Composite cathode and methods for forming the same

Abstract

The present disclosure relates to a composite cathode for a lithium-ion cell comprising a cathode active layer and a cathode coating of zinc oxide. The cathode active layer comprises composite particles, which comprise a core of cathode active material surrounded by a shell of stabilising material. The combination of composite particles and a cathode coating is found to improve the battery performance at higher voltages.

Description

[0001] COMPOSITE CATHODE AND METHODS FOR FORMING THE SAME

[0002] FIELD

[0003] The present description relates to composite cathodes for lithium-ion cells, which have increased stability particularly at high voltage.

[0004] BACKGROUND

[0005] Rechargeable or 'secondary' batteries find widespread use as electrical power supplies and energy storage systems. For example, in automobiles, battery packs formed of a plurality of battery modules, wherein each battery module includes a plurality of electrochemical cells, are provided as a means of effective storage and utilization of electric power.

[0006] Secondary cells such a lithium-ion cells comprise a positive active material, or "cathode active material", and a negative active material or "anode active material" that form the electrochemically active components of the cathode and anode respectively.

[0007] Exemplary cathode active materials (CAM) include layered lithium metal oxide (LiMO?) cathode materials, preferably wherein the metal is nickel. For example, cathode active materials such as lithium nickel manganese cobalt oxide (NMC) is a possible cathode active material for high-energy Li-ion batteries.

[0008] Stability of the cathode active material under repeated charge and discharge is an ongoing challenge. Recently, high-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, typically the stability at high voltage is poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution.

[0009] Therefore, there is an interest in improving the properties of CAM materials to increase the stability especially at high voltage.

[0010] BRIEF DESCRIPTION

[0011] According to an aspect of the present disclosure, there is provided a composite cathode comprising a cathode active layer; and a cathode coating, wherein the cathode active layer comprises: composite particles; binder; and optionally conductive additive, wherein the composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material; and wherein the cathode coating is disposed on the cathode active layer and comprises a passivating material comprising zinc oxide .

[0012] According to a second aspect of the present disclosure, there is provided a method of forming a composite cathode comprising the steps of

[0013] 1) providing a cathode active layer comprising composite particles; and

[0014] 2) depositing a cathode coating on the cathode active layer by atomic layer deposition to form a composite cathode; wherein the composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material; and wherein the cathode coating comprises a passivating material comprising zinc oxide

[0015] According to a third aspect of the present disclosure, there is provided a composite cathode obtained by the above-mentioned method.

[0016] While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example and will be described in detail. It should be understood, however, that other embodiments, beyond the particular embodiments described, are possible as well. All modifications, equivalents, and alternative embodiments falling within the spirit and scope of the appended claims are covered as well.

[0017] The above discussion is not intended to represent every example embodiment or every implementation within the scope of the current or future Claim sets. The figures and Detailed Description that follow also exemplify various example embodiments. Various example embodiments may be more completely understood in consideration of the following Detailed Description. BRIEF DESCRIPTION OF THE FIGURES

[0018] Figures la / lb show the specific discharge capacity and the discharge capacity retention during high-voltage cycling of a full cell comprising a composite cathode of the disclosure.

[0019] Figures 2a / 2b show the specific discharge capacity and the capacity retention during normal-voltage cycling of a coin cell comprising a composite cathode of the disclosure.

[0020] The present invention will in the following be described in more detail.

[0021] DETAILED DESCRIPTION

[0022] The disclosure relates to a composite cathode which comprises a cathode active layer and a cathode coating. The cathode active layer comprises composite particles, which comprise a core of cathode active material and a shell of stabilising material.

[0023] The disclosure further relates to the method of producing said composite cathode.

[0024] CATHODE ACTIVE MATERIAL

[0025] The cathode active material may comprise a suitable material for use as a cathode active material in the cathode of a cell, particularly a lithium ion cell.

[0026] The term "cathode active material" is to be understood as an electrochemical species which can be oxidised and reduced in a system which enables a cell to produce electric energy during discharge. The role of the cathode active material is to reversibly intercalate (or otherwise bind) ions (such as lithium ions) during cell charge and discharge cycles.

