Preparation method of non-stoichiometric iron phosphate materials and cathode materials for sodium-ion batteries

Abstract

This invention discloses a method for preparing a non-stoichiometric iron phosphate material and a cathode material for sodium-ion batteries. The preparation method includes the following steps: S1, preparation of the precursor solution: a buffer solution is added to a phosphoric acid solution to adjust the pH of the reaction system to the range of 0.5-3, ferrous salt is added, and the mixture is stirred to obtain the precursor solution; S2, precipitation and separation of the solid: under controlled temperature conditions, an oxidant is added to the precursor solution, and after the oxidation reaction is completed, the mixture is aged under controlled temperature. Solid-liquid separation is then performed to obtain the non-stoichiometric acid iron phosphate material Fe. x H y (PO m ) (3x+y) / (2m‑5) • ZH₂O solid. This invention features high product yield, low cost, high compaction density, and excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing a non-stoichiometric iron phosphate material and a sodium-ion battery cathode material. Background Technology

[0002] Sodium-ion batteries, especially sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) batteries, are gradually becoming a replacement for lithium iron phosphate (LFP) batteries due to their abundant raw material reserves, low price, high safety, and excellent electrochemical performance. The key cathode material for this type of sodium-ion battery is sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), with an iron-to-phosphorus ratio of 0.75.

[0003] However, the preparation process of Na4Fe3(PO4)2P2O7 mainly uses iron phosphate (FePO4) with an iron-to-phosphorus ratio of 1 as the raw material. Therefore, to balance the elemental ratio, more phosphorus sources, such as dihydrogen phosphates, need to be introduced during the preparation process. Moreover, due to the need for precise control of the iron-to-phosphorus ratio, the purity of the phosphorus source material is required to be high, increasing the material cost of the cathode material and hindering the reduction of the overall cost of sodium-ion batteries. Analyzing the preparation process of iron phosphate, existing methods are mainly chemical methods, using phosphoric acid and iron as raw materials. Based on the co-precipitation reaction of phosphate and ferric iron, iron phosphate (FePO4) is prepared, generating waste liquid that requires subsequent treatment.

[0004] Therefore, the difference in the iron-phosphorus ratio severely limits its widespread application in the synthesis of sodium iron pyrophosphate. In addition, there are additional waste liquid treatment costs, resulting in high process costs, which is not conducive to the low-cost popularization and application of sodium-ion batteries.

[0005] In response to this, Chinese invention patent application CN116409766A discloses a method for controlling the iron-phosphorus ratio in the range of 0.8-1.7. However, the iron-phosphorus ratio of the synthesized iron phosphate material is too high and cannot directly meet the requirement of a low iron-phosphorus ratio for the production of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, with an iron-phosphorus ratio of 0.75).

[0006] Furthermore, when iron phosphate is used as a raw material to produce electrode materials, such as lithium iron phosphate, sodium iron pyrophosphate, and sodium iron phosphate, the particle size and morphology of the iron phosphate precursor particles have a significant impact on the synthesis and electrochemical performance of the final electrode material products. Smaller particle size usually means shorter ion insertion / extraction paths, which is beneficial to improving the specific capacity and rate performance of the electrode material. Specific morphologies, such as spherical shapes, can improve compaction density and optimize the processing performance of the material. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing non-stoichiometric iron phosphate material and a sodium-ion battery cathode material, which has the characteristics of high product yield, low cost, high compaction density and excellent electrochemical performance.

[0008] This invention can be achieved through the following technical solutions:

[0009] This invention discloses a method for preparing non-stoichiometric iron phosphate material, comprising the following steps:

[0010] S1. Preparation of precursor solution: Add buffer solution to phosphorus source solution to adjust the pH range of the reaction system to 1-3, add ferrous salt, mix well to obtain precursor solution;

[0011] S2. Precipitation and Separation of Solids: Under controlled temperature conditions, an oxidant is added to the precursor solution from step S1. After the oxidation reaction is complete, the solution is aged under controlled temperature. Solid-liquid separation yields non-stoichiometric acid ferric phosphate (Fe). x H y (PO m ) (3x+y) / (2m-5) ·zH2O solid;

[0012] S3, High-temperature calcination: The non-stoichiometric acidic ferric phosphate Fe from step S2 is calcined... x H y (PO m ) (3x+y) / (2m-5) ·ZH2O solid is calcined at high temperature to obtain non-stoichiometric iron phosphate material after all or part of the bound water is removed.

