Materials

Abstract

A material having the general formula AX where A = MeSnf[c]d; [c] is a cation vacancy; M = Li, Na or a combination thereof; 0 ≤ e < 1; 0 ≤ f < 1; 0 ≤ d < 1 / 2; and the sum of e, f, d is 1; X = OpSqFlCl

Description

BACKGROUND The performance of the electrodes in lithium-ion batteries can be affected by the chemical composition and structure of the electrode material. Anode materials having different chemical compositions and crystal structures are known, such as graphite and transition metal oxides. Also known are lithium- and tin-containing materials. For example, tin oxides and lithium-tin oxides such as SnO, SnOa, LiSnOa, and SnSiOa. Li2SnOa having a layered structure is also known. These materials are judged on performance and cost. Performance is often assessed based on energy density and stability (i.e. stability over multiple charge-discharge cycles). Cost includes the cost of raw materials and manufacturing. SUMMARY The present invention relates to materials, and methods for preparing the materials. Materials of the present invention may be described, for example, in terms of their chemical composition and their structure. The chemical composition may be described, for example, in terms of a general formula. According to a first aspect, there is provided a material having the general formula AX. wherein: A = MeSnf[c]d; [c] is a cation vacancy M = Li, Na or a combination thereof; 0<e<1;0<f<1;0<d< 1 / 2; and the sum of e, f and d is 1; and wherein: X = OPSqFiClk[a]m; [a] is an anion vacancy; 0<p<1;0<q<1;0<l<1;0<k<1;0<m< 1 / 2; and the sum of p, q, I, k and m is 1; and the material has a structure which is a disordered rock salt structure. Materials of the first aspect may display anode like behaviour during electrochemical cycling and may exhibit surprisingly high capacity compared with known materials having a structure other than a disordered rock salt structure. For example, such materials may have a surprisingly high capacity compared with other known tin-containing anode materials which have a structure other than a disordered rock salt structure, such as a layered structure. In some embodiments of the first aspect, M comprises or consists of Li. Such materials may have a surprisingly high capacity. In some embodiments, M comprises or consists of Li and, in a charge-discharge curve of the material, the capacity after the first discharge is greater than 400 mAh g-1. In some embodiments, M consists of Li and, in a charge-discharge curve of the material, the capacity after the first discharge is greater than 450 mAh g-1. In some embodiments, M comprises or consists of Na. Anode materials comprising sodium are rare and it is surprising that such materials of the first aspect can show anode like behaviour. The replacement of lithium with sodium may reduce the cost of the raw materials needed to make the material according to the first aspect. In some embodiments, M comprises Li and Na. In some examples, such materials may have increased stability compared when M consists of Li. In some embodiments, 1 / 6 <e <2 / 3; 0 <f <1 / 3; 0 <d <1 / 2. In some embodiments, materials of the first aspect do not comprise any vacancies. In such embodiments, materials of the first aspect have the general formula M2SnO3, wherein M is Li, Na ora combination thereof. In some embodiments of the first aspect, (i) k = 0; and / or (ii) p and / or q >0; and / or (iii) p >0; and / or (iv) I >0. When k = 0, the material does not comprise Cl. When p and / or q >0, the material comprises O and / or S. When p >0, the material comprises O. When I >0, the material comprises F. In some embodiments, materials of the first aspect comprise anion and / or cation vacancies. Materials of the first aspect may comprise cation vacancies as a result of an absence of M and / or Sn. In such embodiments, d >0. In some embodiments, d >0, e <2 / 3 and / or f <1 / 3. Materials of the first aspect may comprise anion vacancies. In such embodiments, m >0. In some embodiments, d and / or m = 0. In such embodiments, materials of the first aspect comprise no cation vacancies and / or no anion vacancies. The structure may be a crystalline structure. In some embodiments, the material of the first aspect comprises a single phase. In some embodiments, the material of the first aspect predominantly comprises a single phase. In some embodiments, the material of the first aspect consists of a single phase. In some embodiments, the material of the first aspect is in the form of a single phase. In some embodiments, in an X-ray diffraction (XRD) pattern of the material using a CuKa radiation source, there is no other crystalline phase identifiable in the range 30-85° 20. In some embodiments, in an XRD pattern of the material using a CuKa radiation source, there is no other phase identifiable in the range 30-75° 20. In some embodiments, in an XRD pattern of the material using a CuKa radiation source, there are no peaks below 30° 20. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 36.0° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 41.8° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 60.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 72.7° ±2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has peaks at a 20 value of at least one of 36.0° ± 2.0, 41.8° ± 2.0°, 60.7° ± 2.0° and 72.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has peaks at a 20 value of at least two of, or at least three of, 36.0° ± 2.0, 41.8° ± 2.0°, 60.7° ± 2.0° and 72.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has peaks at a 20 value of 36.0° ± 2.0, 41.8° ± 2.0°, 60.7° ± 2.0° and 72.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 33.4° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 38.8° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 56.3° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 67.2° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 70.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has peaks at a 20 value of at least one of 33.4° ± 2.0°, 38.8° ± 2.0°, 56.3° ± 2.0°, 67.2° ± 2.0° and 70.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has peaks at a 20 value of at least two of, or at least three of, 33.4° ± 2.0°, 38.8° ± 2.0°, 56.3° ± 2.0°, 67.2° ± 2.0° and 70.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has peaks at a 20 value of 33.4° ± 2.0°, 38.8° ± 2.0°, 56.3° ± 2.0°, 67.2° ± 2.0° and 70.7° ± 2.0°. In a second and a third aspect, there are provided methods of making materials according to the first aspect. Both methods comprise at least one step involving ball milling. Such methods are found to produce the materials of the first aspect without showing evidence of the presence of precursors or contaminants. In the second aspect, there is provided a method of preparing a material according to the first aspect, the method comprising the steps of: (a) providing a precursor mixture comprising M and Sn; wherein M and Sn are provided in the form of at least one salt precursor selected from: lithium salt precursors, sodium salt precursors, tin salt precursors and mixed metal salt precursors, or a combination thereof; and wherein the salt of the at least one salt precursor is or comprises an oxide, a sulfide, a fluoride or a chloride; and (b) ball milling the precursor mixture provided (a) for a period of time. In the third aspect, there is provided a method of preparing a material according to the first aspect, wherein the method comprises the steps of: (a) providing a precursor mixture comprising or consisting of at least one precursor having the general formula AX, wherein: A = MeSnf[c]dj [c] is a cation vacancy; M = Li, Na or a combination thereof; 0 <e <1; 0 <f <1; 0 <d <1 / 2; and the sum of e, f and d is 1; and wherein: X = OPSqFiClk[a]m; [a] is an anion vacancy; 0<p<1;0<q<1;0<l<1;0<k<1;0<m< 1 / 2; and the sum of p, q, I, k and m is 1; and (b) ball milling the precursor mixture provided at (a) for a period of time. In some embodiments, the composition of the precursor is defined by the general formula AX, wherein 1 / 6 <e <2 / 3; 0 <f <1 / 3; and 0 <d <1 / 2. In some embodiments of the third aspect, the structure of the precursor having the general formula AX is a layered structure, a spinel structure or a disordered rock salt structure. In some embodiments, the precursor mixture comprises at least one precursor having the general formula AX selected from: Li2SnO3 and Na2SnO3. In other words, the precursor mixture may comprise Li2SnO3 and / or Na2SnO3. In some embodiments, the precursor mixture comprises Li2SnO3 and Na2SnO3. One or both of Li2SnOs and Na2SnOs may have a layered structure. In some embodiments, the precursor mixture further comprises SnO2. In some embodiments, the method of the third aspect further comprises preparing at least one precursor having the general formula AX by solid state synthesis. In such embodiments, the solid state synthesis comprises: (i) providing a precursor mixture comprising M and Sn; wherein M and Sn are provided in the form of at least one salt precursor selected from: lithium salt precursors, sodium salt precursors, tin salt precursors and mixed metal salt precursors, or a combination thereof; and (ii) heating the precursor mixture provided at (i) for a period of time. In some embodiments, the salt of the at least one salt precursor is or comprises an oxide, a sulfide, a fluoride, a chloride, a carbonate, a sulfate, a nitrate, an oxalate or a hydroxide. In some embodiments, the salt of the lithium salt precursor and / or the sodium salt precursor is or comprises a carbonate. In some embodiments, the salt of the lithium salt precursor and / or the sodium salt precursor consists of a carbonate. In some embodiments, the precursor mixture provided at (i) comprises Li2CO3 and / or Na2CO3. In some embodiments, the precursor mixture provided at (i) comprises Li2CO3 and Na2CO3. In some embodiments, the mixture obtained after (ii) is substantially free of carbon i.e. the carbon present in the carbonate is evolved in the form of carbon dioxide during the heating step (ii). In some embodiments, the carbonate salt or carbonate salts are provided in a slight excess with respect to the amount required by the target composition, such as an excess of 1 wt%, or 2 wt%, or 3 wt%, or 4 wt%, or 5 wt%. Heating the precursor mixture at (ii) may convert at least some of the precursors provided at (i) into at least one precursor having the general formula AX. In other words, the mixture obtained after (ii) comprises at least one precursor having the general formula AX and may further comprise: residual salt precursors from the mixture provided at (i) that remain unconverted after (ii); and / or salt precursors that form during step (ii) but do not have the general formula AX. In some embodiments, the method further comprises providing at least one solvent at (i) and / or mixing the precursor mixture prior to step (ii) by ball milling. Suitable solvents may be unreactive with the precursors and have a suitable boiling point to allow them to be removed by evaporation. In some embodiments, the solvent is isopropanol (IPA). In some embodiments of the third aspect, heating the precursor mixture at (ii) is a calcination. In some embodiments, the method comprises a further step (iii) after (ii) of heating the precursor mixture. Step (ii) and (iii) involve maintaining the temperature of the precursor mixture at a particular temperature fora period of time. The temperature in step (ii) may be the same as or different to the temperature in step (iii). In some embodiments, the step (ii) and / or (iii) involve maintain the temperature of the precursor mixture at a temperature in the range of from 500 °C to 1000 °C. In some such embodiments, the temperature is in the range of from 600 °C to 1000 °C, or 700 °C to 1000 °C, or 800 °C to 1000 °C, or 850 °C to 1000 °C. In some embodiments, during the heating in (ii) and / or (iii), the precursor mixture is maintained at a temperature of about 900 °C. In some embodiments, the period of time is in the range of from 5 and 24 hours, or from 10 and 24 hours, or from 12 and 24 hours. In some embodiments, the period of time is greater than 5 hours, or greater than 10 hours, or greater than 12 hours. The steps (ii) and / or (iii) may be carried out under a particular atmosphere. In some embodiments, the steps (ii) and / or (iii) are carried out under air or under an inert atmosphere. In general, the characteristics of the ball milling may apply to step (b) of each of the methods of the second and third aspects. In some embodiments, the period of time in (b) may be in the range of from 1 hour to 300 hours. In some embodiments, ball milling is carried out at a speed of 150 rpm or more and / or 1000 rpm or less. In some embodiments, a weight ratio of milling media to the precursor mixture is in the range of from 100:1 to 1:1. In some embodiments, the ball milling is carried out in a milling jar which is formed from a material comprising zirconia (ZrO2), stainless steel, agate (SiO2), Silicon carbide (SiC), or Tungsten carbide (WC). In a fourth aspect, there is provided a material obtainable by the method according to the second or the third aspect. In a fifth aspect, there is provided an electrode comprising a material according to the first aspect, or a material prepared by the method of the second or the third aspect. In some embodiments, the electrode comprises additives and / or a binder; and / or the electrode is an anode. In a sixth aspect, there is provided an electrochemical cell comprising a material according to the first aspect, or a material prepared by the method of the second or the third aspect, or an electrode according to the fifth aspect. In a seventh aspect, there is provided an electrochemical energy storage device comprising an electrochemical cell according to the sixth aspect. In an eighth aspect, there is provided a use of a material according to the first aspect in an electrode, an electrochemical cell, or an electrochemical energy storage device. