A composition for a particle having core and coating

Abstract

The present disclosure relates to compositions which may be used in a redox flow battery. The compositions comprise an electrolyte comprising particles of positive electrode active material and / or particles of negative electrode active material. The particles of positive or negative electrode active material include a core and a coating which comprise a metal or metal-containing compound. The density of the core and the density of the coating differs by at least 0.1 g / cm3. The disclosure also relates to a redox flow battery comprising such compositions and to a method for its use.

Description

[0001] A COMPOSITION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to compositions for use in flow batteries, more particularly, to compositions containing positive and / or negative electrode active materials and to flow batteries containing such compositions.

[0004] BACKGROUND

[0005] Redox flow batteries, also known as flow batteries, flow cells or redox batteries, are energy storage devices in which soluble metal ions in liquid solution are oxidized or reduced during the operation of the cell. Liquid state redox reactions are carried out at the positive and negative electrodes using two reversible redox couples. A redox flow battery comprises a power-generating assembly which typically includes an ionically permeable membrane separating positive and negative electrode reactants (also called catholyte and anolyte, respectively), and positive and negative current collectors (also called electrodes) which facilitate the transfer of electrons to an external circuit but do not take part in the redox reaction.

[0006] The charged anolyte and catholyte are typically held in storage tanks before being provided to the cell of the flow battery for participation in the redox reaction. Depleted anolyte and catholyte are then removed from the cell and returned to their respective storage tanks. Flow batteries are generally of the "closed-loop" configuration in which the depleted anolyte and catholyte are recharged by returning the solutions to the cell and applying an electrical charge. However, recharge times can be lengthy and limiting on the types of applications in which closed loop flow batteries can be used. In contrast, "open-loop" flow battery systems are those in which the depleted electrode reactants are physically removed and replaced with fresh catholyte and anolyte, thereby avoiding the need for recharging.

[0007] Although flow batteries have several advantages, including their long lifetimes and use of non-volatile electrolytes, they tend to have a relatively low energy density when compared to the batteries typically used in electric vehicles. Flow batteries are therefore generally only used as static power sources for utility grid back-up.

[0008] The present invention seeks to mitigate the aforementioned problems, at least to some extent. SUMMARY OF THE INVENTION

[0009] In accordance with a first aspect of this invention, there is provided a composition comprising: an electrolyte comprising particles of positive electrode active material and / or particles of negative electrode active material; wherein the particles of positive electrode active material and / or the particles of negative electrode active material include a core and a coating, and wherein the coating comprises a metal or metal-containing compound.

[0010] The metal may be selected from lithium, aluminium or sodium. In some embodiments, the metal may be selected from aluminium or sodium.

[0011] In some embodiments, the density of the core and the density of the coating may differ.

[0012] This difference in density of the material of the core and the coating causes particles for which the coating is depleted to separate in the electrolyte from the particles comprising the core and the coating. This facilitates separation of the depleted or spent particles from the electrolyte.

[0013] The materials of the core and coating may be selected to ensure a sufficient density difference between the core and the coating to facilitate the separation of the depleted or spent particles.

[0014] In some embodiments, the density of the core and coating may differ by at least 0.1 g / cm3. In some embodiments, the density of the core and coating may differ by at least 2.5, 2.0, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, or 0.1 g / cm3.

[0015] The composition may be for use in a redox flow battery. When the composition is introduced into a redox flow battery, the particles of positive and / or negative electrode active material act as a high surface area distributed cathode and / or anode, respectively. The high surface areas serve to increase the total electrical charge generated by the flow battery in use.

[0016] The composition may conceptually be regarded as a fuel source for an open-loop redox flow battery in which the redox flow battery can be conveniently "recharged" by replacing the depleted composition with fresh composition. Such batteries may be advantageous for use in electric vehicles, for example, where the process of replacing the depleted composition may be quicker than charging a conventional electric vehicle battery. The composition may be for use in an aluminium or sodium redox flow battery. The use of aluminium or sodium rather than lithium in the electrode active material circumvents problems associated with lithium-based batteries, such as shortages in supply and ethical issues surrounding the mining practices used to extract lithium and metals such as nickel and cobalt which are used in lithium batteries. Furthermore, aluminium and sodium have the advantages of being safe to store and use, non-toxic, non-flammable, stable and in relative abundance. They also have desirable electrochemical attributes such as high energy density, good cycle life, and an ability to sustain high-rate performance. Furthermore, Al3+has a high charge density due to its small ionic radius. Theoretical capacity and energy density for aluminium are 2981 mAhg-1and 4140 Whkg-1respectively. Similarly, sodium has a theoretical capacity of 1171 mAhg-1and energy density of 2930 Whkg-1.

[0017] In some embodiments, the coating on the particles of negative electrode active material may comprise aluminium metal. The coating may comprise an alloy of aluminium metal and one or more elements selected from the group consisting of antimony, zinc, tin, carbon, and combinations thereof. The alloy may inhibit corrosion and the growth of a passivating layer on the aluminium surface. The coating may further comprise a component configured to inhibit corrosion of the aluminium. The component may be a tin salt, which may preferably be selected from KzSnCh and NazSnCh. In these embodiments, the electrolyte may comprise particles of negative electrode active material which may include a core and a coating, wherein the coating comprises aluminium.

