Battery material

Abstract

Provided is a particulate active electrode material having a D50 particle diameter of at least 250 nm, comprising: particles of an oxide comprising Nb, the particles comprising at least a partial coating formed of a coating material; wherein the coating material is selected from: metal oxides, metal sulfates, metal phosphates, metalloid oxides, metalloid sulfates, and metalloid phosphates.

Description

[0001] Battery Material

[0002] Field of the Invention

[0003] The present invention relates to active electrode materials, methods for the manufacture of active electrode materials, and electrodes comprising active electrode materials. Such materials are of interest as active electrode materials in metal-ion batteries, such as lithium-ion or sodium-ion batteries, for example as anode materials.

[0004] Background

[0005] Lithium-ion (Li-ion) batteries are a commonly used type of rechargeable battery. A typical Li-ion battery is composed of multiple cells connected in series or in parallel. Each individual cell is usually composed of an anode (negative polarity electrode) and a cathode (positive polarity electrode), separated by a porous, electrically insulating membrane (called a separator), immersed into a liquid (called an electrolyte) enabling lithium ion transport. In most systems, the electrodes are composed of an active electrode material - meaning that it is able to chemically react with lithium ions to store and release them reversibly in a controlled manner - mixed if necessary with an electrically conductive additive (such as carbon) and a polymeric binder. A slurry of these components is coated as a thin film on a current collector (typically a thin foil of copper or aluminium), forming the electrode upon drying.

[0006] Graphite is the most widely used active electrode material in the anode but has safety limitations which are a serious impediment to its use in high power, fast-charge applications. When charging rate increases, typical graphite voltage profiles are such that there is a high risk that overpotentials lead to the potential of sites on the anode to become < 0 V vs. Li / Li+, which leads to a phenomenon called lithium dendrite electroplating, whereby lithium ions deposit at the surface of the graphite electrode as lithium metal. This leads to irreversible loss of active lithium and hence rapid capacity fade of the cell. Dendrites can pierce the battery separator and lead to a short-circuit of the cell, triggering catastrophic failure leading to fire. Accordingly, the fastest-charging batteries having graphitic anodes are limited to charging rates of 5-7 C, but often much less. Lithium titanate (Li4TisOi2, LTO) and oxides comprising Nb are the main contenders to replace graphite as the active material of choice for high power, fast-charge applications.

[0007] LTO anodes do not suffer from dendrite electroplating at high charging rate thanks to their high potential (1 .6 V vs. Li / Li+) and have excellent cycle life as they do not suffer from significant volume expansion of the active material upon intercalation of Li ions due to their accommodating 3D crystal structure. LTO cells are typically regarded as high safety cells for these two reasons. However, LTO is a relatively poor electronic and ionic conductor, which leads to limited capacity retention at high rate and resultant power performance, unless the material is nanosized to increase specific surface area, and carbon-coated to increase electronic conductivity. This particle-level material engineering increases the porosity and specific surface area of the active material, and results in a significantly lower achievable packing density in an electrode. This is significant because it leads to low density electrodes and a higher fraction of electrochemically inactive material, resulting in much lower energy densities. This leads a high $ / kWh cost in various applications. As a result, LTO batteries / cells are generally limited to specific niche applications, despite their long cycle life, fast-charging capability, and high safety. In contrast, oxides comprising Nb have high capacity and ionic conductivity, with electrical conductivity being high once the material is partially lithiated. As such, a nanosizing and conductive carbon coating is not typically required to improve the power capability.

[0008] Surface reactivity is a key difference between LTO and oxides comprising Nb. LTO typically has a far higher titanium content than oxides comprising Nb. Titanium, especially anatase TiC>2 which is found as an impurity in LTO, is reactive and participates in many types of electrode / electrolyte reactions, having a high catalytic activity towards the degradation of organic compounds and water-splitting. This results in different and more pronounced reactions with the electrolyte for LTO compared to oxides comprising Nb. This is exacerbated by the need to nanosize LTO in view of its low electronic conductivity, increasing the surface area available for reactions, which may not be needed for oxides comprising Nb.

[0009] One of the problems experienced in lithium batteries is reaction between the active material surface and the electrolyte, which can further lead to gas evolution, which can impede long term cycling as well as promote mechanical expansion of cells. The exact mechanism responsible for these interfacial reactions and gas evolution is not fully understood and is specific to the chemical and physical form of each active material, along with its operating redox potential. A problem experienced when using oxides comprising Nb is physical particle cracking during long-term cycling. Cracking can lead to further failure of the battery as it exposes more of the active material surface for reactions.

[0010] Various coatings for anode and cathode active electrode materials have been explored, such as carbon coatings and metal oxide coatings. Cathode materials are unstable during delithiation so carbon coatings have been used to improve electrical or ionic conductivity within the cathode, and some LTO anodes, while protecting the reactive surface. Metal oxide coatings have been used to act as a physical barrier between electrodes and the electrolyte because of their stability and inertness. However, this inertness can also be a deleterious, as they are generally inactive to lithium ions and thus have low lithium-ion conductivity. As such, there is a prejudice in the industry towards the use of carbon coatings, which possess better conductivity among other advantages. Coatings are tailored for the surface reactivity and properties needed of the substrate material. Thus, a coating for one class of material may not be directly transferable to another.

[0011] H.-W. Ha et al. I Electrochimica Acta 52 (2007) 3236-3241 describes how LiMn2C>4 spinel cathodes coated with CeC>2 demonstrated improved cycling stability of the electrode by suppressing the dissolution reaction of manganese-ions at elevated temperatures. Y. Kim, et al. ; Electrochimica Acta 52 (2006) 1316-1322 describes how LiNii / 3Coi / 3Mni / 3C>2 cathode materials coated with AI2O3 nanoparticles using a sol-gel process demonstrated improved electrochemical properties. F. Ma, et al. J Solid State Electrochem (2019) 23, 2213-2222 describes how LiNi08Co01Mn01O2 (NCM81 1) comprising a uniform coating of MgO displayed a significant improvement in retention capacity and better rate properties over its pristine NMC811 counterpart. This is attributed to the MgO coating layer, which helps to suppress deleterious side reactions, lower the overpotential on the surface, and facilitate lithium-ion diffusion. U. Nisar, et al. RSC Adv. (2020) 10, 15274-15281 describes zirconia coated Li1 2Ni0 i6Mn056Co008O2 (NMC) cathodes displaying significantly improved electrochemical performance at a higher C-rate as compared to the pristine equivalent. C. Han, et al.; Electrochimica Acta, Volume 157,1 March 2015, 266-273 describes the use of zinc oxide as a coating for lithium titanate (LTO) anodes for lithium ion batteries. It is postulated that this creates a barrier layer between the anode and the electrolyte solution that stabilises the electrode / electrolyte interface and suppress interfacial reactions between LTO and electrolyte solution.

[0012] WO2021 / 074592, WO2021 / 074593, WO2021 / 074594, W02022 / 043704, WO2021 / 245410, W02022 / 208102, W02022 / 043702, W02022 / 043701 , W02022 / 043705, WO2021 / 245411 , WO2023 / 161641 , WO2023 / 118812, WO2023 / 152505, WO2023 / 214181 , WO2024 / 127010, and WO2024 / 126839 disclose oxides comprising Nb for use as anode materials.

[0013] US2016 / 351973 discloses nano-engineered coatings for cathode active materials, anode active materials, and solid-state electrolyte materials for reducing corrosion and enhancing cycle life of a battery, and processes for applying the disclosed coating. W02021 / 074406 discloses an electrode having a surface layer of an oxides comprising Nb disposed on a secondary active electrode material.

[0014] US20217367224 discloses an anode material coated with a protective coating capable of protecting the anode material from the atmosphere of the environment. US2024 / 066368 discloses a coating of an electronically conductive material deposited on and within the pores of a porous layer of anode material.

[0015] Despite extensive research in this area, there remains a need to provide batteries that address the problems outline above, especially those using oxides comprising Nb as active electrode materials.

[0016] Summary of the Invention

[0017] In a first aspect, the invention provides a particulate active electrode material having a Dso particle diameter of at least 250 nm, comprising: particles of an oxide comprising Nb, the particles comprising at least a partial coating formed of a coating material; wherein the coating material is selected from: metal oxides, metal sulfates, metal phosphates, metalloid oxides, metalloid sulfates, and metalloid phosphates.

[0018] The inventors have found that the coating improves the cycle life and / or gassing a metal ion battery. In particular, a coated oxide comprising Nb according to the invention allows the provision of a metal-ion battery which maintains a higher capacity after being subjected to many hundreds of charge / discharge cycles at high rates (e.g. 2C), compared to an equivalent un-coated oxide comprising Nb. Alternatively or in addition, a coated oxide according to the invention exhibits advantageously reduced gassing compared to an equivalent un-coated oxide comprising Nb.

[0019] In a second aspect, the invention provides an electrode comprising the active electrode material of the first aspect. Preferably the electrode is an anode.

[0020] In a third aspect, the invention provides a metal-ion battery comprising the electrode of the second aspect. Preferably the battery is a lithium-ion battery and the electrode forms the anode. In a fourth aspect, the invention provides a method of preparing a coated active electrode material, the method comprising the steps of: combining particles of an oxide comprising Nb as defined herein with a coating material as defined herein or a precursor thereof. An active electrode material made by this process is also provided.

[0021] In a fifth aspect, the invention provides the use of a coating material as defined herein for forming an active electrode material according to the first aspect. The use may be in connection with the method of the fourth aspect.

[0022] Summary of the Figures

[0023] Figure 1 is a scanning electron microscopy (SEM) image of sample 1 , an uncoated oxide comprising Nb. Figure 2a is an SEM image of sample 2, an oxide comprising Nb with a coating of zinc oxide.

[0024] Figure 2b is an SEM image of sample 4, an oxide comprising Nb with a coating of zirconium oxide.

[0025] Figure 2c is an SEM image of sample 5, an oxide comprising Nb with a coating of magnesium oxide.

[0026] Figure 2d is an SEM image of sample 9, an oxide comprising Nb with a coating of lithium sulfate.

[0027] Figures 3 and 4 show the gassing data obtained in the examples.

[0028] Detailed Description of the Invention

[0029] The oxide comprising Nb is preferably selected from oxides having a Wadsley-Roth (WR) or Tetragonal Tungsten Bronze (TTB) crystal structure, most preferably a WR crystal structure. WR and TTB crystal structures are known to the skilled person.

[0030] WR crystal structures are a crystallographic off-stoichiometry of the MO3 (ReCh) crystal structure containing crystallographic shear, with simplified formula of MOs-x. As a result, these structures typically contain [MOe] octahedral subunits in their crystal structure. The open tunnel-like MO3 crystal structure of these materials makes them ideal candidates for having high capacity for Li-ion storage and high rate intercalation / de-intercalation. WR structures contain crystallographic superstructures due to the off- stoichiometry. These superstructures, compounded by other qualities such as the Jahn-Teller effect and enhanced crystallographic disorder when using multiple mixed cations, stabilise the crystal and keep the tunnels open and stable during intercalation, enabling extremely high rate performance due to high Li-ion diffusion rates (reported as ~10-13cm2s-1).

[0031] Zn2Nb34Os7 or Cu2Nb34Os7 adopt WR crystal structures which can be described as having a 3x4x« crystallographic block structure composed of [MOe] octahedra, where M is Cu, Zn, or Nb. The Cu and Zn octahedra may be randomly distributed in the structure or may have a preference for particular sites such as at the edge, or corner of the blocks. This equates to 2 / 3 of one Zn or Cu cation per block. The crystal structure of Zn2Nb34Os7 can be described as an isostructural phase to Cu2Nb34Os7 with slight differences in some bond lengths and bond enthalpies. Preferably, the oxide comprising Nb has the crystal structure of Zn2Nb34Os7.

[0032] The oxide comprising Nb may have the crystal structure of AINbnC>29, which adopts a WR crystal structure which can be described as having a 3x4x« crystallographic block structure composed of [MOe] octahedra, where M is Al, or Nb. The Al octahedra may be randomly distributed in the structure or may prefer particular sites such as at the edge, or corner of the blocks. This equates to one Al cation per block.

[0033] The oxide comprising Nb may have the crystal structure of PNbgC>25, which adopts a WR crystal structure which can be described as having a 3x3x« crystallographic block structure, with corner-sharing tetrahedra.

[0034] The oxide comprising Nb may have the crystal structure of MoNbi2C>33 or WNbi2C>33, which adopt a WR crystal structure which can be described as having a 3x4x« crystallographic block structure, with cornersharing tetrahedra ([WO4] or [M0O4]).

[0035] The oxide comprising Nb may have the crystal structure of TiNb2O?, which adopts a WR crystal structure which can be described as having a 3x3x« crystallographic block structure composed of [MOe] octahedra, where M is Ti or Nb. The crystal structure is typically monoclinic. The crystal structure of TiNb2O? may be found at PDF card 00-039-1407.

[0036] The oxide comprising Nb may have the crystal structure of WsNbieOss, which adopts a WR crystal structure.

[0037] The oxide comprising Nb may have WR crystal structure having a 4x4 octahedral block structure where each block is connected through edge-sharing octahedra only.

