Pyrophosphate and ferric sodium phosphate composite material as well as preparation method and application thereof

Abstract

The invention belongs to the field of batteries, and particularly discloses a sodium ferric pyrophosphate composite material as well as a preparation method and application thereof. According to the method, the pyrophosphate ferric phosphate sodium material seed crystal containing the solid electrolyte is firstly prepared, and the seed crystal is matched with other raw materials to synthesize the pyrophosphate ferric phosphate sodium composite material, so that the problems of long sintering time and more product impurities in the synthesis of the pyrophosphate ferric phosphate sodium composite material are solved, and the energy consumption is greatly reduced; the finally obtained pyrophosphate ferric phosphate sodium composite material has high compaction density, stability, specific discharge capacity, rate and conductivity, and has a prospect of large-scale production.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of batteries, and particularly relates to a sodium pyrophosphate ferric phosphate composite material and a preparation method and application thereof. BACKGROUND

[0002] Although traditional lithium-ion batteries have been widely used, due to the relative scarcity and uneven distribution of lithium resources, the problems of high cost and unstable supply chain are caused. In contrast, sodium resources are abundant and widely distributed, and the price is also more affordable, so sodium-ion batteries are considered as a potential energy storage technology to replace lithium-ion batteries. Developing high-performance sodium-ion battery materials can not only meet the growing demand for energy storage, but also effectively reduce costs and improve the sustainability of energy systems.

[0003] Among the many polyanion-type sodium-ion battery cathode materials, sodium pyrophosphate ferric phosphate (Na4Fe3(PO4)2P2O7) as a new type of sodium-ion battery cathode material has a broad application prospect due to its stable structure and low cost. Moreover, the raw material for preparing sodium pyrophosphate ferric phosphate is abundant, and the material has the advantage of long cycle performance, and will become the preferred cathode material in the future in the field of sodium energy storage.

[0004] At present, the methods for preparing Na4Fe3(PO4)2P2O7 cathode material mainly include solid phase method and liquid phase method. The liquid phase method atomically mixes raw materials to obtain sodium pyrophosphate ferric phosphate material with high uniformity, but this method needs precise control at each stage, and large-scale production is difficult. The solid phase method is to mix raw materials by mechanical mixing, and then sinter the product at high temperature. This method has low requirements for equipment and the reaction is easy to control, and is suitable for large-scale production. However, the solid phase method for preparing sodium pyrophosphate ferric phosphate material is prone to produce impurities in the finished product, and has the defects of poor compaction density, electrical conductivity, cycle performance and rate performance, which seriously affects the specific capacity of sodium pyrophosphate ferric phosphate material. In addition, this method is difficult to synthesize, and has poor consistency and stability. Therefore, it is still necessary to develop a solid phase method for preparing sodium pyrophosphate ferric phosphate material, so that the sodium pyrophosphate ferric phosphate material has less impurities and also has higher electrical conductivity and compaction density, thereby improving the specific capacity and electronic conductivity of the battery. SUMMARY

[0005] In view of the problems involved in the prior art, the present application provides a sodium pyrophosphate ferric phosphate composite material and a preparation method and application thereof.

[0006] To achieve the above-mentioned purpose, the following technical solutions are specifically included:

[0007] In a first aspect, the application provides a preparation method of a seed crystal of sodium pyrophosphate ferric phosphate material, comprising the following steps:

[0008] (1) grinding and mixing a sodium source, an iron source, a phosphorus source, an optional M metal element source, a solid-state electrolyte material, a reducing agent, and a solvent to obtain a mixed slurry;

[0009] (2) drying the mixed slurry to obtain a mixture material;

[0010] (3) sintering the mixture material under an inert gas atmosphere, and then crushing to obtain a sodium iron pyrophosphate seed crystal.

[0011] The solid phase method can be used to synthesize the seed crystal, and the sodium iron pyrophosphate material and the solid-state electrolyte material can be combined in a solid solution state to construct the seed crystal. The method is simple and has a relatively short cycle.

[0012] Preferably, in step (1), the sodium source includes one or more of sodium bicarbonate, sodium hydroxide, sodium carbonate, sodium nitrate, sodium chloride, sodium dihydrogen phosphate, sodium acetate, sodium acetate, and sodium citrate.

[0013] Preferably, in step (1), the iron source includes one or more of iron hydroxide, iron phosphate, ferrous oxalate, diiron trioxide, triiron tetroxide, and iron acetate.

[0014] Preferably, in step (1), the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate.

[0015] Preferably, in step (1), the reducing agent includes one or more of starch, conductive carbon black, glucose, sucrose, polypropylene, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol (PPG), acetylene black, carbon nanotubes, graphene, citric acid, fructose, maltose, lactose, and ascorbic acid.

