Supercritical preparation method and application of self-repairing and high-compaction lithium iron phosphate
By using a supercritical method to repair the surface of lithium iron phosphate seeds and perform pulsed epitaxial growth, combined with the formation of a carbon coating layer, the problems of low tap density and insufficient electronic conductivity in lithium iron phosphate preparation were solved, thus realizing the preparation of high-performance lithium iron phosphate and the effective utilization of resources.
Patent Information
- Application Number
- CN202511251386.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-03
- Publication Date
- 2025-11-18
AI Technical Summary
Existing lithium iron phosphate preparation processes have difficulty controlling the microstructure and particle packing behavior of the products, resulting in low tap density, waste of unqualified materials, and increased production costs. The materials prepared by traditional methods have low electronic conductivity, which limits the performance of lithium-ion batteries.
A supercritical preparation method was adopted to prepare lithium iron phosphate with high density and excellent electrochemical performance by repairing and activating the surface of unqualified lithium iron phosphate seeds, performing pulsed epitaxial growth under supercritical conditions, and forming a carbon coating layer by subsequent heat treatment.
By effectively utilizing substandard materials, the tap density and electrochemical performance of lithium iron phosphate were improved, production costs were reduced, the volumetric energy density and rate performance of lithium-ion batteries were enhanced, and cycle stability was extended.
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Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of lithium ion battery cathode material, in particular to a supercritical preparation method of self-repairing and high-compaction lithium iron phosphate and application thereof. BACKGROUND
[0002] Lithium iron phosphate has become one of the most widely used cathode materials in the field of lithium ion batteries due to its structural stability, high safety and low cost. With the rapid development of electric vehicles and large-scale energy storage markets, higher requirements are put forward for the energy density of lithium iron phosphate batteries. Since the specific capacity of lithium iron phosphate is close to its theoretical value, improving the tap density and compaction density of the material and thus increasing the volume energy density of the battery have become the key direction of current technology development.
[0003] However, existing lithium iron phosphate preparation processes, such as traditional hydrothermal method or solid phase method, still have challenges in controlling the micro-morphology and particle accumulation behavior of the product. These methods often inevitably produce products with irregular morphology, wide particle size distribution or serious particle agglomeration, which directly leads to low tap density of the material and limits its performance in high-end applications.
[0004] In addition, during the industrialized mass production of lithium iron phosphate, due to fluctuations in process parameters, unqualified products that do not meet the factory standards in terms of physical or electrochemical properties are often produced. These materials are usually treated as industrial waste or at a very low price, not only causing serious resource waste, but also increasing the production cost of enterprises. How to effectively repair and improve the performance of such unqualified lithium iron phosphate so that it regains its value is a technical problem that needs to be solved in the field.
[0005] Therefore, it is of great technical significance and economic value to develop a new method that can not only realize the recycling of low-value materials, but also prepare lithium iron phosphate with high tap density and excellent electrochemical performance. SUMMARY
[0006] In view of the deficiencies of the prior art, the present application provides a supercritical preparation method of self-repairing and high-compaction lithium iron phosphate and application thereof, which solves the problem that in the production process of lithium iron phosphate in the prior art, some materials do not meet the specifications of the final product in terms of physical or chemical properties. Such unqualified materials are usually discarded, leading to resource waste and increased production cost. At the same time, the intrinsic electronic conductivity of the lithium iron phosphate material prepared by the traditional method is low, and the compaction density after particle accumulation is not high, which limits the rate performance and volume energy density of lithium ion batteries.
[0007] The first aspect of the present application provides a supercritical preparation method of self-repairing and high-compaction lithium iron phosphate, comprising the following steps: S1, Preparation of the precursor: A slurry A is prepared by mixing unqualified lithium iron phosphate seed crystals, an in-situ synergistic agent, and deionized water, wherein the mass ratio of the unqualified lithium iron phosphate seed crystals to the in-situ synergistic agent is 1000:(10-50), and the mass ratio of the total mass of the unqualified lithium iron phosphate seed crystals and the in-situ synergistic agent to the deionized water is 1:(1.5-4).
