WO2023150922A1

WO2023150922A1 - Entropy stabilised oxide

Info

Publication number: WO2023150922A1
Application number: PCT/CN2022/075610
Authority: WO
Inventors: Yuguang PU; Peng CAO
Original assignee: Auckland Uniservices Ltd
Current assignee: Auckland Uniservices Ltd
Priority date: 2022-02-09
Filing date: 2022-02-09
Publication date: 2023-08-17
Anticipated expiration: 2024-08-09

Abstract

An entropy stabilised oxide represented by the formula (Mg_vCo_wNi_xCu_yZn_z) O, wherein v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1; and wherein the entropy stabilised oxide is characterised by any one or more of the following: a X-ray powder diffraction spectrum comprising (111) and (200) peaks wherein the relative peak intensity of I₍₁₁₁₎ /I₍₂₀₀₎ is greater than about 1; an average particle size D₅₀ of about 800 nm or less; a particle size D₉₀ of about 1200 nm or less; submicron particles having a rod-like shape; or a particle size distribution having a standard deviation of less than about 250 nm. Also disclosed are methods for preparing an entropy stabilised oxide. Also disclosed are an electrode, e. g. an anode, a catalyst and an electrochemical cell comprising the entropy stabilised oxide.

Description

ENTROPY STABILISED OXIDE

TECHNICAL FIELD

The present invention relates to an entropy stabilised oxide, method for the preparation thereof and the use thereof as an electrode material and a catalyst.

BACKGROUND ART

Entropy stabilised oxides, also known as high entropy oxides, have drawn much attention for their outstanding compositional and structural stability under extreme conditions, such as extreme temperatures and chemical environments. Furthermore, many other appealing and unique properties have been discovered in these materials, such as exceptional superionic conductivity at room temperature, high dielectric constant, and tailorable bandgap. As such, entropy stabilised oxides may be useful as anode materials (e.g. in lithium batteries) , cathode materials, and catalysts.

Solid-state synthesis is the most common and facile method to fabricate entropy stabilised oxides. For example, Qiu et al. (Qiu, N.; Chen, H.; Yang, Z.; Sun, S.; Wang, Y.; Cui, Y., A high entropy oxide (Mg _0.2Co _0.2Ni _0.2Cu _0.2Zn _0.2O) with superior lithium storage performance. Journal of Alloys and Compounds 2019, 777, 767-774) describes a method in which MgO, CoO, NiO, CuO and ZnO were mixed in a planetary ball mill, then pressed into pellets and sintered at 1000℃ for 24 hours. However, the particles synthesised by solid state methods are often large, which adversely impacts the applications of entropy stabilised oxides. For example, large particles tend to slow down catalytic reaction rates due to the limited specific surface area of catalysts. A recently developed variation of the solid-state method used nebulised spray pyrolysis to synthesise (Mg _0.2Co _0.2Ni _0.2Cu _0.2Zn _0.2) O (Sarkar, A.; Velasco, L.; Wang, D.; Wang, Q.; Talasila, G.; de Biasi, L.; Kübel, C.; Brezesinski, T.; Bhattacharya, S. S.; Hahn, H., High entropy oxides for reversible energy storage. Nature communications 2018, 9 (1) , 1-9) . The entropy stabilised oxide particles synthesised by this method were found to be either hollow or solid spheres, and the particle size ranged from nanometre to micrometre. Such a broad size distribution may lead to a high overpotential on large particles when the particles are used as electrode materials.

Alternatively, co-precipitation methods may be used to synthesise nanosized entropy stabilised oxides. These methods generally involve transforming metal cations into a hydroxide precursor and then annealing the precursor to provide an oxide product. Sodium hydroxide and ammonia solution are commonly used to prepare the hydroxide precursors. However, many practical issues arise from these methods. For instance, as sodium hydroxide and ammonia solution react with metal cations rapidly, it is difficult to regulate the morphology and size distribution of the synthesised particles. Hexamethylenetetramine (HMTA) or urea are sometimes used as precipitants for homogeneous deposition in addition to direct hydroxide sources such as NaOH. In these cases, the functional component for precipitation is ammonia generated by the thermal decomposition of the precipitants, which subsequently dissolves into water to generate ammonium hydroxide. The low basicity when stoichiometric ammonia is added leads to incomplete deposition of Mg ²⁺. On the other hand, excessive addition of urea or HMTA would cause re-dissolution of as-precipitated Cu (OH) ₂ by forming copper-ammonia complexes. After co-precipitation, the obtained precursor usually undergoes a subsequent annealing step at elevated temperatures. Such annealing leads to severe agglomeration of ultrafine entropy stabilised oxide particles. Consequently, it is still challenging to synthesise entropy stabilised oxides with a narrow size distribution and a homogeneous composition of the submicron particles.

Accordingly, it is an object of the present invention to go some way to avoiding the above disadvantages; and/or to at least provide the public with a useful choice.

Other objects of the invention may become apparent from the following description which is given by way of example only.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides an entropy stabilised oxide represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, wherein v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1; and wherein the entropy stabilised oxide is characterised by any one or more of the following:

● a X-ray powder diffraction spectrum comprising (111) and (200) peaks wherein the relative peak intensity of I ₍₁₁₁₎ /I ₍₂₀₀₎ is greater than about 1;

● an average particle size D ₅₀ of about 800 nm or less;

● a particle size D ₉₀ of about 1200 nm or less;

● submicron particles having a rod-like shape;

● a particle size distribution having a standard deviation of less than about 250 nm.

In some embodiments, v, w, x, y and z are each about 0.2.

In some embodiments, the relative peak intensity of I ₍₁₁₁₎ /I ₍₂₀₀₎ is about 1.2 to about 2.0, about 1.3 to about 1.9, about 1.4 to about 1.8, or about 1.5 to about 1.7. Preferably, the relative peak intensity of I ₍₁₁₁₎ /I ₍₂₀₀₎ is about 1.6.

In some embodiments, the entropy stabilised oxide is characterised by a X-ray powder diffraction spectrum comprising (111) , (200) and (220) peaks. In some embodiments, the relative peak intensity of I ₍₂₂₀₎ /I ₍₂₀₀₎ is about 0.3 to about 1.1, about 0.4 to about 1.0, about 0.5 to about 0.9, or about 0.6 to about 0.8. Preferably, the relative peak intensity of I ₍₂₀₎ /I ₍₂₀₀₎ is 0.7

In some embodiments, the entropy stabilised oxide is characterised by a X-ray powder diffraction spectrum comprising (111) , (200) , (220) , (311) and (222) peaks. In some embodiments, the relative peak intensity of I ₍₃₁₁₎ /I ₍₂₂₂₎ is less than about 1.

In some embodiments, the entropy stabilised oxide is characterised by an X-ray powder diffraction spectrum substantially as shown in figure 4.

In some embodiments, the entropy stabilised oxide is characterised as being substantially uniform.

In some embodiments, the entropy stabilized oxide has a particle size distribution having a standard deviation of less than about 245 nm, less than about 240 nm, less than about 235 nm, less than about 230 nm, less than about 225 nm, less than about 220 nm, less than about 215 nm, less than about 210 nm, less than about 205 nm, or less than about 200 nm. In some embodiments, the entropy stabilised oxide has a particle size distribution having a standard deviation of about 215 nm and a relative standard deviation of about 0.34.

