Carbon-coated lithium iron phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention discloses a carbon-coated lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery. An energy density synergist is added into a lithium iron phosphate material, uniform anchoring on the surfaces of lithium iron phosphate particles is achieved, the electronic conductivity is improved, the structural stability is enhanced, the first coulombic efficiency is improved, the specific capacity is improved, a boron-oxygen cluster structure in the energy density synergist provides multiple coordination sites, Li < + > is specifically adsorbed through the Lewis acid-base effect, and the specific adsorption capacity is improved. Lithium ion migration energy barriers are reduced, and the charge transmission efficiency is improved. A rigid framework of polyhedral oligomeric silsesquioxane is embedded into a carbon layer to form a nano support body, so that the lattice volume expansion in the charge-discharge process is effectively inhibited, and the structural stability of the material is improved.

Description

Technical Field

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

[0002] As global demand for renewable energy and electric vehicles continues to grow, lithium-ion batteries, due to their high energy density, long cycle life, and environmentally friendly properties, have become the core power source for energy storage systems and electric vehicles. Among them, lithium iron phosphate (LiFePO4), as a cathode material, has attracted widespread attention due to its high thermal stability, excellent safety, low cost, and environmental friendliness.

[0003] However, lithium iron phosphate materials have inherent problems of poor electronic conductivity and low lithium ion diffusion rate, which limit their performance in high power density and fast charge and discharge applications. To overcome these shortcomings, researchers have proposed a variety of improvement methods, including: Carbon coating technology: By coating a conductive carbon layer on the surface of lithium iron phosphate particles, the electronic conductivity is improved. Commonly used carbon sources include organic substances such as glucose, sucrose, and citric acid, and a carbon layer is formed by high-temperature heat treatment. For example, the invention application with publication number CN120149384A discloses a uniformly carbon-coated lithium iron phosphate positive electrode material, wherein the lithium iron phosphate matrix particles are coated with a composite carbon layer, and the composite carbon layer is doped with cerium-modified tantalum carbide nanocrystals. The invention application with publication number CN120089728A discloses a highly conductive carbon-coated lithium iron phosphate positive electrode material, including a spherical lithium iron phosphate matrix, a modified lithium niobium vanadate uniformly coated on the outside of the nano-scale spherical lithium iron phosphate matrix, and a composite carbon layer uniformly coated on the outside of the modified lithium niobium vanadate. The organic carbon source in the outermost composite carbon layer is carbonized to form a continuous amorphous carbon layer.

[0004] Metal ion doping: introduction of Mg 2+ 、Ti 4+ Such metal ions can improve the electrical conductivity and structural stability of the material.

[0005] Nano-processing: Control the size of lithium iron phosphate particles at the nanometer level, shorten the lithium ion diffusion path, and improve rate performance.

[0006] Although the above methods have improved the performance of lithium iron phosphate to a certain extent, there are still some problems: Uneven carbon coating: Traditional carbon coating methods make it difficult to achieve uniformity in the carbon layer, resulting in poor conductivity in some areas and affecting the overall electrochemical performance.

[0007] It is difficult to control the carbon content: too high a carbon content will reduce the energy density of the material, while too low a carbon content will not effectively improve the conductivity, making it difficult to strike a balance between the two.

[0008] Complex process: Some modification methods involve multi-step reactions or high-cost raw materials, which are not conducive to large-scale industrial production.

[0009] Therefore, there is an urgent need for a preparation method with simple process, controllable cost, uniform carbon coating and effective improvement of the electrochemical performance of lithium iron phosphate positive electrode materials to meet the development needs of high-performance lithium-ion batteries. Summary of the Invention

[0010] In view of the above-mentioned deficiencies in the prior art, the present invention provides a carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, and a lithium ion battery.

[0011] The present invention first provides a method for preparing a carbon-coated lithium iron phosphate positive electrode material, comprising the following steps: (1) Adding iron phosphate, lithium source and carbon source into deionized water and stirring uniformly to obtain a slurry; (2) ball milling the slurry and then spray drying it to obtain dry powder; (3) heat-treating the dry powder under an inert atmosphere to pyrolyze the carbon source to form a conductive carbon layer; (4) Adding an energy density enhancer to the heat-treated product and mixing them evenly to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0012] Preferably, in step (1), by weight, the iron phosphate is 85-115 parts, the lithium source is 15-30 parts, the carbon source is 5-15 parts, and the deionized water is 185-245 parts.

