Lithium iron phosphate positive electrode material recovery method and positive electrode material prepared by same

Abstract

The invention relates to a method for recycling a scrapped lithium iron phosphate battery cell by a physical method, and relates to the technical field of lithium iron phosphate recycling, and the method comprises the following steps: collecting the scrapped lithium iron phosphate battery cell, disassembling the lithium iron phosphate battery cell, collecting a positive plate, washing the collected positive plate with water, and drying in an inert gas atmosphere; shearing the dried positive plate into small pole pieces, and sintering the small pole pieces in an inert atmosphere; placing the sintered small pole pieces in a drum screen to peel off coarse lithium iron phosphate particles, and crushing and screening the coarse lithium iron phosphate particles to obtain lithium iron phosphate powder; adding the obtained lithium iron phosphate powder into a carboxymethyl cellulose solution, fully stirring and mixing, carrying out spray drying, and calcining in an inert atmosphere; and demagnetizing and screening the calcined lithium iron phosphate powder to obtain the positive electrode material. The positive electrode material prepared by the invention has a relatively good capacity retention ratio after circulation.

Description

Technical Field

[0001] The present application relates to the technical field of lithium battery cell recycling, and in particular to a method for recycling and reusing positive electrode sheets of lithium iron phosphate battery cells by physical methods and a positive electrode material obtained by the method. Background Art

[0002] Lithium iron phosphate batteries are a high-energy-density electrode material that is also safe because it contains no heavy metals. With the widespread use of lithium-ion batteries, the number of scrapped lithium iron phosphate cells has gradually increased. These scrapped cells contain a large amount of materials, such as lithium metal, electrodes, and electrolyte. However, due to the lack of efficient recycling methods, these resources are often wasted or cause environmental pollution.

[0003] Currently, the primary treatment for scrapped lithium iron phosphate batteries is chemical recycling, which extracts metallic lithium and other valuable materials from scrapped cells through acid-base dissolution and solvent extraction. However, this process is complex, involves the use of numerous chemicals, and carries the risk of environmental pollution and safety hazards.

[0004] In response to the above-mentioned related technologies, people will adopt physical recycling methods to replace chemical recycling methods. Invention patent application publication number CN115939553A discloses a dry-wet mixed physical recycling method for lithium iron phosphate battery positive electrode sheet scraps. Through physical methods, the active substances on the electrode sheets are separated into water and then dehydrated, dried, calcined in an oxygen-free state, crushed and demagnetized to finally obtain lithium iron phosphate positive electrode material powder; but since this method is only applicable to lithium iron phosphate electrode materials that have not yet generated electricity, for used scrapped electrodes, the electrode materials obtained by this method will have too low electrode activity and poor performance due to reasons such as changes in the lithium iron phosphate lattice or aggregation of the electrode materials.

[0005] Invention patent application publication number CN116632398A discloses a method for regenerating waste lithium iron phosphate battery positive electrode materials, which comprises mixing waste lithium iron phosphate powder with lithium carboxymethyl cellulose, and then calcining the mixture at 600°C-700°C for 10-15 hours in an Ar / H2 mixed gas atmosphere, and then cooling the mixture to obtain regenerated lithium iron phosphate powder. However, this method directly mixes the lithium iron phosphate powder with the lithium carboxymethyl cellulose for calcination. Although this method can replenish the lithium source, it is prone to uneven distribution, resulting in uneven coating of the lithium iron phosphate by the carbon layer produced by the lithium carboxymethyl cellulose. Lithium ions cannot quickly enter and exit the electrode material through the carbon layer, which leads to low ion conductivity. After multiple cycles, the electrode capacity decays severely and the performance of the electrode material is poor. Summary of the Invention

[0006] In order to solve the problem of poor performance of electrode materials produced after the recycling of scrapped lithium iron phosphate batteries, the present application provides a physical recycling and reuse method for scrapped lithium iron phosphate batteries, which can make the recycled products have better electrode performance.

