether woven or non-woven, and between the carbon fibre filaments bundled together in each yam. As used herein, graphitic carbon fibre refers to carbon fibre in which the carbon atoms are predominantly in the form of graphitic carbon. The degree of graphitisation corresponds to the Id / Ig ratio as measured by Raman spectroscopy. In some embodiments, the Id / Ig ratio is greater than about 1.2, preferably greater than 1.5. The carbon fibre comprises graphite crystals that are substantially aligned parallel to the long axis of the carbon fibre filaments, with this crystal alignment giving the carbon fiber a high strength-to-volume ratio and excellent electrical conductivity. Graphitic carbon fibre can thus be distinguished from carbon nanotubes, in which a hexagonal two-dimensional lattice of carbon atoms is “rolled up” to form a nanometer-scale, hollow cylindrical structure. Graphitic carbon fibre is generally formed by carbonization of precursor polymeric fibres to remove the non-carbon atoms and to graphitize the residual carbon content. The degree of carbonization can be controlled in this process, with the electrical conductivity of the carbon fibre increasing at higher graphitic carbon content. In some embodiments, the graphitic carbon fibre comprises at least 90 wt.% carbon, or at least 95 wt.% carbon, such as at least 99 wt.% carbon. Surprisingly, it has been found that flexible current collectors formed of woven carbon-based fabrics with a carbon content in excess of 99% are sufficiently flexible for use in a flexible lithium-ion battery, retaining good electrical conductivity and the capability to functionally support an electroactive composition loaded thereon even when subjected to flexing in use. Because of the high carbon content in the graphitic carbon fibre, the fibres themselves are intrinsically electrically conductive and capable of suitable charge transfer to and from the electroactive composition supported on the fabric current collector. There is no need to add a conductive coating or to functionalise the carbon fibres with conductive carbon nanostructures, and the graphitic carbon fibre is therefore typically not functionalised with carbon nanostructures or other conductive compositions. In some embodiments, the carbon-based fabric has an electric resistivity (in the absence of the electroactive composition) of less than 0.1 ohm.cm, or less than 0.01 ohm.cm, such as less than 0.002 ohm.cm. The electrical resistivity of fabrics can be measured using a two-point resistance probe. The carbon-based fabric may comprise the graphitic carbon fibre in an amount of at least 90 wt.% based on the total weight of the carbon-based fabric. In some embodiments, the carbon-based fabric consists of graphitic carbon fibre. In some embodiments, the carbon-based fabric is a woven fabric comprising a weave of graphitic carbon fibre. The woven carbon-based fabric may comprise a weave of graphitic carbon fibre yarns, each yarn comprising a bundle of carbon fibre filaments. Examples of such materials are the HC (>88% carbon) and HCB (>99% carbon) ranges of carbon fabrics, available from AvCarb, USA. The pore structure of woven carbon-based fabrics has been found particularly suitable for penetration by a precursor electrode slurry into the internal porosity of the fabric, including both the pores between the interlocking yarns in the weave and between the individual carbon fibre filaments which form each yarn. As a consequence, the resulting layer of electroactive composition is intimately integrated with, and thus resiliently supported by, the fabric current collector. The flexible current collectors of the cathode and anode may each comprise a terminal portion of the carbon-based fabric which is configured to pass through the flexible casing to provide an electrical terminal for the flexible lithium-ion battery. The terminal portion, which is typically not coated with electroactive composition, may be a strip of the carbon-based fabric extending from the main, electroactive portion, i.e. the fabric portion which supports the electroactive composition. As will be described in greater detail hereafter, an advantage of this arrangement is that the flexible casing may be sealed against ingress of air or egress of the electrolyte where the terminal portion passes through the flexible casing by a polymeric sealant infiltrated through the carbon-based fabric. Electroactive composition The cathode and anode both include an electroactive composition which is supported on the flexible current collector. The electroactive composition is at least partially infiltrated through the porosity of the porous network of the graphitic carbon fibre of the carbon-based fabric. By this it is meant that the electroactive composition penetrates the porosity of the carbon-based fabric to form a layer of electroactive composition intimately associated with and surrounding the carbon fibres present in the carbon-based fabric. This arrangement, which provides excellent mechanical and electronic integration of the electroactive composition with the flexible current collector, can be distinguished from electrodes in which the electroactive composition is present primarily in a discrete layer on top of the current collector surface. The electroactive composition comprises an electrode material, typically in particulate form, and a polymeric binder to consolidate the electrode material and to adhere the electroactive composition to the flexible current collector. The electroactive composition may also comprise a conductive additive, typically also in particulate form, to improve the electrical conductivity of the electroactive composition and thus its electrical contact with the flexible current collector. Any of the known types of anode materials suitable for use in lithium ion batteries may be used as the electrode material for the anode. In some embodiments, the anode material comprises an electroactive substance which is capable of reversibly intercalating / deintercalating lithium ions, such as graphite, graphite composites with silicon, lithium metal, alloys of lithium metal, lithiated carbonaceous materials (such as lithiated graphites, activated carbons, hard carbons and the like), lithium intercalating metal oxide based materials such as lithium titanate (Li4TisOi2; LTO), metal alloys such as Sn-based systems and conducting polymers, such as n-doped polymers, including polythiophene and derivatives thereof. Any of the known types of cathode materials suitable for use in lithium ion batteries may be used as the electrode material for the cathode. In some embodiments, the cathode material comprises an electroactive substance which is capable of reversible lithium intercalation and deintercalation. Of particular interest are lithium intercalating metal oxide and metal phosphate materials. Preferred examples of cathode materials include olivine type materials of general formula LiMPO4, where M is Fe, Co, Mn, and Ti. Specific examples of cathode materials include LiCoO2, LiMn2O4, LiFePO4 (LFP), and LiNiO?. Other examples include LiNiCoAIO2 (NCA), LiNiCoMnO2 (NMC), lithium manganese iron phosphate (LMFP), LiMn2O4 (LMO), LiNio.5Mm.5O4 (LMNO), LiMn2O4, LiMnNiO4 and analogues thereof or conducting polymers, redox conducting polymers, capacitor cathode materials, and combinations thereof. Examples of lithium intercalating conducting polymers are polypyrrole, polyaniline, polyacetylene, polythiophene, and derivatives thereof. Examples of redox conducting polymers are diaminoanthroquinone, poly metal Schiff-base polymers and derivatives thereof. The polymeric binder may in principle be any polymeric material with sufficient thermal and chemical stability to withstand the chemical environment and electrochemical processes in the battery, and which is capable of binding the particulate components of the electroactive composition together and to the flexible current collector. In some embodiments, the polymeric binder comprises one or more selected from the group consisting of polyvinylidene fluoride (PVDF), a polyurethane, styrene butadiene copolymer (SBR), polyvinylacetate, polyvinylalcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, cross linked polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), polyvinylidene fluoride, a copolymer of hexafluoropropylene and poly(ethyl acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, polypyrrole, polythiophene, derivatives thereof, blends thereof, copolymers thereof, and combinations thereof. A common polymeric binder used in lithium ion batteries is polyvinylidene difluoride (PVDF), which is favoured due to its high thermal and electrochemical stability as well as its excellent adhesive properties. The inventors have found that electroactive compositions comprising PVDF binder could be supported on carbon-based fabrics to prepare electrodes with excellent electrochemical performance. However, the flexibility of such electrodes was restricted by the layer of electroactive composition integrated into the carbonbased fabric of the current collector. It was proposed that PVDF, which has a tensile modulus of about 2.5 GPa, is too rigid to accommodate the flexing of the electroactive composition layer preferred for a flexible electrode. In some embodiments, therefore, the polymeric binder has a tensile modulus (Young’s modulus) of less than 1 GPa, or less than 0.5 GPa, or less than 0.1 GPa, for example less than 0.05 GPa. It has been found that the flexibility of electrodes comprising a carbon-based fabric current collector is not substantially restricted by the rigidity of the electroactive composition when using such low modulus polymeric binders. Moreover, the relative elasticity of the binder allows the electrode to flex without compromising the integrity of the layer of electroactive composition supported on the current collector. An example of a low modulus polymeric material suitable for use as the binder is Pellethane 2102-85A, a polyester polycaprolactone polyurethane elastomer available from Dow (tensile modulus of c.a. 0.007 GPa). In some embodiments, therefore, the binder is a polyurethane elastomer, such as a polyester polycaprolactone polyurethane elastomer. The polymeric binder may be present in an amount in the range of 1 wt.