Positive electrode active material for batteries

Abstract

A solid material of formula (I): of formula (I): AaNi1-xMbYz wherein: A is selected from Li, Na, or K or a mixture thereof; a is from 0.75 to 1.25; x is 0.005 < x ≤ 0.500; M is selected from Ti, Zr, Hf, Nb, Ta, Mo, W, V, or Re or a mixture thereof; b is 0.005 ≤ b < 0.500, wherein x > b; Y is selected from O or S or a mixture thereof; and z is from 1.5 to 2.5. The solid material suitably provides a positive electrode active material for use in a battery. A battery comprising the solid material as a positive electrode and a method of preparing the solid material are also disclosed.

Description

[0001]Positive Electrode Active Material for BatteriesFieldThe present invention relates to a solid material, a method of preparing a solid material and the useof said solid material as a positive electrode material. The present invention also relates to a batterycomprising the solid material. The solid materials may be particularly useful as positive electrodeactive materials in lithium-ion batteries and lithium metal batteries.Background Lithium-ion batteries (LIBs) have been the subject of much research and development over recentyears due to their potential to provide next-generation energy storage devices. LIBs comprise anegative electrode, a non-aqueous electrolyte, a separator and a positive electrode. In use, thenegative electrode and positive electrode store lithium ions and the electrolyte carries the positivelycharged lithium ions from the negative electrode to the positive electrode and vice versa through theseparator. During charging, lithium ions are released from the positive electrode and flow to thenegative electrode. During discharging, the lithium ions return to the positive electrode whichgenerates a flow of electrons through a circuit from the negative electrode to the positive electrode,powering an attached device, for example.The further development of LIBs has been limited by currently available positive electrode activematerials. Positive electrode active materials typically comprise metal-oxides. The most commonpositive electrode active materials are, lithium cobalt oxide (LiCoO2), lithium manganese oxide(LiMn2O4), lithium iron phosphate (LiFePO4, or LFP) and lithium nickel manganese cobalt oxide(LiNi1-x-yMnxCoyO2, or NMC). However, bottlenecks in cobalt supply have negatively impactedcommercial battery production. Cobalt extraction and production also have known cost, ethical andenvironmental disadvantages. Therefore, it would be advantageous to use alternative positiveelectrode active materials which are substantially free of cobalt.However, achieving the high capacity and fast-charging properties demonstrated by such cobalt-freepositive electrode active materials remains a challenge due to degradation and capacity loss overelectrochemical charge / discharge cycling. Recent efforts have been directed to maximising theobtainable charge capacity of cobalt-free positive electrode materials. One such Co-free lithiumpositive electrode active material is lithium nickel oxide (LiNiO2), which is an analogue to LiCoO2.The use of LiNiO2-based positive electrode active materials offers potential advantages due to itsfavorable redox properties, allowing for high energy density and desirable voltage characteristics andrelatively low cost. However, LiNiO2 suffers capacity fading, poor rate capability, and severedegradation over long-term electrochemical charge / discharge cycling. These problems have hindered the practical application of this material in lithium-ion batteries [Angew. Chem.2018, 58, 10434-10458]. Further research has been conducted on LiNiO2 materials doped with different d0cations, with the aim to modify its structural stability and to improve the electrochemical properties. Al-doped LiNiO2 has shown enhanced capacity retention and improved cycling stability, attributed to the suppression of structural transformation during cycling. Ti-doped LiNiO2 has shown improved structural stabilityand better cycling performance over 100 cycles and better thermal stability than LiNiO2. Nb and W-doping can efficiently improve the cycling and rate performance of the material [Electrochem. Commun., 2001, 3, 52-55, J. Power Sources, 2006, 161, 606-611, J. Electrochem. Soc., 2022, 169, 4, 040533, J. Electroanal. Chem., 2022, 907, 116034, J. Mater. Chem. A, 2019, 7, 18580-18588, J. Mater. Chem. A, 2022, 10, 7841-7855].In the prior art, LiNi1-xMxO2 (M = dopant element) materials with stoichiometric composition havebeen studied. Fast-charging, and high loading of active materials have not been demonstrated in theprior art [J. Mater. Chem. A, 2019, 7, 18580–18588; Electrochem. Commun.2001, 3, 52-55].The W-doped LiNiO2 showed insufficient capacity retention [J. Mater. Chem. A, 2022, 10, 7841–7855], which dropped below 150 mAh g-1after 100 cycles when measured at 25oC and 112.5 mA g-1between 3 and 4.3 V vs. Li+ / Li. For Nb-doped LiNiO2 [J. Electrochem. Soc., 2022, 169, 4, 040533], the capacity decreased to 135 mAh g-1after 200 cycles when measured at 25oC and 135 mA g-1between 2.7 and 4.3 V vs. Li+ / Li. Considering the materials described above, in order to achieve optimized performance, there remains a need for positive electrode materials having the following key properties; Co-free, highinitial capacity, good capacity retention over long-term cycling, a good rate performance that allowsfast charging, high mass loading of active materials that allow high areal capacity. Summary of the InventionIt is one aim of the present invention, amongst others, to provide a solid material that addresses atleast one disadvantage of the prior art, whether identified here or elsewhere, or to provide analternative to existing solid materials. For instance, it may be an aim of the present invention toprovide a solid material which may be used as a positive electrode active material in lithium-ionbatteries or lithium metal batteries.It may be a further aim of the present invention to provide a solid material which shows enhancedlong-term cycling stability suitably with good capacity retention.It may be a further aim of the present invention to provide a solid material which demonstrates improved fast charging performance.It may be a further aim of the present invention to provide a solid material which shows high loadingin an electrode fabrication.It may be a further aim of the invention to provide such a solid material which is substantially free ofmetal species which are resource-limited, for example cobalt.According to aspects of the present invention, there is provided a solid material, a battery comprisingsuch a solid material, a positive electrode comprising such a solid material, and a method ofpreparing such a solid material as set forth in the appended claims. Other features of the inventionwill be apparent from the dependent claims, and from the description which follows. According to a first aspect of the present invention, there is provided a solid material of formula (I): AaNi1-xMbYz wherein: Ais selected from Li, Na, or K or a mixture thereof;a is from 0.75 to 1.25; 0.005 < x ≤ 0.500;M is selected from Ti, Zr, Hf, Nb, Ta, Mo, W, V, or Re