In-situ doped composite sodium ferric phosphate positive electrode material, preparation method thereof and sodium ion battery

Abstract

The invention belongs to the technical field of sodium ion battery positive electrode material preparation, and relates to an in-situ doped composite sodium ferric phosphate positive electrode material and a preparation method thereof, and a sodium ion battery. The structural general formula of the composite material is Na (4-2x) AxFe3 (PO4) 2P2O7, the general formula of a precursor is Na (1-2x) AxFe3 (PO4) 2P2O7, x is greater than or equal to 0 and less than or equal to 0.2, and an element A is Ca and Ba modified metal. The doped and modified composite sodium ferric phosphate is synthesized by adopting a precursor two-step method, and Na site is replaced by Ca and Ba elements. The in-situ doped composite sodium ferric phosphate positive electrode material prepared by the method can replace inert sodium ions at the Na2 position of composite sodium ferric phosphate, lattice parameters are adjusted by doping elements, the intrinsic conductivity of the composite sodium ferric phosphate is improved, the discharge capacity is further increased, the energy density of a sodium ion battery is improved, and the service life of the sodium ion battery is prolonged. The problems of low specific discharge capacity, poor stability and the like of the composite sodium ferric phosphate material are effectively solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing cathode materials for sodium-ion batteries, and relates to an in-situ doped composite sodium iron phosphate cathode material, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] Sodium-ion batteries have received extensive attention in large-scale energy storage systems and smart grids due to their abundant sodium sources and low costs. Looking ahead, the key lies in developing cathode materials with stable structures and excellent performance to address the challenges posed by the relatively low energy density and slow electrochemical kinetics of sodium-ion batteries. Currently, researchers are focusing on studying three types of cathode materials: layered transition metal oxides, polyanion compounds, and Prussian blue compounds. Among them, polyanion compounds have received research attention due to their high working voltage and stable structure.

[0003] Iron-based polyanion compounds are considered the most commercially promising cathode materials due to their low cost, environmental friendliness, and abundant resources. Due to the successful application of LiFePO4 materials in lithium-ion batteries, NaFePO4 has become a research hotspot. However, olivine-type NaFePO4 must be obtained through the electrochemical reaction of LiFePO4, and this process is not suitable for large-scale preparation. Sodium pyrophosphate Na2FeP2O7 shows an open three-dimensional Na + channel due to the strong inductive effect of the P2O7 group and allows a higher working voltage (~3.0V). However, the high molecular weight of Na2FeP2O7 results in a theoretical capacity of only 97 mAh / g, thus limiting its commercial application. Mixed iron-based polyanion compounds (typically Na4Fe3(PO4)2P2O7, abbreviated as NFPP) have the advantages of phosphates and pyrophosphates, showing higher structural stability and accelerated electrochemical kinetics. In Na4Fe3(PO4)2P2O7, four sodium ions occupy four different coordinations respectively: Na1 (6-coordination), Na2 (7-coordination), Na3 (5-coordination), and Na4 (6-coordination). Among them, Na1, Na3, and Na4 are active sodium ions and will participate in the deintercalation and insertion reactions. However, the sodium ion at the Na2 position has a higher deintercalation reaction barrier due to its high coordination and is inert during the charge and discharge process. Therefore, during charging, Na4Fe3(PO4)2P2O7 will deintercalate three sodium ions to form NaFe3(PO4)2P2O7, and during discharging, NaFe3(PO4)2P2O7 will insert three sodium ions to form Na4Fe3(PO4)2P2O7.

[0004] NFPP is associated with electrochemically inert NaFePO4 impurities during the synthesis process and Na +The slow diffusion kinetics severely limits the rate performance and energy density of NFPP. Existing patents such as CN 117794854 A, CN 118588925 A, and CN117936755 A propose to improve the electrochemical performance of NFPP by element doping to replace part of the iron element or anion group. However, the existing NFPP materials still have problems such as low discharge specific capacity and poor cycle stability. In addition, some patents (CN 118851132 A, CN 117558905 A) also propose to replace part of the sodium element by element doping, but direct sodium element substitution will preferentially replace the sodium ions at the Na1, Na3, and Na4 sites, resulting in a decrease in the content of active sodium ions and further reducing the theoretical specific capacity of NFPP. Therefore, it is necessary to further optimize the material preparation process to achieve the ideal performance indicators for industrial applications. Summary of the Invention