[0007] In the first aspect, the present application provides a physical recycling and reuse method for scrapped lithium iron phosphate batteries using the following technical solutions: A method for recovering lithium iron phosphate cathode material, comprising: Stripping coarse lithium iron phosphate particles from the sintered small pieces of the positive electrode sheet, crushing and sieving the coarse lithium iron phosphate particles to obtain lithium iron phosphate powder, wherein the particle size of the lithium iron phosphate powder ranges from 0.9 to 20 μm; Immersing and suspending the lithium iron phosphate powder in an organic solution, spray drying, and calcining under an inert atmosphere to form a uniform carbon layer on the surface of the lithium iron phosphate powder, wherein the viscosity of the organic solution is in the range of 3500-4500 mPa.s; The calcined lithium iron phosphate powder is demagnetized and sieved to obtain a positive electrode material, wherein the thickness of the carbon layer on the surface of the positive electrode material is 30-400 nm.

[0008] By adopting the above technical solution, since the ionic conductivity of the lithium iron phosphate lattice structure electrode is low, it is necessary to reduce the particle size of the lithium iron phosphate powder so that the lithium iron phosphate powder has a larger specific surface area, which is beneficial to improving the conductivity of ions in the lithium iron phosphate lattice, thereby improving the conductivity of the electrode. In addition, the smaller particle size allows more lithium iron phosphate lattices to be utilized during the charge and discharge process, avoiding some lithium ions trapped inside the lattice from being unable to participate in the charge and discharge process, which is beneficial to improving the compaction density of the lithium iron phosphate electrode, thereby improving the energy density of the electrode. The water-insoluble lithium iron phosphate powder is suspended in an organic solvent with a higher viscosity, and the shear stress of the high-viscosity solution is used to offset the effect of gravity, so that the lithium iron phosphate powder can be better dispersed in the organic solvent. In a short period of time, the lithium iron phosphate powder can remain dispersed in the organic solvent without coagulation, which is conducive to the formation of lithium iron phosphate particles coated with a uniform organic layer through subsequent spray drying, and then after calcination, a uniform and dispersed lithium iron phosphate positive electrode material coated with a carbon layer can be formed; spray drying is carried out by atomizing the organic solvent in which the lithium iron phosphate powder is suspended, so that it is fully exposed to hot air and the water in the mixture is rapidly evaporated to obtain lithium iron phosphate powder coated with an organic layer. Since the drying speed of spray drying is fast, the organic solution is subject to gravity flow for a short time and can be evenly wrapped on the surface of the lithium iron phosphate powder. Moreover, since it is dried after atomization, the organic layer can cover every part as much as possible. A fine lithium iron phosphate powder can form a more uniform carbon layer on the surface of the lithium iron phosphate during the subsequent calcination operation, and the carbon layer can cover the relatively fine lithium iron phosphate powder. Since the ion pass rate of the lithium iron phosphate lattice is low, its own electrical conductivity is poor. Therefore, it is necessary to cover the surface of the lithium iron phosphate with a carbon layer to increase the ion pass rate of the lithium iron phosphate, thereby obtaining better electrical conductivity and improving the charge and discharge performance of the electrode. By evenly covering the surface of the lithium iron phosphate powder with a carbon layer, on the one hand, lithium iron phosphate powder of all coarseness and fineness can be covered with the carbon layer, making full use of the electrical conductivity of the carbon layer and greatly improving the electrical conductivity. On the other hand, the lithium iron phosphate powder can be effectively dispersed, so that lithium ions can be more easily transmitted in the lithium iron phosphate lattice, and the probability of electrode material aggregation during battery use can be reduced, thereby slowing down the rate of battery capacity decline and helping to extend the service life of the electrode. During the high-temperature carbonization of the organic layer of the lithium iron phosphate positive electrode material, the carbon layer will form magnetic covalent bonds with magnetic moments when some free electrons form covalent bonds, and the positive electrode material will finally have some magnetism. These magnetic forces will generate repulsive forces between the positive electrode material powders, resulting in the difficulty of processing the magnetic powders in the subsequent electrode manufacturing process, causing magnetic traces and other phenomena, which in turn leads to a decline in electrode performance. Therefore, demagnetization treatment is required to ensure the normal progress of subsequent electrode manufacturing; screening can discharge defective products that have accumulated during the calcination process. Only the edge parts of the accumulated defective products can participate in the charging and discharging process, but they occupy a larger volume, and the various performance of the electrode will be reduced as a result.

