electrodes

Abstract

An electrochemical device component comprises at least one electro-spun fibre (1) comprising a core (2) and a sheath (3) surrounding the core. The core (2) comprises particles of an electrode active material (4) held together by a first polymeric material (6), and is electrically conductive along its length. The sheath (3) is formed from a second polymeric material, the second polymeric material being electrically insulating. The core (2) of the at least one electro-spun fibre (1) is arranged to provide a first electrode of an electrochemical device. The sheath (3) of the at least one electro- spun fibre (1) may be arranged to provide a separator between the first electrode and a counter electrode of the electrochemical device.

Description

[0001] ELECTRODES

[0002] This invention relates to a new electrode design which may provide an increased energy density, and which may have particular benefits for structural batteries. Batteries using such electrodes and methods of making such electrodes are also disclosed.

[0003] It known to produce nanofibers by electrospinning for use in batteries (see for example US 2018 / 331341 (Al)), and more particularly for use in the electrodes of batteries. A fluid stock for electrospinning is generally prepared by combining a polymer and a precursor of a high energy capacity material. After formation of the fibres by electrospinning, the fibres are calcined. This thermal treatment calcines the precursor to form either a high energy capacity material, or a material that can be converted to a high energy capacity material (e.g. by subsequent reduction of the calcined precursor). Calcination carbonises the polymeric component of the fibre - some of the polymeric material is generally lost, and the remaining carbonised material can serve to improve electrical conductivity of the fibre. The use of electrospun fibres for water management in fuel cells has also been disclosed (see US2013260283 (Al)), with those fibres providing only some proton conduction rather than electronic conduction.

[0004] Structural batteries are batteries able to both store electrical energy in the same way as a lithium-ion battery (for example) and carry a load. Structural batteries comprise multifunctional materials or structures, capable of acting as an electrochemical energy storage system (i.e. cells or batteries) while also possessing mechanical integrity (i.e. being able to bear a load). Such batteries have particular utility in transport applications (e.g. electric vehicles and drones) as they help to reduce weight and can improve system efficiencies. Currently, there are two main types of structural batteries: embedded batteries and batteries with laminated structural electrodes. Embedded batteries are multifunctional structures where state-of-the-art (e.g. lithium-ion) battery cells are embedded into a composite structure. Often, the cells are bonded between two thin and strong face sheets (e.g. aluminium). In-plane and bending loads are carried by these sheets while the battery core can take up transverse shear and compression loads (in addition to storing the electrical energy). This multifunctional structure can then be used as a load-bearing material as well as an energy storage structure. By contrast, in laminated structural electrodes the electrode material itself possesses intrinsic load-bearing and energy storage functions. Structural batteries made with such electrodes are also referred to as “massless” batteries, since they allow vehicle body parts to store energy without needing the additional weight of face sheets as is required for embedded batteries.

[0005] The inventors appreciated that electrospun fibres could be used to form a composite material with suitable structural properties for use in a structural battery, and sought to improve the electrode material-containing fibres so as to decrease the required mass / increase the gravimetric energy density. According to a first aspect of the invention there is provided an electrochemical device component comprising at least one electro-spun fibre comprising a core and a sheath surrounding the core, wherein: the core comprises particles of an electrode active material held together by a first polymeric material, and wherein the core is electrically conductive along its length; and the sheath is formed from a second polymeric material, the second polymeric material being electrically insulating, and wherein the core of the at least one electro-spun fibre is arranged to provide (at least a part of) a first electrode of an electrochemical device.

[0006] The sheath is therefore electrically insulating, whilst the core is electrically conductive.

[0007] As used herein, and as these terms are generally understood, “electrical conduction” / a material being “electrically conductive” refers to the conduction of electrons, and does not include ionic conduction (e.g. proton conduction). The same applies to “electrically insulating” - in particular, an electrically insulating material may still conduct ions.

[0008] In some implementations, one or both of the first polymeric material and the second polymeric material may be ionically conductive.

[0009] The electrode active material may be an inorganic material, and more particularly may be an inorganic solid optionally containing sodium and / or lithium (e.g. lithium titanate).

[0010] The sheath may be coaxial with the core.

[0011] The electrochemical device component may be a battery component, and more specifically may provide part or all of an electrode for a battery, the electrode having an integrated separator provided by the electrically-insulating sheath.

[0012] As, unlike in prior art approaches, an electrode active material is used in the electrospinning fluid (in place of a precursor for making an electrode active material in situ), no thermal treatment (e.g. calcination) is needed to make the material electrode-active after the fibres are formed. As such, improved control over the electrode active material properties is provided - particle sizes and surface modifications are chosen and known prior to electro-spinning, and remain at least substantially unaltered as no calcination is required or performed. The use of a conductive additive, and / or of a conductive electrode active material, so as to provide electrical conduction along the length of the fibre’s core also means that carbonisation of the polymeric component(s) of the electrospun fibre is not needed to provide conductivity. A need for calcination is therefore avoided, so both preserving electrode material properties in the core and allowing for use of the electrospun polymeric sheath as an insulating separator surrounding the core. In summary, inclusion of both active electrode material particles and conductive particles (which may or may not be or comprise the electrode material particles) in the electrospun fibre removes the requirement for calcination after electrospinning to give the fibres the desired electrochemical and electrical properties. This is advantageous as calcination has a high energy cost.

[0013] Removing the need for calcination also allows for the use of the polymer sheath material as a separator in use of the fibres as an electrode, as this would otherwise degrade at calcination temperatures.

[0014] The sheath of the at least one electro-spun fibre may therefore be arranged to act as a separator between the first electrode and a counter electrode of the electrochemical device.

[0015] The formation of this two-part fibre reduces separator mass and thickness, also reduces battery mass and provides a shorter diffusion pathway, so allowing for increased energy density and efficiency.

[0016] The sheath may also be arranged to act as an electrolyte of the electrochemical device, providing functionality of both a separator (preventing direct electron flow between opposing electrodes) and electrolyte. A solid polymer electrolyte, for example, may therefore be provided in the form of the sheath(s). This may reduce or avoid a need for a liquid electrolyte. The second polymeric material may be a solid polymer electrolyte material.

[0017] The second polymeric material may be soluble in a solvent in which the first polymeric material is not soluble. Optionally the solvent in which only the second polymeric material is soluble may be water. This may facilitate removal of one or more portions of the sheath without affecting the core.

[0018] In some embodiments, the second polymeric material is therefore soluble in a solvent in which the first polymeric material is not soluble. The solubility mis-match between the sheath and the core allows for a novel method of current collector attachment - regions of the sheath can be selectively removed with a carefully-chosen solvent without risking damage to the core.

[0019] The creation of the sheath with the core by electrospinning, and without the need for calcination to convert an electrode material precursor into an active electrode material, allows for the production of an ultra-thin separator (e.g. two orders of magnitude thinner than current standard separators), which can therefore significantly increase energy density in use and reduce battery mass by avoiding the need for a traditional separator.

[0020] The invention therefore provides a micro-scale coaxial battery electrode material, where the core material functions as an electrode and the sheath functions as the battery separator.

[0021] In some embodiments, the co-axial fibres may use a sodium battery chemistry for the core and a biodegradable / sustainable plastic sheath - this may allow their use as “disposable” batteries, with most components being biodegradable and the sodium active materials being of a lower priority for reclamation than lithium (for example). In some embodiments, the electrospun fibres may be combined with structural fibres e.g. carbon, glass or natural fibres. This may provide an electrode composite suitable for use in a structural battery; providing a novel architecture for a structural battery. Combination here refers to any suitable means of combining structural fibres with the electrospun fibres - e.g.:

[0022] • direct electrospinning onto a structural fibre - each electrospun fibre may therefore comprise a structural fibre in addition to its core and sheath in such embodiments;

[0023] • stitching to a structural fibre layer as non-crimp construction of a fibrous composites, where the electrospun mat is employed as veil to provide electrical power and control resin flow (a veil material is sometimes placed between fibre layers in carbon fibre composites to improve resin uptake and resist delamination - the electrospun electrode disclosed herein may be used as such a veil. As well as stitching between layers, resin is generally used to improve load transfer between the fibres. Non-crimp fabrics are typically unidirectional; the electrospun mat used here as a veil may be inserted between the unidirectional layers. The electrospun veil may contribute to bifunctionality, providing electrode active material and conductive pathways whilst also forming part of and strengthening the composite structural material, such that it contributes both electrochemically and structurally); or

[0024] • a layering process where mechanical connection of the electrospun mat and the structural fibres is performed via application of a resin, the resin being either pre-impregnated onto the structural fibres or added via resin transfusion or a transfer moulding process.

[0025] The or each fibre may have a diameter in the range from 1000 nm to 2000 nm, and optionally of around 1300 nm.