[0005] Aiming at the problems existing in the above technologies, the purpose of the present invention is to provide a preparation method of in-situ doped composite sodium iron phosphate. In this method, the precursor two-step method is adopted for preparation. The first step includes: mixing an iron source, a phosphorus source, a sodium source, and a modified metal source, drying them, and sintering them at a specified temperature to obtain Na 1-2x A x Fe3(PO4)2P2O7 containing only inert sodium ions, where 0 ≤ x ≤ 0.2, and the A element = Ca, Ba modified metal; the second step includes: mixing the precursor with the remaining sodium source and carbon source, drying them, and then sintering them under an inert atmosphere to obtain a high-performance modified doped composite sodium iron phosphate cathode material.

[0006] To achieve the above invention purpose, the present invention is realized through the following technical solutions:

[0007] A preparation method of in-situ doped composite sodium iron phosphate, the material has a structural general formula of Na 4-2x A x Fe3(PO4)2P2O7, and the precursor general formula is Na 1-2x A x Fe3(PO4)2P2O7, where 0 ≤ x ≤ 0.2, and the A element = Ca, Ba modified metal. The preparation method of synthesizing the in-situ doped composite sodium iron phosphate cathode material for sodium-ion batteries by the precursor two-step method includes the following steps:

[0008] 1) Calculate and weigh a certain amount of iron source, phosphorus source, sodium source, and modified metal source according to the stoichiometry of the precursor, then mix the raw materials with water evenly, and carry out ball milling to obtain a precursor slurry; then dry, crush, and screen the precursor slurry to obtain a powder;

[0009] 2) Carry out the first heat treatment on the powder obtained in step 1) at a certain temperature to obtain a precursor;

[0010] 3) Weigh a certain amount of the precursor obtained in step 2), supplement the sodium source and carbon source, obtain the slurry again, and then dry the slurry.

[0011] 4) Heat-treat the material obtained in step 3) twice under an inert atmosphere to obtain an in-situ doped composite sodium iron phosphate cathode material.

[0012] According to a further embodiment of the present invention, in step 1), the iron source includes any one or a combination of at least two of ferric nitrate, ferrous nitrate, ferrous oxalate, ferric sulfate, ferrous sulfate, iron oxide, ferric citrate, ferrous citrate, or ferric pyrophosphate.

[0013] According to a further embodiment of the present invention, in step 1), the phosphorus source includes any one or a combination of at least two of disodium hydrogen phosphate, sodium dihydrogen phosphate, ferrous phosphate, ferric phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, or sodium pyrophosphate.

[0014] According to a further embodiment of the present invention, in step 1), the sodium source includes any one or a combination of at least two of disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, or sodium pyrophosphate.

[0015] According to a further embodiment of the present invention, in step 1), the modified metal source includes at least one of calcium oxide, calcium carbonate, calcium sulfate, calcium nitrate, calcium hydrogen phosphate, calcium dihydrogen phosphate; magnesium oxide, magnesium carbonate, magnesium sulfate, magnesium nitrate, magnesium hydrogen phosphate, magnesium dihydrogen phosphate, etc.

[0016] According to a further embodiment of the present invention, in step 1), the particle size D50 of the precursor slurry obtained after ball milling is ≤ 0.5 μm.

[0017] According to a further embodiment of the present invention, in step 2), the first heat treatment temperature is 300 - 700 °C (specifically, it can be 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, etc.), and the heat treatment time is 2 - 15 h (specifically, it can be 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, etc.).

[0018] According to a further embodiment of the present invention, in step 3), the carbon source includes at least one of starch, glucose, sucrose, citric acid, or ascorbic acid.

[0019] According to a further embodiment of the present invention, in step 3), the mixing method includes any one or a combination of at least two of stirring, ultrasonic oscillation, mortar grinding, ball milling or sand milling; and / or the drying method includes any one or a combination of at least two of gel drying, vacuum drying, spray drying or freeze drying.

