A preparation method and application of sodium iron pyrophosphate cathode material

Abstract

This application provides a method for preparing and applying sodium iron pyrophosphate cathode material, belonging to the field of sodium-ion battery technology. A carbon source, sodium source, phosphorus source, and mixed iron source are sequentially added to deionized water and stirred to obtain a suspension. The suspension is then milled in a sand mill and spray-dried to obtain a precursor powder. The precursor powder is sintered in an inert atmosphere to obtain the sodium iron pyrophosphate cathode material. This preparation method effectively alleviates the problems associated with using a single iron source in production, improves production efficiency, and allows for flexible adjustment of the proportions to reduce costs and meet the needs of different customers. This process route enhances the ability to cope with raw material supply chain risks and has minimal impact on existing manufacturing processes.

Description

Technical Field

[0001] This application relates to a method for preparing sodium iron pyrophosphate cathode material and its application, belonging to the field of sodium-ion battery technology. Background Technology

[0002] Sodium-ion batteries and lithium-ion batteries work on similar principles; both are intercalation-deintercalation secondary batteries, relying on sodium... + Or Li + Sodium-ion batteries complete charging and discharging by moving between the positive and negative electrodes. Their manufacturing process highly overlaps with that of lithium-ion batteries, and they have a solid foundation for industrialization. Compared with lithium-ion batteries, sodium-ion batteries have advantages such as better safety, lower material cost, wider operating temperature range, excellent high and low temperature performance, and better rate performance.

[0003] Among them, the mainstream cathode materials for sodium-ion batteries include three types: layered oxide materials, Prussian blue-based materials, and polyanionic materials. Layered oxides suffer from irreversible phase transitions leading to battery cycle performance degradation and are susceptible to moisture absorption from the air, affecting electrochemical performance. While Prussian blue-based compounds have low manufacturing costs, they suffer from water of crystallization issues, leading to battery cycle performance and safety problems. Polyanionic materials have an olivine structure similar to lithium iron phosphate, exhibiting not only high structural stability and the longest theoretical cycle life, but also advantages such as low manufacturing costs, environmental friendliness, and high theoretical capacity.

[0004] In the preparation of polyanionic materials using several common iron source processes:

[0005] (1) When FePO4 is used as the iron source, the manufacturing process of this route is simple, the raw materials are easy to mix evenly, and the final carbon coating effect is good. However, the unit price of FePO4 raw material is high, and FePO4 accounts for as much as 70% of the raw material cost, which makes the material cost of this process route high.

[0006] (2) When FeC2O4 is used as the iron source, the iron ion in FeC2O4 has a +2 oxidation state, and no carbon source is required for high-temperature solid-phase carbothermic reduction. The raw material system requires less carbon source and the carbon content is reduced, which can increase the material's specific capacity. Correspondingly, the sugar content of the raw materials is reduced, which helps to prevent the material from sticking to the wall during spray drying and improves the material yield. However, this process is highly dependent on equipment, has a long overall production cycle, and the final material morphology is difficult to control. At the same time, the production process will emit ammonia, which puts great pressure on environmental protection.

[0007] (3) When Fe2O3 is used as the iron source, the process is simple, the morphology of the finished product is controllable, the raw materials are widely available, the output is sufficient, and the price is cheap. However, the raw materials have a high sugar content, which makes them easy to stick to the wall during spray drying, resulting in a low material yield. The Fe2O3 raw material has a high impurity content and the purity is difficult to exceed 99%, resulting in a low specific weight of the final product.

[0008] How to improve the aforementioned problems in the preparation of sodium iron pyrophosphate cathode material using a single iron source is a pressing issue that needs to be addressed in this case. Summary of the Invention

[0009] To address the problems existing in the polyanionic phosphoric acid pyrophosphate iron cathode material in the prior art, this case proposes to achieve the preparation of cathode material by using a mixed iron source.

[0010] Specifically, the technical solution adopted in this application is as follows:

[0011] A method for preparing a sodium iron pyrophosphate cathode material includes the following steps:

[0012] Step 1: Weigh out the sodium source, phosphorus source, carbon source, and mixed iron source according to the stoichiometric ratio, and add them sequentially to deionized water while stirring to disperse, thus obtaining a suspension.

