Sodium manganese ferric pyrophosphate positive electrode material as well as preparation method and application thereof

Abstract

The preparation method comprises the following steps: mixing a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, water and a polyethylene glycol additive with the weight-average molecular weight of 5000-6000, and carrying out sanding, drying, sintering, crushing and other operations on the obtained mixed slurry to obtain the sodium ferromanganese pyrophosphate positive electrode material. The sodium ferromanganese pyrophosphate positive electrode material is obtained. The sodium ferromanganese pyrophosphate positive electrode material is prepared by adopting a wet ball milling-carbon thermal reduction method, and polyethylene glycol with the weight-average molecular weight of 5000-6000 is added as an additive in the preparation process, so that the prepared sodium ferromanganese pyrophosphate positive electrode material is nano spherical particles which are uniform in size distribution and have an excellent carbon conductive network structure. When the material is applied to a sodium-ion battery, the material has good electronic and ionic conductivity, and the electrochemical performance such as discharge capacity, platform voltage, rate capability and cycle performance can be greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and in particular, to a sodium iron manganese pyrophosphate phosphate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] The electrochemical performance of sodium-ion batteries mainly depends on the sodium storage performance of electrode materials. Generally, it is considered that the cathode material is one of the most critical materials affecting the energy density, cycle life, and safety of sodium-ion batteries. At present, the sodium-ion battery cathode materials that have been studied more mainly include three types: transition metal oxides, phosphates, Prussian blue and its analogs. Among them, the phosphate cathode material has excellent properties such as a strong three-dimensional network framework structure, a large sodium-ion diffusion channel, and a volume change of less than 4% during charge and discharge, making the phosphate cathode material very promising to become a sodium-ion battery cathode material with high rate and long life. For example, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 cathode material.

[0003] However, the existing phosphate cathode materials have problems such as poor cycle performance and rate performance, low reversible charge-discharge specific capacity, and low plateau voltage, which limit the further development of phosphate cathode materials in the field of sodium-ion batteries. In order to further improve the electrochemical performance of phosphate materials, so that they can not only have good electronic and ionic conductivity, but also have good cycle performance, rate performance, reversible charge-discharge specific capacity, and high plateau voltage and other comprehensive use performances, and at the same time can meet the requirements of simple preparation method and can realize industrial scale production, the present invention is specifically proposed. Summary of the Invention

[0004] The main object of the present invention is to provide a sodium iron manganese pyrophosphate phosphate cathode material, a preparation method thereof, and an application thereof, so as to solve the problem that the comprehensive performance of the sodium iron manganese pyrophosphate phosphate cathode material in the prior art is poor, and it cannot have both good electronic and ionic conductivity, and can also have good cycle performance, rate performance, reversible charge-discharge specific capacity, and high plateau voltage and other characteristics, and at the same time can meet the requirements of simple preparation method and easy to realize large-scale production.

[0005] To solve the above problems, the present invention provides a method for preparing a sodium iron manganese pyrophosphate cathode material, which includes the following steps: Step (1), mixing a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, water and an additive to obtain a mixed slurry; wherein, the additive is polyethylene glycol with a weight average molecular weight of 5000-6000; Step (2), subjecting the mixed slurry to sanding and drying to obtain a sodium iron manganese pyrophosphate precursor; Step (3), sintering and pulverizing the sodium iron manganese pyrophosphate precursor to obtain a sodium iron manganese pyrophosphate cathode material.

[0006] Further, the addition amount of the additive is 0.05-0.5% of the weight of the sodium source; preferably, the addition amount of the additive is 0.1-0.3% of the weight of the sodium source; preferably, the addition amount of the carbon source is 5-20% of the weight of the sodium source.

[0007] Further, the molar ratio of Na:Fe:Mn:P in the sodium source, the iron source, the manganese source and the phosphorus source is 4:x:(3-x):4, where 0 < x < 3; preferably, the range of x is 1 ≤ x < 3; more preferably, the range of x is 2 ≤ x < 3.

[0008] Further, the solid content of the mixed slurry is 20-60%; preferably, the solid content of the mixed slurry is 40-50%; preferably, the particle size distribution range of the sodium iron manganese pyrophosphate precursor is 10-1000 nm, and 100 ≤ D50 ≤ 300 nm; more preferably, the particle size distribution range of the sodium iron manganese pyrophosphate precursor is 50-500 nm, and 150 ≤ D50 ≤ 200 nm.

[0009] Further, the drying temperature is 60-120 °C; preferably, the drying time is 8-12 h.

[0010] Further, the sintering temperature is 400-600 °C; preferably, the sintering time is 4-15 h.

[0011] Further, the sintering is carried out in an atmosphere of a protective gas, and preferably the protective gas is at least one of argon, nitrogen, a mixture of argon and hydrogen, and a mixture of nitrogen and hydrogen.

[0012] Further, the carbon source is one or more of citric acid, glucose, sucrose, starch, graphite, carbon nanotubes, and cyclodextrin; preferably, the carbon source is one or more of citric acid, glucose, sucrose, starch, and cyclodextrin; preferably, the manganese source is one or more of manganese sulfate, manganese phosphate, manganese chloride, manganese oxalate, and manganese acetate; preferably, the iron source is one or more of iron phosphate, ferrous sulfate, ferrous chloride, ferrous oxalate, and ferrous acetate; preferably, the sodium source is one or more of sodium pyrophosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium oxalate, sodium bicarbonate, sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, sodium citrate, trisodium citrate dihydrate, trisodium citrate pentahydrate, and sodium gluconate; preferably, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, disodium hydrogen phosphate, sodium pyrophosphate, iron phosphate, and manganese phosphate.

[0013] According to the second aspect of the present invention, there is also provided a sodium manganese iron pyrophosphate cathode material, which is prepared by the above preparation method and includes a sodium manganese iron pyrophosphate material substrate and a carbon coating layer coated on the surface of the sodium manganese iron pyrophosphate material substrate. The chemical formula of the sodium manganese iron pyrophosphate cathode material is Na 4 Fe x Mn 3-x (PO 4 ) 2 P 2 O 7 / C, where 0 < x < 3, and the particle size is 200 ≤ D50 ≤ 300 nm; the carbon coating weight is 1 - 2%.

