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

Abstract

The application belongs to the technical field of sodium ion battery cathode material preparation, and relates to an in-situ doped composite sodium iron phosphate cathode material, a preparation method thereof and a sodium ion battery. 4‑2x A x Fe3(PO4)2P2O7, a precursor general formula is Na 1‑2x A x Fe3(PO4)2P2O7, wherein 0<=x<=0.2, and the A element is Ca or Ba modified metal. The in-situ doped composite sodium iron phosphate is synthesized by a two-step precursor method, and Ca and Ba elements are used to replace Na. The in-situ doped composite sodium iron phosphate cathode material prepared by the application can replace inert sodium ions in the Na2 position of the composite sodium iron phosphate, the lattice parameters are adjusted by doping elements, the intrinsic conductivity of the composite sodium iron phosphate is improved, and then the discharge capacity is increased and the energy density of the sodium ion battery is improved, so that the problems of low discharge specific capacity and poor stability of the composite sodium iron phosphate material are effectively solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery cathode material preparation, and relates to an in-situ doped composite sodium iron phosphate cathode material, a preparation method thereof and a sodium ion battery. BACKGROUND

[0002] Sodium ion batteries are widely concerned in large-scale energy storage systems and smart grids due to their abundant sodium source and low cost. Looking forward to the future, the key lies in developing cathode materials with stable structure and superior performance to solve the challenges brought by the relatively low energy density and slow electrochemical kinetics of sodium ion batteries. At present, researchers are focusing on three types of cathode materials: layered transition metal oxides, poly-anion compounds and Prussian blue compounds, among which, poly-anion compounds are concerned due to their high working voltage and stable structure.

[0003] Iron-based poly-anion compounds are considered to be the most promising cathode materials due to their low cost, environmental friendliness and abundant resources. Since the successful application of LiFePO4 material in lithium ion batteries, NaFePO4 has become a research hotspot. However, the olivine-type NaFePO4 must be obtained through the electrochemical reaction of LiFePO4, which is not suitable for large-scale preparation. Pyrophosphate Na2FeP2O7 shows open three-dimensional Na + channels due to the strong inductive effect of P2O7 groups, 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, thereby limiting its commercial application. Mixed iron-based poly-anion compounds (typically such as Na4Fe3(PO4)2P2O7, referred to as NFPP) have the advantages of phosphate and pyrophosphate, and exhibit higher structural stability and accelerated electrochemical kinetics. The four sodium ions in Na4Fe3(PO4)2P2O7 occupy four different coordination sites: Na1 (6-coordinated), Na2 (7-coordinated), Na3 (5-coordinated) and Na4 (6-coordinated), among which Na1, Na3 and Na4 are active sodium ions that will participate in the de-intercalation reaction. The Na2 site sodium ion has a higher de-intercalation reaction barrier due to the presence of high coordination, and is inert during the charging and discharging process. Therefore, Na4Fe3(PO4)2P2O7 will lose three sodium ions during charging to generate NaFe3(PO4)2P2O7, and NaFe3(PO4)2P2O7 will intercalate three sodium ions to generate Na4Fe3(PO4)2P2O7 during discharging.

[0004] NFPP is associated with Na +The diffusion kinetics is slow, which seriously limits the rate performance and energy density of the NFPP. Patents such as CN 117794854 A, CN 118588925 A and CN 117936755 A propose to replace part of the iron elements or anion groups by element doping to improve the electrochemical performance of the NFPP. However, the existing NFPP materials still have problems such as low specific discharge capacity and poor cycle stability. In addition, some patents (CN 118851132 A and CN 117558905 A) also propose to replace part of the sodium elements by element doping, but direct sodium element replacement will preferentially replace the sodium ions at the Na1, Na3 and Na4 positions, resulting in a decrease in the content of active sodium ions and further reducing the theoretical specific capacity of the NFPP. Therefore, it is still necessary to further optimize the material preparation process to achieve the ideal performance indicators for industrial application. SUMMARY

