Method for manufacturing a component for a secondary battery

Abstract

The present disclosure provides a method for manufacturing a component of a secondary battery, a component for a secondary battery, and a battery cell comprising the component. The method comprises: providing an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, wherein a metal anode is provided as a layer of electrode active material on the 3D current collector, and wherein the metal anode comprises an alkali metal or an alkaline earth metal; coating the 3D current collector with a polymer, wherein the polymer is capable of at least partially conducting ionically and / or forming a gel, and / or forming a porous structure; and contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to an ionically conductive gel or to form an ungelled porous structure which at least partially holds the liquid electrolyte.

Description

[0001] METHOD FOR MANUFACTURING A COMPONENT FOR A SECONDARY BATTERY

[0002] FIELD OF THE INVENTION

[0003] This present invention relates to a method for manufacturing a component for a secondary battery, a component for a secondary battery, and a battery cell comprising the component.

[0004] BACKGROUND

[0005] In the field of battery technology, the development of efficient and durable electrodes is important. Newer electrodes for secondary batteries, such as alkali metal (e.g. Na, K and Li) batteries, although providing higher energy capacity, often face performance challenges due to uncontrolled growth of dendrite structures, unstable solid electrolyte interface (SEI) formation and excessive volume change of the metal anode over time.

[0006] To mitigate against some of these issues, there has been a growing interest in recent years to use three-dimensional (3D) electrodes in battery manufacture. 3D electrodes comprise a current collector substrate having a base with a plurality of electrically conductive protrusions extending from it. The protrusions increase the surface area of the electrode, which can enhance performance by providing more active sites for electrochemical reactions. Furthermore, the higher surface area leads to lower overpotential, higher aerial capacity and lower local current density, which can reduce the growth of dendrites. However, the larger anode-electrolyte interface also results in more solid electrolyte interphase (SEI) layer being formed and more lithium and lithium salts being consumed. Strategies to mitigate dendrite formation include using electrolytes with high salt concentration, and charging batteries at high temperatures and / or at lower current densities, but such strategies are not beneficial for high performance cell design.

[0007] A further problem associated with 3D alkali metal electrodes applied in liquid-based battery cells is that there is no hard pressure on the side walls and the valley of the 3D structure. The application of a stack pressure is beneficial for the density and conformality of the plated metal and the absence of such pressure on the side walls and the valley of the protrusions may reduce the performance of electrode material at those locations during battery cycling. For the avoidance of doubt, the term "hard pressure" as used herein with reference to the stack pressure refers to pressure exerted by a "hard" (i.e. at least semisolid) material. For instance, although liquid electrolytes exert hydrostatic pressure on the side walls (for instance, when compressed by the cell stack pressure), dendrites / porous SEI can still grow through a liquid electrolyte as liquid is not a hard / solid material and so does not form a barrier to dendritic growth. Furthermore, a liquid electrolyte may displace while under pressure, because of the inherant flowability of liquid materials. Where this happens the resultant pressure on the side walls and valley becomes zero, facilitating dendrite formation.

[0008] In addition, alkali metal electrodes are highly reactive and in a pristine state they can only be handled in exceptionally dry environments. Often thin inorganic passivation layers are applied to allow handling during production.

[0009] Therefore, there is a need for an electrode component and a method for manufacturing an electrode component for use in alkali metal batteries that alleviates some of the above problems at least to some extent, preferably without negatively affecting the performance of the batteries.

[0010] SUMMARY OF THE INVENTION

[0011] In accordance with a first aspect of this invention, there is provided a method for manufacturing a component for a secondary battery, the method comprising: i) providing an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, ii) coating the 3D current collector with a polymer wherein the polymer is capable of at least partially conducting ionically and / or forming a gel, and / or forming a porous structure, and iii) contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to an ionically conductive gel or to form an ungelled porous structure which at least partially holds the liquid electrolyte.

[0012] In some embodiments, the component may comprise a metal anode comprising an alkali metal or an alkaline earth metal. The metal anode may be provided as a layer of electrode active material on the surface of the 3D current collector. The polymer may be coated over the surface of the metal anode.

[0013] In other embodiments, the component may be anode-less, in which case it does not comprise anode electrode active material. The method may further comprise coating the 3D current collector, or anode electrode, with a monomer and polymerising the monomer to form the polymer. The polymerising may be initiated by UV or heat.

[0014] The method may comprise coating the 3D current collector or anode electrode with a solution comprising the monomer, drying the 3D current collector to remove solvent, and polymerising the monomer to form the polymer. The solution may comprise a volatile solvent which can be evaporated to leave a residue of the monomer on the surface of the 3D current collector or anode electrode. The residue may then be polymerised to form the polymer coating.

[0015] In accordance with a second aspect of this invention, there is provided a method for manufacturing a component for a secondary battery, the method comprising: i) providing an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, ii) heating a polymer to provide an at least partially molten polymer, and iii) coating the 3D current collector substrate with the at least partially molten polymer.

[0016] In some embodiments, the component may comprise a metal anode comprising an alkali metal or an alkaline earth metal. The metal anode may be provided as a layer of electrode active material on the surface of the 3D current collector. The polymer may be coated over the surface of the metal anode.

[0017] In other embodiments, the component may be anode-less, in which case it does not comprise electrode active material on the anode electrode.

[0018] In embodiments of the first and second aspects where the metal anode is present, the metal anode forms a highly reactive, lightweight electrode on the surface of the 3D current collector. It has a high theoretical capacity and energy density, and provides significantly improved performance over conventional anode materials, such as graphite. By forming the metal anode into a three-dimensional shape (typically in the form of a foil) and coating the surface of the metal anode with a polymer, some of the common drawbacks associated with conventional 2D metal anodes, such as dendritic formation and low cycling performance, can be overcome. The higher surface area of the 3D structured anode (compared to a 2D anode) lowers the specific current density. This reduces and slows down interfacial degradation and dendrite growth. Some of the advantages of the polymer coating are that it makes ion flux during cycling more uniform, it protects the metal anode during handling / transport and from impurities during cycling, it is more coordinated to electrolyte solvents which reduces its interaction with the metal anode, and it applies a degree of mechanical pressure on the metal anode.

