A carbon-coated lithium iron phosphate positive electrode material, a preparation method thereof, and a lithium ion battery

Abstract

The application discloses a kind of carbon-coated lithium iron phosphate positive electrode materials and its preparation method, lithium ion battery.Energy density synergist is added to lithium iron phosphate material, it is realized in the uniform anchoring of lithium iron phosphate particle surface, improve electronic conductivity, enhance structural stability, improve the first coulomb efficiency, promote specific capacity, boron oxygen cluster structure in energy density synergist provides multiple coordination sites, through Lewis acid-base effect specific adsorption Li + , reduce lithium ion migration energy barrier, improve charge transport efficiency.Cage structure enhancement: rigid skeleton of cage polysilsesquioxane is embedded into carbon layer, forms "nanometer support", effectively inhibits lattice volume expansion in charge and discharge process, improves material structural stability.

Description

Technical Field

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

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

[0003] However, lithium iron phosphate materials inherently suffer from poor electronic conductivity and low lithium-ion diffusion rate, limiting their performance in high power density and fast charge / discharge applications. To overcome these shortcomings, researchers have proposed several improvement methods, including:

[0004] Carbon coating technology: This technology improves electronic conductivity by coating the surface of lithium iron phosphate particles with a conductive carbon layer. Common carbon sources include organic compounds such as glucose, sucrose, and citric acid, which are used to form the carbon layer through high-temperature heat treatment. For example, patent application CN120149384A discloses a uniformly carbon-coated lithium iron phosphate cathode material, in which 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. Patent application CN120089728A discloses a highly conductive carbon-coated lithium iron phosphate cathode material, comprising a spherical lithium iron phosphate matrix, a modified lithium niobate uniformly coated on the outside of the nanoscale spherical lithium iron phosphate matrix, and a composite carbon layer uniformly coated on the outside of the modified lithium niobate. The organic carbon source in the outermost composite carbon layer is carbonized to form a continuous amorphous carbon layer.

[0005] Metal ion doping: introducing metals such as Mg 2+ Ti 4+ Metal ions improve the electrical conductivity and structural stability of materials.

[0006] Nano-sizing: Controlling the size of lithium iron phosphate particles to the nanoscale shortens the lithium-ion diffusion path and improves rate performance.

[0007] Although the above methods have improved the performance of lithium iron phosphate to some extent, some problems still exist:

[0008] Uneven carbon coating: Traditional carbon coating methods are difficult to achieve uniformity of carbon layer, resulting in poor conductivity in some areas and affecting the overall electrochemical performance.

[0009] Controlling carbon content is difficult: too high a carbon content will reduce the energy density of the material, while too low a carbon content will not effectively improve conductivity, making it difficult to strike a balance between the two.

[0010] Complex processes: Some modification methods involve multi-step reactions or high-cost raw materials, which are not conducive to large-scale industrial production.

[0011] Therefore, there is an urgent need for a simple, cost-controllable preparation method that can achieve uniform carbon coating and effectively improve the electrochemical performance of lithium iron phosphate cathode materials to meet the development needs of high-performance lithium-ion batteries. Summary of the Invention

[0012] To address the aforementioned shortcomings in the prior art, this invention provides a carbon-coated lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery.

[0013] This invention first provides a method for preparing carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0014] (1) Add iron phosphate, lithium source and carbon source to deionized water and stir evenly to obtain slurry;

[0015] (2) The slurry is ball-milled and then spray-dried to obtain a dry powder;

[0016] (3) Heat-treat the dried powder under an inert atmosphere to pyrolyze the carbon source to form a conductive carbon layer;

[0017] (4) Add an energy density enhancer to the heat-treated product and mix evenly to obtain carbon-coated lithium iron phosphate cathode material.

[0018] Preferably, 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.

[0019] Preferably, 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, 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 dodecyl sulfate.