[0038] TTB crystal structures are formed of a framework of [MOe] octahedra sharing corners linked in such a way that three, four and five sided tunnels are formed (Sagrario M. Montemayor et.al., J. Mater. Chem., 8 (1998), 2777-2781 , e.g. M8W9O47). 5-sided tunnels are filled with (Mi, M2, M3...), O, or a suitable cation to form the pentagonal columns. In the structure the pentagonal bipyramid MO7 shares edge with five MOe octahedra. An oxide with a TTB crystal structure typically comprises tungsten but, alternatively, may be free from tungsten. The oxide comprising Nb may have the crystal structure of WieNbisOgs, which adopts a TTB crystal structure.

[0039] The oxide comprising Nb may comprise oxygen vacancies or may have oxygen excess. Oxygen vacancies may be formed by substitution of a cation in the crystal structure by a cation of lower oxidation state. Oxygen excess may be formed substitution by a cation of increased oxidation state. Oxygen vacancies may also be formed by heating an oxide comprising Nb under reducing conditions, which may be termed forming induced oxygen deficiency. The amount of oxygen vacancies and excess may be expressed relative to the total amount of oxygen in the base material, i.e. the amount of oxygen in the unsubstituted material (e.g. Zn2Nb34Os7). Materials comprising oxygen vacancies are preferable, as this is believed to reduce electrical resistance, typically where the oxygen content is reduced by up to 5 at% relative to the amount of oxygen present in the base crystal structure. The crystal structure of a material may be determined by analysis of X-ray diffraction (XRD) patterns, as is widely known. For instance, XRD patterns obtained from a given material can be compared to known XRD patterns to confirm the crystal structure, e.g. via public databases such as the ICDD crystallography database. Rietveld analysis and Pawley analysis can also be used to determine the crystal structure of materials, in particular for the unit cell parameters. Therefore, the oxide comprising Nb may have a WR or TTB, preferably WR, crystal structure as determined by X-ray diffraction. It will be understood that the oxide comprising Nb having a WR or TTB structure does not exclude the presence of other phases with other crystal structures, e.g. minor impurities. The oxide comprising Nb may also be an interpenetrating mixture of two or more WR or TTB crystal structures. A preferred example of material having interpenetrating phases is an interpenetrating mixture of a first phase and a second phase; wherein the first phase has the crystal structure of TiNb2O? and the second phase has the crystal structure of Zn2Nb34Os7.

[0040] Reference patterns for WR crystal structures are available at: Zn2Nb34Os7, ICDD crystallography database entry JCPDS 28-1478 AINbnC>29, ICDD crystallography database entry JCPDS 22-009 FeNb49Oi24, ICDD crystallography database entry JCPDS 22-0351 ZrNb24C>62, ICDD crystallography database entry JCPDS 01-072-1655 PNb9O25, ICDD crystallography database entry JCPDS 81-1304 WNbi2O33, ICDD crystallography database entry JCPDS 73-1322

[0041] When the oxide comprising Nb has a particular crystal structure, it will be understood that peaks in its XRD pattern may be shifted compared to corresponding peaks in a reference XRD pattern for the crystal structure, e.g. due to the presence of cations of different size than the reference material thus leading to a different unit cell size. Typically, the peaks are shifted by no more than 0.5 degrees, or no more than 0.25 degrees, or no more than 0.1 degrees.

[0042] The oxide comprising Nb may comprise < 45 at%, or < 40 at%, preferably < 34 at% Ti relative to the amount of cations in the oxide comprising Nb.

[0043] The oxide comprising Nb may be selected from binary niobium oxides and doped variants thereof. Binary niobium oxides include Nbi2C>29, NbC>2, NbO, and Nb2Os. A doped variant or a material comprising a dopant may be understood as a material where an additional cation and / or anion is present without departing from the crystal structure of the material without the dopant. Typically, a cationic dopant is present at less than 5 at% relative to the cations in the undoped material, and an anionic dopant is present at less than 5 at% relative to the anions in the undoped material.

[0044] Nb2C>5 is the preferred binary niobium oxide, typically having the crystal structure of monoclinic Nb2Os, most preferably the crystal structure of H-Nb2Os. Further information on crystal structures of Nb2Os may be found at Griffith et al., J. Am. Chem. Soc. 2016, 138, 28, 8888-8899. The oxide comprising Nb preferably comprises at least one metal or metalloid in addition to Nb. The oxide comprising Nb may be expressed by the formula [M]x[Nb]y[O]z, wherein: M represents one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; x satisfies 0 < x < 0.5; y satisfies 0.5 < y < 49; and z satisfies 4 < z < 124.

[0045] Preferably, M represents one or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn; or two or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn; or most preferably three or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn. A particularly preferred oxide comprising Nb comprises Nb, Ti, Cr, and Zn and has a WR crystal structure.

[0046] The oxide comprising Nb may further comprise Li and / or Na. For example, Li and / or Na may enter the crystal structure when the oxide comprising Nb is used in a Li- or Na-ion battery anode.

[0047] The oxide comprising Nb may an anionic dopant such as F, Cl, Br, I, N, S, Se, and mixtures thereof. The oxide comprising Nb may comprise a cationic dopant.

[0048] In a preferred implementation, the oxide comprising Nb has the formula M1aM22-aM3bNb34-bO87-c-dQd (Formula 1), wherein:

[0049] M1 and M2 are different;

[0050] M1 is selected from Mg, Ca, Sr, Y, La, Ce, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi and mixtures thereof;

[0051] M2 is Zn or Cu;

[0052] M3 is selected from Mg, Ca, Sr, Y, La, Ce, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi, and mixtures thereof;

[0053] Q is selected from F, Cl, Br, I, N, S, Se, and mixtures thereof;

[0054] 0 < a < 1.0; 0 < b < 3.4; -0.5 < c < 4.35; 0 < d < 4.35; one or more of a, b, c, and d does not equal 0; and when a, b, and d equal zero, c is greater than zero.

[0055] Formula 1 represents an example of an oxide comprising Nb having the crystal structure of Zn2Nb34Os7. By ‘and mixtures thereof, it is intended that M1 , M3, and Q may each represent two or more elements from their respective lists. An example of such a material is Mgo iGeo iZni 8Nb34087 i. Here, M1 is

[0056] MgaGea" (where a’ + a” = a), M2 is Zn, a=0.2, b=0, c=-0.1 , d=0. Here, c has been calculated assuming that each cation adopts its typical oxidation state, i.e. Mg2+, Ge4+, Zn2+, and Nb5+.

[0057] M2 is Zn or Cu. Preferably, M2 is Zn in which case the material is based on Zn2Nb34Os7.

[0058] M1 is a cation which substitutes for M2 in the crystal structure. M1 may be selected from Mg, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, Si, Ge, Sn, P, and mixtures thereof; preferably Mg, Ti, Zr, V, Nb, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Si, Ge, P, and mixtures thereof; most preferably Mg, Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, Ge, P, and mixtures thereof. M1 may have a different valency than M22+. This gives rise to oxygen deficiency or excess. Optionally, M1 has an equal or higher valency than M22+, preferably higher.

[0059] When more than one element is present as M1 or M3 it will be understood that the valency refers to M1 or M3 as a whole. For example, if 25 at% of M1 is Ti and 75 at% of M1 is W the valency M1 is 0.25x4 (the contribution from Ti) + 0.75x6 (the contribution from W).

[0060] The amount of M1 is defined by a, meeting the criterion 0 < a < 1 .0. a may be 0 < a < 0.6, preferably 0 < a < 0.2. Most preferably, a > 0, for example a > 0.01 . Higher values of a may be more readily achieved when M1 has the same valency as M2. When M1 comprises a cation with a 2+ valency (for example Mg) a may be 0 < a < 1 .0. When M1 does not comprise a cation with a 2+ valency a may be 0 < a < 0.15.

[0061] M3 is a cation which substitutes for Nb in the crystal structure. M3 may be selected from Mg, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, Si, Sn, P, and mixtures thereof; preferably Mg, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof; most preferably Ti, Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, P, and mixtures thereof. M3 may have a different valency than Nb5+. This gives rise to oxygen deficiency or excess. Preferably, M3 has a lower valency than Nb5+. This gives rise to oxygen deficiency, i.e. the presence of oxygen vacancies providing the advantages discussed herein.

[0062] Optionally, M1 does not comprise Nb and M3 does not comprise Zn and / or Cu.

[0063] The amount of M3 is defined by b, meeting the criterion 0 < b < 3.4. b may be 0 < b < 1 .5, preferably 0 < b < 0.3. In each of these cases b may be > 0, e.g. b > 0.01 . Higher values of b may be more readily achieved when M3 has the same valency as Nb5+. When M3 comprises a cation with a 5+ valency (for example Ta) b may be 0 < b < 3.4. When M3 does not comprise a cation with a 5+ valency b may be 0 < b < 0.2. c reflects the oxygen content of the oxide comprising Nb. When c is greater than 0, it forms an oxygendeficient material, i.e. the material has oxygen vacancies. Alternatively, c may equal 0, in which it is not an oxygen-deficient material, c may be below 0, which is a material with oxygen-excess, c may be -0.25 < c < 4.35.

[0064] When c is 4.35, the number of oxygen vacancies is equivalent to 5% of the total oxygen in the crystal structure, c may be greater than 0.0435, greater than 0.087, greater than 0.174, or greater than 0.435. c may be between 0 and 2, between 0 and 0.75, between 0 and 0.5, or between 0 and 0.25. For example, c may satisfy 0.01 < c < 4.35. When the material is oxygen-deficient, for example with induced oxygen deficiency, the electrochemical properties of the material may be improved, for example, resistance measurements may show improved conductivity in comparison to equivalent non-oxygen-deficient materials. As will be understood, the percentage values expressed herein are in atomic percent.

[0065] Several methods exist for determining whether oxygen deficiency, e.g. oxygen vacancies, is present in a material. For example, Thermogravimetric Analysis (TGA) may be performed to measure the mass change of a material when heated in air atmosphere. A material comprising oxygen vacancies can increase in mass when heated in air due to the material “re-oxidising” and the oxygen vacancies being filled by oxide anions. The magnitude of the mass increase may be used to quantify the concentration of oxygen vacancies in the material, on the assumption that the mass increase occurs entirely due to the oxygen vacancies being filled. It should be noted that a material comprising oxygen vacancies may show an initial mass increase as the oxygen vacancies are filled, followed by a mass decrease at higher temperatures if the material undergoes thermal decomposition. Moreover, there may be overlapping mass loss and mass gain processes, meaning that some materials comprising oxygen vacancies may not show a mass gain (and sometimes not a mass loss or gain) during TGA analysis.

[0066] Other methods of determining whether oxygen deficiency e.g. oxygen vacancies, is present include Raman spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS, e.g. of oxygen 1s and / or and of cations in a mixed oxide), X-ray absorption near-edge structure (XANES, e.g. of cations in a mixed metal oxide), and TEM (e.g. scanning TEM (STEM) equipped with high-angle annular darkfield (HAADF) and annular bright-field (ABF) detectors). The presence of oxygen deficiency can be qualitatively determined by assessing the colour of a material relative to a non-oxygen-deficient sample of the same material, indicative of changes to its electronic band structure through interaction with light. For example, non-oxygen deficient stoichiometric Zn2Nb34Os7 has a white colour. Zn2Nb34O<87 with induced oxygen deficiency has a grey / black. The presence of vacancies can also be inferred from the properties, e.g. electrical conductivity, of a stoichiometric material compared to those of an oxygendeficient material.

[0067] When d > 0, additional anions Q are introduced into the oxide comprising Nb. Due to their differing electronic structure (e.g. F- vs O2) and differing ionic radii (6-coordinate O2-= 1 .40 A, 6-coordinate F- = 1 .33 A) they may improve electrochemical performance in the active material. This is due to altering unit cell characteristics with differing ionic radii allowing for improved Li ion capacity or improved Coulombic efficiencies by improving reversibility. They may additionally improve electrical conductivity as for oxygen vacancy defects, or sub-valent cation substitutions, by altering the electronic structure of the crystal ( / .e. doping effects), d may be 0 < d < 3.0, or 0 < d < 2.17. In each of these cases d may be > 0. Q may be selected from F, Cl, N, S, and mixtures thereof; or F, N, and mixtures thereof; or Q is F.

[0068] Optionally d = 0, in which case Formula 1 is M1aM22-aM3bNb34-bO87-c where M1 , M2, M3, a, b, and c are as defined herein. Advantageously, materials where d = 0 are free from anion Q and may be easier to synthesise.

[0069] When a > 0 and b = d = 0 Formula 1 is M1aM22-aNb34O87-c where M1 , M2, a, and c are as defined herein, for example 0 < c < 4.35. This represents a material which has been modified at the M2 site and optionally modified by induced oxygen deficiency. Such materials represent a particularly effective way to improve the properties of the ‘base’ oxide M22Nb34Os7 by simple synthetic means. Here, M1 may represent Ti, Mg, V, Cr, W, Zr, Mo, Cu, Ga, Ge, Ni, Al, Hf, Ta, Zn and mixtures thereof; preferably Ti, Mg, V, Cr, W, Zr, Mo, Ga, Ge, Al, Zn, and mixtures thereof. When a = b = d = 0 and c > 0 Formula 1 has the composition M22Nb34Os7-c where M2 and c are as defined herein. This represents a material which has been modified solely by inducing oxygen deficiency. For example, materials where a = b = d = 0 and c > 0 have been found to have surprisingly improved electronic conductivity.

