A method for preparing carbon-coated lithium iron phosphate

Abstract

This invention provides a method for preparing carbon-coated lithium iron phosphate, comprising: S1: providing a lithium source solution, a ferrous source solution, and a phosphorus source solution; S2: mixing the lithium source solution, the ferrous source solution, and the phosphorus source solution to obtain a first solution, adjusting the pH of the first solution to alkaline, to obtain a second solution; S3: performing vacuum distillation on the second solution, during which an organic carbon source solution is added to the second solution until the volume of the reaction solution no longer decreases, to obtain a third solution; S4: heating the third solution to a preset temperature, maintaining the preset temperature, and introducing a coolant into the third solution to obtain a fourth solution; S5: drying the fourth solution to obtain the carbon-coated lithium iron phosphate. This method can reduce energy loss during the preparation process, reduce raw material costs, and improve the cycle life of lithium iron phosphate battery modules.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage material preparation technology, and in particular to a method for preparing carbon-coated lithium iron phosphate. Background Technology

[0002] Lithium iron phosphate (LiFePO4) possesses advantages such as high theoretical capacity, high operating voltage and suitable operating density, low self-discharge, long cycle life, and good cycle performance, making it an ideal cathode material for next-generation lithium-ion batteries. After years of development, lithium-ion batteries are now widely used in electronic devices such as mobile phones, cameras, and laptops, as well as in electric vehicles and hybrid vehicles.

[0003] Current methods for preparing lithium iron phosphate (LFP) primarily involve sand milling of raw materials, followed by spray drying and sintering. This process requires evaporating moisture from the reaction system, and due to the nature of the reaction, significant amounts of residual raw materials remain, reducing utilization and further increasing production costs. Furthermore, this method uses battery-grade raw materials, resulting in excessively high raw material costs. These issues contribute to the high cost of LFP energy storage. Additionally, the high-temperature conditions involved in LFP preparation also exacerbate energy storage costs and waste substantial amounts of energy. Moreover, the resulting lithium-ion batteries have a limited number of cycles, leading to excessively high calendar costs based on calendar life, and the cycle life is insufficient to meet future energy storage requirements.

[0004] Therefore, there is an urgent need to provide a method for preparing lithium iron phosphate to reduce energy loss during the preparation process, reduce raw material costs, and improve the cycle life of lithium iron phosphate battery modules. Summary of the Invention

[0005] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, this invention provides a method for preparing carbon-coated lithium iron phosphate. This method utilizes low-temperature crystallization technology to achieve sintering at a lower temperature, completing the material's shape and reducing sintering costs. Simultaneously, it employs a liquid-phase precipitation method, which can utilize raw materials with high impurities, avoiding the impact of impurities from the solid-phase reaction in traditional sand milling-spray drying-sintering processes on the performance of lithium iron phosphate.

[0006] Therefore, the first aspect of the present invention provides a method for preparing carbon-coated lithium iron phosphate, the method comprising the following steps:

[0007] S1: Provides lithium source solution, ferrous source solution and phosphorus source solution;

[0008] S2: The lithium source solution, ferrous source solution and phosphorus source solution are mixed to obtain a first solution, and the pH of the first solution is adjusted to alkaline to obtain a second solution;

[0009] S3: Perform vacuum distillation on the second solution. During this process, add an organic carbon source solution to the second solution until the volume of the reaction solution no longer decreases, to obtain the third solution.

[0010] S4: Heat the third solution to a preset temperature, maintain the preset temperature, and introduce a coolant into the third solution to obtain a fourth solution;

[0011] S5: The fourth solution is dried to obtain the carbon-coated lithium iron phosphate.

[0012] To address the shortcomings of existing technologies, this invention incorporates vacuum distillation into the preparation method of lithium iron phosphate. This allows for faster evaporation and removal of water from the system at lower temperatures, simultaneously removing dissolved oxygen and preventing oxidation that could affect the material's crystal purity. During this process, the continuous addition of an organic carbon source solution to the reaction solution not only prevents abnormal precipitation of solids after water reduction, ensuring the temperature stability of the entire reaction system, but also coats the lithium iron phosphate particles with a carbon layer, increasing their conductivity. Simultaneously, it prevents particle fusion that could lead to particle growth, resulting in the preparation of nanoscale lithium iron phosphate with higher lithium-ion migration rates. Furthermore, by introducing a coolant into the reaction solution at a predetermined temperature, an interface layer between ultra-high and ultra-low temperatures is created, which is beneficial for preparing highly uniform lithium iron phosphate particles. The porous structure between the particles allows for greater electrolyte retention, and the presence of pores also helps absorb stress during material cycling, preventing electrode pulverization.

[0013] According to an embodiment of the present invention, the lithium source solution, ferrous source solution and phosphorus source solution mentioned in step S1 are solutions containing lithium source compound, ferrous source compound and phosphorus source compound, respectively.

