Electrode material

Abstract

An s-block-metal / transition metal oxide particle having a diameter of at most 4 µm and a circularity of at least 50% is disclosed where the s-block element may be lithium sodium or magnesium and the t

Description

BACKGROUND Lithium-ion secondary batteries are the leading battery technology currently used in applications from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. They contain a plurality of lithium-ion secondary cells, which is one example of an alkali metal ion secondary cell. One type of electrochemically active material commonly used in the cathodes of lithium-ion secondary cells is lithium nickel manganese cobalt oxide (NMC). These materials have a layered structure, and may be described as mixed metal oxides, or more specifically an s-block-metal transition-metal oxide for example. SUMMARY In a first aspect, the present invention provides an s-block-metal transition-metal oxide particle having a diameter of at most 4 pm and a circularity of at least 50%. Cathode active materials are typically synthesised with an average particle size larger than about 8 to 10 pm. The simultaneous use of particles with a different size may be useful in electrode fabrication, in order to improve the packing of particles and therefore the volumetric energy density of the cathode. Denser and less porous cathodes allow for a more efficient utilisation of the electrode volume, but higher material density also typically corresponds with larger ciystal domains, which may pose some challenges in lithium-containing materials due to the diffusion of lithium ions. This is especially the case for materials that are particularly lithium-rich, which have slow lithium diffusion kinetics. It is therefore challenging to achieve satisfying electrochemical performance while maintaining high particle density. The size of the smaller crystallites (primary particles) that form the larger polycrystalline agglomerates (secondary particles) is therefore generally kept relatively small. Attempts have been made to make denser particles by forming so-called single-crystal materials, which are particles of active materials comprising individual crystallites with a significantly larger size. However, this approach also has to deal with the challenge posed by low lithium-ion diffusion. Such an issue may be acted upon by significantly decreasing the average particle size of the agglomerates, but this leads to the formation of a myriad of small crystals with irregular shapes. Small and irregular particles however negatively impact the way that the particles pack. This also contributes to maintaining a high specific surface area, which is detrimental for the long-term stability. The s-block-metal transition-metal oxide particles according to the present invention have a highly regular spherical shape and morphology. When an electrode material is formed from a plurality of the particles of the invention, the material may exhibit excellent electrochemical properties, including high discharge capacity and negligible capacity fade. The regular shape and morphology also allow the particles to be blended with larger porous s-block-metal rich materials to yield an electrode material with a beneficially increased density (e.g. as measurable by TAP density). The diameter of a particle may refer to the longest dimension across the particle (i.e. the maximum distance between any two points on the edge of the particle). The size of a particle may be measured by SEM or laser diffraction. Circularity is a quantification of how similar to a circle that the shape of a particle is when a two-dimensional image of a particle (e.g. a particle within a material) is analysed. A perfect circle has a circularity of 1, or 100% when expressed as a percentage. As particles become less circular, their measured circularity will decrease. Circularity can be determined by static image analysis of a sample of the material by a static image analyser, such as a Malvern PanalyticalC™) Morphologic™) 4, using the method and calculation in ISO 9276-6. To determine the circularity of a material, a sample of the material is analysed using the static image analyser, the circularity of all particles within the frame of the image is determined and then a mean average circularity value is calculated. As described herein, the present invention enables particles of surprisingly high circularity to be obtained. The particle may have a circularity of at least 55%, or at least 60%, or at least 65%, or at least 70%, or at least 75%, or at least 80%. The particle may have a diameter of at least 0.5 pm, or at least 1.0 pm, or at least 1.5 pm, or at least 2.0 pm. The particle may have a diameter of at least 1.0 pm. The particle may have a diameter of at most 3.5 pm, or at most 3.0 pm, or at most 2.5 pm, or at most 1.5 pm. The particle may have a diameter of at most 2.5 pm. The particle may have a diameter in the range from 1.0 to 2.5 pm. Suitably, the s-block-metal transition-metal oxide particle is a secondary particle comprising a plurality of primary particles. The primary particles may have an average diameter of at least 100 nm, or at least 150 nm, or at least 200 nm, or at least 250 nm. The primary particles may have an average diameter of at least 200 nm. The primary particles may have an average diameter of at most 700 nm, or at most 600 nm, or at most 550 nm, or at most 500 nm, or at most 450 nm, or at most 400 nm. The primary particles may have an average diameter of at most 500 nm. The primary particles may have an average diameter in the range from 200 to 500 nm. The present inventors have generally found that the s-block-metal transition-metal oxide particle has a relatively low number of crystal domains, and that those crystal domains are relatively large in size. The s-block-metal transition-metal oxide particle may comprise crystal domains having an average area of at least 0.02 pm2, or at least 0.04 pm2, or at least 0.06 pm2. The s-block-metal transition-metal oxide particle may comprise crystal domains having an average area of at least 0.02 pm2. The s-block-metal transition-metal oxide particle may comprise crystal domains having an average diameter of at least 160 nm, or at least 180 nm, or at least 200 nm, or at least 220 nm. The s-block-metal transition-metal oxide particle may comprise crystal domains having an average diameter of at least 160 nm. The s-block-metal transition-metal oxide particle may comprise crystal domains having an average area of at least 0.02 pm2 and an average diameter of at least 160 nm. The diameter of a crystal domain may refer to the longest dimension across the crystal domain (i.e. the maximum distance between any two points on the edge of the crystal domain). The size (i.e. diameter or area) of a crystal domain may be measured by SEM and / or electron backscatter diffraction (EBSD). By “average” area or diameter, it is meant the mean area or diameter, which may be calculated by observing the crystal domains at a location of the material, such as those described in the Examples section below (for instance, the crystal domains may be observed in a portion of a cross-section of the material, e.g. the portion may have an area of at least 25 pm2). The s-block-metal transition-metal oxide particle comprises at least one s-block metal element, at least one transition metal element, and oxide. The s-block-metal transitionmetal oxide particle may consist of at least one s-block metal element, at least one transition metal element, and oxide. However, in some embodiments, the particle may contain further cationic species and / or further anionic species. The s-block metal of the s-block-metal transition-metal oxide particle may be any suitable metal element in the s-block (i.e. Groups 1 and 2) of the Periodic Table. For instance, the s-block metal may be lithium, sodium, potassium, beryllium, magnesium or calcium; e.g. lithium, sodium or magnesium. As the skilled person will appreciate, although hydrogen and helium may be described as being s-block elements, hydrogen and helium are not s-block metals because they are non-metals. The s-block metal of the s-block-metal transition-metal oxide particle may be a metal element in Group 1 of the Periodic Table (i.e. an alkali metal). Thus, the s-block-metal transition-metal oxide particle may be an alkali-metal transition-metal oxide particle. For instance, the alkali metal may be lithium, sodium or potassium; e.g. lithium or sodium. The s-block metal of the s-block-metal transition-metal oxide particle may be a metal element in Group 2 of the Periodic Table (i.e. an alkaline earth metal). Thus, the s-block-metal transition-metal oxide particle may be an alkaline-earth-metal transition-metal oxide particle. For instance, the s-block metal may be beryllium, magnesium or calcium; e.g. magnesium. The s-block metal of the s-block-metal transition-metal oxide particle may be lithium. Thus, the s-block-metal transition-metal oxide particle may be a lithium transition-metal oxide particle. The s-block metal of the s-block-metal transition-metal oxide particle may be sodium. Thus, the s-block-metal transition-metal oxide particle may be a sodium transition-metal oxide particle. The s-block metal of the s-block-metal transition-metal oxide particle may be magnesium. Thus, the s-block-metal transition-metal oxide particle may be a magnesium transitionmetal oxide particle. The s-block metal of the s-block-metal transition-metal oxide particle may be potassium. Thus, the s-block-metal transition-metal oxide particle may be a potassium transition-metal oxide particle. The transition metal element of the s-block-metal transition-metal oxide particle may comprise or consist of one or more of manganese, nickel, cobalt, iron, titanium, vanadium, tungsten and zirconium. The transition metal element of the s-block-metal transition-metal oxide particle may comprise or consist of one or more of manganese, nickel and cobalt. The transition metal element of the s-block-metal transition-metal oxide particle may comprise or consist of manganese, nickel and cobalt. The transition metal element of the s-block-metal transition-metal oxide particle may comprise or consist of manganese and / or nickel. The s-block-metal transition-metal oxide particle may have a composition according to Formula I: MnxCoyNizMqO2 Formula I wherein 0<x< 1.0; 0<y <1.0; 0<z< 1.0; M is one or more elements selected from Li, Na, K, Be, Ca and Mg; 0 <q <1.0; and x + y + z = 1.0. In some embodiments, 0 <x <1.0. In some embodiments, x = 1.0. In some embodiments, x = 0. In some embodiments, 0 <y <1.0. In some embodiments, y = 1.0. In some embodiments, y = 0. In some embodiments, 0 <z <1.0. In some embodiments, z = 1.0. In some embodiments, z = 0. In some embodiments, at least two of x, y and z are non-zero, and at least