Cathode materials [0001] The present application claims priority to Australian Provisional Patent Application No. 2022903301, filed 4 November 2022, the entire disclosure of which is incorporated herein by cross-reference. Field of the disclosure [0002] The present disclosure broadly relates to high-entropy metal oxides useful as cathode materials. Background of the disclosure [0003] Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field. [0004] Green and renewable energy sources (such as hydro, wind, and solar) have been rapidly adopted around the world, aiming to replace conventional fossil fuel (such as coal, oil and natural gas). However, these renewable energy sources are season/weather dependent and intermittent. For ensuring affordable and continuous energy supply to the customers, the development of large-scale energy storage systems is crucial and necessary. [0005] Lithium-ion batteries are the current front-runner in the energy storage market. However, the uneven worldwide distribution and surging cost of lithium limits the further application of Li-ion batteries in grid-scale energy storage. By contrast, sodium-ion batteries have the potential to become the promising alternatives to replace Li-ion batteries in the future electrical energy storage system due to their abundant and much lower cost electrode element: sodium. [0006] Sodium layered oxide materials are some of the most attractive cathode materials for sodium-ion batteries. Sodium layered oxide materials can be identified as two main types: P2- type and O3-type sodium layered oxides. The P2-type sodium layered oxides can offer a higher rate performance and capacity retention than the O3-type, but their capacities are limited due to their low sodium content (Na ≤ 0.67). The O3-type sodium layered oxides can provide a higher capacity due to their higher sodium content (0.67 ≤ Na ≤ 1), but undergo significant volume changes (≈23%), especially in Mn-based sodium layered oxides, due to O3-P3 phase transitions during charge/discharge. This leads to an unstable structure, and results in poor capacity retention and cycling performance for Na-ion batteries. The current ME_215242160_2 method to stabilise the structure of the O3-type Mn-based sodium layered oxides is to dope with elements such as Ni or Co to provide improved performance in capacity retention. However, these elements are toxic and expensive. Summary of the disclosure [0007] In a first aspect of the disclosure, there is provided a compound of formula (I): Na _σ TMʹ _w TMʹʹ _x TMʹʹʹ _y TMʹʹʹʹ _z Mʹ _t Mʹʹ _u Mʹʹʹ _v O ₂ , (I) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, z is 0 or between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, t and v are 0 or between about 0.01 and about 0.3, w + x + y + z + t+ u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element. [0008] The following options may be used in conjunction with the first aspect of the disclosure, either individually or in any combination. [0009] TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ may be independently selected from the group consisting of Ti, Mn, Zn, Fe, Cu, and Sc. TMʹ may be Cu, TMʹʹ may be Fe, TMʹʹʹ may be Mn, and TMʹʹʹʹ may be Ti. [0010] w may be between about 0.05 and about 0.2. w may be between about 0.1 and about 0.15. x may be between about 0.1 and about 0.4. x may be between about 0.2 and about 0.3. y may be between about 0.1 and about 0.4. y may be between about 0.2 and about 0.3. z may be between about 0.01 and about 0.5. z may be between about 0.05 and about 0.3. z may be 0. [0011] Mʹ, Mʹʹ and Mʹʹʹ may be independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Ti, Sn, Mo, Si, and Sb. Mʹ may be Li, Mʹʹ may be Mg and Mʹʹʹ may be Al. ME_215242160_2 [0012] t may be between about 0.01 and about 0.1. t may be 0.05. t may be 0. u may be between about 0.01 and about 0.1.u may be 0.05. v may be between about 0.01 and about 0.3. v may be between about 0.05 and about 0.2. v may be between about 0.05 and about 0.1. v may be 0.1. v may 0. [0013] σ may be between about 0.75 and about 1. σ may be between about 0.8 and about 1. σ may be between about 0.8 and about 0.9. σ may be 0.85. [0014] In some embodiments, the compound of formula (I) may be selected from the group consisting of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.2 Mn _0.2 Ti _0.3 O ₂ , Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1O2, Na0.85Li0.05Mg0.05Al0.1Cu0.15Fe0.3Mn0.3Ti0.05O2, Na0.85Li0.05Mg0.05Al0.05Cu0.2Fe0.3Mn0.3Ti0.05O2, NaLi0.05Mg0.05Al0.05Cu0.2Fe0.3Mn0.3Ti0.05O2, Na0.85Li0.05Mg0.05Cu0.2Fe0.3Mn0.3Ti0.1O2, NaLi0.05Mg0.05Cu0.2Fe0.3Mn0.3Ti0.1O2, Na0.85Mg0.05Cu0.2Fe0.3Mn0.3Ti0.15O2, and NaMg0.05Cu0.2Fe0.3Mn0.3Ti0.15O2. In some embodiments, the compound of formula (I) may be selected from the group consisting of Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.2Mn0.2Ti0.3O2, Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1O2, and Na0.85Li0.05Mg0.05Al0.1Cu0.15Fe0.3Mn0.3Ti0.05O2. [0015] The compound of formula (I) may be in the O3 structural form. [0016] The compound of formula (I) may be in the form of crystals having a particle size of between about 0.5 and about 20 µm. The compound of formula (I) may be in the form of crystals having a particle size of between about 1 and about 10 µm. [0017] In a second aspect of the disclosure, there is provided a method of preparing a compound of formula (I) as defined in relation to the first aspect of the disclosure, the method comprising: (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ: TMʹʹʹʹ:Mʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:z:t:u:v, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, z is 0 or between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, t and v are 0 or between about 0.01 and about 0.3, ME_215242160_2 w + x + y + z + t+ u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (I). [0018] The following options may be used in conjunction with the second aspect of the disclosure, either individually or in any combination. [0019] In step (ii) of the second aspect of the disclosure the mixture may be sintered at a temperature of about 500 °C to about 1500 °C. In step (ii) the mixture may be sintered for a time of between about 0.5 hours to 60 hours. In step (ii) the mixture may be sintered at a temperature of between about 800 °C and about 1000 °C. In step (ii) the mixture may be sintered for between about 2 hours and about 30 hours. In step (ii) the mixture may be sintered at about 850 °C for about 12 hours. In step (ii) the mixture may be sintered at about 950 °C for about 12 hours. In step (ii) the mixture may be sintered at about 900 °C for about 15 hours. [0020] In step (i) the mixture may be prepared by ball milling. [0021] Step (i) may further comprise adding a further amount of the sodium salt such that it is present in the mixture in a molar