Layered polyanion positive electrode material of high-purity sodium ion battery and preparation method of layered polyanion positive electrode material

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a layered polyanion positive electrode material of a high-purity sodium ion battery and a preparation method of the layered polyanion positive electrode material. The layered polyanion positive electrode material is prepared by performing carbon coating on a polyanion compound, the chemical general formula of the polyanion compound is NaxMyNp (AO4) z, M is a transition metal element of + 2 valence, and M is at least one of Fe, Mn, Ni or Co; n is a doping element and is at least one of Al and Mg; aO4 is a polyanion group, and AO4 is any one of PO4 < 3->, SO4 < 2-> and SiO4 < 4->; the value range of x, y, z and p is any positive number smaller than 10. By optimizing the component design and the preparation process, the ionic conductivity, the structural stability and the electrochemical performance of the material are remarkably improved, and the material is suitable for the high-performance sodium ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a layered polyanion cathode material for a high-purity sodium ion battery and a preparation method thereof. Background Art

[0002] As an alternative to lithium-ion batteries, sodium-ion batteries (SIBs) show great potential for application in large-scale energy storage systems due to their abundant raw materials and low cost. However, the development of SIBs is limited by performance bottlenecks in cathode materials, such as low energy density, short cycle life, and poor conductivity. Layered polyanionic materials, which combine the advantages of layered oxides and polyanionic compounds, possess stable crystal structures and high ionic conductivity, making them a research hotspot for SIB cathode materials.

[0003] Currently, layered polyanionic materials are primarily based on the following systems: The NaFePO4 system: This system has an olivine-type structure, which imparts excellent thermal and chemical stability. However, NaFePO4 inherently has poor electrical conductivity, which limits its application in high-power density devices. To overcome this shortcoming, carbon coating or nanocrystallization techniques are often used to enhance its electronic conductivity and ion diffusion rate, but these improvements increase the complexity and cost of the preparation process. The Na2Fe2(SO4)3 system: This system offers a higher voltage platform and theoretically can achieve higher energy density. However, its synthesis process is relatively complex, involving reaction steps under high temperature and high pressure conditions, which poses challenges for industrial-scale production. Furthermore, sulfate-based materials may undergo phase transitions or dissolution during long-term cycling, affecting the overall performance and lifespan of the battery. The Na3V2(PO4)3 system: Due to its unique NASICON (sodium superionic conductor) structure, Na3V2(PO4)3 exhibits excellent high-voltage characteristics and good cycling stability. Despite this, the high price and resource scarcity of vanadium remain major factors limiting its widespread application. Na2MnSiO4 system: As a potential high-capacity cathode material, Na2MnSiO4 not only has low raw material costs but also a high theoretical specific capacity. However, this material suffers from structural instability, particularly due to its tendency to expand and contract during charge and discharge, leading to particle breakage and thus affecting battery cycle performance.

[0004] The layered polyanion materials and their preparation methods in the existing technology still face several challenges: (1) Insufficient ionic conductivity: Although these materials have a relatively stable structure, the diffusion rate of sodium ions inside them is low, especially in the layered structure, which directly limits the rate performance of the battery, that is, the ability to charge and discharge quickly. (2) Structural stability issues: Under high voltage conditions or after long-term cycling, many layered polyanion materials are prone to phase change or structural collapse, resulting in capacity decay and shortened cycle life. This instability not only affects the overall performance of the battery, but also brings uncertainty to practical applications. (3) Complex preparation process. Current preparation methods such as solid-phase reaction method, hydrothermal / solvothermal method, co-precipitation method, etc., although they can synthesize materials with specific properties, often require harsh synthesis conditions (such as high temperature and high pressure), and strict process control, making it difficult to achieve large-scale industrial production.