[0014] According to embodiments of the present invention, the lithium source compound includes at least one selected from lithium sulfate, lithium carbonate, lithium oxalate, lithium acetate, lithium hydroxide, lithium fluoride, and lithium chloride.

[0015] According to embodiments of the present invention, the ferrous source compound includes at least one selected from ferrous sulfate, ferrous nitrate, ferrous oxalate, ferrous acetate, and ferrous chloride.

[0016] According to embodiments of the present invention, the phosphorus source compound includes at least one selected from diammonium hydrogen phosphate, phosphoric acid, sodium hydrogen phosphate, lithium dihydrogen phosphate, ferrous ammonium phosphate, and ammonium dihydrogen phosphate.

[0017] According to an embodiment of the present invention, in step S2, the lithium source solution, ferrous source solution, and phosphorus source solution are mixed in an elemental molar ratio of lithium, iron, and phosphorus of (1-3):(0.9-1):1 to obtain a first solution. At this point, the raw materials are mixed at the atomic level, resulting in higher consistency and a significant improvement in the cycling stability of the material.

[0018] Preferably, in step S2, the molar ratio of lithium, iron and phosphorus is (1-3):1:1; more preferably, the molar ratio of lithium, iron and phosphorus is 1:1:1.

[0019] According to an embodiment of the present invention, step S2 further includes: adding an alkaline solution to the first solution to adjust the pH of the first solution to 7-11. This improves the utilization efficiency of raw materials and avoids the oxidation of ferrous iron.

[0020] According to an embodiment of the present invention, the alkaline solution includes at least one selected from ammonia, sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, and ammonium carbonate.

[0021] According to an embodiment of the present invention, the vacuum distillation operation in step S3 is carried out at 30-80°C. This ensures that the water in the system is evaporated and removed more quickly during the vacuum distillation operation, while also removing dissolved oxygen contained in the water, thus preventing oxygen from oxidizing the system.

[0022] According to embodiments of the present invention, the organic carbon source solution comprises at least one selected from ethylene glycol, diformate, benzyl alcohol, ethyl benzoate, isophorone, N-methylpyrrolidone, sulfolane, diphenyl ether, dodecane, and glycerol. This allows for the subsequent coating of a carbon layer onto the lithium iron phosphate particles, increasing the conductivity of the lithium iron phosphate while preventing particle fusion that could lead to particle growth.

[0023] According to an embodiment of the present invention, the preset temperature in step S4 is 100-180°C. This forms lithium iron phosphate crystals with a thermodynamically stable state.

[0024] According to an embodiment of the present invention, step S4 further includes heating the third solution to a preset temperature in a protective gas. This avoids the oxidation of the ferrous iron raw material.

[0025] According to an embodiment of the present invention, the protective gas is nitrogen, helium, neon, or argon.

[0026] According to an embodiment of the present invention, step S4 further includes: maintaining the preset temperature and introducing a coolant into the third solution for 1-10 hours to obtain a fourth solution.

[0027] According to an embodiment of the present invention, the coolant in step S4 includes at least one selected from liquid nitrogen, liquid helium, solid ethanol, dry ice, and solid acetone. This achieves the formation of an ultra-low temperature interface layer in the reaction solution.

[0028] According to an embodiment of the present invention, the drying process in step S5 is carried out under vacuum conditions at 200-450°C. This achieves the carbonization of the organic carbon source solution, obtaining a lithium iron phosphate material with carbon coating.

[0029] A second aspect of the present invention provides a positive electrode material for lithium-ion batteries, the positive electrode material comprising carbon-coated lithium iron phosphate obtained by the preparation method described in the first aspect.

[0030] A third aspect of the present invention provides a method for preparing a lithium-ion battery, comprising the following steps:

[0031] (1) Preparation of cathode materials;

[0032] The cathode material includes carbon-coated lithium iron phosphate, a conductive agent, and a binder;

[0033] The carbon-coated lithium iron phosphate is obtained by the preparation method described in the first aspect;

[0034] (2) Assemble the positive electrode material, lithium metal and electrolyte to obtain the lithium-ion battery.

[0035] The carbon-coated lithium iron phosphate material obtained by the first aspect of the present invention is nanoscale and has pores between particles, which can accommodate more electrolyte and bring better cycle performance during cycling. Using it in the preparation of lithium battery cathode material can bring better lithium ion insertion / extraction kinetics and obtain higher specific capacity.

[0036] According to an embodiment of the present invention, the mass ratio of carbon-coated lithium iron phosphate, conductive agent and binder in step (1) is 9:0.5:0.5.

[0037] According to an embodiment of the present invention, the conductive agent is acetylene black.

[0038] According to an embodiment of the present invention, the adhesive is polyvinylidene fluoride.

[0039] According to an embodiment of the present invention, the solute of the electrolyte in step (2) is lithium hexafluorophosphate.