[0020] In step (3), the heat treatment temperature is 700~900℃ and the time is 10~14 hours. After heat treatment, the carbon source is pyrolyzed to form a conductive carbon layer, thus obtaining carbon-coated lithium iron phosphate material. The heat treatment heating rate is 2~10℃ / min.

[0021] 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.

[0022] Preferably, in step (4), the amount of energy density enhancer added is 0.5% to 0.9% of the product after heat treatment. When added, the mixture is mixed for 20 to 40 minutes at a temperature of 70 to 80°C.

[0023] Preferably, in step (4), the preparation method of the energy density enhancer includes the following steps: 2-dihydroxyboryl-3-thiophenic acid, epoxycyclohexyl cage polysilsesquioxane, and organotin are added to a solvent, reacted under an inert atmosphere, and then the solvent is removed to obtain the energy density enhancer.

[0024] More preferably, by weight, 18-36 parts of 2-dihydroxyboryl-3-thiophenic acid, 1-4 parts of epoxycyclohexyl cage-like polysilsesquioxane, and 2-4 parts of organotin. During the preparation of the energy density enhancer, the reaction temperature is 100-110°C, and the reaction time is 5-7 hours. Toluene is used as the solvent, with 200-300 parts of toluene. After the reaction is complete, the mixture is dried to obtain the energy density enhancer.

[0025] Energy density enhancers are uniformly anchored on the surface of lithium iron phosphate particles.

[0026] The present invention further provides a carbon-coated lithium iron phosphate cathode material prepared by the aforementioned preparation method.

[0027] The present invention also provides a lithium-ion battery, including a positive electrode, wherein the positive electrode material used in the positive electrode is the carbon-coated lithium iron phosphate positive electrode material.

[0028] The organotin is selected from at least one of dibutyltin dilaurate, stannous octoate, dibutyltin maleate, and dibutyltin maleate.

[0029] The reaction mechanism is as follows:

[0030] The boron hydroxyl group (-B(OH)2) in 2-dihydroxyboron-3-thiophenecarboxylic acid undergoes a ring-opening esterification reaction with the epoxy group of epoxycyclohexyl cage polysilsesquioxane (POSS) under organotin catalysis to form a boron ester bond (-BOC-). The organotin catalyst activates the epoxy group through a Lewis acid mechanism, promoting ring opening.

[0031] Synergistic effects of the POSS skeleton:

[0032] The cage-like structure of epoxycyclohexyl-POSS provides multiple reaction sites. Its Si-O-Si backbone stabilizes the reaction intermediates through steric hindrance, while the dispersed borate ester bonds can enhance the intermolecular crosslinking density and improve thermal stability.

[0033] Beneficial effects of this invention:

[0034] 1. Improved electronic conductivity and enhanced rate performance: The carbon coating provides an electron transport channel and reduces electron migration resistance, thereby improving the material's performance under high-rate charge and discharge conditions.

[0035] 2. Enhanced structural stability and extended cycle life: High-temperature sintering promotes the perfection of crystal structure, reduces lattice defects, and improves the structural stability of materials, thereby extending the cycle life of batteries.

[0036] 3. Improved initial coulombic efficiency and enhanced electrochemical performance: High-purity raw materials and a uniform carbon coating reduce irreversible reactions during the initial charge and discharge process of the battery, thereby improving the initial coulombic efficiency and enhancing the overall electrochemical performance.

[0037] 4. Improved specific capacity to meet high energy density requirements: Uniform carbon coating and optimized crystal structure improve lithium-ion insertion and extraction efficiency, thereby increasing the specific capacity of the material and meeting the requirements of high energy density batteries.

