A nickel-carbon co-doped iron phosphate and lithium iron phosphate material and its preparation method

Abstract

The present invention discloses a nickel-carbon co-doped iron phosphate and lithium iron phosphate material and a preparation method thereof, belonging to the field of lithium ion battery positive electrode materials. The preparation method of the iron phosphate comprises: preparing an iron salt and a phosphate solution respectively, adjusting the pH constant temperature reaction to prepare dihydrated iron phosphate; taking the dihydrated iron phosphate and an ionic liquid, a nickel-containing metal organic complex Ni-MOF and subjecting it to a microwave-assisted solvent thermal reaction in an alcohol solvent, to achieve the goal of dehydrating the dihydrated iron phosphate and coating the ionic liquid-Ni-MOF complex and supporting the iron phosphate structure; subjecting the obtained nickel-carbon co-doped iron phosphate precursor to a low-temperature ion thermal carbonization treatment, and finally obtaining the nickel-carbon co-doped iron phosphate material. The lithium iron phosphate, a lithium ion battery positive electrode material synthesized using the iron phosphate prepared by this method, has good rate performance and low-temperature performance.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion battery positive electrode materials, and in particular relates to a nickel-carbon co-doped iron phosphate and lithium iron phosphate material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate is one of the most widely used cathode materials in power batteries due to its good cycle stability, safety performance, wide range of raw material sources, non-toxic and environmentally friendly advantages. However, the intrinsic electronic conductivity of lithium iron phosphate is relatively low (about 10 -9 S / cm) and the lithium ion diffusion coefficient (about 10 -16 ~10 -14 cm 2 / s) makes its rate and low-temperature performance have certain defects, limiting its further application in power batteries.

[0003] At present, one of the mainstream preparation methods for commercial lithium iron phosphate is the high-temperature solid-phase reaction method using iron phosphate as a precursor. This method combines components such as iron phosphate, lithium source, and carbon source through liquid phase mixing, grinding, spray drying, etc. to form a macroscopically uniform lithium iron phosphate precursor, and then calcines it at high temperature to obtain the target lithium iron phosphate product. This method is simple in process and can meet high-yield requirements, but it also has many defects that limit the performance of lithium iron phosphate materials. First, the basic principle of this method is the lithiation reaction of iron phosphate materials at high temperatures. The crystal structure, particle size distribution, specific surface area, and morphology of iron phosphate materials will directly affect the various properties of the product lithium iron phosphate, such as compaction density and electrochemical properties. In addition, the uniformity of the high-temperature solid-phase mixing is difficult to reach the level of the liquid phase method. The carbon source that improves electronic conductivity basically exists in the form of particles between the lithium iron phosphate particles, and the uneven distribution of Li, Fe, and P elements in the lithium iron phosphate particles themselves will also block the lithium ion migration channel.

[0004] To address the above technical difficulties, modifying the iron phosphate material is a relatively direct approach. Currently, conventional iron phosphate modification methods such as morphology control, carbon doping, and metal ion doping can improve the various properties of the back-end lithium iron phosphate to a certain extent, but they have also encountered some problems. For example, the carbon-doped dihydrate iron phosphate sample formed by adding a carbon source to the iron phosphate material at the synthesis end needs to be calcined at high temperature to remove the crystal water. During this process, the problem of trivalent iron reduction is prone to occur, resulting in impure iron phosphate crystals. In situ doping of iron phosphate materials with metal ions at the synthesis end will affect the phase formation of iron phosphate, and there is a problem of uniform distribution when using finished iron phosphate materials to dope metal ions ex situ. Summary of the Invention

[0005] In view of the defects of the existing technology, the present invention provides a preparation method of nickel-carbon co-doped iron phosphate and lithium iron phosphate and nickel-carbon co-doped iron phosphate and lithium iron phosphate materials.

[0006] The method for preparing nickel-carbon co-doped ferric phosphate provided by the present invention comprises the following steps:

[0007] (1) Preparation of ferric phosphate dihydrate by liquid phase method;

[0008] (2) weighing the ferric phosphate dihydrate, the ionic liquid, and the nickel-containing metal organic complex Ni-MOF in a certain ratio, mixing them in an alcohol solvent, stirring them evenly to obtain a reaction solution, transferring the reaction solution to a microwave reactor for microwave-assisted solvent thermal reaction, separating the reaction precipitate, washing, and drying to obtain a nickel-carbon co-doped ferric phosphate precursor;

[0009] (3) subjecting the nickel-carbon co-doped iron phosphate precursor to ion thermal carbonization to obtain nickel-carbon co-doped iron phosphate.

