A nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, a preparation method therefor, and an application thereof
By coating the surface of lithium iron phosphate particles with a nitrogen-doped carbon layer through plasma ball milling, the problem of insufficient improvement in the electrochemical performance of regenerated lithium iron phosphate cathode materials in the existing technology has been solved, and the material has achieved high capacity, excellent rate performance and cycle stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- XIAMEN UNIV GULEI PETROCHEMICAL RES INST
- Filing Date
- 2026-03-05
- Publication Date
- 2026-05-29
AI Technical Summary
Existing technologies offer limited improvements in the electrochemical performance of regenerated lithium iron phosphate cathode materials, particularly in electronic conductivity and lithium-ion diffusion rate. This results in difficulties in restoring the rate performance and long-cycle stability of regenerated batteries to or exceeding their initial levels.
Nitrogen-doped carbon coating of regenerated lithium iron phosphate powder was carried out by plasma ball milling. Carbon and nitrogen compounds were coated on the surface of lithium iron phosphate particles under high-energy plasma bombardment, and a nitrogen-doped carbon layer was formed during annealing, which improved the electronic conductivity and lithium-ion diffusion performance of the material.
The electronic conductivity, lithium-ion diffusion coefficient and structural stability of the recycled lithium iron phosphate composite material are significantly improved, enabling the recycled battery to exhibit excellent rate performance and cycle performance, even surpassing the original level.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of lithium-ion battery recycling technology, specifically relating to a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, its preparation method, and its application. Background Technology
[0002] Lithium iron phosphate (LFP) batteries, with their superior safety, ultra-long lifespan, and significant cost advantages, have become the absolute mainstream in the energy storage field and have achieved large-scale application in the electric vehicle sector. Currently, they are experiencing a peak in retirement. The recycling and processing of spent LFP batteries includes two methods: one is to recover individual elements, and the other is to directly regenerate the lithium iron phosphate (LFP) material. Regenerating LFP has two main purposes: first, to replenish lost lithium, and second, to reduce the trivalent iron in FePO4 to divalent iron. The commonly used hydrothermal method involves mixing the cathode material from disassembled spent batteries with a lithium-containing solution and reacting it in a high-temperature, high-pressure hydrothermal environment. Hydrothermal conditions can promote the reinsertion of lithium ions into the crystal structure of the material, while simultaneously repairing lattice defects caused by cycle decay and restoring the stoichiometry of the material.
[0003] Currently, the hydrothermal regeneration of spent lithium iron phosphate cathode materials generally involves adding a lithium source and a reducing agent. For example, patent application CN202311140131 discloses a regeneration method using Li₂SO₄ as the lithium source and hydrazine hydrate as the reducing agent, and patent application CN202111039670 discloses a regeneration method using LiOH as the lithium source and citric acid as the reducing agent. However, the lithium iron phosphate cathode materials regenerated by these methods still show significant differences in electrochemical performance compared to the initial materials. Not only is there a gap in capacity performance, but rate performance and cycle performance also differ significantly from the initial levels. Therefore, there is an urgent need to find a regeneration modification method for spent lithium iron phosphate cathode materials to improve the regeneration value of retired lithium iron phosphate batteries. Summary of the Invention
[0004] The purpose of this invention is to address the problem that existing methods for regenerating lithium iron phosphate cathode materials have limited ability to improve and restore performance, making it difficult to bring the performance of lithium-ion batteries back to their original level. This invention provides a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, its preparation method, and its application. This nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material has excellent electronic conductivity, lithium-ion diffusion coefficient, and structural stability, thereby enabling lithium-ion batteries to exhibit high capacity performance, as well as excellent rate performance and cycle performance.
[0005] After extensive and in-depth research, the inventors of this invention discovered that while the commonly used citric acid hydrothermal regeneration method can effectively repair the crystal structure of retired lithium iron phosphate materials and achieve lithium replenishment, its core limitation lies in the fact that this method can only repair the physicochemical structure and cannot improve the intrinsic electrochemical performance of the material. Specifically, the inherent defects of the regenerated cathode material, such as low electronic conductivity and slow lithium-ion diffusion rate, still exist, making it difficult to restore the rate performance and long-cycle stability of the regenerated battery to or even exceed its initial level. This fundamentally limits the application value of regenerated batteries. Based on this, this invention uses plasma ball milling to nitrogen-doped carbon-coated regenerated lithium iron phosphate powder, overcoming the limitation of the traditional citric acid method, which can only achieve structural repair but cannot improve the intrinsic electrochemical kinetics of the material. The resulting regenerated lithium iron phosphate cathode material not only has its capacity fully restored, but its electronic conductivity, lithium-ion diffusion coefficient, and structural stability are also significantly improved, thus enabling the regenerated battery to exhibit excellent rate performance and high cycle stability.