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a powder XRD pattern of a material having a target composition Li2SnO3. Figure 2a is a scanning electron microscope (SEM) of a material having a target composition Li2SnO3. The scale bar is 80 pm. Figure 2b and 2c are EDX maps of the material. The scale bars are 50 pm. Figure 2b shows oxygen and Figure 2c shows tin. Figure 2d is an EDX spectrum of the material. Figure 3 shows the results of galvanostatic testing between 4.8 - 0.1 V at 30°C carried out on half cells comprising a material having a target composition Li2SnO3. Figure 4 is a powder XRD pattern of a material having a target composition Na2SnO3. Figure 5a is a scanning electron microscope (SEM) of a material having a target composition Na2SnO3. The scale bar is 80 pm. Figure 5b, 5c and 5d are EDX maps of the material. The scale bars are 50 pm. Figure 5b shows sodium, Figure 5c shows oxygen and Figure 5d shows tin. Figure 5e is an EDX spectrum of the material. Figures 6a and 6b show the results of galvanostatic testing between 4.8 - 0.1 V at 30°C carried out on half cells comprising a material having a target composition Na2SnO3. Figure 6a shows the results for cycling against lithium. Figure 6b shows the results for cycling against sodium. Figure 7 is a powder XRD pattern of a material having a target composition Na2SnO3. Figure 8a is a scanning electron microscope (SEM) of a material having a target composition Na2SnO3. The scale bar is 80 pm. Figure 8b, 8c and 8d are EDX maps of the material. The scale bars are 50 pm. Figure 8b shows sodium, Figure 8c shows oxygen and Figure 8d shows tin. Figure 8e is an EDX spectrum of the material. Figures 9a and 9b show the results of galvanostatic testing between 4.8 - 0.1 V at 30°C carried out on half cells comprising a material having a target composition Na2SnO3. Figure 9a shows the results for cycling against lithium. Figure 9b shows the results for cycling against sodium. Figure 10 is a schematic drawing of a method of determining full-width halfmaximum (FWHM) based on a Powder X-ray Diffraction (PXRD) peak. DETAILED DESCRIPTION Methods described herein usually employ ambient temperature of a typical laboratory, which is typically in the range of from 20 to 30°C, such as around 25°C, at atmospheric pressure, unless a different condition is defined herein or is more usually employed e.g. for a particular apparatus. The methods of the second and third aspect may be described as mechanochemical synthesis methods because they use a mechanical method to cause a chemical reaction. The methods use a ball milling process to achieve the mechanochemical synthesis, and not only a mechanical mixing of precursors. Unless indicated otherwise, “%” refers to “weight%”. Similarly, “wt%” denotes “weight%”. First Aspect In a first aspect, there is provided a material having the general formula AX. In some embodiments, e >0 and the material comprises lithium and / or sodium. In some embodiments, the amount of lithium and / or sodium present may be greater than the amount of Sn (i.e. e >f) and the material may be described as lithium-rich. In some embodiments, 0 <e <0.65. In some embodiments, M consists of Li, or is a combination of Li and Na, wherein: Me = LiyNaz; y+z=e, and y >0. In some embodiments, y >z. In some embodiments, e = 0. In some embodiments, e >0, such as 0.005 or more, 0.01 or more, 0.015 or more, 0.02 or more, 0.025 or more, 0.05 or more, 0.075 or more, 0.1 or more, 0.13 or more, or 0.15 or more such as 1 / 3 or more. In some embodiments, e is 0.25 or more, such as 0.27 or more, 0.3 or more, 0.33 or more, or 0.35 or more. In some embodiments, e is 2 / 3 or less, such as 0.65 or less, 0.63 or less, 0.6 or less, or 0.57 or less. Combinations of any of these values may be used to provide exemplary ranges for e. For example, e may be in the range of 0 <e <2 / 3, or 0 <e <0.63, or 0.005 <e <2 / 3, or 0.01 <e <0.63, or 0.015 <e <0.6, or 0.02 <e <0.65, or 0.025 <e <0.065, or 0.05 <e <0.6, or 0.075 <e <0.65, or 0.1 <e <0.63, or 0.13 <e <0.65, or 0.15 <e <0.63, or 0.25 <e <0.57, or 0.27 <e <0.65, or 0.3 <e <0.6, or 0.33 <e <0.65, or 0.35 <e <0.57. In some embodiments, 0.05 <e <2 / 3. In some embodiments, 0.25 <e <2 / 3. In some embodiments, 0.25 <e <0.6. In some embodiments, f <1, such as 0.99 or less, 0.98 or less, 0.97 or less, 0.96 or less, 0.95 or less, 0.94 or less, 0.93 or less, 0.9 or less, 0.88 or less, or 0.85 or less. In some embodiments, f is 0.75 or less, such as 0.73 or less, 0.7 or less, 0.68 or less, or 0.65 or less. In some embodiments, f is 1 / 3 or more, such as 0.35 or more, 0.38 or more, 0.4 or more, or 0.43 or more. Combinations of any of these values may be used to provide exemplary ranges for f. For example, f may be in the range of 1 / 3 <f <1, or 1 / 3 <f <1, or 1 / 3 <f <0.99, or 1 / 3 <f <0.98, or 0.35 <f <1, or 0.35 <f <0.97, or 0.35 <f <0.96, or 0.38 <f <0.96, or 0.38 <f <0.95, or 0.38 <f <0.93, or 0.4 <f <1, 0.4 <f <0.95, 0.4 <f <0.9, 0.4 <f <0.88, 0.4 <f <0.85, 0.43 <f <1, or 0.43 <f <0.85, or 0.43 <f <0.75, or 1 / 3 <f <0.75, or 0.4 <f <0.73, or 1 / 3 <f <0.7, or 1 / 3 <f <0.68, or 0.38 <f <0.68, or 0.43 <f <0.68. In some embodiments, 1 / 3 <f <0.95. In some embodiments, 1 / 3 <f <0.75. In some embodiments, 0.4 <f <0.75. X comprises at least one of O, S, F and Cl. Optionally, X comprises [a]. In some embodiments, X comprises O and / or F. In some embodiments, X comprises O. In some embodiments, X comprises O and does not comprise S, F or Cl. In some embodiments, X consists of O, or X consists of O and [a]. In some embodiments, X comprises O and F. In some embodiments, X comprises O and F, and does not comprise S, or Cl. In some embodiments, X consists of O and F, or X consists of O, F and [a]. In some embodiments, X consists of O and optionally [a]. In some embodiments, X consists of S and optionally [a]. In some embodiments, X consists of F and optionally [a]. In some embodiments, X consists of Cl and optionally [a]. In some embodiments, X consists of O and S and optionally [a]. In some embodiments, X consists of O and F and optionally [a]. In some embodiments, X consists of O and Cl and optionally [a]. In some embodiments, X consists of S and F and optionally [a]. In some embodiments, X consists of S and Cl and optionally [a]. In some embodiments, X consists of O, S and F and optionally [a]. In some embodiments, X consists of O, S and Cl and optionally [a]. In some embodiments, p = 0. In some embodiments, p >0, such as at least 0.01, at least 0.05, at least 0.1, at least 0.2 or at least 0.3. In some embodiments, p is 1. In some embodiments, p <1, such as 0.99 or less, 0.98 or less, 0.97 or less, 0.96 or less, or 0.95 or less. Combinations of these end-points may be combined to form any suitable range. For example, 0<p<1,or0<p< 0.99, or 0.01 <p <0.99, or 0.05 <p <0.99, or 0.1 <p <0.98, or 0.2 <p <0.98, or 0.3 <p <0.95. In some embodiments, q = 0. In some embodiments, q >0 such as at least 0.01, at least 0.02, at least 0.03, at least 0.04 or at least 0.05. In some embodiments, q is 1. In some embodiments, q <1, such as 0.9 or less, 0.5 or less, 0.4 or less, 0.2 or less, or 0.1 or less. Combinations of these end-points may be combined to form any suitable range. For example, 0<q<1,or0<q< 0.9, or 0.01 <q <0.5, or 0.02 <q <0.4, or 0.03 <q <0.5, or 0.04 <q <0.2, or 0.05 <q <0.1. In some embodiments, I = 0. In some embodiments, I >0, such as at least 0.01, at least 0.02, at least 0.03, at least 0.04 or at least 0.05. In some embodiments, I is 1. In some embodiments, I <1, such as 0.9 or less, 0.5 or less, 0.4 or less, 0.2 or less, or 0.1 or less. Combinations of these end-points may be combined to form any suitable range. For example, 0<l<1,or0<l< 0.9, or 0.01 <I <0.5, or 0.02 <I <0.4, or 0.03 <I <0.5, or 0.04 <I <0.2, or 0.05 <I <0.1. In some embodiments, k = 0. In some embodiments, k >0, such as at least 0.01, at least 0.02, at least 0.03, at least 0.04 or at least 0.05. In some embodiments, k is 1. In some embodiments, k <1, such as 0.9 or less, 0.5 or less, 0.4 or less, 0.2 or less, or 0.1 or less. Combinations of these end-points may be combined to form any suitable range. For example, 0<k<1,or0<k< 0.9, or 0.01 <k <0.5, or 0.02 <k <0.4, or 0.03 <k <0.5, or 0.04 <k <0.2, or 0.05 <k <0.1. In some embodiments, the material of the first aspect comprises an anion vacancy [a]. In some embodiments, m = 0. Such embodiments do not have any anion vacancy. In some embodiments, m >0, such as at least 0.01, at least 0.02, at least 0.03, at least 0.04, at least 0.05, or at least 0.08. In some embodiments, m = 0.5. In some embodiments, m <0.5, such as up to 0.48, up to 0.45, up to 0.43 or up to 0.4. Combinations of any of these end-points may be combined to form a suitable range, such as 0 <m <0.5, 0 <m <0.5, 0 <m <0.5, 0.01 <m <0.48, 0.02 <m <0.45, 0.02 <m <0.43, 0.03 <m <0.43, 0.05 <m <0.48, 0.08 <m <0.45, or 0.08 <m <0.4. In some embodiments, m <0.2, or m <0.18, or m <0.15, or m <0.12; or m <0.10. In some embodiments, the material of the first aspect comprises a cation vacancy [c]. In some embodiments, d = 0. Such embodiments do not have any cation vacancy. In some embodiments, d >0, such as at least 0.01, at least 0.02, at least 0.03, at least 0.04, at least 0.05, or at least 0.08. In some embodiments, d = 0.5. In some embodiments, d <0.5, such as up to 0.48, up to 0.45, up to 0.43 or up to 0.4. Combinations of any of these end-points may be combined to form a suitable range, such as 0 <d <0.5, 0 <d <0.5, 0 <d <0.5, 0.01 <d <0.48, 0.02 <d <0.45, 0.02 <d <0.43, 0.03 <d <0.43, 0.05 <d <0.48, 0.08 <d <0.45, or 0.08 <d <0.4. In some embodiments, d <0.2, or d <0.18, or d <0.15, or d <0.12; or d <0.10. In some embodiments, materials of the first aspect may any combination of the above ranges ford and m. For example, m <0.2 and d <0.1, m <0.1 and d <0.2, m <0.15 and d <0.15, m <0.12 and d <0.18, m <0.1 and d <0.1. In some embodiments, p >0. In some such embodiments, e >0 and / or q=l=k=O. In some embodiments, 0 <e <0.75; 0 <f <0.5; 0.01 <g <0.3; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5. In some embodiments, 0 <e <0.65; 0 <f <0.5; 0.01 <g <0.3; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5. In some embodiments, 0 <e <0.65; 0 <f <0.5; 0.01 <g <0.2; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5. In some embodiments, 0 <e <0.65; 0 <f <0.5; 0.01 <g <0.15; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5. In some embodiments, 0 <e <0.6; 0 <f <0.5; 0.01 <g <0.15; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5. In some embodiments, M consists of Mn and q=l=k=O. In some such embodiments: 0 <e <0.75; 0 <f <0.5; 0.01 <g <0.3; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5; or 0 <e <0.65; 0 <f <0.5; 0.01 <g <0.3; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5; or 0 <e <0.65; 0 <f <0.5; 0.01 <g <0.2; 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5; or 0 <e <0.65; 0 <f <0.5; 0.01 <g<0.15;0<d< 0.5; 0.5 <p <1; and 0 <m <0.5; or 0<e<0.6; 0 <f <0.5; 0.01 <g <0.15; 0<d<0.5; 0.5 <p <1; and 0 <m <0.5. In some embodiments, 0 <d <0.3 and 0 <m <0.3; or 0 <d <0.2 and 0 <m <0.2; or 0 <d <0.1 and 0 <m <0.1; or d=m=0. In some embodiments, the material has a face centred cubic (FCC) structure. In such embodiments, the material has a space group Fm-3m. In some embodiments, the material has a unit cell having a dimension (length) of at least a=b=c=4.00 A=0.400 nm, such as at least 0.401 nm (4.01 A), or at least 0.405 nm (4.05 A). In some embodiments, the material has a unit cell dimension of no more than 0.480 nm (4.80 A), no more than 0.470 nm (4.70 A), or no more than 0.465 nm (4.65 A). Suitable combinations of these values may be used to provide exemplary ranges. For example, in some embodiments, the material has a unit cell dimension of in the range of from 0.400 nm (4.00 A) to 0.480 nm (4.80 A), or from 0.405 nm (4.05 A) to 0.470 nm (4.70 A). In these embodiments, o=|3=y=90o. A skilled person will appreciate that disordered rock salt structures have a cubic unit cell size (i.e. a=b=c and a=|3=Y=90o). In some embodiments, any structural disorder will be averaged across the unit cell dimension. In some embodiments, M comprises Li and the material has a unit cell having a dimension (length) of at least 0.410 nm (4.10 A), at least 0.420 nm (4.20 A), or at least 0.425 nm (4.25 A). In some embodiments, the material has a unit cell dimension of no more than 0.450 nm (4.50 A), no more than 0.440 nm (4.40 A), or no more than 0.435 nm (4.35 A). In some embodiments, M comprises Na and the material has a unit cell having a dimension (length) of at least 0.440 nm (4.40 A), at least 0.450 nm (4.50 A), or at least 0.455 nm (4.55 A). In some embodiments, the material has a unit cell dimension of no more than 0.480 nm (4.80 A), no more than 0.470 nm (4.70 A), or no more than 0.465 nm (4.65 A). The structure of the materials may be assessed using XRD techniques. Typically, the materials of the present invention are in the form of a powder, rather than single crystals, and so the structure of the materials may be assessed using powder XRD techniques. Accordingly, where X-ray diffraction or XRD is used herein, it may generally refer to powder X-ray diffraction or powder XRD. The XRD pattern may be measured on any suitable diffractometer. Suitable diffractometers are typically used in reflection geometry. Suitable diffractometers may use CuKa radiation, with an x-ray wavelength of 0.154056 nm (1.54056 A). Suitable diffractometers may operate at 40 kV and 15 mA. A measurement range may be 30-75° 20. Analysis may be performed by any suitable means, such as with appropriate software. For example, a suitable diffractometer may be an Aeris(RTM) Benchtop X-ray diffractometer equipped with a PIXcel(RTM) detector. Any suitable sample preparation method may be used. In some embodiments, the XRD pattern shows a signature of a cubic phase associated with peaks at indicated positions. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has a peak at a 20 value of at least one of: 34.5° ± 5.0°; 39.5 ± 5.0°; 58.5 ± 5.0°; 69.0 ± 5.0° and 70.7 ± 5.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has a peak at a 20 value of two of, or three of, four of or all five of: 34.5° ± 5.0°; 39.5 ± 5.0°; 58.5 ± 5.0°; 69.0 ± 5.0° and 70.7 ± 5.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has peaks at a 20 value of 34.5° ± 5.0°; 39.5 ± 5.0°; 58.5 ± 5.0° and 69.0 ± 5.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has peaks at a 20 value of 34.5° ± 5.0°; 39.5 ± 5.0°; 58.5 ± 5.0°; 69.0 ± 5.0° and 70.7 ± 5.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has a peak at a 20 value of at least one of: 34.5° ± 2.0°; 39.5 ± 2.0°; 58.5 ± 2.0° and 69.0 ± 2.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has a peak at a 20 value of two of, or three of, or four of, or all five of: 34.5° ± 2.0°; 39.5 ± 2.0°; 58.5 ± 2.0°; 69.0 ± 2.0° and 70.7 ± 2.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has peaks at a 20 value of 34.5° ± 2.0°; 39.5 ± 2.0°; 58.5 ± 2.0° and 69.0 ± 2.0°. In some embodiments, an X-ray diffraction pattern of the material using a CuKa radiation source has peaks at a 20 value of 34.5° ± 2.0°; 39.5 ± 2.0°; 58.5 ± 2.0°; 69.0 ± 2.0° and 70.7 ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 36.0° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 36.0° ± 1.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 36.0° ± 0.5°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 36.0° ±0.4°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 41.8° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 41.8° ± 1.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 41.8° ± 0.5°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 41.8° ± 0.4°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 60.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 60.7° ± 1.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 60.7° ± 0.5°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 60.7° ±0.4°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 72.7° ± 2.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 72.7° ± 1.0°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 72.7° ± 0.5°. In some embodiments, an XRD pattern of the material using a CuKa radiation source has a peak at a 20 value of 72.7° ±0.4°. In some embodiments, M consists of Li and an XRD pattern of the material using a CuKa radiation source has three peaks at 20 = 36.0°, 41.8°, 60.7° and 72.7°. In some embodiments, M consists of Na and an XRD pattern of the material using a CuKa radiation source has three peaks at 20 = 33.4°, 38.8°, 56.3°, 67.3° and 70.7° In some embodiments, M consists of Na and an XRD pattern of the material using a CuKa radiation source has three peaks at 20 = 33.3°, 38.8°, 56.2°, 67.2° and 70.6° In some embodiments, an XRD pattern of the material using a CuKa radiation source has an absence of peaks below a 20 value of 30°. In some embodiments, the characteristic XRD pattern has an absence of peaks below a 20 value of 31°, or below a 20 value of 32°, or below a 20 value of 33°, or below a 20 value of 34°, or below a 20 value of 35°. In some embodiments, the characteristic XRD pattern is substantially similar to, or the same as, the XRD pattern shown in any of Figures 1,4 and 7. In some embodiments, the peaks of the characteristic peak XRD pattern with the range 30 to 75° 20 can be indexed to the Miller indices (a) (111); (b) (200) and (c) (220). In some embodiments, the peaks of the characteristic peak XRD pattern with the range 30 to 75° 20 can be indexed to the Miller indices (a) (111); (b) (200); (c) (220) and (d) (311). In some embodiments, the peaks of the characteristic peak XRD pattern with the range 30 to 75° 20 can be indexed to the Miller indices (a) (111); (b) (200); (c) (220); (d) (311) and (e) (222). In some embodiments, the material predominantly comprises a single phase. In some embodiments, the material of the first aspect contains a single phase. In some embodiments, the material of the first aspect consists of a single phase. The single phase may be a disordered rock salt structure. A material predominantly comprising, containing, or consisting of, a single phase may be characterised by an absence of other crystalline phases, such as remaining precursor materials. The absence of other crystalline phases such as remaining precursors may be confirmed by the absence of peaks below 30° 20 in the powder XRD pattern. As such, a material predominantly comprising, containing, or consisting of, a single phase may be characterised by a powder XRD pattern without any peaks below 30° 20. Figure 10 shows a schematic diagram showing the principle of the method of measuring the full-width half-maximum (FWHM) of an XRD peak. The maximum intensity (Imax) of the peak is determined by identifying the peak position at 20 and reading the corresponding intensity value, then subtracting the value of the background. The half-maximum intensity (lmax / 2) can then be determined by dividing the maximum intensity by 2. The width of the peak at this half maximum value is the FWHM value. Typically, this calculation can be done using appropriate software and performing a Rietveld refinement according to standard methods. The reader is directed to e.g. H.M. Rietveld, J. Appt. Cryst. (1969) 2, 65-71. In some embodiments, the FWHM of the material, based on the peaks in an XRD pattern of the material using a CuKa radiation source, is from about 1.0° 20 to about 2.0° 20, or from about 1.10 20 to about 2.0° 20; or from about 1.3° 20 to about 1.9° 20; or from about 1.4° 20 to about 1.8° 20; or from about 1.5° 20 to about 1.7° 20. In some embodiments, the electrochemical properties of the material may be measured. Such measurements may be carried out in a non-aqueous electrolyte containing a Li salt with a Li metal counter / reference electrode. Application of a positive / negative current may be applied and the resulting changes in potential vs Li metal recorded. These measurements may be measured using a cell, such as a coin type cell, assembled in an area containing inert gas, such as argon. Galvanostatic tests may be performed on an appropriate machine, such as a MACCOR, at a desired current density and within a desired voltage range, at any desired temperature and suitable pressure. A suitable pressure may be 101 kPa. Electrodes may be prepared according to any suitable method, and counter electrodes and electrolytes chosen appropriately. These measurements may permit understanding of the electronic structure within the material as a function of the Li content. The resulting voltage vs charge plots may be thought of as correlating to a measurement of the Fermi level within a material as a function of the Li content. These measurements can be used to infer how this material may perform in a practical Li ion cell. Target Composition and Vacancies Materials of the present invention may comprise vacancies, as defined by [c] and [a] in the general formula AX. For convenience, the material may also be described in terms of a target composition rather than the general formula AX. Whereas the general formula AX describes the actual chemical composition of the material and accounts for the presence of vacancies, the target composition is defined by the ratio of elements present in the precursors used to prepare the material. In other words, the target composition is the theoretical composition of the material that would be obtained from a given precursor mixture, assuming that there are no vacancies in the obtained material. For example, the target composition may be defined by the ratio of elements in the precursor mixture provided at (a) in the method of the second or the third aspect disclosed herein. It will be understood that the material obtained by the method of the second or the third aspect may have a chemical composition which is the same as the target composition (i.e. the material may contain no vacancies), or the material may have a composition which is different to the target composition (i.e. the material may contain vacancies). In some embodiments, the presence of cation vacancies [c] in the material can be controlled by appropriate choice of input ratio of M and Sn during preparation of the material. Alternatively, or additionally, the presence of cation vacancies [c] may be induced by a ball milling method, such as a ball milling method described herein, as a result of local imperfections in the resulting crystal. In some embodiments, anion vacancies [a] may be introduced during preparation of the materials, such as during a ball milling step discussed elsewhere herein. In some embodiments, anion vacancies [a] may be introduced during other stages e.g. during fast cooling (quenching) of the materials following heating. In some embodiments, cation vacancies [c] may be introduced by delithiation, such as during use in a lithium-ion battery. In some embodiments, vacancies [a] and [c] may be introduced by careful control of the starting materials. For example, if cation vacancies [c] are desired, using a specific proportion of precursors may suitably be used in a method described elsewhere herein