Sodium ion battery positive electrode material, preparation method therefor, and use thereof

Abstract

A sodium ion battery positive electrode material, a preparation method therefor, and a use thereof, relating to the technical field of sodium ion batteries. The sodium ion battery positive electrode material comprises a polyanionic iron-manganese-based inner core material and a fast ion conductor layer coating the outer surface of the polyanionic iron-manganese-based inner core material; the polyanionic iron-manganese-based inner core material comprises iron and manganese, and the iron and the manganese are non-uniformly distributed in the polyanionic iron-manganese-based inner core material; in the centre of the polyanionic iron-manganese-based inner core material, the manganese content is higher than the iron content; and in the surface layer of the polyanionic iron-manganese-based inner core material, the iron content is higher than the manganese content. The uneven component distribution of the iron and manganese in the inner core material of the positive electrode material can effectively relieve the risk of structural collapse caused by metal ion migration during cycling; the fast ion conductor coating layer reduces the side reactions caused by electrolyte corrosion; and the problems of low capacity, fast attenuation, and low energy density of traditional single-phase materials are effectively solved.

Description

A sodium ion battery positive electrode material and its preparation method and application Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and in particular relates to a sodium ion battery positive electrode material and a preparation method and application thereof. Background Art

[0002] Energy is a vital material foundation for the survival and development of human society. With the increasing depletion of traditional fossil resources, countries around the world have begun to vigorously develop new energy systems. Lithium-ion batteries, currently the most comprehensive secondary battery, dominate the portable electronics and electric vehicle markets and are gradually expanding into the energy storage sector. However, lithium reserves in the Earth's crust are limited, and most of these resources are concentrated in a few countries in South America. This situation has caused global lithium prices to rise rapidly, and the manufacturing costs of lithium-ion batteries have also increased accordingly. Currently, the shortage of lithium resources and the rising cost of lithium battery production make it difficult for lithium-ion batteries to meet the future needs of the electric vehicle and energy storage industries. To overcome the constraints of lithium resource scarcity, it is necessary to develop a new generation of energy storage batteries with greater cost advantages.

[0003] The physical and chemical properties of sodium and lithium are very similar. In addition, metallic sodium is abundant, widely distributed, and inexpensive. Sodium-ion batteries based on the intercalation and deintercalation mechanism have more resource and cost advantages than lithium-ion batteries. This has led to significant progress in sodium-ion battery technology in recent years, making it a research hotspot. The industrialization process is also advancing rapidly, enabling rapid application in energy storage and small power fields. Positive electrode materials are a very important component of sodium-ion batteries. Currently, there are three main technical routes for positive electrode materials for sodium batteries: layered oxides, Prussian blue, and polyanions. Layered oxides have high energy density and compaction density, but their air stability is poor and they are very easy to absorb moisture. In addition, sodium is more active than lithium, resulting in more serious residual alkali on the surface of sodium-ion layered oxides than in lithium batteries. Polyanion compounds are one of the ideal choices for energy storage sodium-ion batteries due to their stable structure, good thermal stability, long cycle life, and low price.

[0004] Iron-based polyanionic compounds are among the most widely studied cathode materials for sodium-ion batteries. The pyrophosphate and phosphate groups in these materials form a stable framework structure, enabling reversible sodium ion shuttle within it. Furthermore, the sodium ion intercalation and deintercalation process is accompanied by minimal lattice distortion, resulting in excellent electrochemical and thermal stability. Furthermore, these materials offer advantages such as safety, non-toxicity, and affordability. However, these materials exhibit low theoretical specific capacity and poor electronic conductivity, resulting in a voltage plateau around 3.2V and a low energy density. Manganese-based materials with the same structure exhibit extremely poor electrochemical performance, with rapid capacity decay during cycling, but a voltage plateau of 3.8V. Consequently, research on iron-manganese-based hybrid materials is growing. While maintaining high specific capacity and excellent cycling performance, they offer significantly higher voltage plateaus and higher energy densities, making them more suitable for commercial applications.

[0005] However, in the current research on the preparation of high-performance polyanionic iron-manganese-based materials, the discharge specific capacity that can be achieved is low, far from the ideal level, and the cycle stability is poor, which affects its application. This is because manganese-based materials are prone to John-Teller distortion during the cycle, and manganese will dissolve in the electrolyte, causing the loss of manganese in the active material, thereby leading to structural decay and capacity decay of the electrode. For example, patent application CN114373923A provides a polycrystalline sodium manganese iron phosphate-carbon composite positive electrode material and its preparation method. The use of carbon-coated sodium manganese iron phosphate cannot prevent the dissolution of manganese, and the structural stability is poor, and the cycle performance is not good. Patent application CN110838576A discloses a doped coated sodium ion battery positive electrode material and its preparation method and use. The material is doped and coated after the core material is obtained by calcination, resulting in poor connection between the coating layer and the core, insufficient coating, and weakened interface protection ability.

[0006] Therefore, it is of great significance to prepare an iron-manganese-based polyanion sodium ion positive electrode material with high specific capacity, high voltage platform and high cycle stability.

[0007] Summary of the Invention

[0008] The purpose of the present invention is to overcome the problems existing in the above-mentioned prior art and provide a sodium ion battery positive electrode material and its preparation method and application.

[0009] The present invention is achieved through the following technical solutions:

[0010] A sodium ion battery cathode material, comprising a polyanion iron-manganese-based core material and a fast ion conductor layer coated on the outer surface of the polyanion iron-manganese-based core material;

[0011] The polyanionic iron-manganese-based core material contains iron and manganese, and the iron and manganese are unevenly distributed in the polyanionic iron-manganese-based core material; in the center of the polyanionic iron-manganese-based core material, the manganese content is higher than the iron content; in the surface layer of the polyanionic iron-manganese-based core material, the iron content is higher than the manganese content.

[0012] In some embodiments, the polyanionic iron-manganese-based core material comprises NaFe x Mn 1-x PO4, Na2Fe x Mn 1-x P2O7、Na3Fe y Mn 2-y (PO4)P2O7, and Na 4-a Fe 2+(a / 2)-z Mn z At least one of (P2O7)2, wherein 0.2≤x≤0.8, 0.2≤y≤1.8, 2 / 3≤a≤7 / 8, 0.2≤z≤1.8.

