WO2025086304A1 - Modified iron phosphate, and preparation method therefor and use thereof - Google Patents

Abstract

Modified iron phosphate, and a preparation method therefor and the use thereof. 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, and performing a heating reaction, so as to obtain zirconium phosphotungstate; (2) mixing the zirconium phosphotungstate serving as a seed crystal with an iron salt, a phosphate, phosphoric acid and an oxidizing agent, and performing a synthesis reaction, so as to obtain a modified material; and (3) sintering the modified material, and then grinding same, so as to obtain modified iron phosphate. In the present disclosure, zirconium phosphotungstate is used as a seed crystal to prepare iron phosphate, which can promote the generation of cracks in secondary particles, making iron phosphate easy to break and an iron phosphate precursor having a smaller particle size be obtained; therefore, a high-energy and high-power-density lithium iron phosphate positive electrode material is prepared.

Description

A modified ferric phosphate and its preparation method and application Technical Field

The present invention belongs to the technical field of battery materials and relates to a modified iron phosphate and a preparation method and application thereof.

Background Art

Lithium iron phosphate, a lithium-ion battery material, has a chemical formula of LiFePO ₄ and is mainly used in various lithium-ion batteries. Since Japan's NTT first revealed the olivine-structured lithium battery positive electrode material in 1996, the University of Texas at Austin also reported the reversible lithium migration and extraction characteristics of LiFePO ₄ in 1997. This material has now been widely used in various lithium-ion batteries.

As the precursor of lithium iron phosphate, iron phosphate determines the performance of lithium iron phosphate. The existing preparation technologies of iron phosphate include solid phase method, particle size grading method, liquid phase synthesis method, etc.

CN105480960A discloses a method for preparing iron phosphate, comprising the following steps: placing iron in a phosphoric acid solution, heating _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 iron 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 a near-neutral value, and drying to obtain solid iron phosphate; and sequentially drying and dehydrating the dried solid iron phosphate to form dehydrated iron phosphate.

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 to 90-100° C., and keeping the temperature until the material turns white; S3, mixing the iron source F2 with 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 the product after the reaction; After calcination, the iron phosphate is obtained.

The iron phosphate products obtained by the above scheme mostly present tightly packed secondary agglomerates. When used for preparing 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

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.

The purpose of the present disclosure is to provide a modified iron phosphate and a preparation method and application thereof. The present disclosure uses zirconium phosphotungstate as a seed 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.

To achieve this purpose, the present invention adopts the following technical solutions:

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

(1) mixing zirconium salt, hydrogen phosphate and tungstate with a solvent to obtain a mixed salt solution, heating the solution for reaction, and obtaining 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) The modified material is sintered and ground to obtain the modified iron phosphate.

The present invention uses zirconium phosphotungstate (Zr ₂ WO ₄ (PO ₄ ) ₂ ) as a seed crystal to prepare iron phosphate. Through 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, which promotes the appearance of cracks in the secondary agglomerated particles and reduces the grinding intensity and time required for preparing small particles of iron phosphate. The prepared modified iron phosphate is used in the subsequent preparation of lithium iron phosphate. The zirconium and tungsten ions are heated in the lithium iron phosphate sintering process. During the bonding process, zirconium ions can be doped into Li ion sites and tungsten ions can be doped into Fe sites, which improves the intrinsic conductivity of the material. In addition, the doping of tungsten and zirconium ions can also increase the compaction density of the material.

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

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.

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

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.

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.

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 separately prepared into solutions in advance and then mixed.

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

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.

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.

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

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

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.

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.

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.

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

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.

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, etc.

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.

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

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

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

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, etc.

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.

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.

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

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.

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.

In a second aspect, the present disclosure provides a modified iron phosphate, wherein the modified iron phosphate is prepared by the method described in the first aspect.

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

The modified iron phosphate disclosed in the present invention is subjected to grinding treatment to obtain modified iron phosphate particles with a median particle size of 0.5 to 1 μm, and a good crushing effect, which also indirectly illustrates the characteristic that the modified iron phosphate prepared in the present application is easy to crush.

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

Compared with the prior art, the present invention has the following beneficial effects:

(1) The present invention uses zirconium tungstate phosphotungstate 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.

(2) The compacted density of the modified iron phosphate prepared by the method disclosed in the present invention can reach above 2.57 g/cm ³ , and the 0.1C discharge capacity of the battery prepared can reach above 156.33 mAh/g.

(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, which is conducive to obtaining lithium iron phosphate with a smaller particle size. Iron-lithium cathode material.

Other aspects will be apparent upon reading and understanding the drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are used to provide further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of the present 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.

FIG. 1 is a SEM image of modified iron phosphate prepared in Example 1 of the present disclosure.