[0070] It will be understood that the discussion of the variables of Formula 1 (M1 , M2, M3, Q, a, b, c, and d) is intended to be read in combination. For example, preferably M1 is selected from Mg, Ti, Zr, V, Nb, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Si, Ge, P, and mixtures thereof and M3 is selected from Mg, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof and Q is selected from F, Cl,

[0071] N, S, and mixtures thereof. Preferably 0 < a < 0.6, 0 < b < 1 .5, 0 < c < 4.35, and 0 < d < 3.0.

[0072] For example, Formula 1 may be M1aM22-aM3bNb34-bO87-c-dQd, wherein:

[0073] M1 and M2 are different;

[0074] M1 is selected from Mg, Ti, Zr, V, Nb, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Si, Ge, P, and mixtures thereof;

[0075] M2 is Zn or Cu;

[0076] M3 is selected from Mg, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof;

[0077] Q is selected from F, N, and mixtures thereof;

[0078] 0 < a < 0.6; 0 < b < 1 .5; -0.5 < c < 4.35; 0 < d < 4.35; one or more of a, b, c, and d does not equal 0; and when a, b, and d equal zero, c is greater than zero.

[0079] In a particularly preferred aspect, Formula 1 may be M1aZn2-aM3bNb34-bO87-c, wherein:

[0080] M1 is selected from Mg, Zr, V, Cr, Mo, W, Fe, Cu, Al, Ge, P, and mixtures thereof; M3 is selected from Ti, Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, P, and mixtures thereof; 0 < a < 1.0; 0 < b < 3.4; -0.5 < c < 4.35.

[0081] Formula 1 may be M1aM22-aM3bNb34-bO87-c, wherein:

[0082] M1 is selected from Cr, Al, Ge, and mixtures thereof, preferably wherein M1 is Cr;

[0083] M2 is Zn or Cu, preferably wherein M2 is Zn;

[0084] M3 is selected from Ti, Zr, Fe, and mixtures thereof and optionally comprises Ti, preferably wherein M3 is selected from Ti, Zr, and mixtures thereof and optionally comprises Ti, most preferably wherein M3 is Ti; 0 < a < 1 .0, preferably 0.01 < a < 1 .0;

[0085] 0 < b < 1 .5, preferably 0.01 < b < 1 .0;

[0086] -0.5 < c < 4.35, preferably -0.5 < c < 2, most preferably c = 0.

[0087] Formula 1 may be CraZn2-aM3bNb34-bO87-c, wherein:

[0088] M3 is selected from Ti, Zr, and mixtures thereof and optionally comprises Ti, preferably wherein M3 is Ti;

[0089] O.01 < a < 1 .0, preferably 0.1 < a < 1 .0;

[0090] 0.01 < b < 1 .0, preferably 0.1 < b < 1 .0;

[0091] -0.5 < c < 2, preferably c = 0. The oxide comprising Nb may have the formula M4aAli-aM5bNbn-bO29-c-dQd (Formula 2), wherein:

[0092] M4 is selected from Mg, Ca, Sr, Y, La, Ce, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi and mixtures thereof;

[0093] M5 is selected from Mg, Ca, Sr, Y, La, Ce, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi, and mixtures thereof;

[0094] Q is selected from F, Cl, Br, I, N, S, Se, and mixtures thereof;

[0095] 0 < a < 0.5; 0 < b < 1 ; -0.5 < c < 1 .45; 0 < d < 1 .45; one or more of a, b, and d does not equal 0.

[0096] Formula 2 represents an example of an oxide comprising Nb having the crystal structure of AINbnO29.

[0097] When a > 0, Formula 2 is modified by partial substitution of Al by M4. When b > 0 Formula 2 is modified by partial substitution of Nb by M5. When c 0, Formula 2 is modified by oxygen deficiency or excess. When d > 0 Formula 2 is modified by partial substitution of O by Q.

[0098] M4, M5, and Q may each represent two or more elements from their respective lists. An example of such a material is Zn005Ga005AI09NbnO28975. Here, M4 is ZnaGaa” (where a’ + a” = a), a=0.1 , b=0, c=0.025, d=0. Here, c has been calculated assuming that each cation adopts its typical oxidation state, i.e. Zn2+Ga3+, APT

[0099] When exchange of the cations or anions in the structure (i.e. Al, Nb, O) have taken place without preserving the initial valency, this can give rise to both oxygen deficiency and excess. For example, a material that substitutes Al3+by Ge4+to some extent will demonstrate minor oxygen excess (i.e. AI2O3 vs GeO2), whereas substitution of Nb5+by Al3+will show a minor oxygen deficiency (i.e. Nb2Os vs AI2O3). Oxygen deficiency can also be induced through thermal treatment in inert or reducing conditions, which results in induced oxygen vacancy defects in the structure.

[0100] There may be partial oxidation or partial reduction to compensate for exchange which does not preserve the initial valency. For example, substitution of Al3+by Ge4+may be compensated at least in part by reduction of some Nb5+to Nb4+.

[0101] M4 is a cation which substitutes for Al in the crystal structure. M4 may be selected from Mg, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Ga, Si, Ge, Sn, P, and mixtures thereof; preferably Mg, Zr,

[0102] V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Ga, Si, Ge, P, and mixtures thereof; most preferably Mg, Zr, Mo,

[0103] W, Cu, Zn, Ga, Ge, P, and mixtures thereof. M4 may have a different valency than Al3+. This gives rise to oxygen deficiency or excess.

[0104] Optionally, M4 has an equal or lower valency than Al3+, preferably lower.

[0105] The amount of M4 is defined by a, meeting the criterion 0 < a < 0.5. a may be 0 < a < 0.4, preferably 0 < a < 0.2. Most preferably, a > 0, for example a > 0.01 . Higher values of a may be more readily achieved when M4 has the same valency as Al3+. When M4 comprises a cation with a 3+ valency (for example Ga) a may be 0 < a < 0.5. When M4 does not comprise a cation with a 3+ valency a may be 0 < a < 0.1 . M5 is a cation which substitutes for Nb in the crystal structure. M5 may be selected from Mg, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, Si, Sn, P, and mixtures thereof; preferably Mg, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof; most preferably Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, P, and mixtures thereof. M5 may have a different valency than Nb5+. This gives rise to oxygen deficiency or excess. Preferably, M5 has a lower valency than Nb5+. This gives rise to oxygen deficiency, i.e. the presence of oxygen vacancies.

[0106] The amount of M5 is defined by b, meeting the criterion 0 < b < 1 . b may be 0 < b < 0.5, preferably 0 < b < 0.1 . In each of these cases b may be > 0, e.g. b > 0.01 . Higher values of b may be more readily achieved when M5 has the same valency as Nb5+. When M5 comprises a cation with a 5+ valency (for example Ta) b may be 0 < b < 1 . When M5 does not comprise a cation with a 5+ valency b may be 0 < b < 0.05.

[0107] Optionally, both a and b are > 0. When both a and b are > 0 the ‘base’ material has been substituted at both the Al site and at the Nb site. c reflects the oxygen content of Formula 2. When c is greater than 0, it forms an oxygen-deficient material, i.e. the material has oxygen vacancies. Alternatively, c may equal 0, in which it is not an oxygen-deficient material, c may be below 0, which is a material with oxygen excess, c may be -0.25 < c < 1.45.

[0108] When c is 1 .45, the number of oxygen vacancies is equivalent to 5at% of the total oxygen in the crystal structure, c may be greater than 0.0145, greater than 0.029, greater than 0.0435, or greater than 0.145. c may be between 0 and 1 , between 0 and 0.75, between 0 and 0.5, or between 0 and 0.25. For example, c may satisfy 0.01 < c < 1 .45.

[0109] When d > 0, additional anions Q are introduced into Formula 2. d may be 0 < d < 1 .0, or 0 < d < 0.7. In each of these cases d may be > 0, for example > 0.01 . Q may be selected from F, Cl, N, S, and mixtures thereof; or F, N, and mixtures thereof; or Q is F.

[0110] Optionally d = 0, in which case Formula 2 is M4aAli-aM5bNbn-bO29-c where M4, M5, a, b, and c are as defined herein. Advantageously, materials where d = 0 are free from anion Q and may be easier to synthesise.

[0111] When a > 0 and b = d = 0 Formula 2 has the composition M4aAli-aNbnO29-c where M4, a, and c are as defined herein, for example 0 < c < 1 .45. Such materials represent a particularly effective way to improve the properties of the ‘base’ oxide AINbnO29 by simple synthetic means. Here, M4 may represent Mg, V, Cr, W, Zr, Mo, Cu, Ga, Ge, Ni, Hf, Ta, Zn and mixtures thereof; preferably Mg, V, Cr, W, Zr, Mo, Ga, Ge, Zn, and mixtures thereof. It will be understood that the discussion of the variables of Formula 2 (M4, M5, Q, a, b, c, and d) is intended to be read in combination. For example, preferably M4 is selected from Mg, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Ga, Si, Ge, P, and mixtures thereof and M5 is selected from Mg, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof and Q is selected from F, Cl, N, S, and mixtures thereof. Preferably 0 < a < 0.4, 0 < b < 0.5, 0 < c < 1 .45, and 0 < d < 1 .0.

[0112] For example, Formula 2 may be M4aAli-aM5bNbn-bO29-c-dQd, wherein:

[0113] M4 is selected from Mg, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Ga, Si, Ge, P, and mixtures thereof; M5 is selected from Mg, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof; Q is selected from F, Cl, N, S and mixtures thereof;

[0114] 0 < a < 0.4; 0 < b < 0.5; -0.25 < c < 1 .45; 0 < d < 1 .45.

[0115] For example, Formula 2 may be M4aAli-aNbnO29-c-dQd, wherein:

[0116] M4 is selected from Mg, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Ga, Si, Ge, P, and mixtures thereof, preferably M4 is selected from Zr, Cr, Zn, Ga, and mixtures thereof;

[0117] Q is selected from F, N, and mixtures thereof, preferably wherein Q is F; 0 < a < 0.5; 0 < c < 1 .45; 0 < d < 1 .45.

[0118] For example, Formula 2 may be M4aAli-aNbnO29-c-dQd, wherein:

[0119] M4 is selected from Mg, Zr, Mo, W, Cu, Zn, Ga, Ge, P, and mixtures thereof, preferably M4 is selected from Zr, Zn, Ga, and mixtures thereof;

[0120] Q is selected from F, N, and mixtures thereof, preferably wherein Q is F; 0 < a < 0.2; 0 < c < 1 .45; 0 < d < 1 .45.

[0121] The oxide of Formula 2 may further comprise Li and / or Na. For example, Li and / or Na may enter the crystal structure when the oxide is used in a metal-ion battery electrode.

[0122] The oxide comprising Nb may have the formula M6aPx-aM7bNb9-bO25-c-dQd (Formula 3) wherein:

[0123] M6 is selected from Na, K, Mg, Ca, Sr, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof;

[0124] M7 is selected from Na, K, Mg, Ca, Sr, Y, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, P, Sb, and mixtures thereof;

[0125] Q is selected from F, Cl, Br, I, N, S, Se, and mixtures thereof;

[0126] 0 < a < 0.5; 0 < b < 2; -0.5 < c < 1 .25; 0 < d < 5; 1 < x < 2; one or more of a, b, c, and d does not equal 0; with the proviso that if M6 consists of Nb and if M7 consists of P then c is > 0.

[0127] Formula 3 represents an example of an oxide comprising Nb having the crystal structure of PNbgO25.

[0128] M6, M7, or Q may each represent two or more elements from their respective lists. An example of such a material is Tioo5Moo o5Po9oNb9025. Here, M6 represents Tia Moa- (where a’ + a” = a), a=0.1 , b=0, c=0, and d=0. Another example of such a material is AI005P095Ti0225Mo0225Nb855O2495. Here, M6 represents Ala, M7 represents Tib MOb (where b’ + b” = b), a=0.05, b=0.45, c=0.05, and d=0. M6 is a cation which substitutes for P in the crystal structure. M6 may be selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof; or Ti, Zr, Hf, Cr, Mo, W, B, Al, Ga, Bi, Sb, and mixtures thereof; or Ti, Mo, Al, B, and mixtures thereof. Preferably M6 is not Nb. Preferably, M6 is not Na. M6 may have a different valency than P5+. This gives rise to oxygen deficiency or excess. Preferably, M6 has a lower valency than P5+. This gives rise to oxygen deficiency, i.e. the presence of oxygen vacancies.

[0129] The amount of M6 is defined by a, meeting the criterion 0 < a < 0.5. a may be 0 < a < 0.3, preferably 0 < a

[0130] < 0.2. In each of these cases a may be > 0, for example > 0.01 .

[0131] M7 is a cation which substitutes for Nb in the crystal structure. M7 may be selected from Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, P, Sb, and mixtures thereof; or Ti, Zr, Hf, Cr, Mo, W, V, Ta, and mixtures thereof; or Ti, Mo, and mixtures thereof. Preferably, M7 is not P. Preferably, M7 is not Na. M7 may have a different valency than Nb5+. This gives rise to oxygen deficiency or excess. Preferably, M7 has a lower valency than Nb5+. This gives rise to oxygen deficiency.

[0132] The amount of M7 is defined by b, meeting the criterion 0 < b < 2. b may be 0 < b < 1 .5, preferably 0 < b < 1 , or 0 < b < 0.9. In each of these cases b may be > 0, for example > 0.01 .