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

[0006] In order to achieve the above application purposes, the present application is realized by the following technical solutions:

[0007] A preparation method of in-situ doped composite sodium iron phosphate, the material has a general structure Na 4-2x A x Fe3(PO4)2P2O7, the general formula of the precursor is Na 1-2x A x Fe3(PO4)2P2O7, wherein 0≤x≤0.2, A element = Ca, Ba modified metal. The preparation method of the in-situ doped composite sodium iron phosphate positive electrode material for sodium ion battery 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 stoichiometric ratio of the precursor, then mix and uniformly add water to each raw material, and perform ball milling to obtain a precursor slurry; then dry, crush and sieve the precursor slurry to obtain a powder;

[0009] 2) The powder obtained in step 1) is subjected to first heat treatment 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 the carbon source, and perform the slurry again to obtain a slurry, and then dry the slurry;

[0011] 4) The material obtained in step 3) is subjected to secondary heat treatment in an inert atmosphere to obtain an in-situ doped composite sodium iron phosphate positive electrode material.

[0012] According to a further embodiment of the present application, 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 application, 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 application, 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 application, 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 application, 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 application, in step 2), the first heat treatment temperature is 300-700℃ (specifically, 300℃, 350℃, 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, etc.), and the heat treatment time is 2-15h (specifically, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, etc.).

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

[0019] According to a further embodiment of the present application, in step 3), the mixing method comprises 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 comprises 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 application, in step 4), the inert atmosphere comprises one of nitrogen, argon, argon-hydrogen mixed gas or nitrogen-hydrogen mixed gas.

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

[0022] According to a further embodiment of the present application, in step 4), the D 50 The particle size is 5-10μm.

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

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

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] (1) The in-situ doped modified composite sodium iron phosphate positive electrode material disclosed in the present application introduces the doping elements Ca or Ba similar in radius to Na ions, which can optimize the NFPP material under the condition that 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] (II) The in-situ doped modified composite sodium iron phosphate material disclosed in the present application is applied to the preparation of electrode materials for sodium ion batteries. The two-step precursor method is used innovatively, and the target material can be obtained in a pure phase, the electrode potential is improved, the energy density of the sodium ion battery is effectively improved, and the electrochemical performance of the battery is obviously improved and enhanced. + The crystal structure is stable during the embedding / extraction process, the stability of the material is improved, the target material in a pure phase is obtained, the electrode potential is improved, the energy density of the sodium ion battery is effectively improved, and the electrochemical performance of the battery is obviously improved and enhanced. BRIEF DESCRIPTION OF DRAWINGS

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

[0030] Figure 3 Charge-discharge curves of the button half-batteries prepared using the composite sodium iron phosphate materials prepared for Example 1-1, Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 3 at a current of 0.1C. DETAILED DESCRIPTION

[0031] The present application relates to an in-situ doped composite sodium iron phosphate positive electrode material and a preparation method. The technical and process preparation schemes in the embodiments of the present application will be described in detail and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor belong to the scope of protection of the present application.

[0032] The raw materials used in the following examples are commercially available or self-made. Unless otherwise specified, the mass percentage is indicated.

[0033] Example 1-1:

[0034] The present embodiment provides a preparation method of an in-situ doped composite sodium iron phosphate positive electrode material. In the method, a two-step “ball milling-calcination” process is used to prepare a composite positive electrode material of formula Na 3.8 Ca 0.1 Fe3(PO4)2P2O7, and the precursor formula is Na 0.8 Ca 0.1 Fe3(PO4)2P2O7. Among them, iron phosphate is used as an iron source, iron phosphate, sodium dihydrogen phosphate and calcium dihydrogen phosphate are used as phosphorus sources, sodium dihydrogen phosphate and sodium carbonate are used as sodium sources, calcium dihydrogen phosphate is used as a calcium source, and anhydrous glucose is used as a carbon source.