[0013] The sodium source is any one of sodium carbonate, sodium bicarbonate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium oxalate, sodium acetate, sodium citrate, sodium oxide, and sodium peroxide.

[0014] The phosphorus source is any one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, and monosodium trihydrogen pyrophosphate.

[0015] The carbon source includes two or more of glucose, sucrose, citric acid, stearic acid, oxalic acid, and cellulose.

[0016] The mixed iron source includes two or more of the following: ferric nitrate, ferric phosphate, ferrous carbonate, ferrous oxalate, ferrous acetate, ferrous citrate, ferric oxide, and iron(II,III) oxide.

[0017] Step 2: The solid particles in the suspension obtained in Step 1 are finely pulverized to make the material uniformly mixed with a particle size of less than 700 nm. After drying, the well mixed solid material is obtained.

[0018] Step 3: To prevent the carbon source from being oxidized and lost at high temperature, the solid material from Step 2 is sintered in a segmented manner in an inert atmosphere to obtain sodium iron pyrophosphate cathode material prepared by mixed iron source.

[0019] Furthermore, as a preferred option:

[0020] Step one also includes supplementing the phosphorus source, which can be any one of ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, or monosodium trihydrogen pyrophosphate.

[0021] It also includes a supplementary sodium source, which can be any one of sodium carbonate, sodium bicarbonate, sodium phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium oxalate, sodium acetate, sodium citrate, sodium oxide, or sodium peroxide.

[0022] In step one, the mixed iron source is Fe2O3 + FeC2O4 or FePO4 + Fe2O3 or FeC2O4 + FePO4, or the mixed iron source is Fe2O3 + FeC2O4 + FePO4.

[0023] When Fe2O3+FeC2O4, FePO4+Fe2O3, or FeC2O4+FePO4 are mixed, the molar ratio of iron in Fe2O3 to FeC2O4, FePO4 to Fe2O3, or FeC2O4 to FePO4 is 0.1~10:0.1~10.

[0024] When using a mixture of Fe2O3, FeC2O4, and FePO4, the molar ratio of iron in Fe2O3, FeC2O4, and FePO4 is 0.1~10:0.1~10:0.1~10.

[0025] In step two, the solid particles are subjected to fine pulverization, preferably by ball milling, rod milling, or sand milling. The mixed material is preferably dried by continuous vacuum drying, freeze drying, or spray drying. More preferably, the sand milling speed is 500-3000 rpm, and the sand milling time is 0.5-2 hours; the particle size D50 of the material after sand milling is required to be 200-700 nm. The spray drying inlet temperature is 100-270℃, and the outlet temperature is 70-150℃.

[0026] In step three:

[0027] The inert sintering atmosphere includes one of nitrogen, a nitrogen-hydrogen mixture, argon, and an argon-hydrogen mixture.

[0028] The segmented sintering process employs a two-stage heating method: first, the temperature is increased from room temperature to 200℃~400℃ at a heating rate of 0.5~5℃ / min, and held for 1~12 hours; then, the temperature is increased to 450~700℃ at a heating rate of 0.5~5℃ / min, and held for 1~24 hours. After the material cools to room temperature, sodium iron pyrophosphate cathode material is obtained.

[0029] The second objective of this application is to provide the application of the above-mentioned sodium iron pyrophosphate cathode material in sodium-ion batteries: the above-mentioned sodium iron pyrophosphate cathode material is uniformly mixed with binder PVDF and conductive agent Super P in a mass ratio of 93~95:2~3:3~4 in NMP, coated on aluminum foil current collector, dried and rolled to become a cathode sheet; after die cutting, it is combined with die-cut negative electrode sheet and separator to form a cell, which is then filled into a casing to form a sodium-ion battery.

[0030] The beneficial technical effects of the technical solution of this invention are as follows:

[0031] (1) When Fe2O3 is used as the single iron source, a large proportion of carbon source is required, resulting in a high sugar content in the raw materials. During spray drying, the material is prone to sticking to the wall, leading to a low material yield and affecting the sugar metering ratio and product performance. Frequent equipment cleaning is also required, which is difficult and seriously hinders production efficiency. Adding a certain proportion of FeC2O4, using Fe2O3 + FeC2O4 as a mixed iron source, can reduce the proportion of carbon source, reduce the carbon content, increase the material's specific capacity, and effectively alleviate material sticking to the wall, thereby improving material yield and batch stability.