DETAILED DESCRIPTION

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 to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

Example 1

This embodiment provides a modified iron phosphate, and the preparation method of the modified iron phosphate is as follows:

(1) Weigh zirconium oxychloride, ammonium hydrogen phosphate and ammonium tungstate respectively, add water to prepare solutions respectively, the solution concentration is 1 mol/L, mix the prepared solutions, control the molar ratio of zirconium, tungsten and phosphorus in the mixed solution to be 2:1:2.1, mix and stir for 20 minutes, transfer the solution to a microwave reactor, heat to 120°C for hydrothermal reaction for 30 minutes, filter and dry in a vacuum drying oven at 60°C for 24 hours to obtain nano zirconium phosphotungstate with a D50 of 150 nm, add the nano zirconium phosphotungstate into deionized water and ultrasonicate for 40 minutes to obtain a zirconium phosphotungstate seed solution;

(2) adding 1.1 mol/L ferrous sulfate solution to a stirred reactor, followed by adding phosphoric acid and hydrogen peroxide (the molar ratio of _H2O2 to Fe is 1.5:1), adjusting the pH to 2.4 by adding phosphoric acid, then adding zirconium phosphotungstate _seed crystals to the reactor, and finally adding 0.9 mol/L sodium phosphate solution, wherein the molar ratio of the Fe element in the ferrous sulfate solution to the P element in the sodium phosphate solution is 1:1.2, and heating the reaction at 90°C for 2h to obtain dihydrate iron phosphate particles grown on the basis of the zirconium phosphotungstate seed crystals;

(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 modified iron phosphate with a D50 of 800 nm. The SEM image of the modified iron phosphate is shown in FIG1 .

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.

Example 2

This embodiment provides a modified iron phosphate, and the preparation method of the modified iron phosphate is as follows:

(1) Weigh zirconium oxychloride, ammonium hydrogen phosphate and ammonium tungstate respectively, add water to prepare solutions respectively, the solution concentration is 1 mol/L, mix the prepared solutions, control the molar ratio of zirconium, tungsten and phosphorus in the mixed solution to be 1.8:1:2, mix and stir for 15 minutes, transfer the solution to a microwave reactor, heat to 100°C for hydrothermal reaction for 40 minutes, filter and dry in a vacuum drying oven at 50°C for 24 hours to obtain nano zirconium phosphotungstate with a D50 of 120 nm, add the nano zirconium phosphotungstate to deionized water and ultrasonicate for 40 minutes to obtain a zirconium phosphotungstate seed solution;

(2) adding 1.1 mol/L ferrous sulfate solution to a stirred reactor, followed by adding phosphoric acid and hydrogen peroxide ₍ the molar ratio of _H2O2 to Fe is 1:1), adjusting the pH to 3 by adding phosphoric acid, then adding zirconium phosphotungstate seed crystals to the reactor, and finally adding 0.5 mol/L sodium phosphate solution, wherein the molar ratio of the Fe element in the ferrous sulfate solution to the P element in the sodium phosphate solution is 1:1, and heating the reaction at 80°C for 3 h to obtain dihydrate iron phosphate particles grown on the basis of the zirconium phosphotungstate seed crystals;

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

Example 3

This embodiment provides a modified iron phosphate, and the preparation method of the modified iron phosphate is as follows:

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

(2) adding 1.1 mol/L ferrous sulfate solution to a stirred reactor, followed by adding phosphoric acid and peracetic acid (the molar ratio of peracetic acid to Fe is 2:1), adjusting the pH to 1 by adding phosphoric acid, then adding zirconium phosphotungstate seed crystals to the reactor, and finally adding 1.5 mol/L sodium phosphate solution, wherein the molar ratio of the Fe element in the ferrous sulfate solution to the P element in the sodium phosphate solution is 1:1.5, and heating the reaction at 80°C for 3 hours to obtain dihydrate iron phosphate particles grown on the basis of the zirconium phosphotungstate seed crystals;

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

Example 4

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

Comparative Example 1

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 D50 of the obtained modified iron phosphate was 3 μm, and the lithium iron phosphate positive electrode material was prepared using the modified iron phosphate, and the D50 of the obtained lithium iron phosphate was 5 μm.

Comparative Example 2

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.

Comparative Example 3

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.

Comparative Example 4

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.

Performance Test:

The iron phosphate prepared in the embodiment and the comparative example is mixed with lithium hydroxide and glucose, and the molar ratio of lithium to iron phosphate is 1 to 1.1: 1. The mass of glucose is 2 to 5% of the mass of the theoretically synthesized lithium iron phosphate;

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 raised from room temperature to 700°C at a rate of 4°C/min, and the temperature is kept for 10 hours. The temperature in the furnace is naturally lowered to room temperature to obtain LiFePO ₄ /C.