[0026] The or each fibre may have a sheath thickness in the range from 150 nm to 450 nm, and optionally of around 300 nm.

[0027] The core may comprise electrically conductive particles held together by the first polymeric material, the conductive particles being held in contact with each other so as to provide a conductive path along the length of the core.

[0028] Optionally, the conductive particles may be electrode active, such that the conductive particles are the particles of an electrode active material.

[0029] The conductive particles may have a conductivity of at least 0.01 S / cm, and optionally of at least 0.02 S / cm or 0.05 S / cm.

[0030] The conductive particles may have a diameter in the range from 40 to 500 nm, and optionally from 40 to 100 nm, or from 80 to 100 nm.

[0031] The conductive particles may comprise carbon black, graphene and / or graphene oxide (e.g. reduced graphene oxide - rGO - or, expanded graphene oxide - eGO). Optionally, the electrode may be an anode. Optionally, the core may comprise conductive particles in addition to particles of an electrode active material, the conductive particles having a higher electrical conductivity than the electrode active material, and the particles all being held together by the first polymeric material.

[0032] The electrode active material may comprise an inorganic solid including at least one of Sodium, Potassium, Magnesium, and Lithium. Optionally, the electrode may be a cathode.

[0033] The electrode active material and the conductive particles may each be provided in the form of nanoparticles.

[0034] The particles of the electrode active material may have a diameter in the range from 40 to 500 nm, and optionally from 40 to 100 nm, or from 80 to 100 nm.

[0035] The electrochemical device component may comprise a plurality of the electro-spun fibres. Optionally, the plurality of fibres may be provided as a mat, having a much greater extent in a plane than in a direction perpendicular to that plane. Such a mat may exhibit alignment of the fibres within the plane such that the fibres extend generally in the same direction.

[0036] According to a second aspect of the invention, there is provided an electrochemical device comprising at least two electrodes, and wherein at least one of the electrodes, and a separator arranged to electrically insulate one electrode from the other one or more electrodes, is provided by at least one electrochemical device component as described with respect to the first aspect.

[0037] Each electrode of the electrochemical device may comprise a separate plurality of electrospun fibres as described with respect to the first aspect.

[0038] Each electrode of the electrochemical device may be provided by one or more electrochemical device components as described with respect to the first aspect. Each electrode may therefore have a separator, so providing some redundancy / increased protection against current leakage / short-circuits.

[0039] In embodiments in which each electrode comprises a separate plurality of electro-spun fibres, each plurality of electro-spun fibres may comprise fibres aligned to be at least substantially planar. The electrochemical device may be formed from multiple such pluralities of aligned fibres stacked together within an electrolyte matrix.

[0040] The electrochemical device may be flexible.

[0041] The electrochemical device may further comprise electrical connectors for each electrode, and optionally for each fibre of each electrode. The material of the sheath of each fibre of the plurality of fibres may be removed in an area to allow electrical connection (directly or indirectly) between the core material and the electrical connector for the respective electrode.

[0042] The sheaths of the fibres may form the only separator(s) of the electrochemical device.

[0043] The electrochemical device may be a battery.

[0044] The or each plurality of electro-spun fibres may be combined with structural fibres, for example so as to form a structural battery. Optionally, the structural fibres may be carbon fibres. The structural fibres may be stiff, so providing some rigidity. In such embodiments, the electrospun fibres may be formed on or around one or more structural fibres.

[0045] Alternatively or additionally, the plurality of electro-spun fibres may be formed as a mat and stitched to a structural fibre layer. Alternatively or additionally, the plurality of electrospun fibres may be formed as a mat and bonded to a structural fibre layer using a resin or adhesive.

[0046] The electrospun fibre battery electrode material can therefore be applied in both “normal” (non-structural) and structural batteries (batteries able to provide electrical power whilst also carrying mechanical loads / forces).

[0047] The electrospun fibres may be attached to stiffer fibres - by any suitable method known in the art - to increase strength for use in structural batteries. The battery may therefore be a structural battery.

[0048] In some embodiments, the electrospun fibres may be attached to natural fibres (e.g. hemp) for additional stiffness. In such embodiments, the battery has potential to be recyclable / compostable / disposable owing to the ability to choose biodegradable polymers for the sheath and core, and the biodegradability of the selected natural fibres. This may be particularly applicable to Na-ion batteries, given the abundance of Sodium, as compared to Lithium.

[0049] In some embodiments, both electrodes of the battery are made from electrospun fibres as described herein. The battery may therefore be described as a fully electrospun battery, and may not have a thick separator as is present in traditional batteries. The sheath of each fibre may provide sufficient electrical separation of the electrodes.

[0050] The or each electrospun electrode of the battery may be made using one or more biopolymers as the polymeric material(s). These biopolymers are biodegradable, so allowing the majority of the battery to be biodegradable. The battery may therefore be more environmentally-friendly than prior art batteries.

[0051] Na+ion active electrode materials may be used as the active electrode materials of the fibre cores. Using an abundant ion like Na+may allow the battery to be disposable.

[0052] According to a third aspect of the invention there is provided a method of manufacturing an electro-spun fibre for use as an electrode of an electrochemical device, the electro-spun fibre comprising a core and a sheath surrounding the core, the method comprising: suspending particles of an electrode active material in a solution or melt of a first polymeric material to form a core electro-spinning medium; dissolving or melting a second polymeric material, the second polymeric material being electrically insulating, to form a sheath electro-spinning medium; feeding the core electro-spinning medium into an inner chamber of a coaxial nozzle; feeding the sheath electro-spinning medium into an outer chamber of the coaxial nozzle; applying a voltage to a tip of the coaxial nozzle to initiate electrospinning of a fibre from the nozzle; controlling feeding rates of the core and sheath electro-spinning media to provide a desired core and sheath thickness of the fibre; and depositing the electrospun fibre onto a collector, and wherein the composition of the core electro-spinning medium is selected to provide electrical conductivity along the length of the electrospun fibre.

[0053] The fibre may be formed by melt electro-spinning; in such implementations, the core electro-spinning medium and the sheath electro-spinning medium may each be provided as a polymer melt, optionally with particles suspended therein. The forming of the core electrospinning medium may comprise melting the first polymeric material and suspending the particles of the electrode active material in the melted polymer to form a core melt. The core electrospinning medium in such implementations may be referred to as a core melt. The forming of the sheath electro-spinning medium may comprise melting the second polymeric material to form a sheath melt. The sheath electro-spinning medium in such implementations may be referred to as a sheath melt.

[0054] The fibre may be formed by solution electro-spinning; in such implementations, the core electro-spinning medium and the sheath electro-spinning medium may each be provided as a solution of a polymer, optionally with particles suspended therein. The forming of the core electro-spinning medium may comprise dissolving the first polymeric material in a solvent and suspending the particles of the electrode active material in the resultant solution to form a core solution. The core electro-spinning medium in such implementations may be referred to as a core solution. The forming of the sheath electro-spinning medium may comprise dissolving the second polymeric material in a solvent to form a sheath solution. The sheath electro-spinning medium in such implementations may be referred to as a sheath solution.

[0055] Different polymers may be used for melt electro-spinning as compared to solution electro-spinning, in some implementations. The same electro-spinning technique may be used for both the core and the sheath to facilitate manufacture as compared to using one melt and one solution.

[0056] In either case, the electro-spinning medium is at least substantially liquid during electrospinning, and dries / solidifies thereafter (e.g. due to solvent evaporation, and / or to cooling).

[0057] For solution electro-spinning, there is therefore provided a method of manufacturing an electro-spun fibre for use as an electrode of an electrochemical device, the electro-spun fibre comprising a core and a sheath surrounding the core, the method comprising: dissolving a first polymeric material in a solvent and suspending particles of an electrode active material in the solution to form a core solution; dissolving a second polymeric material in a solvent, the second polymeric material being electrically insulating, to form a sheath solution; feeding the core solution into an inner chamber of a coaxial nozzle; feeding the sheath solution into an outer chamber of the coaxial nozzle; applying a voltage to a tip of the coaxial nozzle to initiate electrospinning of a fibre from the nozzle; controlling feeding rates of the core and sheath solutions to provide a desired core and sheath thickness of the fibre; and depositing the electrospun fibre onto a collector, and wherein the composition of the core solution is selected to provide electrical conductivity along the length of the electrospun fibre.

[0058] For both melt electro-spinning and solution electrospinning:

[0059] The method may avoid any need for, and indeed may exclude, any high-temperature (e.g. above 400°C, 300°C, or 200°C, or above 100°C) processing such as calcination after the formation of the fibres by electrospinning. The properties of the electrode active material may therefore be maintained, being at least substantially unaffected by the electrode formation process. The method may be performed entirely at temperatures below 400°C, 300°C, 200°C, or 100°C (noting that the lower temperatures, e.g. temperatures of less than 100°C, may be most suitable for solution electro-spinning).