[0005] Chinese patent publication number CN116207267A discloses a carbon-sulfur-coated polyanion-type sodium ion battery positive electrode material and a preparation method thereof. The polyanion-type sodium ion battery positive electrode material includes polyanion positive electrode material particles, and the surface of the polyanion positive electrode material particles is a carbon-sulfur coating layer. The carbon-sulfur-coated polyanion-type sodium ion battery positive electrode material and the preparation method thereof of the present invention have the characteristics of fast sodium ion diffusion rate, stable and uniform interface, good electronic conductivity and excellent processing performance. Summary of the Invention

[0006] The present invention aims to provide a layered polyanion cathode material for a high-purity sodium ion battery and a preparation method thereof.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A layered polyanion cathode material for a high-purity sodium ion battery, wherein the layered polyanion cathode material is prepared by carbon-coating a polyanion compound, wherein the polyanion compound has a general chemical formula of NaxMyNp(AO4)z, wherein M is a +2-valent transition metal element, and M is at least one of Fe, Mn, Ni, or Co; N is a doping element, and N is at least one of Al and Mg; AO is a polyanion group, and AO4 is PO4 3- 、SO4 2- 、SiO4 4- Any of the following; the value range of x, y, z, and p is any positive number less than 10.

[0009] Preferably, the layered polyanion positive electrode material is prepared by first carbon-coating the polyanion compound with a carbon source and then performing a second carbon coating with a conductive additive.

[0010] Preferably, the conductive additive comprises graphene, carbon nanotubes and carbon black in a mass ratio of (1.4-1.6):1:(0.2-0.5).

[0011] The present invention sequentially coats a polyanionic compound with a carbon source and then a conductive additive, creating a synergistic effect through the secondary coatings. This improves the initial cycle capacity and capacity retention of the positive electrode material at room temperature. By compounding different conductive agents, the present invention constructs an efficient three-dimensional conductive path, while also filling tiny gaps in the network and ensuring continuous electron transport.

[0012] Preferably, N is a doping element, N includes Al and Mg, and the molar ratio of M, Al and Mg is (1-3): (0.5-2): (0.5-2).

[0013] The present invention can improve the capacity retention rate of the positive electrode material at high temperatures by adding compounded doping elements; and can improve the capacity retention rate of the positive electrode material at low temperatures by adding doping elements and transition elements in specific proportions. Analysis shows that the present invention can improve the lattice parameter changes caused by lithium ion deintercalation by adding specific amounts of magnesium and aluminum, thereby improving the structural stability of the material, helping to reduce the volume expansion and contraction of the material under high temperature conditions, and thus improving the capacity retention rate at high temperatures; the appropriate amount of Mg and Al doping and transition metals form a synergistic effect, which can optimize the electronic conductivity of the material, improve the migration ability of lithium ions under low temperature conditions, and make the diffusion of lithium ions at low temperatures easier by changing the crystal structure of the material, thereby improving the electrochemical performance at low temperatures.

[0014] Preferably, the chemical formula of the polyanion compound is any one of Na2MnAlMg(PO4)3, NaMn2AlMg2(SO4)6, and Na2MnAl2Mg(PO4)4.

[0015] The present invention provides a method for preparing the layered polyanion cathode material for a high-purity sodium ion battery, comprising the following steps:

[0016] (1) Raw material preparation: weigh the raw materials according to the stoichiometric ratio: sodium source, transition metal source, anion source, carbon source and doping element source; the molar ratio of sodium source, transition metal source and anion source is (1-2): (0.5-4): (3-10);

[0017] (2) Mixing and grinding: Mix the raw materials, add a solvent, and grind them thoroughly using a ball mill or grinding equipment to ensure that the raw materials are evenly dispersed to obtain a mixture;

[0018] (3) sintering or solvothermal reaction: subjecting the mixture to sintering or solvothermal reaction to obtain a product;

[0019] (4) Post-treatment: The product was cooled to room temperature and ball-milled to obtain a polyanionic compound;

[0020] (5) Carbon coating: The polyanion compound is carbon-coated to obtain a layered polyanion positive electrode material for a high-purity sodium ion battery.

[0021] Preferably, the specific steps of carbon coating are:

[0022] S1: Mix the polyanion compound and the carbon source in a mass ratio of (4-8):1, perform ball milling, and vacuum dry the mixed slurry obtained by ball milling to obtain a precursor solid.

[0023] S2: The precursor solid is placed in a tube furnace and introduced with inert gas. The temperature is raised to 300-350°C at 5-8°C / min and kept at this temperature for 1-2 hours. The temperature is then raised to 600-800°C at 2-4°C / min and kept at this temperature for 2-4 hours. The precursor is then naturally cooled to room temperature to obtain a primary coating material.