to introduce cation vacancies [c]. For example, a material having a target composition Li2SnO3 and a structure which is a disordered rock salt structure can be prepared by a ball milling method, such as the method of the second or third aspect. An example of a suitable precursor mixture that could be used to prepare this material is shown in Table 1 below. The elements in the precursor mixture are Li, Sn and O and the ratio of the precursors results in a molar ratio of Li, Sn and O of 2:1:3. This molar ratio corresponds to the ratios in the target formula Li2SnO3. The precursor mixture in Table 1 consists of two metal oxide precursors. It will be understood that other combinations of precursors (i.e. different types, different numbers of precursors etc) could be used to prepare a material having this target composition and the skilled person is capable of determining suitable precursor mixtures. Examples of other suitable precursors that could be used include, for example, mixed metal salt precursors or precursors having a composition according to the general formula AX. This will be described in more detail in relation to the methods of the second and third aspects. Table 1 Precursor mixture used to prepare a material having the target composition Li2SnO3. Metal Oxide Mass / g Li2O 0.4944 SnO2 2.5000 In some examples, the chemical composition of the material obtained after ball milling of the precursor mixture shown in Table 1 is the same as the target composition Li2SnO3. According to the general formula AX, a material having the target composition Li2SnO3(i.e. containing no vacancies) has the following values: e = 2 / 3, f = 1 / 3, p = 1, and d=q=l=k=m=0. In some examples, the chemical composition of the material obtained after ball milling of the precursor mixture shown in Table 1 may be LiSn[c]O3 due to the presence of lithium vacancies. According to the general formula AX, a material having the composition LiSn[c]O3 has the following values: e = 1 / 3, f = 1 / 3, d = 1 / 3, p = 1, and q=l=k=m=0. In another example, the actual chemical composition of a material obtained after ball milling of this precursor mixture may be LiSn[c]O2[a] due to the presence of lithium vacancies (i.e. cation vacancies) and oxygen vacancies (i.e. anion vacancies). According to the general formula AX, a material having the composition LiSn[c]O2[a] has the following values: e = 1 / 3, f = 1 / 3, d = 1 / 3, p = 2 / 3, m = 1 / 3 and q=l=k=0. The above examples are illustrative, and it will be understood that the actual chemical composition of the material prepared from a given precursor mixture may vary due to the presence of different amounts and types of vacancies. The vacancies may include cation vacancies, due to the absence of Li and / or Sn compared with the target composition. Additionally, or alternatively, the vacancies may include anion vacancies, due to the absence of O, S, F and / or Cl compared with the target composition. As described above, the target composition of the precursor mixture in Table 1 is Li2SnO3 (having the following values in the general formula AX: e = 2 / 3, f = 1 / 3, p = 1, and d=q=l=k=m=0). The chemical composition of materials obtainable from ball milling of the precursor mixture in Table 1 may be defined by the general formula AX (in order to account for the presence of vacancies), wherein: 1 / 6 <e <2 / 3; 0 <f <1 / 3; and 0 <d <0.5; 0.5 <p <1; and 0 <m <0.5. Second Aspect In some embodiments of the second aspect, when more than one precursor is used, the salt (i.e. O, S, F or Cl) may be the same or different among the precursors. In some embodiments, at least one salt is an oxide. In some embodiments, at least one salt is a sulfide. In some embodiments, at least one salt is fluoride. In some embodiments, at least one salt is a chloride. In order to prepare materials according to the first aspect, the precursor mixture provided at (a) comprises a suitable combination of precursors selected from: lithium salt precursors, tin salt precursors and mixed metal salt precursors. The combination of precursors (and their relative ratios in the precursor mixture) is selected to achieve the ratio of M, Sn, O, S, F and Cl in the target composition. In some embodiments, the precursor mixture comprises at least one tin salt precursor; and at least one lithium salt precursor and / or at least one sodium salt precursor. In some embodiments, the precursor mixture comprises at least one mixed metal salt precursor. Materials according to the first aspect comprise tin. Therefore, the precursor mixture provided at (a) must comprise at least one tin salt precursor and / or at least one tin-containing mixed metal salt precursor. Suitable tin salt precursors may be selected from: SnO, SnO2, SnS, SnS2, SnF2, SnF4, SnCh, and SnCk. In some such embodiments, the precursor mixture comprises one or more tin salt precursors selected from: SnO and SnO2. Suitable tin-containing mixed metal salt precursors may be selected from Li2SnO3, Li2SnS3, Li2SnF6, Li2SnCl6, Na2SnO3, Na2SnS3, Na2SnFe and Na2SnCle. In some embodiments, the precursor mixture comprises Li2SnOs and / or Na2SnO3. The tin-containing mixed metal salt precursor may have a layered structure. In some embodiments, M comprises or consists of Li and the precursor mixture comprises at least one lithium salt precursor. The at least one lithium salt precursor may be selected from: U2O, Li2S, LiF and LiCL In some such embodiments, the lithium salt precursor is at least one of Li2O and LiF. In some such embodiments, the lithium salt precursor consists of Li2O and LiF. In some embodiments, the lithium salt precursor comprises Li2O. In some embodiments, the lithium salt precursor consists of Li2O. In some embodiments, the precursor mixture comprises at least one lithium-containing mixed metal salt. In some embodiments, the lithium-containing mixed metal salt is Li2SnO3. In some embodiments, the precursor mixture comprises, or consists of, Li2SnO3. In some such embodiments, Li2SnO3 provided at (a) has a structure other than a disordered rock salt structure, such as a layered structure or a spinel structure. In some embodiments, M comprises, or consists of, Na and the precursor mixture comprises at least one sodium salt precursor. The at least one sodium salt precursor may be selected from: Na2O, Na2S, NaF and NaCL In some such embodiments, the sodium salt precursor is at least one of Na2O and NaF. In some such embodiments, the sodium salt precursor consists of Na2O and NaF. In some embodiments, the sodium salt precursor comprises Na2O. In some embodiments, the sodium salt precursor consists of Na2O. In some embodiments, the precursor mixture comprises at least one sodium-containing mixed metal salt. In some embodiments, the sodium-containing mixed metal salt is Na2SnO3. In some embodiments, the precursor mixture comprises, or consists of, Na2SnO3. In some such embodiments, Na2SnOs provided at (a) has a layered structure. In some embodiments, M comprises Li and the precursor mixture comprises a suitable combination of precursors selected from: lithium salts, tin salts and mixed metal salts. In some embodiments, M consists of Li and the precursor mixture consists of a suitable combination of precursors selected from: lithium salts, tin salts and mixed metal salts. In some embodiments, M comprises Na and the precursor mixture comprises a suitable combination of precursors selected from: sodium salts, tin salts and mixed metal salts. In some embodiments, M consists of Na and the precursor mixture consists of a suitable combination of precursors selected from: sodium salts, tin salts and mixed metal salts. In some embodiments, M is a combination of Na and Li, and the precursor mixture comprises a suitable combination of precursors selected from: lithium salts, sodium salts, tin salts and mixed metal salts. In some embodiments, the precursor mixture comprises a lithium salt and a tin salt. In some embodiments, the precursor mixture comprises a sodium salt and a tin salt. In some embodiments, the precursor mixture comprises a lithium salt, a sodium salt and a tin salt. In some embodiments, the lithium salt is Li2O. In some embodiments, the sodium salt is Na2O. In some embodiments, the tin salt is SnO2. In some embodiments, the method comprises, at step (a), providing a precursor mixture comprising M and Sn in a 2:1 ratio. In some embodiments, the method comprises, at step (a), providing a precursor mixture comprising Li and Sn in a 2:1 ratio. In some embodiments, the method comprises, at step (a), providing a precursor mixture comprising Na and Sn in a 2:1 ratio. In some embodiments of the second aspect, the target composition is LixNa2-xSnO3, wherein 0 <x <2. In such embodiments, the method comprises, at step (a), providing a precursor mixture comprising Li, Na and Sn in the ratio x:(2-x):1. In some embodiments, 1 <x <2, such as 1.1 <x <2, or 1.5 <x <2. In some embodiments, 0 <x <1, such as 0 <x <0.9, or 0 <x <0.5. In some embodiments, the method of the second aspect comprises solid state synthesis prior to providing the precursor mixture at (a). The solid state synthesis may be used to prepare at least one mixed metal salt precursor to be provided in the precursor mixture at (a). Solid state synthesis of mixed metal salts is known in the art. The solid state synthesis comprises (at step i) providing a precursor mixture comprising at least two of: Li, Na and Sn. As such, the solid state synthesis may be used to prepare mixed metal salts comprising: Li and Na, Li and Sn; Na and Sn; or Li, Na and Sn. In some embodiments, the solid state synthesis prepares at least one mixed metal salt. In some embodiments, the at least one mixed metal salt is selected from: Na2SnO3 and Li2SnO3. Third Aspect In the third aspect, the precursor mixture comprises or consists of a precursor having the general formula AX. As with the materials according to the first aspect, the precursor may also be described in terms of its chemical composition (e.g. using the general formula, AX) and its structure, such as its crystal structure. The precursor may have a structure which is a disordered rock salt structure, or a structure other than a disordered rock salt structure. In some embodiments, the structure other than a disordered rock salt structure is a layered structure or a spinel structure. It will be understood that the features of the general formula AX described with respect to the material of the first aspect apply equally to the general formula of the precursor used in the method of the third aspect. In some embodiments, the precursor used in the method of the third aspect and the material obtained from the method of the third aspect both have a chemical composition according to the general formula AX, but a different structure (e.g. crystal structure). In some embodiments, the precursor has a composition defined by the general formula AX and a structure which is a disordered rock salt structure. In some embodiments, the precursor mixture provided at (a) consists of the precursor having a disordered rock salt structure. In such embodiments, the precursor having a disordered rock salt structure provided at (a) and the material obtained after the ball milling step (b) have the same target composition (i.e. ratio of elements as defined by the general formula AX). In some examples, vacancies may be introduced during the ball milling step (b) such that the precursor and the material obtained after ball milling have a different amount of vacancies. In some examples, another property may be altered during the ball milling step (b), such as the crystallite size or morphology. For example, the precursor and the material obtained after ball milling may have a different amount of vacancies and / or a different crystallite size and / or a different crystal morphology. In some embodiments, the precursor mixture provided at (a) comprises a precursor having a disordered rock salt structure and one or more additional precursors, such as one or more additional precursors having the general formula AX, or one or more of the precursors described in relation to the second aspect, such as lithium salt precursors, tin salt precursors and mixed metal salt precursors. In such embodiments, the material obtained after the ball milling step (b) will have a target composition based on all of the precursors making up the precursor mixture. Thus, in