[0060] The electrode fibres may therefore be produced as a mat using electrospinning; a process that uses high voltages to draw nanometre-scale fibres. These mats can be stacked / layered to form flexible batteries with micron scale thickness, or combined with stiff fibres such as carbon or glass to create structural batteries.

[0061] The collector may comprise, or have positioned thereon, one or more structural fibres, such that the deposited electrospun fibre makes contact with the one or more structural fibres, and binds thereto as the solvent of the sheath solution dries / as the melted polymer of the sheath melt sets / hardens, with the second polymeric material acting as binder.

[0062] The second polymeric material may be soluble in a solvent in which the first polymeric material is not soluble. The method may further comprise selectively dissolving one or more regions of the sheath so as to expose the core in selected regions.

[0063] The core electro-spinning medium (e.g. core solution or core melt) may have electrically conductive particles suspended therein as well as the electrode active particles, the conductive particles having a higher electrical conductivity than the electrode active material. The method of this third aspect may be used to produce an electrochemical device component according to the first aspect. The method of this third aspect may be used in the production of a battery, or other electrochemical device, according to the second aspect.

[0064] Features described with respect to one aspect of the invention may be applied to other aspects of the invention, mutatis mutandis. Aspects of the invention may be of particular utility in applications such as consumer electronics e.g. laptop and phone batteries that are part of the casing, and in composite structures where energy per unit mass is important.

[0065] The invention will now be described by way of example only with reference to the following Figures in which:

[0066] Figure 1 illustrates two cross-sectional views of an electrospun electrode fibre of various embodiments, at different levels of magnification;

[0067] Figure 2 is a scanning electron microscope (SEM) image of a portion of a mat made of fibres as shown in Figure 1;

[0068] Figure 3 is a lower-resolution SEM image of a less dense mat made of fibres as shown in Figure 1;

[0069] Figure 4 illustrates as electrode mat made of fibres as shown in Figure 1 with a current collector connected thereto;

[0070] Figure 5 illustrates a structural battery comprising an electrode made up of multiple electrode mats of fibres as shown in Figure 1;

[0071] Figure 6 provides a graph of potential vs. capacity for three cycles of an LFP cathode;

[0072] Figure 7 illustrates an apparatus for manufacturing electrospun fibres as described herein;

[0073] Figure 8 is a close-up cross-sectional view of a co-axial nozzle used as part of the apparatus of Figure 7;

[0074] Figure 9 provides four SEM images of fibres as described herein, with a PAN+PEG sheath present in 9A and 9B only (Figures 9C and 9D showing the fibre without the sheath for comparison);

[0075] Figure 10 provides cross-sectional images of an example cathode fibre (10A) in which separate electrode-active and electrically-conductive materials are provided, and an example anode fibre (10B) in which the electrode-active material is also electrically conductive; and Figure 11 is a flowchart illustrating a fibre formation method of various embodiments, using solution electro-spinning.

[0076] Figure 1 of the appended drawings illustrates a cross-section of an electrode fibre 1. The electrode fibre 1 is made by electrospinning, and may therefore be referred to as an electrospun electrode fibre 1. The fibre 1 is generally at least substantially cylindrical in shape, and comprises a core 2 surrounded by a sheath 3. As shown, the sheath 3 is coaxial with the core 2, and the fibre 1 may therefore be referred to as a coaxial electrospun fibre 1. In other embodiments, the core 2 may be offset from the centre of the sheath 3, for example to provide a thicker sheath layer on a side of the fibre 1 arranged to face an opposing electrode in use - the fibres 1 with a core 2 and sheath 3 may therefore be referred to more generally as bi-layer electrospun fibres 1.

[0077] In the embodiment shown, a diameter, D, of the fibre 1 is in the range from 600 nm to 2000 nm, more particularly from 1000 nm to 2000 nm, and more specifically of 1300±600 nm. In various embodiments, the fibre 1 may have a diameter of around 1 pm, although this may vary depending on factors including material selections and the intended product.

[0078] In the embodiment shown, a thickness, T, of the sheath 3 is in the range from 150nm to 600 nm, and more specifically is 300±150 nm. The diameter of the core is around 700 nm in this embodiment, and more generally may be in the range from 100 nm to 1700 nm. Core diameter, D — 2T, and sheath thicknesses, T, may vary in other embodiments, again depending on factors including material selections and the intended product.

[0079] The core 2 comprises particles 4 of an electrode active material - i.e. of a material which is suitable for use as an electrode (anode or cathode). This electrode active material 4 enables the fibre 1 to act as an electrode. In the embodiment shown, the electrode active material 4 is provided in the form of nanoparticles.

[0080] The particles 4 are held together by a first polymeric material 6. In the embodiment shown, the particles of electrode active material 4 are dispersed in a polymer matrix composed of the first polymeric material 6. In alternative embodiments, the first polymeric material 6 may be provided as a binder, coating the particles and connecting them together, rather than as a continuous matrix 6 in which the particles 4 are embedded.

[0081] The amount of polymeric material 6 in the core 2 may therefore vary significantly - for example from the core 2 being 60-80% polymer by mass, and optionally around 67% polymer by mass, when the first polymeric material 6 forms a matrix in which particles are embedded, but only 5-20% polymer by mass when the material 6 acts as a binder or “glue” cementing the particles together, rather than forming the bulk of the core 2.

[0082] The particles 4 may include particles of multiple different electrode active materials in some embodiments.

[0083] The choice of electrode active material 4 depends on whether the electrode is to be used as an anode or cathode, and on the desired electrode reactions. The electrode active material 4 of various embodiments may comprise Lithium (Li), Sodium (Na), and / or Potassium (K), generally in the form of a solid inorganic salt. In the embodiment being described, the electrode fibres 1 are intended to be used as a cathode 10 and the particles 4 are lithium-iron-phosphate (LFP) nanoparticles. It will be appreciated that other inorganic compounds, for example lithium titanate (LizTiCh), and optionally mixtures of inorganic compounds, may be used in other embodiments.

[0084] The core 2 shown in Figure 1 further comprises a conductive additive 5. In the embodiment shown, the conductive additive 5 is in the form of particles 5, and in particular nanoparticles of carbon black. Different conductive additives, for example reduced graphene oxide (rGO), graphene, one or more conductive polymers, and / or carbon nanotubes may be used in other embodiments. In some embodiments, the electrode active material 4 may itself be sufficiently conductive that no additional conductive material is needed. In some embodiments, the electrode active material 4 may itself be electrically conductive, but with an insufficient conductivity alone such that an additive 5 is used to boost the overall electrical conductivity. In other embodiments, the electrode active material 4 may be at least substantially electrically insulating, such that addition of a different conductive material 5 is necessary to provide any significant electrical conduction.

[0085] The conductivity and amount of the conductive additive 5 (and / or the electrode active material 4, in implementations in which the electrode active material 4 has non-negligible conductivity) may be selected such that the core 2 has a conductivity along its length of at least 0.01 Scm1, and optionally of at least 0.02, 0.04, 0.05, or 0.1 Scm1.

[0086] The first polymeric material 6 also holds the conductive additive(s) 5 in place; providing conductive pathways along the length of the fibre 1.

[0087] The first polymeric material 6 may be selected to be ion-conducting, so as to facilitate ions of an electrolyte solution reaching the electrode active material 4. Alternatively or additionally, the core 2 may be porous so as to allow a liquid electrolyte / electrolyte solution to reach the electrode active material 4.

[0088] The sheath 3 is formed from a second polymeric material. In the embodiment shown, no particles are embedded within the second polymeric material. In the embodiment shown, the sheath 3 is composed of a single, at least substantially uniform, polymeric material. Sheath composition may vary in other embodiments.

[0089] The second polymeric material 3 is electrically insulating, and may therefore act as a battery separator when the fibre 1 is in use as an electrode, electrically insulating the electrode material 4 of the core 2 from a counter electrode of the battery. The use of these coaxial fibres 1 therefore avoids the need for a traditional battery separator to electrically isolate the anode from the cathode, so reducing mass.

[0090] The sheath 3 is arranged to allow at least one of (i) an electrolyte (e.g. liquid electrolyte / electrolyte solution) to reach the electrode active material 4 of the core 2, and (ii) ions to travel between the electrolyte and core 2. The sheath 3 may therefore be porous, and / or may be ionconducting (although electrically insulating). For example, the sheath 3 may be made from a proton-conducting polymer. Porosity of the sheath 3 may be provided by including a solvent such as water in the sheath solution from which the sheath 3 is formed, and then allowing that solvent to slowly evaporate off.

[0091] The first 6 and second 3 polymeric materials are different in the example being described, and in particular are selected such that the second polymeric material 3 (which forms the sheath 3) is soluble in a solvent in which the first polymeric material 6 is not soluble. Regions of the sheath 3 may therefore be selectively removed (dissolved using the solvent) without damaging the core 2. This selective removal of the sheath 3 may be used to allow for electrical connection between the core 2 and a current collector 15, as described in more detail below with respect to Figure 4.