or a mixture thereof; 0.005 ≤ b < 0.500wherein x > b; Y is selected from O or S or a mixture thereof; and z is from 1.5 to 2.5.Each of the A, M and Y atoms can be present in mixtures of atoms selected from the lists above andtherefore are not necessarily present in stoichiometric amounts. Furthermore, b and x may have anyvalue between the stated ranges. For example, x may be 0.025, 0.05, 0.075, 0.1, 0.2, 0.3.In the solid material of this first aspect, M in formula (I) is selected from Ti, Zr, Hf, Nb, Ta, Mo, W, V,or Re or a mixture thereof. By a “mixture thereof” we mean that the material may contain at least twoof these different species to provide the one “M” group in the formula (I). In such embodiments, the material may be considered to contain the different “M” species in a random order throughout thesolid material, with their relative abundance as indicated by the formulas disclosed herein. Thismeaning also applies to the species A and Y of the material of formula (I).For the avoidance of doubt, the species mentioned above (Na, Li, K, Ti, Zr, Hf, Nb, Ta, Mo, W, V,Re, O, S) suitably have their normal charges in the solid material of this first aspect, for example Li+,Ti4+, Zr4+, Hf4+, V4+, V5+, Nb5+, Ta5+, Mo6+, W6+, Re7+, S2- and O2-. Suitably the solid material of the firstaspect is overall charge neutral. The material of formula (I) is suitably charge balanced to providesuch a charge neutral material. In particular, the value of a for the amount of lithium present in theformula may vary to account for and balance the charges of the species mentioned above when present in the material, in particular the “M” species. The inventors have found that the solid materials of this first aspect according to formula (I) may exhibit enhanced long-term cycling stability, which may be an improvement on the long-term cyclingstability of LiNiO2 under the same electrochemical test conditions. The solid materials suitably showenhanced long-term cycling stability with good capacity retention. The material also suitablydemonstrates the persistence of a stable layered structure and so may be a suitable improvementcompared with current solid materials for use in batteries.The solid materials of formula (I) contain cation vacancies due to the presence of the dopant M species in the stated relative amounts wherein x > b. Without being bound by theory, it is believed that the said cation vacancies provide or at least contribute to the beneficial properties of the material described herein. These cation vacancies are believed to cause the material to adopt a structure having both a layered / ordered phase with a structure similar to that of crystalline LiNiO2 and a disordered rocksalt-type phase. Furthermore, this structure having the two different phases is believed to provide or at least contribute to the beneficial properties of the material described herein.The inventors have found that the solid material of this first aspect may have relatively fast chargingperformance compared to known LiNiO2-based materials, even at an 8 C (1 C = 200 mA g−1) currentrate, for example by achieving 80 % of theoretical capacity within 6 minutes of charging. Such fastcharging is believed to be advantageous for the use of the solid material as a positive electrode active material in batteries such as lithium-ion batteries, or Li metal batteries. Scanning Electron Microscopy (SEM) studies of the as-prepared powders of the solid material showspherical-ball-like morphology. This may allow high packing density of powders to be achieved,relative to known materials, and may provide structure-related performance enhancements.As such, the solid material of the present invention may combine several key properties such asgood cycling stability, excellent rate performance and high loading in electrode fabrication.Furthermore, the solid material of the present invention suitably achieves these properties withoutthe use of cobalt, which may therefore avoid the supply, cost, ethical and environmental issuesassociated with the extraction and production of cobalt for use in lithium batteries.Suitably the solid material is substantially free of cobalt. Suitably, the solid material of this first aspectdoes not contain any cobalt.In the solid material of formula (I): AaNi1-xMbYz of this first aspect, the following relationship betweenthe values of a, x, b and z suitably applies: a+1-x+b < z. Said relationship suitably further defines thatthe solid material of this first aspect comprise cation vacancies, as discussed above.In the solid material of this first aspect of formula (I): AaNi1-xMbYz, A is selected from Li, Na, or K or amixture thereof. A suitably comprises Li. In some embodiments A is Li. Therefore, the solid materialsuitably has the formula (II): LiaNi1-xMbYz; wherein a, x, b, Y and z are as defined above. In the solid material of formulas (I) or (II), a is from 0.75 to 1.25. Suitably a is from 0.9 to 1.1, suitably from 0.95 to 1.05. Alternatively, a may be from 1 to 1.5, from 1 to 1.25 or from 1 to 1.1. Suitably a isapproximately 1, preferably 1. In some embodiments A is Li and a is 1. Therefore, the solid materialsuitably has the formula (III): LiNi1-xMbYz; wherein x, M, b, Y and z are as defined above.In the solid material of this first aspect of formula (I): AaNi1-xMbYz, Y is O or S and z is from 1.5 to 2.5.In some embodiments, Y is O. Suitably z is from 1.75 to 2.25, from 1.9 to 2.1 or from 1.95 to 2.05.Suitably z is approximately 2. Suitably z is 2. Therefore, the solid material of the first aspect suitablyhas formula (IV): AaNi1-xMbO2wherein A, a, x, M and b are as defined above.In such embodiments, A is suitably Li and a is 1. Therefore, the solid material suitably has the formula(V): LiNi1-xMbO2 wherein x, M and b are as defined above.In the solid material of this first aspect of formula (I): AaNi1-xMbYz, M is selected from Ti, Zr, Hf, Nb,Ta, Mo, W, V, or Re or a mixture thereof, x is 0.005 < x ≤ 0.500 and b is 0.005 ≤ b < 0.500.Suitably 0.010 ≤ x ≤ 0.450, suitably 0.015 ≤ x ≤ 0.400 or 0.020 ≤ x ≤ 0.350. In some embodiments,0.020 ≤ x ≤ 0.300.Suitably 0.005 ≤ b ≤ 0.400, suitably 0.010 ≤ b ≤ 0.350 or 0.015 ≤ b ≤ 0.300. In some embodiments,0.010 ≤ b ≤ 0.250.In some embodiments, 0.015 ≤ x ≤ 0.400 and 0.010 ≤ b ≤ 0.350.In some embodiments, 0.020 ≤ x ≤ 0.350 and 0.010 ≤ b ≤ 0.250.In the solid material of formula (I), x > b. Suitably x-b is at least 0.001, suitably at least 0.002 or at least 0.005. Suitably x-b is from 0.001 to 0.250, suitably from 0.002 to 0.150 or from 0.005 to 0.100.As discussed above, the solid material of formula (I) contains cation vacancies due to the presenceof the dopant M species in the stated relative amounts wherein x > b. The cation vacancies in the solid material are believed to provide or at least contribute to the beneficial properties of the material described herein. Suitably the solid material of formula (I) comprises a stoichiometric amount of vacancies of at least0.001, suitably at least 0.002 or at least 0.005. Suitably the stoichiometric amount of vacancies in thesolid material of formula (I) is from 0.001 to 0.250, suitably from 0.002 to 0.150 or from 0.005 to0.100.In some embodiments, the solid