[0040] According to an embodiment of the present invention, the solvent of the electrolyte is at least one selected from dimethyl carbonate, ethyl methyl carbonate, and ethylene carbonate.

[0041] The advantages of this invention over the prior art are:

[0042] (1) Reduce the raw material cost of lithium iron phosphate synthesis and improve the utilization efficiency of lithium iron phosphate raw materials.

[0043] Traditional lithium iron phosphate (LFP) performance is highly sensitive to impurity content, requiring battery-grade raw materials. This invention, however, can utilize non-battery-grade raw materials, effectively reducing LFP costs and offering significant advantages in raw material cost and sourcing compared to traditional processes, thus avoiding dependence on external suppliers for key raw materials. Furthermore, the use of low-temperature crystallization technology allows for sintering at lower temperatures to achieve material shaping, reducing sintering costs. Additionally, adjusting the pH of the first solution containing lithium, ferrous, and phosphorus sources to alkaline levels also improves raw material utilization efficiency.

[0044] (2) Reduce energy loss during the synthesis of lithium iron phosphate and save synthesis costs.

[0045] The preparation method provided by this invention requires a maximum temperature of 450°C, which is much lower than the sintering temperature (700-800°C) used in the prior art, thus reducing thermal energy costs.

[0046] (3) Improve the cycle life of lithium iron phosphate energy storage battery modules and reduce calendar costs.

[0047] The carbon-coated lithium iron phosphate preparation method provided by this invention employs a liquid-phase precipitation method, which can utilize raw materials with high impurity content and avoid the impact of impurities on the performance of lithium iron phosphate caused by traditional solid-phase reactions such as sand milling, spray drying, and sintering. Simultaneously, it can produce lithium iron phosphate particles with uniform particle size. The porosity between the particles increases the liquid retention capacity, and due to the high uniformity of the particles, the degree of loss of each particle is consistent during cycling, avoiding localized particle deterioration that reduces the overall cycling performance of the material.

[0048] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0049] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0050] Figure 1 The image shows a SEM image of the carbon-coated lithium iron phosphate sample prepared in Example 1 of this invention.

[0051] Figure 2 The image shows a SEM image of the carbon-coated lithium iron phosphate sample prepared in Comparative Example 4 of this invention. Detailed Implementation

[0052] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0053] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0054] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0055] To facilitate understanding of the invention, certain technical and scientific terms are specifically defined below. Unless otherwise expressly defined elsewhere in this document, all other technical and scientific terms used herein have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains.

[0056] In this document, the terms “comprising” or “including” are open-ended expressions, meaning that they include the contents specified in this invention, but do not exclude other aspects.

[0057] In this document, the terms “optionally,” “optionally,” or “optionally” generally refer to an event or condition that may, but may not, occur, and the description includes both cases in which the event or condition occurs and cases in which the event or condition does not occur.

[0058] According to embodiments of the present invention, a first aspect provides a method for preparing carbon-coated lithium iron phosphate, comprising the following steps:

[0059] S1: Provides lithium source solution, ferrous source solution and phosphorus source solution.

[0060] According to specific embodiments of the present invention, the lithium source solution, ferrous source solution, and phosphorus source solution are solutions containing lithium source compounds, ferrous source compounds, and phosphorus source compounds, respectively. Specifically, the source compounds can be dissolved in acidic solvents or organic solvents to prepare the source solutions. The lithium source compounds, ferrous source compounds, and phosphorus source compounds can be made from raw materials with lower purity, such as lithium salts extracted from salt lakes, iron salts from titanium dioxide byproducts, and fertilizer-grade phosphate salts, thus avoiding the difficulty of requiring battery-grade raw materials in the prior art. The lithium source compounds include, but are not limited to, lithium sulfate, lithium carbonate, lithium oxalate, lithium acetate, lithium hydroxide, lithium fluoride, and lithium chloride; the ferrous source compounds include, but are not limited to, ferrous sulfate, ferrous nitrate, ferrous oxalate, ferrous acetate, and ferrous chloride; and the phosphorus source compounds include, but are not limited to, diammonium hydrogen phosphate, phosphoric acid, sodium hydrogen phosphate, lithium dihydrogen phosphate, ferrous ammonium phosphate, and ammonium dihydrogen phosphate.

[0061] S2: Mix the lithium source solution, ferrous source solution and phosphorus source solution to obtain a first solution, and adjust the pH of the first solution to alkaline to obtain a second solution.

[0062] According to a specific embodiment of the present invention, the lithium source solution, ferrous source solution, and phosphorus source solution can be mixed according to a designed ratio, for example, with an elemental molar ratio of lithium, iron, and phosphorus of (1-3):(0.9-1):1. Here, the raw materials are mixed at the atomic level, which provides higher consistency and significantly improves the cycling stability of the material. Then, an alkaline solution can be added to the first solution to adjust its pH to alkaline, preferably 7-11, to improve the utilization efficiency of the raw materials. When the pH of the first solution is greater than 7, lithium, iron, and phosphorus exist in the solution as solid precipitates. As the pH of the solution increases, the number of hydroxide ions in the system increases, forming substances with hydroxide structures with lithium, iron, and phosphorus. This increase leads to a reaction with trace amounts of oxygen in the reaction system, oxidizing ferrous iron to ferric iron, which is detrimental to subsequent reactions. Therefore, if the pH of the first solution is adjusted too high, it will lead to the oxidation of ferrous iron. The alkaline solution can be selected from ammonia, sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, ammonium carbonate, etc.