[0009] Impurities such as binders are removed by sintering to increase the recovery rate of lithium iron phosphate. After the lithium iron phosphate is crushed, it is mixed with a carboxymethyl cellulose solution and then spray-dried so that the surface of the lithium iron phosphate powder can be evenly and as much as possible covered with lithium iron phosphate powder of different particle sizes, completing the preparation of the positive electrode material. This preparation method does not require the use of a large amount of chemical reagents, avoiding the harmful waste and environmental pollution problems caused by the chemical treatment process; the operation does not involve complex chemical reactions and high-temperature and high-pressure treatments, reducing safety risks during the operation; the operation is simple and suitable for large-scale production and application; and because the lithium iron phosphate is highly dispersed in the positive electrode material and the carbon layer is evenly distributed, the prepared positive electrode material has good ion conductivity and thus good electrode performance.

[0010] Optionally, also include: Obtaining a positive electrode sheet from a lithium iron phosphate battery cell, washing it with water and drying it in an inert gas atmosphere to remove the electrolyte on the surface of the positive electrode sheet, and then cutting it into small pieces; Sintering the small electrode piece at 450-550° C. in an inert atmosphere to remove the adhesive; Coarse lithium iron phosphate particles are peeled off from the small electrode pieces using a drum screen, and the coarse lithium iron phosphate particles are crushed and sieved to obtain lithium iron phosphate powder. The particle size of the lithium iron phosphate powder is in the range of 0.9-20 μm.

[0011] By adopting the above technical solution, since the positive electrode sheets in scrapped batteries may be attached with electrode liquid or other impurities due to long-term use, it is necessary to first wash away the washable substances on the positive electrode sheets by water washing, and then dry the positive electrode sheets to preliminarily dry out the moisture on the positive electrode sheets. After drying, the positive electrode sheets are cut into small pieces and sintered. Cutting into small pieces is to enable substances other than active ingredients in the positive electrode sheets, such as binders, to be more thoroughly removed by sintering during the subsequent sintering process, so that more lithium iron phosphate can be recovered during the subsequent recovery of lithium iron phosphate, and to ensure that the performance of the final regenerated electrode material is not affected by substances such as binders; the sintering temperature needs to take into account the degree of mixing of the binder or other liquid impurities in the positive electrode sheets with the lithium iron phosphate, as well as the degree of adhesion of the lithium iron phosphate to the positive electrode sheets. With the general adhesion degree of the positive electrode sheets, 450-550°C can remove impurities such as binders inside the positive electrode sheets, while preventing impurities from affecting subsequent processing, and allowing the lithium iron phosphate to fall off the positive electrode sheets more easily, thereby increasing the recovery rate of the lithium iron phosphate.

[0012] Optionally, 49%-51% of the lithium iron phosphate powder has a particle size of 0.9-1.9 μm, so that the lithium iron phosphate powder has different particle sizes.

[0013] By adopting the above technical solution, the particle size of lithium iron phosphate powder will affect the stacking degree of the positive electrode material, and the stacking degree will affect the energy density of the battery. By generating particle size differences, lithium iron phosphate is divided into different particle sizes, and large and small particles of lithium iron phosphate cooperate with each other. Through good grading, the lithium iron phosphate can be compacted to form a better filling structure. A good filling structure is conducive to improving the energy density of the electrode. Although the particle size difference of lithium iron phosphate is conducive to forming a better filling structure, excessive particle size difference will cause poor grading between particles, and the positive electrode material cannot be effectively compacted and filled, thereby affecting the energy density of the electrode. Therefore, the particle size of 49%-51% lithium iron phosphate needs to be controlled between 0.9-1.9μm. By limiting the partial particle size range, the particle size difference is controlled not to be too large, thereby ensuring that the electrode can have better performance.

[0014] Optionally, the organic solution is a carboxymethyl cellulose solution, the mass fraction of the carboxymethyl cellulose solution is 1%-5%, and the viscosity of the carboxymethyl cellulose solution is 3500-4500 mPa.s.

[0015] By adopting the above technical solution, carboxymethyl cellulose is easily soluble in water, and the aqueous solution is a colloid. By mixing lithium iron phosphate powder into it and stirring it thoroughly, because the carboxymethyl cellulose solution is a colloid, the water-insoluble lithium iron phosphate powder can be dispersed more easily and evenly with a smaller amount of dissolution. If a liquid organic matter is selected, the water-insoluble substance is difficult to disperse evenly therein and is easy to precipitate at the bottom. In the subsequent spray drying process, the insoluble substance in the solution is difficult to disperse evenly under the action of external force and is easy to aggregate. The colloidal structure can keep the lithium iron phosphate powder in a dispersed state during atomization, which is conducive to the formation of a positive electrode material coated with a uniform carbon layer; the viscosity is proportional to the concentration of the solution. By controlling the mass fraction of carboxymethyl cellulose in the solution between 1% and 5%, the solution viscosity can be more conveniently controlled between 3500 and 4500 mPa.s, thereby eliminating the complicated operation of measuring the viscosity and making the production process faster.

