A method for producing a crystalline NFPP material

Abstract

The present disclosure generally relates to a method for producing a crystalline NFPP material comprising: a) providing an NFPP precursor material; b) combusting the NFPP precursor material at a temperature of from 200°C to 700°C for 5 to 90 minutes, preferably from 5 to 30 minutes, to yield a carbon coated NFPP material; c) grinding the carbon coated NFPP material into particles; and d) heating the particles obtained in step c) at a temperature of from 400°C to 800°C to yield a crystalline NFPP material, wherein step a) of providing the NFPP precursor material comprises: i) providing an aqueous mixture by mixing a metal nitrate (MeNO3) with sodium dihydrogen phosphate (NaH2PO4) and ascorbic acid (C6H4O2(OH)4), ii) adding citric acid (C6H8O7) to the aqueous mixture, iii) heating the aqueous mixture obtained in step ii) to yield the NFPP precursor material. The present disclosure also relates to a crystalline NFPP material obtained by the method and to a cathode comprising the crystalline NFPP material. Furthermore, the present disclosure relates to a sodium ion battery cell comprising the cathode.

Description

[0001] A METHOD FOR PRODUCING A CRYSTALLINE NFPP MATERIAL

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to a method for producing a crystalline NFPP material. The present disclosure also relates to a crystalline NFPP material obtained by the method and to a cathode comprising the crystalline NFPP material. Furthermore, the present disclosure relates to a sodium ion battery cell comprising the cathode.

[0004] BACKGROUND

[0005] Lithium ion-based batteries dominate the market for rechargeable batteries. However, the technology has drawbacks, not at least due to the relatively scarce resources of lithium. Although better than previous generations of secondary battery technologies, the lithium ion-based batteries are not considered to be environmentally friendly and are also costly from a recycling perspective.

[0006] These drawbacks have triggered the exploration of alternatives to lithium ion batteries. Sodium ion batteries (SIB) represent attractive alternatives due to their low cost and wide availability of sodium resources.

[0007] The performance of sodium ion batteries is greatly dependent on the properties of the electrode materials.

[0008] Na4Fe3(PO4)2P2O? (NFPP) is a cathode material which has a great potential for sodium ion batteries (SIBs) due to its thermal stability, considerable theoretical capacity and small volume change. However, its poor intrinsic conductivity leads to low discharge capacity and poor cycling stability, which limits its practical application.

[0009] The poor performance, e.g. poor conductivity of NFPP is typically due to the generation of impurities; i.e. maricite (NaFePCL) impurity during synthesis.

[0010] To date, NFPP may be synthesized via a sol -gel route starting from either a mixture of a metal and citric acid, or from a mixture of a metal nitrate, sodium diphosphate and ascorbic acid. In order to enhance the electronic conductivity of the NFPP material, a carbon coating may be provided on the NFPP material. In order to provide a carbon coating, the NFPP material must typically be heated at high temperatures for a long period of time, typically for at least five hours. The lengthy heating step may be referred to as a calcination step. Despite the generation of a carbon coating, maricite (NaFePCU) impurities still arise, which, as mentioned hereinbefore, results in a material with insufficient cathode characteristics, such as poor conductivity and slow Na+diffusion kinetics.

[0011] Accordingly, there is a need to provide an NFPP cathode material having an improved cycling capacity and conductivity, and which generates less maricite impurities during production.

[0012] SUMMARY

[0013] In view of above-mentioned and other drawbacks of the prior art, it is an object of the present disclosure to provide improvements in relation to NFPP cathode materials.

[0014] According to a first aspect of the present disclosure, there is provided a method for producing a crystalline NFPP material comprising: a) providing an NFPP precursor material; b) combusting the NFPP precursor material at a temperature of from 200°C to 700°C for 5 to 90 minutes, preferably for 5 to 30 minutes to yield a carbon coated NFPP material; c) grinding the carbon coated NFPP material into particles; and d) heating the particles obtained in step c) at a temperature of from 400°C to 800°C to yield a crystalline NFPP material, wherein step a) of providing the NFPP precursor comprises: i) providing an aqueous mixture by mixing a metal nitrate (MeNCh) with sodium dihydrogen phosphate (NaEfePC ) and ascorbic acid (CeFUCh OH^) ii) adding citric acid (CeHsO?) to the aqueous mixture iii) heating the aqueous mixture obtained in step ii) to yield the NFPP precursor material.