ratio of at least about 1.01 × σ. [0022] TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ may be independently selected from the group consisting of Ti, Mn, Zn, Fe, Cu, and Sc. TMʹ may be Cu, TMʹʹ may be Fe, TMʹʹʹ may be Mn, and TMʹʹʹʹ may be Ti. In this case, the oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ may be CuO, Fe2O3, MnO2 and TiO2. [0023] w may be between about 0.05 and about 0.2. w may be between about 0.1 and about 0.15. x may be between about 0.1 and about 0.4. x may be between about 0.2 and about 0.3. y may be between about 0.1 and about 0.4. y may be between about 0.2 and about 0.3. z may be between about 0.01 and about 0.5. z may be between about 0.05 and about 0.3. z may be 0. [0024] Mʹ, Mʹʹ and Mʹʹʹ may be independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Ti, Sn, Mo, Si, and Sb. Mʹ may be Li, Mʹʹ may be Mg and Mʹʹʹ may be Al. In this case, the oxides or carbonates of Mʹ, Mʹʹ, and Mʹʹʹ may be Li2CO3, MgO, and Al2O3. [0025] t may be between about 0.01 and about 0.1. t may be 0.05. t may be 0. u may be between about 0.01 and about 0.1. u may be 0.05. v may be between about 0.01 and about ME_215242160_2 0.3. v may be between about 0.05 and about 0.2. v may be between about 0.05 and about 0.1. v may be 0.1. v may be 0. [0026] The sodium salt may be Na ₂ CO ₃ . [0027] σ may be between about 0.75 and about 0.1. σ may be between about 0.8 and about 1. σ may be between about 0.8 and about 0.9. σ may be 0.85. [0028] In a third aspect of the disclosure there is provided a sodium-ion battery comprising: (i) a cathode comprising a compound of formula (I) as defined in the first aspect of the disclosure, (ii) an anode comprising a conductive material, (iii) a separator; and (iv) a non-aqueous electrolyte comprising sodium ions. [0029] In a fourth aspect of the disclosure there is provided use of the compound of formula (I) as defined in the first aspect of the disclosure as a cathode material in a sodium-ion battery. [0030] In a fifth aspect of the disclosure there is provided a method of fabricating a cathode for a sodium-ion battery, the method comprising: combining a compound of formula (I) as defined in the first aspect of disclosure, a conductive material, and a binder to form a mixture. [0031] In a sixth aspect of the disclosure there is provided cathode material for a sodium-ion battery, comprising: (i) a compound of formula (I) as defined in the first aspect of the disclosure, (ii) a conductive material, and (iii) a binder. [0032] The following options may be used in conjunction with the third, fourth, fifth, and/or sixth aspects of the disclosure, either individually or in any combination. [0033] The conductive material may be a carbon material. The carbon material may be selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, expanded graphite, carbon fibre, glassy carbon, shredded carbon felt, and carbon foam. [0034] The binder may be polyvinylidene fluoride. ME_215242160_2 Definitions [0035] The following are some definitions that may be helpful in understanding the description of the present disclosure. These are intended as general definitions and should in no way limit the scope of the present disclosure to those terms alone, but are put forth for a better understanding of the following description. [0036] Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. [0037] The terms "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element. [0038] In the context of this specification the term "about" is understood to refer to ± 10% of the recited value. [0039] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of 1.0 to 5.0 is intended to include all sub-ranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 5.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 5.0, such as 2.1 to 4.5. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein. [0040] Any description of prior art documents herein, or statements herein derived from or based on those documents, is not an admission that the documents or derived statements are part of the common general knowledge of the relevant art. [0041] For the purposes of description, all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise stated Brief Description of the Drawings [0042] Figure 1. XRD spectra of Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.2Mn0.2Ti0.3O2, Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1O2, and Na0.85Li0.05Mg0.05Al0.1Cu0.15Fe0.3Mn0.3Ti0.05O2. ME_215242160_2 [0043] Figure 2. XRD spectrum of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.3 Mn _0.3 Ti _0.1 (F2). [0044] Figure 3. Scanning electron micrograph of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.3 Mn _0.3 Ti _0.1 (F2). [0045] Figure 4. Scanning electron micrograph of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.3 Mn _0.3 Ti _0.1 (F2). [0046] Figure 5. Energy Dispersive X-ray Spectroscopy (EDS) mapping of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.3 Mn _0.3 Ti _0.1 (F2) [0047] Figure 6. Scanning electron micrograph of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.2 Mn _0.2 Ti _0.3 (F1) with calcination temperature of 850 °C and calcination time of 12 hours. [0048] Figure 7. Scanning electron micrograph of Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.2Mn0.2Ti0.3 (F1) with calcination temperature of 950 °C and calcination time of 12 hours. [0049] Figure 8. Internal impedance of a Na-ion battery with Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1 (F2) cathode after 1 cycle at 0.1C, exhibiting a charge transfer resistance of ~380 Ω. [0050] Figure 9. Comparison of the cycling performance of F1, F2, and F3. Where the F1, F2, and F3 solid markers represent cycling performance, and the F1-Efficiency, F2-Efficiency, and F3-Efficiency hollow markers represent the corresponding Coulombic efficiencies. [0051] Figure 10. Galvanostatic charge/discharge performance of sodium-ion batteries using the as-prepared F2 cathode at a current density of 0.1C (1C corresponds to the current that the battery can be fully charged at 1h) in the voltage range of 2 – 4 V. [0052] Figure 11. Long-term cycling performance of sodium-ion batteries using the as- prepared F2 cathode at a current density of 0.5 C in the voltage range of 2 – 4 V. The calculation of specific capacity and current density is based on the mass of high entropy sodium layered oxide except for binder and carbon black additive. Detailed Description [0053] The inventors have surprisingly found that high entropy sodium layered oxides are highly stable and effective cathode materials for Na-ion batteries, and provide one or more of the following advantages: ME_215242160_2 • The high entropy sodium layered oxides of formula (I) described below provide a highly stable layered structure owing to their high molar configurational entropy and low Gibbs free energy. Due to the several (for example 5 to 7) elements mixed inside the oxide layers, the compounds of formula (I) have large tensile and compressing strains inside the lattice, which can effectively resist structure change during sodium intercalation/deintercalation, and thus provide high capacity and stable cycling performance during the charge/discharge of a Na-ion battery. • The main elements used in compounds of formula (I) are cheap, earth abundant, low toxicity and easily obtained. • The solid-state synthesis method for compounds of formula (I) is simple, low-cost, and easy to commercialise. Compounds [0054] In a first aspect of the disclosure, there is provided a compound of formula (I): Na _σ TMʹ _w TMʹʹ _x TMʹʹʹ _y TMʹʹʹʹ _z Mʹ _t Mʹʹ _u Mʹʹʹ _v O ₂ , (I) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, z is 0 or between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, t and v are 0 or between about 0.01 and about 0.3, w + x + y + z + t+ u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element. ME_215242160_2 [0055] TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ may be independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. In some embodiments, TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ may be independently selected from the group consisting of Ti, Mn, Zn, Fe, Cu, and Sc. In some embodiments, TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ may be independently selected from the group consisting of Ti, Mn, Fe, and Cu. In some embodiments, TMʹ is Cu, TMʹʹ is Fe, TMʹʹʹ is Mn, and TMʹʹʹʹ is Ti. [0056] TMʹ is present in the compound of formula (I) in a molar ratio of w. The molar ratio w may be between about 0.01 and about 0.5, or between about 0.01-0.05, 0.01-0.1, 0.01-0.15, 0.01-0.2, 0.01-0.3, 0.01-4, 0.05-0.1, 0.05-0.15, 0.05-0.2, 0.05-0.3, 0.05-0.4, 0.05-0.5, 0.1- 0.15, 0.1-0.2, 0.1-0.3, 0.1-0.4, 0.1-0.5, 0.15-0.2, 0.15-0.3, 0.15-0.4, 0.15-0.5, 0.2-0.3, 0.2-0.4, 0.2-0.5, 0.3-0.4, 0.3-0.5, or between about 0.4 and about 0.5. The molar ratio w may be between about 0.05 and about 0.2. The molar ratio w may be between about 0.1 and about 0.15. The molar ratio w may be about 0.1 and about 0.15. The molar ratio w may be about 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or about 0.5. The molar ratio w may be about 0.1. The molar ratio w may be about 0.15. The molar ratio w may be about 0.2. [0057] TMʹʹ is present in the compound of formula (I) in a molar ratio of x. The molar ratio x may be between about 0.01 and about 0.5, or between about 0.01-0.05, 0.01-0.1, 0.01-0.2, 0.01-0.3, 0.01-4, 0.05-0.1, 0.05-0.2, 0.05-0.3, 0.05-0.4, 0.05-0.5, 0.1-0.2, 0.1-0.3, 0.1-0.4, 0.1- 0.5, 0.2-0.3, 0.2-0.4, 0.2-0.5, 0.3-0.4, 0.3-0.5, or between about 0.4 and about 0.5. The molar ratio x may be between about 0.1 and about 0.4. The molar ratio x may be between about 0.2 and about 0.3. The molar ratio x may be about 0.01, 0.05, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or about 0.5. The molar ratio x may be about 0.2. The molar ratio x may be about 0.3. [0058] TMʹʹʹ is present in the compound of formula (I) in a molar ratio of y. The molar ratio y may be between about 0.01 and about 0.5, or between about 0.01-0.05, 0.01-0.1, 0.01-0.2, 0.01-0.3, 0.01-4, 0.05-0.1, 0.05-0.2, 0.05-0.3, 0.05-0.4, 0.05-0.5, 0.1-0.2, 0.1-0.3, 0.1-0.4, 0.1- 0.5, 0.2-0.3, 0.2-0.4, 0.2-0.5, 0.3-0.4, 0.3-0.5, or between about 0.4 and about 0.5. The molar ratio y may be between about 0.1 and about 0.4. The molar ratio y may be between about 0.2 and about 0.3. The molar ratio y may be about 0.01, 0.05, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or about 0.5. The molar ratio y may be about 0.2. The molar ratio y may be about 0.3. [0059] TMʹʹʹʹ is present in the compound of formula (I) in a molar ratio of z. The molar ratio z may be between about 0.01 and about 0.5, or between about 0.01-0.05, 0.01-0.1, 0.01-0.2, 0.01-0.3, 0.01-4, 0.05-0.1, 0.05-0.2, 0.05-0.3, 0.05-0.4, 0.05-0.5, 0.1-0.2, 0.1-0.3, 0.1-0.4, 0.1- 0.5, 0.2-0.3, 0.2-0.4, 0.2-0.5, 0.3-0.4, 0.3-0.5, or between about 0.4 and about 0.5. The molar ratio z may be between about 0.05 and about 0.3. The molar ratio z may be about 0.01, 0.05, ME_215242160_2 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or about 0.5. The molar ratio z may be about 0.1. The molar ratio z may be about 0.05. The molar ratio z may be about 0.3. Alternatively, the molar ratio z may be 0. That is, the compound of formula (I) may not contain any of the element TMʹʹʹʹ. [0060] Mʹ, Mʹʹ and Mʹʹʹ may be independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb. In some embodiments, Mʹ, Mʹʹ and Mʹʹʹ may be independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Ti, Sn, Mo, Si, and Sb. In some embodiments, Mʹ, Mʹʹ and Mʹʹʹ may be independently selected from the group consisting of Li, Mg, and Al. In some embodiments, Mʹ is Li, Mʹʹ is Mg and Mʹʹʹ is Al. [0061] Mʹ is present in the compound of formula (I) in a molar ratio of t. The molar ratio t may be between about 0.01 and about 0.3, or between about 0.01-0.05, 0.01-0.1, 0.01-0.15, 0.01- 0.2, 0.01-0.25, 0.05-0.1, 0.05-0.15, 0.05-0.2, 0.05-0.25, 0.05-0.3, 0.1-0.15, 0.1-0.2, 0.1-0.25, 0.1-0.3, 0.15-0.2, 0.15-0.25, 0.15-0.3, 0.2-0.25, 0.2-0.3, or between about 0.25-0.3. The molar ratio t may be between about 0.01 and about 0.1. The molar ratio t may be about 0.05, or about 0.1, 0.15, 0.2, 0.25 or about 0.3. The molar ratio t may be about 0.05. Alternatively, the molar ratio t may be 0. That is, the compound of formula (I) may not contain any of the element Mʹ. [0062] Mʹʹ is present in the compound of formula (I) in a molar ratio of u. The molar ratio u may be between about 0.01 and about 0.3, or between about 0.01-0.05, 0.01-0.1, 0.01-0.15, 0.01- 0.2, 0.01-0.25, 0.05-0.1, 0.05-0.15, 0.05-0.2, 0.05-0.25, 0.05-0.3, 0.1-0.15, 0.1-0.2, 0.1-0.25, 0.1-0.3, 0.15-0.2, 0.15-0.25, 0.15-0.3, 0.2-0.25, 0.2-0.3, or between about 0.25-0.3. The molar ratio u may be between about 0.01 and about 0.1. The molar ratio u may be about 0.05, or about 0.1, 0.15, 0.2, 0.25 or about 0.3. The molar ratio u may be about 0.05. [0063] Mʹʹʹ is present in the compound of formula (I) in a molar ratio of v. The molar ratio v may be between about 0.01 and about 0.3, or between about 0.01-0.05, 0.01-0.1, 0.01-0.15, 0.01- 0.2, 0.01-0.25, 0.05-0.1, 0.05-0.15, 0.05-0.2, 0.05-0.25, 0.05-0.3, 0.1-0.15, 0.1-0.2, 0.1-0.25, 0.1-0.3, 0.15-0.2, 0.15-0.25, 0.15-0.3, 0.2-0.25, 0.2-0.3, or between about 0.25-0.3. The molar ratio v may be between about 0.05 and about 0.15. The molar ratio v may be between about 0.05 and about 0.1. The molar ratio v may be about 0.05, or about 0.1, 0.15, 0.2, 0.25 or about 0.3. The molar ratio v may be about 0.1. The molar ratio v may be about 0.05. Alternatively, the molar ratio v may be 0. That is, the compound of formula (I) may not contain any of the element Mʹʹʹ. [0064] Sodium