[0133] Preferably, at least one of a and b is > 0. Both of a and b can be > 0. c reflects the oxygen content of the active electrode material, c may be -0.25 < c < 1 .25. Preferably c is 0

[0134] < c < 1 .25. Optionally, if a = b = 0 then c > 0; preferably if a = b = 0 then c > 0.

[0135] When c is 1 .25, the number of oxygen vacancies is equivalent to at5% of the total oxygen in the crystal structure, c may be greater than 0.0125 (0.05% oxygen vacancies), greater than 0.025 (0.1 % oxygen vacancies), greater than 0.05 (0.2% oxygen vacancies), or greater than 0.125 (0.5% oxygen vacancies), c may be between 0 and 1 (4% oxygen vacancies), between 0 and 0.75 (3% oxygen vacancies), between 0 and 0.5 (2% oxygen vacancies), or between 0 and 0.25 (1 % oxygen vacancies). For example, c may satisfy 0.01 < c < 1 .25.

[0136] Formula 3 may be PxNbgO25-c-dQd where x, c, d, and Q are as defined herein. The amount of oxygen vacancies and excess may be expressed relative to the total amount of oxygen in the base material, i.e. the amount of oxygen in the un-substituted material (e.g. PNbgO2s) or the material before heating under reducing conditions.

[0137] When d > 0, additional anions Q are introduced into the phosphorus niobium oxide, d may be 0 < d < 2.5, or 0 < d < 1 . In each of these cases d may be > 0. Q may be selected from F, Cl, N, S, and mixtures thereof; or F, N, and mixtures thereof; or Q is N. Optionally d = 0, in which case the material has the composition M6aPx-aM7bNb9-bO25-c where M6, M7, a, b, c, and x are as defined herein. Advantageously, materials where d = 0 are free from anion Q and may be easier to synthesise. x reflects the amount of phosphorus in the material, meeting the criterion 1 < x < 2. x may be 1 < x < 1 .25. Preferably, x = 1 . When x = 1 Formula 3 is based on PNbgO25.

[0138] It will be understood that the discussion of the variables of the composition (M6, M7, Q, a, b, c, d, and x) is intended to be read in combination. For example, preferably M6 is selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof and M7 is selected from Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, P, Sb, and mixtures thereof. M6 may be selected from Ti, Zr, Hf, Cr, Mo, W, B, Al, Ga, Bi, Sb, and mixtures thereof and M7 may be selected from Ti, Zr, Hf, Cr, Mo, W, V, Ta, and mixtures thereof. M6 may be selected from Ti, Mo, Al, B, and mixtures thereof and M7 may be selected from Ti, Mo, and mixtures thereof. M6 is preferably not Nb and M7 is preferably not P. M6 and M7 are preferably not Na. M6 and M7 may be different, a may be 0 < a < 0.3 and b may be 0 < b < 1 .5. Preferably 0 < a < 0.2 and 0 < b < 1 . In each of these cases a and / or b may be > 0.

[0139] For example, Formula 3 may be M6aPx-aM7bNb9-bO25-c-dQd, wherein:

[0140] M6 is selected from Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof;

[0141] M7 is selected from Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof;

[0142] Q is selected from F, Cl, N, S, and mixtures thereof;

[0143] 0 < a < 0.3; 0 < b < 1 .5; -0.25 < c < 1 .25; 0 < d < 2.5; 1 < x < 1 .25; one or more of a, b, c, and d does not equal 0.

[0144] For example, Formula 3 may be M6aPi-aM7bNb9-bO25-c-dQd, wherein:

[0145] M6 is selected from Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof;

[0146] M7 is selected from Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Ga, Si, Ge, Sn, Bi, Sb, and mixtures thereof;

[0147] Q is selected from F, N, and mixtures thereof; 0 < a < 0.3; 0 < b < 1.5; 0 < c < 1 .25; 0 < d < 2.5; one or more of a, b, c, and d does not equal 0.

[0148] For example, Formula 3 may be M6aPi-aM7bNb9-bO25-c-dQd, wherein:

[0149] M6 is selected from Ti, Zr, Hf, Cr, Mo, W, B, Al, Ga, Ge, Bi, Sb, and mixtures thereof;

[0150] M7 is selected from Ti, Zr, Hf, Cr, Mo, W, V, Ta, Ga, Ge, and mixtures thereof;

[0151] Q is selected from F, N, and mixtures thereof; 0 < a < 0.2; 0 < b < 1 ; 0 < c < 1 .25; 0 < d < 2.5; wherein at least one of a and b is > 0. In another preferred implementation, the oxide comprising Nb also comprises Ti and further comprises M(lll) and / or M(l I); wherein M(lll) is selected from Cr, Al, Ga, and mixtures thereof, preferably Cr, Al, and mixtures thereof, most preferably Cr; M(ll) is selected from Zn, Cu, Mg, and mixtures thereof, preferably Zn, Cu, and mixtures thereof, most preferably Zn; wherein the oxide comprising Nb comprises an interpenetrating mixture of a first phase and a second phase; wherein the first phase has the crystal structure of TiNb2O7and the second phase has the crystal structure of Zn2Nb34Os7. The first phase and second phase may be identified by XRD, typically by a peak A attributed to the first phase at 20 = 26.0±0.1 ° and a peak B attributed to the second phase at 20 = 24.9±0.1 °. The ratio of the intensity IB of peak B to the intensity IA of peak A may be 0 < IB / IA 0.4, or 0.01 < IB / IA 0.25, or 0.05 < IB / IA 0.22, or 0.07 < IB / IA 0.16. Preferably the first phase forms at least 85 wt%, or at least 90 wt%, or at least 92 wt% of the oxide comprising Nb. Nb and Ti are the major cations, typically such that the oxide comprising Nb comprises 66-80 at% Nb, 33-17 at% Ti, and further comprising >0-1 at% M(lll) and / or >0-2 at% M(ll) relative to all cations. M(lll) is preferably present, and most preferably both M(lll) and M(l I) are present.

[0152] The oxide comprising Nb may be a lithium-free mixed Ti and Nb oxide, comprising at least one trivalent metal M (e.g. selected from the group consisting of Fe, Ga, Mo, Al, B, and mixtures thereof) and having a molar ratio Nb / Ti greater than 2, wherein the oxide is selected from a material of formula (I) and a material of formula (II):

[0153] MxTii-2xNb2+xO7±6(I) where 0<x<0.20; -0.3<6<0.3; MxTi2-2xNbio±x029±6(II) where 0<x<0.40; -0.3<6<0.3.

[0154] The oxide comprising Nb may be made by: processing a precursor mixture comprising a Nb precursor, a Ti precursor, and a metal halide to form the active electrode material; wherein the atomic ratio of Nb:Ti in the precursor mixture is > 2; wherein the atomic ratio of cations:anions in the precursor mixture does not correspond to the atomic ratio of cations:anions of TiNb2O7, Ti2NbwO29, TiNbuOs?, or TiNb24G62; wherein the oxide comprising Nb has the crystal structure of TiNb2O7, Ti2NbwO29, TiNbuOs?, and / or TiNb24O62; most preferably TiNb2O7.

[0155] The metal halide may be a metal fluoride, metal chloride, metal bromide; or a metal fluoride or a metal chloride; most preferably a metal fluoride. Preferably the metal of the metal halide is not Nb or Ti. The metal of the metal halide may be Mg, Al, Zn, Cr, Ni, Nb, Cu, Mn, Fe, Zr, Ga, Ge, Sn, and mixtures thereof; or Al, Zn, Cr, Fe, Zr, Nb, and mixtures thereof; preferably Al, Zn, and mixtures thereof. The metal halide may be present in an amount such that the metal of the metal halide is at present at >0.1 , 0.2-5, 0.3-3, or <6 at% relative to the amount of Nb and Ti in the precursor mixture.

[0156] The coating is formed of a coating material selected from: metal oxides, metal sulfates, metal phosphates, metalloid oxides, metalloid sulfates, and metalloid phosphates; preferably from metal oxides or metalloid oxides. Metalloids will be understood to mean B, Si, Ge, As, Sb, and Te. When the coating material is a sulfate or phosphate, preferably it is lithium sulfate or lithium phosphate. The coating material may comprise a metal or metalloid selected from: Al, B, P, Mg, Zn, Ce, Li, Ti, S, Zr, Fe, Co, Ni, Cu, Ca, Sr, Ba, Sn, Y, W, Mn, V, Cr, Ga, In, Si, Mo, Ta, Sc, Pb, and mixtures thereof; or Al, B, P, Mg, Zn, Ce, Li, Ti, S, Zr, Fe, Cu, Ca, Sr, Ba, Sn, Y, W, Si, Ta, and mixtures thereof; or Al, B, P, Mg, Zn, Ce, Li, Ti, S, Zr, and mixtures thereof; or Al, B, P, Mg, Zn, Ce, Ti, S, Zr, and mixtures thereof; or Al, B, Mg, Zn, Ce, Ti, Zr, and mixtures thereof; or preferably Zn, Mg, B, and mixtures thereof; most preferably Zn. Oxides are preferred as the coating material. For example, the coating material may be expressed by the formula: [A]a[O]b, wherein [A] is a metal or metalloid as defined herein. It will be understood that the ratio of the atomic amounts of a general formula are such that the coating material is charge-balanced. The coating material may comprise an anionic and / or cationic dopant. It will be understood that an oxide comprising an anionic dopant is still considered to be an oxide coating material.

[0157] Reactions may occur at the interface between the oxide comprising Nb and the coating material or precursor thereof, e.g. during preparation of the active electrode material. This may result in the formation of interface phases comprising one or more cations from the the oxide comprising Nb (typically Nb) and one or more cations from the coating material or precursor. For example, zinc niobate phases (e.g.

[0158] Zn2Nb34Os7) may be formed by a reaction between the oxide comprising Nb and a coating material or precursor comprising Zn. Similarly, titanium niobate phases (e.g. TiNb2O?, Ti2NbwO29, TiNbuCh?, or TiNb24C>62) may be formed when the coating material or precursor comprises Ti. Thus, the active electrode material may comprise an interface phase formed from a reaction between the oxide comprising Nb and the coating material or precursor thereof. In some cases, the interface phase is the coating material.

[0159] As specific examples, the coating material can be selected from zinc oxide, aluminium oxide, boron oxide, magnesium oxide, cerium oxide, lithium oxide, titanium oxide, zirconium oxide, lithium sulfate, lithium phosphate, and mixtures thereof. Typically, these are in the common form as would be known to the skilled person (e.g. ZnO, B2O3, MgO, CeC>2, Li2O, TiC>2, ZrC>2, Li2SC>4, and LisPC ) but some off- stoichiometry and / or doping may be present. Preferably the coating material is selected from zinc oxide, magnesium oxide, lithium sulfate, and mixtures thereof; more preferably zinc oxide, lithium sulfate, and mixtures thereof.

[0160] Zinc oxide is the most preferred coating material. The coating material may comprise at least 75 wt% zinc oxide relative to the mass of the coating material, or consist essentially of zinc oxide, or consist of zinc oxide. As mentioned, the zinc oxide may be doped, e.g. with Ga, Al, B, Be, Cr, Ce, Ti, In, Co, Ni, Cu, Mn, Ge, and mixtures thereof.

[0161] The coating material may comprise at least 75 wt% magnesium oxide relative to the mass of the coating material, or consist essentially of magnesium oxide, or consist of magnesium oxide.

[0162] The coating material may comprise at least 75 wt% boron oxide relative to the mass of the coating material, or consist essentially of boron oxide, or consist of boron oxide. The coating material may comprise at least 75 wt% lithium sulfate relative to the mass of the coating material, or consist essentially of lithium sulfate, or consist of lithium sulfate.

[0163] Other coating materials for use in the invention include the following: silicates such as Li2SiOs, Li2SisOn , Li2Si2Os, Li2SiOe, LiAISiO4, Li4SiO4, LiAISi2Oe; oxides such as a coating of AI2O3, LiNbOs; fluorides such as AIF3, LaFs, CaF2, LiF, CeFs; compounds of the anti-perovskite type chosen from: U3OA with A a halide or a mixture of halides, preferably at least one of the elements chosen from F, Cl, Br, I or a mixture of two or three or four of these elements; Li<3-x)Mx / 2OA with 0<x<3, M a divalent metal, preferably at least one of the elements Mg, Ca, Ba, Sr or a mixture of two or three or four of these elements, A a halide or a mixture of halides, preferably at least one of the elements F, Cl, Br, I or a mixture of two or three or four of these elements; Lip-xjMs-x / sOA with 0<x<3, M3 a trivalent metal, A a halide or a mixture of halides, preferably at least one of the elements F, Cl, Br, I or a mixture of two or three or four of these elements; or LiCOXzY(i-z), with X and Y halides such as mentioned above in relation with A, and 0<z<1 ; tin oxide, zinc oxide, indium oxide, gallium oxide, a mixture of two of these oxides such as indium-tin oxide corresponding to a mixture of indium oxide and tin oxide, a mixture of three of these oxides or a mixture of four of these oxides; doped indium oxides, the doping being preferably with Sn, and / or Ga and / or Cr and / or Ce and / or Ti and / or In and / or Co and / or Ni and / or Cu and / or Mn and / or Ge; doped tin oxides, the doping being preferably with As and / or F and / or N and / or Nb and / or P and / or Sb and / or Al and / or Ti, and / or Ga and / or Cr and / or Ce and / or In and / or Co and / or Ni and / or Cu and / or Mn and / or Ge.