[0024] S3: Mix 100 parts by mass of the primary coating material with 5-10 parts by mass of the conductive additive, ball mill, sieve, and place in a tubular furnace again. Pass an inert gas, heat to 400-500°C at 2-5°C / min, keep warm for 1-2 hours, cool naturally, and pass through a 400-mesh sieve to obtain a high-purity layered polyanion positive electrode material for sodium ion batteries.

[0025] Preferably, in step (1), the sodium source is selected from at least one of sodium carbonate and sodium hydroxide; the transition metal source is selected from at least one of ferrous sulfate and manganese sulfate; the anion source is selected from at least one of phosphoric acid and sulfuric acid; the carbon source is selected from at least one of glucose and sucrose; and the doping element source is selected from at least one of aluminum oxide and magnesium oxide.

[0026] Preferably, the sintering in step (3) comprises the following steps: placing the ground mixture in a high-temperature furnace and sintering it under an inert atmosphere.

[0027] Preferably, the solvent reaction in step (3) comprises the following steps: transferring the mixture into an autoclave, adding a solvent, and reacting at 180-200° C. for 12-24 hours.

[0028] When the carbon source is mixed with the polyanionic compound, the distribution may be uneven, resulting in a local carbon layer that is too thick or too thin, affecting the electrical conductivity and ion diffusion properties.

[0029] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0030] 1. This invention provides a high-purity layered polyanion cathode material suitable for sodium-ion batteries and a method for preparing the same. By optimizing the component design and preparation process, this material significantly improves its ionic conductivity, structural stability, and electrochemical performance, making it suitable for high-performance sodium-ion batteries.

[0031] 2. The present invention is based on a stable framework structure formed by transition metal ions (M) and polyanionic groups (AO4), in which sodium ions (Na) act as charge carriers. By adjusting the stoichiometric ratio of transition metals, polyanionic groups, and sodium ions, the crystal structure and ion transport path of the material are optimized. By doping modification and introducing Al and Mg doping elements, the structure of the material can be stabilized, and the phase change and volume expansion during the charge and discharge process can be reduced. The present invention uses carbon coating to use the carbon layer as a conductive network, significantly improving the electronic conductivity of the material while suppressing side reactions between the electrode and the electrolyte.

[0032] 3. The present invention coats the polyanion compound with a carbon source and a conductive additive once, and produces a synergistic effect through secondary coating, thereby improving the first-week capacity and capacity retention rate of the positive electrode material at room temperature.

[0033] 4. The present invention can improve the capacity retention rate of the positive electrode material at high temperature by adding compounded doping elements; the present invention can improve the capacity retention rate of the positive electrode material at low temperature by adding doping elements and transition elements in specific proportions. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the scanning electron microscope (10 μm) of the positive electrode material of Example 1.

[0035] Figure 2 Schematic diagram of the scanning electron microscope (50 μm) of the positive electrode material of Example 1. DETAILED DESCRIPTION

[0036] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0037] The raw materials used in the following examples of the present invention are all commercially available commodities:

[0038] Graphene, Chengdu Jiacai Technology Co., Ltd., Specifications: Single-layer graphene oxide powder (size: 0.5-3μm.

[0039] Carbon nanotubes, Tianjin Jinglin New Materials Technology Co., Ltd., single-walled carbon nanotubes SG-100, tube diameter, 1-2 nm; length 5-30 μm.

[0040] Carbon black, CABOT, model conductive carbon black VULCANXC305, iodine value 72 mg / g.

[0041] Example 1

[0042] This embodiment provides a layered polyanion cathode material for a high-purity sodium ion battery, and a preparation method thereof comprises the following steps:

[0043] (1) Raw material preparation: Weigh the raw materials according to the stoichiometric ratio: sodium source: sodium carbonate; transition metal source: manganese sulfate; anion source: phosphoric acid; carbon source: glucose; doping element sources: aluminum oxide and magnesium oxide. The molar ratio of the sodium source, transition metal source, and anion source is 2:1:3, and the molar ratio of manganese, Al, and Mg is 1:1:1.