[0006] In a first aspect, the present invention provides a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material. The preparation method includes the following steps: S1. The waste lithium iron phosphate cathode sheet is first treated with ultrasonic water bath, and then the cathode material is separated from the current collector by sieving. The obtained cathode material is ball-milled and then washed with alkaline solution to obtain cathode powder. S2. The positive electrode powder obtained in step S1 is mixed with citric acid, LiOH, and water and then subjected to hydrothermal treatment. The resulting hydrothermal reaction product is regenerated lithium iron phosphate. The regenerated lithium iron phosphate and carbonitride compounds are subjected to plasma ball milling under the condition of passing a reactive gas or inert gas I. The resulting ball-milled mixture is annealed under an inert gas II atmosphere to obtain a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material. The nitrogen content in the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material is 0.5~3.5wt%.
[0007] In a preferred embodiment, in step S1, the conditions for the ultrasonic water bath treatment include: a temperature of 70~90℃ and a time of 0.5~2h.
[0008] In a preferred embodiment, in step S1, the conditions for ball milling include: a rotation speed of 200-300 rpm and a time of 4-8 h.
[0009] In a preferred embodiment, in step S1, the alkaline solution is a NaOH solution and / or a KOH solution.
[0010] In a preferred embodiment, in step S1, the concentrations of the NaOH solution and the KOH solution are each independently 0.1~0.3M.
[0011] In a preferred embodiment, in step S1, the conditions for the alkaline washing treatment include: a temperature of 50~70℃ and a time of 0.5~2h.
[0012] In a preferred embodiment, in step S2, the mass ratio of the positive electrode powder to citric acid is 1:(1.5~2).
[0013] In a preferred embodiment, in step S2, the solid-liquid ratio of the solution formed by the positive electrode powder, citric acid, LiOH, and water is 20-40 g / L.
[0014] In a preferred embodiment, in step S2, the concentration of LiOH in the solution formed by citric acid, LiOH, and water is 0.7~0.9M.
[0015] In a preferred embodiment, in step S2, the conditions for hydrothermal treatment include: pH 5-6, temperature 180-220°C, and time 4-8 hours.
[0016] In a preferred embodiment, in step S2, the amount of the carbon-nitrogen compound used is 1 to 5 wt% of the mass of the regenerated lithium iron phosphate.
[0017] In a preferred embodiment, in step S2, the conditions for plasma ball milling include: a rotation speed of 200-300 rpm, a time of 4-8 h, a ball-to-material ratio of (10-20):1, and a discharge frequency of 10-50 kHz.
[0018] In a preferred embodiment, in step S2, the annealing conditions include: a temperature of 500~700℃ and a time of 2~6h.
[0019] In a preferred embodiment, in step S2, the carbon-nitrogen compound is selected from at least one of melamine, urea, acetonitrile, glucosamine, polyaniline, and polypyrrole.
[0020] In a preferred embodiment, in step S2, the reactive gas is ammonia.
[0021] In a preferred embodiment, in step S2, the inert gas I and inert gas II are each independently nitrogen and / or argon.
[0022] Secondly, the present invention provides a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material prepared by the above method.
[0023] Thirdly, the present invention also provides the application of the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material in lithium-ion batteries.