[0015] The inventors have surprisingly found that a method according to the present disclosure results in less maricite (NaFePCU) impurities during synthesis. Furthermore, when the crystalline NFPP material obtained by the method described hereinabove is utilized as a cathode material in a sodium ion battery cell, a high specific capacity, an improved cycling performance, superior rate capability, and a high coulombic efficiency is obtained. Accordingly, the crystalline NFPP material may successfully be used as a cathode material in sodium ion battery applications. In conventional NFPP synthesis routes, the provision of a carbon coating must typically be accomplished by means of a lengthy heating step in an inert environment and in the presence of a reducing gas. In contrast, the method of the present disclosure allows for a carbon coating to be obtained quickly (5 to 90 minutes), and the final crystalline NFPP material displays improved cathode characteristics.

[0016] The NFPP precursor material is formed by: i) providing an aqueous mixture by mixing a metal nitrate (MeNCh) with sodium dihydrogen phosphate (NaFfcPCU) and ascorbic acid (CeH4O2(OH)4); ii) adding citric acid (CeHsO?) to the aqueous mixture; iii) heating the aqueous mixture obtained in step ii) to yield the NFPP precursor material.

[0017] In the heating step iii), excess water evaporates, which leads to the formation of a solid gel NFPP precursor material.

[0018] The citric acid of the NFPP precursor material acts as a “fuel” in the subsequent step b) of combusting the NFPP precursor material. The citric acid undergoes a redox reaction with atmospheric oxygen. Hence, citric acid oxidizes and breaks down into carbon dioxide (CO2) and water (H2O). The decomposition of citric acid leaves behind carbonaceous residues, which form a thin carbon coating on the NFPP material.

[0019] After the combustion step b), the resulting material is a carbon-coated NFPP precursor, which contains both amorphous and crystalline phases.

[0020] In step c), the carbon-coated material is ground into particles to improve the homogeneity of the final product and to ensure optimal particle size distribution for the next step. Grinding enhances the surface area and ensures that the heating in the subsequent step (step d) is uniform, which allows for an efficient crystallization of the NFPP material.

[0021] In step d), the amorphous NFPP material is converted into its final crystalline form. The result is a carbon-coated crystalline NFPP material having an improved conductivity, specific capacity and cycling stability.

[0022] The method of the present disclosure allows for a quicker, yet improved provision of a cathode material suitable for large-scale cathode production.

[0023] The NFPP precursor material in step a) may be in the form of particles or pellets.

[0024] Particles have a large surface area, which may facilitate a fast and efficient combustion step. An efficient contact between the reactants is provided during the combustion step such that a thin, dense, and uniform carbon coating is obtained. Pellets are compacted or compressed aggregates of the NFPP precursor material, which may be formed by pressing particles together into a dense, homogenous form. An even distribution of reactants is thus obtained, which reduces the likelihood of inhomogeneities (which could potentially lead to the formation of unwanted maricite impurity). Furthermore, the provision of the NFPP precursor material in the form of pellets allows for a carbon coating to be formed homogenously on the particles present in the pellets. This may improve the morphology, purity, physical stability, and performance (e.g. electrical conductivity) of the final crystalline NFPP material.

[0025] The provision of the NFPP precursor material in the form of particles of pellets secures that the subsequent combustion step may be carried out in a quicker and more efficient manner.

[0026] The NFPP precursor material in step a) is typically substantially amorphous.

[0027] In the subsequent combustion step (step b), the carbon coated NFPP material, typically comprises mixed phases of amorphous NFPP, and crystalline NFPP, respectively.

[0028] After the final heating step (step d), a crystalline NFPP material is obtained.

[0029] The step d) of heating may be performed at a heating time of at least 5 hours, preferably from 5 to 12 hours.

[0030] This allows for a fully crystalline NFPP material to be obtained.

[0031] Typically, the step d) of heating is performed in the presence of a reducing gas, preferably a hydrogen gas.

[0032] In manufacturing of sodium iron phosphate pyrophosphate (NFPP) materials, it is important to maintain Fe2+rather than allow oxidation to Fe3+. The Fe2+ion is active in the battery cycling process, whereas Fe3+does not contribute to the capacity as efficiently.