[0013] In some embodiments, the manganese content of the center of the polyanionic ferromanganese-based core material is greater than the manganese content of the surface layer of the polyanionic ferromanganese-based core material.

[0014] In some embodiments, the surface layer of the polyanionic ferromanganese-based core material has an iron content greater than the iron content of the center of the polyanionic ferromanganese-based core material.

[0015] In some embodiments, the polyanionic iron-manganese-based core material is NaFe x Mn 1-x PO4, where 0.4≤x≤0.6, for example, x is 0.5.

[0016] In some embodiments, the polyanionic iron-manganese-based core material is Na2Fe x Mn 1-x P2O7, wherein 0.4≤x≤0.6, for example, x is 0.5.

[0017] In some embodiments, the polyanionic iron-manganese-based core material is Na3Fe y Mn 2-y (PO4)P2O7, wherein 0.4≤y≤1.6, for example, y is 0.6, 0.8, 1, 1.2, or 1.4.

[0018] In some embodiments, the polyanionic iron-manganese-based core material is Na 4-a Fe 2+(a / 2)-z Mn z(P2O7)2, wherein 0.67≤a≤0.8, 0.5≤z≤1.5; preferably, 0.68≤a≤0.8 or 0.68≤a≤0.7; preferably, 0.8≤z≤1.3 or 1≤z≤1.2.

[0019] In some embodiments, the polyanionic iron-manganese-based core material is selected from Na3FeMn(PO4)P2O7, N a3 Fe 0.4 Mn 1.6 (PO4)P2O7、Na3Fe 1.6 Mn 0.4 (PO4)P2O7、N a3.32 Fe 1.17 Mn 1.17 (P2O7)2 and NaFe 0.5 Mn 0.5 One or more of PO4.

[0020] In the present invention, the manganese content of the polyanionic iron-manganese-based core material refers to the ratio of the amount of manganese element to the total amount of manganese element and iron element, and the iron content of the polyanionic iron-manganese-based core material refers to the ratio of the amount of iron element to the total amount of manganese element and iron element.

[0021] In some embodiments, the polyanionic iron-manganese-based core material comprises NaFe x Mn 1-x PO4, Na2Fe x Mn 1-x P2O7、Na3Fe x Mn 2-x (PO4)P2O7、Na 4-y Fe 2+y Mn 2+y At least one of (P2O7)2, wherein 0.2≤x≤0.8, 2 / 3≤y≤7 / 8.

[0022] The sodium ion battery positive electrode material of the present invention uses a polyanion iron-manganese-based material as the core material, and the outer layer has a fast ion conductor coating. The fast ion conductor coating can achieve the doping of metal ions on the surface of the core material, so that the surface coating layer and the core material particles have a tighter connection and are not easy to fall off; it can also reduce the erosion of the electrolyte on the core material, and can prevent parasitic reactions at the interface while stabilizing the crystal structure of the core material; at the same time, the fast ion conductor layer has a good sodium ion conductivity and will not cause capacity loss. Compared with metal oxide coating, the effect is better. Metal oxide has poor conductivity, and the polyanion material itself has poor conductivity. The combination of the two may cause the capacity to decay inevitably; compared with conductive polymers with better conductivity, the effect is also better. Usually, the conductive polymer coating layer is thicker, which increases the migration distance of sodium ions and reduces the kinetic performance. Moreover, the central manganese content of the polyanionic iron-manganese-based core material is higher than the iron content, and the surface iron content is higher than the manganese content, that is, the core part of the polyanionic iron-manganese-based material is rich in manganese, and the shell is rich in iron. This uneven component distribution can effectively alleviate the problems of manganese dissolution and iron ion migration during the cycle, which not only ensures the stability of the structure, alleviates the stress changes of the material during the charging and discharging process, avoids the performance deterioration caused by the crushing of particles, but also improves the problem of poor electrochemical performance of manganese-based materials.

[0023] The fast ion conductor is Na a M b+ c O (a+bc) / 2 , wherein M comprises at least one of Zr, Al, V, and Si, and a, b, and c satisfy (a+bc) / 2 and are positive integers. In some embodiments, a is selected from 1 to 5, for example, 2, 3, or 4. In some embodiments, b is selected from 2 to 5, for example, 3 or 4. In some embodiments, c is selected from 1 to 3, for example, 2. In some embodiments, the fast ion conductor is selected from one or more of Na2Si2O5, Na3VO4, and NaAlO2.

[0024] As a preferred embodiment of the sodium ion battery positive electrode material of the present invention, the mass of the fast ion conductor layer is 0.5% to 6% of the mass of the polyanionic iron-manganese-based core material, for example, 0.5% to 5%; or the thickness of the fast ion conductor layer is 1 nm to 10 nm. In some embodiments, the mass of the fast ion conductor layer is 0.5% to 6% of the mass of the polyanionic iron-manganese-based core material, for example, 1%, 2%, 3%, 4%, or 5%.

[0025] As a preferred embodiment of the sodium ion battery positive electrode material of the present invention, a carbon layer is further contained between the polyanionic iron-manganese-based core material and the fast ion conductor layer; the mass of the carbon layer is 0.1% to 4% of the mass of the polyanionic iron-manganese-based core material, for example, 0.2%, 0.5%, 1%, 2%, or 3%.

[0026] Another object of the present invention is to provide a method for preparing the sodium ion battery positive electrode material, comprising the following steps:

[0027] (1) dissolving a metal chelating agent in a solvent to obtain solution A, dissolving an iron source in a solvent to obtain solution B, and dissolving a manganese source in a solvent to obtain solution C; dropping solution B and solution C into solution A respectively, with solution C added first and solution B added later; then adding a phosphorus source, a sodium source, and a carbon source; and evaporating the resulting mixed solution to a gel, drying, grinding, calcining, and grinding to obtain a polyanionic iron-manganese-based core material precursor;

[0028] (2) dispersing a fast ion conductor in a solvent to obtain a solution D, dispersing the polyanion iron-manganese-based core material precursor obtained in step (1) in a solvent to obtain a solution E, adding solution D dropwise to solution E, and then evaporating and drying the resulting mixed solution to obtain a fast ion conductor-coated polyanion iron-manganese-based core material precursor;

[0029] (3) Grinding and calcining the fast ion conductor-coated polyanion iron-manganese-based core material precursor obtained in step (2) to obtain the sodium ion battery positive electrode material.

