A method for manufacturing a composite of sodium-vanadium fluorophosphate with amorphous carbon, a positive electrode material for sodium-ion cells manufactured using this method and a sodium-ion cell using this positive electrode material

Abstract

A method for manufacturing a composite of sodium-vanadium fluorophosphate with amorphous carbon, crystallizing in the NASICON structure, consisting in preparing a solid, homogeneous and stoichiometric mixture of precursors containing Na+, VO3-, PO43- and F- ions, and counterions undergoing decomposition into gaseous products at high temperature, which mixture is subjected to thermal treatment and subsequent homogenisation with a carbon material, characterized in that the solid homogeneous mixture of precursors, additionally containing an oxidizer and a fuel, is subjected to self-combustion, and the obtained material is subjected to grinding with amorphous carbon with subsequent thermal treatment, wherein: an oxidizer solution is prepared containing NH4F, (NH4)2HPO4 and NaNO3 in a 3:2:3 molar ratio in deionized water in a PTFE beaker, with nitrate anions as the oxidizer, and a fuel solution is prepared containing NH4VO3 and citric acid in a 3:2 molar ratio in deionized water in a glass beaker, with citric acid as the fuel, and after which the oxidizer solution and the fuel solution are simultaneously stirred and heated at temperature of 80°C until the fuel solution turns blue, at which time the fuel solution is added drop by drop to the oxidizer solution with continuous stirring, maintaining a 1:1 molar ratio of NH4VO3 to (NH4)2HPO4, and then the resulting solution is heated at temperature of 80°C until the solvent is completely evaporated, and the resulting precipitate is heated to a temperature of 150°C until it spontaneously ignites, wherein the green powder obtained after the combustion process is dried in a vacuum at a temperature of 120°C for 12 hours, and the dried material is ground with amorphous carbon in a mass ratio of 1:14 in a ball mill at 500 rpm for 5 hours, and the grinding product is subjected to heat treatment in a tube furnace in an inert atmosphere at a temperature of 300°C for 4 hours and then at a temperature of 550°C for 8 hours, as a result of which a composite material is obtained containing crystallographically pure, single-phase sodium-vanadium fluorophosphate, the crystallites of which are covered with amorphous carbon, forming composite particles with a size of 50-200 nm, wherein the carbon coating has a thickness in the range of 2-20 nm. The sodium-vanadium fluorophosphate-amorphous carbon composite obtained by the method of the invention is phase-pure and has a uniform morphology. The self-combustion method allows for obtaining a powder with excellent properties for use in sodium-ion systems, demonstrating high specific capacity and excellent cyclicity.

Description

[0001] A method for manufacturing a composite of sodium-vanadium fluorophosphate with amorphous carbon, a positive electrode material for sodium-ion cells manufactured using this method and a sodium-ion cell using this positive electrode material

[0002] The invention provides a method for synthesizing a carbon composite of sodium vanadium fluorophosphate, with the NASICON structure and the molecular formula Nas^O-i.x^PO^iF-i+ix (0 < x < 1). The invention also relates to the use of the resulting sodium vanadium fluorophosphate composite with amorphous carbon as an active material in the positive electrode of a sodium-ion cell. Furthermore, the invention relates to a sodium-ion cell utilizing this electrode material.

[0003] Sodium-ion (Na-ion) cells are a promising alternative to lithium-ion cells. Despite their lower charge density resulting from the less favourable charge-to-mass ratio of the cation, sodium-ion cells are being considered for widespread commercial use due to the high availability of sodium in nature and its low cost [Chem. Rev. 144 (2014) 11636-11682], Currently, intensive research is underway to develop and optimize new electrode materials, electrolytes, and technical solutions aimed at improving the operating parameters of this type of cells [Electrochem. Energy Rev. 7 (2024) 17] and, in the longer term, introducing Na-ion cells to the market.

[0004] Positive electrode materials for sodium-ion cells fall into three main categories: layered oxides, Prussian blue analogues, and polyanionic compounds. Their action is based on the oxidation / reduction of redox centers in the form of transition metal cations, and the differences between their properties result from the different chemical environment of these centers. Layered oxides are characterized by high theoretical specific capacities (150-250 mAh / g), however, due to their low structural stability, these materials undergo numerous, often irreversible, phase transformations in the first cycles of the cell operation, which results in a significant reduction in specific capacity [Energy Mater. 8 (2018) 1702619], Prussian blue analogues exhibit moderate theoretical specific capacities (120-170 mAh / g) but are easy to obtain, however, their cyclic resistance is also unsatisfactory due to the large number of Fe(CN)6 defects and coordinated water in the crystal structure [Acc. Chem. Res. 5 (2023) 284—296], Moreover, materials belonging to both of these classes are sensitive to moisture, and Prussian blue analogues are also sensitive to light. In turn, polyanionic compounds, despite lower theoretical specific capacities (typically 90-160 mAh / g), exhibit excellent structural and electrochemical stability and are also less sensitive to environmental factors [Mater. Chem. Front. 5 (2021) 3735-3764], Therefore, they constitute a promising class of active materials for the construction of the positive electrode of sodium-ion cells.

[0005] Sodium-vanadium phosphate (NajVi^O^s, NVP) is one of the most widely described positive polyanionic electrode materials (materials for the construction of the positive electrode) used in sodium-ion batteries [Electrochem. Commun. 14 (2012) 86-89], It crystallizes in the NASICON structure (space group no. 63, C mcm), which provides good transport properties of Na+ions through the crystalline framework and excellent structural, thermal, and chemical stability in Na-ion cells. However, due to the presence of the bulky phosphate anion (PCV-) and the heavy vanadium redox center (V47V3+), NVP is characterized by moderate specific capacity (ca. 118 mAh / g) and low operating potential (ca. 3.4 V versus Na7Na°), which results in a low specific energy of this material.

[0006] One way to increase the potential of the vanadium redox center (V47V3+) in polyanionic compounds is to introduce fluoride anions (F“) into the crystal structure. The compounds obtained in this way form a new family of chemical compounds with the general formula Na3(VOi.x)2(PO4)2Fi+2x (0 < x < 1). Due to the strong inductive effect of the F“ and PO43-groups on the vanadium redox center, the exploitation potential of these materials is increased in comparison to the starting sodium-vanadium phosphate [Adv. Funct. Mater. 24 (2014) 4603-4614], The fully fluorinated compound, sodium- vanadium fluorophosphate (Na3V2(PO4)2p3, NVPF) is characterized by an operating potential of about 4.2 V versus Na7Na° and a higher specific capacity of about 128 mAh / g than sodium-vanadium phosphate [Chem. Mater. 28 (2016) 7683-7692],