In some embodiments, the entropy stabilised oxide is characterised by an average particle size D ₅₀ of about 700 nm or less. In some embodiments, the entropy stabilised oxide is characterised by an average particle size D ₅₀ of about 600 nm or less. In some embodiments, the entropy stabilised oxide is characterised by an average particle size D ₅₀ of about 600 nm. In some embodiments, the entropy stabilised oxide is characterised by a particle size D ₉₀ of about 1100 nm or less. In some embodiments, the entropy stabilised oxide is characterised by a particle size D ₉₀ of about 1000 nm or less. In some embodiments, the entropy stabilised oxide is characterised by a particle size D ₉₀ of about 1000 nm.

In some embodiments, the length : width ratio of the submicron particles is about 1: 15 to about 1: 35. In some embodiments, the length : width ratio of the submicron particles is about 1:2 to about 1: 3. In some embodiments, the length : width ratio of the submicron particles is about 1: 2.5. In some embodiments, the average length of the submicron particles is about 400 nm to about 1000 nm. In some embodiments, the average length of the submicron particles is about 500 nm to about 900 nm. In some embodiments, the average length of the submicron particles is about 630 nm. In some embodiments, the average width of the submicron particles is about 150 nm to about 450 nm, about 200 nm to about 400 nm or about 250 nm to about 350 nm. In some embodiments, the average width of the submicron particles is about 300 nm.

In some embodiments, the entropy stabilised oxide is substantially non-porous. In some embodiments, the entropy stabilised oxide is non-porous.

In some embodiments, the entropy stabilised oxide is substantially homogeneous. In some embodiments, the entropy stabilised oxide is a single phase.

In some embodiments, the entropy stabilised oxide has a lattice parameter of at least about

preferably between about

to about

more preferably about

to about

In some embodiments, the lattice parameter is about

preferably the lattice parameter is

In a second aspect, the present invention is directed to a method of preparing an entropy stabilised oxide, the method comprising:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(b) mixing the Mg salt, the Co salt, the Ni salt, the Cu salt and the Zn salt in a solvent to form a solution;

(c) mixing the solution obtained in step (b) with a precipitating agent to obtain a precipitate;

(d) calcining the precipitate obtained in step (c) to obtain an oxide intermediate; and

(e) annealing the oxide intermediate obtained in step (d) in the presence of a solid-state dispersant to provide the entropy stabilised oxide;

wherein the entropy stabilised oxide is represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, and v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1.

In some embodiments, the precipitating agent is a hydroxide compound (e.g. NaOH) , an organic compound (e.g. an oxalate compound) or a source of ammonia (e.g. hexamethylenetetramine or urea) . In some embodiments, the oxalate compound is ammonium oxalate monohydrate.

In some embodiments, the amount of the solid-state dispersant is greater than about 5 times (by weight) the amount of the oxide intermediate. In some embodiments, the amount of the solid-state dispersant is greater than about 10 times (by weight) the amount of the oxide intermediate. In some embodiments, the amount of the solid-state dispersant is about 5 to about 15 times (by weight) the amount of the oxide intermediate. In some embodiments, the amount of the solid-state dispersant is about 10 times (by weight) the amount of the oxide intermediate.

In some embodiments, the solid-state dispersant is a material having a melting point above the annealing temperature of step (e) . In some embodiments, the solid-state dispersant is water soluble. In some embodiments, the solid-state dispersant is inert. Preferably, the solid-state dispersant is selected from the group consisting of potassium sulfate, potassium phosphate, sodium phosphate, sodium aluminate and a combination of any two or more thereof. In some embodiments, the solid-state dispersant is potassium sulfate.

In some embodiments, the method further comprises washing the product obtained from annealing step (e) with water to remove the solid-state dispersant.

In a third aspect, the present invention is directed to a method of preparing an entropy stabilised oxide, the method comprising:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(c) mixing the solution obtained in step (b) with precipitating agent to obtain a precipitate, wherein the precipitating agent is an oxalate compound;

(e) annealing the oxide intermediate obtained in step (d) to provide the entropy stabilised oxide;

In some embodiments, the Mg salt, the Co salt, the Ni salt, the Cu salt and the Zn salt are each independently a chloride salt, a nitrate salt, a sulfate salt or an acetate salt. In some embodiments, the Mg salt, the Co salt, the Ni salt, and the Cu salt are each independently a chloride salt, a nitrate salt, a sulfate salt or an acetate salt. In some embodiments, the Mg salt, the Co salt, the Ni salt, and the Cu salt are chloride salts. In some embodiments, the Zn salt is a nitrate salt, a sulfate salt or an acetate salt. In some embodiments, the Zn salt is a nitrate salt.

In some embodiments, the solvent in step (b) is water. In some embodiments, the solvent is a mixture of water and an organic solvent, such as ethylene glycol. In some embodiments, the solvent is an about 1: 2 mixture (by volume) of water and ethylene glycol.

In some embodiments, the precipitating agent is dissolved in a solvent before mixing with the solution obtained in step (b) . In some embodiments, the solvent is water. In some embodiments, the solvent is a mixture of water and an organic solvent, such as ethylene glycol. In some embodiments, the solvent is an about 1: 2 mixture (by volume) of water and ethylene glycol.

In some embodiments, the calcining step (d) is performed at a temperature of about 300℃ to about 500℃. In some embodiments, the calcining step (d) is performed at a temperature of about 350℃ to about 450℃. In some embodiments, the calcining step (d) is performed at a temperature of about 400℃.

In some embodiments, the calcining step (d) comprises heating the precipitate for a period of at least about 30 minutes. In some embodiments, the calcining step (d) comprises heating the precipitate for a period of at least about 1 hour, about 2 hours or about 3 hours. In some embodiments, the calcining step (d) comprising heating the precipitate for a period of about 1 hour, about 2 hours or about 3 hours. In some embodiments, the calcining step (d) comprising heating the precipitate for a period of about 3 hours.

In some embodiments, the annealing step (e) is performed at a temperature of at least about 900℃. In some embodiments, the annealing step (c) is performed at a temperature of at least about 900℃, at least about 950℃, at least about 1000℃, at least about 1050℃, at least about 1100℃, at least about 1150℃ or at least about 1200℃. In some embodiments, the annealing step (e) is performed at a temperature of about 1000℃.

In some embodiments, the annealing step (e) comprises heating the oxide intermediate for a period of at least about 30 minutes. In some embodiments, the annealing step (e) comprises heating the oxide intermediate for a period of at least about 1 hour, about 2 hours or about 3 hours. In some embodiments, the annealing step (e) comprises heating the oxide intermediate for a period of about 1 hour, about 2 hours or about 3 hours. In some embodiments, the annealing step (e) comprises heating the oxide intermediate for a period of about 3 hours. In some embodiments, the calcining step (d) comprising heating the precipitate for a period of less than about 3 hours. In some embodiments, the calcining step (d) comprising heating the precipitate for a period of less than about 2 hours.

In some embodiments, the annealing step (e) is performed immediately after the calcining step (d) , e.g. in the same apparatus without isolating the oxide intermediate. In some embodiments, the oxide intermediate is cooled (e.g., to room temperature) before annealing step (e) .