[0019] Unlike traditional layered graphite anodes in lithium-ion batteries, metal anodes do not intercalate metallic ions or particles. Instead, their operation is based on the plating and stripping of M+ions at the anode, rather than the intercalation and de-intercalation process observed in graphite anodes during charging and discharging. A problem associated with metal anodes is that they tend to be less uniform in plating and stripping than conventional anodes. However, in the present invention, the inclusion of a polymer coating can help to mitigate against this problem.

[0020] The alkali metal may be selected from the group consisting of lithium, sodium, potassium and rubidium, preferably from lithium and sodium, or more preferably may be lithium. The alkaline earth metal may be selected from the group consisting of magnesium, calcium and strontium, preferably from magnesium and calcium, or more preferably may be calcium. In some preferable embodiments, the metal anode may comprise lithium metal.

[0021] The methods of the first and second aspects produce a component for a secondary battery which contains a 3D current collector having a polymer coating, which is typically several microns thick. The polymeric layer enforces a stack pressure on the side walls and the valley of the 3D current collector during cell operation, whilst at the same time acting as a protective passivation layer during manufacturing and handling of the electrode and battery cell. The presence of the polymeric layer, provides for a stack pressure which is a "hard pressure" as described above. The hard pressure may be provided by the polymer which is generally a solid or semi-solid material; the terms "solid" and "semi-solid" having their usual meanings in the art. It will often be the case that the polymer will form a gel, which like other semi-solids provides for some flow of the material, whilst maintaining shape. Where the polymer is semi-solid, this provides for conditions where the dendrites / SEI are unable to penetrate the electrolyte. In addition, the higher viscosity of the semi-solid reduces flowability at the electrode / electrolyte interace, ensuring that the stack pressure is maintained.

[0022] In the first and second aspects, the 3D current collector substrate may comprise a plurality of electrically conductive protrusions extending from two opposite sides of the base of the substrate. A metal anode may be provided as a layer of electrode active material on the 3D current collector on both sides of the base. The method may comprise coating the metal anode on both sides of the base with the polymer. The polymer may be alkali / alkaline earth metal ion conductive so that alkali / alkaline earth metal ions are able to pass through it during cell operation.

[0023] The method offers a way to coat the 3D current collector (CC) substrate with the polymer. In some embodiments, the coating may be applied to the 3D structure via melt coating which ensures that the entire surface, including the areas between the protrusions, is fully covered. The polymer coating reduces material loss by minimizing SEI formation during initial and subsequent plating / stripping cycles. It also limits reactions between the electrolyte solvent and substrate by reducing the solvent access and decreases substrate corrosion by reducing contact between salt anions (e.g., -FSI-, -TFSI-) and the substrate.

[0024] The polymer may be in the form of solid or viscous liquid and may be compressible and elastic so that it is able to adapt to volume changes of the anode during cycling. The polymer may be selected to have strong adhesion to the 3D current collector to ensure that the polymer maintains conformational interfacial contact throughout the cycling process, regardless of volume fluctuations. Additionally, it can be beneficial if the polymer has sufficient ionic conductivity to enable the transfer of ions, particularly alkali / alkaline earth metal ions, between the electrolyte and electrode to facilitate battery operation.

[0025] As noted above, the polymer may be capable of forming a gel, although in some instances there may be components of the polymer which are ungellable. The polymer may be ungellable but porous so that it can hold liquid within its pores and thereby achieve ionic conductivity (where it may act as a porous separator). In some embodiments the ungelled porous polymer, may be contacted with a liquid electrolyte that optionally could contain monomers, initiators and cross-linkers, which may then form a thermally cross-linked gel inside the pores (e.g. upon subsequent activation by heating, such as at 70 °C for 2 hours, or by exposure to radiation).

[0026] The method may further comprise contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to a gel, in many instances the gel will be an ionically conductive gel. During cell assembly, when liquid electrolyte is introduced into the cell, the polymer layer makes contact with and may absorb the electrolyte at least to some extent. This interaction may lead to the polymer becoming gelated or gel-like and thus often ionically conductive. In this process, it may be that a part of the liquid electrolyte, for instance the electrolyte close to or in direct contact with the polymer layer of the anode (anode electrolyte) interacts with the coating to form part of the gel, such that at least a portion of the electrolyte becomes gelated or gel-like. This will typically be the anode electrolyte, and it will generally be the case that where the anode electrolyte becomes gelated or gel-like, the rest of the electrolyte in the battery cell (the non-anode electrolyte) remains in liquid or hybrid form. For instance, the anode electrolyte, namely the electrolyte in contact with the polymer coating, may form a gel, whilst the electrolyte inside the separator and inside the pores of the cathode may remain as a liquid. As such, the electrolyte, or more specifically the anode electrolyte, may comprise a gel, often an ionically conductive gel.

[0027] It will often be the case that the anode electrolyte will form a layer of depth in the range 2 - 40 pm, often 5 - 30 pm or 10 - 20 pm.

[0028] The method may further comprise depositing an electrode active layer on the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. The anode electrode may preferably comprise lithium, sodium, magnesium, potassium, calcium or combinations thereof. Often the anode electrode will comprise lithium, sodium, silicon, potassium or combinations thereof.

[0029] The method may further comprise actively depositing an artificial solid electrolyte interphase layer (ASEI) on the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. The ASEI is in addition to the naturally occurring SEI layers described above. A stable ASEI layer can restrict dendrite penetration from the metal anode, reduce electrolyte reactions with the anode, and ensure consistent homogeneous stack pressure in the 3D electrode. It can also adjust the surface energy of the 3D current collector to enhance polymer wettability.

