A modified ferric phosphate and its preparation method and application

Abstract

The present disclosure provides a modified iron phosphate and its preparation method and application. The preparation method comprises the following steps: (1) mixing a zirconium salt, a hydrogen phosphate, and a tungstate with a solvent to obtain a mixed salt solution, heating the solution to obtain zirconium phosphotungstate; (2) using the zirconium phosphotungstate as a seed crystal, mixing it with an iron salt, a phosphate, phosphoric acid, and an oxidant, and performing a synthesis reaction to obtain a modified material; (3) sintering the modified material and grinding it to obtain the modified iron phosphate. The present disclosure uses zirconium phosphotungstate as a seed crystal to prepare iron phosphate, which can promote the generation of secondary particle cracks, facilitate crushing, obtain an iron phosphate precursor with a smaller particle size, and further produce a high-energy, high-power density lithium iron phosphate positive electrode material.

Description

Technical Field

[0001] The present disclosure belongs to the technical field of battery materials and relates to a modified iron phosphate and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate, a lithium-ion battery material with the chemical formula LiFePO4, is primarily used in various lithium-ion batteries. Since Japan's NTT first unveiled an olivine-structured lithium battery cathode material in 1996, Texas State University in the United States also reported the reversible lithium migration and removal properties of LiFePO4 in 1997. This material has now been widely used in various lithium-ion batteries.

[0003] Iron phosphate, as the precursor of lithium iron phosphate, determines the performance of lithium iron phosphate. Existing preparation technologies of iron phosphate include solid phase method, particle size grading method, liquid phase synthesis method, etc.

[0004] CN105480960A discloses a method for preparing ferric phosphate, comprising the following steps: placing iron in a phosphoric acid solution, heating it to perform an iron oxidation reaction, and obtaining a reaction liquid containing Fe(H2PO4)2; adding hydrogen peroxide to the filtrate of the reaction liquid, performing an oxidation reaction under stirring, and then adding polyethylene glycol and continuing to stir to react Fe(H2PO4)2 to generate ferric phosphate, and obtaining an oxidized liquid; adding distilled water to the oxidized liquid to perform a hydrolysis reaction; performing solid-liquid separation on the hydrolyzed liquid, washing the separated solid phase discharge with water until the pH value of the washed liquid reaches near neutral, and drying to obtain solid ferric phosphate; and sequentially drying and dehydrating the dried solid ferric phosphate to form dehydrated ferric phosphate.

[0005] CN111704121A discloses a method for preparing iron phosphate and lithium iron phosphate, comprising the following steps: S1, preparing an iron source and a phosphorus source, dividing the iron source into two parts F1 and F2, and dividing the phosphorus source into two parts P1 and P2; S2, adding the phosphorus source P1 to the iron source F1, heating the mixture to 90-100°C, and keeping the temperature until the material turns white; S3, mixing the iron source F2 and the phosphorus source P2, and adding sulfuric acid to obtain a mixed solution; S4, adding the mixed solution obtained in step S3 to the material treated in step S2, reacting at 90-100°C for 1-3 hours, and washing and calcining the reaction product to obtain the iron phosphate.

[0006] The iron phosphate products obtained by the above scheme mostly present tightly packed secondary agglomerates. When used to prepare lithium iron phosphate, in order to obtain finer iron phosphate particles, greater grinding intensity and longer grinding time are required, which leads to increased costs. Summary of the Invention

[0007] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0008] The purpose of the present disclosure is to provide a modified iron phosphate and its preparation method and application. The present disclosure uses zirconium tungstate phosphotungstate as a seed crystal to prepare iron phosphate, which can promote the generation of secondary particle cracks, make the iron phosphate easier to break, reduce the difficulty of grinding the iron phosphate, and obtain an iron phosphate precursor with a smaller particle size, thereby preparing a high-energy, high-power density lithium iron phosphate positive electrode material.

[0009] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a method for preparing modified ferric phosphate, the preparation method comprising the following steps:

[0011] (1) mixing zirconium salt, hydrogen phosphate and tungstate with a solvent to obtain a mixed salt solution, and heating the solution to react to obtain zirconium phosphotungstate;

[0012] (2) using the zirconium phosphotungstate as a seed crystal, mixing it with an iron salt, a phosphate, phosphoric acid and an oxidant, and performing a synthesis reaction to obtain a modified material;

[0013] (3) The modified material is sintered and ground to obtain the modified iron phosphate.