is present in the compound of formula (I) in a molar ratio of σ. The molar ratio σ may be between about 0.67 and about 0.1, or between about 0.67-0.7, 0.67-0.75, 0.67-0.8, ME_215242160_2 0.67-0.85, 0.67-0.9, 0.67-0.95, 0.7-0.75, 0.7-0.8, 0.7-0.85, 0.7-0.9, 0.7-0.95, 0.7-1, 0.75-0.8, 0.75-0.85, 0.75-0.9, 0.75-0.95, 0.75-1, 0.8-0.85, 0.8-0.9, 0.8-0.95, 0.8-1, 0.85-0.9, 0.85-0.95, 0.85-1, 0.9-0.95, 0.9-1, or between about 0.95 and about 1. The molar ratio σ may be between about 0.75 and about 1. The molar ratio σ may be between about 0.8 and about 1. The molar ratio σ may be between about 0.8 and about 0.9. The molar ratio σ may be about 0.67, or about 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or about 1. The molar ratio σ may be about 0.85. The molar ratio σ may be about 1. [0065] In some embodiments of the first aspect of the disclosure the compound of formula (I) may contain 7 elements, that is, none of z, t, or v are 0. In other embodiments, the compound of formula (I) may contain 6 elements, that is, one of z, t, or v is 0. In other embodiments, the compound of formula (I) may contain 5 elements, That is, two of z, t, or v are 0. In this case, z and t may be 0. Alternatively, z and v may be 0. Alternatively, t and v may be 0. [0066] In one embodiment, there is provided a compound of formula (Ia): NaσTMʹwTMʹʹxTMʹʹʹyTMʹʹʹʹzMʹtMʹʹuMʹʹʹvO2, (Ia) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, t, u, and v are between about 0.01 and about 0.3, w + x + y + z + t + u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element. [0067] In one embodiment, there is provided a compound of formula (Ib): NaσTMʹwTMʹʹxTMʹʹʹyTMʹʹʹʹzMʹtMʹʹuO2, (Ib) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, ME_215242160_2 Mʹ and Mʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, t and u are between about 0.01 and about 0.3, w + x + y + z + t + u = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, and Mʹʹ represent a different element. [0068] In one embodiment, there is provided a compound of formula (Ic): Na _σ TMʹ _w TMʹʹ _x TMʹʹʹ _y Mʹ _t Mʹʹ _u Mʹʹʹ _v O ₂ , (Ic) wherein: TMʹ, TMʹʹ, and TMʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, t, u, and v are between about 0.01 and about 0.3, w + x + y + t + u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element. [0069] In one embodiment, there is provided a compound of formula (Id): Na _σ TMʹ _w TMʹʹ _x TMʹʹʹ _y Mʹ _t Mʹʹ _u O ₂ , (Id) wherein: TMʹ, TMʹʹ, and TMʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹ and Mʹʹ and are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, ME_215242160_2 t and u are between about 0.01 and about 0.3, w + x + y + t + u = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, Mʹ, and Mʹʹ represent a different element. [0070] In one embodiment, there is provided a compound of formula (Ie): NaσTMʹwTMʹʹxTMʹʹʹyTMʹʹʹʹzMʹʹuMʹʹʹvO2, (Ie) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, u and v are between about 0.01 and about 0.3, w + x + y + z + u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹʹ, and Mʹʹʹ represent a different element. [0071] In one embodiment, there is provided a compound of formula (If): NaσTMʹwTMʹʹxTMʹʹʹyMʹʹuMʹʹʹvO2, (If) wherein: TMʹ, TMʹʹ, and TMʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, u and v are between about 0.01 and about 0.3, w + x + y + u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, Mʹʹ, and Mʹʹʹ represent a different element. [0072] In one embodiment, there is provided a compound of formula (Ig): ME_215242160_2 Na _σ TMʹ _w TMʹʹ _x TMʹʹʹ _y TMʹʹʹʹ _z Mʹʹ _u O ₂ , (Ig) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹʹ and is independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, w + x + y + z + u = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, and Mʹʹrepresent a different element. [0073] In some embodiments, the compound of formula (I) may be selected from the group consisting of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.2 Mn _0.2 Ti _0.3 O ₂ , Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.3 Mn _0.3 Ti _0.1 O ₂ , Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.15 Fe _0.3 Mn _0.3 Ti _0.05 O ₂ , Na _0.85 Li _0.05 Mg _0.05 Al _0.05 Cu _0.2 Fe _0.3 Mn _0.3 Ti _0.05 O ₂ , NaLi _0.05 Mg _0.05 Al _0.05 Cu _0.2 Fe _0.3 Mn _0.3 Ti _0.05 O ₂ , Na _0.85 Li _0.05 Mg _0.05 Cu _0.2 Fe _0.3 Mn _0.3 Ti _0.1 O ₂ , NaLi _0.05 Mg _0.05 Cu _0.2 Fe _0.3 Mn _0.3 Ti _0.1 O ₂ , Na _0.85 Mg _0.05 Cu _0.2 Fe _0.3 Mn _0.3 Ti _0.15 O ₂ , and NaMg _0.05 Cu _0.2 Fe _0.3 Mn _0.3 Ti _0.15 O _2. . In some embodiments, the compound of formula (I) may be selected from the group consisting of Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.2 Mn _0.2 Ti _0.3 O ₂ , Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.3 Mn _0.3 Ti _0.1 O ₂ , and Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.15 Fe _0.3 Mn _0.3 Ti _0.05 O ₂ , The compound of formula (I) may be a compound wherein TMʹ is Cu, TMʹʹ is Fe, TMʹʹʹ is Mn, TMʹʹʹʹ is Ti, Mʹ is Li, Mʹʹ is Mg and Mʹʹʹ is Al, and σ is 0.85, w is 0.1, x is 0.2, y is 0.2, z is 0.3, t is 0.05, u is 0.05 and v is 0.1. The compound of formula (I) may be a compound wherein TMʹ is Cu, TMʹʹ is Fe, TMʹʹʹ is Mn, TMʹʹʹʹ is Ti, Mʹ is Li, Mʹʹ is Mg and Mʹʹʹ is Al, and σ is 0.85, w is 0.1, x is 0.3, y is 0.3, z is 0.1, t is 0.05, u is 0.05 and v is 0.1. The compound of formula (I) may be a compound wherein TMʹ is Cu, TMʹʹ is Fe, TMʹʹʹ is Mn, TMʹʹʹʹ is Ti, Mʹ is Li, Mʹʹ is Mg and Mʹʹʹ is Al, and σ is 0.85, w is 0.15, x is 0.3, y is 0.3, z is 0.05, t is 0.05, u is 0.05 and v is 0.1. [0074] The compound of formula (I) may be in the O3 structural form. Layered transition metal oxides may have one of two structural forms, referred to as Pn and On (as classified in C. Delmas, C. et al., Physica B+C, 1980, 99, 81–85). The designation O or P refers to the position of sodium ions in the edge-sharing MO6 octahedra, which represents the octahedral (O) or trigonal prismatic (P) coordination. The n refers to the number of repetitions of the transition metal layer in the stacking repeat unit. Thus compounds having the O3 structure have ME_215242160_2 octahedral co-ordination and three repetitions of the transition metal layer in the stacking repeat unit. [0075] The compound of formula (I) may be in the form of crystals. Crystals of the compound of formula (I) may have a particle size of between about 0.5 and about 20 µm, or about 0.5-1, 0.5-5, 0.5-10, 0.5-15, 1-5, 1-10, 1-15, 1-20, 5-10, 5-15, 5-20, 10-15, 10-20, or between about 15 and about 20 µm. Crystals of the compound of formula (I) may have a particle size of between about 1 and about 10 µm. Crystals of the compound of formula (I) may have a particle size of between about 1 and about 5 µm. Crystals of the compound of formula (I) may have a particle size of between about 5 and about 10 µm. Crystals