% to 15 wt.%, preferably in the range of 2 wt.% to 12 wt.%, based on the total weight of the electroactive composition. Higher amounts of binder may undesirably affect the electrical conductivity of the electroactive composition, while lower amounts may be insufficient to form a network of binder capable of consolidating and adhering the particulate components to the flexible current collector. The electroactive composition may also comprise a conductive additive, which is generally in particulate form. The conductive additive may be a conductive carbon additive, such as carbon black. In some embodiments, the electroactive composition comprises the electrode material in an amount in the range of 50 wt.% to 94 wt.%, the polymeric binder in an amount in the range of 1 wt.% to 15 wt.% and the conductive additive in an amount in the range of 5 wt.% to 20 wt.%, each based on the total weight of the electroactive composition. In some embodiments, the cathode and the anode both have an electrical resistivity of less than 10 ohm / cm, such as less than 5 ohm / cm. The electrical resistivity of the electrodes can be measured across the portion of the electrodes on which the electroactive composition is supported using a two-point resistance probe. The electroactive composition may be supported on the flexible current collector by applying an electrode precursor slurry to the flexible current collector, for example by coating, doctor blading, spraying, printing or other suitable techniques. The electrode precursor slurry typically comprises the electrode material (or precursors thereof which can form the electrode material in situ), the polymeric binder and optionally the conductive additive dispersed in a solvent. Battery configuration The flexible lithium-ion battery comprises the cathode and the anode, as described herein, and an electrolyte comprising lithium ions contained within a flexible casing. The flexible lithium-ion battery may comprise one cell or multiple cells, so that more than two electrodes may be enclosed within the flexible casing. The flexible casing comprises, and is typically formed from, a flexible barrier film. The flexible barrier film may be in the form of a multi-layer laminate comprising one or more polymeric layers and one or more water-impervious metallic or inorganic film layers. The water-impervious film layers may be relatively thick (e.g. >10 pm) metallic (e.g. aluminium) foils or relatively thin e.g. (e.g. <150 nm) coatings of a metal or inorganic compound (e.g. metal oxides or metal nitrides) supported on a polymeric film substrate. The multi-layer laminate may thus have a water vapour transmission rate (WVTR) of less than 200 mg / m2 / day, such as less than 100 mg / m2 / day, as measured by ASTM F3299-18. In some embodiments, the multi-layer laminate comprises one, two, or more barrier layers, each barrier layer comprising a semi-crystalline polymeric film, for example a biaxially-oriented polyethylene terephthalate (BOPET) film, and a water-impervious metallic or inorganic film, for example a metal, a metal oxide or a metal nitride. The metallic or inorganic film may be formed as a coating on the semicrystalline polymeric film with a thickness of less than about 150 nm. Such coatings may be produced by vapor deposition, for example by a vapor deposition technique selected from vacuum deposition, electron beam deposition and plasma enhanced chemical vapour deposition. This arrangement can be distinguished from foil laminates where one or more relatively thick (e.g. >10 pm), self-supporting metallic foils is used as the water-impervious metallic or inorganic film layer. While such foil laminates can provide excellent barrier properties, they are less flexible and can thus restrict the flexibility of the lithium-ion battery as a whole. Where two barrier layers are used in combination, they may be adhered together by an adhesive interlayer, for example a two part polyurethane adhesive, with their respective water-impervious metallic or inorganic films facing each other. Typically, the barrier film includes a polymeric seal layer as a continuous layer across one surface of the multi-layered laminate, which will thus be present on the entire inside surface of the flexible casing. The seal layer is configured to seal liquid electrolytes within the flexible lithium-ion battery, to avoid or limit electrolyte-induced degradation of other layers in the laminate, to avoid or limit contamination of the electrolyte by other layers in the laminate and to electrically insulate the flexible lithium-ion battery. The polymeric seal layer may comprise a heat-activatable adhesive polymer, for example linear low density polyethylene (LLDPE) or other suitable heat-activatable polymers known in the art, which can be used to heat-seal portions of the laminate together to form the flexible case. The flexible casing may be in the form of a substantially flat (i.e. planar) pouch produced from sheets of the flexible barrier film. Two sheets of the flexible barrier film, or one sheet folded in half, may be heat sealed together around the periphery to form the pouch. The flexible lithium-ion battery may comprise a separator to separate the cathode and the anode. Where the electrodes are configured as flexible sheets, the separator may be an electrically insulating but porous flexible sheet interposed between the electrodes. Suitable separators, such as porous polymeric membranes and non-woven fabrics, are well-known in the battery art. It will be appreciated that a separator may not be required in all embodiments, for example where a solid polymeric electrolyte is used. The flexible lithium-ion battery comprises an electrolyte which comprises lithium ions. Both electrodes are in contact with the electrolyte, which thus provides lithium ion conductivity between the electrode materials of the cathode and anode during charging and discharging of the battery. The electrolyte may generally be any electrolyte suitable for lithium ion batteries. The electrolyte may be a liquid electrolyte, including liquid electrolytes comprising aprotic molecular solvents, such as carbonates, water or ionic liquids, such as the pyrrolidinium-based ionic liquids disclosed in WO2004 / 082059. In some embodiments, the electrolyte is a non-aqueous electrolyte. The lithium ions may be provided by dissolution of a lithium salt such as LiPFe in the electrolyte medium. Tab seal As already described, the flexible current collectors of the cathode and anode may each comprise a terminal portion of the carbon-based fabric which passes through the flexible casing to provide an electrical terminal for the flexible lithium-ion battery. Typically, the terminal portions extend through one sealed edge of the planar pouch formed by the two overlying sheets of flexible barrier film. Using a portion of the conductive carbon-based fabric for the battery terminals in this way avoids the need to pass electrical wires through the flexible casing. However, it is still necessary to create an effective seal where the terminal portion of the carbon-based fabric current collectors pass through the flexible casing. The inventors have found that an effective seal against ingress of air or egress of the electrolyte can be created where the terminal portion passes through the flexible casing by infiltrating the carbon-based fabric of the tab portion with a polymeric sealant. The polymeric sealant may generally be any hydrophobic polymeric material which is capable of infiltrating the porosity of the carbon-based fabric and sealing around the graphitic carbon fibre of the carbonbased fabric. In some embodiments, the polymeric sealant is a heat-activated thermoplastic, such as a polyolefin, for example a polyethylene such as linear low density polyethylene. In some embodiments, as already disclosed herein, the carbon-based fabric is a woven fabric comprising a weave of graphitic carbon fibre, for example a weave of graphitic carbon fibre yarns where each yarn comprises a bundle of carbon fibre filaments. Carbon-based fabrics of this type have been found well suited to sealing because hydrophobic polymeric sealants, such as heat-activated polyolefins, can penetrate through the pore structure of the fabric, including both the pores between