material has a structure comprising cation vacancies having astoichiometry of x-b in relation to formula (I). In such embodiments, the value of “a” is suitably 1.In such embodiments, x-b is suitably at least 0.001, suitably at least 0.002 or at least 0.005.In such embodiments, x-b is suitably from 0.001 to 0.250, suitably from 0.002 to 0.150 or from 0.005to 0.100.In some embodiments, the solid material has a structure comprising cation vacancies in astoichiometric amount less than x-b in relation to formula (I). In such embodiments, the value of “a”may be greater than 1. In such embodiments, some of the vacancies which the solid material mayhave otherwise contained may be taken up by additional A species, for example Li ions. In such embodiments, the stoichiometry of the vacancies in the solid material of formula (I) may be approximated to (x-b)-(a-1). In such embodiments, the stoichiometric amount of vacancies in the solid material of formula (I) is suitably from 0.001 to 0.200, suitably from 0.002 to 0.100 or from 0.005 to 0.075. The values of x and b may depend on the particular M species present in the solid material. Suitable values of x and b for different M species are discussed below.In some embodiments, M is selected from Ti, Zr, V, or Hf or a mixture thereof, suitably wherein V isin the form V4+. In such embodiments, b may be related to x as follows: x / 4 ≤ b ≤ 3x / 4. Suitably b =3x / 4.M in formula (I) may be TidZreHffVg, wherein d, e and f are each independently 0 to 0.500 and d + e+ f + g = b, and therefore 0.010 ≤ d + e + f + g < 0.500.In some embodiments, M is Ti and therefore the solid material may have the formula (VI):AaNi1-xTibYzwherein A, a, x, b, Y and z are as defined above and suitably wherein x / 4 ≤ b ≤ 3x / 4.In such embodiments, the solid material suitably has the formula (VII):LiaNi1-xTibO2wherein a, x and b are as defined above and suitably wherein x / 4 ≤ b ≤ 3x / 4.Suitably b = 3x / 4 and therefore the solid material has the formula (VIII):LiaNi1-xTi3x / 4O2. In such embodiments, x is suitably from 0.015 to 0.400 or from 0.02 to 0.350, or from 0.025 to 0.3. In such embodiments wherein a = 1, the solid material has the formula (IX): LiNi1-xTi3x / 4O2.In some embodiments, M is selected from V, Nb or Ta or a mixture thereof, suitably wherein V is inthe form V5+. In such embodiments, b may be related to x as follows: 2x / 5 ≤ b ≤ 3x / 5. Suitably b =3x / 5.M in formula (I) may be VhNbiTaj, wherein g and h are each independently 0 to 0.5 and h + i + j = b,and therefore 0.010 ≤ h + i + j < 0.5.In some embodiments, M is selected from Mo or W or a mixture thereof. In such embodiments, bmay be related to x as follows: x / 3 ≤ b ≤ x / 2. Suitably b = x / 2.M in formula (I) may be MokWl, wherein k and l are each independently 0 to 0.500 and k + l = b, andtherefore 0.010 ≤ k + l < 0.5.In some embodiments, M is Re. In such embodiments, b may be related to x as follows: 2x / 7 ≤ b ≤3x / 7. Suitably b = 3x / 7.The solid material of this first aspect suitably comprises a highly ordered structure forming a crystallattice extending in three dimensions. The solid material is suitably formed from a unit cell which repeats in three dimensions to form a three-dimensional lattice structure. As will be known by theskilled person, a unit cell is characterized by six parameters. These parameters are three edges (a,b and c) and angles between them (α, β and γ). Suitably, using these parameters, a unit cell may bedefined as cubic, tetragonal, orthorhombic, monoclinic, hexagonal or triclinic.As discussed above, the solid materials of this first aspect contain cation vacancies which are believed to cause the material to adopt a structure having both a layered / ordered phase with a structure similar to that of crystalline LiNiO2 and a disordered rocksalt-type phase.In some embodiments, the solid materials of this first aspect are crystalline. Therefore the solidmaterial may be referred to as a solid crystalline material. The solid materials of this aspect may beconsidered to be nanocomposites of the layered and disordered rocksalt structures. Suitably the solidmaterial of this first aspect has a crystal structure comprising a layered structure (R3^m) and a disordered rocksalt structure (Fm3^m). The crystal structure of the solid materials suitably comprises at least 1% of the disordered rocksalt structure (Fm3^m), suitably at least 5% or at least 10%. Suitably the crystal structure of the solid materials comprises up to 50% of the disordered rocksalt structure (Fm3^m), suitably up to 45% or up to 40%. Suitably the crystal structure of the solid materials comprises from 1 to 50% of the disordered rocksalt structure (Fm3^m), suitably from 5 to 45% or from 10 to 40%. The remainder of the crystal structure is suitably provided by the ordered, layered structure (R3^m). According to a second aspect of the present invention, there is provided a positive electrode comprising a solid material of the first aspect.The solid material may have any of the suitable features and advantages discussed above in relationto the first aspect.The positive electrode suitably comprises a positive electrode active material and a collector material.The solid material of the first aspect suitably provides the positive electrode active material. Thepositive electrode active material is suitably arranged on a collector material to provide the positiveelectrode. Suitable collector materials are known in the art.The solid active material is suitably combined with further components to provide a positive electrodecomposite material, for example a mixture with a conductive agent and / or a binder. Therefore thepositive electrode suitably comprises a positive electrode composite material and a collector, the positive electrode composite material comprising the solid material of the present invention (as positive electrode active material) and a conductive agent and / or a binder.The positive electrode composite material suitably comprises a conductive agent, for examplecarbon. The positive electrode composite material suitably comprises from 0.1 to 20 wt% of theconductive agent, based on the total weight of the positive electrode composite material (namely, thetotal weight of the positive electrode active material and the carbon, with or without binder), suitablyfrom 1 to 10 wt% or from 2 to 8 wt%.The positive electrode composite material suitably comprises a binder, suitably a polymeric binder,for example polyvinylidene fluoride (PVDF). The positive electrode composite material suitablycomprises from 0.1 to 20 wt% of the binder, based on the total weight of the positive electrode(namely, the total weight of the positive electrode active material, carbon, and binder), suitably from1 to 10 wt% or from 2 to 8 wt%.The positive electrode composite material suitably comprises from 60 to 100 wt% of the solid materialof the first aspect, suitably from 80 to 98 wt% or from 84 to 96 wt%, based on the total weight of thepositive electrode composite material (namely, the total weight of the positive electrode activematerial, carbon, and binder).The positive electrode composite material is suitably arranged on the collector material