In some embodiments, the oxide intermediate is mixed with a solid-state dispersant before the annealing step (e) . In some of these embodiments, the oxide intermediate is suspended in a solvent (e.g., water) with the solid-state dispersant, then the suspension is dried and the resulting powder ground before the annealing step (e) .

In another aspect, the present invention is directed to a method of controlling dispersity of a particle size of an entropy stabilised oxide, the method comprising:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(e) annealing the oxide intermediate obtained in step (d) in the presence of an amount of solid-state dispersant and at a temperature and for a time sufficient to provide the entropy stabilised oxide having a desired dispersity of the particle size;

wherein the entropy stabilised oxide is represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, and v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z =1.

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(e) annealing the oxide intermediate obtained in step (d) at a temperature and for a time sufficient to provide the entropy stabilised oxide having a desired dispersity of the particle size;

In another aspect, the present invention is directed to an entropy stabilised oxide obtained by a method according to the second or third aspect of the invention.

In still another aspect, the present invention is directed to an electrode, e.g. an anode or a cathode, comprising the entropy stabilised oxide according to the invention.

In some embodiments, the electrode comprises at least about 70% (by weight) of the entropy stabilised oxide. In some embodiments, the electrode comprises at least about 80% (by weight) of the entropy stabilised oxide. In some embodiments, the electrode comprises about 80% (by weight) of the entropy stabilised oxide.

In some embodiments, the electrode is an anode. In some embodiments, the anode further comprises a conductive additive, such as carbon black (e.g. Super P carbon black) , acetylene black, conductive graphite, Ketjen black ^TM, carbon nanotubes or a combination of any two or more thereof. In some embodiments, the anode comprises about 5%to about 15% (by weight) of the conductive additive. In some embodiments, the anode comprises about 10% (by weight) of the conductive additive. In some embodiments, the anode further comprises a binder, such as polyvinylidene fluoride (PVDF) carboxymethyl cellulose (CMC) or a combination thereof. In some embodiments, the anode comprises about 5%to about 15% (by weight) of the binder. In some embodiments, the anode comprises about 10% (by weight) of the binder. In some embodiments, the anode further comprises another anode material.

In some embodiments, the anode has a specific capacity of at least about 600 mAh/g. In some embodiments, the anode has a specific capacity of about 600 mAh/g to about 1200 mAh/g. In some embodiments, the anode has a specific capacity of at least about 700 mAh/g, about 800 mAh/g, about 900 mAh/g, about 100 mAh/g, about 1100 mAh/g or about 1200 mAh/g. In some embodiments, the anode has a specific capacity of about 800 mAh/g at a current rate of 0.2 A/g. In some embodiments, the anode has a specific capacity of at least about 800 mAh/g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 900 mAh/g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 1000 mAh/g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 1100 mAh/g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 800 mAh/g after 470 cycles. In some embodiments, the anode has a specific capacity of at least about 900 mAh/g after 470 cycles. In some embodiments, the anode has a specific capacity of at least about 1000 mAh/g after 470 cycles. In some embodiments, the anode has a specific capacity of at least about 1100 mAh/g after 470 cycles.

In yet another aspect, the present invention is directed to a catalyst comprising the entropy stabilised oxide according to the invention. In some embodiments, the catalyst is useful for catalysing a reaction selected from the group consisting of water-gas-shift, steam-reforming, Fischer-Tropsch synthesis or higher alcohol synthesis from CO ₂ reduction

In a further aspect, the present invention is directed to an electrochemical cell comprising, an anode, a cathode, a separator between the anode and cathode, and an electrolyte, wherein the anode comprises the entropy stabilised oxide according to the invention.

In some embodiments, the electrochemical cell is comprised in a lithium-ion battery.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

In addition, where features or aspects of the invention are described in terms of Markush groups, those persons skilled in the art will appreciate that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

As used herein “ (s) ” following a noun means the plural and/or singular forms of the noun.

As used herein the term “and/or” means “and” or “or” or both.

The term “comprising” as used in this specification means “consisting at least in part of”. When interpreting each statement in this specification that includes the term “comprising” , features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

Although the present invention is broadly as defined above, those persons skilled in the art will appreciate that the invention is not limited thereto and that the invention also includes embodiments of which the following description gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the Figures in which:

Figure 1 shows (a) a SEM image of as-precipitated oxalate precursor, and (b) a TEM bright-field image of a few oxalate precursor bundles.

Figure 2 shows a) STEM-HAADF image with a scale bar of 200 nm, and b-f) EDS elemental mappings of Cu, Co, Ni, Zn, and Mg.

Figure 3 shows a thermogravimetric analysis (TGA) , and differential scanning calorimetry (DSC) and derivative thermogravimetry (DTG, dotted line) plots of an oxalate precursor of the invention.

Figure 4 shows XRD patterns of the entropy stabilised oxides annealed from room temperature to different terminal temperatures and then held at each temperature for 3 hours. From bottom to top: 700℃, 800℃, 900℃ and 1000℃. Square, asterisk, triangle, and hash indicate rocksalt, tenorite (CuO) , spinel (Co ₃O ₄) , wurtzite (ZnO) phases, respectively.

Figure 5 shows an XRD pattern of the entropy stabilised oxide prepared via solid-state dispersant assisted annealing. The projections of different crystal planes are exhibited next to their corresponding diffraction peaks.

Figure 6 shows a Rietveld refinement of corresponding XRD patterns.

Figure 7 shows (a) a SEM image of entropy stabilised oxide rods, (b) a BF-TEM image of entropy stabilised oxide rods, and (c) HR-TEM image of an entropy stabilised oxide rod showing rocksalt lattice fringes of (111) planes parallel to the longitudinal axis of the rod, while the top-left corner inset is the corresponding FFT patterns.

Figure 8 shows (a-c) a HAADF-STEM image, an ABF image, and a contrast-reversed image of the ABF image taken along [100] orientation (metal cations and oxygen anions are depicted as large and small spheres, respectively) , and (d) ABF image taken along [110] orientation (drifts of O atomic columns are demonstrated as arrows, and the direction of an arrow indicates the drifting orientation) .

Figure 9 shows (a) the CV curves in the first 5 cycles, (b) the CV curves at crescent scan rates from 0.3 mV/Sto 1.0 mV/S, and (c) the voltage profiles in different cycles at a current rate of 0.2 A/g.

Figure 10 shows the cycling performance of an entropy stabilised oxide anode in 470 cycles at 0.2 A/g.

Figure 11 shows (a) the rate performance, (b) the voltage profiles at different current rates, and (c) the cycling performance of conventionally annealed (CA) entropy stabilised oxide anode at 0.1·A/g.

Figure 12 shows the first discharge profiles of the entropy stabilised oxide anode prepared by conventional annealing (dashed) and dispersant assisted annealing (solid) at a current rate of 0.1 A/g, and inset image is the differential capacity plots (dQ/dV) of two discharge profiles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an entropy stabilised oxide represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, wherein v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1. Preferably, the entropy stabilised oxide is (Mg _0.2Co _0.2Ni _0.2Cu _0.2Zn _0.2) O.