[0164] The active electrode material may further comprise a different coating, such as a carbon coating. Alternatively, the active electrode material may be free from carbon coatings. The coating may consist essentially of the coating material or consist of the coating material.

[0165] The coating may be present in any morphology. For example, the coating may cover substantially all, or all, of the particles of the oxide comprising Nb. A coating covering substantially all, or all, of the surface may have a substantially uniform thickness (e.g. a variation in thickness of less than about 20%, preferably of less than about 15% and more preferably of less than about 10%). The coating may be a conformal coating, e.g. as made by ALD.

[0166] Alternatively, the coating is a partial coating, preferably an island coating. An island coating is a preferred form of coating which may be formed by a dry or wet coating process, e.g. using a particulate coating material or precursor. The term “island coating” would be known to the skilled person and is commonly used, such as in Energy Storage Materials, Volume 38, June 2021 , Pages 309-328. In particular, an island coating comprises discrete regions of coating material on the surface of the particles of oxide comprising Nb, typically at least 10 discrete regions per particle of oxide comprising Nb when viewing at least 5 particles by SEM. A partial coating can also be in the form of a continuous coating having gaps, which may be formed by a wet coating process, e.g. from salts deposited during wet mixing with subsequent spray drying and / or annealing.

[0167] The coverage of a partial coating may be such that at least 50%, or at least 60%, preferably at least 75%, or most preferably at least 85% of the surface of the particles of the oxide comprising Nb is not covered by the coating material. The coverage refers to the proportion of the surface of the particles which is not in contact with the coating material, e.g. the area between islands for an island coating. A partial coating may be formed of particles of the coating material, where the particles have a median diameter of 1-1 ,000 nm, or 5-750 nm, preferably 10-500 nm, most preferably 50-250 nm.

[0168] It has surprisingly been found that a partial coating can provide a significant improvement to the cycle life and / or gassing of a metal-ion battery comprising the active electrode material, compared to an equivalent uncoated active electrode material. Reactive species in the electrolyte are theorised to react preferentially with the coating material, minimising unwanted reactions which could degrade the underlying oxide comprising Nb. Accordingly, the coating is believed to act as a scavenger rather than as physical barrier, advantageously having little effect on the conductivity of the particles and not hindering lithium intercalation. Advantageously, partial coatings can be made using cheap, scalable synthesis methods.

[0169] The morphology of the coating may be assessed by SEM image analysis, as is widely known. Various open-source image analysis software are available including ilastik, scikit-image, and Imaged. The particle size of the coating material can be expressed as the number-weighted median diameter of a circle having the same area as a particle of coating material for a sample size of at least 200 particles of coating material viewed by SEM. The coverage of the coating material can be determined by using image analysis to identify coating material and oxide comprising Nb and calculating: observed area of coating material - - - x 100% observed area of coating material + observed area of oxide comprising Nb

[0170] The magnification and field of view for the SEM images can be chosen based on the particle size of the active electrode material. A field of view of 14.1 pm and a magnification of 9,000x may be suitable e.g. for active electrode material particles having a Dso particle diameter of about 5 pm.

[0171] The coating material can be present in an amount of 0.1-15 wt%, preferably 0.1-10 wt%, more preferably 0.3-10 wt%, or most preferably 0.5-5 wt%, or 0.7-1 .3 wt%, or about 1 wt% relative to the combined mass of the oxide comprising Nb and coating material. Surprisingly, these low amounts of coating, even as partial coatings, have been found to provide a significant benefit in cycle life and / or gassing. The amount of the coating material can be achieved by controlling the amounts of precursors and process used for making the active electrode material. The amount of the coating material can be verified by elemental analysis of the active electrode material, e.g. by ICP-OES, based on knowledge of the composition of the oxide comprising Nb. The active electrode material has a Dso particle diameter of at least 250 nm, or 0.5-100 pm, or 0.75-50 pm, preferably 1.0-20 pm. It will be understood that physical parameters, e.g. particle size parameters, of the active electrode material refer to the particles of oxide comprising Nb and the coating. These particle sizes are advantageous because they are easy to process and fabricate into electrodes. Moreover, these particle sizes avoid the need to use complex and / or expensive methods for providing nanosized particles. Nanosized particles (e.g. particles having a Dso particle diameter of 100 nm or less) are typically more complex to synthesise and require additional safety considerations.

[0172] The active electrode material may have a D particle diameter of at least 0.05 pm, or at least 0.1 pm, or at least 0.5 pm, or at least 1 pm. By maintaining a Dw particle diameter within these ranges, the potential for parasitic reactions in a Li ion cell is reduced from having reduced surface area, and it is easier to process with less binder in the electrode slurry.

[0173] The active electrode material may have a D90 particle diameter of no more than 200 pm, no more than 100 pm, no more than 50 pm, or no more than 20 pm. By maintaining a D90 particle diameter within these ranges, the proportion of the particle size distribution with large particle sizes is minimised, making the material easier to manufacture into a homogenous electrode.

[0174] The term “particle diameter” refers to the equivalent spherical diameter (esd), i.e. the diameter of a sphere having the same volume as a given particle, where the particle volume is understood to include the volume of any intra-particle pores. The terms “Dn” and “Dnparticle diameter” refer to the diameter below which n% by volume of the particle population is found, i.e. the terms “D50” and “D50 particle diameter” refer to the volume-based median particle diameter below which 50% by volume of the particle population is found. Where a material comprises primary crystallites agglomerated into secondary particles, it will be understood that the particle diameter refers to the diameter of the secondary particles. Dnparticle diameters are preferably determined by laser diffraction. Particle diameters can be determined in accordance with ISO 13320:2020, for example using Mie theory.

[0175] The active electrode material may have a BET surface area in the range of 0.1-100 m2 / g, or 0.2-50 m2 / g, or 0.5-20 m2 / g. In general, a low BET surface area is preferred to minimise the reaction of the oxide comprising Nb with the electrolyte, e.g. minimising the formation of solid electrolyte interphase (SEI) layers during the first charge-discharge cycle of an electrode comprising the material. However, a BET surface area which is too low results in unacceptably low charging rate and capacity due to the inaccessibility of the bulk of the oxide comprising Nb to metal ions in the surrounding electrolyte.

[0176] The term “BET surface area” refers to the surface area per unit mass calculated from a measurement of the physical adsorption of gas molecules on a solid surface, using the Brunauer-Emmett-Teller theory. For example, BET surface areas can be determined in accordance with ISO 9277:2022.

[0177] The coating may be applied by a variety of methods involving combining an oxide comprising Nb as defined herein with a coating material as defined herein or a precursor thereof. Typically, the method comprises annealing the resulting combination. The annealing step improves the adhesion between the oxide comprising Nb and coating material. When precursors are used, the annealing step converts the precursor to the coating material. Annealing is typically performed at a temperature of at least 150 °C, or 175-900 °C, or 200-800 °C, or 250-750 °C, or 300-500 °C. Annealing is preferably carried out in air or in an oxidising atmosphere, thereby facilitating the formation of an oxide coating material. However, annealing is not essential. The coating material may be adequately adhered to the oxide comprising Nb by high-energy milling or mechanofusion. The particle size of the oxide comprising Nb used in the method may be defined by any of the particle size parameters above for the active electrode material, as the coating typically has minimal effect on the particle size.

[0178] Following coating, optional post-processing steps may be implemented, such as de-agglomeration, milling, classification.

[0179] A preferred class of method is a solvent-free or “dry” coating method. Here, the oxide comprising Nb is combined with the coating material or precursor thereof typically by dry mixing such as mechanofusion, low or high shear mixing, impaction mixing, and / or manual mixing. This method is advantageous as it avoids the need for additional reagents, e.g. mixing solvents, thus reducing costs and minimising manufacturing complexity. Preferably, dry coating methods use the coating material itself (e.g. a metal or metalloid oxide) rather than a precursor, avoiding the risk of introducing impurities from the precursor into the active electrode material.

[0180] For example, dry coating can be achieved by mixing particles of an oxide comprising Nb with particles of metal or metalloid oxide (e.g. zinc oxide) in a high shear impaction mixer for at least 5 minutes, followed by annealing at least at 300 °C for at least 2 hours.

[0181] Solvent-based or “wet” coating methods can be used. Here, the oxide comprising Nb is combined with the coating material or precursor thereof dispersed in a solvent, aiding the mixing between the two. The resulting combination may be mixed, e.g. by high shear mixing. The resulting combination is annealed to form the active electrode material. A spray-drying step may be used prior to annealing. For making oxide coatings, a precursor which is soluble in the solvent and which forms the oxide upon annealing is preferred.

[0182] For example, wet coating can be achieved by mixing particles of an oxide comprising Nb with a precursor salt of the coating material (e.g. zinc acetate), using 10 wt% water to aid dispersion, in a high shear impaction mixer for at least 5 minutes. The result is annealed at least at 300 °C for at least 2 hours then deagglomerated in a low intensity mixer for at least 2 minutes.

[0183] In another example, wet coating can be achieved by mixing particles of an oxide comprising Nb with a precursor salt of the coating material (e.g. zinc nitrate), using 80 wt% waterto aid dispersion, in a high intensity mixer for at least 15 minutes. The result is spray dried e.g. with 220-240 °C inlet temperature and 110 °C constant outlet temperature, with sample collected by cyclonic separation. The result is annealed at least at 300 °C for at least 2 hours. Chemical vapour deposition (CVD) may be used to make the active electrode material, such as low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), and atomic layer deposition (ALD). CVD is performed at elevated temperatures which constitutes the annealing step of the coating method. An active electrode material made by CVD can be subjected to a further annealing step.

[0184] Further methods for making the coating on the oxide comprising Nb include vacuum thermal evaporation, electron beam evaporation, laser beam evaporation, arc evaporation, molecular beam epitaxy, vapor phase epitaxy, ion plating, sputtering (e.g. direct current sputtering and radio frequency sputtering), solgel techniques, chemical bath deposition, spray pyrolysis, electroplating, electroless deposition, chemical solution deposition, spin-coating, dip-coating, molecular layer deposition, and physical vapour deposition.

[0185] The coating material may be combined with the oxide comprising Nb directly or in the form of a precursor which can be converted to the coating material, e.g. by annealing. The precursor is preferably selected from inorganic or organic salts such as alcoholates, carbonates, nitrates, hydroxides, hydrides, sulphates, carboxylates, halides, oxalates, bisoxalatoborates, bis(trifluoromethanesulfonyl)imides, citrates, phosphates, complexes comprising EDTA, complexes comprising rotaxanes, and mixtures thereof; preferably alcoholates, oxalates, acetates, and nitrates. When CVD is used, a suitable gaseous precursor may be chosen as is known in the art. When the coating material is combined directly it is used in the form in which it is expected to adopt in the active electrode material, e.g. a metal or metalloid oxide coating material may be combined directly by combining the oxide comprising Nb with the metal or metalloid oxide.

[0186] The coating material or precursor thereof may be used in particulate form. Typically, the coating material or precursor thereof has a Dso particle diameter of less than 500 nm, or less than 250 nm, preferably less than 150 nm. The coating material or precursor thereof may have a number-weighted median diameter of less than 150 nm or less than 100 nm, referring to a circle having the same area as a particle for a sample size of at least 200 particles viewed by SEM.

[0187] A particular example of the first aspect is a particulate active electrode material having a Dso particle diameter of at least 250 nm, comprising: particles of an oxide comprising Nb having a Wadsley-Roth crystal structure, the particles comprising a partial coating formed of a coating material; wherein the coating material is selected from zinc oxide, magnesium oxide, lithium sulfate, and mixtures thereof (preferably wherein the coating material is zinc oxide); wherein the coating material is present in an amount of 0.1-10 wt% (preferably 0.3-10 wt%), relative to the combined mass of the oxide comprising Nb and coating material.

[0188] Another particular example of the first aspect is a particulate active electrode material having a Dso particle diameter of 0.5-100 pm, comprising: particles of an oxide comprising Nb having a Wadsley-Roth crystal structure, the particles comprising a partial coating formed of a coating material; wherein the coating material is selected from zinc oxide, magnesium oxide, lithium sulfate, and mixtures thereof (preferably wherein the coating material is zinc oxide); wherein the coating material is present in an amount of 0.1-10 wt% (preferably 0.5-10 wt%), relative to the combined mass of the oxide comprising Nb and coating material; wherein at least 60% of the surface of the particles of the oxide comprising Nb is not covered by the coating material.

[0189] Another particular example of the first aspect is a particulate active electrode material having a Dso particle diameter of 0.75-50 pm, comprising: particles of an oxide comprising Nb and Ti having a Wadsley-Roth crystal structure, the particles comprising an island coating formed of a coating material; wherein the coating material is selected from zinc oxide, magnesium oxide, lithium sulfate, and mixtures thereof (preferably wherein the coating material is zinc oxide); wherein the coating material is present in an amount of 0.3-10 wt% (preferably 0.5-5 wt%), relative to the combined mass of the oxide comprising Nb and Ti and coating material; wherein at least 75% of the surface of the particles of the oxide comprising Nb and Ti is not covered by the coating material; and the coating is formed of particles of the coating material having a number-weighted median diameter of 10-500 nm.

[0190] Another particular example of the first aspect is a particulate active electrode material having a Dso particle diameter of 1 .0-20 pm, comprising: particles of an oxide having a Wadsley-Roth crystal structure and comprising Nb and Ti (and preferably Cr and Zn), the particles comprising an island coating of zinc oxide; wherein the zinc oxide is present in an amount of 0.5-5 wt% relative to the combined mass of the oxide comprising Nb and the zinc oxide; wherein at least 85% of the surface of the particles of the oxide comprising Nb is not covered by the zinc oxide; and the coating is formed of particles of zinc oxide having a number-weighted median diameter of 50-250 nm.