[0063] S3: Perform vacuum distillation on the second solution. During this process, add an organic carbon source solution to the second solution until the reaction solution no longer decreases, to obtain the third solution.

[0064] According to a specific embodiment of the present invention, since trace amounts of oxygen still exist in the system, the oxidation degree will intensify as the temperature of the reaction system continues to rise, which is detrimental to subsequent reactions. Therefore, it is necessary to evaporate and remove the water in the system more quickly at a lower temperature, while also removing the dissolved oxygen contained in the water to prevent oxygen from oxidizing the system. Vacuum distillation can achieve the above objective and improve the crystal purity of the material. Simultaneously, evaporation at low temperature avoids high-temperature crystallization in the presence of water, preventing the formation of crystalline materials with an aqueous phase and avoiding the formation of impurity phases. The specific vacuum distillation temperature can be reasonably selected according to the preparation conditions, for example, 30-80℃.

[0065] During vacuum distillation, it is necessary to continuously replenish the organic carbon source solution, such as one or more of ethylene glycol, diformate, benzyl alcohol, ethyl benzoate, isophorone, N-methylpyrrolidone, sulfolane, diphenyl ether, dodecane, and glycerol, to prevent abnormal precipitation of solids after water loss and to ensure the temperature stability of the entire reaction system. Simultaneously, the organic carbon source solution can also coat the lithium iron phosphate particles with a carbon layer, increasing the conductivity of lithium iron phosphate while preventing particle growth caused by particle fusion, resulting in nanoscale particles with higher lithium-ion migration rates. The specific operating steps are as follows: During vacuum distillation, add the organic carbon source solution to maintain the liquid level in the distillation vessel at the same height. Continue vacuum distillation until the liquid level in the vessel no longer decreases, then stop adding the organic carbon source solution and stop the vacuum distillation process.

[0066] S4: Heat the third solution to a preset temperature, maintain the preset temperature, and introduce a coolant into the third solution to obtain a fourth solution.

[0067] According to a specific embodiment of the present invention, the third solution is heated. Under heating, the solid mixture in the reaction system forms lithium iron phosphate crystals with a thermodynamically stable state. However, the crystal morphology and state of these lithium iron phosphate crystals are uncontrolled, resulting in large and unevenly sized particles. By adding a coolant to the reaction system, such as liquid nitrogen, liquid helium, solid ethanol, dry ice, solid acetone, or one or more other substances, an interface layer between ultra-high and ultra-low temperatures is created during the reaction. The lithium iron phosphate particles near the interface undergo a rapid temperature change from ultra-high to ultra-low, resulting in significant internal stress. Simultaneously, the low-temperature substance collides with the high-temperature liquid, undergoes a phase transition, and becomes a gaseous substance, escaping from the system and forming numerous uniform bubbles. These bubbles collide with the particles, and the particles collide with each other, increasing the internal energy of the system. Under these numerous collisions, the energy within the lithium iron phosphate particles is released, causing the large lithium iron phosphate particles to shatter. Furthermore, due to the formation of numerous uniform bubbles during the low-temperature substance conversion, particles of suitable size enter the bubbles after shattering. The stability of the bubble's internal environment prevents further particle breakage, thereby improving material consistency. Meanwhile, because the bubbles produced by the decomposition of materials at low temperatures are spherical, there is a huge temperature difference between the inside and outside of the bubbles. This huge temperature difference causes the large particles that have entered the bubble locally to break apart. Combined with the high-intensity collision, this results in lithium iron phosphate particles that are highly uniform spherical. When these spherical particles are stacked in subsequent battery manufacturing, there are pores between the particles, which can hold more electrolyte and have a higher electrolyte retention capacity. At the same time, the presence of pores can absorb the stress of the material during the material cycling process, thus avoiding electrode pulverization.

[0068] According to a specific embodiment of the present invention, the third solution can be heated to 100-180°C in a protective gas atmosphere, maintained at this temperature, and a coolant can be continuously introduced into the third solution for 1-10 hours to obtain a fourth solution containing lithium iron phosphate. Nitrogen can be selected as the protective gas, and the heating temperature and reaction time can be adjusted appropriately according to the actual preparation conditions.

[0069] S5: The fourth solution is dried to obtain the carbon-coated lithium iron phosphate.

