Active electrode material

Abstract

The invention relates to an active electrode material having a formula according to formula (I): NaAnFe1-x-yM1xM2yPO4F (I) wherein A is Li, Na, K, or a combination thereof; M1 is Mn, Co, or a combination thereof; M2 is Mg, Ca, Zn, or a combination thereof; 0 ≤ n ≤ 1; 0 < x ≤ 0.3; 0 < y ≤ 0.4; and x + y < 0.5.

Description

[0001] Active Electrode Material

[0002] Field of the Invention

[0003] The present invention relates to active electrode materials and their use in metal-ion batteries, in particular active cathode materials for use in sodium-ion batteries.

[0004] Background

[0005] In most systems, metal-ion battery electrodes are composed of an active electrode material (the term “active electrode material” meaning a material which can chemically react with metal ions such as lithium and / or sodium, preferably sodium, to store and release them reversibly in a controlled manner) mixed if necessary with an electrically conductive additive (such as carbon) and a polymeric binder. A slurry of these components is coated as a thin film on a current collector (typically a thin foil of copper or aluminium), thus forming the electrode upon drying.

[0006] One of the most promising active electrode materials for use in metal-ion battery cathodes are iron flouro phosphates. These are often employed in sodium-ion batteries and are sometimes referred to as “NFPF” systems (short for Na Fe P F). The materials involved in their manufacture are an attractive alternative to many existing cathode materials. Moreover, the energy density, comparative cost with respect to other cathode materials, and their fast-charging potential makes them very desirable candidates for a host of different battery applications.

[0007] Various attempts have been made to optimise the behaviour of these materials. For instance, it is often the case that conductive agents such as graphite are incorporated with NFPF materials to enhance conductivity, or that conductive carbon coatings are utilised. Such composite materials are sometimes referred to as “NFPF / C” materials.

[0008] In recent years, various organisations have explored substituted versions of the base NFPF materials (the term “base” meaning a material without partial substitution, e.g. Na2FePC>4F for NFPF), exploring how replacement of the iron atoms within the NFPF crystal structure can alter the properties of these materials.

[0009] Ellis et al., Chem. Mater. 2010, 22, 1059-1070 describes how the Fe in Na2FePC>4F can be substituted with Mg, Co, and / or Ni.

[0010] Rechem et al., J. Electrochem. Soc. 2009, Volume 156, Issue 12, Pages A993-A999, discloses the low temperature synthesis of highly divided Na-based fluorophosphates in ionic liquid media.

[0011] Deng et al., ACS Appl. Mater. Interfaces 2017, 9, 19, 16280-16287, discloses a green route strategy for fabrication of carbon-coated Na2FePC>4F cathode forSIBs.

[0012] Juwita, “Towards Affordable Sodium-ion Batteries: Mechanochemical Synthesis and Electrochemical Assessment of Iron-Based Fluorophosphate Cathode Material” KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, 2023 discloses the characterisation andperformance evaluation of Na2FePC>4F (NFPF) and NFPF / C positive electrode materials forsodium-ion batteries (SIBs).

[0013] Guo et al., Electrochemistry, 90(11), 117003 (2022) describes Na2Feo8Mgo i5Nioo5PC>4F / C.

[0014] Kacemi et al., Computational Materials Science 206 (2022) 111292 investigates the impact of iron replacement with manganese on behaviour and crystal structure using computational methods.

[0015] Kawabe et al.; Electrochemistry, 80(2), 80-84 (2012) describes situations where half of the molar iron content of NFPF is replaced with manganese.

[0016] US 11 ,196,046 and EP 4253 330 each describe sodium-ion battery materials not based upon NFPF systems but which introduce a range of different dopants in an attempt to enhance electrochemical properties. WO2024235662A1 relates to an electrode comprising one or more active materials, wherein one of the active materials is a lithium manganese iron phosphate of formula LixMni-y-zFeyM2PO4 (LMFP) where 0.8<x<1.2; 0.5<1-y-z<1 ; 0<y<0.5; 0<z<0.2.

[0017] Lui et al., Energy Storage Materials 68 (2024) 103319 describes how magnesium can be introduced into NFPF / C systems to alter the behaviour of such cathode materials.

[0018] Despite extensive research in this area, there remains a need to provide improved NFPF active electrode materials. For example, it is desirable to obtain materials with excellent fast-charging / high-power properties, energy density, cycle-life, safety and that employ materials that are cost effective and readily available.