The entropy stabilised oxide may have a preferential crystallographic orientation of (111) . For example, the entropy stabilised oxide may be characterised by a X-ray powder diffraction spectrum comprising (111) and (200) peaks wherein the relative peak intensity of I ₍₁₁₁₎ /I ₍₂₀₀₎ is greater than about 1. In some embodiments, the entropy stabilised oxide is characterised by a X-ray powder diffraction spectrum comprising (111) , (200) , (220) , (311) and (222) peaks.

One particular advantage of the present invention is the unexpected ability to form an entropy stabilized oxide with a relatively uniform particle size distribution. It is desirable for commercial entropy stabilised oxides to have a relatively uniform distribution, i.e. be substantially uniform. Particle size is typically measured by electron microscope image analysis, where the particle sizes are measured using image processing software, such as Image J. More than 100 particles from the image are typically measured manually. Size distribution analysis may also be conducted using dynamic light scattering (DLS) method.

A relatively short duration of the annealing process and/or a relatively high amount of solid dispersant in the mixture during the annealing process can decrease the standard deviation of the particle size distribution. Without wishing to be bound by theory, it is believed the decrease in standard deviation is due to constrained interparticle agglomeration. However, the annealing time should be at least 30 minutes to ensure there is sufficient crystallisation of the entropy stabilised oxide.

Accordingly, those persons skilled in the art will appreciate that the methods of the invention enable control of the dispersity of the particle size of an entropy stabilized oxide.

More specifically, in one aspect, the present invention is directed to a method of controlling dispersity of a particle size of an entropy stabilised oxide, the method comprising:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(d) calcining the precipitate obtained in step (c) to obtain an oxide intermediate;

(e) annealing the oxide intermediate obtained in step (d) in the presence of an amount of solid-state dispersant and at a temperature and for a time sufficient to provide the entropy stabilised oxide having a desired dispersity of the particle size; wherein the entropy stabilised oxide is represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, and v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z =1.

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

Additionally or alternatively, the entropy stabilised oxide may be characterised by an average particle size or a particle size distribution (dispersity) . “Dispersity” is the IUPAC-approved measure of the heterogeneity of sizes of molecules or particles in a given mixture. A uniform particle size distribution i.e. one with low heterogeneity, is referred to herein as being “substantially uniform” and has a very low dispersity (close to 1) . It is accepted in the art that an alternative measure of particle size distribution is the standard deviation. In some embodiments, the standard deviation of the entropy stabilized oxide is less than about 250 nm and the relative standard deviation is less than about 0.34.

In some embodiments, the entropy stabilised oxide may have an average particle size D ₅₀ of about 800 nm or less, about 700 nm or less, or about 600 nm or less. Additionally or alternatively, the entropy stabilised oxide may have a particle size D ₉₀ of about 1200 nm or less, about 1100 nm or less, or about 1000 nm or less. These D ₅₀ and D ₉₀ values may be based on the length of the particles.

Additionally or alternatively, the entropy stabilised oxide may be characterised by submicron particles having a rod-like shape, e.g. a submicron particle in which the length of the submicron particle is greater than the width of the submicron particle. For example, the length : width ratio of the submicron particles may be about 1: 15 to about 1: 35, about 1: 2 to about 1: 3 or about 1: 2.5.

In some embodiments, the entropy stabilised oxide is substantially homogeneous. In some embodiments, the entropy stabilised oxide is substantially pure. In some embodiments, the entropy stabilised oxide is greater than about 90%pure, greater than about 95%pure, or greater than about 99%pure. In some embodiments, the entropy stabilised oxide is a single phase.

The entropy stabilised oxide may be prepared by a co-precipitation method. The method comprises precipitating a precursor of the entropy stabilised oxide from a solution of salts of Mg, Co, Ni, Cu and Zn. The precipitate is formed by mixing a solution of the metal salts with a precipitating agent. Suitable precipitating agents include, but are not limited to, a hydroxide compound (e.g. NaOH) , an organic compound (e.g. an oxalate compound) or a source of ammonia (e.g. hexamethylenetetramine or urea) . The precipitating agent may be dissolved in a solvent before mixing with the solution of metal salts. Advantageously, the use of an oxalate compound as the precipitating agent may provide an entropy stabilised oxide having a controlled morphology, e.g. an entropy stabilised oxide comprising submicron particles having a rod-like structure.

Accordingly, in some embodiments, the co-precipitation method of preparing an entropy stabilised oxide, the method comprises:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(c) mixing the solution obtained in step (b) with a precipitating agent to obtain a precipitate, wherein the precipitating agent is an oxalate compound, such as ammonium oxalate monohydrate;

The solution comprising the metal salts and precipitating agent is mixed to form a suspension. The reaction solution may be stirred at an elevated temperature, e.g., above about 30℃, above about 40℃, above about 50℃ or above about 60℃. In some embodiments, the solution is stirred at a temperature of about 40℃ to about 60℃. In some embodiments, the solution is stirred at a temperature of about 50℃. The reaction may be stirred for a period of at least about 1 hour, about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours. In some embodiments, the solution is stirred for about 5 to about 10 hours. In some embodiments, the solution is stirred for about 7 to about 9 hours. In some embodiments, the solution is stirred for about 8 hours. The precipitated precursor may then be isolated from the reaction solution, e.g., by centrifugation or filtration. The precipitated precursor may be washed one or more times with a solvent, e.g., water and/or an organic solvent such as ethanol, and dried.

Without wishing to be bound by theory, the inventors hypothesise that, in some embodiments, the precipitated precursor may form in one dimensional chains by axial polymerisation between the metal cations and anionic ligands. The inventors further hypothesise that this axial polymerisation progresses step-wise based on the well-known Irving-Williams series. The dissimilarity in complex stabilities may determine different rates of nucleation progress and crystal growth upon different cations. According to the Irving-Williams series, the stability of complexes formed by divalent 3d transition metal cations increases steadily to reach a maximum at copper, regardless of ligand. For example, the stability of metal-oxalate complexes follows a sequence as below:

[Cu (C ₂O ₄) ] ²⁺>> [Ni (C ₂O ₄) ] ²⁺ > [Zn (C ₂O ₄) ] ²⁺> [Co (C ₂O ₄) ] ²⁺>> [Mg (C ₂O ₄) ] ²⁺

The co-precipitation method further comprises calcining the precipitated precursor at a high temperature to obtain an oxide intermediate. For example, the precipitated precursor may be calcined at a temperature of at about 300℃ to about 500℃, about 350℃ to about 450℃ or about 400℃.

The calcining step may be performed for at an hour. For example, the calcining process may comprise heating the precipitated precursor for a period of at least about an hour, about 2 hours, about 3 hours, about 4 hours or about 5 hours. In some embodiments, the calcining step comprising heating the mixture for a period of about 3 hours

In some embodiments, the precipitated precursor is calcined at a temperature of about 400℃ for about 3 hours.

In some embodiments, the calcining process is performed in air.

The co-precipitation method further comprises annealing the oxide intermediate at a high temperature. For example, the oxide intermediate may be annealed at a temperature of at least about 900℃. In some embodiments, the oxide intermediate is annealed at a temperature of at least about 950℃, at least about 1000℃, at least about 1050℃, at least about 1100℃, at least about 1150℃ or at least about 1200℃.