[0018] The particles of negative electrode active material may further comprise TiC , TiC - graphene, MoOs, and / or chloroaluminate. These components may act as catalysts on the surface of the negative electrode active material, where they assist in accelerating oxidation of the aluminium and release of aluminium ions.

[0019] The positive electrode active material may comprise an oxygen reduction catalyst. The oxygen reduction catalyst may assist in catalysing the reduction of oxygen, for example, where the cell comprises an aluminium-air redox couple. The oxygen reduction catalyst may be selected from a platinum, manganese oxide, iron-nitrogen-carbon (Fe-N-C), or palladium-based catalyst.

[0020] The positive electrode active material may comprise a substrate configured to take up (intercalate) or release (de-intercalate) metal ions, such as lithium, aluminium or sodium ions, preferably aluminium or sodium ions, in use, i.e. during discharging or charging. The substrate may comprise one or more of graphite, carbon blacks, Ketjen® Black (conductive acetylene carbon black), V2O5, FeV04, VOPO4, CuFe(CN)e, Fe4[Fe(CN)e]3, K2CoFe(CN)6, MnO2, Bi2O3and WO3.

[0021] In some embodiments, the coating on the particles may comprise sodium. In this embodiment, the composition may be for use in a sodium redox flow battery. The electrolyte may comprise particles of positive electrode active material and particles of negative electrode active material, wherein the particles of negative electrode active material include a core and a coating comprising sodium ions. The particles of negative electrode active material may include a core and a coating which comprises sodium metal.

[0022] In some embodiments, the coating on the particles may comprise lithium. In this embodiment, the composition may be for use in a lithium redox flow battery.

[0023] The negative electrode active material may further comprise one or more materials selected from carbon, graphite, graphene, titanium dioxide and Na2MnFe(CN)e. These materials may be configured to release or take up metals, such as lithium, aluminium or sodium, preferably aluminium or sodium, in use, i.e. during discharging or charging.

[0024] The electrolyte may comprise a fluid medium and a salt. The salt may be present in a greater volume and / or weight than the fluid medium. For example, the composition may comprise a water in salt electrolyte ("WISE") which may comprise more salt than water by volume and / or weight. In WISE electrolytes the salts may not be fully solvated, which may lead to the formation of inter-ionic pairs, higher ionic conductivity and an electrochemical potential range which may be much wider than the 1.23V normally available in aqueous electrolytes.

[0025] Where the coating comprises lithium, the salt may comprise lithium ions. The salt may comprise one or more of LiCI, LiF, Lil, Li(OTf), Li2(SO4), Li(NO3) and Li(CFsSO3). The salt may preferably comprise LiCI or LiF, more preferably, LiCI.

[0026] Where the coating comprises aluminium, the salt may comprise aluminium ions. The salt may comprise one or more of AICI3, AIF3, AII3, AI(OTf)s, Al2(SO4)3, AI(NO3)3 and A^CFsSOs). The salt may preferably comprise AICI3 or AIF3, more preferably, AICI3.

[0027] Where the coating comprises sodium, the salt may comprise sodium ions. The salt may comprise one or more of NaCI, NaF, NaBr, Nal, Na2SO4, NaNOs, NaNHz, CHsCOONa, or sodium chlorate. The electrolyte may comprise a room temperature ionic liquid (RTIL).

[0028] The particles of positive electrode active material and / or the particles of negative electrode active material may be suspended in the electrolyte.

[0029] The coating may be porous.

[0030] The particles of positive electrode active material and / or the particles of negative electrode active material may have a density of from 1.0 to 1.4 g / cm3, preferably from 1.1 to 1.25 g / cm3. The porosity may increase the surface area of the particles in contact with the electrolyte, and the porosity and density of the particles may assist in maintaining the particles in suspension.

[0031] The core of the particles of positive electrode active material may be a microparticle or a nanoparticle. The microparticle or nanoparticle size of the core may provide for a greater surface area of the distributed cathode and anode.

[0032] The core may comprise graphene, titanium dioxide, a titanium suboxide (i.e. TinCkn-i where n = integer greater than one), silicon carbide, silicon nitride, titanium dioxide, silicon oxide, carbon, graphite, hollow glass, solid glass, polypropylene, acrylonitrile butadiene styrene (ABS), polyether ether ketone (PEEK), polystyrene, or epoxy resin. These materials may be useful for forming microparticles or nanoparticles.

[0033] The composition may further comprise KOH, NaOH and / or NaCI at a concentration of from 1 M to 8 M, preferably from 4 M to 5 M.

[0034] In accordance with a second aspect of this invention, there is provided a redox flow battery comprising the composition of the first aspect.

[0035] The particles of positive electrode active material and / or particles of negative electrode active material may be separated from each other by an ion-permeable medium. The ion- permeable medium may be a membrane.

[0036] In some embodiments, the redox flow battery of the second aspect is an aluminium-air redox flow battery. In some embodiments, the aluminium-air redox flow battery comprises a positive electrode (cathode), for example a non-particulate positive electrode, and a composition according to the first aspect in which the electrolyte comprises particles of negative electrode active material having a core and a coating comprising aluminium metal.

[0037] In some embodiments, the aluminium-air redox flow battery comprises an oxygen reduction cathode. The oxygen reduction cathode may comprise platinum and / or manganese oxide oxygen reduction catalysts. The oxygen reduction cathode may comprise iron-nitrogen-carbon (Fe-N-C).