[0016] The carbon-based reducing agent provides a reducing condition and acts as a reducing agent for reducing non-Fe2+ ions in the iron source. In addition, since the carbon-based reducing agent is used, even if there is a small amount of residual reducing agent, it can act as a carbon source for carbon-coating the sodium iron pyrophosphate material and the solid-state electrolyte material. The interaction between the solid-state electrolyte material in the seed crystal and the outer carbon source improves the conductivity of the sodium iron pyrophosphate material seed crystal, and the seed crystal is more conducive to improving the electrical properties of the subsequently prepared sodium iron pyrophosphate composite material. However, the carbon content of the residual reducing agent as part of the carbon source should be ≤0.02wt%. A low carbon content can improve the solid solution combination of the solid-state electrolyte and the sodium iron pyrophosphate material in the seed crystal, and is conducive to the electrical properties of the subsequently synthesized sodium iron pyrophosphate composite material.

[0017] Preferably, in step (1), the solid-state electrolyte material includes sodium zirconium silicon phosphate (NZSP). The sodium zirconium silicon phosphate (NZSP) can be purchased or prepared by the following preparation method:

[0018] a. Sodium carbonate, silicon oxide, zirconium oxide and ammonium dihydrogen phosphate are weighed according to the stoichiometric ratio, and then fully mixed for 2-6 h. The mixed raw materials are heat treated at a temperature of 1100-1300 °C for 6-24 h to obtain NZSP blocks; b. The NZSP blocks are broken into micron-sized powders by ball milling, and then the powders are dispersed in an aqueous solution. The slurry is obtained by ball milling at a speed of 200-500 rpm for 4-24 h, and the target particle size is adjusted as needed; c. The slurry is dried and then ground to obtain sodium zirconium silicon phosphate.

[0019] Preferably, in step (1), the source of M metal elements comprises an oxide of M metal elements.

[0020] Preferably, in step (1), the source of sodium, the source of iron, the source of phosphorus and the source of M metal elements are in a molar ratio of (3-6):(2-4):(0.8-2):(0.01-1) based on Na element, Fe element, P element and M metal elements, respectively.

[0021] Preferably, in step (1), the source of iron and the reducing agent are in a molar ratio of (2-4):(0.1-0.5) based on Fe element.

[0022] Preferably, in step (1), the mass percentage of the solid-state electrolyte material is 1-5% based on the total mass of the source of sodium, the source of iron and the source of phosphorus.

[0023] Preferably, in step (1), the average particle size of the mixed slurry is ≤1.0 μm, and further preferably 0.3-0.5 μm.

[0024] Preferably, in step (1), the solvent comprises water.

[0025] Preferably, in step (2), the drying comprises spray drying.

[0026] Preferably, in step (3), the sintering temperature is 350-650 °C, the sintering time is 3-20 h, and the rate of temperature increase to the sintering temperature is 1-10 °C / min.

[0027] Further preferably, in step (3), the sintering temperature is 400-550 °C, and the sintering time is 5-8 h.

[0028] Preferably, in step (3), the inert gas comprises at least one of nitrogen, argon or helium.

[0029] Preferably, in step (3), the average particle size of the seed crystal of sodium iron pyrophosphate is ≤ 5 μm, and further preferably 1-4 μm.

[0030] In a second aspect, the application provides a seed crystal of sodium iron pyrophosphate material prepared by the method.

[0031] Preferably, the seed crystal of sodium iron pyrophosphate material is composed of sodium iron pyrophosphate material and solid electrolyte material, the chemical formula of the sodium iron pyrophosphate material is Na4Fe 3-x M x (PO4)2P2O7, 0≤x≤0.9, 2.1≤3-x≤3, M includes but is not limited to one or more of La, Ce, Nd, Pr, Ti, Mg, Al, Ni, Nb, Mn, Zr, Ta, Mo, W, Eu, and in the seed crystal of sodium iron pyrophosphate material, the solid electrolyte material accounts for 0.1%-5.0% of the mass of the sodium iron pyrophosphate material.

[0032] The seed crystal of the application includes a solid solution of sodium iron pyrophosphate material and solid electrolyte material, which is used as a seed crystal for subsequent synthesis of sodium iron pyrophosphate composite material, and is beneficial to improve the tap density, electrical conductivity, cycle performance, rate performance and discharge specific capacity of the sodium iron pyrophosphate composite material.

[0033] Preferably, the solid electrolyte material includes a NASICON structure solid electrolyte material; the NASICON structure solid electrolyte material includes sodium zirconium silicon phosphate (NZSP); and the sodium zirconium silicon phosphate includes a material with a chemical formula of Na3Zr2Si2PO 12 The doping amount (%) of the doping metal element M in the sodium iron pyrophosphate material in the seed crystal of the application is calculated as follows: n M / (n M +n Fe )*100, n M is the molar amount of the doping metal element M, and n Fe is the molar amount of iron, the doping amount of the doping metal element M is 0.1%-5%, and further preferably 1%-3%; and the percentage (%) of the solid electrolyte material in the mass of the sodium iron pyrophosphate material in the seed crystal of the application is calculated as follows: M2 (mass of the solid electrolyte material in the seed crystal) / M1 (mass of the sodium iron pyrophosphate material in the seed crystal)*100, and the percentage of the solid electrolyte material in the mass of the sodium iron pyrophosphate material is 0.1%-5%, and further preferably 1%-3%.