[0014] According to the third aspect of the present invention, there is also provided a sodium-ion battery, and the positive electrode sheet of the sodium-ion battery includes the above sodium manganese iron pyrophosphate cathode material.

[0015] The present invention mainly provides a sodium manganese iron pyrophosphate cathode material, its preparation method and application. The sodium manganese iron pyrophosphate cathode material is prepared by a wet ball milling - carbothermal reduction method, and polyethylene glycol with a weight average molecular weight of 5000 - 6000 is added as an additive during the preparation process. The addition of each component and the cooperation with the preparation method in the above preparation process make the prepared sodium manganese iron pyrophosphate cathode material be nano-spherical particles with uniform size distribution. The material structure has an excellent carbon conductive network structure, and when in use, it not only has good electron and ion conductivity but also can greatly improve the electrochemical properties such as discharge capacity, plateau voltage, rate performance, and cycle performance. In addition, the preparation method has a simple process and is suitable for industrial scale production. Description of the Drawings

[0016] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0017] Figure 1 The SEM image of the sodium iron manganese pyrophosphate cathode material obtained from the preparation of Example 1 of the present invention is shown;

[0018] Figure 2 The rate performance data graph when the sodium iron manganese pyrophosphate cathode materials obtained from the preparations of Examples 1 to 3 and Comparative Example 4 are applied to sodium ion batteries is shown;

[0019] Figure 3 The 0.2C rate first cycle charge-discharge curves when the sodium iron manganese pyrophosphate cathode materials obtained from the preparations of Examples 1 to 3 and Comparative Example 4 are applied to sodium ion batteries are shown;

[0020] Figure 4 It is the state after drying the mixed slurry during the process of preparing the sodium iron manganese pyrophosphate cathode material in Example 1;

[0021] Figure 5 It is the state after drying the mixed slurry during the process of preparing the sodium iron manganese pyrophosphate cathode material in Comparative Example 4;

[0022] Figure 6 It is the SEM-EDS elemental analysis cross-sectional view of the sodium iron manganese pyrophosphate cathode material obtained from the preparation of Comparative Example 5. Detailed embodiments

[0023] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0024] As described in the background art section, the phosphate cathode material has excellent characteristics such as a robust three-dimensional network framework structure, a large sodium ion diffusion channel, and small volume change during charge and discharge, and has great potential to become a cathode material for sodium ion batteries with high rate and long life. However, in the existing modification schemes for phosphate cathode materials, it is impossible to meet the requirements of having good electronic and ionic conductivity, good cycle performance, rate performance, reversible charge-discharge specific capacity, and high plateau voltage during use, which limits the further development of the sodium iron pyrophosphate material.

[0025] To solve the above problems, the present invention provides a method for preparing sodium iron manganese pyrophosphate cathode material, which comprises the following steps: Step (1), mixing a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, water and an additive to obtain a mixed slurry; wherein, the additive is polyethylene glycol with a weight average molecular weight of 5000-6000; Step (2), subjecting the mixed slurry to grinding and drying to obtain a sodium iron manganese pyrophosphate precursor; Step (3), sintering and pulverizing the sodium iron manganese pyrophosphate precursor to obtain the sodium iron manganese pyrophosphate cathode material.

[0026] Specifically, when preparing the sodium iron manganese pyrophosphate cathode material of the present invention, first mix a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source and an additive to obtain a mixed slurry, wherein the added additive is polyethylene glycol with a weight average molecular weight of 5000-6000; then subject the mixed slurry to grinding and drying to obtain a sodium iron manganese pyrophosphate precursor, and finally obtain the sodium iron manganese pyrophosphate cathode material by sintering and pulverizing the sodium iron manganese pyrophosphate precursor.

[0027] In the process of preparing the sodium iron manganese pyrophosphate cathode material of the present invention, in addition to adding a sodium source, an iron source, a manganese source and a phosphorus source, a carbon source is also added to achieve the purpose of further carbon coating the sodium iron manganese pyrophosphate material during the preparation process, and further improve the ionic and electronic conductivity of the sodium iron manganese pyrophosphate cathode material. However, due to the relatively complex raw material components in the preparation of battery cathode materials, the conductivity, cycle stability and charge capacity of the sodium iron manganese pyrophosphate cathode material are restricted by a series of heterogeneous structure transformations in the material, making the sodium iron manganese pyrophosphate cathode material unable to achieve the ideal effect during use. To solve the above problems, the present invention specifically prepares a sodium iron manganese pyrophosphate precursor in the preparation process, and then prepares the sodium iron manganese pyrophosphate material by operations such as sintering and pulverizing the sodium iron manganese pyrophosphate precursor. On the one hand, the carbon source can have a better coating effect on the sodium iron manganese pyrophosphate material, and on the other hand, the heterogeneous structure system of the obtained sodium iron manganese pyrophosphate precursor can be improved, making it have a higher tap density, which is more conducive to improving the comprehensive performance of the sodium iron manganese pyrophosphate cathode material.

[0028] More importantly, when preparing the above-mentioned sodium iron manganese pyrophosphate precursor according to the technical solution provided by the present invention, polyethylene glycol with a weight average molecular weight of 5000-6000 is also added as an additive. Adding polyethylene glycol when mixing the carbon source, sodium source, iron source, manganese source, and phosphorus source can better improve the heterogeneous structure of the sodium iron manganese pyrophosphate cathode material and enhance the compaction performance of the material. Moreover, through a large number of experimental explorations, the inventor found that adding polyethylene glycol with a weight average molecular weight in the range of 5000-6000 results in better electronic and ionic conductivity of the obtained sodium iron manganese pyrophosphate cathode material, and the performance of the corresponding sodium-ion battery in terms of cycle performance, rate performance, reversible charge-discharge specific capacity, and higher plateau voltage is also better. Adding polyethylene glycol with a weight average molecular weight of 5000-6000 when preparing the sodium iron manganese pyrophosphate cathode material of the present invention can further improve the comprehensive electrochemical performance of the obtained material.