one of x, y and z is zero. In some embodiments, x and z are non-zero, and y is zero. The s-block-metal transition-metal oxide particle may be an s-block-metal nickel manganese cobalt oxide particle. The s-block-metal transition-metal oxide particle may be a lithium nickel manganese cobalt oxide particle. The s-block-metal transition-metal oxide particle may be an s-block-metal nickel manganese oxide particle. The s-block-metal transition-metal oxide particle may be a lithium nickel manganese oxide particle. A second aspect of the invention provides a method of manufacturing the particle of the first aspect, the method comprising: mixing a transition metal salt with a second salt in a liquid vehicle, wherein the second salt is selected from one or more of a carbonate salt and a bicarbonate salt; carrying out a precipitation reaction to form a slurry comprising a transition metal carbonate precursor particle in the liquid vehicle; mixing the transition metal carbonate precursor particle with an s-block metal source, and calcining the mixture of the transition metal carbonate precursor particle and the s-block metal source to form the s-block-metal transition-metal oxide particle; wherein the calcination comprises a high-temperature calcination step performed at a temperature of greater than 850 °C. The transition metal salt may comprise or consist of one or more of transition metal sulfate salts, transition metal nitrate salts, transition metal oxalate salts and transition metal acetate salts. The transition metal salt may comprise or consist of a transition metal sulfate salt. The identity of the transition metal or transition metals within the transition metal salt is not particularly limited. If the s-block-metal transition-metal oxide material is later mixed with a porous particulate mixed metal oxide material in a blended material as described herein, the s-block-metal transition-metal oxide material will generally make up a small proportion of the mass of the blended material, because the Dv50 of the particulate mixed metal oxide material is greater than the Dv50 of the s-block-metal transition-metal oxide material. The choice of transition metal or transition metals within the transition metal salt can be used to influence certain aspects of the size and morphology of the s-block-metal transition-metal oxide particle. The transition metal salt may comprise or consist of one or more transition metal salt species, i.e. one or more distinct transition metal salt species. The transition metal salt may comprise or consist of a single transition metal salt species. The transition metal salt may comprise or consist or two or more transition metal salt species. The transition metal salt may comprise or consist or three or more transition metal salt species. The transition metal salt may comprise or consist or four or more transition metal salt species. The transition metal salt may comprise or consist of two transition metal salt species. The use of two distinct transition metal salt species will provide a diameter for the s-block-metal transition-metal oxide particle that is slightly larger than when a single species of transition metal salt is present, which may be desirable in some embodiments. The transition metal salt may comprise or consist of more than two transition metal salt species. The use of more than two distinct transition metal salt species will provide a diameter for the s-block-metal transition-metal oxide particle that is slightly larger than when two distinct species of transition metal salt are present, which may be desirable in some embodiments. For example, the transition metal salt may comprise or consist of three or four transition metal salt species. It is therefore possible to choose the number of distinct species of transition metal salt within the transition metal salt depending on the desired particle size of the s-block-metal transition-metal oxide particle. The transition metal salt may comprise or consist of one or more of a manganese salt, a nickel salt, a cobalt salt, an iron salt, a titanium salt, a vanadium salt, a tungsten salt, and a zirconium salt. The transition metal salt may comprise or consist of one or more of an iron salt, a manganese salt, a nickel salt, and a cobalt salt. The transition metal salt may comprise or consist of a manganese salt. The transition metal salt may comprise or consist of an iron salt. The transition metal salt may comprise or consist of a nickel salt. The transition metal salt may comprise or consist of a cobalt salt. The transition metal salt may comprise or consist of a manganese salt and a nickel salt. The transition metal salt may comprise or consist of a manganese salt and an iron salt. The transition metal salt may comprise or consist of a manganese salt, a nickel salt, and a cobalt salt. The nickel salt may be a nickel sulfate salt (e.g. NiSO4), and / or the cobalt salt may be a cobalt sulfate salt (e.g. COSO4), and / or the manganese salt may be a manganese sulfate salt (e.g. MnSO4). The nickel salt may be a nickel sulfate salt (e.g. NiSO4), and the cobalt salt may be a cobalt sulfate salt (e.g. COSO4), and the manganese salt may be a manganese sulfate salt (e.g. MnSO4). The transition metal salt may comprise or consist of an iron salt, a manganese salt, a nickel salt, and a cobalt salt. The second salt may have a solubility in water of at least 90 g / L at 20 °C; for example at least 100 g / L, at least 120 g / L, at least 140 g / L, at least 160 g / L, at least 180 g / L, at least 200 g / L, at least 210 g / L or at least 215 g / L. The second salt may have a solubility in water of from 90 g / L to 250 g / L at 20 °C, for example from 100 g / L to 250 g / L, from 120 g / L to 250 g / L, from 140 g / L to 250 g / L, from 160 g / L to 250 g / L, from 180 g / L to 250 g / L, from 200 g / L to 250 g / L or from 200 ^L to 220 g / L. The second salt may consist of either a carbonate salt or a bicarbonate salt. The second salt may comprise or consist of a bicarbonate salt. The second salt may consist of a single species of bicarbonate salt. Therefore, in some embodiments, the transition metal salt is mixed with a bicarbonate salt. The bicarbonate salt may comprise or consist of one or more of ammonium bicarbonate and sodium bicarbonate. The bicarbonate salt may comprise or consist of ammonium bicarbonate. Ammonium bicarbonate results in an s-block-metal transition-metal oxide material having a particularly desirable particle size distribution with low agglomeration and a narrow distribution. The second salt may have a solubility in water of at least 90 g / L at 20 °C, and the second salt may comprise or consist of ammonium bicarbonate. In the step of mixing the transition metal salt with the second salt in the liquid vehicle, a further metal source may also be mixed in the liquid vehicle. The further metal source may comprise one or more of magnesium, boron, aluminium, and tin. For example, the further metal source may be a salt such as a sulfate. The metal of the further metal source may act as a dopant element. The precipitation reaction may be performed at a temperature within the range of 15 to 80 °C; for example 15 to 70 °C, 15 to 60 °C, 15 to 55 °C, 15 to 50 °C, 15 to 45 °C, 15 to 40 °C, 15 to 35 °C, 15 to 30 °C, 15 to 25 °C, or 20 to 25 °C, or at room temperature. The precipitation reaction may be performed at a temperature with the range of 15 to 50 °C. The liquid vehicle may consist of water. The liquid vehicle may consist of water, and the transition metal salt and the second salt (and optionally, where present, the further metal source) may be dissolved in the water before the precipitation reaction. The molar ratio of the transition metal salt to the second salt may be from 1:2 to 1:80; for example from 1:2 to 1:70, from 1:2 to 1:60, from 1:2 to 1:50, from 1:2 to 1:40, from 1:2 to 1:30, from 1:2 to 1:20, from 1:4 to 1:80, from 1:4 to 1:50 or from 1:4 to 1:20. The liquid vehicle may be water, and the transition metal salt and the second salt may be dissolved in the water before the precipitation reaction; and the molar ratio of the transition metal salt to the second salt may be from 1:4 to 1:80. Particularly good results are observed when the molar ratio of the transition metal salt to the second salt is from 1:8 to 1:45; for example from 1:10 to 1:40, from 1:15 to 1:40, from 1:20 to 1:40, from 1:25 to 1:40, from 1:30 to 1:40, or about 1:33. Such molar ratios have been observed to lead to particularly spherical particles with high circularity, a narrow particle size distribution and few agglomerates. The transition metal salt may be mixed with the second salt in the liquid vehicle by different methods. For example, the transition metal salt and the second salt, both in powder form, may be added to water to form an aqueous solution or suspension. Alternatively, an aqueous solution of the second salt may be provided and the transition metal salt in solid form may be added to the aqueous solution of the second salt. However, more preferably, an aqueous solution of the transition metal salt may be provided and the second salt in solid form may be added to the aqueous solution of the transition metal salt. It has been found that mixing an aqueous solution of the transition metal salt with the second salt in solid (powder) form produces a product material with fewer agglomerates, smaller particle size and higher particle circularity. Even more preferably, separate aqueous solutions of the transition metal salt and the second salt may be prepared before being mixed together to form a mixed aqueous solution as a reaction mixture. Such embodiments have been shown to produce a product material with the lowest proportion of agglomerates, and optimal particle size and circularity. The method may comprise: preparing a first solution of the one or more transition metal salts in water, wherein the total concentration of the one or more transition metal sails in the first solution is from 0.8 M to 3.5 M, for example from 0.8 M to 2.0 M; preparing a second solution of the second salt in water, wherein the total concentration of the second salt in the second solution is from 2.0 M to 4.0 M; and combining the first solution with the second solution, such that the molar ratio of the transition metal precursor salt to the second salt is from 1:2 to 1:80, for example from 1:4 to 1:20. The total concentration of the one or more transition metal salts in the first solution may be from 0.8 M to 1.5 M; for example from 0.8 M to 1.2 M, or about 1.0 M. The total concentration of the second salt in the second solution may be from 2.0 M to 3.5 M; for example from 2.0 M to 3.0 M, from 2.0 M to 2.5 M, or about 2.2 M. Mixing the first solution with the second solution may be such that the molar ratio of the transition metal precursor salt to the second