[0024] Beneficial effects: The key to this invention lies in the re-lithiation and regeneration of old lithium iron phosphate materials using a citric acid hydrothermal method, followed by coating the surface of the regenerated lithium iron phosphate particles with carbon and nitrogen compounds under plasma ball milling, and then obtaining a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material through annealing. This regenerated lithium iron phosphate composite material has significantly improved and enhanced electronic conductivity, lithium-ion diffusion coefficient and structural stability. Not only can the capacity be restored to the initial level, but the regenerated lithium-ion battery also exhibits excellent or even better rate performance and cycle performance than the original. The reasons for this are speculated to be as follows: First, plasma ball milling can refine LFP particles through plasma bombardment, generating numerous active sites such as point defects, lattice distortions, and dangling bonds on the LFP particle surface. These "active sites" will become preferential sites for subsequent C and N atom doping, significantly reducing the energy barrier required for doping. Second, it enables uniform nanoscale mixing of LFP particles and carbonitride compounds. Third, under the bombardment of high-energy plasma particles (reactive gases or inert gas I are excited to form plasma gas), carbonitride compounds may partially decompose or generate active C / N-containing groups. These groups can directly combine with the activated LFP surface to form a kind of "precursor state" C / N bond, making the subsequent annealing reaction easier and more uniform. Therefore, compared to ordinary ball milling, plasma ball milling offers several advantages. Ball milling not only achieves more uniform coating but also creates doping modification at the interface between LFP particles and the coating layer, thus endowing the recycled lithium iron phosphate composite material with superior electrochemical kinetic performance. The carbonitrides coating the LFP particle surface pyrolyze during annealing to form a carbon layer, which helps improve interparticle conductivity and inhibit particle agglomeration at high temperatures, maintaining the material's high specific surface area and short ion diffusion path. Simultaneously, the pyrolysis of carbonitrides releases abundant small-molecule active nitrogen species (such as NH3, HCN, and nitrogen-containing free radicals). At high temperatures, these active nitrogen species and the "precursor state" C / N bonds can be incorporated into the carbon layer and even the interior of the lithium iron phosphate lattice (nitrogen atoms are most likely to replace phosphorus (P) sites in the lattice). The strong electronegativity of nitrogen can stabilize the lattice structure and may slightly broaden the Li-N crystal. + A one-dimensional diffusion channel is created, thereby increasing ionic conductivity. Detailed Implementation
[0025] 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. Furthermore, unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present invention.
[0026] The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material provided by the present invention includes the following steps: S1. The waste lithium iron phosphate cathode sheet is first treated with ultrasonic water bath, and then the cathode material is separated from the current collector by sieving. The obtained cathode material is ball-milled and then washed with alkaline solution to obtain cathode powder. S2. The positive electrode powder obtained in step S1 is mixed with citric acid, LiOH and water and then subjected to hydrothermal treatment. The hydrothermal reaction product is regenerated lithium iron phosphate. The regenerated lithium iron phosphate and carbon-nitrogen compounds are subjected to plasma ball milling under the condition of passing a reactive gas or inert gas I. The ball-milled mixture is then annealed under an inert gas II atmosphere to obtain a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material.
[0027] In this invention, the nitrogen content in the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material can be 0.5 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, or any value between them. When the nitrogen content is too low, the performance improvement effect on the cathode material is minimal; when the nitrogen content is too high, it may lead to excessive graphitization of the carbon layer, increased structural disorder, or the generation of numerous defects that hinder ion transport, ultimately completely offsetting or even reversing the conductivity advantage brought by nitrogen doping and severely blocking the lithium-ion transport channels. Therefore, controlling the nitrogen content in the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material within the above-mentioned range can effectively achieve the improvement effect of nitrogen doping modification on the electronic conductivity, lithium-ion diffusion coefficient, and structural stability of the cathode material.
[0028] In this invention, the purpose of the ultrasonic water bath treatment in step S1 is to break the bond between the adhesive (such as PVDF) and the current collector, thereby achieving rapid and uniform peeling of the positive electrode material. The current collector is the carrier used to load the positive electrode material in the preparation of the positive electrode sheet; it generally refers to aluminum foil, but may also be other current collectors used as the carrier of the positive electrode sheet. Before the ultrasonic water bath treatment, the positive electrode sheet can be processed into fragments to better separate the positive electrode material from the current collector through the ultrasonic water bath treatment. The size of the fragments can be 2cm*2cm, 1.5cm*1.5cm, 2cm*1.5cm, 1cm*1cm, etc. The preferred conditions for the ultrasonic water bath treatment include: a temperature of 70~90℃, such as 70℃, 75℃, 80℃, 85℃, 90℃ or any value between them; and a time of 0.5~2h, such as 0.5h, 1h, 1.5h, 2h or any value between them.
[0029] In this invention, the ball milling conditions in step S1 preferably include: a rotation speed of 200~300 rpm, such as 200 rpm, 220 rpm, 250 rpm, 280 rpm, 300 rpm or any value between them; and a time of 4~8 h, such as 4 h, 5 h, 6 h, 7 h, 8 h or any value between them.
