Method for preparing carbon-coated lithium iron phosphate anode material based on straw and retired lithium iron phosphate recovery material

Abstract

The application discloses a method for preparing carbon-coated lithium iron phosphate positive electrode material based on straw and retired lithium iron phosphate recovery material, and belongs to the technical field of new energy lithium ion battery waste resource recycling. The method is based on the black powder obtained from the retired lithium iron phosphate battery, and the purified leaching solution is obtained after impurity removal by acid leaching, and then the lithium iron phosphate preparation mother liquor or the lithium iron phosphate, lithium carbonate recovery material obtained by step-by-step precipitation, and the activated biochar or straw-based ordered mesoporous carbon obtained based on the straw; the lithium iron phosphate recovery material and the biomass material carbon source are jointly converted, so that the lithium iron phosphate positive electrode material is prepared by recycling multiple sources of waste resources.

Description

Technical Field

[0001] This invention relates to a green synthesis method for battery materials, and more particularly to a method for preparing carbon-coated lithium iron phosphate cathode materials based on straw and recycled retired lithium iron phosphate materials, belonging to the field of resource recycling technology for retired lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries possess significant advantages such as high energy density and long cycle life, making them the most widely used power source for new energy vehicles. Based on battery lifespan projections, my country is about to enter a phase of large-scale retirement of its power batteries. Improper disposal of retired lithium-ion batteries will pose a threat to human health and sustainable environmental development. Furthermore, power batteries contain key metal resources such as lithium; efficient recycling of these resources will reduce waste and effectively alleviate resource shortages.

[0003] Lithium iron phosphate (LFP) batteries are a representative type of power battery for new energy vehicles. Due to their robust bonding structure, LFP batteries exhibit excellent stability and safety, making them widely used in large electric vehicles, hybrid electric vehicles, and some low-cost small electric vehicles.

[0004] Retired lithium iron phosphate (LFP) batteries are primarily recycled for the production of LFP cathode materials, with main processes including high-temperature solid-state methods, hydrothermal methods, and spray drying methods. Among these, the carbothermal reduction method, a derivative of the high-temperature solid-state method, offers advantages such as low raw material costs and simple processing. Furthermore, the introduction of a carbon source forms a carbon coating on the LFP surface, effectively addressing the low conductivity of LFP batteries and improving their electrochemical performance and cycle life.

[0005] Research on the carbothermic reduction synthesis of lithium iron phosphate cathode materials generally uses glucose as a carbon source, which is mixed with lithium carbonate or lithium hydroxide and iron phosphate and calcined. In addition, other organic materials such as citric acid, sucrose, starch, and polymers such as polyacrylic acid, polyvinyl alcohol, and polypropyleneamine can also be used as carbon sources for the synthesis of lithium iron phosphate cathode materials. Based on this, for the recycling of retired lithium iron phosphate batteries, most research uses carbothermic reduction synthesis raw materials where only the lithium and iron precursors are recycled products, while the carbon source is a chemical raw material. If waste biomass is used as a carbon source, it is possible to achieve the regeneration of multiple waste resources to prepare lithium iron phosphate cathode materials.

[0006] In recent years, an increasing amount of biomass has been directly used in the preparation of lithium-ion battery materials, including eggs, plant leaves, and animal shells. This is because biomass, as a carbon source, is relatively inexpensive and more environmentally friendly compared to chemical raw materials. However, much biomass remains underutilized. As an important biomass resource, most straw is not properly developed and utilized, but rather discarded or burned, resulting in resource waste.

[0007] Existing lithium iron phosphate (LFP) synthesis methods mostly use chemical raw materials. Even when biomass is used as the carbon source, the lithium or iron precursors are not recycled materials. Furthermore, although many biomasses have the potential to serve as carbon sources, different biomasses vary significantly in their microscopic physical structure and elemental composition. This means that the specific processing technology and biomass composition used in LFP synthesis have a significant impact on the electrochemical performance of the final LFP cathode material. This invention focuses on the natural physical properties of biomass and recycled LFP materials. Through appropriate processing, carbon-coated LFP is prepared to limit the disordered growth of LFP particles, forming a regular ion transport pathway, thereby enhancing its electrochemical performance. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing carbon-coated lithium iron phosphate cathode materials based on straw and recycled retired lithium iron phosphate materials. This method co-converts the recycled retired lithium iron phosphate materials and straw to achieve the goal of regenerating lithium iron phosphate cathode materials from multiple sources of waste, thereby reducing carbon emissions during the recycling process of retired lithium batteries and solving the problem of low utilization rate of high-value straw.