[0019] Summary of the Invention

[0020] The invention provides an active electrode material having a formula according to formula (I):

[0021] NaAnFei-x-yM1xM2yPO4F (I)

[0022] wherein A is Li, Na, K, or a combination thereof; M1 is Mn, Co, or a combination thereof; M2 is Mg, Ca, Zn, or a combination thereof; 0 < n < 1; 0 < x< 0.3; 0 < y < 0.4; and x + y < 0.5.

[0023] The invention also provides an active electrode material having the crystal structure of orthorhombic Na2FePC>4F and comprising Mn and Mg, wherein the combined amount of Mn and Mg in the active electrode material is no more than 6.25 at.%.

[0024] As shown in the examples, materials according to the invention have been found to provide a surprising improvement in performance at high rates when utilised in sodium-ion batteries.

[0025] Summary of the Figures

[0026] Figure 1 is a powder XRD pattern of samples 1 and 2.

[0027] Figure 2 is cell cycling performance of full cells utilising Samples 2, 5, 8, and 9.

[0028] Figure 3 is cell cycling performance of full cells utilising Samples 2 and 9-11 .

[0029] Figure 4 is a powder XRD pattern of samples 2, 9, 12, 13 and 14.

[0030] Figure 5 shows discharge specific capacity vs. cycle index for asymmetric discharge rate testing of half cells utilising samples 2, 9, 12, 13 and 14.Figure 6 shows discharge capacity retention vs. rate for half cells utilising samples 2, 9, 12, 13 and 14.

[0031] Detailed Description of the Invention

[0032] M1 is Mn, Co, or a combination thereof. These elements typically adopt a 2+ oxidation state and so can be readily substituted for Fe in NFPF, where the Fe is in its 2+ oxidation state. Like Fe, Mn and Co are redox-active (e.g. they have an accessible 2+ / 3+ redox couple). Partially substituting redox-active Fe with an alternative redox-active metal is believed to reduce the bandgap of NFPF, improving electronic conductivity and utility as an active electrode material. M1 preferably contains Mn. For example, at least 75 at.% of M1 can be Mn, or at least 95 at.% of M1 can be Mn, or most preferably M1 is Mn. It will be understood that when at least 75 at.% of M1 is Mn the remaining amount of M1 , less than 25 at.%, is Co. The variable x defines the amount of element M1. x can be 0.001 < x < 0.25, preferably 0.005 < x < 0.2, most preferably 0.01 < x < 0.1 , for example, 0.02 < x< 0.1. An active electrode material of the invention preferably comprises least 0.02 at.% Mn, or at least 0.1 at.% Mn, or most preferably at least 0.2 at.% Mn.

[0033] M2 is Mg, Ca, Zn, or a combination thereof. M2 preferably contains Mg. For example, at least 75 at.% of M2 may be Mg, or at least 95 at.% of M2 is may be Mg, or most preferably M2 is Mg. It will be understood that when at least 75 at.% of M2 is Mg the remaining amount of M2, less than 25 at.%, is Ca and / or Zn. The M2 elements, Mg, Ca, and Zn, typically adopt a 2+ oxidation state so can be readily substituted for Fe in NFPF. However, unlike the M1 elements, the M2 elements are not redox active, not having an accessible 2+Z3+ redox couple. It is believed that partially substituting Fe with the non-redox active metal M2 stabilises a desirable crystal structure of NFPF and facilitates the partial substitution with the redoxactive metal M1. There is thus a synergistic effect when substituting Fe with both M1 and M2. The variable y defines the amount of element M2, y can be 0.001 < y < 0.3, preferably 0.005 < y < 0.25, most preferably 0.01 < y < 0.1 , for example, 0.01 < y < 0.05.

[0034] x + y defines the amount of Fe substituted by M1 and M2 combined, x + y can be 0.002 < x + y < 0.4, preferably 0.01 < x + y < 0.3, most preferably 0.02 < x + y < 0.2, for example, 0.04 < x + y < 0.2,, or as a further example, 0.04 < x + y < 0.15.

[0035] The base NFPF material is believed to be more tolerant to partial substitution by M2 than M1 while maintaining the desirable crystal structure, which is why a higher amount of M2 is permitted in formula (I) than M1. In a further example, 0.001 < y < 0.4 and 0.001 < x < 0.25; preferably 0.005 < y < 0.25 and 0.005 < x< 0.1 ; more preferably 0.02 < x< 0.1 and 0.01 < y < 0.1 ; most preferably 0.02 < x< 0.1 and 0.01 < y < 0.05.