in a loadingof at least 1 mg cm-2 of positive electrode active materials per unit area of the collector material,suitably at least 5 mg cm-2, at least 10 mg cm-2 or at least 20 mg cm-2.It is believed that the solid material of the present invention allows the use of a relatively high loadingof positive electrode active material on said positive electrode whilst maintaining stability andproviding a high areal charge capacity, compared to known materials such as LiCoO2. The solid material of the present invention suitably has spherical-ball-like morphology, suitably as determined by Scanning Electron Microscopy (SEM). This suitably allows a relatively high packing density of powders to be achieved, relative to known materials, and may provide structure-related performance enhancements.The solid material of the present invention, suitably having the morphology discussed above, suitablyhas a primary particle size of 10 to 500 nm, from 10 to 200 nm, or from 50 to 200 nm. Suitably theprimary particle size is as measured by SEM as discussed below.According to a third aspect of the present invention, there is provided a battery comprising a solidmaterial of the first aspect. The solid material is suitably a positive electrode active material.The solid material is suitably able to release electrons to the external circuit and may be oxidizedduring an electrochemical reaction.The solid material may also be used as a component in a composite electrode in the battery.The solid material may be used in a positive electrode in the battery. Therefore, the battery of thissecond aspect suitably comprises: a negative electrode; apositive electrode comprising the solid material of the first aspect; andan electrolyte.The positive electrode is suitably as described in relation to the second aspect of the presentinvention.In such embodiments, the electrolyte is suitably arranged between the positive electrode and thenegative electrode. Suitably the electrolyte is a liquid electrolyte or a solid-state electrolyte. Suitablythe battery is a primary battery or a rechargeable (or “secondary”) battery. The battery may be anysuitable battery that uses Li, Na, or K ions or a mixture thereof.Suitable materials for the negative electrode and electrolyte may be known in the art.Suitable constructions of such a battery are known in the art.Suitable materials for the negative electrode are a lithium host material capable of incorporating andsubsequently releasing the lithium ion such as graphite, a lithium metal oxide (e.g. lithium titanium oxide), carbon, a tin / cobalt alloy, a Li / In alloy or silicon / carbon composite material. The negativeelectrode material can be a mixture of any number of these negative electrode materials.Alternatively, pure Li metal may provide the negative electrode. Preferably, the negative electrodeis formed of a lithium metal negative electrode which may advantageously allow the battery to providerelatively high energy densities. Suitable materials for the electrolyte are media that allow charge carrier transport inside a battery. The electrolyte material can comprise an ionic polymer. The electrolyte material can be aqueous electrolytes, or the electrolyte material can be a molten salt or a solid electrolyte. Suitably the electrolyte material can be a lithium salt dissolved in a solution. The electrolyte material can be a lithium salt dissolved in organic solution or aqueous solution. The electrolyte can be a lithium salt dissolved in a mixture of solvents. The electrolyte material can be LiPF6 in a carbonate solvent. The carbonate solvent can be, for example, ethyl carbonate, dimethyl carbonate, vinylene carbonate, ethyl methyl carbonate.In some embodiments, the battery of this third aspect is a solid-state battery, suitably a solid-statelithium metal battery.According to a fourth aspect of the invention, there is provided a method of preparing a solid materialaccording to the first aspect, the method comprising the steps of: (a) admixing a source of A, a source of Ni, a source of M and a source of Y; (b) heating the mixture obtained in step (a). The method may be a solid-state (or dry) procedure i.e., a method that does not require solvent.The solid material prepared by this fourth aspect may be used in the positive electrode of the secondaspect and / or the battery of the third aspect.Suitably the steps of the method of this third aspect are carried out in the order of step (a) followed by step (b). Step (a) involves admixing a source of A, a source of Ni, a source of M and a source of Y. Suitably the molar ratios of the sources of A, Ni, M and / or Y are selected to provide the desired stoichiometry of the formulas of solid material described in relation to the first aspect. The source of A may be any suitable material comprising one or more alkali metals selected from Li,Na, or K. In some embodiments, Li, Na or K, or a mixture thereof, may be present in the source of Mand / or Y.Suitably, the source of A is an alkali metal hydroxide. For example, LiOH, NaOH, and / or KOH.Suitably the source of A is an alkali metal hydroxide hydrate. For example, LiOH·H2O and / orKOH·xH2O. Suitably the source of A is LiOH.The source of Ni may be any suitable material comprising Ni. For example, NiO, NiCO3, NiCl2 andNi(OH)2. Suitably the source of Ni is Ni(OH)2.The source of M may be any suitable material comprising Ti, Zr, Hf, Nb, Ta, Mo, W, V, or Re or amixture thereof. Suitably the source of M may be a material comprising an oxide of Ti, Zr, Hf, Nb, Ta,Mo, W, V, or Re or a mixture thereof. Suitably the source of M may be a material comprising TiO2,ZrO2, HfO2, VO2, V2O5, Nb2O5, Ta2O5, MoO3(H2O)n, MoO3, WO3·H2O, WO3·2H2O, WO3, Re2O7, or amixture thereof.The source of Y may be any suitable material comprising O and / or S. Suitably the source of Y maybe elemental sulfur. Suitably the source of O may be O2 gas. Suitably, the source of O may be amaterial comprising a metal oxide. Preferably, the source of O may be a material comprising TiO2,ZrO2, HfO2, VO2, V2O5, Nb2O5, Ta2O5, MoO3(H2O)n, MoO3, WO3·H2O, WO3·2H2O, WO3, Re2O7 or amixture thereof. In embodiments wherein the solid material is of formula (V): LiNi1-xMbO2, step (a) of the methodsuitably involves admixing LiOH, Ni(OH)2 and one or more of TiO2, ZrO2, HfO2, VO2, V2O5, Nb2O5,Ta2O5, MoO3(H2O)n, MoO3, WO3·H2O, WO3·2H2O, WO3, Re2O7, according to the M species required in the material.In embodiments wherein the solid material is of formula(VII): LiaNi1-xTibO2, step (a) of the methodsuitably involves admixing LiOH, Ni(OH)2 and TiO2. In embodiments wherein the solid material is of formula (VIII): LiaNi1-xTi3x / 4O2, step (a) suitablyinvolves admixing 1 eq LiOH, 1-x eq Ni(OH)2 and 3x / 4 eq TiO2., wherein x is as defined in relation tothe first aspect, suitably wherein a = 1. Suitably the sources of A, Ni, M and / or Y are provided as powders. The method may be a solution-based synthetic procedure or a chemical vapour transport procedure. However, in preferredembodiments the method of the third aspect is a solid-based synthetic procedure. Once the solidmaterial is made using a suitable method, a physical vapour deposition procedure can be applied forthe deposition