[0034] In a third aspect, the application provides a preparation method of sodium iron pyrophosphate composite material, comprising the following steps:

[0035] S1, a sodium source, an iron source, a phosphorus source, an optional M metal element source, a carbon source, a solvent and the seed crystal of the sodium iron phosphate pyrophosphate material are ground and mixed to obtain a mixed slurry;

[0036] S2, the mixed slurry is dried to obtain a mixture;

[0037] S3, the mixture is sintered under an inert gas atmosphere, and then crushed to obtain a sodium iron phosphate pyrophosphate composite material.

[0038] In the method of the present application, a special seed crystal is used to guide the synthesis of the sodium iron phosphate pyrophosphate composite material. The seed crystal guides the rapid growth of the sodium iron phosphate pyrophosphate material, effectively shortening the time required for the synthesis of the sodium iron phosphate pyrophosphate composite material, reducing the temperature required for the synthesis of the material, and making the inter-particle gap smaller and the particles more compact. The compactness, stability and consistency of the sodium iron phosphate pyrophosphate composite material are effectively improved, and the cycle stability, specific discharge capacity and rate performance of the material are improved. The solid-state electrolyte exists in the seed crystal, and the seed crystal is carbon-coated during the synthesis of the sodium iron phosphate pyrophosphate material. The overall particle of the obtained sodium iron phosphate pyrophosphate composite material has higher electrical conductivity. In summary, the solid-state electrolyte material is introduced into the sodium iron phosphate pyrophosphate material seed crystal during the synthesis of the sodium iron phosphate pyrophosphate composite material. The final obtained sodium iron phosphate pyrophosphate composite material has high compactness, cycle stability, specific discharge capacity, rate and electrical conductivity. In addition, the seed crystal and the sodium iron phosphate pyrophosphate material can be doped with metal elements, which can further significantly improve the electrical conductivity and improve the compactness, cycle stability, specific discharge capacity and rate performance of the material to a certain extent.

[0039] Preferably, in step S1, the sodium source includes one or more of sodium bicarbonate, sodium hydroxide, sodium carbonate, sodium nitrate, sodium chloride, sodium dihydrogen phosphate, sodium acetate, sodium acetate, sodium citrate.

[0040] Preferably, in step S1, the iron source includes one or more of iron hydroxide, iron phosphate, ferrous oxalate, diiron trioxide, triiron tetroxide, iron acetate.

[0041] Preferably, in step S1, the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate.

[0042] Preferably, in step S1, the carbon source includes one or more of starch, conductive carbon black, glucose, sucrose, polypropylene, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol (PPG), acetylene black, carbon nanotube, graphene, citric acid, fructose, maltose, lactose, ascorbic acid.

[0043] Preferably, in step S1, the M metal element source comprises an oxide of the M metal element.

[0044] Preferably, in step S1, the molar ratio of the sodium source calculated as Na element, the iron source calculated as Fe element, the phosphorus source calculated as P element, and the M metal element source calculated as M metal element is (4-10):(4-6):(1.5-3):(0.05-0.5).

[0045] Preferably, in step S1, the molar ratio of the iron source calculated as Fe element and the carbon source is (4-6):(0.5-1).

[0046] Preferably, in step S1, the mass percentage of the seed crystal of the sodium iron pyrophosphate material is 5-80%, further preferably 10-45%, based on the total mass of the sodium source, the iron source, and the phosphorus source; or, the mass percentage of the seed crystal is 5-80%, further preferably 10-45%, based on the total mass of the sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, M n = 623.39 g / mol) and the seed crystal; wherein, when calculating the mass of the sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, M n = 623.39 g / mol), the influence of the doped metal element M and the carbon doping is ignored, and the mass of the seed crystal is the actual mass of the added seed crystal.

[0047] Preferably, in step S1, the mass percentage of the carbon source is 0.1-20%, further preferably 2-15%, based on the total mass of the sodium source, the iron source, and the phosphorus source.

[0048] Preferably, in step S1, the molar percentage of the M metal element source is 0.1-10%, further preferably 2-5%, based on the total molar amount of the sodium source, the iron source, the phosphorus source, and the M metal element source. Preferably, in step S1, the average particle size of the mixed slurry is ≤1.0 μm, further preferably 0.3-0.5 μm.

[0049] Preferably, in step S1, the solvent comprises water.

[0050] Preferably, in step S2, the drying comprises spray drying.

[0051] Preferably, in step S3, the sintering temperature is 350-650°C, the sintering time is 3-20 h, and the rate of temperature increase to the sintering temperature is 1-10°C / min.