[0010] Preferably, the specific preparation method of step (1) is as follows: prepare an iron salt solution and a phosphate solution respectively, mix and stir the iron salt solution and the phosphate solution, add a pH regulator, react at a constant temperature, separate the reaction precipitate, wash, and dry to obtain ferric phosphate dihydrate.

[0011] Furthermore, the iron salt is at least one of FeCl3, FeCl3·6H2O, Fe(NO3)3, Fe(NO3)3·9H2O, etc., the solvent of the iron salt solution is water, and the molar concentration is 0.5mol / L to 2mol / L.

[0012] Furthermore, the phosphate is at least one of NH4H2PO4, (NH4)2HPO4, (NH4)3PO4, H3PO4, etc., the phosphate solution solvent is water, and the molar concentration is 0.75mol / L to 5mol / L.

[0013] Furthermore, the molar ratio of the iron salt in the iron salt solution to the phosphate in the phosphate solution is 1:1.5 to 1:2.5, the stirring speed is 300 to 500 rpm, the pH adjuster is at least one of H2SO4, HCl, and HNO3, the target pH range is 1 to 2, the reaction temperature is 70 to 90°C, the reaction time is 8 to 12 hours, the separation method is centrifugation or filtration, washing is carried out 3-5 times with deionized water, and the drying temperature is 100-120°C.

[0014] Preferably, the ionic liquid in step (2) is at least one of [BMIm]Cl, [BMIm][AcO], [BMIm][BF4], etc.;

[0015] Preferably, the nickel-containing metal organic composite Ni-MOF in step (2) is prepared by the following method: adding NiCl2·6H2O and triethylamine to a mixture of terephthalic acid, ethanol, and deionized water, stirring at 200 rpm until the solution is clear, ultrasonically treating at a frequency of 30 Hz and a power of 200 W for 5 h, centrifuging, washing with ethanol three times, and drying at 80-100° C. to obtain Ni-MOF; wherein the mass ratio of terephthalic acid, ethanol, deionized water, NiCl2·6H2O, and triethylamine used in the reaction is 1:10:10:1:5;

[0016] Preferably, in step (2), the mass ratio of ferric phosphate dihydrate, ionic liquid, and Ni-MOF is 1:0.3-0.4:0.1-0.2, the alcohol solvent is at least one of ethylene glycol, isopropanol, n-butanol, etc., the stirring speed is 300-500 rpm, the microwave reactor can be a Teflon microwave reactor, the microwave equipment model used for the microwave-assisted solvent thermal reaction is MDS-6, the reaction temperature is 150-180°C, the reaction time is 1-2h, the separation method is centrifugation or filtration, washing is performed 3-5 times with ethanol, and the drying temperature is 80-100°C;

[0017] Preferably, the heat temperature of the ion thermal carbonization treatment in step (3) is 200-300°C, and the heat treatment time is 8-10h;

[0018] According to the above method, the present invention disperses ferric phosphate dihydrate in an alcohol solvent containing an ionic liquid and Ni-MOF, and replaces the crystal water in the ferric phosphate dihydrate with the ionic liquid through a solvent thermal reaction. At the same time, a composite reaction between Ni-MOF and the ionic liquid adsorbed on the surface of the ferric phosphate particles is utilized to obtain ferric phosphate coated with a Ni-MOF-ionic liquid complex, i.e., the nickel-carbon co-doped ferric phosphate precursor. The Ni-MOF-ionic liquid complex has the characteristics of an ionic liquid and can be carbonized at low temperature by ionic thermal treatment to obtain the target nickel-carbon co-doped ferric phosphate.

[0019] The nickel-carbon co-doped ferric phosphate material prepared by the above-mentioned method for preparing the nickel-carbon co-doped ferric phosphate material also falls within the scope of protection of the present invention.