the interlocking yams in the weave and between the individual carbon fibre filaments which form each yarn. The flexible casing may be sealed at the location where the terminal portion passes through the casing by a polymeric seal (e.g. a heat seal) between the flexible casing and the polymeric sealant-infiltrated terminal portions of the flexible current collectors. The seal may be produced when constructing the flexible pouch casing by heat-sealing the two constituent sheets of flexible barrier film together at the periphery. The heat-sealing will heat-activate the polymeric seal layer of the flexible barrier film sheets, causing the molten polymer to flow and bond to the polymeric sealant previously infiltrated through the carbon-based fabric. After cooling, a resilient and hermetic seal is created, effectively sealing the flexible casing so that the long term cycling performance of the lithium-ion battery is not affected by wicking of the electrolyte or ingress of air or moisture where the terminal portions pass through the flexible casing. As an alternative, it is envisaged that the carbon-based fabric terminal portions may be infiltrated by the polymeric sealant simultaneously as the heatsealing of the flexible casing, i.e. by providing sufficient heat-activatable polymer at the location where the terminal portion passes between the sheets of barrier film to simultaneously infiltrate the carbon-based fabric and seal the barrier film sheets to each other and the interposed terminal portions of the current collectors. Battery performance The flexible lithium-ion battery according to the present disclosure may provide excellent cycling performance, despite being configured for flexibility. In some embodiments, the flexible lithium-ion battery has an initial areal capacity of at least 70%, or at least 80%, of theoretical capacity, based on the loading of electrode materials on the electrodes. As used herein, the initial areal capacity is defined as the areal capacity measured in the first cycle when cycling the battery at C / 10 between 1.0 and 2.6 V. In some embodiments, the flexible lithium-ion battery exhibits a capacity retention of at least 90% after 36 cycles. As used herein, the capacity retention is defined as the capacity retention of the 36th cycle, relative to the first cycle, when cycling the battery at C / 10 between 1.0 and 2.6 V. In the case of a lithium-ion battery comprising LTO as the cathode material and LFP as the anode material, the initial areal capacity may be at least 0.5 mAh / cm2, such as at least 1 mA / cm2, when cycled at C / 10 between 1.0 and 2.6 V. Method of producing a flexible lithium-ion battery The invention further relates to a method of producing a flexible lithium-ion battery. The method comprises producing each of a cathode and an anode by the following steps (i) to (iii). In step (i), a flexible current collector comprising, and typically formed of, a carbon-based fabric is provided. The carbon-based fabric comprises a porous network of graphitic carbon fibre with a carbon content of at least 85 wt.%. In step (ii), an electrode precursor slurry is applied to the flexible current collector so that the electrode precursor slurry is at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre of the carbon-based fabric. The electrode precursor slurry comprises an electrode material and a polymeric binder dispersed in a solvent. The electrode material of the cathode is a lithium-intercalating cathode material and the electrode material of the anode is a lithium-intercalating anode material. In step (iii), the electrode precursor slurry is dried to provide an electroactive composition comprising the electrode material and the polymeric binder supported on the flexible current collector. The electroactive composition is thus at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre, so that the electroactive composition is intimately integrated with the carbon-based fabric of the flexible current collector. Using the cathode and anode thus produced, the method then comprises a step of functionally arranging the cathode, the anode and an electrolyte comprising lithium ions within a flexible casing. The carbon-based fabric, the electrode materials of the two electrodes, the polymeric binder, the electrolyte and the flexible casing are generally as described herein in the context of the flexible lithium-ion battery. The electrode precursor slurry for preparation of each electrode comprises the electrode material (either a cathode material or an anode material as required), a polymeric binder and optionally a conductive additive dispersed in a solvent. The dispersed components are added in proportion to the required composition of the electroactive composition, while the solids concentration of the slurry may be optimised, based on the slurry application methodology, for a desirable viscosity and solids loading rate. The electrode materials and the conductive additive may be homogenised to reduce the particle size and to ensure a homogeneous dispersion of particulate components in the slurry. The solvent may be selected for its capability to dissolve the polymeric binder, to suspend the particulate components and to provide a suitable drying rate during the drying step. An example of a suitable solvent is dimethylformamide (DMF). In some embodiments, the precursor electrode slurry comprises a solids component comprising the electrode material in an amount in the range of 50 wt.% to 94 wt.%, the polymeric binder in an amount in the range of 1 wt.% to 15 wt.% and the conductive additive in an amount in the range of 5 wt.% to 20 wt.%, each based on the total weight of the solids component. To provide a composition suitable for application to the flexible current collector, the concentration of this solids component in the precursor electrode slurry may be in the range of 10 to 50 wt.%, such as in the range of 20 to 35 wt.%. In one exemplary embodiment, a suitable viscosity for spray application is obtained with a solids component concentration of between 20 and 35 wt% and DMF as the solvent. The electrode precursor slurry may be applied to the flexible current collector by any suitable technique, including spraying, coating, doctor blading, printing and the like. However, it is desirable that the electrode precursor slurry is applied uniformly and at a controllable loading rate such that the electrode precursor slurry is able to penetrate throughout the porosity of the carbon-based fabric and dry therein to produce a consistent layer of electroactive composition which is infiltrated through the porosity of the carbon-based fabric and which does not adversely affect the electrical conductivity of the electrode. The inventors have found that this can be achieved by spraying the electrode precursor slurry onto the flexible current collector. In some embodiments, therefore, the electrode precursor slurry is applied to the flexible current collector by spraying. The flexible current collector may be mounted vertically and the electrode precursor slurry is sprayed horizontally onto the carbon-based fabric. The slurry may be sprayed via a sequence of continuous, contiguous sweeps over the electroactive portion of the current collector surface to provide a substantially continuous loading rate of slurry per unit area. The slurry discharge rate and the sweep speed during each sweep may be optimised to allow suitable penetration of the fabric without run-off of excess slurry. To increase the total loading of electroactive composition on the flexible current collector, the electrode precursor slurry may be sprayed in this manner via two or more passes (each covering the entire electroactive portion of the current collector surface), applied to each side of the flexible current collector. Drying of the electrode precursor slurry results in the formation of an electroactive composition comprising the electrode material, the polymeric binder and optionally the conductive additive supported on the flexible current collector. A significant portion of the drying may take place during the application of slurry to the flexible current collector, since the solvent will begin to evaporate immediately after spraying a portion of the fabric. The drying process may be assisted by blowing dry gas and / or heating the wet flexible current collector current between spray passes and / or after completing the spraying. As disclosed herein, the carbon-based fabric may be a woven fabric comprising a weave of graphitic carbon fibre, for example a weave of graphitic carbon fibre yarns where each yarn comprises a bundle of carbon fibre filaments. By controlling the spray and drying parameters, including the composition of the electrode precursor slurry, the spray application protocol and the drying rate as disclosed herein, the inventors have found that the electrode precursor slurry can be infiltrated into and dried within the internal porosity of such fabrics, including both the pores between the interlocking yarns in the weave and between the individual carbon fibre filaments which form each yarn. As a consequence, the resulting layer of electroactive composition is intimately integrated with, and thus resiliently supported by, the fabric current collector and the electrodes