[0052] Further preferably, in step S3, the sintering temperature is 400-550℃, and the sintering time is 5-8h.

[0053] In a fourth aspect, the present application provides a sodium iron pyrophosphate composite material prepared by the method for preparing sodium iron pyrophosphate material.

[0054] The raw materials for preparing the sodium iron pyrophosphate composite material of the present application include seeds, a carbon source, a sodium source, an iron source, a phosphorus source, and a doping metal M source. In the sodium iron pyrophosphate composite material finally prepared, there are three parts: the seeds (including the sodium iron pyrophosphate material and the solid electrolyte), the newly grown sodium iron pyrophosphate material (the part of the sodium iron pyrophosphate material is consistent with the sodium iron pyrophosphate material in the seeds) in the preparation process, and the carbon coating layer (evolved from the carbon source). Part of the sodium iron pyrophosphate material particles continue to grow on the basis of the seeds, and the size of this part of the sodium iron pyrophosphate material particles is relatively large. There are also part of the sodium iron pyrophosphate material particles that are not obtained by growing on the seeds, and the size of this part of the particles is relatively small. The size of the large and small particles is matched, which can significantly improve the electrical properties of the sodium iron pyrophosphate composite material. 3-x M x The chemical formula of the sodium iron pyrophosphate material in the composite material is Na4Fe (PO4) 2P2O7, 0≤x≤0.9, 2.1≤3-x≤3, wherein the doping amount (%) of the doping metal element M is calculated as follows: n M / (n M +n Fe )*100, n M is the molar amount of the doping metal element M, and n Fe is the molar amount of the iron element.

[0055] In the sodium iron pyrophosphate composite material of the present application, the doping amount of the doping metal element M is 0.1%-5%, and further preferably 1%-3%.

[0056] In the sodium iron pyrophosphate composite material of the present application, the carbon content is 0.5-15%, and further preferably 2%-5%.

[0057] In a fifth aspect, the present application provides a battery, wherein the positive electrode sheet includes a current collector and an active material coating layer on at least one side surface of the current collector, and the active material coating layer includes the sodium iron pyrophosphate composite material.

[0058] Compared with the prior art, the present application has the following beneficial effects: the present application first prepares sodium iron pyrophosphate seed material containing solid-state electrolyte, and synthesizes sodium iron pyrophosphate composite material by combining the seed material with the remaining raw materials, solves the problems of long sintering time and many impurities of the synthesized sodium iron pyrophosphate material, greatly reduces the consumption of energy, and can make the finally obtained sodium iron pyrophosphate composite material have high compactness, stability, specific discharge capacity, rate and conductivity, and has the prospect of large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0059] Figure 1 is the XRD pattern of the sodium iron pyrophosphate composite material in application example 1.

[0060] Figure 2 is the formation diagram of the sodium iron pyrophosphate composite material in application example 1.

[0061] Figure 3 is the 2.0V-4.0V, 1C charge-discharge cycle diagram of the sodium iron pyrophosphate composite material in application example 1. DETAILED DESCRIPTION

[0062] In order to better illustrate the purpose, technical scheme and advantages of the present application, the present application will be further described below through specific examples. Unless otherwise specified, the test methods used in the examples and / or comparative examples are all conventional methods; and the materials, reagents, etc. used are all commercially available unless otherwise specified.

[0063] Example 1

[0064] A preparation method of sodium iron pyrophosphate seed material, comprising the following steps:

[0065] (1) 2 mol of Na2CO3, 2.94 mol of FePO4, 1.06 mol of NH4H2PO4, 0.06 mol of MgO, 85.51 g of sucrose, 12.47 g of Na3Zr2Si2PO 12 and 2L of pure water are added into a sand mill and ground for 3h, and the particle size of the slurry is ground to 300nm (0.3μm), to obtain mixed slurry 1;

[0066] (2) the mixed slurry 1 is spray dried, and the particle size of the material after spray drying is controlled to be about 30μm, to obtain a mixture;

[0067] (3) the mixture is placed in a box furnace under nitrogen protection, and is subjected to first sintering at 550℃ for 5h, and the cooled material is broken to 4μm, to obtain sodium iron pyrophosphate seed material.

[0068] Example 2-3

[0069] Example 2-3 is the same as Example 1 except that the amount of MgO added is different, as shown in Table 1.

[0070] Example 4

[0071] Example 4 is the same as Example 1 except that TiO2 is used in equimolar amount to replace MgO, as shown in Table 1.

[0072] Examples 5-6

[0073] Examples 5-6 are the same as Example 1 except that the amount of Na3Zr2Si2PO 12 added is different, as shown in Table 1.

[0074] Examples 7-8

[0075] Examples 7-8 are the same as Example 1 except that the temperature and time of the first sintering are different, as shown in Table 1.

[0076] Examples 9-10

[0077] Examples 9-10 are the same as Example 1 except that the types of sodium source, iron source, phosphorus source and reducing agent are different, as shown in Table 1.

