EP4634117A1

EP4634117A1 - Active electrode material

Info

Publication number: EP4634117A1
Application number: EP23833186.2A
Authority: EP
Inventors: Wanwei ZHANG; Joshua Lewis; Alexander GROOMBRIDGE
Original assignee: Echion Technologies Ltd
Current assignee: Echion Technologies Ltd
Priority date: 2022-12-14
Filing date: 2023-12-14
Publication date: 2025-10-22
Also published as: JP2025542005A; WO2024127010A1; KR20250124810A; CN120303217A; AU2023392658A1; GB202218870D0

Abstract

The invention relates to a method of making an active electrode material, the method comprising processing a precursor mixture comprising a niobium precursor, a titanium 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, Ti2Nb10O29, TiNb14O37, or TiNb24O62; and wherein the active electrode material has the crystal structure of TiNb2O7, Ti2Nb10O29, TiNb14O37, and/or TiNb24O62.

Description

Active electrode material

Field of the Invention

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.

Background

Lithium-ion (Li-ion) batteries are a commonly used type of rechargeable battery with a global market predicted to grow to $200bn by 2030. Li-ion batteries are the technology of choice for electric vehicles that have multiple demands across technical performance to environmental impact, providing a viable pathway for a green automotive industry.

A typical lithium-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 ions 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), thus forming the electrode upon drying.

In the known Li-ion battery technology, the safety limitations of graphite anodes upon battery charging is a serious impediment to its application in high-power electronics, automotive and industry. Linder nominal conditions, lithium ions are inserted into the anode active material upon charging. 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 instead 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. In some cases, these dendritic deposits can grow to such large sizes that they pierce the battery separator and lead to a short-circuit of the cell. This can trigger a catastrophic failure of the cell leading to a fire or an explosion. Accordingly, the fastest-charging batteries having graphitic anodes are limited to charging rates of 5-7 C, but often much less.

Among a wide range of potential alternatives proposed recently, lithium titanate (Li4TisOi2, LTO) and mixed niobium oxides (MNOs - defined herein as oxides comprising niobium and at least one other cation) are the main contenders to replace graphite as the active material of choice for high power, fastcharge applications. 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. Carbon-coatings can be used to increase electronic conductivity and/or avoid reactions with electrolyte (He, YB. Et al., Sci Rep 2, 913 (2012) and Han, C. et al., J. Mater. Chem. A, 2017,5, 6368-6381). 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 (e.g. binder, carbon additive), resulting in much lower gravimetric and volumetric energy densities.

A key measure of anode performance is the electrode volumetric capacity (mAh/cm³), that is, the amount of electric charges (that is lithium ions) that can be stored per unit volume of the anode. This is an important factor to determine the overall battery energy density on a volumetric basis (Wh/L) when combined with the cathode and appropriate cell design parameters. Electrode volumetric capacity can be approximated as the product of electrode density (g/cm³), active material specific capacity (mAh/g), and fraction of active material in the electrode. LTO anodes typically have relatively low specific capacities (~165 mAh/g, to be compared with ~330 mAh/g for graphite) which, combined with their low electrode densities (typically <2.0 g/cm3) and low active material fractions (<90%) discussed above, lead to very low volumetric capacities (<300 mAh/cm³) and therefore low battery energy density and high $/kWh cost in various applications. As a result, LTO batteries are generally limited to specific niche applications, despite their long cycle life, fast-charging capability, and high safety.

MNOs have attracted interest in replacing LTO since they have shown similar properties in fast-charging, long cycle life, etc. Moreover, comparing to LTO, MNOs have higher capacity and therefore higher battery energy density. However, MNOs can suffer from other drawbacks. Titanium niobium oxides, such as TiNb2O? and Ti2NbwO29, suffer problems of gassing, where gaseous species are generated by interfacial reaction with the electrolyte, reported by a few studies (Buannic et al., J. Mater. Chem. A, 2016,4, 11531- 11541 ; Parikh et al., ACS Appl. Mater. Interfaces 2021 , 13, 46, 55145-55155; and Wu et al. , ACS Appl. Mater. Interfaces 2018, 10, 32, 27056-27062). The gassing mechanism for TiNb20? is still unclear, researchers have suggested possibilities of trace water, catalytic effect from Ti⁴⁺, etc. Moreover, MNOs, due to their metal oxide nature, require improvements in ionic/electronic conductivity to get best performance at high power (GB2598438B, GB2588254B). Furthermore, the formation of titanium niobium oxides via solid-state reaction is very energy intense. It generally requires 1200°C reaction temperature for extended periods for the complete formation of TiNb2O?, which is challenging for cost-effective mass production. Accordingly, areas for improvements in titanium niobium oxides are: (i) processing - it would be desirable to reduce the reaction energy, resulting in lower synthesis temperature and time; (ii) ionic/electronic conductivity - higher conductivity benefits higher Li-ion cell performance at high charge/discharge rates; (iii) reduced reaction with electrolytes, improving first cycle columbic efficiency and cycle life.

US20200140339A1 claims doped titanium niobium oxides having chemical composition of Ti<i-_X)M1 _xNb<2- y)M2_yO(7-z)Qz or Ti(2-_X')M1_xNb(io-y)M2y O(29-z’)Qz', wherein M1 is Li, Mg, or a combination thereof; M2 is Fe, Mn, V, Ni, Cr, or a combination thereof; Q is F, Cl, Br, I, S, or a combination thereof; 0<x<0.15; 0<y<0.15; 0.01 <z<2; 0<x’<0.3; 0<y’<0.9; and 0.01<z’<8. The doped titanium niobium oxides in US20200140339A1 thus require the same cation:anion ratio to the base material TiNb2O? or Ti2NbioC>29.

US20210296631 A1 claims an active material composite of titanium niobium oxide with fluorine atoms on at least part of a surface, the fluorine level on the surface of the electrode satisfying: 3.5<AF/(ATi+ANb) <50. The fluorine source of the electrode is from the electrolyte, forming after the assembly of the battery by post treatment, e.g. pulse charging treatment.

AIF3 has been used as a coating to improve Li-ion diffusion in LTO and modify surface reactivity (Li, et al., Electrochimica Acta, 2014, 139, 104; Chung, et al., Journal of Electroanalytical Chemistry, 2019, 837, 240).

W02008100002A1 claims an active anode material with a fluorine-based surface coating for reducing electrolyte side reactions. The materials are prepared by dispersing the anode material in a metal salt and fluorine precursor solution followed by calcination to form a fluorine-based coating. The examples are of graphite-based anode active materials where electrolyte side reactions and Solid Electrolyte Interphase (SEI) optimization is key.

Summary of the Invention

In a first aspect the invention provides a method of making an active electrode material, the method comprising: processing a precursor mixture comprising a niobium precursor, a titanium 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 TiNb2O?, Ti2NbioC>29, TiNbuOs?, or TiNb24C>62; wherein the active electrode material has the crystal structure of TiNb2O?, Ti2NbioC>29, TiNbuChy, and/or TiNb24O62.