[0009] The technical solution adopted by this invention to solve its technical problem is: A method for preparing carbon-coated lithium iron phosphate cathode materials based on straw and decommissioned lithium iron phosphate recycled materials includes the following steps: The retired lithium iron phosphate cells are discharged, crushed, sieved, and roasted to obtain black powder. The black powder was leached with dilute sulfuric acid, and ascorbic acid was added in an amount equal to the amount of iron in the black powder to inhibit the oxidation of iron during the leaching process, and finally a leachate containing lithium, aluminum, phosphorus, iron and copper was obtained. The leachate was purified by removing copper with sodium sulfide, removing aluminum by hydrolysis precipitation, and removing sodium by freezing. Straw is pretreated by one or more of the following methods: baking, activation, and carbonization to obtain biochar; the resulting biochar is activated biochar or straw-based ordered mesoporous carbon. The mother liquor for preparing lithium iron phosphate is obtained from the purified leachate. It is then combined with the straw-based ordered mesoporous carbon via a hydrothermal synthesis method to obtain carbon-coated lithium iron phosphate cathode material. Alternatively, the purified leachate is precipitated stepwise to obtain iron phosphate and lithium carbonate, respectively. The activated biochar and the recovered iron phosphate and lithium carbonate are then subjected to mechanical ball milling and high-temperature calcination to obtain carbon-coated lithium iron phosphate cathode material.

[0010] In the above technical solution, furthermore, the purification of the leachate by removing copper with sodium sulfide, removing aluminum by hydrolysis precipitation, and removing sodium by freezing is specifically as follows: Sodium sulfide is added to the leachate to make the molar ratio of sodium sulfide to copper in the leachate 3:1. The reaction is carried out at room temperature to remove the precipitate and achieve copper removal. Add 3 mol / L sodium hydroxide solution dropwise to the leaching solution after copper removal to adjust the pH value to 3.7, thereby achieving aluminum precipitation.

[0011] The leachate obtained after aluminum removal was frozen, and sodium sulfate decahydrate was separated by crystallization; the freezing temperature was -2℃ and the freezing time was 15h to achieve sodium removal; Furthermore, the straw is one or more of rice straw, corn straw, wheat straw, and highland barley straw.

[0012] Furthermore, using straw as raw material, an activator is added for pyrolysis. The mass ratio of activator to straw is 1-4:1. The activator is either ferric chloride or phosphoric acid. During pyrolysis, the nitrogen purging flow rate is 50-500 ml / min, the heating rate is 3-10 °C / min, the pyrolysis holding temperature is 600-900 °C, and the time is 30-120 min, resulting in activated biochar. In this invention, the activation in the pretreatment step weakens the original binding force in the biochar and forms pores. The activated biochar has mesopores and micropores, while unactivated biochar only has pure micropores, thereby improving the ion diffusion of the lithium iron phosphate cathode material. The pore structure of the biochar can improve the conductivity and ion diffusion of the carbon-coated lithium iron phosphate cathode material. Potassium hydroxide, as the most widely used activator, has a good activation effect, but it has the problems of high acid washing dosage and significant equipment corrosion in subsequent steps. This invention prefers ferric chloride or phosphoric acid as the activator, which can obtain a relatively excellent pore structure and avoid introducing new cationic impurities. During high-temperature pyrolysis, the presence of biochar / carbon creates a confined space, reducing the excessive growth and agglomeration of lithium iron phosphate particles, and improving the conductivity of the cathode material by increasing the contact between particles.

[0013] In addition, before adding the activator for pyrolysis, the straw is first roasted to obtain roasted material. Specifically, the straw is placed in a tube furnace and heated to 200 °C at a heating rate of 5 °C / min under a nitrogen protective atmosphere of 50 ml / min, and held for 120 min to obtain roasted material. This roasting treatment can selectively remove components with low thermal stability from the straw, release moisture, reduce volatile matter, and increase carbon yield, forming a synergy with subsequent high-temperature pyrolysis, resulting in a larger specific surface area and better pore structure of biochar. On the other hand, it can also weaken the connection between hemicellulose and cellulose, making the straw fully brittle, which is more conducive to the full integration of the two in the subsequent mechanical ball milling and high-temperature roasting co-conversion process with recycled materials, thereby improving the performance of the obtained cathode material.

[0014] Furthermore, the method for preparing the straw-based ordered mesoporous carbon includes the following: S1: Preparation of straw lignin: Straw is placed in a Soxhlet extractor, ethanol is added, a condenser is installed, and the mixture is refluxed at 100°C to extract straw lignin. S2: Preparation of lignin oligomers: Using an alkaline catalytic process, lignin is added to an aqueous sodium hydroxide solution and stirred. Formaldehyde solution is added dropwise, and the temperature is raised to react the lignin with formaldehyde. The resulting oligomers are then prepared into an aqueous solution. S3: Preparation of ordered mesoporous carbon: The template agent is added to the aqueous solution of lignin oligomer, stirred, and the resulting mixture is dried and then carbonized under an argon atmosphere to obtain lignin-based ordered mesoporous carbon.