[0038] The positive electrode active material may comprise an oxygen reduction catalyst. The oxygen reduction catalyst may be selected from a platinum, manganese oxide, iron- nitrogen-carbon (Fe-N-C), or palladium-based catalyst.

[0039] In some embodiments, the positive electrode active material may be in the form of a composition according to the first aspect in which the electrolyte comprises particles of positive electrode active material.

[0040] In some embodiments, the redox flow battery of the second aspect is a sodium redox flow battery. In some embodiments, the sodium redox flow battery comprises a composition according to the first aspect in which the electrolyte comprises particles of negative electrode active material having a core and a coating comprising sodium and / or particles of positive electrode active material having a core and a coating comprising sodium.

[0041] In some embodiments, the sodium redox flow battery comprises a positive electrode active material comprising a coating which comprises sodium ions, and a negative electrode active material comprising a coating which comprises sodium metal.

[0042] In accordance with a third aspect of this invention, there is provided an electric vehicle comprising the redox flow battery of the second aspect.

[0043] In accordance with a fourth aspect of this invention, there is provided a method of operating a redox flow battery, the method comprising: providing a redox flow battery comprising the composition according to the first aspect, and discharging the redox flow battery to produce an electrical current.

[0044] The method may further comprise, after discharging, replacing the composition with fresh composition, wherein the fresh composition has a higher chemical potential than the replaced composition. The method may further comprise recharging the replaced composition to increase its chemical potential.

[0045] In order that the invention may be more readily understood, it will be described further with reference to the figures and to the specific examples hereinafter.

[0046] BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In the accompanying Figures:

[0048] Figure 1 is a schematic representation of a positive electrode active material particle according to the first aspect of the invention which is suitable for reversible take up or release of aluminium (Al3+) ions during charge and discharge.

[0049] Figure 2 is a schematic representation of a negative electrode active material including a particle core and a coating comprising aluminium or aluminium alloy surrounding the core, according to the first aspect of the invention. The coating is depleted as the battery discharges, resulting in a change of particle density allowing for ready separation of the depleted particle from the composition.

[0050] Figure 3 is a schematic diagram of a positive electrode active material according to the first aspect of the invention, comprising a particle core coated with active electrode material suitable for the reversible take up or release of sodium (Na+) ions during charge and discharge.

[0051] Figure 4 is a graph illustrating changes in density (g / cm3) of the negative electrode particles as a function of conversion (%).

[0052] Figure 5 is a schematic representation of a flow cell comprising the composition of the first aspect, which optionally contains a separator for separating the positive and negative electrode active material.

[0053] DETAILED DESCRIPTION

[0054] As used herein and in the accompanying claims, unless the context requires otherwise, "comprise" or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0055] Although all aspects of the invention described herein preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

[0056] Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0057] "Negative electrode active material" refers to material which in use forms an anode of a redox flow cell.

[0058] "Positive electrode active material" refers to material which in use forms a cathode of a redox flow cell.

[0059] "Nanoparticles" are particles in which at least half of the particles in a number size distribution have lengths in two or three dimensions which measure from 1 nm to 100 nm.

[0060] "Microparticles" are particles in which at least half of the particles in a number size distribution have lengths in two or three dimensions which measure from 1 pm to 100 pm.

[0061] "Fresh" with respect to the particles, electrolyte, composition etc. described herein refers to the particles, electrolyte, composition etc. having a greater chemical potential (i.e. being more charged) than "depleted" particles, electrolyte, composition etc. which have a lower chemical potential.

[0062] "Density" as used herein is provided in units of g / cm3and measurable according to MPIF test method 42 (for determination of density of compacted or sintered powder metallurgy (PM) products), International Standards Organization test method ISO 2738 (for sintered metal materials, excluding hardmetals), ASTM B311 (for PM materials containing less than two percent porosity), or ASTM D6683-01 (for measuring bulk density values of powders and other bulk solids as a function of compaction pressure). The present disclosure relates to a redox flow battery, compositions for use in the redox flow battery, articles that include the redox flow battery, and methods for their use. The redox flow battery has enhanced performance in terms of charge-discharge rates, cycling stability, rate capability and lifespan.

[0063] In a first aspect, the present disclosure relates to a composition. The composition can be for use in a redox flow battery. The composition can be introduced into an electroactive zone of the flow battery in which it undergoes an electrochemical reaction to produce electrical charge. The composition comprises an electrolyte comprising particles of positive electrode active material and / or particles of negative electrode active material. The particles of positive electrode active material and / or the particles of negative electrode active material include a core and a coating. The coating comprises a metal or metalcontaining compound, wherein the metal or metal of the metal-containing compound may be selected from lithium, aluminium or sodium, or preferably from aluminium or sodium.

[0064] The density of the core and the density of the coating may differ. In some embodiments, the density of the core and the density of the coating may differ by at least 0.1 g / cm3.

[0065] The particles of positive electrode active material and / or particles of negative electrode active material may be suspended in the electrolyte. The densities of the particles may be configured to be within a threshold range of the density of the electrolyte to ensure that the particles remain in suspension. Accordingly, the particles may have a density which is within ±1.4, ±1.3, ±1.2, ±1.1, ±1.0, ±0.9, ±0.8, ±0.7, ±0.6, ±0.5, ±0.4, ±0.3, ±0.2, ±0.1, ±0.05, ±0.01 g / cm3of the density of the electrolyte.