of the compound of formula (I) may have a particle size of about 0.5 µm, or about 1, 5, 10, 15, or about 20 µm. Particle size may also refer to average particle size. Particle size may refer to the measurement along the longest axis of a crystal. Particle size may be influenced by preparing the compound of formula (I) at varying temperatures. Methods [0076] In a second aspect of the disclosure, there is provided a method of preparing a compound of formula (I) as defined in the first aspect of the disclosure, the method comprising: (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ: TMʹʹʹʹ:Mʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:z:t:u:v, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, z is 0 or between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, t and v are 0 or between about 0.01 and about 0.3, w + x + y + z + t + u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (I). [0077] Step (i) of the method of the second aspect of the disclosure involves preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ. That is, each metal TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ may be provided as either ME_215242160_2 its oxide or its carbonate. TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ are as described above in respect of the first aspect of the disclosure. In some embodiments, the oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ are CuO, Fe ₂ O ₃ , MnO ₂ and TiO ₂ . In some embodiments, the oxides or carbonates of Mʹ, Mʹʹ, and Mʹʹʹ are Li ₂ CO ₃ , MgO, and Al ₂ O ₃ . The mixture may be prepared by any suitable method, for example ball milling, sand milling and high speed blending. [0078] In the mixture of step (i), a sodium salt and the oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ are present in the molar ratio of a sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:TMʹʹʹʹ:Mʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:z:t:u:v. The molar ratios σ, w, x, y, z, t, u, and v are as described above in respect of the first aspect of the disclosure. [0079] In the mixture of step (i), the sodium salt may be Na2CO3. [0080] In one embodiment, there is provided a method of preparing a compound of formula (Ia) as defined in the first aspect of the disclosure, the method comprising: (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:TMʹʹʹʹ:Mʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:z:t:u:v, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, t, u, and v are between about 0.01 and about 0.3, w + x + y + z + t + u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (Ia). [0081] In one embodiment, there is provided a method of preparing a compound of formula (Ib) as defined in the first aspect of the disclosure, the method comprising: (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, and Mʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:TMʹʹʹʹ:Mʹ:Mʹʹ = σ:w:x:y:z:t:u, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, t and u are between about 0.01 and about 0.3, w + x + y + z + t + u = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, and Mʹʹ represent a different element; and ME_215242160_2 (ii) Sintering the mixture to provide a compound of formula (Ib). [0082] In one embodiment, there is provided a method of preparing a compound of formula (Ic) as defined in the first aspect of the disclosure, the method comprising: (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:Mʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:t:u:v, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, t, u, and v are between about 0.01 and about 0.3, w + x + y + t + u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (Ic). [0083] In one embodiment, there is provided a method of preparing a compound of formula (Id) as defined in the first aspect of the disclosure, the method comprising: (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, Mʹ, and Mʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:Mʹ:Mʹʹ = σ:w:x:y:t:u, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, t and u are between about 0.01 and about 0.3, w + x + y + t + u = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, Mʹ, and Mʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (Id). [0084] In one embodiment, there is provided a method of preparing a compound of formula (Ie) as defined in the first aspect of the disclosure, the method comprising (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹʹ and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:TMʹʹʹʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:z:u:v, σ is between about 0.67 and about 1, ME_215242160_2 w, x, y, and z are between about 0.01 and about 0.5, u and v are between about 0.01 and about 0.3, w + x + y + z + u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (Ie). [0085] In one embodiment, there is provided a method of preparing a compound of formula (If) as defined in the first aspect of the disclosure, the method comprising (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, Mʹʹ and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:u:v, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, u and v are between about 0.01 and about 0.3, w + x + y + u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (If). [0086] In one embodiment, there is provided a method of preparing a compound of formula (Ig) as defined in the first aspect of the disclosure, the method comprising (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, and Mʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ:TMʹʹʹʹ:Mʹʹ = σ:w:x:y:z:u, σ is between about 0.67 and about 1, w, x, y, and z are between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, w + x + y + z + u = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, and Mʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (Ig). [0087] Step (i) may further comprise adding a further amount of the sodium salt, for example Na2CO3, such that it is present in a molar ratio of at least about 1.01 × σ, or at least about 1.025, 1.05, or at least about 1.1 × σ. Step (i) may further comprise adding a further amount of the sodium salt such that it is present in a molar ratio of between about 1.01 and about 1.1 × σ, or between about 1.01-1.025, 1.01-1.05, 1.01-1.1, 1.025-1.05, 1.025-1.1, or between ME_215242160_2 about 1.05 and about 1.1 × σ. Step (i) may further comprise adding a further amount of the sodium salt such that it is present in a molar ratio of about 1.01 × σ, or about 1.025, 1.05, or about 1.1 × σ. Step (i) may further comprise adding a further amount of the sodium salt such that it is present in a molar ratio of about 1.05 × σ. The sodium salt may be provided in an excess to the molar ratio of σ in order to compensate for the evaporation of sodium at high temperature, such that the molar ratio of sodium in the resulting compound of formula (I) is σ. [0088] In