[0015] Furthermore, the sodium hydroxide aqueous solution has a mass fraction of 5%, the stirring temperature is 40°C, the formaldehyde solution has a mass fraction of 37%, the heating temperature is 70°C, and the oligomer is prepared into an aqueous solution with a mass fraction of 20%.

[0016] Furthermore, after obtaining biochar, it can be first subjected to acid washing to remove impurities, including: soaking the biochar in hydrochloric acid under water bath heating conditions, filtering, washing with water until the solution is neutral, and vacuum drying for more than 12 hours; the water bath heating temperature is 50~90 ℃, the hydrochloric acid concentration is 1~5M, and the acid leaching time is 1~12 hours.

[0017] Furthermore, the specific method for preparing the lithium iron phosphate preparation mother liquor is as follows: ferrous sulfate, phosphoric acid, and lithium hydroxide are added to the purified leachate, and Fe... 2+ The concentration was adjusted to 0.5 M, and the Li:Fe:P molar ratio was made to 2:1:1 to obtain the mother liquor for lithium iron phosphate preparation.

[0018] Furthermore, the hydrothermal synthesis method specifically involves: dispersing straw-based ordered mesoporous carbon in deionized water, ultrasonically treating it to obtain a black suspension; adding it to the lithium iron phosphate preparation mother liquor, with the addition amount at a mass ratio of mesoporous carbon to expected LFP cathode material of 8:100, wherein the mass of the expected LFP cathode material is based on Fe in the lithium iron phosphate preparation mother liquor; heating the resulting mixture to 200°C in a high-pressure reactor for 6 hours, filtering and drying it to obtain a solid powder; finally, calcining the solid powder at 200°C for 6 hours to obtain lithium iron phosphate coated with ordered mesoporous carbon.

[0019] Further, the mechanical ball milling and high-temperature calcination specifically involves: weighing the recovered lithium carbonate and iron phosphate at a Li:Fe molar ratio of 1~1.05, placing them in a ball mill jar, adding activated biochar at an amount of 10~27 wt.% of the iron phosphate, and then dripping an appropriate amount of anhydrous ethanol into the ball mill jar; mixing uniformly by wet milling; immediately after ball milling, placing the ball mill jar and the mixture together in a vacuum drying oven for vacuum drying; then transferring the dried mixture to a graphite crucible and calcining it at a high temperature of 650~800 ℃ under a nitrogen atmosphere for 7~12 h; and obtaining carbon-coated lithium iron phosphate after natural cooling. More preferably, the anhydrous ethanol added to the ball mill jar is 1~1.5 times the total mass of the solid raw materials; the ball milling frequency is 3~30 Hz; the ball milling time is 1~10 h; and the vacuum drying temperature is 60~80 ℃ for 2~12 h. During high-temperature calcination, the flow rate of nitrogen gas introduced is 50~200 ml / min, and the heating rate is 5~10 ℃ / min.

[0020] The beneficial effects of this invention are: (1) In the method of this invention, activated biochar or straw-based ordered mesoporous carbon is formed by specific pretreatment of straw, which is then co-converted with lithium iron phosphate recycled material. This method effectively increases the mesopore and micropore structure of biochar, which can reduce the excessive growth and agglomeration of lithium iron phosphate particles during high-temperature roasting. At the same time, it can effectively adsorb the residual trace heteroatoms in the recycled material after specific impurity removal treatment, forming a continuous conductive network, effectively enriching the ion diffusion channels of lithium iron phosphate cathode material, enhancing its conductivity and ion diffusion. Moreover, the controllable residual heteroatoms can also reduce the lithium ion transport energy barrier through lattice distortion and lithium vacancies, improving the material stability and capacity characteristics. The solution of this invention can effectively solve the problem of performance degradation of recycled materials.

[0021] (2) This invention uses biomass resources with low utilization rate as carbon source. Compared with high-purity chemical raw materials (such as glucose, citric acid, etc.), the cost of waste biomass resources is relatively low. Applying it to lithium-ion battery cathode materials can effectively utilize waste resources and achieve the goal of regenerating lithium iron phosphate cathode materials from multiple waste resources.