[0191] A mixture may be formed of a first active electrode material according to the first aspect and a second active electrode material according to the first aspect, where the first and second materials have a different coating material, a different coating morphology, a different oxide comprising Nb, and combinations thereof. A mixture may be formed of an active electrode material according to the first aspect and an uncoated oxide comprising Nb as described herein. The oxide comprising Nb may be the same for the coated and uncoated materials. A mixture may be formed of an active electrode material according to the first aspect and a different active electrode material which is not according to the first aspect. The different material may be lithium titanate (LTO), graphite, hard carbon, soft carbon, silicon, silicon oxide, doped versions thereof, and mixtures thereof.

[0192] The electrode of the second aspect is typically of the form of an electrode composition in electrical contact with a current collector, where the electrode composition comprises the active electrode material. A current collector is typically a metal foil, e.g. copper or aluminium foil.

[0193] Optionally, the active electrode material forms at least 25 wt.%, at least 50 wt.%, or at least 75 wt.% of the total active electrode material in the electrode. The active electrode material may form the sole active electrode material in the electrode. The active electrode material of the first aspect may consist of the particles of the oxide comprising Nb, the particles comprising the at least partial coating formed of the coating material; this active electrode material may form part of a composition, e.g. an electrode composition, comprising at least one other component. The electrode composition may further comprise at least one other component selected from a binder, a conductive additive, an active electrode material other than that of the first aspect, and mixtures thereof. For instance, one electrode composition comprises about 92 wt% active electrode material of the first aspect, about 5 wt% conductive additives (e.g. carbon black), and about 3 wt% binder (e.g. poly(vinyldifluoride)), based on the total dry weight of the electrode composition. An active electrode material other than that of the first aspect may be selected from LTO, titanium niobium oxide, graphite, hard carbon, soft carbon, silicon, silicon oxide, doped variants thereof, and mixtures thereof.

[0194] Examples of suitable binders include polyvinylidene fluoride and its copolymers (PVDF), polytetrafluoroethylene (PTFE) and its copolymers, polyacrylonitrile (PAN), poly(methyl)methacrylate or poly(butyl)methacrylate, polyvinyl chloride (PVC), polyvinyl fomal, polyetheramide, polymethacrylic acid, polyacrylamide, polyitaconic acid, polystyrene sulfonic acid, polyacrylic acid (PAA) and alkali metal salts thereof, modified polyacrylic acid (mPAA) and alkali metal salts thereof, cellulose-based polymers, carboxymethylcellulose (CMC), modified carboxymethylcellulose (mCMC), sodium carboxymethylcellulose (Na-CMC), polyvinylalcohol (PVA), alginates and alkali metal salts thereof, butadieneacrylonitrile rubber (NBR), hydrogenated form of NBR (HNBR), styrene-butadiene rubber (SBR) and polyimide. The binder may be present in the electrode composition at 0-30 wt%, or 0.1-10 wt%, or 0.1-5 wt%, based on the total dry weight of the electrode composition.

[0195] Conductive additives are preferably non-active materials which are included to improve electrical conductivity between the active electrode material and the current collector. The conductive additives may suitably be selected from graphite, carbon black, carbon fibers, vapor-grown carbon fibres (VGCF), carbon nanotubes, graphene, acetylene black, ketjen black, metal fibers, metal powders and conductive metal oxides. Preferred conductive additives include carbon black and carbon nanotubes. Conductive additives may be present in the electrode composition at 0-20 wt%, 0.1-10 wt%, or 0.1-5 wt%, based on the total dry weight of the electrode composition.

[0196] The active electrode material may be present in the electrode composition at 100-50 wt%, 99.8-80 wt%, or 99.8-90 wt%, based on the total dry weight of the electrode composition. When the active electrode material is present at 100 wt.% of the electrode composition it may be used as a solid-state electrode. An electrode may be made by forming a slurry of the active electrode material and a solvent. The slurry may comprise at least one other component selected from a binder, a conductive additive, a different active electrode material, and mixtures thereof. The slurry may be deposited onto a current collector and the solvent removed, thereby forming an electrode composition on the current collector. Further steps, such as heat treatment to cure any binders and / or calendaring of the electrode layer may be carried out as appropriate. For example, the solvent may be removed by drying e.g. at temperatures of 30-100°C. The electrode may be calendared to a density of 2-3.5 or 2.6-2.9 g cm3. The electrode layer may have a thickness in the range of from 5 pm to 2 mm, preferably 5 pm to 1 mm, preferably 5 pm to 500 pm, preferably 5 pm to 200 pm, preferably 5 pm to 100 pm, preferably 5 pm to 50 pm. Alternatively, the slurry may be formed into a freestanding film or mat comprising the active electrode material, for instance by casting the slurry onto a suitable casting template, removing the solvent and then removing the casting template. The resulting film or mat is in the form of a cohesive, freestanding mass which may then be bonded to a current collector by known methods.

[0197] In the third aspect, the active electrode materials defined herein are used in a metal-ion battery, preferably in the anode of a lithium-ion battery. Lithium-ion batteries include liquid-based batteries, polymer-based batteries, semi-solid-based batteries and all solid-state-based batteries. The electrolyte of the battery of the third aspect may include any material suitable for metal-ion battery operation, preferably lithium-ion battery operation. For example, the electrolyte may be a non-aqueous solution (e.g., an organic electrolytic solution). The electrolyte may include one or more non-aqueous solvents and a salt that is at least partially dissolved in the solvent. For example, the solvent may include an organic solvent, such as, e.g., ethylene carbonate (EC) and / or other carbonate based solvents, or butyrate, or acetate, or mixtures thereof. The solvent may include 1 M LiPFe dissolved in an aprotic solvent mixture, such as a 1 :1 by weight of a mixture of ethylene carbonate and other carbonate based solvents or butyrate or acetate.

[0198] Salts suitable for use in the invention include LiPFe, LiSbFe, LiBF4, LiTFSI, LiFSI, LiAICk, LiAsFe, LiCIC , LiGaCU, LiC(SC>2CF3)3, LiN(CF3SC>2)2, Li(CF3SC>3), LiB(CeH4O2)2, LiBOB (lithium bis(oxalate) borate), and LiDFOB (lithium difluoro (oxalate) borate). Low-viscosity solvents (e.g., organic solvents) suitable for use in the electrolyte may include, but are not limited to ethyl methyl carbonate (EMC), dioxlane (DOL), ethyl acetate (EA); propylene acetate (PA); butyl acetate (BA); methyl butyrate (MB); ethyl butyrate (EB); dimethyl carbonate (DMC); diethyl carbonate (DEC); 1 ,2-dimethoxyethane (DME); tetrahydrofuran (THF); methyl acetate (MA); diglyme (DGL); triglyme; tetraglyme; cyclic carbonates; cyclic esters; cyclic amides; propylene carbonate (PC); methyl propyl carbonate (MPC); acetonitrile; dimethyl sulfoxide (DMS); dimethyl formamide; dimethyl acetamide; gamma-butyrolactone (GBL); and N-methyl-pyrrolidinone (NMP); as well as various mixtures or combinations thereof.

[0199] The oxide comprising niobium may be synthesised by conventional ceramic techniques, e.g. by solid- state synthesis or sol-gel synthesis. Oxides comprising niobium may additionally be synthesised by hydrothermal or microwave hydrothermal synthesis, solvothermal or microwave solvothermal synthesis, coprecipitation synthesis, spark or microwave plasma synthesis, combustion synthesis, electrospinning, spray pyrolysis, chemical vapour deposition, atomic layer deposition, and mechanical alloying.

[0200] The oxide comprising niobium may be synthesised by a method comprising steps of providing one or more precursor materials; mixing said precursor materials to form a precursor material mixture; and heat treating the precursor material mixture in a temperature range from 400-1350 °C or 800-1250 °C, thereby providing the oxide comprising niobium.

[0201] To provide an oxide comprising niobium and an additional electronegative anion other than oxygen, i.e. an anionic dopant, the method may further comprise the steps of: mixing the oxide comprising niobium with a precursor comprising an additional electronegative anion to provide a further precursor material mixture; and heat treating the further precursor material mixture in a temperature range from 300-1200 °C or 800-1100 °C optionally under reducing conditions, thereby providing the oxide comprising niobium and an additional electronegative anion.

[0202] For example, to provide an oxide comprising niobium and N, the method may further comprise the steps of: mixing the oxide comprising niobium with a precursor comprising N (for example melamine or urea) to provide a further precursor material mixture; and heat treating the further precursor material mixture in a temperature range from 300-1200 °C under reducing conditions (for example under N2), thereby providing the oxide comprising niobium and N.

[0203] For example, to provide an oxide comprising niobium and F, the method may further comprise the steps of: mixing the oxide comprising niobium with a precursor comprising F (for example polyvinylidene fluoride or NF F) to provide a further precursor material mixture; and heat treating the further precursor material mixture in a temperature range from 300-1200 °C under oxidising conditions (for example in air), thereby providing the oxide comprising niobium and F.

[0204] The method may comprise the further step of heat treating the oxide comprising niobium in a temperature range from 400-1350 °C or 800-1250 °C under reducing conditions, thereby inducing oxygen vacancies in the oxide comprising niobium.

[0205] The precursor materials for making the oxide comprising niobium may include one or more metal oxides, metal hydroxides, metal salts or ammonium salts. For example, the precursor materials may include one or more metal oxides or metal salts of different oxidation states and / or of different crystal structure. Examples of suitable precursor materials include but are not limited to: Nb2Os, Nb(OH)s, Niobic Acid, NbO, Ammonium Niobate Oxalate, NH4H2PO4, (NH4)2PO4, (NH4)3PO4, P2O5, H3PO3, Ta2Os, WO3, ZrO2, TiO2, M0O3, V2O5, ZrO2, CuO, ZnO, AI2O3, K2O, KOH, CaO, GeO2, Ga2O3, SnO2, CoO, C02O3, Fe2O3, Fe3O4, MnO, MnO2, NiO, Ni2Os, H3BO3, ZnO, U2CO3, Na2CO3, H3BO3, NiO, Mgs(CO3)4(OH)2.5H2O, and and MgO. The precursor materials may not comprise a metal oxide or may comprise ion sources other than oxides. For example, the precursor materials may comprise metal salts (e.g. NO3; SO3 ) or other compounds (e.g. oxalates, carbonates). For the substitution of the oxygen anion with other electronegative anions, the precursors may include one or more organic compounds, polymers, inorganic salts, organic salts, gases, or ammonium salts; examples include but are not limited to: melamine, NH4HCO3, NH3, NH4F, PVDF, PTFE, NH4CI, NH4Br, NH4I, Br2, Cl2, 12, ammonium oxychloride amide, and hexamethylenetetramine.

[0206] Some or all of the precursor materials may be particulate materials. Where they are particulate materials, preferably they have a D50 particle diameter of less than 20 pm in diameter, for example from 10 nm to 20 pm. Providing particulate materials with such a particle diameter can help to promote more intimate mixing of precursor materials, thereby resulting in more efficient solid-state reaction during the heat treatment step. However, it is not essential that the precursor materials have an initial particle size of <20 pm in diameter, as the particle size of the one or more precursor materials may be mechanically reduced during the step of mixing said precursor materials to form a precursor material mixture. The step of mixing the precursor materials to form a precursor material mixture and / or further precursor material mixture may be performed by a process selected from: dry or wet / solvated planetary ball milling, rolling ball milling, high energy ball milling, bead milling, pin milling, a classification step, high shear milling, air jet milling, steam jet milling, planetary mixing, powder blending, and / or impact milling. The force used for mixing / milling may depend on the morphology of the precursor materials. For example, where some or all of the precursor materials have larger particle sizes (e.g. a Dso particle diameter of greater than 20 pm), the milling force may be selected to reduce the particle diameter of the precursor materials such that the particle diameter of the precursor material mixture is reduced to 20 pm in diameter or lower. When the particle diameter of particles in the precursor material mixture is 20 pm or less, this can promote a more efficient solid-state reaction of the precursor materials in the precursor material mixture during the heat treatment step. The solid-state synthesis may also be undertaken in pellets formed at high pressure (>10 MPa) from the precursor powders.

[0207] The step of heat treating the precursor material mixture and / or the further precursor material mixture may be performed for a time of from 1 hour to 24 hours, more preferably from 3 hours to 18 hours. For example, the heat treatment step may be performed for 1 hour or more, 2 hours or more, 3 hours or more, 6 hours or more, or 12 hours or more. The heat treatment step may be performed for 24 hours or less, 18 hours or less, 16 hours or less, or 12 hours or less.

[0208] The step of heat treating the precursor material mixture may be performed in a gaseous atmosphere, preferably air. Suitable gaseous atmospheres include air, N2, Ar, He, CO2, CO, O2, H2, NH3 and mixtures thereof. The gaseous atmosphere may be a reducing atmosphere. Where it is desired to make an oxygen-deficient material, preferably the step of heat treating the precursor material mixture is performed in an inert or reducing atmosphere.