[0078] Example 11

[0079] Example 11 is the same as Example 1 except that the particle size of the mixed slurry 1 is different, as shown in Table 1.

[0080] Comparative Example 1

[0081] Comparative Example 1 is the same as Example 1 except that no MgO and Na3Zr2Si2PO 12 is added, as shown in Table 1.

[0082] Comparative Example 2

[0083] Comparative Example 2 is the same as Example 1 except that no MgO is added, as shown in Table 1.

[0084] Comparative Example 3

[0085] Comparative Example 3 is the same as Example 1 except that the amount of MgO added is different, as shown in Table 1.

[0086] Comparative Example 4

[0087] Comparative Example 4 is the same as Example 1 except that no Na3Zr2Si2PO 12 is added, as shown in Table 4.

[0088] Comparative Example 5

[0089] Comparative Example 5 is the same as Example 1 except that the amount of Na3Zr2Si2PO 12 is different, as shown in Table 1, and the rest is the same.

[0090] Comparative Examples 6-7

[0091] Comparative Examples 6-7 are the same as Example 1 except that the first sintering temperature and time are different, as shown in Table 1, and the rest is the same.

[0092] Comparative Example 8

[0093] Comparative Example 8 is the same as Example 1 except that the amount of carbon added is different, as shown in Table 1, and the rest is the same.

[0094] Comparative Example 9

[0095] Comparative Example 9 is the same as Example 1 except that the particle size of the mixed slurry 1 is different, as shown in Table 1, and the rest is the same.

[0096] The seed crystal of the application is a solid solution composed of sodium pyrophosphate iron phosphate material and solid electrolyte material. The chemical formula of the sodium pyrophosphate iron phosphate material in the seed crystal is Na4Fe 3-x M x (PO4)2P2O7, 0≤x≤0.9, 2.1≤3-x≤3, wherein the doping amount (%) of the doping metal element M is calculated as follows: n M / (n M +n Fe )*100, n M is the molar amount of the doping metal element M, and n Fe is the molar amount of iron; in the seed crystal, the solid electrolyte material accounts for a percentage (%) of the mass of the sodium pyrophosphate iron phosphate material, which is calculated as follows: M2 (mass of solid electrolyte material in the seed crystal) / M1 (mass of sodium pyrophosphate iron phosphate material in the seed crystal) * 100. The carbon content in the sodium pyrophosphate iron phosphate seed crystal prepared in the above examples and comparative examples is detected by a carbon-sulfur detector, and the specific results are shown in Table 1.

[0097] Table 1

[0098]

[0099]

[0100] Application Example 1

[0101] A preparation method of a sodium pyrophosphate iron phosphate composite material, comprising the following steps:

[0102] S1, 4 mol of Na2CO3, 5.88 mol of FePO4, 2.12 mol of NH4H2PO4, 0.12 mol of MgO, 255 g of sucrose, 316 g of the crystal seeds of Example 1, and 2.5 L of pure water were added into a sand mill and ground for 4 h, and the particle size of the slurry reached 400 nm to obtain a mixed slurry 2;

[0103] S2, the mixed slurry 2 was spray dried, and the particle size of the sprayed material was controlled to be about 24 μm to obtain a mixed material;

[0104] S3, the mixed material was placed in a box furnace under nitrogen protection and sintered at 500°C for 4 h, and the material after cooling was broken to below 12 μm, and the final sodium iron pyrophosphate material was obtained after sieving and removing water.

[0105] It is calculated and tested that the carbon coating amount, the doping element content, and the proportion of the crystal seeds in the sodium iron pyrophosphate composite material are 2%, 2%, and 20%, respectively, and the details are shown in Table 2.

[0106] Application Example 2-11

[0107] The difference between Application Example 2-11 and Application Example 1 is that the crystal seeds in Application Example 2-11 are replaced with the crystal seeds of Example 2-11 in equal amounts, and the rest is the same as Application Example 1, and the details are shown in Table 2.

[0108] Application Example 12-20

[0109] The difference between Application Example 12-20 and Application Example 1 is that the crystal seeds in Application Example 12-20 are replaced with the crystal seeds of Comparative Example 1-9 in equal amounts, and the rest is the same as Application Example 1, and the details are shown in Table 2.

[0110] Application Example 21-24

[0111] The difference between Application Example 21-24 and Application Example 1 is that the addition amount of the crystal seeds in Application Example 21-24 is different, and the rest is the same as Application Example 1, and the details are shown in Table 2.

[0112] Application Example 25-27

[0113] The difference between Application Example 25-27 and Application Example 1 is that the addition amount of MgO in Application Example 25-27 is different, and the rest is the same as Application Example 1, and the details are shown in Table 2.

[0114] Application Example 28

[0115] The difference between Application Example 28 and Application Example 1 is that TiO2 is used to replace MgO in equal amounts in Application Example 28, and the rest is the same as Application Example 1, and the details are shown in Table 2.