[0022] (3) The recycled material in this invention contains a certain amount of Al impurities, because Al 3+ Radius smaller than Fe 2+ and Fe 3+Al can induce local lattice distortion, altering the energy levels of lattice regions and thus increasing lithium-ion diffusion channels. Simultaneously, Al can introduce lithium vacancies into the crystal, lowering the lithium-ion transport energy barrier and improving electrochemical performance. It also reduces the cell volume, increases internal stability, and enhances capacity characteristics. When the amount of Al reaches a certain level, it can balance material performance and stability. This not only solves the impurity problem associated with using recycled materials as raw materials but also utilizes the characteristics of heteroions to improve the capacity characteristics of regenerated LFP cathode materials. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 This is a process flow example of an element recovery method for full-element leaching and impurity removal in lithium iron phosphate batteries, wherein lithium carbonate and iron phosphate recycled materials are obtained by this method; Figure 2 This is a schematic diagram of the process flow for preparing carbon-coated lithium iron phosphate cathode material by mixing straw and activator through impregnation in Embodiment 2 of the present invention, followed by pyrolysis activation and carbothermal reduction. Figure 3 These are the first charge-discharge curves of batteries made from lithium iron phosphate cathode materials prepared according to the methods in Examples 1-4 of this invention. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below through specific embodiments. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0026] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art.

[0027] Lithium, iron, and phosphorus are mainly obtained from the recycling of retired lithium iron phosphate battery cells, such as... Figure 1 As shown, the present invention obtains recycled material from decommissioned lithium iron phosphate through the following processing: The retired lithium iron phosphate cells are discharged, crushed, sieved, and roasted to obtain black powder. The black powder was leached with dilute sulfuric acid, and ascorbic acid was added in an amount equal to the amount of iron in the black powder to inhibit the oxidation of iron during the leaching process, and finally a leachate containing lithium, aluminum, phosphorus, iron and copper was obtained. The leachate was purified by removing copper with sodium sulfide, removing aluminum by hydrolysis precipitation, and removing sodium by freezing. The mother liquor for preparing lithium iron phosphate is obtained from the purified leachate, or the purified leachate is precipitated stepwise to obtain iron phosphate and lithium carbonate respectively. In some embodiments of the present invention, the specific steps are as follows: 1. Retired lithium iron phosphate cells are immersed in a 10% sodium chloride solution for 72 hours to ensure complete discharge; 2. The retired lithium iron phosphate cells after immersion discharge are placed into a dual-shaft shredder for crushing. After the cells are crushed and disassembled, the crushed materials of the whole battery components are obtained, mainly including the positive and negative electrode black powder of lithium iron phosphate batteries, crushed shell, separator, conductive agent, binder, current collector, etc., and the diameter of the crushed material is no more than 12 mm. 2. The crushed material of the entire battery cell is not screened and is placed in a steel strip furnace for roasting. The temperature is controlled at 600-650℃ and the roasting time is 120-240 min. The oxygen content in the roasting section is not greater than 0.3 vol%, and the remainder is nitrogen. 3. The calcined battery cell components were sieved using a sieve with a mesh size greater than 400 to obtain battery cell black powder and calcination residue calcined under nitrogen atmosphere. ICP-MS analysis of the calcined black powder revealed that it contained 2.25 wt% lithium, 8.57 wt% phosphorus, 14.23 wt% iron, 0.58 wt% aluminum, and 3.02 wt% copper, with the remainder being oxygen, carbon, and other trace elements.

[0028] 4. Acid leaching: The roasted black powder is added to 1 mol / L dilute sulfuric acid, with a solid-liquid ratio of black powder to dilute sulfuric acid of 120 g / L; at the same time, ascorbic acid with an equimolar amount of iron is added to inhibit the oxidation of iron during the leaching process; the black powder and sulfuric acid mixture is stirred and reacted in a reaction vessel at 35°C, and the solution is taken out after 180 minutes to obtain a leaching solution containing lithium, aluminum, phosphorus, iron and copper; 5. Copper removal by sulfide: Sodium sulfide is added to the leachate at a molar ratio of 3:1 to copper. The resulting copper sulfide is insoluble in water at room temperature, and almost all copper is removed as a precipitate. 6. Aluminum removal by hydrolysis precipitation: Add 3 mol / L sodium hydroxide solution to the leachate after copper removal to adjust the pH of the solution to 3.7, thereby precipitating aluminum and removing most of the aluminum. 7. Sodium removal by freezing: Utilizing the principle that the solubility of sodium sulfate decreases with decreasing temperature, a freezing separation process is designed. The leachate is frozen, and sodium sulfate decahydrate is crystallized and separated. The freezing temperature is -2℃, and the freezing time is 15 hours, resulting in the removal of sodium sulfate. The purified leachate is divided into two parts. One part of the purified leachate is used to remove Fe by adding ferrous sulfate, phosphoric acid, and lithium hydroxide. 2+ The concentration was adjusted to 0.5 M, and the Li:Fe:P molar ratio was made to 2:1:1 to obtain the mother liquor for lithium iron phosphate preparation; another portion was used for recycling to prepare iron phosphate and lithium carbonate, as follows: 8. To precipitate iron and phosphorus, hydrogen peroxide solution is slowly added dropwise. The molar ratio of hydrogen peroxide to iron is 1:2. This oxidizes the ferrous iron in the solution, and the iron ions combine with phosphate ions to form a precipitate. The precipitate is stirred continuously at 90°C for 3 hours to form a white precipitate. After filtration, washing, and drying, a white powder is obtained. This powder is then calcined at 600°C for 4 hours to recover ferric phosphate. 9. To precipitate lithium, heat the lithium-containing leachate and evaporate and concentrate it at 95°C. Then add sodium carbonate with a sodium carbonate:lithium molar ratio of 1.2:1. After sufficient precipitate is formed, filter the solution, wash the precipitate with deionized water, dry it at 60°C, and recover lithium carbonate.