[0070] According to a specific embodiment of the present invention, the fourth solution can be washed and dried sequentially to obtain carbon-coated lithium iron phosphate. Specifically, the drying process can be carried out under vacuum conditions at 200-450°C to carbonize the organic carbon source solution and obtain carbon-coated lithium iron phosphate.

[0071] According to an embodiment of the present invention, a second aspect of the present invention provides a positive electrode material for lithium-ion batteries, the positive electrode material comprising carbon-coated lithium iron phosphate obtained by the preparation method described in the first aspect.

[0072] The carbon-coated lithium iron phosphate provided in this application has smaller particle size, more uniform carbon coating, does not involve high-temperature sintering, and has a crystal structure closer to the theory.

[0073] According to an embodiment of the present invention, a third aspect of the present invention provides a method for preparing a lithium-ion battery, comprising the following steps:

[0074] (1) Preparation of positive electrode material.

[0075] The cathode material includes carbon-coated lithium iron phosphate, a conductive agent, and a binder;

[0076] The carbon-coated lithium iron phosphate is obtained using the preparation method described in the first aspect.

[0077] According to a specific embodiment of the present invention, the mass ratio of carbon-coated lithium iron phosphate, conductive agent, and binder in step (1) can be 9:0.5:0.5. The conductive agent can be acetylene black, and the binder can be polyvinylidene fluoride.

[0078] (2) Assemble the positive electrode material, lithium metal and electrolyte to obtain the lithium-ion battery.

[0079] According to a specific embodiment of the present invention, the solute of the electrolyte may be lithium hexafluorophosphate. The solvent of the electrolyte may be at least one selected from dimethyl carbonate, ethyl methyl carbonate, and ethylene carbonate.

[0080] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0081] Example 1

[0082] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0083] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 1 mol of lithium sulfate solid to prepare a 1 mol / L solution; weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; weigh 2 mol of diammonium hydrogen phosphate to prepare a 2 mol / L solution;

[0084] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 8, and obtain a mixed solution containing solids.

[0085] S3: Place the mixed solution prepared in step S2 into a vacuum distillation container and carry out vacuum distillation at 40°C. At the same time, continuously add liquid ethylene glycol during the distillation process. Control the rate of addition of ethylene glycol during the distillation process to keep the liquid level at the same height. Continue vacuum distillation until the liquid volume inside no longer decreases, then stop adding ethylene glycol and stop vacuum distillation.

[0086] S4: The mixed solution obtained in step S3 is heated to 150°C under nitrogen protection. After the temperature reaches 150°C, liquid nitrogen is added to the mixed solution through a pipeline. At this time, a large number of bubbles will be generated in the reaction system. The temperature is maintained and the mixture is heated continuously for 5 hours. The synthesis of lithium iron phosphate is then completed.

[0087] S5: After washing and drying the mixed solution obtained in step S4, it is vacuum dried at 450°C and the surface organic solvent is carbonized to obtain a lithium iron phosphate cathode material with carbon coating.

[0088] Sample SEM image as follows Figure 1 As shown, the lithium iron phosphate cathode material sample is generally spherical, with some porosity between the stacked particles. This porosity can accommodate more electrolyte, resulting in better cycle performance during cycling. The particle size distribution is around 100 nm. The nanoscale lithium iron phosphate cathode material can bring better lithium-ion insertion / extraction kinetics, achieving higher specific capacity.

[0089] Example 2

[0090] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0091] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 2 mol of industrial grade lithium carbonate solid, add an appropriate amount of nitric acid to dissolve it, and add an appropriate amount of deionized water to prepare a 2 mol / L solution; Weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; Weigh 2 mol of phosphoric acid to prepare a 2 mol / L solution.

[0092] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 7, and obtain a mixed solution containing solids.

[0093] S3: Place the mixed solution prepared in step S2 into a vacuum distillation container and carry out vacuum distillation at 40°C. At the same time, continuously add liquid ethylene glycol during the distillation process. Control the rate of addition of ethylene glycol during the distillation process to keep the liquid level at the same height. Continue vacuum distillation until the liquid volume inside no longer decreases, then stop adding ethylene glycol and stop vacuum distillation.

[0094] S4: Heat the mixed solution obtained in step S3 to 150°C under nitrogen protection. After the temperature reaches 150°C, add liquid nitrogen to the mixed solution through a pipeline. At this time, a large number of bubbles will be generated in the reaction system. Continue heating for 5 hours while maintaining the temperature. The synthesis of lithium iron phosphate is completed.

[0095] S5: After washing and drying the mixed solution obtained in step S4, it is vacuum dried at 450°C and the surface organic solvent is carbonized to obtain a lithium iron phosphate cathode material with carbon coating.

[0096] Example 3

[0097] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0098] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 2 mol of industrial grade lithium carbonate solid, add an appropriate amount of nitric acid to dissolve it, and add an appropriate amount of deionized water to prepare a 2 mol / L solution; Weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; Weigh 2 mol of phosphoric acid to prepare a 2 mol / L solution.