[0013] In non-stoichiometric acid ferric phosphate Fe x H y (PO m ) (3x+y) / (2m-5) In stoichiometric iron phosphate materials with zH2O and all or part of the bound water removed, the iron-hydrogen ratio x / y ranges from 1 / 3 to 17 / 3, x ranges from 0.5 to 0.95, y ranges from 0.15 to 1.5, z ranges from 1 to 4, and m ranges from 3.5 to 4.

[0014] Furthermore, since phosphoric acid is a moderately strong acid, it has a significant impact on the pH of the solution system. The buffering capacity of a buffer solution is directly related to its total concentration. In the same buffer system, when the buffer ratio remains constant, the higher the total concentration of the buffer solution, the greater the amount of its acid-resistant and alkali-resistant components. This means that after adding the same amount of acid or alkali, the smaller the change in the buffer ratio, the greater the buffer capacity, thus exhibiting a stronger buffering ability. Conversely, if the total concentration is low, the buffer capacity is also low, and the buffering ability weakens accordingly. Considering the concentration of phosphoric acid and the solubility of the effective components in the buffer solution, the higher the concentration of phosphoric acid in the solution, the higher the concentration of the buffer solution required to maintain pH stability. However, if the buffer solution concentration is too high, the viscosity of the solution increases, which is detrimental to the reaction process and subsequent washing process. The concentration of ferrous ions is adjusted according to the concentration of phosphate ions added to ensure the iron-phosphorus ratio of the product. If the concentration of ferrous ions is too high, it will lead to an even higher iron-phosphorus ratio in the product. Therefore, the final concentration range of the phosphoric acid solution is 0.1-12 wt%, the concentration range of the buffer solution is 0.01-1 mol / L, and the ferrous ion concentration of the ferrous salt is 0.1-8 wt%. Through these controls, the pH of the reaction solution is kept at a low level, and the control of the non-stoichiometric iron-phosphorus ratio (0.2-0.85) based on pH and the feed iron-phosphorus ratio can effectively regulate the non-stoichiometric iron phosphate (Fe3+). x H y (PO m ) (3x+y) / (2m-5) The hydrogen content in zH2O is controlled to adjust the iron-hydrogen ratio x / y to 1 / 3-17 / 3. Based on charge conservation, this effectively ensures the preparation of non-stoichiometric iron phosphate materials. Meanwhile, the lower pH is unfavorable for the precipitation of other phosphates (such as titanium phosphate, magnesium phosphate, and aluminum phosphate). Therefore, recycled phosphoric acid, which is more cost-effective, can be used to obtain high-purity non-stoichiometric acid iron phosphate.

[0015] Further, in step S2, an oxidant is added to the precursor solution, and the reaction to rapidly nucleate and form an iron phosphate complex through an oxidative co-precipitation reaction is exothermic. Higher reaction temperatures result in faster reaction rates, which is beneficial for rapidly generating a solid with a concentrated and uniform particle size distribution. However, excessively high reaction temperatures can lead to excessively rapid reaction rates, severe exothermic reactions, and safety hazards. Therefore, the oxidation reaction temperature is 30-80℃. Aging mainly involves two reactions: the transformation and dissolution of the precipitate, and the aggregation and growth of the precipitate particles. Higher temperatures result in a higher degree of dissociation of the polyphosphoric acid, which easily leads to a decrease in the pH of the solution, and consequently a decrease in the iron-phosphorus ratio of the product. At high aging temperatures, particle aggregation is rapid, easily forming large particles. At lower aging temperatures, particles slowly aggregate and grow densely, ultimately yielding a spherical, high-compact-density material. The reaction temperature is 30-80℃; the aging conditions are: an aging temperature range of 10-80℃ and an aging time of 2-10 hours.