[0007] The materials Na3(VOi.x)2(PO4)2Fi+2x (0 < x < 1) with the NASICON structure can be treated as a solid solution of Na3V2(PO4)2p3 and Na3(VO)2(PO4)2F. The skeleton of the Na3V2(PO4)2p3 structure consists of two V2O8F3 octahedra, in the centers of which vanadium is located surrounded by four oxygen atoms in the a-b plane and three fluorine atoms along the c axis, one of which is common to both octahedra (Fig. 6). This mutual arrangement of vanadium, oxygen, and fluorine is called a dioctahedron. The dioctahedra are connected to each other by PC ” groups in the a-b plane. Oxygen can partially replace the apical fluorine in dioctahedra, changing the oxidation state of vanadium from 3+ to 4+ with the formation of a short covalent double bond V=O [Adv. Funct. Mater. 24 (2014) 4603-4614], This leads to a slight distortion of the dioctahedra and, consequently, to compression of the unit cell along the c-axis, resulting in a decrease of the unit cell volume. Any symmetry changes in the unit cell are recorded in X-ray diffractograms, which makes it possible to determine the chemical composition based on the c-axis parameter and the unit cell volume [Energy Storage Mater. 20 (2019) 324-334], Numerous methods for obtaining NVPF are known. The best-known is the solid-state synthesis method [RSC Adv. 9 (2019) 30628-30636], This method involves homogenizing salts containing phosphate, vanadium, fluoride, and sodium ions, followed by thermal treatment of the resulting mixture. Despite its simplicity, this method typically results in materials with low phase purity, characterized by a high sodium-vanadium phosphate content.

[0008] A method for synthesizing NVPF using the sol-gel method is known [CrystEngComm 24 (2022) 4519- 4526], In a typical procedure, a solution containing the appropriate ions and a gelling agent is obtained. After evaporation of the solvent, the resulting gel is then dried, ground, and subjected to thermal treatment. Unfortunately, NVPF obtained by this method are usually characterized by low specific capacities and unsatisfactory performance under high current loads. Improving these properties requires the use of expensive carbon nanomaterials, e.g. nanotubes [ACS Appl. Energy Mater. 3 (2020) 3845-3853] or graphene [Ceram. Int. 46 (2020) 9170-9175],

[0009] A method for synthesising NVPF by electrospinning is known. [ACS Appl. Mater. Interfaces 12 (2020) 25920-25929], This method involves preparing a solution of appropriate salts with an organic polymer. Thus prepared mixture is then placed in a spinneret in the form of a metal needle. As a result of applying a high voltage between the nozzle and the collector in the form of a metallic foil (typically 10-30 kV), the solution at the tip of the needle assumes a Taylor cone shape and is then injected onto the collector, with simultaneous evaporation of the solvent. The precursor thus produced is then subjected to a thermal treatment process, which results in the formation of an NVPF composite with carbon in the form of nanofibres. The materials obtained in this way are typically characterized by good electrochemical properties. Nevertheless, the usefulness of this method is limited due to difficulties in applying it on a technical scale and the need for expensive equipment.

[0010] A method for synthesising NVPF using hydrothermal and solvothermal methods is known [ / . Electrochem. Sci. Technol. 10 (2019) 1-13], which have been successfully adapted to synthesize NVPF with good electrochemical properties in sodium-ion batteries. These methods involve reactions in aqueous (hydrothermal) or organic (solvothermal) solutions at high temperatures and pressures. For this purpose, devices called autoclaves are used. Synthesis using a pressure reactor offers several advantages, particularly in the context of synthesising materials with high purity, well-defined particle sizes and morphology, and the process is also scalable. This method also poses challenges related to the complexity of the equipment, which must withstand high temperatures and pressures, as well as relatively long reaction time, which can take up to several days. Precise control of pressure and temperature can be a limitation, affecting the quality and properties of the material produced, and furthermore, the synthetic product may require additional procedures such as washing, drying, and further thermal processing. The limited solubility of precursors in water or selected organic solvents also limits the applicability of hydrothermal and solvothermal methods. Balancing the advantages and disadvantages is crucial for optimizing the synthesis process in research applications and when scaling up production.

[0011] There is an unmet need to develop a simpler and cheaper method of manufacturing sodiumvanadium fluorophosphate as a positive electrode material for sodium-ion cells, which would exhibit single-phase, morphological homogeneity and high reversible specific capacity relative to metallic sodium (Na+|Na°), and would also allow its direct use in the construction of sodium-ion cells characterized by high operating parameters such as specific capacity, operating voltage, high resistance to repeated charge and discharge cycles and high resistance to the application of high charge and discharge currents.

[0012] Summary of the Invention

[0013] A method for manufacturing a composite of sodium-vanadium fluorophosphate with amorphous carbon, crystallizing in the NASICON structure, consisting in preparing a solid, homogeneous and stoichiometric mixture of precursors containing Na+, VO3 , PO43-and F“ ions, and counterions undergoing decomposition into gaseous products at high temperature, which mixture is subjected to thermal treatment and subsequent homogenisation with a carbon material, characterized in that the solid homogeneous mixture of precursors, additionally containing an oxidizer and a fuel, is subjected to self-combustion, and the obtained material is subjected to grinding with amorphous carbon with subsequent thermal treatment, wherein: an oxidizer solution is prepared containing NH4F, (NH4)2HPO4 and NaNOs in a 3:2:3 molar ratio in deionized water in a PTFE beaker, with nitrate anions as the oxidizer, and a fuel solution is prepared containing NH4VO3 and citric acid in a 3:2 molar ratio in deionized water in a glass beaker, with citric acid as the fuel, and after which the oxidizer solution and the fuel solution are simultaneously stirred and heated at temperature of 80°C until the fuel solution turns blue, at which time the fuel solution is added drop by drop to the oxidizer solution with continuous stirring, maintaining a 1:1 molar ratio of NH4VO3 to (NFUjiHPC^, and then the resulting solution is heated at temperature of 80°C until the solvent is completely evaporated, and the resulting precipitate is heated to a temperature of 150°C until it spontaneously ignites, wherein the green powder obtained after the combustion process is dried in a vacuum at a temperature of 120°C for 12 hours, and the dried material is ground with amorphous carbon in a mass ratio of 1:14 in a ball mill at 500 rpm for 5 hours, and the grinding product is subjected to heat treatment in a tube furnace in an inert atmosphere at a temperature of 300°C for 4 hours and then at a temperature of 550°C for 8 hours, as a result of which a composite material is obtained containing crystallographically pure, single-phase sodium-vanadium fluorophosphate, the crystallites of which are covered with amorphous carbon, forming composite particles with a size of 50-200 nm, wherein the carbon coating has a thickness in the range of 2-20 nm.

[0014] According to the invention, the composite material containing sodium-vanadium fluorophosphate with the molecular formula Na3(VOo, 79)2^04^1, 42 is obtained.

[0015] The oxidizer solution comprises 0.084-8.378 g of NH4F, preferably 0.838 g of NH4F, 0.190-18.966 g of (NH4)2HPO4, preferably 1.897 g of (NH4)2HPO4, and 0.183-18.308 g of NaNOs, preferably 1.831 g of NaNOs, and the volume of deionized water is 5-500 ml, preferably 50 ml.

