Preparation method for improving compaction density of regenerated lithium iron phosphate and product application

Abstract

The invention provides a preparation method for improving the compaction density of regenerated lithium iron phosphate and a product application. Aiming at the problem that the compaction density of the retired lithium iron phosphate material is low due to particle mechanical property degradation, particle size distribution imbalance and conductive structure damage, a cooperative regulation and control mechanism of body strengthening, interface plasticizing and conductive reconstruction is constructed, and a low-cycle atomic layer deposition and fused salt treatment combined process is adopted, so that the compaction density of the retired lithium iron phosphate material is improved. And the structure integrity, the interface compactness and the conductive network of the regenerated lithium iron phosphate particles are subjected to system reconstruction, so that the stacking efficiency and the compaction performance of the material are remarkably improved. The method is simple in technological process, low in energy consumption, capable of being directly integrated to an existing regeneration production line, suitable for lithium iron phosphate materials with different degradation degrees, environmentally friendly and high in resource utilization rate due to the adoption of a closed molten salt circulation system. The treated regenerated lithium iron phosphate material can meet the application requirements of high-compaction-density and high-energy-density batteries, and has a good industrialization prospect.

Description

Technical Field

[0001] This invention relates to the field of deep processing technology of lithium-ion battery recycled materials, and in particular to a preparation method and product application for improving the compaction density of recycled lithium iron phosphate. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) is widely used in power batteries and energy storage batteries due to its high thermal stability, excellent intrinsic safety, and long cycle life. Especially in recent years, with the continuous optimization of cell structure design, battery pack integration methods, and system-level energy management technologies, the lithium iron phosphate system has, to some extent, compensated for its relatively low theoretical energy density, leading to a continuous expansion of its application in long-range electric vehicles and a sustained increase in its market share.

[0003] However, as national and industry standards place higher demands on the driving range, energy efficiency, and overall performance of new energy vehicles, simply relying on structural optimization is gradually approaching technological bottlenecks. Further improving the volumetric energy density of lithium iron phosphate cathode materials has become one of the core directions for industry development. Among these, increasing the compaction density of lithium iron phosphate materials is widely recognized as a key technological path to achieving high-energy-density battery cells.

[0004] From the perspective of industry development trends, the compaction density requirements for lithium iron phosphate cathode materials are showing a continuous upward trend: around 2018, the compaction density of mainstream products was approximately 2.2 g / cm³. 3 By 2021, it had increased to 2.5 g / cm³. 3 By around 2025, the market has generally demanded a compaction density of 2.6 g / cm³ for lithium iron phosphate materials. 3 Even higher than 2.7 g / cm³ 3 This change clearly demonstrates that high actual density has become an important technical indicator and market trend for lithium iron phosphate materials.

[0005] On the other hand, against the backdrop of the rapid development of the new energy industry, a large number of early-stage lithium iron phosphate batteries are gradually entering the scrapping and retirement stage. High-value repair and regeneration, and the realization of resource recycling, have become a key focus of the industry. However, the lithium iron phosphate materials recovered and regenerated from existing retired lithium iron phosphate batteries generally suffer from low compaction density, making it difficult to meet the current application requirements of high-energy-density battery cells.

[0006] Analysis revealed that the fundamental reasons for the insufficient compaction density of regenerated lithium iron phosphate are mainly twofold: Firstly, after multiple charge-discharge cycles and complex service environments, the mechanical strength and structural integrity of the material particles degrade, making them prone to breakage or irreversible deformation during compaction. Secondly, the regeneration process inevitably introduces problems such as unbalanced particle size distribution, roughened particle surfaces, and increased agglomeration, further weakening the packing efficiency and densification capacity between particles. Furthermore, the disruption of the conductive network and the decline in interfacial bonding also limit the electrochemical performance under high compaction conditions to some extent.

[0007] Existing lithium iron phosphate repair and regeneration technologies mostly focus on component compensation, surface carbon coating, or simple particle size control, which are difficult to fundamentally improve the mechanical properties, interfacial plasticity, and conductive structure synergy of particles. Therefore, they cannot effectively overcome the application bottleneck of regenerated lithium iron phosphate materials under high pressure density conditions.