[0027] The cathode active material may comprise an intercalation material, such as a lithium intercalation material, for example a lithium metal oxide which may include lithium and a transition metal.

[0028] The cathode active material may comprise any one or a mixture of two or more of lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel manganese oxide (LNMO), lithium nickel cobalt oxide, lithium nickel manganese cobalt (NMC) oxide, lithium iron phosphate (LFP), lithium iron manganese phosphate (LFMP) and lithium nickel cobalt aluminium oxide (NCA). Preferably, the cathode active material is a transition metal complex such as layered lithium metal oxide (LiMO2) cathode materials, wherein the metal is typically nickel. Even more preferably, the cathode active material is a lithium nickel manganese cobalt oxide (NMC), such as a high nickel lithium nickel manganese cobalt oxide (high Ni NMC) or lithium rich lithium nickel manganese cobalt oxide (Li rich NMC).

[0029] In some embodiments, the cathode active material comprises lithium nickel cobalt manganese oxides (NMC) (LibNii-x-yCOxMnyAzO2(0<x+y< l)), where A is an element other than Li, Ni, Co, Mn or O and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0030] In preferred embodiments, the NMC cathode materials are lithium rich. As such, the cathode active material typically comprises lithium nickel cobalt manganese oxides (NMC) represented by the formula LibNii-x-yCOxMnyAzO2(0<x+y< l), where A is an element other than Li, Ni, Co, Mn or O and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 1.05<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0031] In preferred embodiments, the NMC cathode materials are high in nickel. As such, the cathode active material typically comprises lithium nickel cobalt manganese oxides (NMC) represented by the formula LibNii-x-yCOxMnyAzO2, wherein 0<x+y<0.4, preferably 0<x+y<0.25, and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0032] The ratio of the nickel of the high-nickel NMC material may range from 33 mol% to 98 mol % in terms of non-lithium metals. Preferably, the ratio may range from 60 mol% to 95 mol% in terms of non-lithium metals. Even more preferably, the ratio may range from 80 mol% to 95 mol% in terms of non-lithium metals.

[0033] In preferred embodiments, the NMC cathode materials is defined as LibNii-x-yCOxMnyAzO2, wherein 0<x+y<0.4, preferably 0<x+y<0.25, and wherein 0<z<0.05, preferably 0.002 <z< 0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.1. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0034] COMPOSITE PARTICLES

[0035] The composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material.

[0036] This shell comprising stabilising material provides multiple advantageous properties. For example, the shell protects the cathode active material from cracking during cycling at high voltage. Further, the shell typically mitigates side reactions and improves the conductivity of lithium ions.

[0037] Suitable stabilising materials comprise boron-containing materials. For example, the stabilising material may be LiBOx, for example LiBO?.

[0038] The stabilising material may be coated on the cathode active material by any suitable means. Preferably, a solid state deposition method is used.

[0039] The size of the composite particles varies from about 0.5 pm to about 20 pm in diameter, such as from about 1 pm to about 15 pm in diameter, for example from about 2 pm to about 14 pm in diameter.

[0040] Preferably, the composite particles comprise a bimodal blend of small particles having a diameter between about 2 pm and about 5 pm and large particles having a diameter between about 12 pm and about 14 pm.

[0041] The particle diameter may be measured using a Mastersizer 3000 from Malvern Panalytical Ltd, for instance using the D[4,3] particle size.

[0042] The shell comprising stabilising material may have a thickness of less than 3 nm, preferably less than 2 nm, such as from 0.1 nm to 2 nm, more preferably from 0.5 nm to 1.5 nm.

[0043] The shell thickness may be measured using the same techniques as used to measure the depth of the cathode coating, as detailed below. CATHODE ACTIVE LAYER

[0044] In the context of this disclosure, a 'cathode active layer' is a layer comprising binder, composite particles, and optionally a conductive additive.

[0045] To produce a cathode active layer, composite particles, a binder and optionally a conductive additive are typically dispersed in a dispersant to form a slurry. The slurry is then deposited on a current collector to form the cathode active layer.