[0038] 5. Boron-oxygen cluster active sites: The boron-oxygen cluster structure in the synergist provides multiple coordination sites, enabling specific adsorption of Li via Lewis acid-base interactions. + This reduces the lithium-ion migration energy barrier and improves charge transport efficiency. Cage structure reinforcement: The rigid framework of cage-shaped polysilsesquioxane is embedded in the carbon layer to form a "nanosupport", which effectively suppresses lattice volume expansion during charging and discharging and improves the structural stability of the material. Detailed Implementation

[0039] To facilitate understanding of the present invention, a more complete description is provided below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0040] Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Raw materials whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0042] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0043] Example 1

[0044] 1) Preparation of lithium iron phosphate precursor: Add 100g iron phosphate (FePO4), 20g lithium carbonate (purity 99.5%) and 10g glucose to 200g deionized water and stir to form a uniform slurry.

[0045] 2) Ball milling treatment: Add the above slurry into a ball mill jar, add 0.3% of polyethylene glycol-400 (polyethylene glycol with a molecular weight of 400) as a dispersant, and ball mill at 800 rpm for 8 hours.

[0046] 3) Drying treatment: The ball-milled slurry is spray-dried at a temperature of 180℃ to obtain dried powder.

[0047] 4) Preparation of energy density enhancer: 25g of 2-dihydroxyboryl-3-thiophenecarboxylic acid (CAS No.: 519054-53-6) and 2g of epoxycyclohexyl-cage-like polysilsesquioxane (CAS No.: 187333-74-0) were added to 250g of toluene. Nitrogen gas was introduced into the reactor, and 3g of dibutyltin dilaurate was added. The reaction was carried out at 105℃ for 6 hours. After the reaction was completed, toluene was removed by vacuum distillation, and then the mixture was dried under vacuum at 65℃ for 10 hours to obtain the energy density enhancer.

[0048] 5) Carbon Coating Treatment: The dried powder is heat-treated under a nitrogen atmosphere, heated to 800℃ at a heating rate of 5℃ / min, and held at that temperature for 12 hours to allow the carbon source to pyrolyze and form a conductive carbon layer. After cooling, an energy density enhancer (0.5% by mass of the heat-treated product) is added at 75℃, and the mixture is high-speed mixed for 30 minutes to obtain carbon-coated lithium iron phosphate cathode material.

[0049] Example 2

[0050] 1) Preparation of lithium iron phosphate precursor: Add 85g of iron phosphate (FePO4), 30g of lithium hydroxide (purity 99.2%) and 5g of sucrose to 245g of deionized water and stir to form a uniform slurry.

[0051] 2) Ball milling treatment: Add the above slurry into a ball mill jar, add 0.1% sodium dodecyl sulfate by weight of the slurry as a dispersant, and ball mill at 400 rpm for 12 hours.

[0052] 3) Drying treatment: The ball-milled slurry is spray-dried at a temperature of 150°C to obtain dried powder.

[0053] 4) Preparation of energy density enhancer: 18g of 2-dihydroxyboryl-3-thiophenecarboxylic acid and 4g of epoxycyclohexyl-cage polysilsesquioxane were added to 200g of toluene. Nitrogen gas was introduced into the reactor, and 4g of stannous octoate was added. The reaction was carried out at 100℃ for 7 hours. After the reaction was completed, toluene was removed by vacuum distillation, and then the mixture was dried under vacuum at 60℃ for 12 hours to obtain the energy density enhancer.

[0054] 5) Carbon Coating Treatment: The dried powder was heat-treated under an argon atmosphere, heated to 900℃ at a heating rate of 2℃ / min, and held at that temperature for 10 hours to allow the carbon source to pyrolyze and form a conductive carbon layer. After cooling, an energy density enhancer (0.65% by mass of the heat-treated product) was added at 70℃, and the mixture was high-speed mixed for 20 minutes to obtain carbon-coated lithium iron phosphate cathode material.

[0055] Example 3

[0056] 1) Preparation of lithium iron phosphate precursor: 115g of iron phosphate (FePO4), 15g of lithium acetate (purity 99.8%) and 15g of citric acid were added to 185g of deionized water and stirred to form a uniform slurry.

[0057] 2) Ball milling treatment: Add the above slurry into a ball mill jar, add 0.6% of the slurry mass of polyethylene glycol-400 as a dispersant, and ball mill at 1200 rpm for 4 hours.