[0008] A solution B is prepared by dissolving at least one of a lithium source, an iron source, a phosphorus source, and a metal ion additive in water as raw materials, wherein the mass ratio of the total mass of the raw materials to the water is 1:(2.3-19).
[0009] S2, Repair and activation of the seed crystals: The slurry A is continuously pumped into a reaction zone one in a tubular reactor, and in the reaction zone one, the temperature is controlled at 250-300°C and the pressure is controlled at 20-25 MPa, under which the water is in a supercritical state, and the in-situ synergistic agent is hydrolyzed to selectively etch the surface of the unqualified lithium iron phosphate seed crystals to remove the defect structure and amorphous phase on the surface, thereby obtaining an activated slurry containing lithium iron phosphate seed crystals with activated surfaces.
[0010] S3, Injection of the raw materials: The solution B is injected into the activated slurry in a pulse feeding manner at the connection between the reaction zone one and a reaction zone two in the tubular reactor.
[0011] S4, Epitaxial growth: In the reaction zone two, the temperature is controlled at 280-400°C and the pressure is controlled at 25-30 MPa, and the raw materials in the pulse-injected solution B undergo heterogeneous nucleation and epitaxial growth on the surface of the activated lithium iron phosphate seed crystals to form the final lithium iron phosphate material.
[0012] S5, Separation and drying of the product: The final lithium iron phosphate material is separated and dried.
[0013] In a specific embodiment, the in-situ synergistic agent includes at least one of triethyl phosphate and phosphinic acid.
[0014] In a specific embodiment, the slurry A further contains a comb-shaped copolymer as a stabilizer, and the mass ratio of the unqualified lithium iron phosphate seed crystals to the comb-shaped copolymer is 1000:(5-20).
[0015] In a specific embodiment, the comb-shaped copolymer is synthesized by the following steps: Polyvinylpyrrolidone and polyethylene glycol monomethyl ether are dissolved in N, N-dimethylformamide, and a free radical grafting reaction is carried out at 70.0±2.0 DEG C under the condition of azobisisobutyronitrile as an initiator.
[0016] In a specific embodiment, the frequency of the pulse feeding is 1.0-20.0 Hz, and the pulse width is 10.0-100.0 ms.
[0017] In a specific embodiment, the method further comprises a subsequent treatment step after step S5: The separated and dried final lithium iron phosphate material is mixed with a carbon precursor, and heat treatment is carried out under an inert atmosphere, during which the carbon precursor pyrolyzes to form a carbon coating layer on the surface of the particles of the final lithium iron phosphate material.
[0018] In a further embodiment, the heat treatment temperature is 650-750 DEG C, and the holding time is 2-6 hours.
[0019] In a further embodiment, the carbon precursor comprises at least one of glucose, sucrose and citric acid. The mass ratio of the final lithium iron phosphate material to the carbon precursor is 100:(2-5).
[0020] In a specific embodiment, the lithium source is lithium carbonate, the iron source is iron nitrate nonahydrate, the phosphorus source is phosphoric acid, and the metal ion additive is titanium tetraisopropoxide.
[0021] The second aspect of the application provides a self-repairing and high-compaction lithium iron phosphate prepared by the preparation method of the first aspect for use in the preparation of a positive electrode of a lithium ion battery.
[0022] The application provides a supercritical preparation method of a self-repairing and high-compaction lithium iron phosphate and applications thereof. 1. The application can effectively utilize materials that are regarded as low-value or waste in conventional production by repairing and activating the surface of unqualified lithium iron phosphate seeds under supercritical hydrothermal conditions and then performing subsequent epitaxial growth, thereby directly reducing the dependence on new raw materials, reducing environmental problems and costs caused by waste treatment, and thus reducing the total production cost of lithium iron phosphate materials.
[0023] 2、The application injects the raw material solution into the activated seed slurry in a pulse feeding mode, and performs epitaxial growth under specific supercritical conditions, so that the controlled feeding and growth mode inhibits free homogeneous nucleation in the solution, promotes ordered lattice matching growth of the raw material on the seed surface, and is beneficial to generating lithium iron phosphate particles with high crystallinity, regular morphology and concentrated particle size distribution, so that the final material powder can realize higher compaction density when stacking, and then helps to improve the volume energy density in the application of lithium ion battery cathode.