[0209] The step of heat treating the further precursor material mixture may be performed under reducing conditions. Reducing conditions include under an inert gas such as nitrogen, helium, argon; or under a mixture of an inert gas and hydrogen; or under vacuum. Preferably, the step of heat treating the further precursor material mixture comprises heating under inert gas.

[0210] The further step of heat treating the oxide comprising niobium and / or the oxide comprising niobium and additional electronegative anions optionally under reducing conditions may be performed for a time of from 0.5 hour to 24 hours, more preferably from 2 hours to 18 hours. For example, the heat treatment step may be performed for 0.5 hour or more, 1 hours or more, 3 hours or more, 6 hours or more, or 12 hours or more. The further step heat treating may be performed for 24 hours or less, 18 hours or less, 16 hours or less, or 12 hours or less. Reducing conditions include under an inert gas such as nitrogen, helium, argon; or under a mixture of an inert gas and hydrogen; or under vacuum. Preferably heating under reducing conditions comprises heating under inert gas.

[0211] In some methods it may be beneficial to perform a two-step heat treatment. For example, the precursor material mixture and / or the further precursor material mixture may be heated at a first temperature for a first length of time, follow by heating at a second temperature for a second length of time. Preferably the second temperature is higher than the first temperature. Performing such a two-step heat treatment may assist the solid-state reaction to form the desired crystal structure. This may be carried out in sequence or may be carried out with an intermediate re-grinding step.

[0212] The method may include one or more post-processing steps after formation of the oxide comprising niobium. In some cases, the method may include a post-processing step of heat treating the oxide comprising niobium, sometimes referred to as ‘annealing’. This post-processing heat treatment step may be performed in a different gaseous atmosphere to the step of heat treating the precursor material mixture to form the oxide comprising niobium. The post-processing heat treatment step may be performed in an inert or reducing gaseous atmosphere. Such a post-processing heat treatment step may be performed at temperatures of above 500 °C, for example at about 900 °C. Inclusion of a postprocessing heat treatment step may be beneficial to e.g. form deficiencies or defects in the oxide comprising niobium, for example to induce oxygen deficiency; or to carry out anion exchange on the formed oxide comprising niobium e.g. N exchange for the O anion.

[0213] Examples

[0214] Materials

[0215] An oxide having a Wadsley-Roth crystal structure containing niobium, chromium, zinc, and titanium was synthesised by a solid state route. In a first step, precursor materials (Nb2Os, ZnO, TiC>2, and CrzOs) were milled to a D50 below 20 pm. The materials were then mixed in stoichiometric proportions (50 g total) and combined in a homogeneous powder mixture by an impact mill at 20,000 rpm. The resulting powders were heat treated in an alumina crucible in a muffle furnace in air at 600-1350 °C for 0.5-24 h, providing the desired Wadsley-Roth phase. The material was then de-agglomerated. This material was used as comparative sample 1 (uncoated) and as the base material for preparing the coated particles listed in Table 1 . All other materials were obtained from commercial suppliers, such as Thermo Scientific and Sigma Aldrich.

[0216] * Comparative sample - uncoated.

[0217] Table 1. Summary of the samples prepared. Particle size distributions were obtained with a Mastersizer 3000 laser diffraction particle analyser and Hydro MV dispersing unit for wet powder dispersions with tetrasodium pyrophosphate solution additive. Measurements were run after a 60 s sonication then a 90 s delay pretest delay.

[0218] Dry coating preparation method

[0219] Samples of coated particles were prepared using a dry intensive mixing process in a 1 L high shear impaction mixer by mixing particulate precursors of oxides and uncoated base material of the type and amount in Table 2 for 10 minutes at 25 m / s anticlockwise tool rotation speed. The samples were then annealed at 400 °C for 4 h in air at a heating rate of 240 °C / h to target temperature. Details of the synthesis of the sample prepared by a dry coating method are provided in the table below.

[0220] Table 2. Summary of samples prepared by dry coating method.

[0221] SEM images of samples 1 and 2 are provided in Figures 1 and 2a respectively. SEM images were interpreted by segmenting into background / base material / coating particle using a random forest pixel classifier trained on operator-drawn image labels. This was implemented using the open-source program ilastik. From these segmented images, individual coating particles were extracted using a watershed algorithm implemented on the open source scikit-image python module. Size and coverage metrics were calculated based on contours of the segmented coating particles using the open source OpenCV python module. The zinc oxide forms an island coating of particles having a number-weighted median diameter of 124 nm covering 6% of the surface of the base material. By the same analysis of SEM of Sample 4 (Figure 2b), the zirconium oxide forms an island coating of particles having a number-weighted median diameter of 116 nm covering 6% of the surface of the base material. By the same analysis of SEM of Sample 5 (Figure 2c), the magnesium oxide forms an island coating of particles having a number- weighted median diameter of 97 nm covering 4% of the surface of the base material. l / l / ef coating method

[0222] Samples of coated particles of niobium-containing oxide were prepared using a water-based mixing process. The precursors listed below were added to water together with niobium-containing oxide in amounts calculated to achieve 1 wt% of the coating in the final material, relative to the weight of the base oxide plus coating. The dispersion was subjected to high shear mixing for 10 minutes at 25 m / s anticlockwise tool rotation speed with a 30° tilt. The result was then annealed at 400 °C for 4 h in air at a heating rate of 240 °C / h to target temperature. The result was then deagglomerated. A list of samples prepared is shown below in Table 3. SEM images of sample 9 (Figure 2d) showed that the wet method provides an island coating of similar morphology to the dry method. By the same analysis as for Figures 2a-2c, the Li2SC>4 forms a coating of particles having a number-weighted median diameter of 155 nm covering 3% of the surface of the base material.

[0223] Table 3. Summary of samples prepared by wet coating method

[0224] ALP coating method

[0225] ALD coated particles were prepared using a two-step gas phase chemical deposition process.

[0226] During this two-step gas pulse sequence, in each cycle, a first chemical precursor is introduced to form a single monolayer across the substrate surface. Following this, an inert gas purge removed any unreacted precursor and by-products. Next, a second precursor is delivered, reacting with the first to complete one atomic layer of the target material. After another purge, the process repeats ensuring uniform film growth with angstrom-level thickness control. Samples 10 and 11 were prepared in this way, targeting a conformal coating of 1 wt% AI2O3 and ZrC>2, respectively, relative to the combined mass of the oxide comprising Nb and coating material.

[0227] Electrochemical Characterisation

[0228] Li-ion cell charge rate is usually expressed as a “C-rate”. A 1 C charge rate means a charge current such that the cell is fully charged in 1 h, 10C charge means that the battery is fully charged in 1 / 1 Oth of an hour (6 minutes). C-rate hereon is defined from the reversible capacity observed of the anode within the voltage limits applied in its second cycle de-lithiation, i.e. for an anode that exhibits 1 .0 mAh cm-2electrode areal capacity within the voltage limits of 1 .1 - 3.0 V, a 1 C rate corresponds to a current density applied of 1 .0 mA cm2. In a typical material as described herein, this could correspond to ~200 mA / g of active material as an example. Electrochemical tests were carried out on full coin cells (CR2032 size). In the below examples, the active material composition to be tested was combined with N-methyl pyrrolidone (NMP), carbon black (C65T) acting as a conductive additive, and poly(vinyldifluoride) (PVDF) binder and mixed to form a slurry using a lab-scale centrifugal planetary mixer (THINKY). The non-NMP composition of the slurries was 92 wt% active material, 3 wt% conductive additive, 5 wt% binder. The slurry was coated on an Al foil current collector to the desired loading of 67 - 73 g nr2by doctor blade coating and dried by heating on a hot plate. The electrodes were then calendered to a density of 2.6 - 2.9 g cm-3at 80°C to achieve targeted porosities of 30 - 35%. Electrodes were punched out at the desired size and used as the anode combined with a separator (Celgard porous PP / PE), cathode layer, and electrolyte (1 .3 M LiPFe in 3:7 ethylene carbonate (EC) : ethyl methyl carbonate (EMC)) inside a steel coin cell casing and sealed under pressure.

[0229] The active cathode material was a nickelate (NMC532) and the cathode composition is the same as the anode composition detailed above with a target porosity of 25 - 30%, except that the active material is NMC532. The full cell anode coatings had a loading of 67 - 73 g m-2matched to the cathode loadings of 77 - 83 g nr2.

[0230] The full cells were measured on Neware cyclers at 45 °C. The specific capacities shown represent the cathode specific capacity, and were calculated from the overall full cell capacity, based on the weight of the 14 mm cathode disc, and the NMC532 nominal specific capacity. The formation cycling protocol was two cycles at 0.1 C / 0.1C CC / CC between 1 .0 - 3.05 V. Full cells using selected samples were subjected to a life cycling protocol of cycles 2C / 2C CC / CC with a voltage window of 1 .0-3.15 V, giving the results in Table 4.

[0231] Table 4. Results of cycle life performance test at 1, 100, 200 and 300 cycles at 45 °C. The standard deviation error of the last digit is shown in brackets. An average of four or five cells was used, as indicated.

[0232] It can be seen that sample 2 (ZnO coating) has a surprisingly improved capacity at the beginning of the life cycling test compared to the uncoated comparative sample and to the other coated samples. Moreover, the magnitude of the improvement increases as the number of cycles increases. The coating improves the lifetime of the cell, resulting in a slower rate of capacity loss with increasing cycles compared to the uncoated sample. Lithium phosphate did not provide an improvement, demonstrating the unpredictable nature of the effect of incorporating coatings onto oxides comprising Nb. DCIR

[0233] In a further experiment, full cells using selected samples were tested for their direct current internal resistance (DCIR) after 100 cycles 2C / 2C CC / CC with a voltage window of 1 .0-3.15 V, giving the results in Table 5.

[0234] Table 5. Results of test for DCIR after 100 2C / 2C cycles at 45 °C.

[0235] A low DCIR after 100 cycles is desirable, indicating a good cell lifetime. It was found that a ZnO coating provided the lowest DCIR, followed by IJ2SO4 and B2O3.

[0236] Gassing experiments

[0237] The ability of the coating materials to interrupt the electrolyte-surface reactions that cause gassing was tested. Sealed pouches of the samples in powder form and electrolyte (1.3M LiPFe in ethylene carbonate (EC) : ethyl methyl carbonate (EMC) 3:7) were stored at high temperature and measured over time. For each pouch, 2 g of material was dried under vacuum at 120 °C for 12 hours, before transferring to the glovebox. Then the pouch was filled with the powder and 2.4 g of electrolyte. The pouches were vacuum- sealed and stored at 60 °C to accelerate gas generation. 3 pouches were used per sample to verify for reproducibility. The volume of the pouches was measured with a densitometer after sealing, and the measurement was repeated every two weeks to monitor the volume change, which corresponds to the amount of gas generated. The results are shown in Figure 3. (NB: for Samples 3* (TiC>2 coating) and 6* (AI2O3) coating, due to availability, the same procedure as summarised in Table 2 was used but the TiC>2 and AI2O3 precursors were sourced from a different supplier).

[0238] It was found that coatings of ZnO and IJ2SO4 resulted in effectively zero gassing during the testing period, which is a significant result. The MgO coating also resulted in advantageously reduced gassing compared to the comparative uncoated sample. Notably, the other coatings deleteriously increased gassing compared to the comparative uncoated sample, demonstrating the unpredictable nature of the effect of incorporating coatings onto oxides comprising Nb. Further gassing experiments were performed using samples prepared in the same way as sample 2 but with 0.2 wt%, 0.5 wt%, or 2 wt% of the zinc oxide coating (relative to the amount of the base oxide and zinc oxide coating). The results are shown in Figure 4. It was found that a coating loading of 0.2 wt% still resulted in some improvement, showing reduced gassing compared to the uncoated sample. Coating loadings of 0.5 wt% and 2 wt% resulted in effectively zero gassing, the same as at 1 wt%.

[0239] Experiments with further oxides comprising Nb

[0240] Experiments were performed on three further oxides having a Wadsley-Roth crystal structure. The oxides contained (i) niobium, titanium, aluminium, and iron; or (ii) phosphorus and niobium; or (iii) titanium and niobium. The oxides were synthesised by a solid-state procedure analogous to that given above. The oxides were coated with 1 wt% ZnO relative to the combined mass of the oxide comprising Nb and coating material using the same procedure at that for sample 2 above.

[0241] Full cells were prepared and tested using the same procedure as above. The number of cycles at 2C / 2C CC / CC with a voltage window of 1 .0-3.15 V completed before the capacity dropped to 80% of the initial capacity was determined. The results are shown in Table 6.

[0242] Table 6. Results of test for number of cycles before capacity drops to 80% of the initial capacity.

[0243] For each of the base oxides comprising Nb, it was found that the ZnO coating led to an increased number of cycles until the capacity had dropped to 80%, compared to the comparative uncoated samples. This demonstrates the improvement in lifetime provided by the coating for several different oxides comprises Nb.

[0244] Further experiments with a zinc oxide coating

[0245] Experiments were performed on samples made using the same base oxide containing Nb and procedure as for sample 2 above but varying the amount of zinc oxide precursor to achieve 0.2-2 wt% of the zinc oxide coating, relative to the combined mass of the oxide comprising Nb and coating material. Full cells were prepared in same way as above but with minor modifications to the electrode formulation such that the results here are not directly comparative with those above. The number of cycles at 2C / 2C CC / CC with a voltage window of 1 .0-3.15 V completed before the capacity dropped to 80% of the initial capacity was determined. The results are shown in Table 7. Significant improvements in lifetime were observed at above 0.2 wt% zinc oxide coating.