[0036] The base NPFP formula is Na2FePC>4F. The NPFP structure is stable provided that there is at least one sodium ion in the empirical formula (e.g. NaFePC F). Alternative alkali metals can be present in the NFPF structure. In use in a sodium-ion battery, the material loses and gains sodium ions as the battery is cycled. Accordingly, in formula (I) there is at least one sodium ion. The variable n defines the amount of element A, which is Li, Na, K, or combinations thereof. Preferably A comprises Na. For example, at least 50 at.% of A can be Na, preferably at least 75 at.% of A is Na, most preferably A is Na. n can be 0 < n <1 , preferably 0.5 < n < 1. Most preferably n = 1. When n = 1 and A is Na formula (I) simplifies to Na2Fei-x-yM1 xM2yPO4F.

[0037] In a preferred implementation of the invention, the active electrode material has a formula according to formula (II):

[0038] NaAnFei-x-yMnxMgyPO4F (II)

[0039] wherein A is Li, Na, K, or a combination thereof; 0 < n < 1 ; 0.001 < x < 0.25; 0.001 < y < 0.4; and x + y < 0.5. It will be understood that the variables in formula (II) may be limited in the same way as the equivalent variables for formula (I). For example, 0.5 < n < 1 ; 0.005 < x < 0.25; and 0.005 < y < 0.25. In a preferred further example, A is Na; n = 1 ; 0.01 < x< 0.1 ; and 0.01 < y < 0.1. In a more preferred further example, A is Na; n = 1 ; 0.02 < x < 0.1 ; 0.01 < y < 0.1 ; and 0.04 < x + y < 0.2. In a most preferred example, A is Na; n = 1 ; 0.02 < x< 0.1 ; 0.01 < y < 0.05; and 0.04 < x + y < 0.15.

[0040] Iron fluoro phosphates can adopt several crystal structures which facilitate (de)intercalation of alkali metals, making them useful as active electrode materials for metal-ion batteries. Na2FePC>4F adopts an orthorhombic crystal structure which is believed to be particularly advantageous. Thus, preferably the active electrode material has the crystal structure of orthorhombic Na2FePC>4F.

[0041] The crystal structure of orthorhombic Na2FePC>4F is described in Ellis et al., Chem. Mater. 2010, 22, 1059-1070 as having space group Pbcn. It is available at ICSD code 167044. The orthorhombic crystal structure has face sharing Fe octahedra, each coordinated to four oxygen and two fluorine ions. There are two Na sites, one of which is electrochemically active. The unit cell parameters are typically a = 5.21-5.25 A, b = 13.83-13.87 A, c = 11.76-11.80 A.

[0042] Other crystal structures may be adopted, e.g. depending on the stoichiometry of sodium and element A in the formula. For example, NaisFePO4F has been reported as having a monoclinic crystal structure with space group P2 / c, which may be adopted.

[0043] The crystal structure of a material may be determined by analysis of X-ray diffraction (XRD) patterns, typically obtained from a Cu Ka source, as is widely known. For instance, XRD patterns obtained from a given material can be compared to known XRD patterns to confirm the crystal structure, e.g. via public databases such as the ICDD crystallography database. Rietveld analysis and Pawley analysis can also be used to determine the crystal structure of materials, in particular the unit cell parameters.

[0044] The invention also provides an active electrode material having the crystal structure of orthorhombic Na2FePC>4F and comprising Mn and Mg, wherein the combined amount of Mn and Mg in the active electrode material is no more than 6.25 at.%, or no more than 5.5 at.%, or 0.02-4.4 at.%, preferably 0.1-3.3 at.%, most preferably 0.2-2.2 at.%, for example, 0.4-2.2 at.%, or as a further example, 0.4-1.6 at.%.

[0045] It will be understood that amounts in at.% refer to the number of atoms in the empirical formula of the material. For example, for Na2Feo95Mnoo25Mgoo25PC>4F, the at.% of Mn is (0.025 / 9) x 100%.The active electrode material is typically provided in particulate form, although it could be provided in the form of a sintered layer. When in particulate form, the active electrode material may have a Dso particle diameter of 0.05-50 pm, or 0.1-25 pm, preferably 0.15-15 pm. Particle diameter is usually determined by laser diffraction.