retain good electrical conductivity. Flexible article of manufacture The invention further relates to a flexible article of manufacture which comprises the flexible lithium-ion battery as disclosed herein. The flexible article of manufacture may comprise, or be formed primarily of, one or more layers of flexible fabric or polymer sheeting which may be required to flex in use. The flexible lithium-ion battery, optionally in the form of a planar pouch, may thus be integrated into the article of manufacture by placing it against or between these layer(s). The flexible lithium-ion battery of the present disclosure is particularly suited to such applications because of the balance of properties provided by the flexible electrodes comprising carbon-based fabric flexible current collectors. The flexible article of manufacture may be any manufactured object containing a flexible portion into which it may be advantageous to integrate a power source. Non-limiting examples of suitable flexible article of manufacture include garments, such as jackets, wearable devices, personal monitoring devices for medical, health or fitness applications, medical equipment, military equipment, portable electronic devices, tents, flexible wall panels and so forth. In the case of garments, the flexible lithium-ion battery should not adversely affect the textile properties of the host garment, compared to the host garment in the absence of the flexible energy storage device. Properties of the host garment that should be retained are thermal properties, mechanical properties of the fabric of the host garment, moisture management, barrier and airflow properties. This may be achieved by appropriate placement of the flexible lithium-ion battery, but alternatively or additionally by design features of the flexible lithium-ion battery. EXAMPLES The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein. Example 1. Preparation of electrode precursor slurries The following general method was used to prepare the electrode precursor slurries. Differences in the specific components, amounts or procedures will be noted in later examples as deviations from this method. A slurry was prepared by first dissolving the binder polyurethane (Pellethane 2102-85A, available from Dow) in dimethylformamide (DMF) (1 g in 60 ml) by continuous stirring at 50 °C for 2 hours until complete dissolution. Then conductive carbon black additive (Denka Black, 2 g) was added followed by homogenisation (Polytron, Kinematica, type PT 45 / 50, dial speed 3) for 1 hour, and probe sonication (Branson Digital Sonifier, model 450, BRANSON Ultrasonics Corporation (US)) for 1.25 hours at a 50 % amplitude (Factory-tuned 20 kHz ultrasonic design). The dispersion was kept cool during probe sonication. For the anode precursor slurry, lithium iron phosphate (LiFePO4 or LFP) (17g) was then added to the mixture, followed by additional homogenisation for 30 minutes, and then additional probe sonication for a further 1.25 hours. Finally, the mixture was sonicated using a bath sonicator (50 Hz) for 1 hour before application to the carbon cloth current collector. For the cathode precursor slurry, lithium titanate (Li4TisOi2 or LTO) (17g) was instead added to the mixture, followed by additional homogenisation for 30 minutes, and then additional probe sonication for a further 1.25 hours. Finally, the mixture was sonicated using a bath sonicator (50 Hz) for 1 hour before application to the carbon cloth current collector. Example 2. Preparation of electrodes at small scale One of the precursor electrode slurries (60 ml), as produced by the method of Example 1, was sprayed onto a 30 x 30 cm sheet of carbon cloth. Carbon cloth from AvCarb (HCB 1170, plain weave, electric resistivity 1.1x10-3 ohm-cm, 99.5% carbon content) was used unless indicated otherwise. The spraying was performed using an automatic spraying technique (15 ml / min fluid flowrate, 2 bar air pressure, 80 mm fan width, 60 mm / s application rate (horizontal), 15 ml solution per pass). The carbon cloth was mounted vertically using magnets around the edges on an open backed stand (to allow free passage of the slurry through the cloth), and the spray arm was moved vertically from bottom to top in a series of sweeps, maintaining a constant fixed distance from the cloth. The slurry adhered to the cloth without running or pooling due to the ratio of solids to liquids in the slurry and the selection of solvent which has a suitable evaporation rate. After each spraying pass, the prepared sheet was dried in the oven at 50 °C for 15 minutes. Spraying was performed by either applying two passes on one side, drying then spraying another two passes on the other side followed by drying or alternatively, by applying one pass followed by drying and then repeating the same process to achieve four passes in total. Example 3. Preparation of electrodes at larger scale, with polyethylene tab seals As schematically represented in Figure 3, sheets of carbon cloth 702 (270 x 280 mm) (Avcarb HCB 1170) were prepared with heat-activatable linear low density polyethylene (PE) tab seal strips 704 (one strip on each side of the carbon cloth) applied at the top and bottom. The tab seal strips were cut from a plastic shopping bag. The PE tab seals, which were about 100 pm thick and 10 mm wide (arrow 706), were positioned on carbon cloth 702 so as to provide terminal portions 708 with a width of 50 mm (arrow 710). The distance between the two PE strips (arrow 712) was 120 mm for LFP-coated electrodes and 130 mm for LTO-coated electrodes. The PE tab seals were then heat activated using a heat sealer to infiltrate the molten LLDPE into the weave of the carbon cloth. The carbon cloth was then held in place vertically using magnetic strips 750 to support the cloth against a backing board and to mask the terminal portions, as shown schematically in Figure 4, and sprayed with an automatic spraying technique using one of the precursor electrode slurries, as produced by the method of Example 1. The spraying was conducted in a vertical direction, flowing the precursor slurry from solution feeder 751 at a flow rate of 115 ml / min, with the robot moving the spray nozzle 752 from bottom to top at 5mm / sec when spraying with anode precursor slurry containing LFP or at 6 mm / sec when spraying with cathode precursor slurry containing LTO slurry. The stand-off distance between the nozzle tip and the fabric surface was 275 mm. Three passes were made on the first side, with an intermediate step of blowing hot air onto the coating after each pass (a hot air gun set at 250°C approximately 30 cm from the substrate, moving at the same speed as the spray arm). The same three-pass spray application was then used on the opposite side. The coated cloth was then dried in an oven at 70°C for one hour. Eight individual flexible electrodes were then cut from each coated cloth 702 using a hydraulic press, in a two x 4 array. The dimensions of the coated portion of the electrodes were 60 mm x 60 mm for the LFP-based anodes and 65 mm x 65 mm for the LTO-based cathodes, plus a tab portion of 30 mm length and 15 mm width. The electrodes had the geometrical configuration of electrodes 110, 112 depicted in Figure 2. Example 4. Characterisation of the electrodes Electrodes produced in Example 2 were characterised by scanning electron microscopy (SEM) to investigate the integration between the electroactive composition and the carbon cloth. As seen in Figure 5, the electroactive composition was provided as a uniform layer supported on the carbon cloth with the composition intimately interspersed through the weave of graphitic carbon fibre in the carbon cloth. The electroactive composition penetrated into each yam of carbon fibres (i.e. between the individual filaments), as well as in the gaps between the woven yarns. The mechanical stability of the electrodes produced in Example 2 was assessed using a peel test, performed by an Instron instrument according to ASTM D 3330. A piece of Scotch tape was thus applied against the LTO or LFP-coated electrode and pressed with a 1 kg flat surface metallic weight for 1 min. The tape was then gently removed from the surface. On both sets of electrodes, very little material was removed on the tape, showing a strong adhesion of the electroactive composition within the graphitic fibres of the carbon cloth substrate. The electrical resistivity of the electrodes produced in Example 3 was then measured using a 2 point probe with a 1 kg weight across the contacts to ensure even pressure, with comparison against the uncoated Avcarb HCB 1170 carbon cloth. The results are shown in Table 1, including measurements of multiple identical cathodes and anodes. The resistivity of the coated carbon cloth electrodes remained very low, indicating a consistent coating which is expected to reduce the self-discharge rate of the battery once charged. Table 1. Electrode Electroactive layer Composition Resistivity (Q / cm) Uncoated carbon cloth <1 3-1 Anode (#1) 17g LFP, 1g binder, 2g carbon black 2 3-2 Anode (#2) 17g LFP, 1g binder, 2g carbon black 2 3-2 Anode (#3) 17g LFP, 1g binder, 2g carbon black 3 3-4 Cathode (#1) 17g LTO, 1g binder, 2g carbon black 4 3-5 Cathode (#2) 17g LTO, 1g binder, 2g carbon black 7 Example 5. Fabrication of batteries