[0029] The reasons mainly include the following aspects: First, in the process of grinding and mixing the mixed slurry in the present invention, in order to enable the carbon source to coat the sodium iron manganese pyrophosphate material more and more evenly, the overall carbon coating effect is improved by pursuing the nanosization of the particle size of the grinding particles during the grinding process. However, during the gradual refinement of the particle size of the grinding particles, the nanoscale particles therein will re-aggregate under the influence of van der Waals forces, double-layer electrostatic interactions, etc., resulting in the phenomena of "coarsening back" and "reverse grinding". As a non-ionic surfactant, polyethylene glycol has good solubility and compatibility. Adding polyethylene glycol with a weight-average molecular weight of 5000-6000 to the mixed slurry with acidic characteristics can utilize the effects of polyethylene glycol on the surface tension and interfacial tension in the system to affect the van der Waals forces, double-layer electrostatic forces, solvation membranes, and steric repulsion forces of the adsorption layer between the particles during the grinding process, thereby further dispersing the nanoscale particles in the system and achieving the purpose of improving the "coarsening back" problem in the system. Second, polyethylene glycol can also act together with the added carbon source to achieve the purpose of coating the sodium iron manganese pyrophosphate cathode material. Polyethylene glycol contains terminal hydroxyl groups. The presence of hydroxyl groups enables polyethylene glycol to better interact with the sodium source, iron source, manganese source, and phosphorus source in the mixed slurry during the grinding process through the hydrogen bonds formed by its own hydroxyl groups, and then cooperate with the carbon source added during the preparation process to achieve better coating of the material. The reason why choosing polyethylene glycol with a weight-average molecular weight of 5000-6000 in the technical solution for preparing the sodium iron manganese pyrophosphate cathode material in the present application has a better effect may be that the intermolecular and intramolecular hydrogen bond forces in polyethylene glycol with a weight-average molecular weight of 5000-6000 are more conducive to exerting its dispersing and coating effects in the preparation system described in the present invention, and synergistically promoting the performance of the sodium iron manganese pyrophosphate cathode material. Larger or smaller molecular weight polyethylene glycols cannot achieve the effects that can be achieved by the technical solution of the present invention due to their water solubility, solubility, and dispersing effects not being suitable for the components, addition amounts, particle sizes, etc. in the system.

[0030] In summary, when preparing the sodium iron manganese pyrophosphate cathode material, the technical solution provided by the present invention, by adding polyethylene glycol with a weight-average molecular weight of 5000-6000 as an additive, can not only make the obtained material more evenly dispersed and the carbon coating more sufficient, but also make the obtained sodium iron manganese pyrophosphate cathode material have better electrochemical properties such as electron and ion conductivity, cycle performance, rate performance, and reversible charge-discharge specific capacity during use. At the same time, this method is simple to operate and can also meet the requirements of large-scale industrial production.

[0031] In a preferred embodiment, the addition amount of the additive is 0.05-0.5% of the weight of the sodium source, specifically, for example, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, or any value between any of the above percentages. Polyethylene glycol plays an important role in improving the dispersibility in the system and the carbon coating effect of the material. Therefore, controlling the addition amount of the polyethylene glycol additive within the above range can further improve the comprehensive performance of the sodium iron manganese pyrophosphate cathode material. Preferably, the addition amount of the additive is 0.1-0.3% of the weight of the sodium source, specifically, for example, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, 0.3%, or any value between any of the above percentages. Controlling the content of polyethylene glycol in the system within the above preferred range has a better effect. Preferably, the addition amount of the carbon source is 5-20% of the weight of the sodium source, specifically, for example, 5%, 8%, 10%, 12%, 15%, 18%, 20%, or any value between any of the above percentages. Controlling the addition amount of the carbon source within the above range can enable the added carbon source to better cooperate with the added polyethylene glycol additive, which is beneficial to further improving the dispersibility among various components, improving the "coarsening" problem in the system, and can also better carbon coat the sodium iron manganese pyrophosphate cathode material, achieving the purpose of further improving the comprehensive performance of the material.

[0032] In a preferred embodiment, the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source and phosphorus source is 4:x:(3-x):4, where 0 < x < 3. Controlling the addition amounts of the sodium source, iron source, manganese source and phosphorus source within the above range during preparation will have a beneficial effect on improving the comprehensive performance of the sodium iron manganese pyrophosphate cathode material. Preferably, the range of x is 1 ≤ x < 3; controlling the addition ratio of the above components within the above range is beneficial to further improving the comprehensive performance of the sodium iron manganese pyrophosphate cathode material. More preferably, the range of x is 2 ≤ x < 3, and controlling the addition ratio of the above components within the above range has a better effect.

[0033] In a preferred embodiment, the solid content of the mixed slurry is 20-60%; controlling the solid content of the slurry within the above range during mixing can make the slurry system mix more uniformly. Preferably, the solid content of the mixed slurry is 40-50%. Preferably, the particle size distribution range of the sodium iron manganese pyrophosphate precursor is 10-1000 nm, and it satisfies 100 ≤ D50 ≤ 300 nm; the milling process is not only to mix the above materials more uniformly, but also to make the carbon source better coat the materials in the system. Controlling the milling time and the particle size within the above range during milling can make the coating effect better, and each component in the system is more uniformly dispersed, which is beneficial to further improving the comprehensive performance of the sodium iron manganese pyrophosphate cathode material. More preferably, the particle size distribution range of the sodium iron manganese pyrophosphate precursor is 50-500 nm, and it satisfies 150 ≤ D50 ≤ 200 nm. Controlling the particle size distribution range of the sodium iron manganese pyrophosphate precursor within the preferred range can achieve better above effects.