[0029] After obtaining the recycled lithium iron phosphate material from decommissioned lithium iron phosphate plants, it is then co-converted with biochar derived from straw to prepare carbon-coated lithium iron phosphate cathode material. Specifically, the following method can be used to prepare it: Example 1 A method for preparing ordered mesoporous carbon-coated lithium iron phosphate cathode material from straw and decommissioned lithium iron phosphate recycled materials, the method comprising the following steps: S1: Preparation of straw lignin: Wheat straw was placed in a Soxhlet extractor, ethanol was added, a condenser was installed, and the mixture was refluxed at 100°C for 4-8 hours. Then, 5% nitric acid was added to adjust the pH to 2-5. The mixture was then centrifuged at 3000-6000 rpm for 5 minutes and washed with water until the pH reached 7. Finally, it was dried at 120°C for 3-5 hours to obtain straw lignin. S2: Preparation of lignin oligomers: Lignin was reacted with formaldehyde via an alkaline catalytic process. 10 g of lignin was added to 70 g of a 5% sodium hydroxide aqueous solution, and the mixture was stirred at 40°C for 10 minutes. Then, 16 g of a 37% formaldehyde solution was slowly added dropwise, and the mixture was heated to 70°C and reacted for 60 minutes. The pH was adjusted to 5 by adding 1% nitric acid solution. The resulting oligomers were then prepared as a 20% aqueous solution. S3: Preparation of ordered mesoporous carbon: 3 g of Pluronic F127 template agent was added to an aqueous solution containing 40 g of lignin oligomers (20%). After stirring for 2 hours, the resulting brown mixture was dried at 50°C for 24 hours, followed by raising the temperature to 120°C and maintaining it for 24 hours. Finally, the mixture was heated to 600°C under an argon atmosphere at a heating rate of 1°C / min and carbonized at this temperature for 3 hours to obtain lignin-based ordered mesoporous carbon.

[0030] S4: Disperse ordered mesoporous carbon in deionized water at a ratio of 1:60 and sonicate for 2 hours to obtain a black suspension. Then, add this suspension to the lithium iron phosphate (LFP) preparation mother liquor, with the addition amount calculated based on a mesoporous carbon / LFP cathode material mass ratio of 8:100. Heat the mixture to 200°C in a high-pressure reactor and maintain for 6 hours. After filtration, dry in a 50°C drying oven for 8 hours to obtain a solid powder. Finally, place the solid powder in a tube furnace and calcine at 200°C for 6 hours to obtain ordered mesoporous carbon-coated lithium iron phosphate.

[0031] Example 2 A method for preparing carbon-coated lithium iron phosphate from straw, lithium carbonate, and recycled iron phosphate materials is shown in the schematic diagram below. Figure 2 The steps of this method are as follows: S1: High-temperature roasting of wheat straw. Dry straw is placed in a tube furnace and heated to 200°C at a heating rate of 5°C / min under a nitrogen protective atmosphere of 50 ml / min. The temperature is held for 120 min to obtain the roasted material. S2: The roasting material is ground and then mixed with the activator. The roasting material is soaked in a 1 mol / L H3PO4 solution, with a straw to H3PO4 solution mass ratio of 1:2. The mixture is stirred in a 90 ℃ water bath until the solution evaporates to dryness; then it is ground evenly in an agate mortar. S3: Biochar preparation. The mixed sample was transferred to a nickel reaction boat, placed in a tube furnace, heated to 600 °C at a heating rate of 10 °C / min under a nitrogen atmosphere of 50 ml / min and held for 90 min. After cooling to room temperature, biochar was obtained. S5: Carbon / lithium / iron precursors are mixed in a specific ratio. The recovered lithium carbonate and iron phosphate materials are weighed according to the molar ratio of Li:Fe=1.02, placed in a ball mill jar, and 20 wt% of biochar of the raw material iron phosphate is added.