[0016] Further, in step S3, the high-temperature calcination conditions are: calcination temperature of 100-700℃ and calcination time of 1-10h. This step aims to remove the bound water from the prepared iron phosphate complex. Lower temperatures and shorter calcination times cannot completely remove the bound water, but can achieve partial removal of the water of crystallization. Higher temperatures and longer calcination times, however, will cause the iron phosphate complex to agglomerate and pyrophosphoricate. Specifically, the Fe from step S2... x H y (PO m ) (3x+y) / (2m-5) Solid zH2O is calcined at high temperatures of 100-800℃. As the calcination temperature increases, bound water is gradually removed. By controlling the calcination temperature, non-stoichiometric acidic iron phosphate materials with different water contents can be produced. The x / y ratio in the non-stoichiometric acidic iron phosphate material ranges from 1 / 3 to 17 / 3. Phosphate complexes prepared by current co-precipitation methods often carry 1-4 bound water molecules. For ease of general expression, 3x + y = 3 is defined, meaning the coefficient of phosphate is 1. Based on the iron-hydrogen ratio test of the product, the range of x is 0.5-0.95, y is 0.15-1.5, and z is 1-4. During the reaction heating process, aging heating process, and calcination process, phosphate pyrophosphorylation may occur. If pyrophosphate is present in the raw material, it will also affect the Fe... x H y (PO m ) (3x+y) / (2m-5) When pyrophosphate is introduced into zH2O, the range of m is 3.5-4. The iron-to-phosphorus ratio of the non-stoichiometric acidic iron phosphate and the non-stoichiometric iron phosphate material is 0.5-0.95.

[0017] Further, the buffer solution is one or more of the following: disodium hydrogen phosphate-citric acid buffer, phthalic acid-hydrochloric acid buffer, glycine-sulfuric acid buffer, phthalic acid-sulfuric acid buffer, phosphate-sodium dihydrogen phosphate, and oxalic acid-sodium hydrogen oxalate.

[0018] Further, the buffer solution is one or more of the following: disodium hydrogen phosphate-citric acid buffer, phthalic acid-hydrochloric acid buffer, glycine-sulfuric acid buffer, phthalic acid-sulfuric acid buffer, phosphate-sodium dihydrogen phosphate, and oxalic acid-sodium hydrogen oxalate.

[0019] Furthermore, the ferrous salt is one or more of ferrous sulfate, ferrous sulfate heptahydrate, ferrous chloride, ferrous nitrate, and sodium ferrous sulfate.

[0020] Furthermore, the phosphorus source is one or more of the following phosphates and pyrophosphates: crude phosphoric acid, refined phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, sodium pyrophosphate, etc.

[0021] Furthermore, the oxidant is one or more of the following: sodium persulfate, hydrogen peroxide, oxygen, ozone, air, sodium hypochlorite, peracetic acid, etc., with the concentration of hydrogen peroxide ranging from 1 to 30 wt%. Hydrogen peroxide is an easily explosive oxidant. When used as an oxidant in this invention, if the concentration is too low, it will lead to excessive dilution of the solution and a slow reaction rate, which is not conducive to the rapid formation of a solid with a concentrated and uniform particle size distribution. If the concentration is too high, it will lead to an overly violent reaction, posing a safety hazard.

[0022] Another aspect of the present invention is to protect a sodium-ion battery cathode material, which is prepared by mixing non-stoichiometric acid iron phosphate obtained above as a precursor with a sodium source and sintering at high temperature.

[0023] Furthermore, the high-temperature sintering conditions are: high-temperature calcination at 600-700℃ for 8-10 hours, with an argon and / or nitrogen protective atmosphere. At high temperatures, the phosphorus, iron, and sodium sources tend to form carbon-coated sodium iron pyrophosphate crystals. If the calcination temperature is too low or the calcination time is too short, the crystal transformation will be insufficient and the crystal phase will be impure. If the calcination temperature is too high or the calcination time is too long, the crystals will agglomerate, reducing their electrochemical performance.

[0024] Furthermore, the sodium salt is one or more of sodium carbonate, sodium dihydrogen phosphate, and sodium nitrate.

[0025] This invention discloses a method for preparing a non-stoichiometric iron phosphate material and a sodium-ion battery cathode material, which has the following beneficial effects:

[0026] First, the product yield is high. This invention introduces a buffer solution into the reaction system, controlling the pH value of the reaction system to remain stable in the range of 1-3. This effectively controls the iron-hydrogen ratio (x / y) of non-stoichiometric acid iron phosphate materials within the range of 1 / 3-17 / 3. This method of controlling the iron-hydrogen ratio is precise, stable, and reproducible, while also achieving a high yield. x H y (PO m ) (3x+y) / (2m-5) After further calcination and dehydration of zH2O, non-stoichiometric iron phosphate material with partial or complete dehydration can be obtained;

[0027] Secondly, the cost is low. In this invention, based on the adjustment of the entire system, there is no high requirement for the purity of the phosphorus source. Low-cost recycled phosphoric acid can be used as the phosphorus source. Even if the recycled phosphoric acid contains a lot of other impurities, this invention can effectively avoid introducing other impurities from the recycled acid into the precipitate product by adjusting the pH value, thereby improving the purity of the product and expanding the range of low-cost phosphorus sources.