[0044] (2) Mixing and grinding: The raw materials were mixed, and ethanol 3 times the weight of the raw materials was added, and the mixture was fully ground using a ball mill to ensure that the raw materials were evenly dispersed to obtain a mixture;

[0045] (3) sintering: sintering the mixture, specifically, placing the ground mixture in a high-temperature furnace and sintering it under an inert atmosphere of nitrogen at a sintering temperature of 600° C. for 10 hours to obtain a product;

[0046] (4) Post-treatment: The product was cooled to room temperature and ball-milled at a speed of 550 rpm for 25 h, with a mass ratio of product to beads of 1:12, to obtain a polyanion compound; the general chemical formula of the polyanion compound was Na2MnAlMg(PO4)3.

[0047] (5) Carbon coating: The polyanion compound is carbon-coated to obtain a layered polyanion positive electrode material for a high-purity sodium ion battery.

[0048] The specific steps of carbon coating are:

[0049] S1: A polyanion compound and a carbon source were mixed in a mass ratio of 6:1, added to a ball mill, and anhydrous ethanol was added (the ratio of the weight of anhydrous ethanol to the total weight of the polyanion compound + carbon source was 3:1). The ball milling conditions were: 300 rpm, ball milling time 10 h, and a zirconia grinding ball to material mass ratio of 10:1. The mixed slurry was dried in a vacuum drying oven at 80°C for 12 h to obtain a precursor solid.

[0050] S2: The precursor solid was placed in a tube furnace and introduced with inert nitrogen gas at a flow rate of 100 mL / min. The temperature was raised to 300°C at a rate of 5°C / min and kept at that temperature for 1 hour; the temperature was further raised to 700°C at a rate of 3°C / min and kept at that temperature for 3 hours. The product was then naturally cooled to room temperature to obtain a primary coating material.

[0051] S3: 100 parts by mass of the primary coating material is mixed with 7 parts by mass of a conductive additive, wherein the conductive additive includes graphene, carbon nanotubes and carbon black in a mass ratio of 1.5:1:0.4; ball milling is performed at 200 rpm for 2 hours, sieved through 500 mesh after ball milling, and placed in a tubular furnace again under Ar atmosphere protection; the temperature is increased to 400°C at 2°C / min, kept warm for 2 hours, and naturally cooled and sieved through a 400 mesh sieve to obtain a high-purity layered polyanion positive electrode material for sodium ion batteries.

[0052] Example 2

[0053] This embodiment provides a layered polyanion cathode material for a high-purity sodium ion battery, and a preparation method thereof comprises the following steps:

[0054] (1) Raw material preparation: weigh the raw materials according to the stoichiometric ratio: sodium source: sodium carbonate; transition metal source: manganese sulfate; anion source: sulfuric acid; carbon source: glucose; doping element sources: aluminum oxide and magnesium oxide; the molar ratio of the sodium source, transition metal source and anion source is 1:2:6, and the molar ratio of manganese, Al and Mg is 2:1:2;

[0055] (2) Mixing and grinding: The raw materials were mixed, and 3 times the weight of ethanol was added, and the mixture was fully ground using a ball mill to ensure that the raw materials were evenly dispersed to obtain a mixture;

[0056] (3) Sintering: The mixture is sintered, specifically, the ground mixture is placed in a high-temperature furnace and sintered under an inert nitrogen atmosphere. The sintering temperature is usually 600°C and the time is 10 hours to obtain the product;

[0057] (4) Post-treatment: The product was cooled to room temperature and ball-milled at a speed of 550 rpm for 25 h, with a mass ratio of product to beads of 1:12, to obtain a polyanion compound; the general chemical formula of the polyanion compound was NaMn2AlMg2(SO4)6.

[0058] (5) Carbon coating: The polyanion compound is carbon-coated to obtain a layered polyanion positive electrode material for a high-purity sodium ion battery.

[0059] The specific steps of carbon coating are:

[0060] S1: A polyanion compound and a carbon source were mixed in a mass ratio of 6:1, added to a ball mill, and anhydrous ethanol was added (the ratio of the weight of anhydrous ethanol to the total weight of the polyanion compound + carbon source was 3:1). The ball milling conditions were: 300 rpm, ball milling time 10 h, and a zirconia grinding ball to material mass ratio of 10:1. The mixed slurry was dried in a vacuum drying oven at 80°C for 12 h to obtain a precursor solid.