[0046] The binder adhesively connects all the electrode materials for long-term charge / discharge cycling. The role of the optional conductive additive is to improve the electronic properties of the cathode and to provide an electrical connection between the particles of cathode active material in the cathode.

[0047] Suitable binders are well known in the art and may be water-insoluble or water-soluble.

[0048] Examples of suitable water-insoluble binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an ethylene oxide-containing polymer, an ethylene propylene copolymer, polystyrene, polyvinylpyrrolidone, polyurethane, polytetrafluoro ethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0049] Examples of suitable water-soluble binders include a rubber binder or a polymer resin binder. The rubber binder may be selected from a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an acrylonitrile-butadiene rubber, an acrylic rubber, a butyl rubber, a fluororubber, and a combination thereof. The polymer resin binder may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, an ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, a polyester resin, an acrylic resin, a phenol resin, an epoxy resin, polyvinyl alcohol, and a combination thereof.

[0050] The cathode active layer may comprise, by weight, 0.01-10 wt% binder, for example from about 0.02-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt% binder. Preferably the cathode active layer comprises from about 0.1 to 3 wt% binder, even more preferably 0.2 wt% to 2 wt% binder, such as from 0.5 wt% to 1.5 wt% binder. Most preferably the cathode active layer comprises about 1 wt% binder. Suitable conductive additives include acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitch-based carbon, coke powders, carbon nanotubes or metallic powders. These conductive additives may be used alone or in combination.

[0051] Preferred conductive additives are selected from carbon black, graphite, carbon nanotubes, or mixtures thereof.

[0052] In an embodiment, the cathode active layer may comprise 0.01-10 wt% conductive additive, for example from about 0.02-8 wt%, from about 0.05-6 wt%, or from about 0.06- 4 wt% conductive additive. Preferably the cathode active layer comprises from about 0.1 to 3 wt% conductive additive, even more preferably 0.2 wt% to 2 wt% conductive additive, such as from 0.5 wt% to 1.5 wt% conductive additive. Most preferably the cathode active layer comprises about 1 wt% conductive additive.

[0053] In the cathode active layer, the cathode active material is typically present in an amount of, by weight, from about 60-99.9 wt% active material, for example from about 70-99.9 wt%, from about 80-99.8 wt%, from about 90-99.6 wt%, or from about 95-99.5 wt% active material. Preferably the cathode active layer comprises from about 96-99.5 wt% active material, even more preferably the cathode active layer comprises about 98 wt% active material.

[0054] The cathode active layer is typically formed by forming a slurry of cathode active material, binder and optional conductive agent; and depositing the slurry on a current collector. Any suitable dispersing medium may be used for the slurry.

[0055] CATHODE COATING

[0056] The composite cathode of the disclosure comprises a cathode coating.

[0057] In the context of the disclosure, "coating" or "coated" refers to a substrate wherein the substrate surface comprises an added layer.

[0058] In an embodiment of the invention, a cathode active layer is provided, which comprises composite particles, binder and optionally conductive additives. Said cathode active layer is coated with a cathode coating.

[0059] In the context of the disclosure, a "cathode coating" is a coating deposited on the surface of the cathode. The cathode coating comprises a passivating material comprising zinc oxide (ZnO).

[0060] The cathode coating may function to protect the cathode active material. Typically, at higher voltage a variety of side reactions may occur. When a cathode coating comprising a passivating material is present, the cathode coating will preferentially react thereby protecting the cathode active material. For example, the cathode coating may function as a HF scavenger.

[0061] A reaction scheme showcasing the HF scavenging effect of ZnO is shown below:

[0062] ZnO+2HF^ZnF2+H2O

[0063] As such, the sacrifice of the cathode coating mitigates otherwise detrimental side reactions which enables high voltage cycling.

[0064] It has surprisingly been found that a cathode coating comprising zinc oxide (ZnO) provides particularly good protection at higher voltage.