[0024] 3、The application can form a layer of conductive carbon network with uniform thickness and physical adhesion on the surface of the lithium iron phosphate particles with pure surface and regular morphology prepared in the foregoing steps through subsequent heat treatment and in-situ carbon coating steps, greatly improves the intrinsic electronic conductivity of the lithium iron phosphate material, provides an effective path for the rapid transmission of lithium ions and electrons, thereby improves the electrochemical reaction kinetics of the material, and finally reflects the improvement of the rate performance and cycle stability of the material. DETAILED DESCRIPTION
[0025] The technical solutions in the embodiments of the application will be described below in combination with the examples and test examples of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.
[0026] Experimental raw materials: The main raw materials and reagents used in the following examples and comparative examples are as follows, and the reagents not specifically described are commercially available analytical pure or higher grade products.
[0027] Triethyl phosphate, CAS: 78-40-0; Phosphinic acid, CAS: 6303-21-5; Lithium carbonate, CAS: 554-13-2; Iron nitrate nonahydrate, CAS: 7782-61-8; Phosphoric acid, CAS: 7664-38-2; Titanium tetraisopropoxide, CAS: 546-68-9; Polyvinylpyrrolidone, CAS: 9003-39-8; Polyethylene glycol monomethyl ether, CAS: 9004-74-4; N,N-dimethylformamide, CAS: 68-12-2; Azobisisobutyronitrile, CAS: 78-67-1; Diethyl ether, CAS: 60-29-7; Glucose, CAS: 50-99-7; Sucrose, CAS: 57-50-1; Citric acid, CAS: 77-92-9; Anhydrous ethanol, CAS: 64-17-5; High purity nitrogen, CAS: 7727-37-9; Deionized water, CAS: 7732-18-5.
[0028] Example 1-3: Example 1:
[0029] The embodiment provides a preparation method of self-repairing and high-compaction lithium iron phosphate, and specific steps are as follows: Synthesis of comb-shaped copolymer: polyvinylpyrrolidone (PVP K30), polyethylene glycol monomethyl ether (mPEG, MW=2000), azobisisobutyronitrile (AIBN) and N,N-dimethylformamide (DMF) are proportioned according to a mass ratio of 10:20:0.15:250, PVP and mPEG are dissolved in DMF in a three-necked flask provided with a reflux condenser, nitrogen is introduced for 30 minutes, then the system is heated to 70.0 DEG C, AIBN is added, and reaction is carried out for 24.0 hours under the protection of nitrogen, after reaction, the mixture is dropped into excessive diethyl ether for precipitation, the precipitate is collected by filtration, washed with diethyl ether for three times, and dried in a vacuum oven at 60.0 DEG C for 24.0 hours, to obtain comb-shaped copolymer powder.
[0030] Preparation of slurry A: unqualified lithium iron phosphate seed crystals, triethyl phosphate (TEP) and prepared comb-shaped copolymer are mixed according to a mass ratio of 1000:20:10, and the above solid-phase mixture is added into deionized water to prepare slurry A with a solid-phase total mass to deionized water mass ratio of 30:70.
[0031] Preparation of solution B: lithium carbonate, iron nitrate nonahydrate and titanium tetraisopropoxide are dissolved in deionized water to prepare solution B with a raw material total mass to deionized water mass ratio of 15:85.
[0032] Supercritical synthesis: slurry A is continuously pumped into the reaction zone one of the tubular reactor at a flow rate of 30.0 mL / min, the temperature of the reaction zone one is controlled to be 280.0 DEG C, and the pressure is controlled to be 22.0 MPa, at the connection between the reaction zone one and the reaction zone two, solution B is pulsedly injected at a frequency of 10.0 Hz and a pulse width of 50.0 ms. The temperature of the reaction zone two is controlled to be 350.0 DEG C, and the pressure is controlled to be 28.0 MPa.