[0016] Optionally, the amount of carboxymethyl cellulose in the carboxymethyl cellulose solution is such that the mass ratio of lithium iron phosphate to carboxymethyl cellulose is 100:(2-15), so that the thickness of the carbon layer on the surface of the positive electrode material is 30-400 nm.

[0017] By adopting the above technical solution, carboxymethyl cellulose is used to form a carbon layer on the surface of lithium iron phosphate powder after high-temperature carbonization. The amount of carboxymethyl cellulose cannot be too much, because too much will cause the carbon layer on the outer layer of lithium iron phosphate to be too thick. Although the carbon layer can promote the conduction of ions between the lithium iron phosphate lattices, the excessively thick carbon layer will play an obstructive role and cannot achieve the purpose of improving the ion conductivity; the amount of carboxymethyl cellulose cannot be too little, because too little will result in fewer carbon layers between the lithium iron phosphate. Without the conduction of the carbon layer, lithium ions cannot be efficiently transferred between the lithium iron phosphate lattices, resulting in a slow discharge rate of the electrode, and fewer carbon layers will cause the lithium iron phosphate to easily aggregate during operation, resulting in a reduction in the sites for accommodating lithium ions, thereby shortening the life of the battery. Therefore, it is necessary to control the amount of carboxymethyl cellulose so that the carbon layer formed by the carboxymethyl cellulose can just evenly wrap the lithium iron phosphate, thereby improving the ion conductivity and giving play to the high specific capacity of lithium iron phosphate and improving the working performance of the electrode.

[0018] Optionally, the spray drying adopts centrifugal spray drying, the air temperature of the centrifugal spray drying is 80-90° C., the negative pressure is 400-600 kPa, the calcination temperature is 600-800° C., and the calcination time is 3.5-4.5 hours.

[0019] By adopting the above technical solution, compared with other spray drying methods, the implementation method of centrifugal spray drying is simpler. It only needs to control the rotation speed and the atomization aperture, which is convenient to operate. In addition, since the mass of the lithium iron phosphate powder is relatively small, in the colloidal carboxymethyl cellulose solution, the lithium iron phosphate powder can remain dispersed under the action of the centrifuge due to the hindrance of the colloid. Centrifugal spray drying sprays the mixture of lithium iron phosphate and carboxymethyl cellulose into hot air, so that the water in the mixture quickly absorbs heat from the hot air and then evaporates and separates from the mixture, thereby forming lithium iron phosphate powder uniformly coated with carboxymethyl cellulose. Since the carboxymethyl cellulose and lithium iron phosphate can withstand high temperatures, and the colloidal carboxymethyl cellulose solution has unstable thermodynamic properties and tends to automatically aggregate, the evaporation rate of the water in the mixture is accelerated by adopting a higher air temperature than that of general spray drying. , which is more conducive to the formation of uniformly dispersed lithium iron phosphate powder coated with carboxymethyl cellulose; operating under a negative pressure environment is also more conducive to water evaporation and accelerates the drying speed, but since the generation of negative pressure is more difficult than heating the air, higher requirements are required for the equipment, so the commonly used spray drying negative pressure is selected; calcination will convert the coated carboxymethyl cellulose into a carbon layer coated on the surface of the lithium iron phosphate powder. During the calcination process, high temperature will destroy the covalent bonds in the carboxymethyl cellulose, causing the oxygen and hydrogen elements to separate and finally form a uniform carbon layer. This process requires a higher temperature and a longer time, and after the carbon layer is formed, a period of high temperature is also required to form a good crystal morphology. The calcination temperature of 600-800℃ and the calcination time of 3.5-4.5h are conducive to the formation of a good morphology of the carbon layer, which is conducive to the improvement of ion conductivity, thereby improving the performance of the positive electrode material.