of a thin film. The resulting material can then find applications in thin film batteries.Suitably in step (a) the sources of A, Ni, M and / or Y are ground together and then heated in step (b).The sources of A, Ni, M and / or Y may be ball-milled together. Suitably the sources of A, Ni, M and / orY may be ball-milled together at 250 rpm to 450 rpm for 1.5 hours to 3 hours, suitably 350 rpm for 2.5 hours, suitably under argon atmosphere. Suitably the method of the third aspect involves a step (a1) of drying the sources of A, Ni, M and / or Y, suitably under vacuum, suitably with heating to 150°C or above. Step (a1) suitably occurs before step (a).Step (b) involves heating the mixture obtained in step (a). Suitably step (b) involves heating themixture obtained in step (a) and step (b) to a temperature from 600 ºC to 800 ºC, suitably to atemperature from 650 ºC to 750 ºC. Step (b) may involve heating the mixture to a set temperature in the ranges noted above at a rate offrom 1 to 30°C / min, suitably from 1 to 10°C / min or from 5 to 10°C / min. In some embodiments, theheating rate may be from 15 to 30°C / min, for example approximately 20°C / min. Following step (b), the mixture is suitably cooled at a rate of from 1 to 10°C / min, suitably from 2 to 8°C / min or approximately 5°C / min, suitably to room temperature. Suitably step (b) involves heating, suitably at said temperatures, the sources of A, Ni, M and / or Y for at least 20 hours. Suitably step (b) involves heating for at least 1 day, at least 2 days or at least 3days. Suitably step (b) involves heating for up to 6 days, up to 5 days or up to 4 days. For example,the source of the sources of A, Ni, M and / or Y may be heated to a temperature of from 600 to 800°Cfor from 20 hours to 6 days. Suitably the heating of step (b) is carried out under a flow of dry O2,suitably with a flow rate between 50 mL / min to 150 mL / min, suitably with a flow rate between 75mL / min to 125 mL / min, for example with a flow rate of 100 mL / min.Prior to the heating of step (b), the mixture obtained in step (a) may be placed in a crucible, which isin turn placed inside a quartz tube with both ends sealed with ball valves, suitably with both ends sealed with Swagelok ball valves. Suitably the heating of step (b) and the reaction to form the solid material provides the product as a powder. The product may then be manipulated into the desired format for the uses described herein, suitably using techniques known in the art.Using this method of the fourth aspect, the materials of the first aspect may be prepared withoutinvolving complicated nanoengineering steps require for the synthesis of known materials used aspositive electrodes in batteries. The synthesis of the materials according to this fourth aspect maybe simpler than the synthesis of such known materials. The synthesis of the materials according tothis fourth aspect may involve lower costs than the synthesis of such known materials.Moreover, the method of this fourth aspect may advantageously utilise only elements with a high earth-abundance and / or which are easily obtainable in large quantities, suitably from extraction and production processes which are relatively low in energy consumption and which do not produce problematic waste materials. Examples Synthesis of LiNi1-xTi3x / 4O2 PrecursorsTo synthesize the LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.3), the starting materials TiO2 (99.7%, Sigma Aldrich),LiOH•H2O (99.995%, Thermoscientific) and Ni(OH)2 (Sigma Aldrich) were used. LiOH•H2O was driedovernight under dynamic vacuum (<10–4 mbar) at 170 °C to yield anhydrous LiOH. TiO2 was driedovernight at 200 °C in air. Following these drying procedures, all starting materials were stored insidean argon-filled glovebox (O2 < 0.1 ppm, H2O < 0.1 ppm). Ball-milling and post-sinteringLiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.3) were synthesized by ball-milling stoichiometric amounts of the startingmaterials (no Li excess used) to get a total mass of 3 g of the target compositions (Table 1). Theprecursors were transferred into 45 mL sealed zirconia jars with 7 zirconia balls (10 mm in diameter). Ball milling was performed at 350 rpm for 2.5 h under Ar atmosphere. The powders were placed in a crucible, which was then placed inside a quartz tube with both ends sealed with Swagelok ballvalves. Final powders were obtained by annealing the ball-milled mixtures at 700 °C for 20 h underflowing dry O2 gas with a flow rate of 100 mL / min.Table 1 Amounts of the starting materials used for the synthesis of LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.3)anhydrous LiOH Ni(OH)2 TiO2Starting material weight molar amount weight molar amount weight molar amount / g / mol / g / mol / g / mol LiNiO2 (x = 0) 0.735892 0.030726 2.848659 0.030726 - -LiNi0.975Ti0.01875O2 0.74021 0.030906 2.793739 0.030134 0.046293 0.00058(x = 0.025)LiNi0.95Ti0.0375O2 (x 0.744579 0.031089 2.738171 0.029535 0.093132 0.001166= 0.05) LiNi0.925Ti0.05625O2 0.749 0.03127 2.681943 0.028928 0.140527 0.001759(x = 0.075)LiNi0.9Ti0.075O2 (x =0.753473 0.03146 2.625044 0.028315 0.188489 0.002360.1) LiNi0.8Ti0.15O2 (x =0.771915 0.03223 2.390482 0.025785 0.386204 0.0048350.2)LiNi0.7Ti0.225O2 (x =0.791281 0.03304 2.14415 0.023127 0.59384 0.0074350.3) Characterization of LiNi1-xTi3x / 4O2The as-prepared LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.3) were analyzed by powder X-ray diffraction (XRD),neutron powder diffraction (ND), and atomic resolution scanning transmission microscopy (STEM). The morphology and size of the primary particles were observed using scanning electron microscopy (SEM). Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine cation contents and Transmission Electron Microscopy Energy-dispersive X-ray spectroscopy (TEM-EDX) was used to confirm the Ti:Ni ratio. Samples were prepared by dissolving 10 mg of each powder into 2 mL concentrated HCl and diluted to 50 mL with ultra-pure water. Samples were dispersed on carbon coated copper TEM grids. Small, isolated particles were picked for EDX analysis to avoid collecting data from agglomerates and 10 particles were measured to determine homogeneity. Correction factors were determined measuring proper standards for each chemical element.Electrochemistry of LiNi1-xTi3x / 4O2Preparation of Electrodes LiNi1-xTi3x / 4O2 (x = 0, 0.025, 0.5, 0.075, 0.1) were mixed with carbon (Super C, Timcal) and polyvinylidene fluoride (PVDF, Kynar) with a ratio of 90:5:5 (wt%) in N-methyl-2-pyrrolidone (NMP, anhydrous, Sigma Aldrich) solvent. Carbon was dried at 250oC under vacuum for one day. Thepositive electrode slurry was cast onto Al current collector inside the Ar-filed glovebox, and dried at80 oC under vacuum. The average loading amount of LiNi1-xTi3x / 4O2 are 2 - 5 mg / cm2. For the higherloading tests, the positive electrode loading amount is 13.6 mg / cm2.Electrolytes 1 M LiPF6 dissolved in carbonate solvent of ethyl carbonate and dimethyl carbonate (EC / DMC, 1: 1 in v / v) was used as a baseline electrolyte.A second electrolyte, 1 M LiPF6 with 2 wt% vinylene carbonate as additive was dissolved in carbonatesolvent of ethyl carbonate and ethyl methyl carbonate (EC / EMC, 3: 7 in v / v) were used. Charge / discharge cycling tests All the tests are conducted as coin cells with Li metal foils (thickness: 256 µm) as counter and reference electrode. All electrochemical tests were done at 30oC. The charge / discharge was performed between 3 and 4.3 V vs. Li+ / Li. Pre-cycling were done for 1 cycle with 0.1C (1C = 200 mA / g) for low-rate (0.5C) and low-loading (2- 5 mg / cm2) cell tests, 3 cycles with 0.1C for high-rate cycle tests (8C), 3 cycles with 0.075C for high-loading (13.6 mg / cm2) cell tests.Rate capability (0.1 C to 8 C, 1C = 200 mA g-1) tests for LiNiO2 and LiNi0.925Ti0.05625O2 (x = 0.075)were performed under two different protocols: (1) constant current (CC), and (2) constant current-constant voltage (CCCV). In the CCCV protocol, the constant voltage processes were added after4.3V charging (by constant current), until the current reach 0.025 C. Ex situ analysisEx situ samples after the electrochemical cycling tests for SXRD studies were collected at the I11beamline at the Diamond Light Source (Oxfordshire, U.K.), on transmission mode using a position sensitive detector (PSD, λ = 0.823899 Å).STEM images were collected for cycled LiNiO2 and LiNi0.925Ti0.05625O2 (x = 0.075), which weredischarged to 3 V after 50 cycles. For both samples, lamella has been prepared on a Thermo-Fisher Helios G5 Cx Ga-FIB-SEM system operating at 30 keV. A layer of ~200 nm of carbon was deposited with the electron beam (E-beam) to protect the surface from the Ion beam (I-beam). Then, layers of carbon and Pt (20×4×1 ^m) were deposited on the selected area I beam (30 keV, 230 pA). A primary thinning was done (30 keV, 0.23 nA) to obtain a ~ 2 ^m thickness lamella. The lamella was then connected to the TEM grid and thinned at lower voltages and currents (down to 2 keV) to obtain a thin lamella. A Thermo-Fisher vacuum transfer holder was used to transfer the samples between the glovebox and the Ga-FIB to avoid air exposure. ResultsFigure 1 shows the synchrotron powder X-ray diffraction patterns XRD data (Beamline I11, λ =0.825005(1) Å) for LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.3). For LiNi0.7Ti0.225O2 (x=0.3) the enlarged (104) peakbetween 22oand 24o(at about 23°) starts splitting indicating the limit of the formation of solid solution,with peak splitting observed for x = 0.3.Table 2 shows the target compositions of LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.1) and the compositions analysedby means of ICP-MS (analysis of Li, Ni and Ti) and TEM-EDX (Ti / Ni ratio). The results are close to the target compositions.Table 2. Composition analysis of LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.1) obtained by ICP-MS and TEM-EDX.ICP-MS TEM-EDX (Ti: Ni)Target composition measured expectedLiNiO2 (x = 0) Li1.026Ni1.000O2 - -LiNi0.975Ti0.01875O2 (x = 0.025) Li1.023Ni0.975Ti0.017O2 0.022 (5) 0.019LiNi0.95Ti0.0375O2 (x = 0.05) Li1.043Ni0.950Ti0.0381O2 0.043 (9) 0.039LiNi0.925Ti0.05625O2 (x = 0.075) Li1.040Ni0.925Ti0.0554O2 0.049 (7) 0.060LiNi0.9Ti0.075O2 (x = 0.1) Li1.040Ni0.900Ti0.0745O2 0.092 (9) 0.083Synchrotron XRD data were used for Rietveld refinements. Structures have been solved with a mixture of two related structures. The first is a layered structure (R3^m) and the second is a cubic disordered rocksalt (Fm3^m). Atomic positions were fixed to the desired compositions with the cations and vacancy occupying the 4a site for the disordered structure and Li fully occupying the 3a site and Ni and Ti occupying the site 3b with a vacancy for the layered structure. The phases percentages between the layered and the disordered rocksalt structures are presented in Figure 2. Figure 2 shows Phase fraction (layered and disordered rocksalt phases) based on the Rietveldrefinement of the synchrotron powder X-ray diffraction patterns of LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.2) inFigure 1.For both LiNi0.95Ti0.0375O2 (x = 0.05, Figure 3) and LiNi0.9Ti0.075O2 (x = 0.1, Figure 4), a combinedRietveld refinement of against synchrotron XRD and ND data was performed. A composition restraintwas introduced to allow the compositions to refine freely between the two phases but kept the global compositions to be the same as expected. Using this restraint, weighted residue values (Rwp) of2.69% and 2.75% were obtained for LiNi0.95Ti0.0375O2 (x = 0.05) and LiNi0.9Ti0.075O2 (x = 0.1),respectively (Figure 3, Figure 4).Figure 3 shows the combined Rietveld refinement of LiNi0.95Ti0.0375O2 (x=0.05) against powder X-raydiffraction (figure 3a), and the time-of-flight neutron powder diffraction data collected from Bank 5 (2θ= 31°) of NOMAD at room temperature (figure 3b). The combined refinement including X-ray andneutron data across four detector banks has a Rwp of 2.69%, Rexp of 0.39%, and GoF of 6.89%.Figure 4 shows the combined Rietveld refinement of LiNi0.9Ti0.075O2 (x=0.1) against powder X-raydiffraction (figure 4a), and time-of-flight neutron powder diffraction data collected from Bank 5 (2θ =31°) of NOMAD at room temperature (figure 4b). The combined refinement including X-ray andneutron data across four detector banks has a Rwp of 2.75%, Rexp of 0.4%, and GoF of 6.77%. Thephase fraction for is summarized in Figure 2.The crystal structure can also be described by a multidomain single-phase hexagonal (R3^m)structural model. This model employs a single set of unit cell parameters alongside distinct peakprofiles to simultaneously characterize reflections from two related structures. The first is a layeredstructure (R3^m) and the second is a cubic disordered rocksalt (Fm3^m). Using this model, thecombined refinement including X-ray and neutron data across four detector banks has a Rwp of 3.23and 3.3%, Rexp of 0.4 % and GoF of 8.44 and 7.94 for LiNi0.95Ti0.0375O2 (x = 0.05, Fig.3c and 3d) andLiNi0.9Ti0.075O2 (x = 0.1, Fig.4c and 4d) respectively.Figure 5 shows SEM images comparing the primary particles sizes of pristine materials LiNi1-xTi3x / 4O2(0 ≤ x ≤ 0.1). The SEM images demonstrate a decrease in the crystallite sizes with increasing x valuein LiNi1-xTi3x / 4O2 (0 ≤ x ≤ 0.1). The SEM image shows that the primary particle size is ~50-200 nm forthe Ti-substituted samples (0 < x ≤ 0.1) which is significantly lower than the ~200-600 nm for LiNiO2(x = 0).Figure 6 shows STEM analysis on LiNi0.925Ti0.05625O2 (x = 0.075). Specifically, figure 6 shows a STEM-HAADF image and selected areas FFT for LiNi0.925Ti0.05625O2 (x = 0.075) showing co-existence of thelayered (indicated with “L”, dashed rectangle) and disordered rocksalt (indicated with “D.R.”, solidrectangle) domains. The STEM analysis shown in figure 6 reveals the intergrowth of a layered anddisordered rock salt domains. Figure 7 shows (a) Electrochemical performances of LiNi1-xTi3 / 4xO2 (x = 0, 0.025, 0.05, 0.075 and 0.1)with a 0.5C current (i.e., 100 mA g-1), Voltage profile of (b) LiNiO2, (c) LiNi0.925Ti0.05625O2 (x = 0.075).Electrolyte: 1 M LiPF6 in EC / DMC. Galvanostatic charge / discharge cycling between 3 and 4.3 V vs. Li+ / Li at a 0.5 C (i.e., 100 mA g-1)rate and at 30 oC of LiNiO2 and LiNi1-xTi3x / 4O2 (x = 0.025, 0.05, 0.075 and 0.1) reveals that all Tisubstituted materials show similar initial