Pouch-cell type lithium-ion batteries of the configuration depicted in Figures 1 and 2 were then fabricated using electrodes produced by the method of Example 3, using one LFP-based electrode as the anode and one LTO-based electrode as the cathode. Pouch-type flexible casings were prepared using a commercially available battery-grade aluminium laminated film with a polypropylene heat seal layer on the inside surface (available from Gelon Lib Group, China). One rectangular piece of the aluminium laminated film was folded in half and heat sealed together opposite the fold using a heat sealer, to provide a pouch with a dimension of approximately 90 mm x 90 mm. One of the electrodes was positioned in the pouch, spaced apart from the edges by about 10 mm. A microporous polyethylene flexible separator (Solupor membrane 3P07A from Lydall Performance Materials), slightly larger in size than the electrodes, was then placed on top of the electrode in the pouch and the other electrode was then placed on top of the separator. The uncoated carbon cloth tabs of the two electrodes, positioned apart from each other as seen in Figure 2, extended through one opening of the pouch such that the heat-activated PE tab seals were just inside the pouch opening and aligned with the edges of the aluminium laminated films. The opening through which the tabs extended was then heat sealed by applying the heat sealer on both sides of the pouch. The heat and pressure applied by the heat sealer resulted in heat-activation of the polypropylene heat seal layer of the aluminium laminated film and thus heatsealing of the pouch film laminate to the polyethylene tab seal (which was already infiltrated through the weave of the carbon cloth; see further details in Example 12). Thus, after cooling and re-solidification of the heat-activated adhesives, the pouch was hermetically sealed against air or moisture ingress or electrolyte escape via the carbon cloth tabs. The pouch was then moved to the antechamber of a glovebox and kept there under vacuum at 50 °C for 5 days to remove traces of moisture. The electrolyte was then added to the pouch cell via the remaining opening, inside the glovebox, making sure to wet both sides of the electrodes. Unless otherwise specified, the electrolyte was 3.5 ml of 1:1 ethylene carbonate (EC) I dimethyl carbonate (DMC) containing 1M lithium bis(trifluoromethane)sulfonimide (LiTFSI). The pouch was then placed into a vacuum sealing machine and de-gassed three times to infiltrate the electrolyte into the pores of the electroactive coating and the fabric current collector. The final opening of the pouch was then sealed in the glovebox using a vacuum heat sealer. Example 6. Cycling performance of the batteries All cycling tests were performed on a MACCOR battery tester series 4000. A C / 10 current rate was used for all tested batteries, with a potential window from 1.0 to 2.6 V. Three similar batteries produced according to Example 5 (cells 6-1 to 6-3) were tested, with the only variation being the small differences in the loading of LFP and LTO sprayed onto the electrodes. The results are shown in Table 2, with the capacity as a function of cycle number for one cell shown in Figure 6. It can be seen that excellent cycling performance was obtained, with more than 90% capacity retention after 36 cycles. Table 2. Cell LFP (g) LTO (g) Theoretical capacity (mAh / cm2) Discharge capacity at cycle 1 (mAh / cm2) Discharge capacity at cycle 36 (mAh / cm2) Capacity retention (%) Charge voltage (V) Discharge voltage (V) Voltage difference (V) 6-1 0.414 0.632 1.84 1.31 1.18 90 1.91 1.82 0.09 6-2 0.439 0.705 1.95 1.48 1.38 93 1.91 1.82 0.09 6-3 0.444 0.714 1.97 1.61 1.46 91 1.91 1.82 0.09 Example 7. Different loadings of the electrode materials A further series of batteries were produced according to the methods of Examples 3 and 5, except that the loading of LTO and LFP were varied on 5 cathode and anode respectively, and 4 ml of the electrolyte was loaded in each pouch. The loading of LFP on the anode ranged from low (~ 6.5 mg / cm2 LFP in cells 8-1 to 8-3) to medium (-18.5 mg / cm2 LFP in cells 8-8 to 8-8) to high (~ 31 mg / cm2 LFP in cells 8-7 to 8-9). Cycling tests were again performed on a MACCOR battery tester 10 series 4000 at C / 10 current rate and a potential window from 0.5 or 1.0 to 2.6 V. The results are shown in Table 3. High loadings of electrode materials lead to a higher absolute capacity, albeit with some loss relative to the theoretical capacity of LFP (c.a. 70% of the theoretical 160 mA / g). By contrast, at the lower loadings the capacity was close to 100% of theoretical. Excellent capacity retention was 15 again obtained, particularly at the lower loadings. It is proposed that capacity retention could be increased at the higher loadings by increasing the amount of electrolyte. Table 3. Cell LFP (g) LTO (g) Theoretical capacity (mAh / cm2) Discharge capacity at cycle 1 (mAh / cm2) Discharge capacity at cycle 16 (mAh / cm2) Capacity retention (%) Charge voltage (V) Discharge voltage (V) 8-1 0.203 0.230 0.9 0.73 0.70 96 1.9 1.82 8-2 0.250 0.312 1.11 0.93 0.93 100 1.91 1.82 8-3 0.253 0.392 1.24 0.94 0.90 96 1.91 1.82 8-4 0.635 0.684 2.82 2.13 2.17 102 1.95 1.78 8-5 0.649 0.688 2.88 2.26 2.05 91 1.99 1.73 8-6 0.681 0.696 3.02 2.41 2.41 100 1.95 1.78 8-7 1.086 1.107 4.83 4.02 3.30 82 1.98 1.75 8-8 1.088 1.138 4.84 3.84 3.48 91 2.02 1.71 8-9 1.171 1.232 5.02 3.87 3.35 87 2.16 1.58 Example 8. Selection and amount of binder The role of the binder is to form a stable network of particulate components of the electroactive composition (electrode material and conductive additives), thus placing and retaining the electroactive composition in intimate contact with the current collector. Polyvinylidene fluoride (PVDF) is commonly used as a binder in lithium battery formulations due to its high thermal and electrochemical stability as well as its excellent adhesive properties (to bind electrode films and current collectors). Electrode precursor slurries (LTO-based and LFP-based) were prepared as per Example 1, except that the binder was either PVDF (1g), 1:1 PVDF I polyurethane (Pll) (1g) or PU (1g). The PVDF used was Kureha Polymer L#9305 in NMP, available from KUREHA Corporation, and the PU was Pellethane 2102-85A, a polyester polycaprolactone-based polyurethane elastomer available from Dow. Cathodes and anodes were prepared with the precursor slurries as described in Example 3, and batteries fabricated with these electrodes as described in Example 5. Cycling tests were performed on a MACCOR battery tester series 4000 at C / 10 current rate and a potential window from 1.0 to 2.6 V. The results are shown in Table 4. Table 4. Cell Binder in anode and cathode LFP (g) LTO (g) Discharge capacity at cycle 1 (mAh / cm2) Discharge capacity at cycle 16 (mAh / cm2) Capacity retention (%) 9-1 PVDF 0.553 0.581 1.97 2.01 102 9-2 1:1 PVDF / PU 0.421 0.631 1.80 1.72 95.5 9-3 PU 0.469 0.519 1.69 1.80 106.5 The electrochemical cycling results were thus surprisingly similar for the PU binder and the PVDF binder. However, the 100% PU binder (as used in cell 9-3) advantageously provided a much more flexible cell compared to the 100% PVDF or 1:1 PVDF / PU binders. The improved flexibility of the electrodes containing PU binder can be ascribed to the low modulus (i.e. improved elastic properties) of the PU compared to PVDF, with recorded tensile modulus values of 0.0072 GPa and 2.43 GPa respectively. Having determined that the PU binder was more suited for flexible electrodes than PVDF, a study was conducted to determine the required amount of the PU binder in the slurry formulation. Electrode precursor slurries (LTO-based and LFP-based) were thus prepared as per Example 1, except that the PU binder was used in amounts of 5% (1 g binder, 2g carbon black, 17g LFP or LTO), 10% (2g binder, 2g carbon black, 16g LFP or LTO) or 15% (3g binder, 2g carbon black, 15g LFP or LTO). Cathodes and anodes were prepared with these precursor slurries as described in Example 3, and batteries fabricated with these electrodes as described in Example 5). Cycling tests were performed on a MACCOR battery tester series 4000 at C / 10 current rate and a potential window from 1.0 to 2.6 V. The electrodes with 15% binder had the best mechanical stability but inferior cycling performance due to the lower conductivity. Good cycling performance was obtained at 5% and 10% PU content and the mechanical properties were found to be acceptable. Example 9. Slurry application methodology Electrode precursor slurries were also applied to carbon cloth current collectors by doctor-blading instead of spraying. While functional electrodes could be produced by this application technique, the doctor-blading tended to produce a non-uniform film of electroactive composition on the current collector so that the resultant electrodes had poor electrical conductivity. By contrast, the spraying method described in Examples 2 and 3 resulted in improved penetration of the electroactive composition into the weave of the carbon cloth, thus providing electrodes with good mechanical properties (flexibility, durability) and high electrical conductivity. Example 10. Raman characterisation of carbon-cloth current collector A range of carbon-based fabrics were