[0061] S2: The precursor solid was placed in a tube furnace and introduced with inert nitrogen gas at a flow rate of 100 mL / min. The temperature was raised to 300°C at a rate of 5°C / min and kept at that temperature for 1 hour; the temperature was further raised to 700°C at a rate of 3°C / min and kept at that temperature for 3 hours. The product was then naturally cooled to room temperature to obtain a primary coating material.

[0062] S3: 100 parts by mass of the primary coating material is mixed with 7 parts by mass of a conductive additive, wherein the conductive additive includes graphene, carbon nanotubes and carbon black in a mass ratio of 1.4:1:0.5; ball milling is performed at 200 rpm for 2 hours, sieved through 500 mesh after ball milling, and placed in a tubular furnace again under Ar atmosphere protection; the temperature is increased to 400°C at 2°C / min, kept warm for 2 hours, and naturally cooled and sieved through a 400 mesh sieve to obtain a high-purity layered polyanion positive electrode material for sodium ion batteries.

[0063] Example 3

[0064] This embodiment provides a layered polyanion cathode material for a high-purity sodium ion battery, and a preparation method thereof comprises the following steps:

[0065] (1) Raw material preparation: weigh the raw materials according to the stoichiometric ratio: sodium source: sodium carbonate; transition metal source: manganese sulfate; anion source: phosphoric acid; carbon source: glucose; doping element sources: aluminum oxide and magnesium oxide; the molar ratio of sodium source, transition metal source and anion source is 2:1:4, and the molar ratio of manganese, Al and Mg is 1:2:1;

[0066] (2) Mixing and grinding: The raw materials were mixed, and ethanol in an amount of 3 times the weight of the raw materials was added, and the mixture was fully ground using a ball mill to ensure that the raw materials were evenly dispersed to obtain a mixture;

[0067] (3) Sintering: The mixture is sintered, specifically, the ground mixture is placed in a high-temperature furnace and sintered under an inert nitrogen atmosphere. The sintering temperature is usually 600°C and the time is 10 hours to obtain the product;

[0068] (4) Post-treatment: The product was cooled to room temperature and ball-milled at a speed of 550 rpm for 25 h, with a mass ratio of product to beads of 1:12, to obtain a polyanion compound; the general chemical formula of the polyanion compound was Na2MnAl2Mg(PO4)4.

[0069] (5) Carbon coating: The polyanion compound is carbon-coated to obtain a layered polyanion positive electrode material for a high-purity sodium ion battery.

[0070] The specific steps of carbon coating are:

[0071] S1: The polyanion compound and the carbon source were mixed in a mass ratio of 6:1, added to a ball mill, and anhydrous ethanol was added (the ratio of the weight of anhydrous ethanol to the total weight of the polyanion compound + carbon source was 3:1). The ball milling conditions were: 300 rpm, ball milling time 10 h, and the mass ratio of zirconia grinding balls to materials was 10:1. The mixed slurry was dried in a vacuum drying oven at 80°C for 12 hours to obtain a precursor solid.

[0072] S2: The precursor solid was placed in a tube furnace and introduced with inert nitrogen gas at a flow rate of 100 mL / min. The temperature was raised to 300°C at a rate of 5°C / min and kept at that temperature for 1 hour; the temperature was further raised to 700°C at a rate of 3°C / min and kept at that temperature for 3 hours. The product was then naturally cooled to room temperature to obtain a primary coating material.

[0073] S3: 100 parts by mass of the primary coating material is mixed with 7 parts by mass of a conductive additive, wherein the conductive additive includes graphene, carbon nanotubes and carbon black in a mass ratio of 1.6:1:0.2; ball milling is performed at 200 rpm for 2 hours, and after ball milling, the mixture is sieved through 500 mesh and placed in a tubular furnace again under Ar atmosphere protection; the temperature is increased to 400°C at 2°C / min, kept warm for 2 hours, and naturally cooled and sieved through a 400-mesh sieve to obtain a high-purity layered polyanion positive electrode material for sodium ion batteries.

[0074] Example 4

[0075] The difference between this embodiment and embodiment 1 is that the doping element source is aluminum oxide, and the molar ratio of manganese to Al is 1:2.

[0076] Example 5

[0077] The difference between this embodiment and embodiment 1 is that the doping element source is magnesium oxide, and the molar ratio of manganese to Mg is 1:2.