[0028] Third, it has high compaction density. The oxidation coprecipitation reaction of this invention selects a relatively high temperature range, which is beneficial for rapidly generating Fe with concentrated particle size distribution and uniform particle distribution at a higher temperature in the initial stage of the reaction. x H y (PO m ) (3x+y) / (2m-5) • zH2O; The aging temperature was set in a relatively low range to promote the slow aggregation and compact growth of particles, ultimately resulting in a spherical material with high compaction density.

[0029] Fourth, it exhibits excellent electrochemical performance. The non-stoichiometric iron phosphate material prepared by the method of this invention has small size and good dispersibility. Based on the genetic characteristics between the precursor and the product, that is, the morphology and structure of the product can inherit the morphology and structure of the precursor, when the non-stoichiometric iron phosphate material precursor is used as the raw material for the synthesis of sodium iron pyrophosphate, a morphology and structure similar to the precursor can be obtained. The sodium iron pyrophosphate cathode material prepared from this precursor has the advantages of uniform phase distribution, high rate capability, high specific capacity, high compaction density, and good processability. Attached Figure Description

[0030] Appendix Figure 1 The charge-discharge curves are for the positive electrode material in Example 1. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0032] This invention discloses a method for preparing non-stoichiometric iron phosphate material, comprising the following steps:

[0033] S1. Preparation of precursor solution: Add buffer solution to phosphoric acid solution to adjust the pH range of the reaction system to 1-3, add ferrous salt, mix well to obtain precursor solution;

[0034] S2. Precipitation and Separation of Solids: Under controlled temperature conditions, an oxidant is added to the precursor solution from step S1. After the oxidation reaction is complete, the solution is aged under controlled temperature, and solid-liquid separation yields Fe. x H y (PO m ) (3x+y) / (2m-5) ·zH2O solid;

[0035] S3, High-temperature calcination: The Fe from step S2 is... x H y (PO m ) (3x+y) / (2m-5) · Non-stoichiometric iron phosphate material obtained by calcining zH2O solid at high temperature to remove all or part of the bound water;

[0036] In Fe xH y (PO m ) (3x+y) / (2m-5) The non-stoichiometric iron phosphate material obtained by removing all or part of the water of crystallization from zH2O has an iron-hydrogen ratio x / y ranging from 1 / 3 to 17 / 3, x ranging from 0.5 to 0.95, y ranging from 0.15 to 1.5, z ranging from 1 to 4, and m ranging from 3.5 to 4.

[0037] Further, in step S1, the concentration range of the phosphorus source solution is 0.1-12 wt%, the concentration range of the buffer solution is 0.01-1 mol / L, the ferrous ion concentration of the ferrous salt is 0.1-8 wt%, and the iron-phosphorus ratio of the feed is 0.2-0.85.

[0038] Furthermore, in step S2, the reaction temperature is 30-80℃; the aging conditions are: the aging temperature range is 10-80℃, and the aging time is 2-10h.

[0039] Furthermore, in step S3, the high-temperature calcination conditions are: calcination temperature of 100-700℃ and calcination time of 1-10h.

[0040] Further, the buffer solution is one or more of the following: disodium hydrogen phosphate-citric acid buffer, phthalic acid-hydrochloric acid buffer, glycine-sulfuric acid buffer, phthalic acid-sulfuric acid buffer, phosphate-sodium dihydrogen phosphate, and oxalic acid-sodium hydrogen oxalate.

[0041] Furthermore, the ferrous salt is one or more of ferrous sulfate, ferrous sulfate heptahydrate, ferrous chloride, ferrous nitrate, and sodium ferrous sulfate.

[0042] Furthermore, the phosphorus source is one or more of the following phosphates and pyrophosphates: crude phosphoric acid, refined phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, sodium pyrophosphate, etc.

[0043] Furthermore, the oxidant is one or more of the following: sodium persulfate, hydrogen peroxide, oxygen, ozone, air, sodium hypochlorite, peracetic acid, etc., and the concentration of hydrogen peroxide is 1-30 wt%.