[0016] The fuel solution contains 0.168-16.800 g of NH4VO3, preferably 1.680 g of NH4VO3, and 0.201- 20.119 g of citric acid, preferably 2.012 g of citric acid, and the volume of deionized water is 5-500 ml, preferably 50 ml.

[0017] The grinding is carried out in a vessel made of tungsten carbide with 250 zirconium balls, each 5 mm in diameter.

[0018] A positive electrode material for sodium-ion cells comprising sodium-vanadium fluorophosphate is characterized in that it is a composite of sodium-vanadium fluorophosphate with amorphous carbon, manufactured by the method described above.

[0019] The sodium-vanadium fluorophosphate-amorphous carbon composite comprises crystallographically pure, single-phase sodium-vanadium fluorophosphate, the crystallites of which are coated with amorphous carbon to form composite particles of 50-200 nm in size, wherein the carbon coating has a thickness in the range of 2-20 nm.

[0020] The sodium-vanadium fluorophosphate composite with amorphous carbon has the molecular formula Na3(VOo, 79)2^04^1 ,42.

[0021] A sodium-ion cell having a negative electrode comprising metallic sodium, a positive electrode comprising sodium-vanadium fluorophosphate, a separator, and an electrolyte, sealed under an inert atmosphere is characterized in that the positive electrode material is a sodium-vanadium fluorophosphate-amorphous carbon composite as described above, manufactured by the method as described above.

[0022] The method for synthesizing a composite of sodium-vanadium fluorophosphate with amorphous carbon using the self-combustion method allows for the obtaining a powder product with the NASICON structure, characterized by nanometric grain sizes. The sodium-vanadium fluorophosphate obtained by the method of the invention is phase-pure and has a homogeneous morphology, which is extremely important for its use as an active material for the positive electrode of a sodium-ion cell.

[0023] Furthermore, the self-combustion method used allows for obtaining an active material with high specific capacity and cyclic resistance. The sodium-ion cell, using the carbon composite sodiumvanadium fluorophosphate (NVPF@C) obtained by the method of the invention as the active material for the positive electrode, is characterized by high resistance to cyclic operation, allowing the cell to retain 97.1% ± 1.6% of its initial capacity after 50 consecutive charge / discharge cycles at a current of 1 C (one hour), as well as high efficiency, maintaining 94.3% ± 0.3% of its initial capacity after a series of cycles under high current loads of up to 10 C (six-minute current).

[0024] A method of manufacturing a composite of sodium-vanadium fluorophosphate with amorphous carbon, a positive electrode material for sodium-ion cells manufactured by this method, and a sodium-ion cell using this electrode material are described in detail below in embodiments with reference to the accompanying drawing, in which:

[0025] Fig. 1 shows a diagram of the procedure for manufacturing sodium-vanadium fluorophosphate, which provides for the following sequence: inorganic synthesis in an aqueous medium followed by solvent evaporation, self-ignition of the resulting solid precipitate followed by vacuum drying, homogenisation with amorphous carbon, and calcination of the obtained powder in a protective atmosphere, as described in Example 1.

[0026] Fig. 2 shows a photograph of positive electrodes in the form of flat disks with a diameter of 9 mm, prepared in Example 7 from sodium-vanadium fluorophosphate obtained by the method of the invention in Example 1, used in Example 8 to create a Swagelok® sodium-ion cell, tested in Examples 9 and 10.

[0027] Fig. 3 shows the result of indexing the reflections of the powder diffractogram measured for sodium-vanadium fluorophosphate, Nas^O-i.x^PO^iF-i+ix , homogenized with amorphous carbon (NVPF@C), obtained in Example 1, where the markings showing the h k I values of each of the reflections indexed in the C man space group (no. 63) are visible, with only one crystalline phase detected in the tested sample (Example 2);

[0028] Fig. 4 shows the result of Rietveld analysis of the powder diffractogram measured for sodium-vanadium fluorophosphate, Nas^O-i.x^PO^iF-i+ix, homogenized with amorphous carbon (NVPF@C), obtained in Example 1, indexed in Example 2, where the following are visible: the experimental curve (black), the curve generated on the basis of the crystal structure (red), the positions of possible reflections in the C mcm space group (No. 63) (green) and the difference curve between the experimental line and the theoretical line (blue), where the experimental data correspond to a unit cell with dimensions: a = 9.0234 A, b = 9.0253 A, c = 10.6369 A, V = 866.26 A3, Z = 4 (Example 2);

[0029] Fig. 5 shows a graph of the dependence of the unit cell dimensions and stoichiometric unit volume on the parameter x, which determines the level of oxygen doping of the fluorineoccupied positions by in the sodium-vanadium fluorophosphate structure, Na3(VOi.x)2(PO4)2Fi+2x, with experimental points from the prior art [Energy Storage Mater. 20 (2019) 324] and a point determined for sodium-vanadium fluorophosphate obtained in Example 1 , and tested in Example 2;

[0030] Fig. 6 shows a visualisation of the unit cell of sodium-vanadium fluorophosphate,

[0031] Na3(VOi.x)2(PO4)2Fi+2x, containing sodium (yellow), vanadium (blue), phosphorus (green), fluorine (gray), and oxygen (red) atoms, where the coordination polyhedra of sodium, vanadium, and potassium are marked with the colours assigned to these atoms;

[0032] Fig. 7 shows images of sodium-vanadium fluorophosphate homogenized with carbon, NVPF@C, obtained by the method of the invention in Example 1, where:

[0033] A. the SEM image shows the material at 25,000 times magnification, with the overall morphology of the material visible.

[0034] B. the SEM image shows the material at 100,000 times magnification, with visible grains of 50-100 nm in size.

[0035] C. the HR-TEM image shows the material at 4,000,000 times magnification, with a visible fragment of a crystalline NVPF particle with visible crystallographic planes spaced 0.536 nm apart, corresponding to the crystallographic planes 002, which particle is covered on the surface with a 2-20 nm thick layer of amorphous carbon.

[0036] Fig. 8 shows an HR-TEM image of sodium-vanadium fluorophosphate homogenized with carbon, NVPF@C, at 80,000x magnification, showing an NVPF particle coated with amorphous carbon, and EDS maps at the same magnification, showing the uniform distribution of the elements Na (red), V (purple), P (blue), O (green), and F (turquoise) within the NVPF particle and carbon aggregates (yellow) on its surface.