[0020] According to a further embodiment of the present invention, in step 4), the inert atmosphere includes one of nitrogen, argon, argon-hydrogen mixture, nitrogen-hydrogen mixture.

[0021] According to a further embodiment of the present invention, the secondary heat treatment in step 4) includes pre-sintering and phase-forming sintering (the phase-forming sintering temperature must be higher than the pre-sintering temperature). The pre-sintering temperature is 300-400 °C (specifically, it can be 300 °C, 310 °C, 320 °C, 330 °C, 340 °C, 350 °C, 360 °C, 370 °C, 380 °C, 390 °C, 400 °C, etc.), and the sintering time is 2-6 h (specifically, it can be 2 h, 3 h, 4 h, 5 h, 6 h, etc.); the phase-forming sintering temperature is 400-700 °C (specifically, it can be 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, etc.), and the sintering time is 5-15 h (specifically, it can be 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, etc.); the sintering heating rate is 1-10 °C / min (specifically, it can be 1 °C / min, 2 °C / min, 3 °C / min, 4 °C / min, 5 °C / min, 6 °C / min, 7 °C / min, 8 °C / min, 9 °C / min, 10 °C / min, etc.).

[0022] According to a further embodiment of the present invention, the D 50 particle size of the in-situ doped composite sodium iron phosphate cathode material obtained in step 4) is 5-10 μm.

[0023] This application also protects the in-situ doped composite sodium iron phosphate material directly obtained by the above method.

[0024] The present invention also provides a sodium ion battery, in which the in-situ doped composite sodium iron phosphate cathode material prepared by the above method is used.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] (1) The in-situ doped and modified composite sodium iron phosphate cathode material disclosed and provided by the present invention introduces a doping element Ca element or Ba element with a radius similar to that of Na ions, which can optimize the NFPP material on the premise of ensuring the proportion of transition metal elements remains unchanged, improve the intrinsic conductivity, and effectively solve the problem of low discharge specific capacity of the NFPP material;

[0027] (2) The in-situ doped and modified composite sodium iron phosphate material disclosed and provided by the present invention is applied to the preparation of sodium-ion battery electrode materials. The precursor two-step method is innovatively adopted, which can stabilize the crystal structure during the process of Na + insertion / extraction, improve the stability of the material, is conducive to obtaining a more pure-phase target substance, increase its electrode potential, effectively improve the energy density of the sodium-ion battery, and significantly improve and enhance the electrochemical performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 XRD patterns of the composite sodium iron phosphate materials prepared in Example 1-1, Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 3;

[0029] Figure 2 Scanning electron microscope images of the composite sodium iron phosphate materials prepared in Example 1-1, Example 2 and Comparative Example 1;

[0030] Figure 3 Charge-discharge curves of the coin-type half-cells prepared using the composite sodium iron phosphate materials prepared in Example 1-1, Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 3 at a current of 0.1C. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The present invention relates to an in-situ doped composite sodium iron phosphate cathode material and a preparation method thereof. The technical and technological preparation schemes in the embodiments of the present invention will be described in detail and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0032] The raw materials used in the following examples are all commercially available or self-made; unless otherwise specified, their mass percentages are indicated.

[0033] Example 1-1:

[0034] This example provides a preparation method of an in-situ doped composite sodium iron phosphate cathode material. In this method, a two-step "ball milling - calcination" process is adopted to prepare a composite cathode material with the chemical formula Na 3.8 Ca 0.1 Fe3(PO4)2P2O7, and its precursor chemical formula is Na 0.8 Ca 0.1 Fe3(PO4)2P2O7. Among them, iron phosphate is used as the iron source, iron phosphate, sodium dihydrogen phosphate and calcium dihydrogen phosphate are used as the phosphorus source, sodium dihydrogen phosphate and sodium carbonate are used as the sodium source, calcium dihydrogen phosphate is used as the calcium source, and anhydrous glucose is used as the carbon source.