[0020] In a second aspect, the present application provides a lithium iron phosphate positive electrode material adopting the following technical solution: A lithium iron phosphate positive electrode material is prepared by the above-mentioned lithium iron phosphate positive electrode material recovery method.

[0021] By adopting the above technical solution, the lithium iron phosphate positive electrode material prepared by the above method has a uniform carbon layer and a good degree of dispersion, thereby having better discharge performance and cycle performance. The evenly distributed carbon layer can better improve the ion conductivity of the positive electrode material and can also adapt to conditions with large currents. In addition, the carbon layer with moderate thickness and uniform arrangement is beneficial to the positive electrode material to maintain the stability of the lithium iron phosphate lattice after multiple charge and discharge cycles, retain the embedding sites of lithium ions, and enable the positive electrode material to maintain good performance after long-term use.

[0022] In a third aspect, the present application provides a lithium-ion battery using the following technical solution: A lithium-ion battery adopts the lithium iron phosphate positive electrode material.

[0023] By adopting the above technical solution, the lithium-ion battery prepared using the above-mentioned lithium iron phosphate positive electrode material has good initial efficiency and capacity retention rate after cycling due to the discharge performance and cycle performance of the positive electrode material, and can maintain good performance under long-term discharge conditions.

[0024] In summary, this application includes at least one of the following beneficial technical effects: 1. By dispersing lithium iron phosphate powder in a colloidal carboxymethyl cellulose solution, spray drying and calcining, a uniform carbon layer is coated on the outer layer of the lithium iron phosphate powder, and the formed positive electrode material is evenly dispersed, with good ion conductivity and anti-aggregation performance, thereby obtaining better charge and discharge capacity and service life.

[0025] 2. By controlling the amount of carboxymethyl cellulose and the concentration of the carboxymethyl cellulose solution, the lithium iron phosphate powder can be better distributed in the carboxymethyl cellulose solution, and the final positive electrode material has a carbon layer with an appropriate thickness. DETAILED DESCRIPTION

[0026] The present application is further described in detail below in conjunction with Examples 1-7 and Comparative Examples 1-2.

[0027] Since the embodiments involve controlling the concentration of the carboxymethyl cellulose solution, carboxymethyl cellulose solutions of various concentrations are first prepared, and then the ratio of lithium iron phosphate to carboxymethyl cellulose is controlled by controlling the amount of the carboxymethyl cellulose solution; and the lithium iron phosphate powder used in each embodiment is lithium iron phosphate powder recovered from the same batch to ensure that the difference in the particle size of the lithium iron phosphate powder will not affect subsequent performance changes.

[0028] Preparation Examples 1-3 The preparation of carboxymethyl cellulose solution comprises the following steps: Measure carboxymethyl cellulose and deionized water, dissolve the carboxymethyl cellulose in the deionized water, and stir until the carboxymethyl cellulose is dissolved to form a colloidal solution.

[0029] Table 1 Mass fractions of carboxymethyl cellulose and deionized water in Preparation Examples 1-3 Examples 1-7 The preparation of lithium iron phosphate positive electrode material includes the following steps: (a) collecting scrapped lithium iron phosphate batteries, disassembling the lithium iron phosphate batteries and collecting the positive electrode sheets, washing the collected positive electrode sheets with water, and drying them under an inert gas atmosphere; (b) Cutting the dried positive electrode sheet into small pieces, and sintering the small pieces at 500°C in an inert atmosphere; (c) Placing the sintered small electrode pieces in a drum screen to remove coarse lithium iron phosphate particles, crushing and sieving the coarse lithium iron phosphate particles to obtain lithium iron phosphate powder, wherein the lithium iron phosphate powder has a particle size D50 of 0.9-1.9 μm and a D100 of less than 20 μm; (d) Adding the lithium iron phosphate powder obtained in (c) to the carboxymethyl cellulose solution prepared in the preparation example, stirring and mixing, and centrifugal spray drying at an atomizer speed of 18,000 r / min, a hot air temperature of 85° C., and a negative pressure of 500 kPa to obtain carboxymethyl cellulose-coated lithium iron phosphate powder; (e) calcining the carboxymethyl cellulose-coated lithium iron phosphate powder obtained in (d) at 700° C. under an inert atmosphere for 4 h to obtain a carbon layer-coated lithium iron phosphate powder; (f) The carbon layer-coated lithium iron phosphate powder obtained in (e) is demagnetized and sieved to obtain a positive electrode material.

