Phosphoric acid-doped sodium ferric pyrophosphate positive electrode material and preparation method and application thereof

Abstract

The invention discloses a ferric sodium pyrophosphate-doped positive electrode material, which is characterized in that layered double hydroxides are used as an iron source and a doped metal source, and iron and doped metal elements are uniformly distributed; doped metal elements in the positive electrode material are one or more of magnesium, zinc, nickel, cobalt, calcium, copper and manganese. The positive electrode material is used as a positive electrode active material and used for preparing a sodium ion battery positive electrode. According to the invention, through the innovation of precursor design, namely the utilization of the structural characteristics of layered double hydroxides, the uniform distribution of doping elements in ferric sodium pyrophosphate is realized, and then through the synergistic effect of optimizing an electronic structure, promoting ion transmission and stabilizing a crystal frame, the purpose of improving the specific capacity and rate capability of the material is finally achieved.

Description

TECHNICAL FIELD

[0001] The application relates to the field of batteries, in particular to a doped sodium iron phosphate pyrophosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium resources are abundant and widely distributed, and sodium-ion batteries have the advantage of being compatible with the production equipment and process of lithium-ion batteries. Sodium-ion batteries have gradually become the focus of new energy technology competition in countries around the world. The cost of the positive electrode material accounts for the highest proportion in the quality of sodium-ion batteries, and the development of the positive electrode material will directly affect the further popularization and application of sodium-ion batteries. Among the three major positive electrode material systems of Prussian blue analogs, layered oxides and polyanion compounds, polyanion compounds are widely concerned due to their high working voltage and good cycle stability. Sodium iron phosphate pyrophosphate, as a typical representative of polyanion compound positive electrode materials, also has the advantage of low cost and is considered to be one of the ideal positive electrode materials for sodium-ion batteries. However, the low conductivity and specific capacity of the material are two problems that cannot be ignored in the development of the material.

[0003] Ion doping and carbon compounding of sodium iron phosphate pyrophosphate material are effective means to improve the above problems. For example, patent CN118841531A discloses a doped sodium iron phosphate pyrophosphate positive electrode material, a preparation method and an application thereof. The electronic transmission network is constructed by a C&A coating layer to improve the electronic conductivity of the material, and the electrochemical capacity of the material is improved by element doping; patent CN116154121A discloses a Na4Fe3-xMx(PO4)2P2O7 / C composite material and a preparation and application thereof in sodium-ion batteries. The combination control of the coating layer material and the element doping and doping content of the core realizes the improvement of the electrochemical performance of the material.

[0004] The existing technology usually needs to physically mix iron source, sodium source, phosphorus source, carbon source and additional one or more doping metal sources to form a precursor through a certain means, and then the precursor is subjected to subsequent calcination treatment. The physical mixing of the iron source and the doping metal source in the precursor can easily lead to uneven distribution of metal elements in the sodium iron phosphate pyrophosphate product, thereby affecting the improvement of the electrochemical performance of the material.

[0005] Therefore, it is necessary to design a doped sodium iron phosphate pyrophosphate positive electrode material and a preparation method and application thereof. SUMMARY

[0006] In order to overcome the defects in the prior art, a doped sodium iron phosphate pyrophosphate positive electrode material and a preparation method and application thereof are provided.

[0007] The application is achieved by the following scheme: A doped sodium iron pyrophosphate cathode material, wherein the cathode material uses a layered double hydroxide as the iron source and the doped metal source, and the iron and doped metal elements are uniformly distributed; the doped metal elements in the cathode material are one or more selected from magnesium, zinc, nickel, cobalt, calcium, copper and manganese.

[0008] A method for preparing a sodium iron pyrophosphate cathode material, the method comprising the following steps: Step 1: Mix the iron source and the doped metal source with the alkaline solution, react, separate, wash, and dry to obtain a layered double hydroxide; Step 2: The precursor obtained by ball milling the layered double hydroxide, sodium source, phosphorus source and carbon source is calcined at high temperature to obtain the doped sodium iron pyrophosphate cathode material.