[0030] The polymer may comprise a laminate structure having a plurality of layers. The laminate structure may comprise a hard first layer proximate to or in contact with the 3D current collector, and a soft second layer distal from the 3D current collector. The soft second layer may be capable of forming an ionically conductive gel when contacted with a liquid electrolyte to a greater extent than the hard first layer so that the outer second layer in contact with the solvent is selectively gelated and the hard first layer in contact with the 3D current collector remains largely or entirely ungelated. In view of this, it is possible that in some examples, the hard first layer is formed from a material which cannot form gels, such that the hard first layer is ungellable.

[0031] The laminate structure may comprise organic and / or inorganic layers. Typically, the laminate structure will include an inorganic layer within the structure. The surface layer (i.e. outer second layer) may be organic or inorganic. The polymer may fill from 5% to 100% of a volume of the 3D current collector substrate, wherein the volume is defined by a space between the base, side walls of the protrusions and outer ends of the protrusions. The polymer coating may fill the spaces around the protrusions as well as covering the tops and side walls of the protrusions.

[0032] The polymer coating may have a gradient hardness in which a polymer surface closest to the 3D current collector substrate is harder than a polymer surface furthest away from the 3D current collector substrate. By having a harder polymer layer closer to the 3D surface, dendrite penetration during battery cycling may be minimised.

[0033] The polymer coating may have a gradient gelability, wherein a polymer surface closest to the 3D current collector substrate has a lower percentage swelling than a polymer surface furthest away from the 3D current collector substrate. For instance, the gradient gelability may arise as the polymer surface closest to the 3D current collector substrate has a percentage swelling of <50% (in the range 0.5% - 49.5%, often in the range 10% - 30%), whereas the polymer surface furthest away from the 3D current collector substrate has a percentage swelling >50% (in the range 50.5% - 200%, often in the range 70% - 80%). This differential in swellability (a lower solvent uptake capacity of the polymer where this is close to the 3D current collector substrate relative to where it is further away), contributes to the hard pressure on the 3D current collector surface.

[0034] The gradient gelability is often achieved by stepwise melting of polymer layers of different composition, or through the provision of a stack of polymer layers of differing compositions (two or more layers e.g. 2, 3, 4 or 5 layers) and applying a single stage melting process. As polymer layers of different compositions will generally have different physical properties, and can be selected for those differences in physical properties (such as swellability), the gelability of each layer can be controlled, and the desired gradient gelability provided.

[0035] The polymer may comprise a homopolymer formed from any one of the polymers below, mixtures of these homopolymers or co-polymers comprising a mixture of polymer types, wherein the polymer types include polycarbonate, polyester, polyolefins (e.g. polyethylene or polypropylene), polyether (e.g. PEO), polyvinylidene fluoride, polyurethane, polyrotaxane, polyphosphazene and polysiloxane. Optionally the polymers are present as binary, ternary or tertiary co-polymers comprising one or more of the polymer types, such that they may comprise one or more of the listed polymers with other polymer types, or they may consist primarily of one or more of the listed polymers (i.e. polycarbonate, polyester, polyolefins (e.g. polyethylene or polypropylene), polyether (e.g. PEO), polyvinylidene fluoride, polyurethane, polyrotaxane, polyphosphazene and polysiloxane).

[0036] The polymer coating may have a thickness that progressively increases away from the base, or progressively decreases away from the base. For example, the coating on terminal ends of the protrusions may be thicker than the polymer coating on the base, and / or the polymer coating on the side walls of the protrusions may progressively increase from the base to the terminal ends. This arrangement may assist in minimising dendrite formation at the ends of the protrusions.

[0037] The polymer coating may be porous or non-porous.

[0038] The polymer coating may have a gradient porosity in which a lower porosity is present on a surface closest to the 3D current collector substrate and a higher porosity is present furthest away from the 3D current collector substrate. The gradient porosity can be achieved through a variety of methods, including, for instance melting the porous polymer, polymer phase inversion, or the selection of porogens of different types and applying these to different regions of the polymer coating. By having an arrangement in which a lower porosity is present closest to, and a higher porosity is present furthest away from, the 3D substrate / anode, dendrite penetration can be reduced. Furthermore, a higher porosity further away from the 3D substrate / anode may provide for improved alkali / alkaline earth metal ion conduction.

[0039] The polymer may be elastomeric, thermo-plastic, a thermoset-plastic, or a combination thereof. In addition, the polymer may be cross-linkable (either at monomer level or of the pre-formed polymer), and cross-linking may be facilitated by the application of a radiation source, such as UV light, by oxidation, or free radical reactions. Generally coatings containing cross-linked polymers will have greater rigidity than non-cross-linked polymer coatings.

[0040] The polymer may comprise one or more metal-salts, solvents, additives, liquid metals (such as Galistan™) and / or particles, for enhancing the polymer's alkali / alkaline earth metal ion conductivity.

[0041] The method may further comprise applying a wetting layer to the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. Many polymeric materials, particularly polyolefins and their derivatives, present a low surface energy which can cause them to have poor wettability, limiting processes such as adhesive bonding or metallizing. The application of a wetting layer may therefore strengthen bonding between the polymer layer and the substrate.

[0042] In some examples, the method will comprise applying both an electrode active layer and a wetting layer, in other examples the wetting layer may be present without the electrode active layer. Where an electrode active layer is present, for instance in metal batteries, the cycle life of the battery can be improved. However, in the absence of the electrode active layer, such that the wetting layer is present alone, the energy density of the battery is improved and the manufacturing processes simplified.

[0043] The method may further comprise applying a seeding layer to the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. The seeding layer may enable uniform nucleation of an alkali metal (e.g. lithium, potassium or sodium metal) or alkaline earth metal anode on the current collector during charging.

[0044] In accordance with a third aspect of this invention, there is provided a component for a secondary battery, the component comprising: i) an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, ii) a metal anode provided as a layer of electrode active material on the 3D current collector, wherein the metal anode comprises an alkali metal or an alkaline earth metal, and iii) a polymer coating on the electrode active material.

[0045] In accordance with a fourth aspect of this invention, there is provided a component for a secondary battery, the component comprising: i) an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, and ii) a polymer coating on the 3D current collector substrate; wherein the component does not comprise anode active material.