[0066] Where the coating comprises aluminium metal (which has a density of 2.7 g / cm3), the composition of the first aspect can be for use in an aqueous rechargeable aluminium ion battery (AIB). In this embodiment, the flow battery can comprise an aluminium-air redox couple comprising the following reactions:

[0067] Anode:

[0068] Cathode:

[0069] Overall:

[0070] The Al-air redox battery may comprise a conventional non-particulate positive electrode (cathode) and a composition according to the first aspect in which the electrolyte comprises particles of negative electrode active material having a core and a coating comprising aluminium metal, wherein the particles are suspended in an electrolyte. The composition may be pumped through the flow battery to act as a high surface area distributed anode. Once depleted the anode suspension may be pumped out of the battery and fresh suspension pumped in. An advantage of the present embodiment is that removal of the depleted suspension removes the need for complex sealing arrangements around the electrodes, and removes the needs to remove and replace mechanical parts. This embodiment differs from known aluminium-air batteries / cells which rely on "mechanical" recharging of aluminium anodes (the anodes generally being in the form of a sheet or foil) by swapping out depleted / corroded anodes and replacing them with fresh electrodes. Such batteries / cells are described, for example, in W02016 / 178017 and WO2019 / 092732.

[0071] The coating on the particles of negative electrode active material can comprise an alloy of aluminium and one or more elements selected from the group consisting of antimony, zinc, tin, carbon, and combinations thereof, to inhibit corrosion and growth of a passivating layer on the aluminium surface. For example, the aluminium can preferably be alloyed with antimony or zinc. In use, the aluminium can be oxidised and released from the coating as Al3+ions during discharge of the redox flow battery. During charging, aluminium can be taken up through adsorption and stored, for example in an electroplating process.

[0072] The particles can further comprise TiC (including anatase and rutile in the form of particles or nanotubes), TiC -graphene, MoOs, and / or chloroaluminate. These components can act as catalysts on the negative electrode active material, where they assist in accelerating oxidation of the aluminium and release of aluminium ions. The coating can optionally be pre-treated with a chloroaluminate melt.

[0073] The positive electrode active material can comprise an oxygen reduction catalyst. The oxygen reduction catalyst can assist in catalysing the reduction of oxygen, for example, where the cell comprises an aluminium-air redox couple. The oxygen reduction catalyst can be selected from a platinum, manganese oxide, iron-nitrogen-carbon (Fe-N-C), or palladium-based catalyst. Platinum and manganese oxide oxygen reduction catalysts can be preferred due to their commercial availability. A catalyst comprising an iron-nitrogen- carbon (Fe-N-C) catalyst can provide a cell voltage of 1.7-1.8 V.

[0074] As shown schematically in Figure 1, the positive electrode active material can comprise a substrate configured to take up (intercalate) or release (de-intercalate) aluminium ions in use. For example, in the right-hand direction in Figure 1, Al3+ions can be taken up by the positive electrode active material, and in the left-hand direction in Figure 1 the Al3+ions can be released. The substrate can comprise one or more of graphite, carbon black, Ketjen® Black (conductive acetylene carbon black), V2O5, FeV04, VOPO4, CuFe(CN)e, iron hexacyanidoferrate (Fe4[Fe(CN)e]3), K2CoFe(CN)e, MnCh, Bi20s and WO3.

[0075] The electrolyte can comprise a fluid medium selected from a polar solvent (e.g. alcohols), or an organic solvent, and a salt. The organic solvent can be a cyclic carbonate ester or an acyclic dialkyl carbonate ester. In embodiments in which the coating comprises aluminium, the electrolyte can comprise water. The electrolyte may comprise a room temperature ionic liquid (RTIL). RTILs are a class of organic salts whose melting temperature is below 100 °C, and are described in Hallett, J.P. and Welton T., Chem. Rev. 2011, 111, 5, 3508-3576, the contents of which is incorporated herein by reference. The RTIL may be selected from imidazolium compounds including l,2-dimethyl-3- propylimidazolium chloride, l-ethyl-3-methylimidazolium chloride ([EMIM]CI, density = 1.435 g / cm3), l-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PFe, density = 1.38 g / cm3), and l-ethyl-3-methylimidazolium acetate ([EMIM]Ac, density = 1.027 g / cm3); pyridinium compounds such as l-butyl-3,5-dimethylpyridinium bromide ([BMPy]Br); as well as ammonium based and pyrrolidinium based RTILs.

[0076] The electrolyte can further comprise one or more of KOH, NaOH and NaCI, typically each of which is at a concentration in the range of 1-8 M, preferably 4-6 M. Maximum conductance of the electrolyte is achieved at a concentration of approximately 6M (density of 1.25 g / cc). The density of 8M KOH is around 1.32 g / cc. Where the coating on the negative electroactive material comprises aluminium metal, the electrolyte can optionally contain additives, such as tin salts (e.g. K2SnO3 and Na2SnO3), to further inhibit corrosion of the aluminium.