[0020] The preparation method of lithium iron phosphate provided by the present invention comprises the following steps: mixing the nickel-carbon co-doped iron phosphate prepared by the above method with a lithium source and a carbon source, and then preparing lithium iron phosphate by a carbothermal reduction method.

[0021] The detailed steps of the lithium iron phosphate preparation method are as follows: at room temperature and under stirring conditions, the nickel-carbon co-doped iron phosphate is fully mixed with a lithium source and a carbon source in an aqueous solution to obtain a mixed suspension, the suspension is spray-dried to obtain a dried material, the dried material is treated by a carbothermal reduction method, and calcined at high temperature under nitrogen protection to obtain lithium iron phosphate.

[0022] Preferably, in the preparation method of the lithium iron phosphate, the lithium source is a mixture of one or two of lithium carbonate and lithium hydroxide, the carbon source is a mixture of one or two of glucose and polyethylene glycol, the molar ratio of the nickel-carbon co-doped iron phosphate to Li in the lithium source is 1:1.005-1.030, the mass ratio of the nickel-carbon co-doped iron phosphate to the carbon source is 1:0.10-0.15, the high temperature calcination temperature in the carbon thermal reduction treatment is 700-800°C, and the calcination time is 8-12h.

[0023] The lithium iron phosphate material prepared by the above-mentioned preparation method of the lithium iron phosphate material also falls within the protection scope of the present invention.

[0024] The present invention also protects the application of lithium iron phosphate materials.

[0025] The application described in the present invention is the application of lithium iron phosphate material in the preparation of lithium ion battery positive electrode material.

[0026] The present invention also protects a positive electrode plate, which includes the lithium iron phosphate material of the present invention.

[0027] The present invention also protects a lithium-ion battery, which comprises the positive electrode sheet of the present invention.

[0028] Compared with the prior art, the present invention has the following beneficial technical effects:

[0029] (1) Using an ex-situ liquid phase method to dope nickel and carbon with iron phosphate avoids both the problem of impure iron phosphate phase produced by the in-situ liquid phase method and the problem of uneven mixing by the conventional solid phase method;

[0030] (2) By utilizing the replacement of ionic liquid and crystal water during the solvothermal reaction and the subsequent composite reaction of ionic liquid and Ni-MOF, a scaffold and coating layer are formed inside and on the surface of the iron phosphate particles, avoiding the problem of internal structure collapse of the iron phosphate during conventional dehydration. The stable structure and rich conductive channels formed after sintering enable the lithium iron phosphate prepared at the back end to obtain excellent rate and low-temperature performance;

[0031] (3) Using the principle of low-temperature ionic thermal carbonization of Ni-MOF by ionic liquid, the Fe 3+ The problem of being restored;

[0032] (4) The lithium iron phosphate material made using nickel-carbon co-doped iron phosphate material has excellent rate performance and low temperature performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 These are the SEM morphology test results of nickel-carbon co-doped iron phosphate prepared in Example 1. DETAILED DESCRIPTION

[0034] The present invention will be further described in detail below in conjunction with specific embodiments. The examples provided are only for illustrating the present invention and are not intended to limit the scope of the present invention. The examples provided below can serve as a guide for further improvements by those skilled in the art and are not intended to limit the present invention in any way.

[0035] Unless otherwise specified, the experimental methods in the following examples are conventional methods and were performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials and reagents used in the following examples, unless otherwise specified, were all commercially available.

[0036] Example 1. Preparation of nickel-carbon co-doped iron phosphate and lithium iron phosphate

[0037] 1) A method for preparing nickel-carbon co-doped ferric phosphate, comprising the following steps:

[0038] Step 1: Prepare 0.5 mol / L FeCl3 aqueous solution and 1 mol / L NH4H2PO4 aqueous solution respectively, take 20 mL of FeCl3 aqueous solution and 20 mL of NH4H2PO4 aqueous solution and stir them at 300 rpm, add H2SO4 to adjust the pH to 1.2, and react at 80°C for 10 hours. The reaction precipitate is centrifuged, washed with deionized water three times, and dried at 100°C to obtain ferric phosphate dihydrate;