salt is from 1:2 to 1:70; for example from 1:2 to 1:60, from 1:2 to 1:50, from 1:2 to 1:40, from 1:2 to 1:30, from 1:2 to 1:20, from 1:4 to 1:80, from 1:4 to 1:50, or from 1:4 to 1:20. The total concentration of the one or more transition metal salts in the first solution may be from 0.8 M to 1.2 M; the total concentration of the second salt in the second solution may be from 2.0 M to 2.5 M; and mixing the first solution with the second solution may be such that the molar ratio of the transition metal precursor salt to the second salt is from 1:4 to 1:40. Such concentrations and ratios have been observed to lead to particularly spherical particles with high circularity, a narrow particle size distribution and few agglomerates. The total concentration of the one or more transition metal salts in the first solution may be about 1.0 M; the total concentration of the second salt in the second solution may be about 2.2 M; and mixing the first solution with the second solution may be such that the molar ratio of the transition metal precursor salt to the second salt is about 1:33. Such concentrations and ratios have been observed to lead to particularly spherical particles with high circularity, a narrow particle size distribution and few agglomerates. After combining the first and second solution, the combined solution may be left to react without any mixing, stirring or agitation. After combining the first and second solution, the combined solution may be mixed or stirred. After combining the first and second solution, the combined solution may be subjected to sonication at ultrasound frequencies. Sonication may be carried out for a period of from 2 to 30 minutes, optionally 2 to 10 minutes. Sonication may be performed at a temperature within the range of 15 to 30 °C, optionally 20 to 25 °C. Sonication may be performed within a round-bottom reaction vessel. The precipitation reaction may be carried out under sonication at ultrasound frequencies, or under stirring. The precipitation reaction may be carried out under sonication at ultrasound frequencies. The UP400St Powerful Ultrasonicator from Hielsche^™'1 is one example of a suitable sonicator. The amplitude range for the sonication may be from 20 to 75 % and the pulse range may be from 30 to 100 %. Performing the precipitation under ultrasonic sonication has been found to provide highly spherical particles, with a circularity of at least 50%; for example at least 60% or at least 70%, along with a small diameter or Dv50 particle size. The precipitation reaction under sonication may be carried out for a period of from 2 to 30 minutes, for example from 2 to 10 minutes. The precipitation reaction under sonication may be performed at a temperature of from 15 to 36 °C; for example, from 15 to 30 °C or from 20 to 25 °C. The precipitation reaction under sonication may be performed within a round-bottom reaction vessel. The precipitation reaction may be carried out under stirring. The stirring may be at a rate of from 150 rpm to 5000 rpm; e.g. from 500 rpm to 5000 rpm, or from 500 rpm to 2000 rpm. In the step of mixing the transition metal carbonate precursor particle with an s-block metal source, an excess of the s-block metal source may be used. For example, the molar ratio of the s-block metal of the s-block metal source to the total transition metals of the transition metal carbonate precursor particle may be at least 1.10:1.00; e.g. at least 1.15:1.00, or at least 1.20:1.00, or at least 1.22:1.00, or at least 1.24:1.00. The molar ratio of the s-block metal of the s-block metal source to the total transition metals of the transition metal carbonate precursor particle may be at least 1.20:1.00. The s-block metal source may comprise or consist of one or more s-block metal salts. For example, the s-block metal source may comprise or consist of one or more s-block metal carbonates and / or s-block metal hydroxides. The s-block metal of the s-block metal source may be any suitable metal element in the s-block (i.e. Groups 1 and 2) of the Periodic Table. The aforementioned preferences relating to the s-block metal in the s-block-metal transition-metal oxide particle apply equally to the s-block metal in the s-block-metal source. The s-block metal source may comprise or consist of lithium carbonate, lithium hydroxide, sodium carbonate, sodium hydroxide, magnesium carbonate, or magnesium hydroxide. The s-block metal of the s-block metal source may comprise or consist of lithium, such that the s-block-metal transition-metal oxide particle is a lithium transition-metal oxide particle. The step of mixing the transition metal carbonate precursor particle with an s-block metal source may be carried out by any suitable mixing method and apparatus. For example, a ball mill may be used (e.g. at about 100 rpm for at least 30 minutes). Alternatively, acoustic mixing may be used (e.g. with a Resodyn1^™^ acoustic mixer). Alternatively, the mixing may comprise precipitating the s-block metal source on the transition metal carbonate precursor particle, e.g. in a stirred tank reactor. The calcining of the mixture of the transition metal carbonate and the s-block metal source may be performed over one or more steps. Where there is more than one calcination step, the steps may be performed at different temperatures. For example, where there are two calcination steps, there may be a low-temperature calcination step and a high-temperature calcination step. For example, where there are three calcination steps, there may be a low-temperature calcination step, a medium-temperature calcination step and a high-temperature calcination step. The calcination steps may be performed in order of increasing temperature. The calcination comprises a step performed at a temperature (Th) of greater than 850 °C. This step, which may be described as a “high-temperature calcination step”, may be the only calcination step or one step within multiple calcination steps. The high-temperature calcination step may be performed at a temperature (Th) of at least 900 °C, or at least 950 °C, or at least 975 °C, or at least 1000 °C. The high-temperature calcination step may be performed at a temperature (Th) of at least 900 °C or at least 950 °C. The high-temperature calcination step may be performed for at least 1 hour, or at least 2 hours, or at least 3 hours, or at least 4 hours, or at least 5 hours. The high-temperature calcination step may be performed for at most 24 hours, or at most 20 hours, or at most 16 hours, or at most 12 hours, or at most 8 hours, or at most 6 hours. The high-temperature calcination step may be performed for a time in the range of from 3 hours to 8 hours. The high-temperature calcination step may be performed at a temperature (Th) of at least 900 °C (such as at least 950 °C or at least 1000 °C) for a time in the range of from 3 hours to 8 hours. The high-temperature calcination step may be performed in an oxygen-enriched atmosphere. For example, an oxygen-enriched atmosphere may have an oxygen content of at least 30%, or at least 40%, or at least 50%, or at least 60 %, or at least 70%, or at least 75%, or at least 80%, or at least 85%, or at least 90%, or at least 95%, or at least 97%, or at least 99% (molar percentage). The oxygen-enriched atmosphere may consist essentially of pure oxygen. The high-temperature calcination step may be performed in an oxygen-enriched atmosphere having an oxygen content of at least 75% or at least 90%. The high-temperature calcination step may be performed at a temperature (Th) of at least 950 °C (e.g. at least 1000 °C) for about 5 hours in an oxygen-enriched atmosphere (e.g. having an oxygen content of at least 75% or at least 90%). The high-temperature calcination step yields an s-block-metal transition-metal oxide particle having a particular structure and morphology, and thus beneficial electrochemical properties. The calcining of the mixture of the transition metal carbonate and the s-block metal source may comprise a low-temperature calcination step performed before the high-temperature calcination step, wherein the low-temperature calcination step is performed at a lower temperature than the high-temperature calcination step. The low-temperature calcination step may be performed at a temperature (Tl) of at least 200 °C, such as at least 250 °C, or at least 300 °C, or at least 350 °C, or at least 400 °C, or at least 450 °C, or at least 500 °C, or at least 550 °C, or at least 600 °C. The low-temperature calcination step may be performed at a temperature (Tl) of at most 900 °C, such as at most 850 °C, or at most 800 °C, or at most 750 °C, or at most 700 °C, or at most 650 °C, or at most 600 °C, or less than 600 °C, or at most 550 °C, or at most 500 °C. Where the calcining of the mixture of the transition metal carbonate and the s-block metal sources comprises a low-temperature calcination step performed before the high-temperature calcination step, the heating to Tl and / or the heating from Tl to Th may be performed at a ramping rate in the range of from 0.5 to 10 °C / min; such as from 0.5 to 5 °C / min, or from 0.5 to 2 °C / min, or about 1 °C / min. In addition to the high-temperature calcination step and the low-temperature calcination step, the calcining of the mixture of the transition metal carbonate and the s-block metal source may comprise a medium-temperature calcination step, wherein the mediumtemperature calcination step is performed at a higher temperature than the low-temperature calcination step and a lower temperature than the high-temperature calcination step. The medium-temperature calcination step may be performed after the low-temperature calcination step and / or before the high-temperature calcination step. The medium-temperature calcination step may be performed at a temperature (Tm) of at least 600 °C, such as at least 650 °C, or at least 700 °C, or at least 750 °C, or at least 800 °C, or at least 850 °C. The medium-temperature calcination step may be performed at a temperature (Tm) of at most 950 °C, or at most 900 °C, or less than 900 °C, or at most 850 °C, or at most 800 °C, or at most 750 °C, or at most 700 °C. Where the calcining of the mixture of the transition metal carbonate and the s-block metal sources comprises a low-temperature calcination step, followed by a medium-temperature calcination step, followed by a high-temperature calcination step, the heating to Tl, the heating from Tl to Tm and / or the heating from Tm to Th may be performed at a ramping rate in the range of from 0.5 to 10 °C / min; such as from 0.5 to 5 °C / min, or from 0.5 to 2 °C / min, or about 1 °C / min. The low-temperature calcination step and / or the medium-temperature calcination step may each be performed for at least 1 hour, or at least 2 hours, or at least 3 hours, or at least 4 hours, or at least 5 hours. The low-temperature calcination