Lithium iron phosphate positive electrode material, preparation method and application thereof

Abstract

This invention provides a lithium iron phosphate cathode material, its preparation method, and its application. The preparation method includes the following steps: (1) mixing ferrous salt and phosphorus source with an alcohol solvent, heating and stirring, and adding a first lithium source solution to obtain a mixed solution; adjusting the pH of the mixed solution to obtain a mixed precursor; (2) mixing a complexing agent, ferric salt, a second lithium source, phosphorus source, and surfactant with water, stirring to form a sol solution; mixing the mixed precursor obtained in step (1) with the sol solution, heating and stirring to obtain a gel precursor; (3) sintering the gel precursor to obtain the lithium iron phosphate cathode material; wherein the mixed precursor includes lithium phosphate and ferrous phosphate. This invention can achieve lithium iron phosphate gradation of different particle sizes through a single production line, improving the uniformity and compaction density of the material, and avoiding performance differences and poor stability between different batches.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a lithium iron phosphate cathode material, its preparation method and application. Background Technology

[0002] In recent years, with the development of new energy vehicles, they have become increasingly popular.

[0003] Lithium iron phosphate (LiFePO4) is a type of lithium... + Lithium iron phosphate (LFP) cathode materials are attracting attention due to their good thermal stability, high safety performance, long cycle life, abundant raw material reserves, and lack of environmental pollution. Therefore, they have become a key focus of production and development in the field of lithium-ion batteries for power storage and are considered one of the most ideal cathode materials for new energy vehicles.

[0004] However, due to certain structural characteristics of LiFePO4 itself, the electronic conductivity of LiFePO4 and Li... + The low diffusion rate, low tap density, and poor low-temperature discharge performance all limit the application areas of LiFePO4 cathode materials.

[0005] CN114141990A discloses a method for preparing a high-compact lithium iron phosphate electrode sheet. The lithium iron phosphate material is composed of large and small particles of different sizes, including nano-sized initial raw materials, pretreatment of raw materials, and high-temperature sintering. The active material of lithium iron phosphate, conductive agent and binder are mixed and slurryed according to the ratio. The slurry is coated on aluminum foil with a fixed thickness to obtain a high-compact positive electrode sheet.

[0006] CN114314541A discloses a high-pressure sealed ferric phosphate and its preparation method. The method includes: preparing ferric phosphate dihydrate and grinding it to obtain ferric phosphate seed crystals; reacting the ferric phosphate seed crystals with an iron salt raw material solution, a phosphate raw material solution, phosphoric acid, and an oxidant to obtain ferric phosphate dihydrate particles grown on the basis of the ferric phosphate seed crystals; sintering and sieving the grown ferric phosphate dihydrate particles to obtain anhydrous ferric phosphate.

[0007] In the process of preparing lithium iron phosphate using the above method, the tap density and specific capacity are improved by gradation. Therefore, large and small particles need to be prepared separately by different production lines, which increases the process flow and may lead to performance differences and poor stability between different batches. Summary of the Invention

[0008] The purpose of this invention is to provide a lithium iron phosphate cathode material, its preparation method, and its application. This invention can achieve lithium iron phosphate gradation of different particle sizes through a single production line, improve the uniformity and compaction density of the material, and avoid problems such as performance differences and poor stability between different batches.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0011] (1) Mix ferrous salt and phosphorus source with alcohol solvent, heat and stir and add first lithium source solution to obtain mixed solution, adjust the pH of mixed solution to obtain mixed precursor;

[0012] (2) Mix the complexing agent, trivalent iron salt, second lithium source, phosphorus source, surfactant and water, stir to form a sol solution, mix the mixture precursor obtained in step (1) with the sol solution, heat and stir to obtain a gel precursor;

[0013] (3) The gel precursor is sintered to obtain the lithium iron phosphate cathode material;

[0014] The precursors of the mixture include lithium phosphate and ferrous phosphate.

[0015] This invention utilizes a co-precipitation method to prepare a mixed precursor containing ferrous phosphate and lithium phosphate; then, a lithium iron phosphate sol solution is prepared using a sol-gel method. After the sol is formed, the mixed precursor of ferrous phosphate and lithium phosphate obtained by the co-precipitation method is added, and stirring continues to ensure uniform mixing and the formation of a gel precursor. This invention leverages the difference in particle size between the co-precipitation and sol-gel methods for preparing lithium iron phosphate, resulting in lithium iron phosphate material with a bimodal particle size distribution. This eliminates the need for additional gradation and improves the tap density of the lithium iron phosphate material.