[0039] Step 2: Add 1g NiCl2·6H2O and 5g triethylamine to a mixture of 1g terephthalic acid, 10g ethanol, and 10g deionized water, stir at 200rpm until the solution is clear, and ultrasonicate at 30Hz and 200W for 5h. Then, centrifuge, wash 3 times with ethanol, and dry at 80℃ to obtain Ni-MOF. Subsequently, weigh 1g ferric phosphate dihydrate, 0.35g [BMIm]Cl, and 0.2g of the Ni-MOF are mixed in 80mL ethylene glycol solvent and stirred at 400rpm to obtain a reaction solution. The reaction solution is transferred to a Teflon microwave reactor at 180℃ for microwave-assisted solvothermal reaction for 2h. The reaction precipitate is centrifuged, washed 3 times with ethanol, and dried at 80℃ to obtain a nickel-carbon co-doped ferric phosphate precursor.

[0040] Step 3: subjecting the nickel-carbon co-doped iron phosphate precursor to ion thermal carbonization treatment at 280° C. for 8 h to obtain nickel-carbon co-doped iron phosphate.

[0041] 2) A method for preparing lithium iron phosphate, comprising the following steps:

[0042] Take 10.00g of the nickel-carbon co-doped iron phosphate, 2.48g of lithium carbonate, and 1.20g of glucose, and fully mix them in an aqueous solution to obtain a mixed suspension. The suspension is spray-dried to obtain a dry material, and the dry material is calcined at 730°C in a nitrogen atmosphere for 10 hours to obtain a lithium iron phosphate material.

[0043] Example 2: Preparation of nickel-carbon co-doped iron phosphate and lithium iron phosphate

[0044] 1) A method for preparing nickel-carbon co-doped ferric phosphate, comprising the following steps:

[0045] Step 1: Prepare 2 mol / L FeCl3 aqueous solution and 5 mol / L H3PO4 aqueous solution respectively, take 20 mL of FeCl3 aqueous solution and 20 mL of H3PO4 aqueous solution, stir and mix at 500 rpm, add HCl to adjust the pH to 1.5, react at 90°C for 12 hours, centrifuge the reaction precipitate, wash with deionized water three times, and dry at 120°C to obtain ferric phosphate dihydrate;

[0046] Step 2: Add 1g NiCl2·6H2O and 5g triethylamine to a mixture of 1g terephthalic acid, 10g ethanol, and 10g deionized water, stir at 200rpm until the solution is clear, and ultrasonicate at 30Hz and 200W for 5h, centrifuge, wash 3 times with ethanol, and dry at 100℃ to obtain Ni-MOF. Subsequently, weigh 1g ferric phosphate dihydrate, 0.4g [BMIm][AcO], and 0.15g of the Ni-MOF are mixed in 80mL of isopropanol solvent and stirred at 500rpm to obtain a reaction solution. The reaction solution is transferred to a Teflon microwave reactor at 170℃ for microwave-assisted solvothermal reaction for 2h. The reaction precipitate is centrifuged, washed 3 times with ethanol, and dried at 100℃ to obtain a nickel-carbon co-doped ferric phosphate precursor.

[0047] Step 3: subjecting the nickel-carbon co-doped iron phosphate precursor to ion thermal carbonization treatment at 270° C. for 9 h to obtain nickel-carbon co-doped iron phosphate.

[0048] 2) A method for preparing lithium iron phosphate, comprising the following steps:

[0049] Take 10.00 g of the nickel-carbon co-doped iron phosphate, 2.50 g of lithium carbonate, and 1.30 g of glucose, and fully mix them in an aqueous solution to obtain a mixed suspension. The suspension is spray-dried to obtain a dry material, and the dry material is calcined at 700° C. for 11 hours in a nitrogen atmosphere to obtain a lithium iron phosphate material.