[0030] In this invention, in step S1, the alkaline solution is preferably a NaOH solution and / or a KOH solution. The concentrations of the NaOH solution and the KOH solution are each preferably 0.1~0.3M, such as 0.1M, 0.15M, 0.2M, 0.25M, 0.3M, or any value between them.
[0031] In this invention, the purpose of alkaline washing in step S1 is to further remove residual impurities (such as aluminum in the aluminum foil current collector) from the positive electrode material, thereby reducing the adverse effects of impurities on the performance of the recycled material. The preferred conditions for the alkaline washing treatment include: a temperature of 50~70℃, such as 50℃, 55℃, 60℃, 65℃, 70℃ or any value between them; and a time of 0.5~2h, such as 0.5h, 1h, 1.5h, 2h or any value between them.
[0032] In this invention, in step S2, the mass ratio of the positive electrode powder to citric acid is preferably 1:(1.5~2), such as 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2 or any ratio therebetween.
[0033] In this invention, in step S2, the solid-liquid ratio of the solution formed by the positive electrode powder, citric acid, LiOH, and water is preferably 20~40 g / L, such as 20 g / L, 25 g / L, 30 g / L, 35 g / L, 40 g / L, or any value between them.
[0034] In this invention, in step S2, the concentration of LiOH in the solution formed by citric acid, LiOH, and water is preferably 0.7~0.9M, such as 0.7M, 0.75M, 0.8M, 0.85M, 0.9M, or any value between them.
[0035] In this invention, the hydrothermal treatment conditions in step S2 preferably include: pH 5~6, such as 5, 5.2, 5.5, 5.8, 6 or any value between them. This is more conducive to providing a buffer environment for the lithium iron phosphate (LFP) cathode powder to be regenerated by heavy lithiation, avoiding excessive leaching of Li and Fe caused by direct mixing of LFP and citric acid, while avoiding corrosion of the LFP surface in an overly alkaline environment, reducing the adverse effects on the electrochemical performance of the regenerated LFP; temperature 180~220℃, such as 180℃, 190℃, 200℃, 210℃, 220℃ or any value between them; and time 4~8h, such as 4h, 5h, 6h, 7h, 8h or any value between them. Before hydrothermal treatment, it is preferable to ultrasonically treat the mixture of cathode powder, citric acid, LiOH, and water. This is more conducive to the uniform mixing of the components and the uniform dispersion of the cathode powder in the liquid, thereby improving the efficiency of the hydrothermal reaction and the effect of heavy lithiation, which in turn helps to improve the electrochemical performance of regenerated lithium iron phosphate.
[0036] In this invention, in step S2, the amount of the carbon and nitrogen compound is preferably 1 to 5 wt% of the mass of the regenerated lithium iron phosphate, such as 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, or any value between them.
[0037] In this invention, the plasma ball milling conditions in step S2 preferably include: a rotation speed of 200~300 rpm, such as 200 rpm, 220 rpm, 250 rpm, 280 rpm, 300 rpm or any value between them; a time of 4~8 h, such as 4 h, 5 h, 6 h, 7 h, 8 h or any value between them; a ball-to-material ratio of (10~20):1, such as 10:1, 12:1, 15:1, 18:1, 20:1 or any value between them; and a discharge frequency of 10~50 kHz, such as 10 kHz, 20 kHz, 30 kHz, 40 kHz, 50 kHz or any value between them.
[0038] In this invention, the annealing conditions in step S2 preferably include: a temperature of 500~700℃, such as 500℃, 550℃, 600℃, 650℃, 700℃ or any value between them; and a time of 2~6h, such as 2h, 3h, 4h, 5h, 6h or any value between them.
[0039] In this invention, in step S2, the carbon-nitrogen compound can be any compound containing carbon and nitrogen elements and capable of forming a nitrogen-doped carbon layer through pyrolysis. Specific examples include, but are not limited to, at least one selected from melamine, urea, acetonitrile, glucosamine, polyaniline, and polypyrrole.
[0040] In this invention, in step S2, the reactive gas can be ammonia. The inert gas I and inert gas II can each be independently nitrogen and / or argon.
[0041] The present invention will be described in detail below through specific embodiments. These embodiments are intended to explain the invention and should not be construed as limiting it. 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 art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.