The annealing step may be performed for at least 30 minutes. For example, the annealing process may comprise heating the oxide intermediate for a period of at least about an hour, about 2 hours, about 3 hours, about 4 hours or about 5 hours. In some embodiments, the annealing step comprises heating the oxide intermediate for a period of about 3 hours.

In some embodiments, the oxide intermediate is annealed at about 1000℃ for about 3 hours.

In some embodiments, the annealing process is performed in air.

In some embodiments, the calcining step and annealing step are performed together. For example, following the calcining step, the temperature may be increased to a temperature suitable for annealing the oxide intermediate (e.g., at least about 900℃) in the same apparatus. Alternatively, the calcining and annealing steps may be performed separately. For example, the oxide intermediate may be processed (e.g., cooled, ground and/or mixed with a solid-state dispersant) between the calcining step and the annealing step.

In some embodiments, the annealing step is performed in the presence of a solid-state dispersant. In these embodiments, the oxide intermediate is mixed with the solid-state dispersant before the annealing step. For example, the oxide intermediate obtained from the calcining step may be suspended in a solvent (e.g., water) with the solid-state dispersant. This suspension may be dried and the resulting powder ground to provide a mixture that is then used in the annealing step.

The solid-state dispersant may be a material having a melting point above the temperature at which the mixture is annealed. Advantageously, the use of a solid-state dispersant may reduce agglomeration during the annealing process. As a result, the entropy stabilised oxide formed in the annealing process may have a narrower particle size distribution compared to an entropy stabilised oxide formed in an annealing process without a solid-state dispersant.

The solid-state dispersant may be inert. The term “inert” as used in this context, refers to a material that does not react, e.g. chemically react, substantially or at least only reacts minimally, with the precipitated precursor. Preferably, the inert solid-state dispersant does not react with the precipitated precursor. In some embodiments, the solid-state dispersant is water soluble. Advantageously, a water-soluble solid-state dispersant may be removed from the mixture after annealing by washing the mixture with water. Examples of suitable solid-state dispersants include, but are not limited to, potassium sulfate, potassium phosphate, sodium phosphate, sodium aluminate and a combination of any two or more thereof. In some embodiments, the solid-state dispersant is potassium sulfate.

Various steps of the co-precipitation method may involve the use of a solvent, e.g., to dissolve and/or suspend components. Those persons skilled in the art may select a suitable solvent based on factors such as the desired solubility of the components. In some steps of the method, water is a suitable solvent. In some steps of the method, a mixture of water and an organic solvent, such as ethylene glycol, is a suitable solvent.

The entropy stabilised oxide according to the present invention may be useful in an electrode in an electrochemical cell, e.g., as an anode in a lithium-ion battery.

The anode may further comprise conventional anode materials including, for example, a conductive additive, such as carbon black (e.g. Super P carbon black) , acetylene black, conductive graphite, Ketjen black, carbon nanotubes or a combination of any two or more thereof, and/or a binder, such as PVDF, CMC or a combination thereof.

The anode may have a specific capacity of at least about 600 mAh/g, e.g., at least about 700 mAh/g, about 800 mAh/g, about 900 mAh/g, about 100 mAh/g, about 1100 mAh/g or about 1200 mAh/g. In some embodiments, the anode has a specific capacity of about 800 mAh/g at a current rate of 0.2 A/g. Advantageously, the anode material may maintain a high specific capacity after repeated cycles. For example, the anode may have a specific capacity of at least about 800 mAh/g after 400 cycles, and preferably after 470 cycles.

The entropy stabilised oxide according to the present invention may also be useful in a catalyst. Examples of processes in which the entropy stabilised oxide may be useful as a catalyst include, but are not limited to, water-gas-shift, steam-reforming, Fischer-Tropsch synthesis or higher alcohol synthesis from CO2 reduction

The following non-limiting examples are provided to illustrate the present invention and in no way limit the scope thereof.

EXAMPLES

Characterization methods

The composition of the samples was determined by ICP emission spectroscopy (ICP-OES Agilent 5110) . About 10 mg of respective samples were dissolved in 10 ml aqua regia at 200 ℃ in a PTFE beaker. Analysis was undertaken using five different calibration solutions. The structures and phase purity of different samples were characterized by X-ray diffraction (X’Pert Pro MPD with Cu Kα radiation) . The thermal analysis was performed on a simultaneous thermal analyzer (Netzsch STA 449 F3 Jupiter) in a Pt crucible with a ramp rate of 5 ℃/min in flowing air. X-ray photoelectron spectroscopy measurements were performed on a Thermo Escalab 250 XI with a monochromatic Al Kα source and a spot size of 400 μm. All spectra were calibrated with the C 1s peak of adventitious hydrocarbons at 284.8 eV before fitting.

SEM images were taken on a field-emission scanning electron microscope (Thermo Fisher Scientific Apreo S) operating at 30 kV and 0.4 nA. Bright-field TEM images and EDS linear scanning results were obtained on a transmission electron microscope (FEI Tecnai F30) equipped with an XFlash 6T-60 EDS detector (Bruker) . The atomic-scale characterizations of individual entropy stabilised oxide nanorods were conducted on an aberration-corrected S/TEM (FEI Titan Cubed Themis G2 300, FEI) at an accelerating voltage of 300 kV with a convergence semi-angle of 25 mrad. The scanning/TEM was equipped with a monochromator, an EDS detector (Bruker) , a Gatan imaging filter (GIF Quantum ER/965, Gatan) of high-resolution electron energy loss spectrometer, and a high-speed K2 camera (Gatan) . The multiple-inelastic-scattering background in the core-loss region was removed by Fourier ratio deconvolution of the low energy-loss signal. Line profiles were collected from EDS mappings of each element cation using Gatan DigitalMicrograph GMS3 software. The profiles were converted into text files via the script (Export Profile as Tabbed Text) created by Dave Mitchell, which is available on the website: www. dmscripting. com.

All chemicals were purchased from Sigma Aldrich and were used without further purification.

Example 1: Co-precipitation of an oxalate precursor of the entropy stabilised oxide

Oxalate anions were used to achieve near-equimolar deposition of different cations by forming corresponding complexes in solution. MgCl ₂·6H ₂O (99%, 0.55 mmol) , CuCl ₂·6H ₂O (99%, 0.5 mmol) , CoCl ₂·6H ₂O (99%, 0.5 mmol) , NiCl ₂·6H ₂O (99.9%, 0.5 mmol) , and Zn(NO ₃) ₂·6H ₂O (98%, 0.5 mmol) were dissolved in a mixed solution of 10 ml deionized water and 20 ml ethylene glycol, marked as solution A. Zinc nitrate was preferred as a zinc source. Zinc chloride is less desirable as a zinc source because it forms insoluble zinc oxychloride in an aqueous solution. An excess of magnesium chloride was added because the magnesium oxalate precipitate is slightly soluble. Then, ammonium oxalate monohydrate ( (NH ₄) ₂C ₂O ₄·H ₂O, 99%, 2.55 mmol) was dissolved into another mixed solution of 10 ml deionized water and 20 ml ethylene glycol at 50℃, marked as solution B. Ammonium oxalate was the preferred precipitating agent because sodium oxalate is insoluble in ethylene glycol and the oxalic acid may give rise to a dissolution of as-precipitated oxalates. Both solutions A and B were heated up to 50℃ under stirring. After that, solution B (oxalate ions) was rapidly poured into solution A (metal ions) under vigorous stirring. The suspension was further stirred at 50℃ for 8 hours, followed by separating the oxalate precursor from the reaction solution by centrifugation. The precursor was washed with water and absolute ethanol several times before drying at 70℃overnight.