[0033] Oxidation of Fe2+to Fe3+can occur during heating, i.e. step d) if oxygen or ambient air is present. The use of a reducing gas prevents this oxidation by creating a reducing environment, which maintains or restores the Fe2+oxidation state. Accordingly, the sodium-ion battery performance, e.g. the specific capacity is improved since the amount of sodium that can be intercalated is increased.

[0034] Furthermore, the reducing gas aids in maintaining the integrity of the carbon coating formed during the combustion step. During the heating step, the carbon coating can be oxidized if the material is exposed to oxygen or air, which may result in degradation of the coating. In addition, the reducing gas advantageously decreases the risk of formation of Fe2C>3 impurities. The reducing gas controls the heating atmosphere and may also decrease or minimize the formation of maricite.

[0035] In exemplary embodiments, the heating step d) is performed in the presence of Hydrogen gas and Argon gas.

[0036] The step d) of heating may be performed in an inert atmosphere, e.g. in the presence of Argon (Ar).

[0037] Accordingly, potential unreacted components are removed. The presence of unreacted components during synthesis may yield an inactive NFPP material with impaired cathode characteristics.

[0038] As mentioned hereinbefore, step a) of providing an NFPP precursor material comprises: i) providing an aqueous mixture by mixing a metal nitrate (MeNCh) with sodium dihydrogen phosphate (Na^PC ) and ascorbic acid (CeFUCh OH^) ii) adding citric acid (CeHsO?) to the aqueous mixture iii) heating the aqueous mixture obtained in step ii) to yield the NFPP precursor material.

[0039] The metal nitrate (MeNCh) may be iron nitrate (Fe(NOs)3), nickel nitrate (Ni(NOs)2), or manganese nitrate (Mn(NOs)2).

[0040] The metal nitrate may be mixed with sodium dihydrogen phosphate and ascorbic acid until dissolution. The obtained aqueous mixture may subsequently be mixed with citric acid, which acts as a “fuel” for gas evolution and generation of a carbon coating during the subsequent combustion step (step b).

[0041] The step iii) of heating may be performed at a temperature of from 60 to 120°C, e.g. from 70 to 90°C. The step iii) of heating may be performed in an oil bath.

[0042] The NFPP precursor material obtained is typically in the form of a solid gel, e.g. a xerogel.

[0043] The method may further comprise a step of: iv) grinding the NFPP precursor material into particles.

[0044] The NFPP precursor material obtained in step iii) of the method may be dried prior to the step iv) of grinding the NFPP precursor material into particles.

[0045] The method may further comprise a step of: v) compressing the particles into pellets.

[0046] Hence, the particles may be compressed together into a dense, homogenous form. An even distribution of reactants is thus obtained, which reduces the likelihood of inhomogeneities (which could potentially form unwanted maricite impurity). Furthermore, the provision of the NFPP precursor material in the form of pellets allows for a carbon coating to be formed homogenously on the particles present in the pellets. This may improve the morphology, purity, physical stability, and performance (e.g. electrical conductivity) of the final crystalline NFPP material.

[0047] According to another aspect, there is provided a crystalline NFPP material obtained by the method as described hereinbefore.

[0048] The crystalline NFPP material comprises less than 4%, preferably less than 1% by weight of maricite (NaFePCU).

[0049] According to a further aspect, there is provided a cathode comprising the crystalline NFPP material as described hereinbefore or produced by the method described hereinbefore.

[0050] According to yet another aspect, there is provided a sodium ion battery cell comprising the cathode defined hereinabove.

[0051] The sodium ion battery cell may have a capacity above 95 m Ah / g at a C-rate of 0.1C.

[0052] Further features of, and advantages with, the present disclosure will become apparent when studying the appended claims and the following description. The skilled addressee realizes that different features of the present disclosure may be combined to create embodiments other than those described in the following, without departing from the scope of the present disclosure.

[0053] BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The various aspects of the present disclosure, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

[0055] Figure 1 schematically illustrates the steps of the method of the present disclosure.

[0056] Figure 2 schematically illustrates a sodium ion battery cell according to an exemplary embodiment of the present disclosure.

[0057] Figure 3 illustrates XRD patterns of an NFPP sample synthesized by a prior art method (referred to as “Calcination”), and an NFPP sample synthesized according to an exemplary embodiment of the present disclosure (referred to as “Combustion”), respectively. Figure 4 illustrates the galvanostatic charge / discharge profiles for NFPP cathodes, wherein the NFPP has been synthesized by a prior art method (“Calcination”), and an NFPP sample synthesized according to an exemplary embodiment of the present disclosure (“Combustion”), respectively.