[0046] The term “particle diameter” refers to the equivalent spherical diameter (esd), i.e. the diameter of a sphere having the same volume as a given particle, where the particle volume is understood to include the volume of any intra-particle pores. The terms “Dn” and “Dnparticle diameter” refer to the diameter below which n% by volume of the particle population is found, i.e. the terms “Dso” and “Dso particle diameter” refer to the volume-based median particle diameter below which 50% by volume of the particle population is found. Where a material comprises primary crystallites agglomerated into secondary particles, it will be understood that the particle diameter refers to the diameter of the secondary particles. Particle diameters can be determined in accordance with ISO 13320:2020, for example using Mie theory.

[0047] The active electrode material may comprise a carbon coating, e.g. an amorphous carbon coating, to further improve its electronic conductivity. The carbon coating may be present at 0.5-25 wt.%, or 0.75-20 wt.%, or 1-15 wt.% based on the weight of the active electrode material and carbon coating. A carbon coating can be formed by mixing (e.g. ball milling) the active electrode material with a carbon coating precursor (e.g. citric acid, ascorbic acid, oxalic acid, carbohydrates (such as sucrose, glucose, lactose) polyacrylonitrile, hydrocarbons (such as pitch carbons), polymers (such as cellulose-based polymers, polyvinyl alcohol), graphite, graphene oxide, graphene etc.) then calcining typically under inert or reducing atmosphere (e.g. N2, Ar, H2, N2 / H2, Ar / H2, etc.). The calcination can be performed at least at 300°C. A carbon coating can also be formed via chemical vapour deposition, e.g. using acetylene gas.

[0048] The active electrode material is intended for use in electrodes. Thus, the invention provides an electrode comprising the active electrode material. The electrode is typically of the form of an electrode composition in electrical contact with a current collector, where the electrode composition comprises the active electrode material. A current collector is typically a metal foil, e.g. copper or aluminium foil.

[0049] Optionally, the active electrode material forms at least 5 wt.%,10 wt.%, or 50 wt.% of the total active electrode material in the electrode. The active electrode material may form the sole active electrode material in the electrode.

[0050] The electrode composition may further comprise at least one other component selected from a binder, a conductive additive, a further active electrode material (e.g. a further active electrode material of the invention or an active electrode material not according to the invention), and mixtures thereof. The active electrode material not according to the invention can be selected from layered transition metal oxides comprising sodium (e.g. Na2 / 3Fei / 2M / 2C>2, NaaNi(i-x-y-z)MnxMgyTizO2, Nao67Nio3-xCuxMno7O2, etc.), oxyanion materials (e.g. sodium vanadium phosphates and fluorophosphates, sodium ferric sulfate, etc.), Prussian blue and Prussian blue analogues, quinones, and mixtures thereof. For instance, one electrode composition comprises at least 80 wt% active electrode material of the invention, at least 5 wt% conductive additive, and at least 2 wt% binder, based on the total dry weight of the electrode composition.Examples of suitable binders include polyvinylidene fluoride and its copolymers (PVDF), polytetrafluoroethylene (PTFE) and its copolymers, polyacrylonitrile (PAN), poly(methyl)methacrylate or poly(butyl)methacrylate, polyvinyl chloride (PVC), polyvinyl fomal, polyetheramide, polymethacrylic acid, polyacrylamide, polyitaconic acid, polystyrene sulfonic acid, polyacrylic acid (PAA) and alkali metal salts thereof, modified polyacrylic acid (mPAA) and alkali metal salts thereof, cellulose-based polymers, carboxymethylcellulose (CMC), modified carboxymethylcellulose (mCMC), sodium carboxymethylcellulose (Na-CMC), polyvinylalcohol (PVA), alginates and alkali metal salts thereof, butadieneacrylonitrile rubber (NBR), hydrogenated form of NBR (HNBR), styrene-butadiene rubber (SBR) and polyimide, polyurethane, polyethylene and polystyrene. The binder may be present in the electrode composition at 0-30 wt%, or 0.1-10 wt%, or 0.1-5 wt%, based on the total dry weight of the electrode composition.

[0051] Conductive additives are preferably non-active materials which are included to improve electrical conductivity between the active electrode material and between the active electrode material and the current collector. The conductive additives may suitably be selected from graphite, carbon black, carbon fibers, vapor-grown carbon fibres (VGCF), carbon nanotubes, graphene, graphene oxide, acetylene black, ketjen black, metal fibers, metal powders and conductive metal oxides. Preferred conductive additives include carbon black and carbon nanotubes. Conductive additives may be present in the electrode composition at 0-20 wt%, 0.1-10 wt%, or 0.1-5 wt%, based on the total dry weight of the electrode composition.