[0032] S6: Ball milling. Add anhydrous ethanol, 1.1 times the solid mass, to the ball mill jar. Set the ball milling frequency to 3 Hz and mill for 1 hour to mix the materials uniformly using wet milling. After ball milling, immediately place the ball mill jar and the mixture into a vacuum drying oven and vacuum dry at 70 ℃ for 5 hours. S7: High-temperature preparation of carbon-coated lithium iron phosphate cathode material. After drying, the mixed powder was removed and placed in a graphite crucible. The graphite crucible was placed in a tube furnace, and nitrogen gas was introduced into the tube furnace at a flow rate of 50 ml / min. The temperature was increased to 750 °C at a rate of 10 °C / min and maintained at this temperature for calcination for 12 h. After that, it was allowed to cool naturally to obtain carbon-coated lithium iron phosphate.

[0033] Example 3 A method for preparing carbon-coated lithium iron phosphate from straw, lithium carbonate, and recycled iron phosphate materials, comprising the following steps: S1: Phosphoric acid impregnated straw. Wheat straw was crushed and passed through a 60-mesh sieve, dried in the air at room temperature, and then impregnated in a 1 mol / L H3PO4 solution. The mass ratio of straw to H3PO4 solution was 1:2. The mixture was stirred in a 90 ℃ water bath until the solution evaporated to dryness. S2: Straw pyrolysis activation. The impregnated sample was transferred to a nickel crucible and placed in a tube furnace. Under nitrogen protection at a rate of 100 ml / min, the temperature was increased to 600 °C at a rate of 5 °C / min and held for 60 min. The sample was then allowed to cool naturally to room temperature to obtain biochar.

[0034] S4: Carbon / lithium / iron precursors are mixed in a specific ratio. The recovered lithium carbonate and iron phosphate materials are weighed according to the molar ratio of Li:Fe=1.05, placed in a ball mill jar, and 20 wt% of biochar of the raw material iron phosphate is added.

[0035] S5: Ball milling. Add anhydrous ethanol dropwise to the ball mill jar at 1.3 times its solid mass. Set the ball milling frequency to 30 Hz and mill for 10 hours to mix the materials uniformly using wet milling. After ball milling, immediately place the ball mill jar and the mixture into a vacuum drying oven and vacuum dry at 60°C for 3 hours. S6: High-temperature preparation of carbon-coated lithium iron phosphate cathode material. After drying, the mixed powder was removed and placed in a graphite crucible. The graphite crucible was placed in a tube furnace, and nitrogen gas was introduced into the tube furnace at a flow rate of 100 ml / min. The temperature was increased to 700 °C at a rate of 5 °C / min and maintained at this temperature for 10 h. After that, it was allowed to cool naturally to obtain carbon-coated lithium iron phosphate.

[0036] Example 4 A method for preparing carbon-coated lithium iron phosphate by pyrolysis of straw with lithium carbonate and iron phosphate recycled materials, the method comprising the following steps: S1: Iron chloride solution impregnates straw. Wheat straw is crushed, passed through a 60-mesh sieve, and dried in the air at room temperature. 1 mol / L FeCl3 solution is added, with a straw to FeCl3 solution mass ratio of 1:2. The mixture is stirred in a 90 ℃ water bath until the solution evaporates to dryness. S2: Straw pyrolysis activation. The impregnated sample was transferred to a nickel reaction boat and placed in a tube furnace. Under nitrogen protection at a rate of 100 ml / min, the temperature was increased to 600 °C at a rate of 5 °C / min and held for 60 min. The sample was then allowed to cool naturally to room temperature to obtain biochar.

[0037] S4: Carbon / lithium / iron precursors are mixed in a specific ratio. The recovered lithium carbonate and iron phosphate materials are weighed according to the molar ratio of Li:Fe=1.05, placed in a ball mill jar, and 20 wt% of biochar of the raw material iron phosphate is added.