[0030] Preparation method of the present invention utilizes the difference of the chelation effect of metal chelator to iron and manganese metal ions, in the process of dripping solution, adopts early stage to add more manganese source to carry out chelation first, makes the central manganese content near polyanion iron manganese-based core material higher than iron content, after the iron with stronger chelation effect enters, more will be distributed on the surface of manganese, makes the surface iron content of polyanion iron manganese-based core material higher than manganese content, obtains the polyanion iron manganese-based core material with uneven distribution of iron and manganese, this center is rich in manganese, the surface layer is rich in iron and can effectively alleviate the risk of structural collapse brought by metal ion migration in the circulation process, ensures the stability of structure, is conducive to the improvement of cycle performance. Add carbon source in the process of preparing polyanion iron manganese-based core material, on the one hand can ensure to obtain divalent iron ions, on the other hand can enhance the conductivity of material, this is because carbon source can form in situ carbon layer on the surface of material when high temperature synthesizes material body. Coating the surface of the polyanionic iron-manganese-based core material with a fast ion conductor layer can reduce the occurrence of side reactions between the core material and the electrolyte, effectively protecting the core material, while ensuring that the fast ion transmission channel is not blocked, effectively improving the voltage platform of the positive electrode material, and improving the energy density, discharge specific capacity and cycle performance of the positive electrode material.

[0031] As a preferred embodiment of the method for preparing the positive electrode material for sodium ion batteries of the present invention, in step (1), the molar ratio of the iron in the iron source to the manganese in the manganese source is Fe:Mn=(1-4):(1-4). For example, Fe:Mn can be 1:1, 2:1, 1:2, 1:3, 3:1, 1:4, 4:1, etc. In some embodiments, the molar ratio of the iron in the iron source to the manganese in the manganese source is Fe:Mn=1:(0.25-4), for example, 1:0.5, 1:1, 1:2, 1:3.

[0032] In the step (1), the molar ratio of iron in the iron source to sodium in the sodium source can be Fe:Na=1:(1-20), preferably 1:(2-15), for example 1:4, 1:6, 1:10, 1:12.

[0033] In the step (1), the molar ratio of iron in the iron source to phosphorus in the phosphorus source can be Fe:P=1:(1-20), preferably 1:(2-15), for example 1:4, 1:6, 1:10, 1:12.

[0034] As a preferred embodiment of the method for preparing the positive electrode material of a sodium ion battery according to the present invention, in the step (1), the metal chelating agent is a polymer compound composed of C, H, and O; and is capable of chelating with iron and manganese; the mass of the metal chelating agent is 1% to 5% of the sum of the masses of the iron source and the manganese source, for example, 2%, 3%, or 4%.

[0035] In the step (1), the amount of the carbon source added is determined according to the mass ratio of the carbon element in the polyanionic iron-manganese-based core material.

[0036] As a preferred embodiment of the method for preparing the sodium ion battery positive electrode material of the present invention, in step (1), the dripping rate ratio of solution B to solution C is 1:(2-5), for example, 1:3 or 1:4. When the volumes of solution B and solution C are equal, the preparation of the core material with uneven iron and manganese distribution of the present invention is achieved by controlling the dripping rates of solution B and solution C.

[0037] Preferably, in step (1), the dripping rate ratio of solution B to solution C is 1:(2-3). In some embodiments, the dripping rate of solution B is 0.05 mL / s to 0.2 mL / s, preferably 0.1 mL / s to 0.2 mL / s, for example, 0.12 mL / s, 0.14 mL / s, 0.16 mL / s, and 0.18 mL / s. In some embodiments, the dripping rate of solution C is 0.1 mL / s to 1 mL / s, preferably 0.2 mL / s to 1 mL / s, for example, 0.4 mL / s, 0.6 mL / s, and 0.8 mL / s.

[0038] In some embodiments, the metal chelating agent is dissolved under heating conditions to obtain solution A; preferably, the heating temperature is 80°C to 100°C.

[0039] In some embodiments, after solution B and solution C are dropped into solution A, the resulting mixture is continuously stirred in a water bath at 80° C. to 100° C., and then a phosphorus source, a sodium source, and a carbon source are added.

[0040] In some embodiments, in step (1), the drying temperature is 100° C. to 150° C., preferably 120° C. to 150° C. In some embodiments, in step (1), the drying time is 10 h to 18 h, preferably 12 h to 15 h.

[0041] Preferably, in step (1), the calcination temperature is 300° C. to 400° C., and the calcination time is 5 hours to 8 hours. In some embodiments, in step (1), the calcination temperature is 350° C. to 400° C., and the calcination time is 6 hours to 8 hours.

[0042] In step (2), the amount of the fast ion conductor added is determined according to the mass ratio of the fast ion conductor layer in the polyanionic iron-manganese-based core material.

[0043] Preferably, in the step (2), the evaporation is carried out at 60°C to 90°C, preferably at 70°C to 80°C.

[0044] Preferably, in step (2), the drying temperature is 100° C. to 150° C., preferably 100° C. to 120° C. Preferably, in step (2), the drying time is 3 h to 10 h, preferably 5 h to 10 h.

[0045] Preferably, in step (3), the calcination temperature is 500°C to 650°C, for example, 500°C to 600°C, and the calcination time is 9h to 12h. In some embodiments, in step (3), the calcination temperature is 550°C to 650°C.

[0046] Preferably, in step (3), the calcination atmosphere is a mixed atmosphere of hydrogen and argon.

[0047] Preferably, the calcination atmosphere consists of 5% to 10% hydrogen and 90% to 95% argon by volume.

[0048] In some embodiments, in step (3), the heating rate of the calcination temperature is 1°C min -1 ~3℃min -1Preferably, in step (3), the heating rate of the calcination temperature is 2°C min -1 .