[0065] Without wishing to be bound by theory, it is believed that the lower oxidation state of zinc in zinc oxide (ZnO) compared to for example aluminium in aluminium oxide (AI2Os) (also referred to as alumina) results in the formation of a more stable cathode coating.

[0066] Preferably, the cathode coating is thin, such as less than 10 nm.

[0067] To achieve thin and uniform layers, atomic layer deposition (ALD) is used for the coating process.

[0068] ALD is based on sequential deposition of monolayers on a surface using a gas-phase chemical process. The surface on which the film is to be deposited is sequentially exposed to different precursors followed by purging of the growth reactor to remove any residual chemically active source gas or by-products. When the growth surface is exposed to a precursor, it gets completely saturated by a monolayer of that precursor. The thickness of a monolayer depends on the reactivity of that precursor with the growth surface. This results in a number of advantages such as excellent uniformity, and easy and accurate film thickness control.

[0069] Generally, between 1 and 5 cycles of ALD, for example 1 to 3, are used for the cathode coating. Preferably, 1 to 2 cycles of ALD are used for the cathode coating. It is found that a thin layer provides better properties.

[0070] In the context of the disclosure, "thin" with respect to a cathode coating refers to a surface coating which may be about 10 nm or less, for instance about 6 nm or less, preferably about 3 nm or less, such as about 2 nm or less, more preferably about 1.5 nm or less.

[0071] Additionally, the thickness of the cathode coating may be about 0.1 nm or more, such as about 0.2 nm or more, preferably about 0.5 nm or more.

[0072] Typically, the cathode coating is therefore from about 0.1 nm to 10 nm, preferably from about 0.1 nm to 6 nm, more preferably from about 0.2 nm to 3 nm, more preferably about 0.2 nm to 2 nm, most preferably about 0.5 nm to 1.5 nm.

[0073] To measure the depth of the cathode coating, different methods can be applied.

[0074] An exemplary method includes depth profiling. Depth profiling is a method based on the sequential surface analysis and surface etching.

[0075] Various methods can be used for the surface analysis. Exemplary analysis methods include X-ray photoelectron spectroscopy (XPS); Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). XPS is the preferred method of surface analysis.

[0076] The surface etching is generally performed using an ionic beam, preferably an argon ion beam. Generally, an ion gun is used to produce said ion beam and direct the beam towards the surface. By bombarding the surface with an ion beam, a small layer will be removed, which allows for analysis of different layers of a sample.

[0077] Alternatively, the surface etching is performed using plasma laser rastering. Plasma laser rastering ensures that only a small layer will be removed, which allows for analysis of different layers of a sample.

[0078] The sequential surface analysis and surface etching allows for analysis of different layers within the sample to reveal a depth profile of the sample. The depth profile shows the composition of different elements at different depths, which can be converted into thicknesses of each layer.

[0079] COMPOSITE CATHODE The composite cathode of the disclosure comprises a cathode active layer and a cathode coating.

[0080] In an embodiment of the invention, a composite cathode comprises a cathode active layer; and a cathode coating, wherein the cathode active layer comprises: composite particles; binder; and optionally conductive additives, wherein the composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material; and wherein the cathode coating is disposed on the cathode active layer and comprises a passivating material comprising zinc oxide.

[0081] Without wishing to be bound by theory, it is thought that the combination of a stabilising coating on the CAM particles and a cathode coating on the cathode active layer provides surprisingly good stability at higher voltage. Moreover, the combination of the two coatings provides a significant reduction in the amount of transition metal ion leaching as compared to either coating alone.

[0082] It has surprisingly been found that the combination of a stabilising coating on the CAM particles and a cathode coating comprising ZnO on the cathode active layer increases the discharge capacity retention, when the battery is cycled at higher voltage.

[0083] Overall, the combination of the two coatings may result in a variety of benefits including: stabilising the cathode - especially at higher voltage; preventing side reactions between cathode active material and the electrolyte; preventing transition metal dissolution; increasing the conductivity; scavenging HF; and improving cycling performance.