[0042] Example 1 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Waste lithium iron phosphate cathode sheets were cut into 2cm*2cm fragments and placed in an ultrasonic water bath at 80℃ for 1 hour. After filtering out the liquid, the cathode material was separated from the aluminum foil by sieving. The sieved cathode material was then placed in a ball mill and ball-milled at 150 rpm for 2 hours to obtain cathode powder. The cathode powder was then placed in a 0.1M NaOH solution at 60℃ and stirred for 1 hour. After filtration, washing with deionized water multiple times, and vacuum drying, the waste lithium iron phosphate cathode powder to be processed was obtained, denoted as S-LFP.
[0043] S-LFP cathode powder was placed in a buffer solution of citric acid monohydrate and LiOH and ultrasonically treated for 30 min, followed by hydrothermal treatment at 200℃ for 6 h. After filtration, washing, and vacuum drying at 80℃ for 6 h, regenerated lithium iron phosphate, denoted as R-LFP, was obtained. R-LFP and melamine were added to a ball mill jar for plasma ball milling at a ball-to-material ratio of 20:1, with nitrogen gas introduced and a discharge frequency of 20 kHz at 250 rpm for 6 h. Then, annealing was performed at 600℃ for 4 h under an argon atmosphere. After cooling, nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-1, was obtained. The nitrogen content in R-LFP@C&N-1 was 0.6 wt% as determined by an oxygen, nitrogen, and hydrogen analyzer.
[0044] In this embodiment, the mass ratio of S-LFP cathode powder to citric acid monohydrate is 1:1.5, the concentration of lithium hydroxide in the buffer solution is 0.72 mol / L, the pH of the buffer solution is 5.2, the solid-liquid ratio of S-LFP cathode powder to buffer solution is 20 g / L, the melamine content in the ball mill jar is 1 wt% of the mass of R-LFP, and the heating rate of the annealing treatment is 5.7 °C / min.
[0045] Example 2 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated recycled lithium iron phosphate composite material was prepared according to the method of Example 1, with the following differences: the waste lithium iron phosphate cathode sheet was cut into 2cm*1.5cm fragments; the mass ratio of S-LFP cathode powder to citric acid monohydrate was 1:1.5; the concentration of lithium hydroxide in the buffer solution was 0.85mol / L; the pH of the buffer solution was 5.4; the solid-liquid ratio of S-LFP cathode powder to buffer solution was 20g / L; the melamine content in the ball mill jar was 2wt% of the mass of R-LFP; and the heating rate of the annealing treatment was 6℃ / min. All other conditions were the same as in Example 1. Thus, the nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, denoted as R-LFP@C&N-2, was prepared. The nitrogen content in R-LFP@C&N-2 material was 1.2wt% as determined by an oxygen, nitrogen, and hydrogen analyzer.
[0046] Example 3 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, with the following differences: the mass ratio of S-LFP cathode powder to citric acid monohydrate was 1:1.5; the concentration of lithium hydroxide in the buffer solution was 0.87 mol / L; the pH of the buffer solution was 5.6; the solid-liquid ratio of S-LFP cathode powder to buffer solution was 20 g / L; the melamine content in the ball mill jar was 3 wt% of the mass of R-LFP; and the heating rate of the annealing treatment was 6 °C / min. All other conditions were the same as in Example 1. Thus, nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-3, was prepared. The nitrogen content in R-LFP@C&N-3 was 1.7 wt% as determined by an oxygen, nitrogen, and hydrogen analyzer.
[0047] Example 4 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, with the following differences: the mass ratio of S-LFP cathode powder to citric acid monohydrate was 1:1.5; the concentration of lithium hydroxide in the buffer solution was 0.87 mol / L; the pH of the buffer solution was 5.8; the solid-liquid ratio of S-LFP cathode powder to buffer solution was 20 g / L; the melamine content in the ball mill jar was 5 wt% of the mass of R-LFP; and the heating rate of the annealing treatment was 6 °C / min. All other conditions were the same as in Example 1. Thus, nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-4, was prepared. The nitrogen content in R-LFP@C&N-4 material was 3.2 wt% as determined by an oxygen, nitrogen, and hydrogen analyzer.
[0048] Example 5 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Waste lithium iron phosphate cathode sheets were cut into 2cm*2cm fragments and placed in an ultrasonic water bath at 80℃ for 1 hour. After filtering out the liquid, the cathode material was separated from the aluminum foil by sieving. The sieved cathode material was then placed in a ball mill and ball-milled at 150 rpm for 2 hours to obtain cathode powder. The cathode powder was then placed in a 0.1M NaOH solution at 60℃ and stirred for 1 hour. After filtration, washing with deionized water multiple times, and vacuum drying, the waste lithium iron phosphate cathode powder to be processed was obtained, denoted as S-LFP.