[0034] In a preferred embodiment, the drying temperature is 60-120 °C and the drying time is 8-12 h. In the preparation process of the present invention, the mixed slurry after milling is dried to remove the solvent therein. Compared with the way of drying the slurry by spray drying, which will cause uneven distribution of elements on the surface and inside of the obtained spherical particles, the present invention uses drying as the way to remove the solvent, so that the particle size of the obtained nanoscale particles can still maintain uniform element distribution on the particle surface and inside the particles after removing the solvent, which is beneficial to further improving the performance of the sodium iron manganese pyrophosphate cathode material. Controlling the drying temperature within the above range can further avoid the carbonization precipitation phenomenon of the carbon source in the precursor material due to too high drying temperature, which is beneficial to further improving the electrochemical performance of the sodium iron manganese pyrophosphate cathode material.

[0035] In a preferred embodiment, the sintering temperature is 400-600 °C, the sintering time is 4-15 h, and the sintering process is carried out in a protective gas atmosphere. Preferably, the protective gas is at least one of argon, nitrogen, a mixture of argon and hydrogen, and a mixture of nitrogen and hydrogen. Sintering in the above inert gas atmosphere can prevent the metal elements from being further oxidized, which is beneficial to further improving the electrochemical performance of the material. In addition, controlling the sintering temperature and sintering time within the above range can make the performance of the obtained sodium iron manganese pyrophosphate cathode material better. Among them, when using a mixture of argon and hydrogen or a mixture of nitrogen and hydrogen, the hydrogen can be in any proportion of the mixed gas.

[0036] In a preferred embodiment, the carbon source is one or more of citric acid, glucose, sucrose, starch, graphite, carbon nanotubes, and cyclodextrin; preferably, the carbon source is one or more of citric acid, glucose, sucrose, starch, and cyclodextrin. In the technical solution of the present invention, the polyethylene glycol additive and the carbon source act together to achieve the carbon coating process of the sodium iron manganese pyrophosphate cathode material. The inventors have found through a large number of experiments that the cooperation effect of carbon sources within the preferred range, such as citric acid, glucose, sucrose, and starch, and polyethylene glycol with a weight average molecular weight of 5000 - 6000 is better. This is because the carbon source types within the preferred range are all macromolecular compound types of carbon sources, which are more conducive to synergistic effects with the polyethylene glycol with a weight average molecular weight of 5000 - 6000 provided in the solution of the present invention, thereby making the obtained sodium iron manganese pyrophosphate cathode material have better effects. In particular, citric acid among them is an organic acid and has good reducibility; glucose and sucrose have better coating effects; cyclodextrin has a special network structure, which can provide a conductive network and improve the electronic conductivity of the sodium iron manganese pyrophosphate cathode material. Selecting a carbon source within the preferred range is beneficial to further improving the electrochemical performance of the sodium iron manganese pyrophosphate cathode material.

[0037] By way of example but not limitation, the manganese source is one or more of manganese sulfate, manganese phosphate, manganese chloride, manganese oxalate, and manganese acetate; preferably, the iron source is one or more of iron phosphate, ferrous sulfate, ferrous chloride, ferrous oxalate, and ferrous acetate; preferably, the sodium source is one or more of sodium pyrophosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium oxalate, sodium bicarbonate, sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, sodium citrate, trisodium citrate dihydrate, trisodium citrate pentahydrate, and sodium gluconate; preferably, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, disodium hydrogen phosphate, sodium pyrophosphate, iron phosphate, and manganese phosphate. The selection of the sodium source, iron source, manganese source, and phosphorus source is not limited, and using the above specific types will make the comprehensive performance of the prepared sodium iron manganese pyrophosphate cathode material better.

[0038] According to another aspect of the present invention, there is provided a sodium iron manganese pyrophosphate cathode material, which is prepared by the above preparation method. The sodium iron manganese pyrophosphate cathode material includes a sodium iron manganese pyrophosphate material substrate and a carbon coating layer coated on the surface of the sodium iron manganese pyrophosphate material substrate. The chemical formula of the sodium iron manganese pyrophosphate cathode material is Na 4 Fe x Mn 3-x (PO 4 ) 2 P 2 O 7 / C, 0 < x < 3, particle size of 200 ≤ D50 ≤ 300 nm; the carbon coating amount is 1 - 2%.

[0039] It should be specifically noted here that due to the particularity of the material field and the limitations of existing testing and characterization methods, it is impossible to comprehensively characterize the materials obtained by the above preparation method. However, it has been experimentally confirmed that the sodium iron manganese pyrophosphate cathode material can significantly improve the electrochemical properties such as discharge capacity, plateau voltage, rate performance, and cycling performance, indicating the improvement of the preparation method of the present invention for the material itself.

[0040] According to another aspect of the present invention, a sodium ion battery is also provided. The positive electrode sheet of the sodium ion battery includes the above-mentioned sodium iron manganese pyrophosphate cathode material.

[0041] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0042] Example 1

[0043] A sodium iron manganese pyrophosphate cathode material, its molecular formula is: Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O 7 / C, its carbon content is 1.8%, the particle size distribution range is 50 - 500 nm, and its particle size satisfies 150 ≤ D50 ≤ 200 nm. Among them, the sodium source used for preparing the sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is iron phosphate, the manganese source is manganese phosphate, the phosphorus source is phosphoric acid, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source, and phosphorus source is 4:2:1:4. The addition amount of the carbon source is 8% of the weight of the sodium source, and the addition amount of the additive is 0.05% of the weight of sodium oxalate.

[0044] The preparation method of the above-mentioned sodium iron manganese pyrophosphate cathode material is as follows:

[0045] Mix a sodium source, an iron source, a manganese source, a phosphorus source, water, and an additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing. The solid content in the mixed slurry is 40%; transfer the above mixed slurry into a nanoscale sand mill and grind it until the D50 of the particle size reaches 200 nm, then transfer it to an oven and dry it at 90 °C for 10 h to obtain a sodium iron manganese pyrophosphate precursor; sinter the obtained sodium iron manganese pyrophosphate precursor in a nitrogen atmosphere at 530 °C for 10 h, and crush the sintered material to obtain a sodium iron manganese pyrophosphate cathode material. Among them, the state of the mixed slurry after drying in the preparation process of the sodium iron manganese pyrophosphate cathode material is as Figure 4 shown.