The inventors have found that active electrode materials made according to the first aspect show excellent retention of capacity when de-lithiated at rates up to 10C, and high first cycle columbic efficiency, as shown by the present examples. The active electrode materials are improved when compared with pure titanium niobium oxides. These are important results in demonstrating the advantages of the metal halide modified titanium niobium oxide of the invention for use in high-power batteries designed for fast charge/discharge. The inventors have found that when the atomic ratio of cations:anions does not correspond to the ratio of the base materials (TiNb2Oy, Ti2NbioC>29, TiNbuOs?, or TiNb24C>62), i.e. an “off-stoichiometric ratio”, using a metal halide surprisingly allows the base crystal structure to be obtained in the resulting active electrode material. The “off-stoichiometry” of the active electrode is believed to induce disorder and/or defects into the crystal structure, which is believed to contribute to the improved properties observed. Moreover, the use of the metal halides was found to favour the formation of the desired crystal structures, allowing for the use of shorter synthesis times at lower temperatures, thus providing for a more economical active electrode material.

In a second aspect the invention provides an active electrode material obtainable by the method of the first aspect.

In a third aspect the invention provides a composition comprising the active electrode material of the second aspect and at least one other component; optionally wherein the at least one other component is selected from a binder, a solvent, a conductive additive, a different active electrode material, and mixtures thereof.

In a fourth aspect the invention provides an electrode comprising the active electrode material of the second aspect; optionally wherein the active electrode material is deposited on a current collector.

In a fifth aspect the invention provides a metal-ion battery comprising the electrode of the fourth aspect, optionally wherein metal-ion battery is a lithium-ion battery or sodium-ion battery and the electrode forms the anode.

In a sixth aspect the invention provides use of an active electrode material according to the second aspect in a metal-ion battery; optionally in an anode of a lithium-ion battery or sodium-ion battery.

Summary of the Figures

Figure 1 : XRD patterns of Examples 1-4

Figure 2: XRD patterns of Examples 5-7

Figure 3: XRD pattern of Comparative Study A

Figure 4: XRD pattern of Comparative Study B Figure 5: XRD pattern of Comparative Study C Figure 6: XRD pattern of Comparative Study D Figure 7: XRD pattern of Comparative Study E Figure 8: XRD pattern of Comparative Study F Figure 9: XRD pattern of Example 8

Detailed Description of the Invention

The active electrode material has the crystal structure of TiNb2Oy, Ti2NbioC>29, TiNbuOsy, and/or TiNb24C>62. These crystal structures belong to the Wadsley-Roth family. Wadsley-Roth crystal structures are considered to be a crystallographic off-stoichiometry of the MO3 (ReOs) crystal structure containing crystallographic shear, with simplified formula of MCh-x. As a result, these structures typically contain [MOe] octahedral subunits in their crystal structure. Phases with these structures are believed to have advantageous properties for use as active electrode materials, e.g. in lithium-ion batteries. The open tunnel-like crystal structure is an ideal candidate for high capacity Li-ion storage and high-rate intercalation/de-intercalation. The crystallographic off-stoichiometry present in the crystal structure causes the Wadsley-Roth crystallographic superstructure. These superstructures, compounded by other qualities such as the Jahn-Teller effect and enhanced crystallographic disorder by making use of multiple mixed cations, stabilise the crystal and keep the tunnels open and stable during intercalation, enabling extremely stable and high rate performance due to high Li-ion diffusion rates (reported as ~10-13 cm² s^-1). Mixed niobium oxides have a high redox voltage vs. Lithium >0.8V, enabling safe and long lifetime operation, crucial for fast charging battery cells. Moreover, niobium cations can have two redox reactions per atom, resulting in higher theoretical capacities than, for example, LTO.

The TiNb2C>7 crystal structure belongs to the monoclinic crystal system with A2/m space group. It can be described as a 3x3x« crystallographic block structure composed of [MOe] octahedra, where M is Ti or Nb. The crystal structure of TiNb2O? can be found at PDF card [01-072-0116] (Wadsley, A.D., Acta Crystallogr., 14, 660, (1961)). The unit cell parameters are typically within the following ranges: a is 11 .90-11 .94 A, b is 3.80-3.82 A, c is 20.38-20.45 A. The unit cell may have a = y = 90° and p may be 120.10-120.30°. Materials having the TiNb2O? crystal structure may be identified by having a characteristic peak at 20 = 26.0 ± 0.2 in the Cu K-a XRD pattern.

The Ti2NbioC>29 crystal structure can be monoclinic or orthorhombic or can be considered as a mixture of both. It can be described as a 3x4x« crystallographic block structure composed of [MOe] octahedra, where M is Ti or Nb. The crystal structure of monoclinic Ti2NbwO29 can be found in ICSD with identifier 15474 (Wadsley, A.D., Acta Crystallogr., 14, 664, (1961)). The crystal structure of orthorhombic Ti2NbwO29 can be found in ICSD with identifier 22000 (R.B. von Dreele, A.K. Cheetham, Proceedings of the Royal Society London, Series A, 338, 311 ,(1974)). The unit cell parameters are typically within the following ranges for the orthorhombic unit cell: a is 28.30-28.70 A, b is 3.78-3.83 A, c is 20.35-20.70 A. The unit cell may have a = p = y = 90°. Typical ranges for the monoclinic unit cell are: a is 20.54-20.57 A, b is 3.80-3.82 A, c is 15.52-15.55 A. The unit cell may have a = y = 90° and p may be 1 13.00-113.70°. Materials having the Ti2NbwO29 crystal structure may be identified by having a characteristic peak at 20 = 24.9 ± 0.2 in the Cu K-a XRD pattern.

The TiNbuOsy crystal structure belongs to monoclinic crystal system. It can be described as a 3x5x« crystallographic block structure composed of [MOe] octahedra, where M is Ti or Nb. The crystal structure is reported in Brunner, H., et.al., Zeitschrift fur Naturforschung B, 31 ,5, 549,(1976). The unit cell parameters are typically within the following ranges: a is 20.00-21 .60 A, b is 3.81-3.83 A, c is 29.82-30.15 A, a = y = 90°, and p is 94.50-95.50°. Materials having the TiNbuC>37 crystal structure may be identified by having a characteristic peak at 20 = 23.8 ± 0.2 in the Cu K-a XRD pattern.

The TiNb24C>62 crystal structure belongs to monoclinic crystal system with C2 space group. It can be described as a 3x4x0.5 crystallographic block structure composed of blocks of 3x4 [MOe] octahedra, where M is Ti or Nb, and 0.5 NbC tetrahedra per block. The crystal structure of TiNb24C>62 can be found at PDF card [01 -072-1655]( Roth, R.S., Wadsley, A.D., Acta Crystallogr., 18, 724, (1965)). The unit cell parameters are typically within the following ranges: a is 29.59-29.98 A, b is 3.80-3.84 A, c is 20.91 -21 .29 A. The unit cell may have a = y = 90° and p may be 94.2-95.6°. Materials having the TiNb24O62 crystal structure may be identified by having a characteristic peak at 20 = 24.7 ± 0.2 in the Cu K-a XRD pattern.

The crystal structure adopted by the active electrode material can be controlled by controlling the atomic Nb:Ti ratio in the precursor mixture. Preferably, the active electrode material has the crystal structure of TiNb2C>7 and/or Ti2NbwO29, most preferably TiNb2O₇. The crystal structure may be determined by analysis of X-ray diffraction (XRD) patterns obtained using Cu K-a radiation, 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 crystal structure of the active electrode material may be determined by XRD.