[0058] Figure 5 illustrates the cycling performances of NFPP cathodes, wherein the NFPP has been synthesized by a prior art method (“Calcination”), and an NFPP sample synthesized according to an exemplary embodiment of the present disclosure (“Combustion”), respectively, at 0.1 C over 100 cycles.

[0059] Figure 6 illustrates the rate capability of the NFPP cathodes, wherein the NFPP has been synthesized by a prior art method (“Calcination”), and an NFPP sample synthesized according to an exemplary embodiment of the present disclosure (“Combustion”), respectively, at various C-rates from 0.05 C to 2 C for 120 cycles.

[0060] DETAILED DESCRIPTION

[0061] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the present disclosure to the skilled person.

[0062] Figure la schematically outlines the main steps of the method (100) for method for producing a crystalline NFPP material comprising: a) providing an NFPP precursor material (step 101), b) combusting the NFPP precursor material at a temperature of from 200°C to 700°C for 5 to 90 minutes to yield a carbon coated NFPP precursor material (step 102), c) grinding the carbon coated NFPP precursor material into particles (step 103), and d) heating the particles obtained in step c) at a temperature of from 400°C to 800°C to yield a crystalline NFPP material (step 104).

[0063] The step a) of providing the NFPP precursor material comprises: i) providing an aqueous mixture by mixing a metal nitrate (MeNCE) with sodium dihydrogen phosphate (NaEhPC ) and ascorbic acid (CeELC^OETL), ii) adding citric acid (CeHsO?) to the aqueous mixture, and iii) heating the aqueous mixture obtained in the step ii) to form the NFPP precursor material.

[0064] As used herein, the term “NFPP precursor material” means a composite material formed from the reaction between metal nitrate (MeNCh), sodium dihydrogen phosphate (NaEEPCU), ascorbic acid (CeH4O2(OH)4), and citric acid (CeHsO?). The NFPP precursor material is typically substantially amorphous.

[0065] As used herein, the term “substantially amorphous” means a material that lacks a defined crystalline structure. In a substantially amorphous material, the atoms or molecules are arranged in a disordered manner, without the regular, repeating pattern characteristic of a crystalline structure.

[0066] In step b) (step 102), the NFPP precursor material is subject to combustion.

[0067] “Combustion” refers to a high-temperature exothermic redox chemical reaction between a reductant and an oxidant (e.g. atmospheric oxygen). The reaction results in the rapid release of heat and the formation of gaseous by-products, e.g. carbon dioxide and water through oxidation of the reductant (fuel).

[0068] In the context of the present disclosure, the combustion step b) involves the reaction of citric acid (as the fuel) with oxygen from the surrounding atmosphere, which leads to the in situ formation of an amorphous carbon coating on the surface of the NFPP precursor material.

[0069] “Calcination” refers to a thermal process in which a material is heated to a high temperature, typically in the absence of oxygen. Calcination is generally a slower process compared to combustion and is used to modify the chemical composition or structure of the material without engaging in a full redox reaction with atmospheric oxygen. Unlike combustion, calcination is performed in a controlled environment, often in an inert or reducing atmosphere.

[0070] Step b) of combusting the NFPP precursor material is carried out at a temperature of from 200°C to 700°C, e.g. from 250°C to 500°C, preferably from 300°C to 500°C.

[0071] Step b) is typically carried out in the presence of air. Air comprises oxygen, which acts as an oxidant during combustion.

[0072] Step b) of combusting the NFPP precursor material is carried out during a time period of from 5 to 90 minutes, e.g. from 5 to 30 minutes. The present inventors have demonstrated that a combustion step for only 10 minutes is sufficient to yield a crystalline NFPP material with excellent electrochemical properties (see Example section). The step b) of combusting the NFPP precursor material is typically carried out until gas evolution.

[0073] During the combustion step, a carbon coating is provided on the NFPP material.

[0074] The carbon coating may be a continuous or a discontinuous coating. In other words, the NFPP material may be partially coated or fully coated with carbon.

[0075] As mentioned hereinbefore, the generation of a carbon coating is typically accomplished by means of a calcination step; i.e. a step requiring heating at high temperatures for several hours in an inert and reducing environment.