[0033] Product collection and post-treatment: The reaction product was collected by a separator, washed with deionized water by centrifugation for three times, washed with anhydrous ethanol once, and then dried in a vacuum oven at 100.0 °C for 24.0 hours to obtain the lithium iron phosphate raw powder.
[0034] Carbon coating: The obtained lithium iron phosphate raw powder was uniformly mixed with anhydrous glucose according to a mass ratio of 100:3, and the mixed powder was heated to 700.0 °C at a heating rate of 5.0 °C / min under a nitrogen atmosphere, and kept for 4.0 hours. After natural cooling, the final lithium iron phosphate material was obtained, denoted as A1.
[0035] Example 2:
[0036] The preparation method of this example is basically the same as that of Example 1, except that: In the preparation step of slurry A, phosphinic acid was used instead of triethyl phosphate as an in-situ synergistic agent, and the mass ratio of unqualified lithium iron phosphate seed crystal to phosphinic acid was 1000:30.
[0037] In the carbon coating step, sucrose was used instead of anhydrous glucose as a carbon precursor, and the mass ratio of lithium iron phosphate raw powder to sucrose was 100:4.
[0038] The obtained final lithium iron phosphate material was denoted as A2.
[0039] Example 3:
[0040] The preparation method of this example is basically the same as that of Example 1, except that: In the preparation step of slurry A, the mass ratio of unqualified lithium iron phosphate seed crystal to comb copolymer was 1000:20.
[0041] In the supercritical synthesis step, the temperature of reaction zone one was controlled at 300.0 °C, and the pressure was 25.0 MPa; the temperature of reaction zone two was controlled at 400.0 °C, and the pressure was 30.0 MPa.
[0042] The obtained final lithium iron phosphate material was denoted as A3.
[0043] Comparative Examples 1-4: Comparative Example 1: The preparation method of this comparative example is basically the same as that of Example 1, except that: in the preparation step of slurry A, triethyl phosphate (TEP) was not added, and the remaining steps and parameters were exactly the same as those of Example 1. The obtained final material was denoted as D1.
[0044] Comparative Example 2: The preparation method of the present comparative example is basically the same as that of Example 1, except that in the supercritical synthesis step, a continuous constant flow pump is used instead of a pulse pump to inject solution B, with an average flow rate identical to the average flow rate corresponding to the pulse feeding in Example 1, and the remaining steps and parameters are exactly the same as those in Example 1. The final material obtained is denoted as D2.
[0045] Comparative Example 3: The preparation method of the present comparative example is basically the same as that of Example 1, except that after completing step 5 (product collection and post-treatment), it is immediately terminated without performing step 6 (carbon coating) operation, and the obtained uncoated lithium iron phosphate material is denoted as D3.
[0046] Comparative Example 4: The present comparative example is prepared by a conventional batch hydrothermal method, and the unqualified lithium iron phosphate seed crystals and all the raw materials contained in solution B in Example 1 are added into deionized water at one time, and after being mixed uniformly, it is placed in a batch hydrothermal reactor and reacted at 220°C for 12 hours. After the reaction is completed, the product is separated and dried to obtain lithium iron phosphate raw powder. The subsequent carbon coating step is exactly the same as step 6 of Example 1. The final material obtained is denoted as D4.
[0047] Test Examples 1-4: Test Example 1: Physical property characterization Test method: Tap density test: According to national standard GB / T 5162-2006, using Quantachrome Autotap type powder tap density instrument for testing.
[0048] Accurately weigh 50.0 g of the sample to be tested, and place it into a clean and dry 100 mL measuring cylinder. Fix the measuring cylinder on the instrument and vibrate it at a frequency of 300 times per minute for 2500 times. Record the volume of the powder after tapping, and calculate the tap density.
[0049] Particle size distribution test: According to international standard ISO 13320:2009, using Malvern Mastersizer 3000 type laser particle size analyzer for testing.
[0050] Add a small amount of the sample to be tested into anhydrous ethanol as a dispersion medium, and ultrasonically treat it in the ultrasonic dispersion unit of the instrument for 180 seconds to ensure that the particles are fully dispersed. Set the pump speed to 2000 rpm for circulation testing, record the particle size distribution data D10, D50 and D90, and calculate the particle size distribution width according to the formula Span = (D90-D10) / D50.