[0046] Perform SEM testing on the prepared sodium iron manganese pyrophosphate cathode material, and the results are shown in Figure 1 shown. It can be seen from the figure that the sodium iron manganese pyrophosphate cathode material prepared by the present invention has a uniform structure, round particles and a small particle size.

[0047] Example 2

[0048] A sodium iron manganese pyrophosphate cathode material, whose molecular formula is: Na 4 FeMn 2 (PO 4 ) 2 P 2 O 7 / C, its carbon content is 1.8%, the particle size distribution range is 50 - 500 nm, and its particle size satisfies 150 ≤ D50 ≤ 200 nm. Among them, the sodium source used in the preparation of the sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is iron phosphate, the manganese source is manganese chloride, the phosphorus source is phosphoric acid, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source, and phosphorus source is 4:1:2:4. The addition amount of the carbon source is 8% of the weight of the sodium source, and the addition amount of the additive is 0.5% of the weight of the sodium oxalate.

[0049] The preparation method of the above sodium iron manganese pyrophosphate cathode material is as follows:

[0050] Mix a sodium source, an iron source, a manganese source, a phosphorus source, water, and an additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing. The solid content in the mixed slurry is 40%; transfer the above mixed slurry into a nanoscale sand mill and grind it until the D50 of the particle size reaches 200 nm, then transfer it to an oven and dry it at 90 °C for 10 h to obtain a sodium iron manganese pyrophosphate precursor; sinter the obtained sodium iron manganese pyrophosphate precursor in a nitrogen atmosphere at 530 °C for 10 h, and crush the sintered material to obtain a sodium iron manganese pyrophosphate cathode material.

[0051] Example 3

[0052] A sodium iron manganese pyrophosphate cathode material, with the molecular formula: Na 4 Fe 2.8 Mn 0.2 (PO 4 ) 2 P 2 O 7 / C, with a carbon content of 1.8%, a particle size distribution range of 50 - 500 nm, and its particle size satisfying 150 ≤ D50 ≤ 200 nm. Among them, the sodium source used in preparing this sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is ferrous sulfate, the manganese source is manganese phosphate, the phosphorus source is phosphoric acid, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source and phosphorus source is 4:2.8:0.2:4. The addition amount of the carbon source is 8% of the weight of the sodium source, and the addition amount of the additive is 0.05% of the weight of sodium oxalate.

[0053] The preparation method of the above sodium iron manganese pyrophosphate cathode material is as follows:

[0054] Mix the sodium source, iron source, manganese source, phosphorus source, water and additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing, where the solid content in the mixed slurry is 40%; when the D50 of the particle size of the above mixed slurry is ground to 200 nm in a nanoscale sand mill, transfer it to an oven and dry it at 90 °C for 10 h to obtain a sodium iron manganese pyrophosphate precursor; sinter the obtained sodium iron manganese pyrophosphate precursor in a nitrogen atmosphere at 530 °C for 10 h, and crush the sintered material to obtain the sodium iron manganese pyrophosphate cathode material.

[0055] Example 4

[0056] A sodium iron manganese pyrophosphate cathode material, with the molecular formula: Na 4 Fe 0.8 Mn 2.2 (PO 4 ) 2 P 2 O 7 / C, with a carbon content of 1.8%, a particle size distribution range of 50 - 500 nm, and its particle size satisfying 150 ≤ D50 ≤ 200 nm. Among them, the sodium source used in preparing the sodium manganese iron pyrophosphate cathode material is sodium carbonate, the iron source is iron phosphate, the manganese source is manganese phosphate, the phosphorus source is phosphoric acid, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source, and phosphorus source is 4:0.8:2.2:4. The addition amount of the carbon source is 8% of the weight of the sodium source, and the addition amount of the additive is 0.05% of the weight of sodium carbonate.

[0057] The preparation method of the above sodium manganese iron pyrophosphate cathode material is as follows:

[0058] Mix the sodium source, iron source, manganese source, phosphorus source, water, and additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing, where the solid content in the mixed slurry is 40%; transfer the above mixed slurry to a nanoscale sand mill and grind it until the D50 of the particle size is 200 nm, then transfer it to an oven and dry it at 90°C for 10 h to obtain a sodium manganese iron pyrophosphate precursor; sinter the obtained sodium manganese iron pyrophosphate precursor in a nitrogen atmosphere at 530°C for 10 h, and pulverize the sintered material to obtain the sodium manganese iron pyrophosphate cathode material.

[0059] Example 5

[0060] The difference from Example 1 is that the addition amount of the additive (polyethylene glycol with a weight average molecular weight of 5000) is 0.1% of the weight of the sodium source.

[0061] Example 6

[0062] The difference from Example 1 is that the addition amount of the additive (polyethylene glycol with a weight average molecular weight of 5000) is 0.3% of the weight of the sodium source.

[0063] Example 7

[0064] The difference from Example 1 is that the carbon source used in preparing the sodium manganese iron pyrophosphate cathode material is citric acid.

[0065] Example 8

[0066] The difference from Example 1 is that the carbon source used in preparing the sodium manganese iron pyrophosphate cathode material is sucrose.

[0067] Example 9

[0068] The difference from Example 1 is that the carbon source used in preparing the sodium manganese iron pyrophosphate cathode material is starch.

[0069] Example 10

[0070] The difference from Example 1 is that the carbon source used in preparing the sodium iron manganese pyrophosphate cathode material is cyclodextrin.

[0071] Example 11

[0072] The difference from Example 1 is that the carbon source used in preparing the sodium iron manganese pyrophosphate cathode material is graphite.