[0030] Table 2 Amount of lithium iron phosphate used in Examples 1-7 and the type and amount of preparation examples Among them, the mass ratio of lithium iron phosphate: carboxymethyl cellulose in Example 1, Example 4 and Example 7 is 100:9. However, due to the different preparation examples used, the mass fraction of the carboxymethyl cellulose solution is different, resulting in different amounts of carboxymethyl cellulose solution. In Examples 2 to 6, the mass ratios of lithium iron phosphate: carboxymethyl cellulose are 100:2, 5, 9, 12 and 15, respectively.

[0031] Comparative Example 1 The lithium iron phosphate positive electrode material prepared in this comparative example is different from that in Example 1 in that the carboxymethyl cellulose powder is fully mixed with the lithium iron phosphate powder obtained in the above step (c), calcined at 700° C. for 4 h, and then demagnetized and sieved to obtain the lithium iron phosphate positive electrode material.

[0032] Comparative Example 2 The lithium iron phosphate positive electrode material prepared in this comparative example is different from that in Example 1 in that carboxymethyl cellulose is replaced with glucose in equal parts by mass, and other preparation conditions remain unchanged.

[0033] The lithium iron phosphate cathode materials obtained in Examples 1-7 and Comparative Examples 1-2 were subjected to performance tests using the following test methods: Electrochemical performance test method: lithium iron phosphate positive electrode material: polyvinylidene fluoride (PVDF): acetylene black = 80:10:10, mixed and ground according to the proportion and evenly coated on aluminum foil as the positive electrode, the metal lithium sheet was used as the negative electrode, and the polypropylene porous membrane was used as the separator. The electrolyte was 1 mol / L LiPF6 conductive salt and DMC:DEC:EC (wt%) = 1:1:1 solvent. CR2032 button batteries were assembled in a dry glove box filled with argon, and the assembled button batteries were charged and discharged using the Xinwei battery testing system, with a voltage range of 2.0-3.75V.

[0034] First efficiency = first discharge capacity / first charge capacity.

[0035] Cycle capacity retention rate = 3000 cycle discharge capacity / first cycle discharge capacity.

[0036] Table 3 Performance test data of Examples 1-7 and Comparative Examples 1-2 Combined with Tables 1, 2 and 3 to explore the performance of lithium iron phosphate positive electrode materials Comparing Example 1, Example 4 and Example 7, the difference lies in the mass fraction of the carboxymethyl cellulose solution during the preparation process. The first discharge capacity of Example 1, Example 4 and Example 7 decreases successively, among which the gap between Example 1 and Examples 4 and 7 is larger, indicating that the first discharge capacity of the carboxymethyl cellulose solution with a mass fraction of 1% is the best; but in terms of the first efficiency, Example 4 is the best, Example 7 is slightly lower, and the gap between Example 1 and Example 7 is large, indicating that after the first discharge, the lithium iron phosphate positive electrode material in Example 1 has more lithium ions that cannot return to the positive electrode material. On the one hand, it may be that the formation of the SEI film consumes too many lithium ions. On the other hand, it may be that the lattice morphology in the positive electrode material changes greatly after the first discharge, and more lithium ions cannot be accommodated. However, in general, the first efficiency of the positive electrode material prepared with a mass fraction of 1% of the carboxymethyl cellulose solution is relatively poor; in terms of the capacity retention rate after 3000 times, Example 4 is the best, Example 7 decreases more, and Example 1 decreases more. It is lower than that of Example 7, indicating that the positive electrode material prepared with a mass fraction of 3% of the carboxymethyl cellulose solution has better cycle performance, indicating that the positive electrode material of Example 4 has the most uniform carbon layer protection, and the lithium iron phosphate is highly dispersed under the coating of the carbon layer. After 3000 cycles, it still maintains a good lattice morphology, retains more lithium ion embedding sites, and thus has a higher capacity retention rate. However, in Example 1 and Example 7, because the mass fraction of the carboxymethyl cellulose solution is larger or smaller during the preparation process, the carbon layer formed is not uniformly coated, and the lithium iron phosphate is not dispersed enough. After multiple cycles of charge and discharge, the lattice morphology of the positive electrode material changes greatly and can no longer accommodate more lithium ions, thereby reducing the capacity and the capacity retention rate. In comprehensive comparison, although Example 1 has the best first discharge capacity, the more practical first efficiency and capacity retention rate of Example 4 are better, indicating that a mass fraction of 3% of the carboxymethyl cellulose solution is a better preparation scheme.