[0014] The present invention uses zirconium tungstate phosphotungstate (Zr2WO4(PO4)2) as a seed crystal to prepare iron phosphate. Through the negative thermal expansion of zirconium tungstate phosphotungstate, the inner core of the iron phosphate precursor can shrink during synthesis and expand after cooling to room temperature, causing cracks to appear in the secondary agglomerated particles, reducing the grinding intensity and time required for the preparation of small-particle iron phosphate. In the subsequent application process of preparing lithium iron phosphate, the zirconium and tungsten ions can achieve the doping of zirconium ions on Li ion sites and tungsten ions on Fe sites during the sintering process of lithium iron phosphate, thereby improving the intrinsic conductivity of the material. In addition, the doping of zirconium tungsten ions can also increase the compaction density of the material.

[0015] In one embodiment, the zirconium salt in step (1) comprises zirconium oxychloride.

[0016] In one embodiment, the hydrogen phosphate includes any one of ammonium hydrogen phosphate, sodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate, or a combination of at least two thereof.

[0017] In one embodiment, the tungstate salt includes sodium tungstate and / or ammonium tungstate.

[0018] In one embodiment, the molar ratio of zirconium, tungsten and phosphorus in the mixed salt solution of step (1) is (1.8-2.3):1:(2-2.5), for example: 1.8:1:2, 1.9:1:2.1, 2:1:2.2, 2.1:1:2.4 or 2.3:1:2.5, etc.

[0019] In one embodiment, the mixing time in step (1) is 15 to 30 minutes, for example, 15 minutes, 18 minutes, 20 minutes, 25 minutes or 30 minutes.

[0020] The mixing in step (1) of the present disclosure may be a mixture of zirconium salt, hydrogen phosphate and tungstate with a solvent, or zirconium salt, hydrogen phosphate and tungstate may be prepared into solutions separately in advance and then mixed.

[0021] In one embodiment, the heating reaction in step (1) comprises a microwave hydrothermal reaction.

[0022] In one embodiment, the temperature of the heating reaction is 100-150°C, for example, 100°C, 110°C, 120°C, 140°C or 150°C.

[0023] In one embodiment, the heating reaction time is 20 to 40 minutes, for example, 20 minutes, 25 minutes, 30 minutes, 35 minutes or 40 minutes.

[0024] In one embodiment, the heating reaction is followed by vacuum drying.

[0025] In one embodiment, the vacuum drying treatment is carried out at a temperature of 50-80°C, for example, 50°C, 55°C, 60°C, 70°C or 80°C.

[0026] In one embodiment, the vacuum drying treatment time is 12 to 24 hours, for example, 12 hours, 15 hours, 18 hours, 20 hours or 24 hours.

[0027] In one embodiment, the median particle size D50 of the zirconium phosphotungstate is 100-200 nm, for example, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm or 200 nm.

[0028] In one embodiment, the iron salt in step (2) includes any one of ferrous sulfate, ferrous chloride or ferrous oxalate, or a combination of at least two thereof.

[0029] In one embodiment, the phosphate includes any one of sodium phosphate, potassium phosphate or ammonium phosphate, or a combination of at least two thereof.

[0030] In one embodiment, the oxidant includes any one of hydrogen peroxide, sodium peroxide, peracetic acid or ammonium persulfate, or a combination of at least two thereof.

[0031] In one embodiment, the molar ratio of the oxidant to the iron element in the iron source is (1-2):1, for example: 1:1, 1.2:1, 1.5:1, 1.8:1 or 2:1.

[0032] In one embodiment, the mixing in step (2) includes mixing an iron salt solution with an oxidant, adding phosphoric acid to adjust the pH, adding zirconium phosphotungstate seed crystals, and then adding a phosphate solution.

[0033] In one embodiment, the concentration of the iron salt solution is 0.5 to 1.5 mol / L, for example, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L or 1.5 mol / L.

[0034] In one embodiment, the pH is 1 to 3, for example, 1, 1.5, 2, 2.5 or 3.