[0009] The molar ratio of the iron source to the doped metal source is 29.9:0.1-0.1:29.9, the concentration of the iron source is 0.1-4 mol / L, the concentration of the doped metal source is 0.1-4 mol / L, and the concentration of the alkaline solution is 0.5-10 mol / L. The molar ratio of Na and Fe to doped metal and P in the iron source, doped metal ion source, sodium source, and phosphorus source is 3.75-4.25:2.75-3.25:3.75-4.25.

[0010] The amount of carbon source added accounts for 1-25 wt% of the total mass of the precursor.

[0011] The iron source is one or more of ferrous nitrate, ferrous chloride, ferrous sulfate, ferrous acetate, ferrous perchlorate, ferrous bromide, ferrous iodide, ferric nitrate, ferric chloride, ferric sulfate, ferric acetate, ferric perchlorate, ferric bromide, ferric iodide, and their possible hydrates.

[0012] The doped metal source is one or more of the following: magnesium nitrate, magnesium chloride, magnesium sulfate, magnesium acetate, magnesium perchlorate, magnesium bromide, magnesium iodide, zinc nitrate, zinc chloride, zinc sulfate, zinc acetate, zinc perchlorate, zinc bromide, zinc iodide, nickel nitrate, nickel chloride, nickel sulfate, nickel acetate, nickel bromide, nickel iodide, cobalt nitrate, cobalt chloride, cobalt sulfate, cobalt acetate, cobalt bromide, cobalt iodide, calcium nitrate, calcium chloride, calcium acetate, calcium perchlorate, calcium bromide, calcium iodide, copper nitrate, copper chloride, copper sulfate, copper acetate, copper perchlorate, copper bromide, manganese chloride, manganese nitrate, manganese sulfate, manganese acetate, manganese bromide, manganese iodide, and their possible hydrates.

[0013] The alkaline solution is one or more of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, potassium carbonate, and ammonium carbonate.

[0014] The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, sodium sulfate, sodium chloride, sodium oxalate, sodium citrate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate. The phosphorus source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, ferric pyrophosphate, and ferrous pyrophosphate. The carbon source is one or more of glucose, sucrose, starch, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, citric acid, ethylenediaminetetraacetic acid, pyromellitic acid, cyclodextrin, graphene, graphene oxide, carbon nanotubes, and Ketjen black. The pH value of the reaction is 8-13, the reaction temperature is room temperature-90℃, and the reaction time is 4-72 h; The separation process can be centrifugation or filtration, washing with one or more of deionized water and ethanol, drying temperature of 60-150℃, and drying time of 8-48 h; The ball-to-material ratio in the ball milling process is 35:1-4:1, the ball milling speed is 150-850 rpm, and the ball milling time is 1-48h. The high-temperature calcination atmosphere is one or more of nitrogen, argon, and hydrogen-argon mixture, the calcination temperature is 400-750℃, and the calcination time is 4-24 h.

[0015] This cathode material is used as a cathode active material and for preparing the cathode of sodium-ion batteries.

[0016] The beneficial effects of this invention are as follows: 1. This invention achieves atomically uniform distribution of iron and doped metal elements in sodium iron pyrophosphate cathode materials by employing layered double hydroxides as both the iron source and the dopant metal source. This uniform distribution stems from the crystal structure characteristics of the layered double hydroxides; the arrangement of metal ions on the layers is regulated by the minimum lattice energy effect and the lattice positioning effect, ensuring that each tiny structural unit has a constant chemical composition. When this layered double hydroxide participates in subsequent solid-state reactions as a precursor, it fundamentally guarantees the uniformity of the doped metal elements in the final product, avoiding elemental segregation or localized enrichment that may occur with traditional physical mixing methods.