[0044] Another aspect of the present invention is to protect a sodium-ion battery cathode material, wherein the non-stoichiometric acid iron phosphate Fe obtained as described above is used. x H y (PO m ) (3x+y) / (2m-5) The zH2O material or non-stoichiometric iron phosphate material is prepared by high-temperature sintering of a precursor with a sodium source.

[0045] Furthermore, the conditions for high-temperature sintering are: high-temperature calcination at 600-700℃ for 8-10 hours, with a protective atmosphere of argon and / or nitrogen.

[0046] Furthermore, the sodium salt is one or more of sodium carbonate, sodium dihydrogen phosphate, and sodium nitrate.

[0047] Example 1

[0048] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0049] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0050] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0051] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0052] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material, the charge-discharge curve of which is shown below. Figure 1 As shown, the charging specific capacity is as high as 108.2 mAh g. -1 It is evident that using non-stoichiometric acidic iron phosphate materials as iron-phosphorus sources can enable the preparation of high-performance sodium iron pyrophosphate cathode materials.

[0053] Example 2

[0054] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0055] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0056] S2. Precipitation and separation of solids: At 50°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation occurs rapidly through an oxidative coprecipitation reaction; after the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0057] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 8 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0058] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0059] Example 3

[0060] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0061] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0062] S2. Precipitation and separation of solids: At 30°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction; after the oxidation reaction is completed, the solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0063] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 6 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0064] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0065] Example 4

[0066] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0067] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (0.8 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (1.1 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0068] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation occurs rapidly through an oxidative coprecipitation reaction; after the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0069] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 4 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0070] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0071] Example 5

[0072] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0073] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.2 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, add ferrous salt (1.65 wt%) at an iron-phosphorus ratio of 0.75:1, mix well to obtain a homogeneous solution;

[0074] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation occurs rapidly through an oxidative coprecipitation reaction; after the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0075] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0076] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0077] Example 6

[0078] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0079] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (3.2 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize in the range of 2, and add ferrous salt (4.4 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0080] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation occurs rapidly through an oxidative coprecipitation reaction; after the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0081] S3. High-temperature calcination: The solid from step S2 above is calcined at 100°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0082] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0083] Example 7

[0084] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0085] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0086] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0087] S3. High-temperature calcination: The solid from step S2 above is calcined at 400°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0088] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0089] Example 8

[0090] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0091] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0092] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0093] S3. High-temperature calcination: The solid from step S2 above is calcined at 450°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0094] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0095] Example 9

[0096] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0097] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0098] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0099] S3. High-temperature calcination: The solid from step S2 above is calcined at 600°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0100] S4. High-temperature sintering: The solid sodium source from step S3 above is mixed and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as iron-phosphorus source can realize the preparation of high-performance sodium iron pyrophosphate cathode material.

[0101] Example 10

[0102] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0103] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0104] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0105] S3. High-temperature calcination: The solid from step S2 above is calcined at 800°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0106] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0107] Example 11

[0108] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0109] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.5 wt%) at an iron-phosphorus ratio of 0.85:1 and mix well to obtain a homogeneous solution.

[0110] S2. Precipitation and separation of solids: At 0°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation occurs rapidly through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 30°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0111] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0112] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can realize the preparation of high-performance sodium iron pyrophosphate cathode material.

[0113] Example 12

[0114] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0115] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (0.6 wt%) at an iron-phosphorus ratio of 0.2:1 and mix well to obtain a homogeneous solution.

[0116] S2. Precipitation and separation of solids: At 0°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation occurs rapidly through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 30°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0117] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0118] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0119] Example 13

[0120] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0121] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L phthalic acid-hydrochloric acid buffer solution to the dilute phosphoric acid solution as a buffer solution system, adjust the pH value of the reaction system to stabilize at about 2, and add ferrous salt (2.2 wt%) at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0122] S2. Precipitation and separation of solids: At 70°C, oxygen is introduced under high pressure into the homogeneous solution of step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction; after the oxidation reaction is completed, the reaction solution is aged at 30°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0123] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0124] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0125] Example 14

[0126] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0127] S1. Preparation of precursor solution: A reaction solution with a phosphorus concentration of 1.6 wt% was prepared using phosphoric acid and sodium iron pyrophosphate solution as phosphorus sources. A 0.5 mol / L phthalic acid-hydrochloric acid buffer solution was added to the reaction solution as a buffer solution system to adjust the pH of the reaction system to be stable at about 2. Ferrous salt (2.2 wt%) was added at an iron-to-phosphorus ratio of 0.75:1 and mixed to obtain a homogeneous solution.