step (ii) of the method of the second aspect of the disclosure, the mixture is sintered. That is, the mixture is formed into the compound of formula (I) at high temperature. The mixture may be sintered at a temperature of between about 500 °C to about 1500 °C, or between about 500 to 1000, or 1000 to 1500 °C. The mixture may be sintered at a temperature of between about 800 °C to about 1000 °C, or between about 800-850, 800-900, 800-950, 850- 900, 850-950, 850-1000, 900-950, 900-1000, or between about 950 and about 1000 °C. The mixture may be sintered at a temperature of between about 800 °C and about 900 °C. The mixture may be sintered at a temperature of between about 850 °C and about 950 °C. The mixture may be sintered at a temperature of about 800 °C, or about 850, 900, 950, or about 1000 °C. The mixture may be sintered at a temperature of about 850 °C. The mixture may be sintered at a temperature of about 950 °C. The mixture may be sintered at a temperature of about 900 °C. The mixture may be sintered for a time of between about 0.5 to about 60 hours, or between about 0.5 to 10, 0.5 to 20, 0.5 to 40, 00.5 to 60, 1 to 20, 1 to 40, 1 to 60, 10 to 20, 10 to 40, or between about 10 to about 60 hours. The mixture may be sintered for a time of between about 2 and about 30 hours, or between about 2-5, 2-10, 2-15, 2-20, 2-25, 5-10, 5- 15, 5-20, 5-25, 5-30, 10-15, 10-20, 10-25, 10-30, 15-20, 15-25, 15-30, 20-25, 20-30, or between about 25 and about 30 hours. The mixture may be sintered for a time of between about 10 and about 15 hours. The mixture may be sintered for a time of between about 10 and about 20 hours. The mixture may be sintered for a time of about 2 hours, or about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 25, or about 30 hours. The mixture may be sintered for a time of about 12 hours. The mixture may be sintered for a time of about 15 hours. [0089] The sintering may be performed in a furnace, such a muffle furnace. The temperature increase rate may be about 5 °C/min, or about 10, 15, or 20 °C/min. The temperature increase rate may be about 10 °C/min. The sintering may be performed under an air atmosphere. The sintering may be performed under an inert atmosphere, such as a nitrogen or argon atmosphere. Varying the sintering temperature may result in a variation of the particle size of crystals of the compound of formula (I). Sintering at higher temperatures results in a larger particle size. Sintering at lower temperatures results in a smaller particle size. In one embodiment, sintering at a temperature of about 800 °C results in crystals of the compound ME_215242160_2 of formula (I) having a particle size of about 3 to 5 µm. In one embodiment, sintering at a temperature of about 950 °C results in crystals of the compound of formula (I) having a particle size of about 8 to 10 µm. Cathode material and battery [0090] In a third aspect of the disclosure, there is provided a sodium-ion battery comprising: (i) a cathode comprising a compound of formula (I) as defined in the first aspect of the disclosure, (ii) an anode comprising a conductive material, (iii) a separator; and (iv) an electrolyte comprising sodium ions. [0091] The components of the battery such as a binder, anode, separator, electrolyte, collectors and housing may be of any conventional sort known to the skilled person. For example, the cathode and anode may comprise a binder such as polyvinylidene fluoride (PVDF). The cathode may also comprise carbon black. The anode may be sodium metal, for example a sodium metal foil. The separator may be, for example, glass fibers or glass mat. The electrolyte may be, for example, a non-aqueous electrolyte such as sodium perchlorate in ethylene carbonate and/or propylene carbonate. The electrolyte may comprise an additive such as fluoroethylene carbonate. [0092] In a fourth aspect of the disclosure, there is provided a use of the compound of formula (I) as defined in the first aspect of the disclosure as a cathode material in a sodium-ion battery. [0093] In a fifth aspect of the disclosure, there is provided a method of fabricating a cathode for a sodium-ion battery, the method comprising combining a compound of formula (I) as defined in the first aspect of the disclosure, a conductive material, and optionally a binder to form a mixture. [0094] In a sixth aspect of the disclosure, there is provided a cathode material for a sodium- ion battery, comprising a compound of formula (I) as defined in the first aspect of the disclosure, a conductive material, and optionally a binder. [0095] In the fifth and sixth aspects of the disclosure, the conductive material may be a carbon material. The carbon material may be selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, expanded graphite, carbon fibre, ME_215242160_2 glassy carbon, shredded carbon felt, and carbon foam. The binder, if present, may be polyvinylidene fluoride. [0096] The present disclosure may be described by reference to the following numbered forms. 1. A compound of formula (I): Na _σ TMʹ _w TMʹʹ _x TMʹʹʹ _y TMʹʹʹʹ _z Mʹ _t Mʹʹ _u Mʹʹʹ _v O ₂ , (I) wherein: TMʹ, TMʹʹ, TMʹʹʹ and TMʹʹʹʹ are independently selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Cr, Ti, Sn, V, Mo, Si, and Sb, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, z is 0 or between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, t and v are 0 or between about 0.01 and about 0.3, w + x + y + z + t+ u + v = 1; and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element. 2. The compound of form 1, wherein TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ are independently selected from the group consisting of Ti, Mn, Zn, Fe, Cu, and Sc. 3. The compound of form 1 or form 2, wherein TMʹ is Cu, TMʹʹ is Fe, TMʹʹʹ is Mn, and TMʹʹʹʹ is Ti. 4. The compound of any one of forms 1 to 3, wherein w is between about 0.05 and about 0.2. 5. The compound of any one of forms 1 to 4, wherein w is between about 0.1 and about 0.15. ME_215242160_2 6. The compound of any one of forms 1 to 5, wherein x is between about 0.1 and about 0.4. 7. The compound of any one of forms 1 to 6, wherein x is between about 0.2 and about 0.3. 8. The compound of any one of forms 1 to 7, wherein y is between about 0.1 and about 0.4. 9. The compound of any one of forms 1 to 8, wherein y is between about 0.2 and about 0.3. 10. The compound of any one of forms 1 to 9, wherein z is between about 0.01 and about 0.5. 11. The compound of any one of forms 1 to 10, wherein z is between about 0.05 and about 0.3. 12. The compound of any one of forms 1 to 9, wherein z is 0. 13. The compound of any one of forms 1 to 12, wherein Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Ti, Sn, Mo, Si, and Sb. 14. The compound of any one of forms 1 to 13, wherein Mʹ is Li, Mʹʹ is Mg and Mʹʹʹ is Al. 15. The compound of any one of forms 1 to 14, wherein t is between about 0.01 and about 0.1. 