[0017] 2. Because the doped metal elements achieve uniform substitution within the sodium iron pyrophosphate lattice, we can homogenize the electronic structure of the transition metal elements. This uniform electronic structure adjustment helps optimize the intrinsic electronic conductivity of the material and promotes electron migration during charging and discharging. Simultaneously, uniform doping also helps lower the sodium ion migration barrier, providing a smoother path for sodium ion diffusion in the lattice channels, thereby effectively improving the ionic conductivity of the material.

[0018] 3. The sodium iron pyrophosphate cathode material prepared by this invention has a uniform crystal framework that helps suppress phase transitions and volume changes during repeated sodium ion insertion / extraction. The uniformly distributed doped metal elements can stabilize the crystal structure, reducing capacity decay caused by uneven local stress or structural distortion during cycling, thereby improving the material's cycling stability.

[0019] 4. Compared with conventional solid-state methods that directly and physically mix iron sources, doped metal sources, and other raw materials, the core advantage of this invention, which utilizes layered double hydroxides as the metal source, lies in ensuring the uniformity of metal element mixing from the source. Conventional methods, even after prolonged ball milling, struggle to achieve atomic-level mixing. This invention, however, naturally achieves a uniform distribution of iron and doped metals at the molecular level through the chemical preparation process of layered double hydroxides, laying the foundation for the subsequent formation of uniform, high-performance cathode materials. Attached Figure Description

[0020] Figure 1 XRD pattern of sodium iron pyrophosphate cathode material; Figure 2 SEM image of sodium iron pyrophosphate cathode material; Figure 3 The charging and discharging curves are shown. Detailed Implementation

[0021] The present invention will be further described below with reference to specific embodiments: A doped sodium iron pyrophosphate cathode material, wherein the cathode material uses a layered double hydroxide as the iron source and the doped metal source, and the iron and doped metal elements are uniformly distributed; the doped metal elements in the cathode material are one or more selected from magnesium, zinc, nickel, cobalt, calcium, copper and manganese.

[0022] This invention primarily addresses the problem of limited electrochemical performance improvement caused by uneven mixing / distribution of metal elements in metal-doped sodium iron pyrophosphate cathode materials. Using layered double hydroxides as both the iron and doping metal sources, and combining sodium, phosphorus, and carbon sources, the doped sodium iron pyrophosphate cathode material is prepared in a solid-state manner. The atomic-level uniform distribution of metal elements in the layered double hydroxides allows for a more uniform distribution of the doping metal elements within the sodium iron pyrophosphate. By homogenizing and adjusting the electronic structure of the transition metal elements, optimizing the sodium ion migration barrier, and stabilizing the crystal framework, the electronic conductivity and sodium ion diffusion rate are synergistically improved, while suppressing phase transitions and volume changes during cycling, ultimately enhancing the specific capacity and rate performance of the product.

[0023] A method for preparing a sodium iron pyrophosphate cathode material, the method comprising the following steps: Step 1: Mix the iron source and the doped metal source with the alkaline solution, react, separate, wash, and dry to obtain a layered double hydroxide; Step 2: The precursor obtained by ball milling the layered double hydroxide, sodium source, phosphorus source and carbon source is calcined at high temperature to obtain the doped sodium iron pyrophosphate cathode material.

[0024] The molar ratio of the iron source to the doped metal source is 29.9:0.1-0.1:29.9, the concentration of the iron source is 0.1-4 mol / L, the concentration of the doped metal source is 0.1-4 mol / L, and the concentration of the alkaline solution is 0.5-10 mol / L. The molar ratio of Na and Fe to doped metal and P in the iron source, doped metal ion source, sodium source, and phosphorus source is 3.75-4.25:2.75-3.25:3.75-4.25.

[0025] The amount of carbon source added accounts for 1-25 wt% of the total mass of the precursor.