[0046] In the third and fourth aspects, the 3D current collector substrate may comprise a plurality of electrically conductive protrusions extending from two opposite sides of a base of the substrate. The 3D current collector or anode active material may be coated with the polymer on both sides of the base. In accordance with a fifth aspect of this invention, there is provided a battery cell comprising a component as defined above and a liquid electrolyte or a hybrid (i.e. semisolid) electrolyte. As described above, it will often be the case that once the electrode is assembled into a battery, that an ionically conductive gel is formed on and / or within the polymer coating.

[0047] In accordance with a sixth aspect of this invention, there is provided a battery cell comprising a component manufactured according to the above defined method, and a liquid electrolyte or a hybrid electrolyte.

[0048] Unless otherwise stated, each of the integers described may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the invention preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

[0049] As used herein and in the accompanying claims, unless the context requires otherwise, "comprise" or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0050] The term "consist(s) / (ing) essentially of", with respect to the components of a composition or mixture, means the composition or mixture contains the indicated components and may contain minor additional components in an amount less than 1 wt% based on the total weight of the composition or mixture, and provided that the additional components do not substantially alter the reactivity of the composition or mixture.

[0051] Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0052] In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about". As used herein, the term "about" means that the stated value can vary by ± 10%. For example, about 90 wt% means 90±9 wt%, and about 0.1 wt% means 0.1±0.01 wt%. When used with reference to a range, the term "about" applies to all values in the range.

[0053] "Alkali metal" refers to the elements Li, Na, K, Rb, Cs and Fr, and in particular, Li, K and Na. The alkali metal can be in ionic form or neutral form.

[0054] "Alkaline earth metal" refers to the elements Br, Mg, Ca, Sr, Ba, Ra, and in particular Mg and Ca. The alkaline earth metal can be in ionic form or neutral form.

[0055] "Hardness" refers to a material property that measures a polymer's resistance to indentation, scratching, or deformation when a force is applied. Hardness testing can be used to characterise the mechanical and physical properties of polymers and to rank the hardness of different polymers. The hardness value of a polymer can be used to predict how well the polymer will recover after being indented. The most common methods for measuring polymer hardness are the Rockwell and Shore (durometer) hardness tests. The Shore A and Shore D scales are often used in to compare the hardness of polymers. The Shore A scale ranges from 0 to 100, Shore D ranges from 40 to 90, and Rockwell ranges from 50 to 150, with higher values indicating a harder material. Standard protocols for testing the hardness of polymers are ASTM D785, ASTM D2240, ISO 868, ASTM D2240 and ASTM D1414.

[0056] "Gelability" refers to the extent to which a polymer is able to form a gel upon contact with a liquid. Solvent uptake tests are typically used to measure the gelability of a polymer, and are the measure used herin with reference to this feature.

[0057] "Electrode" refers to a component of a battery at which an electrochemical reaction occurs, such as an anode (negative electrode) and a cathode (positive electrode). The electrode structure can include a current collector, electrode active material, coating materials, and optionally one or more additional layers.

[0058] The present invention will be better understood in light of the following examples and the accompanying figures, which are given in an illustrative manner only and should not be interpreted in a restrictive manner.

[0059] BRIEF DESCRIPTION OF THE FIGURES

[0060] In the accompanying Figures: Figure 1 is a schematic representation of an embodiment of the method according to the present invention.

[0061] Figure 2 is a schematic representation of an embodiment of an electrode produced according to the method of the present invention.

[0062] Figure 3 is a schematic representation of a further embodiment of an electrode produced according to the method of the present invention.

[0063] Figure 4 shows cycling performance of cells with pristine 3D lithium metal anode and Pl polymer coated 3D lithium metal anode at 1C Ch / DCh.

[0064] DETAILED DESCRIPTION

[0065] The present invention provides a method for manufacturing a component for a secondary battery, particularly an alkali metal battery such as a lithium ion, potassium ion, or sodium ion battery, or an alkaline earth metal battery.

[0066] The method may comprise providing an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, coating the 3D current collector with a polymer wherein the polymer is capable of at least partially conducting ionically and / or forming a gel, and contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to an ionically conductive gel or to form an ungelled porous structure which at least partially holds the liquid electrolyte. The component may further comprise a metal anode comprising an alkali metal or an alkaline earth metal.

[0067] In some embodiments, the method may further comprise coating the 3D current collector or metal anode with a pre-polymer solution containing one or more monomer, crosslinker and initiator and polymerising the monomer to form the polymer. The coating step may comprise applying the pre-polymer solution in a solvent to the 3D current collector (or where a metal anode is present, to a surface of the metal anode), polymerizing the monomer in the presence of a solvent or, alternatively, the solvent may be at least partially removed prior to polymerization. The selection and sequence of solvent removal and polymerization steps may be varied to achieve the desired material properties. The solvent may be selected from the organic solvents comprising alcohols (e.g. Cl-5 alcohols), esters (such as methyl acetate and ethyl acetate), ethers (such as diethyl ether), ketones (e.g. acetone, butanone), and aromatic solvents such as benzene and toluene. In certain embodiments, volatile solvents may be employed to facilitate evaporation prior to, during, or post polymerization. Suitable monomers may include, but are not limited to vinyl-based monomers (e.g., acrylates, methacrylates), ethylene oxide-containing monomers (e.g., epoxy, oxetane, or polyethylene glycol derivatives), cyclic / acyclic carbonate monomers (e.g., ethylene carbonate, vinylene carbonate derivatives), ionic liquid monomers and ionomer-forming monomers, single ion conducting monomers, nitrile-functional monomers (e.g., acrylonitrile derivatives), sulfonyl fluoride- and sulfonate-containing monomers, fluorinated monomers (e.g., vinylidene fluoride derivatives), urethane- and carbonate-forming monomers, siloxane-containing monomers or any monomer capable of forming an ion-conductive or protective polymer matrix for lithium metal anodes. Polymerization may be initiated by any suitable technique, including radiation-induced (e.g., UV) initiation or thermal initiation.