step and / or the mediumtemperature calcination step may each be performed for at most 24 hours, or at most 20 hours, or at most 16 hours, or at most 12 hours, or at most 8 hours, or at most 6 hours. The low-temperature calcination step and / or the medium-temperature calcination step may each be performed for a time in the range of from 3 hours to 8 hours. The calcination may comprise a low-temperature calcination step performed at a temperature (Tl) in the range of at least 200 °C to less than 600 °C, followed by a mediumtemperature calcination step performed at a temperature (Tm) in the range of at least 600 °C to less than 900 °C, followed by the high-temperature calcination step; wherein the high-temperature calcination step is performed at a temperature (Th) of at least 900 °C (such as at least 950 °C or at least 1000 °C). The heating to Tl, the heating from Tl to Tm, and / or the heating from Tm to Th may each be performed at a ramping rate in the range of from 0.5 to 10 °C / min; such as 0.5 to 5 °C / min, or 0.5 to 2 °C / min, or about 1 °C / min. One or more of the calcination steps may be performed in an oxygen-enriched atmosphere, such as at least the high-temperature calcination step or all the calcination steps. Any one or more of the calcination steps may be performed in any suitable calciner or furnace, e.g. a muffle furnace, rotary furnace or roller hearth kiln (RHK). Thus, the particle of the first aspect may be obtained or obtainable by the method according to the second aspect. A third aspect of the invention provides an s-block-metal transition-metal oxide material comprising a plurality of particles according to the first aspect. The s-block-metal transition-metal oxide material may be a particulate material, i.e. made up of a plurality of particles. Suitably, the s-block-metal transition-metal oxide material may be an electrochemically active material. The material may be described as an electrode material, such as a cathode material (e.g. positive active material). The material may be defined as having a layered structure. The s-block-metal transition-metal oxide material may have a TAP density of at least 1.75 g / cm3. The TAP density may be relatively high, due to efficient packing of the particles resulting from their regular, highly spherical shape. TAP density refers to density determined by the application of trans-axial pressure (TAP). Such TAP density may be measured using methods and apparatus known to the skilled person, for example using a Micrometries^™) GeoPyc^™) 1365 Envelope and Density Analyzer. Such values measured by TAP in this way tend to be comparable with the tapped density of the material, i.e. the final density of the material after repeatedly mechanically tapping the material. A fourth aspect of the invention provides a blended material comprising a mixture of: (i) the s-block-metal transition-metal oxide material according to the third aspect; and (ii) a particulate mixed metal oxide material comprising an intra-particle porosity of from 2 to 35%, as determined by mercury intrusion porosimetry before mixing with the s-block-metal transition-metal oxide material; wherein the Dv50 (volumetric median particle diameter) of the particulate mixed metal oxide material is greater than the Dv50 of the s-block-metal transition-metal oxide material; and wherein the TAP density of the blended material is greater than the TAP density of the particulate mixed metal oxide material; and the intra-particle porosity of the blended material is less than the intraparticle porosity of the particulate mixed metal oxide material, as determined by mercury intrusion porosimetry. As the particles of the s-block-metal transition-metal oxide material are small and have a regular shape and morphology, these particles may suitably occupy the pores of such a porous particulate mixed metal oxide material, when the two types of material are mixed to yield a blended material. Thus, the blended material may have a higher density, which can beneficially lead to improved electrochemical properties. The term “intra-particle porosity” refers to the porosity within the particle structure, and excludes porosity arising from the spaces between the particles of the material. The particulate mixed metal oxide material is particulate, i.e. made up of a plurality of discrete particles. The particles may comprise primary particles and / or secondary particles formed from the agglomeration of a plurality of primary particles. The Dv50 (volumetric median particle diameter) of the particulate mixed metal oxide material is greater than the Dv50 of the s-block-metal transition-metal oxide material. Dv50 is the volumetric median particle size. In other words, it represents the particle size in microns which splits the volume distribution of that population of particles in half, with 50 vol% of the particles having a particle size below that value and 50 vol% having a particle size above that value. The skilled person will appreciate that the volume median particle size Dv50 can be measured using a Malvern^™) Mastersizer1^™) 3000 using the light scattering method set out in ASTM B822-20, applying the Mie scattering theory. The particulate mixed metal oxide material may comprise a Dv50 of at least 1 pm, for example at least 2 pm, at least 3 pm, at least 4 pm, at least 5 pm, at least 6 pm, or at least 7 pm. The particulate mixed metal oxide material may comprise a Dv50 of at least 7 pm, for example at least 10 pm, at least 12 pm, at least 15 pm, at least 20 pm, or at least 21 pm. The particulate mixed metal oxide material may comprise a Dv50 of up to 100 pm, for example up to 90 pm, up to 80 pm, up to 70 pm, up to 60 pm, up to 50 pm, up to 40 pm, up to 30 pm or up to 25 pm. The particulate mixed metal oxide material may comprise a Dv50 of from 1 to 100 pm, for example from 2 to 100 pm, from 2 to 90 pm, from 2 to 80 pm, from 2 to 70 pm, from 2 to 60 pm, from 2 to 50 pm, from 3 to 50 pm, from 4 to 50 pm, from 2 to 40 pm, from 3 to 40 pm, from 4 to 40 pm, from 4 to 30 pm, from 5 to 30 pm, from 5 to 25 pm or from 7 to 25 pm. The particulate mixed metal oxide material may be a lithium nickel manganese oxide. The particulate mixed metal oxide material may comprise a composition which is a mixed metal oxide comprising lithium, nickel and manganese, optionally further comprising cobalt. The particulate mixed metal oxide material may be a lithium nickel manganese cobalt oxide. The particulate mixed metal oxide material may be a lithium nickel manganese oxide that does not contain any cobalt. The particulate mixed metal oxide material may be a lithium nickel manganese oxide or a lithium nickel manganese cobalt oxide, further comprising one or more dopant elements. The particulate mixed metal oxide material may comprise particles comprising or consisting of a different composition from that of the s-block-metal transition-metal oxide material in the blended material. The particulate mixed metal oxide material may comprise particles comprising or consisting of a composition according to Formula II: LiaNixCoyMnzAqOi+dFe Formula II wherein A is one or more dopant elements selected from Na, Mg, Al, B, Ti, V, W, Sn, Zr, and Fe; 0.1 <x<0.4; 0<y<0.1; 1.0 <a< 1.3; 0.33 <z <0.66; 0<q<0.10; d is within the range 0 to ±0.2; and e is within the range 0 to 0.1. In some embodiments, a = 1.33 - (2x / 3) - (y / 3) ±5; and z = 0.66 — (x / 3) - (2y / 3) ±O wherein 3 and O are each independently selected from within the range 0 to 0.1. In some embodiments, 8 and O are each zero. In some embodiments, 0.1 <x <0.4, for example 0.11 <x <0.4, 0.12 <x <0.4, 0.15 <x <0.4, 0.2 <x <0.4, 0.25 <x <0.4, 0.3 <x <0.4 or 0.35 <x <0.4. In some embodiments, x = 0.4. In some embodiments, 0 <y <0.1, for example 0 <y <0.09, 0 <y <0.08, 0 <y <0.09, 0 <y <0.07, 0 <y <0.06, 0 <y <0.05, 0 <y <0.04, 0 <y <0.03, 0 <y <0.02 or 0 <y <0.01. In some embodiments, y = 0. In some embodiments, 1.0 <a <1.3, for example 1.0 <a <1.25, 1.0 <a <1.2, 1.06 <a <1.14,1.06<a< 1.13,1.0<a< 1.15,1.0<a< 1.1, 1.0<a< 1.07,1.02<a< 1.07,1.04<a <1.07,1.05 <a< 1.07 or 1.06<a< 1.07. In some embodiments, a = 1.0667. In some embodiments, a = 1.13. In some embodiments, 0.35 <z <0.65, for example 0.40 <z <0.65, 0.45 <z <0.65,0.45 <z <0.60, 0.50 <z <0.60, 0.50 <z <0.55,0.51 <z <0.55, 0.51 <z <0.54,0.52 <z <0.55, 0.53 <z <0.55 or 0.53 <z <0.54. In some embodiments, z = 0.5333. In some embodiments, z = 0.5167. In some embodiments, z = 0.541. In some embodiments, 0 <q <0.09, for example 0 <q <0.08, 0 <q <0.07, 0 <q <0.06, 0 <q <0.05, 0 <q <0.04, 0 <q <0.03, 0 <q <0.02 or 0 <q <0.01. In some embodiments, q = 0. In some embodiments, d is within the range 0 to ±0.19, for example 0 to ±0.18, 0 to ±0.17, Oto±0.16, Oto±0.15, Oto±0.14, Oto±0.13, Oto±0.12, Oto±0.11 or0to±0.10. In some embodiments, d = 0. In some embodiments, 0 <e <0.09, for example 0 <e <0.08, 0 <e <0.07, 0 <e <0.06, 0 <e <0.05, 0 <e <0.04, 0 <e <0.03,0 <e <0.02 or 0 <e <0.01. In some embodiments, e = 0. The intra-particle porosity of the particulate mixed metal oxide material, as determined by mercury intrusion porosimetry, may be from 3 to 35%, for example from 4 to 35%, from 5 to 35%, from 6 to 35%, from 7 to 35%, from 8 to 35%, from 9 to 35%, from 10 to 35%, from 15 to 35%, from 20 to 35%, from 25 to 35%, from 30 to 35%, from 31 to 35%, from 32 to 35%, from 33 to 35% or from 34 to 35%. The intra-particle porosity of the particulate mixed metal oxide material, as determined by mercury intrusion porosimetry, may be from 5 to 34%, from 5 to 33%, from 5 to 32%, from 5 to 31%, from 5 to 30%, from 6 to 30%, from 7 to 30%, from 8 to 30%, from 9 to 30%, from 9 to 26%, from 10 to 30%, from 10 to 29%, from 10 to 28%, from 10 to 27%, from 10 to 26% or from 10 to 25%. The intraparticle porosity of the blended material is less than the intra-particle porosity of the particulate mixed metal oxide material, as determined by mercury intrusion porosimetry. The use of mercury intrusion porosimetry to determine the characteristics of material porosity is well-known. The method involves the gradual increase in the pressure applied to a column of mercury which is in contact with the material to be tested. As the pressure is increased, larger voids and pores will fill with mercury first, followed by smaller pores at higher pressures. For any material, inter-particle voids are larger than intra-particle pores. So, for a material with both inter-particle voids and intra-particle pores, it is possible to distinguish these during mercury intrusion porosimetry due to the higher pressure necessary to impregnate the intra-particle pores with mercury. The result is one or more low-pressure volume intrusion peaks for the filling of the larger inter-particle voids, along with one or more separate high-pressure volume intrusion peaks for the filling of the smaller