[0035] In one embodiment, the concentration of the phosphate solution is 0.5 to 1.5 mol / L, for example, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L or 1.5 mol / L.

[0036] In one embodiment, the molar ratio of the iron element in the iron salt solution to the phosphorus element in the phosphate solution is 1:(1-1.5), for example: 1:1, 1:1.1, 1:1.2, 1:1.4 or 1:1.5.

[0037] In one embodiment, the temperature of the synthesis reaction in step (2) is 80-100°C, for example, 80°C, 85°C, 90°C, 95°C or 100°C.

[0038] In one embodiment, the synthesis reaction time is 1 to 3 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours.

[0039] In one embodiment, the temperature of the sintering treatment in step (3) is 400-700°C, for example, 400°C, 450°C, 500°C, 600°C or 700°C.

[0040] In one embodiment, the sintering treatment time is 3 to 6 hours, for example, 3 hours, 3.5 hours, 4 hours, 5 hours or 6 hours.

[0041] In one embodiment, the grinding time is 1 to 3 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours.

[0042] In a second aspect, the present disclosure provides a modified ferric phosphate, which is prepared by the method described in the first aspect.

[0043] In one embodiment, the median particle size D50 of the modified ferric phosphate is 0.5 to 1 μm, for example, 0.5 μm, 0.6 μm, 0.8 μm, 0.9 μm or 1 μm.

[0044] The modified iron phosphate disclosed in the present invention is ground to obtain modified iron phosphate particles with a median particle size of 0.5 to 1 μm, which has a good crushing effect. This also indirectly illustrates the characteristic that the modified iron phosphate prepared in the present application is easy to crush.

[0045] In a third aspect, the present disclosure provides a lithium iron phosphate positive electrode material, which is prepared by mixing and sintering the modified iron phosphate as described in the second aspect and a lithium source.

[0046] Compared with the prior art, the present disclosure has the following beneficial effects:

[0047] (1) The present invention uses zirconium tungstate as a seed crystal to prepare iron phosphate, which can promote the generation of secondary particle cracks, making the iron phosphate easier to break, reducing the difficulty of grinding the iron phosphate, and obtaining an iron phosphate precursor with a smaller particle size, thereby preparing a high-energy, high-power density lithium iron phosphate positive electrode material.

[0048] (2) The compacted density of the modified iron phosphate prepared by the method disclosed in the present invention can reach 2.57 g / cm 3 The above results show that the 0.1C discharge capacity of the battery can reach more than 156.33mAh / g.

[0049] (3) The outer layer of the iron phosphate precursor prepared in the present invention is coated with a small amount of zirconium phosphotungstate, which can limit the growth of lithium iron phosphate particles during the synthesis of lithium iron phosphate, and is conducive to obtaining lithium iron phosphate positive electrode materials with smaller particle size.

[0050] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0052] Figure 1 This is an SEM image of the modified iron phosphate prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0053] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0054] Example 1

[0055] This embodiment provides a modified ferric phosphate, and the preparation method of the modified ferric phosphate is as follows:

[0056] (1) Weigh zirconium oxychloride, ammonium hydrogen phosphate, and ammonium tungstate, respectively, and add water to prepare solutions. The solution concentration is 1 mol / L. The prepared solutions are mixed to control the molar ratio of zirconium, tungsten, and phosphorus in the mixed solution to be 2:1:2.1. After mixing and stirring for 20 minutes, the solution is transferred to a microwave reactor, heated to 120°C for hydrothermal reaction for 30 minutes, filtered, and dried in a vacuum drying oven at 60°C for 24 hours to obtain nano-zirconium phosphotungstate with a D50 of 150 nm. The nano-zirconium phosphotungstate is added to deionized water and ultrasonicated for 40 minutes to obtain a zirconium phosphotungstate seed solution;

[0057] (2) A 1.1 mol / L ferrous sulfate solution was added to a stirred reactor, followed by the addition of phosphoric acid and hydrogen peroxide (the molar ratio of H2O2 to Fe was 1.5:1), and the pH was adjusted to 2.4 by the addition of phosphoric acid. Then, zirconium phosphotungstate seed crystals were added to the reactor, and finally, a 0.9 mol / L sodium phosphate solution was added. The molar ratio of the Fe element in the ferrous sulfate solution to the P element in the sodium phosphate solution was 1:1.2. After heating at 90°C for 2 hours, dihydrate iron phosphate particles grown on the zirconium phosphotungstate seed crystals were obtained.