these embodiments, the precursor having a disordered rock salt structure provided at (a) and the material obtained after the ball milling step (b) have a different composition. In some embodiments, the precursor has a composition defined by the general formula AX and a structure other than a disordered rock salt structure, such as a layered structure or a spinel structure. In such embodiments, the ball milling step (b) converts the structure of the precursor from a structure other than a disordered rock salt structure into a rock salt structure, thus obtaining a material having a composition defined by the general formula AX and a structure which is a disordered rock salt structure. Methods of making a precursor having the general formula AX and a structure other than a disordered rock salt structure are known in the art. Such a precursor may be made by solid state synthesis. The solid state synthesis may be used to prepare a precursor with a particular target composition. In some embodiments, the precursor mixture provided at (i) comprises M and Sn in the ratio required for a target composition of Li2SnO3. In some embodiments, the precursor mixture provided at (i) comprises M and Sn in the ratio required for a target composition of Na2SnO3. Solid State Synthesis Some embodiments of the second or the third aspect comprise solid state synthesis. Solid state synthesis comprises one or more heating steps. The heating steps are carried out at high temperatures. For example, the heating step may be a calcination at a temperature in the range of from 500 °C to 1000 °C. In some such embodiments, the temperature may be in the range of from 600 °C to 1000 °C, or 700 °C to 1000 °C, or 800 °C to 1000 °C, or 850 °C to 1000 °C. When more than one heating step is included, the temperatures used in each heating step may be the same or different. In some embodiments, the precursors are mixed prior to the heating step of the solid state synthesis. The mixing may be carried out by hand (e.g. with a pestle and mortar) or it may involve ball milling. In some examples, the speed of ball milling used to mix the precursors prior to step (ii) if the solid state synthesis may be lower than the speed used in the ball milling step (b). In some embodiments, the precursor mixture provided at (i) in the solid state synthesis further comprises at least one solvent. The addition of solvent may prevent caking of the precursor mixture, which can occur during ball milling of dry precursors. In the solid state synthesis, the salt of the at least one salt precursor may be or may comprise an oxide, a sulfide, a fluoride, a chloride, a carbonate, a sulfate, a nitrate, an oxalate or a hydroxide. As the solid state synthesis involves a heating step, salts such as carbonates, sulfates, nitrates, oxalates and hydroxides may be converted to oxides. For example, an oxide may be generated from a carbonate salt by evolution of carbon dioxide gas. When the precursors are mixed by ball milling prior to step (ii) of the solid state synthesis, the solid state synthesis comprises, first, providing a precursor mixture comprising M and Sn; then, ball milling the precursor mixture for a period of time; and then, heating the precursor mixture for a period of time. In some embodiments of the second or the third aspect, the precursor mixture provided at (i) for solid state synthesis comprises one or more carbonate salts. In some such embodiments, the one or more carbonate salts are provided in a slight excess with respect to the amount of the one or more carbonate salts required by the target composition. For example, the carbonate salt may be a lithium carbonate and may be provided to give an excess of Li in the precursor mixture compared with the amount of Li required by the target composition. The slight excess may account for loss of the carbonate salt during the solid state synthesis. For example, the one or more carbonate salts may be provided in an excess of 1 wt%, or 2 wt%, or 3 wt%, or 4 wt%, or 5 wt%. In some examples, the excess may be a molar excess of 1 mol%, or 2 mol%, or 3 mol%, or 4 mol%, 5 mol%. When the method of the second or the third aspect comprises solid state synthesis, the mixture obtained after the solid state synthesis may comprise one or more precursors that were provided in the solid state synthesis precursor mixture at (i) and remain unreacted after the heating step (ii). Such unreacted precursors may be referred to as residual precursors. In some examples of the second aspect, the mixture obtained after the solid state synthesis comprises residual precursors and one or more mixed metal salt precursors having a layered structure. In some examples of the third aspect, the mixture obtained after the solid state synthesis comprises residual precursors and one or more precursors having the general formula AX. In some such embodiments, it may be advantageous to minimise the amount of residual precursors present after solid state synthesis in order to successfully prepare the material according to the first aspect. In some embodiments, the mixture obtained after the solid state synthesis comprises up to 10 wt% residual precursors; or up to 5 wt%; or up to 3 wt%; or up to 2 wt%; or up to 1 wt%. Delithiation In some embodiments of the method of the second and third aspect, the method further comprises delithiating the material obtained after step (b). In some embodiments, delithiating the material comprises applying a potential difference to an electrode comprising a material of the first aspect, so as to cause delithiation of the material originally produced following step (b). In some embodiments, delithiation can be achieved using chemical means, such as with iodine. In some embodiments, the delithiating comprises reducing the amount of lithium in the material. In some embodiments, the delithiating is carried out until e=0. In some embodiments, the delithiating is carried out until the amount of lithium is reduced, but e >0. Ball Milling Step (b) of the methods according to the second and third aspects comprises ball milling the precursor mixture for a period of time. Unless otherwise stated, the following description of the ball milling in step (b) applies equally to step (b) of the second and third aspects. In some embodiments, in step (b), the period of time is measured in hours (h). In some embodiments, in step (b), the period of time is at least 1 h, such as at least 3 h, at least 5 h, at least 7 h, or at least 10 h. In some embodiments, in step (b), the period of time is up to 300 h, such as up to 280 h, up to 250 h, up to 220 h, up to 200 h, or up to 150 h. Combinations of any of these values may be used to provide exemplary ranges for the period of time. For example, the period of time may be in the range of from 1 h to 300 h, from 3 h to 280 h, from 3 h to 250 h, from 7 h to 200 h, or from 10 h to 150 h. In some embodiments, in step (b), the period of time corresponds with a time at which no crystalline precursor peaks can be observed in a product of the ball milling, the crystalline precursor peaks being determined by taking an XRD pattern using a CuKa radiation source as described elsewhere herein. In some embodiments, the period of time is less than 100 h, or less than 90 h, or less than 80 h. In some embodiments, the period of time is in the range of from 1 h to 100 h, or from 10 h to 100 h, or from 40 to 80 h, or from 40 h to 70 h. In some embodiments, the period of time is in the range of from 45 to 65 h, or is about 60 h. For example, ball milling may be performed for a certain time, then paused, to form a single cycle. In some embodiments, the period of time may correspond to the total milling time taken for a particular number of such cycles, excluding any pauses. In some embodiments, there is no pause. In some embodiments, in step (b), the ball milling is carried out at a speed of 150 revolutions per minute (rpm) or more, such as 250 rpm or more, 350 rpm or more, 450 rpm or more, or 500 rpm or more such as 550 rpm. In some embodiments, in step (b), the ball milling is carried out at a speed of 1000 rpm or less, such as 950 rpm or less, 900 rpm or less, 850 rpm or less, or 800 rpm or less. Combinations of any of these values may be used to provide exemplary ranges for the ball milling speed. For example, ball milling may be carried out at a speed in the range of from 150 rpm to 1000 rpm, or from 250 rpm to 950 rpm, or from 350 rpm to 900 rpm, or from 450 rpm to 850 rpm, or from 550 rpm to 800 rpm. In some embodiments, in step (b), a speed of the ball milling may be varied over the period of time. For example, ball milling may be carried out at 350 rpm for part of the period of time and increased to one or more higher speeds for the remainder of the period of time. In some embodiments, in step (b), a milling media is used to assist or cause the mechanochemical reaction. The milling media may comprise beads and / or balls. The milling media may be placed inside the ball mill to affect, cause or enhance the mechanochemical reaction. In some embodiments, the milling media has a diameter in the range of from 0.6-20 mm, such as from 3-10 mm, such as from 4-7 mm. Any suitable known milling media may be used, for example yttrium stabilized ZrO2 (YSZ). In some embodiments, in step (b), a weight ratio of the precursor mixture to the milling media is at least 1:1, such as at least 1:2, at least 1:4, at least 1:5, at least 1:6, or at least 1:7. The ratio refers to the sum of the weight of all of the precursors in the precursor mixture provided at (b). In some embodiments of the third aspect, in step (b), the weight ratio of the precursor having the general formula AX, or the precursor mixture comprising a precursor having the general formula AX, to the milling media is up to 1:100, up to 1:50, up to 1:20, or up to 1:10. Combinations of any of these values may be used to provide exemplary ranges for the weight ratio. For example, the weight ratio may be 1:1 to 1:100,1:2 to 1:100,1:4 to 1:50, 1:5 to 1:20 or 1:7 to 1:10. In some embodiments, in step (b), the ball milling is carried out in a milling jar which is formed from a material comprising ZrO2, stainless steel, agate (SiO2), SiC, or WC. Such milling jars are considered to be capable of achieving the mechanochemical reaction while not causing unwanted contamination. In some embodiments, a hard and / or dense material is chosen for the jar material. In some embodiments, the milling jar is formed from a material comprising or consisting of ZrO2 or stainless steel. In some embodiments, the milling jar is formed from ZrO2. The precursor mixture may be added to the jar under an inert atmosphere. For example, the precursor mixture may be added to the jar in an argon-filled glovebox. In some embodiments, the milling jar has a volume of at least 20 mL, at least 45 mL, at least 50 mL, at least 80 mL or at least 100 mL. In some embodiments, the milling jar has a volume of at least 250 mL or at least 500 mL. In some embodiments, a larger milling jar may allow a lower rpm to be used to achieve the mechanochemical synthesis. In some embodiments, variation of the ball milling conditions in step (b) may provide the materials with one or more anion vacancies. In some embodiments, one or more anion vacancies can be introduced by considering aspects of the ball milling conditions of step (b) such as the period of time or speed of ball milling. In some embodiments, anion and / or cation vacancies can be measured by X-ray absorption spectroscopy (XAS). Fitting the extended X-ray absorption fine structure (EXAFS) region results in defining coordination numbers of scattering shells surrounding the central absorbing atom. The number of vacancies can be determined by comparing the expected coordination number (based on the chemical composition) and the actual coordination number determined by EXAFS. Further characterisation techniques, such as neutron diffraction can also be used. In some embodiments, the amount of anion vacancies can be measured using TGA or XPS. In some embodiments, the amount of anion vacancies can be measured using TGA. In some such embodiments, the material may be heated in air at any appropriate heating rate. In principle, any anion vacancies are filled by oxide anions from air during the heating process. Therefore, a mass increase recorded by the TGA in the region 0 to 450°C may be ascribed to the