[0073] Example 12

[0074] The difference from Example 1 is that the carbon source used in preparing the sodium iron manganese pyrophosphate cathode material is carbon nanotubes.

[0075] Example 13

[0076] The difference from Example 3 is that when preparing the sodium iron manganese pyrophosphate cathode material, the solid content in the mixed slurry is 50%.

[0077] Example 14

[0078] The difference from Example 3 is that when preparing the sodium iron manganese pyrophosphate cathode material, the solid content in the mixed slurry is 20%.

[0079] Example 15

[0080] The difference from Example 3 is that when preparing the sodium iron manganese pyrophosphate cathode material, the solid content in the mixed slurry is 60%.

[0081] Example 16

[0082] A sodium iron manganese pyrophosphate cathode material, whose molecular formula is: Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O 7 / C, its carbon content is 1%, the particle size distribution range is 10 - 1000 nm, and its particle size satisfies 100 ≤ D50 ≤ 300 nm. Among them, the sodium source used in preparing this sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is iron phosphate, the manganese source is manganese phosphate, the phosphorus source is ammonium dihydrogen phosphate, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source and phosphorus source is 4:2:1:4. The addition amount of the carbon source is 5% of the weight of the sodium source, and the addition amount of the additive is 0.05% of the weight of sodium oxalate.

[0083] The preparation method of the above-mentioned sodium iron manganese pyrophosphate cathode material is as follows:

[0084] Mix a sodium source, an iron source, a manganese source, a phosphorus source, water and an additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing, wherein the solid content in the mixed slurry is 40%; transfer the above mixed slurry to a nanoscale sand mill and grind it until the D50 of the particle size is 200 nm, then transfer it to an oven and dry it at 120 °C for 8 h to obtain a sodium iron manganese pyrophosphate precursor; sinter the above obtained sodium iron manganese pyrophosphate precursor in an argon atmosphere at 400 °C for 15 h, and crush the sintered material to obtain a sodium iron manganese pyrophosphate cathode material.

[0085] Example 17

[0086] A sodium iron manganese pyrophosphate cathode material, whose molecular formula is: Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O 7 / C, its carbon content is 2%, the particle size distribution range is 10 - 1000 nm, and its particle size satisfies 100 ≤ D50 ≤ 300 nm. Among them, the sodium source used in preparing this sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is iron phosphate, the manganese source is manganese phosphate, the phosphorus source is phosphoric acid, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source and phosphorus source is 4:2:1:4, the addition amount of the carbon source is 20% of the weight of the sodium source, and the addition amount of the additive is 0.05% of the weight of sodium oxalate.

[0087] The preparation method of the above sodium iron manganese pyrophosphate cathode material is as follows:

[0088] Mix a sodium source, an iron source, a manganese source, a phosphorus source, water and an additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing, wherein the solid content in the mixed slurry is 40%; transfer the above mixed slurry to a nanoscale sand mill and grind it until the D50 of the particle size is 200 nm, then transfer it to an oven and dry it at 60 °C for 12 h to obtain a sodium iron manganese pyrophosphate precursor; sinter the above obtained sodium iron manganese pyrophosphate precursor in an argon and hydrogen (v:v = 10:1) atmosphere at 600 °C for 4 h, and crush the sintered material to obtain a sodium iron manganese pyrophosphate cathode material.

[0089] Example 18

[0090] A sodium iron manganese pyrophosphate cathode material, whose molecular formula is: Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O7 / C, with a carbon content of 0.8%, a particle size distribution range of 50 - 500 nm, and its particle size satisfying 150 ≤ D50 ≤ 200 nm. Among them, the sodium source used in preparing the sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is iron phosphate, the manganese source is manganese phosphate, the phosphorus source is phosphoric acid, the carbon source is glucose, the additive is polyethylene glycol with a weight average molecular weight of 5000, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source, and phosphorus source is 4:2:1:4. The addition amount of the carbon source is 3% of the weight of the sodium source, and the addition amount of the additive is 1% of the weight of sodium oxalate.

[0091] The preparation method of the above sodium iron manganese pyrophosphate cathode material is as follows:

[0092] Mix the sodium source, iron source, manganese source, phosphorus source, water, and additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing, where the solid content in the mixed slurry is 20%; when the D50 of the particle size of the above mixed slurry is ground to 200 nm in a nanoscale sand mill, transfer it to an oven and dry it at 130 °C for 20 h to obtain a sodium iron manganese pyrophosphate precursor; sinter the obtained sodium iron manganese pyrophosphate precursor in a nitrogen atmosphere at 610 °C for 15 h, and crush the sintered material to obtain the sodium iron manganese pyrophosphate cathode material.

[0093] Comparative Example 1

[0094] The difference from Example 1 is that when preparing the sodium iron manganese pyrophosphate cathode material, the additive used is polyethylene glycol with a weight average molecular weight of 3000.

[0095] Comparative Example 2

[0096] The difference from Example 1 is that when preparing the sodium iron manganese pyrophosphate cathode material, the additive used is polyethylene glycol with a weight average molecular weight of 400.

[0097] Comparative Example 3

[0098] The difference from Example 1 is that when preparing the sodium iron manganese pyrophosphate cathode material, the additive used is polyethylene glycol with a weight average molecular weight of 8000.

[0099] Comparative Example 4

[0100] The difference from Example 1 is that no additive is added when preparing the sodium iron manganese pyrophosphate cathode material. Among them, the state of the mixed slurry after drying in the preparation process of the sodium iron manganese pyrophosphate cathode material is as Figure 5 shown.

[0101] Comparative Example 5

[0102] Instead of using the wet ball milling-carbothermal reduction method described in the present invention to prepare the sodium iron manganese pyrophosphate cathode material, the spray drying method is used to prepare the sodium iron manganese pyrophosphate cathode material.