[0016] Preferably, the ferrous salt in step (1) includes ferrous sulfate and / or ferrous chloride.

[0017] Preferably, the phosphorus source includes any one or a combination of at least two of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, or ammonium phosphate. Typical but non-limiting combinations include combinations of phosphoric acid and ammonium hydrogen phosphate, combinations of ammonium hydrogen phosphate and ammonium dihydrogen phosphate, or combinations of phosphoric acid and ammonium phosphate.

[0018] Preferably, the alcohol solvent includes anhydrous alcohol or an aqueous solution of alcohol.

[0019] Preferably, the alcohol in the alcohol solvent includes any one or a combination of at least two of methanol, ethanol, isopropanol or ethylene glycol. Typical but non-limiting combinations include combinations of methanol and ethanol, methanol and isopropanol, ethanol and ethylene glycol, etc.

[0020] Preferably, the molar volume ratio of the ferrous salt to the alcohol solvent is 0.5 to 1 mol / L, for example: 0.5 mol / L, 0.6 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] Preferably, the heating and stirring temperature in step (1) is 60-70°C, for example: 60°C, 62°C, 65°C, 68°C or 70°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0022] Preferably, the heating and stirring speed is 600-800 rpm, for example: 600 rpm, 650 rpm, 700 rpm, 750 rpm or 800 rpm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] Preferably, the solute in the first lithium source in step (1) includes lithium hydroxide and / or lithium nitrate.

[0024] Preferably, the molar ratio of lithium ions, ferrous ions and phosphorus in the mixed solution is (2.5-3.5):(0.8-1.2):1, for example: 2.5:0.8:1, 2.8:0.9:1, 3:1:1, 3.2:0.8:1 or 3.5:1.2:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the pH is 7 to 9, for example: 7, 7.5, 8, 8.5 or 9, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Preferably, the molar ratio of the complexing agent and the iron element in the ferric salt in step (2) is (2-3):1, for example: 2:1, 2.2:1, 2.5:1, 2.8:1 or 3:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the complexing agent in step (2) includes any one or a combination of at least two of citric acid, oxalic acid, lauric acid, sorbic acid, benzoic acid or oleic acid. Typical but non-limiting combinations include combinations of citric acid, oxalic acid and lauric acid, combinations of citric acid and oxalic acid, or combinations of lauric acid and benzoic acid.

[0028] Preferably, the trivalent iron salt includes ferric nitrate and / or ferric chloride.

[0029] Preferably, the phosphorus source includes any one or a combination of at least two of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, or ammonium phosphate. Typical but non-limiting combinations include combinations of phosphoric acid and ammonium hydrogen phosphate, combinations of ammonium hydrogen phosphate and ammonium dihydrogen phosphate, or combinations of phosphoric acid and ammonium phosphate.

[0030] Preferably, in step (2), the second lithium source includes any one or a combination of at least two of lithium carbonate, lithium hydroxide, or lithium acetate. Typical but non-limiting combinations include combinations of lithium carbonate and lithium hydroxide, combinations of lithium carbonate and lithium acetate, or combinations of lithium hydroxide and lithium acetate.

[0031] Preferably, the molar ratio of iron in the trivalent iron salt, lithium in the second lithium source, and phosphorus in the phosphorus source is (0.8-1.2):(0.8-1.2):1, for example: 0.8:0.8:1, 1:0.8:1, 1:1:1, 1.2:1:1, or 0.8:1:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, the surfactant comprises any one or a combination of at least two of lauric acid, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, sodium dodecyl sulfonate, or polyethylene glycol. Typical but non-limiting combinations include combinations of lauric acid and tetradecyltrimethylammonium bromide, combinations of lauric acid and hexadecyltrimethylammonium bromide, or combinations of sodium dodecyl sulfonate and polyethylene glycol, etc.

[0033] Preferably, the mass ratio of the surfactant to the ferric salt is (1-5):100, for example: 1:100, 2:100, 3:100, 4:100 or 5:100, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] Preferably, the molar ratio of iron in the mixed precursor and iron in the ferric salt in step (2) is (1-4):6, for example: 1:6, 1.5:6, 2:6, 3:6 or 4:6, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0035] Preferably, the heating and stirring temperature is 70-90°C, for example: 70°C, 75°C, 80°C, 85°C or 90°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] Preferably, a drying process is performed before the sintering process described in step (3).