[0037] Fig. 9 shows the results of an experiment aimed at determining the pore size of the

[0038] NVPF@C composite material obtained in Example 1, where: A. the graph shows the nitrogen absorption and desorption isotherms,

[0039] B. the graph shows the pore size distribution of the material;

[0040] Fig. 10 shows:

[0041] A. the results of thermogravimetric measurements of the NVPF@C composite material obtained in Example 1, where oxidation of amorphous carbon is visible at temperatures of 350-450°C with a loss of 6.9% of the composite mass;

[0042] B. cyclic voltammogram (CV) measured at a rate of 0.1 mV / s in a sodium-ion cell system prepared in Example 8, in which the electrode made of compressed NVPF@C powder, prepared in Example 7, from the NVPF@C composite material obtained in Example 1, was the positive electrode, while metallic sodium was used as the negative electrode, and a 1 M NaCIO4 solution in a mixture of ethylene carbonate and propylene carbonate in a 1:1 volume ratio was used as the electrolyte (the voltammogram shows oxidation peaks at 3.655 V and 4.065 V and reduction peaks at 3.570 V and 3.990 V);

[0043] Fig. 11 shows the cumulative test results of the sodium-ion cell prepared in Example 8 and tested in Example 9, where:

[0044] A. the graph shows the changes in the specific capacity of the NVPF@C composite material determined during discharge of the test cell (with a current of 1 C, 2 C, 5 C, 10

[0045] C. and 1 C) as a function of the operating cycle from 1 to 25.

[0046] B. the graph shows the curves of the change in the potential of the test cell during discharge (with a current of 1 C, 2 C, 5 C, 10 C, and 1 C) as a function of the specific capacity of the electrode made of the NVPF@C composite material.

[0047] C. the graph of the stability of changes in the specific capacity of the NVPF@C composite material determined during charge / discharge tests (with a current of 1 C) as a function of the number of the operating cycle from 1 to 50, wherein the lightly coloured area indicates the standard deviation resulting from the differences in the results obtained in different tests.

[0048] D. the graph shows the curves of the test cell potential change during charging and discharging (with a current of 1 C) as a function of the specific capacity of the electrode made of NVPF@C composite material for cycles from 1 and 50. Detailed Description of the Invention

[0049] The growing demand for electricity sources and batteries for applications in portable electronics, the automotive industry, and renewable energy sources necessitates the use of efficient and affordable energy storage systems. Currently, the most commonly used systems for the above applications are lithium-ion cells. However, due to the increasing demand for these systems and the related concerns about the availability of deposits and the costs of obtaining lithium and cobalt, new technologies are being sought to replace lithium-ion cells in the battery market. One possible solution is to replace lithium ions as charge carriers in the cell with sodium ions, creating new sodium-ion systems.

[0050] The present invention concerns a new method for the synthesizing a carbon composite of sodiumvanadium fluorophosphate, NVPF@C, its use as a positive electrode material for sodium-ion cells, and a sodium-ion cell utilizing this electrode material.

[0051] The NVPF@C composite material obtained by the method of the invention is characterized by phase purity and is free from the most commonly observed impurity, sodium-vanadium phosphate. It also demonstrates excellent operating parameters in a sodium-ion cell. When discharged at a current of 1 C, it exhibits a specific capacity of 110.5 ± 0.3 mAh / g, close to the theoretical capacity, and when the discharge current is increased tenfold to 10 C, it retains 94.3 ± 0.3% of its initial specific capacity. Another advantage of this composite is its high resistance to subsequent charge / discharge cycles. After 50 operating cycles, the material retains 97.1 ± 1.6% of its initial specific capacity. The synthesis method used is easily translated from laboratory to pilot and technical scales. Moreover, such excellent performance parameters were achieved without the use of expensive carbon nanomaterials (such as nanotubes or graphene), which are often used as a conductive additive in composite materials containing sodium-vanadium fluorophosphate. Instead, only amorphous carbon is used (e.g. Vulcan® XC72R carbon, Cabot), the price of which is many times lower.

[0052] Method of manufacturing NVPF@C composite material

[0053] The method for manufacturing a sodium-vanadium fluorophosphate composite with amorphous carbon involves preparing a solid, homogeneous mixture of precursors containing Na+, VO3-, Poland F“ ions and counterions that decompose into gaseous products at high temperature, as well as an oxidizer and fuel, and subjecting it to self-combustion. The resulting material is then milled with amorphous carbon followed by thermal treatment, and the resulting material can be used as a positive electrode material in a sodium-ion cell. The synthesis scheme is shown in Fig. 1.

[0054] According to the invention, the precursor mixture is obtained by preparing an aqueous solution containing the appropriate components. For process safety reasons, separate solutions containing the oxidizer and fuel are prepared. An oxidizer solution is prepared, containing NH4F, (NH^iHPC^ and NaNOs in a 3:2:3 molar ratio in deionized water in a PTFE beaker. The oxidizer is nitrate anions. The oxidizer solution contains 0.084-8.378 g of NH4F, preferably 0.838 g of NH4F, 0.190-18.966 g of (NH4)2HPO4, preferably 1.897 g of (NF^iHPC and 0.183-18.308 g of NaNOs, preferably 1.831 g of NaNOj. The volume of deionized water is 5-500 ml, preferably 50 ml. Separately, a fuel solution is prepared containing NH4VO3 and citric acid in a 3:2 molar ratio dissolved in deionized water in a glass beaker. The fuel is citric acid. The fuel solution contains 0.168-16.800 g of NH4VO3, preferably 1.680 g of NH4VO3, and 0.201-20.119 g of citric acid, preferably 2.012 g of citric acid. The volume of deionized water is 5-500 ml, preferably 50 ml.

[0055] The oxidizer solution and the fuel solution are mixed separately and heated at temperature of 80°C until the fuel solution turns blue, which indicates a change in the vanadium oxidation state from 5+ to 4+. The fuel solution is then added drop by drop to the oxidizer solution with constant stirring, maintaining a 1:1 molar ratio of NH4VO3 to (NH4)IHPO4 ,and then heated at temperature of 80°C until the solvent is completely evaporated. This process typically takes about 30-1200 minutes, depending on the solution volume (solutions with small volumes evaporate faster, while larger ones take longer). The resulting precipitate is heated to temperature of 150°C, which causes it to spontaneously ignite. As a result of combustion, a green powder is obtained, which is then subjected to further mechanical and thermal processing.

[0056] The green powder obtained after the combustion process is dried in a vacuum at temperature of 120°C for 12 hours, after which the dried material is homogenized with amorphous carbon, for example Vulcan® XC72R carbon (Cabot), in a powder-to-carbon mass ratio of 1:14. Homogenisation is carried out by energetic grinding, preferably in a ball or disk mill. Grinding in the ball mill is preferably carried out at a rotational speed of 500 rpm for 5 hours, preferably in a tungsten carbide vessel containing 250 zirconium balls, although grinding in WC vessels with WC grinding elements or in zirconium vessels with zirconium grinding elements is equally preferred. The milling product is then heat treated in a tube furnace in an inert atmosphere, preferably nitrogen or argon, at temperature of 300°C for 4 hours and then at temperature of 550°C for 8 hours.