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

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

[0037] 2) All the materials weighed in step 1) are added to a ball mill tank, and deionized water is added. After stirring and mixing uniformly, the slurry is ball milled at a speed of 300 rpm for 8 hours, so that the slurry with a particle size D50≤0.5 μm is obtained;

[0038] 3) The slurry obtained by ball milling is transferred to an oven at 80℃ for drying to completely remove the water therein. After drying, the material is broken by a high-speed grinder, and the material is sieved by a 300 mesh screen to collect the undersize material;

[0039] 4) The material obtained in step 3) is loaded in a crucible, and is heat-treated at 550℃ at a heating rate of 5℃ / min for 10 hours. After the material is naturally cooled to room temperature, the precursor Na 0.8 Ca 0.1 Fe3(PO4)2P2O7 is obtained;

[0040] 5) 0.1 mol of the precursor obtained in step 4), 0.15 mol of sodium carbonate, and a certain amount of anhydrous glucose are weighed in a ball mill tank for ball milling (the amount of glucose is 12% of the theoretical yield of the composite sodium iron phosphate), so that the slurry with a particle size D50≤0.5 μm is obtained. The slurry is dried, broken, and sieved by a 300 mesh screen to collect the undersize material;

[0041] 6) The undersize material obtained in step 5) is loaded in a crucible, and is heat-treated at 350℃ at a heating rate of 5℃ / min for 4 hours under the protection of nitrogen atmosphere, and then is heat-treated at 500℃ at a heating rate of 5℃ / min for 10 hours. After the material is naturally cooled to room temperature, the Na 3.8 Ca 0.1 Fe3(PO4)2P2O7 positive electrode material is obtained.

[0042] Example 1-2:

[0043] The specific preparation method steps of this example are similar to those of Example 1-1, except that the chemical formula of the precursor 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) 0.3 mol of iron phosphate, 0.09 mol of sodium dihydrogen phosphate, 0.005 mol of calcium dihydrogen phosphate were weighed respectively.

[0044] Example 1-3:

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

[0046] Example 1-4:

[0047] The specific preparation method steps of this example are similar to those of Example 1-1, except that the step 4) is: the material obtained in step 3) is loaded in a crucible, and is heated at 450℃ for 14h at a heating rate of 5℃ / min. After the material is naturally cooled to room temperature, Na 0.8 Ca 0.1 Fe3(PO4)2P2O7precursor. The rest of the process is consistent with 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 the step 4) is: the material obtained in step 3) is loaded in a crucible, and is heated at 650℃ for 6h at a heating rate of 5℃ / min. After the material is naturally cooled to room temperature, Na 0.8 Ca 0.1 Fe3(PO4)2P2O7precursor. The rest of the process is consistent with that in Example 1-1.

[0050] Example 2:

[0051] This example provides a preparation method of an in-situ doped composite sodium iron phosphate positive electrode material, in which a two-step "ball milling-calcination" process is adopted to prepare a composite positive electrode material with a chemical formula of Na 3.8 Ba 0.1 Fe3(PO4)2P2O7, the chemical formula of the precursor 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 precursor Na 0.8 Ba 0.1 Fe3(PO4)2P2O7material, 0.3 mol of iron phosphate, 0.08 mol of sodium dihydrogen phosphate and 0.01 mol of barium dihydrogen phosphate are weighed;

[0054] 2) All the materials weighed in step 1) are added into a ball mill tank, and deionized water is added, and after stirring and mixing uniformly, the slurry with a particle size D50≤0.5 μm is obtained by ball milling at a rotating speed of 300 rpm for 8 h;

[0055] 3) The slurry obtained by ball milling is transferred to a drying oven at 80 ℃ to completely remove the water therein. After drying, the material is crushed by using a high-speed grinding machine, and the material is sieved by using a 300-mesh screen to collect the undersize material;

[0056] 4) The material obtained in step 3) is loaded into a crucible, and is kept at 550 ℃ at a temperature increasing rate of 5 ℃ / min for 10 h. After the material is naturally cooled to room temperature, the precursor Na 0.8 Ba 0.1 Fe3(PO4)2P2O7material is obtained;