[0026] The iron source is one or more of ferrous nitrate, ferrous chloride, ferrous sulfate, ferrous acetate, ferrous perchlorate, ferrous bromide, ferrous iodide, ferric nitrate, ferric chloride, ferric sulfate, ferric acetate, ferric perchlorate, ferric bromide, ferric iodide, and their possible hydrates.

[0027] The doped metal source is one or more of the following: magnesium nitrate, magnesium chloride, magnesium sulfate, magnesium acetate, magnesium perchlorate, magnesium bromide, magnesium iodide, zinc nitrate, zinc chloride, zinc sulfate, zinc acetate, zinc perchlorate, zinc bromide, zinc iodide, nickel nitrate, nickel chloride, nickel sulfate, nickel acetate, nickel bromide, nickel iodide, cobalt nitrate, cobalt chloride, cobalt sulfate, cobalt acetate, cobalt bromide, cobalt iodide, calcium nitrate, calcium chloride, calcium acetate, calcium perchlorate, calcium bromide, calcium iodide, copper nitrate, copper chloride, copper sulfate, copper acetate, copper perchlorate, copper bromide, manganese chloride, manganese nitrate, manganese sulfate, manganese acetate, manganese bromide, manganese iodide, and their possible hydrates.

[0028] The alkaline solution is one or more of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, potassium carbonate, and ammonium carbonate.

[0029] The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, sodium sulfate, sodium chloride, sodium oxalate, sodium citrate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate. The phosphorus source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, ferric pyrophosphate, and ferrous pyrophosphate. The carbon source is one or more of glucose, sucrose, starch, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, citric acid, ethylenediaminetetraacetic acid, pyromellitic acid, cyclodextrin, graphene, graphene oxide, carbon nanotubes, and Ketjen black. The pH value of the reaction is 8-13, the reaction temperature is room temperature-90℃, and the reaction time is 4-72 h; The separation process can be centrifugation or filtration, washing with one or more of deionized water and ethanol, drying temperature of 60-150℃, and drying time of 8-48 h; The ball-to-material ratio in the ball milling process is 35:1-4:1, the ball milling speed is 150-850 rpm, and the ball milling time is 1-48h. The high-temperature calcination atmosphere is one or more of nitrogen, argon, and hydrogen-argon mixture, the calcination temperature is 400-750℃, and the calcination time is 4-24 h.

[0030] The method of this invention enables more uniform doping of metal elements in sodium iron pyrophosphate, and can essentially achieve synergistic optimization of the entire crystal framework. By homogenizing and adjusting the electronic structure of transition metal elements, optimizing the sodium ion migration barrier and stabilizing the crystal framework, the electronic conductivity and sodium ion diffusion rate are synergistically improved and phase transitions and volume changes during cycling are suppressed, ultimately improving the electrochemical performance of the product.

[0031] This cathode material is used as a positive electrode active material and for preparing the cathode of sodium-ion batteries. Layered double hydroxides are compounds assembled from positively charged host layers and interlayer anions through non-covalent interactions. They possess characteristics such as adjustable chemical composition of the host layers and adjustable types and quantities of interlayer anions, and can be obtained through a simple co-precipitation method. In the crystal structure of layered double hydroxides, due to the influence of the minimum lattice energy effect and its lattice positioning effect, metal ions are uniformly distributed on the layers in a certain manner; that is, the chemical composition remains unchanged in each tiny structural unit on the layer. This invention proposes a doped sodium iron pyrophosphate cathode material and its preparation method: using layered double hydroxides as the iron source and doping metal source, combined with sodium, phosphorus, and carbon sources, the doped sodium iron pyrophosphate cathode material is prepared in a solid phase. The atomic-level uniform distribution of metal ions in the layered double hydroxides promotes more uniform doping of metal elements in the sodium iron pyrophosphate, thereby further improving the specific capacity and rate performance of the product.

[0032] The present invention will be described in detail below through examples and comparative examples, but the scope of protection of the present invention is not limited to these examples. Unless otherwise specified, the chemical reagents and raw materials used in the following examples and comparative examples are all conventional commercially available products.