obtained from commercial sources and characterised by Raman Spectroscopy, which is an important tool to deduce the structure in particular the defects and disordered nature of graphitebased materials. Table 5 shows the materials studied, the suppliers and source of the materials, the degree of graphitisation as described by the Id / Ig ratio determined from Raman Spectroscopy and the resistivity of each of the samples (Q / cm). The Raman spectra of the samples exhibit two peaks at around 1340 and 1500 cm'1 corresponding to the well-defined D band and G band, respectively. The graphitic peak (G band) at 1500-1510 cm'1 is due to the E2g vibrational mode of the C-C bond stretching and the disorder peak (D-band) at 1330-1345 cm'1 is due to the Aig vibrational mode. The intensity ratio of the D and G bands i.e (Id / Ig ratio) helps to estimate the degree of graphitisation of the materials as compared to more amorphous or disordered carbon materials. The fabrics with a higher Id / Ig ratio (corresponding to higher degree of graphitisation) generally had a lower resistivity, due to the higher electrical conductivity of graphite as compared to the amorphous carbon phase. Table 5. Sample Supplier Id / Ig ratio Resistivity CFF-2 Activated Carbon Fibre Felt 1000 CM Carbon Co Ltd (China) 1.349 2.8 Q / cm 6 cm wide CFF-1 Activated Carbon Fibre Felt 1000 CM Carbon Co Ltd (China) 1.683 2.8 Q / cm 6 cm wide CCSW-1 Activated carbon fibre cloth (single weave) Charcoal House LLC (USA) 1.176 20 Q / cm 6 cm wide CCSW-2 Activated carbon fibre cloth (single weave) Charcoal House LLC (USA) 1.140 20 Q / cm 6 cm wide CC-1 Carbon cloth AvCarb (USA) 1.663 1.5 Q / cm 6 cm wide CC-2 Carbon cloth AvCarb (USA) 1.709 1.5 Q / cm 6 cm wide 6F-1 (GF-1) BGF-1 Graphite felt CM Carbon Co Ltd (China) 1.267 1.6 Q / cm 6 cm wide 6F-2 (GF-2) BGF-1 Graphite felt CM Carbon Co Ltd (China) 1.228 1.6 Q / cm 6 cm wide CF-220-1 Activated Carbon Fibre Cloth (SA 300-400 m2 / g) Shanghai Liso Composite Material Technology, Co. Ltd. (China) 1.093 5 Q / cm 6 cm wide CF-220-2 Activated Carbon Fibre Cloth (SA 300-400 m2 / g) Shanghai Liso Composite Material Technology, Co. Ltd. (China) 1.122 5 Q / cm 6 cm wide CF-170-1 Activated Carbon Fibre Cloth (SA 700-800 m2 / g) Shanghai Liso Composite Material Technology, Co. Ltd. (China) 1.095 67 Q / cm 6 cm wide CF-170-2 Activated Carbon Fibre Cloth (SA 700-800 m2 / g) Shanghai Liso Composite Material Technology, Co. Ltd. (China) 1.151 67 Q / cm 6 cm wide AEROSPCC-1 AC220127 - CARBON WOVEN FABRIC PLAIN 200G / M2 1270MM Golan Products Pty Limited (Australia) 0.854 1.9 Q / cm 6 cm wide AEROSPCC-2 AC220127 - CARBON WOVEN FABRIC PLAIN 200G / M2 1270MM Colan Products Pty Limited (Australia) 0.904 1.9 Q / cm 6 cm wide ACN-1 ACN-211-20 Activated Carbon Felt Kynol (Japan) 1.073 8.7 Q / cm 6 cm wide ACN-2 ACN-211-20 Activated Carbon Felt Kynol (Japan) 1.155 8.7 Q / cm 6 cm wide ACC3-Plain-1 ACC-5092-20, Activated Carbon Fabric Kynol (Japan) 1.182 11.0 Q / cm 6 cm wide ACC3-Plain-2 ACC-5092-20, Activated Carbon Fabric Kynol (Japan) 1.178 11.0 Q / cm 6 cm wide Example 11. Comparative results with carbon-nanotube based current collector A15cm x 6cm sample of a non-woven carbon-based fabric formed from 5 carbon nanotubes was obtained from Tortech for a comparison trial against the Avcarb HCB1170 fabric. The uncoated fabrics was tested before coating for resistivity using a 2 point probe with results being 0.8 Q / cm for Avcarb HCB1170 carbon fibre fabric and 5.36 Q / cm for Tortech carbon nanotube fabric. The non-woven carbon nanotube fabric was found to be unsuitable as 10 a current collector for the following reasons. A suitably integrated and resilient layer of electroactive composition could not be supported on the non-woven carbon nanotube fabric by spray application with the precursor electrode slurries of Example 1. Instead, the layer that formed on the fabric cracked and was susceptible to delamination when flexed. It appears that the electroactive 15 composition could not be intimately interspersed through the carbon nanotubes forming the non-woven fabric. Furthermore, it was concluded that a hermetically sealed battery could not be produced when terminal tab portions of the non-woven carbon nanotube fabric extend through the flexible pouch to provide battery terminals. Attempts to produce a tab seal by penetrating the uncoated fabric with molten LLDPE in a heat sealer (as described in Example 3 and Example 12 below) were unsuccessful. Again, it appears that the carbon nanotubes forming the nonwoven fabric were not susceptible to penetration by a liquid composition which should be intimately interspersed through the fabric. Example 12. Development of the tab sealing methodology To provide electrical terminals for a flexible battery, it is necessary to provide conductive electrical conduits from the electrodes which pass through the flexible casing. A significant issue with many flexible battery designs is the vulnerability to air and moisture ingress or electrolyte leakage through the flexible casing at the location where these electrical connections pass through the casing. One approach is to use an uncoated terminal portion (a tab) of each electrode’s flexible current collector as the electrical conduit which protrudes the barrier film of the flexible casing. In initial attempts to fabricate cells using carbon-cloth supported electrodes (of the type described in Example 3), the pouch was sealed by heatsealing the aluminium laminate film directly onto the uncoated carbon cloth tabs of the electrodes. However, the resultant cells performed poorly in cycling tests and it was observed that the electrolyte was being lost. Further investigations revealed that the electrolyte was escaping the cell by wicking though the carbon cloth tabs, and it is believed that oxygen and moisture also entered the cell via this pathway. To address this wicking issue, and also to better resist air / moisture ingress, it was proposed to infiltrate the fibres of the carbon cloth with a hydrophobic polymer at the location of heat sealing to the barrier film (i.e. where the tabs passed through the flexible casing). Heat-activatable linear low density polyethylene (PE) tab seal strips (85 pm thick and 10 mm wide; cut from a plastic shopping bag) were thus placed on either side of an AvCarb HCB 1170 carbon cloth (in similar arrangement to that seen in Figure 3) and heat activated using a platinum press at 130-140 °C for 3-4 mins with a 60 pm shim (strip of stainless steel on each side to improve heat distribution). After cooling, strips of the carbon cloth (dimensions 80 mm x 20 mm) were cut, with the heat-activated activated tab seal located in the middle of the strips. The bottom half of a strip was then immersed in electrolyte (1:1 EC / DMC containing 1M LiTFSI) in a vial, with the top half dangling out of the vial. It was observed that electrolyte wicking was blocked by the activated PE tab seal. Carbon cloth strips produced in this manner were cut through the tab seal and imaged by scanning electron microscopy (SEM). The SEM images demonstrated that the heat-activated LLDPE had penetrated into the weave of the carbon cloth, forming a good seal around the yarns but not into the bundles of fibres in each yarn. Further development efforts were therefore conducted to improve the penetration. The total amount of heat-activated polymer was doubled by placing two PE tab seal strips on each side of the carbon cloth (4 x 85 pm thick in total). The PE tab seals were then subjected to an initial adhesion step using an impulse heat sealer for a short time, followed by a sustained heat-sealing step using a continuous heat sealer at 140 °C or 150 °C for periods of time ranging from 1 to 5 min. This was done by sandwiching the carbon cloth with adhered tab seals between two Teflon sheets and then two strips of stainless steel to keep it in place. The whole assembly was placed between the jaws of the continuous heat sealer for the desired time and temperature. The best sealing results were observed after 2 min of application at either 140 °C or 150 °C, where complete penetration of the PE within the woven structure and between the individual fibres in the yams was obtained. In the low magnification SEM image shown in Figure 7, it can be seen that the heat-activated PE tab seal formed a cohesive polymer layer 802 completely surrounding and sealing the woven yarns 804. In the high magnification SEM image shown in Figure 8, it can be seen that the heat-activated PE has penetrated into the porosity of each yarn, surrounding the individual carbon fibre filaments. These results demonstrate an improved methodology for creating an impervious tab seal, which is particularly compatible with carbon fiber fabric current collectors. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically 5 described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention. Future patent applications may be filed in the UK or overseas on the basis of or claiming priority from the present application. It is to be understood that the following provisional claims are provided by way of example only, and are 10 not intended to limit the scope of what may be claimed in any such future application. Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions.