[0092] In some embodiments, the solvent may be water - for example, the sheath 3 may be water- soluble and the polymeric material 6 of the core 2 may not be water-soluble, or vice versa. Alternatively, the solvent may be an organic solvent. It will be appreciated that there may be a class of solvents in which the sheath 3 is soluble and the polymeric material 6 of the core 2 is not soluble, and that some solvents may dissolve both in some embodiments - the requirement in implementations with selectively-soluble sheaths 3 is simply that at least one solvent is known which dissolves the sheath 3 but not the core 2, so as to allow selective removal of the sheath 3 only.

[0093] In the embodiment being described, the first polymeric material 6, which may be descried as a core polymer binder, is polyacrylonitrile (PAN), and the second polymeric material 3, which forms the sheath 3, is polyvinyl acetate (PVA) or polyethylene oxide (PEO / PEG).

[0094] The core 2 of this example comprises active material particles 4 (which store energy) and conductive additive particles 5 (which allow for electron transport) embedded in a polymer binder 6, and the sheath 3 is a different polymer.

[0095] Other examples of pairs of suitable polymers for each (core 2 and sheath 3), and a suitable solvent to dissolve the sheath 3 only for each pair, are shown below in Table 1.

[0096] Table 1: Polymer and solvent options The same polymer can therefore be used as either core 2 or sheath 3, in various implementations. DMF, a very common electrospinning solvent, can be a suitable solvent for the processes described herein, depending on polymer choice. In addition, sustainable materials such as cellulose acetate can be used in some implementations.

[0097] The particle concentration in the core 2 is controlled to be above the percolation threshold, meaning that particles contact their neighbouring particles to form an electrically conductive network throughout the core / polymer matrix. In implementations in which the electrode active material 4 is electrically conductive, this conductive network may be provided partially or completely by the electrode active material 4. In implementations in which the electrode active material 4 is insufficiently electrically conductive, particles 5 of an electrically conductive material may provide some or all of the electrically conductive network.

[0098] In use in an electrochemical device, the sheath 3 acts as an ultrathin separator, creating an electrically-insulating barrier between the electrospun electrode and a counter electrode (which may or may not also be electrospun, in various embodiments). This thinness of the separator is advantageous as it reduces the mass and the volume of the battery, as well as reducing resistance to ion transfer (shorter diffusion pathway for ions to travel between the electrolyte and the electrode active material 4). A typical thickness of a battery separator is around 20 microns. An “ultrathin” separator as provided by a sheath 3 as described herein may be at least one order of magnitude thinner, and optionally may be around two orders of magnitude thinner; e.g. below 2 pm, and optionally thinner still - e.g. in the range from 150 nm to 450 nm, and optionally around 200 nm or 300 nm. Separator thickness using the present invention may therefore be around two orders of magnitude thinner than current standards.

[0099] The sheath 3 may be made from a suitable material to form a solid polymer electrolyte. Such a sheath 3 would act as both electrolyte and separator. Solid polymer electrolytes are normally avoided as their ionic conductance is low compared to liquid electrolytes. However, as the sheath 3 is thin, ionic conductance is unlikely to be the limiting factor in many implementations so rendering the use of such materials suitable. In electrochemical devices in which both electrodes are made of electrospun fibres, the total anode-cathode distance may be just the sheath thickness (if only one electrode’s fibres are sheathed) or twice the sheath thickness (if both electrodes have sheathed cores), and may still be less than the traditional separator thickness by around an order of magnitude.

[0100] A “solid polymer electrolyte” is an electrolyte where the polymer matrix is the ion transport medium. The sheath 3 may also be used as a solid composite electrolyte and / or gel polymer electrolyte. A “solid composite electrolyte” is an electrolyte wherein either: (i) the polymer matrix is the ion transport medium and has reduced crystallinity because of ceramic particles embedded in the matrix; or (ii) both the polymer matrix and the ceramic particles embedded in the matrix are the ion transport medium. A gel polymer electrolyte material could also work as a sheath 3 provided that appropriate polymers were chosen, e.g. longer chain and more interlinked polymers, as this would improve stiffness.

[0101] Examples of polymers that may be used in a solid polymer electrolyte, solid composite electrolyte, or gel polymer electrolyte and which are also suitable for electrospinning include: polyethylene oxide (PEO), polyacrylonitrile (PAN), and poly(methyl methacrylate) (PMMA). In such implementations, the polymer may be described as a “polymer host” of the electrolyte, and may contain one or more inclusions or additions such as salts and / or metallic or ceramic particles. The “polymer host” is simply a term used for a polymer that provides a matrix for any other materials (e.g. ceramic particles), in this context in the electrolyte.

[0102] In implementations using solid polymer electrolytes, there is no liquid electrolyte to enter pores within the core 2. An ionically-conducting polymer (whether or not the same polymer as that used to form the sheath 3) may therefore be incorporated into the core 2, as some or all of the first polymeric material 6, to provide ion conduction pathways through the polymer 6 in the core 2 to the embedded particles 4 within the core.

[0103] In some implementations, the polymer(s) for each of the core 2 and sheath 3 are selected to have different solubilities. The sheath polymeric material 3 being soluble in a solvent in which the core polymeric material 6 is insoluble allows the sheath 3 to be dissolved off in one or more selected areas, so as to provide an electrical connection to the core electrode 2.

[0104] The high surface area to volume ratio of the fibres 1 increases the volumetric current density as compared to traditional electrode designs, as it exposes more of the active material, providing more sites for ion transfer.

[0105] In general, a single electrode fibre 1 as described herein would not provide sufficient electrode active material 4 for use as an electrode 10 alone - a plurality of such fibres 1 is therefore generally used (although it will be appreciated that a single fibre 1 can have significant length, and that one fibre 1 could be used alone in some embodiments).

[0106] The plurality of fibres 1 may be at least partially aligned in a plane so as to provide a substantially flat layer, or mat, comprising the fibres 1. SEM images of two example mats are shown in Figures 2 and 3. Mat thickness and density of fibres 1 may be adjusted as desired, for example based on the electrolyte to be used in the electrochemical device (e.g. battery) and the selected materials.

[0107] A plurality of fibres 1 may therefore be provided as a mat, having a much greater extent in a plane than in a direction perpendicular to that plane. The plane may be flat or curved. In some implementations, the mat may exhibit alignment of the fibres 1 within the plane of the mat, such that the fibres extend generally in the same direction, optionally being at least substantially parallel to each other. Such an arrangement may facilitate provision of electrical interconnects 15 (as discussed in more detail below), if, for example, an interconnect along one edge of a mat of fibres as shown in Figure 4 could be guaranteed to cross all fibres of the mat. A spinning collector may be used during deposition of electrospun fibres 1 to draw out fibres 1 with an overall direction as opposed to a mat with at least substantially randomly oriented fibres within a plane - some direction / alignment of the fibres 1 within a plane may therefore be provided as part of the manufacturing process.

[0108] It will be appreciated that use of a large number of narrow fibres 1 to form an electrode 10 can provide a large surface area for electrode reactions within a small volume, with a minimal amount of electrode active material 4 being unavailable, so improving both gravimetric and volumetric energy densities.

[0109] To make an electrical connection between the electrode active material 4 / core 2 and an external circuit, an electrical contact 15, which may be referred to as a connector or current collector, is needed. Contact must be made with each core 2 when multiple fibres 1 are used, optionally in multiple places along each core 2 to improve current collection efficiency (especially in embodiments wherein the connector 15 has a higher conductivity than the core 2). Figure 4 illustrates a portion of a mat of fibres 1 with a single, linear, electrical connector 15 lying across the mat. Selective dissolution of the sheaths 3 may be performed along the intended line of the electrical connector 15, and the electrical connector 15 may then be held in place so as to contact each fibre 1 which crosses it. Any suitable technique known in the art may be used to bond or otherwise hold the fibres 1 to the connector 15, so providing reliable electrical connections. Multiple connectors 15 may be provided per mat of fibres 1, for example being regularly spaced along the mat.

[0110] A single layer, or mat, may be used as an electrode 10, or a multi-layer electrode 10 comprising multiple such mats may be provided as illustrated in Figure 5.

[0111] Figure 5 shows a battery 100 comprising two electrodes 10, 20 and an electrolyte matrix 30 surrounding the electrodes. One of the electrodes 20 is the anode and the other electrode 10 is the cathode. The first electrode 10 comprises a plurality of mats of fibres 1 as described above. The second electrode 20 comprises a plurality of layers of carbon fibres. The layers 10, 20 are interleaved such that anode and cathode layers alternate. An ion transfer path length through the electrolyte matrix 30, to get from one electrode to the other, may therefore be minimised.