[0033] Example 1 This embodiment discloses a doped sodium iron pyrophosphate cathode material, using ferric nitrate nonahydrate as the iron source, nickel nitrate hexahydrate as the doping metal source, sodium hydroxide solution as the alkali solution, sodium dihydrogen phosphate as the sodium and phosphorus sources, and glucose as the carbon source to prepare the doped sodium iron pyrophosphate cathode material. Specifically, (I) Prepare 200 mL of aqueous solution containing 2.0 mol / L ferric nitrate nonahydrate and 1.0 mol / L nickel nitrate hexahydrate, add 2 mol / L sodium hydroxide solution dropwise until the pH is 12, continue stirring for 12 h, filter, wash with deionized water, and dry at 80℃ for 15 h to obtain layered double hydroxide; (II) The layered double hydroxide from step (I), 0.8 mol sodium dihydrogen phosphate, and 10 wt% glucose were poured into a ball mill and ball-milled (ball-to-material ratio of 15:1, ball milling speed of 550 rpm, and ball milling time of 10 h) to obtain a precursor; the obtained precursor was calcined at 550 °C for 12 h in a nitrogen atmosphere to obtain a doped sodium iron pyrophosphate cathode material.

[0034] This embodiment also discloses a battery cathode, including the sodium iron pyrophosphate cathode material of this embodiment.

[0035] Figure 1 The XRD pattern shows that the material obtained in Example 1 matches the standard spectrum of Na4Fe3(PO4)2P2O7, and no other impurity phases are generated; Figure 2 SEM images show that the material obtained in Example 1 has a blocky structure and a uniform particle size distribution; Figure 3 The charge-discharge curves show a typical sodium iron pyrophosphate charge-discharge platform of Example 1, with a specific capacity of 115 mAh / g at 0.2C.

[0036] Example 2-14 Examples 2-14 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in that the iron source is different, while the other experimental parameters and procedures are the same as in Example 1. The iron sources in Examples 2-14 are detailed in Table 1 below.

[0037] Examples 2-14 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 2-14 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 2-14.

[0038] Examples 15-57 Examples 15-57 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in that the doping metal source is different, while the other experimental parameters and procedures are the same as in Example 1. The doping metals in Examples 15-57 are detailed in Table 2 below.

[0039] Examples 15-57 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 15-57 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 15-57.

[0040] Examples 58-65 Examples 58-65 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in the molar ratio of iron source to doped metal source. All other experimental parameters and procedures are the same as in Example 1. The molar ratio of iron source to doped metal source in Examples 58-65 is detailed in Table 3 below.

[0041] Examples 58-65 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 58-65 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 58-65.

[0042] Examples 66-71 Examples 66-71 disclose a series of doped sodium iron pyrophosphate cathode materials. The difference between these and Example 1 lies in the concentrations of the iron source and the doping metal source used to prepare the layered double hydroxides. All other experimental parameters and procedures are the same as in Example 1. The concentrations of the iron source and the doping metal source used to prepare the layered double hydroxides in Examples 66-71 are detailed in Table 4 below.

[0043] Examples 66-71 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 66-71 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 66-71.

[0044] Examples 72-76 Examples 72-76 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in that the pH used for preparing the layered double hydroxides is different; all other experimental parameters and steps are the same as in Example 1. The pH values ​​used for preparing the layered double hydroxides in Examples 72-76 are detailed in Table 5 below.

[0045] Examples 72-76 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 72-76 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 72-76.

[0046] Examples 77-81 Examples 77-81 disclose a series of doped sodium iron pyrophosphate cathode materials. The difference between these and Example 1 lies in the alkaline solution used to prepare the layered double hydroxides; all other experimental parameters and procedures are the same as in Example 1. Details of the alkaline solutions used in Examples 77-81 for preparing the layered double hydroxides are shown in Table 6 below.