[0037] Comparing Examples 2-6, the difference lies in the amount of carboxymethyl cellulose used; from the perspective of the first discharge capacity, there is little difference between Example 2, Example 3 and Example 6, among which the first discharge capacity of Example 4 and Example 5 is relatively low. In the range of the mass ratio of lithium iron phosphate: carboxymethyl cellulose = 100:2-15, the first discharge capacity first decreases and then increases with the increase of the amount of carboxymethyl cellulose used; from Example 2 to Example 5, the first efficiency gradually increases, while the first efficiency of Example 6 decreases compared with Example 5, and the first efficiency of Example 5 is the best, indicating that within a certain range, the first efficiency increases with the increase of the amount of carboxymethyl cellulose. This is because as the amount of carboxymethyl cellulose increases, the thicker the carbon layer coated on the surface of the lithium iron phosphate, to a certain extent, the thicker the carbon layer is, the more conducive it is to maintaining the dispersion and lattice morphology of the lithium iron phosphate, and can reduce the amount of lithium ions forming the SEI film, but compared with Example 5 and Example 6, as the amount of carboxymethyl cellulose increases, the first efficiency begins to decrease, and the lithium ions are initially After the first discharge and charge cycle, it can no longer return to the positive electrode material; the capacity retention rate of Examples 3, 4 and 5 after 3000 cycles is good, among which Example 5 is the best, while Examples 2 and 6 are relatively poor, indicating that when the amount of carboxymethyl cellulose is large or small, it is difficult for the positive electrode material to maintain a good lattice morphology after multiple cycles, and when the amount of carboxymethyl cellulose is moderate, the positive electrode material maintains a good lattice morphology after multiple cycles. This is because when the carbon layer is too thin, during the process of multiple conduction of lithium ions and continuous erosion of the electrolyte, the carbon layer gradually falls off, resulting in a decrease in the ion conductivity of the positive electrode material, and some lithium iron phosphate will also lose the embedding site of lithium ions. When the carbon layer is too thick, the conduction of lithium ions and the erosion of the electrolyte will cause the outer layer of the carbon layer to lose its graphite structure, thereby causing the carbon layer to change from promoting ion conduction to hindering ion conduction. When the carbon layer is moderate, even after losing the graphite structure, because the carbon layer is thin, the ion conductivity can still be maintained, so that the positive electrode material has a good capacity retention rate.

[0038] Comparing Example 4 and Comparative Example 1, the difference is that Comparative Example 1 directly synthesizes the positive electrode material by calcining the mixture of carboxymethyl cellulose and lithium iron phosphate powder. The first discharge capacity of Comparative Example 1 is better, the first efficiency of Example 4 is better, and the capacity retention rate after 3000 cycles is higher than that of Example 4, and the difference is obvious, indicating that the positive electrode material prepared by the method of the present application has a better cycle capacity retention rate even when the component dosage is the same, and thus has a better service life. This is because the lithium iron phosphate powder is dispersed into the carboxymethyl cellulose colloid and then calcined after spray drying, so that the carbon layer formed by the carboxymethyl cellulose can be evenly coated on the outside of the lithium iron phosphate powder. The formed positive electrode material does not have excessive aggregation of lithium iron phosphate in some parts or excessive aggregation of the carbon layer. When the positive electrode material is prepared by the method of Comparative Example 1, the probability of this situation increases. After multiple cycles, the aggregated lithium iron phosphate and carbon layer will cause the positive electrode material to lose more lithium ion embedding sites, thereby causing the capacity retention rate to decrease. By comparing Example 4 and Comparative Example 1, it can be seen that the positive electrode material synthesized by the preparation method of the present application has better electrode performance.