filling of anion vacancies, and thus the amount of vacant anion sites may be calculated. In some embodiments, the mass increase is the maximum mass increase, which may be identified by a peak position in that region. In some embodiments, other subsequent mass changes may be ascribed to thermal decomposition processes, such as mass changes occurring above 450°C or above 500°C. In some embodiments, the mass change is calculated from the peak position, or the largest mass change observed in the region 0 to 450°C. In some embodiments, the amount of anion vacancies that can be measured using TGA is the amount of oxygen anion vacancies. In some embodiments, induced oxygen deficiency may be calculated from TGA. In some embodiments, the calculation of induced oxygen deficiency may involve converting the wt% mass change to O2 (g), and using oxygen deficiency = (O2 mass x 2) / (mol[AX] x molecular mass [AX]). Corresponding calculations may be done for F, S and Cl. Fifth Aspect The fifth aspect provides an electrode comprising a material according to the first aspect, or a material prepared by a method according to the second or the third aspect. In some embodiments, the electrode comprises additives and / or a binder. In some embodiments, the electrode is an anode. In some embodiments, the electrode comprising the material of the present invention comprises three fractions: a first fraction consisting of one or more materials according to the present invention; a second fraction consisting of one or more additives; and a third fraction consisting of one or more binders. The first fraction may be present in an amount in the range of from 60-98 wt% with respect to the total combined weight of the three fractions. In some embodiments, the material is present at about 70 wt%, or 75 wt%, 80 wt%, 90 wt% and 95 wt%). The first fraction may consist of one material according to the present invention. In some embodiments, the first fraction consists of one material according to the first aspect, or one material prepared by a method according to the second or the third aspect. The second fraction of the electrode comprises additives such as carbon, for example, carbon black. In the electrode, the second fraction may make up 50-90 % of the remaining mass of the electrode, excluding the mass of the first fraction. For example, when the first fraction is present in an amount of 80 wt% relative to the total mass of the electrode, the second fraction may be present in an amount in the range of from 10 wt% to 18 wt%. In some embodiments, the second fraction may be present in an amount of at least 1 wt%, based on the total mass of the electrode (the total mass of the electrode being 100 wt%), for example at least 2 wt%, at least 3 wt%, or at least 4 wt%. In some embodiments, the second fraction may be present in an amount of up to 20 wt%, based on the total mass of the electrode, such as up to 15 wt%, or up to 12 wt%. Combinations of any of these values may be used to provide exemplary ranges. For example, in some embodiments the second fraction is present in the electrode in an amount in the range of from 1-20 wt%, such as from 1-12 wt%, 2-15 wt%, 2-15 wt%, 3-15 wt%, 4-20 wt%, or 4-12 wt%. The third fraction of the electrode comprises binders. In some embodiments, the third fraction comprises one or more binders selected from: a solvent, a plasticiser and a polymeric binder. In some embodiments, the third fraction may comprise or consist of one or more selected from the group of: poly(vinylidene difluoride) (PVDF), poly(methyl methacrylate) (PMMA), carboxymethyl cellulose (CMC), poly(ethylene oxide) (PEO), or polytetrafluoroethylene (PTFE). In some embodiments, the third fraction makes up the remaining mass of the electrode (i.e. the electrode consists of the three fractions described above). In some embodiments, the electrode comprises the three fractions described above along with further components. In some embodiments, the third fraction may be present in an amount of at least 1 wt%, based on the total mass of the electrode (the total mass of the electrode being 100 wt%), for example at least 2 wt%, at least 3 wt%, or at least 4 wt%. In some embodiments, the third fraction may be present in an amount of up to 20 wt%, based on the total mass of the electrode, such as up to 15 wt%, or up to 12 wt%. Combinations of any of these values may be used to provide exemplary ranges. For example, in some embodiments the third fraction is present in the electrode in an amount in the range of from1-20 wt%, such as from 1-12 wt%, 2-15 wt%, 2-15 wt%, 3-15 wt%, 4-20 wt%, or 4-12 wt%. In some embodiments, the second fraction and the third fraction are included in the electrode in roughly equal amounts. Further Aspects The sixth aspect provides an electrochemical cell comprising a material according to the first aspect, or a material prepared by the method of the second or the third aspect, or an electrode according to the fifth aspect. In some embodiments the electrochemical cell comprises a first electrode and a second electrode, wherein the first electrode is an anode according to the fifth aspect and the second electrode is a cathode. In some embodiments, the electrochemical cell may also comprise an electrolyte between the cathode and the anode. In some embodiments, the electrolyte is a liquid electrolyte. In some embodiments, the liquid electrolyte comprises or is a solution comprising an alkali metal salt. For example, the alkali metal salt may comprise LiPFe and / or NaPFe. In some embodiments, the alkali metal salt may comprise or consist of a lithium salt. The lithium salt may comprise one or more of LiPFe, LiBF4, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate and lithium bis(oxalato) borate. In some embodiments the electrochemical cell comprises an electrode according to the invention laminated with a current collector, for example a metallic foil. The seventh aspect provides an electrochemical energy storage device comprising an electrochemical cell according to the sixth aspect. In some embodiments, the electrochemical energy storage device is a battery. In some embodiments, the electrochemical energy storage device is a lithium-ion battery. The eighth aspect provides a use of a material according to the first aspect in an electrode, or in an electrochemical cell, or in an electrochemical energy storage device. EXAMPLES The present invention will now be described by way of examples, which are intended to be illustrative and not limiting on the present invention. Examples 1 to 3 are examples of methods a making a material according to the first aspect. The following materials were prepared using ball milling using magnesia stabilised zirconia (MgSZ) jars with 5 mm diameter yttria stabilised zirconia (YSZ) balls as the milling media. A 10:1 ratio by mass of the milling media to the precursors was used. The Milling was performed in a Fritsch(RTM) Pulverisette(RTM) 7. PXRD patterns were measured with an Aeris(RTM) Benchtop X-ray diffractometer equipped with a PIXcel(RTM) detector in reflection geometry using CuKa radiation, with an x-ray wavelength of 0.154056 nm (1.54056A), operating at 40 kV and 15 mA. The measurement range was 30-75° 20. SEM and EDX was performed on a Phenom XL G2 Desktop SEM from Thermofisher Scientific(RTM). For electrochemical testing, the prepared materials were mixed inside a glovebox with a carbon conductive additive and PTFE as binder in a weight ratio of 80:10:10. The charge discharge curve was measured from a standard CR 2032 coin type cell assembled in an argon filled glove box (H2O content <1 ppm). The charge-discharge cycle tests were performed at a current density of 30 mA / g cycled between 4.8 - 0.1 V vs. Li / Li+ or Na / Na+ on a MACCOR. 1 M LiPFe or NaPFe solutions were obtained by dissolving in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1 was used as electrolyte. Electrodes were prepared by combining the material, carbon black, and PTFE respectively in the ratio 80:10:10 into a pellet. Metallic lithium was used as the counter electrode. The chargedischarge cycle tests were performed at 30°C (303 K) unless stated otherwise, and ambient pressure (101 kPa). Example 1 In this example, a material with a target composition of Li2SnO3 and disordered rock salt structure was prepared. The method comprised solid state synthesis to prepare a precursor having the target composition and a layered structure. A precursor mixture consisting of the following precursors was provided for solid state synthesis: Li2CO3 (2.0457 g); SnO2 (4.1727 g). The precursors were mixed manually with a pestle and mortar and then fired in a muffle furnace, in an alumina crucible, under air. The precursor mixture was calcined using the following heating profile: - the temperature was increased from ambient temperature to reach 800°C, at a rate of 5 °C / min; - the temperature was maintained at 800°C for 12 hours; - the temperature was reduced to ambient temperature at a rate of 10 °C / min (until natural cooling is not sufficient enough to follow this trend). The material was collected and grinded with pestle and mortar. The composition and structure of the material were analysed using pXRD analysis. The analysis confirmed the presence of layered Li2SnO3 (97.8 wt%) and residual SnO2 (2.2 wt%). A precursor mixture consisting of this material was used to prepare the material having the disordered rock salt structure. A 20 mL magnesia stabilised zirconia (MgSZ) jar was filled with 40 g of milling media consisting of yttria stabilised zirconia (YSZ) balls of 5 mm diameter. 4 g of the above precursor mixture was added to the jar (on top of the balls) to achieve a 10:1 ball to powder ratio. Loading of the precursors was done in an argon filled glovebox, to ensure an inert environment. The jar was then sealed and removed from the glovebox for milling to be performed. The jar was milled at 400 rpm for 5 hours (10 cycles; 1 cycle = 30 minutes milling and 30 minutes break). The jar was returned to the glovebox for sampling and manual homogenisation of the material. Then the jar was further milled at 700 rpm for 10 hours (8 cycles; 1 cycle = 75 minutes milling and 15 minutes break). The jar was returned again to the glovebox for sampling and manual homogenisation of the material. This process was repeated as many times as necessary to obtain a material having a DRS structure. In this example, obtaining the material having a DRS structure required a total of 25 hours of ball milling. After obtaining the material having the DRS structure, the material (in the form of a powder) was collected by sieving and stored under inert atmosphere (argon). Powder XRD analysis of the final samples gave the pattern shown in Figure 1 and revealed four peaks, at 20 = 35.9563°, 41.7873°, 60.6648°, and 72.6672°. These peaks were indexed (h,k,l) as (1,1,1), (2,0,0), (2,2,0), and (3,1,1) respectively. This matched a cubic structure of the space group Fm-3m, which is a disordered rock salt structure. Rietveld refinement analysis was used to calculate the FWHM and unit cell values. The FWHM was calculated to be 1.5894° 20. The unit cell length was calculated to be 4.3056 A. No evidence of crystalline precursors was observed. Figure 2a shows an SEM image of the final sample, showing a particulate material. Figures 2b and 2c show EDX maps of oxygen and tin, respectively. These EDX maps show a homogeneous distribution of the metals throughout the sample (the lighter areas indicate the presence of the respective elements). The peaks in the EDX spectrum of the material shown in Figure 2d correspond to the characteristic peaks for oxygen and tin, and are labelled accordingly. There is no peak associated with lithium, as the energy associated with lithium is too low to be detected. The EDX spectrum also indicates the absence of any other elements i.e. the material is free from other contaminants. Powder electrodes were made to test electrochemical properties. They consist of a mixture of active material (AM) which is the material to be tested, carbon black, and PTFE respectively in the ratio 80:10:10. That mixture was pressed together in a pellet and assembled in a half cell in a coin cell format. Two different tests were performed: 1. Cycling against lithium: Whatman glass fibre was used as the separator, lithium metal foil was used as the anode. The electrolyte consisted of 1M LiPF6 salt dissolved in a mixture of solvent EC / DMC in a ratio of 1:1. 