16. The compound of any one of forms 1 to 15, wherein t is 0.05. 17. The compound of any one of forms 1 to 14, wherein t is 0. 18. The compound of any one of forms 1 to 17, wherein u is between about 0.01 and about 0.1. 19. The compound of any one of forms 1 to 18, wherein u is 0.05. 20. The compound of any one of forms 1 to 19, wherein v is between about 0.01 and about 0.3. 21. The compound of any one of forms 1 to 20, wherein v is between about 0.05 and about 0.2. ME_215242160_2 22. The compound of any one of forms 1 to 21, wherein v is between about 0.05 and about 0.1. 23. The compound of any one of forms 1 to 22, wherein v is 0.1. 24. The compound of any one of forms 1 to 19, wherein v is 0. 25. The compound of any one of forms 1 to 24, wherein σ is between about 0.75 and about 1. 26. The compound of any one of forms 1 to 25, wherein σ is between about 0.8 and about 1. 27. The compound of any one of forms 1 to 26, wherein σ is between about 0.8 and about 0.9. 28. The compound of any one of forms 1 to 27, wherein σ is 0.85. 29. The compound of any one of forms 1 to 28, selected from the group consisting of Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.2Mn0.2Ti0.3O2, Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1O2, Na0.85Li0.05Mg0.05Al0.1Cu0.15Fe0.3Mn0.3Ti0.05O2, Na0.85Li0.05Mg0.05Al0.05Cu0.2Fe0.3Mn0.3Ti0.05O2, NaLi0.05Mg0.05Al0.05Cu0.2Fe0.3Mn0.3Ti0.05O2, Na0.85Li0.05Mg0.05Cu0.2Fe0.3Mn0.3Ti0.1O2, NaLi0.05Mg0.05Cu0.2Fe0.3Mn0.3Ti0.1O2, Na0.85Mg0.05Cu0.2Fe0.3Mn0.3Ti0.15O2, and NaMg0.05Cu0.2Fe0.3Mn0.3Ti0.15O2. 30. The compound of any one of forms 1 to 29, selected from the group consisting of Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.2Mn0.2Ti0.3O2, Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1O2, and Na0.85Li0.05Mg0.05Al0.1Cu0.15Fe0.3Mn0.3Ti0.05O2. 31. The compound of any one of form 1 to 30, wherein the compound is in the O3 structural form. 32. The compound of any one of forms 1 to 31, wherein the compound is in the form of crystals having a particle size of between about 0.5 µm and about 20 µm. 33. The compound of any one of forms 1 to 32, wherein the compound is in the form of crystals having a particle size of between about 1 µm and about 10 µm. 34. A method of preparing a compound of formula (I) as defined in any one of forms 1 to 33, the method comprising: ME_215242160_2 (i) Preparing a mixture comprising a sodium salt and oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ, wherein the molar ratio of sodium salt:TMʹ:TMʹʹ:TMʹʹʹ: TMʹʹʹʹ:Mʹ:Mʹʹ:Mʹʹʹ = σ:w:x:y:z:t:u:v, σ is between about 0.67 and about 1, w, x, and y are between about 0.01 and about 0.5, z is 0 or between about 0.01 and about 0.5, u is between about 0.01 and about 0.3, t and v are 0 or between about 0.01 and about 0.3, w + x + y + z + t+ u + v = 1, and each of TMʹ, TMʹʹ, TMʹʹʹ, TMʹʹʹʹ, Mʹ, Mʹʹ, and Mʹʹʹ represent a different element; and (ii) Sintering the mixture to provide a compound of formula (I). 35. The method of form 34, wherein in step (ii) the mixture is sintered at a temperature of between about 500 °C to 1500 °C. 36. The method of form 34 or form 35, wherein in step (ii) the mixture is sintered for a time of about 0.5 hours to about 60 hours. 37. The method of any one of forms 34 to 36, wherein in step (ii) the mixture is sintered at a temperature of between about 800 °C and about 1000 °C. 38. The method of any one of forms 34 to 37, wherein in step (ii) the mixture is sintered for between about 2 hours and about 30 hours. 39. The method of any one of forms 34 to 38, wherein in step (ii) the mixture is sintered at about 850 °C for about 12 hours. 40. The method of any one of forms 34 to 39, wherein in step (ii) the mixture is sintered at about 950 °C for about 12 hours. ME_215242160_2 41. The method of any one of forms 34 to 40, wherein in step (i) the mixture is prepared by ball milling. 42. The method of any one of forms 34 to 41, wherein step (i) further comprises adding a further amount of the sodium salt such that it is present in the mixture in a molar ratio of at least about 1.01 × σ. 43. The method of any one of forms 34 to 42, wherein TMʹ is Cu, TMʹʹ is Fe, TMʹʹʹ is Mn, and TMʹʹʹʹ is Ti. 44. The method of form 43, wherein in step (i) the oxides or carbonates of TMʹ, TMʹʹ, TMʹʹʹ, and TMʹʹʹʹ are CuO, Fe2O3, MnO2 and TiO2. 45. The method of any one of forms 34 to 44, wherein w is between about 0.05 and about 0.2. 46. The method of any one of forms 34 to 45, wherein w is between about 0.1 and about 0.15. 47. The method of any one of forms 34 to 46, wherein x is between about 0.1 and about 0.4. 48. The method of any one of forms 34 to 47, wherein x is between about 0.2 and about 0.3. 49. The method of any one of forms 34 to 48, wherein y is between about 0.1 and about 0.4. 50. The method of any one of forms 34 to 49, wherein y is between about 0.2 and about 0.3. 51. The method of any one of forms 34 to 50, wherein z is between about 0.01 and about 0.5. 52. The method of any one of forms 34 to 51 wherein z is between about 0.05 and about 0.3. 53. The method of any one of forms 34 to 50, wherein z is 0. 54. The method of any one of forms 34 to 53, wherein Mʹ, Mʹʹ and Mʹʹʹ are independently selected from the group consisting of Li, Mg, Ca, Ba, Sr, Al, B, Ti, Sn, Mo, Si, and Sb. ME_215242160_2 55. The method of any one of forms 34 to 54, wherein Mʹ is Li, Mʹʹ is Mg and Mʹʹʹ is Al. 56. The method of form 51, wherein in step (i) the oxides or carbonates of Mʹ, Mʹʹ, and Mʹʹʹ are Li ₂ CO ₃ , MgO, and Al ₂ O ₃ . 57. The method of any one of forms 34 to 55, wherein t is between about 0.01 and about 0.1. 58. The method of any one of forms 34 to 56, wherein t is 0.05. 59. The method of any one of forms 34 to 56, wherein t is 0. 60. The method of any one of forms 34 to 59, wherein u is between about 0.01 and about 0.1. 61. The method of any one of forms 34 to 60, wherein u is 0.05. 62. The method of any one of forms 34 to 61, wherein v is between about 0.01 and about 0.3. 63. The method of any one of forms 34 to 62, wherein v is between about 0.05 and about 0.2. 64. The method of any one of forms 34 to 63, wherein v is between about 0.05 and about 0.1. 65. The method of any one of forms 34 to 64, wherein v is 0.1. 66. The method of any one of forms 34 to 61, wherein v is 0. 67. The method of any one of forms 34 to 66, wherein the sodium salt is Na2CO3. 68. The method of any one of forms 34 to 67, wherein σ is between about 0.75 and about 0.1. 69. The method of any one of forms 34 to 68, wherein σ is between about 0.8 and about 1. 70. The method of any one of forms 34 to 69, wherein σ is between about 0.8 and about 0.9. 