[0077] The electrolyte may have a density of from 1.0 to 2.5 g / cm3, from 1.0 to 2.0 g / cm3, from from 1.0 to 1.8 g / cm3, from 1.0 to 1.6 g / cm3, from 1.0 to 1.5 g / cm3, 1.2 to 2.2 g / cm3, from 1.2 to 2.0 g / cm3, from 1.2 to 1.8 g / cm3, or from 1.3 to 1.5 g / cm3. Preferably the density of the electrolyte is from 1.0 to 1.5 g / cm3.

[0078] In embodiments in which the coating comprises aluminium, the electrolyte can comprise a salt selected from AICI3, AIF3, AII3, AI(OTf)3 (where OTf = trifluoromethanesulfonate), Al2(SO4)3, AI(NO3)3 and AI(CF3SO3). The salt can preferably comprise AICI3 or AIF3, more preferably, AICI3. In some embodiments, the salt can comprise a chloroaluminate melt, which can be selected from one or more of l-ethyl-3-methylimidazolium chloride:aluminium trichloride, urea :AICl3, acetamide:AICl3, and NaCI :AICl3. The pH of the electrolyte can be adjusted using standard methods, for example by adding OH-containing salts to raise pH, or acids to lower pH, in order to adjust the voltage stability window of the electrolyte or to reduce degradation by proton exchange of certain active materials. The pH may optimally have a range of 4-10, preferably 5-9 or 6-8.

[0079] Figure 2 shows a schematic representation of the process of the aluminium coating being released from the particle core during discharge. As the aluminium is released, the particle coating is depleted.

[0080] Where the coating comprises sodium, the composition of the first aspect can be for use in an aqueous rechargeable sodium ion battery (ARSIB). In these embodiments, the composition comprises an electrolyte which comprises particles of positive electrode active material and particles of negative electrode active material, both of which have a core and a coating. The positive electrode active material has a coating which comprises sodium ions and the negative electrode active material has a coating which comprises sodium metal.

[0081] In this embodiment, the salt can comprise one or more of NaCI, NaF, NaBr, Nal, NazSC , NaNOs, NaNH2, CHsCOONa and sodium chlorate. The salt can preferably comprise NaCI. The pH of the electrolyte can be adjusted using standard methods, for example by adding OH-containing salts to raise pH, or acids to lower pH, in order to adjust the voltage stability window of the electrolyte or to reduce degradation by proton exchange of certain active materials.

[0082] The particles of positive electrode active material can be configured to take up (intercalate) aluminium or sodium ions during discharge and release (de-intercalate) aluminium or sodium ions during charging. The coating of the particles of positive electrode active material can comprise multiple layers to facilitate intercalation of aluminium or sodium ions therebetween. Where the positive electrode active material is configured to take up and release sodium ions, the coating can comprise one or more materials selected from nickel oxides (NaaNii-x-y-zM1xM2yM3zO2, where X, Y and Z are each independently selected from 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, and M1, M2and M3are each independently a transition metal), phosphates (such as Na7M4(P2O7)4PO4, Na?M3(P2O7)3, and Na4M3(PO4)2P2O?), layered antimony (such as NasNizSbOe), layered tellurium (such as Na2Ni2TeOe), layered / orthorhombic NaMC (birnessite), Na2MO?, NaMP04, NaM2(PO4)3, and Na2MPO4F, where M is a transition metal.

[0083] The density of the coating may be from 1 to 4 g / cm3, from 1 to 3 g / cm3, from 1 to 2.7 g / cm3, from 1.2 to 3 g / cm3, from 1.2 to 2.7 g / cm3, from 1.5 to 3 g / cm3, or from 1.5 to 2.7 g / cm3. The density of the core may be from 0.05 to 5 g / cm3, preferably from 0.1 to 4.5 g / cm3. Where the core is selected to have a lower density than the coating, the core may have a density of from 0.05 to 1.3 g / cm3, from 0.1 to 1.1, or from 0.1 to 1 g / cm3. Where the core is selected to have a higher density than the coating, the core may have a density of from 1.3 to 5 g / cm3, from 1.3 to 4.5 g / cm3, or from 2.2 to 4.5 g / cm3.

[0084] The intercalation and release of sodium ions by the positive electrode active material during charging and discharging is represented schematically in Figure 3 which shows that in the right-hand direction, Na+ions are taken up (intercalated) by the particles of the positive electrode material, and in the left-hand direction, the Na+ions are released (deintercalated) from the positive electrode active material.

[0085] The negative electrode active material can comprise a material which is configured to store aluminium or sodium reversibly through surface adsorption and desorption during discharge and charging. Suitable materials for this purpose include activated carbon, mesoporous carbon, carbon nanotubes (single wall or multiwall), graphite, graphene, titanium dioxide and Na2MnFe(CN)e. The negative electrode active material can be in nanoparticulate form.

[0086] In use, the composition can be pumped through the flow battery to act as a high surface area distributed anode and / or cathode. During discharge, cations of a metal such as lithium, aluminium or sodium in the coating of the negative electrode active material (the anode) are released and the positive electrode active material (i.e. the cathode) takes them up (intercalates the cations). Once the metal in the coating of the anode has been depleted (i.e. when the chemical potential of the composition has been lowered), the composition can be removed (e.g. by pumping out) from the battery and fresh composition introduced (e.g. by pumping in). The depleted composition can be recharged to restore its chemical potential.