The active electrode material may have a mixed crystal structure where more than one crystal structure is present. For example, both the TiNb2O₇ and Ti2NbioC>29 crystal structures may be present. However, preferably the active electrode material has a single predominant crystal structure (e.g. TiNb2O₇ or Ti2NbioC>29), for example with other crystal structures being present at <10 wt%, <5 wt%, or <1 wt% as determined by Cu K-a XRD analysis. The active electrode material may have a single phase crystal structure consisting of the crystal structure of TiNb2O₇, Ti2NbioC>29, TiNbuOsy, or TiNb24C>62; or TiNb20₇ or Ti2NbioC>29; most preferably TiNb2O₇. When the active electrode material has a single phase crystal structure, this can be understood such that other phases are not detectable by Cu K-a XRD analysis. It will be understood that a single phase active electrode material of the invention may subsequently be mixed with a further material of a different crystal structure, e.g. to form an electrode composition as described herein.

The skilled person would understand that a single phase typically refers to a material entity with a uniform chemical makeup and structure. Practically, a single phase can be described by the average relative and identities of the ions in the crystal structure, and their characteristic range of deviation as determined by a measurement of a representative sample of the whole material. The chemical makeup and structure can never approach total uniformity due to defects, impurities, disorder and distortions which are a result of unavoidable practical and physical limitations, so acceptable tolerances have to be determined based on the limits of the available measurements such as by XRD analysis.

The atomic ratio of cations:anions in the precursor mixture does not correspond to the atomic ratio of cations:anions of TiNb2O₇, Ti2NbioC>29, TiNb Osy, or TiNb24C>62. The atomic ratio of cations:anions of TiNb₂O₇ is 3:7 (1 xTi + 2xNb : 7x0); of Ti₂Nbi₀O₂9 is 12:29, of TiNbuOsy is 15:37; of TiNb₂4O₈2 is 25:62. For example, the atomic ratio of cations:anions for a precursor mixture containing Nb2Os:TiO2:AIF3 in a molar ratio of 1 :0.94:0.01 is 2.95:6.91 (1 x2xNb + 0.94x1 xTi + 0.01 xl xAl : 1 x5xQ + 0.94x2x0 + 0.01 x3xF). With the metal halide in the precursor mixture, it has surprisingly been found that the crystal structure of TiNb2O?, Ti2NbwO29, TiNbuOs?, and/or TiNb24O62 can be obtained, even in a single phase, despite the use of an “off-stoichiometric” precursor mixture.

The atomic ratio of Nb:Ti in the precursor mixture is > 2. Optionally, the atomic ratio of Nb:Ti in the precursor mixture is not 5, 14, or 24 (the ratios for Ti2NbwO29, TiNbuOs?, and TiNb24O62).

When keeping the cations:anions ratio the same as the base material composition and partially substituting an element with another, the material is likely to preserve the base crystal structure and compensate possible valence change by partially oxidizing or reducing other elements. For example, when substituting a 4+ cation with a 3+ cation, a 5+ cation in the system may be partially reduced to 4+. In this invention, the atomic ratio of cations:anions is purposefully set to be off-stoichiometric, i.e. not the same ratio as the base titanium niobium material. Combined with the high electronegativity of halide anions, this is believed to induce stronger Jahn-Teller effect creating stronger distortion of octahedra, increasing the entropy of system and therefore decreasing the reaction energy. In this way, the active electrode material is more economical to synthesise, as shown by the examples. Additionally, with the extra valence electron from the halide anions, the occupation state of Nb 4d and O 2p orbitals may change and the Fermi level may shift towards the conduction band, indicating higher electronic conductivity (El-Shazly, et al., Applied Physics A, 2016, 122, 859). In this way, the properties of the active electrode material for use in lithium-ion battery anodes are improved, such as the capacity and capacity retention at high de-lithiation rates, as shown by the examples.

The precursor mixture comprises a niobium precursor, a titanium precursor, and a metal halide. The precursors may include one or more metal oxides, metal hydroxides, metal salts (e.g. NOs", SO3 ) or ammonium salts. Preferably the niobium precursor and the titanium precursor are not metal halides.

Examples of suitable niobium precursors include Nb2Os, Nb(OH)s, niobic acid, NbO, ammonium niobate oxalate, NbC>2, NbC>2F, NbsOyF, niobium chloride, niobium fluoride, niobium bromide. Typically the oxidation state of Nb in the niobium precursor is 5+. Preferably the niobium precursor is a niobium oxide such as Nb2C>5.

Examples of suitable titanium precursors include TiC>2, titanium chloride, titanium fluoride, titanium bromide, titanium oxalate, ammonium titanyl oxalate or nitrate, titanyl nitrate, titanyl sulfate, titanyl hydroxide, and ammonium bis(oxolato)oxotitanate. Typically the oxidation state of Ti in the titanium precursor is 4+. Preferably the titanium precursor is TiC>2.

Examples of suitable metal halides include MgNbuO35F2, ZnNbi4O3sF2, zirconium chloride, zirconium fluoride, zirconium bromide, copper chloride, copper fluoride, copper bromide, zinc chloride, zinc fluoride, zinc bromide, aluminium chloride, aluminium fluoride, aluminium bromide, germanium chloride, germanium fluoride, germanium bromide, gallium chloride, gallium fluoride, gallium bromide, tin bromide, tin chloride, tin fluoride, iron chloride, iron fluoride, iron bromide, manganese bromide, manganese chloride, manganese fluoride, magnesium fluoride, magnesium chloride, magnesium bromide, nickel fluoride, nickel chloride, nickel bromide, chromium fluoride, chromium chloride, and chromium bromide. Preferably the metal of the metal halide is not Nb or Ti. In this way, the base niobium titanium oxide will be substituted by a different cation, contributing to the presence of disorder and/or defects. The oxidation state of the metal of the metal halide may be +4 or less or +3 or less, such as +3 or +2. When the oxidation state is +3 or less the base niobium titanium oxide will be substituted by a cation with lower oxidation state than the base Ti⁴⁺ and Nb⁵⁺ cations, further contributing to the presence of disorder and/or defects. The metal of the metal fluoride 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, or Al, Zn, and mixtures thereof. The metal halide may be a metal fluoride, metal chloride, metal bromide; or a metal fluoride or a metal chloride; most preferably the metal halide is a metal fluoride. For example, the metal fluoride may be selected from ZnF2 and/or AIF3

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.

The precursor materials may not comprise a metal oxide, or may comprise ion sources other than oxides. For example, the niobium and/or titanium precursors may comprise metal salts (e.g. NO3; SO3 ) or other compounds (e.g. oxalates, carbonates).

The precursor mixture may contain further precursors other than the niobium precursor, titanium precursor, and the metal halide. For substitution of the base material by additional cations, the precursor mixture may include one or more metal oxides or metal salts. Examples of cation substitution precursor materials include but are not limited to: NH4H2PO4, (NH4)2PO4, (NH4)3PO4, P2O5, H3PO3, Ta2Os, WO3, ZrO2, TiO2, M0O3, V2O5, ZrO2, CuO, Cr2O3, ZnO, AI2O3, K2O, KOH, CaO, GeO2, Ga2O3, SnO2, CoO, C02O3, Fe2O3, Fe3O4, MnO, MnO2, NiO, Ni2Os, H3BO3, I 2CO3, Na2CO3, Mgs(CO3)4(OH)2.5H2O, and MgO. For the further 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 melamine, NH4HCO3, NH₃, NH₄F, PVDF, PTFE, NH4CI, NH₄Br, NH₄I, Br₂, Cl₂, I2, ammonium oxychloride amide, and hexamethylenetetramine. The precursor mixture may comprise less than 10 wt%, less than 5 wt%, or less than 1 wt% further precursors. The precursor mixture may consist of a niobium precursor, a titanium precursor, and a metal halide.