[0037] Preferably, the atmosphere for the sintering process includes an inert atmosphere.

[0038] Preferably, the inert atmosphere includes any one or a combination of at least two of argon, nitrogen, or helium. Typical but non-limiting combinations include combinations of argon and nitrogen, argon and helium, or argon and helium.

[0039] Preferably, the sintering process includes a first-stage sintering and a second-stage sintering.

[0040] Preferably, the sintering temperature of the first stage is 200-350℃, for example: 200℃, 220℃, 250℃, 300℃ or 350℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, the sintering time is 4 to 6 hours, for example: 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] Preferably, the temperature of the two-stage sintering is 700-750℃, for example: 700℃, 710℃, 720℃, 740℃ or 750℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0043] Preferably, the sintering time for the two stages is 8 to 12 hours, for example: 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] In a second aspect, the present invention provides a lithium iron phosphate cathode material, which is prepared by the method described in the first aspect.

[0045] Thirdly, the present invention provides a positive electrode sheet comprising the lithium iron phosphate positive electrode material as described in the second aspect.

[0046] Fourthly, the present invention provides a lithium-ion battery comprising a positive electrode as described in the third aspect.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) The present invention can obtain a lithium phosphate and ferrous phosphate mixture precursor with a particle size of 1-1.2 μm by co-precipitation method, and prepare a lithium iron phosphate sol solution with a particle size of 50-150 nm by sol method. After mixing the two to form a gel, sintering can obtain lithium iron phosphate material with bimodal particle size distribution. The particle size of the material is controllable and different particle sizes of lithium iron phosphate can be achieved through a single production line, which improves the uniformity and compaction density of the material and avoids problems such as performance differences and poor stability between different batches.

[0049] (2) The tap density of the lithium iron phosphate cathode material prepared by the method of the present invention can reach 1.37 g / cm³. 3 The resulting batteries can achieve a capacity of over 161 mAh / g at 0.2C, over 153 mAh / g at 1C, over 146 mAh / g at 2C, over 137 mAh / g at 5C, and over 125 mAh / g at 10C. After 500 cycles, the capacity retention rate can reach over 98.7%. Attached Figure Description

[0050] Figure 1 This is a SEM image of the lithium iron phosphate cathode material prepared in Example 1 of the present invention.

[0051] Figure 2 This is a particle size distribution diagram of the lithium iron phosphate cathode material prepared in Example 1 of the present invention. Detailed Implementation

[0052] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0053] Example 1

[0054] This embodiment provides a lithium iron phosphate cathode material, which is prepared by the following method:

[0055] (1) Ammonium hydrogen phosphate and ferrous sulfate were added to a 50wt% ethanol aqueous solution at a molar ratio of 1:1 to prepare a mixed solution of ammonium hydrogen phosphate and ferrous sulfate with a total concentration of 1mol / L (the concentrations of ammonium hydrogen phosphate and ferrous sulfate were both 0.5mol / L). After stirring evenly, the solution was heated to 65℃. Under a nitrogen atmosphere, the prepared lithium hydroxide solution was added to make the lithium salt solution concentration 1.5mol / L. The solution was then stirred at 700rpm for 3h. The pH of the reaction solution was maintained at 8 using hydrochloric acid and sodium hydroxide. After precipitation, the solution was filtered, washed and dried to obtain the mixed precursor of lithium phosphate and ferrous phosphate prepared by coprecipitation method.

[0056] (2) Add citric acid, ferric chloride and lithium acetate to deionized water and dissolve them. The concentration of ferric chloride in the solution is 1.5 mol / L, the molar ratio of citric acid to ferric chloride is 2.5:1, and the molar ratio of lithium acetate to ferric chloride is 1:1. After stirring evenly, add ammonium hydrogen phosphate so that the molar ratio of it to ferric chloride is 1:1. After stirring evenly, add lauric acid of 4% by mass of iron salt and continue stirring for 2 hours to form a sol solution. Then mix the mixture precursor prepared by the coprecipitation method obtained above into it (the molar ratio of iron element in the mixture precursor to iron element in ferric chloride is 2:6). Stir for 2 hours, then raise the temperature to 80℃ and continue stirring until a gel precursor is formed.