[0013] Preferably, in step (2), a dispersant is added before ball milling, with the mass of the dispersant being 0.1% to 0.6% of the slurry; the ball milling time is 4 to 12 hours, the ball milling speed is 400 to 1200 rpm, and the spray drying temperature is 150 to 200°C. The dispersant is preferably at least one of polyethylene glycol and sodium lauryl sulfate.

[0014] In step (3), the heat treatment temperature is 700-900°C for 10-14 hours. After the heat treatment, the carbon source is pyrolyzed to form a conductive carbon layer, thereby obtaining a carbon-coated lithium iron phosphate material. The heat treatment heating rate is 2-10°C / min.

[0015] Preferably, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate and lithium nitrate; and the carbon source is selected from at least one of glucose, sucrose, citric acid, phenolic resin, graphite and carbon nanotubes.

[0016] Preferably, in step (4), the amount of the energy density enhancer added is 0.5% to 0.9% of the heat-treated product, and the mixture is mixed at a temperature of 70 to 80° C. for 20 to 40 minutes.

[0017] Preferably, in step (4), the preparation method of the energy density enhancer comprises the following steps: adding 2-dihydroxyboryl-3-thiophenecarboxylic acid, epoxycyclohexyl cage-shaped polysilsesquioxane, and organotin to a solvent, reacting under an inert atmosphere, and then removing the solvent to obtain the energy density enhancer.

[0018] More preferably, the composition comprises 18-36 parts by weight of 2-dihydroxyboryl-3-thiophenecarboxylic acid, 1-4 parts of epoxycyclohexyl cage-type polysilsesquioxane, and 2-4 parts of organotin. The energy density enhancer is prepared at a reaction temperature of 100-110°C for 5-7 hours. Toluene is used as the solvent in an amount of 200-300 parts. After the reaction is complete, the mixture is dried to obtain the energy density enhancer.

[0019] The energy density enhancer is evenly anchored on the surface of lithium iron phosphate particles.

[0020] The present invention further provides a carbon-coated lithium iron phosphate positive electrode material prepared by the preparation method.

[0021] The present invention also provides a lithium ion battery comprising a positive electrode, wherein the positive electrode material used in the positive electrode is the carbon-coated lithium iron phosphate positive electrode material.

[0022] The organic tin is at least one selected from dibutyltin dilaurate, stannous octoate, dibutyltin maleate and dibutyltin maleate.

[0023] The reaction mechanism is: The boronate hydroxyl group (-B(OH)2) in 2-dihydroxyboryl-3-thiophenecarboxylic acid undergoes a ring-opening esterification reaction with the epoxy group of epoxycyclohexyl cage-type polysilsesquioxane (POSS) under the catalysis of an organotin catalyst, forming a borate ester bond (-BOC-). The organotin catalyst activates the epoxy group via a Lewis acid mechanism, promoting the ring opening.

[0024] ‌Poss skeleton synergy‌: The cage-like structure of epoxycyclohexyl-POSS provides multiple reaction sites, and its Si-O-Si skeleton stabilizes the reaction intermediates through steric hindrance. At the same time, the dispersed borate bonds can enhance the intermolecular cross-linking density and improve thermal stability.

[0025] Beneficial effects of the present invention: 1. Improve electronic conductivity and enhance rate performance: The carbon coating provides an electron transmission channel, reduces electron migration resistance, and thus improves the performance of the material under high-rate charge and discharge conditions.

[0026] 2. Enhance structural stability and extend cycle life: High-temperature sintering promotes the improvement of crystal structure, reduces lattice defects, and improves the structural stability of the material, thereby extending the cycle life of the battery.

[0027] 3. Improve the initial coulombic efficiency and electrochemical performance: High-purity raw materials and uniform carbon coating reduce irreversible reactions during the initial charge and discharge of the battery, improve the initial coulombic efficiency, and enhance the overall electrochemical performance.