[0057] The product obtained in the above process is a composite material containing crystallographically pure, single-phase sodium-vanadium fluorophosphate (Fig. 3 and Fig. 4), the crystallites of which are covered with amorphous carbon, forming composite particles with a size of 50-200 nm (Fig. 7B), where the carbon coating has a thickness in the range of 2-20 nm (Fig. 7C). The product of the above synthesis is single-phase, crystallizing in the space group no. 63 C mcm, characteristic for NASICON (Fig. 3), identified as Na3(VOi.x)2(PO4)2Fi+2x, where 0 < x < 1, by Rietveld refinement to the known structure of a material of this type [Energy Storage Mater. 20 (2019) 324-334] (Fig. 4). The crystal parameters of the obtained sodium-vanadium fluorophosphate are a = 9.0234 A; b = 9.0253; c = 10.6369 A; V = 866.26 A3(Table 1). The molecular formula of sodium-vanadium fluorophosphate, determined based on the c parameter of the unit cell [Energy Storage Mater. 20 (2019) 324-334], is Na3(VO G,79)2(PO4)2FI,42 (parameter value x 0.21, Fig. 5), which means that 52.67 % of the fluorine atoms in the vanadium coordination zone were substituted with oxygen atoms (Fig. 5).

[0058] Amorphous carbon is not an admixture for sodium-vanadium fluorophosphate obtained by the method according to the invention, but only constitutes a matrix in which this compound was dispersed, creating together with it a homogeneous NVPF@C composite material (Fig. 8). The thickness of the carbon coating is 2-20 nm (Fig. 7C). The homogeneity of the material results from the dispersion of the precursor in the carbon matrix, which prevents the formation of agglomerates of the product crystallites, simultaneously limiting their growth during the subsequent heat treatment.

[0059] The NVPF@C composite material obtained by the method of the invention is porous, with pores mainly in the micropore and mesopore range [Pure Appl. Chem. 54 (1982) 2201; Pure Appl. Chem. 87 (2015) 1051], which suggests the shape of the adsorption isotherm corresponding to mixed IUPAC type ll / IV curves (Fig. 9A). The pore distribution (Fig. 9B) shows that the dominant pore sizes are in the range of 0.6-0.7 nm, 3-4 nm and 9-10 nm. The specific surface area calculated by the BET method based on nitrogen adsorption / desorption is 19.37 ± 0.08 m2 / g, of which 3.32 m2 / g accounts for the specific surface area of the micropores (calculated by the T-plot method). The average grain size calculated from BET experiments is approximately 98 nm, which is consistent with the observations in SEM images (Fig. 7).

[0060] The NVPF@C composite material obtained by the method of the invention is stable at temperatures up to 300°C (Fig. 10A). Above 300°C, oxidation of amorphous carbon occurs, associated with a 6.9% mass loss at temperatures of 300-550°C. Above 550°C, oxidation of vanadium to vanadium oxides occurs.

[0061] NVPF@C composite material as the positive electrode material of a sodium-ion cell.

[0062] According to the invention, a composite of sodium-vanadium fluorophosphate with amorphous carbon (NVPF@C), obtained by the method of the invention described above, can be used as an active material of the positive electrode of a sodium-ion cell.

[0063] Electrodes containing Na3(VOo,79)2(P04)2Fi,42 are prepared by a standard procedure involving grinding

[0064] NVPF@C powder with amorphous conducting carbon (e.g. Vulcan® XC72R, Cabot) in an agate mortar for 20 minutes. Next, a solution of polyvinylidene fluoride (PVDF) in N-methyl-2 - pyrrolidone (NMP) at a concentration of 5% by weight is added to the thus prepared material, and the resulting mixture is homogenized, preferably using a centrifugal mixer in two 5-minute cycles at 3000 rpm. Preferably, the mass ratio of NVPF@C:PVDF:Vulcan® is 8:1:1. The obtained suspension is then applied to the surface of the current collector, preferably in the form of aluminum, copper, or titanium foil, using a squeegee with an appropriate setting, preferably 200 pm. The collector with the applied layer is pre-dried at temperature of 55°C in air to remove NMP, and then dried under vacuum at temperature of 120°C for 12 hours. Typically, electrodes with an active material layer thickness of 10-40 pm are obtained. Electrodes are then cut from the foil to a shape appropriate for the geometry of the sodium-ion cells being created; for example, circular electrodes with a diameter of 9 mm are cut for button cells. The obtained electrodes are pressed in a hydraulic press at a pressure of 6 tons for 15 seconds, weighed and dried again at temperature of 120°C under vacuum for at least 12 hours, and transferred to a glove box filled with an inert gas, preferably argon or nitrogen, preferably of at least 5.0 purity, with water and oxygen contamination levels below 0.5 ppm, where sodium-ion cells are assembled. The electrodes thus obtained (Fig. 2) can be used in sodium-ion cells.

[0065] Sodium-ion cells containing positive electrodes with a sodium vanadium fluorophosphate / amorphous carbon composite can adopt any geometry, for example, prismatic cells, button cells, coin cells, pillow cells, or Swagelok®-type three-electrode test cells. The negative electrode can be metallic sodium or another electrode material capable of accumulating sodium ions. A standard separator (e.g., Whatman GF / F) is used, preferably soaked with a 1 M NaCIOi solution in ethylene carbonate / propylene carbonate in a 1:1 volume ratio.

[0066] The theoretical capacity of the NVPF@C composite containing Na3(VOo, 79)2^04^1, 42 is 128 mAh / g, which was calculated from the chemical formula, knowing the Faraday constant and the molar mass of the obtained compound. The NVPF@C composite material provides a high discharge capacity of 110.5 ± 0.3 mAh / g at a discharge current of 1 C in relation to the mass of the entire composite. When the discharge current increases, the capacity of the positive electrode slightly decreases to 108.3 ± 0.2 mAh / g at 2 C, 105.2 ± 0.1 mAh / g at 5 C and 101.0 ± 0.4 mAh / g at 10 C, and the capacity after reducing the discharge current again to 1 C is 108.76 ± 0.22 mAh / g, which means that the NVPF@C composite is resistant to operation under high current load, maintaining 94.3 ± 0.3% of the initial capacity (Table 2, Fig. 11A and Fig. 11 B). The NVPF@C composite also exhibits high resistance to repeated charge / discharge cycles in a sodium-ion cell, as it retains 97.1 ± 1.6% of its initial capacity after 50 cycles of operation under a current load of 1 C (Table 2, Fig. 11C and Fig.

[0067] 11 D).

[0068] The charge-discharge curves of the NVPF@C composite have two plateaus at potentials of 3.60 V and 4.05 V versus Na7Na°, which are associated with the reversible electrochemical intercalation / deintercalation of sodium ions into / from the NVPF matrix (Fig. 11 B, Fig. 11D). These potentials do not coincide with the theoretical values for the pure NasV^PCX^Fs phase of 3.7 V and 4.2 V, which confirms the accuracy of the determined molecular formula Na3(VOo, 79)2^04^1 ,42. There is also no potential plateau at 3.4 V associated with sodium-vanadium phosphate, Na3V2(PO4)3, which confirms the lack of contamination of the NVPF@C composite obtained by the method according to the invention with this compound. The two pairs of redox peaks visible in the cyclic voltammograms of this material represent oxidation / reduction processes of the V47V3+redox couple during deintercalation / intercalation of sodium ions from / into the NVPF crystal matrix (Fig. 10B). The peak positions correspond well to the charge / discharge plateau observed in constant current, CP measurements (Fig. 11 B and Fig. 11 D). A method for manufacturing a sodium-vanadium fluorophosphate-amorphous carbon composite, a positive electrode material for sodium-ion cells manufactured by this method, and a sodium-ion cell using this electrode material are described below in the embodiments.