[0052] The active electrode material may be present in the electrode composition at 100-50 wt%, 99.8-80 wt%, or 99.8-90 wt%, based on the total dry weight of the electrode composition. When the active electrode material is present at 100 wt.% of the electrode composition it may be for a solid-state electrode.

[0053] The electrode is for incorporation into an electrochemical cell, typically a metal-ion battery, preferably a sodium-ion battery. An electrochemical cell comprises an anode, an electrolyte, and a cathode. The electrode of the invention preferably forms the cathode. Thus, the invention also provides the use of the active electrode material as an active cathode material for a sodium-ion battery.

[0054] Active anode materials for use in electrochemical cells may be selected from hard carbon; graphene; graphite; modified graphite; carbon arsenide; metals, intermetallics, and semi-metals which can form alloys with sodium (e.g. Pb, P, Si, Sn, Ge, Sb, and alloys thereof); sodium metal, metal oxides (e.g. TiO2, Fe3O4, V2O3, V2O5, NiO, MnsO4); sodium titanates (e.g. Na2TisO7 or NaTiO2); and sulfides (e.g. MnS, FeS, M0S2, TiS2). Hard carbon is the preferred active anode material for use in electrochemical cells incorporating the material of the invention as an active cathode material.

[0055] The active electrode material of the invention enables the provision of a sodium-ion battery having particularly advantageous properties. Once such battery comprises an anode comprising hard carbon as an active anode material, wherein the battery has a gravimetric capacity of at least 74 mAh / g at a specific current relative to the cathode active material of 62 mA / g (a current density which is equivalent to 0.5C), wherein the battery can be charged and discharged at specific currents relative to the cathode activematerial of 124 mA / g or more, or 248 mA / g or more, or 620 mA / g or more, or 1240 mA / g or more whilst retaining greater than 82% of the initial cell capacity at 62 mA / g.

[0056] A further such battery, which can be a solid-state battery utilising a sodium metal anode, comprises an anode comprising sodium metal as an active anode material, wherein the battery has a gravimetric capacity of at least 96 mAh / g at a specific current relative to the cathode active material of 62 mA / g, wherein the battery can be charged and discharged at specific currents relative to the cathode active material of 124 mA / g or more, or 248 mA / g or more, or 620 mA / g or more, or 1240 mA / g or more whilst retaining greater than 68% of the initial cell capacity at 62 mA / g.

[0057] The active electrode material of the invention may be made by a variety of methods including solid-state synthesis, ionothermal synthesis, and hydrothermal synthesis. For example, precursors for Na, A, Fe, M1 , M2, PO4, and F can be mixed (e.g. by ball milling) then calcined to form the desired material. The calcination is typically under inert or reducing atmosphere (e.g. N2, Ar, H2, N2 / H2, Ar / H2, etc.) and typically at above 500 °C. Calcination may be performed for at least 0.5 hours, or 1-24 hours, or 2-12 hours.

[0058] Materials may be subjected to further milling and calcinations steps to improve phase purity. The synthesised material may be subjected to further steps of milling and / or classifying (e.g. impact milling, jet milling, steam jet milling, high energy milling, high shear milling, pin milling, air classification, wheel classification, sieving, cyclonic separation, bead milling) to provide a material with any of the particle size parameters given above. The active electrode material of the invention may comprise low concentrations of further elements, for example derived from unavoidable impurities in the precursors.

[0059] Electrodes may be made by providing an active electrode material as defined herein and depositing the active electrode material onto a current collector, thereby forming the electrode. The depositing step may include forming a slurry of the active electrode material and a solvent. The slurry typically further comprises a binder, a conductive additive, and mixtures thereof. The slurry may be deposited onto a current collector and the solvent removed, thereby forming an electrode layer on the current collector. Further steps, such as heat treatment to cure any binders and / or calendaring of the electrode layer may be carried out as appropriate. For example, the solvent may be removed by drying e.g. at temperatures of 30-100°C. The electrode layer may be calendared to a porosity of 20-60%, preferably 25-40%. Electrode layer porosity may be determined by the sum of the true density of each component of the electrode layer, divided by the measured calendared density, with 0% representing an electrode with no porosity. The electrode layer may have a density of 1-3.5 g cm-3. The electrode layer may have a thickness in the range of from 5 pm to 2 mm, preferably 5 pm to 1 mm, preferably 5 pm to 500 pm, preferably 5 pm to 200 pm, preferably 5 pm to 100 pm, preferably 5 pm to 50 pm.