[0028] 4. Improve specific capacity to meet high energy density requirements: Uniform carbon coating and optimized crystal structure improve the efficiency of lithium ion insertion and extraction, increase the specific capacity of the material, and meet the needs of high energy density batteries.

[0029] 5. Boron-oxygen cluster active site: The boron-oxygen cluster structure in the synergist provides multiple coordination sites, which specifically adsorb Li through Lewis acid-base interaction. + , reducing the energy barrier for lithium-ion migration and improving charge transfer efficiency. Cage structure enhancement: The rigid skeleton of the cage-shaped polysilsesquioxane is embedded in the carbon layer to form a "nano-support", which effectively inhibits the lattice volume expansion during the charge and discharge process and improves the structural stability of the material. DETAILED DESCRIPTION

[0030] For ease of understanding of the present invention, the present invention will be described more fully below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0031] If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in this field or the product instructions were used. Raw materials used without manufacturer specified are all commercially available conventional products.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0033] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0034] Example 1

[0035] 1) Preparation of lithium iron phosphate precursor: Add 100 g of iron phosphate (FePO4), 20 g of lithium carbonate (purity 99.5%) and 10 g of glucose to 200 g of deionized water and stir to form a uniform slurry.

[0036] 2) Ball milling: The above slurry was added to a ball mill, and 0.3% of the slurry weight of polyethylene glycol-400 (polyethylene glycol with a molecular weight of 400) was added as a dispersant, and ball milling was performed at a speed of 800 rpm for 8 hours.

[0037] 3) Drying: The ball-milled slurry was spray-dried at a temperature of 180°C to obtain dry powder.

[0038] 4) Preparation of an energy density enhancer: 25 g of 2-dihydroxyboryl-3-thiophenecarboxylic acid (CAS No. 519054-53-6) and 2 g of epoxycyclohexyl-cage polysilsesquioxane (CAS No. 187333-74-0) were added to 250 g of toluene. The reactor was purged with nitrogen. 3 g of dibutyltin dilaurate was added and the mixture was reacted at 105°C for 6 hours. After the reaction, the toluene was removed by vacuum distillation, and the mixture was then dried under vacuum at 65°C for 10 hours to obtain an energy density enhancer.

[0039] 5) Carbon Coating: The dried powder was heat-treated in a nitrogen atmosphere at a heating rate of 5°C / min to 800°C and held at this temperature for 12 hours to pyrolyze the carbon source and form a conductive carbon layer. After cooling, an energy density enhancer (0.5% by weight of the heat-treated product) was added at 75°C and mixed at high speed for 30 minutes to obtain a carbon-coated lithium iron phosphate cathode material.

[0040] Example 2

[0041] 1) Preparation of lithium iron phosphate precursor: 85 g of iron phosphate (FePO4), 30 g of lithium hydroxide (purity 99.2%) and 5 g of sucrose were added to 245 g of deionized water and stirred to form a uniform slurry.

[0042] 2) Ball milling: The above slurry was added to a ball mill, and 0.1% of the slurry mass of sodium lauryl sulfate was added as a dispersant, and the mixture was ball milled at a speed of 400 rpm for 12 hours.

[0043] 3) Drying: The ball-milled slurry was spray-dried at a temperature of 150°C to obtain dry powder.

[0044] 4) Preparation of an energy density enhancer: 18 g of 2-dihydroxyboryl-3-thiophenecarboxylic acid and 4 g of epoxycyclohexyl-cage polysilsesquioxane were added to 200 g of toluene. The reactor was purged with nitrogen and 4 g of stannous octoate was added. The reaction was carried out at 100°C for 7 hours. After the reaction, the toluene was removed by vacuum distillation, and the mixture was then dried under vacuum at 60°C for 12 hours to obtain an energy density enhancer.