When it is desired to make an active electrode material comprising a cation of a specific oxidation state a precursor comprising that cation at that oxidation state may be selected. For example, when making an active electrode material comprising Mn²⁺, MnO may be used as the precursor. When making an active electrode material comprising Mn⁴⁺, MnO2 may be used as the precursor.

It will be understood that the active electrode material may further comprise Li and/or Na, which may reversibly intercalate in situ when the active electrode material is in a metal-ion battery. Some or all of the precursors 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 precursors have an initial particle size of <20 pm in diameter, as the particle size of the precursors may be mechanically reduced by milling, for example during a step of mixing the precursor materials to form a precursor material mixture.

Milling of precursors to achieve desired particle sizes may be performed by a process selected from: grinding milling, impact milling, air jet milling, steam jet milling, high energy milling, high shear milling, pin milling, air classification, wheel classification, sieving, cyclonic separation, and/or bead milling. Mixing the precursors to form the precursor 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, high shear milling, planetary mixing, powder blending, impaction milling, high shear and/or intensive mixer mixing. The force used for mixing and/or milling may depend on the morphology of the precursor materials. For example, where some or all of the precursors have larger particle sizes (e.g. a D50 particle diameter of greater than 20 pm), the milling force may be selected to reduce the particle diameter of the precursors such that the such that the particle diameter of the precursor mixture is reduced to 20 pm in diameter or lower. When the particle diameter of particles in the precursor mixture is 20 pm or less, this can promote a more efficient reaction of the precursors during the step of processing to form the active electrode material. Processing the precursor mixture may comprise solid-state synthesis , for example undertaken in pellets formed at high pressure (e.g. >10 MPa) from the precursor powders.

Processing the precursor mixture preferably comprises heating the precursor mixture. The heating may be performed for 1-48 hours, 2-24 hours, or preferably 3-18 hours. For example, the heating 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 heating may be performed for 24 hours or less, 18 hours or less, 16 hours or less, or 12 hours or less. The heating may be performed for at 400-1350°, or 800-1250°C, or preferably 1000-1125°C. It has been found that the use of the precursor mixture in accordance with the invention allows for a more economical synthesis of an active electrode material having the desired crystal structure.

Heating the precursor mixture may be performed in a gaseous atmosphere, preferably N2 or air, preferably in the absence of water. 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 mixture is performed in an inert or reducing atmosphere.

In some methods a two-step heat treatment may be performed. For example, the precursor 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. Processing the precursor mixture to form the active electrode material encompasses conventional ceramic synthetic techniques. For example, the active electrode material may be formed by one or more of solid-state synthesis, sol-gel synthesis, 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, and atomic layer deposition. Optionally, processing the precursor mixture comprises solid-state synthesis, hydrothermal or microwave hydrothermal synthesis, solvothermal or microwave solvothermal synthesis, coprecipitation synthesis, and/or chemical vapour deposition. Most preferably, processing the precursor mixture comprises solid-state synthesis.

Solid-state synthesis is a widely used method comprising a chemical reaction from solid starting materials to form a new solid. It typically involves measuring solid precursors to achieve an intended elemental ratio, mixing the precursors (e.g. by wet or dry milling), and heating the precursor mixture to facilitate a solid-state reaction to achieve the desired product. It may include further steps such as spray drying (e.g. after mixing the precursors) and deagglomeration (e.g. after heating).

To provide an active electrode material comprising an additional electronegative anion than oxygen and the halide from the metal halide the method may further comprise the steps of: mixing the active electrode material with a precursor comprising an additional electronegative anion to provide a further precursor mixture; and heat treating the further precursor mixture in a temperature range from 300 - 1200 °C or 800 - 1100 °C optionally under reducing conditions, thereby providing the active electrode material comprising an additional electronegative anion.

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

For example, to provide an active electrode material comprising F (e.g. if the metal halide does not comprise F), the method may further comprise the steps of: mixing the active electrode material with a precursor comprising F (for example polyvinylidene fluoride or NF F) to provide a further precursor mixture; and heat treating the further precursor mixture in a temperature range from 300 - 1200 °C under oxidising conditions (for example in air), thereby providing the active electrode material comprising F.

The method may comprise the further step of heat treating the active electrode material in a temperature range from 400 - 1350 °C or 800 - 1250 °C under reducing conditions, thereby inducing oxygen vacancies in the active electrode material.

The method may include one or more post-processing steps after formation of the active electrode material. In some cases, the method may include a post-processing step of heat treating the active electrode material, sometimes referred to as ‘annealing’. This post-processing heat treatment step may be performed in a different gaseous atmosphere to the step of processing the precursor mixture to form the active electrode material. 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 post-processing heat treatment step may be beneficial to e.g. form further disorder or defects in the active electrode material, for example to change the electron distribution and electronic band structures; or to carry out anion exchange on the active electrode material e.g. N exchange for the O anion.

The method may include a step of milling and/or classifying the active electrode material (e.g. impact milling, jet milling, steam jet milling, high energy milling, high shear milling, pin milling, air classification, wheel classification, sieving, cyclonic separation, bead milling) to provide a material with any of the particle size parameters given herein.

The active electrode material is preferably in particulate form. The active electrode material may have a D50 particle diameter in the range of 0.1 -100 pm, or 0.5-50 pm, or 1-20 pm. 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 D50 particle diameter of 100 nm or less) are typically more complex to synthesise and require additional safety considerations.

The active electrode material may have a Dio 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 D10 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.

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.

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 “D_n” and “D_n particle 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. Particle diameters can be determined by laser diffraction. Particle diameters can be determined in accordance with ISO 13320:2009, for example using Mie theory. 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 in orderto minimise the reaction of the active electrode material 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 active electrode material to metal ions in the surrounding electrolyte.

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 9277:2010.

The active electrode material may be coated with carbon, e.g. to improve its surface electronic conductivity and/or to prevent reactions with electrolyte. Accordingly, the method may include a further step of forming a carbon coating on the active electrode material.

The active electrode material may have a protective coating; optionally the protective coating comprises niobium oxide, aluminium oxide, zirconium oxide, organic or inorganic fluorides, organic or inorganic phosphates, titanium oxide, lithiated versions thereof, and mixtures thereof.

The atomic ratio of the metal of the metal halide : titanium in the precursor mixture is preferably < 0.3.

The invention provides an active electrode material obtainable by the method of the first aspect. Elemental analysis may be performed on the active electrode material, e.g. by ICP-OES, ICP-MS, XRF, EDS/X from SEM or TEM, and/or XPS, to confirm the presence of the off-stoichiometry in accordance with the invention,

The active electrode material may be part of a composition comprising the active electrode material and at least one other component; optionally wherein the at least one other component is selected from a binder, a solvent, a conductive additive, a different active electrode material, and mixtures thereof.

The active electrode material is typically incorporated into an electrode. The electrode 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.

Accordingly, the invention also provides a method of making an electrode, comprising making an active electrode material by the method of the first aspect of the invention, and forming an electrode comprising the active electrode material. Preferably forming the electrode comprises depositing the active electrode material on a current collector.

The depositing step may include 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 layer on the current collector. Dry processes that do not use a solvent to coat the current collector may also be used, such as by extrusion methods. 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 2.4-2.9 g cm ³. 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.