[0068] In some embodiments, as shown in Figure 1, the method (100) may comprise providing an electrically conductive 3D current collector substrate (105) comprising a plurality of electrically conductive protrusions (110) extending from a base of the substrate (115), heating a polymer (120) to provide an at least partially molten polymer (125), and coating the 3D current collector substrate with the at least partially molten polymer.

[0069] The 3D current collector substrate (also referred to as a scaffold) can be prepared in several steps to ensure optimal performance. Initially, a suitable substrate material, such as nickel foam or copper foam, may be selected for its high conductivity and structural integrity. This substrate may then be cleaned and treated to remove any impurities that could affect the scaffold's performance. Next, a conductive layer, often composed of materials like graphene or carbon nanotubes, may be deposited onto the substrate using techniques such as chemical vapor deposition (CVD) or electrochemical deposition. This layer enhances the electrical conductivity and provides a large surface area for active material deposition. The scaffold may then be subjected to a series of thermal treatments to improve its mechanical stability and adhesion properties.

[0070] The 3D current collector substrate can be manufactured from a variety of materials such as Al, Cu, Ni, stainless steel, carbon, metalized polymer and combinations thereof. The electrically conductive protrusions are designed to maximize the surface area of the current collector. This increased surface area allows for more efficient electron transfer and improved overall conductivity. The protrusions also provide additional mechanical stability to the current collector, ensuring that it maintains its structural integrity during operation. The protrusions can be of various shapes and sizes (including ribs, pillars, domes, craters and the like), depending on the specific application and desired properties of the current collector.

[0071] The 3D current collector may be metallised to enhance its electrical conductivity and mechanical strength. The process may comprise coating a base scaffold with a thin layer of metal, for example using techniques such as electroplating, evaporation, spray coating, sputtering, or chemical vapor deposition (CVD). These metallisation techniques may ensure a uniform and adherent metal coating, which may significantly improve the scaffold's electrical properties and structural integrity. The metallised 3D scaffold may then be characterized to confirm the quality and uniformity of the metal coating.

[0072] The metallised 3D current collector can function independently as an electrode, i.e. it may be "anode-free" or "anode-less", i.e. free of anode electrode active material. Alternatively, a separate metal anode may be secured to the 3D current collector as a layer of electrode active material (130) in electrical contact with the 3D current collector, as shown in Figure 1. The electrode active material may comprise anode (negative) electrode material. Preferably, the electrode active material is deposited on the 3D current collector before the polymer is coated on the 3D current collector so that the polymer also coats the electrode material. The anode electrode material may comprise an alkali metal or an alkaline earth metal. For example, the anode electrode material may comprise a metal selected from the group consisting of lithium, sodium, magnesium, potassium, calcium, and combinations thereof.

[0073] The component may further comprise a cathode (positive) electrode. Suitable materials for the cathode electrode may, for example, be selected from a group consisting of metal oxides, silicon, graphitic materials, sulfur, phosphates, oxygen, and air. For Li-ion batteries it may for example comprise LiCoO?, MnO?, LiMn2O4, LiNiO?, Lix(MnyNii-y)2-xO2, LiNii-xCoxCh, LiNixCoyAlzC>2, Li(Nii / 3Mni / 3Coi / 3)O2, lithium nickel manganese cobalt oxide (LiNiMnCoO2 or NMC), LiFePO4, LiFexMnyPO4(x+y= l), Li2FePO4F, V2O5, V2O5-TeO2, WO3-V2O5, TiSxOy, MOx, MSx or Li-V2O. For other alkali metal ion batteries, the positive electrode active layer may for example comprise similar materials as listed above for Li-ion batteries, but with the Li being substituted by the other ion (e.g. K or Na).

[0074] The polymer used in the coating can comprise a wide range of materials, and will be formulated for the specific application and properties desired. By using a combination of different polymers, it is possible to tune the physical, chemical, electrochemical and mechanical properties of any gel that is formed. The polymers in the coating may be homopolymers, polymer mixtures (physical mixture without chemical bonding or a homopolymer or co-polymer or combination thereof), co-polymers, graft copolymers or mixtures of these. The polymers may include, but are not limited to polyethers such as polyethylene oxide (PEO), fluoropolymers such as polyvinylidene fluoride (PVDF), polyolefins such as polypropylene (PP), and polyethylene (PE); often polyethers will be present, although this is not essential. Where present, polyethers and polyethylene oxides can absorb more liquid electrolyte compared to polyvinylidene fluoride or polyolefins. As a result, where the gel is ionically conductive, the conductivity is increased. This effect is further enhanced with polyethylene oxides as the ethylene oxide can dissociate metal-ion salts, which further improves the conductivity of the gel. Further, inorganic polymers such as polysiloxane, polyphosphazene, polycarbonates such as poly(ethylene carbonate) (PEC) or poly(trimethylene carbonate) (PTMC), polyesters such as poly(pentyl malonate), polyurethane, polyrotaxane, ionomers (anionic, cationic or zwitterionic polymers) or mixtures thereof may also be present, in combination with any of the polymers listed, or other polymers as would be understood to the skilled reader. It may be the case that the polymer coating includes a polysiloxane, a polyrotaxane, a polycarbonate or combination of these, in particular a combination of polysiloxane and polycarbonate. This can be beneficial as polysiloxanes / polyrotaxanes can offer excellent mechanical properties in terms of compressibility, strength, stretchability. In addition, polysiloxanes are a non-polar polymer, this lack of polarity helps in restricting solvent access to the anode and hence prevents side reactions between the electrolyte solvent and anode, thus improving cell longevity.