[0069] Example 1. (NVPF synthesis) Stoichiometric amounts of 0.838 g of NH4F (Sigma-Aldrich), 1.897 g of (NH4)2HPO4(Chempur), and 1.831 g of NaNOs (Chempur) (stoichiometric ratio 3 / 2 / 3) were dissolved in 50 ml of deionized water in a PTFE beaker to obtain an oxidizer solution. Simultaneously, stoichiometric amounts of 1.680 g of NH4VO3 (Sigma-Aldrich) and 2.012 g of citric acid (POCH) (molar ratio of citric acid to vanadium ions 2 / 3) were dissolved in 50 ml of deionized water in a glass beaker to obtain a fuel solution. Both solutions were mixed on a magnetic stirrer while heating to temperature of 80°C until the fuel solution turned blue (approx. 30 min). Then, it was added dropwise to the oxidizer solution, stirred continuously, and heated until the solvent completely evaporated (approx. 120 min). The resulting precipitate was heated to 150°C, which initiated self-combustion. The self-combustion of the powder turned it green. The green powder thus obtained was dried under vacuum at temperature of 120°C for 12 hours and then homogenized with Vulcan® XC72R amorphous carbon (Cabot) (powder / carbon mass ratio of 1 / 14) in a Fritsch Pulverisette 7 Premium Line planetary ball mill for 5 hours at 500 rpm. This process was carried out in a dedicated tungsten carbide grinding vessel containing 250 zirconium beads. Finally, the powder was heat-treated in a quartz tube furnace under an argon protective atmosphere, initially at temperature of 300°C for 4 hours and then at temperature of 550°C for 8 hours. The product obtained was a homogeneous composite material consisting of sodium-vanadium fluorophosphate coated with amorphous carbon (NVPF@C). The obtained product was subjected to analyses in further examples to determine its composition, morphology, specific surface area, thermal properties, and electrochemical characteristics. The synthesis procedure is schematically shown in Fig. 1.

[0070] Example 1. (X-ray studies) Crystallographic studies of the NVPF@C composite material obtained in Example 1 were performed. Powder diffractograms in Bragg- Brentano geometry were obtained using a Panalytical Empyrean diffractometer with Cu KQradiation (KQI = 1.54060 A, KQ2 = 1.54443 A) in the angular range 20 of 5-115°, at a rate of 250 steps per second, using a step size of 0.02°. The obtained data were analyzed using HighScore 4.0, Match!® and FullProf software to determine the unit cell parameters and the phase purity of the obtained product. The reflections were indexed in the space group no. 63 C man, characteristic for NASICON (Fig. 3). One phase was detected in the synthesis product from Example 1. The diffractogram was refined by the Rietveld method to the NASICON structure, of the formula Nas^O-i.x^PO^iF-i+ix , where 0 < x < 1, [Energy Storage Mater. 20 (2019) 324-334] obtaining a good fit and determining the crystallographic parameters of the material, presented in Table 1 (Fig. 4). Amorphous carbon was not an admixture for sodiumvanadium fluorophosphate obtained by the method according to the invention, but only constituted a matrix in which this compound was dispersed, together forming a homogeneous NVPF@C composite material. Based on the known dependence of the degree of substitution of fluorine atoms with oxygen atoms in the vanadium coordination zone, causing compression of the unit cell in the c direction [Energy Storage Mater. 20 (2019) 324-334], the value of the parameter x = 0.21 was determined (Fig. 5), which means that 52.67% of the fluorine atoms in the vanadium coordination zone were substituted with oxygen atoms, and the molecular formula of the material obtained by the method according to the invention is Na3(VOo, 79)2^04^1, 42 (Fig. 5).

[0071] Table 1. Refinement parameters of the powder diffractogram recorded for the NVPF@C composite material obtained in Example 1. Example 3. (SEM imaging) Morphology studies of the NVPF@C composite material obtained in Example 1 were carried out. SEM images were obtained with a ZEISS Merlin electron microscope using a 3 kV electron beam (Fig. 7). The tested material consisted of fine particles of indefinite shapes, with grain sizes in the range of approximately 50-200 nm, with a slight tendency to agglomerate into larger structures with a diameter of approximately 1 pm. The primary particles were homogeneous, which confirms the homogeneity of the NVPF@C composite material obtained by the method according to the invention.

[0072] Example 4 (TEM imaging) Morphology studies of the NVPF@C composite material obtained in Example 1 were carried out using a TEM Thermo Scientific TALOS F200X transmission electron microscope with an EDS attachment. A material particle with a size of approximately 1500 nm was examined. The analysis of the distribution maps of the elements Na, V, P, O, F and C confirms the homogeneity of the distribution of the elements Na, V, P, O, F, with no visible precipitations or agglomerates; the grains are covered with amorphous carbon, locally forming larger aggregates on their surface (Fig. 8).

[0073] Example 5. (determination of porosity) Porosity studies were carried out on the NVPF@C composite material obtained in Example 1. Nitrogen (N2) adsorption / desorption experiments were carried out in a MICROMERITICS ASAP 2060 apparatus at a temperature of 77.349 K in the relative pressure range of 0.01-0.995 p(p°)“1.The microporosity of the NVPF@C powder was analyzed by introducing fixed amounts of nitrogen to obtain a relative pressure of 0.01 p(p°)“1. The specific surface area (BET), specific micropore surface area, and average grain size (assuming ideally spherical particle geometry and known NVPF crystal density of 3.167 g / cm3) were calculated using ASAP 2060 software. The pore distribution was calculated using the BJH (Barrett-Joyner-Halenda) model in the mesopore range and the Horvath-Kawazoe (H-K) method in the micropore range. During the nitrogen adsorption / desorption experiment, capillary condensation hysteresis was observed. The shape of the adsorption isotherm corresponds to mixed IUPAC type ll / IV curves, suggesting that the analyzed material is porous, with pores mainly in the micropore and mesopore range [Pure Appl. Chem. 54 (1982) 2201; Pure Appl. Chem. 87 (2015) 1051] (Fig. 9A). The pore distribution presented in Fig. 9B shows that the dominant pore sizes are in the range of 0.6-0.7 nm, 3-4 nm, and 9-10 nm. The BET specific surface area calculated based on nitrogen adsorption / desorption was 19.37 ± 0.08 m2 / g, of which 3.32 m2 / g is accounted for by the micropore surface area (calculated by the T-plot method). The obtained results are consistent with the SEM observations in Example 3. Furthermore, the experimentally calculated average grain size was approximately 98 nm, which is within the range of particle diameters observed in the SEM images (Fig 7). Example 6. (thermogravimetry) Thermogravimetric studies were carried out on the NVPF@C composite material obtained in Example 1. The measurements were carried out in the temperature range of 25-850°C, with a heating rate of 10°C / min and an oxygen flow of 5 ml / min (Fig. 10A). An initial loss of 1% of mass was observed at temperatures up to 300°C, resulting mainly from the desorption of water from the material surface. Above 300°C, oxidation of amorphous carbon was observed, associated with a loss of 6.9% of mass at temperatures of 300-550°C. Above 550°C, a slight increase in the sample mass was observed, most likely related to the oxidation processes of vanadium to vanadium oxides.