[0246] Table 7. Results of test for number of cycles before capacity drops to 80% of the initial capacity

[0247] Numbered embodiments

[0248] 1 . A particulate active electrode material having a Dso particle diameter of at least 250 nm, comprising: particles of an oxide comprising Nb, the particles comprising at least a partial coating formed of a coating material; wherein the coating material is selected from: metal oxides, metal sulfates, metal phosphates, metalloid oxides, metalloid sulfates, and metalloid phosphates.

[0249] 2. The active electrode material according to embodiment 1 , wherein the coating material comprises a metal or metalloid selected from:

[0250] Zn, Al, B, P, Mg, Ce, Li, Ti, S, Zr, Fe, Co, Ni, Cu, Ca, Sr, Ba, Sn, Y, W, Mn, V, Cr, Ga, In, Si, Mo, Ta, Sc, Pb, and mixtures thereof; or

[0251] Zn, Al, B, P, Mg, Ce, Li, Ti, S, Zr, Fe, Cu, Ca, Sr, Ba, Sn, Y, W, Si, Ta, and mixtures thereof; or

[0252] Zn, Al, B, P, Mg, Ce, Li, Ti, S, Zr, and mixtures thereof; or

[0253] Zn, Al, B, P, Mg, Ce, Ti, S, Zr, and mixtures thereof; or

[0254] Zn, Al, B, Mg, Ce, Ti, Zr, and mixtures thereof; or

[0255] Zn, B, Mg, and mixtures thereof; or Zn.

[0256] 3. The active electrode material according to any preceding embodiment, wherein the coating material is selected from: zinc oxide, aluminium oxide, boron oxide, magnesium oxide, cerium oxide, lithium oxide, titanium oxide, zirconium oxide, lithium sulfate, lithium phosphate, and mixtures thereof; or zinc oxide, boron oxide, magnesium oxide, cerium oxide, titanium oxide, zirconium oxide, and mixtures thereof; or zinc oxide, boron oxide, magnesium oxide, and mixtures thereof.

[0257] 4. The active electrode material according to any preceding embodiment, wherein the coating material is zinc oxide. 5. The active electrode material of any preceding embodiment, wherein the coating material comprises an anionic and / or cationic dopant.

[0258] 6. The active electrode material of any preceding embodiment, comprising an interface phase formed from a reaction between the oxide comprising Nb and the coating material or a precursor thereof.

[0259] 7. The active electrode material of any preceding embodiment, which is free from carbon coatings.

[0260] 8. The active electrode material according to any preceding embodiment, wherein the oxide comprising Nb is selected from: oxides having a Wadsley-Roth crystal structure, oxides having a Tetragonal Tungsten Bronze crystal structure, and binary niobium oxides and doped variants thereof; or oxides having a Wadsley-Roth crystal structure and oxides having a Tetragonal Tungsten Bronze crystal structure; or oxides having a Wadsley-Roth crystal structure.

[0261] 9. The active electrode material according to any preceding embodiment, wherein the oxide comprising Nb comprises at least one metal or metalloid in addition to Nb.

[0262] 10. The active electrode material according to embodiment 9, wherein the oxide comprising Nb is expressed by the formula [M]x[Nb]y[O]z, wherein:

[0263] M represents one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; x satisfies 0 < x < 0.5; y satisfies 0.5 < y < 49; and z satisfies 4 < z < 124.

[0264] 11 . The active electrode material according to embodiment 10, wherein M represents: one or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn; or two or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn; or three or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn.

[0265] 12. The active electrode material according to any preceding embodiment, wherein the oxide comprising Nb comprises an anionic and / or cationic dopant.

[0266] 13. The active electrode material according to any preceding embodiment, having a Dso particle diameter of 0.5-100 pm, or 0.75-50 pm, or 1 .0-20 pm.

[0267] 14. The active electrode material according to any preceding embodiment, wherein the coating is a partial coating; optionally wherein the coating is an island coating. 15. The active electrode material according to embodiment 14, wherein the coating is formed of particles of the coating material, said particles having a median diameter of 1-1 ,000 nm, or 5-750 nm, or 10-500 nm, or 50-250 nm.

[0268] 16. The active electrode material according to embodiment 14 or 15, wherein at least 50%, or at least 60%, or at least 75%, or at least 85% of the surface of the particles of the oxide comprising Nb is not covered by the coating material.

[0269] 17. The active electrode material according to any of embodiments 1-13, wherein the coating covers substantially all of, or all of, the surface of the particles of the oxide comprising Nb; optionally wherein the coating is a conformal coating.

[0270] 18. The active electrode material according to any preceding embodiment, wherein the coating material is present in an amount of 0.1-10 wt%, or 0.5-5 wt%, or 0.7-1 .3 wt%, or about 1 wt% relative to the combined mass of the oxide comprising Nb and coating material.

[0271] 19. An electrode comprising the active electrode material according to any preceding embodiment; optionally wherein the electrode is an anode.

[0272] 20. A metal-ion battery comprising the electrode of embodiment 19; optionally wherein the battery is a lithium-ion battery and the electrode forms the anode.

[0273] 21 . A composition for preparing an electrode, the composition comprising the active electrode material of any of embodiments 1-18 and at least one other component selected from a binder, a solvent, a conductive additive, an active electrode material other than as defined in any of embodiments 1-18, and mixtures thereof.

[0274] 22. A method of preparing a coated active electrode material, the method comprising the steps of: combining particles of an oxide comprising Nb as defined in any of embodiments 1-18 with a coating material as defined in any of embodiments 1-18 or a precursor thereof.

[0275] 23. The method of embodiment 22, wherein the combining step comprises mixing the oxide comprising Nb with the coating material or precursor thereof.

[0276] 24. The method of embodiment 22 or embodiment 23, wherein the combining step comprises dispersing the oxide comprising Nb and the coating material or precursor thereof in a solvent.

[0277] 25. A method according to any of embodiments 22-24, wherein the combining step comprises chemical vapour deposition (CVD), optionally atomic-layer deposition (ALD).

[0278] 26. A method according to any of embodiments 22-25, wherein the combining step comprises dry mixing the oxide comprising Nb with an oxide coating material. 27. A method according to any of embodiments 22-26, comprising annealing the combination of the particles of the oxide comprising Nb and the coating material or precursor thereof; optionally wherein the annealing is performed at a temperature of at least 150 °C, or 175-900 °C, or 200- 800 °C, or 250-750 °C, or 300-500 °C.

[0279] 28. A method according to any of embodiments 22-27, wherein the coating material or precursor thereof has a Dso particle diameter of less than 500 nm, or less than 250 nm, or less than 150 nm. 29. An active electrode material made by the process of any of embodiments 22-28.

[0280] 30. Use of a coating material as defined in any of embodiments 1-18 for forming an active electrode material according to any of embodiments 1-18.

Claims

Claims1 . A particulate active electrode material having a Dso particle diameter of at least 250 nm, comprising: particles of an oxide comprising Nb, the particles comprising at least a partial coating formed of a coating material; wherein the coating material is selected from: metal oxides, metal sulfates, metal phosphates, metalloid oxides, metalloid sulfates, and metalloid phosphates.

2. The active electrode material according to any preceding claim, wherein the coating material is selected from zinc oxide, lithium sulfate, magnesium oxide, and mixtures thereof; or zinc oxide, lithium sulfate, and mixtures thereof.

3. The active electrode material according to any preceding claim, wherein the coating material is zinc oxide.

4. The active electrode material of any preceding claim, wherein the coating material comprises an anionic and / or cationic dopant.

5. The active electrode material of any preceding claim, comprising an interface phase formed from a reaction between the oxide comprising Nb and the coating material or a precursor thereof.

6. The active electrode material of any preceding claim, which is free from carbon coatings.

7. The active electrode material according to any preceding claim, wherein the oxide comprising Nb is selected from: oxides having a Wadsley-Roth crystal structure, oxides having a Tetragonal Tungsten Bronze crystal structure, and binary niobium oxides and doped variants thereof; or oxides having a Wadsley-Roth crystal structure and oxides having a Tetragonal Tungsten Bronze crystal structure; or oxides having a Wadsley-Roth crystal structure.

8. The active electrode material according to any preceding claim, wherein the oxide comprising Nb has a crystal structure of Zn2Nb34Os7 or TiNb2O?9. The active electrode material according to any preceding claim, wherein the oxide comprising Nb comprises at least one metal or metalloid in addition to Nb.

10. The active electrode material according to claim 9, wherein the oxide comprising Nb is expressed by the formula [M]x[Nb]y[O]z, wherein:M represents one or more of Ti, Mg, V, Cr, W, Zr, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; x satisfies 0 < x < 0.5;y satisfies 0.5 < y < 49; and z satisfies 4 < z < 124.11 . The active electrode material according to claim 10, wherein M represents: one or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn; or two or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn; or three or more of Ti, Cr, W, Zr, Mo, Cu, Fe, Ni, Co, Al, and Zn.

12. The active electrode material according to any preceding claim, wherein the oxide comprising Nb comprises an anionic and / or cationic dopant.

13. The active electrode material according to any preceding claim, having a Dso particle diameter of 0.5-100 pm, or 0.75-50 pm, or 1 .0-20 pm.

14. The active electrode material according to any preceding claim, wherein the coating is a partial coating; optionally wherein the coating is an island coating.

15. The active electrode material according to claim 14, wherein the coating is formed of particles of the coating material, said particles having a number-weighted median diameter of 1-1 ,000 nm, or 5-750 nm, or 10-500 nm, or 50-250 nm.

16. The active electrode material according to claim 14 or 15, wherein at least 50%, or at least 60%, or at least 75%, or at least 85% of the surface of the particles of the oxide comprising Nb is not covered by the coating material.

17. The active electrode material according to any preceding claim, wherein the coating material is present in an amount of 0.1-15 wt%, or 0.3-10 wt%, or 0.5-5 wt% relative to the combined mass of the oxide comprising Nb and coating material.

18. The active electrode material according to any preceding claim, wherein: the Dso particle diameter is 0.5-100 pm; the particles of the oxide comprising Nb comprise a partial coating formed of the coating material; the oxide comprising Nb has a Wadsley-Roth crystal structure; the coating material is selected from zinc oxide, magnesium oxide, lithium sulfate, and mixtures thereof; the coating material is present in an amount of 0.1-10 wt%, optionally 0.3-10 wt%, relative to the combined mass of the oxide comprising Nb and coating material; wherein at least 60% of the surface of the particles of the oxide comprising Nb is not covered by the coating material.

19. The active electrode material according to any preceding claim, wherein: the D50 particle diameter is 0.75-50 pm;the oxide comprising Nb further comprises Ti; the particles of the oxide comprising Nb comprise an island coating formed of the coating material; the oxide comprising Nb has a Wadsley-Roth crystal structure; the coating material is selected from zinc oxide, magnesium oxide, lithium sulfate, and mixtures thereof; the coating material is present in an amount of 0.3-10 wt%, optionally 0.5-5 wt%, relative to the combined mass of the oxide comprising Nb and coating material; wherein at least 75% of the surface of the particles of the oxide comprising Nb is not covered by the coating material; the coating is formed of particles of the coating material having a number-weighted median diameter of 10-500 nm.

20. The active electrode material according to any preceding claim, wherein: the Dso particle diameter is 1 .0-20 pm; the oxide comprising Nb further comprises Ti, and optionally Cr and Zn; the particles of the oxide comprising Nb comprise an island coating formed of the coating material; the oxide comprising Nb has a Wadsley-Roth crystal structure; the coating material is zinc oxide; the coating material is present in an amount of 0.5-5 wt% relative to the combined mass of the oxide comprising Nb and coating material; wherein at least 85% of the surface of the particles of the oxide comprising Nb is not covered by the coating material; the coating is formed of particles of the coating material having a number-weighted median diameter of 50-250 nm.

21. An electrode comprising the active electrode material according to any preceding claim; optionally wherein the electrode is an anode.

22. A metal-ion battery comprising the electrode of claim 21 ; optionally wherein the battery is a lithium-ion battery and the electrode forms the anode.

23. A composition for preparing an electrode, the composition comprising the active electrode material of any of claims 1 -20 and at least one other component selected from a binder, a solvent, a conductive additive, an active electrode material other than as defined in any of claims 1-20, and mixtures thereof.

24. A method of preparing a coated active electrode material as defined in any of claims 1-20, the method comprising the steps of: combining particles of an oxide comprising Nb as defined in any of claims 1-20 with a coating material as defined in any of claims 1-20 or a precursor thereof.

25. The method of claim 24, wherein the combining step comprises mixing the oxide comprising Nb with the coating material or precursor thereof.

26. The method of claim 24 or claim 25, wherein the combining step comprises dispersing the oxide comprising Nb and the coating material or precursor thereof in a solvent.

27. A method according to any of claims 24-26, wherein the combining step comprises dry mixing the oxide comprising Nb with an oxide coating material.

28. A method according to any of claims 24-27, comprising annealing the combination of the particles of the oxide comprising Nb and the coating material or precursor thereof; optionally wherein the annealing is performed at a temperature of at least 150 °C, or 175-900 °C, or 200-800 °C, or 250- 750 °C, or 300-500 °C.

29. A method according to any of claims 24-28, wherein the coating material or precursor thereof has a Dso particle diameter of less than 500 nm, or less than 250 nm, or less than 150 nm.

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