[0038] S5: Ball milling. Add anhydrous ethanol dropwise to the ball mill jar at 1.3 times its solid mass. Set the ball milling frequency to 30 Hz and mill for 10 hours to mix the materials uniformly using wet milling. After ball milling, immediately place the ball mill jar and the mixture into a vacuum drying oven and vacuum dry at 60°C for 3 hours. S6: High-temperature preparation of carbon-coated lithium iron phosphate cathode material. After drying, the mixed powder was removed and placed in a graphite crucible. The graphite crucible was placed in a tube furnace, and nitrogen gas was introduced into the tube furnace at a flow rate of 100 ml / min. The temperature was increased to 800 °C at a rate of 5 °C / min and maintained at this temperature for 10 h. After that, it was allowed to cool naturally to obtain carbon-coated lithium iron phosphate.

[0039] The specific surface area of ​​the biochar obtained in each embodiment, the Al content in the obtained cathode material, and the specific capacity are shown in the table below. Figure 3 The first charge-discharge curves are for batteries made from cathode materials prepared based on the various embodiments. This invention utilizes low-utilization biomass resources as a carbon source, combined with recycled materials from retired lithium iron phosphate batteries, to jointly convert and apply them to lithium-ion battery cathode materials. This effectively utilizes waste resources and achieves the goal of regenerating lithium iron phosphate cathode materials from multiple waste sources. The scheme employs activated biochar or straw-based ordered mesoporous carbon, exhibiting multiple functions and achieving multifunctional effects. First, its reducing properties can replace the reducing agent in the carbothermic reduction method, reducing iron in iron phosphate from trivalent to divalent. Second, it can simultaneously achieve carbon coating of lithium iron phosphate particles, realizing both reduction and coating effects in one step. Third, the numerous mesopores and micropores in the obtained biochar increase the specific surface area of ​​the material, enriching the ion diffusion channels of the lithium iron phosphate cathode material; thereby enhancing the conductivity and ion diffusion of the lithium iron phosphate cathode material and improving its electrochemical performance. Furthermore, compared to traditional methods that easily lead to excessive growth and agglomeration of lithium iron phosphate particles, resulting in performance degradation, this invention uses porous biochar obtained through a specific method as a carbon source, which can reduce particle agglomeration, optimize inter-particle contact, improve electronic conductivity, and enhance rate performance. Existing technologies, such as using polymer materials as coating materials, only produce a simple coating layer, which is not very effective in preventing the aggregation of lithium iron phosphate particles. Furthermore, the biochar used in this invention is inferior in terms of physicochemical properties such as specific surface area and pore structure.

[0040] Unlike traditional methods that strictly remove impurities, this invention retains a certain amount of impurities such as Al in the recycled material. This fully utilizes the potential positive effects of Al and other impurities on the crystal structure and electrochemical performance. 3+ Radius smaller than Fe 2+ and Fe 3+ Al can cause local lattice distortion, altering the energy levels of lattice regions and thus increasing lithium-ion diffusion channels. Simultaneously, Al can introduce lithium vacancies into the crystal, lowering the lithium-ion transport energy barrier and improving electrochemical performance. It also reduces the cell volume, increases internal stability, and enhances capacity characteristics. When the amount of Al reaches a certain level, it can balance material performance and stability. This not only solves the impurity problem associated with using recycled materials as raw materials but also utilizes the characteristics of heteroions to improve the capacity characteristics of lithium iron phosphate cathode materials.

[0041] Each embodiment in this specification focuses on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to in the method section.

[0042] The method for preparing carbon-coated lithium iron phosphate cathode material from straw and decommissioned lithium iron phosphate recycled materials provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing carbon-coated lithium iron phosphate cathode material based on straw and decommissioned lithium iron phosphate recycled materials, characterized in that, Includes the following steps: The retired lithium iron phosphate cells are discharged, crushed, sieved, and roasted to obtain black powder. The black powder was leached with dilute sulfuric acid, and ascorbic acid was added in an amount equal to the amount of iron in the black powder to inhibit the oxidation of iron during the leaching process, and finally a leachate containing lithium, aluminum, phosphorus, iron and copper was obtained. The leachate was purified by removing copper with sodium sulfide, removing aluminum by hydrolysis precipitation, and removing sodium by freezing. Straw is pretreated by one or more of the following methods: baking, activation, and carbonization to obtain biochar; the resulting biochar is activated biochar or straw-based ordered mesoporous carbon. The mother liquor for preparing lithium iron phosphate is obtained from the purified leachate. It is then combined with the straw-based ordered mesoporous carbon via a hydrothermal synthesis method to obtain carbon-coated lithium iron phosphate cathode material. Alternatively, the purified leachate is precipitated stepwise to obtain iron phosphate and lithium carbonate, respectively. The activated biochar and the recovered iron phosphate and lithium carbonate are then subjected to mechanical ball milling and high-temperature calcination to obtain carbon-coated lithium iron phosphate cathode material.