[0039] Comparing Example 4 with Comparative Example 2, the difference is that Comparative Example 2 uses glucose instead of carboxymethyl cellulose. The first discharge capacity of Comparative Example 2 is slightly better, and the gap is not obvious. The first efficiency of Example 4 is slightly better, and the cycle 3000 capacity retention rate of Example 4 is significantly better than that of Comparative Example 2. This shows that the positive electrode material prepared by the lithium iron phosphate recovery method of the present application has better cycle performance, that is, longer service performance. On the one hand, this is because the proportion of carbon in carboxymethyl cellulose is higher. Under equal mass, carboxymethyl cellulose has more carbon elements than glucose, and thus can form more carbon layers. According to the above analysis, the carbon layer content in Example 4 is better than that in Example 2. When the capacity retention rate of the positive electrode material is in the range near Example 4, the increase of the carbon layer increases. On the other hand, compared with carboxymethyl cellulose, the viscosity of the solution prepared by glucose is low, which makes it difficult to maintain the dispersion of lithium iron phosphate powder when it is dispersed therein. After a period of static stirring, the lithium iron phosphate powder will begin to sink, and the glucose coating layer of the lithium iron phosphate powder prepared by subsequent spray drying is not uniform, and a good carbon layer cannot be formed after calcination, resulting in a faster decrease in the capacity retention rate after the cycle. In contrast, carboxymethyl cellulose is a better raw material for the coating carbon layer of the positive electrode material prepared by the lithium iron phosphate recovery method of this application.

[0040] The examples of this specific embodiment are all preferred embodiments of this application and are not intended to limit the scope of protection of this application. Identical components are represented by the same reference numerals. Therefore, any equivalent changes made based on the structure, shape, and principle of this application should be included in the scope of protection of this application.

Claims

1. A method for recovering lithium iron phosphate cathode material, characterized in that: include: Stripping coarse lithium iron phosphate particles from the sintered small pieces of the positive electrode sheet, crushing and sieving the coarse lithium iron phosphate particles to obtain lithium iron phosphate powder, wherein the particle size of the lithium iron phosphate powder ranges from 0.9 to 20 μm; Immersing and suspending the lithium iron phosphate powder in an organic solution, spray drying, and calcining under an inert atmosphere to form a uniform carbon layer on the surface of the lithium iron phosphate powder, wherein the viscosity of the organic solution is in the range of 1500-2500 mPa.s; The calcined lithium iron phosphate powder is demagnetized and sieved to obtain a positive electrode material, wherein the thickness of the carbon layer on the surface of the positive electrode material is 30-400 nm.

2. The method for recycling lithium iron phosphate cathode materials according to claim 2, characterized in that: include Obtaining a positive electrode sheet from a lithium iron phosphate battery cell, washing it with water and drying it in an inert gas atmosphere to remove the electrolyte on the surface of the positive electrode sheet, and then cutting it into small pieces; Sintering the small electrode piece at 450-550° C. in an inert atmosphere to remove the adhesive; Coarse lithium iron phosphate particles are peeled off from the small electrode pieces using a drum screen, and the coarse lithium iron phosphate particles are crushed and sieved to obtain lithium iron phosphate powder. The particle size of the lithium iron phosphate powder is in the range of 0.9-20 μm.

3. The lithium iron phosphate cathode material recycling method according to claim 1, wherein 49%-51% of the lithium iron phosphate powder has a particle size of 0.9-1.9 μm, so that the lithium iron phosphate powder has different particle sizes.

4. The method for recycling lithium iron phosphate cathode materials according to claim 1, characterized in that: The organic solution is a carboxymethyl cellulose solution, the mass fraction of the carboxymethyl cellulose solution is 1-5%, and the viscosity of the carboxymethyl cellulose solution is 1500-2500 mPa.s.

5. The method for recycling lithium iron phosphate cathode materials according to claim 4, characterized in that: The amount of carboxymethyl cellulose in the carboxymethyl cellulose solution is such that the mass ratio of lithium iron phosphate to carboxymethyl cellulose is 100:(2-15), so that the thickness of the carbon layer on the surface of the positive electrode material is 30-400 nm.

6. The method for recycling lithium iron phosphate cathode materials according to claim 1, characterized in that: The calcination temperature is 600-800° C., the calcination time is 3.5-4.5 hours, the spray drying adopts centrifugal spray drying, the air temperature of the centrifugal spray drying is 80-90° C., and the negative pressure is 400-600 kPa.

7. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the method for recovering the lithium iron phosphate positive electrode material according to any one of claims 1 to 6.

8. A lithium-ion battery, characterized in that: The lithium iron phosphate positive electrode material according to claim 7 is used.

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