[0008] Therefore, there is an urgent need to propose a new technical approach that systematically improves the compaction performance and comprehensive electrochemical performance of regenerated lithium iron phosphate from multiple levels, such as material microstructure, particle mechanical properties, and interfacial conductive networks, by constructing a synergistic regulation mechanism of "bulk strengthening-interface plasticization-conductive reconstruction". This will enable the high-value application of regenerated lithium iron phosphate materials in high compaction density and high energy density batteries. Summary of the Invention

[0009] Purpose of the invention: The purpose of this invention is specifically to address the low compaction density (generally ≤2.0 g / cm³) of recycled lithium iron phosphate (LFP). 3 Addressing issues such as severe particle breakage and high interfacial impedance, this study employs a triple innovative process: nano-reinforced core-shell construction, surface plasticization treatment, and in-situ forming of a conductive-bonding dual-network, achieving a compaction density of recycled LFP ≥2.45 g / cm³. 3 This simultaneously improves cycle stability and rate performance.

[0010] In view of the above reasons, the present invention provides a preparation method for improving the compaction density of recycled lithium iron phosphate and its product application.

[0011] In a first aspect, the method for improving the compaction density of regenerated lithium iron phosphate includes the following steps: S1. Powder particle purification: The lithium iron phosphate powder obtained from waste recycling is subjected to oxygen-free heating treatment to obtain powder particles; S2. Constructing a nano-reinforced core-shell: Powder particles are placed in a fluidized bed reactor, and trimethylaluminum and water vapor are introduced and heated to deposit atomic layers on the surface of the powder particles; then, non-regenerated nano-lithium iron phosphate particles are formulated into a sol, impregnated with the powder particles, and heat-treated to obtain regenerated lithium iron phosphate powder particles with a core-shell structure. S3. Molten salt layer coating: The double lithium salt eutectic mixture and nano diamond powder are added to the fluidized bed to melt and form molten salt, which is directly sprayed onto the surface of the core-shell constructed regenerated lithium iron phosphate powder particles, and then placed under microwave treatment to form a molten salt layer structure. S4. Construction of conductive-bonding dual network: Powder particles forming a molten salt layer structure are dispersed in a mixed solution of lithium polyacrylate, carbon nanotubes and ethanol, stirred evenly, and polymerized and cured by ultraviolet light to obtain high-density regenerated lithium iron phosphate powder.

[0012] In some embodiments, during the oxygen-free heating process, the heating rate is 2-5°C / min, the temperature is raised to 385-750°C, and the temperature is maintained for 4-6 hours. This step aims to increase the purity of the lithium iron phosphate powder, preparing it for the next process.

[0013] In some embodiments, the molar ratio of trimethylaluminum to water vapor is 2:3-5; the heating deposition conditions are a temperature of 80-220°C and a pressure of 0.8-2 kPa; the deposition is performed 2-5 times until an atomic layer is formed. The thickness of the atomic layer is 2-5 nm. The coating uses lithium iron phosphate as the treatment target, and trimethylaluminum and water vapor as the atomic deposition coating atmosphere; when the atmosphere is insufficient for atomic deposition coating, trimethylaluminum and water vapor need to be continuously added; the Al2O3 atomic layer deposition formed by the coating can inhibit crack propagation and reduce particle breakage rate through the pinning effect.

[0014] In some embodiments, the non-regenerated nano-lithium iron phosphate particles have a particle size of 50-100 nm; the solid-liquid ratio of the sol is 1:3-7; further, the solid-liquid ratio of the sol is 1:5.

[0015] In some embodiments, the heat treatment atmosphere is an inert gas atmosphere, the heat treatment temperature is 220°C, and the heat treatment time is 1-4 hours.

[0016] In some embodiments, the lithium salt in the dual lithium salt eutectic mixture is selected from any two combinations of lithium hexafluorophosphate, lithium perchlorate, lithium oxalate borate, lithium bis(trifluorosulfonyl)imide, lithium tetrafluoroborate, lithium acetate, and lithium oxalate. For example, the dual lithium salt eutectic mixture can be a lithium nitrate-lithium hydroxide eutectic mixture with a molar ratio of 6:4.