Without wishing to be bound by theory, the inventors believe the complex was formed in a step-wise polymerisation. The copper-oxalate complex was first formed, followed by forming the complexes of other three transition metals ions (that is, Ni ²⁺, Zn ²⁺, and Co ²⁺) . Due to a low value of the critical stability constant (log K) , Mg ²⁺ cations are most difficult to coordinate with oxalate ions. Therefore, the chains of the magnesium-oxalate complexes were formed last. This process provided a precipitated oxalate precursor having hierarchical bundle structure.

Figure 1a illustrates a scanning electron microscopic (SEM) image of the as-prepared oxalate precursor. The precursor precipitated according to the process above showed bundle-like morphology. The transmission electron microscopic (TEM) image in Figure 1b demonstrates that the oxalate bundles were monodispersed. Each bundle was 500 nm to 1 μm long and with an average cross-section of 180 × 180 nm ². The TEM image also revealed that the as-precipitated precursor was dense and solid. Energy-dispersive X-ray spectroscopic (EDS) analysis was conducted on one of these bundles. The signal of Cu was more intense at the centre. In contrast, the Mg signal was relatively stronger on the periphery of the bundle. The signals of Ni, Zn, Co were almost uniform across the entire oxalate bundle. Hence, the results of the EDS analysis corroborated the above hypothesis on the hierarchical structure.

Figure 2a shows a high-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) image of an oxalate bundle. In Figure 2b-f, the elemental mappings of Cu, Co, Ni, Zn, and Mg are displayed separately. At a glance, the overall distribution of 5 metal cations across the bundle suggests the successful co-precipitation of a multi-component system on a sub-micron scale. Additionally, a close-up observation (Figure 1b and f) indicates that the Mg concentration was more intensive on the periphery while those of Cu were denser at the core.

The chemical composition of oxalate precursor was further determined by inductively coupled plasma optical emission spectrometry (ICP-OES) . The results indicated that molar percentages of each metal cation are relatively close to 20%, i.e., they are close to equimolar.

Example 2: High temperature annealing of precipitated precursor

To prepare the oxide intermediate, the dried oxalate precursor was calcined in a muffle furnace at 400℃ for 3 hours with a ramp rate of 10℃/min. Two different annealing approaches were used to transform the calcined oxide intermediate into the entropy stabilised oxides. In a conventional approach, the as-calcined black powder was annealed in a muffle furnace at different temperatures for 3 hours. The ramp rates of all processes were 10℃/min. The second approach was an annealing process assisted by solid-salt dispersants. 0.2 g oxide intermediate was re-dispersed in 40 ml deionized water with 2 g K ₂SO ₄ under stirring for 30 minutes. After being sonicated for 20 minutes, the suspension was put into an oven and heated up to 120℃. After the water was completely removed, the solid product was finely ground in a mortar. The powder was subsequently placed in ceramic crucibles and annealed at 1000℃ for 3 hours with the same ramp rate used previously. After annealing, K ₂SO ₄ was dissolved in water, while the solid product was separated out via vacuum filtration. The entropy stabilised oxide rods were obtained after washing the product with adequate deionized water and drying the sample in the oven.

Thermogravimetric (TG) analysis in air from 30℃ to 1000℃ at a ramp rate of 5℃/min showed two notable weight-loss stages between 100℃ and 400℃ (Figure 3) . The weight loss at a lower temperature, represented by the inflexion point at 169℃ in derivative thermogravimetry (DTG) , corresponds to water loss. The weight loss at a higher temperature corresponds to the decomposition of oxalate precursor, which is represented by the inflexion point at about 326℃ in DTG. At 1000℃, around 38%of the mass remains as the entropy stabilised product. Differential scanning calorimetry (DSC) further confirms that the water loss process is endothermic while the decomposition of oxalates is exothermic. Additionally, an enormous endothermic peak positioned at 740℃ is observed. This peak is indicative of the entropy-driven solid solution process, including the incorporation of Zn ²⁺ into rocksalt structure and the conversion of spinel Co ₃O ₄ into CoO. It is believed that the mixing of various components on a sub-micron or even a nanometre scale in oxalate precursor facilitates the solid-solution process at a lower annealing temperature relative to known process. Furthermore, according to the X-ray diffraction (XRD) patterns in Figure 4, the Bragg peaks indexed to tenorite CuO can be observed after heating the precursor at 800℃ for 3 hours. Those peaks disappear after further escalating the temperature to above 900℃, as tenorite CuO is gradually incorporated into the rocksalt structure, coinciding with the subtle endothermic peak in the DSC curve centred at 830℃.

Conventional annealing treatments at 1000℃ may lead to severe aggregation of particles. Therefore, to circumvent the formation of large aggregates, a solid-state dispersant was used during high-temperature annealing. Without wishing to be bound by theory, it is believed that the solid-state dispersant suppresses the aggregation and crystallite growth. Considering that the annealing temperature could be as high as 1000℃, potassium sulfate was selected as the dispersant since its melting point is 1069℃. Specifically, the oxalate precursor was first annealed at 400 ℃ for 3 hours to covert oxalates into a mixed-oxides intermediate. The phases in the intermediate are confirmed by XRD. Despite the poor crystallinity, the phases in the intermediate could be indexed to rocksalt NiO, tenorite CuO, rocksalt MgO, spinel Co ₃O ₄, and wurtzite ZnO. SEM and TEM images revealed that the bundle-like shape is preserved after moderate-temperature annealing. Interestingly, the intermediate rods have a mesoporous structure. This is because the thermal decomposition of oxalate precursor leaves substantial inner voids within these rods. The as-annealed intermediate was finely dispersed in K ₂SO ₄, followed by further annealing the mixture at 1000 ℃ for 3 hours.

The XRD pattern in Figure 5 demonstrates that the (Mg _0.2Co _0.2Ni _0.2Cu _0.2Zn _0.2) O entropy stabilised oxide prepared via a solid-state dispersant assisted annealing is a single-phase compound without any impure phases. The Rietveld refinement results in Figure 6 show good convergence and low R-factors, validating that the (Mg _0.2Co _0.2Ni _0.2Cu _0.2Zn _0.2) O entropy stabilised oxide prepared via solid-state dispersant assisted annealing has an fcc cubic crystal structure with the Fm3m space group. The refined lattice parameters are

α=β=γ=90°,

In such a structure, oxygen anions occupy the 4a sites, whereas the octahedral 4b sites are randomly co-occupied by Co, Cu, Ni, Zn, and Mg ions with a coordination number of 6.