[0116] Application Example 29-30

[0117] The application examples 29-30 differ from the application example 1 in that the temperature and time of the second sintering are different in the application examples 29-30, and the rest is the same as the application example 1, and details are shown in Table 2.

[0118] Application example 31

[0119] The application example 31 differs from the application example 1 in that the types of the sodium source, the iron source, the phosphorus source and the carbon source are different in the application example 31, and the rest is the same as the application example 1, and details are shown in Table 2.

[0120] Application example 32

[0121] The application example 32 differs from the application example 1 in that the mixed slurry 2 is different in the application example 32, and the rest is the same as the application example 1, and details are shown in Table 2.

[0122] Application example 33

[0123] The application example 33 differs from the application example 1 in that the carbon coating amount is different in the application example 33, and the rest is the same as the application example 1, and details are shown in Table 2.

[0124] The preparation raw materials of the sodium iron pyrophosphate phosphate composite material in the application examples 1-33 of the application include seeds, a carbon source, a sodium source, an iron source, a phosphorus source and a doped metal M source. In the finally prepared sodium iron pyrophosphate phosphate composite material, there are three parts of components, i.e., seeds (including sodium iron pyrophosphate phosphate material and solid electrolyte), new sodium iron pyrophosphate phosphate material grown in the preparation process (the part of the sodium iron pyrophosphate phosphate material is consistent with the composition of the sodium iron pyrophosphate phosphate material in the seeds) and a carbon coating layer (evolved from the carbon source), and part of the sodium iron pyrophosphate phosphate material particles are obtained by further growth based on the seeds, and the size of the part of the sodium iron pyrophosphate phosphate material particles is relatively large. There are also part of the sodium iron pyrophosphate phosphate material particles which are not obtained by growth based on the seeds, and the size of the part of the particles is relatively small. The chemical formula of the sodium iron pyrophosphate phosphate material in the composite material is Na4Fe 3-x M x (PO4)2P2O7, 0≤x≤0.9, 2.1≤3-x≤3, wherein the doping amount (%) of the doped metal element M is calculated in the following manner: n M / (n M +n Fe )*100, n M is the molar amount of the doped metal element M, and n Fe is the molar amount of the iron element. The mass percentage content of the added seeds is that the seeds account for the predicted synthesis chemical formula Na4Fe3(PO4)2P2O7(M nThe percentage of the mass of sodium pyrophosphate iron phosphate of the total mass of the sodium pyrophosphate iron phosphate (not considering the influence of the doping metal element M and carbon doping) is calculated, for example, in application example 1, according to the molar amount of the added Na source, it is expected to synthesize 2 mol of sodium pyrophosphate iron phosphate, then the mass percentage of the added seed crystal is = 316 g / (2 mol * 623.39 g / mol + 316 g) * 100 = 20%. The amount of carbon coating in the composite material prepared in the above examples and comparative examples is detected by a carbon-sulfur detector, and the specific results are shown in Table 1 in wt.%.

[0125] Table 2

[0126]

[0127]

[0128]

[0129] Performance test

[0130] The tap density and conductivity of the sodium pyrophosphate iron phosphate composite powder in application examples 1-33 are measured by a PRCD3100 type powder conductivity & tap density instrument.

[0131] The sodium pyrophosphate iron phosphate composite in application examples 1-33 is used as an active material in a positive electrode sheet of a sodium ion battery, and the electrochemical performance of the prepared sodium ion battery is measured by a blue electricity test system, and the preparation method and test process of the sodium ion battery are as follows:

[0132] The sodium pyrophosphate iron phosphate composite, the conductive agent conductive carbon black (SP) and the binder polyvinylidene fluoride (PVDF) prepared in the above application examples are weighed according to a mass ratio of 8:1:1, N-methyl pyrrolidone (NMP) is used as a solvent, and the positive electrode slurry is prepared by ball milling in a planetary ball mill for 2 h, then coated on an aluminum foil current collector, dried, rolled and punched into a positive electrode sheet, dried in a vacuum drying box for 12 h, and then placed in a glove box under an inert gas protective atmosphere. The negative electrode uses a metal sodium sheet, the electrolyte is a mixture of 1 mol / L sodium perchlorate (NaClO4), 1 mol / L ethylene carbonate (EC) and 1 mol / L propylene carbonate (PC), and the separator is glass fiber, and a CR2032 type button cell is assembled. Finally, the charge and discharge test is carried out on the battery test cabinet, and the test rates are 0.2C / 0.2C and 10C / 10C, and the test voltage range is 2.0-4.0V, wherein 10C / 10C represents the performance of the battery under a large rate current. The test results are shown in Table 3 and part of the results are shown in Figures 2-3

[0133] ​The sodium pyrophosphate ferric phosphate composite material in application example 1 is tested by XRD, and the test result is shown in Table 1. Figure 1 As shown in Table 1, the sodium pyrophosphate ferric phosphate composite material is successfully prepared, and has no obvious impurities and high purity. Figure 1