[0103] A sodium iron manganese pyrophosphate cathode material, whose molecular formula is: Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O 7 / C, its carbon content is 1.8%, the particle size distribution range is 50 - 500 nm, and its particle size satisfies 150 ≤ D50 ≤ 200 nm. Among them, the sodium source used in preparing this sodium iron manganese pyrophosphate cathode material is sodium oxalate, the iron source is iron phosphate, the manganese source is manganese phosphate, the phosphorus source is phosphoric acid, the carbon source is glucose, and the molar ratio of Na:Fe:Mn:P in the sodium source, iron source, manganese source and phosphorus source is 4:2:1:4, and the addition amount of the carbon source is 8% of the weight of the sodium source.

[0104] The preparation method of the above sodium iron manganese pyrophosphate cathode material is as follows:

[0105] Mix the sodium source, iron source, manganese source, phosphorus source, water and additive, place them in a vertical stirrer for mixing, and obtain a mixed slurry after uniform mixing, where the solid content in the mixed slurry is 40%; transfer the above mixed slurry into a nanoscale sand mill and grind it until D50 is 200 nm, then use a spray dryer for spray granulation, with an inlet temperature of 230 °C and an outlet temperature of 110 °C, to obtain a sodium iron manganese pyrophosphate precursor; sinter the obtained sodium iron manganese pyrophosphate precursor in a nitrogen atmosphere at 530 °C for 10 h, and crush the sintered material to obtain the sodium iron manganese pyrophosphate cathode material.

[0106] Perform SEM-EDS tests on the prepared sodium iron manganese pyrophosphate cathode material, and the results are shown in Figure 6 As shown. From the SEM-EDS element analysis cross-sectional diagram in Figure 6 , it can be seen that the particle element distribution in the obtained sodium iron manganese pyrophosphate cathode material is uneven, and the sodium content inside the material is relatively low. This may be because during the spray drying process, it is mainly a process of water diffusing to the outside of the particles and solutes diffusing to the inside. Affected by the difference in the diffusion ability of the iron source and the sodium source, the uneven distribution of sodium element content in the material occurs.

[0107] Perform relevant performance tests on the sodium iron manganese pyrophosphate cathode materials prepared in Examples 1 to 18 and Comparative Examples 1 to 5, make them into battery positive electrode sheets and apply them to batteries, test their relevant performance, and record the results in Table 1. Among them, the preparation method of the sodium ion battery is as follows:

[0108] Preparation of the positive electrode of the battery: Conductive carbon black (SP) is used as the conductive agent, and polyvinylidene fluoride (PVDF) is used as the binder. The current collector of the electrode is carbon-coated aluminum foil. By weight percentage, 80% of the above-mentioned sodium manganese iron pyrophosphate positive electrode material, 10% of SP, 10% of PVDF, and N-methylpyrrolidone are stirred to form a slurry (the weight is accurate to one-thousandth). The slurry is evenly coated on the carbon-coated aluminum foil and dried in an oven at 120°C. Strictly control the mixing and coating process. The coating thickness of the measured electrode should be uniform. Cut it into a positive electrode with a diameter of ф12mm and a thickness of 14.5μm.

[0109] Preparation of the sodium-ion battery: In an inert gas glove box with the content of water and oxygen less than or equal to 0.0005%, a sodium metal sheet is used as the negative electrode material, glass fiber is used as the separator, and 1mol / L NaClO 4 / EC+DEC (where the mass ratio of EC and DEC is 1:1, EC is ethylene carbonate, and DEC is diethyl carbonate) is used as the electrolyte. Assemble them into a test battery. After the battery is sealed, a sodium-ion battery is obtained.

[0110] The testing process of the sodium-ion battery is as follows: At the start of the test (the test time changes by 1s): First step: Stand still for 1s → Charge at a rate of 0.2C until the voltage reaches 4V → Charge at a constant voltage of 4V until the current rate is 0.05C → Stand still for 1s → Discharge at a rate of 0.2C until the voltage reaches 1.5V, and repeat the above cycle 3 times; Second step: Stand still for 1s → Charge at a rate of 0.5C until the voltage reaches 4V → Charge at a constant voltage of 4V until the current rate is 0.05C → Stand still for 1s → Discharge at a rate of 0.5C until the voltage reaches 1.5V, and repeat the above cycle 3 times; Third step: Stand still for 1s → Charge at a rate of 1C until the voltage reaches 4V → Charge at a constant voltage of 4V until the current rate is 0.05C → Stand still for 1s → Discharge at a rate of 1C until the voltage reaches 1.5V, and repeat the above cycle 3 times; Fourth step: Stand still for 1s → Charge at a rate of 0.2C until the voltage reaches 4V → Charge at a constant voltage of 4V until the current rate is 0.05C → Stand still for 1s → Discharge at a rate of 0.2C until the voltage reaches 1.5V, and repeat the above cycle 3 times; The test ends.

[0111] Test conditions for powder resistivity: Pressure 200MPa, pressure holding time 10s.

[0112] Table 1

[0113]

[0114]

[0115] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0116] Examples 1 to 18 are for preparing the sodium manganese iron phosphate cathode material using the method provided by the present invention. After using the prepared sodium manganese iron phosphate cathode material in a sodium-ion battery, the electrochemical properties such as the conductivity, rate performance, and average voltage of the sodium-ion battery are all improved. In particular, when controlling each parameter in the preparation process of the sodium manganese iron phosphate cathode material within the preferred range, the comprehensive performance of the corresponding sodium-ion battery is better.

[0117] In Comparative Examples 1 to 3, if the molecular weight of the polyethylene glycol additive added during the preparation process is too large or too small, it will have an adverse effect on the structure of the prepared sodium manganese iron phosphate cathode material, and the electrochemical performance parameters of the corresponding sodium-ion battery will also be worse compared to the sodium-ion batteries in the examples of the present invention. In Comparative Example 4, no polyethylene glycol additive was added during the preparation of the sodium manganese iron phosphate cathode material. Comparative Example 5 is for preparing the sodium manganese iron pyrophosphate cathode material by spray drying method, and the comprehensive performance of its corresponding sodium-ion battery is even worse.