Optionally, the active electrode material forms at least 5 wt.%,10 wt.%, or 50 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 electrode composition may further comprise at least one other component selected from a binder, a conductive additive, a different active electrode material (e.g. a further active electrode material of the invention), and mixtures thereof. For instance, one electrode composition comprises about 92 wt% active electrode material of the invention, about 5 wt% conductive additive (e.g. carbon black), and about 3 wt% binder (e.g. poly(vinyld ifluoride)) , based on the total dry weight of the electrode composition.

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.

Conductive additives are preferably non-active materials which are included so as to improve electrical conductivity between the active electrode material and 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. 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 for a solid-state electrode.

When a different active electrode material is present in addition to the active electrode material, it may be selected from lithium titanium oxide, a mixed niobium oxide such as a titanium niobium oxide, a different active electrode material of the invention, graphite, hard carbon, soft carbon, silicon, doped versions thereof, and mixtures thereof.

The active electrode material may be in combination with a lithium titanium oxide to form an electrode composition.

The lithium titanium oxide preferably has a spinel or ramsdellite crystal structure, e.g. as determined by X-ray diffraction. An example of a lithium titanium oxide having a spinel crystal structure is Li4TisOi2. An example of a lithium titanium oxide having a ramsdellite crystal structure is Li2TisO7. These materials have been shown to have good properties for use as active electrode materials. Therefore, the lithium titanium oxide may have a crystal structure as determined by X-ray diffraction corresponding to Li4TisOi2 and/or Li2TisO7. The lithium titanium oxide may be selected from Li4TisOi2, Li2TisO7, and mixtures thereof.

The lithium titanium oxide may be doped with additional cations or anions. The lithium titanium oxide may be oxygen deficient. The lithium titanium oxide may comprise a coating, optionally wherein the coating is selected from carbon, polymers, metals, metal oxides, metalloids, phosphates, and fluorides.

The lithium titanium oxide may be synthesised by conventional ceramic techniques, for example solid- state synthesis or sol-gel synthesis. Alternatively, the lithium titanium oxide may be obtained from a commercial supplier.

The lithium titanium oxide is in preferably in particulate form. The lithium titanium oxide may have a Dso particle diameter in the range of 0.1-50 pm, or 0.25-20 pm, or 0.5-15 pm. The lithium titanium oxide may have a Dw particle diameter of at least 0.01 pm, or at least 0.1 pm, or at least 0.5 pm. The lithium titanium oxide may have a D90 particle diameter of no more than 100 pm, no more than 50 pm, or no more than 25 pm. By maintaining a D90 particle diameter in this range the packing of lithium titanium oxide particles in the mixture with the active electrode material particles is improved.

Lithium titanium oxides are typically used in battery anodes at small particle sizes due to the low electronic conductivity of the material. In contrast, the active electrode material of the invention may be used at larger particle sizes since it typically has a higher lithium-ion diffusion coefficient than lithium titanium oxide. Advantageously, in the electrode composition the lithium titanium oxide may have a smaller particle size than the active electrode material, for example such that the ratio of the D50 particle diameter of the lithium titanium oxide to the D50 particle diameter of the active electrode material is in the range of 0.01 :1 to 0.9:1 , or 0.1 :1 to 0.7:1 . In this way, the smaller lithium titanium oxide particles may be accommodated in the voids between the larger active electrode material particles, increasing the packing efficiency of the composition. The lithium titanium oxide may have a BET surface area in the range of 0.1 -100 m²/g, or 1-50 m²/g, or 3- 30 m²/g.

The ratio by mass of the lithium titanium oxide to the active electrode material may be in the range of 0.5 : 99.5 to 99.5 : 0.5, preferably in the range of 2 : 98 to 98 : 2. In one implementation the electrode composition comprises a higher proportion of the lithium titanium oxide than the active electrode material, e.g. the ratio by mass of at least 2:1 , at least 5:1 , or at least 8:1 . Advantageously, this allows the active electrode material to be incrementally introduced into existing electrodes based on lithium titanium oxides without requiring a large change in manufacturing techniques, providing an efficient way of improving the properties of existing electrodes. In another implementation the electrode composition has a higher proportion of the active electrode material than the lithium titanium oxide, e.g. such that the ratio by mass of the lithium titanium oxide to the active electrode material is less than 1 :2, or less than 1 :5, or less than 1 :8. Advantageously, this allows for the cost of the electrode composition to be reduced by replacing some of the active electrode material with lithium titanium oxide.

The active electrode material may be in combination with a niobium oxide to form an electrode composition. The niobium oxide may be selected from Nbi2C>29, NbC>2, NbO, and Nb2Os. Preferably, the niobium oxide is Nb2Os.

The niobium oxide may be doped with additional cations or anions, for example provided that the crystal structure of the niobium oxide corresponds to the crystal structure of an oxide consisting of Nb and O, e.g. Nbi₂O₂9, NbC>2, NbO, and Nb2Os. The niobium oxide may be oxygen deficient. The niobium oxide may comprise a coating, optionally wherein the coating is selected from carbon, polymers, metals, metal oxides, metalloids, phosphates, and fluorides.

The niobium oxide may have the crystal structure of Nbi2O29, NbO2, NbO, or Nb2Os as determined by X-ray diffraction. For example, the niobium oxide may have the crystal structure of orthorhombic Nb2Os or the crystal structure of monoclinic Nb2Os. Preferably, the niobium oxide has the crystal structure of monoclinic Nb2Os, most preferably the crystal structure of /7-Nb2Os. Further information on crystal structures of Nb2Os may be found at Griffith et al., J. Am. Chem. Soc. 138, 28, 8888-8899 (2016).

The niobium oxide may be synthesised by conventional ceramic techniques, for example solid-state synthesis or sol-gel synthesis. Alternatively, the niobium oxide may be obtained from a commercial supplier.

The niobium oxide is in preferably in particulate form. The niobium oxide may have a Dso particle diameter in the range of 0.1 -100 pm, or 0.5-50 pm, or 1-20 pm. The niobium oxide may have a D particle diameter of at least 0.05 pm, or at least 0.5 pm, or at least 1 pm. The niobium oxide may have a D90 particle diameter of no more than 100 pm, no more than 50 pm, or no more than 25 pm. By maintaining a D90 particle diameter in this range the packing of niobium oxide particles in the mixture with active electrode material particles is improved. The niobium oxide may have a BET surface area in the range of 0.1-100 m²/g, or 1 -50 m²/g, or 1-20 m²/g.

The ratio by mass of the niobium oxide to the active electrode material may be in the range of 0.5 : 99.5 to 99.5 : 0.5, or in the range of 2 : 98 to 98 : 2, or preferably in the range of 15 : 85 to 35 : 55.

The invention also provides the use of the active electrode material of the invention in an anode for a metal-ion battery, optionally wherein the metal-ion battery is a lithium-ion or sodium-ion battery, preferably a lithium-ion battery. Lithium-ion batteries include liquid-based batteries, polymer-based batteries, semi- solid-based batteries and full solid-state-based batteries. Accordingly, the invention also provides a method of making a metal-ion battery, comprising making an active electrode material by the method of the first aspect of the invention, and forming an electrode comprising the active electrode material, and forming a metal-ion battery comprising the electrode. Preferably the electrode forms the anode of the metal-ion battery.