[0057] (3) The gel precursor was dried in a forced-air drying process at 70°C to obtain a dry gel, which was then ground into powder and calcined in a nitrogen environment at 300°C for 4 hours. The temperature was then increased to 700°C and calcined for 10 hours to obtain the lithium iron phosphate material.

[0058] The SEM image of the lithium iron phosphate cathode material is as follows: Figure 1 As shown, by Figure 1 It can be seen that the lithium iron phosphate material prepared by this invention exists in two different particle sizes.

[0059] The particle size distribution diagram of the lithium iron phosphate cathode material is as follows: Figure 2 As shown, by Figure 2 It can be seen that the particle size distribution of the lithium iron phosphate cathode material prepared by this invention is bimodal.

[0060] Example 2

[0061] This embodiment provides a lithium iron phosphate cathode material, which is prepared by the following method:

[0062] (1) Add ammonium hydrogen phosphate and ferrous sulfate in a molar ratio of 1:1 to a 50wt% aqueous ethanol solution to prepare a mixed solution of ammonium hydrogen phosphate and ferrous sulfate with a total concentration of 1mol / L (the concentrations of ammonium hydrogen phosphate and ferrous sulfate are both 0.5mol / L). After stirring evenly, heat to 60℃ and add the prepared lithium hydroxide solution under a nitrogen atmosphere to make the lithium salt solution concentration 1.25mol / L. Then stir the reaction at 600rpm for 4h. Use hydrochloric acid and sodium hydroxide to maintain the pH of the reaction solution at 9. After the reaction precipitates, filter, wash and dry to obtain a mixed precursor of lithium phosphate and ferrous phosphate.

[0063] (2) Lauric acid, ferric chloride and lithium carbonate were added to deionized water and dissolved therein. The concentration of ferric chloride in the solution was 3 mol / L, the molar ratio of lauric acid to ferric chloride was 3:1, and the molar ratio of lithium carbonate to ferric chloride was 1:1. After stirring evenly, ammonium hydrogen phosphate was added so that the molar ratio of ferric chloride to ferric chloride was 1:1. After stirring evenly, 5% by mass of hexadecyltrimethylammonium bromide was added. After stirring for 4 hours, a sol solution was formed. Then the mixed precursor prepared by the coprecipitation method was mixed into it (the molar ratio of iron in the mixed precursor to iron in ferric chloride was 1:6). After stirring for 2 hours, the temperature was raised to 90°C and stirring was continued until a gel precursor was formed.

[0064] (3) The gel precursor was dried in an air dryer at 80°C to obtain a dry gel, which was then ground into powder and calcined in a helium atmosphere at 200°C for 6 hours. The temperature was then increased to 700°C and calcined for 12 hours to obtain lithium iron phosphate material.

[0065] Example 3

[0066] This embodiment provides a lithium iron phosphate cathode material, which is prepared by the following method:

[0067] (1) Add ammonium hydrogen phosphate and ferrous sulfate in a molar ratio of 1:1 to an 80wt% methanol aqueous solution to prepare a mixed solution of ammonium hydrogen phosphate and ferrous sulfate with a total concentration of 1mol / L (the concentrations of ammonium hydrogen phosphate and ferrous sulfate are both 0.5mol / L). After stirring evenly, heat to 60℃ and add the prepared lithium hydroxide solution under a nitrogen atmosphere to make the lithium salt solution concentration 1.75mol / L. Then stir the reaction at 800rpm for 3h. Use hydrochloric acid and sodium hydroxide to maintain the pH of the reaction solution at 7. After the reaction precipitates, filter, wash and dry to obtain a mixed precursor of lithium phosphate and ferrous phosphate.

[0068] (2) Oxalic acid, ferric chloride and lithium acetate were added to deionized water and dissolved therein. The concentration of ferric chloride in the solution was 3 mol / L, the molar ratio of oxalate to ferric chloride was 2.5:1, and the molar ratio of lithium acetate to ferric chloride was 1:1. After stirring evenly, ammonium hydrogen phosphate was added so that the molar ratio of phosphate to ferric chloride was 1:1. After stirring evenly, tetradecyltrimethylammonium bromide with a mass of 1% of iron salt was added. After stirring for 4 hours, a sol solution was formed. Then the mixture precursor prepared by the coprecipitation method was mixed into it (the molar ratio of iron in the mixture precursor to iron in ferric chloride was 1:4). The mixture was stirred for 2 hours, and then the temperature was raised to 70°C. The mixture was stirred until a gel precursor was formed.