The prepared LiFePO ₄ /C was mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride PVDF at a mass ratio of 90:5:5, and N-methylpyrrolidone NMP was used as a solvent. The mixture was evenly coated on aluminum foil, dried, and rolled to form a simulated battery positive electrode. The negative electrode was a metal lithium sheet, the separator was Celgard2400, and the electrolyte was 1 mol/L LiPF ₆ /DMC+DEC (volume ratio was 1:1), forming a CR2025 type simulated battery. The charge and discharge voltage range was 2.9 to 3.7V, and the electrochemical performance data of lithium iron phosphate, a positive electrode material of a lithium-ion battery, was obtained. The test results are shown in Table 1:

Table 1

As can be seen from Table 1, from Examples 1-3, the compaction density of the modified iron phosphate prepared by the method of the present disclosure can reach more than 2.57 g/cm ³ , and the 0.1C discharge capacity of the prepared battery can reach more than 156.33 mAh/g.

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.

By comparing Example 1 with Comparative Example 1, it can be seen that the present disclosure uses zirconium phosphotungstate as a seed to prepare iron phosphate. Through 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, which promotes cracks in the secondary agglomerated particles and reduces the grinding intensity and time required for preparing small particles of iron phosphate. In addition, the crushing and grinding of the secondary agglomerated particles can make part of the zirconium phosphotungstate core coated on the particle surface, which can limit the growth of its particles during the synthesis of lithium iron phosphate, and obtain small particles of lithium iron phosphate positive electrode material.

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.

From the comparison between Example 1 and Comparative Examples 3-4, it can be seen that zirconium and tungsten ions can The doping of zirconium ions to Li ion sites and tungsten ions to Fe sites improves the intrinsic conductivity of the material. In addition, the doping of tungsten and zirconium ions can also increase the compaction density of the material.

Claims (18)

A method for preparing modified ferric phosphate comprises 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) The modified material is sintered and ground to obtain the modified iron phosphate. The preparation method according to claim 1, wherein the zirconium salt in step (1) comprises zirconium oxychloride. The preparation method according to claim 1 or 2, wherein the hydrogen phosphate comprises 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. The preparation method according to any one of claims 1 to 3, wherein the tungstate comprises sodium tungstate and/or ammonium tungstate. The preparation method according to any one of claims 1 to 4, 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). The preparation method according to any one of claims 1 to 5, wherein the mixing time is 15 to 30 minutes. The preparation method according to any one of claims 1 to 6, wherein the heating reaction in step (1) comprises a microwave hydrothermal reaction; Optionally, the temperature of the heating reaction is 100-150°C; Optionally, the heating reaction time is 20 to 40 minutes; Optionally, vacuum drying treatment is performed after the heating reaction. The preparation method according to claim 7, wherein the temperature of the vacuum drying treatment is 50～80℃; Optionally, the vacuum drying treatment is performed for 12 to 24 hours. The preparation method according to any one of claims 1 to 8, wherein the median particle size D50 of the zirconium phosphotungstate is 150 to 300 nm. The preparation method according to any one of claims 1 to 9, wherein the iron salt in step (2) comprises any one of ferrous sulfate, ferrous chloride or ferrous oxalate, or a combination of at least two thereof. The preparation method according to any one of claims 1 to 10, wherein the phosphate comprises any one of sodium phosphate, potassium phosphate or ammonium phosphate, or a combination of at least two of them. The preparation method according to any one of claims 1 to 11, wherein the oxidant comprises any one of hydrogen peroxide, sodium peroxide, peracetic acid or ammonium persulfate, or a combination of at least two thereof. The preparation method according to any one of claims 1 to 12, wherein the molar ratio of the oxidant to the iron element in the iron source is (1 to 2):1. The preparation method according to any one of claims 1 to 13, wherein the mixing in step (2) comprises 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; Optionally, the concentration of the iron salt solution is 0.5 to 1.5 mol/L; Optionally, the pH is 1 to 3; Optionally, the concentration of the phosphate solution is 0.5 to 1.5 mol/L; Optionally, 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). The preparation method according to any one of claims 1 to 14, wherein the temperature of the synthesis reaction in step (2) is 80 to 100° C.; Optionally, the synthesis reaction time is 1 to 3 hours. The preparation method according to any one of claims 1 to 15, wherein the temperature of the sintering treatment in step (3) is 400 to 700° C.; Optionally, the sintering treatment time is 3 to 6 hours; Optionally, the grinding time is 1 to 3 hours. A modified ferric phosphate prepared by the method according to any one of claims 1 to 16, wherein the median particle size D50 of the modified ferric phosphate is 0.5 to 1 μm. A lithium iron phosphate positive electrode material prepared by mixing and sintering the modified iron phosphate as claimed in claim 17 and a lithium source.