[0057] 5) 0.1 mol of the precursor obtained in step 4), 0.15 mol of sodium carbonate and a certain amount of anhydrous glucose are weighed in a ball mill tank for ball milling (the amount of glucose is 12% of the theoretical yield of the composite sodium iron phosphate), to obtain a slurry with a particle size D50≤0.5 μm, and the slurry is dried, crushed and sieved by 300 meshes to collect the undersize material;

[0058] 6) The undersize material obtained in step 5) is loaded into a crucible, and is kept at 350 ℃ at a temperature increasing rate of 5 ℃ / min for 4 h, and then is kept at 500 ℃ at a temperature increasing rate of 5 ℃ / min for 10 h, and after the material is naturally cooled to room temperature, the Na 3.8 Ba 0.1 Fe3(PO4)2P2O7positive electrode material is obtained.

[0059] Comparative Example 1:

[0060] The Na 3.8 Ca 0.1 Fe3(PO4)2P2O7is prepared by using a one-step “ball milling-calcining” process, wherein the iron phosphate is used as the iron source, the iron phosphate, the sodium dihydrogen phosphate and the calcium dihydrogen phosphate are used as the phosphorus source, the sodium dihydrogen phosphate and the sodium carbonate are used as the sodium source, the calcium dihydrogen phosphate is used as the calcium source, and the anhydrous glucose is used as the carbon source.

[0061] The specific preparation steps are as follows:

[0062] 1) According to Na 3.8 Ca 0.1 Fe3(PO4)2P2O7, 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 is 12% of the theoretical yield of the composite iron sodium phosphate) are weighed according to the stoichiometric ratio;

[0063] 2) The materials weighed in step 1) are added to a ball mill tank, and deionized water is added. After stirring and mixing uniformly, the slurry is ball milled at a speed of 300 rpm for 8 h, and a slurry with a particle size D50≤0.5 μm is obtained;

[0064] 3) The slurry obtained by ball milling is transferred to a 80℃ oven for drying to completely remove the water therein. After drying, the material is crushed by a high-speed grinder, and the material is sieved by a 300 mesh screen to collect the undersize material;

[0065] 4) The material obtained in step 3) is loaded into a crucible and heated at a rate of 5℃ / min under nitrogen atmosphere protection at 350℃ for 4h, and then heated at a rate of 5℃ / min at 500℃ for 10h. After the material is naturally cooled to room temperature, Na 3.8 Ca 0.1 Fe3(PO4)2P2O7 cathode material is obtained.

[0066] Comparative Example 2:

[0067] Na4Fe3(PO4)2P2O7 is prepared by 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 consistent with that of Comparative Example 1.

[0068] Comparative Example 3:

[0069] Undoped Na4Fe3(PO4)2P2O7 is prepared by 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 consistent with that of Example 1-1.

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

[0071] Table 1

[0072]

[0073]

[0074] To verify the use effect of the positive electrode material described in the present application, the materials prepared in each of Examples 1-2 and Comparative Examples 1-3 were prepared into positive electrode sheets. The preparation method was as follows: the positive electrode material, acetylene black and PVDF were mixed into a uniform slurry at a mass ratio of 94:3:3, then the slurry was uniformly coated on an aluminum foil using a 200 μm four-side coater, and then the film was placed in a 100°C air drying oven for drying for 8 hours. The electrode film was punched into a 14 mm diameter round sheet using a sheet punching machine. The cut electrode round sheet was placed in a 100°C vacuum drying oven for drying for 4 hours and then transferred to a glove box. A CR2016 type button cell was assembled in the glove box using a metal Na round sheet as the counter electrode, NaPF6 as the electrolyte and Whatman GF / D glass fiber separator, and constant current charge and 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 to 3

[0076]

[0077]