[0076] In the method of the present disclosure, the calcination step is excluded and replaced with a combustion step, which results in a quicker process suitable for large-scale production. Furthermore, the final crystalline NFPP material is associated with improved electrochemical characteristics; i.e. and improved electronic conductivity and cycling capacity.

[0077] As used herein, the term "crystalline NFPP material" refers to a sodium iron phosphate pyrophosphate material having the formula Na4Fe3(PO4)2(P2O?). The material exhibits a well-defined crystalline structure, with an open framework composed of [Fe3P20i3] layers interconnected by (P2O?)4' groups. The crystalline NFPP material incorporates both phosphate (PO43) and pyrophosphate (P2O74) groups, which contribute to its high thermal and structural stability and electrochemical performance when used as a cathode material in sodium-ion batteries. The crystallinity enhances the material's cycling stability, specific capacity, and coulombic efficiency.

[0078] Surprisingly, as demonstrated in the Example section, the method of the present disclosure yielded a reduced carbon content compared to the carbon content achieved with the conventional “sol-gel calcination” synthesis. Despite the lower carbon content, the obtained crystalline NFPP material displayed significantly better electrochemical performance compared to the “Sol -gel-calcination” synthesized NFPP material (see Examples, and figures 3-6).

[0079] The NFPP precursor material in step a) is typically in the form of particles or pellets.

[0080] The particles or pellets may have an average size of from 2 to 30 mm, preferably from 8 to 20 mm. The intermediate NFPP material; i.e. the carbon coated NFPP material, obtained in step b) typically comprises mixed phases of amorphous NFPP, and crystalline NFPP, respectively.

[0081] Subsequently, the intermediate product (the partially carbon coated NFPP precursor material) obtained after the combustion step is ground into particles (step c).

[0082] The average diameter of the particles in step c) may be from 20 to 300 nm, such as from 30 nm to 170 nm.

[0083] An average diameter in the above-mentioned range may lead to a better performance due to an increased conductivity.

[0084] A Barrett- Joy ner-Halenda desorption average pore diameter of the particles in the step c) may be in the from 3 to 30 nm, such as from 5 to 25 nm.

[0085] The specific Barrett- Joy ner-Halenda desorption average pore diameter may allow for faster ion transport.

[0086] A specific surface area of the particles in step c) may be from 20 to 50 m2g such as 25 to 40 m2g as determined by nitrogen gas adsorption and Braunauer-Emmett- Teller theory.

[0087] The specific surface area may facilitate the contact between electrode and electrolytes shortening the diffusion pathway for Na+, thereby enhancing the electrochemical performance.

[0088] After step c) and prior to step d), the particles may be compressed into pellets.

[0089] The pellets may have an average diameter of from 2 to 30 mm, e.g. from 8 to 20 mm.

[0090] In order to generate a crystalline NFPP material, the particles (or pellets) are subject to a heating step (step d). The temperature may be from 400°C to 800°C, e.g. from 400°C to 600°C, preferably from 450°C to 600°C.

[0091] The heating time may be at least 5 hours, preferably from 5 to 12 hours.

[0092] The heating step d) is preferably performed in the presence of a reducing gas, e.g. a hydrogen gas.

[0093] In exemplary embodiments, the heating step d) is performed in the presence of Hydrogen gas and Argon gas.

[0094] Preferably, the heating step is carried out in an inert atmosphere, preferably in the presence of Argon (Ar).

[0095] This is to secure that all potential unreacted components, which may negatively affect the capacity and conductivity of the final NFPP product, are eliminated. As mentioned hereinbefore, step a) of providing the NFPP precursor material comprises: i) providing an aqueous mixture by mixing a metal nitrate (MeN03) with sodium dihydrogen phosphate (NaH2PO4) and ascorbic acid (CeH4O2(OH)4), ii) adding citric acid (CeHsO?) to the aqueous mixture, and iii) heating the aqueous mixture obtained in the step ii) to form the NFPP precursor material.

[0096] Typically, the method further comprises a step of: iv) grinding the NFPP precursor material into particles after step iii).

[0097] The method may further comprise a step of: v) compressing the particles into pellets after step iv).

[0098] The pellets may have an average diameter of from 2 to 30 mm, e.g. from 8 to 20 mm.

[0099] The use of a metal nitrate (MeN03) as a starting material is associated with low costs. Hence, the synthesis of the final crystalline NFPP material is cost-efficient.