[0035] The specific preparation method includes the following steps:

[0036] 1) According to the elemental stoichiometry of the precursor material Na 0.8 Ca 0.1 Fe3(PO4)2P2O7, weigh 0.3 mol of iron phosphate, 0.08 mol of sodium dihydrogen phosphate, and 0.01 mol of calcium dihydrogen phosphate respectively;

[0037] 2) Add all the materials weighed in step 1) into the ball milling tank, add deionized water, stir and mix evenly, and then ball mill at a speed of 300 rpm for 8 h to obtain a slurry with a slurry particle size D50 ≤ 0.5 μm;

[0038] 3) Transfer the slurry obtained by ball milling to an oven at 80 °C for drying to completely remove the moisture therein. After drying, use a high-speed grinder to crush the material, and use a 300-mesh sieve to screen the material to collect the undersize material;

[0039] 4) Load the material obtained in step 3) into a crucible, heat it at a heating rate of 5 °C / min and hold it at 550 °C for 10 h. After the material naturally cools to room temperature, Na 0.8 Ca 0.1 Fe3(PO4)2P2O7 precursor can be obtained;

[0040] 5) Weigh 0.1 mol of the precursor obtained in step 4), 0.15 mol of sodium carbonate, and a certain amount of anhydrous glucose into the ball milling tank for ball milling (the dosage of glucose is 12% of the theoretical output of sodium iron phosphate composite), obtain a slurry with a particle size D50 ≤ 0.5 μm, dry, crush the slurry and screen it through 300 meshes to collect the undersize material;

[0041] 6) Load the undersize material obtained in step 5) into a crucible, heat it at a heating rate of 5 °C / min under a nitrogen atmosphere protection and hold it at 350 °C for 4 h, then heat it at a heating rate of 5 °C / min and hold it at 500 °C for 10 h. After the material naturally cools to room temperature, Na 3.8 Ca 0.1 Fe3(PO4)2P2O7 cathode material can be obtained.

[0042] Examples 1-2:

[0043] The specific preparation method steps of this example are similar to those of Example 1-1, the difference is that: the chemical formula is Na 3.9 Ca 0.05 Fe3(PO4)2P2O7, and the chemical formula of the precursor is Na 0.9 Ca 0.05Fe3(PO4)2P2O7. Step 1): Weigh 0.3 mol of iron phosphate, 0.09 mol of sodium dihydrogen phosphate, and 0.005 mol of calcium dihydrogen phosphate respectively.

[0044] Examples 1 - 3:

[0045] The specific preparation method steps of this example are similar to those of Example 1 - 1, except that: the chemical formula is Na 3.7 Ca 0.15 Fe3(PO4)2P2O7, and the precursor chemical formula is Na 0.7 Ca 0.15 Fe3(PO4)2P2O7. Step 1): Weigh 0.3 mol of iron phosphate, 0.07 mol of sodium dihydrogen phosphate, and 0.015 mol of calcium dihydrogen phosphate respectively.

[0046] Example 1 - 4:

[0047] The specific preparation method steps of this example are similar to those of Example 1 - 1, except that in step 4): Load the material obtained in step 3) into a crucible, and keep it at 450 °C for 14 h at a heating rate of 5 °C / min. After the material is naturally cooled to room temperature, the Na 0.8 Ca 0.1 Fe3(PO4)2P2O7 precursor can be obtained. The rest of the process is the same as that in Example 1 - 1.

[0048] Example 1 - 5:

[0049] The specific preparation method steps of this example are similar to those of Example 1 - 1, except that in step 4): Load the material obtained in step 3) into a crucible, and keep it at 650 °C for 6 h at a heating rate of 5 °C / min. After the material is naturally cooled to room temperature, the Na 0.8 Ca 0.1 Fe3(PO4)2P2O7 precursor can be obtained. The rest of the process is the same as that in Example 1 - 1.

[0050] Example 2:

[0051] This example provides a preparation method of an in - situ doped composite sodium iron phosphate cathode material. In this method, a two - step "ball milling - calcination" process is used to prepare a composite cathode material with the chemical formula of Na 3.8 Ba 0.1 Fe3(PO4)2P2O7, and its precursor chemical formula is Na 0.8 Ba 0.1 Fe3(PO4)2P2O7. Among them, iron phosphate is used as the iron source, iron phosphate, sodium dihydrogen phosphate, and barium dihydrogen phosphate are used as the phosphorus source, sodium dihydrogen phosphate and sodium carbonate are used as the sodium source, barium dihydrogen phosphate is used as the barium source, and anhydrous glucose is used as the carbon source.