[0060] Alternatively, the slurry may be formed into a freestanding film or mat comprising the active electrode material, for instance by casting the slurry onto a suitable casting template, removing the solvent and then removing the casting template. The resulting film or mat is in the form of a cohesive, freestanding mass which may then be bonded to a current collector by known methods.Examples

[0061] Material synthesis

[0062] The active electrode materials were synthesised using a solid-state route. Stoichiometric amounts of precursors for Na, Fe, M1 , M2, PO4, and F (selected from NaCHsCOO, FeC2C>4'2H2O, Mn(NO3)2'4H2O, Mg(CH3COO)2'4H2O, Ni(NC>3)2'6H2O, NH4H2PO4, and NaF) were ground with zirconia balls at a rotational speed of 500 rpm for 2 h. After grinding, the precursors were dried at room temperature and further calcined at 350 °C for 5 h under flowing N2 to decompose the precursors. Then, the sample was further ball milled for 2 h, dried and pelletised, followed by a final calcination treatment at 600 °C for 6 h under N2 atmosphere to obtain the desired material. To synthesise carbon coated materials, citric acid was added in a weight ratio of 20 % into the mixture after calcination at 350 °C and the same further steps performed (ball milling, pelletisation, and calcination). The materials synthesised are shown in Table 1.

[0063]

[0064] Table 1 - Materials synthesised

[0065] XRD

[0066] The XRD pattern and Rietveld refinement of the synthesised NFPF powder (Sample 1) was consistent with earlier studies of Na2FePC>4F. An XRD pattern of the NFPF / C powder (Sample 2) (Figure 1) overlapped with the uncoated NFPF, suggesting the presence of an amorphous carbon coating as no additional peaks were observed. Samples 3-14 were isostructural to NFPF / C, and this is seen in the XRD patterns for samples 2, 9, 12, 13 and 14 (Figure 4).

[0067] Particle size distribution

[0068] Laser diffraction was used to measure the particle size distribution of selected samples.

[0069]

[0070] Table 2 - D50 particle diameter

[0071] Observations by SEM indicated that Samples 3-14 had similar particle size distributions.Electrochemical performance evaluation

[0072] The active materials were coated on carbon-coated aluminium foil using NMP / PVDF-based inks (85:10:5, active electrode material : carbon black : binder). Sodium-ion half-cells were assembled inside a glovebox under argon atmosphere. Glass microfibre (Whatman GF / F grade, diameter 18 mm) was used as the separator. 1 M NaCIO4 with ethylene carbonate:propylene carbonate in 1 :1 weight ratio was used as the electrolyte (100 pl).

[0073] Full cells were assembled by pairing the cathodes with hard carbon anode. Hard carbon electrodes (diameter 15 mm) obtained commercially (coating density: 8 mg cm'2, 94.8 % active material) were used for the full cell study for consistency. The mass balancing was carried out by considering 250 mAh / g of reversible capacity for hard carbon anode and the n / p ratio was kept as 1 .2. For the cathodes, the reversible capacities from half-cell studies were used for the n / p ratio calculation. The cathode materials were coated in appropriate thickness (90:5:5; active electrode material : carbon : binder) to achieve the target mass loading. A single layer of Celgard H1960 was used as the separator. 1 M NaPFe with ethylene carbonate:diethyl carbonate:propylene carbonate in 1 :2:1 weight ratio was used as the electrolyte (80 l).

[0074] The rate performance in half cells (Table 3) and full cells (Table 4) was tested, giving the following results for the discharge capacity at the given C-rate:

[0075]

[0076]

[0077] Table 3 - Half-cell rate performance

[0078] To collect the data in Table 3, half cells were subjected to CC / CC cycling at a fixed charge rate of 0.2C and a sequentially increasing discharge rate in a voltage window of 2.5-4.0 V.

[0079]

[0080] Table 4 - Full-cell rate performance

[0081] It was surprisingly found that Sample 9, according to the invention and representing base Na2FePC>4F cosubstituted with Mn and Mg, gave the best performance at the high rates of 5C and 10C. The improvement is particularly pronounced in the full-cell tests. This is an important result indicating improvements in active cathode materials for sodium-ion batteries intended for high power / fast charge / discharge applications. Notably, the performance at high rates is better than would have been expected based on the performance of the samples substituted with either Mn or Mg alone (e.g. Samples 5 and 8), demonstrating the synergistic effect of the co-substitution of the invention. Moreover, even compared to co-substituted samples outside the scope of the invention (Samples 10 and 11 , substituted with Mg and Ni), Sample 9 shows a remarkable improvement.