[0100] The metal nitrate (MeN03) is typically iron nitrate (Fe(NOs)3), nickel nitrate (Ni(NO3)2), or manganese nitrate (Mn(NOs)2). Preferably, iron nitrate is used as the starting material.

[0101] The metal nitrate, e.g. iron nitrate is mixed with sodium dihydrogen phosphate and ascorbic acid.

[0102] The ascorbic acid secures that the metal ion precursor, e.g. Fe3+is converted to the Fe2+state.

[0103] Citric acid is preferably added to the aqueous solution to trigger an exothermic reaction during the subsequent combusting step (step b). The citric acid acts as a “fuel” in the combusting step and triggers gas evolution and an efficient provision of a carbon coating on the NFPP particles.

[0104] The following reaction may occur in the step b) of combusting the composite NFPP material:

[0105] 4NaH2PO4.2H2O + 3Fe3+(NO3)3.9H2O + 9 / 4C6H8O7Na4Fe2+3(PO4)2(P2O7) / C + NOX$ +

[0106] CO2T + H2OT

[0107] The present disclosure also relates to a crystalline NFPP material obtained by the method as described hereinabove. The XRD pattern in figure 1 demonstrates that the NFPP material obtained by the method has an orthorhombic crystal structure with a. Pn2ia space group crystalline NFPP material.

[0108] The crystalline NFPP material comprises less than 4%, preferably less than 1 % by weight of maricite (NaFePCU).

[0109] The present disclosure further relates to a cathode comprising the crystalline NFPP material as defined hereinabove or produced by the method described hereinbefore.

[0110] In a further aspect, there is provided a sodium ion battery cell comprising the cathode described hereinbefore.

[0111] The sodium ion battery cell preferably has a capacity above 95 m Ah / g at a C- rate of 0.1 C. in a voltage range of from 1.8 to 3.8 V in a half cell.

[0112] Figure 2 illustrates the schematic principles of a sodium ion battery 200 utilizing sodium ions 201 as charge carriers. The battery stores energy in chemical bonds of the negative electrode, i.e. the anode 202. Charging the battery 200 forces Na+ions 201 to deintercalate from the positive electrode; i.e. the cathode 203 and migrate towards the anode 202. During discharge, the process reverses. Once a circuit is completed, electrons pass back from the anode 202 to the cathode 203 and the Na+ions 201 travel back to the cathode 203. During discharge of the battery, as illustrated in figure 2, oxidation takes place at the anode 202, while reduction takes place at the cathode 203. The current flow is determined by the potential difference between the cathode 203 and the anode 202, the cell voltage.

[0113] The two electrodes are separated by a separator 205 infiltrated with an electrolyte 204.

[0114] As used herein the term “battery” means a device comprising one or more battery cells.

[0115] A “battery cell” comprises a positive electrode; i.e. a cathode, a negative electrode; i.e. an anode and a separator. In the battery cell, chemical energy is transformed into electricity by reduction and oxidation (redox) reactions at the electrodes.

[0116] The cathode 203 utilized in the sodium ion battery of the present disclosure comprises a current collector coated with the crystalline NFPP material as described hereinbefore.

[0117] The negative electrode material, i.e. the anode 202, is not particularly limited as long as it is a material capable of storing / releasing sodium or potassium ions. Examples include metal composite oxides, sodium metal, sodium alloys, silicon, silicon-based alloys, tin-based alloys, bismuth-based alloys, metal oxides, conductive polymers, Na-Co-Ni -based materials, hard carbon and the like. The anode may be a metal foil coated with an active anode material.

[0118] Examples

[0119] Synthesis of Na4Fe3(PO4)2(P2O7)

[0120] NFPP was synthesized by a flash-combustion process using Fe(NO3)3 9H2O (>99.99%), ascorbic acid (C6H8O6) (>99%), NaH2PO4.2H2O (>98%) and citric acid (C6H8O7) (, >99.5%) as precursors. First, 0.045 mol (18.18 gram) Fe(NO3)3 9H2O (oxidant) as Fe3+precursor was dissolved in 150 mL distilled water at room temperature and was completely converted into Fe2+state by the addition of a pinch of ascorbic acid (CeEfcOe) resulting a color change from brown to colorless jade-green.

[0121] After that, 0.06 mol (9.36 gram) NaH2PO4.2H2O was added and stirred until dissolution. Finally, 0.03375 mol (6.48 gram) citric acid (fuel) was added to the above solution to trigger the exothermic reaction.