[0049] S-LFP cathode powder was placed in a buffer solution of citric acid monohydrate and LiOH and ultrasonically treated for 30 min, followed by hydrothermal treatment at 180℃ for 8 h. After filtration, washing, and vacuum drying at 80℃ for 6 h, regenerated lithium iron phosphate, denoted as R-LFP, was obtained. R-LFP and melamine were added to a ball mill jar for plasma ball milling at a ball-to-material ratio of 20:1, with nitrogen gas introduced and a discharge frequency of 30 Hz at 300 rpm for 4 h. Then, it was annealed at 500℃ for 4 h under an argon atmosphere. After cooling, nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-5, was obtained. The nitrogen content in R-LFP@C&N-5 was 2.6 wt% as determined by an oxygen, nitrogen, and hydrogen analyzer.
[0050] In this embodiment, the mass ratio of S-LFP cathode powder to citric acid monohydrate is 1:1.5, the concentration of lithium hydroxide in the buffer solution is 0.87 mol / L, the pH of the buffer solution is 5.6, the solid-liquid ratio of S-LFP cathode powder to buffer solution is 40 g / L, the melamine content in the ball mill jar is 4 wt% of the mass of R-LFP, and the heating rate of the annealing treatment is 6 °C / min.
[0051] Example 6 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Waste lithium iron phosphate cathode sheets were cut into 2cm*2cm fragments and placed in an ultrasonic water bath at 80℃ for 1 hour. After filtering out the liquid, the cathode material was separated from the aluminum foil by sieving. The sieved cathode material was then placed in a ball mill and ball-milled at 150 rpm for 2 hours to obtain cathode powder. The cathode powder was then placed in a 0.1M NaOH solution at 60℃ and stirred for 1 hour. After filtration, washing with deionized water multiple times, and vacuum drying, the waste lithium iron phosphate cathode powder to be processed was obtained, denoted as S-LFP.
[0052] S-LFP cathode powder was placed in a buffer solution of citric acid monohydrate and LiOH and ultrasonically treated for 30 min, followed by hydrothermal treatment at 220℃ for 4 h. After filtration, washing, and vacuum drying at 80℃ for 6 h, regenerated lithium iron phosphate, denoted as R-LFP, was obtained. R-LFP and melamine were added to a ball mill jar for plasma ball milling and mixing at a ball-to-material ratio of 20:1. Nitrogen gas was introduced, the discharge frequency was 30 kHz, and the mixture was ball milled at 200 rpm for 8 h. Then, it was annealed at 700℃ for 4 h under an argon atmosphere. After cooling, nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-6, was obtained. The nitrogen content in R-LFP@C&N-6 was 1.8 wt% as determined by an oxygen, nitrogen, and hydrogen analyzer.
[0053] In this embodiment, the mass ratio of S-LFP cathode powder to citric acid monohydrate is 1:1.5, the concentration of lithium hydroxide in the buffer solution is 0.87 mol / L, the pH of the buffer solution is 5.6, the solid-liquid ratio of S-LFP cathode powder to buffer solution is 30 g / L, the melamine content in the ball mill jar is 3 wt% of the mass of R-LFP, and the heating rate of the annealing treatment is 6 °C / min.
[0054] Example 7 This embodiment illustrates a method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, except that the pH of the buffer solution was adjusted to 10, while all other conditions were the same as in Example 1. The resulting nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was designated R-LFP@C&N-5. Analysis using an oxygen, nitrogen, and hydrogen analyzer showed that the nitrogen content in R-LFP@C&N-5 was 0.5 wt%.
[0055] Comparative Example 1 This comparative example illustrates a method for preparing a reference recycled lithium iron phosphate material, the specific process of which is as follows: Waste lithium iron phosphate cathode sheets were cut into 1.5cm*1.5cm fragments and placed in an ultrasonic water bath at 80℃ for 1 hour. After filtering out the liquid, the cathode material was separated from the aluminum foil by sieving. The sieved cathode material was then placed in a ball mill and ball-milled at 150 rpm for 2 hours to obtain cathode powder. The cathode powder was then placed in a 0.1M NaOH solution at 60℃ and stirred for 1 hour. After filtration, washing with deionized water multiple times, and vacuum drying, the waste lithium iron phosphate cathode powder to be processed was obtained, denoted as S-LFP.