intra-particle voids. For example, the presence of peaks at a pressure greater than 400 psia may be indicative of the presence of intra-particle porosity. A material lacking intra-particle porosity would only exhibit the low-pressure volume intrusion peak(s) for the filling of the larger inter-particle voids. As used herein, “intra-particle porosity” refers to the volume of “open” pores within the particles as a percentage of the total volume occupied by the particles. Only the open pores are taken into account for intra-particle porosity, since mercury is unable to intrude into closed pores, so such pores are “invisible” to mercury intrusion porosimetry. For the avoidance of doubt, the “total volume occupied by the particles” for the purposes of calculating intra-particle porosity does not include the voids between particles. In other words, the intra-particle porosity is an intrinsic property of the material and is independent of properties of the bulk material such as particle packing or bulk density. At least 50% of the total pore volume of the intra-particle porosity may be open to the particle surface, for example at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% or at least 95% of the total pore volume. This can be determined by comparing the total intra-particle porosity determined by mercury intrusion porosity with the total porosity determined through image analysis of a cross section of the particle in an SEM image. The porosity calculated by such SEM image analysis will include all intra-particle porosity, including both open and closed pores. The porosity determined by mercury intrusion porosity will only factor in open pores which are able to be intruded by mercury. The particulate mixed metal oxide material may provide high proportions of open porosity, as may be evident from an analysis of SEM images of the particles. The intra-particle pore structure of the particulate mixed metal oxide material may result at least in part from the use of a mixed metal carbonate precursor material during synthesis. When such a carbonate precursor is subsequently calcined to form the oxide material of the invention, carbon dioxide is evolved during the calcination, creating a network of pores through the entire body of the particle, the pores being open to the particle surface and extend radially from the surface towards the particle centre, as observed by a qualitative assessment of the pores in an SEM image. Such pores have low tortuosity and form an essentially continuous network of intra-particle porosity. By contrast, materials made by an analogous synthesis route which uses a mixed metal hydroxide precursor material instead of a mixed metal carbonate precursor material would produce dense particles with little or no intra-particle porosity. The particulate mixed metal oxide material may comprise a TAP density of from 1.0 to 4.0 g / cm3, for example from 1.0 to 2.5 g / cm3, from 1.0 to 2.0 g / cm3, from 1.0 to 1.8 g / cm3, from 1.0 to 1.6 g / cm3, from 1.1 to 1.6 g / cm3, or from 1.2 to 1.6 g / cm3. The particulate mixed metal oxide material may comprise a TAP density of from 1.4 to 2.2 g / cm3. The TAP density of the blended material is greater than the TAP density of the particulate mixed metal oxide material. For example, the TAP density of the blended material may be at least 5% greater, or at least 7% greater, or at least 9% greater, or at least 11% greater, or at least 13% greater, or at least 15% greater, or at least 17% greater, or at least 19% greater than the TAP density of the particulate mixed metal oxide material. The TAP density of the blended material may be at most 40% greater, or at most 35% greater, or at most 30% greater, or at most 25% greater, or at most 20% greater than the TAP density of the particulate mixed metal oxide material. The particulate mixed metal oxide material may comprise particles having a shape which is near-spherical, or potato-like. The particulate mixed metal oxide material may comprise particles having a sphericity of at least 60%, for example at least 70%, at least 75%, at least 80%, at least 85%, at least 90% or at least 95%. The blended material may comprise at least 5 wt%, or at least 10 wt%, or at least 15 wt%, or at least 20 wt%, or at least 25 wt% of the s-block-metal transition-metal oxide material, based on the total weight of the blended material. The blended material may comprise at most 40 wt%, or at most 35 wt%, or at most 30 wt%, or at most 25 wt%, or at most 20 wt% of the s-block-metal transition-metal oxide material, based on the total weight of the blended material. The blended material may comprise at least 60 wt%, or at least 65 wt%, or at least 70 wt%, or at least 75 wt%, or at least 80 wt% of the particulate mixed metal oxide material, based on the total weight of the blended material. The blended material may comprise at most 95 wt%, or at most 90 wt%, or at most 85 wt%, or at most 80 wt%, or at most 75 wt% of the particulate mixed metal oxide material, based on the total weight of the blended material. A fifth aspect of the invention provides an electrode comprising the s-block-metal transition-metal oxide material according to the third aspect, or the blended material according to the fourth aspect. The electrode may comprise the s-block-metal transition-metal oxide material according to the third aspect and a binder. The binder may comprise a polymer (such as PVDF, PTFE, NaCMC or NaAlginate). The s-block-metal transition-metal oxide material may make up at least 50 vol% of the electrode, based on the total volume of electrode, for example at least 55 vol%, at least 60 vol%, at least 62 vol%, at least 64 vol%, or at least 65 vol%. The s-block-metal transition-metal oxide material may make up at most 80 vol% of the electrode, based on the total volume of electrode. The s-block-metal transition-metal oxide material may make up from 64 vol% to 80 vol% of the electrode, based on the total volume of electrode. The electrode may be a positive electrode (cathode). The cathode may be a solid, solvent-cast cathode. Alternatively, the cathode may be a polymer gel cathode, comprising a gelled polymer matrix made up of a polymer-electrolyte gel matrix phase and a dispersed phase, wherein the dispersed phase comprises the s-block-metal transition-metal oxide material (positive active material). The polymer-electrolyte gel matrix phase may be formed from one or more electrolyte components and at least one gelling polymer. The one or more electrolyte components may include a solvent suitable for use as an electrolyte solvent in a gel electrode, for example an organic solvent. The one or more electrolyte components may include a salt. The one or more electrolyte components may constitute an electrolyte salt solution or liquid electrolyte. The one or more electrolyte components may comprise a solvent comprising one or more cyclic or linear carbonate compounds. The solvent may comprise one or more cyclic carbonate compounds. The solvent may comprise one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. The solvent may comprise a blend of at least two different compounds, for example at least three or at least four different compounds. The solvent may comprise a blend of at least two different organic carbonate compounds, for example at least three or at least four different organic carbonate compounds. The electrolyte component(s) may comprise a solvent with low vapour pressure and high flash point to enable safe processing. An example of a solvent fulfilling these criteria is propylene carbonate. Accordingly, the one or more electrolyte components may comprise or consist of propylene carbonate, or a blend of propylene carbonate with one or more of the above listed solvents. The one or more electrolyte components may comprise an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). The alkali metal salt may be a lithium, sodium, or potassium salt. The anion of the alkali metal salt may be any suitable anion. Typical anions are known to the skilled person and may be chosen based on the kind of alkali metal. For example, when the alkali metal is lithium, the anion of the salt comprises a halogen such as fluorine. Examples include BF4-, PFe”, TFSI-, FSI-, OTf, DFOB- and TDL. The one or more electrolyte components may comprise a lithium salt. The electrolyte may comprise a mixture of at least two different lithium salts. Examples of suitable lithium salts include LiPFe, LiBF4, LiTFSI, LiFSI, LiOTf, LiDFOB and LiTDI. The salt may be a thermally stable salt. It has been found that LiPFe has relatively low thermal stability relative to other available lithium salts, and accordingly use of LiPFe may be avoided - that is, in some instances, the electrolyte component(s) do not include LiPFe. One or more kinds of alkali metal salt may be used in accordance with the present invention. Typically, but not exclusively, when more than one kind of alkali metal salt is used, they share a common alkali metal. The polymer-electrolyte gel matrix phase may comprise a gel matrix formed by the gelling of one or more gelling polymers when the polymer(s) absorb a liquid electrolyte. The polymer-electrolyte gel matrix phase may therefore comprise a gel comprising the polymer(s) and absorbed liquid electrolyte. The gelling polymer may comprise one or more gelling polymers independently selected from carboxymethyl cellulose (CMC), poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA), polystyrene (PS), poly(ethyleneglycol dimethylether), poly(ethyleneglycol diethylether), poly[bis(methoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene-substituted polystyrene, ferrocene-substituted polyethylene, carbazole-substituted polyethylene, polyoxyphenazine, poly(heteroacene), poly[(4-styrenesulfonyl)(frifluoromethanesulfonyl)imide-co-methoxy- polyethyleneglycolacrylate] (Li[PSTFSI-co-MPEGA]), sulfonated poly(phenylene oxide) (PPO), N,N-dimethylacryl amide (DMAAm), lithium 2-acrylamido-2-methyl-l-propane sulfonate (LiAMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co-Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, PEO terpolymer, and NIPPON SHOKUBAI® polymer, or mixtures or co-polymers thereof. In some examples, the gelling polymer may comprise one or more gelling polymers independently selected from carboxymethyl cellulose (CMC), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA) and polystyrene (PS) The polymer-electrolyte gel matrix phase may make up from 20 vol% to 50 vol% of the cathode, for example from 25 vol% to 45 vol%, from 28 vol% to 42 vol%, from 30 vol% to 40 vol%, from 31 vol% to 39 vol% or from 32 vol% to 38 vol%. Suitably, the polymerelectrolyte gel matrix phase may make up about 33.52 vol%, about 34.11 vol%, or about 34.72 vol% of the cathode. The gel cathode may be produced by processing a cathode precursor composition to