[0122] Then, the resultant solution was mixed in oil bath at 80 °C until the gel formation. After the evaporation of excess water, the precursor powder was dried in in vacuum oven at 80°°C overnight. Next, the dry metal-citro-nitrate complex gel was pressed into cylindrical pellets, which hereinafter may be referred to as sol-gel precursor pellets.

[0123] A first set of sol -gel precursor pellets were fired in tubular furnace at 300° C for 10 min under an air atmosphere to initiate the combustion which resulted in NOXtype gas evolution. The final reaction and the calculation for the required minimum citric acid can be written as follows by using the combustion index:

[0124] 4NaH2PO4.2H2O + 3Fe3+(NO3)3.9H2O + 9 / 4C6H8O7Na4Fe2+3(PO4)2(P2O7) / C + NOX$ +

[0125] CC + H2O$

[0126] For comparative tests, a second set of sol -gel precursor pellets were precalcinated at 300 C for 5 h under Ar / H2gas (95:5 vol.%) flow with a heating rate of 3oCmin_1to eliminate organic components.

[0127] The amorphous intermediate products obtained after combustion, and calcination, respectively, were ground in an agate mortar and pelletized again using a discshaped mold (0 = 13 mm). The pellets were further annealed at 500°C for 10 h under flowing Ar / H2gas (95:5 vol.%) as a reducing atmosphere with a heating rate of 3 °C min'1. During calcination at high temperature, carbonaceous sources are thermally decomposed to form an in situ amorphous carbon coating layer on the NFPP surface. After naturally cooling down to room temperature, nanosized NFPP / CC product was obtained.

[0128] Material Characterization

[0129] X-ray powder diffraction (XRPD) was used to determine the crystal structure and purity of the “Sol -gel-combustion” samples, and the “Sol -gel-calcination samples”, respectively, using Bruker D8 Twin-Twin diffractometer equipped with Cu Ka (X = 1.5406 A) source operated at 40 kV and 15 mA over the range of 5-70 ° (20) with a scanning step of 0.01 ° s'1. The57Fe Mossbauer spectra were recorded in transmission mode using a57Co Rh source of constant acceleration at room temperature, covering a velocity span of ±5 mm / s. 10 mg sample was mixed with 5 mg boron nitride to form absorbers with a concentration of about 10 mg / cm2of the actual substance under argon. This was then spread evenly over a 0.8 cm diameter absorber disk. The carbon contents were found by using an elemental analyzer (FLASH EA 1112 CHNS-O).

[0130] Electrochemical Measurements

[0131] The electrochemical performance of NFPP cathodes, prepared via “Sol -gel combustion”, vs “Sol-gel calcination”, respectively was investigated in pouch cells assembled in an Ar-filled glovebox in which O2 and H2O levels were below 1 ppm. Electrodes were fabricated by mixing the active material with conductive carbon black (C65) and polyvinylidene fluoride (PVDF) binder in N-methyl-2 -pyrrolidone (NMP) () in 80: 10: 10 weight ratio. Electrode slurry was cast onto carbon-coated aluminum foil on a bar coater with a thickness of 400 pm using doctor blade. Electrodes were dried in vacuum oven at 80°C overnight and further punched into 20 mm diameter discs. Additionally, punched electrodes were dried in a Buchi oven under vacuum at 120 °C overnight inside the glove-box. The loading of active material for the galvanostatic cycling was approximately 4-5 mg cm'2after drying.

[0132] Pouch cells were assembled in an argon filled glove-box using glass fiber membrane (26 mm diameter, 240 pm thick)) separators, 22 mm of Na disks punched from metallic Na cubes serving as a counter and reference electrode, and 200-250 pL of 1 M NaPFe (99%) in diethylene glycol dimethyl ether (diglyme) (>99.99%) as the electrolyte.

[0133] The galvanostatic charge-discharge cycling tests of half-cells were carried out using a Neware battery tester BTS4000 in a potential window of 1.8-3.8 V vs. Na / Na+at 25 C after 12 h rest time to ensure proper wetting of electrodes. Results

[0134] The XRD patterns illustrated in figure 2 indicate that NFPP materials show an orthorhombic crystal structure with a Pn2ia space group. In addition to the NFPP main phase, maricite (NaFePCf) impurities were observed for the NFPP synthesized via sol-gel-calcination (see asterix* in figure 1). On the other hand, the NFPP synthesized with combustion resulted in less maricite phase in the NFPP compound compared to the NFPP which was calcinated at 300 C for 5 h under reducing atmosphere.