[0056] S-LFP cathode powder was placed in a buffer solution of citric acid monohydrate and LiOH, ultrasonically treated for 30 min, and then hydrothermally treated at 200℃ for 6 h. After filtration, washing, and vacuum drying at 80℃ for 6 h, regenerated lithium iron phosphate, denoted as R-LFP, was obtained. The obtained R-LFP was directly annealed at 600℃ for 4 h in an argon atmosphere. After cooling, a reference regenerated lithium iron phosphate composite material, denoted as R-LFP-D1, was obtained. The N content in R-LFP-D1 material was 0 wt%.
[0057] Comparative Example 2 This comparative example illustrates a method for preparing a reference nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, except that conventional ball milling was used instead of plasma ball milling, and the melamine content in the ball mill jar was 1 wt% of the mass of R-LFP. All other conditions were the same as in Example 1. A reference nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-D2, was thus prepared. The nitrogen content in R-LFP@C&N-D2 was determined to be 0.3 wt% by an oxygen, nitrogen, and hydrogen analyzer.
[0058] Comparative Example 3 This comparative example illustrates a method for preparing a reference nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, except that the melamine content in the ball mill jar was 0.5 wt% of the mass of R-LFP, and all other conditions were the same as in Example 1. A reference nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-D3, was thus prepared. The nitrogen content in R-LFP@C&N-D3 material was determined to be 0.3 wt% by an oxygen, nitrogen, and hydrogen analyzer.
[0059] Comparative Example 4 This comparative example illustrates a method for preparing a reference nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, except that the melamine content in the ball mill jar was 10 wt% of the mass of R-LFP, and all other conditions were the same as in Example 1. A reference nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-D4, was thus prepared. The nitrogen content in R-LFP@C&N-D4 material was determined to be 6.1 wt% by an oxygen, nitrogen, and hydrogen analyzer.
[0060] Comparative Example 5 This comparative example illustrates a method for preparing a reference carbon-coated recycled lithium iron phosphate composite material, the specific process of which is as follows: Nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material was prepared according to the method of Example 1, except that glucose was used instead of melamine, i.e., the glucose content in the ball mill jar was 1 wt% of the mass of R-LFP. All other conditions were the same as in Example 1. A reference nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material, denoted as R-LFP@C&N-D5, was thus prepared. Oxygen, nitrogen, and hydrogen analysis showed that the nitrogen content in R-LFP@C&N-D5 was 0 wt%.
[0061] Test case The recycled lithium iron phosphate materials prepared in the above examples and comparative examples were assembled into CR2032 coin cells according to the following methods, and then subjected to a series of electrochemical performance tests as described below. The specific test methods are as follows, and the test results are shown in Table 1: (1) Assembly of CR2032 coin cell: The recycled lithium iron phosphate material, conductive carbon black, and polyvinylidene fluoride (PVDF) binder prepared in the above examples and comparative examples were mixed in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added to prepare a uniform slurry, which was then coated onto aluminum foil and dried in a vacuum oven at 120°C for 12 hours. The slurry was then stamped into a positive electrode sheet with a diameter of 14 mm. The loading of active material (recycled lithium iron phosphate material) was controlled at 2.0±0.2 mg / cm³. 2 Using lithium foil as the counter electrode, 1M LiPF6 solution as the electrolyte (solvent: EC:DEC:EMC=1:1:1), and Celgard 2325 (PP / PE / PP) as the separator, the CR2032 coin cell was assembled with the positive electrode prepared by the above method in an argon-protected glove box (H2O, O2<0.1ppm).
[0062] (2) Charge transfer resistance (Rct) and lithium-ion diffusion coefficient (DLi) + The test: Electrochemical impedance spectroscopy (EIS) was obtained using the Shanghai Chenhua electrochemical workstation. Parameter settings: testing was conducted at the open-circuit potential of the battery, with a frequency range of 100kHz to 10mHz and an AC perturbation amplitude of 5mV. Data analysis: ZView software was used to fit an equivalent circuit model (usually R(QR)(QR)W) to obtain the ohmic resistance (Rs), electrode / electrolyte interface film resistance (Rf), and charge transfer resistance (Rct). The Warburg coefficient (σ) was then calculated based on the slope in the low-frequency region.