[0047] Examples 77-81 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 77-81 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 77-81.

[0048] Examples 82-88 Examples 82-88 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in that they have different high-temperature calcination temperatures, while the other experimental parameters and procedures are the same as in Example 1. The high-temperature calcination temperatures in Examples 82-88 are detailed in Table 7 below.

[0049] Examples 82-88 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 82-88 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 82-88.

[0050] Examples 89-116 Examples 89-116 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in the types and amounts of sodium and phosphorus sources used. All other experimental parameters and procedures are the same as in Example 1. The types and amounts of sodium and phosphorus sources used in Examples 89-116 are detailed in Table 8 below.

[0051] Examples 89-116 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 89-116 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 89-116.

[0052] Examples 117-121 Examples 117-121 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in the amount of carbon source used. All other experimental parameters and procedures are the same as in Example 1. The amounts of carbon source used in Examples 117-121 are detailed in Table 9 below.

[0053] Examples 117-121 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 117-121 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 117-121.

[0054] Examples 122-134 Examples 122-134 disclose a series of doped sodium iron pyrophosphate cathode materials, which differ from Example 1 in that they use different types of carbon sources. All other experimental parameters and procedures are the same as in Example 1. The types of carbon sources used in Examples 122-134 are detailed in Table 10 below.

[0055] Examples 122-134 also disclose a series of battery cathodes, which differ from Example 1 only in that the battery cathodes of Examples 122-134 respectively include the doped sodium iron pyrophosphate cathode material prepared in Examples 122-134.

[0056] Comparative Example 1 This comparative example discloses a doped sodium iron pyrophosphate cathode material prepared by a conventional solid-state method. The preparation process includes: weighing 0.4 mol of ferric nitrate nonahydrate, 0.2 mol of nickel nitrate hexahydrate, 0.8 mol of sodium dihydrogen phosphate, and 10 wt% glucose and pouring them into a ball mill for ball milling (ball-to-material ratio of 10:1, ball milling speed of 550 rpm, and ball milling time of 5 h) to obtain a precursor; calcining the obtained precursor at 550 °C for 12 h in a nitrogen atmosphere to obtain the doped sodium iron pyrophosphate cathode material.

[0057] Comparative Example 1 discloses a sodium-ion battery cathode, corresponding to the sodium iron pyrophosphate cathode material prepared in this comparative example.

[0058] Performance Test Results and Analysis Performance tests were conducted on the half-cells of each embodiment and comparative example, specifically including: The preparation process of a sodium-ion half-cell is as follows: Positive electrode: The doped sodium iron pyrophosphate positive electrode material prepared in Examples 1-134 and Comparative Example 1 is provided with carbon black and polyvinylidene fluoride (mass ratio 8:1:1), and N-methylpyrrolidone is added to form positive electrode active material. The positive electrode current collector is coated with carbon aluminum foil (16 μm thick) and cleaned. Then, the positive electrode active material is uniformly coated on the surface of the positive electrode current collector (coating thickness 200 μm). After the positive electrode active material layer is completely dry, it is cut to obtain the battery positive electrode of the required size. Counter electrode: Sodium metal sheet (thickness 400-500 μm); Electrolyte: 1 mol / L NaPF6 / PC-EC (1:1, v / v) + 5% FEC; Assembly (assembling a half-cell according to the sodium-ion battery assembly method) includes: assembling the positive electrode and the counter electrode, and injecting electrolyte to obtain a half-cell. The half-cells obtained in Examples 1-134 are numbered B1-B134 sequentially, and the half-cell obtained in Comparative Example 1 is numbered C1.