[0112] When selecting an electrolyte, a key compatibility criterion is that the sheath polymer, and preferably also the core polymer, should be insoluble in the solvent(s) used in the electrolyte. Common solvents used in electrolytes are the organic carbonates; in particular ethylene carbonate (EC), but with propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) also being important. The fibre polymers may therefore be selected not to be soluble in these solvents.

[0113] Whilst the specific electrolyte used in the experiments described below - 0.4 M LiBoB and 0.6 M LiTf (Lithium Tritiate) in EC:PC (50:50 wt.%) - is not a widely-used electrolyte (selected here for ease of comparison to tests reported in the literature using similar electrolytes in structural batteries), the solvents used (EC:PC) are very typical.

[0114] Four layers are provided for each electrode 10, 20 in the embodiment 100 shown; it will be appreciated that this may vary in other embodiments. Further, the layers 10, 20 are flat in the embodiment shown, but it will be appreciated that another shape - e.g. a curved shape - may be adopted in other embodiments, for example to conform to the shape of a product into which the battery 100 is to be integrated.

[0115] In the embodiment shown in Figure 5, the carbon fibre layers 20 provide the anode 20 and the electrospun mats 10 provide the cathode 10. Structural supports / ties 16 hold the layers together. In alternative embodiments, the anode may be provided by the electrospun fibres 1 instead of the cathode, or both electrodes may be provided by (different sets of) the electrospun fibres 1, so allowing for the creation of a fully electrospun battery 100. The electrode active material(s) 4 may be selected as appropriate for the desired electrode reaction(s). The same approach described above for producing a cathode 10 can be used to produce coaxial electrospun anodes simply by swapping the cathode active material (e.g. the LFP cathode nanoparticles 4) for an anode active material (e.g. nanoparticles of LTO (lithium -titanium-oxide)). Coaxial electrospun cathodes and anodes can then stacked together to produce a fully electrospun battery 100.

[0116] It will be appreciated that in embodiments with both electrodes 10, 20 being formed from electrospun fibres 1, the separator sheaths on one electrode’s fibres may be redundant, as the sheaths on the other electrode’s fibres may suffice as a separator. In some embodiments, one electrode may be made of bi-layer (e.g. coaxial core 2 and sheath 3) fibres as described with respect to Figure 1, and the other electrode may be made from single-layer fibres 1, the fibres of this electrode effectively only having the core 2 (lacking a polymeric sheath). Removing the sheaths 3 from one electrode may reduce battery mass, and may also facilitate ion transport through the electrolyte between electrodes (removing a diffusion pathway). The sheath 3 may however play a useful role in surrounding and protecting the core 2 even if it is not needed as a separator, so may still be included.

[0117] Experimental trials demonstrated that the sheath 3 successfully insulated the fibre cores 2 of the electrode 10, providing a working battery 100 (in this case, with the electrospun fibre electrode 10 as the cathode and carbon fibres as the anode, as shown in Figure 5). An initial prototype showed a gravimetric capacity of 38 mAh g1(as compared to 165 mAh g1for current commercial batteries) despite the prototype battery having a much lower gravimetric loading of electrode active material in the cathode (17% of the fibre mat by mass, as compared to 80% of the commercial battery’s cathode by mass), demonstrating the effectiveness of the new electrode design. The capacity and active material fraction of the fibre cathode 10 could be increased significantly by increasing porosity of the core 2 (e.g. by reducing the amount of the first polymeric material 6 used - e.g. moving to use of a polymer as a binder rather than to provide a matrix), and tuning fractions of active cathode material 4 and conductive additive 5 in the core 2.

[0118] In these experimental trials, the coaxial electrospun cathodes 1 consisted of a PVA (PolyVinyl Alcohol - the second polymeric material 3) sheath and a core comprising 66.6% PAN (PolyAcryloNitrile - the first polymeric material 6), 19.8% LFP (lithium ferrophosphate - the active electrode material 4), and 13.2% CB (carbon black - the electrically conductive additive 5) - all percentages here being wt.%. These electrospun fibres 1 were placed directly against a lithium metal counter electrode 20 inside of a pouch cell and charged / discharged for 10 cycles, and their capacity and Coulombic efficiency (CE) were measured. This demonstrated the ability of the electrospun cathode 1 to reversibly store energy, and also that the cell does not short- circuit, showing that the sheath 3 is working as a separator and that dissolving away part of the sheath 3 to connect a current collector 15 was effective.

[0119] In addition to providing higher gravimetric capacities, the electrodes 10 as described herein may also facilitate the development of biodegradable batteries 100, so allowing batteries to be disposable with a reduced environmental impact. For example, the sheath 3 may be made from a Poly (Lactic Acid) - PLA - polymer, which is biodegradable. Any suitable sustainable polymer may be selected for the sheath 3 and / or the polymeric material 6 of the core 2. Na- based electrode active materials may be favoured over Li-based materials in such batteries as the higher environmental abundance of Na (e.g. in sea water) avoids a need for recovery of the active material. Batteries 100 may therefore be made more environmentally-friendly, for example by using biopolymers (e.g. ethyl cellulose) as the first and second polymeric materials 6, 3. In addition, using Na-ion electrodes (e.g. polyanion cathodes) and hard carbon anodes could allow for electrospun batteries 100 that can be easily recycled.

[0120] In additional or alternative embodiments, the electrospun electrodes 1 can be designed for use in structural batteries 100. In some such embodiments, the coaxial fibres 1 are electrospun directly onto a stiff structural fibre, such as a carbon fibre, or onto a mat of structural fibres, as described in more detail below. This structural fibre may be advantageous to the performance of the electrospun electrode 1, 10 - for example by providing additional conductivity as described below, and also provide structural reinforcement.

[0121] Figures 7 and 8 show an apparatus 1000 that can be used to make electrospun fibres 1 as described herein, and the flowchart of Figure 11 summarises a manufacturing method 1100 that may be performed using this apparatus 1000. The apparatus may be described as an electrospinner 1000. Figure 11 is specifically directed to solution electro-spinning for clarity, but it will be appreciated that the same principles can be used with melt electro-spinning.

[0122] The apparatus 1000 comprises a coaxial nozzle 1020 arranged to form coaxial streams of two solutions - a first solution to form the core 2 and a second solution to form the sheath 3. As shown in Figure 8, the coaxial nozzle 1020 has an inner chamber 1024 arranged to receive the core solution, C. The inner chamber 1024 is located centrally with respect to the nozzle 1020 in this arrangement, and is straight and substantially cylindrical in shape, having an inlet at its upper end (in the orientation shown), and an outlet at its lower end, adjacent to a tip 1022 of the nozzle 1020. The inner chamber 1024 has an initial, upper, portion arranged to receive the core solution, and then narrows in diameter to a second, lower, portion - the stream of the core solution leaving the nozzle 1020 may therefore be finer / thinner than the stream of the core solution entering the nozzle 1020. The core solution, C, flows downwardly through the nozzle 1020.

[0123] The coaxial nozzle 1020 also comprises an outer chamber 1026 arranged to receive the sheath solution, S. The outer chamber 1026 surrounds at least a lower part of the inner chamber 1024, and is again at least substantially cylindrical in shape. The sheath solution, S, is introduced from one side of the nozzle 1020 - this different position may allow more space for other apparatus components 1030, 1035 as described below than if both solutions S, C were introduced from above. The inner and outer chambers 1024, 1026 are coaxial / have concentric crosssections, so forming a coaxial flow arrangement for the two solutions S, C in use, with a layer of the sheath solution surrounding the stream of the core solution.

[0124] In the implementation shown, the walls of the inner chamber 1024 terminate before the tip 1022 of the nozzle 1020 - pressure, speed, miscibility, and viscosity of the solutions S,C may be considered and controlled so as to minimise or avoid mixing.

[0125] The nozzle 1020 has a tip 1022 through which the solutions are arranged to emerge, in a coaxial arrangement as controlled by the internal nozzle shape. The outer chamber 1026 narrows at the tip - flow speed of the solutions S, C is therefore increased by the narrowing pipe diameter, providing a relatively fast and thin stream at the nozzle tip 1022. For example, from an initial internal chamber diameter of 7 mm, the inner chamber 1024 may go down to a diameter of 1 mm - this narrower portion may be referred to as a core exit pipe, and its lower end may be referred to as a core nozzle. Similarly, the outer chamber 1026 may have an annular cross-section with a width of 1.5 mm and an outer diameter of 7 mm around the lower portion of the inner chamber 1024 (the inner chamber and its walls taking up the central 4 mm diameter), before narrowing to an internal diameter of only 1.5 mm in the tip 1022. The outer chamber 1026 therefore continues beyond the inner chamber 1024 and provides the nozzle tip 1022. In the example shown, the tip 1022 - which may also be described as the sheath exit pipe - has a height of around 4 mm and a wall thickness of around 1.5 mm. These dimensions are provided by way of example only; many different coaxial nozzle designs exist, and dimensions may be adjusted based on desired fibre thicknesses and flow rates.