[0051] Test results: The materials (A1, A2, A3, D1, D2, D3, D4) prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to physical property characterization according to the above-mentioned test methods, and the test results are recorded in Table 1.
[0052] Table 1: Test results of physical properties of each sample
[0053] The data in Table 1 shows that the tap density of the materials prepared by the methods of Examples A1, A2 and A3 are all higher than 2.1 g / cm 3 , and the span values of the particle size distribution of the materials are all lower than 0.9, which indicates that the particles have high packing efficiency and size uniformity, and this result is attributed to the two-stage supercritical reaction design in the technical solution.
[0054] In reaction zone one, the surface of the unqualified seed crystals is repaired in situ with the synergistic reagent in the supercritical water environment, eliminating the original surface defects and forming uniform and highly active growth elementary points. Subsequently, in reaction zone two, the instantaneous concentration of the raw material is controlled through pulse feeding, inhibiting homogeneous nucleation in the solution, so that the raw material undergoes ordered epitaxial growth on the activated seed crystal surface, thereby forming final particles with regular morphology and high crystallinity.
[0055] In contrast, the materials prepared in Comparative Examples D1, D2 and D4 exhibit significant differences in physical properties. Comparative Example D1 lacks the repair step of the in-situ synergistic reagent, so the defects on the surface of the unqualified seed crystals directly serve as sites for subsequent growth, leading to disordered growth and irregular morphology, and the tap density is only 1.15 g / cm 3， . Comparative Example D2 uses continuous feeding, which cannot effectively control the supersaturation at the reaction interface, resulting in the coexistence of partial homogeneous nucleation and non-uniform growth, and the tap density is 1.63 g / cm 3 , which is lower than that of the examples. Comparative Example D4 uses the traditional batch hydrothermal method, and the nucleation and growth processes during the reaction are difficult to control, leading to serious particle agglomeration and irregular morphology, and the tap density is only 1.02 g / cm 3 .
[0056] In summary, the data clearly confirms the combined effect of the two consecutive steps of repair activation and pulse growth in the technical solution. It is through the first step of pretreatment of the seed crystal substrate and the second step of precise control of the growth process that the regular morphology and high tap density of the final product particles are ensured. The data of Comparative Example D3 shows that the subsequent carbon coating step has no significant effect on the tap density and particle size distribution of the particles, further confirming that the physical morphology of the particles is determined in the supercritical synthesis stage. This method provides a specific technical path for preparing high-tap-density lithium iron phosphate materials from unqualified raw materials.
[0057] Test Example 2: Electrochemical basic performance characterization Test battery preparation: Each active material powder prepared in the examples and comparative examples, Super P conductive carbon black and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 90:5:5 in N-methyl pyrrolidone (NMP) solvent, stirred uniformly using a planetary ball mill, to prepare a positive electrode slurry, the slurry was uniformly coated on an aluminum foil current collector, dried in a vacuum oven at 120.0°C for 12.0 hours, the dried electrode was punched into a circular electrode with a diameter of 14.0mm, and was pressed to a compacted density of 3.2g / cm 3 In an argon-filled glove box (H2O <0.1 ppm, O2<0.1 ppm), a CR2032 type button cell was assembled with the positive electrode as the working electrode, a lithium metal sheet as the counter electrode, Celgard 2400 as the separator, and a 1M LiPF6 solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1) as the electrolyte.
[0058] First charge-discharge performance test method: The assembled button cell was tested using a NEWARE BTS-4000 battery test system under constant temperature conditions at 25.0°C.
[0059] The test system was as follows: a complete charging and discharging cycle was performed at a constant current of 0.1C (1C=170mA / g) in the voltage range of 2.5V to 4.2V (vs. Li / Li + ). The first charge specific capacity, the first discharge specific capacity, and the first coulombic efficiency (first discharge specific capacity / first charge specific capacity x 100%) were recorded.