capacities (230-240 mAh / g) but significantly enhancedcapacity retention, as shown in Figure 7a. Among these materials, LiNi0.925Ti0.05625O2 (x = 0.075)shows the best cycle capacity retention over 200 cycles of 77% with 168.2 mAh / g after 200 cycles, in contrast to 97.3 mAh / g (45% retention) for LiNiO2 under the same conditions.While the successive plateau in the load curve of LiNiO2 are clearly visible, as shown in Figure 7b,they are fully suppressed in LiNi0.925Ti0.05625O2 (x = 0.075), as shown in Figure 7c. In addition, a clearcapacity loss and discharge voltage fade over 200 cycles can be seen in LiNiO2, as shown in Figure7b. In contrast, LiNi0.925Ti0.05625O2 (x = 0.075), as shown in Figure 7c, shows good retention in bothdischarge capacity and the discharge voltage.In Figure 8, rate capability (0.1 to 8 C, 1C = 200 mA g-1) tests for LiNiO2 and LiNi0.925Ti0.05625O2 (x =0.075) were performed under two different protocols for charging: constant current (CC), andconstant current, and constant voltage (CCCV). The constant voltage step upon charge enablesdeep delithiation. Electrolyte: 1 M LiPF6 in EC / DMC.LiNiO2 shows poor capacity retention at high current rates at both test protocols. The deep delithiation causes structural degradation and accordingly capacity loss. In contrast, LiNi0.925Ti0.05625O2 shows high specific capacity at all current rates. The deep delithiation (CCCV protocol) can further improve the reversible discharge capacity, which is directly related to structural stability over cycling. For LiNi0.925Ti0.05625O2 (x = 0.075), a high reversible capacity of 181mAh / g at 8C (or at 1600 mA g-1) has been achieved with the CCCV protocol. In Figure 8, the goodcapacity retention of the LiNi0.925Ti0.05625O2 (x = 0.075) under CCCV tests at each C-rate (0.1 C to 8C) suggests that the LiNi0.925Ti0.05625O2 (x = 0.075) with a high Ni content can maintain well thestructure upon deep-delithiation and upon holding at a high charge potential of 4.3 V vs. Li+ / Li.In addition, compared to the use of 1 M LiPF6 in EC / DMC electrolyte, the use of a different electrolyte,1 M LiPF6 in EC / EMC with 2wt% vinylene carbonate (VC) as additive (labelled as,“LiNi0.925Ti0.05625O2+ VC”), can help to improve the cycling stability of LiNi0.925Ti0.05625O2 (x = 0.075) over long cycling (Figure 9a). Figure 9 shows (a) the influence of the use of two different electrolytes (1 M LiPF6 in EC / DMC, and1 M LiPF6 in EC / EMC with 2wt% vinylene carbonate (VC) additive) on the cycling performance(tested at 0.1C, 1C, 4C and 8C) of LiNiO2 and LiNi0.925Ti0.05625O2 (x = 0.075) with an active material loading of 3.76 mg / cm2. (b) Cycling performance (tested at 0.075C, 0.5C, and 2C) ofLiNi0.925Ti0.05625O2 (x = 0.075) with an active material loading of 13.6 mg / cm2, using 1 M LiPF6 inEC / EMC with 2wt% VC additive as electrolyte.1C = 200 mA g-1.For LiNi0.925Ti0.05625O2 (x = 0.075), the cycling performance of the positive electrode active materialwith a high loading of 13.6 mg / cm2was performed, using 1 M LiPF6 in EC / EMC with 2wt% VC additiveas electrolyte (Figure 9b). At 0.5 and 2C (1C = 200 mA g-1), the LiNi0.925Ti0.05625O2 (x = 0.075) deliversa high initial capacity of about 205 and 140 mAh g-1, respectively. Figure 10 shows selected Q (Q = 4πsin(θ) / λ) ranges of synchrotron XRD patterns for pristine and cycled (discharged to 3V after 50 cycles) samples of (a) LiNiO2 and (b) LiNi0.925Ti0.05625O2 (x=0.075).Ex situ SXRD collected for both LiNiO2 (x = 0) and LiNi0.925Ti0.05625O2 (x = 0.075) discharged to 3Vafter 50 cycles shows that the LiNiO2 transforms to a mixture of the monoclinic phase (M, diffraction peak position indicated with a star in Figure 10a) and the hexagonal (H1) phase, while theLiNi0.925Ti0.05625O2 (x = 0.075) maintains its initial crystal structure (Figure 10b).Figure 11 shows STEM-HAADF images for ex situ samples discharged to 3V after 50 cycles withinserts showing corresponding FFT emphasizing. (a) the structure change of the LiNiO2 to themonoclinic (indicated with “M”) phase and (b) the structure stability of LiNi0.925Ti0.05625O2 (x = 0.075)(the hexagonal phase indicated with “H1”).STEM images collected after 50 cycles demonstrate the structural change to the monoclinic phasefor LiNiO2 (Figure 11a). STEM images for LiNi0.925Ti0.05625O2 (x = 0.075) demonstrates the persistenceof stable layered structure (Figure 11b). These results demonstrate that the present invention provides solid materials with advantageous high initial capacity, good capacity retention, structural stability and fast charging performance, whichmay be suitable for use as positive electrode active materials in batteries. Furthermore, theachievement of these advantageous properties has proved to be possible using materialssubstantially free of cobalt and a simple, low cost synthesis procedure. Therefore, the solid materialsof the present invention may provide positive electrodes with improved performance in batterieswhilst providing cost benefits. Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. Throughout this specification, the term “comprising” or “comprises” means including thecomponent(s) specified but not to the exclusion of the presence of other components. The term“consisting essentially of” or “consists essentially of” means including the components specified but excluding other components except for materials present as impurities, unavoidable materialspresent as a result of processes used to provide the components, and components added for apurpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1% by weight of non-specified components. The term “consisting of” or “consists of” means including the components specified but excluding addition of other components. Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising”may also be taken to encompass or include the meaning “consists essentially of” or “consistingessentially of”, and may also be taken to include the meaning “consists of” or “consisting of”.For the avoidance of doubt, wherein amounts of components in a composition are described in wt%, this means the weight percentage of the specified component in relation to the whole composition referred to. The optional features set out herein may be used either individually or in combination with each otherwhere appropriate and particularly in the combinations as set out in the accompanying claims. Theoptional features for each aspect or exemplary embodiment of the invention as set out herein arealso to be read as applicable to any other aspect or exemplary embodiments of the invention, whereappropriate. In other words, the skilled person reading this specification should consider the optionalfeatures for each exemplary embodiment of the invention as interchangeable and combinablebetween different exemplary embodiments. Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims1. A solid material of formula (I):AaNi1-xMbYz wherein: Ais selected from Li, Na or K or a mixture thereof;a is from 0.75 to 1.25; xis 0.005 < x ≤ 0.500;M is selected from Ti, Zr, Hf, Nb, Ta, Mo, W, V, or Re or a mixture thereof;b is 0.005 ≤ b < 0.500wherein x > b; Y is selected from O or S or a mixture thereof; and z is from 1.5 to 2.5.