[0050] Example 3: Preparation of nickel-carbon co-doped iron phosphate and lithium iron phosphate

[0051] 1) A method for preparing nickel-carbon co-doped ferric phosphate, comprising the following steps:

[0052] Step 1: prepare 1 mol / L Fe(NO3)3 aqueous solution and 2 mol / L (NH4)2HPO4 aqueous solution respectively, take 20 mL of Fe(NO3)3 aqueous solution and 20 mL of (NH4)2HPO4 aqueous solution and stir them at 300 rpm, add HCl to adjust the pH to 2, and react at 70°C for 9 hours. The reaction precipitate is centrifuged, washed with deionized water 3 times, and dried at 100°C to obtain ferric phosphate dihydrate;

[0053] Step 2: Add 1g NiCl2·6H2O and 5g triethylamine to a mixture of 1g terephthalic acid, 10g ethanol, and 10g deionized water, stir at 200rpm until the solution is clear, ultrasonicate at 30Hz frequency and 200W power for 5h, centrifuge, wash 3 times with ethanol, and dry at 90℃ to obtain Ni-MOF. Subsequently, weigh 1g ferric phosphate dihydrate, 0.3g [BMIm][BF4], and 0.2g of the Ni-MOF are mixed in 80mL n-butanol solvent and stirred at 300rpm to obtain a reaction solution. The reaction solution is transferred to a Teflon microwave reactor at 150℃ for microwave-assisted solvothermal reaction for 1h. The reaction precipitate is centrifuged, washed 3 times with ethanol, and dried at 80℃ to obtain a nickel-carbon co-doped ferric phosphate precursor.

[0054] Step 3: subjecting the nickel-carbon co-doped iron phosphate precursor to ion thermal carbonization treatment at 200° C. for 8 h to obtain nickel-carbon co-doped iron phosphate.

[0055] 2) A method for preparing lithium iron phosphate, comprising the following steps:

[0056] Take 10.00g of the nickel-carbon co-doped iron phosphate, 1.61g of lithium hydroxide monohydrate, and 1.25g of glucose, and fully mix them in an aqueous solution to obtain a mixed suspension. The suspension is spray-dried to obtain a dry material, and the dry material is calcined at 800°C in a nitrogen atmosphere for 12h to obtain a lithium iron phosphate material.

[0057] Example 4: Preparation of nickel-carbon co-doped iron phosphate and lithium iron phosphate

[0058] 1) A method for preparing nickel-carbon co-doped ferric phosphate, comprising the following steps:

[0059] Step 1: prepare 0.5 mol / L Fe(NO3)3 aqueous solution and 0.75 mol / L (NH4)2HPO4 aqueous solution respectively, take 20 mL of Fe(NO3)3 aqueous solution and 20 mL of (NH4)2HPO4 aqueous solution and stir them at 500 rpm, add HNO3 to adjust the pH to 1, and react at 80°C for 10 hours. The reaction precipitate is centrifuged, washed with deionized water 3 times, and dried at 100°C to obtain ferric phosphate dihydrate;

[0060] Step 2: Add 1g NiCl2·6H2O and 5g triethylamine to a mixture of 1g terephthalic acid, 10g ethanol, and 10g deionized water, stir at 200rpm until the solution is clear, and ultrasonicate at 30Hz and 200W for 5h. Then, centrifuge, wash 3 times with ethanol, and dry at 100℃ to obtain Ni-MOF. Subsequently, weigh 1g ferric phosphate dihydrate, 0.38g [BMIm]Cl, and 0.18g of the Ni-MOF are mixed in 80mL of isopropanol solvent and stirred at 300rpm to obtain a reaction solution. The reaction solution is transferred to a Teflon microwave reactor at 180℃ for microwave-assisted solvothermal reaction for 1.8h. The reaction precipitate is centrifuged, washed 3 times with ethanol, and dried at 100℃ to obtain a nickel-carbon co-doped ferric phosphate precursor.

[0061] Step 3: subjecting the nickel-carbon co-doped iron phosphate precursor to ion thermal carbonization treatment at 300° C. for 10 h to obtain nickel-carbon co-doped iron phosphate.

[0062] 2) A method for preparing lithium iron phosphate, comprising the following steps:

[0063] Take 10.00 g of the nickel-carbon co-doped iron phosphate, 1.63 g of lithium hydroxide monohydrate, and 1.15 g of polyethylene glycol, and fully mix them in an aqueous solution to obtain a mixed suspension. The suspension is spray-dried to obtain a dry material, and the dry material is calcined at 710° C. in a nitrogen atmosphere for 8 h to obtain a lithium iron phosphate material.