[0078] As can be seen from Table 2, the amount of the doping element varies, and the electrochemical performance varies. Among them, Na 3.8 Ca 0.1 Fe3(PO4)P2O7(Example 1-1) compared with Na 3.9 Ca 0.05 Fe3(PO4)P2O7(Example 1-2) and Na 3.7 Ca 0.15The 0.1C discharge specific capacity of Fe3(PO4)P2O7 (Example 1-3) is higher, 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 sample prepared by the technical route of in-situ doped composite sodium iron phosphate using a two-step method (Example 1-1, Example 2) has a reversible capacity higher than that of the sample without in-situ doping (Comparative Example 3). Compared with doping Ba (Example 2), the composite sodium iron phosphate 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 Ca, the sample prepared by the two-step method process (Example 1-1) has significantly better charge-discharge performance than that of the one-step method process (Comparative Example 1). In addition, even without any doping, comparing the results of Comparative Example 3 and Comparative Example 2, it can also be found that the sample prepared by the two-step method process has better performance than that of the one-step method. Obviously, the in-situ doped composite sodium iron phosphate produced by the technical route provided by the present application can improve the activity of the material, increase the reversible capacity, and improve the energy density of the sodium ion battery.

[0079] Figure 1 The XRD patterns of the composite sodium iron phosphate materials prepared for Example 1-1, Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 3. It can be seen from Figure 1 that, compared with the PDF card of Na4Fe3(PO4)2(P2O7), the diffraction peaks of the samples of different examples and comparative examples correspond to the PDF card one by one, and there are no obvious impurity peaks, indicating that the examples and comparative examples can successfully synthesize composite sodium iron phosphate, and there is no obvious impurity phase.

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

[0081] Figure 3 The charge-discharge curves at 0.1C current of the button half-batteries prepared using the composite sodium iron phosphate materials prepared for Example 1-1, Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 3.

[0082] The foregoing basic examples and each further selected example of the present application can be freely combined to form a plurality of examples, all of which are examples that can be used and claimed by the present application. In the present application scheme, each selected example can be arbitrarily combined with any basic example and selected example. Those skilled in the art can know that there are numerous combinations.

[0083] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing an in-situ doped composite sodium iron phosphate cathode material, characterized in that: The general structural 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, wherein 0<x≤0.1, element A=Ca or Ba; a precursor two-step method is used to synthesize an in-situ doped composite sodium iron phosphate positive electrode material for a sodium ion battery, and the preparation method comprises the following steps: 1) Calculate and weigh a certain amount of iron source, phosphorus source, sodium source and modified metal source according to the stoichiometric ratio of the precursor, then mix each raw material with water uniformly, and perform ball milling to obtain a precursor slurry; then dry, crush and sieve the precursor slurry to obtain a powder; 2) Perform 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 with sodium source and carbon source, and obtain a slurry again, then dry the slurry; 4) Perform second heat treatment on the material obtained in step 3) under an inert atmosphere to obtain an in-situ doped composite sodium iron phosphate positive electrode material.

2. The method of claim 1, wherein: 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; 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; 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; and 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 or magnesium dihydrogen phosphate.

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

4. The method of claim 1, wherein: The first heat treatment temperature in step 2) is 300-700 ℃, and the heat treatment time is 2-15 h.

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

6. The method of claim 1, wherein: The mixing method in steps 1) and 3) includes any one or a combination of at least two of stirring, ultrasonic oscillation, mortar grinding, ball milling or sand milling.

7. The method of claim 1, wherein: The inert atmosphere in step 4) includes one of nitrogen, argon, argon-hydrogen mixed gas or nitrogen-hydrogen mixed gas.

8. The method of claim 1, wherein: The secondary heat treatment in step 4) comprises pre-sintering and phase sintering, the pre-sintering temperature is 300-400 ℃, the sintering time is 2-6 h; the phase sintering temperature is 400-700 ℃, the sintering time is 5-15 h; the sintering heating rate is 1-10 ℃ / min; the D 50 particle size is 5-10 μm.

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

10. A sodium ion battery comprising the in-situ doped composite sodium iron phosphate positive electrode material of claim 9.

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