2. Cycling against sodium: Whatman glass fibre was used as the separator, sodium metal foil was used as the anode. The electrolyte consisted of 1M NaPFe salt dissolved in a mixture of solvent EC / DMC in a ratio of 1:1. Galvanostatic testing was performed on half cells between 4.8 - 0.1 V (vs Li as described above) at 30°C and showed a capacity of first discharge of 450 mAh / g against Li. The charge-discharge curves for the first 5 cycles against Li are shown in Figure 3. Example 2 In this example, a material with a target composition of Na2SnO3 and disordered rock salt structure was prepared. The method comprised solid state synthesis to prepare a precursor having the target composition and a layered structure. In this example, the solid state synthesis comprised two heating stages (calcinations). A precursor mixture consisting of the following precursors was provided for solid state synthesis: Na2CO3 (2.6162 g); SnO2 (3.5429 g). This represents a 5 wt% excess of Na2CO3 compared to the stoichiometric amount required by the target composition Na2SnO3. This excess was used to compensate possible loss during the solid-state synthesis. The precursors were mixed manually with a pestle and mortar and then fired in a muffle furnace, in an alumina crucible, under air. A first calcination was carried out, using the following heating profile: - the temperature was increased from ambient temperature to reach 800°C, at a rate of 5 °C / min; - the temperature was maintained at 800°C for 12 hours; - the temperature was reduced to ambient temperature at a rate of 10 °C / min (until natural cooling is not sufficient enough to follow this trend). The material was collected and grinded with pestle and mortar. A second calcination was then carried out, using the following heating profile: - the temperature was increased from ambient temperature to reach 900°C, at a rate of 5 °C / min; - the temperature was maintained at 900°C for 12 hours; - the temperature was reduced to ambient temperature at a rate of 10 °C / min (until natural cooling is not sufficient enough to follow this trend). The material was collected and grinded with pestle and mortar. The composition and structure of the material were analysed using pXRD analysis. The analysis confirmed the presence of layered Na2SnO3 and showed no evidence of residual SnO2. A precursor mixture consisting of this material was used to prepare the material having the disordered rock salt structure. A 20 mL magnesia stabilised zirconia (MgSZ) jar was filled with 40 g of milling media consisting of yttria stabilised zirconia (YSZ) balls of 5 mm diameter. 4 g of the above precursor mixture was added to the jar (on top of the balls) to achieve a 10:1 ball to powder ratio. Loading of the precursors was done in an argon filled glovebox, to ensure an inert environment. The jar was then sealed and removed from the glovebox for milling to be performed. The jar was milled at 700 rpm for 10 hours (8 cycles; 1 cycle = 75 minutes milling and 15 minutes break). The jar was returned to the glovebox for sampling and manual homogenisation of the material. Then the jar was further milled at 700 rpm for 10 hours (8 cycles; 1 cycle = 75 minutes milling and 15 minutes break). The jar was returned again to the glovebox for sampling and manual homogenisation of the material. This process was repeated as many times as necessary to obtain a material having a DRS structure. In this example, obtaining the material having a DRS structure required a total of 20 hours of ball milling. After obtaining the material having the DRS structure, the material (in the form of a powder) was collected by sieving and stored under inert atmosphere (Argon). Powder XRD analysis of the final samples gave the pattern shown in Figure 4 and revealed five peaks, at 20 = 33.3715°, 38.8037°, 56.2809°, 67.267° and 70.723. These peaks were indexed as (1,1,1), (2,0,0), (2,2,0), (3,1,1) and (2,2,2) respectively. This matched a cubic structure of the space group Fm-3m, which is a disordered rock salt structure. Rietveld refinement analysis was used to calculate the FWHM and unit cell values. The FWHM was calculated to be 1.5263° 20. The unit cell length was calculated to be 4.5905 A. No evidence of crystalline precursors was observed. Figure 5a shows an SEM image of the final sample, showing a particulate material. Figures 5b, 5c and 5d show EDX maps of sodium, oxygen and tin, respectively, in the material. These EDX maps show a homogeneous distribution of the metals throughout the sample (the lighter areas indicate the presence of the respective elements). The peaks in the EDX spectrum of the material shown in Figure 5e correspond to the characteristic peaks for carbon, oxygen, sodium and tin, and are labelled accordingly. For EDX analysis, the sample is deposited onto a carbon tape which gives rise to the peak labelled as carbon i.e. the peak labelled as carbon does not originate from the sample. The EDX spectrum also indicates the absence of any other elements i.e. the material is free from other contaminants. Powder electrodes were made to test electrochemical properties. They consist of a mixture of active material (AM) which is the material to be tested, carbon black, and PTFE respectively in the ratio 80:10:10. That mixture was pressed together in a pellet and assembled in a half cell in a coin cell format. Two different tests were performed: 1. Cycling against lithium: Whatman glass fibre was used as the separator, lithium metal foil was used as the anode. The electrolyte consisted of 1M LiPF6 salt dissolved in a mixture of solvent EC / DMC in a ratio of 1:1. 2. Cycling against sodium: Whatman glass fibre was used as the separator, sodium metal foil was used as the anode. The electrolyte consisted of 1M NaPFe salt dissolved in a mixture of solvent EC / DMC in a ratio of 1:1. Galvanostatic testing was performed on half cells between 4.8 - 0.1 V (vs Li or Na as described above) at 30°C and showed a capacity of first discharge of 45 mAh / g against Li and 21 mAh / g against Na. The charge-discharge curves for the first 5 cycles against Li and Na are shown in Figure 6a and 6b, respectively. Example 3 In this example, a material with a target composition of Na2SnO3 and disordered rock salt structure was prepared. The method comprised solid state synthesis to prepare a precursor having the target composition and a layered structure. In this example, the precursors were mixed by ball milling prior to the heating step of the solid state synthesis. A precursor mixture consisting of the following precursors was provided for solid state synthesis: Na2CO3 (4.1859 g); SnO2 (5.6687 g). This represents a 5 wt% excess of Na2CO3 compared to the stoichiometric amount required by the target composition Na2SnO3. This excess was used to compensate possible loss during the solid-state synthesis. The precursors were mixed using ball milling by filling a 50 mL yttria stabilised zirconia (YSZ) jar with 80 g of milling media consisting of yttria stabilised zirconia (YSZ) balls of 5 mm diameter. 15 mL of isopropyl alcohol (IPA) was added to the jar. The above precursor mixture was added to the jar (on top of the balls) to achieve a 10:1 ball to powder ratio. Loading of the precursors was done in an argon filled glovebox, to ensure an inert environment. The volume of IPA added to the jar was sufficient to cover the milling media and the precursor mixture. The jar was then sealed and removed from the glovebox for milling to be performed. The jar was milled at 400 rpm for 10 hours (60 cycles; 1 cycle = 10 minutes milling and 20 minutes break). After milling, a slurry of the precursor mixture and the solvent was obtained. The slurry was sieved to separate it from the milling media. The sieved slurry was dried overnight (18 hours at 50°C) to remove the IPA. The dried precursor mixture was fired in a muffle furnace, in an alumina crucible, under air. The precursor mixture was calcined using the following heating profile: - the temperature was increased from ambient temperature to reach 800°C, at a rate of 5 °C / min; - the temperature was maintained at 800°C for 12 hours; - the temperature was reduced to ambient temperature at a rate of 10 °C / min (until natural cooling is not sufficient enough to follow this trend). The material was collected and grinded with pestle and mortar. The composition and structure of the material were analysed using pXRD analysis. The analysis confirmed the presence of layered Na2SnO3 (99 wt%) and residual SnO2 (1 wt%). A precursor mixture consisting of this material was used to prepare the material having the disordered rock salt structure. A 20 mL magnesia stabilised zirconia (MgSZ) jar was filled with 40 g of milling media consisting of yttria stabilised zirconia (YSZ) balls of 5 mm diameter. 4 g of the above precursor mixture was added to the jar (on top of the balls) to achieve a 10:1 ball to powder ratio. Loading of the precursors was done in an argon filled glovebox, to ensure an inert environment. The jar was then sealed and removed from the glovebox for milling to be performed. The jar was milled at 700 rpm for 10 hours (8 cycles; 1 cycle = 75 minutes milling and 15 minutes break). The jar was returned to the glovebox for sampling and manual homogenisation of the material. Then the jar was further milled at 700 rpm for 10 hours (8 cycles; 1 cycle = 75 minutes milling and 15 minutes break). The jar was returned again to the glovebox for sampling and manual homogenisation of the material. This process was repeated as many times as necessary to obtain a material having a DRS structure. In this example, obtaining the material having a DRS structure required a total of 20 hours of ball milling. After obtaining the material having the DRS structure, the material (in the form of a powder) was collected by sieving and stored under inert atmosphere (Argon). Powder XRD analysis of the final samples gave the pattern shown in Figure 7 and revealed five peaks, at 20 = 33.3277°, 38.7544°, 56.2125°, 67.1851° and 70.6365°. These peaks were indexed (h,k,l) as (1,1,1), (2,0,0), (2,2,0), (3,1,1) and (2,2,2) respectively. This matched a cubic structure of the space group Fm-3m, which is a disordered rock salt structure. Rietveld refinement analysis was used to calculate the FWHM and unit cell values. The FWHM was calculated to be 1.6212° 20. The unit cell length was calculated to be 4.5948 A. No evidence of crystalline precursors was observed. Figure 8a shows an SEM image of the final sample, showing a particulate material. Figures 8b, 8c and 8d show EDX maps of sodium, oxygen and tin, respectively. These EDX maps show a homogeneous distribution of the metals throughout the sample (the lighter areas indicate the presence of the respective elements). The peaks in the EDX spectrum of the material shown in Figure 8e correspond to the characteristic peaks for carbon, oxygen, sodium and tin, and are labelled accordingly. For EDX analysis, the sample is deposited onto a carbon tape which gives rise to the peak labelled as carbon i.e. the peak labelled as carbon does not originate from the sample. The EDX spectrum also indicates the absence of any other elements i.e. the material is free from other contaminants. Powder electrodes were made to test electrochemical properties. They consist of a mixture of active material (AM) which is the material to be tested, carbon black, and PTFE respectively in the ratio 80:10:10. That mixture was pressed together in a pellet and assembled in a half cell in a coin cell format. Two different tests were performed: 1. Cycling against lithium: Whatman glass fibre was used as the separator, lithium metal foil was used as the anode. The electrolyte consisted of 1M LiPFe salt dissolved in a mixture of solvent EC / DMC in a ratio of 1:1. 2. Cycling against sodium: Whatman glass fibre was used as the separator, sodium metal foil was used as the anode. The electrolyte consisted of 1M NaPFe salt dissolved in a mixture of solvent EC / DMC in a ratio of 1:1. 5 Galvanostatic testing was performed on half cells between 4.8 - 0.5 V (vs Li or Na as described above) at 30°C and showed a capacity of first discharge of 42 mAh / g against Li and 21 mAh / g against Na. The charge-discharge curves for the first 5 cycles against Li and Na are shown in Figure 9a and 9b, respectively. 10