[0064] Comparative Example 1

[0065] 1) A method for preparing nickel-carbon co-doped ferric phosphate, comprising the following steps:

[0066] Step 1: Prepare 0.7 mol / L FeCl3 aqueous solution and 1.2 mol / L NH4H2PO4 aqueous solution respectively, take 20 mL of FeCl3 aqueous solution and 20 mL of NH4H2PO4 aqueous solution, stir and mix at 300 rpm, add H2SO4 to adjust the pH to 1.5, react at 80°C for 10 hours, centrifuge the reaction precipitate, wash with deionized water 3 times, and dry at 100°C to obtain ferric phosphate dihydrate;

[0067] Step 2: Add 1g NiCl2·6H2O and 5g triethylamine to a mixture of 1g terephthalic acid, 10g ethanol, and 10g deionized water, stir at 200rpm until the solution is clear, ultrasonicate at 30Hz frequency and 200W power for 5h, centrifuge, wash with ethanol three times, and dry at 80℃ to obtain Ni-MOF. Subsequently, weigh 1g ferric phosphate dihydrate, 0.35g glucose, and 0.2g of the Ni-MOF and mix them for ball milling. The ball milling medium is anhydrous ethanol, the solid-liquid mass ratio of the ball milling is 1:1.5, the ball-to-material mass ratio is 10:1, the ball milling speed is 500r / min, the ball milling time is 5h, and the ball milled material is dried at 100℃ to obtain a nickel-carbon co-doped ferric phosphate precursor.

[0068] Step 3: Sintering the nickel-carbon co-doped iron phosphate precursor at 650° C. in a nitrogen furnace for 10 h to obtain nickel-carbon co-doped iron phosphate.

[0069] 2) A method for preparing lithium iron phosphate, comprising the following steps:

[0070] Take 10.00 g of the nickel-carbon co-doped iron phosphate, 2.50 g of lithium carbonate, and 1.13 g of glucose, and fully mix them in an aqueous solution to obtain a mixed suspension. The suspension is spray-dried to obtain a dry material, and the dry material is calcined at 750° C. for 11 hours in a nitrogen atmosphere to obtain a lithium iron phosphate material.

[0071] To verify the electrochemical performance of the lithium iron phosphate cathode materials obtained in Examples 1-4 and Comparative Example 1, the materials were mixed in a mass ratio of lithium iron phosphate: conductive agent Super P: binder PVDF = 8:1:1 to prepare a battery cathode slurry. This slurry was coated onto 12 μm thick aluminum foil, vacuum dried, and roll-pressed to form a positive electrode sheet. A button cell was constructed using lithium metal as the negative electrode and lithium hexafluorophosphate as the electrolyte. Rate and low-temperature performance were tested at 25°C within the voltage range of 2.0-4.0 V. The test results are shown in Table 1 below.

[0072] Table 1: Buckling test results of lithium iron phosphate cathode materials prepared in Examples 1 to 4 and Comparative Example 1

[0073]

[0074] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for preparing nickel-carbon co-doped ferric phosphate, comprising the following steps: (1) Preparation of ferric phosphate dihydrate by liquid phase method; (2) mixing the ferric phosphate dihydrate with an ionic liquid and a nickel-containing metal organic complex Ni-MOF in an alcohol solvent, stirring the mixture to obtain a reaction solution, transferring the reaction solution to a microwave reactor for microwave-assisted solvent thermal reaction, separating the reaction precipitate, washing, and drying the reaction precipitate to obtain a nickel-carbon co-doped ferric phosphate precursor; (3) subjecting the nickel-carbon co-doped iron phosphate precursor to ion thermal carbonization to obtain nickel-carbon co-doped iron phosphate.

2. The preparation method according to claim 1, wherein: The specific method for preparing ferric phosphate dihydrate by the liquid phase method in step (1) is as follows: respectively preparing an iron salt solution and a phosphate solution, mixing and stirring the iron salt solution and the phosphate solution, adding a pH regulator, reacting at a constant temperature, separating the reaction precipitate, washing, and drying to obtain ferric phosphate dihydrate.