[0017] In some embodiments, the nanodiamond powder has a particle size of 20-50 nm; the amount of nanodiamond powder added accounts for 0.8-1.5 wt% of the molten salt.

[0018] In some embodiments, the reaction temperature of the fluidized bed is 220°C, the residence time of the molten salt is 30-120 min, and the thickness of the formed molten salt layer structure is 100-200 nm.

[0019] In some embodiments, the microwave conditions for microwave processing are 800W power, 2.45 GHz microwave field, and processing for 30-120 seconds.

[0020] In some embodiments, the mass ratio of the powder particles of the molten salt layer structure to the mixed solution is 1:3-5; the mass ratio of lithium polyacrylate to carbon nanotubes added in the mixed solution is 3-8:1-3; and the solid content of the mixed solution is 10%.

[0021] In some embodiments, the polymerization and curing conditions are a wavelength of 365 nm and an energy density of 300-500 mJ / cm². 2 .

[0022] In a second aspect, the present invention provides the application of the high-density regenerated lithium iron phosphate powder in lithium battery regeneration materials.

[0023] In some embodiments, the application includes batch mixing high-density regenerated lithium iron phosphate powder with conventional low-density regenerated lithium iron phosphate powder to obtain regenerated lithium iron phosphate material.

[0024] In some embodiments, the high-pressure dense regenerated lithium iron phosphate powder has a particle size D10 ≥ 3 μm and D90 ≥ 18 μm.

[0025] In some embodiments, the density of the conventional low-compact density regenerated lithium iron phosphate powder is ≤2.0 g / cm³. 3 .

[0026] In some embodiments, the mass ratio of the high-density regenerated lithium iron phosphate powder to the conventional low-density regenerated lithium iron phosphate powder is one of the following: 1:9, 2:8, 3:7, or 4:6. Beneficial effects

[0027] The technical solution provided by this invention repairs cracks and improves the compressive strength of individual particles through nano-reinforced core-shell construction, reduces rolling stress through surface plasticization treatment, and constructs a "point-line" composite conductive path through in-situ forming of a conductive-bonding dual-network to reduce interfacial resistance. These three innovative processes work synergistically to break through the performance limits of recycled LFP, achieving a batch-stable recycled lithium iron phosphate compaction density ≥2.45 g / cm³, compressive strength ≥210 MPa, and interfacial resistance reduced to 8 Ω·cm. The process is highly compatible and lower in cost, making it suitable for large-scale recycling scenarios. Attached Figure Description

[0028] Figure 1 This is an SEM image of the high-density regenerated lithium iron phosphate powder prepared in Example 1 of the present invention.

[0029] Figure 2 The rate performance diagram of compacting the high-density regenerated lithium iron phosphate powder prepared in Example 1 into a 10Ah soft package.

[0030] Figure 3 The cycling performance diagram of a 10Ah soft-pack battery cell made by compacting the high-density regenerated lithium iron phosphate powder prepared in Example 2. Detailed Implementation

[0031] The present invention will be described below with reference to specific embodiments. It should be noted that the following embodiments are examples of the present invention and are used only to illustrate the invention, not to limit it. Other combinations and various modifications within the scope of the present invention can be made without departing from its spirit or scope.

[0032] Unless otherwise specified, all chemical reagents used in this invention are commercially available analytical grade reagents.

[0033] In the following experiments, the conventional regenerated lithium iron phosphate was purchased from Guangdong Wentai New Energy, model WT01.