[0059] The sodium-ion half-cells of Examples 1-134 and Comparative Example 1 were subjected to performance tests, including specific capacity (mAh / g) at 0.2C (1C = 129 mA / g) and 5C. It was found that the specific capacity of the doped sodium iron pyrophosphate was as high as 125 mAh / g at 0.2C and as high as 88 mAh / g at 5C, which is superior to the 103 mAh / g (0.2C) and 66 mAh / g (5C) of Comparative Example 1. This indicates that the doped sodium iron pyrophosphate prepared in this invention has excellent capacity and rate performance in sodium-ion half-cells.

[0060] In addition, as shown in Table 1-10, the type of iron source has little impact on the material properties in this invention; among the doped metal sources, cobalt source and manganese source have the best doping effect; the molar ratio and concentration of iron source and doped metal source, the type of alkali solution, the pH of the preparation of layered double hydroxide, the type and amount of sodium source and phosphorus source, the calcination temperature, and the type and amount of carbon source all affect the material properties, and precise control is required during the preparation process to achieve the best sodium storage performance.

[0061] The specific test results are shown in Table 1-10: Table 1 ; Table 2 ; Table 3 ; Table 4 ; Table 5 ; Table 6 ; Table 7 ; Table 8 ; Table 9 ; Table 10

[0062] The present invention provides a sodium iron pyrophosphate doped cathode material with good uniformity and more even distribution of dopant elements in the product. When the cathode active material is used to prepare the cathode of a sodium-ion battery, the sodium-ion battery exhibits high specific capacity and rate performance. The preparation method of the sodium-ion battery cathode includes the following steps: weighing the cathode active material, conductive agent, and binder in a certain proportion, adding them to an appropriate solvent, and mixing thoroughly to form a uniform slurry to form a cathode active material layer; cleaning the cathode current collector, then uniformly coating the cathode active material layer onto the surface of the cathode current collector; after the cathode active material layer is completely dry, cutting it to obtain the desired battery cathode size. The obtained sodium-ion battery cathode can be used to prepare sodium-ion batteries.

[0063] The positive electrode active material is selected from one of the doped sodium iron pyrophosphate positive electrode materials; the positive electrode current collector is one of aluminum foil, carbon-coated aluminum foil, iron foil, tin foil, zinc foil, nickel foil, titanium foil, and manganese foil; the content of the doped sodium iron pyrophosphate positive electrode material is 60-95 wt%.

[0064] This invention uses layered double hydroxides as the iron source and doping metal source, combined with sodium, phosphorus, and carbon sources to prepare doped sodium iron pyrophosphate cathode materials in the solid phase. The uniform atomic-level distribution of each metal element in the layered double hydroxides allows for more uniform doping of the metal elements in the sodium iron pyrophosphate, fundamentally achieving synergistic optimization of the entire crystal framework. By homogenizing and adjusting the electronic structure of the transition metal elements, optimizing the sodium ion migration barrier, and stabilizing the crystal framework, the electronic conductivity and sodium ion diffusion rate are synergistically improved, while suppressing phase transitions and volume changes during cycling, ultimately enhancing the specific capacity and rate performance of the product.

[0065] This invention uses layered double hydroxides as the iron source and doping metal source, and combines sodium, phosphorus and carbon sources to prepare doped sodium iron pyrophosphate cathode materials in the solid phase. The uniform atomic-level distribution of metal ions in the layered double hydroxides promotes more uniform doping of metal elements in sodium iron pyrophosphate, further improving the specific capacity and rate performance of the product.

[0066] Although the technical solutions of the present invention have been described and enumerated in detail, it should be understood that modifications to the above embodiments or the adoption of equivalent alternatives are obvious to those skilled in the art. Such modifications or improvements made without departing from the spirit of the present invention are all within the scope of protection claimed by the present invention.

Claims

1. A sodium iron phosphate-doped pyrophosphate cathode material, characterized in that, The positive electrode material uses a layered double hydroxide as an iron source and a doping metal source, and iron and doping metal elements are uniformly distributed; the doping metal element in the positive electrode material is one or more of magnesium, zinc, nickel, cobalt, calcium, copper and manganese.