[0078] Comparative Example 1

[0079] The difference between this comparative example and Example 1 is that the molar ratio of manganese, Al and Mg is 1:0.2:0.2.

[0080] Comparative Example 2

[0081] The difference between this comparative example and Example 1 is that the molar ratio of manganese, Al and Mg is 1:4:3.

[0082] Comparative Example 3

[0083] The difference between this comparative example and Example 1 is that the conductive additive is graphene.

[0084] Comparative Example 4

[0085] The difference between this comparative example and Example 1 is that the conductive additives are graphene and carbon nanotubes in a mass ratio of 1.5:1.

[0086] Comparative Example 5

[0087] The difference between this comparative example and Example 1 is that the conductive additive includes graphene, carbon nanotubes and carbon black in a mass ratio of 0.4:1.6:1.

[0088] Comparative Example 6

[0089] The difference between this comparative example and Example 1 is that carbon coating is performed only once.

[0090] The specific steps of carbon coating are:

[0091] S1: The polyanion compound and the carbon source were mixed in a mass ratio of 6:1, added to a ball mill, and anhydrous ethanol was added (the ratio of the weight of anhydrous ethanol to the total weight of the polyanion compound + carbon source was 3:1). The ball milling conditions were: 300 rpm, ball milling time 10 h, and the mass ratio of zirconia grinding balls to materials was 10:1. The mixed slurry was dried in a vacuum drying oven at 80°C for 12 hours to obtain a precursor solid.

[0092] S2: Place the precursor solid in a tubular furnace and introduce inert nitrogen gas at a flow rate of 100 mL / min. The heating program is: heat to 300°C at 5°C / min and keep warm for 1 hour; continue to heat to 700°C at 3°C / min, keep warm for 3 hours, and naturally cool to room temperature to obtain a primary coating material; after natural cooling, pass through a 400-mesh sieve to obtain a high-purity layered polyanion positive electrode material for sodium ion batteries.

[0093] Comparative Example 7

[0094] This comparative example is the positive electrode material prepared in Example 1 of the Chinese patent publication number CN116207267A, which discloses a carbon-sulfur-coated polyanion sodium ion battery positive electrode material and a preparation method thereof.

[0095] Performance Testing

[0096] The performance of the layered polyanion positive electrode materials for high-purity sodium ion batteries prepared in Examples 1-5 and Comparative Examples 1-7 was tested.

[0097] After mixing high-purity sodium-ion battery layered polyanion cathode material, acetylene black, and PVDF in a mass ratio of 8:1:1 into a homogenous slurry, the black slurry was coated on aluminum foil and dried in a vacuum drying oven at 100°C for 12 hours. The electrode film was punched into discs with a radius of 0.6 mm using a punching machine, and the active material loading was approximately 2.0 mg / cm2 Coin cells were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄ (a 1:1 volume ratio mixture of ethylene carbonate (EC) and diethyl carbonate (DEC)) as the electrolyte, and 5 vol% FEC (fluoroethylene carbonate) as the separator. The electrical performance of the cells was tested.

[0098] Table 1 Performance test results

[0099]

[0100] It can be seen from Table 1 that the first-week capacity of the layered polyanion positive electrode material of the high-purity sodium ion battery of Examples 1-5 is higher than the first-week capacity of the positive electrode material of the prior art in Comparative Example 7, and the results show that the positive electrode materials of Examples 1-5 of the present invention have excellent cycle retention rates of 200 weeks at room temperature, while the 200-week cycle retention rate of the positive electrode material in the prior art is less than 95%, and it can be seen from Examples 1-3 that when the compounded doping elements are added, the positive electrode materials of Examples 1-3 have excellent cycle retention rates at high and low temperatures.

[0101] In Comparative Example 1 and Comparative Example 2, the ratio of the doping element and the transition element added is too high or too low, and the capacity retention rate of the positive electrode material at low temperature decreases.

[0102] In Comparative Examples 3-5, the types and proportions of the conductive additives were changed, and the first-week capacity and the capacity retention rate of the positive electrode material at room temperature decreased.

[0103] In Comparative Example 6, carbon coating was performed only once, and the capacity of the positive electrode material decreased significantly in the first week, and the capacity retention rate was also unsatisfactory.