[0034] Example 1 S1. Powder particle purification: The lithium iron phosphate powder obtained from the waste recycling (recycled lithium iron phosphate powder from retired power batteries (D50=4.2μm, residual lithium 3.2wt%)) was heated in a nitrogen environment at a temperature rise rate of 2℃ / min to 385℃ and held for 6h for purification. S2. Construction of nano-reinforced core-shell: The purified powder particles were placed in a fluidized bed reactor, and a mixture of trimethylaluminum and water vapor with a molar ratio of 2:4 was introduced. The mixture was heated at 100℃ and 0.1kPa for deposition, and the process was repeated 3 times to form a 3nm atomic layer deposition on the surface of the powder particles. Then, non-regenerated lithium iron phosphate with a D50 particle size of 50nm was taken, and a mixture of water and ethanol with a volume ratio of 2:1 was used as the solvent. The solid content of lithium iron phosphate was 20%, and 0.2% of polyvinylpyrrolidone PVP-K30 was added as a dispersant. After mixing, the mixture was placed in a nano-lithium iron phosphate sol to agglomerate. After stirring and dispersing evenly, a non-regenerated nano-lithium iron phosphate sol was formed. The sol and powder particles were impregnated at a volume ratio of 5:1. After centrifugation, the mixture was heat-treated in nitrogen at 220℃ for 1h to obtain regenerated lithium iron phosphate powder particles with a core-shell structure. S3. Molten Salt Layer Coating: A double lithium salt eutectic mixture (lithium nitrate-lithium hydroxide eutectic mixture, molar ratio 6:4, melting point 200℃) is mixed with 1 wt% of 50 nm nanodiamond powder, and then added to a fluidized bed to melt and form molten salt. The molten salt is then sprayed directly onto the surface of the core-shell constructed regenerated lithium iron phosphate powder particles at 220℃. The residence time in the fluidized bed reactor is controlled to be 100 min, and the molten salt layer thickness is 200 nm. The mixture is then placed in a 2.45 GHz microwave field for 30 seconds (power 800W) to form a molten salt layer structure. S4. Construction of conductive-bonding dual network: Powder particles forming a molten salt layer structure are dispersed in a mixed solution at a mass ratio of 1:3. The mixed solution is obtained by mixing lithium polyacrylate, carbon nanotubes and ethanol (LiPAA:CNT=3:1, solid content 10%). After stirring evenly, polymerization and solidification are initiated by ultraviolet light at a wavelength of 365 nm and an energy density of 300 mJ / cm² to obtain high-density regenerated lithium iron phosphate powder.

[0035] Example 2 S1. Powder particle purification: The lithium iron phosphate powder obtained from the waste recycling (recycled lithium iron phosphate powder from retired energy storage batteries (D50=6.5μm, Fe / P deviation ratio 8%)) was heated in a nitrogen environment at a temperature rise rate of 2℃ / min to 385℃ and held for 6h for purification. After purification, it was mixed with 0.5 wt% lithium dihydrogen phosphate to obtain powder particles. S2. Construction of nano-reinforced core-shell: The purified powder particles were placed in a fluidized bed reactor, and a mixture of trimethylaluminum and water vapor with a molar ratio of 2:4 was introduced. The mixture was heated at 100°C and 0.1 kPa for deposition, and the process was repeated 5 times to form a 5 nm atomic layer on the surface of the powder particles. Then, non-regenerated lithium iron phosphate with a D50 particle size of 50 nm was taken, and a mixture of water and ethanol with a volume ratio of 2:1 was used as the solvent. The solid content of lithium iron phosphate was 20%, and 0.2% of polyvinylpyrrolidone (PVP-K30) was added as a dispersant. After mixing, the mixture was placed in a nano-lithium iron phosphate sol to agglomerate. After stirring and dispersing evenly, a non-regenerated nano-lithium iron phosphate sol was formed. The sol and powder particles were impregnated at a volume ratio of 5:1. After centrifugation, the mixture was heat-treated in nitrogen at 220°C for 1 h to obtain regenerated lithium iron phosphate powder particles with a core-shell structure. The subsequent steps are the same as in Example 1, and high-density regenerated lithium iron phosphate powder is prepared.

[0036] Comparative Example Comparative examples were prepared using the technical solution provided by invention patent CN119079960A.