[0049] Preferably, the sodium source comprises at least one of sodium carbonate, sodium bicarbonate, sodium acetate, and sodium nitrate. In some embodiments, the sodium source is sodium nitrate.

[0050] Preferably, the iron source comprises at least one of ferrous oxalate dihydrate, ferrous phosphate, and ferric nitrate nonahydrate. In some embodiments, the iron source is ferric nitrate nonahydrate.

[0051] Preferably, the manganese source comprises at least one of manganese chloride, manganese acetate, and manganese nitrate. In some embodiments, the manganese source is manganese acetate.

[0052] Preferably, the phosphorus source comprises at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate. In some embodiments, the phosphorus source is ammonium dihydrogen phosphate.

[0053] Preferably, the carbon source comprises at least one of glucose, sucrose, and citric acid. In some embodiments, the carbon source is sucrose.

[0054] Preferably, the metal chelating agent includes at least one of polyvinyl alcohol and polymethacrylic acid.

[0055] Preferably, the fast ion conductor is selected from one or more of Na2Si2O5, Na3VO4 and NaAlO2.

[0056] Another object of the present invention is to provide a sodium ion battery positive electrode slurry, wherein the sodium ion battery positive electrode slurry comprises the sodium ion battery positive electrode material of the present invention. The positive electrode slurry optionally further comprises a solvent, a conductive agent, and a binder.

[0057] Another object of the present invention is to provide a sodium ion battery positive electrode sheet, comprising the sodium ion battery positive electrode material of the present invention. The sodium ion battery positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer can be obtained by coating a positive electrode slurry onto the positive electrode current collector, followed by drying and pressing.

[0058] Another object of the present invention is to provide a sodium ion battery, comprising a positive electrode plate comprising the sodium ion battery positive electrode material of the present invention or the sodium ion battery positive electrode material prepared by the preparation method. The sodium ion battery also comprises a negative electrode plate, a separator, an electrolyte, and the like commonly used in the art.

[0059] The sodium ion battery cathode material of the present invention is based on a polyanionic iron-manganese-based core material with an open three-dimensional framework structure. It has better ion transport channels and lower ion migration barriers, which facilitates the rapid migration of sodium ions. It also combines the stability of iron-based materials with the high voltage platform of manganese-based materials. When applied to the positive electrode sheet of a sodium ion battery, it can be stably cycled and has a high capacity and high voltage platform. The sodium ion battery obtained by using the sodium ion battery cathode material of the present invention has a high specific capacity and high voltage platform, and has stable performance, strong environmental adaptability, and is environmentally friendly.

[0060] The present invention has the following beneficial effects:

[0061] (1) The sodium ion battery positive electrode material of the present invention comprises a polyanion iron-manganese-based core material and a fast ion conductor coating layer. The iron and manganese in the polyanionic iron-manganese-based core material are unevenly distributed, with the manganese content higher than the iron content in the center and the iron content higher than the manganese content in the surface. The uneven component distribution can effectively alleviate the problems of manganese dissolution and iron ion migration during the cycle, ensuring the stability of the structure, alleviating the stress changes of the material during the charge and discharge process, avoiding the performance deterioration caused by the crushing of particles, and improving the problem of poor electrochemical performance of manganese-based materials; the presence of the fast ion conductor coating layer further protects iron and manganese, can protect manganese dissolution, inhibit iron migration, and reduce the side reactions caused by electrolyte corrosion, and the coating is carried out during the preparation stage of the polyanionic iron-manganese-based precursor, so that during the secondary sintering process, the metal of the fast ion conductor coating layer enters the lattice of the polyanionic iron-manganese-based core material, undergoes an intercalation effect, and realizes the doping of metal ions on the surface of the core material, so that the coating layer and the core form a more stable interlayer effect, improve the integrity and tightness of the coating, and achieve more stable interface protection capability, while ensuring that the fast ion transmission channel is not blocked, maintaining good sodium ion conductivity, and will not cause capacity loss. The sodium ion battery positive electrode material of the present invention effectively protects the manganese-based material and improves the voltage platform of the iron-based material, solving the problems of low capacity, fast decay and low energy density of traditional single-phase materials, and providing an iron-manganese-based material that can be stably circulated and has a high capacity and voltage platform as a positive electrode material for sodium ion batteries.

[0062] (2) The present invention adopts a dripping solvothermal method to prepare a polyanion iron-manganese-based core material for a positive electrode material of a sodium ion battery. A metal chelating agent is added during the preparation process. The strength of the metal chelating agent's chelation effect on different metal ions is utilized to achieve an uneven distribution of metals in the polyanion iron-manganese-based material. The metal chelating agent can simultaneously chelate with both iron and manganese, and has a stronger chelation effect on iron. In the process of dripping the iron and manganese solutions, a large amount of manganese source is added in the early stage for chelation, so that the manganese content near the center of the polyanion iron-manganese-based core material is higher than the iron content. Later, the iron with a stronger chelation effect enters and is more distributed on the surface of the manganese, so that the iron content of the surface of the polyanion iron-manganese-based core material is higher than the manganese content, thereby obtaining a polyanion iron-manganese-based core material with uneven iron and manganese distribution.

[0063] (3) A carbon source is added during the preparation of the polyanionic iron-manganese-based core of the sodium ion battery positive electrode material of the present invention. On the one hand, a carbon thermal reduction reaction can occur during the calcination process to reduce metal ions and form the required divalent metal compound; on the other hand, the conductivity of the sodium ion battery positive electrode material can be enhanced, thereby improving the problem of low conductivity of existing sodium ion battery positive electrode materials.

[0064] (4) The preparation method of the present invention has a simple process, readily available materials, low cost, and easy-to-achieve conditions. The prepared positive electrode material has excellent electrochemical properties, is conducive to large-scale industrial applications, reduces the production costs of enterprises, and has great development prospects in the field of large-scale energy storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG1 is a charge and discharge curve of the positive electrode material prepared in Example 1.

[0066] FIG2 is a charge and discharge curve of the positive electrode material prepared in Comparative Example 1.