[0128] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0129] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0130] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0131] Example 15

[0132] This embodiment relates to a non-stoichiometric acid-type iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0133] S1. Preparation of precursor solution: For the reaction solution containing ammonium dihydrogen phosphate with a phosphorus concentration of 1.6 wt%, 0.5 mol / L phthalic acid-hydrochloric acid buffer solution was added as a buffer solution system to adjust the pH value of the reaction system to be stable at about 2. Ferrous salt (2.2 wt%) was added at an iron-to-phosphorus ratio of 0.75:1 and mixed well to obtain a homogeneous solution.

[0134] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the reaction solution is aged at 40°C for 4 hours, and the solid obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0135] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain non-stoichiometric acidic iron phosphate material.

[0136] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material. Its electrochemical performance is shown in Table 1. It can be seen that using non-stoichiometric acid iron phosphate material as an iron-phosphorus source can achieve the preparation of high-performance sodium iron pyrophosphate cathode material.

[0137] Comparative Example 1

[0138] This embodiment relates to an iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0139] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%). Add 0.5 mol / L acetate-sodium acetate buffer solution to the dilute phosphoric acid solution as a buffer solution system. Adjust the pH value of the reaction system to stabilize in the range of 3.5. Add ferrous salt at an iron-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0140] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative co-precipitation reaction. After the oxidation reaction is completed, the solution is aged at 40°C for 4 hours, and the iron phosphate composite material obtained from solid-liquid separation is dried overnight in a 100°C oven.

[0141] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water. However, due to the excessively high pH of the reaction solution, some ferric hydroxide precipitates out, and the product is no longer a pure ferric phosphate precursor.

[0142] S4. High-temperature sintering: The solid from step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material, the electrochemical performance of which is shown in Table 1.

[0143] Comparative Example 2

[0144] This embodiment relates to a low iron-to-phosphorus ratio iron phosphate material and a sodium-ion battery cathode material prepared using it as a precursor. The preparation method includes the following steps:

[0145] S1. Preparation of precursor solution: Dilute crude phosphoric acid to obtain dilute phosphoric acid (1.6 wt%), adjust the pH of the reaction system to 0.4, add ferrous salt at an iron-to-phosphorus ratio of 0.75:1 and mix well to obtain a homogeneous solution.

[0146] S2. Precipitation and separation of solids: At 70°C, an oxidant is added to the homogeneous solution from step S1 above, and nucleation is rapidly achieved through an oxidative coprecipitation reaction. After the oxidation reaction is completed, the solution is aged at 40°C for 4 hours. The phosphate composite material with bound water obtained by solid-liquid separation is dried overnight in a 100°C oven.

[0147] S3. High-temperature calcination: The solid from step S2 above is calcined at 300°C for 10 hours to remove bound water and obtain the iron phosphate composite material.

[0148] S4. High-temperature sintering: The iron phosphate composite material obtained in step S3 above is mixed with a sodium source and calcined under an inert atmosphere to obtain sodium iron pyrophosphate cathode material, the electrochemical performance of which is shown in Table 1.

[0149] Table 1 Performance Test Results

[0150]

[0151] The test results in Table 1 show that:

[0152] The Fe provided by this invention x H y (PO m ) (3x+y) / (2m-5) The ·zH₂O preparation method can prepare an iron-phosphorus precursor with an iron-phosphorus ratio of 0.75, with a yield as high as 97%. As the reaction temperature increases, the prepared Fe... x H y (PO m ) (3x+y) / (2m-5) ·zH2O has a higher compaction density.

[0153] As the pH, regulated by the buffer solution, decreases, the aging temperature increases, and the reactant concentration increases, the iron-hydrogen ratio (x / y) changes, directly causing the iron-phosphorus ratio of the prepared non-stoichiometric acid ferric phosphate to also change, thereby achieving the goal of controlling the iron-phosphorus ratio by controlling the iron-hydrogen ratio.

[0154] As can be seen from Examples 1 and 2, 3, 13, raising the initial temperature of the reaction to a higher level rapidly generates particles with concentrated particle size distribution and uniform particle distribution; selecting a lower temperature range for aging promotes the slow aggregation and compact growth of particles, ultimately resulting in a spherical material with high compaction density.

[0155] As can be seen from Examples 1, 4, 5, and 6, as the reactant concentration increases, the iron-hydrogen ratio (x / y) of the prepared non-stoichiometric iron phosphate decreases, and the iron-phosphorus ratio of the corresponding product also shows a downward trend.