[0037] S1. The lithium iron phosphate powder obtained from the waste recycling was heated in a nitrogen atmosphere at a temperature rise rate of 2℃ / min to 385℃ and held for 6h for purification. S2. Prepare materials according to the formula ratio: 98% purified recovered lithium iron phosphate powder, 0.2% lithium dihydrogen phosphate, 0.2% ferric oxide, 0.8% titanium dioxide, 1% glucose, and 25% solid content; S3. Pour water into a ball mill jar, add purified recovered lithium iron phosphate powder and stir for 30 minutes to obtain a slurry; S4. Add lithium dihydrogen phosphate, glucose, and ferric oxide to the above slurry and continue stirring for 1 hour to obtain a slurry. S5. Transfer the ball-milled slurry to a sand mill for grinding. At the same time, add titanium dioxide to the sand mill jar for grinding, so that the product particle size D50 = 0.7 ± 0.05 μm; S6. Spray dry the slurry obtained after grinding to obtain a yellow powder; S7. The sprayed material is fed into a kiln and sintered at a constant temperature of 750°C for 6 hours in a nitrogen-sealed atmosphere for 10 hours. Finally, it is cooled down and removed from the kiln to obtain a black powder. S8. The sintered powder is subjected to air jet milling, and the target particle size is achieved by adjusting the air jet milling parameters. S9. The pulverized black powder is sieved, demagnetized, and packaged to obtain recycled lithium iron phosphate powder.

[0038] Performance testing The regenerated lithium iron phosphate powders prepared in the above embodiments and comparative examples were used to prepare soft-pack batteries (the positive electrode material was a mixture of the regenerated lithium iron phosphate powder provided in the embodiments or comparative examples and conventional low-compaction-density regenerated lithium iron phosphate powder with a compaction density of 1.8 g / cm³ at a mass ratio of 3:7; the negative electrode material was shanshan graphite FSN-1; the separator was a PP base film; the electrolyte was 1M LiPF6 / EC-DMC; and the coating surface density was 18 mg / cm²). The following performance tests were conducted, and the test results are shown in Table 1.

[0039] 1. The compaction density of lithium iron phosphate powder was tested using an electronic pressure testing machine (model UTM7305Z09) with a pressure parameter of 30 kN.

[0040] 2. Use a powder resistivity tester to test the interface contact resistance.

[0041] 3. Electrode porosity was tested using a static adsorption surface area analyzer; 4. The 0.2C discharge capacity and the capacity retention rate at 5C and 100 cycles were tested using the Blue Electric Tester equipment in accordance with the national standard GB / T 31484-2015 "Requirements and Test Methods for Cycle Life of Power Batteries for Electric Vehicles".

[0042] Table 1 Test Results

[0043] Figure 2 To obtain Example 1, according to 2.48 g / cm 3 The compaction ratio of the 10Ah soft pack shows that its performance is similar to that of virgin material. Figure 3 The regeneration and compaction of lithium iron phosphate powder from retired energy storage batteries in Example 2 resulted in cell cycle performance similar to that of virgin materials. This demonstrates that the technical solution of the present invention can simultaneously improve the cycle stability and rate performance of regenerated lithium-ion batteries.

[0044] This invention achieves precise enhancement of the structure and interface of regenerated lithium iron phosphate particles by introducing a low-cycle atomic layer deposition (ALD) synergistic molten salt conditioning process. The ALD process requires only 2-5 deposition cycles to complete the construction of the effective functional layer, with a single batch processing time of less than 5 minutes. This significantly reduces equipment operating time and energy consumption while ensuring improved material performance. Simultaneously, the use of a molten salt system for particle densification and morphology optimization replaces traditional methods relying on costly spheroidization equipment, effectively reducing overall equipment investment and operating costs. Compared to conventional particle size distribution or multiple mechanical shaping processes, this invention reduces overall energy consumption by approximately 40%, demonstrating outstanding scalability and economic efficiency.

[0045] The technical solution of this invention can be directly integrated into the end of existing retired lithium iron phosphate battery material regeneration lines. It only requires the addition of a fluidized bed treatment unit and an ultraviolet heating curing unit to the existing process, without large-scale modification of the main production line, demonstrating good engineering scale-up feasibility and process compatibility. Furthermore, this solution exhibits good adaptability to lithium iron phosphate materials with different degrees of degradation, suitable for regenerated lithium iron phosphate raw materials with an initial specific capacity of not less than 120 mAh / g, and can stably achieve increased compaction density, enhancing the application range and product consistency of the regenerated materials.