[0058] 3) Drying treatment: The ball-milled slurry is spray-dried at a temperature of 200℃ to obtain dried powder.

[0059] 4) Preparation of energy density enhancer: 36g of 2-dihydroxyboryl-3-thiophenecarboxylic acid and 1g of epoxycyclohexyl-cage polysilsesquioxane were added to 300g of toluene. Nitrogen gas was introduced into the reactor, and 2g of dibutyltin maleate was added. The reaction was carried out at 110℃ for 5 hours. After the reaction was completed, toluene was removed by vacuum distillation, and then the mixture was dried under vacuum at 70℃ for 8 hours to obtain the energy density enhancer.

[0060] 5) Carbon Coating Treatment: The dried powder is heat-treated under a nitrogen atmosphere, heated to 700℃ at a heating rate of 10℃ / min, and held at that temperature for 14 hours to allow the carbon source to pyrolyze and form a conductive carbon layer. After cooling, an energy density enhancer (0.75% by mass of the heat-treated product) is added at 80℃, and the mixture is high-speed mixed for 40 minutes to obtain carbon-coated lithium iron phosphate cathode material.

[0061] Example 4

[0062] 1) Preparation of lithium iron phosphate precursor: 95g of iron phosphate (FePO4), 25g of lithium oxalate (purity 99.3%) and 8g of phenolic resin were added to 220g of deionized water and stirred to form a uniform slurry.

[0063] 2) Ball milling treatment: Add the above slurry into a ball mill jar, add 0.4% sodium dodecyl sulfate by weight of the slurry as a dispersant, and ball mill at 600 rpm for 10 hours.

[0064] 3) Drying treatment: The ball-milled slurry is spray-dried at a temperature of 170℃ to obtain dried powder.

[0065] 4) Preparation of energy density enhancer: 30g of 2-dihydroxyboryl-3-thiophenic acid and 3g of epoxycyclohexyl-cage polysilsesquioxane were added to 280g of toluene. Nitrogen gas was introduced into the reactor, and 3g of dibutyltin maleate was added. The reaction was carried out at 108℃ for 5.5 hours. After the reaction was completed, toluene was removed by vacuum distillation, and then the mixture was dried under vacuum at 68℃ for 9 hours to obtain the energy density enhancer.

[0066] 5) Carbon Coating Treatment: The dried powder was heat-treated under an argon atmosphere, heated to 750℃ at a heating rate of 7℃ / min, and held at that temperature for 13 hours to allow the carbon source to pyrolyze and form a conductive carbon layer. After cooling, an energy density enhancer (0.9% by mass of the heat-treated product) was added at 72℃, and the mixture was high-speed mixed for 35 minutes to obtain carbon-coated lithium iron phosphate cathode material.

[0067] Comparative Example 1

[0068] 1) Preparation of lithium iron phosphate precursor: Add 100g iron phosphate (FePO4), 20g lithium carbonate (purity 99.5%) and 10g glucose to 200g deionized water and stir to form a uniform slurry.

[0069] 2) Ball milling treatment: Add the above slurry into a ball mill jar, add 0.3% of the slurry mass of polyethylene glycol-400 as a dispersant, and ball mill at 800 rpm for 8 hours.

[0070] 3) Drying treatment: The ball-milled slurry is spray-dried at a temperature of 180℃ to obtain dried powder.

[0071] 4) Carbon coating treatment: The dry powder is heat-treated in a nitrogen atmosphere, heated to 800℃ at a heating rate of 5℃ / min, and held for 12 hours to obtain carbon-coated lithium iron phosphate cathode material.

[0072] Comparative Example 2

[0073] 2-Dihydroxyboryl-3-thiophenic acid was not added, and everything else was the same as in Example 1, because the reaction could not be carried out. In fact, the energy density enhancer was epoxycyclohexyl-cage polysilsesquioxane.