3. The preparation method according to claim 2, wherein: The iron salt is at least one of FeCl3, FeCl3·6H2O, Fe(NO3)3, and Fe(NO3)3·9H2O, and the solvent of the iron salt solution is water, with a molar concentration of 0.5 mol / L to 2 mol / L; Or, in step (1), the phosphate is at least one of NH4H2PO4, (NH4)2HPO4, (NH4)3PO4, and H3PO4, the solvent of the phosphate solution is water, and the molar concentration is 0.75 mol / L to 5 mol / L; Or, in step (1), the molar ratio of the iron salt in the iron salt solution to the phosphate in the phosphate solution is 1:1.5 to 1:2.5; Or, in step (1), the stirring speed is 300 to 500 rpm; Alternatively, in step (1), the pH adjuster is at least one of H2SO4, HCl, and HNO3, and the target pH range is 1 to 2; Alternatively, in step (1), the reaction temperature is 70-90° C., and the reaction time is 8-12 h; Alternatively, in step (1), the separation method is centrifugation or filtration, washing is performed 3-5 times with deionized water, and the drying temperature is 100-120°C.

4. The preparation method according to claim 1, wherein: In the step (2), the ionic liquid is at least one of [BMIm]Cl, [BMIm][AcO], and [BMIm][BF4]; Or, in step (2), the mass ratio of the ferric phosphate dihydrate, the ionic liquid, and the Ni-MOF is 1:0.3-0.4:0.1-0.2; Or, in step (2), the alcohol solvent is at least one of ethylene glycol, isopropanol, and n-butanol; Or, in step (2), the stirring speed is 300-500 rpm; Alternatively, in step (2), the reaction temperature is 150-180° C., and the reaction time is 1-2 h; Alternatively, in step (2), the separation method is centrifugation or filtration, washing is performed 3-5 times with ethanol, and the drying temperature is 80-100°C.

5. The preparation method according to claim 1 or 4, characterized in that: The nickel-containing metal organic composite Ni-MOF is prepared by the following method: NiCl2·6H2O and triethylamine are added to a mixture of terephthalic acid, ethanol, and deionized water, stirred at 200 rpm until the solution is clear, ultrasonically treated at a frequency of 30 Hz and a power of 200 W for 5 hours, centrifuged, washed with ethanol three times, and dried at 80-100°C to obtain the Ni-MOF; wherein the mass ratio of terephthalic acid, ethanol, deionized water, NiCl2·6H2O, and triethylamine used in the reaction is 1:10:10:1:

5.

6. The preparation method according to claim 1, wherein: In the step (3), the heat temperature of the ion thermal carbonization treatment is 200-300° C., and the heat treatment time is 8-10 hours.

7. A nickel-carbon co-doped ferric phosphate material prepared by the method for preparing a nickel-carbon co-doped ferric phosphate material according to any one of claims 1 to 6.

8. A method for preparing lithium iron phosphate, comprising: The nickel-carbon co-doped iron phosphate according to claim 7 is mixed with a lithium source and a carbon source, and then lithium iron phosphate is prepared by a carbothermal reduction method.

9. The preparation method according to claim 8, characterized in that: The lithium iron phosphate preparation method includes the following steps: at room temperature and under stirring conditions, fully mixing the nickel-carbon co-doped iron phosphate with a lithium source and a carbon source in an aqueous solution to obtain a mixed suspension, spray-drying the suspension to obtain a dried material, treating the dried material with a carbothermal reduction method, and calcining the dried material at high temperature under nitrogen protection to obtain lithium iron phosphate.

10. The preparation method according to claim 8 or 9, characterized in that: The lithium source is one of lithium carbonate and lithium hydroxide or a mixture of the two; Alternatively, the carbon source is glucose, polyethylene glycol, or a mixture of the two; Or, the molar ratio of the nickel-carbon co-doped iron phosphate to Li in the lithium source is 1:1.005-1.030, and the mass ratio of the nickel-carbon co-doped iron phosphate to the carbon source is 1:0.10-0.15; Alternatively, the high temperature calcination temperature in the carbothermal reduction treatment is 700-800° C., and the calcination time is 8-12 hours.

11. A lithium iron phosphate material prepared by the method for preparing a lithium iron phosphate material according to any one of claims 8 to 10.

12. Use of the lithium iron phosphate material according to claim 11 in preparing positive electrode materials for lithium-ion batteries.

13. A positive electrode plate, characterized in that: The positive electrode plate includes the lithium iron phosphate material according to claim 11.

14. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to claim 13.

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