Claims

1. A flexible lithium-ion battery comprising a cathode 110, an anode 112 and an electrolyte 120 comprising lithium ions contained within a flexible casing 118, wherein each of the cathode 110 and the anode 112 independently comprises:a flexible current collector 122, 126 comprising a carbon-based fabric comprising a porous network of graphitic carbon fibre, wherein the graphitic carbon fibre comprises at least 85 wt.% carbon; andan electroactive composition 124, 128 supported on the flexible current collector, the electroactive composition 124, 128 comprising an electrode material and a polymeric binder, wherein the electrode material of the cathode 110 is a lithium-intercalating cathode material and the electrode material of the anode 112 is a lithium-intercalating anode material,wherein the electroactive composition 124, 128 is at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre.

2. The flexible lithium-ion battery according to claim 1, wherein the carbonbased fabric is a woven fabric comprising a weave of graphitic carbon fibre.

3. The flexible lithium-ion battery according to claim 2, wherein the weave of graphitic carbon fibre comprises a weave of carbon fibre yarns, each yarn comprising a bundle of carbon fibre filaments, and wherein the electroactive composition is interspersed between the carbon fibre filaments of the carbon fiber yarns.

4. The flexible lithium-ion battery according to any one of claims 1 to 3, wherein the graphitic carbon fibre comprises at least 90 wt.% carbon, preferably at least 95 wt.% carbon, more preferably at least 99 wt.% carbon.