[0052] The preparation method comprises the following steps:

[0053] 1) According to the elemental stoichiometry of the precursor Na 0.8 Ba 0.1 Fe3(PO4)2P2O7 material, 0.3 mol of iron phosphate, 0.08 mol of sodium dihydrogen phosphate, and 0.01 mol of barium dihydrogen phosphate are weighed respectively;

[0054] 2) Add all the materials weighed in step 1) into a ball milling tank, add deionized water, stir and mix evenly, and then ball mill at a rotation speed of 300 rpm for 8 h to obtain a slurry with a slurry particle size D50 ≤ 0.5 μm;

[0055] 3) Transfer the slurry obtained by ball milling to an oven at 80 °C for drying to completely remove the moisture therein. After drying, use a high-speed grinder to crush the material, and use a 300-mesh sieve to screen the material to collect the undersize;

[0056] 4) Load the material obtained in step 3) into a crucible, heat it at a heating rate of 5 °C / min and hold it at 550 °C for 10 h. After the material is naturally cooled to room temperature, the Na 0.8 Ba 0.1 Fe3(PO4)2P2O7 precursor material can be obtained;

[0057] 5) Weigh 0.1 mol of the precursor obtained in step 4), 0.15 mol of sodium carbonate, and a certain amount of anhydrous glucose into a ball milling tank for ball milling (the dosage of glucose is 12% of the theoretical yield of sodium iron phosphate composite), obtain a slurry with a particle size D50 ≤ 0.5 μm, dry, crush the slurry and screen it through 300 meshes, and collect the undersize;

[0058] 6) Load the undersize obtained in step 5) into a crucible, heat it at a heating rate of 5 °C / min under a nitrogen atmosphere protection and hold it at 350 °C for 4 h, then heat it at a heating rate of 5 °C / min and hold it at 500 °C for 10 h. After the material is naturally cooled to room temperature, the Na 3.8 Ba 0.1 Fe3(PO4)2P2O7 cathode material can be obtained.

[0059] Comparative example 1:

[0060] Use a one-step "ball milling - calcination" process to prepare Na 3.8 Ca 0.1 Fe3(PO4)2P2O7, where iron phosphate is used as the iron source, iron phosphate, sodium dihydrogen phosphate, and calcium dihydrogen phosphate are used as the phosphorus source, sodium dihydrogen phosphate and sodium carbonate are used as the sodium source, calcium dihydrogen phosphate is used as the calcium source, and anhydrous glucose is used as the carbon source.

[0061] The specific preparation steps are as follows:

[0062] 1) According to the stoichiometric ratios of Na 3.8 Ca 0.1 Fe3(PO4)2P2O7, weigh 0.3 mol of iron phosphate, 0.08 mol of sodium dihydrogen phosphate, 0.01 mol of calcium dihydrogen phosphate, 0.15 mol of sodium carbonate and a certain amount of anhydrous glucose (the amount of glucose used is 12% of the theoretical yield of sodium iron phosphate composite) respectively;

[0063] 2) Add all the materials weighed in step 1) into the ball milling tank, add deionized water, stir and mix evenly, and then ball mill at a rotation speed of 300 rpm for 8 h to obtain a slurry with a slurry particle size D50 ≤ 0.5 μm;

[0064] 3) Transfer the slurry obtained by ball milling to an oven at 80 °C for drying to completely remove the moisture therein. After drying, use a high-speed grinder to crush the materials, and use a 300-mesh sieve to screen the materials to collect the materials passing through the sieve;

[0065] 4) Load the materials obtained in step 3) into a crucible, heat at a heating rate of 5 °C / min under a nitrogen atmosphere protection at 350 °C for 4 h, and then heat at a heating rate of 5 °C / min at 500 °C for 10 h. After the materials are naturally cooled to room temperature, the Na 3.8 Ca 0.1 Fe3(PO4)2P2O7 cathode material can be obtained.

[0066] Comparative Example 2:

[0067] Prepare Na4Fe3(PO4)2P2O7 by a one-step "ball milling - calcination" process. The specific preparation process is similar to that of Comparative Example 1, except that: no doping metal source is added, and only iron phosphate, sodium dihydrogen phosphate, sodium carbonate and anhydrous glucose are used as raw materials, and the rest of the process is the same as that in Comparative Example 1.