A further implementation of the invention is an electrochemical device comprising an anode, a cathode, and an electrolyte disposed between the anode and the cathode, wherein the anode comprises an active electrode material according to the invention; optionally wherein the electrochemical device is metal-ion battery such as a lithium-ion battery or a sodium-ion battery. Preferably, the electrochemical device is a lithium-ion battery having a reversible anode active material specific capacity of greater than 225 mAh/g at 20 mA/g, wherein the battery can be charged and discharged at current densities relative to the anode active material of 200 mA/g or more, or 1000 mA/g or more, or 2000 mA/g or more, or 4000 mA/g or more whilst retaining greater than 70% of the initial cell capacity at 20 mA/g. It has been found that use of the active electrode materials of the invention can enable the production of a lithium-ion battery with this combination of properties, representing a lithium-ion battery that is particularly suitable for use in applications where high charge and discharge current densities are desired. Notably, the examples have shown that active electrode materials according to the invention have excellent capacity retention at high C-rates.

Examples

Synthesis and Materials Characterisation

Metal halide modified titanium niobium oxides can be synthesised via a solid-state reaction in N2 or dry air atmosphere. The precursor mixtures are shown in Tables 1 and 2. Precursors of Examples 2, 3, 4, 6, and 7 are mixed using impaction milling with blade rotation speed of 15000 rpm for 4 min and then heated at 1100°C for 12 hr in N2. Example 1 was prepared by a rolling ball mill of 200 g of precursors composing TiC>2 and Nb2Os for 3 h at 400 rpm and then heated at 1200°C for 12 h in air. Example 5 was prepared by impaction mill with blade rotation speed of 10000 rpm for 2 min and then 20000 rpm for 4 min. The mixed powder was heated at 1100°C for 12 h in air. Example 8 was prepared by roller ball milling in ethanol for 24hr. The obtained mixture was then dried into powder form at 80°C on a hot plate and then put into a crucible and sintered at 1150°C for 12 h in air. The powder of Examples 1 and 5 showed an off-white colour and the powder of Example 8 showed a white colour, while the powder of Examples 2, 3, 4, 6, and 7 showed a blue-grey colour indicating an alteration in the band gap of the material of the invention, implying increased electrical conductivity. The as-synthesised powder was then de-agglomerated using impaction milling with blade rotation speed of 20000 rpm for 4 min to obtain fine sized particles.

The phase purity of samples was analysed using a Rigaku Miniflex powder X-ray diffractometer in the 20 range (10-70°) at 1 min scan rate. The instrument has an instrumental shifting error of 0.1 °, the peaks in one XRD result could have a constant shift of up to 0.2°. These or equivalent conditions may be used to determine if a material has a single phase.

Particle Size Distributions were obtained with a Horiba laser diffraction particle analyser LA-960 with dry powder feeder. Air pressure was kept at 0.3 MPa.

The XRD pattern of metal halide modified TiNb2O? matches pure TiNb2O? found at PDF card [01-072- 0116] (Wadsley, A.D., Acta Crystallogr., 14, 660, (1961)) with peak intensity difference at 2theta = 27.07°, corresponding with the (60-1) plane (Figures 1 and 9).

The XRD pattern of metal halide modified Ti2NbioC>29 is a mixture of monoclinic and orthorhombic phase of Ti2NbioC>29. The reference of monoclinic Ti2NbioC>29 is from Wadsley, A.D., Acta Crystallogr., 14, 664, (1961). The reference of orthorhombic Ti2NbioC>29 is from R.B. von Dreele, A.K. Cheetham, Proceedings of the Royal Society London, Series A, 338, 311 ,(1974) (Figure 2).

Rietveld refinement of all the examples was performed using HighScore Plus based on the reference patterns noted above. Goodness of fit of all the refinement performed are below 10.

Table 1: Examples with TiNb2O7 crystal structure and their particle size and unit cell data

★Comparative example Table 2 Examples with Ti2NbwO29 crystal structure and their unit cell data regarding orthorhombic region ★Comparative example

Table 3: Examples with Ti2NbwO29 crystal structure and their unit cell data regarding monoclinic region ★Comparative example

Electrochemical Characterisation

Li-ion cell charge rate is usually expressed as a “C-rate”. A 1C 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 capacity within the voltage limits of 1 .1 - 3.0 V, a 1C rate corresponds to a current applied of 1 .0 mA. In a typical material as described herein, this corresponds to ~225 mA/g of active material.

Electrochemical tests were carried out in half-coin cells (CR2032 size) for analysis. In half-coin tests, the active material is tested in an electrode versus a Li metal electrode to assess its fundamental performance. In the below examples, the active material composition to be tested was combined with N- Methyl Pyrrolidone (NMP), carbon black (Super P) acting as a conductive additive, and poly(vinyldifluoride) (PVDF) binder and mixed to form a slurry using a lab-scale centrifugal planetary mixer. The non-NMP composition of the slurries was 92 wt% active material, 5 wt% conductive additive, 3 wt% binder. The slurry was coated on an Al foil current collector to the desired loading of 67 - 73 g nr² by doctor blade coating and dried by heating. The electrodes were then calendared to a density of 2.6 - 2.9 g cm ³ at 80°C to achieve targeted porosities of 30-35%. Electrodes were punched out at the desired size and combined with a separator (Celgard porous PP/PE), Li metal, and electrolyte (1 .3 M LiPFe in EC/DEC) inside a steel coin cell casing and sealed under pressure. Cycling was then carried out at 25°C at low current (C/10) for 2 full cycles of lithiation and de-lithiation between 1.1 - 3.0 V. Afterwards, the cells were tested for their performance at increasing current. During these tests, the cells were cycled asymmetrically at 25°C, with a slow lithiation (C/5) followed by increasing de-lithiation rates (e.g. 1C, 5C, 10C) to test the capacity retention at various currents.

Data has been averaged from 3 to 5 cells prepared from the same electrode coating, with the error shown from the standard deviation. Accordingly, the data represent a robust study showing the improvements achieved by the materials according to the invention compared to prior materials. These data are shown in Tables 4-6.

Homogeneous, smooth coatings on both Cu and Al current collector foils, the coatings being free of visible defects or aggregates may also be prepared as above for these samples with a centrifugal planetary mixer to a composition of up to 94 wt% active material, 4 wt% conductive additive, 2 wt% binder. These can be prepared with both PVDF (/.e. NMP-based) and CMC:SBR-based (/.e. waterbased) binder systems. The coatings can be calendared at 80°C for PVDF and 50°C for CMC:SBR to porosities of 30-40% at loadings from 1 .0 to 5.0 mAh cm ². This is important to demonstrate the viability of these materials in both high energy and high-power applications, with high active material content.

Table 4 Results of electrochemical characterisation of examples 1*, 2-4, and 8*

Table 5: Results of electrochemical characterisation of examples 1*, 2-4, and 8* Table 6: Results of electrochemical characterisation of examples 1*, 2-4, and 8*

Table 7; Results of electrochemical characterisation of examples 6 and 7

Table 8: Results of electrochemical characterisation of examples 6 and 7

Table 9: Results of electrochemical characterisation of examples 6 and 7

Solid State NMR

Fluorine-19 NMR spectra were recorded at 379.60 MHz using A Bruker Advance III HD spectrometer and a 3.2 mm magic-angle spinning probe. The spectra were obtained using direct polarization. A recycle delay of 2s was used. Samples were acquired at a sample spin-rate of 20kHz. Spectral referencing is with respect to CFCh, carried out by setting the signal from an external sample of 50% CF3COOH in H2O to - 76.54 ppm.