[0069] (3) The gel precursor was dried in an air dryer at 80°C to obtain a dry gel, which was then ground into powder and calcined in a helium atmosphere at 350°C for 4 hours. The temperature was then increased to 750°C and calcined for 8 hours to obtain lithium iron phosphate material.

[0070] Example 4

[0071] The only difference between this embodiment and embodiment 1 is that the molar ratio of iron in citric acid and ferric chloride in step (2) is 1:1, while the other conditions and parameters are exactly the same as in embodiment 1.

[0072] Example 5

[0073] The only difference between this embodiment and embodiment 1 is that the molar ratio of iron in citric acid and ferric chloride in step (2) is 4:1, while the other conditions and parameters are exactly the same as in embodiment 1.

[0074] Example 6

[0075] The only difference between this embodiment and embodiment 1 is that the molar ratio of iron in the mixture precursor and iron in ferric chloride in step (2) is 0.5:6. All other conditions and parameters are exactly the same as in embodiment 1.

[0076] Example 7

[0077] The only difference between this embodiment and embodiment 1 is that the molar ratio of iron in the mixture precursor and iron in ferric chloride in step (2) is 6:6. All other conditions and parameters are exactly the same as in embodiment 1.

[0078] Comparative Example 1

[0079] This comparative example provides a lithium iron phosphate cathode material, which is prepared by the following method:

[0080] (1) Ammonium hydrogen phosphate and ferrous sulfate were added to a 50 wt% ethanol solution at a molar ratio of 1:1 to prepare a 0.5 mol / L mixed solution of ammonium hydrogen phosphate and ferrous sulfate. After stirring evenly, the solution was heated to 65°C. Under a nitrogen atmosphere, a prepared lithium hydroxide solution was added to the solution to make the lithium salt concentration 1.5 mol / L. The reaction was then stirred at 700 rpm for 3 h, and the pH of the reaction solution was maintained at 7-9 using hydrochloric acid and sodium hydroxide. The resulting precipitate was filtered, washed, and then mixed with 5 wt% glucose. After drying, the lithium iron phosphate precursor was obtained.

[0081] (2) Then the lithium iron phosphate precursor was calcined at 750℃ in an inert atmosphere for 10h to obtain lithium iron phosphate with a particle size D50 of 1μm.

[0082] Comparative Example 2

[0083] This comparative example provides a lithium iron phosphate cathode material, which is prepared by the following method:

[0084] (1) Add citric acid, ferric chloride and lithium acetate to deionized water and dissolve them. The concentration of ferric chloride in the solution is 1.5 mol / L, the molar ratio of citric acid to ferric chloride is 2.5:1, and the molar ratio of lithium acetate to ferric chloride is 1:1. After stirring evenly, add ammonium hydrogen phosphate so that the molar ratio of ammonium hydrogen phosphate to ferric chloride is 1:1. After stirring evenly, add lauric acid of 4% by weight of ferric salt and continue stirring for 3 hours to form a sol. Raise the temperature to 80℃ and continue stirring until a gel precursor is formed.

[0085] (2) The gel precursor was dried in a forced-air drying process at 70°C to obtain a dry gel. It was then ground into powder and calcined in a nitrogen environment at 300°C for 4 hours. The temperature was then increased to 700°C and calcined for 10 hours to obtain lithium iron phosphate material with a particle size distribution of 50-150 nm.

[0086] Comparative Example 3

[0087] The only difference between this comparative example and Example 1 is that ferric salts are used in step (1), while the other conditions and parameters are exactly the same as in Example 1.

[0088] Performance testing:

[0089] (1) Weigh the mass m1 of the dry graduated cylinder. Place the lithium iron phosphate cathode materials prepared in the examples and comparative examples into the graduated cylinders (fill to about 5 ml). Seal the cylinder opening and vibrate it vertically until the sample volume no longer decreases. Record the sample volume V (ml). Weigh the mass m2 (g) of the graduated cylinder + sample. Calculate ρ (g / cm³) using the formula ρ=(m2-m1) / V. 3 The tap density of the sample to be tested is obtained.