[0032] (2) When FeC2O4 is used as the single iron source, it is difficult to control the morphology of the final product, and ammonia is generated during the production process, which has high environmental protection requirements. Adding a certain proportion of FePO4, and using FeC2O4 + FePO4 as a mixed iron source, can make the raw materials more uniformly mixed, make the production simpler, reduce ammonia emissions during the production process, and alleviate environmental pressure.

[0033] (3) When FePO4 is used as a single carbon source, the raw material price is higher, the production cost is higher, and the cost-effectiveness is low. Adding a certain proportion of Fe2O3, and using FePO4+Fe2O3 as a mixed iron source, can reduce the raw material cost and make the final product morphology easier to control.

[0034] (4) When two or more iron sources are used as mixed iron sources in a certain proportion, the characteristics of different iron source materials can be combined to greatly alleviate the problems of a single iron source in the production process and play a synergistic role.

[0035] (5) The proportion of mixed iron sources can be flexibly adjusted according to existing equipment conditions or different customer requirements for product performance. At the same time, different proportions of mixed iron sources can accelerate the differentiated development of products and quickly seize applications in some special fields.

[0036] (6) The above-mentioned preparation process of mixed iron source requires no additional preparation process or production equipment, has minimal impact on existing manufacturing processes, and is fully compatible with production equipment. It enhances the ability to cope with raw material supply chain risks, and can select the process route with the lowest raw material cost according to changes in market raw material prices, thereby reducing costs and improving product competitiveness. Attached Figure Description

[0037] Figure 1 This is a SEM image of the sodium ion cathode material prepared in Example 1;

[0038] Figure 2 The image shows the XRD pattern of the sodium ion cathode material prepared in Example 1.

[0039] Figure 3 The charge-discharge curves are for the sodium iron pyrophosphate cathode material prepared in Example 1.

[0040] Figure 4 The image shows the XRD pattern of the sodium ion cathode material prepared in Example 10.

[0041] Figure 5 The charge-discharge curves are for the sodium iron pyrophosphate cathode material prepared in Example 10. Implementation

[0042] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention. Example

[0043] This embodiment describes a method for preparing a sodium iron pyrophosphate cathode material, using Fe2O3 + FeC2O4 as a mixed iron source, with a molar ratio of iron oxide to iron oxalate of 3:1. Sodium dihydrogen phosphate is used as both the sodium and phosphorus source, and glucose and citric acid are used as the carbon source. The steps are as follows:

[0044] (1) 957.60g of iron oxide, 575.20g of ferrous oxalate, 2557.78g of sodium dihydrogen phosphate, 432.13g of glucose and 614.47g of citric acid were added to 9014.23mL of deionized water and stirred to obtain a suspension.

[0045] (2) Transfer the suspension to a sand mill for sand milling, using 0.3mm~0.4mm zirconia beads as the sand milling medium, with a sand mill speed of 1000rpm and sand milling for 1h;

[0046] (3) The slurry after sand milling is spray-dried at an inlet air temperature of 150°C and an outlet air temperature of 86°C to obtain precursor powder.

[0047] (4) The precursor was sintered under the protective atmosphere of argon-hydrogen mixture (5% H2). The heating rate of the first stage was 1.5℃ / min and the heating rate of the second stage was 2℃ / min. The temperature of the first stage was 350℃ and held for 6h; the temperature of the second stage was 550℃ and held for 15h. After the material was cooled down, sodium ion cathode material, i.e. sodium iron pyrophosphate cathode material, was obtained.

[0048] Figure 1 This is a scanning electron microscope image of the cathode material prepared in this embodiment. Figure 2 The XRD pattern of the cathode material prepared in this embodiment shows that the powder obtained in step four has a spherical morphology. The particle size of the product can be controlled, but there is some agglomeration, which is related to the high sugar content.