[0045] 5) Carbon Coating: The dried powder was heat-treated under an argon atmosphere at a heating rate of 2°C / min to 900°C and held at this temperature for 10 hours to pyrolyze the carbon source and form a conductive carbon layer. After cooling, an energy density enhancer (0.65% by weight of the heat-treated product) was added at 70°C and mixed at high speed for 20 minutes to obtain a carbon-coated lithium iron phosphate cathode material.

[0046] Example 3

[0047] 1) Preparation of lithium iron phosphate precursor: 115 g of iron phosphate (FePO4), 15 g of lithium acetate (purity 99.8%) and 15 g of citric acid were added to 185 g of deionized water and stirred to form a uniform slurry.

[0048] 2) Ball milling: The above slurry was added to a ball mill, and 0.6% of the slurry weight of polyethylene glycol-400 was added as a dispersant, and the mixture was ball milled at a speed of 1200 rpm for 4 hours.

[0049] 3) Drying: The ball-milled slurry was spray-dried at a temperature of 200°C to obtain dry powder.

[0050] 4) Preparation of an energy density enhancer: 36 g of 2-dihydroxyboryl-3-thiophenecarboxylic acid and 1 g of epoxycyclohexyl-cage polysilsesquioxane were added to 300 g of toluene. The reactor was purged with nitrogen and 2 g of dibutyltin maleate was added. The reaction was carried out at 110°C for 5 hours. After the reaction, the toluene was removed by vacuum distillation, and the mixture was then vacuum dried at 70°C for 8 hours to obtain an energy density enhancer.

[0051] 5) Carbon Coating: The dried powder was heat-treated in a nitrogen atmosphere at a heating rate of 10°C / min to 700°C and held at this temperature for 14 hours to pyrolyze the carbon source and form a conductive carbon layer. After cooling, an energy density enhancer (0.75% by weight of the heat-treated product) was added at 80°C and mixed at high speed for 40 minutes to obtain a carbon-coated lithium iron phosphate cathode material.

[0052] Example 4

[0053] 1) Preparation of lithium iron phosphate precursor: 95 g of iron phosphate (FePO4), 25 g of lithium oxalate (purity 99.3%) and 8 g of phenolic resin were added to 220 g of deionized water and stirred to form a uniform slurry.

[0054] 2) Ball milling: The slurry was added to a ball mill, and sodium lauryl sulfate (0.4% by weight of the slurry) was added as a dispersant. The mixture was ball milled at a speed of 600 rpm for 10 hours.

[0055] 3) Drying: The ball-milled slurry was spray-dried at a temperature of 170°C to obtain a dry powder.

[0056] 4) Preparation of an energy density enhancer: 30 g of 2-dihydroxyboryl-3-thiophenecarboxylic acid and 3 g of epoxycyclohexyl-cage polysilsesquioxane were added to 280 g of toluene. The reactor was purged with nitrogen and 3 g of dibutyltin maleate was added. The reaction was continued at 108°C for 5.5 hours. After the reaction, the toluene was removed by vacuum distillation, and the mixture was then vacuum dried at 68°C for 9 hours to obtain an energy density enhancer.

[0057] 5) Carbon Coating: The dried powder was heat-treated under an argon atmosphere at a heating rate of 7°C / min to 750°C and held at this temperature for 13 hours to pyrolyze the carbon source and form a conductive carbon layer. After cooling, an energy density enhancer (0.9% by weight of the heat-treated product) was added at 72°C and mixed at high speed for 35 minutes to obtain a carbon-coated lithium iron phosphate cathode material.

[0058] Comparative Example 1 1) Preparation of lithium iron phosphate precursor: Add 100 g of iron phosphate (FePO4), 20 g of lithium carbonate (purity 99.5%) and 10 g of glucose to 200 g of deionized water and stir to form a uniform slurry.

[0059] 2) Ball milling: The above slurry was added to a ball mill, and 0.3% of the slurry weight of polyethylene glycol-400 was added as a dispersant, and the mixture was ball milled at a speed of 800 rpm for 8 hours.

[0060] 3) Drying: The ball-milled slurry was spray-dried at a temperature of 180°C to obtain dry powder.