[0090] (2) The lithium iron phosphate cathode material prepared in the examples and comparative examples was mixed with acetylene black and PVDF at a mass ratio of 75:15:10 to form a uniform slurry, which was then uniformly coated onto an aluminum foil substrate as the cathode of the simulated battery. The cathode of the simulated battery used a lithium sheet, a polypropylene porous membrane as the separator, and 1 mol of LiPF6 dissolved in 1 L of a mixed solvent of EC and DMC (volume ratio 1:1). The cathode, cathode, electrolyte, and separator were assembled into a battery in an argon-protected glove box. First, it was charged at a constant current to 4.2V, then discharged at a higher rate to 2.0V. The capacity discharged was the discharge capacity at that rate. After discharge, it was discharged at a constant current to 2.0V again. Then, the next rate test was performed. The test results of this simulated battery are shown in Table 1.

[0091] Table 1

[0092]

[0093] As shown in Table 1, based on Examples 1-3, the tap density of the lithium iron phosphate cathode material prepared by the method of the present invention can reach 1.37 g / cm³. 3 The resulting batteries can achieve a capacity of over 161 mAh / g at 0.2C, over 153 mAh / g at 1C, over 146 mAh / g at 2C, over 137 mAh / g at 5C, and over 125 mAh / g at 10C. After 500 cycles, the capacity retention rate can reach over 98.7%.

[0094] A comparison of Examples 1 and 4-5 shows that during the preparation of the lithium iron phosphate cathode material of the present invention, the molar ratio of citric acid to iron in the ferric salt affects its performance. Controlling the molar ratio of citric acid to iron in the ferric salt at (2-3):1 results in a lithium iron phosphate cathode material with better performance. If the amount of citric acid added is too large, it will lead to excessive carbon content, which will affect the specific capacity. If the amount of citric acid added is too small, it will lead to insufficient carbon content, which will affect the rate performance.

[0095] A comparison of Examples 1 and 6-7 shows that, in the preparation process of the lithium iron phosphate cathode material of the present invention, the particle size distribution of lithium iron phosphate is controlled by the molar ratio of iron in the mixed precursor and iron in the trivalent iron salt. Controlling the molar ratio of iron in the mixed precursor and iron in the trivalent iron salt to (1-4):6 results in a lithium iron phosphate cathode material with better performance. If the amount of trivalent iron salt added is too large, it will lead to an increase in the proportion of lithium iron phosphate used in the sol-gel process, meaning that the proportion of small-diameter lithium iron phosphate will be too large, affecting the cycle stability of the cathode material and the carbon content. Similarly, if the amount of trivalent iron salt added is too small, the proportion of large-diameter lithium iron phosphate will be too large, affecting the rate performance, and the increased carbon content will further affect the specific capacity.

[0096] As can be seen from the comparison between Example 1 and Comparative Example 1, the lithium iron phosphate cathode material prepared by the co-precipitation method has a larger particle size, smaller capacity, and relatively lower rate performance.

[0097] A comparison of Example 1 and Comparative Example 2 reveals that the lithium iron phosphate cathode material prepared by the sol-gel method has a smaller particle size, resulting in a larger specific surface area, lower nanoparticle packing density, and lower volumetric energy density. The extremely large specific surface area of ​​the nanoparticles requires more binders and dispersants, which is detrimental to electrode processing, and also leads to more side reactions between the active material surface and the electrolyte. During battery charging and discharging, the nanoparticles are prone to agglomeration. After agglomeration, the internal particles may lose electrical contact, significantly reducing the advantages in kinetics and cycle stability.

[0098] As can be seen from the comparison between Example 1 and Comparative Example 3, the present invention uses ferrous salt in the co-precipitation process and ferric salt in the sol-gel method. If ferric salt is used in the co-precipitation process, it will lead to the formation of ferric phosphate instead of ferrous phosphate, which will affect the particle size of lithium iron phosphate formed in the precipitation process.

[0099] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The preparation method includes the following steps: (1) Mix ferrous salt and phosphorus source with alcohol solvent, heat and stir and add first lithium source solution to obtain mixed solution, adjust the pH of mixed solution to obtain mixed precursor; (2) Mix the complexing agent, ferric salt, second lithium source, phosphorus source, surfactant and water, stir to form a sol solution, mix the mixture precursor obtained in step (1) with the sol solution, heat and stir to obtain a gel precursor; (3) The gel precursor is sintered to obtain the lithium iron phosphate cathode material; The mixture precursor includes lithium phosphate and ferrous phosphate; The molar ratio of iron in the mixed precursor and iron in the ferric salt in step (2) is (1~4):6; The heating and stirring temperature in step (1) is 60~70℃; The heating and stirring temperature in step (2) is 70~90℃; A lithium iron phosphate and ferrous phosphate mixture precursor with a particle size of 1-1.2 μm was obtained by co-precipitation. A lithium iron phosphate sol solution with a particle size of 50-150 nm was prepared by sol-gel method. The two were mixed to form a gel and then sintered to obtain a lithium iron phosphate material with a bimodal particle size distribution.