[0068] Comparative Example 3:

[0069] Prepare undoped Na4Fe3(PO4)2P2O7 by a two-step "ball milling - calcination" process. The specific preparation process is similar to that of Example 1-1, except that: no doping metal source is added, and only iron phosphate, sodium dihydrogen phosphate, sodium carbonate and anhydrous glucose are used as raw materials, and the rest of the process is the same as that in Example 1-1.

[0070] The detailed ingredient information is shown in Table 1:

[0071] Table 1

[0072]

[0073]

[0074] To verify the usage effect of the positive electrode material of the present invention, the materials prepared in each of Examples 1-2 and Comparative Examples 1-3 were respectively prepared into positive electrode sheets. The preparation method was to mix the positive electrode material, acetylene black, and PVDF in a mass ratio of 94:3:3 to form a homogeneous slurry, and then use a 200-μm four-sided spreader to uniformly coat the slurry on the aluminum foil. Then, the film was placed in a blast drying oven at 100°C and dried for 8 hours. The electrode film was punched into a circular sheet with a diameter of 14 mm using a punching machine. The cut electrode circular sheet was placed in a vacuum drying oven at 100°C and dried for 4 hours, and then transferred to a glove box. Using a metal Na circular sheet as the counter electrode, NaPF6 as the electrolyte, and Whatman GF / D glass fiber diaphragm as the separator, a CR2016 type button battery was assembled in the glove box, and a constant current charge-discharge test was carried out at 0.1C. The specific results are shown in Table 2 and Figures 1-3 .

[0075] Table 2 Comparison of Electrochemical Performance of Examples 1-1 to 5, Example 2 and Comparative Examples 1-3

[0076]

[0077]

[0078] As can be seen from the comparison in Table 2, when the dosages of the doping elements are different, the electrochemical performances they exhibit are different. Among them, Na 3.8 Ca 0.1 Fe3(PO4)P2O7 (Example 1-1) compared to Na 3.9 Ca 0.05 Fe3(PO4)P2O7 (Example 1-2) and Na 3.7 Ca 0.15Fe3(PO4)P2O7 (Examples 1 - 3) has a higher 0.1C discharge specific capacity, reaching 111.98 mAh / g. At different sintering temperatures, the discharge specific capacity of Example 1 - 1 at 0.1C is also better than that of Example 1 - 4 and Example 1 - 5. The samples prepared by the technical route of in-situ doping of sodium iron phosphate composite (Example 1 - 1, Example 2) using a two-step method have a reversible capacity exceeding that of the samples without in-situ doping (Comparative Example 3). Compared with the sample doped with Ba (Example 2), the sodium iron phosphate composite doped with Ca (Example 1 - 1) exhibits the highest charge-discharge performance (at 0.1C, the reversible capacity is 111.98 mAh / g). In the case of doping with Ca, the samples prepared by the two-step process (Example 1 - 1) have significantly better charge-discharge performance than the one-step process (Comparative Example 1). In addition, even without any doping, by comparing the results of Comparative Example 3 and Comparative Example 2, it can also be found that the performance of the samples prepared by the two-step process is better than that of the one-step process. Obviously, producing in-situ doped sodium iron phosphate composite by the technical route provided by the present invention can improve the material activity, increase the reversible capacity, and enhance the energy density of sodium-ion batteries.

[0079] Figure 1 XRD patterns of the sodium iron phosphate composite materials prepared for Example 1 - 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3. As Figure 1 can be seen, by comparing with the PDF card of Na4Fe3(PO4)2(P2O7), the diffraction peaks of the samples in different examples and comparative examples correspond one by one to the PDF card, and there are no obvious impurity peaks, indicating that the examples and comparative examples can all successfully synthesize sodium iron phosphate composite without obvious impurity phases.

[0080] Figure 2 Scanning electron microscope images of the sodium iron phosphate composite materials prepared for Example 1 - 1, Example 2, and Comparative Example 1. As Figure 2 can be seen, Example 1 - 1 and Example 2 are in the shape of irregular particles, and the particle size is distributed between 1 - 2 μm. The particles in Comparative Example 1 are smaller, and the particle size is distributed between 0.1 - 0.5 μm.