[0067] FIG3 is a charge and discharge curve of the positive electrode material prepared in Comparative Example 2.

[0068] FIG4 is a SEM image of the positive electrode material prepared in Example 1.

[0069] FIG5 is an EDS element distribution diagram of the positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0070] To better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0071] Unless otherwise specified, the experimental methods used in the examples are conventional methods; the materials, reagents, etc. used are all available from commercial sources unless otherwise specified.

[0072] The element distribution of the polyanionic iron-manganese-based core material was analyzed by EDS element distribution diagram.

[0073] In the present invention, the mass ratio of the fast ion conductor layer, the carbon layer and the polyanion iron-manganese-based core material is calculated based on the feed ratio in the preparation process.

[0074] Example 1

[0075] A method for preparing a sodium ion battery positive electrode material is as follows:

[0076] (1) Dissolve polyvinyl alcohol (number average molecular weight 25,000-35,000) in 10 mL of deionized water, heat to 80°C and stir constantly until completely dissolved to obtain solution A; dissolve ferric nitrate nonahydrate in 10 mL of deionized water and stir to dissolve to obtain solution B (concentration 0.1 mol / mL); dissolve manganese acetate in 10 mL of deionized water and stir to dissolve to obtain solution C. Solution B and solution C were pumped into solution A through a peristaltic pump, and the dripping rate of solution B was controlled to be 0.1 mL / s and the dripping rate of solution C was controlled to be 0.2 mL / s. After all the solutions were added, the obtained mixture was continuously stirred in a water bath at 80°C, and then diammonium dihydrogen phosphate, sodium nitrate and sucrose were added in sequence. After the solution evaporated to a gel state, heating was stopped, and the solution was transferred to a constant temperature oven at 120°C and dried for 12 hours. After being taken out, it was ground into powder, and then the obtained powder was calcined at 350°C for 6 hours. After cooling, it was further ground to obtain a mixed sodium iron manganese pyrophosphate precursor (Na3FeMn(PO4)P2O7).

[0077] Among them, the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate, and ammonium dihydrogen phosphate is 1:1:3:3; the mass of polyvinyl alcohol is 3% of the sum of the mass of the manganese source and the iron source, and the amount of sucrose added is calculated based on the mass ratio of carbon to core material in the final sodium ion positive electrode material.

[0078] (2) Disperse NaAlO2 in deionized water and stir continuously at 25°C for 30 minutes to obtain a mixed solution D (concentration of 1 mol / mL); disperse the mixed sodium manganese iron pyrophosphate precursor prepared in step (1) in anhydrous ethanol to obtain a mixed solution E (concentration of 0.1 mol / mL). Add the mixed solution D to the mixed solution E through a peristaltic pump. After the addition is completed, stir and heat the new mixed solution in a 70°C water bath to quickly evaporate the solution. After the liquid is completely evaporated, dry it in a constant temperature oven at 120°C for 5 hours to obtain a NaAlO2-coated mixed sodium manganese iron pyrophosphate precursor. The amount of NaAlO2 added is calculated based on the mass ratio of NaAlO2 to the core material in the final sodium ion positive electrode material.

[0079] (3) The NaAlO2-coated mixed sodium manganese iron pyrophosphate precursor obtained in step (2) was ground and further calcined at 550°C for 12 hours in a calcination atmosphere of 5% hydrogen plus 95% argon (volume fraction) at a heating rate of 2°C min -1 , a NaAlO2-coated Na3FeMn(PO4)P2O7 sodium-ion battery cathode material is obtained. In the resulting sodium-ion cathode material, the mass of the NaAlO2 coating layer is 1% of the mass of the Na3FeMn(PO4)P2O7 core material, the thickness of the NaAlO2 coating layer is 2-3 nm, and the mass of carbon is 2% of the mass of the Na3FeMn(PO4)P2O7 core material. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than that in the surface layer, and the iron content in the surface layer of the core material is greater than that in the center.

[0080] The SEM image and EDS element distribution diagram of the positive electrode material prepared in Example 1 are shown in Figures 4 and 5.

[0081] Example 2

[0082] The difference between this embodiment and embodiment 1 is only that the amount of NaAlO2 added in step (2) is changed so that in the final sodium ion positive electrode material, the mass of the NaAlO2 coating layer is 0.5% of the mass of the Na3FeMn(PO4)P2O7 core material, and the thickness of the NaAlO2 coating layer is 1-2 nm. The rest is the same as in embodiment 1, thereby obtaining the Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material coated with NaAlO2 in this embodiment. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0083] Example 3

[0084] The difference between this embodiment and embodiment 1 is only that the amount of NaAlO2 added in step (2) is changed so that in the final sodium ion positive electrode material, the mass of the NaAlO2 coating layer is 2% of the mass of the Na3FeMn(PO4)P2O7 core material, and the thickness of the NaAlO2 coating layer is 4-5 nm. The rest is the same as in embodiment 1, thereby obtaining the Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material coated with NaAlO2 in this embodiment. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0085] Example 4

[0086] The only difference between this embodiment and embodiment 1 is that the amount of NaAlO2 added in step (2) is changed so that in the final sodium ion positive electrode material, the mass of the NaAlO2 coating layer is 5% of the mass of the Na3FeMn(PO4)P2O7 core material, and the thickness of the NaAlO2 coating layer is 7-8 nm. The rest is the same as in embodiment 1, thereby obtaining the Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material coated with NaAlO2 in this embodiment. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0087] Example 5

[0088] The difference between this embodiment and embodiment 1 is that in step (1), the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate and ammonium dihydrogen phosphate is 1:4:15:15, and the rest is the same as embodiment 1, to obtain Na3Fe2 coated NaAlO2 of this embodiment. 0.4 Mn 1.6 (PO4)P2O7 sodium-ion battery cathode material. The manganese content in the core material is higher than the iron content; in the core material's surface layer, the iron content is higher than the manganese content. The manganese content in the core material's center is greater than that in the surface layer, and the iron content in the core material's surface layer is greater than that in the center.