[0074] Example 7. (preparation of electrode from NVPF@C) The NVPF@C composite material obtained in Example 1 was used to fabricate electrodes for a sodium-ion cell. Na3(VOo, 79)2^04^1 ,42 powder was first mixed in an agate mortar with Cabot Vulcan® XC72R amorphous conducting carbon for 20 minutes. Then, a 5 wt.% solution of polyvinylidene fluoride (PVDF, Alfa Aesar) in N-methyl-2- pyrrolidone (NMP, Sigma-Aldrich) was added to the NVPF@C powder, and the resulting mixture was then homogenized using a HAUSCHILD Speed Mixer® DAC 150.3 FVZ centrifugal mixer in two 5-minute cycles at 3000 rpm. The NVPF@C:PVDF:Vulcan® ratio was 8:1:1 wt%. The obtained suspension was then applied to the surface of an aluminum foil using a 200 pm squeegee, dried at temperature of 55°C in air to remove excess NMP, and then dried under vacuum at temperature of 120°C for 12 hours. Round / circular electrodes with a diameter of 9 mm were then cut from the foil, pressed in a hydraulic press under 6 tons of pressure for 15 seconds, weighed, and dried at temperature of 120°C under vacuum for at least 12 hours. Coatings of electrode material with a thickness of 25 pm were obtained. The finished electrodes were transferred to an mBRAUN glove box filled with 5.0 purity argon, with water and oxygen contamination levels below 0.5 ppm. The resulting electrodes are shown in Fig. 2.

[0075] Example 8. (preparation of a cell with an NVPF@C electrode) Sodium-lithium cells were constructed in a three-electrode Swagelok® geometry, with the working electrode made in Example 7, counter and reference electrodes made of metallic sodium (Sigma-Aldrich) and a Whatman GF / F separator soaked in 1 M NaCIO4 solution (Sigma-Aldrich) in ethylene carbonate / propylene carbonate (1:1 v / v, Sigma-Aldrich).

[0076] Example 9. (chronopotentiometry, CP) Galvanostatic charge / discharge cycles of the cell prepared in Example 8 were carried out using a SOLLICH ATLAS 0961 multi-channel battery tester in the potential range of 2-4.5 V versus Na7Na°. A series of cells (at least 3) were subjected to cyclic tests and tests under increasing current load (from 1 C to 10 C), in which the first, initial cycle was carried out at a current intensity of 0.1 C (10-hour discharge) with a theoretical working electrode capacity of 128 mA / g. After conditioning, the cells were charged / discharged at a current of 1 C for 50 consecutive cycles during the assessment of their cyclicity. During high-current testing, the cells were charged at 1 C and discharged at varying currents: 1 C, 2 C, 5 C, 10 C, and again 1 C, for 5 consecutive cycles at each current. The CP measurement results are presented in Table 2.

[0077] Table 2. Results of chronopotentiometric measurements of the NVPF@C composite material (obtained in Example 1) in a sodium-ion cell based on subsequent charge / discharge cycles and discharge tests with currents of 1 C, 2 C, 5 C and 10 C.

[0078] Example 10. (cyclic voltammetry, CV) The cells tested in Example 9, immediately after completion of the tests, were subjected to CV analysis performed on a SOLARTRON SI 1287 potentiostat / galvanostat in the potential range of 2—4.5 V versus Na7Na°. The working electrodes were polarized at a rate of 0.1 mV / s. During the CV experiments, the oxidation and reduction processes of sodium-vanadium fluorophosphate were observed. The voltammogram shows two very well-defined pairs of redox peaks, originating from the oxidation / reduction of the V47V3+redox couple during deintercalation / intercalation of sodium ions into the NVPF crystalline / crystal matrix (Fig. 10B). The peak positions correspond well to the charge / discharge plateau observed in the CP measurements (Fig. 11 B and Fig. 11 D). The presence of two pairs of redox peaks results from changes in the chemical potential of Na+, resulting from the rearrangement of sodium ions in the crystal lattice after deintercalation of 1 mole of Na+ions per stoichiometric unit, in order to minimize the Na+-Na+Coulomb interaction forces occurring in the crystal [Adv. Funct. Mater. 24 (2014) 4603-4614], Example 11. (NVPF@C synthesis on a smaller scale) The synthesis of the NVPF@C composite was carried out as in Example 1, with the difference that the oxidizer solution contained 0.084 g of NH4F, 0.190 g of (NH4)2HPO4, 0.183 g of NaNOs, and 5 ml of deionized water, while the fuel solution contained 0.168 g of NH4VO3, 0.201 g of citric acid, and 5 ml of deionized water. These solutions were heated and stirred at temperature of 80°C for approximately 30 minutes until the fuel solution turned blue, and after mixing, complete evaporation of the obtained solution took approximately 30 minutes. The remaining synthesis steps were as in Example 1. The obtained material was subjected to studies as in Examples 2-6. It was confirmed that a homogeneous NVPF@C composite material with the NASICON structure was obtained with the molecular formula Na3(VOo,79)2(P04)2Fi,42 (space group C mcm; a = 9.0234 A; b = 9.0253; c = 10.6369 A; V = 866.26 A3; general formula Na3(VOi.x)2(PO4)2Fi+2x, 0 < x < 1; parameter value x = 0.21; 52.67% of the fluorine atoms in the vanadium coordination zone substituted with oxygen atoms). The NVPF@C composite consisted of fine, homogeneous particles of indefinite shape, with grain sizes in the range of approx. 50-200 nm, with a slight tendency to agglomerate into larger structures with a diameter of approx. 1 pm, analogously to Fig. 7. The elements Na, V, P, O, F were distributed homogeneously in the particles with no visible precipitations or agglomerates, and the grains were covered with amorphous carbon, locally forming larger aggregates on their surface, analogously to Fig. 8. The NVPF@C composite was porous, with pores mainly in the micropore and mesopore range, with a distribution analogous to that in Fig. 9B. The NVPF@C composite was thermally stable up to 300°C, while at temperatures of 300-550°C, oxidation of amorphous carbon was observed, associated with a 6.9% mass loss, and above 550°C, a slight increase in the sample mass was observed, analogously to Fig. 10A. The NVPF@C composite was used to manufacture electrodes and cells as in Examples 7-8. The theoretical capacity of the NVPF@C composite calculated from the chemical formula was 128 mAh / g. The NVPF@C composite was characterized by resistance to operation under high current load and high resistance to repeated charge / discharge cycles in a sodium-ion cell, analogously to Examples 9-10.