[0099] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 7, and obtain a mixed solution containing solids.

[0100] S3: Place the mixed solution prepared in step S2 into a vacuum distillation vessel and carry out vacuum distillation at 80°C. At the same time, continuously add liquid glycerol during the distillation process. Control the rate of addition of glycerol during the distillation process to keep the liquid level at the same height. Continue vacuum distillation until the liquid volume inside no longer decreases, then stop adding glycerol and stop vacuum distillation.

[0101] S4: The mixed solution obtained in step S3 is heated to 150°C under nitrogen protection. After the temperature reaches 150°C, liquid nitrogen is added to the mixed solution through a pipeline. At this time, a large number of bubbles will be generated in the reaction system. The temperature is maintained and the mixture is heated continuously for 5 hours. The synthesis of lithium iron phosphate is then completed.

[0102] S5: After washing and drying the mixed solution obtained in step S4, it is vacuum dried at 450°C and the surface organic solvent is carbonized to obtain a lithium iron phosphate cathode material with carbon coating.

[0103] Example 4

[0104] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0105] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 2 mol of industrial grade lithium carbonate solid, add an appropriate amount of nitric acid to dissolve it, and add an appropriate amount of deionized water to prepare a 2 mol / L solution; Weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; Weigh 2 mol of phosphoric acid to prepare a 2 mol / L solution.

[0106] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 7, and obtain a mixed solution containing solids.

[0107] S3: Place the mixed solution prepared in step S2 into a vacuum distillation container and carry out vacuum distillation at 40°C. At the same time, continuously add liquid ethylene glycol during the distillation process. Control the rate of addition of ethylene glycol during the distillation process to keep the liquid level at the same height. Continue vacuum distillation until the liquid volume inside no longer decreases, then stop adding ethylene glycol and stop vacuum distillation.

[0108] S4: The mixed solution obtained in step S3 is heated to 100°C under nitrogen protection. After the temperature reaches 100°C, liquid nitrogen is added to the mixed solution through a pipeline. At this time, a large number of bubbles will be generated in the reaction system. The temperature is maintained and the mixture is heated continuously for 10 hours. The synthesis of lithium iron phosphate is then completed.

[0109] S5: After washing and drying the mixed solution obtained in step S4, it is vacuum dried at 450°C and the surface organic solvent is carbonized to obtain a lithium iron phosphate cathode material with carbon coating.

[0110] Comparative Example 1

[0111] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0112] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 1 mol of lithium sulfate solid to prepare a 1 mol / L solution; weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; weigh 2 mol of diammonium hydrogen phosphate to prepare a 2 mol / L solution;

[0113] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1 and mix them evenly to obtain a mixed solution;

[0114] S3: Place the mixed solution prepared in step S2 into a vacuum distillation vessel and perform vacuum distillation at 40°C. Simultaneously, continuously add liquid ethylene glycol during the distillation process, controlling the rate of addition to maintain a constant liquid level. Continue vacuum distillation until the liquid volume no longer decreases, then stop adding ethylene glycol. After stopping vacuum distillation, a small amount of precipitate will appear in the solution, and the supernatant will turn light blue.

[0115] S4: The mixed solution obtained in step S3 is heated to 150°C under nitrogen protection. After the temperature reaches 150°C, liquid nitrogen is added to the mixed solution through a pipeline. At this time, a large number of bubbles will be generated in the reaction system. The temperature is maintained and the mixture is heated continuously for 5 hours. The synthesis of lithium iron phosphate is then completed.

[0116] S5: After washing and drying the mixed solution obtained in step S4, it is vacuum dried at 450°C and the surface organic solvent is carbonized to obtain a lithium iron phosphate cathode material with carbon coating.

[0117] Comparative Example 2

[0118] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0119] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 1 mol of lithium sulfate solid to prepare a 1 mol / L solution; weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; weigh 2 mol of diammonium hydrogen phosphate to prepare a 2 mol / L solution;

[0120] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 8, and obtain a mixed solution containing solids.

[0121] S3: The mixed solution prepared in step S2 was distilled at 100℃. Ethylene glycol was continuously added during the distillation process. The rate of addition of ethylene glycol was controlled to keep the liquid level at the same height. Due to the high temperature, there was still some dissolved oxygen in the water. The high temperature distillation caused the solid precipitate to oxidize. The solid substance changed from gray to brownish-brown, and the experiment failed.

[0122] S4: After washing and drying the solid sample obtained in step S3, it was vacuum dried at 450℃ to obtain a brownish-red solid powder.

[0123] Therefore, it can be seen that when atmospheric pressure high-temperature distillation is used in the vacuum distillation step, the reaction system is too hot and contains a large amount of water. The ferrous ions react with the trace dissolved oxygen in the water and are oxidized to ferric ions, causing the experiment to fail.