[0126] In this example, the inner diameter of the core exit pipe is 1.0 mm and the inner diameter of the sheath exit pipe is 1.5 mm. This coaxial nozzle 1022 is a “middle nozzle” system, in that the exit point of the core nozzle 1022c is further up than the exit point of the sheath nozzle 1022. This allows the core solution to be enveloped in the sheath solution before reaching the extrusion point 1022. This may improve jet stability and allow a stable Taylor cone to form with a lower ratio of sheath solution than if the nozzles 1022c, 1022 had the same exit point. This coaxial nozzle 1020 may also be referred to as a double nozzle, as it has two solution exit points / extrusion points 1022c, 1022.

[0127] The apparatus 1000 is arranged to apply a voltage to a tip 1022 of the coaxial nozzle 1020 so as to initiate electrospinning of a fibre 1 from the nozzle 1020. The fibre 1 is coaxial as the nozzle 1020 causes the sheath solution to surround the core solution, so forming a coaxial stream.

[0128] Each solution is introduced into the coaxial nozzle 1020 via a respective syringe 1030c, 1030s. In the implementation shown, the syringe 1030c for the core solution is arranged vertically, directing the core solution downwardly into the coaxial nozzle 1020, and the syringe 1030s for the sheath solution is arranged horizontally, directing the sheath solution into the coaxial nozzle 1020 from the side. It will be appreciated that arrangements may differ in other implementations.

[0129] Each syringe 1030s, c is controlled by a corresponding pump 1035s, c. Two separate pumps 1035 are provided in the implementation shown; one for each syringe 1030. This may allow the feed rates of the two solutions to be individually controlled and adjusted. A single pump 1035 may be used to control both syringes 1030s,c in other implementations. The pump(s) 1035 allow feed rates of the core and sheath solutions into the nozzle 1020 to be adjusted so as to provide a desired core and sheath thickness of the resultant electrospun fibre 1.

[0130] Each syringe and pump 1030s, c, 1035s, c is supported by a corresponding support 1040s, c - the support 1040 may be adjustable so as to allow a position of the nozzle 1020 to be changed, for example to accommodate different locations of deposition surfaces onto which the electrospun fibre 1 is arranged to be deposited.

[0131] Extrusion of material is therefore controlled by two syringe pumps 1035, one for the core 2 and one for the sheath 3, in this implementation. In experiments performed, these syringes and pumps 1030, 1035 were assembled from 3D-printed parts based on an existing syringe pump design.

[0132] It will be appreciated that a melt (with suspended particles therein as applicable, in particular for the core melt) could be used for electro-spinning in place of either or both solutions in other implementations. In such embodiments, one or more heaters or heating elements (not shown) may be added to the apparatus 100 to bring the melt to a suitable temperature / viscosity for melt electrospinning. Temperatures of 150-350 °C, and optionally up to around 400°C depending on polymer choice, may be used to ensure a molten polymer, all the way to the tip of the spinneret. Temperatures for melt electrospinning can nonetheless be kept low enough not to oxidise or otherwise degrade the electrode-active material particles and / or the conductive particles suspended in the molten polymer. As polymer melts for electro-spinning are generally more viscous than electro-spinning solutions, an additional or extended mixing step may be introduced to ensure proper dispersion of the suspended particles when using melts (e.g. by sonication).

[0133] The apparatus 1000 comprises a collector 1050 arranged to receive the electrospun fibre 1 (whether made by melt electro-spinning or solution electro-spinning). In the example shown, the collector 1050 comprises a rotating drum 1050 - the fibre 1 is deposited onto the surface of the drum 1050. A motor and controller 1052 are provided to move the drum 1050 as desired. The collection surface may therefore move with respect to the nozzle 1020 during deposition. In some implementations, the nozzle 1020 may be moved with respect to the collector 1050 during deposition, instead of, or as well as, the collector moving with respect to the nozzle. For example, the collector 1050 was mounted on a platform with adjustable height, allowing for a nozzle to collector distance, H, of 10-20 cm for the tests described herein.

[0134] The nozzle tip 1022 and the collector 1050 are both connected to a power supply (not pictured) to allow a suitable voltage to be applied for electrospinning. In tests performed, a power supply capable of up to 40 kV was used, and the entire apparatus 1000 was placed inside an insulating box (Perspex® was used) for electrical safety reasons, and to prevent any materials from the electrospinning process contaminating other parts of the laboratory.

[0135] In some implementations, the collector 1050 simply provides a surface for the fibres 1 to land on and dry, and the fibres 1 / mat of fibres 1 can then be lifted off the surface for use. A non-stick surface may be used (properties selected as appropriate in light of knowledge of the sheath solution or melt, S). In other implementations, the collector 1050 may comprise, or have arranged thereon, one or more stiff structural fibres - e.g. an array of positioned structural fibres, or a mat of structural fibres (e.g. carbon fibres). When the electrospun fibres 1 are first deposited, the sheath may not be entirely dry / may still be sticky, and may act as a binder, sticking the electrospun fibre(s) 1 to the structural fibre(s). The coaxial fibres 1 may therefore be electro -spun directly onto a stiff structural fibre, such as a carbon fibre, or onto a mat of structural fibres, and this composite material may be removed from a collector 1050 / from a bed arranged to support the mat of fibres used as a collector 1050 once the sheath solution or melt, S, has fully dried / set.

[0136] In this implementation, the sheath 3 may or may not act as a separator. For example, if the structural fibres are electrically-insulating and / or if the structural fibres only sink into an outer portion of the sheath’s thickness but are still insulated from the core 2 by the rest of the sheath’s thickness, the sheath 3 may still serve to electrically insulate the core 2 from other components. The sheath 3 therefore acts as both binder and separator in such implementations. By contrast, if the structural fibres are electrically-conducting and in contact with the core 2 (i.e. passing all the way through the sheath 3 in at least some locations), the sheath 3 acts only as a binder and no longer (fully) electrically insulates the core 2 - an additional separator may be provided to shield one electrode from the other in some such embodiments. In such implementations, the high surface area of the carbon fibres may be beneficial to maintain high interconnectivity between the particles of the core 2 and a current collector.

[0137] In experimental tests, coaxial electrospun cathodes 1 deposited directly onto carbon fibre were found to have a reversible capacity of 68 mAh / g after 10 cycles, as compared to 19 mAh / g after 10 cycles when aluminium is used as a substrate, indicating that electrospinning onto carbon fibre is beneficial.

[0138] The method 1100 of manufacturing an electro-spun fibre 1 comprising a core 2 and a sheath 3 surrounding the core 2 described in Figure 11 may be performed using the apparatus 1000 described above. For implementations using solution electro-spinning, the method 1100 may comprise dissolving 1102c a first polymeric material in a solvent and suspending electrodeactive particles, and optionally also electrically conductive particles of a different material, in the solution to form a core solution, C. The method 1100 may comprise dissolving 1102s a second polymeric material in a solvent, the second polymeric material being electrically insulating, to form a sheath solution, S. The two solutions S, C may be made 1102 in either order, or simultaneously. The solvent used may be the same for each solution, or may differ. The solvent may be selected based on the polymeric material to be dissolved, and also on consideration of miscibility with the other solution.

[0139] The method 1100 then comprises feeding 1104c the core solution into an inner chamber 1024 of a coaxial nozzle 1020 and feeding 1104s the sheath solution into an outer chamber 1026 of the coaxial nozzle 1020. The two solutions S,C are generally fed to the nozzle 1020 simultaneously, so as to facilitate the formation of a coaxial flow of the two different solutions.

[0140] It will be appreciated that the same process can be applied for core and solution melts for melt electro-spinning, simply substituting the dissolving of polymeric materials with the melting of the polymeric materials.

[0141] The method 1100 then comprises applying 1106 a voltage to a tip 1022 of the coaxial nozzle 1020 to initiate electrospinning of a fibre 1 from the nozzle 1020. Knowledge of the electro-spinning media and feed rates may be used in selecting the voltage to apply 1106, amongst other parameters.

[0142] The method 1100 comprises controlling 1108 feeding rates of the core and sheath electrospinning media to provide a desired core and sheath thickness of the resultant fibre 1. The feed rates 1108 may be selected - e.g. by calculation or by trial and error - at any point during the process 1100, and may be dynamically adjusted during the process. The order of steps shown is by way of example only and is not intended to be limiting.

[0143] The electrospun fibre 1 is then deposited 1110 onto a collector 1050. The composition of the core electro-spinning medium is selected to provide electrical conductivity along the length of the electrospun fibre 1, whether or not that conductivity is provided wholly or in part by the particles of the electrode-active material (a conductive additive 5 may be used in addition).