[0058] (3) The iron phosphate dihydrate particles grown on the basis of zirconium phosphotungstate seed crystals were sintered at 500°C for 4 hours, and the sintered iron phosphate was ball-milled for 1.5 hours to obtain a modified iron phosphate with a D50 of 800 nm. The SEM image of the modified iron phosphate is shown in FIG. Figure 1 shown.

[0059] The modified iron phosphate is used to prepare a lithium iron phosphate positive electrode material, and the D50 of the obtained lithium iron phosphate positive electrode material is about 1 μm.

[0060] Example 2

[0061] This embodiment provides a modified ferric phosphate, and the preparation method of the modified ferric phosphate is as follows:

[0062] (1) Weigh zirconium oxychloride, ammonium hydrogen phosphate, and ammonium tungstate, respectively, and add water to prepare solutions. The solution concentration is 1 mol / L. The prepared solutions are mixed to control the molar ratio of zirconium, tungsten, and phosphorus in the mixed solution to be 1.8:1:2. After mixing and stirring for 15 minutes, the solution is transferred to a microwave reactor, heated to 100°C for hydrothermal reaction for 40 minutes, filtered, and dried in a vacuum drying oven at 50°C for 24 hours to obtain nano-zirconium phosphotungstate with a D50 of 120 nm. The nano-zirconium phosphotungstate is added to deionized water and ultrasonicated for 40 minutes to obtain a zirconium phosphotungstate seed solution;

[0063] (2) A 1.1 mol / L ferrous sulfate solution was added to a stirred reactor, followed by the addition of phosphoric acid and hydrogen peroxide (the molar ratio of H2O2 to Fe was 1:1), the pH was adjusted to 3 by the addition of phosphoric acid, and then zirconium phosphotungstate seed crystals were added to the reactor, and finally a 0.5 mol / L sodium phosphate solution was added, the molar ratio of the Fe element in the ferrous sulfate solution to the P element in the sodium phosphate solution was 1:1, and the mixture was heated at 80°C for 3 h to obtain dihydrated iron phosphate particles grown on the zirconium phosphotungstate seed crystals;

[0064] (3) The dihydrate iron phosphate particles grown on the basis of zirconium phosphotungstate seed crystals were sintered at 400°C for 6 hours, and the sintered iron phosphate was ball-milled for 1 hour to obtain modified iron phosphate with a D50 of 950 nm.

[0065] Example 3

[0066] This embodiment provides a modified ferric phosphate, and the preparation method of the modified ferric phosphate is as follows:

[0067] (1) Weigh zirconium oxychloride, sodium hydrogen phosphate, and sodium tungstate, respectively, and add water to prepare solutions. The solution concentration is 1 mol / L. The prepared solutions are mixed to control the molar ratio of zirconium, tungsten, and phosphorus in the mixed solution to be 2.3:1:2.5. After mixing and stirring for 30 minutes, the solution is transferred to a microwave reactor, heated to 150°C for hydrothermal reaction for 20 minutes, filtered, and dried in a vacuum drying oven at 50°C for 24 hours to obtain nano-zirconium phosphotungstate with a D50 of 110 nm. The nano-zirconium phosphotungstate is added to deionized water and ultrasonicated for 40 minutes to obtain a zirconium phosphotungstate seed solution;

[0068] (2) A 1.1 mol / L ferrous sulfate solution was added to a stirred reactor, followed by the addition of phosphoric acid and peracetic acid (the molar ratio of peracetic acid to Fe was 2:1), the pH was adjusted to 1 by the addition of phosphoric acid, and then zirconium phosphotungstate seed crystals were added to the reactor, and finally a 1.5 mol / L sodium phosphate solution was added, the molar ratio of the Fe element in the ferrous sulfate solution to the P element in the sodium phosphate solution was 1:1.5, and the mixture was heated at 80°C for 3 h to obtain dihydrated iron phosphate particles grown on the zirconium phosphotungstate seed crystals;

[0069] (3) The iron phosphate dihydrate particles grown on the basis of zirconium phosphotungstate seed crystals were sintered at 700°C for 3 hours, and the sintered iron phosphate was ball-milled for 1 hour to obtain modified iron phosphate with a D50 of 900 nm.