[0049] from Figure 3 It can be seen that the sodium iron pyrophosphate cathode material prepared in this embodiment, when used as the cathode material, results in a coin cell with an operating voltage between 2.0V and 4.2V, an initial coulombic efficiency >97%, and a reversible capacity >103mAh / g at 0.1C. The assembled coin cell has a reversible capacity >92mAh / g at 1C.

[0050] Comparative Example 1

[0051] The settings in this embodiment are the same as in Embodiment 1, except that the iron source is as shown in Table 1.

[0052] Table 1: Effect of different iron sources on preparation results

[0053] .

[0054] As can be seen from Table 1, under the same conditions, using a mixed iron source can effectively improve the wall adhesion of spray drying, control the product morphology, and keep the cost at a low level, while there is no pollutant emission during the production process.

[0055] Comparative Example 2

[0056] This embodiment describes a method for preparing a sodium-ion cathode material, using a mixed iron source of FeCO3+C6H8FeO7 and nitrous carbonate.

[0057] The molar ratio of iron to ferrous citrate is 3:1, sodium dihydrogen phosphate is used as both the phosphorus and sodium source, and glucose is used as the carbon source. The steps are as follows:

[0058] (1) 694.80g of ferrous carbonate, 495.60g of ferrous citrate, 1539.50g of sodium dihydrogen phosphate and 173.42g of glucose were added to deionized water in sequence and stirred to obtain a suspension;

[0059] (2) Transfer the suspension to a sand mill for sand milling, using 0.3mm~0.4mm zirconia beads as the sand milling medium, with a sand mill speed of 1000rpm and sand milling for 1h;

[0060] (3) The slurry after sand milling is spray-dried at an inlet air temperature of 178°C and an outlet air temperature of 97°C to obtain precursor powder.

[0061] (4) The precursor was sintered under the protective atmosphere of argon-hydrogen mixture (5% H2). The first stage temperature was 350℃ and the holding time was 6h; the second stage temperature was 550℃ and the holding time was 15h. After the material was cooled down, sodium ion cathode material was obtained.

[0062] In this embodiment, the button cell assembled using the material as the positive electrode material has an operating voltage between 2.0V and 4.2V, a reversible capacity of 78mAh / g at 0.1C, and a reversible capacity of 65mAh / g at 1C.

[0063] Comparative Example 3

[0064] The process flow in this case is the same as that in Comparative Example 2, except that the molar ratio of ferrous carbonate to ferrous citrate is 2:1.

[0065] Comparative Examples 2 and 3 show that when the two iron sources are used as mixed iron sources to prepare cathode materials, no new experimental phenomena are observed in the different proportions of the preparation process, and the materials are quite similar in morphology, physicochemical data, and electrical properties.

[0066] Comparative Example 4

[0067] The process flow in this embodiment is the same as that in Comparative Example 2, except that the molar ratio of ferrous carbonate to ferrous citrate is 1:1. The coin cell assembled using these materials as the positive electrode material in this embodiment has an operating voltage between 2.0V and 4.2V, a reversible capacity of 75mAh / g at 0.1C, and a reversible capacity of 67mAh / g at 1C.

[0068] In this case, further changes in the ratio of the two iron sources did not significantly alter the overall material performance. When these two iron sources are used as single iron sources to prepare sodium-ion cathode materials, their material properties, required auxiliary materials, and preparation processes are highly similar. Compared to Example 1, even with similar preparation processes, in Comparative Examples 2 to 4, when these two iron sources were used as mixed iron sources to prepare sodium-ion cathode materials, controlling different ratios still made it difficult to modify and improve the material's properties.

[0069] Example 2

[0070] The process flow in this embodiment is the same as that in embodiment 1, except that the molar ratio of iron in iron oxide to ferrous oxalate is 2:1. The proportion of iron oxide is reduced, and the carbon source required for carbothermal reduction is reduced accordingly. Consequently, the amount of glucose and citric acid added needs to be reduced.

[0071] In Example 1, some adhesion to the spray drying wall still occurred, while in this example, the adhesion phenomenon was significantly reduced. Furthermore, the material capacity in Example 1 was approximately 103 mAh / g, while in this example it was approximately 105 mAh / g. Therefore, this example demonstrates that by increasing the proportion of ferrous oxalate, reducing the overall carbon source and sugar content, it can improve spray drying wall adhesion and increase the material capacity.