[0144] In forming the core solution or melt, C, the percolation threshold may be considered - the loading of conductive particles may be set to be equal to or above the percolation threshold - a conductive network is formed once the conductive particle loading is above the percolation threshold (noting that both the conductive additive and the electrode active material may be classed as conductive particles in implementations in which the electrode active material itself has an acceptable conductivity). The percolation threshold can be determined using standard techniques; typically based mainly on particle volume but optionally also on one or more other factors such as the particle shape and the medium the particles are suspended in.

[0145] Electrical conductivity is impacted by conductive particle diameter and conductive particle volume fraction in the mixture used to form the core. If optimising the electro-spinning medium to be very close to the percolation threshold, then the interaction between the particles and the polymer may be important. In the example implementation described herein, concentrations were selected to be comfortably above the percolation threshold. By contrast, ionic conductivity is impacted by active material particle fraction in the mixture, and, for implementations using liquid electrolytes, also by porosity of the polymer (as pores can fill with liquid electrolyte). In some implementations, the polymer(s) used to form the core (the first polymeric material) may also be ionically conductive. Any of these conductivities can be the limiting factor, depending on system parameters.

[0146] Whilst the core 2 was around 17% electrode active material in the proof-of-concept cell for which tests are described herein, the loading is likely to be increased, e.g. to at least 50% electrode active material by mass, for commercial applications.

[0147] As described above, the collector 1050 may comprise, or have positioned thereon, one or more structural fibres, such that the deposited electrospun fibre 1 makes contact with the one or more structural fibres, and binds thereto as the solvent of the sheath solution, S, dries (or equivalently as the melted polymer of the sheath melt solidifies on cooling, for melt electrospinning). The second polymeric material may therefore act as a binder.

[0148] In solution electro-spinning, manufacturing may all be performed at or near room temperature, although slightly elevated temperatures may be used for some stages in some implementations - for example, vacuum oven drying at 60 °C, and / or a sonicator may heat the solution(s) to a few degrees above room temperature when dissolving polymers / dispersing particles in the solvent. The maximum temperature during manufacturing is below 100 °C, and optionally below 80 °C, in many implementations. In solution electro-spinning, higher temperatures may be needed to ensure that the polymer melts are not too viscous. Nonetheless, manufacturing may all be performed below 500 °C, and optionally below 450 °C, 400 °C, 350 °C, 300 °C, or 250 °C.

[0149] The maximum temperature in use of the fibres 1 may be around 60 °C (typical maximum operating temperature for a Li-ion based battery).

[0150] Electrospinning can therefore be used to produce a fibrous mat 10 of controllable thickness, with each fibre 1 having a core 2 and a coaxial sheath 3. A rotating drum collector 1050 may provide some alignment of fibres 1 within a mat.

[0151] The second polymeric material may be selected to be soluble in a solvent in which the first polymeric material is not soluble. In such implementations, the method 1100 may further comprise selectively dissolving one or more regions of the sheath 3 so as to expose the core 2 in selected regions. These selected regions may be used for current collectors, to allow current to flow from the core 2 to an external circuit. This may provide a larger area for current collection contact than if an end of the fibre was used.

[0152] In implementations in which the electrospun fibre 1 is arranged to form an electrode of an electrochemical device, the core electro-spinning medium, C, may have particles of an electrically insulating electrode active material suspended therein as well as the conductive particles. In other such implementations, the electrode active material 4 may itself be electrically conducting - in some such implantations, no additional conductive particles 5 may be needed to provide sufficient conductivity along the fibre 1.

[0153] Figure 10 shows cross-sectional images of two such examples - the example cathode fibre la of Figure 10A comprises LFP nanoparticles 4 as the electrode active material, and conductive particles 5 of carbon black to boost the electrical conductivity of the fibre la. The proportion of conductive particles 5 within the core electro-spinning medium is chosen to be sufficient to reliably provide a continuous conductive path along the full length of the fibre la. The example anode fibre lb of Figure 10B is able to use carbon black as the electrode active material 4. As carbon black is also conductive, the electrode-active particles 4 also provide the conductive particles 5. Just a single particle type 4,5 is therefore used in this implementation.

[0154] The end-to-end conductivity of the anode fibre lb may therefore be higher than that of the cathode fibre la in at least some implementations (depending on the particles and particle density chosen).

[0155] In some implementations, the first polymeric material 6, which may be descried as a core polymer binder, is polyacrylonitrile (PAN), and the second polymeric material 3, which forms the sheath 3, is polyvinyl acetate (PVA) or polyethylene oxide (PEO / PEG). The electrospinning solvent selected for the manufacture of the coaxial fibre 1 comprising these materials was dimethyl sulfoxide (DMSO) - it will be appreciated that an electrospinning solvent should be selected for compatibility with the fibre materials. For one example of cathode fibre la production, polymers used included PAN (150,000 g mol"1), PVA (125,000 g mol"1), and PEG (600 g mol"1). LFP particles with a diameter of 80-100 nm were used as the electrode-active (cathode) material 4. Carbon black was used as a conductive additive 5. DMSO was used as a solvent for electrospinning.

[0156] The carbon black used herein had a min. - max. particle size range of 10-500 nm. Different particle sizes may be used in other implementations.

[0157] In various implementations tested, sheath solutions were made by dissolving PVA, or PAN and PEG, in DMSO. Core solutions were made by dissolving PAN in DMSO and then suspending LFP and carbon black particles or rGO particles in the solution. Sonication was used to disperse the particles in the solution.

[0158] The sheath and core polymeric materials were chosen such that water could be used as a solvent for removal of the sheath 3, without dissolving the core 2.

[0159] For the electrospinning process 1100 (in these examples, solution electro-spinning), the voltage was set to 10 kV and the tip-to-collector distance, H, set to 20 cm (in some instances, the tip-to-collector distance was set to 17 cm due to spacing constraints). A baseline flow rate was established by holding a small droplet of sheath solution, S, on the end of the nozzle 1022. Voltage was then increased in increments of 50 V until electrospinning started. The flow rate was then slowly raised in steps of 0.1 ml / hr until the Taylor cone was stable. The core flow rate was then increased (in steps of 10% of the sheath flow), and the sheath flow was reduced to maintain a stable Taylor cone. This was continued until the desired ratio of core: sheath was achieved.

[0160] In the implementations described herein, the determinations were done visually. The identification of the point of formation of a stable Taylor cone may be slightly subjective, but a general definition of what constitutes a stable Taylor cone was followed: no dripping, the cone was not growing or shrinking, and no intermittent ‘spraying’.

[0161] This core: sheath ratio used herein is based on volumes. In general, the ideal ratio would have as little sheath as possible whilst still providing sufficient sheath material around the core to be fully insulating. A 66:33 core to sheath volume ratio may be taken as a suitable target, as this is core-heavy whilst the one-third sheath can still reliably provide full insulation. Desired ratios may be varied based on material selections and specific details of the manufacturing technique, among other variables.

[0162] Two coaxial electrospun cathodes were spun, with the parameters shown in Table 2, where “CB” indicates carbon black particles. The ratio of LFP to CB in the core was 50:50 by mass in both cathode cores. Similarly, the ratio of PAN to PEG was 50:50 by mass in both cathode sheaths. Table 2: Coaxial cathode parameters. materials. Starting at 50:50, the flow rates of core and sheath were changed incrementally until a stable Taylor cone was formed. The choice of starting points in these tests is why the resultant ratios generally lie around 50:50. A typical range would be from 40:60 core: sheath at the low core volume end, to 66:33 core:sheath at the high core volume end. The low end is chosen because reducing core thickness / increasing sheath thickness further could cause conductivity to drop excessively. The high end is chosen because it is desirable to have as much core volume as possible without compromising the insulating properties of the sheath; a 66:33 core to sheath ratio is a reasonably conservative estimate of where this point might be (thinner sheaths may be possible whilst still providing sufficient insulation, but this was chosen as a limit to provide a safety margin). Anode and cathode fibres should be approximately the same in this regard, as the spinnability properties that change when the different materials are swapped in are minimal.

[0163] Although the assessments above are made in terms of a volume ratio between the core and sheath, other approaches may instead look at a minimum sheath thickness required to provide sufficient insulation (irrespective of core size). The minimum sheath thickness may define if the layer will work as an insulator / separator, whilst the ratio itself may be more important for overall optimisation of performance.

[0164] Removal of the sheath 3 was carried out by submerging an edge of the electrode 1 in water for 24 hours, followed by drying in a vacuum oven (at around 60 °C). Due to the high porosity of the fibre mat, a solvent front moved up the mat during the sheath removal process, removing sheath material further up the cathode than just the part submerged in water. These excess sections were cut away so that only 5 mm of unsheathed region was left for current collector attachment.