[0075] In some embodiments, the polymer coating may comprise co-polymers having a mixture of the above mentioned polymer types (for instance, polycarbonate, polyester, polyether, polyurethane and siloxane), and may include polyvinyl ethylene carbonate-co- hydroxyethyl methacrylate copolymer. As noted above, the polymer may be a thermoplastic, a thermoset-plastic, or a combination thereof. As also noted above, the choice of polymer can vary depending on the specific application and desired properties of the final coating. The polymer may also contain metal-salts such as LiTFSI, LiFSI, LiNOs and combinations of these; solvents such as ethylene carbonate, propylene carbonate, dimethoxy ethane, diethoxy ethane and combinations thereof; additives such as dioxolane vinyl ethylene carbonate and / or fluoroethylene carbonate; and / or particles such as LLZO and / or LATP to enhance the alkali / alkaline earth metal ion conductivity of the polymer coating. The polymer may intrinsically be ionically conductive, and if this is desirable will be selected for this property. Often, where ionic conductivity is desired, polyethers (PEO) or polycarbonates (PTMC) will be selected. The polymer coating may have a thickness of from 0.1 pm to 100 pm, from 0.5 pm to 50 pm, from 1 pm to 20 pm, from 1 pm to 5 pm, or from 0.1 pm to 10 pm.

[0076] The polymer may be capable of forming a gel. The method may further comprise contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to an ionically conductive gel. The contacting may occur during assembly of a cell or battery which incorporates the electrode when liquid electrolyte is introduced into the cell or battery. The contacting may result in the polymer at least partially absorbing electrolyte and becoming gelated. The polymer gel volume may swell by an amount of from 20% to 200%, from 50% to 150%, or from 50% to 100% relative to the ungelated polymer.

[0077] The polymer may fill from 5% to 100%, from 20% to 80%, from 30% to 70%, or from 40% to 60% of a volume of the 3D current collector substrate. The volume may be defined by a space between the base, side walls of the protrusions and outer ends of the protrusions. The coating may cover the protrusions and substantially fill the gaps and spaces between the protrusions so that none of the surface of the 3D current collector is exposed.

[0078] The polymer coating may have a laminate structure with multiple layers. For example, it can comprise a hard first layer in contact with the 3D current collector and a soft second layer distal from the 3D current collector. The soft second layer may be capable of forming an ionically conductive gel when contacted with a liquid electrolyte to a greater extent than the hard first layer, and the hard first layer may be incapable of forming a gel because of the composition of this layer, or it may be gellable but gels to a lesser extent than the soft second layer because it is proximate to the current collector. The soft layer may be thicker than the hard layer. The soft layer may have a thickness in the range 1 pm to 10 pm, the hard layer may independently have a thickness in the range 0.1 pm to 10 pm. The laminate may comprise different polymer layers. The layers may optionally be crosslinked and / or have different thicknesses.

[0079] The polymer coating may have a gradient hardness. For example, a polymer surface or layer closest to the 3D current collector substrate may be harder than a polymer surface or layer furthest away from the 3D current collector substrate. The polymer closest to the 3D CC may have a hardness of from 50 to 90 on the Shore D scale (when tested according to ASTM D2240 or ISO 868), whereas the polymer furthest away from the 3D CC may have a hardness of from 20 to 80 on the Shore A scale (when tested according to ASTM D2240 or ASTM D1414). The hard layer may comprise a composite containing inorganic particles, for instance the inorganic particles may be LLZO, LATP and / or AI2O3. The properties of the hard layer may be similar to those of the ASEI layer optionally deposited on the 3D current collector, as described below. The soft layer and hard layer may be tuned to have different Young's moduli.

[0080] In some alternative embodiments, a polymer surface or layer furthest away from the 3D current collector substrate may be harder than a polymer surface or layer closest to the 3D current collector substrate.

[0081] As noted previously, the polymer coating may exhibit a gradient in gelability, wherein the polymer surface or layer nearest to the 3D current collector substrate has lower percentage swelling compared to the polymer surface or layer further away. This configuration may enable the polymer to maintain adhesion to the 3D CC surface while simultaneously forming a gel on its outer layer to promote ion transport across the polymer coating during operation.

[0082] As shown in Figure 2, the polymer coating (225) may have a thickness that progressively increases away from the base. In some embodiments, the polymer coating may progressively decreases away from the base. For example, the polymer coating covering the tops of the protrusions may be thicker than the polymer coating covering the base of the 3D CC. Alternatively, the coating on the tops of the protrusions may be thinner than the coating on the base. In a further embodiment, the polymer may not fill in-between the 3D protrusions, may rather be a conformal or semi-conformal coating along the 3D protrusions.

[0083] In varying embodiments, the polymer coating may be porous or non-porous. A high porosity provides for good deposition of the polymer, and rapid gelation as, during battery assembly, the liquid electrolyte can flow into the pores. Preferably, the polymer is alkali / alkaline earth metal ion conductive. The polymer coating may have a gradient porosity in which a lower porosity is present on surface closest to the 3D current collector substrate and a higher porosity is present furthest away from the 3D current collector substrate. This may assist in minimising dendrite formation and porous anode metal deposition on the side walls and in between the base of the protrusions.

[0084] The heating step of the method may ensure that the polymer reaches a viscosity suitable for coating the 3D current collector substrate. The heating step can include heating the polymer at a temperature proximate to or above the melting point of at least one of the co-polymers. The heating temperature will therefore depend upon the polymer selected. For example, if the polymer comprises polyethylene (PE), the temperature can be in the range of 120 °C to 130 °C. If the polymer comprises polyethylene glycol (PEG), the temperature can be in the range of 50 °C to 70 °C, depending on the length of the polymer. If the polymer comprises polypropylene (PP), the temperature can be in the range of 160 °C to 171 °C. If the polymer comprises polyvinyl alcohol (PVA), the temperature can be in the range of 150 °C to 200 °C. The temperature of this aspect of the method can be in the range of 50 °C to 250 °C. The polymer may be placed on the 3D current collector and heated while in position (in situ heating) until it at least partially melts. The heating may be carried out in an oven or furnace or by using direct or indirect heating methods. Alternatively, the polymer may be heated apart from the 3D current collector (i.e. ex situ heating) prior to being coated on the surface of the 3D current collector.