form a film or coating. The processing may comprise thermal processing and / or extrusion. The gel cathode may be an extruded cathode. Alternatively, the gel cathode may be a hot-rolled cathode. Alternatively, the gel cathode may be prepared by extruding a cathode precursor composition through a die to form a film. The gel cathode may be both extruded and hot-rolled, for example by a process which comprises first extruding the cathode through a die followed by hot-rolling the cathode down to a desired thickness. The cathode may be a solid, solvent-cast cathode. Such solid, solvent-cast cathodes may be prepared by mixing the positive active material, binder and optional further additive(s) with a solvent, before casting onto a foil layer and drying to remove solvent. Calendaring may then be performed after drying. The cathode may comprise the positive active material, a conductive additive and a binder. The conductive additive may comprise or consist of carbon, for example carbon black and / or carbon nanotubes. The polymer may comprise or consist of one or more of the polymers set out in the list of gelling polymers above. The polymer may comprise or consist of PvDF, or PvDF-HFP. The cathode may be for a lithium-ion secondary electrochemical cell or battery. A sixth aspect of the invention provides an electrochemical secondary cell or battery comprising the electrode according to the fifth aspect. The electrochemical secondary cell or battery may comprise the electrode according to the fifth aspect, a second electrode, and an electrolyte (between the two electrodes). The electrode according to the fifth aspect may be a positive electrode (cathode) and the second electrode is a negative electrode (anode). The cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell. The cell may be a lithium-ion secondary cell. The electrochemical secondary cell may comprise an electrode according to the fifth aspect that is a cathode laminated with a current collector, for example a metallic foil. A seventh aspect of the invention provides an electrical device or a vehicle comprising the electrochemical secondary cell or battery according to the sixth aspect. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows SEM images of Inventive Example 1 (images (a) to (c)) and Comparative Example 1 (images (d) to (f)). Figure 2 shows the XRD patterns for Comparative Example 1 (top pattern) and Inventive Example 1 (bottom pattern). Figure 3 shows EBSD analysis for Inventive Example 1 at a first location (see image (a) and graph (c)) and a second location (see image (b) and graph (d)) of the material. Figure 4 shows the impact of the particle size and morphology on the electrochemically active surface area (ESA), as measured by EIS on Comparative Example 1 (left bar), Comparative Example 2 (middle bar) and Inventive Example 1 (right bar). Figure 5 shows a comparison of the electrochemical behaviour of standard porous Li-rich material (Comparative Example 1, light grey lines) with that of densified Li-rich cathodes calcined in pure O2 (Inventive Example 1, dark grey lines) and in air (Inventive Example 2, black lines). Graph (a) shows the first cycle, while graph (b) shows the second cycle at C / 10. Figure 6 shows energy density (in graph (a)) and energy retention (in graph (b)) as a function of the number of cycles. Comparative Example 1 corresponds to the lines with points shown with circles; Inventive Example 1 corresponds to the lines with points shown with squares; and Inventive Example 2 corresponds to the lines with points shown with triangles. EXAMPLES &DETAILED DESCRIPTION Preparation of Materials A 2 L mixed transition metal solution of total concentration 2.0 M was prepared by adding NiSO4, MnSO< and CoSO4 to 2 L of water. 250 mL of the mixed metal sulfate solution was mixed with 500 mL of a 2.2 M aqueous solution of ammonium bicarbonate at 50 °C in a conical flask. The mixture was stirred at 500 rpm using a magnetic stirrer bar. The reaction was allowed to proceed for 30 minutes, after which time it was considered complete. A precipitate (slurry) of transition metal carbonate precursor particles that was formed during the reaction was collected by filtration, and elemental analysis was performed by using X-ray fluorescence in order to determine the content of transition metals. LiiCCh was then added in an amount such that the molar ratio of the lithium to transition metal carbonate precursor particle was 1.24:1.00 (i.e. a 10% excess in order to achieve a ratio of at least 1.13:1.00). This was done in order to compensate for possible Li evaporation at high temperature, and thus avoid under-lithiation of the final active material. The two powders were blended using a resonant acoustic mixer (Resodyn) for 2 minutes - this step was repeated twice. The resultant mixture was then transferred into a cylindrical alumina crucible for high temperature calcination. As shown in Table 1 below, different calcination protocols and atmospheres were tested. In Inventive Example 1, the calcination took place in an oxygen-enriched atmosphere (pure O2) and comprised a high-temperature calcination step performed at 1000 °C. The calcination in Inventive Example 2 was the same as Inventive Example 1, except that the calcination took place in air. The calcination in Comparative Example 1 was the same as Inventive Example 2, except that there was no high-temperature calcination step performed at 1000 °C. Table 1 - Calcination protocols for Inventive Examples 1&2, and Comparative Example 1 Tl temp. (°C) Tl dwell (hours) Tm temp. (°C) Tm dwell (hours) Th temp. (°C) Th dwell (hours) Atmosphere Inv. 1 240 5 850 5 1000 5 o2 Inv. 2 240 5 850 5 1000 5 Air Comp. 1 240 5 850 5 N / A N / A Air Calcination in an oxygen-enriched atmosphere was performed inside a muffle furnace equipped with an air-tight retort which allowed for control of the atmosphere. The standard O2 flow was 5 L / min. Once the material reached room temperature, the crucible was removed from the furnace and gently ground with a pestle and mortar to break any possible agglomerates. The ground powder was then sieved with a 45 pm sieve to remove larger agglomerates. The powder could then be used for the preparation of electrodes to be used in electrochemical tests, as discussed below. Analysis of Materials Scanning electron microscopy (SEM) analysis was performed on the materials of Inventive Example 1 and Comparative Example 1. Cross-sections for SEM analysis were prepared using a JEOL^™) IB-19520CCP ion bean polisher. A JEOL^™) 7900F scanning electron microscope was then used to investigate the material morphology. Figure 1 shows SEM images of the materials and cross-sections of Inventive Example 1 (images (a) to (c)) and Comparative Example 1 (images (d) to (f)). As shown in Figure 1, the material of the invention comprises small spherical secondary particles with a diameter of about 2 to 3 pm (referred to as “2 pm” in the Figures 5 and 6). The primary particles that constitute the agglomerates have a diameter ranging from about 200 to 500 nm. The core of these spheres is very dense, with only few gaps visible. Due to the larger crystallite size, very few grain boundaries could be observed. The material of the invention can be described as a “densified” Li-rich material. Conversely, the material of Comparative Example 1 can be described as a “standard porous” Li-rich material. As shown in Figure 1, such a material has a significantly different morphology. The particles are larger (having a diameter of about 8 pm) and appear to have a rougher surface, which is the result of the agglomeration of smaller crystallites (having a diameter of <100 nm). Cavities on the surface leading towards the centre of the particles were observed. Such porous structure is not limited to the surface, but spread across the whole particle. The more porous structure has the disadvantageous consequence of decreasing material density and increasing the surface area of the material. Powder X-ray diffraction (XRD) analysis was performed using a Malvern Panalytical^™^ Aeris^™) Powder X-ray Diffractometer, using CuKa with an X-ray wavelength of 1.54056 A. The XRD pattern was collected at angles between 15 and 75 26. Figure 2 shows the XRD patterns for Comparative Example 1 (top pattern) and Inventive Example 1 (bottom pattern). It can be seen that the reflections of the densified Li-rich material are much sharper than those of the standard porous Li-rich material of Comparative Example 1. Narrower diffraction peaks can generally be attributed to larger crystal domains. In order to investigate the ciystal domain size, SEM imaging and electron backscatter diffraction (EBSD) analysis was performed on the material of Inventive Example 1. A polished cross-section (1 mm width) was prepared using a JEOL IB-19520CCP ion bean polisher, before a JEOL^™} 7900F scanning electron microscope was then used (25 kV electron beam, medium probe current size 8, working distance 15.0 mm, sample tilt 70°, image magnification xlOk). EBSD data were then collected using an Oxford Instruments^™} Symmetry detector (speed acquisition 1-5 Hz, exposure time 500 ms, hit rate 15-25%, crystal indexation 225 space group, m-3m Laue group). EBSD analysis was performed at two different locations of the material (a first and second location). The raw unprocessed data were filtered to remove artefacts that were not representative of crystal domains. Figure 3 shows the processed data for the first and second location, in image (a) and image (b) respectively. In these images, the crystal domains used for the analysis of the crystal domain area are represented as patches on a dark background. The analysis of the crystal domain area for the first and second location are shown in graph (c) and graph (d) respectively of Figure 3. These bar graphs depict the number of crystal domains having an area falling in particular ranges as shown. The material of Inventive Example 1 was found to have crystal domains with an area particularly in the range of 0.02 to 0.13 pm2. The diameter of the crystal domains was found to be particularly in the range of 0.16 to 0.41 pm. Therefore, the size of the crystal domains was similar to those of a commercial (not “densified”) NMC material, which was found to have crystal domains with an area particularly in the range of 0.03 to 0.22 pm2 and a diameter particularly in the range of 0.2 to 0.5 pm (as measured by the same technique). Preparation of Electrodes The calcined and sieved powders (prepared as described above) were used for the preparation of electrodes (cathodes). This process involved the production of thick slurry containing 94 wt% of cathode active material (CAM), 3 wt% of polyvinylidene fluoride (PVDF, Solef 5140, Solvay) which acts as a binder to provide the desired mechanical strength to the cathode, and 3 wt% of carbon black (Super C65, Imerys). In particular, the procedure involved pouring the required amount of a 6 wt% of PVDF in NMP (N-methyl-2-pyrrolidone) inside a plastic pot. The required amount of carbon black was then added to the same plastic pot. The pot was then transferred into a mixer (Thinky^™), ARE-250) and mixed for 30 minutes at 1500 rpm. The CAM was then added to the pot, which was then mixed for an additional 30 minutes at 1500 rpm. Pure NMP was then added to the pot in small aliquots, and the pot was mixed for 15 minutes at 1500 rpm after each addition. The process was repeated until the desired viscosity was achieved. Once the slurry was sufficiently fluid, it was poured onto a drawdown table (MTI) equipped with a doctor blade (Zehntnei^1™^). The cast material was then transferred onto a hot plate set at 90 °C, where the electrode was left to dry for 30 minutes. The electrodes were calendared to achieve the desired density of 2 g / cm3. Disc electrodes with a diameter of 12.7 mm were then punched out, before being dried in a vacuum at 120 °C for 12 hours inside a glass oven (Biichi^™) B-585). Finally, the airtight glass oven was transferred inside an Ar-filled glovebox, where the subsequent battery assembly could take place. Electrochemical Testing Conditions Electrodes (cathodes) were electrochemically tested in half-cell configuration. 