[0165] A piece of aluminium foil current collector was then attached to the exposed core region 2 of the coaxial fibres 1 using silver paint. The cathode 10 was assembled into a pouch cell, placing the cathode 10 directly against a lithium metal counter electrode, with LiBoB:LiTf (50:50 wt%) used as an electrolyte. A vacuum sealer was used to seal the sides of the pouch cell. Cycling was carried out with a C-rate of 0.2 C.

[0166] The potential vs. capacity plots for three cycles are shown in Figure 6. The plateau regions at 3.6 V during charge and 3.35 V during discharge are characteristic of LFP cathodes, providing further evidence that the electrospun cathode 1 is working as part of a battery. There is an overpotential of about 0.1 V on both the charge and discharge (i.e. the potential is 0.1 V higher / lower than where it ideally should be), which is likely due to the difficulty for Li+ions to migrate through the polymer bulk 6 in which the LFP particles 4 are embedded. This could be reduced by increasing the fraction of LFP particles 5 in the core 2.

[0167] Figure 9 shows SEM images of fibres 1 with a PAN+PEG sheath 3. Figures 9A and 9B show the fibres 1 at 5, OOOx magnification (Figure 9A) and at 50,000x magnification (Figure 9B), showing the smooth sheath surface 3.

[0168] Figures 9C and 9D show fibres with the core 2 made in the same way as for Figures 9A and 9D, but without the sheath - at 5, OOOx magnification (Figure 9C) and at 30, OOOx magnification (Figure 9D). The core surface 2 is rough - being made up of a large number of individual particles 4,5 bound together with a polymeric binder. The volume of the core 2 provided by the particles 4,5 may therefore be much higher than the volume provided by the polymeric material.

[0169] In terms of example possible ranges of constituents (wt.%) for solutions and for finished fibres, for the solutions used in electrospinning ranges of 10-30% solid material (polymer and electrode-active and / or conductive particles) in the solvent are likely. For the finished fibres, the limiting factor is generally how high the particle content of the solution can be whilst still being spinnable. The active material mass ratio can potentially be very high due to high density.

[0170] It will be appreciated that the embodiments described in detail herein are given by way of illustrative example only, and not intended to be limiting. The scope of the invention is to be limited only by the appended claims.

Claims

CLAIMS1. An electrochemical device component comprising at least one electro-spun fibre comprising a core and a sheath surrounding the core, wherein: the core comprises particles of an electrode active material held together by a first polymeric material, and wherein the core is electrically conductive along its length; and the sheath is formed from a second polymeric material, the second polymeric material being electrically insulating, and wherein the core of the at least one electro-spun fibre is arranged to provide a first electrode of an electrochemical device.

2. The electrochemical device component of Claim 1, wherein the second polymeric material is soluble in a solvent in which the first polymeric material is not soluble, and wherein optionally the solvent in which only the second polymeric material is soluble is water.

3. The electrochemical device component of Claim 1 or Claim 2, wherein, in use, the sheath of the at least one electro-spun fibre is arranged to act as a separator between the first electrode and a counter electrode of the electrochemical device, and optionally wherein the sheath is also arranged to act as an electrolyte of the electrochemical device.

4. The electrochemical device component of any preceding claim, wherein the or each fibre has a diameter in the range from 1000 nm to 2000 nm, and optionally of around 1300 nm.

5. The electrochemical device component of any preceding claim, wherein the or each fibre has a sheath thickness in the range from 150 nm to 450 nm, and optionally of around 300 nm.

6. The electrochemical device component of any preceding claim, wherein the core comprises electrically conductive particles held together by the first polymeric material, the conductive particles being held in contact with each other so as to provide a conductive path along the length of the core, and wherein optionally the conductive particles are electrode active, such that the conductive particles are the particles of an electrode active material.

7. The electrochemical device component of Claim 6, wherein the conductive particles have a conductivity of at least 0.01 S / cm, and optionally of at least 0.02 S / cm or 0.05 S / cm.

8. The electrochemical device component of Claim 6 or Claim 7, wherein the conductive particles have a diameter in the range from 40 to 500 nm, and optionally from 40 to 100 nm, or from 80 to 100 nm.

9. The electrochemical device component of any of Claims 6 to 8, wherein the conductive particles comprise carbon black, graphene, or graphene oxide, and wherein optionally the electrode is an anode.

10. The electrochemical device component of any preceding claim, wherein the core comprises conductive particles in addition to particles of an electrode active material, the conductive particles having a higher electrical conductivity than the electrode active material, and the particles all being held together by the first polymeric material.

11. The electrochemical device component of Claim 10, wherein the electrode active material comprises an inorganic solid including at least one of Sodium, Potassium, Magnesium, and Lithium, and wherein optionally the electrode is a cathode.

12. The electrochemical device component of Claim 10 or Claim 11, wherein the electrode active material and the conductive particles are each provided in the form of nanoparticles.

13. The electrochemical device component of any preceding claim, wherein the particles of an electrode active material have a diameter in the range from 40 to 500 nm, and optionally from 40 to 100 nm, or from 80 to 100 nm.

14. The electrochemical device component of any preceding claim, comprising a plurality of the electro-spun fibres, and wherein the plurality of fibres is provided as a mat, having a much greater extent in a plane than in a direction perpendicular to that plane, the mat optionally exhibiting alignment of the fibres within the plane such that the fibres extend generally in the same direction.

15. An electrochemical device comprising at least two electrodes, and wherein at least one of the electrodes, and a separator arranged to electrically insulate one electrode from the other one or more electrodes, is provided by an electrochemical device component as claimed in any of Claims 1 to 14.

16. The electrochemical device of Claim 15, wherein each electrode of the electrochemical device comprises a separate plurality of the electro-spun fibres.

17. The electrochemical device of Claim 16, wherein each plurality of electro-spun fibres comprises fibres aligned to be at least substantially planar, and wherein the electrochemical device is formed from multiple such pluralities of aligned fibres stacked together within an electrolyte matrix, and optionally wherein the electrochemical device is flexible.

18. The electrochemical device of any of Claims 15 to 17, further comprising electrical connectors for each electrode, and wherein the material of the sheath of each fibre of the plurality of fibres is removed in an area to allow electrical connection between the core material and the electrical connector for the respective electrode.

19. The electrochemical device of any of Claims 15 to 18, wherein the sheaths of the fibres form the only separator(s) of the electrochemical device.

20. The electrochemical device of any of Claims 15 to 19, wherein the electrochemical device is a battery, and the or each plurality of electro-spun fibres is combined with structural fibres so as to form a structural battery, and wherein optionally the structural fibres are carbon fibres.

21. The battery of Claim 20, wherein at least one of the following applies:(i) the electro-spun fibres are formed on or around a structural fibre;(ii) the plurality of electro-spun fibres is formed as a mat and stitched to a structural fibre layer;(iii) the plurality of electro-spun fibres is formed as a mat and bonded to a structural fibre layer using a resin or adhesive.

22. A method of manufacturing an electro-spun fibre for use as an electrode of an electrochemical device, the electro-spun fibre comprising a core and a sheath surrounding the core, the method comprising: suspending particles of an electrode active material in a solution or melt of a first polymeric material to form a core electro-spinning medium; dissolving or melting a second polymeric material, the second polymeric material being electrically insulating, to form a sheath electro-spinning medium; feeding the core electro-spinning medium into an inner chamber of a coaxial nozzle; feeding the sheath electro-spinning medium into an outer chamber of the coaxial nozzle; applying a voltage to a tip of the coaxial nozzle to initiate electrospinning of a fibre from the nozzle;controlling feeding rates of the core and sheath electro-spinning media to provide a desired core and sheath thickness of the fibre; and depositing the electrospun fibre onto a collector, and wherein the composition of the core electro-spinning medium is selected to provide electrical conductivity along the length of the electrospun fibre.

23. The method of Claim 22, wherein solution electrospinning is used to form the fibre, and wherein: the forming of the core electro-spinning medium comprises dissolving the first polymeric material in a solvent and suspending the particles of the electrode active material in the solution to form a core solution; and the forming of the sheath electro-spinning medium comprises dissolving the second polymeric material in a solvent to form a sheath solution.

24. The method of Claim 22 or Claim 23, wherein the collector comprises or has positioned thereon one or more structural fibres, such that the deposited electrospun fibre makes contact with the one or more structural fibres, and binds thereto as the sheath electro-spinning medium dries, with the second polymeric material acting as binder.

25. The method of any of Claims 22to 24, wherein at least one of the following applies:(i) the second polymeric material is soluble in a solvent in which the first polymeric material is not soluble, and the method further comprises selectively dissolving one or more regions of the sheath so as to expose the core in selected regions; and(ii) the core electro-spinning medium has electrically conductive particles suspended therein as well as the electrode active particles, the conductive particles having a higher electrical conductivity than the electrode active material.

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