[0089] Example 6

[0090] The difference between this embodiment and embodiment 1 is that in step (1), the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate and ammonium dihydrogen phosphate is 4:1:15:15, and the rest is the same as embodiment 1, to obtain Na3Fe2 coated NaAlO2 of this embodiment. 1.6 Mn 0.4 (PO4)P2O7 sodium-ion battery cathode material. The manganese content in the core material is higher than the iron content; in the core material's surface layer, the iron content is higher than the manganese content. The manganese content in the core material's center is greater than that in the surface layer, and the iron content in the core material's surface layer is greater than that in the center.

[0091] Example 7

[0092] The only difference between this embodiment and embodiment 1 is that in step (2), Na2Si2O5 is used instead of NaAlO2. The rest is the same as embodiment 1, and the Na2Si2O5-coated Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material of this embodiment is prepared. The mass of the Na2Si2O5 coating layer is 1% of the mass of the Na3FeMn(PO4)P2O7 core material, and the thickness of the coating layer is 2 to 3 nm. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0093] Example 8

[0094] The only difference between this embodiment and embodiment 1 is that in step (2), Na3VO4 is used instead of NaAlO2. The rest is the same as embodiment 1, and the Na3VO4-coated Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material of this embodiment is prepared. The mass of the Na3VO4 coating layer is 1% of the mass of the Na3FeMn(PO4)P2O7 core material, and the thickness of the NaAlO2 coating layer is 2-3nm. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0095] Example 9

[0096] The only difference between this embodiment and embodiment 1 is that in step (1), the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate, and ammonium dihydrogen phosphate is 1.17:1.17:3.32:4; in step (3), the obtained dry powder is ground and further calcined at 600°C for 12 hours (heating rate is 2°C min -1 ), the rest are the same as in Example 1, and the NaAlO2 coated Na 3.32 Fe 1.17 Mn 1.17 (P2O7)2 sodium-ion battery cathode material. The manganese content in the core material is higher than the iron content; in the core material's surface layer, the iron content is higher than the manganese content. The manganese content in the core material's center is greater than that in the surface layer, and the iron content in the core material's surface layer is greater than that in the center.

[0097] Example 10

[0098] The only difference between this embodiment and embodiment 1 is that in step (1), the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate and ammonium dihydrogen phosphate is 0.5:0.5:1:1; in step (3), the obtained dry powder is ground and further calcined at 650°C for 12 hours (heating rate is 2°C min -1 ), the rest are the same as in Example 1, to obtain NaFe coated with NaAlO2 in this embodiment. 0.5 Mn 0.5 PO4 sodium-ion battery positive electrode material. The manganese content in the core material is higher than the iron content; in the core material's surface layer, the iron content is higher than the manganese content. The manganese content in the core material's center is greater than that in the surface layer, and the iron content in the surface layer is greater than that in the core material.

[0099] Example 11

[0100] The only difference between this embodiment and embodiment 1 is that the amount of sucrose added in step (1) is changed so that the mass of carbon in the final sodium ion positive electrode material is 0.1% of the mass of the Na3FeMn(PO4)P2O7 core material. The rest is the same as in embodiment 1, and the Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material coated with NaAlO2 in this embodiment is obtained. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0101] Example 12

[0102] The difference between this embodiment and embodiment 1 is only that the amount of sucrose added in step (1) is changed so that the mass of carbon in the final sodium ion positive electrode material is 4% of the mass of the Na3FeMn(PO4)P2O7 core material. The rest is the same as in embodiment 1, and the Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material coated with NaAlO2 in this embodiment is obtained. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0103] Example 13

[0104] The only difference between this embodiment and Example 1 is that in step (1), an equal amount of sodium polymethacrylate (number average molecular weight of approximately 9500) is used to replace polyvinyl alcohol. The rest is the same as in Example 1, to produce the NaAlO2-coated Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material of this embodiment. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0105] Example 14

[0106] The only difference between this embodiment and embodiment 1 is that in step (1), the dripping rate of solution B is 0.1 mL / s, and the dripping rate of solution C is 0.3 mL / s. The rest is the same as in embodiment 1, and the NaAlO2-coated Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material of this embodiment is prepared. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0107] Example 15

[0108] The only difference between this embodiment and embodiment 1 is that in step (1), the dripping rate of solution B is 0.1 mL / s, and the dripping rate of solution C is 0.5 mL / s. The rest is the same as in embodiment 1, and the NaAlO2-coated Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material of this embodiment is prepared. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0109] Example 16

[0110] The only difference between this embodiment and embodiment 1 is that the amount of NaAlO2 added in step (2) is changed so that in the final sodium ion positive electrode material, the mass of the NaAlO2 coating layer is 6% of the mass of the Na3FeMn(PO4)P2O7 core material, and the thickness of the NaAlO2 coating layer is 8-10 nm. The rest is the same as in embodiment 1, thereby obtaining the NaAlO2-coated Na3FeMn(PO4)P2O7 sodium ion battery positive electrode material of this embodiment. In the center of the core material, the manganese content is higher than the iron content; in the surface layer of the core material, the iron content is higher than the manganese content. The manganese content in the center of the core material is greater than the manganese content in the surface layer, and the iron content in the surface layer of the core material is greater than the iron content in the center.

[0111] Comparative Example 1

[0112] A method for preparing a sodium ion battery positive electrode material is as follows:

[0113] (1) Dissolve polyvinyl alcohol (number average molecular weight 25,000-35,000) in 10 mL of deionized water, heat to 80°C and stir until completely dissolved to obtain solution A; dissolve ferric nitrate nonahydrate in 10 mL of deionized water, stir and dissolve to obtain solution B (concentration 0.1 mol / mL); dissolve manganese acetate in 10 mL of deionized water, stir and dissolve to obtain solution C. Pump solution B and solution C into solution A through a peristaltic pump, control the drip rate of solution B to 0.1 mL / s and the drip rate of solution C to 0.2 mL / s. After all the solution is added, the obtained mixture is continuously stirred in an 80°C water bath, and then sodium nitrate, ammonium dihydrogen phosphate and sucrose are added in sequence. After the solution evaporates to a gel state, stop heating, transfer to a 120°C constant temperature oven and dry for 12 hours. After taking out, grind into powder, and then calcine the obtained powder at 350°C for 6 hours. After cooling, further grind to obtain a mixed sodium manganese iron pyrophosphate precursor.