2032 stainless steel coincell (Hohsen Corp) were assembled. Glass fibre (Whatman^™)) was used as separator and a 1 mm stainless steel disc as spacer. A Li chip (PI-KEM) was used as counter-electrodes. LP40 (Tomiyama^™)) was used as liquid electrolyte. During coincell assembly, the various components were stacked inside the coincell base which was pre-equipped with a plastic gasket, in order to guarantee complete air-tightness after the coincell was sealed. The different components were stacked in the following order: cathode with the current collector side facing the coincell base, glass fibre separator, Li chip, spacer, and spring. A 1.4 mm stainless steel spring was required to guarantee sufficient stack pressure. 90 pL of electrolyte were pipetted onto the glass fibre separator during stack assembly, before loading the Li chip. The coincell was then sealed by crimping a lid onto it (Hohsen Corp). All these steps were performed inside an Ar-filled glovebox with <0.5 ppm of O2 and <0.5 ppm of H2O. Once assembled, the coincell was connected to a battery cycler (MPG2, Biologic*^™)) which was operated with EC-Lab software (vll.43, Biologic). All coincells were connected using a coincell holder which was kept inside an incubator throughout the duration of the experiment, in order to avoid fluctuation of the temperature. All experiments were performed at 30 °C. The typical experiment comprises a 10 h resting step, which allows for proper wetting by the electrolyte of the various components built inside the coincell. The cell was then cycled within the voltage window of 2.0 and 4.8 V vs Li+ / Li. In a typical cycle, during charge, a positive constant current density of 30 mA / g was applied to the cell up to an upper cutoff voltage of 4.8 V vs Li+ / Li. Once the cell voltage reached this value, it was held at 4.8 V until the current density reached a value of 6 mA / g. A constant negative current density of 30 mA / g was then applied until a lower voltage cutoff of 2.0 V was reached. This whole process, which corresponds to a full cycle, was then repeated. Electrochemically active surface area (ESA) was determined using electrochemical impedance spectroscopy (EIS). The EIS method was based on a method disclosed in Oswald et al., “Novel method for monitoring the electrochemical capacitance by in situ impedance spectroscopy as indicator for particle cracking of nickel-rich NCMs: Part I. Theory and Validation”, Journal of the Electrochemical Society, 2020. Electrochemical Testing Results Figure 4 shows the impact of the particle size and morphology on the electrochemically active surface area (ESA), as measured by EIS on Comparative Example 1 (left bar), Comparative Example 2 (middle bar) and Inventive Example 1 (right bar). Comparative Example 2 is a material comprising particles having a diameter of about 2 to 3 pm (similar to Inventive Example 1), but with greater porosity (similar to Comparative Example 1). Comparative Example 2 is synthesised from the same source material used for Inventive Example 1. Comparative Example 2 is synthesised with a 2-step calcination protocol in air, involving a 5 h step at 240 °C followed by 5 h at 850 °C. Sphericity and size of Comparative Example 2 were similar to those of Inventive Example 1. They differ for their internal porosity and surface roughness. The ESA of the standard porous material of Comparative Example 1, having a particle diameter of about 8 pm, was found to be 0.61 m2 / g. When the particle diameter is decreased to about 2 to 3 pm whilst maintaining porosity, the ESA increases to 1.14 m2 / g (Comparative Example 2), i.e. an 87% increase. Such a higher surface area represents a significant drawback for the long-term cycling performance, because there is more contact area between the active material and the electrolyte, which may result in more pronounced side reactions. However, when the porous material of approximately 2-3 pm particles is densified via the higher temperature calcination process, the ESA drops to 0.70 m2 / g (Inventive Example 1), which is only a 15% increase from the standard porous material. Lower ESA is expected to have a beneficial impact on electrochemical stability. Figure 5 shows a comparison of the electrochemical behaviour of standard porous Li-rich material (Comparative Example 1, light grey lines) with that of densified Li-rich cathodes calcined in pure O2 (Inventive Example 1, dark grey lines) and in air (Inventive Example 2, black lines). Graph (a) shows the first cycle, while graph (b) shows the second cycle at C / 10. Both densified Li-rich cathodes displayed higher overpotentials than the standard porous Li-rich cathode during charge and discharge. Such overpotential difference was especially marked during the first formation cycle, whereas it decreased from the second cycle onwards. It was less pronounced for the Ch-synthesised material. The first cycle discharge capacity for the inventive cathodes (approximately 250 mAhg-1) was lower than that of the comparative cathode (280 mAhg-1). Such observations seem to fit the slow Li-ion diffusion kinetics of Li-rich materials, which are exacerbated in the case of densified materials by the lower porosity and larger average crystallite size. On the other hand, the low surface area and porosity appear to provide a significantly higher cycling stability. Figure 6 shows energy density (in graph (a)) and energy retention (in graph (b)) as a function of the number of cycles. Comparative Example 1 corresponds to the lines with points shown with circles; Inventive Example 1 corresponds to the lines with points shown with squares; and Inventive Example 2 corresponds to the lines with points shown with triangles. The first 20 cycles are shown in the voltage range of 2.0 to 4.8 V. Cycling was performed at a rate of C / 10 with a constant voltage step at 4.8 V with a current cut-off of C / 50. All tests were performed at 30 °C. While the initial energy density was about 15% lower for the densified Li-rich cathode, the energy retention was markedly enhanced. The standard porous Li-rich material was observed to retain only 87% of the initial energy, whereas the densified cathode synthesised in pure O2 demonstrated negligible energy fade. Meanwhile, the densified cathode synthesised in air maintained lower initial fade rate compared to the standard porous material, while showing fewer advantages in terms of long-term stability. After only 20 cycles, representing a relatively short stability test, the standard porous Li-rich material and the densified Li-rich material synthesised in O2 were observed to have the same energy density. Moreover, the outlook of energy density suggested that the denser cathode would be able to outperform the standard porous Li-rich cathode in the longer term. A more stable material is appealing because it allows for a balanced full Li-ion cell to be properly designed and built. Therefore, despite the lower initial energy, the densified Li-rich material appears to have significant potential. Blended Material Cathode active material with different sizes and internal porosities were blended to improve the packing efficiency of material. Specifically, the densified Li-rich material of Inventive Example 1 was blended with a standard porous active material of Comparative Example 2. The standard porous (Li-rich) active material of Comparative Example 2 was made by suspending a particulate transition metal carbonate material in a liquid vehicle, and initiating a precipitation reaction involving transition metal sources in the liquid vehicle, before calcining together with an alkali metal source. Using the techniques described herein, the resultant material of Comparative Example 2 was found to comprise secondary particles having an average diameter that is at least three times greater than the average diameter of the secondary particles of Inventive Example 1. In order to make the blend of the materials of Inventive Example 1 and Comparative Example 2, the desired amounts of materials were weighed inside a plastic pot. The pot was then loaded onto a resonant acoustic mixer (RAM, Resodyn*^™)) and mixed for 2 minutes with an acceleration of 70g. This process was repeated twice. A sample of the homogeneous powder was then transferred into a graduated glass cylindrical chamber (having an internal diameter of 12.7 mm), which was then placed in an apparatus (Micromeritics^™) GeoPyc^™) 1365) that physically tapped the cylinder, causing the material to settle. This was consolidated by applying a plunger for a total of 10 repetitions, which repeatedly compacted the powder. The final volume was then used to determine the TAP density from the mass of powder that had been loaded into the chamber (i.e. mass divided by volume). As shown in Table 2 below, the blending of the small, spherical particles with the larger cathode material yielded a blended material with increased TAP density (a 15% increase from 1.9 to 2.2 g / cm3). This is believed to be due to improved packing of different 5 particles, allowing the available volume to be utilised more efficiently. Advantageously, this increased TAP density is in turn expected to correlate with increased volumetric energy density. Table 2 - TAP density ofpure and blended standard porous Li-rich material TAP density (g / cm3) Proportion of Inventive Example 1 particles Pure material (Comparative Example 2) 1.9 0 wt% Blended material (Comparative Example 2 and Inventive Example 1) 2.2 20 wt% 10