2. A method for preparing the doped sodium iron phosphate pyrophosphate positive electrode material according to claim 1, characterized in that: The method comprises the following steps: Step one, mixing the iron source and the doping metal source with an alkali solution, separating, washing and drying after reaction to obtain a layered double hydroxide; Step two, high-temperature calcination of a precursor obtained by ball milling the layered double hydroxide, a sodium source, a phosphorus source and a carbon source to obtain the doped phosphoric acid pyrophosphate iron sodium positive electrode material.

3. The preparation method of the doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The molar ratio of the iron source to the doping metal source is 29.9:0.1-0.1:29.9, the concentration of the iron source is 0.1-4 mol / L, the concentration of the doping metal source is 0.1-4 mol / L, and the concentration of the alkali solution is 0.5-10 mol / L; The molar ratio of Na, Fe and doping metal and P in the iron source, the doping metal ion source, the sodium source and the phosphorus source is 3.75-4.25:2.75-3.25:3.75-4.

25.

4. The preparation method of the doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The addition amount of the carbon source accounts for 1-25 wt% of the total mass of the precursor.

5. The preparation method of the doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The iron source is one or more of ferrous nitrate, ferrous chloride, ferrous sulfate, ferrous acetate, ferrous perchlorate, ferrous bromide, ferrous iodide, ferric nitrate, ferric chloride, ferric sulfate, ferric acetate, ferric perchlorate, ferric bromide, ferric iodide and possible hydrates thereof.

6. The method for preparing a doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The doping metal source is one or more of magnesium nitrate, magnesium chloride, magnesium sulfate, magnesium acetate, magnesium perchlorate, magnesium bromide, magnesium iodide, zinc nitrate, zinc chloride, zinc sulfate, zinc acetate, zinc perchlorate, zinc bromide, zinc iodide, nickel nitrate, nickel chloride, nickel sulfate, nickel acetate, nickel bromide, nickel iodide, cobalt nitrate, cobalt chloride, cobalt sulfate, cobalt acetate, cobalt bromide, cobalt iodide, calcium nitrate, calcium chloride, calcium acetate, calcium perchlorate, calcium bromide, calcium iodide, copper nitrate, copper chloride, copper sulfate, copper acetate, copper perchlorate, copper bromide, manganese chloride, manganese nitrate, manganese sulfate, manganese acetate, manganese bromide, manganese iodide and possible hydrates thereof.

7. The method for preparing a doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The alkali solution is one or more of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, potassium carbonate, ammonium carbonate.

8. The method for preparing a doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, sodium sulfate, sodium chloride, sodium oxalate, sodium citrate, sodium phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium pyrophosphate; The phosphorus source is one or more of sodium dihydrogen phosphate, sodium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, ferric pyrophosphate and ferrous pyrophosphate; The carbon source is one or more of glucose, sucrose, starch, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, citric acid, ethylenediaminetetraacetic acid, pyromellitic acid, cyclodextrin, graphene, graphene oxide, carbon nanotube and Ketjen black.

9. The method for preparing a doped sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized by: The pH value of the reaction is 8-13, the reaction temperature is normal temperature-90℃, and the reaction time is 4-72 h; The separation process can be centrifugation or suction filtration, the washing is performed with one or more of deionized water and ethanol, the drying temperature is 60-150℃, and the drying time is 8-48 h; The ball-to-material ratio of the ball milling process is 35:1-4:1, the ball milling rotation speed is 150-850 rpm, and the ball milling time is 1-48 h; The high-temperature calcination atmosphere is one or more of nitrogen, argon, hydrogen-argon mixed gas, the calcination temperature is 400-750 DEG C, and the calcination time is 4-24 h.

10. Use of a doped sodium iron phosphate pyrophosphate positive electrode material prepared according to any one of claims 2 to 9, characterized in that: The positive electrode material is used as a positive electrode active material and for preparing a sodium ion battery positive electrode.

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