[0087] The material of the core and the coating may be selected to ensure that the density of the core and coating differ, for example by at least 0.1 g / cm3. This density difference is provided so that when the coating is depleted, the cores can be separated from the electrolyte based on their different densities with respect to that of the electrolyte. In some embodiments, the density of the core and coating may differ by at least 2.5, 2.0, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, or 0.1 g / cm3. The coating may have a higher or a lower density than the core. In embodiments in which the coating of the negative electrode active material has a higher density than the core, depletion of the metal (e.g. lithium, aluminium or sodium) content during discharge causes the density of the particles to decrease, resulting in the particles rising to the surface of the electrolyte. Concurrently, the density of the particles of the positive electrode active material increases as they take up ionic metal (e.g. lithium, aluminium or sodium), causing the particles to fall within the electrolyte.

[0088] In embodiments in which the coating of the negative electrode active material has a lower density than the core, the opposite process may occur such that depletion of the metal content in the coating during discharge causes the density of the particles to increase, resulting in the negative electrode active particles falling within the electrolyte. Concurrently, the density of the particles of the positive electrode active material can increase or decrease as they take up metallic cations, depending on the density of the core or substrate material and also on the density of the electrolyte, causing the particles to rise or fall within the electrolyte.

[0089] In these embodiments, the particles can be separated from the electrolyte based on their respective lower and higher densities and replaced with fresh particles as an alternative to replacing the entire composition.

[0090] The core of the particles may comprise graphene, titanium dioxide (rutile or anatase), a titanium suboxide (i.e. TinCkn-i where n = integer greater than one), silicon carbide, silicon nitride, titanium dioxide, silicon oxide, carbon, graphite, hollow glass, solid glass, polypropylene, acrylonitrile butadiene styrene (ABS), polyether ether ketone (PEEK), polystyrene, or epoxy resin. These materials may be compatible with a metallic (e.g. lithium, aluminium or sodium) coating and / or useful for forming particles, for example nanoparticles or microparticles. The density of the core may be from 0.05 to 5 g / cm3, preferably from 0.1 to 4.5 g / cm3. Where the core is selected to have a lower density than the coating, the core may have a density of from 0.05 to 1.3 g / cm3, from 0.1 to 1.1, or from 0.1 to 1 g / cm3. Where the core is selected to have a higher density than the coating, the core may have a density of from 1.3 to 5 g / cm3, from 1.3 to 4.5 g / cm3, or from 2.2 to 4.5 g / cm3. Alternatively, the density range may be selected from any two values provided in the table below, which provides the densities of a selection of suitable materials for the core.

[0091] The core may be considered to be a nanoparticle or microparticle. The small size of the core can provide for a greater surface area of the distributed cathode and anode than would be achieved using larger particle sizes. In some embodiments, the core may have an average core diameter in the range 0.1-1000 pm, 0.1-500 pm, 1-500 pm, 1-10 pm, 2- 100 pm, 2-50 pm, 5-100 pm, or 5-50 pm.

[0092] The coating may have a thickness in the range 1-100 pm, 1-50 pm, 5-20 pm, 5-10 pm, or 10-15 pm thick. The total average diameter of the particles can be from 1.1-1100 pm, 1.1-600 pm, 2-110 pm, 2-60 pm, 7-25 pm, 7-15 pm or 12-20 pm.

[0093] The particles of electrode active material can be porous. The coating can have a controlled level of porosity both to adjust the particle density to the required value and to increase active surface area. The particles of the negative electrode active material can have a high surface area to volume ratio thereby providing a highly electrochemically active suspension.

[0094] The particles of positive electrode active material and / or the particles of negative electrode active material can have a density of from 1.0 to 1.3 g / cm3, or from 1.1 to 1.2 g / cm3. The particles of electrode active material can have a porosity greater than 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 30%, or 50% by volume. The porosity can increase the surface area of the particles in contact with the electrolyte which can increase the electrochemical reactivity of the particles, and the porosity and density of the particles can assist in maintaining the particles in suspension.

[0095] The total average density of the composition can be from about 1.0 to 2.5 g / cm3, 1.0 to 2.0 g / cm3, 1.0 to 1.8 g / cm3, 1.0 to 1.5 g / cm3, 1.0 to 1.3 g / cm3, 1.1 to 1.2 g / cm3, 1.3 to 2.0 g / cm3, 1.5 to 2.0 g / cm3, or 1.5 to 2.5 g / cm3. The positive and / or negative electrode active materials can be applied to a particle core, for example a nanoparticle or microparticle core by any suitable coating method. For example, the positive and negative electrode active materials can be applied by spray drying, chemical vapour deposition (CVD), or physical vapour deposition (PVD) (e.g. thermal evaporation, sputter deposition or arc vapor deposition), electroplating, electroless deposition, or co-mixing / grinding. These coating methods can permit the porosity of the coating to be controlled to a desired amount.

[0096] The composition can comprise a water in salt electrolyte ("WISE"). This refers to an electrolyte which contains more salt than water (as measured by volume and / or weight). For example, the composition can comprise 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100% more salt than water by volume or by mass. This excess of salt ensures that the salt is not fully solvated which can lead to the formation of inter-ionic pairs. This has several advantages such as higher ionic conductivity and an electrochemical potential range which can be much wider than the 1.23V normally available in aqueous electrolytes.