[0118] To further illustrate the technical effects of the present invention, the test performances of the sodium-ion batteries corresponding to the sodium manganese iron pyrophosphate cathode materials in Examples 1 to 3 and Comparative Example 4 are plotted in a graph. Among them, Figure 2 shows the rate performance data when the sodium manganese iron pyrophosphate cathode materials prepared in Examples 1 to 3 and Comparative Example 4 are applied to the battery; as can be seen from the graph, the rate performance of the sodium-ion batteries corresponding to Examples 1 to 3 is better, and there is a large gap between Comparative Example 4 and Examples 1 to 3. It can be seen that by adding polyethylene glycol with a suitable molecular weight during the preparation process of the sodium manganese iron pyrophosphate cathode material, the uniformity of material particles and carbon coating can be improved, which is beneficial to improving the rate performance of the sodium manganese iron pyrophosphate cathode material. Figure 3 shows the first-cycle charge-discharge curves of the sodium-ion batteries corresponding to the sodium manganese iron pyrophosphate cathode materials prepared in Examples 1 to 3 and Comparative Example 4; as can be seen from the graph, during the preparation process of the sodium manganese iron pyrophosphate cathode material in Examples 1 to 3, the addition of a polyethylene glycol additive with a suitable molecular weight can effectively increase the capacity of the sodium manganese iron pyrophosphate cathode material. Figure 4 and Figure 5 respectively show the states of the mixed slurry after drying during the preparation process of the sodium manganese iron pyrophosphate cathode material in Example 1 and Comparative Example 4; as can be seen from the graph, when adding an appropriate amount of polyethylene glycol additive during the preparation process of the sodium manganese iron pyrophosphate cathode material, the mixed slurry is not easily broken after drying and has better integrity, indicating that the addition of the polyethylene glycol additive is beneficial to improving the precipitation phenomenon of the slurry and making the components in the mixed slurry mix better.

[0119] In summary, when the sodium manganese iron pyrophosphate cathode material prepared by using the method for preparing the sodium manganese iron pyrophosphate cathode material proposed in the present invention is used in a sodium-ion battery, it can effectively improve the electronic and ionic conductivity of the sodium-ion battery, and can also greatly improve the electrochemical properties such as discharge capacity, average voltage, rate performance, and cycle performance. In addition, the preparation method has a simple process and is suitable for industrial scale production.

[0120] The foregoing are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing sodium ferromanganese pyrophosphate positive electrode material, characterized in that: The preparation method comprises the following steps: Step (1), mixing a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, water and an additive to obtain a mixed slurry; wherein the additive is polyethylene glycol having a weight average molecular weight of 5000 to 6000; Step (2), sand-milling and drying the mixed slurry to obtain a sodium manganese iron pyrophosphate precursor; Step (3), sintering and crushing the sodium manganese iron pyrophosphate precursor to obtain the sodium manganese iron pyrophosphate positive electrode material.

2. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to claim 1, characterized in that: The additive is added in an amount of 0.05 to 0.5% by weight of the sodium source; Preferably, the additive is added in an amount of 0.1 to 0.3% by weight of the sodium source; Preferably, the amount of the carbon source added is 5-20% by weight of the sodium source.

3. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to claim 1, characterized in that: The molar ratio of Na:Fe:Mn:P in the sodium source, the iron source, the manganese source and the phosphorus source is 4:x:(3-x):4, ​​wherein 0<x<3; Preferably, the range of x is 1≤x<3; More preferably, the range of x is 2≤x<3.

4. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The solid content of the mixed slurry is 20-60%; Preferably, the solid content of the mixed slurry is 40-50%; Preferably, the particle size distribution range of the sodium manganese iron pyrophosphate precursor is 10 to 1000 nm, and 100≤D50≤300 nm; More preferably, the particle size distribution range of the sodium manganese iron pyrophosphate precursor is 50-500 nm, and 150≤D50≤200 nm.

5. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The drying temperature is 60-120°C; Preferably, the drying time is 8 to 12 hours.

6. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The sintering temperature is 400-600°C; Preferably, the sintering time is 4 to 15 hours.

7. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The sintering is carried out in a protective gas atmosphere, and the protective gas is preferably at least one of argon, nitrogen, a mixture of argon and hydrogen, and a mixture of nitrogen and hydrogen.

8. The method for preparing the sodium ferromanganese pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The carbon source is one or more of citric acid, glucose, sucrose, starch, graphite, carbon nanotubes and cyclodextrin; Preferably, the carbon source is one or more of citric acid, glucose, sucrose, starch and cyclodextrin; Preferably, the manganese source is one or more of manganese sulfate, manganese phosphate, manganese chloride, manganese oxalate and manganese acetate; Preferably, the iron source is one or more of ferrous phosphate, ferrous sulfate, ferrous chloride, ferrous oxalate and ferrous acetate; Preferably, the sodium source is one or more of sodium pyrophosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium oxalate, sodium bicarbonate, sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, sodium citrate, sodium citrate dihydrate, sodium citrate pentahydrate and sodium gluconate; Preferably, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, disodium hydrogen phosphate, sodium pyrophosphate, iron phosphate and manganese phosphate.

9. A sodium manganese iron pyrophosphate positive electrode material, characterized in that: The sodium manganese iron pyrophosphate positive electrode material is prepared by the preparation method according to any one of claims 1 to 8, wherein the sodium manganese iron pyrophosphate positive electrode material comprises a sodium manganese iron pyrophosphate material base and a carbon coating layer coated on the surface of the sodium manganese iron pyrophosphate material base, and the chemical formula of the sodium manganese iron pyrophosphate positive electrode material is Na4Fe x Mn 3-x (PO4)2P2O7 / C, 0<x<3, particle size is 200≤D50≤300nm; the carbon coating weight is 1-2%.

10. A sodium ion battery, characterized in that: The positive electrode plate of the sodium ion battery comprises the sodium iron manganese pyrophosphate positive electrode material according to claim 9.

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