2. The solid material according to any preceding claim, wherein A is Li.

3. The solid material according to any preceding claim, wherein Y is O.

4. The solid material according to any preceding claim, wherein the solid material is substantiallyfree of Co.

5. The solid material according to any preceding claim, wherein M is selected from Ti, Zr, V orHf, or a mixture thereof and x / 4 ≤ b ≤ 3x / 4.

6. The solid material according to any preceding claim, wherein M is selected from V, Nb or Taor a mixture thereof and 2x / 5 ≤ b ≤ 3x / 5.

7. The solid material according to any preceding claim, wherein M is selected from Mo or W or amixture thereof and x / 3 ≤ b ≤ x / 2.

8. The solid material according to any preceding claim, wherein M is Re and 2x / 7 ≤ b ≤ 3x / 7.

9. The solid material according to any one of claim 1 to 5 having formula (VII): LiaNi1-xTibO2.

10. The solid material according to claim 1 having formula (VIII): LiaNi1-xTi3x / 4O2.

11. The solid material according to any preceding claim, wherein 0.015 < x ≤ 0.400.

12. The solid material according to any preceding claim, comprising a crystal structure comprisingcation vacancies having stoichiometry x−b in formula (I).

13. The solid material according to any preceding claim, having a crystal structure comprising alayered rhombohedral structure and a cubic structure or a mixture thereof.

14. The solid material according to any preceding claim, having a crystal structure comprising alayered structure (R3^m) and a disordered rocksalt structure (Fm3^m) or a mixture thereof.

15. A positive electrode comprising a solid material according to any one of claims 1 to 14.

16. The positive electrode in a battery according to claim 15, comprising a positive electrode activematerial and a conducting material, wherein the positive electrode active material comprisesfrom 60 to 100 wt% of the solid material according to any one of claims 1 to 14.

17. A battery comprising:a negative electrode; apositive electrode comprising a solid material according to any of claims 1 to 14; andan electrolyte.

18. A method of preparing a solid material according to any of claims 1 to 14, the methodcomprising the steps of: (a) admixing a source of A, a source of Ni, a source of M and a source of Y;(b) heating the mixture obtained in step (a).

19. The method according to claim 18, wherein step (b) involves heating the mixture obtained instep (b) to a temperature of 600 ºC to 800 ºC under a flow of O2.

20. The method according to claim 18 or claim 19, wherein step (a) involves admixing LiOH·H2O,Ni(OH)2, TiO2; wherein the solid material has formula (VII): LiaNi1-xTibO2.

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