Claims

1. A material having the general formula AX, wherein:A = MeSnf[c]d;[c] is a cation vacancy;M = Li, Na or a combination thereof;0 <e <1; 0 <f <1; 0 <d <1 / 2; and the sum of e, f and d is 1; and wherein:X = OPSqFiClk[a]m;[a] is an anion vacancy;0 <p <1; 0 <q <1; 0 <I <1; 0 <k <1; 0 <m <1 / 2; and the sum of p, q, I, k and m is 1; and the material has a structure which is a disordered rock salt structure.

2. The material according to claim 1, wherein 1 / 6 <e <2 / 3; 0 <f <1 / 3; and 0 <d <1 / 2.

3. The material according to claim 1 or claim 2, wherein the material predominantly comprises a single phase.

4. The material according to any one of the preceding claims, wherein M comprises or consists of Li.

5. The material according to any one of the preceding claims, wherein:(i) k = 0; and / or(ii) p and / or q >0; and / or(iii) p >0; and / or(iv) I >0.

6. The material according to any one of the preceding claims, wherein d and / or m = 0.

7. The material according to any one of the preceding claims, wherein M comprises or consists of Li and an X-ray diffraction pattern of the material using a CuKa radiation source has a peak at a 20 value of at least one of, and optionally all of: (a) 36.0° ± 2.0°;(b) 41.8 ±2.0°;(c) 60.7 ± 2.0°;(d) 72.7 ± 2.0°.

8. The material according to any one of the preceding claims, wherein M comprises or consists of Na and an X-ray diffraction pattern of the material using a CuKa radiation source has a peak at a 20 value of at least one of, and optionally all of: (a) 33.4° ± 2.0°;(b) 38.8 ±2.0°;(c) 56.3 ± 2.0°;(d) 67.2 ± 2.0°;(e) 70.7 ± 2.0°.

9. The material according to any one of the preceding claims, wherein an X-ray diffraction pattern of the material using a CuKa radiation source has an absence of peaks below a 20 value of 30°.

10. A method of preparing a material according to any one of the preceding claims, the method comprising the steps of:(a) providing a precursor mixture comprising M and Sn;wherein M and Sn are provided in the form of at least one salt precursor selected from:lithium salt precursors, sodium salt precursors, tin salt precursors and mixed metal salt precursors, or a combination thereof; andwherein the salt of the at least one salt precursor is or comprises an oxide, a sulfide, a fluoride or a chloride; and(b) ball milling the precursor mixture provided at (a) for a period of time.

11. A method of preparing a material according to any one of claims 1 to 9, the method comprising the steps of:(a) providing a precursor mixture comprising or consisting of at least one precursor having the general formula AX, wherein:A = MeSnf[c]d;[c] is a cation vacancy;M = Li, Na or a combination thereof;0<e<1;0<f<1;0<d< 1 / 2; and the sum of e, f and d is 1; and wherein:X = OPSqFiClk[a]m;[a] is an anion vacancy;0<p<1;0<q<1;0<l<1;0<k<1;0<m< 1 / 2; and the sum of p, q, I, k and m is 1; and(b) ball milling the precursor mixture provided at (a) for a period of time.

12. The method according to claim 11, wherein the structure of the precursor having the general formula AX is a disordered rock salt structure or a structure other than a disordered rock salt structure; optionally, wherein the structure other than a disordered rock salt structure is a layered structure or a spinel structure.13.The method according to claim 11 or claim 12, wherein, in the general formula AX of the precursor, M consists of Li or M consists of Na.

14. The method according to any of claims 11 to 13, wherein the at least one precursor having the general formula AX is prepared by solid state synthesis, wherein the solid-state synthesis comprises:(i) providing a precursor mixture comprising M and Sn;wherein M and Sn are provided in the form of at least one salt precursor selected from:lithium salt precursors, sodium salt precursors, tin salt precursors and mixed metal salt precursors, or a combination thereof; and(ii) heating the precursor mixture provided at (i) for a period of time.

15. The method according to claim 14, wherein the precursor mixture provided at (i) comprises Li2CO3 and / or Na2CO3.16.The method according to claim 14 or claim 15, wherein the precursor mixture provided at (i) further comprises at least one solvent and / or the precursors are mixed prior to step (ii) by ball milling.

17. The method according to any one of claims 10 to 16, wherein the period of time in (b) is 1 hour to 300 hours.

18. The method according to any one of claims 10 to 17, wherein ball milling is carried out at a speed of 150 rpm or more and / or 1000 rpm or less.

19. The method according to any one of claims 10 to 18, wherein a weight ratio of milling media to the precursor mixture is in the range of from 100:1 to 1:1.

20. The method according to any one of claims 10 to 19, wherein the ball milling is carried out in a milling jar which is formed from a material comprising ZrO2, stainless steel, agate (SiO2), SiC, or WC.

21. An electrode comprising a material according to any one of claims 1 to 9, or a material prepared by the method of any one of claims 10 to 20.22.The electrode according to claim 21, wherein the electrode comprises additives and / or a binder; and / or wherein the electrode is an anode.

23. An electrochemical cell comprising a material according to any one of claims 1 to 9, or a material prepared by the method of any one of claims 10 to 20, or an electrode according to claim 21 or claim 22.

24. An electrochemical energy storage device comprising an electrochemical cell according to claim 23.

25. Use of a material according to any one of claims 1 to 9 in an electrode, an 5 electrochemical cell, or an electrochemical energy storage device.

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