2. The method according to claim 1, characterized in that, The process of removing copper with sodium sulfide, removing sodium by freezing, and removing aluminum by hydrolysis precipitation yields a purified leachate, specifically: Sodium sulfide is added to the leachate to make the molar ratio of sodium sulfide to copper in the leachate 3:

1. The reaction is carried out at room temperature to remove the precipitate and achieve copper removal. Add sodium hydroxide aqueous solution dropwise to the leachate after sodium removal to adjust the pH value to 3.7, thereby achieving aluminum precipitation; The leachate obtained after copper removal was frozen, and sodium sulfate decahydrate was separated by crystallization. The freezing temperature was -2℃ and the freezing time was 15h to achieve sodium removal.

3. The method according to claim 1, characterized in that, The straw is one or more of the following: rice straw, corn straw, wheat straw, and highland barley straw.

4. The method according to claim 1, characterized in that, Using straw as raw material, an activator is added for pyrolysis. The mass ratio of activator to straw is 1~4:

1. The activator is ferric chloride or phosphoric acid. During the pyrolysis process, the nitrogen purging flow rate is 50~500 ml / min, the heating rate is 3~10 ℃ / min, the pyrolysis holding temperature is 600~900 ℃, and the time is 30~120 min to obtain activated biochar.

5. The method according to claim 1, characterized in that, The method for preparing the straw-based ordered mesoporous carbon includes the following: S1: Preparation of straw lignin: Straw is placed in a Soxhlet extractor, ethanol is added, a condenser is installed, and the mixture is refluxed at 100°C to obtain straw lignin; S2: Preparation of lignin oligomers: Using an alkaline catalytic process, lignin is added to an aqueous sodium hydroxide solution and stirred. Formaldehyde solution is added dropwise, and the temperature is raised to react the lignin with formaldehyde. The resulting oligomers are then prepared into an aqueous solution. S3: Preparation of ordered mesoporous carbon: The template agent is added to the aqueous solution of lignin oligomer, stirred, and the resulting mixture is dried and then carbonized under an argon atmosphere to obtain lignin-based ordered mesoporous carbon.

6. The method according to claim 1, characterized in that, The sodium hydroxide aqueous solution has a mass fraction of 5%, the stirring temperature is 35~45℃, the formaldehyde solution has a mass fraction of 37%, the heating temperature is 65~75℃, and the oligomer is prepared into an aqueous solution with a mass fraction of 20%.

7. The method according to claim 1, characterized in that, After obtaining biochar, it is subjected to acid washing to remove impurities, including: soaking the biochar in hydrochloric acid under water bath heating, filtering, washing with water until the solution is neutral, and vacuum drying for more than 12 hours; the water bath heating temperature is 50~90 ℃, the hydrochloric acid concentration is 1~5M, and the acid leaching time is 1~12 hours.

8. The method according to claim 1, characterized in that, The specific method for preparing the lithium iron phosphate preparation mother liquor is as follows: ferrous sulfate, phosphoric acid, and lithium hydroxide are added to the purified leachate, and Fe... 2+ The concentration was adjusted to 0.5 M, and the Li:Fe:P molar ratio was made to 2:1:1 to obtain the mother liquor for lithium iron phosphate preparation.

9. The method according to claim 1, characterized in that, The hydrothermal synthesis method specifically involves: dispersing straw-based ordered mesoporous carbon in deionized water, ultrasonically treating it to obtain a black suspension; adding it to the lithium iron phosphate preparation mother liquor, with the addition amount at a mass ratio of mesoporous carbon to expected LFP cathode material of 8:100, wherein the mass of the expected LFP cathode material is based on Fe in the lithium iron phosphate preparation mother liquor; heating the resulting mixture to 200°C in a high-pressure reactor for 6 hours, filtering and drying it to obtain a solid powder; and finally calcining the solid powder at 200°C for 6 hours to obtain lithium iron phosphate coated with ordered mesoporous carbon.

10. The method according to claim 1, characterized in that, The mechanical ball milling and high-temperature calcination process is as follows: The recovered lithium carbonate and iron phosphate are weighed at a molar ratio of Li:Fe = 1~1.05 and placed in a ball mill jar. Activated biochar is added at a rate of 10~27 wt.% of the iron phosphate. Then, an appropriate amount of anhydrous ethanol is added dropwise to the ball mill jar. The mixture is then wet-milled until homogeneous. After ball milling, the ball mill jar and the mixture are immediately placed in a vacuum drying oven for vacuum drying. The dried mixture is then transferred to a graphite crucible and calcined at a high temperature of 650~800 ℃ under a nitrogen atmosphere for 7~12 h. After natural cooling, carbon-coated lithium iron phosphate is obtained.

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