Discussion

Compared to the comparative examples, the metal halide modified titanium niobium oxides according to the invention exhibited remarkable improvement in the electrochemical properties. Example 1 * is a comparative example without the use of a metal halide, corresponding to TiNb2Oy. Example 8* is a comparative example where a metal halide was used in the precursor mixture but where the atomic ratio of cations:anions in the precursor mixture corresponds to the atomic ratio of cations:anions of TiNb2Oy. Examples 2, 3, and 4 use a metal halide in the precursor mixture and the atomic ratio of cations:anions in the precursor mixture does not correspond to the atomic ratio of cations:anions of TiNb2Oy, Ti2NbioC>29, TiNb Osy, or TiNb24O62. Higher 1 ^st cycle coulombic efficiency and higher de-lithiation capacity and capacity retention at 5C and 10C were demonstrated for examples 2, 3, and 4 compared to examples 1* and 8*. The improvement of rate performance can be explained by the better electrical conductivity caused by the Fermi level of the material shifted towards conductive band due to the halide anions and metal cations incorporated in the crystal structure, and the off-stoichiometric composition. This can be generalised for other titanium niobium oxides, e.g., Ti2NbioC>29 and TiNbuOsy, as they have similar block structures.

Furthermore, the reaction energy can be significantly reduced by incorporation of metal halides. Nonnanosized TiNb2Oy is only possible to obtain at 1200°C via solid state reaction while by incorporation of metal halides, the reaction temperature can be reduced to 1100°C, providing an economic benefit for industrial scale processes.

Solid-state NMR confirmed the chemical environment of F in example 6. A signal was observed at -161 .6 ppm corresponding to F atoms in a bridging position between two Al atoms and bonded to Al in an octahedral environment. This shows that the Al and F from the metal halide used in the precursor mixture have entered the Wadsley-Roth crystal structure.

Synthetic Studies

Synthetic studies were carried out to assess the importance of the metal halide and the off-stoichiometry in the precursor mixture to the formation of the desired crystal structure. Comparative studies A, B, C, E syntheses were carried under identical conditions: mixed using impaction milling with blade rotation speed of 15000 rpm for 4 min and then heated at 1 100°C for 12 hr in N2. Comparative study D syntheses was carried out with same milling conditions above but heated in 1150°C for 12 hr in air. Comparative study F syntheses was carried out with same milling conditions above but heated at 1150°C for 12 hr in N₂.

Comparative Study A

The precursor mixture was Nb2Os:TiO2 in a molar ratio of 1 :0.9. An XRD pattern of the resulting material showed peaks attributed to TiNb2Oy crystal structure but with additional peaks attributed to Ti2NbwO29 (see arrow in Figure 3). In contrast, Example 4 (Nb2Os:TiO2:AIF3 = 1 :0.9:0.05) was able to form single phase TiNb2Oy. This demonstrates that, without the use of the metal halide, an off-stoichiometric precursor mixture with Nb/Ti>2 was not able to make single phase TiNb2Oy. Comparative Study B

The precursor mixture was Nb2O5:TiO2:aluminium oxalate in a molar ratio of 1 :0.9:0.05. An XRD pattern of the resulting material showed high levels of Ti2NbioC>29 and TiC>2 and broad peaks (Figure 4). In contrast, Example 4 (Nb2Os: TiC>2:AIF3 = 1 :0.9:0.05) was able to form single phase TiNb2Oy. This demonstrates that the metal halide as used in the invention provides for the formation of a single phase, and that replacement by an alternative metal precursor (in this study, aluminium oxalate) does not.

Comparative Study C

The precursor mixture was Nb2Os:TiO2:AIF3 in a molar ratio of 1 :2:0.1 . An XRD pattern of the resulting material showed peaks attributed to TiNb2Oy but with additional peaks attributed to TiC>2 (see arrow in Figure 5). No peaks attributed to AIF3 were observed. This demonstrates that having an Nb:Ti ratio of 2, in the presence of a metal halide, does not allow for the formation of a single phase. It is believed that AIF3 takes priority reacting to Nb2Os which resulted in residual TiC>2 in the synthesised material.

Comparative Study D

The precursor mixture was Nb2Os:TiO2:ZnF2 in a molar ratio of 1 :0.9:0.1 (i.e. corresponding to the stoichiometric ratio for TiNb20y). An XRD pattern of the resulting material showed peaks attributed to TiNb2<Dy crystal structure but with additional peaks attributed to Ti2NbwO29 (see arrow in Figure 6). This demonstrates that even with higher temperature 1150°C, composition with stoichiometric substitution of TiC>2 with ZnF2, does not allow for the formation of a single phase. In contrast, Example 3 (Nb2Os: TiC>2:ZnF2 = 1 :0.94:0.01) was able to form single phase TiNb2<Dy. This demonstrates that an off-stoichiometric precursor mixture in accordance with the invention is required for single-phase formation.

Comparative Study E

The precursor mixture was Nb2Os:TiO2:AIF3 in a molar ratio of 0.9:1 .1 :0.1 (i.e. corresponding to the stoichiometric ratio for TiNb20y). An XRD pattern of the resulting material showed peaks attributed to TiNb2<Dy but with additional peaks attributed to TiC>2 (see arrow in Figure 7). Since Al is a 3+ cation, both Nb and Ti are adjusted to make a stoichiometric composition. In contrast, Example 4 (Nb2Os:TiO2:AIF3 = 1 :0.9:0.05) was able to form single phase TiNb2<Dy This demonstrates that an off-stoichiometric precursor mixture in accordance with the invention is required for single-phase formation.

Comparative Study F

The precursor mixture was Nb2Os:TiO2:NH4F in a molar ratio of 1 :0.95:0.1 (i.e. not corresponding to the stoichiometric ratio for TiNb2<Dy but not using a metal halide). An XRD pattern of the resulting material showed high levels of Ti2NbwO29 impurities (see arrow in Figure 8) even with higher temperature synthesis (1150°C). In contrast, Examples 3 and 4 were able to form single phase TiNb2<Dy at 1100°C This demonstrates that a metal halide must be present in the precursor mixture to form a single-phase material. The simultaneous anion and cation substitution provided by the metal halide is leads to singlephase formation.

Claims

Claims:

1 . A method of making an active electrode material, the method comprising: processing a precursor mixture comprising a niobium precursor, a titanium 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 TiNb2Oy, Ti2NbioC>29, TiNb Os?, or TiNb24C>62; wherein the active electrode material has the crystal structure of TiNb2Oy, Ti2NbioC>29, TiNbuOs?, and/or TiNb24C>62.

2. The method of claim 1 , wherein the active electrode material has the crystal structure of TiNb2Oy, optionally wherein an X-ray diffraction pattern of the active electrode material has a peak at 20 = 26.0 ± 0.2 assigned to the crystal structure of TiNb2Oy.

3. The method of claim 2, wherein the crystal structure is monoclinic, optionally wherein the unit cell parameters are: a is 20.38-20.45 A, b is 3.80-3.82 A, c is 1 1 .90-11 .94 A, a = y = 90°, and p is 120.10-120.30°.