[0134] Table 3

[0135]

[0136] As shown in application example 1, application example 2, application example 3, application example 4, application example 12, application example 13 and application example 14, the conductivity of the composite material can be effectively improved by adding different contents of the doped metal element Mg in the sodium pyrophosphate ferric phosphate seed, but the influence of the conductivity of the composite material gradually decreases when the content of the doped metal element is too much, and the doped metal element also has an influence on other electrical properties, therefore, when the doping amount of the doped metal element is 1-3%, the compactness, the conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0137] As shown in application example 1, application example 5, application example 6, application example 12, application example 15 and application example 16, the rate performance of the composite material can be improved by increasing the content of the solid-state electrolyte material in the sodium pyrophosphate ferric phosphate seed, but the improvement effect is not great when the content is too much.

[0138] As shown in application example 1, application example 7, application example 8, application example 17 and application example 18, the sintering temperature and time of the sodium pyrophosphate ferric phosphate seed have a certain influence on the performance of the composite material, especially on the compactness, therefore, the sintering temperature of the first time is preferably 400-550 DEG C, the sintering time of the first time is preferably 5-8h, and the compactness, the conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0139] As shown in application example 1, application example 9 and application example 10, the influence of the composite material is not great by using different raw materials to mix and sinter with the solid-state electrolyte material.

[0140] As shown in application example 1, application example 11 and application example 20, the grinding particle size of the sodium pyrophosphate ferric phosphate seed has an influence on the performance of the sodium pyrophosphate ferric phosphate seed, and further has a certain influence on the performance of the composite material, therefore, when the particle size of the mixed slurry 1 during the preparation of the seed is preferably 0.3-0.5um, the compactness, the conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0141] ​It can be seen from application example 1 and application example 19 that the carbon content in the sodium pyrophosphate ferric phosphate seed is too high, which can cause the combination state of each component in the seed to be poor, the melting state required by the seed to be not reached, the growth of the seed to be limited, the performance of the seed to be changed, and the electrical performance of the composite material to be affected to a certain extent. Therefore, when the carbon content in the seed is preferably ≤0.01%, the compactness, the electrical conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0142] It can be seen from application example 1, application example 21, application example 22, application example 23 and application example 24 that the more the addition amount of the sodium pyrophosphate ferric phosphate seed is, the higher the compactness of the composite material is, and the better the rate performance is, but the discharge specific capacity is reduced. Therefore, when the addition amount of the seed is preferably 10-30%, the compactness, the electrical conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0143] It can be seen from application example 1, application example 25, application example 26, application example 27 and application example 28 that with the increase of the doping amount of the metal element, the electrical conductivity of the composite material is slightly increased; different kinds of doped metal elements have little influence.

[0144] It can be seen from application example 1, application example 29 and application example 30 that the sintering temperature and the sintering time have certain influence on the material performance when the composite material is prepared. Therefore, when the temperature of the second sintering is preferably 400-550°C and the time of the second sintering is preferably 5-8h, the compactness, the electrical conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0145] It can be seen from table application example 1 and application example 31 that the replacement of the types of raw materials has little influence on the performances of the composite material.

[0146] It can be seen from application example 1 and application example 32 that when the particle size of the mixed slurry 2 is preferably 0.3-0.5μm when the composite material is prepared, the compactness, the electrical conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0147] It can be seen from application example 1 and application example 33 that the more the content of the carbon coating layer is, the lower the compactness of the composite material is. Therefore, when the content of the carbon coating layer in the composite material is preferably ≤10%, the compactness, the electrical conductivity, the cycle performance, the rate performance and the discharge specific capacity performance of the composite material are more excellent.

[0148] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, but not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A method for preparing seed crystals of sodium iron phosphate pyrophosphate material, characterized in that, Includes the following steps: (1) Sodium source, iron source, phosphorus source, optional M metal element source, solid electrolyte material, reducing agent and solvent are ground and mixed to obtain a mixed slurry; (2) The mixed slurry is dried to obtain a mixed material; (3) The mixture is sintered in an inert gas atmosphere and then crushed to obtain sodium iron phosphate pyrophosphate seed crystals.