[0135] The Mdssbauer spectroscopy was further performed to determine the oxidation state and coordination of Fe within the NFPP compound, which has three different Fe2+sites (Fel, Fe2, Fe3). The Mdssbauer spectrum further confirms that NFPP synthesized by sol-gel- calcination using Felll precursor contains 5.34% Fe3+impurity whereas performing combustion results < 1% impurity showing the complete transformation of Fe3+—> Fe2+.

[0136] For the NFPP synthesized by sol-gel-calcination, the carbon content was found to be 10.55 % from CHNS analysis. The atomic ratio between Na Fe P was calculated as 4.0, 2.64 and 3.84 (calculated through ICP). The NFPP synthesized with flash combustion has a lower carbon content around 5.09%. The ratio between Na, Fe, and P was calculated as 4, 3.39 and 4.15.

[0137] As illustrated in figure 3 (galvanostatic charge / discharge profiles) the sol-gel- combustion synthesized NFPP sample (referred to as “Combustion” in figure 3) shows a higher specific capacity than the sol -gel -cal cination synthesized NFPP sample (referred to as “Calcination” in figure 3).

[0138] Furthermore, an improved cycling performance is obtained with the “Combustion” synthesized NFPP sample (see figure 4). The “Combustion” synthesized sample displays a higher specific discharge capacity and also a higher capacity retention ratio after 100 cycles (99.7% compared to 98.7 % for the sol-gel-calcination synthesized sample).

[0139] As illustrated in figure 5 (rate capability at various C-rates) the combustion synthesized NFFP sample shows a higher specific discharge capacity at various C-rates over 120 cycles compared to its sol-gel-calcination counterpart (“Calcination”).

[0140] Terms, definitions and embodiments of all aspects of the present disclosure apply mutatis mutandis to the other aspects of the present disclosure.

[0141] Even though the present disclosure has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the present disclosure, from a study of the drawings, the disclosure, and the appended claims. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

Claims

CLAIMS1. A method for producing a crystalline NFPP material comprising: a) providing an NFPP precursor material; b) combusting said NFPP precursor material at a temperature of from 200°C to 700°C for 5 to 90 minutes, preferably from 5 to 30 minutes, to yield a carbon coated NFPP material; c) grinding said carbon coated NFPP material into particles; and d) heating said particles obtained in said step c) at a temperature of from 400°C to 800°C to yield a crystalline NFPP material, wherein said step a) of providing said NFPP precursor material comprises: i) providing an aqueous mixture by mixing a metal nitrate (MeNCh) with sodium dihydrogen phosphate (NaFLPCh) and ascorbic acid (CeH4O2(OH)4), ii) adding citric acid (CeHsO?) to said aqueous mixture, iii) heating said aqueous mixture obtained in said step ii) to yield said NFPP precursor material.

2. The method according to claim 1, wherein said step b) is performed in the presence of air.

3. The method according to claim 1 or claim 2 , wherein said NFPP precursor material of said step a) is in the form of particles or pellets.

4. The method according to any one of the preceding claims, wherein said NFPP precursor material of said step a) is substantially amorphous.

5. The method according to any one of the preceding claims, wherein said step d) of heating is performed at a heating time of at least 5 hours, preferably from 5 to 12 hours.

6. The method according to any one of the preceding claims, wherein said step d) of heating is performed in the presence of a reducing gas, preferably a hydrogen gas.

7. The method according to any one of the preceding claims, wherein said step d) of heating is performed in an inert atmosphere, preferably in the presence of Argon (Ar).

8. The method according to any one of the preceding claims, further comprising a step of: iv) grinding said NFPP precursor material into particles.

9. The method according to claim 8, further comprising a step of: v) compressing said particles into pellets.

10. A crystalline NFPP material obtained by the method according to any one of claims 1-9.

11. The crystalline NFPP material according to claim 10 comprising less than 4%, preferably less than 1% by weight of maricite (NaFePCU).

12. A cathode comprising the crystalline NFPP material according to claim 10 or claim 11, or produced by the method according to claims 1-9.

13. A sodium ion battery cell comprising the cathode according to claim 12.

14. The sodium ion battery cell according to claim 13 having a capacity above 95 m Ah / g at aC-rate of 0.1 C.

Images