4. The method of claim 1 , wherein the active electrode material has the crystal structure of Ti2NbioC>29, optionally wherein an X-ray diffraction pattern of the active electrode material has a peak at 20 = 24.9 ± 0.2 assigned to the crystal structure of Ti2NbioC>29.

5. The method of claim 4, wherein the crystal structure is orthorhombic, optionally wherein the unit cell parameters are: a is 28.30-28.70 A, b is 3.78-3.83 A, c is 20.35-20.70 A, and a = p = y = 90°.

6. The method of claim 4, wherein the crystal structure is monoclinic, optionally wherein the unit cell parameters of the active electrode material are: a is 20.54-20.57 A, b is 3.80-3.82 A, c is 15.52- 15.55 A, a = y = 90°, and p is 113.00-113.70°.

7. The method of claim 1 , wherein the active electrode material has the crystal structure of TiNbuChy, optionally wherein an X-ray diffraction pattern of the active electrode material has a peak at 20 = 23.8 ± 0.2assigned to the crystal structure of TiNbuOs?.

8. The method of claim 7, wherein the crystal structure is monoclinic, optionally wherein the unit cell parameters of the active material are: a is 20.00-21.60 A, b is 3.81 -3.83 A, c is 29.82-30.15 A, a = y = 90°, and p is 94.50-95.50°

9. The method of claim 1 , wherein the active electrode material has the crystal structure of TiNb24C>62, optionally wherein an X-ray diffraction pattern of the active electrode material has a peak at 20 = 24.7 ± 0.2 assigned to the crystal structure of TiNb24O62.

10. The method of claim 9, wherein the crystal structure is monoclinic, optionally wherein the unit cell parameters are a is 29.59-29.98 A, b is 3.80-3.84 A, c is 20.91-21 .29 A, a = Y = 90°, and p is 94.2-95.6°.

11 . The method of any preceding claim, wherein the active electrode material has the crystal structure of TiNb2Oy and/or Ti2NbioC>29; or wherein the active electrode material has the crystal structure of TiNb2Oy. The method of any preceding claim, wherein the active electrode material has a single phase crystal structure consisting of the crystal structure of TiNb2Oy, Ti2NbioC>29, TiNbuOs?, or TiNb24C>62; or consisting of the crystal structure 0f TiNb2Oy or Ti2NbioC>29; or consisting of the crystal structure of TiNb2Oy. The method of any preceding claim, wherein the niobium precursor is selected from Nb2Os, Nb(OH)s, niobic acid, NbO, ammonium niobate oxalate, NbC>2, NbC>2F, NbsOyF, niobium chloride, niobium fluoride, niobium bromide, and mixtures thereof; or wherein the niobium precursor is a niobium oxide; or wherein the niobium precursor is Nb2Os. The method of any preceding claim, wherein the titanium precursor is a selected from TiC>2, titanium chloride, titanium fluoride, titanium bromide, titanium oxalate, ammonium titanyl oxalate or nitrate, titanyl nitrate, titanyl sulfate, titanyl hydroxide, and ammonium bis(oxolato)oxotitanate, and mixtures thereof; or wherein the titanium precursor is TiC>2. The method of any preceding claim, wherein the oxidation state of the metal of the metal halide is +4 or less or +3 or less. The method of any preceding claim, wherein the metal of the metal halide is 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; or Al, Zn, and mixtures thereof. The method of any preceding claim, wherein the metal halide is a metal fluoride, metal chloride or metal bromide; optionally wherein the metal halide is a metal fluoride. The method of any preceding claim, wherein the metal of the metal halide is not Nb or Ti. The method of any preceding claim, wherein the niobium precursor and titanium precursor are not metal halides. The method of any preceding claim, wherein the metal fluoride is ZnF2 and/or AIF3. The method of any preceding claim, wherein the niobium precursor, titanium precursor, and metal halide are in particulate form; optionally wherein the niobium precursor, titanium precursor, and metal halide have a D50 particle diameter of < 20 pm. The method of any preceding claim, wherein the atomic ratio of the metal of the metal precursor : titanium in the precursor mixture is < 0.3. The method of any preceding claim, wherein the metal halide is 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. The method of any preceding claim, wherein the active electrode material is in particulate form, optionally wherein the active electrode material has a D50 particle diameter in the range of 0.1-100 pm, or 0.5-50 pm, or 1-20 pm. The method of any preceding claim, wherein the active electrode material has a D10 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. The method of any preceding claim, wherein the active electrode material has a D90 particle diameter of no more than 200 pm, or no more than 100 pm, no more than 50 pm, or no more than 20 pm. The method of any preceding claim, wherein the active electrode material has a BET surface area in the range of 0.1-100 m²/g, or O.25-50 m²/g, or 0.5-20 m²/g. The method of any preceding claim, wherein the active electrode material is coated with carbon. The method of any preceding claim, wherein the active electrode material comprises a protective coating, optionally wherein the protective coating comprises niobium oxide, aluminium oxide, zirconium oxide, organic or inorganic fluorides, organic or inorganic phosphates, titanium oxide, lithiated versions thereof, and mixtures thereof. The method of any preceding claim, wherein processing the precursor mixture comprises solid- state synthesis, 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, and/or atomic layer deposition; optionally wherein processing the precursor mixture comprises solid- state synthesis. The method of any preceding claim, wherein processing the precursor mixture comprises heating the precursor mixture. The method of claim 31 , wherein the heating is performed at 400-1350°, or 800-1250°C, or 1000- 1125°C. The method of claim 31 or 32, wherein the heating is performed for 1-48 hours, or 2-24 hours, or 3-18 hours. The method of any of claims 31-33, wherein the heating is performed in a gaseous atmosphere, optionally under nitrogen or air, optionally in the absence of water. An active electrode material obtainable by the method of any of claims 1-34. A composition comprising the active electrode material of claim 35 and at least one other component; optionally wherein the at least one other component is selected from a binder, a solvent, a conductive additive, a different active electrode material, and mixtures thereof. An electrode comprising the active electrode material of claim 35; optionally wherein the active electrode material is deposited on a current collector. The electrode of claim 37, wherein the active electrode material according to claim 35 forms at least 5 wt.%, at least 10 wt.%, or at least 50 wt% of the total active electrode material in the electrode; or wherein the active electrode material according to claim 35 is the sole active electrode material in the electrode. The electrode of claim 38, further comprising at least one other component selected from a binder, a conductive additive, a different active electrode material, and mixtures thereof. The electrode according to claim 39, wherein the different active electrode material is selected from lithium titanium oxide, titanium niobium oxide, a niobium oxide, a different active electrode material obtained by the method of any of claims 1-35, graphite, hard carbon, soft carbon, silicon, doped and/or carbon-coated versions thereof, and mixtures thereof. The electrode of any of claims 37-40, wherein the active electrode material is 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. A metal-ion battery comprising the electrode of any of claims 37-41 , optionally wherein metal-ion battery is a lithium-ion battery or sodium-ion battery and the electrode forms the anode. The metal-ion battery according to claim 42, which is a lithium-ion battery having a reversible anode active material specific capacity of greater than 225 mAh/g at 20 mA/g, wherein the battery can be charged and discharged at current densities relative to the anode active material of 200 mA/g or more, or 1000 mA/g or more, or 2000 mA/g or more, or 4000 mA/g or more whilst retaining greater than 70% of the initial cell capacity at 20 mA/g. Use of an active electrode material according to claim 35 in a metal-ion battery; optionally in an anode of a lithium-ion battery or sodium-ion battery.