[0072] Example 3

[0073] The process flow in this embodiment is the same as that in embodiment 1, except that the molar ratio of iron in ferric oxide to ferrous oxalate is 1:1, the proportion of ferric oxide is further reduced, and the amount of glucose and citric acid added is reduced accordingly.

[0074] In this embodiment, the proportion of ferrous oxalate is further increased, while the carbon source and carbon content are further reduced. However, the price of ferrous oxalate is about 16% higher than that of iron oxide. Furthermore, excessive addition of ferrous oxalate makes it difficult to control the final material morphology. The coin cell assembled using this material as the positive electrode material operates at a voltage between 2.0V and 4.2V, with a reversible capacity of only 85mAh / g at 0.1C and 71mAh / g at 1C.

[0075] Comparing Examples 1, 2, and 3, it can be seen that increasing the proportion of ferrous oxalate and reducing the carbon source and carbon content to a certain extent can improve the adhesion to the spray drying wall and the material's specific capacity. However, when the proportion of ferrous oxalate exceeds 40%, it is easy to cause uneven carbon layer coating during spray drying, resulting in poor morphology of the final sintered material. The coated carbon layer is easily damaged during pulverization, which leads to a significant decrease in the overall performance of the material. The specific capacity of the material cannot be fully utilized, reaching only 85 mAh / g, and the cycle decay is severe.

[0076] Example 4

[0077] This embodiment describes a method for preparing a sodium-ion cathode material. The mixed iron source is FeC2O4 + FePO4, the molar ratio of ferrous oxalate to iron in ferric phosphate is 3:1, ammonium dihydrogen phosphate is used as a supplementary phosphorus source, sodium carbonate is used as a sodium source, and glucose and citric acid are used as carbon sources. The steps are as follows:

[0078] (1) 862.80g ferrous oxalate, 301.54g ferric phosphate, 919.86g ammonium dihydrogen phosphate, 565.05g sodium carbonate, 168.07g glucose and 102.43g citric acid were added to deionized water in sequence and stirred to obtain a suspension.

[0079] (2) Transfer the suspension to a sand mill for sand milling, using 0.3mm~0.4mm zirconia beads as the sand milling medium, with a sand mill speed of 1000rpm and sand milling for 1h;

[0080] (3) The slurry after sand milling is spray-dried at an inlet air temperature of 170°C and an outlet air temperature of 94°C to obtain precursor powder.

[0081] (4) The precursor was sintered under the protective atmosphere of argon-hydrogen mixture (5% H2). The first stage temperature was 350℃ and the holding time was 6h; the second stage temperature was 550℃ and the holding time was 15h. After the material was cooled down, sodium ion cathode material was obtained.

[0082] In this embodiment, the button cell assembled using the material as the positive electrode material has an operating voltage between 2.0V and 4.2V, a reversible capacity of 84mAh / g at 0.1C, and a reversible capacity of 70mAh / g at 1C.

[0083] Example 5

[0084] The process flow in this embodiment is the same as that in embodiment 4, except that the molar ratio of iron in ferrous oxalate to iron phosphate is 2:1, the proportion of iron phosphate in the iron source is increased, and iron phosphate has a certain phosphorus content, so the amount of phosphorus source, namely ammonium dihydrogen phosphate, is reduced accordingly. At the same time, the proportion of iron phosphate is increased, the number of ferric ions increases, and more carbon source is needed for reduction, so the amount of glucose and citric acid needs to be increased accordingly.

[0085] Comparing Examples 4 and 5, it can be seen that reducing the amount of ammonium dihydrogen phosphate added helps to reduce ammonia emissions.

[0086] Example 6

[0087] The process flow in this embodiment is the same as that in embodiment 4, except that the molar ratio of ferrous oxalate to ferric phosphate is 1:1, the proportion of ferric phosphate is further increased, the amount of ammonium dihydrogen phosphate added is reduced accordingly, and the amount of glucose and citric acid added is increased.