[0070] Example 4

[0071] The only difference between this embodiment and embodiment 1 is that the microwave hydrothermal treatment in step (1) is replaced by conventional hydrothermal treatment, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0072] Comparative Example 1

[0073] In this comparative example, ferrous phosphate was used as a seed crystal to prepare modified iron phosphate, and other conditions and parameters were exactly the same as those in Example 1. The obtained modified iron phosphate had a D50 of 3 μm, and the lithium iron phosphate positive electrode material was prepared using the modified iron phosphate, and the obtained lithium iron phosphate had a D50 of 5 μm.

[0074] Comparative Example 2

[0075] The only difference between this comparative example and Example 1 is that the ball milling time is 5 h, and the other conditions and parameters are exactly the same as those in Example 1.

[0076] Comparative Example 3

[0077] The only difference between this comparative example and Example 1 is that no zirconium source is added, and other conditions and parameters are exactly the same as those in Example 1.

[0078] Comparative Example 4

[0079] The only difference between this comparative example and Example 1 is that no tungsten source is added, and other conditions and parameters are exactly the same as those in Example 1.

[0080] Performance testing:

[0081] The iron phosphate prepared in the examples and comparative examples was mixed with lithium hydroxide and glucose, with a molar ratio of lithium to iron phosphate of 1 to 1.1:1. The mass of glucose was 2 to 5% of the theoretical mass of the synthesized lithium iron phosphate.

[0082] The mixed material is placed in a sintering device, and a protective or reducing gas is introduced at a gas flow rate of 0.08 L / min. The temperature is increased from room temperature to 700°C at a rate of 4°C / min, and the temperature is kept at this temperature for 10 hours. The temperature in the furnace is then naturally lowered to room temperature to obtain LiFePO4 / C.

[0083] The prepared LiFePO4 / C was mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 90:5:5, using N-methylpyrrolidone (NMP) as the solvent. The mixture was then evenly coated on aluminum foil. After drying, it was rolled to form a simulated battery positive electrode. The negative electrode was a metal lithium sheet, the separator was Celgard 2400, and the electrolyte was 1 mol / L LiPF6 / DMC+DEC (volume ratio of 1:1). A CR2025-type simulated battery was formed. The charge and discharge voltage range was 2.9 to 3.7 V, and the electrochemical performance data of lithium iron phosphate, a lithium-ion battery positive electrode material, was obtained. The test results are shown in Table 1:

[0084] Table 1

[0085]

[0086]

[0087] As can be seen from Table 1, from Examples 1-3, the compacted density of the modified ferric phosphate prepared by the method of the present disclosure can reach 2.57 g / cm 3 The above results show that the 0.1C discharge capacity of the battery can reach more than 156.33mAh / g.

[0088] From the comparison between Example 1 and Example 4, it can be seen that the present disclosure can obtain an iron phosphate precursor with a finer particle size through microwave hydrothermal treatment, which is beneficial to the improvement of compaction density and discharge capacity.

[0089] By comparing Example 1 and Comparative Example 1, it can be seen that the present disclosure uses zirconium phosphotungstate as a seed crystal to prepare iron phosphate. Due to the negative thermal expansion of zirconium phosphotungstate, the core of the iron phosphate precursor can shrink during synthesis and expand after cooling to room temperature, causing cracks to appear in the secondary agglomerated particles, reducing the grinding intensity and time required to prepare small-particle iron phosphate. In addition, the crushing and grinding of the secondary agglomerated particles can cause part of the zirconium phosphotungstate core to coat the particle surface, which can limit its particle growth during the lithium iron phosphate synthesis process, resulting in small-particle lithium iron phosphate positive electrode material.

[0090] From the comparison between Example 1 and Comparative Examples 1-2, it can be seen that the method described in the present application can significantly reduce the accumulation of secondary agglomerates in the iron phosphate product, and lithium iron phosphate particles with smaller particle size can be obtained in a shorter grinding time. Compared with Comparative Example 2, the grinding time is increased (exceeding the grinding time of the present application), but the particle size of the material does not change significantly.