2. The method for preparing seed crystals of sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, Includes at least one of the following: A. In step (1), the sodium source includes one or more of sodium bicarbonate, sodium hydroxide, sodium carbonate, sodium nitrate, sodium chloride, sodium dihydrogen phosphate, sodium acetate, sodium citrate, and sodium carbonate. B. In step (1), the iron source includes one or more of the following: ferric hydroxide, ferric phosphate, ferrous oxalate, ferric oxide, ferric tetroxide, and ferric acetate. C. In step (1), the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate; D. In step (1), the reducing agent includes one or more of starch, conductive carbon black, glucose, sucrose, polypropylene, polyvinylpyrrolidone, polyethylene glycol, polypropylene glycol, acetylene black, carbon nanotubes, graphene, citric acid, fructose, maltose, lactose, and ascorbic acid. E. In step (1), the solid electrolyte material includes sodium silicon zirconium phosphate; F. In step (1), the source of the M metal element includes an oxide of the M metal element; G. In step (1), the sodium source is calculated as Na element, the iron source as Fe element, the phosphorus source as P element, and the M metal element source as M metal element, and the molar ratio of the sodium source, iron source, phosphorus source, and M metal element source is (3-6):(2-4):(0.8-2):(0.01-1); I. In step (1), the molar ratio of the iron source (Fe element) to the reducing agent is (2-4):(0.1-0.5); J. In step (1), based on the total mass of the sodium source, iron source and phosphorus source, the mass percentage of the solid electrolyte material is 1-5%.

3. The method for preparing seed crystals of sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, In step (3), the sintering temperature is 350-650℃, the sintering time is 3-20h, and the rate of heating to the sintering temperature is 1-10℃ / min.

4. A seed crystal of sodium iron phosphate pyrophosphate material prepared by the method for preparing seed crystals of sodium iron phosphate pyrophosphate material according to any one of claims 1-3.

5. The seed crystal of sodium iron phosphate pyrophosphate material as described in claim 4, characterized in that, It is composed of sodium iron phosphate pyrophosphate and a solid electrolyte material, wherein the chemical formula of the sodium iron phosphate pyrophosphate is Na₄Fe₂O₃. 3-x M x (PO4)2P2O7, 0≤x≤0.9, 2.1≤3-x≤3, M includes, but is not limited to, one or more of La, Ce, Nd, Pr, Ti, Mg, Al, Ni, Nb, Mn, Zr, Ta, Mo, W, and Eu. In the seed crystals of the sodium iron phosphate pyrophosphate material, the solid electrolyte material accounts for 0.1%-5.0% of the mass of the sodium iron phosphate pyrophosphate material.

6. A method for preparing a sodium iron phosphate pyrophosphate composite material, characterized in that, Includes the following steps: S1. The sodium source, iron source, phosphorus source, optional M metal element source, carbon source, solvent and the seed crystals of sodium iron phosphate pyrophosphate material as described in claim 4 or 5 are ground and mixed to obtain a mixed slurry. S2. The mixed slurry is dried to obtain a mixed material; S3. The mixture is sintered in an inert gas atmosphere and then crushed to obtain a sodium iron phosphate pyrophosphate composite material.

7. The method for preparing the sodium iron phosphate pyrophosphate composite material as described in claim 6, characterized in that, Includes at least one of the following: K. In step S1, the sodium source includes one or more of sodium bicarbonate, sodium hydroxide, sodium carbonate, sodium nitrate, sodium chloride, sodium dihydrogen phosphate, sodium acetate, sodium citrate, and sodium carbonate. L. In step S1, the iron source includes one or more of ferric hydroxide, ferric phosphate, ferrous oxalate, ferric oxide, ferric oxide, and ferric acetate. In step S1, the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate. N. In step S1, the carbon source includes one or more of starch, conductive carbon black, glucose, sucrose, polypropylene, polyvinylpyrrolidone, polyethylene glycol, polypropylene glycol, acetylene black, carbon nanotubes, graphene, citric acid, fructose, maltose, lactose, and ascorbic acid. O. In step S1, the source of the M metal element includes an oxide of the M metal element; In step S1, the sodium source is calculated as Na element, the iron source as Fe element, the phosphorus source as P element, and the M metal element source as M metal element, and the molar ratio of the sodium source, iron source, phosphorus source, and M metal element source is (4-10):(4-6):(1.5-3):(0.05-0.5). Q. In step S1, the molar ratio of the iron source (Fe element) to the carbon source is (4-6):(0.5-1). R. In step S1, based on the total mass of the sodium source, iron source and phosphorus source, the mass percentage of the seed crystals of the sodium iron phosphate pyrophosphate material is 5-80%. S. In step S1, based on the total mass of the sodium source, iron source and phosphorus source, the mass percentage of the carbon source is 1-20%. In step S1, based on the total molar amount of the sodium source, iron source, phosphorus source, and M metal element source, the molar percentage of the M metal element source is 0.1-10%.

8. The method for preparing the sodium iron phosphate pyrophosphate composite material as described in claim 6, characterized in that, In step S3, the sintering temperature is 350-650℃, the sintering time is 3-20h, and the rate of heating to the sintering temperature is 1-10℃ / min.

9. A sodium iron phosphate pyrophosphate composite material, characterized in that, It is prepared by the method for preparing sodium iron phosphate pyrophosphate composite material according to any one of claims 6-8.

10. A battery, comprising a positive electrode, characterized in that, The positive electrode includes a current collector and an active material coating on at least one side surface of the current collector, wherein the active material coating includes the sodium iron phosphate pyrophosphate composite material as described in claim 9.

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