[0074] Comparative Example 3

[0075] Without adding epoxycyclohexyl-cage polysilsesquioxane, everything else is the same as in Example 1, because the reaction cannot proceed. In fact, the energy density enhancer is 2-dihydroxyboryl-3-thiophenic acid.

[0076] Detection Example 1

[0077] The testing method involved in this invention is as follows:

[0078] Electrochemical performance testing: CR2032 coin cells were assembled according to SJ / T 11793-2022 standard, and the specific capacity, cycle performance and rate performance of the cells were tested.

[0079] Cathode materials: lithium iron phosphate (LiFePO4) active material, conductive agent (acetylene black), binder (polyvinylidene fluoride PVDF).

[0080] Negative electrode material: lithium metal sheet, approximately 15.8 mm in diameter, must be handled in a glove box to avoid oxidation.

[0081] Separator: Polyethylene (PE) microporous membrane, approximately 16 mm in diameter.

[0082] Electrolyte: 1 mol / L LiPF6 dissolved in a mixed solvent of EC:DMC (1:1 volume ratio).

[0083] Other components: CR2032 battery casing (positive and negative electrode casing), gaskets, and springs.

[0084] The test results are shown in Table 1.

[0085] Table 1

[0086]

[0087] In summary, the carbon-coated lithium iron phosphate cathode material preparation method provided by this invention significantly improves the electrochemical performance of the material through reasonable raw material ratios and process parameter control, and has good application prospects.

Claims

1. A method for preparing a carbon-coated lithium iron phosphate anode material, characterized in that, The preparation method comprises the following steps: (1) adding iron phosphate, a lithium source and a carbon source into deionized water, stirring to obtain a slurry; (2) ball-milling the slurry and then performing spray drying to obtain dry powder; (3) heat-treating the dry powder in an inert atmosphere to make the carbon source pyrolyze to form a conductive carbon layer; (4) adding an energy density enhancer into the heat-treated product, mixing to obtain a carbon-coated lithium iron phosphate positive electrode material. In step (4), the preparation method of the energy density enhancer comprises the following steps: adding 2-dihydroxyboron-3-thiophene carboxylic acid, epoxy cyclohexyl cage polysilsesquioxane and organic tin into a solvent, reacting in an inert atmosphere, and then removing the solvent to obtain the energy density enhancer; when the energy density enhancer is prepared, the reaction temperature is 100-110 ℃, and the reaction time is 5-7 hours.

2. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and a carbon source to form a mixture; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. In step (1), the iron phosphate is 85-115 parts by mass, the lithium source is 15-30 parts by mass, the carbon source is 5-15 parts by mass, and the deionized water is 185-245 parts by mass.

3. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and carbon black to form a mixture; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. In step (2), a dispersant is added before ball-milling, and the added mass of the dispersant is 0.1%-0.6% of the slurry; The ball-milling time is 4-12 hours, and the ball-milling rotation speed is 400-1200 rpm; The spray drying temperature is 150-200 ℃; In step (3), the heat-treatment temperature is 700-900 ℃, and the heat-treatment time is 10-14 hours.

4. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and carbon black to form a mixture; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. 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 and phenolic resin.

5. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and carbon black; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. In step (4), the added amount of the energy density enhancer is 0.5%-0.9% of the heat-treated product; when added, the temperature is 70-80 ℃, and the mixing time is 20-40 minutes.

6. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and carbon black; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. The 2-dihydroxyboron-3-thiophene carboxylic acid is 18-36 parts by mass, the epoxy cyclohexyl cage polysilsesquioxane is 1-4 parts by mass, and the organic tin is 2-4 parts by mass.

7. The carbon-coated lithium iron phosphate positive electrode material prepared by the preparation method in any one of claims 1-6.

8. A lithium-ion battery, characterized by The battery comprises a positive electrode, and the positive electrode material used by the positive electrode is the carbon-coated lithium iron phosphate positive electrode material in claim 7.

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