71. The method of any one of forms 34 to 70, wherein σ is 0.85. ME_215242160_2 72. A sodium-ion battery comprising: (i) a cathode comprising a compound of formula (I) as defined in any one of forms 1 to 33, (ii) an anode comprising a conductive material, (iii) a separator; and (iv) a non-aqueous electrolyte comprising sodium ions. 73. Use of the compound of formula (I) as defined in any one of forms 1 to 33 as a cathode material in a sodium-ion battery. 74. A method of fabricating a cathode for a sodium-ion battery, the method comprising: combining a compound of formula (I) as defined in any one of forms 1 to 33, a conductive material, and a binder to form a mixture. 75. A cathode material for a sodium-ion battery, comprising: (i) a compound of formula (I) as defined in any one of forms 1 to 33, (ii) a conductive material, and (iii) a binder. 76. The sodium ion battery of form 72, the method of form 74 or the cathode material of form 75, wherein the conductive material is a carbon material. 77. The method or cathode material of form 76, wherein the carbon material is selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, expanded graphite, carbon fibre, glassy carbon, shredded carbon felt, and carbon foam. 78. The method or cathode material of any one of forms 74 to 77, wherein the binder is polyvinylidene fluoride. Examples [0097] The present disclosure is further described below by reference to the following non- limiting examples. ME_215242160_2 Example 1 [0098] A mixture of 4,928.21 mg Na ₂ CO ₃ (inclusive of an additional 5% Na), 202.1 mg Li ₂ CO ₃ , 210.0 mg MgO, 531.2 mg Al ₂ O ₃ , 828.8 mg CuO, 1,663.9 mg Fe ₂ O ₃ , 1,645.0 mg MnO ₂ , and 2,496.5 mg TiO ₂ was prepared by ball milling. Then the mixture was transferred into a muffle furnace for annealing at 850 °C for 12 hours with a ramping rate of 10 °C/min, to provide Na _0.85 Li _0.05 Mg _0.05 Al _0.1 Cu _0.1 Fe _0.2 Mn _0.2 Ti _0.3 (F1). An XRD spectrum of F1 is shown in Figure 1. Example 2 [0099] A mixture of 4,852.1 mg Na ₂ CO ₃ , 199.0 mg Li ₂ CO ₃ , 206.7 mg MgO, 523.0 mg Al ₂ O ₃ , 816.0 mg CuO, 2,457.3 mg Fe2O3, 2,429.4 mg MnO2, and 819.3 mg TiO2 was prepared by ball milling. The powder was transferred into a muffle furnace for annealing at 850 °C for 12 hours with a temperature ramping rate of 10 °C/min, to provide Na0.85Li0.05Mg0.05Al0.1Cu0.1Fe0.3Mn0.3Ti0.1 (F2). An XRD spectrum of F2 is shown in Figure 1 and Figure 2. Scanning electron micrographs of F2 are shown in Figures 3 and 4. Energy Dispersive X-ray Spectroscopy (EDS) mapping of F2 is shown in Figure 5, which indicates the uniform distribution of Na, Li, Al, Mg, Cu, Fe, Mn and Ti elements in the high entropy oxide. Example 3 [00100] A mixture of 4,813.4 mg Na2CO3, 197.4 mg Li2CO3, 205.1 mg MgO, 518.8 mg Al2O3, 1,214.3 mg CuO, 2,437.7 mg Fe2O3, 2,410.0 mg MnO2, and 406.4 mg TiO2 was prepared by ball milling. The mixture was transferred into a muffle furnace for annealing at 850 °C for 12 hours with a ramping rate of 10 °C/min, to provide Na0.85Li0.05Mg0.05Al0.1Cu0.15Fe0.3Mn0.3Ti0.05 (F3). An XRD spectrum of F3 is shown in Figure 1. Example 4 [00101] A mixture of 4,928.21 mg Na2CO3, 202.1 mg Li2CO3, 210.0 mg MgO, 531.2 mg Al2O3, 828.8 mg CuO, 1,663.9 mg Fe2O3,1,645.0 mg MnO2, and 2,496.5 mg TiO2 was prepared by ball milling. The mixture was transferred into a muffle furnace for annealing in the conditions set out in Table 1. [00102] Table 1. ME_215242160_2 [00103] As shown in Figure 6, the morphology of the compound F1 sintered at 800 °C for 12 hours shows an average particle size in the range of 3~5 µm. As shown in Figure 7, the morphology of F1 sintered at 950 °C shows an average particle size of 8~10 µm. Generally, the higher temperature sintering and longer heat treatment lead to bigger particle sizes and better crystallinity, however, large particle size may negatively influence rate performance (corresponding to the power density of resulting batteries). Further optimisation identified suitable sintering conditions to be 900 °C for 15 hours. Example 5 [00104] The high entropy sodium layered oxide cathodes were tested in CR2032 coin-type cells with sodium metal foils as anodes. First, the as-prepared compounds of formula (I) were mixed with carbon black and polyvinylidene fluoride (PVDF) binder in a ratio of 84: 8: 8, and then a suitable quantity of N-methyl-2-pyrrolidone (NMP) was added to the mixture under grinding to form a homogenous slurry. The slurry was then uniformly cast onto a carbon- coated aluminum foil, followed by drying in a vacuum oven at 120 °C for 12 hours. The dried electrode was pressed by using a rolling machine (MTI Corporation, MSK-HRP-01) at room temperature under a pressure of ~ 90 MPa, punched into discs (φ=12 or 14 mm) and transferred into a glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm, MBRAUN Glove Box) for cell assembly. The typical loading of the active material was approximately 1.5 – 3 mg/cm. Sodium metal foil was applied as the counter electrode, glass fibers (GF, Filtech, 0.7 µm) were used as separators, and 1 M sodium perchlorate (NaClO4) in ethylene carbonate (EC)/propylene carbonate (PC) (v/v = 1: 1) with 5 vol% fluoroethylene carbonate (FEC) additive was employed as electrolyte. Battery performance was measured using a LAND CT2001A battery testing system. [00105] The testing result of internal impedance of a Na-ion battery using F2 is shown in Figure 8. The cycling performance and the Coulombic efficiency comparison of F1, F2, and F3 are depicted in Figure 9. Figure 10 represents the galvanostatic charge/discharge performance of the sodium-ion batteries using the as-prepared F2 cathode. Figure 11 represents the long-term cycling performance of a sodium-ion battery using the as-prepared F2 cathode. [00106] The performance of Na-ion batteries comprising compounds of formula (I) as the cathode material, compared to conventional cathode materials, can be summarised as follows. ME_215242160_2 [00107] Table 2. [00108] In Table 2 the energy density is based on the mass of the cathode, for example comprising a compound of formula (I). When incorporated into a sodium-ion battery device, the mass of the anode, separator, electrolyte, package, and other components must be considered in the calculation. The energy density of the final device is estimated to be in the range of 110-150 Wh/kg, compared to a typical lithium-ion battery 120–260 Wh/kg. The energy density of sodium-ion batteries is lower than that of lithium-ion batteries on this basis. Sodium-ion batteries are advantageous for large-scale energy storage, considering cost, safety, power density, cycle life, and low-temperature performance. The cathode material of the present disclosure does not contain Ni and Co, providing a somewhat lower energy density for a considerably low cost. [00109] Those skilled in the art will appreciate that the disclosure described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of an two or more of said steps, features, compositions and compounds. ME_215242160_2