[0124] Comparative Example 3

[0125] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0126] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 1 mol of lithium sulfate solid to prepare a 1 mol / L solution; weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; weigh 2 mol of diammonium hydrogen phosphate to prepare a 2 mol / L solution;

[0127] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 8, and obtain a mixed solution containing solids.

[0128] S3: Place the mixed solution prepared in step S2 in a vacuum distillation container and perform vacuum distillation at 40°C until the water evaporates. During the evaporation process, the volume of the mixed solution continuously decreases, and the solid matter in the solution becomes viscous. As the evaporation continues, the solid matter begins to solidify, resulting in uneven heating. As the water continues to decrease, the part in contact with the air is oxidized, and the solid matter changes from gray to brownish-brown. The experiment fails.

[0129] S4: After washing and drying the solid sample obtained in step S3, it was vacuum dried at 450℃ to obtain a brownish-red solid powder.

[0130] Therefore, it can be seen that if ethylene glycol organic solvent is not added in the vacuum distillation step, the water in the reaction system will evaporate, causing the solid powder to be exposed to the air. At the same time, as the water decreases, it becomes difficult to disperse the solid material, resulting in uneven heating and oxidation of the reaction, leading to experimental failure.

[0131] Comparative Example 4

[0132] A method for preparing carbon-coated lithium iron phosphate includes the following steps:

[0133] S1: Preparation of lithium source solution, ferrous source solution and phosphorus source solution: Weigh 1 mol of lithium sulfate solid to prepare a 1 mol / L solution; weigh 2 mol of ferrous sulfate solid to prepare a 2 mol / L solution; weigh 2 mol of diammonium hydrogen phosphate to prepare a 2 mol / L solution;

[0134] S2: According to the elemental molar ratio of lithium, iron and phosphorus in the solution being 1:1:1, take 100ml of the lithium, iron and phosphorus solution prepared in step S1, mix them evenly, add an appropriate amount of ammonia water to adjust the pH to 8, and obtain a mixed solution containing solids.

[0135] S3: Place the mixed solution prepared in step S2 into a vacuum distillation container and carry out vacuum distillation at 40°C. At the same time, continuously add liquid ethylene glycol during the distillation process. Control the rate of addition of ethylene glycol during the distillation process to keep the liquid level at the same height. Continue vacuum distillation until the liquid volume inside no longer decreases, then stop adding ethylene glycol and stop vacuum distillation.

[0136] S4: The mixed solution obtained in step S3 is heated to 150°C under nitrogen protection. After the temperature reaches 150°C, it is heated continuously for 5 hours while maintaining the temperature. The synthesis of lithium iron phosphate is then completed.

[0137] S5: After washing and drying the mixed solution obtained in step S4, it is vacuum dried at 450°C and the surface organic solvent is carbonized to obtain a lithium iron phosphate cathode material with carbon coating.

[0138] Sample SEM image as follows Figure 2 As shown in Example 1, this comparative example did not add liquid nitrogen during the heating reaction stage, which resulted in the particles failing to undergo effective collision and internal stress release during the reaction, leading to uneven particle size distribution. Consequently, the SEM images of the prepared lithium iron phosphate samples showed lithium iron phosphate particles with different particle sizes and poor consistency.

[0139] Example 5

[0140] The carbon-coated lithium iron phosphate samples obtained in Examples 1-4 and Comparative Examples 1 and 4 were assembled into lithium-ion batteries, respectively, using the following methods:

[0141] The positive electrode sheet is composed of carbon-coated lithium iron phosphate positive electrode material, acetylene black, and polyvinylidene fluoride (mass ratio 9:0.5:0.5). The positive electrode slurry is mixed at a concentration of 3–4 mg / cm³. 2 The CR2025 coin-shaped battery was assembled in an Ar atmosphere glove box by coating the current collector aluminum foil with a density of [missing information - likely a specific coating material]. Lithium metal was used as the negative electrode, and the electrolyte was a 1M lithium hexafluorophosphate solution with a solvent of DMC (dimethyl carbonate) / EMC (ethyl methyl carbonate) / EC (ethylene carbonate) (volume ratio 1:1:1). The electrochemical performance of the material was studied using the Xinwei Battery Testing System at different rates (1C = 170 mAh / g), and the results are shown in Table 1.

[0142] Table 1 Electrochemical performance of lithium-ion batteries in each group

[0143]

[0144] The 0.1C capacity data is obtained by testing at a current density of 17 mA / g. The data shows the specific capacity of the prepared lithium iron phosphate at this current density. The higher the value, the better the electrical performance of the battery made from the material.

[0145] The 25℃ 500-cycle retention data is tested under constant temperature conditions of 25℃ with a current density of 170mA / g. The data shows the capacity retention rate of the prepared lithium iron phosphate after 500 charge-discharge cycles at this current density. The higher the value, the better the cycle stability of the battery made from the material.