[0079] Example 12. (NVPF@C synthesis on a larger scale) The synthesis of the NVPF@C composite was carried out as in Example 1, with the difference that the oxidizer solution contained 8.378 g of NH4F, 18.966 g of (NH4)2HPO4, 18.308 g of NaNOs, and 500 ml of deionized water, while the fuel solution contained 16.800 g of NH4VO3, 20.119 g of citric acid, and 500 ml of deionized water. These solutions were heated and stirred at temperature of 80°C for approximately 30 min until the fuel solution turned blue, and after mixing, complete evaporation of the obtained solution took approximately 1200 min. The remaining synthesis steps were the same as in Example 1. The obtained material was subjected to tests as in Examples 2-6. It was confirmed that a homogeneous NVPF@C composite material with the NASICON structure was obtained with the molecular formula Na3(VOo,79)2(P04)2Fi,42 (space group C mcm; a = 9.0234 A; b = 9.0253; c = 10.6369 A; V = 866.26 A3; general formula Na3(VOi.x)2(PO4)2Fi+2x, 0 < x < 1; parameter value x = 0.21; 52.67% of the fluorine atoms in the vanadium coordination zone substituted with oxygen atoms). The NVPF@C composite consisted of fine, homogeneous particles of indefinite shapes, with grain sizes in the range of approx. 50-200 nm, with a slight tendency to agglomerate into larger structures with a diameter of approx. 1 pm, analogously to Fig. 7. The elements Na, V, P, O, F were distributed homogeneously in the particles with no visible precipitations or agglomerates, and the grains were covered with amorphous carbon, locally forming larger aggregates on their surface, analogously to Fig. 8. The NVPF@C composite was porous, with pores mainly in the micropore and mesopore range, with a distribution analogous to Fig. 9B. The NVPF@C composite was thermally stable up to 300°C, while at temperatures of 300-550°C, oxidation of amorphous carbon was observed, associated with a 6.9% mass loss, and above 550°C, a slight increase in the sample mass was observed, analogously to Fig. 10A. The NVPF@C composite was used to manufacture electrodes and cells as in Examples 7-8.

[0080] The theoretical capacity of the NVPF@C composite calculated from the chemical formula was 128 mAh / g. The NVPF@C composite was characterized by resistance to operation under high current load and high resistance to repeated charge / discharge cycles in a sodium-ion cell, analogously to Examples 9-10.

Claims

Claims1. A method for manufacturing a composite of sodium-vanadium fluorophosphate with amorphous carbon, crystallizing in the NASICON structure, consisting in preparing a solid, homogeneous and stoichiometric mixture of precursors containing Na+, VO3 , PO43-and F“ ions, and counterions undergoing decomposition into gaseous products at high temperature, which mixture is subjected to thermal treatment and subsequent homogenisation with a carbon material, characterized in that the solid homogeneous mixture of precursors, additionally containing an oxidizer and a fuel, is subjected to self-combustion, and the obtained material is subjected to grinding with amorphous carbon with subsequent thermal treatment, wherein:- an oxidizer solution is prepared containing NH4F, (NH4)IHPO4 and NaNOs in a 3:2:3 molar ratio in deionized water in a PTFE beaker, with nitrate anions as the oxidizer,- a fuel solution is prepared containing NH4VO3 and citric acid in a 3:2 molar ratio in deionized water in a glass beaker, with citric acid as the fuel,- the oxidizer solution and the fuel solution are simultaneously stirred and heated at temperature of 80°C until the fuel solution turns blue,- the fuel solution is added drop by drop to the oxidizer solution with continuous stirring, maintaining a 1:1 molar ratio of NH4VO3 to (NH^iHPC^, and then heated at temperature of 80°C until the solvent is completely evaporated,- the resulting precipitate is heated to a temperature of 150°C until it spontaneously ignites,- the green powder obtained after the combustion process is dried in a vacuum at a temperature of 120°C for 12 hours,- the dried material is ground with amorphous carbon in a mass ratio of 1:14 in a ball mill at 500 rpm for 5 hours,- the grinding product is subjected to heat treatment in a tube furnace in an inert atmosphere at a temperature of 300°C for 4 hours and then at a temperature of 550°C for 8 hours,as a result of which a composite material is obtained containing crystallographically pure, singlephase sodium-vanadium fluorophosphate, the crystallites of which are covered with amorphous carbon, forming composite particles with a size of 50-200 nm, wherein the carbon coating has a thickness in the range of 2-20 nm.

2. The method according to claim 1, characterized in that the composite material containing sodium-vanadium fluorophosphate with the molecular formula Na3(VOo, 79)2^04^1, 42 is obtained.

3. The method according to claim 1 or 2, characterized in that the oxidizer solution comprises 0.084-8.378 g of NH4F, preferably 0.838 g of NH4F, 0.190-18.966 g of (NH4)2HPO4, preferably 1.897 g of (NH4)2HPO4, and 0.183-18.308 g of NaNOs, preferably 1.831 g of NaNOs, and the volume of deionized water is 5-500 ml, preferably 50 ml.

4. The method according to claim 1 or 2 or 3, characterized in that the fuel solution contains 0.168-16.800 g of NH4VO3, preferably 1.680 g of NH4VO3, and 0.201-20.119 g of citric acid, preferably 2.012 g of citric acid, and the volume of deionized water is 5-500 ml, preferably 50 ml.

5. The method according to any one of claims 1-4, characterized in that the grinding is carried out in a vessel made of tungsten carbide with 250 zirconium balls, each 5 mm in diameter.

6. A positive electrode material for sodium-ion cells comprising sodium-vanadium fluorophosphate, characterized in that it is a composite of sodium-vanadium fluorophosphate with amorphous carbon, manufactured by the method described in claims 1-5.

7. A positive electrode material according to claim 6, characterized in that the sodium- vanadium fluorophosphate-amorphous carbon composite comprises crystallographically pure, single-phase sodium-vanadium fluorophosphate, the crystallites of which are coated withamorphous carbon to form composite particles of 50-200 nm in size, wherein the carbon coating has a thickness in the range of 2-20 nm.

8. A positive electrode material according to claim 6, characterized in that the sodiumvanadium fluorophosphate composite with amorphous carbon has the molecular formula Na3(VOo,79)2(P04)2Fl,42.

9. A sodium-ion cell having a negative electrode comprising metallic sodium, a positive electrode comprising sodium-vanadium fluorophosphate, a separator, and an electrolyte, sealed under an inert atmosphere, characterized in that the positive electrode material is a sodiumvanadium fluorophosphate-amorphous carbon composite as described in claims 6-8, manufactured by the method as described in claims 1-5.

Images