[0097] The composition may not be suitable for use in a lithium-ion redox flow battery.

[0098] In some embodiments, the composition may be suitable for use in a lithium-ion redox flow battery.

[0099] The composition can be configured to be introduced into a single flow path through the electroactive zone of the flow battery, or it can be configured to be introduced into the electroactive zone through separate flow paths for each of the negative electroactive material and positive electroactive material, as illustrated in Figure 5.

[0100] The present disclosure further relates to a redox flow battery comprising the composition. The redox flow battery can comprise a power-generating assembly, including positive and negative current collectors (electrodes) to facilitate the transfer of electrons from the flow battery to an external circuit, and at least one flow path through an electroactive zone in which the electrochemical reaction takes place.

[0101] The electroactive zone can be defined as a region of the battery at which the electrochemical reaction occurs and which contains a positive electrode current collector, a negative electrode current collector, a fluid receptacle in which the composition is receivable, and optionally an ion permeable separator for separating the positive and negative electrode active particles. The composition can be transported to and from the electroactive zone during discharge and charging.

[0102] The battery can define a single flow path through the electroactive zone. Alternatively, the battery can define separate flow paths for the negative electroactive material and the positive electroactive material in which the flow paths are separated from each other by an ion-permeable medium. The ion-permeable medium can be a membrane or separator. During operation, the positive and negative electroactive materials undergo reduction and oxidation, ions move across the ion permeable medium, and electrons flow through an external circuit to generate current.

[0103] The redox flow battery can be included in an electric vehicle, a watercraft, a drone, or an aircraft. Preferably the redox flow battery is included in an electric vehicle.

[0104] The present disclosure further relates to a method of operating a redox flow battery. The method comprises providing a redox flow battery comprising the composition and discharging the redox flow battery to produce an electrical current.

[0105] The method can further comprise, after discharging, replacing the composition with fresh composition, wherein the fresh composition has a higher chemical potential than the replaced composition. After discharging, the composition will typically be depleted of its chemical potential. The depleted composition can be removed from the battery, for example by a pump, and replaced with the fresh composition, which can also be introduced into the battery by a pump.

[0106] The method can further comprise recharging the replaced composition to increase its chemical potential. In some embodiments, the recharging can be carried out in the redox flow battery by applying an external current to the battery while the composition is in the electroactive zone and reversing the electrochemical reaction. In these embodiments, the electrolyte may comprise a room temperature ionic liquid. When the composition is in a flow battery in an electric vehicle, the recharging may be performed in-vehicle rather than externally. In other embodiments, the recharging can be carried out externally to the battery, for example in a device specifically configured for such a purpose.

[0107] EXAMPLES

[0108] Example 1: To prepare a composition which is suitable for use in a rechargeable sodium ion redox flow cell, a negative active material comprising sodium manganese hexacyanoferrate (Na2MnFe(CN)e) is prepared by a coprecipitation reaction of MnSC and Na4Fe(CN)e followed by washing and drying under vacuum to produce the desired sodium manganese hexacyanoferrate (Na2MnFe(CN)e) in high yield and purity (according to the method described in Nano Res 2022, 15(6): 5072-5080). Similarly, a positive active material comprising sodium titanium phosphate (NaTi2(PO4)s) is produced by a hydrothermal reaction of phosphoric acid, metatitanic acid and sodium hydrogen phosphate. The positive and negative active materials are then applied, respectively, to a suitable core material by spray drying optionally incorporating an inert binder, such as polyvinylidene fluoride (PVDF), chemical vapour deposition, physical vapour deposition, electroplating, electroless deposition or co-mixing / grinding. Preferably, the coating is applied by spray drying or co- mixing / grinding. The core material is selected to have a density which differs from the density of the Na2MnFe(CN)e and NaTi2(PO4)3 by greater than 0.1 g / cm3, but which produces particles that have a density within 0.3 g / cm3of the electrolyte (in this case, 1- ethyl-3-methylimidazolium chloride, [EMIMJCI, density = 1.435 g / cm3). The coated particles are then suspended in the electrolyte to produce the composition.

[0109] Example 2:

[0110] To prepare a composition which is suitable for use in a rechargeable aluminium ion redox flow cell, a negative active material may be prepared by the following method. Hollow glass microspheres having a true density of 0.6 g / cm3, mean particle size of 100 microns and crush strength of 3.5 MPa may be flame sprayed using an aluminium source material which has been pre-alloyed with zinc and tin. Microspheres which have a greater or smaller diameter than 100 microns may also be utilised. The microspheres may be passed through the flame repeatedly until the desired coating of thickness is obtained. A coating thickness of 100 microns or more may be achieved. The resulting aluminium alloy coated hollow glass microsphere will have an apparent density of around 2.4 g / cm3and may be suspended in the electrolyte solution by agitation or pumping. As the aluminium alloy is depleted by electron transfer at the negative electrode its apparent density will decrease such that it will eventually become naturally buoyant in the electrolyte solution. For example if the electrolyte solution in use is 6 Molar potassium hydroxide solution the density of the particle will become less than that of the solution at an aluminium depletion level of greater than 90%, as shown in Figure 4. The particles will be naturally buoyant in the solution at this point and may be separated from non-depleted particles by means of a weir or hydrocyclone. The separated depleted particles may be trapped within a cartridge which may be removed from the vehicle and a new cartridge containing fresh particles may be inserted.