[0146] The 60℃ 168h storage data is obtained by placing the battery in a constant temperature environment of 60℃ for 168 hours. Before placement, the battery capacity is fully charged at a current density of 170mA / g. After placement, the battery is discharged at a current density of 170mA / g. The discharge capacity is compared with the charging capacity to obtain the 60℃ 168h storage data. The higher the value, the better the calendar storage performance of the battery made of the material.

[0147] Therefore, it can be seen that the batteries prepared using the lithium iron phosphate samples obtained in Examples 1-4 have better electrical performance, cycle stability, and calendar storage performance than the samples prepared in Comparative Examples 1 and 4, indicating that:

[0148] (1) Replacing the battery-grade raw materials with industrial-grade raw materials of lower purity can also produce high-performance lithium iron phosphate cathode materials, indicating that the preparation method provided by the present invention has strong visibility and can effectively save the synthesis cost of lithium iron phosphate (corresponding to Example 2).

[0149] (2) By increasing the vacuum distillation temperature to 80°C, high-performance lithium iron phosphate cathode materials can be prepared (corresponding to Example 3);

[0150] (3) The synthesis temperature is reduced to 100℃, which can produce high-performance lithium iron phosphate cathode materials. The lower temperature synthesis saves heating costs (corresponding to Example 4).

[0151] Meanwhile, in Comparative Example 1, the absence of an alkaline solution to adjust the pH of the reaction system led to incomplete precipitation of raw materials, resulting in waste and increased costs. Furthermore, the incomplete precipitation caused an imbalance in the elemental composition of the synthesized material, leading to decreased battery performance. In Comparative Example 4, the lack of a coolant during the heating reaction stage resulted in significant differences in the particle size of the obtained lithium iron phosphate particles, further degrading battery performance. All of the above demonstrates that the lithium iron phosphate preparation method provided by this invention is more advantageous.

[0152] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0153] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing carbon-coated lithium iron phosphate, characterized in that, Includes the following steps: S1: Provides lithium source solution, ferrous source solution and phosphorus source solution; S2: The lithium source solution, ferrous source solution and phosphorus source solution are mixed to obtain a first solution, and the pH of the first solution is adjusted to alkaline to obtain a second solution; S3: Perform vacuum distillation on the second solution. During this process, add an organic carbon source solution to the second solution until the volume of the reaction solution no longer decreases, to obtain a third solution. The organic carbon source solution includes at least one selected from ethylene glycol, diformate, benzyl alcohol, ethyl benzoate, isophorone, N-methylpyrrolidone, sulfolane, diphenyl ether, dodecane, and glycerol. S4: Heat the third solution to a preset temperature, maintain the preset temperature, and introduce a coolant into the third solution to obtain a fourth solution; The coolant mentioned in step S4 includes at least one selected from liquid nitrogen, liquid helium, solid ethanol, dry ice, and solid acetone. S5: The fourth solution is dried to obtain the carbon-coated lithium iron phosphate. The drying process described in step S5 is carried out under vacuum conditions at 200-450°C.

2. The preparation method according to claim 1, characterized in that, The lithium source solution, ferrous source solution, and phosphorus source solution mentioned in step S1 are solutions containing lithium source compound, ferrous source compound, and phosphorus source compound, respectively. Optionally, the lithium source compound includes at least one selected from lithium sulfate, lithium carbonate, lithium oxalate, lithium acetate, lithium hydroxide, lithium fluoride, and lithium chloride; Optionally, the ferrous source compound includes at least one selected from ferrous sulfate, ferrous nitrate, ferrous oxalate, ferrous acetate, and ferrous chloride; Optionally, the phosphorus source compound includes at least one selected from diammonium hydrogen phosphate, phosphoric acid, sodium hydrogen phosphate, lithium dihydrogen phosphate, ferrous ammonium phosphate, and ammonium dihydrogen phosphate.

3. The preparation method according to claim 1, characterized in that, In step S2, the lithium source solution, ferrous source solution and phosphorus source solution are mixed to obtain a first solution with an elemental molar ratio of lithium, iron and phosphorus of (1~3): (0.9~1):

1. Optionally, step S2 further includes: adding an alkaline solution to the first solution to adjust the pH of the first solution to 7-11; Optionally, the alkaline solution includes at least one selected from ammonia, sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, and ammonium carbonate.

4. The preparation method according to claim 1, characterized in that, The vacuum distillation operation described in step S3 is carried out at 30-80°C.

5. The preparation method according to claim 1, characterized in that, The preset temperature mentioned in step S4 is 100-180℃; Optionally, step S4 further includes: heating the third solution to a preset temperature in a protective gas; Optionally, the protective gas is nitrogen, helium, neon, or argon; Optionally, step S4 further includes: maintaining the preset temperature and introducing a coolant into the third solution for 1-10 hours to obtain a fourth solution.

6. A positive electrode material for lithium-ion batteries, characterized in that, The positive electrode material for the lithium-ion battery comprises carbon-coated lithium iron phosphate obtained by the preparation method according to any one of claims 1-5.

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