5. The flexible lithium-ion battery according to any one of claims 1 to 4, wherein the graphitic carbon fibre is not functionalised with carbon nanostructures.

6. The flexible lithium-ion battery according to any one of claims 1 to 5, wherein the carbon-based fabric comprises the graphitic carbon fibre in an amount of at least 90 wt.% based on the total weight of the carbon-based fabric, preferably wherein the carbon-based fabric consists of the graphitic carbon fibre.

7. The flexible lithium-ion battery according to any one of claims 1 to 6, wherein the carbon-based fabric has an electric resistivity of less than 0.1 ohm.cm, preferably less than 0.01 ohm.cm, more preferably less than 0.002 ohm.cm.

8. The flexible lithium-ion battery according to any one of claims 1 to 7, wherein the polymeric binder has a tensile modulus of less than 1 GPa, preferably less than 0.5 GPa, more preferably less than 0.1 GPa, most preferably less than 0.05 GPa.

9. The flexible lithium-ion battery according to any one of claims 1 to 8, wherein the polymeric binder is a polyurethane elastomer, preferably a polyester polycaprolactone polyurethane elastomer.

10. The flexible lithium-ion battery according to any one of claims 1 to 9, wherein the polymeric binder is present in an amount in the range of 1 wt.% to 15 wt.%, preferably in the range of 2 wt.% to 12 wt.%, based on the total weight of the electroactive composition.11 .The flexible lithium-ion battery according to any one of claims 1 to 10, wherein the electroactive composition 124, 128 comprises a conductive additive, preferably a conductive carbon additive.

12. The flexible lithium-ion battery according to claim 11, wherein the electroactive composition 124, 128 comprises the electrode material in an amount in the range of 50 wt.% to 94 wt.%, the polymeric binder in an amount in the range of 1 wt.% to 15 wt.% and the conductive additive in an amount in the range of 5 wt.% to 20 wt.%, each based on the total weight of the electroactive composition.

13. The flexible lithium-ion battery according to any one of claims 1 to 12, wherein the lithium-intercalating cathode material is selected from the group consisting of graphite, graphite composites with silicon, lithium metal, alloys of lithium metal, lithiated carbonaceous materials, and lithium titanate, preferably wherein the lithium-intercalating cathode material is lithium titanate.

14. The flexible lithium-ion battery according to any one of claims 1 to 13, wherein the lithium-intercalating anode material is a lithium metal oxide or lithium metal phosphate, preferably wherein the lithium-intercalating anode material is lithium iron phosphate.

15. The flexible lithium-ion battery according to any one of claims 1 to 14, wherein each of the cathode and the anode has an electrical resistivity of less than 10 ohm / cm, preferably less than 5 ohm / cm.

16. The flexible lithium-ion battery according to any one of claims 1 to 15, wherein the flexible current collectors 122, 126 of the cathode 110 and anode 112 each comprise a terminal portion 708 of the carbon-based fabric which passes through the flexible casing 118 to provide an electrical terminal for the flexible lithium-ion battery, wherein the flexible casing 118 is sealed against ingress of air or egress of the electrolyte 120 where the terminal portion 708 passes through the flexible casing 118 by a polymeric sealant infiltrated through the porosity of the porous network of graphitic carbon fibre in the carbon-based fabric.

17. The flexible lithium-ion battery according to claim 16, wherein the carbonbased fabric is a woven fabric comprising a weave of graphitic carbon fibre, and wherein the polymeric sealant is infiltrated through the weave of the carbon-based woven fabric.

18. The flexible lithium-ion battery according to claim 17, wherein the weave of graphitic carbon fibre comprises a weave of carbon fibre yarns, each yarn comprising a bundle of carbon fibre filaments, and wherein the polymeric sealant is infiltrated between the carbon fibre filaments of the carbon fiber yarns.

19. The flexible lithium-ion battery according to any one of claims 16 to 18, wherein the polymeric sealant 704 is a heat-activated thermoplastic.

20. The flexible lithium-ion battery according to claim 19, wherein the heat-activated thermoplastic is a polyolefin, preferably a polyethylene, most preferably linear low density polyethylene.21 .The flexible lithium-ion battery according to any one of claims 1 to 20, which has an initial areal capacity of at least 70% of theoretical capacity, preferably at least 80% of theoretical capacity, when cycled at C / 10 between 1.0 and 2.6 V, based on the loading of electrode material on the cathode 110 and the anode 112.

22. The flexible lithium-ion battery according to any one of claims 1 to 21, which exhibits a capacity retention of at least 90% after 36 cycles when cycled at C / 10 between 1.0 and 2.6 V.

23. A method of producing a flexible lithium-ion battery, the method comprising: producing a cathode 110 and an anode 112 by:(i) providing a flexible current collector 122, 126 comprising a carbon-based fabric comprising a porous network of graphitic carbon fibre 114, wherein the graphitic carbon fibre comprises at least 85 wt.% carbon;(ii) applying an electrode precursor slurry to the flexible current collector 122, 126 so that the electrode precursor slurry is at least partially infiltrated through the porosity of the porous network of graphitic carbon fibre, the electrode precursor slurry comprising an electrode material and a polymeric binder dispersed in a solvent, wherein the electrode material of the cathode is a lithium-intercalating cathode material and the electrode material of the anode is a lithium-intercalating anode material; and(iii) drying the electrode precursor slurry to provide an electroactive composition 124, 128 comprising the electrode material and the polymeric binder supported on the flexible current collector 122, 126; and functionally arranging the cathode 110, the anode 112 and an electrolyte comprising lithium ions within a flexible casing 118.

24. The method according to claim 23, wherein the carbon-based fabric is a woven fabric comprising a weave of graphitic carbon fibre.

25. The method according to claim 24, wherein the weave of graphitic carbon fibre comprises a weave of carbon fibre yarns, each yarn comprising a bundle of carbon fibre filaments, and wherein the electrode precursor slurry is interspersed between the carbon fibre filaments of the carbon fiber yarns.

26. The method according to any one of claims 23 to 25, wherein the electrode precursor slurry is applied to the flexible current collector 122, 126 by spraying, preferably wherein the flexible current collector 122, 126 is mounted vertically and the electrode precursor slurry is sprayed horizontally onto the flexible current collector 122, 126 via two or more passes applied to each side of the flexible current collector 122, 12646.

27. The method according to any one of claims 23 to 26, wherein the solvent is dimethylformamide.

28. The method according to any one of claims 23 to 27, wherein the graphitic carbon fibre comprises at least 90 wt.% carbon, preferably at least 95 wt.% carbon, more preferably at least 99 wt.% carbon.

29. The method according to any one of claims 23 to 28, wherein the polymeric binder has a tensile modulus of less than 1 GPa, preferably less than 0.5 GPa, more preferably less than 0.1 GPa, most preferably less than 0.05 GPa.

30. A flexible article of manufacture comprising a flexible lithium-ion battery according to any one of claims 1 to 22.

31. A flexible article of manufacture according to claim 30, wherein the flexible lithium-ion battery is integrated into a flexible fabric portion of the flexible article of manufacture.

32. A flexible article of manufacture according to claim 30 or claim 31, which is a garment.

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