[0114] Among them, the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate, and ammonium dihydrogen phosphate is 1:1:3:3; the mass of polyvinyl alcohol is 3% of the sum of the mass of the manganese source and the iron source, and the amount of sucrose added is calculated based on the mass ratio of carbon to core material in the final sodium ion positive electrode material.

[0115] (2) The mixed sodium manganese iron pyrophosphate precursor obtained in step (1) was ground and further calcined at 550°C for 12 hours in a calcination atmosphere of 5% hydrogen plus 95% argon by volume, with a heating rate of 2°C min -1 , a pure-phase Na3FeMn(PO4)P2O7 sodium-ion battery cathode material is obtained; the mass of carbon in the resulting sodium-ion cathode material is 3% of the mass of Na3FeMn(PO4)P2O7. In the core material's center, the manganese content is higher than the iron content; in the core material's surface, the iron content is higher than the manganese content. The manganese content in the core material's center is greater than that in the surface layer, and the iron content in the core material's surface layer is greater than that in the center.

[0116] Comparative Example 2

[0117] A method for preparing a sodium ion battery positive electrode material is as follows:

[0118] (1) Dissolve ferric nitrate nonahydrate in 10 mL of deionized water, stir and dissolve to obtain solution B (concentration of 0.1 mol / mL), dissolve manganese acetate in 10 mL of deionized water, stir and dissolve to obtain solution C, mix solution B and solution C, and continuously stir the obtained mixture in a water bath at 80°C, then add ammonium dihydrogen phosphate, sodium nitrate and sucrose in sequence, wait for the solution to evaporate to a gel state, stop heating, transfer to a constant temperature oven at 120°C and dry for 12 hours, take out and grind into powder, then calcine the obtained powder at 350°C for 6 hours, and further grind after cooling to obtain a mixed sodium manganese ferric pyrophosphate precursor.

[0119] Among them, the molar ratio of ferric nitrate nonahydrate, manganese acetate, sodium nitrate, and ammonium dihydrogen phosphate is 1:1:3:3, and the amount of sucrose added is calculated based on the mass ratio of carbon to core material in the final sodium ion positive electrode material.

[0120] (2) NaAlO2 was first dispersed in deionized water and stirred continuously at 25°C for 30 minutes to obtain a mixed solution D (concentration of 1 mol / mL). The mixed sodium manganese iron pyrophosphate precursor prepared in step (1) was dispersed in anhydrous ethanol to obtain a mixed solution E (concentration of 0.1 mol / mL). The mixed solution D was slowly added to the mixed solution E through a peristaltic pump. After the dropwise addition was completed, the new mixed solution was stirred and heated in a 70°C water bath to quickly evaporate the solution. After the liquid was completely evaporated, it was dried in a constant temperature oven at 120°C for 5 hours to obtain a NaAlO2-coated mixed sodium manganese iron pyrophosphate precursor. The amount of NaAlO2 added was calculated based on the mass ratio of NaAlO2 to the core material in the final sodium ion positive electrode material.

[0121] (3) After grinding the NaAlO2-coated mixed sodium manganese iron pyrophosphate precursor obtained in step (2), it was further calcined at 550°C for 12 hours in a calcination atmosphere of 5% hydrogen plus 95% argon at a heating rate of 2°C min -1 The NaAlO2-coated Na3FeMn(PO4)P2O7 sodium-ion battery cathode material is obtained. In this sodium-ion cathode material, the mass of the NaAlO2 coating layer is 1% of the mass of the Na3FeMn(PO4)P2O7 core material, the thickness of the NaAlO2 coating layer is 2-3 nm, and the mass of carbon is 3% of the mass of the Na3FeMn(PO4)P2O7 core material. The manganese content in both the core material's center and surface is equal to the iron content. The manganese content in the core material's center is equal to that in the surface layer, and the iron content in the surface layer is equal to that in the core material.

[0122] Performance testing:

[0123] The sodium ion battery positive electrode material obtained in the embodiment and comparative example is mixed with conductive carbon and PVDF solution (dissolved in n-methylpyrrolidone solution) to make a slurry, which is then evenly coated on aluminum foil to obtain a positive electrode sheet, which is then combined with a Na sheet in a glove box to form a button half-cell. Electrochemical data were collected from a half-cell constructed using a 2016 battery casing. The specific charge and discharge curves and 0.1C specific capacity data were obtained by charging and discharging at a rate of 0.1C in a constant temperature box at 25°C using a blue electric test device (voltage range of 1.6-4.3V). After cycling 100th at a rate standard of 0.5C / 0.5C, the corresponding cycle capacity retention rate (the percentage of the 100th cycle capacity relative to the first cycle capacity) was recorded. The electrochemical impedance spectroscopy (EIS) was obtained using an electrochemical workstation (CHI650D), and the test frequency was set to 0.01-10 5 Hz. See Table 1 and Figures 1, 2, and 3 for details.

[0124] Table 1 Performance test results of batteries assembled in the embodiment and comparative example

[0125] Figure 1 is the charge and discharge curve of the positive electrode material prepared in Example 1, Figure 2 is the charge and discharge curve of the positive electrode material prepared in Comparative Example 1, Figure 3 is the charge and discharge curve of the positive electrode material prepared in Comparative Example 2, Figure 4 is the SEM image of the positive electrode material prepared in Example 1, and Figure 5 is the EDS element distribution diagram of the positive electrode material prepared in Example 1.