[0084] An exemplary method of forming a composite cathode comprises the steps of:

[0085] 1) providing a cathode active layer comprising composite particles; and

[0086] 2) depositing a passivating material on the cathode to form a cathode coating by atomic layer deposition; wherein the composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material; and wherein the cathode coating comprises a passivating material comprising zinc oxide. CELLS

[0087] The present disclosure also relates to cells comprising the composite cathode of the disclosure, for example a secondary lithium-ion cell. Such cells typically additionally comprise an anode, a separator disposed between the anode and composite cathode, said composite cathode, anode and separator forming an electrode assembly, the cell further comprising a housing for the electrode assembly. The cell will typically comprise an electrolyte to facilitate the transport of lithium ions between the composite cathode and anode. The housing is typically sealed to ensure the electrolyte is retained within the housing. Said housing usually includes terminals in electrical contact with the anode and composite cathode.

[0088] In an embodiment of the invention, the cell is suitable for use at high voltage. By high voltage is meant cycling at higher voltage than regular operating conditions.

[0089] For example, regular operating conditions for a full cell comprising a graphite anode are between 2.8 and 4.2 V. Similarly, a coin half-cell is typically cycled between 3.0 and 4.3 V.

[0090] For a full cell comprising a graphite anode, exemplary high voltage conditions may include cycling between 2.8 and 4.35 V.

[0091] These cells may be combined to form a battery system (i.e. an array of cells).

[0092] The disclosure also relates to an electrical device comprising a cell of the disclosure. For instance, the disclosure relates to a vehicle comprising a cell (or battery system) of the disclosure. The vehicle is preferably an electric vehicle, such as a car, truck, bus, scooter, motorbike, bicycle or the like, preferably a car, truck or bus.

[0093] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. EXAMPLES

[0094] EXAMPLE 1

[0095] A full 2032 button cell battery was prepared using the composite cathode described herein. Slurry compositions were prepared using either water or NMP as dispersing media. A NMC cathode active material comprising a shell of lithium boron oxide was used as cathode active material. Polyvinylidene fluoride (PVDF) was used as binder. Carbon nanotubes (CNT) and carbon black was used as conductive additive in a 1 : 1 ratio. ZnO or AI2O3 (deposited by ALD) was used as cathode coating. Further, a cathode without cathode coating ('a non-coated cathode') was prepared as a reference.

[0096] The anode used in the cell was graphite. A ceramic-coated separator was used to separate the anode and cathode. Stainless steel was used for the positive and negative casing elements of the cell.

[0097] The formed cell was cycled between 2.8 V and 4.35 V at room temperature. The rate was 1 C for both charging and discharging.

[0098] The discharge capacity retention of the coin cells prepared as described above is seen in Figure 1. Further, selected discharge capacity retention values are presented in the Table 1 below:

[0099] Table 1. Discharge capacity retention results for coin cells prepared using either a noncoated cathode, a ZnO coated cathode and an AI2O3 coated cathode.

[0100] The discharge results clearly show improved discharge capacity retention at high voltage for a composite cathode comprising composite particles and a cathode coating compared to a non-coated cathode.

[0101] This example further shows that a ZnO cathode coating provides superior results compared to an AI2O3 cathode coating or no cathode coating at all. EXAMPLE 2

[0102] A coin half-cell was prepared using the composite cathode described herein. Slurry compositions were prepared using either water or NMP as dispersing media. A NMC cathode active material comprising a shell of lithium boron oxide was used as cathode active material. Polyvinylidene fluoride (PVDF) was used as binder. Carbon nanotubes (CNT) and carbon black was used as conductive additive in a 1: 1 ratio. ZnO and AI2O3 was used as cathode coating. Further, a cathode without cathode coating ('a non-coated cathode') was prepared as a reference.

[0103] The anode used in the coin half-cell was lithium. A ceramic-coated separator was used to separate the anode and cathode. Stainless steel was used for the positive and negative casing elements of the cell.

[0104] The formed cell was cycled between 3.0 V and 4.3 V at 45 °C. The rate was 0.5 C for charging and 1 C for discharging.