[0061] 4) Carbon coating treatment: The dry powder was heat treated in a nitrogen atmosphere, heated to 800°C at a heating rate of 5°C / min, and kept at this temperature for 12 hours to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0062] Comparative Example 2 2-dihydroxyboryl-3-thiophenecarboxylic acid was not added, and the other steps were the same as in Example 1 because the reaction could not proceed. In fact, the energy density enhancer was epoxycyclohexyl-cage polysilsesquioxane.

[0063] Comparative Example 3 Without adding epoxycyclohexyl-cage polysilsesquioxane, the other steps were the same as in Example 1 because the reaction could not proceed. In fact, the energy density enhancer was 2-dihydroxyboryl-3-thiophenecarboxylic acid.

[0064] Test Example 1 The testing method involved in the present invention is as follows: Electrochemical performance test: According to the SJ / T 11793-2022 standard, CR2032 button cells were assembled and the battery's specific capacity, cycle performance, and rate performance were tested.

[0065] Positive electrode material: lithium iron phosphate (LiFePO4) active material, conductive agent (acetylene black), binder (polyvinylidene fluoride PVDF).

[0066] Negative electrode material: metallic lithium sheet, approximately 15.8 mm in diameter, must be handled in a glove box to avoid oxidation.

[0067] Diaphragm: Polyethylene (PE) microporous membrane, diameter about 16mm.

[0068] Electrolyte: 1 mol / L LiPF6 dissolved in a mixed solvent of EC:DMC (1:1 volume ratio).

[0069] Other components: CR2032 battery case (positive and negative cases), gasket, and shrapnel.

[0070] The test results are shown in Table 1.

[0071] Table 1

[0072] In summary, the method for preparing the carbon-coated lithium iron phosphate positive electrode material provided by the present invention significantly improves the electrochemical performance of the material through reasonable raw material ratio and process parameter control, and has good application prospects.

Claims

1. A method for preparing a carbon-coated lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: (1) Adding iron phosphate, lithium source and carbon source into deionized water and stirring uniformly to obtain a slurry; (2) ball milling the slurry and then spray drying it to obtain dry powder; (3) heat-treating the dry powder under an inert atmosphere to pyrolyze the carbon source to form a conductive carbon layer; (4) Adding an energy density enhancer to the heat-treated product and mixing them evenly to obtain a carbon-coated lithium iron phosphate positive electrode material.

2. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (1), by mass, the iron phosphate is 85-115 parts, the lithium source is 15-30 parts, the carbon source is 5-15 parts, and the deionized water is 185-245 parts.

3. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (2), a dispersant is added before ball milling, and the mass of the dispersant added is 0.1% to 0.6% of the slurry; The ball milling time is 4 to 12 hours, and the ball milling speed is 400 to 1200 rpm; The spray drying temperature is 150~200℃; In step (3), the heat treatment temperature is 700-900°C and the time is 10-14 hours.

4. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate and lithium nitrate; The carbon source is selected from at least one of glucose, sucrose, citric acid, phenolic resin, graphite and carbon nanotubes.

5. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (4), the amount of energy density enhancer added is 0.5% to 0.9% of the heat-treated product; when added, the temperature is 70 to 80° C. and mixed for 20 to 40 minutes.

6. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (4), the preparation method of the energy density enhancer includes the following steps: adding 2-dihydroxyboryl-3-thiophenecarboxylic acid, epoxycyclohexyl cage-shaped polysilsesquioxane, and organic tin to a solvent, reacting under an inert atmosphere, and removing the solvent to obtain the energy density enhancer.

7. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 6, characterized in that: By mass, the ingredients include 18 to 36 parts of 2-dihydroxyboryl-3-thiophenecarboxylic acid, 1 to 4 parts of epoxycyclohexyl cage-shaped polysilsesquioxane, and 2 to 4 parts of organotin.

8. The method for preparing the carbon-coated lithium iron phosphate positive electrode material according to claim 6, characterized in that: When preparing the energy density enhancer, the reaction temperature is 100-110° C. and the reaction time is 5-7 hours.

9. A carbon-coated lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: It comprises a positive electrode, and the positive electrode material used in the positive electrode is the carbon-coated lithium iron phosphate positive electrode material according to claim 9.

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