Claims

1. An s-block-metal transition-metal oxide particle having a diameter of at most 4 pm and a circularity of at least 50%.

2. The s-block-metal transition-metal oxide particle according to claim 1, having a diameter in the range from 1.0 to 2.5 pm.

3. The s-block-metal transition-metal oxide particle according to either claim 1 or claim 2, wherein the s-block-metal transition-metal oxide particle is a secondary particle comprising a plurality of primary particles, wherein the primary particles have an average diameter in the range of from 200 to 500 nm.

4. The s-block-metal transition-metal oxide particle according to any one of claims 1 to 3, wherein the s-block-metal transition-metal oxide particle comprises crystal domains having an average area of at least 0.02 pm2 and / or an average diameter of at least 160 nm.

5. The s-block-metal transition-metal oxide particle according to any one of claims 1 to 4, wherein the s-block-metal transition-metal oxide particle is a lithium transition-metal oxide particle.

6. A method of manufacturing an s-block-metal transition-metal oxide particle according to any one of claims 1 to 5, the method comprising:mixing a transition metal salt with a second salt in a liquid vehicle, wherein the second salt is selected from one or more of a carbonate salt and a bicarbonate salt;carrying out a precipitation reaction to form a slurry comprising a transition metal carbonate precursor particle in the liquid vehicle;mixing the transition metal carbonate precursor particle with an s-block metal source, and calcining the mixture of the transition metal carbonate precursor particle and the s-block metal source to form the s-block-metal transition-metal oxide particle;wherein the calcination comprises a high-temperature calcination step performed at a temperature of greater than 850 °C.

7. The method according to claim 6, wherein the transition metal salt comprises or consists of a transition metal sulfate salt.

8. The method according to either claim 6 or claim 7, wherein the transition metal salt consists of two or more transition metal salt species; optionally wherein the transition metal salt comprises or consists of one or more of a manganese salt, a nickel salt, a cobalt salt, an iron salt, a titanium salt, a vanadium salt, a tungsten salt, and a zirconium salt.

9. The method according to any one of claims 6 to 8, wherein the transition metal salt comprises a nickel salt, a cobalt salt and a manganese salt; optionally wherein the nickel salt is a nickel sulfate salt, the cobalt salt is a cobalt sulfate salt, and the manganese salt is a manganese sulfate salt.

10. The method according to any one of claims 6 to 9, wherein, in the step of mixing the transition metal salt with the second salt in the liquid vehicle, a further metal source is also mixed in the liquid vehicle, wherein the further metal source comprises one or more of magnesium, boron, aluminium, and tin.

11. The method according to any one of claims 6 to 10, wherein the second salt has a solubility in water of at least 90 g / L at 20 °C, and / or wherein the second salt comprises or consists of ammonium bicarbonate.

12. The method according to any one of claims 6 to 11, wherein the liquid vehicle is water, and the transition metal salt and the second salt are dissolved in the water before the precipitation reaction; and / or wherein the molar ratio of the transition metal salt to the second salt is from 1:4 to 1:80.

13. The method according to any one of claims 6 to 12, wherein the precipitation reaction is performed at a temperature within the range of 15 to 50 °C.

14. The method according to any one of claims 6 to 13, wherein, in the step of mixing the transition metal carbonate precursor particle with an s-block metal source, the molar ratio of the s-block metal of the s-block metal source to the total transition metals of the transition metal carbonate precursor particle is at least 1.20:1.00.

15. The method according to any one of claims 6 to 14, wherein the s-block metal source comprises or consists of lithium carbonate, lithium hydroxide, sodium carbonate, sodium hydroxide, magnesium carbonate, or magnesium hydroxide.

16. The method according to any one of claims 6 to 15, wherein the s-block metal of the s-block metal source comprises or consists of lithium, such that the s-block-metal transition-metal oxide particle is a lithium transition-metal oxide particle.

17. The method according to any one of claims 6 to 16, wherein the high-temperature calcination step is performed at a temperature of at least 900 °C, or at least 950 °C.

18. The method according to any one of claims 6 to 17, wherein the high-temperature calcination step is performed in an oxygen-enriched atmosphere, optionally having an oxygen content of at least 75%, or at least 90%.

19. The method according to any one of claims 6 to 18, wherein the calcination comprises a low-temperature calcination step performed at a temperature (Tl) in the range of at least 200 °C to less than 600 °C, followed by a medium-temperature calcination step performed at a temperature (Tm) in the range of at least 600 °C to less than 900 °C, followed by the high-temperature calcination step; wherein the high-temperature calcination step is performed at a temperature (Th) of at least 900 °C; and wherein heating to Tl, heating from Tl to Tm, and heating from Tm to Th are each performed at a ramping rate in the range of from 0.5 to 10 °C / min.

20. An s-block-metal transition-metal oxide particle obtained or obtainable by the method according to any one of claims 6 to 19.

21. An s-block-metal transition-metal oxide material comprising a plurality of particles according to any one of claims 1 to 5 and claim 20; optionally wherein the s-block-metal transition-metal oxide material has a TAP density of at least 1.75 g / cm3.

22. A blended material comprising a mixture of: (i) the s-block-metal transition-metal oxide material according to claim 21; and (ii) a particulate mixed metal oxide material comprising an intra-particle porosity of from 2 to 35%, as determined by mercury intrusion porosimetry before mixing with the s-block-metal transition-metal oxide material;wherein the Dv50 (volumetric median particle diameter) of the particulate mixed metal oxide material is greater than the Dv50 of the s-block-metal transition-metal oxide material; andwherein the TAP density of the blended material is greater than the TAP density of the particulate mixed metal oxide material; and the intra-particle porosity of the blended material is less than the intra-particle porosity of the particulate mixed metal oxide material, as determined by mercury intrusion porosimetry.

23. An electrode comprising the s-block-metal transition-metal oxide material according to claim 21, or the blended material according to claim 22.

24. An electrochemical secondary cell or battery comprising the electrode according to claim 23.

25. An electrical device or a vehicle comprising the cell or battery according to claim 24.

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