[0081] Figure 3 Charge-discharge curves of the coin-type half-cells prepared using the sodium iron phosphate composite materials prepared for Example 1 - 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3 at a current of 0.1C.

[0082] The basic examples of the present invention and their respective further selected examples can be freely combined to form multiple examples, all of which are examples that can be adopted and claimed by the present invention. In the solution of the present invention, each selected example can be arbitrarily combined with any basic example and selected example. Those skilled in the art know that there are numerous combinations.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of an in-situ doped composite sodium iron phosphate cathode material, characterized in that: The structural general formula of the material is Na 4-2x A x Fe3(PO4)2P2O7; the general formula of the precursor is Na 1-2x A x Fe3(PO4)2P2O7, where 0 ≤ x ≤ 0.2, and the A element = Ca, Ba modified metal; the precursor two-step method is used to synthesize the in-situ doped composite sodium iron phosphate cathode material for sodium ion batteries, and the preparation method includes the following steps: 1) Calculate and weigh a certain amount of iron source, phosphorus source, sodium source and modified metal source according to the stoichiometry of the precursor, then mix the raw materials with water evenly and carry out ball milling to obtain a precursor slurry; then dry, crush and screen the precursor slurry to obtain a powder; 2) Carry out the first heat treatment on the powder obtained in step 1) at a certain temperature to obtain a precursor; 3) Weigh a certain amount of the precursor obtained in step 2), supplement the sodium source and carbon source, and obtain a slurry again, and then dry the slurry; 4) Carry out secondary heat treatment on the material obtained in step 3) under an inert atmosphere to obtain an in-situ doped composite sodium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that: In step 1), the iron source includes any one or at least two combinations of ferric nitrate, ferrous nitrate, ferrous oxalate, ferric sulfate, ferrous sulfate, iron oxide, ferric citrate, ferrous citrate or ferric pyrophosphate; the phosphorus source includes any one or at least two combinations of disodium hydrogen phosphate, sodium dihydrogen phosphate, ferrous phosphate, ferric phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate or sodium pyrophosphate; the sodium source includes any one or at least two combinations of disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium pyrophosphate; the modified metal source includes at least one of calcium oxide, calcium carbonate, calcium sulfate, calcium nitrate, calcium hydrogen phosphate, calcium dihydrogen phosphate; magnesium oxide, magnesium carbonate, magnesium sulfate, magnesium nitrate, magnesium hydrogen phosphate, magnesium dihydrogen phosphate.

3. The preparation method according to claim 1, wherein: In step 1), the particle size D50 of the precursor slurry obtained after ball milling is ≤ 0.5 μm.

4. The preparation method according to claim 1, wherein: In step 2), the first heat treatment temperature is 300 - 700 °C, and the heat treatment time is 2 - 15 h.

5. The preparation method according to claim 1, wherein: In step 3), the carbon source includes at least one of starch, glucose, sucrose, citric acid, ascorbic acid; the drying method includes any one or at least two combinations of gel drying, vacuum drying, spray drying or freeze drying.

6. The preparation method according to claim 1, characterized in that: The mixing methods described in step 1) and step 3) include any one or at least two combinations of stirring, ultrasonic oscillation, mortar grinding, ball milling or sand milling.

7. The preparation method according to claim 1, wherein: The inert atmosphere in step 4) includes one of nitrogen, argon, argon - hydrogen mixture, nitrogen - hydrogen mixture.

8. The preparation method according to claim 1, wherein: The secondary heat treatment in step 4) includes pre-sintering and phase-forming sintering. The pre-sintering temperature is 300 - 400 °C, and the sintering time is 2 - 6 h; the phase-forming sintering temperature is 400 - 700 °C, and the sintering time is 5 - 15 h; the sintering heating rate is 1 - 10 °C / min; the D 50 particle size of the in-situ doped composite sodium iron phosphate cathode material is 5 - 10 μm.

9. An in-situ doped composite sodium iron phosphate cathode material prepared by the method according to any one of claims 1 - 8.

10. A sodium ion battery, comprising the in-situ doped composite sodium iron phosphate cathode material according to claim 9.

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