[0046] Meanwhile, this invention employs a closed-loop molten salt circulation system for processing, allowing the molten salt to be reused within the system. This avoids the generation of wastewater and high-salt waste liquids in traditional wet processes, significantly reducing the environmental burden. The nanodiamond material introduced as a conductive and structurally reinforcing component exhibits good stability and recyclability during the process, with a recovery rate exceeding 95%. This effectively reduces material consumption and the risk of secondary pollution, aligning with the development direction of green manufacturing and a circular economy for new energy materials.

[0047] This invention can also be implemented in various other ways. Without departing from the spirit and essence of this invention, those skilled in the art can make various corresponding changes and modifications according to this invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A method for preparing recycled lithium iron phosphate with improved compaction density, characterized in that, The preparation method includes the following steps: S1. Powder particle purification: The lithium iron phosphate powder obtained from waste recycling is subjected to oxygen-free heating treatment to obtain powder particles; S2. Constructing a nano-reinforced core-shell: Powder particles are placed in a fluidized bed reactor, and trimethylaluminum and water vapor are introduced and heated to deposit atomic layers on the surface of the powder particles; then, non-regenerated nano-lithium iron phosphate particles are formulated into a sol, impregnated with the powder particles, and heat-treated to obtain regenerated lithium iron phosphate powder particles with a core-shell structure. S3. Molten salt layer coating: The double lithium salt eutectic mixture and nano diamond powder are added to the fluidized bed to melt and form molten salt, which is directly sprayed onto the surface of the core-shell constructed regenerated lithium iron phosphate powder particles, and then placed under microwave treatment to form a molten salt layer structure. S4. Construction of conductive-bonding dual network: Powder particles forming a molten salt layer structure are dispersed in a mixed solution of lithium polyacrylate, carbon nanotubes and ethanol, stirred evenly, and polymerized and cured by ultraviolet light to obtain high-density regenerated lithium iron phosphate powder.

2. The preparation method according to claim 1, characterized in that, In the oxygen-free environment heating, the heating rate is 2-5℃ / min, the temperature is heated to 385-750℃, and the temperature is maintained for 4-6 hours.

3. The preparation method according to claim 1, characterized in that, The molar ratio of trimethylaluminum to water vapor is 2:3-5; the heating deposition conditions are a temperature of 80-220℃ and a pressure of 0.8-2 kPa; the deposition is performed 2-5 times, and the thickness of the atomic layer is 2-5 nm.

4. The preparation method according to claim 1, characterized in that, The non-regenerated nano-lithium iron phosphate particles have a particle size of 50-100 nm; the solid-liquid ratio of the sol is 1:3-7.

5. The preparation method according to claim 1, characterized in that, The heat treatment temperature is 220℃, and the heat treatment time is 1-4 hours.

6. The preparation method according to claim 1, characterized in that, The lithium salt in the eutectic mixture is selected from any two combinations of lithium hexafluorophosphate, lithium perchlorate, lithium oxalate borate, lithium bis(trifluorosulfonyl)imide, lithium tetrafluoroborate, lithium acetate, and lithium oxalate.

7. The preparation method according to claim 1, characterized in that, The nanodiamond powder has a particle size of 20-50 nm; the amount of nanodiamond powder added accounts for 0.8-1.5 wt% of the molten salt; the reaction temperature of the fluidized bed is 220℃, the residence time of the molten salt is 30-120 min, and the thickness of the molten salt layer structure formed is 100-200 nm.

8. The preparation method according to claim 1, characterized in that, The mass ratio of the powder particles of the molten salt layer structure to the mixed solution is 1:3-5; the mass ratio of lithium polyacrylate to carbon nanotubes in the mixed solution is 3-8:1-3; and the solid content of the mixed solution is 10%.

9. The application of the high-density regenerated lithium iron phosphate powder prepared by the preparation method according to any one of claims 1-8 in lithium battery regeneration materials.

10. The application of the high-density regenerated lithium iron phosphate powder according to claim 9 in lithium battery regeneration materials includes batch mixing the high-density regenerated lithium iron phosphate powder with conventional low-density regenerated lithium iron phosphate powder to obtain regenerated lithium iron phosphate materials.

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