[0085] The coating step can be performed using various techniques, such as dipping, spraying, blade casting, slot die coating, or brushing, to ensure a uniform and consistent coating. In some embodiments in which a thin coating layer is required, the coating may be applied by initiated chemical vapour deposition (iCVD). Preferably, however, the coating may be performed by allowing the at least partially molten polymer to melt over the 3D current collector. In this way, the at least partially molten polymer may flow around the protrusions, creating a continuous and adherent layer. The at least partially molten polymer may be pressed onto the 3D CO to improve adhesion. The at least partially molten state of the polymer ensures it flows smoothly over the substrate, filling any pores or gaps and providing a uniform coating.

[0086] The method may further comprise a step of depositing an artificial solid electrolyte interphase layer (ASEI) on the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. The ASEI layer may be deposited by dip-coating with subsequent solvent evaporation, polymer melt infusion, by initiated Chemical Vapor Deposition (iCVD) or by sequential Atomic Layer Deposition (sALD). The ASEI layer may comprise a stiff, hard polymer with a high degree of crosslinking, a single-ion conducting polymer, a ceramic and polymer composite layer, or a ceramic layer.

[0087] The method may further comprise applying a wetting layer to the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. The application of a wetting layer to the 3D current collector substrate may improve adhesion and provide a uniform coating when subsequently applying the at least partially molten polymer. This may be performed by cleaning the 3D current collector substrate to remove any contaminants and applying the wetting layer to the cleaned substrate. The wetting layer may be a thin film of material that has good adhesion properties and is compatible with both the substrate and the polymer. Common materials for the wetting layer include silanes, titanates, or other adhesion promoters. The wetting layer may be applied using techniques such as dip-coating, spin-coating, or spraying, ensuring a uniform and thin coverage. Once applied, the wetting layer may be dried and cured to form a stable and adherent film. This step may involve heating the substrate to a specific temperature for a set period, depending on the material used for the wetting layer. Once the wetting layer is in place and properly cured, the 3D current collector substrate may be ready for the polymer coating. In some examples, the method will comprise applying both an electrode active layer and a wetting layer, in other examples the wetting layer may be present without the electrode active layer.

[0088] The method may further comprise applying a seeding layer to the 3D current collector substrate before coating the 3D current collector substrate with the at least partially molten polymer. The seeding layer may comprise fine particles or a thin film of a material that promotes nucleation and adhesion of the subsequent polymer layer. Common materials for the seeding layer may include nanoparticles of metals, metal oxides, or conductive polymers. For instance, the seeding layer may comprise tin, tin oxide, alumina, zinc oxide, Ag-C composite or combinations thereof. The seeding layer may be applied using techniques such as dip-coating, spin-coating, or spraying, ensuring a uniform and thin coverage. After the seeding layer may been applied, it may be dried and cured.

[0089] The present invention extends to a component for a secondary battery. The component comprises an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, a metal anode comprising electrode active material, and a polymer coating on the 3D current collector substrate and electrode active material. The electrode active material of the metal anode comprises an alkali metal or an alkaline earth metal. The polymer coating comprises an ionically conductive gel, which assists in transfer of ions across the coating during operation of the battery. The component may be manufactured according to the method described above.

[0090] As shown in Figure 3, an embodiment of the component (300) comprises a 3D CC substrate (105) comprising a plurality of electrically conductive protrusions (110) extending from a base of the substrate (115). A layer of electrode active material (130) is secured to the substrate (115) and is covered by an ASEI layer (135). A polymer coating (125) covers both the electrode active material (130) and ASEI layer (135). The present invention further extends to a battery cell comprising the component described above and a liquid electrolyte or a hybrid electrolyte. The battery cell may be manufactured by a process involving combining the component and liquid electrolyte to permit the polymer layer to at least partially absorb the electrolyte and form a gel on its outer surface. The battery may further be sealed, degassed and aged to produce the completed product.

[0091] The component described herein may be suitable for use in a secondary battery. It may also be used as a component of a fuel cell or electrolyser, in electrocatalysis or electrochemical sensors, or in any relevant field in which a protected 3D electrode is required.

[0092] The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the technology to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

[0093] The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the present disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of any accompanying claims.

[0094] Examples

[0095] Fabrication of polymer protected 3D metal anode

[0096] A preformed polymer sheet was prepared by solution casting a mixture comprising monomer, crosslinker and initiator followed by polymerization as described above, followed by solvent removal under vacuum. The polymer sheet possessed sufficient thermal softening properties to enable conformal coating. The polymer sheet was placed in direct contact with a 3D electrically conductive lithium metal anode comprising protrusions extending from a supporting base. The lithium metal anode overlaid a 3D current collector substrate with which it was in intimate contact. The resulting assembly was heated at predetermined temperature in an inert environment. The polymer softened and flowed to envelop the protrusions and fully cover the exposed surfaces of the substrate, forming a continuous polymer layer. After cooling, the coating remained mechanically stable.

[0097] Assembly of secondary battery cell and evaluation of charge / discharge cycle performance

[0098] A secondary cell was assembled in an inert atmosphere. The polymer-coated 3D lithium metal anode assembly was used as the negative electrode. A commercially available cathode was employed as the positive electrode, separated by a microporous polyolefin separator. The polymer-coated substrate was subsequently contacted with home grown liquid electrolyte, resulting in at least partial conversion of the polymer layer into an ionically conductive gel structure capable of conducting corresponding alkali metal ion. Cells were allowed to rest prior to testing. The cells were cycled between 4.3V -3.0V with 1C charge and 1C discharge.

[0099] The data illustrated in Figure 4 show that the secondary battery cell including the polymer- coated 3D metal anode exhibits an increased initial discharge capacity, which is attributed to the formation of a more stable and uniform native solid electrolyte interphase (SEI) at the negative electrode surface. The presence of the polymer coating further promotes dense and controlled lithium deposition during cycling, thereby suppressing the growth of lithium dendrites. As a result, the polymer-coated 3D lithium metal anode demonstrates improved capacity retention and operational stability relative to a secondary battery cell incorporating an uncoated 3D lithium metal anode.