[0060] Test results: The materials (A1, A2, A3, D1, D2, D3, D4) prepared in Examples 1-3 and Comparative Examples 1-4 were each prepared into a battery according to the above method and tested, and the test results are recorded in Table 2.
[0061] Table 2: Electrochemical basic performance test results of each sample
[0062] The data in Table 2 shows that the materials prepared in Examples A1, A2 and A3 all exhibit high first discharge specific capacity and high first coulombic efficiency. This result is a direct manifestation of the high crystallinity and structural integrity of the materials.
[0063] The two-stage reaction in the technical solution first removes the crystal lattice defects hindering lithium ion intercalation / deintercalation by repairing and activating the surface of the seed crystal, providing a uniform substrate for subsequent growth; then, the controlled epitaxial growth realized by pulse feeding ensures that the newly grown lithium iron phosphate has a highly ordered crystal structure. This structural integrity allows most of the active sites in the material to participate in electrochemical reactions, thereby releasing high specific capacity. At the same time, the regular particle surface and the uniform carbon layer formed subsequently jointly construct a stable electrode / electrolyte interface, reducing the occurrence of irreversible side reactions in the first cycle, thus achieving high coulombic efficiency.
[0064] In contrast, the electrochemical performance indicators of Comparative Examples D1, D2 and D4 are lower than those of the examples. Comparative Example D1 lacks repair of unqualified seeds, and its material inherits a large number of structural defects, which limit the transport of lithium ions and solid phase transformation, resulting in low active material utilization and a discharge capacity of only 121.7 mAh / g.
[0065] The continuous feeding method used in Comparative Example D2 results in a decrease in the crystallinity of the final product and the presence of amorphous phase due to uneven control of reactant concentration, resulting in a lower capacity than the examples. The traditional batch hydrothermal method used in Comparative Example D4 results in disordered structure and severe agglomeration of the final product due to the uncontrolled nucleation and growth process, resulting in low electrochemical activity and a discharge capacity of only 118.9 mAh / g.
[0066] The results of Comparative Example D3 further clarify the function of the carbon coating layer. After going through the same supercritical synthesis steps as Example 1, the sample only lacks the final carbon coating heat treatment, and its discharge specific capacity drops to 95.3 mAh / g. This directly indicates that the electronic conductivity of lithium iron phosphate material itself is extremely low, and without the presence of a surface conductive network, the internal charge transport process will be severely hindered, making most of the active material unable to effectively participate in electrochemical reactions.
[0067] Therefore, the structurally regular lithium iron phosphate particles prepared by the technical solution, together with the high-quality carbon coating layer formed on their surface, are two necessary components for achieving high specific capacity and high coulombic efficiency.
[0068] Test Example 3: Rate performance characterization Test method: CR2032 button cells prepared in Test Example 2 were used. Before rate capability test, all cells were first subjected to a full charge-discharge cycle at 25.0 °C with a constant current of 0.1 C to complete activation. Subsequently, the cells were charged to 4.2 V with a constant current of 0.5 C, and then charged at a constant voltage until the current was less than 0.05 C. After that, the cells were discharged to a cut-off voltage of 2.5 V with a constant current of 0.2 C, 0.5 C, 1 C, 2 C and 5 C, respectively. Each discharge rate was cycled for 3 times, and the stable discharge specific capacity of the 3rd cycle was recorded.
[0069] Test results: The materials (A1, A2, A3, D1, D2, D3, D4) prepared in Examples 1-3 and Comparative Examples 1-4 were tested according to the above methods, respectively, and the test results are recorded in Table 3.
[0070] Table 3: Test results of rate capability of each sample
[0071] The data in Table 3 clearly shows that the discharge specific capacity of the materials prepared by the methods of Examples A1, A2 and A3 is significantly higher than that of all the comparative materials at all tested current densities. Especially at a high rate of 5 C, the capacity retention rate of the example materials is all above 74%, while the capacity retention rate of Comparative Examples D1, D3 and D4 is all below 35%. This result indicates that the material prepared by the technical solution has the ability to effectively charge and discharge at high current density, which is directly related to the transmission dynamics characteristics of the internal charge and ions.