Figure 7a displays an SEM image of the as-annealed entropy stabilised oxide rods, validating that the uniformity and bundle-like shape of the oxalate precursor are preserved to a large extent after annealing. Unlike the porous intermediate, the entropy stabilised oxide nanorods were fairly dense, evidenced in the bright-field TEM image (Figure 7b) . As shown in Figure 7c, the high-resolution TEM image exhibits the (111) lattice fringes of the entropy stabilised oxide with an interplanar spacing of 0.246 nm. The axial (longitudinal) direction of the nanosized rod is along [111] . The corresponding Fast Fourier Transform (FFT) patterns are represented in the top-left corner. The EDS linear scanning across the width of an entropy stabilised oxide rod showed the distribution of 5 cations in entropy stabilised oxide become more uniform across the rod when compared with the linear scanning results for the oxalate precursor.

Figure 8a shows the atomic-resolution HAADF-STEM image of an entropy stabilised oxide nanorod projected along the [100] orientation. The image explicitly reveals that the entropy stabilised oxide has an fcc sublattice of metal cations with oxygen anions residing at the octahedral holes, indicated by four large spheres (Me) and 12 small spheres (O) . Figure 8b shows an annular bright-field (ABF) image, indicating that the atomic columns of metals are axis-aligned with O atomic columns along [100] orientation. The ABF image along [110] direction (Figure 8d) shows that some O anions exhibit a subtle drift from their perfect octahedral sites. This drift is believed to arise from the anion sublattice distortion caused by the Jahn-Teller effects on tetrahedrally coordinated Cu ²⁺ in an octahedral configuration.

Example 3: Electrochemical performance

The obtained entropy stabilised oxides were used to assemble half cells, where lithium discs were employed as both counter and reference electrodes. The working electrodes were prepared via a typical slurry method. The synthesised entropy stabilised oxide powder and carbon black were mixed with PVDF binder with a mass ratio of 8: 1: 1 in N-methyl-2-pyrrolidinone (NMP) . The obtained slurry was coated onto a copper foil. Then, the film was heated on a hotplate at 80℃ to evaporate the NMP, followed by completely drying the film under vacuum at 80℃ overnight. CR2032 coin cells were assembled in a glove box under a pure argon atmosphere. In a coin cell 2-electrode configuration, a Li disc was used as the other electrode and separated from the working electrode by a polymer separator. The electrolyte was 1 M LiPF ₆ dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1: 1. The mass loading of the active materials on the working electrodes was 0.924-1.052 mg/cm ².

Figure 9a presents the cyclic voltammetry (CV) curves of the cell at a scan rate of 0.1 mV·S ^-1. The curves are similar to those of the previously reported entropy stabilised oxides (Sarkar et al. ) . The intensive cathodic peak at 0.52 V is reduced remarkably after first cycling, indicating solid electrolyte interphase (SEI) formation and initial reduction of transition metal oxides into metals and Li ₂O. A subtle cathodic peak at about 1.2 V in the first lithiation can be attributed to the Cu ²⁺/Cu ⁺ transformation. In the following cycles, four redox peaks are detectable from the CV results. One pair of intensive redox peaks centred at 1.2 V (cathodic) and 1.8 V (anodic) can be ascribed to the reduction of transition metal oxides and re-oxidation of metals. Additionally, a pair of minor redox peaks positioned at 0.375 V and 0.80 V may stem from a combined effect of the alloying and dealloying processes of Zn metal with Li and the spin-polarised surface capacitance of Co and Ni nanoparticles.

In the previous studies (Sarkar et al. and Ghigna, P.; Airoldi, L.; Fracchia, M.; Callegari, D.; Anselmi-Tamburini, U.; D'Angelo, P.; Pianta, N.; Ruffo, R.; Cibin, G.; de Souza, D.O.; Quartarone, E., Lithiation Mechanism in High-Entropy Oxides as Anode Materials for Li-Ion Batteries: An Operando XAS Study. ACS Applied Materials &Interfaces 2020, 12 (45) , 50344-50354) , multiple additional cathodic peaks were commonly observed in the first lithiation process, suggesting multiple individual lithiation reactions of the involved cations. The multiple cathodic peaks imply that known entropy stabilised oxide materials may still preserve a small amount of binary or ternary oxides at some microdomains. The CV curves for subsequent cycles are almost identical after 2 cycles, indicating a superior capacity retention ability of the entropy stabilised oxide electrodes. Moreover, the CV curves at increasing scan rates from 0.3 to 1 mV·S ^-1 show similar redox trends in Figure 9b, which demonstrates a good electrochemical response to different current rates.

Figure 9c exhibits the galvanostatic charge/discharge profiles of some representative cycles of the half-cell. The entropy stabilised oxide electrode delivers a relatively high discharge capacity of 1639 mAh·g ^-1 at a current of 0.2 A·g ^-1 during the first cycle. The mechanism of lithium storage in entropy stabilised oxides is through conversion type reaction and, therefore, the initial Coulombic efficiency of the entropy stabilised oxide anode merely reaches 50.4%. The lithiation capacity then drops to 650 mAh·g ^-1 at the 30 ^th cycle, before it increases to 1170 mAh·g ^-1 at the 400 ^th cycle progressively. It is worth noting that the voltage profiles of 400 ^th and 470 ^th are nearly overlapped, indicating that the capacity of the entropy stabilised oxide anode is stabilised after 400 cycles. Likewise, Figure 10 shows the impressive cyclability of the entropy stabilised oxide anode, demonstrating that the entropy stabilised oxide of the present invention is impressively stable. Furthermore, the entropy stabilised oxide anode displays excellent rate performance at increasing current rates and impressive capacity retention (Figure 11a) . More specifically, the entropy stabilised oxide anode delivers high specific capacities of 545, 470, 407, and 308 mAh/g at 0.2, 0.5, 1, and 3 A/g, respectively. The capacity is stabilised at around 510 mAh/g in post cycles at 0.2 A/g, indicating superior structural stability of the entropy stabilised oxide anode under electrochemical operating conditions. Figure 11b demonstrates the charge/discharge profiles of the entropy stabilised oxide anode at different current rates. The discharge profile at 0.2 A·g ^-1 after high-rate cycles is superimposed in Figure 11c. It appears to be overlapped with a discharge profile at 0.2 A/g before high-rate cycles (solid yellow curve) from 1.5 to 1 V. Similarly, the discharge profiles in the same range display minor changes in capacity upon cycling in Figure 9c, implying that this conversion reaction process is highly reversible.

We compared the electrochemical performance of entropy stabilised oxide materials prepared according to the present invention and conventional annealing (CA-ESO) . The entropy stabilised oxide particles obtained after conventional annealing were also assembled into half-cells. Figure 12 displays the cycling performance of the CA-ESO anode at a low current rate of 0.1 A/g. Despite a low current rate, the CA-ESO anode exhibits a sudden capacity decay after 280 cycles. In addition, by comparing the first discharge profiles of both entropy stabilised oxide anodes, despite similar discharge capacities, the lithiation plateau observed in the entropy stabilised oxide rod anode was lower than that of the CA-ESO anode. There is a potential difference of around 46.8 mV between the plateaus of the two entropy stabilised oxide electrodes. Without wishing to be bound by theory, it is believed this lithiation potential difference may arise from the following two aspects. Firstly, the broad particle size distribution of CA-ESO may create uneven surface overpotentials and kinetics of lithiation/de-lithiation. Secondly, lithiation rates are dependent on the crystallographic directions. The entropy stabilised oxide rods synthesised in this study have an axial direction of <111>, and the sidewall planes of the rod are {110} and {112} . These non-closely packed planes are kinetically favourable for lithiation. Even though the fine size inevitably leads to a low initial Coulombic efficiency (that is, around 50%) caused by the enormous SEI formation, the short diffusion paths and stable 1D structure enable the entropy stabilised oxide rods anode a superior long-term cyclability and rate performance.