[0105] The discharge capacity retention of the coin half-cells prepared as described above is seen in Figure 2. Further, selected discharge capacity retention values are presented in the Table 2 below:

[0106] Table 2. Discharge capacity retention results for coin half-cells prepared using either a noncoated cathode, a ZnO coated cathode and an AI2O3 coated cathode.

[0107] The discharge results show similar discharge capacity retention at normal voltage for a non-coated cathode and a composite cathode comprising composite particles and a cathode coating.

[0108] Based on the two examples, it is clear that the voltage range influences the capacity retention. In Example 2, a coin half-cell is cycled within a normal voltage range which leads to a slight decrease in capacity retention for all the cells whether or not a cathode coating is provided. However, when cycling at high voltage such as in Example 1, a cathode lacking a cathode coating experiences a marked decrease in capacity retention, while those cathodes which comprise a cathode coating only experience small decreases in capacity retention.

[0109] Hence, the examples underline that a composite cathode comprising a cathode active layer and a cathode coating improves the stability of a battery during cycling at high voltage. In particular, the irreversible capacity loss is diminished when a composite cathode according to the disclosure is cycled at high voltage.

[0110] All embodiments of the invention and particular features mentioned herein may be taken in isolation or in combination with any other embodiments and / or particular features mentioned herein (hence describing more particular embodiments and particular features as disclosed herein) without departing from the disclosure of the invention.

[0111] As used herein, the term "comprises" will take its usual meaning in the art, namely indicating that the component includes but is not limited to the relevant features (i.e. including, among other things). As such, the term "comprises" will include references to the component consisting essentially of the relevant substance(s).

[0112] Wherever the word 'about' is employed herein in the context of amounts, for example absolute amounts, weights, volumes, sizes, diameters etc., or relative amounts (e.g. percentages) of individual constituents in a composition or a component of a composition (including concentrations and ratios), timeframes, and parameters such as temperatures etc., it will be appreciated that such variables are approximate and as such may vary by ±10%, for example ±5% and preferably ±2% (e.g. ±1%) from the actual numbers specified herein. This is the case even if such numbers are presented as percentages in the first place (for example 'about 10%' may mean ±10% about the number 10, which is anything between 9% and 11%).

Claims

CLAIMS1. A composite cathode comprising a cathode active layer; and a cathode coating, wherein the cathode active layer comprises: composite particles; binder; and optionally conductive additives, wherein the composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material; and wherein the cathode coating is disposed on the cathode active layer and comprises a passivating material comprising zinc oxide.

2. The composite cathode of claim 1, wherein the stabilising material comprise boron- containing materials.

3. The composite cathode of any of the preceding claims, wherein the stabilising material is LiBOx.

4. The composite cathode of any of the preceding claims, wherein the cathode active material comprises layered lithium metal oxide, preferably wherein the metal comprises nickel.

5. The composite cathode of any of the preceding claims, wherein the cathode active material comprises lithium nickel manganese cobalt oxide.

6. The composite cathode of claim 5, wherein the lithium nickel manganese cobalt oxide comprises 80 mol% to 95 mol% nickel in terms of non-lithium metals.

7. The composite cathode of any of the preceding claims, wherein the cathode coating has a thickness from 0.1 nm to 10 nm.

8. The composite cathode of any of the preceding claims, wherein the shell comprising the stabilising material has a thickness of from 0.1 to 2 nm.

9. A method of forming a composite cathode comprising the steps of1) providing a cathode active layer comprising composite particles; and2) depositing a cathode coating on the cathode active layer by atomic layer deposition to form a composite cathode; wherein the composite particles comprise a core comprising cathode active material and a shell comprising a stabilising material; and wherein the cathode coating comprises a passivating material comprising zinc oxide10. The method of claim 9, wherein the cathode coating is applied by atomic layer deposition, preferably wherein the cathode coating is applied by 1 to 2 cycles of atomic layer deposition.

11. A composite cathode obtained by any of the methods of claims 9 or claim 10.

12. A cell, for example a secondary lithium-ion cell, comprising the composite cathode of any of claims 1 to 8, or 11.

13. A vehicle comprising the cell of claim 12.

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