[0072] The electrochemical performance of the material at high rate depends on the diffusion rate of lithium ions inside the solid phase particles and the transmission rate of electrons in the electrode material. The technical solution prepares lithium iron phosphate particles with high crystallinity and low defect concentration by repairing the surface of unqualified seeds and subsequent pulse epitaxial growth. This highly ordered crystal structure reduces the hindrance to lithium ion diffusion, providing a structural basis for the rapid migration of lithium ions inside the particles. In contrast, Comparative Examples D1, D2 and D4 cannot effectively control the crystallization process during preparation, and there are many lattice defects and disordered structures in the product, which will seriously hinder the solid-phase diffusion of lithium ions, leading to increased polarization at high current and rapid capacity decay.
[0073] The comparison between Comparative Example D3 and Example A1 clearly defines the role of the surface carbon coating layer. The core particles of the material of Comparative Example D3 are the same as A1, but due to the lack of a carbon coating layer, its 5C capacity retention is only 16.6%. This confirms that the low electronic conductivity of the lithium iron phosphate material itself is another key factor limiting its rate performance. The subsequent heat treatment step in the examples forms a uniform carbon conductive network on the surface of the particles. This network provides the necessary electron transport path for the electrochemical reaction, ensuring that electrons can quickly and uniformly reach every reaction site on the surface of the particles. Therefore, the synergistic effect of optimizing the internal structure of the particles to accelerate ion transport and building a surface conductive network to accelerate electron transport is the fundamental reason for ultimately achieving high rate performance of the material.
[0074] Test Example 4: Cycle stability characterization Test method: CR2032 button cells prepared in Test Example 2 were used. All the batteries were first subjected to a complete charge-discharge cycle at a constant current of 0.1C at 25.0°C to complete activation, and the discharge specific capacity of the 1st cycle was recorded. Subsequently, the batteries were charged to 4.2V at a constant current of 1C and then discharged to 2.5V at a constant current of 1C at a constant temperature of 25.0°C, which was one cycle. This process was repeated 500 times. The discharge specific capacity at the end of the 500th cycle was recorded, and the capacity retention was calculated according to the formula (discharge specific capacity of the 500th cycle / discharge specific capacity of the 1st cycle x 100%).
[0075] Test results: The materials (A1, A2, A3, D1, D2, D3, D4) prepared in Examples 1-3 and Comparative Examples 1-4 were tested according to the above method, and the test results are recorded in Table 4.
[0076] Table 4: Cycle stability test results of each sample
[0077] The data in Table 4 shows that the materials prepared in Examples A1, A2 and A3 have a capacity retention of more than 96% after 500 cycles. This result indicates that the material has high structural stability and interface stability. The present technical solution prepares lithium iron phosphate particles with high crystalline integrity by surface repair of unqualified seeds and subsequent pulse epitaxial growth. This inherent structural stability enables the material to maintain the integrity of its crystal lattice during repeated lithium ion intercalation and deintercalation processes, effectively inhibiting the generation and expansion of particle microcracks caused by lattice stress due to phase transition, thereby reducing the loss of active material.
[0078] In contrast, the materials of Comparative Examples D1, D2 and D4 showed obvious capacity fading in long-term cycling. The materials of Comparative Examples D1 and D4 had more lattice defects and irregular morphology due to inheritance or during preparation, and their particles were more prone to structural degradation and pulverization during cycling, leading to the gradual loss of electrical contact between the active material and the conductive network, and continuous capacity decline. The material of Comparative Example D2 had lower crystallinity than the examples, and its structural stability was between that of the examples and D1 and D4, so its cycling performance was also at an intermediate level. These comparative data confirm that the intrinsic structural stability of the particles obtained by the present technical solution is the basis for achieving long cycle life.