[0088] In this embodiment, the increased amount of iron phosphate makes the raw materials easier to mix evenly, resulting in better carbon coating and easier control of material morphology. The coin cell assembled using this material as the positive electrode operates at a voltage between 2.0V and 4.2V, with a reversible capacity of 95mAh / g at 0.1C and 83mAh / g at 1C. Compared to the material in Example 4, the electrochemical performance is significantly improved.

[0089] Comparing Examples 4, 5, and 6, it can be seen that reducing the amount of ammonium dihydrogen phosphate helps reduce ammonia emissions, while increasing the amount of ferric phosphate allows for better mixing of raw materials, resulting in a better final material morphology and improved material performance. However, on the other hand, the relatively increased amount of ferric phosphate, which is about 36% more expensive than ferrous oxalate, leads to higher production costs.

[0090] Example 7

[0091] This embodiment describes a method for preparing a sodium-ion cathode material, using FePO4 + Fe2O3, with a molar ratio of iron in ferric phosphate to iron oxide of 3:1, sodium dihydrogen phosphate as the sodium and phosphorus source, sodium carbonate as a supplementary sodium source, and glucose and citric acid as carbon sources. The steps are as follows:

[0092] (1) Add 904.62g of ferric phosphate, 159.60g of ferric oxide, 319.71g of sodium dihydrogen phosphate, 423.81g of sodium carbonate, 240.11g of glucose and 409.72g of citric acid to deionized water in sequence and stir to obtain a suspension;

[0093] (2) Transfer the suspension to a sand mill for sand milling, using 0.3mm~0.4mm zirconia beads as the sand milling medium, with a sand mill speed of 1000rpm and sand milling for 1h;

[0094] (3) The slurry after sand milling is spray-dried at an inlet air temperature of 160°C and an outlet air temperature of 91°C to obtain precursor powder.

[0095] (4) The precursor was sintered under the protective atmosphere of argon-hydrogen mixture (5% H2). The first stage temperature was 350℃ and the holding time was 6h; the second stage temperature was 550℃ and the holding time was 15h. After the material was cooled down, sodium ion cathode material was obtained.

[0096] Example 8

[0097] The process flow in this embodiment is the same as that in Embodiment 7, the difference being that the molar ratio of iron in ferric phosphate to ferric oxide is 2:1, the ferric phosphate content is reduced, and the amount of sodium dihydrogen phosphate (the phosphorus source) added is increased accordingly, while the amount of sodium carbonate added is reduced. The production cost using ferric phosphate as the iron source is approximately RMB 15,300 / ton, while the production cost using ferric oxide as the iron source is approximately RMB 13,100 / ton, a difference of about 14%. Using the mixed iron source in this embodiment, the cost can be controlled at approximately RMB 14,550 / ton, reducing the production cost by about 5%. Furthermore, the proportion of ferric oxide can be further increased to further reduce the production cost.

[0098] Comparing Examples 7 and 8: The reduction in the proportion of iron phosphate helps to reduce production costs.

[0099] Example 9

[0100] The process flow in this embodiment is the same as that in embodiment 7, except that the molar ratio of iron in ferric phosphate and ferric oxide is 1:1, and the amount of sodium dihydrogen phosphate added is increased accordingly while the amount of sodium carbonate added is decreased.

[0101] Comparing Examples 7, 8, and 9, it can be seen that reducing the proportion of iron phosphate helps to reduce production costs, but the high content of iron oxide impurities and excessive addition will reduce the product's specific volume.

[0102] Example 10

[0103] The process flow of this embodiment is the same as that of Embodiment 1, except that: the mixed iron source is Fe2O3+ FeC2O4+FePO4, the molar ratio of iron oxide, ferrous oxalate and iron phosphate is 1:1:1, ammonium dihydrogen phosphate is used as the phosphorus source, sodium carbonate is used as the sodium source, and glucose and citric acid are used as the carbon source.

[0104] Figure 4 The image shows the XRD pattern of the sodium-ion cathode material prepared in Example 10, compared with Example 1 and... Figure 2 The material prepared in this embodiment contains a relatively high amount of impurities, which results in a lower specific capacity. Figure 5 It can be seen that the cathode material obtained in Example 10 has a reversible capacity of 101 mAh / g at 0.1C. The assembled coin cell has a reversible capacity of 94 mAh / g at 1C, which is better than that in Example 1, indicating that the three mixed iron sources may have better rate performance.