[0082] The performance of Sample 9 also far exceeds that reported in the art for half cells comprising Na2Feo8oMgoi5Nioo5P04F / C as an active electrode material (88, 81.6, 73.6, and 53 mAh / g at 0.5C, 1C, 2C, and 5C, respectively - see Guo et al., Electrochemistry, 90(11), 117003 (2022)). The improvement is particularly notable at the highest C-rates (e.g. at 5C, 80.30 ± 2.04 mAh / g for Sample 11 vs. 53 mAh / g in Guo).

[0083] Full cell cycle life tests were carried out at 1C / 1C rate at 25 °C using Samples 2, 5, and 8-11 as the active material (Figures 2 and 3). The cells based on Sample 9 maintained the highest capacity over the duration of the test and maintained ~96 % capacity after 100 cycles. Compared to commercial sodium cathodes (sodium vanadium phosphate I NVP and sodium ferric sulfate I NFS) which showed ~ 92 % retention in hard carbon full cells, these results indicate the promising performance provided by the invention.A literature report on carbon-coated ix^Feo sMno sPC F showed that half-cells with this cathode material retained just 80% of their capacity after just 20 cycles at 0.05C / 0.05C, inferior to that observed for Sample 9 under the far more stringent conditions of 100 cycles at 1C / 1C. (Kawabe et al.; Electrochemistry, 80(2), 80-84 (2012)). The capacity reported for carbon-coated ix^Feo sMno sPC F at 0.5C and 1C was respectively ~90 and ~75 mAh / g (capacity retention of ~83%), which is inferior to that observed for Sample 9 (96.50 ± 0.21 and 94.21 ± 0.30 mAh / g (capacity retention of 97.6%)).

[0084] Half cells having samples 2, 9, and 12-14 as the active material were subjected to an asymmetric charge-discharge rate capability protocol involving CC / CC cycling at a fixed charge rate of 0.2C and a sequentially increasing discharge rate in a voltage window of 2.0-4.2 V. The asymmetric chargedischarge rate capability test gave the following results for the discharge capacity at the given C-rate (shown in Figures 5 and 6):

[0085]

[0086] Table 5 - Further tests of half-cell rate performance

[0087] Each of samples 9 and 12-14, according to the invention and representing base Na2FePC>4F cosubstituted with Mn and Mg, outperform comparative sample 2 at certain rates. In particular, samples 9, 13 and 14 show a significantly improved capacity at high rates of 5C and 10C, and improved capacity retention at all rates (relative to capacity at 0.5C) compared to sample 2. This can be seen in Figure 6. Sample 12 shows an improved or similar capacity to sample 2 at all tested rates. Thus, these results demonstrate the benefits of the claimed materials for use in sodium ion batteries, especially for high power and fast charge / discharge applications. As the voltage window used to collect the data in Table 5 is different to the voltage window used to collect the data in Table 3, the data in Tables 3 and 5 cannot be directly compared. The wider voltage window used to collect the data in Table 5 accesses additional reversible capacity and therefore results in higher measured discharge capacities across the rate series.

[0088] It would be expected that similar improvements would be seen for further active electrode materials within the scope of the invention.

Claims

Claims:

1. An active electrode material having a formula according to formula (I):NaAnFei-x-yM1xM2yPO4F (I)whereinA is Li, Na, K, or a combination thereof;M1 is Mn, Co, or a combination thereof;M2 is Mg, Ca, Zn, or a combination thereof;0 < n < 1 ;0 < x< 0.3;0 < y < 0.4; andx + y < 0.5.

2. An active electrode material according to claim 1 , wherein 0.001 < x < 0.25, or 0.005 < x < 0.2, or 0.01 < x < 0.1 , or 0.02 < x< 0.1 .

3. An active electrode material according to any preceding claim, wherein 0.001 < y < 0.3, or 0.005 < y < 0.25, or 0.01 < y < 0.1.

4. An active electrode material according to any preceding claim, wherein 0.002 < x + y < 0.4, or 0.01 < x + y < 0.3, or 0.02 < x + y < 0.2, or 0.04 < x + y < 0.2.

5. An active electrode material according to any preceding claim, wherein 0 < n < 1 , or 0.5 < n < 1 , or n = 1 .

6. An active electrode material according to any preceding claim, wherein at least 75 at.% of M1 is Mn, or wherein at least 95 at.% of M1 is Mn, or wherein M1 is Mn.