[0079] The results of Comparative Example D3 clearly demonstrate the role of the surface carbon layer in cycle stability. The core particles of this material have the same structure as Example A1, but due to the lack of a carbon coating layer, its capacity retention rate is the lowest among all samples. This indicates that, during cycling, uncoated lithium iron phosphate particles directly contact the electrolyte, which can trigger continuous side reactions and form an unstable solid-state electrolyte interface film (SEI), which not only consumes active lithium ions but also increases the interface impedance. The carbon coating step in the present technical solution forms a layer of stable physical and chemical properties on the surface of the particles. This layer effectively isolates the active material from the electrolyte, inhibits side reactions, and maintains the stability of the interface. Therefore, the combination of structural stability within the particle and interface stability on the surface of the particle is a decisive factor in achieving high cycle stability of the material.
Claims
1. A supercritical method for the preparation of self-healing, high-compactable lithium iron phosphate, characterized in that, The preparation method comprises the following steps: S1, mixing unqualified lithium iron phosphate seed crystals, an in-situ synergistic agent and deionized water to prepare slurry A, wherein the mass ratio of the unqualified lithium iron phosphate seed crystals to the in-situ synergistic agent is 1000:(10-50), the total mass of the unqualified lithium iron phosphate seed crystals and the in-situ synergistic agent to the mass of the deionized water is 1:(1.5-4), and at least one of a lithium source, an iron source, a phosphorus source and a metal ion additive is dissolved in water as a raw material to prepare solution B, wherein the mass ratio of the total mass of the raw material to the mass of the water is 1:(2.3-19); S2, continuously pumping the slurry A into a tubular reactor comprising reaction zone one and reaction zone two in series, in the reaction zone one, controlling the temperature to be 250-300 DEG C and the pressure to be 20-25 MPa, so that the in-situ synergistic agent is hydrolyzed, and the surface of the unqualified lithium iron phosphate seed crystals is repaired and activated to obtain slurry A comprising activated lithium iron phosphate seed crystals; S3, at the connection between the reaction zone one and the reaction zone two, the solution B is injected into the slurry A comprising activated lithium iron phosphate seed crystals in a pulse feeding mode; S4, in the reaction zone two, controlling the temperature to be 280-400 DEG C and the pressure to be 25-30 MPa, so that the raw material in the solution B grows on the surface of the activated lithium iron phosphate seed crystals to form a final lithium iron phosphate material; S5, separating and drying the final lithium iron phosphate material.
2. The supercritical preparation method of self-repairing, high-compacted lithium iron phosphate according to claim 1, characterized in that, The in-situ synergistic agent comprises at least one of triethyl phosphate and phosphinic acid.
3. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 1, characterized in that, The slurry A further comprises a comb-shaped copolymer as a stabilizer, and the mass ratio of the unqualified lithium iron phosphate seed crystals to the comb-shaped copolymer is 1000:(5-20).
4. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 3, characterized in that, The comb-shaped copolymer is synthesized by the following steps: Polyvinylpyrrolidone and polyethylene glycol monomethyl ether are dissolved in N,N-dimethylformamide, and a free radical grafting reaction is carried out at 70.0±2.0 DEG C under the condition of azobisisobutyronitrile as an initiator.
5. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 1, characterized in that, The frequency of the pulse feeding is 1.0-20.0 Hz, and the pulse width is 10.0-100.0 ms.
6. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 1, characterized in that, The step S5 further comprises: Mixing the final lithium iron phosphate material after separation and drying with a carbon precursor, and performing heat treatment under an inert atmosphere to form a carbon coating layer on the surface of the final lithium iron phosphate material.
7. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 6, characterized in that, The temperature of the heat treatment is 650-750 DEG C, and the holding time is 2-6 hours.
8. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 6, characterized in that, The carbon precursor comprises at least one of glucose, sucrose and citric acid, and the mass ratio of the final lithium iron phosphate material to the carbon precursor is 100:(2-5).
9. The supercritical preparation method of self-repairing, high-compactness lithium iron phosphate according to claim 1, characterized in that, The lithium source is lithium carbonate, the iron source is iron nitrate nonahydrate, the phosphorus source is phosphoric acid, and the metal ion additive is titanium tetraisopropoxide.
10. The use of self-repaired and high-compacted lithium iron phosphate prepared by the preparation method in any one of claims 1-9 in preparing a positive electrode of a lithium ion battery.