[0104] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A layered polyanion cathode material for a high-purity sodium ion battery, characterized in that: The layered polyanion positive electrode material is prepared by carbon coating a polyanion compound, wherein the chemical formula of the polyanion compound is Na x M y N p (AO4) z , M is a +2 valence transition metal element, M is at least one of Fe, Mn, Ni or Co; N is a doping element, N is at least one of Al and Mg; AO4 is a polyanion group, AO is PO4 3- 、SO4 2- 、SiO4 4- Any of the following; the value range of x, y, z, and p is any positive number less than 10.

2. The layered polyanion cathode material for a high-purity sodium ion battery according to claim 1, wherein The layered polyanion positive electrode material is prepared by firstly carbon-coating the polyanion compound with a carbon source and then performing a second carbon coating with a conductive additive.

3. The layered polyanion cathode material for a high-purity sodium ion battery according to claim 2, wherein: The conductive additives include graphene, carbon nanotubes and carbon black in a mass ratio of (1.4-1.6):1:(0.2-0.5).

4. The layered polyanion cathode material for a high-purity sodium ion battery according to claim 3, wherein: N is a doping element, N includes Al and Mg, and the molar ratio of M, Al and Mg is (1-3): (0.5-2): (0.5-2).

5. The layered polyanion cathode material for a high-purity sodium ion battery according to claim 4, characterized in that: The general chemical formula of the polyanion compound is any one of Na2MnAlMg(PO4)3, NaMn2AlMg2(SO4)6, and Na2MnAl2Mg(PO4)4.

6. A method for preparing a high-purity layered polyanion cathode material for a sodium ion battery according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) Raw material preparation: weigh the raw materials according to the stoichiometric ratio: sodium source, transition metal source, anion source, carbon source and doping element source; the molar ratio of sodium source, transition metal source and anion source is (1-2): (0.5-4): (3-10); (2) Mixing and grinding: mixing the raw materials, adding a solvent, and grinding to obtain a mixture; (3) sintering or solvothermal reaction: subjecting the mixture to sintering or solvothermal reaction to obtain a product; (4) Post-treatment: The product was cooled to room temperature and ball-milled to obtain a polyanionic compound; (5) Carbon coating: The polyanion compound is carbon-coated to obtain a layered polyanion positive electrode material for a high-purity sodium ion battery.

7. The method for preparing a layered polyanion cathode material for a high-purity sodium ion battery according to claim 6, wherein: The specific steps of carbon coating are: S1: Mix the polyanion compound and the carbon source in a mass ratio of (4-8):1, perform ball milling, and vacuum dry the mixed slurry obtained by ball milling to obtain a precursor solid. S2: placing the precursor solid in a tube furnace, introducing inert gas, heating to 300-350°C, and keeping warm for 1-2 hours; further heating to 600-800°C, keeping warm for 2-4 hours, and naturally cooling to room temperature to obtain a primary coating material; S3: 100 parts by mass of the primary coating material is mixed with 5-10 parts by mass of the conductive additive, ball-milled, sieved, and placed in a tubular furnace again. Inert gas is introduced, the temperature is raised to 400-500°C, kept warm for 1-2 hours, and naturally cooled and passed through a 400-mesh sieve to obtain a high-purity layered polyanion positive electrode material for sodium ion batteries.

8. The method for preparing a layered polyanion cathode material for a high-purity sodium ion battery according to claim 6, wherein: In step (1), the sodium source is selected from at least one of sodium carbonate and sodium hydroxide; the transition metal source is selected from at least one of ferrous sulfate and manganese sulfate; the anion source is selected from at least one of phosphoric acid and sulfuric acid; the carbon source is selected from at least one of glucose and sucrose; and the doping element source is selected from at least one of aluminum oxide and magnesium oxide.

9. The method for preparing a layered polyanion cathode material for a high-purity sodium ion battery according to claim 6, wherein: The sintering in step (3) includes the following steps: placing the ground mixture in a high-temperature furnace and sintering it under an inert atmosphere.

10. The method for preparing a layered polyanion cathode material for a high-purity sodium ion battery according to claim 6, wherein: Solvent reaction in step (3) The method comprises the following steps: transferring the mixture into an autoclave, adding a solvent, and reacting at 180-200° C. for 12-24 hours.

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