2. The preparation method according to claim 1, characterized in that, The ferrous salt in step (1) includes ferrous sulfate and / or ferrous chloride.

3. The preparation method according to claim 1, characterized in that, The phosphorus source in step (1) includes any one or a combination of at least two of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, or ammonium phosphate.

4. The preparation method according to claim 1, characterized in that, The alcohol solvent in step (1) includes anhydrous alcohol or an aqueous solution of alcohol.

5. The preparation method according to claim 1, characterized in that, The alcohol in the alcohol solvent includes any one or a combination of at least two of methanol, ethanol, isopropanol, or ethylene glycol.

6. The preparation method according to claim 1, characterized in that, The molar volume ratio of the ferrous salt and the alcohol solvent in step (1) is 0.5~1 mol / L.

7. The preparation method according to claim 1, characterized in that, The heating and stirring speed in step (1) is 600~800 rpm.

8. The preparation method according to claim 1, characterized in that, Step (1) The solute in the first lithium source solution includes lithium hydroxide and / or lithium nitrate.

9. The preparation method according to claim 1, characterized in that, The molar ratio of lithium ions, ferrous ions and phosphorus in the mixed solution in step (1) is (2.5~3.5):(0.8~1.2):

1.

10. The preparation method according to claim 1, characterized in that, The pH value in step (1) is 7-9.

11. The preparation method according to claim 1, characterized in that, The molar ratio of iron in the complexing agent and the ferric salt in step (2) is (2~3):

1.

12. The preparation method according to claim 1, characterized in that, The complexing agent in step (2) includes any one or a combination of at least two of citric acid, oxalic acid, lauric acid, sorbic acid, benzoic acid or oleic acid.

13. The preparation method according to claim 1, characterized in that, The ferric salts mentioned in step (2) include ferric nitrate and / or ferric chloride.

14. The preparation method according to claim 1, characterized in that, The phosphorus source in step (2) includes any one or a combination of at least two of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, or ammonium phosphate.

15. The preparation method according to claim 1, characterized in that, Step (2) The second lithium source includes any one or a combination of at least two of lithium carbonate, lithium hydroxide or lithium acetate.

16. The preparation method according to claim 1, characterized in that, The molar ratio of iron in the trivalent iron salt, lithium in the second lithium source, and phosphorus in the phosphorus source in step (2) is (0.8~1.2):(0.8~1.2):

1.

17. The preparation method according to claim 1, characterized in that, The surfactant in step (2) includes any one or a combination of at least two of the following: lauric acid, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, sodium dodecyl sulfonate, or polyethylene glycol.

18. The preparation method according to claim 1, characterized in that, The mass ratio of the surfactant and the ferric salt in step (2) is (1~5):

100.

19. The preparation method according to claim 1, characterized in that, Before the sintering process described in step (3), a drying process is performed.

20. The preparation method according to claim 1, characterized in that, The atmosphere for the sintering process in step (3) includes an inert atmosphere.

21. The preparation method according to claim 20, characterized in that, The inert atmosphere gas in step (3) includes any one or a combination of at least two of argon, nitrogen or helium.

22. The preparation method according to claim 1, characterized in that, The sintering process in step (3) includes a first-stage sintering and a second-stage sintering.

23. The preparation method according to claim 22, characterized in that, The sintering temperature of the first section is 200~350℃.

24. The preparation method according to claim 22, characterized in that, The sintering time for the first stage is 4-6 hours.

25. The preparation method according to claim 22, characterized in that, The sintering temperature for the two stages is 700~750℃.

26. The preparation method according to claim 22, characterized in that, The sintering time for the two stages is 8-12 hours.

27. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by the method described in any one of claims 1-26.

28. A positive electrode sheet, characterized in that, The positive electrode comprises the lithium iron phosphate positive electrode material as described in claim 27.

29. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode as described in claim 28.

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