[0091] Comparison of Example 1 and Comparative Examples 3-4 shows that during the sintering of lithium iron phosphate, zirconium and tungsten ions can achieve doping of the material with zirconium ions at the Li ion sites and tungsten ions at the Fe sites, thereby improving the intrinsic conductivity of the material. Furthermore, doping with tungsten and zirconium ions can also increase the compaction density of the material.

Claims

1. A method for preparing modified ferric phosphate, comprising the following steps: (1) mixing zirconium salt, hydrogen phosphate and tungstate with a solvent to obtain a mixed salt solution, and heating the solution to obtain zirconium phosphotungstate; (2) using the zirconium phosphotungstate as a seed crystal, mixing it with an iron salt, a phosphate, phosphoric acid and an oxidant, and performing a synthesis reaction to obtain a modified material; (3) sintering the modified material and grinding it to obtain the modified iron phosphate; The heating reaction in step (1) includes a microwave hydrothermal reaction, and the temperature of the heating reaction is 100-150°C.

2. The preparation method according to claim 1, wherein The zirconium salt in step (1) includes zirconium oxychloride.

3. The preparation method according to claim 1, wherein The hydrogen phosphate includes any one of ammonium hydrogen phosphate, sodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate, or a combination of at least two thereof.

4. The preparation method according to claim 1, wherein The tungstate includes sodium tungstate and / or ammonium tungstate.

5. The preparation method according to claim 1, wherein The molar ratio of zirconium element, tungsten element and phosphorus element in the mixed salt solution of step (1) is (1.8-2.3):1:(2-2.5).

6. The preparation method according to claim 1, wherein The mixing time is 15 to 30 minutes.

7. The preparation method according to claim 1, wherein The heating reaction time is 20 to 40 minutes.

8. The preparation method according to claim 1, wherein The heating reaction is followed by vacuum drying.

9. The preparation method according to claim 8, wherein The temperature of the vacuum drying treatment is 50-80°C.

10. The preparation method according to claim 8, wherein The vacuum drying process lasts for 12 to 24 hours.

11. The preparation method according to claim 1, wherein The median particle size D50 of the zirconium phosphotungstate is 150-300 nm.

12. The preparation method according to claim 1, wherein The iron salt in step (2) includes any one of ferrous sulfate, ferrous chloride or ferrous oxalate, or a combination of at least two of them.

13. The preparation method according to claim 1, wherein The phosphate includes any one of sodium phosphate, potassium phosphate or ammonium phosphate, or a combination of at least two of them.

14. The preparation method according to claim 1, wherein The oxidant includes any one of hydrogen peroxide, sodium peroxide, peracetic acid or ammonium persulfate, or a combination of at least two thereof.

15. The preparation method according to claim 1, wherein The molar ratio of the oxidant to the iron element in the iron source is (1-2):

1.

16. The preparation method according to claim 1, wherein The mixing in step (2) includes mixing the iron salt solution with the oxidant, adding phosphoric acid to adjust the pH, adding zirconium phosphotungstate seed crystals and then adding the phosphate solution.

17. The preparation method according to claim 16, wherein The concentration of the iron salt solution is 0.5-1.5 mol / L.

18. The preparation method according to claim 16, wherein The pH is 1-3.

19. The preparation method according to claim 16, wherein The concentration of the phosphate solution is 0.5-1.5 mol / L.

20. The preparation method according to claim 16, wherein The molar ratio of the iron element in the iron salt solution to the phosphorus element in the phosphate solution is 1:(1-1.5).

21. The preparation method according to claim 1, wherein The temperature of the synthesis reaction in step (2) is 80-100°C.

22. The preparation method according to claim 1, wherein The synthesis reaction time is 1 to 3 hours.

23. The preparation method according to claim 1, wherein The temperature of the sintering treatment in step (3) is 400-700°C.

24. The preparation method according to claim 1, wherein The sintering treatment time is 3 to 6 hours.

25. The preparation method according to claim 1, wherein The grinding time is 1 to 3 hours.

26. A modified ferric phosphate prepared by the method according to any one of claims 1 to 25, wherein the median particle size D50 of the modified ferric phosphate is 0.5 to 1 μm.

27. A lithium iron phosphate positive electrode material prepared by mixing and sintering the modified iron phosphate according to claim 26 and a lithium source.

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