A comparison of the entropy stabilised oxide prepared according to the present invention with two known entropy stabilised oxide anodes is shown in Table 1. The entropy stabilised oxide anode according to the present invention delivers the most impressive electrochemical performance with the highest ratio of active material in anode. Furthermore, it is believed the low initial Coulombic efficiency can be effectively overcome by various pre-lithiation methods.

Table 1

It is not the intention to limit the scope of the invention to the abovementioned examples only. As would be appreciated by a skilled person in the art, many variations are possible without departing from the scope of the invention as set out in the accompanying claims.

Claims

An entropy stabilised oxide represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, wherein v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1; and wherein the entropy stabilised oxide is characterised by any one or more of the following:

· a X-ray powder diffraction spectrum comprising (111) and (200) peaks wherein the relative peak intensity of I ₍₁₁₁₎ /I ₍₂₀₀₎ is greater than about 1;

· an average particle size D ₅₀ of about 800 nm or less;

· a particle size D ₉₀ of about 1200 nm or less;

· submicron particles having a rod-like shape;

· a particle size distribution having a standard deviation of less than about 250 nm.
The entropy stabilised oxide of claim 1, wherein v, w, x, y and z are each about 0.2.
The entropy stabilised oxide of claim 1 or 2, where the relative peak intensity of I ₍₁₁₁₎ /I ₍₂₀₀₎ is about 1.2 to about 2.0.
The entropy stabilised oxide of any one of claims 1 to 3, wherein the entropy stabilised oxide is characterised by a X-ray powder diffraction spectrum comprising (111) , (200) and (220) peaks and the relative peak intensity of I ₍₂₂₀₎ /I ₍₂₀₀₎ is about 0.3 to about 1.1
The entropy stabilised oxide of any one of claims 1 to 4, wherein the entropy stabilised oxide is characterised by a X-ray powder diffraction spectrum comprising (111) , (200) , (220) , (311) and (222) peaks and the relative peak intensity of I ₍₃₁₁₎ /I ₍₂₂₂₎ is less than about 1.
The entropy stabilised oxide of any one of claims 1 to 5, wherein the entropy stabilised oxide is characterised by an average particle size D ₅₀ of about 600 nm or less.
The entropy stabilised oxide of any one of claims 1 to 6, wherein the entropy stabilised oxide is characterised by a particle size D ₉₀ of about 1000 nm or less.
The entropy stabilised oxide of any one of claims 1 to 7, wherein the length : width ratio of the submicron particles is about 1: 15 to about 1: 35.
The entropy stabilised oxide of any one of claims 1 to 8, wherein the average length of the submicron particles is about 400 nm to about 1000 nm and the average width of the submicron particles is about 150 nm to about 450 nm.
The entropy stabilised oxide of any one of claims 1 to 8, wherein the entropy stabilised oxide has a particle distribution having a standard deviation of about 215 nm with a relative standard deviation of about 0.34.
A method of preparing an entropy stabilised oxide, the method comprising:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(b) mixing the Mg salt, the Co salt, the Ni salt, the Cu salt and the Zn salt in a solvent to form a solution;

(c) mixing the solution obtained in step (b) with a precipitating agent to obtain a precipitate;

(d) calcining the precipitate obtained in step (c) to obtain an oxide intermediate; and

(e) annealing the oxide intermediate obtained in step (d) in the presence of a solid-state dispersant to provide the entropy stabilised oxide;

wherein the entropy stabilised oxide is represented by the formula (MgvCowNixCuyZnz) O, and v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1.
The method of claim 11, wherein the precipitating agent is a hydroxide compound, an organic compound or a source of ammonia.
The method of claim 12, wherein the oxalate compound is ammonium oxalate monohydrate.
The method of any one of claims 11 to 13, wherein the solid-state dispersant is selected from the group consisting of potassium sulfate, potassium phosphate, sodium phosphate, sodium aluminate and a combination of any two or more thereof.
A method of preparing an entropy stabilised oxide, the method comprising:

(a) providing a Mg salt, a Co salt, a Ni salt, a Cu salt and a Zn salt;

(b) mixing the Mg salt, the Co salt, the Ni salt, the Cu salt and the Zn salt in a solvent to form a solution;

(c) mixing the solution obtained in step (b) with precipitating agent to obtain a precipitate, wherein the precipitating agent is an oxalate compound;

(d) calcining the precipitate obtained in step (c) to obtain an oxide intermediate; and

(e) annealing the oxide intermediate obtained in step (d) to provide the entropy stabilised oxide;

wherein the entropy stabilised oxide is represented by the formula (Mg _vCo _wNi _xCu _yZn _z) O, and v, w, x, y and z are each independently about 0.05 to about 0.30, provided that v + w + x + y + z = 1.
The method of any one of claims 11 to 15, wherein the Mg salt, the Co salt, the Ni salt, the Cu salt and the Zn salt are independently a chloride salt, a nitrate salt, a sulfate salt or an acetate salt.
The method of any one of claims 11 to 16, wherein the Mg salt, the Co salt, the Ni salt, and the Cu salt are chloride salts and the Zn salt is a nitrate salt.
The method of any one of claims 11 to 17, wherein the precipitating agent is dissolved in a solvent before mixing with the solution obtained in step (b) .
The method of any one of claims 11 to 18, wherein the solvent (s) is a mixture of water and ethylene glycol.
The method of any one of claims 11 to 19, wherein the calcining step (d) is performed at a temperature of about 300℃ to about 500℃.
The method of any one of claims 11 to 20, wherein the annealing step (e) is performed at a temperature of at least about 900℃.
The method of any one of claims 11 to 21, wherein the annealing step (e) comprises heating the oxide intermediate for a period of at least about 30 minutes.
An entropy stabilised oxide obtained by a method of any one of claims 11 to 22.
An electrode comprising the entropy stabilised oxide of any one of claims 1 to 10 and 23.
The electrode of claim 24, wherein the electrode comprises at least about 70% (by weight) of the entropy stabilised oxide.
The electrode of claim 24 or 25, wherein the electrode is an anode.
The electrode of claim 26, wherein the anode further comprises a conductive additive and/or a binder.
The electrode of claim 26 or 27, wherein the anode has a specific capacity of at least about 600 mAh/g to about 1200 mAh/g.
The electrode of any one of claims 26 to 28, wherein the anode has a specific capacity of at least about 800 mAh/·g after 400 cycles.
A catalyst comprising the entropy stabilised oxide of any one of claims 1 to 10 and 23.
An electrochemical cell comprising, an anode, a cathode, a separator between the anode and cathode, and an electrolyte, wherein the anode comprises the entropy stabilised oxide of any one of claims 1 to 10 and 23.