Claims

CLAIMS1. A composition for a redox flow battery, the composition comprising : an electrolyte comprising particles of positive electrode active material and / or particles of negative electrode active material, wherein the particles of positive electrode active material and / or the particles of negative electrode active material include a core and a coating, wherein the coating comprises a metal or metal-containing compound, and wherein the density of the core and the density of the coating differs by at least 0.1 g / cm3.

2. The composition according to claim 1, wherein the particles of positive electrode active material and / or particles of negative electrode active material are suspended in the electrolyte.

3. The composition according to claim 1 or 2, wherein the coating of the particles of negative electrode active material comprises lithium, aluminium or sodium metal, such as aluminium or sodium metal.

4. The composition according to claim 3, wherein the coating of the particles of negative electrode active material comprises an alloy of aluminium metal and one or more elements selected from the group consisting of antimony, zinc, tin, carbon, and combinations thereof.

5. The composition according to claim 3 or 4, wherein the coating of the particles of negative electrode active material further comprises TiO2, TiC -graphene, MoOs, and / or chloroaluminate.

6. The composition according to any of claims 3 to 5, wherein the positive electrode active material comprises one or more materials selected from the group consisting of graphite, V2O5, FeV04, V0P04, CuFe(CN)6, Fe4[Fe(CN)6]3, K2CoFe(CN)6, MnO2, Bi2O3and WO3.

7. The composition according to any of claims 3 to 6, wherein the positive electrode active material comprises an oxygen reduction catalyst.

8. The composition according to claim 7, wherein the oxygen reduction catalyst is selected from a platinum, manganese oxide, iron-nitrogen-carbon (Fe-N-C), and palladiumcontaining catalyst.

9. The composition according to any of claims 1 to 8, wherein the electrolyte comprises a salt comprising aluminium ions.

10. The composition according to claim 9, wherein the salt comprises AICI3, AIF3, AII3, AI(OTf)3, Al2(SO4)3, AI(NO3)3, AI(CF3SO3) or a combination thereof.

11. The composition according to claim 1, wherein the coating comprises sodium.

12. The composition according to claim 11, wherein the negative electrode active material comprises one or more materials selected from the group consisting of carbon black, graphite, graphene, titanium dioxide and Na2MnFe(CN)e.

13. The composition according to claim 11 or 12, wherein the electrolyte comprises a salt comprising sodium ions.

14. The composition according to any preceding claim, wherein the electrolyte comprises a room temperature ionic liquid.

15. The composition according to any preceding claim, wherein the core comprises graphene, titanium dioxide, a titanium suboxide, silicon carbide, silicon nitride, titanium dioxide, silicon oxide, carbon, graphite, hollow glass, solid glass, polypropylene, acrylonitrile butadiene styrene (ABS), polyether ether ketone (PEEK), polystyrene or epoxy resin.

16. The composition according to any preceding claim, wherein the core is a nanoparticle.

17. The composition according to any preceding claim, wherein the coating is porous, preferably wherein the particles of positive electrode active material and / or the particles of negative electrode active material have a density of from 1.0 to 1.3 g / cm3, preferably wherein the density is from 1.1 to 1.2 g / cm3.

18. The composition according to any preceding claim, wherein the electrolyte comprises a salt and a fluid medium and wherein the salt is present in a greater volume and / or weight than the fluid medium.

19. The composition according to any preceding claim further comprising KOH, NaOH and / or NaCI at a concentration of from 1 M to 8 M, preferably from 4 M to 5 M.

20. The composition according to any of claims 3-10, further comprising a component configured to inhibit corrosion of the coating of the negative electrode active material, preferably wherein the component is a tin salt, preferably wherein the tin salt is selected from KzSnCh and NazSnCh.

21. A redox flow battery comprising the composition according to any of claims 1 to 20.

22. The redox flow battery according to claim 21, wherein the electrolyte comprises particles of positive electrode active material and particles of negative electrode active material, and wherein the particles of positive electrode active material and particles of negative electrode active material are separated from each other by an ion- permeable medium.

23. A redox flow battery according to claim 21, wherein redox flow battery is an aluminium- air redox flow battery comprising a positive electrode (cathode), for example a nonparticulate positive electrode, and a composition according any one of claims 1 to 20, wherein the electrolyte comprises particles of negative electrode active material having a core and a coating comprising aluminium.

24. A redox flow battery according to claim 21, wherein the redox flow battery of the second aspect is a sodium redox flow battery comprising a composition according to any one of claims 1 to 20, wherein the electrolyte comprises particles of negative electrode active material having a core and a coating comprising sodium and / or particles of positive electrode active material having a core and a coating comprising sodium.

25. An electric vehicle comprising the redox flow battery according to any one of claims 21 to 24.

26. A method of operating a redox flow battery, the method comprising: providing a redox flow battery comprising the composition according to any of claims 1 to 20; and discharging the redox flow battery to produce an electrical current.

27. The method of claim 26, comprising, after discharging, replacing the composition with fresh composition, wherein the fresh composition has a higher chemical potential than the replaced composition.

28. The method according to claim 27, comprising recharging the replaced composition to increase its chemical potential.

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