[0156] As can be seen from Examples 1 and 7, 8, 9, and 10, the water content of the prepared non-stoichiometric iron phosphate gradually decreases with increasing calcination temperature between 300 and 800 degrees Celsius, but it is still suitable as a raw material for the synthesis of sodium iron phosphate pyrophosphate.

[0157] As can be seen from Examples 1 and 11, 12, as the iron-phosphorus ratio of the feed decreases, the iron-hydrogen ratio (x / y) of the prepared non-stoichiometric iron phosphate system also decreases, and the iron-phosphorus ratio of the corresponding product also shows a downward trend.

[0158] As can be seen from Examples 14 and 15, when other phosphorus sources or mixed phosphorus sources are used, non-stoichiometric iron phosphate with a low iron-hydrogen ratio and similar compaction density characteristics can still be prepared by adjusting other parameters. The electrochemical performance of sodium iron pyrophosphate synthesized with it as a precursor also maintains a high level.

[0159] As can be seen from Examples 1, 2, 3 and Comparative Examples 1, 2, it is beneficial to stabilize the iron-phosphorus ratio of the obtained product by using a buffer solution to control the pH at around 2. Lowering the pH of the reaction solution significantly reduces the iron-phosphorus ratio and yield of the prepared iron phosphate precursor. On the other hand, raising the pH of the reaction will cause the precipitation of iron hydroxide, which will lead to an increase in the iron-phosphorus ratio of the product, making it unsuitable for the synthesis of sodium iron pyrophosphate.

[0160] The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A method of preparing a non-metered ratio iron phosphate material, characterized by: The method comprises the following steps: S1, preparation of a precursor solution: a buffer solution is added to a phosphorus source solution to adjust the pH of the reaction system to a range of 1-3, and a ferrous salt is added, and the precursor solution is obtained after mixing; S2, precipitation and separation of solid: under temperature control, an oxidizing agent is added to the precursor solution of step S1, and after the completion of the oxidation reaction, the temperature is controlled for aging, and solid-liquid separation is performed to obtain Fe x H y (PO m ) (3x+y) / (2m-5) ·zH2O solid; In Fe x H y (PO m ) (3x+y) / (2m-5) The iron-hydrogen ratio x / y ranges from 1 / 3 to 17 / 3, x ranges from 0.5 to 0.95, y ranges from 0.15 to 1.5, z ranges from 1 to 4, and m ranges from 3.5 to 4. In step S1, the concentration of the phosphorus source solution is 0.1-12wt%, the concentration of the buffer solution is 0.01-1 mol / L, the concentration of ferrous ions in the ferrous salt is 0.1-8wt%, and the iron-phosphorus ratio of the raw materials is 0.2-0.85; the buffer solution is one or more of the following: disodium hydrogen phosphate-citric acid buffer, phthalic acid-hydrochloric acid buffer, glycine-sulfuric acid buffer, phthalic acid-sulfuric acid buffer, phosphoric acid-sodium dihydrogen phosphate, oxalic acid-sodium hydrogen oxalate; In step S2, the reaction temperature is 30-90℃; the aging conditions are: the aging temperature is 10-90℃, and the aging time is 2-10h.

2. The method of claim 1, wherein the non-metered ratio iron phosphate material is characterized by: The ferrous salt is one or more of the following: ferrous sulfate, ferrous sulfate heptahydrate, ferrous chloride, ferrous nitrate, and sodium ferrous sulfate; the phosphorus source is one or more of the following: crude phosphoric acid, refined phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

3. The method of claim 1, wherein the non-metered ratio iron phosphate material is prepared by: The oxidizing agent is one or more of the following: sodium persulfate, hydrogen peroxide, oxygen, ozone, air, sodium hypochlorite, and peroxoacetic acid, and the concentration of the hydrogen peroxide is 1-30wt%.

4. The method of claim 1, wherein the non-metered ratio iron phosphate material is characterized by: The method further comprises the following steps: S3, high temperature calcination: Fe x H y (PO m ) (3x+y) / (2m-5) solid at high temperature to obtain a non-stoichiometric iron phosphate material with all or part of the bound water removed.

5. The method of claim 4, wherein the non-metered ratio iron phosphate material is prepared by: In step S3, the high-temperature calcination conditions are: the calcination temperature is 100-700℃, and the calcination time is 1-10h.

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