[0126] It can be seen from the comparison between Example 1 and Comparative Example 1 that the NaAlO2 coating layer can improve the capacity of the battery and maintain the cyclic stability of the material. It can be seen from the comparison between Example 1 and Comparative Example 2 that the positive electrode material with uneven manganese iron distribution of the present invention can improve the structural stability of the battery, thereby improving the electrochemical performance of the battery. Figures 4 and 5 show the particle morphology of the sodium ion battery positive electrode material obtained in Example 1, wherein the EDS element distribution diagram of Figure 5 shows the element distribution. It can be seen that the density of the iron element in the surface layer of the polyanionic iron-manganese-based core material of the sodium ion battery positive electrode material is higher than that of the manganese element, thereby indicating that the iron content is higher than the manganese content in the surface layer of the polyanionic iron-manganese-based core material of the sodium ion battery positive electrode material.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A sodium ion battery positive electrode material, characterized in that: The positive electrode material comprises a polyanion iron-manganese-based core material and a fast ion conductor layer coated on the outer surface of the polyanion iron-manganese-based core material; The polyanion iron-manganese-based core material contains iron and manganese, and the iron and manganese are non-uniformly distributed in the polyanion iron-manganese-based core material; in the center of the polyanion iron-manganese-based core material, the manganese content is higher than the iron content; in the surface layer of the polyanion iron-manganese-based core material, the iron content is higher than the manganese content; The polyanionic iron-manganese-based core material includes NaFe x Mn 1-x PO4、Na2Fe x Mn 1-x P2O7、Na3Fe y Mn 2-y (PO4)P2O7, and Na 4-a Fe 2+(a / 2)-z Mn z At least one of (P2O7)2, wherein 0.2≤x≤0.8, 0.2≤y≤1.8, 2 / 3≤a≤7 / 8, 0.2≤z≤1.

8.

2. The sodium ion battery positive electrode material according to claim 1, characterized in that 0.4≤x≤0.6。 3. The sodium ion battery positive electrode material according to claim 1, characterized in that The fast ion conductor is Na a M b+ c O (a+bc) / 2 , a, b, c are positive integers, wherein M includes at least one of Zr, Al, V, and Si.

4. The sodium ion battery positive electrode material according to claim 1, characterized in that The mass of the fast ion conductor layer is 0.5% to 6% of the mass of the polyanion iron-manganese-based core material.

5. The sodium ion battery positive electrode material according to claim 1, characterized in that The mass of the fast ion conductor layer is 0.5% to 5% of the mass of the polyanion iron-manganese-based core material.

6. The sodium ion battery positive electrode material according to claim 1, characterized in that A carbon layer is also contained between the polyanion iron-manganese-based core material and the fast ion conductor layer; the mass of the carbon layer is 0.1% to 4% of the mass of the polyanion iron-manganese-based core material.

7. The sodium ion battery positive electrode material according to claim 1, characterized in that The manganese content in the center of the polyanionic iron-manganese-based core material is greater than the manganese content in the surface layer of the polyanionic iron-manganese-based core material, and the iron content in the surface layer of the polyanionic iron-manganese-based core material is greater than the iron content in the center of the polyanionic iron-manganese-based core material.

8. The method for preparing the positive electrode material for a sodium ion battery according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) dissolving a metal chelating agent in a solvent to obtain a solution A, dissolving an iron source in a solvent to obtain a solution B, and dissolving a manganese source in a solvent to obtain a solution C, and respectively dropping the solution B and the solution C into the solution A, wherein the solution C is added first and the solution B is added later; Then add phosphorus source, sodium source and carbon source; The obtained mixed solution is then evaporated to a gel state, dried, ground, calcined, and ground to obtain a polyanion iron-manganese-based core material precursor; (2) dispersing a fast ion conductor in a solvent to obtain a solution D, dispersing the polyanion iron-manganese-based core material precursor obtained in step (1) in a solvent to obtain a solution E, adding solution D dropwise to solution E, and then evaporating and drying the obtained mixed solution to obtain a fast ion conductor-coated polyanion iron-manganese-based core material precursor; (3) Grinding and calcining the fast ion conductor-coated polyanion iron-manganese-based core material precursor obtained in step (2) to obtain the sodium ion battery positive electrode material.

9. The method for preparing a positive electrode material for a sodium ion battery according to claim 8, characterized in that: In the step (1), the molar ratio of iron in the iron source to manganese in the manganese source is Fe:Mn=(1-4):(1-4).

10. The method for preparing a positive electrode material for a sodium ion battery according to claim 8, characterized in that: In the step (1), the metal chelating agent is a polymer compound composed of C, H, and O, and can chelate with iron and manganese; the mass of the metal chelating agent is 1% to 5% of the sum of the mass of the iron source and the manganese source.

11. The method for preparing a positive electrode material for a sodium ion battery according to claim 8, characterized in that: In the step (1), the dripping rate ratio of the solution B to the solution C is 1:(2-5).

12. The method for preparing a positive electrode material for a sodium ion battery according to claim 8, characterized in that: Satisfy at least one of the following: (1) In the step (1), the dripping rate ratio of the solution B to the solution C is 1:(2-3); (2) In step (1), the calcination temperature is 300° C. to 400° C. and the calcination time is 5 h to 8 h; (3) In the step (3), the calcination temperature is 500° C. to 650° C. and the calcination time is 9 h to 12 h; (4) In step (3), the calcination atmosphere is a mixed atmosphere of hydrogen and argon; (5) The sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium acetate, and sodium nitrate; (6) The iron source includes at least one of ferrous oxalate dihydrate, ferrous phosphate, and ferric nitrate nonahydrate; (7) The manganese source includes at least one of manganese chloride, manganese acetate and manganese nitrate; (8) The phosphorus source includes at least one of diammonium phosphate, diammonium hydrogen phosphate, and ammonium phosphate; (9) The carbon source includes at least one of glucose, sucrose, and citric acid; (10) The metal chelating agent includes at least one of polyvinyl alcohol and polymethacrylic acid.

13. The method for preparing a positive electrode material for a sodium ion battery according to claim 8, characterized in that: In the step (3), the calcination temperature is 500°C to 600°C.

14. A positive electrode slurry or positive electrode sheet for a sodium ion battery, characterized in that: The sodium ion battery positive electrode slurry or positive electrode sheet comprises the sodium ion battery positive electrode material according to any one of claims 1 to 7 or the sodium ion battery positive electrode material prepared by the preparation method according to any one of claims 8 to 13.

15. A sodium ion battery, characterized in that: The invention comprises a positive electrode plate, wherein the positive electrode plate comprises the positive electrode material for a sodium ion battery according to any one of claims 1 to 7 or the positive electrode material for a sodium ion battery prepared by the preparation method according to any one of claims 8 to 13.

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