Claims

CLAIMS1. A method for manufacturing a component of a secondary battery, the method comprising: i) providing an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, wherein a metal anode is provided as a layer of electrode active material on the 3D current collector, and wherein the metal anode comprises an alkali metal or an alkaline earth metal, iv) coating the metal anode with a polymer wherein the polymer is capable of at least partially conducting ionically and / or forming a gel, and / or forming a porous structure, and ii) contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to an ionically conductive gel or to form an ungelled porous structure which at least partially holds the liquid electrolyte.

2. The method according to claim 1, comprising coating the 3D current collector with a monomer and polymerising the monomer to form the polymer; optionally wherein the polymerising is initiated by UV or heat.

3. The method according to claim 2, comprising coating the 3D current collector with a solution comprising the monomer, drying the 3D current collector to remove solvent, and polymerising the monomer to form the polymer.

4. The method according to claim 1, wherein the polymer coating is formed by a method comprising: i) heating a polymer to provide an at least partially molten polymer, and ii) coating the 3D current collector substrate with the at least partially molten polymer.

5. A method for manufacturing a component for a secondary battery, the method comprising: i) providing an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, wherein a metal anode is provided as a layer of electrode active material on the 3D current collector, and wherein the metal anode comprises an alkali metal or an alkaline earth metal, ii) heating a polymer to provide an at least partially molten polymer, andiii) coating the metal anode with the at least partially molten polymer.

6. The method according to any preceding claim, wherein the 3D current collector substrate comprises a plurality of electrically conductive protrusions extending from two opposite sides of the base of the substrate, wherein a metal anode is provided as a layer of electrode active material on the 3D current collector on both sides of the base, and wherein the method comprises coating the metal anode on both sides of the base with the polymer.

7. The method according to any preceding claim, further comprising contacting the polymer coating with a liquid electrolyte to at least partially convert the polymer to an ionically conductive gel or to form an ungelled porous structure which at least partially holds the liquid electrolyte.

8. The method according to any preceding claim, further comprising depositing an artificial solid electrolyte interphase layer (ASEI) or a passivation layer on the 3D current collector substrate before coating the 3D current collector substrate with the polymer.

9. The method according to any preceding claim, wherein the alkali metal or alkaline earth metal is selected from the group consisting of lithium, sodium, magnesium, potassium, calcium and combinations thereof.

10. The method according to claim 9, wherein the alkali metal or alkaline earth metal is lithium or sodium or magnesium.

11. The method according to any preceding claim, wherein the polymer comprises a laminate structure having a plurality of layers, preferably wherein the laminate structure comprises a hard first layer proximate to or in contact with the 3D current collector, and a soft second layer distal from the 3D current collector, preferably wherein the soft second layer is capable of forming an ionically conductive gel when contacted with a liquid electrolyte to a greater extent than the hard first layer.

12. The method according to claim 11, wherein the laminate structure comprises an inorganic layer.

13. The method according to any preceding claim, wherein the polymer fills from 5% toby a space between the base, side walls of the protrusions and outer ends of the protrusions.

14. The method according to any preceding claim, wherein the polymer coating has a gradient hardness, wherein a polymer surface closest to the 3D current collector substrate is harder than a polymer surface furthest away from the 3D current collector substrate.

15. The method according to any preceding claim, wherein the polymer coating has a gradient gelability, wherein a polymer surface closest to the 3D current collector substrate has a lower gelability than a polymer surface furthest away from the 3D current collector substrate.

16. The method according to any preceding claim, wherein the polymer comprises copolymers comprising a mixture of polymer types including one or more of polycarbonate, polyester, polyolefins, polyether, polyvinylidene fluoride, polyurethane polyrotaxane, polyphosphazene and polysiloxane.

17. The method according to any preceding claim, wherein the polymer coating has a thickness that progressively increases away from the base, or progressively decreases away from the base.

18. The method according to any preceding claim, wherein a polymer surface closest to the 3D current collector substrate has a percentage swelling of from 0.5% - 50% by volume and a polymer surface furthest away from the 3D current collector substrate has a percentage swelling of 50% - 200% by volume.

19. The method according to any preceding claim, wherein the polymer coating is porous or non-porous.

20. The method according to any preceding claim, wherein the polymer coating has a gradient porosity in which a lower porosity is present on surface closest to the 3D current collector substrate and a higher porosity is present furthest away from the 3D current collector substrate.

21. The method according to any preceding claim, wherein the polymer is elastomeric, thermo-plastic, a thermoset-plastic, or a combination thereof.

22. The method according to any preceding claim, wherein the polymer is alkali / alkaline earth metal ion conductive.

23. The method according to any preceding claim, wherein the polymer comprises one or more metal-salts, solvents, additives, liquid metals and / or particles, for enhancing the polymer's alkali / alkaline earth metal ion conductivity.

24. The method according to any preceding claim, further comprising applying a wetting layer to the 3D current collector substrate before coating the 3D current collector substrate with the polymer.

25. The method according to any preceding claim, further comprising applying a seeding layer to the 3D current collector substrate before coating the 3D current collector substrate with the polymer or an electrode active layer.

26. A component for a secondary battery, the component comprising: i) an electrically conductive 3D current collector substrate comprising a plurality of electrically conductive protrusions extending from a base of the substrate, ii) a metal anode provided as a layer of electrode active material on the 3D current collector, wherein the metal anode comprises an alkali metal or an alkaline earth metal, and iii) a polymer coating on the electrode active material.

27. A battery cell comprising the component of claim 26 and a liquid electrolyte or a hybrid electrolyte.

28. A battery cell comprising a component manufactured according to the method of any of claims 1 to 25, and a liquid electrolyte or a hybrid electrolyte.

29. A battery cell according to claim 27 or claim 28, comprising an anode electrolyte, wherein the anode electrolyte forms an ionically conductive gel with the polymer coating, and non-anode electrolyte in the battery cell is in liquid or hybrid form.

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