[0105] The solution provided in this case uses a specially selected mixed iron source to prepare the cathode material. Different iron sources complement each other in performance, thereby increasing product yield, reducing production costs, optimizing production processes, improving production efficiency, enhancing product quality, and reducing environmental pressure.

[0106] The above description is merely a preferred embodiment of this application and is not intended to limit the invention. Various modifications and variations can be made to this invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a sodium iron phosphate pyrophosphate positive electrode material, characterized in that, Comprising the following steps: Step one: according to the stoichiometric ratio, respectively, sodium source, phosphorus source, carbon source and mixed iron source, and sequentially added to the deionized water stirring dispersion, get the suspension, The sodium source is any one of sodium carbonate, sodium bicarbonate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium oxalate, sodium acetate, sodium citrate, sodium oxide and sodium peroxide, The phosphorus source is any one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium pyrophosphate, disodium pyrophosphate and monosodium pyrophosphate, The carbon source is two or more of glucose, sucrose, citric acid, stearic acid, oxalic acid and cellulose, The mixed iron source is Fe2O3+FeC2O4, FePO4+Fe2O3, FeC2O4+FePO4 mixed mode, the molar ratio of iron in Fe2O3 and FeC2O4, FePO4 and Fe2O3 or FeC2O4 and FePO4 is 0.1~10:0.1~10; or the mixed iron source is Fe2O3+FeC2O4+FePO4 mixed mode, the molar ratio of iron in Fe2O3, FeC2O4 and FePO4 is 0.1~10:0.1~10:0.1~10; Step two: the suspension obtained in step one is subjected to fine crushing treatment to a solid particle size of less than 700 nm, and dried; Step three: the solid material obtained by drying in step two is subjected to stepwise sintering in an inert atmosphere to obtain a sodium iron phosphate pyrophosphate positive electrode material prepared by mixed iron source.

2. The preparation method of the sodium ferric phosphate pyrophosphate positive electrode material according to claim 1, characterized in that: In step one, the sodium source is any one of sodium carbonate, sodium bicarbonate, sodium phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium oxalate, sodium acetate, sodium citrate, sodium oxide and sodium peroxide.

3. The preparation method of the sodium ferric phosphate pyrophosphate positive electrode material according to claim 1, characterized by comprising the following steps: In step one, the phosphorus source is any one of ammonium dihydrogen phosphate, disodium hydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium pyrophosphate, disodium pyrophosphate and monosodium pyrophosphate.

4. The method of claim 1, wherein: In step two, the method of fine crushing treatment is ball mill, rod mill or sand mill sand grinding treatment, and drying is carried out by continuous vacuum drying, freeze drying or spray drying.

5. The method of claim 4, wherein: The sand mill sand grinding speed is 500~3000rpm, and the sand grinding time is 0.5h~2h; the inlet temperature of spray drying is 100~270℃, and the outlet temperature is 70~150℃.

6. The method of claim 1, wherein: In step three, the inert atmosphere is any one of nitrogen, nitrogen-hydrogen mixed gas, argon and argon-hydrogen mixed gas.

7. The method of claim 1, wherein: In step three, two-stage temperature rising is adopted for stepwise sintering: first, the temperature is raised from room temperature to 200℃~400℃ at a rate of 0.5~5℃ / min, and then the temperature is raised to 450~700℃ at a rate of 0.5~5℃ / min after holding for 1~12h.

8. The use of the sodium iron phosphate pyrophosphate positive electrode material prepared by the method of claim 1, characterized in that: The sodium iron phosphate positive electrode material is mixed with a binder PVDF and a conductive agent Super P in a mass ratio of 93-95:2-3:3-4 in NMP, coated on an aluminum foil current collector, and dried and rolled to form a positive electrode sheet; after die cutting, the positive electrode sheet is combined with a die-cut negative electrode sheet and a separator to form a battery cell, and the battery cell is put into a shell and filled with liquid to form a sodium ion battery.

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