7. An active electrode material according to any preceding claim, wherein at least 75 at.% of M2 is Mg, or wherein at least 95 at.% of M2 is Mg, or wherein M2 is Mg.

8. An active electrode material according to any preceding claim, wherein at least 50 at.% of A is Na, or wherein at least 75 at.% of A is Na, or wherein A is Na.

9. An active electrode material according to claim 1 having a formula according to formula (II):NaAnFei-x-yMnxMgyPO4F (II)whereinA is Li, Na, K, or a combination thereof;0 < n < 1 ;0.001 < x < 0.25;0.001 < y < 0.4; andx + y < 0.5.

10. An active electrode material according to claim 9 wherein:0.5 < n < 1 ;0.005 < x < 0.25; and0.005 < y < 0.25.

11. An active electrode material according to claim 10 wherein:A is Na;n = 1 ;0.01 < x < 0.1 ; and0.01 < y < 0.1.

12. An active electrode material according to claim 11 wherein:A is Na;n = 1 ;0.02 < x < 0.1 ;0.01 < y < 0.1 ; and0.04 < x + y < 0.2.

13. An active electrode material according to claim 12 wherein:A is Na;n = 1 ;0.02 < x < 0.1 ;0.01 < y < 0.05; and0.04 < x + y < 0.15.

14. An active electrode material according to any preceding claim, having the crystal structure of orthorhombic Na2FePC>4F.

15. An active electrode material having the crystal structure of orthorhombic Na2FePC>4F and comprising Mn and Mg, wherein the combined amount of Mn and Mg in the active electrode material is no more than 6.25 at.%.

16. An active electrode material according to claim 15, wherein the combined amount of Mn and Mg in the active electrode material is no more than 5.5 at.%, or 0.02-4.4 at.%, or 0.1-3.3 at.%, or 0.2- 2.2 at.%, or O.4-2.2 at.%.

17. An active electrode material according to any preceding claim, comprising at least 0.02 at.% Mn, or at least 0.1 at.% Mn, or at least 0.2 at.% Mn.

18. An active electrode material according to any preceding claim which is in particulate form; optionally wherein the active electrode material has a D50 particle diameter of 0.05-50 pm, or 0.1-25 pm, orO.15-15 pm.

19. An active electrode material according to any preceding claim comprising a carbon coating; optionally wherein the carbon coating is present at 0.5-25 wt.%, or 0.75-20 wt.%, or 1-15 wt.% based on the weight of the active electrode material and carbon coating.

20. An electrode comprising an active electrode material according to any preceding claim.

21. An electrochemical cell comprising an electrode according to claim 20; optionally wherein the electrochemical cell comprises an anode, an electrolyte, and a cathode which is an electrode according to claim 20.

22. An electrochemical cell according to claim 21 which is a metal-ion battery, optionally a sodium-ion battery.

23. An electrochemical cell according to claim 22, which is a sodium-ion battery comprising an anode comprising hard carbon as an active anode material, wherein the battery has a gravimetric capacity of at least 74 mAh / g at a specific current relative to the cathode active material of 62 mA / g, wherein the battery can be charged and discharged at specific currents relative to the cathode active material of 124 mA / g or more, or 248 mA / g or more, or 620 mA / g or more, or 1240 mA / g or more whilst retaining greater than 82% of the initial cell capacity at 62 mA / g.

24. An electrochemical cell according to claim 22, which is a sodium-ion battery comprising an anode comprising sodium metal as an active anode material, wherein the battery has a gravimetric capacity of at least 96 mAh / g at a specific current relative to the cathode active material of 62 mA / g, wherein the battery can be charged and discharged at specific currents relative to the cathode active material of 124 mA / g or more, or 248 mA / g or more, or 620 mA / g or more, or 1240 mA / g or more whilst retaining greater than 68% of the initial cell capacity at 62 mA / g.

25. Use of an active electrode material according to any of claims 1-19 as an active cathode material for a sodium-ion battery.

26. A method of making an electrode, comprising depositing an active electrode material according to any of claims 1-19 onto a current collector.

27. A method of making an electrochemical cell, comprising depositing an active electrode material according to any of claims 1-19 onto a current collector to form an electrode, and forming an electrochemical cell comprising the electrode; optionally wherein the electrochemical cell is a sodium-ion battery and the electrode forms the cathode.

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