[0063] Based on the Warburg coefficient (σ) obtained from the EIS test, DLi is calculated using the following formula. + :DLi + = R 2 T 2 / (2A 2 n 4 F 4 C 2 σ 2 ), where R is the ideal gas constant, T is the absolute temperature, A is the electrode area, n is the number of reaction electrons (n=1 for LiFePO4), F is the Faraday constant, and C is the lithium ion bulk concentration.
[0064] (3) Rate performance and cycle performance tests: all were tested on the Xinwei Battery Test System.
[0065] Rate performance test: Within the voltage range of 2.5-4.2V, constant current charge and discharge processes were performed at 1C and 10C rates, with 5 cycles at each rate. The discharge specific capacity at each rate was recorded, and the average value of the five cycles was taken as the discharge capacity at 1C and 10C rates.
[0066] Cyclic performance test: Continuous constant current charge-discharge cycles were performed at 1C rate. The specific capacity of the first discharge and the specific capacity of the discharge after the 100th cycle were recorded, and the capacity retention rate of the battery after the 100th cycle was calculated.
[0067] Table 1
[0068]
[0069] As shown in Table 1, compared with Comparative Examples 1-5, the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material prepared by the method of this invention has a higher lithium-ion diffusion coefficient (DLi). + It exhibits higher capacity performance and excellent rate and cycle performance due to its lower charge transfer resistance (Rct) and lower charge transfer resistance.
[0070] 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 without departing from the principles and spirit of the present invention.
Claims
1. A method for preparing a nitrogen-doped carbon-coated recycled lithium iron phosphate composite material, characterized in that, The preparation method includes the following steps: S1. The waste lithium iron phosphate cathode sheet is first treated with ultrasonic water bath, and then the cathode material is separated from the current collector by sieving. The obtained cathode material is ball-milled and then washed with alkaline solution to obtain cathode powder. S2. The positive electrode powder obtained in step S1 is mixed with citric acid, LiOH, and water and then subjected to hydrothermal treatment. The resulting hydrothermal reaction product is regenerated lithium iron phosphate. The regenerated lithium iron phosphate and carbonitride compounds are subjected to plasma ball milling under the condition of passing a reactive gas or inert gas I. The resulting ball-milled mixture is annealed under an inert gas II atmosphere to obtain a nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material. The nitrogen content in the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material is 0.5~3.5wt%.
2. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S1, the conditions for the ultrasonic water bath treatment include: a temperature of 70~90℃ and a time of 0.5~2h; Preferably, the ball milling conditions include: a rotation speed of 200~300 rpm and a time of 4~8 h.
3. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S1, the alkaline solution is a NaOH solution and / or a KOH solution; Preferably, the concentrations of the NaOH solution and the KOH solution are each independently 0.1~0.3M; Preferably, the alkaline washing treatment conditions include: a temperature of 50~70℃ and a time of 0.5~2h.
4. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S2, the mass ratio of the positive electrode powder to citric acid is 1:(1.5~2); Preferably, the solid-liquid ratio of the solution formed by the positive electrode powder, citric acid, LiOH, and water is 20-40 g / L; Preferably, the concentration of LiOH in the solution formed by citric acid, LiOH, and water is 0.7~0.9M.
5. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S2, the conditions for hydrothermal treatment include: pH 5-6, temperature 180-220℃, and time 4-8h.
6. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S2, the amount of the carbon and nitrogen compound used is 1-5 wt% of the mass of the recycled lithium iron phosphate. Preferably, the conditions for plasma ball milling include: a rotation speed of 200-300 rpm, a time of 4-8 h, a ball-to-material ratio of (10-20):1, and a discharge frequency of 10-50 kHz.
7. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S2, the annealing conditions include: a temperature of 500~700℃ and a time of 2~6h.
8. The method for preparing nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 1, characterized in that, In step S2, the carbon-nitrogen compound is selected from at least one of melamine, urea, acetonitrile, glucosamine, polyaniline, and polypyrrole; Preferably, the reactive gas is ammonia; Preferably, the inert gas I and inert gas II are each independently nitrogen and / or argon.
9. A nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material prepared by the method according to any one of claims 1 to 8.
10. The application of the nitrogen-doped carbon-coated regenerated lithium iron phosphate composite material according to claim 9 in lithium-ion batteries.