Method for reversely analyzing particle size of lithium iron phosphate of positive pole piece

Abstract

The invention discloses a method for reversely analyzing the particle size of lithium iron phosphate of a positive pole piece, and relates to the technical field of new energy batteries, and the method comprises the following steps: S1, soaking the positive pole piece in a first organic solvent; s2, drying the positive pole piece; s3, soaking the positive pole piece in a second organic solvent to obtain positive pole powder; s4, adding into a strong alkali solution, and performing high-temperature stirring, dispersing and refluxing; s5, stirring and dispersing based on a test type planetary vacuum stirrer; s6, carrying out titration neutralization; s7, removing reaction by-products, namely salt inorganic matters and PVDF; s8, performing high-speed centrifugation; and S9, obtaining lithium iron phosphate positive electrode powder, and carrying out particle size test. According to the method disclosed by the invention, PVDF, a conductive agent and side reaction products are thoroughly removed on the premise of not damaging the original crystal structure of lithium iron phosphate through a multi-step process of cleaning with an organic solvent, destroying the binder with strong base and the like, so that the lithium iron phosphate positive electrode powder with high purity and high dispersity is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy batteries, in particular to a method for reverse analysis of lithium iron phosphate particle size of positive electrode sheet. BACKGROUND

[0002] With the increasing proportion of renewable energy such as solar energy and wind energy in the power system, lithium ion batteries, as a secondary energy storage power source that can convert electrical energy and chemical energy into each other, can regulate and balance the unstable clean energy generated. More and more countries have introduced relevant policies to support the development of energy storage batteries.

[0003] Energy storage batteries need to be more economical in application scenarios, which requires continuous improvement in energy density, cycle performance, charge and discharge efficiency, and degree of electrical cost. As the electrode that provides energy for energy storage batteries, the particle size of the positive electrode material directly affects the lithium ion diffusion rate, electrode interface reaction uniformity and roll pressing process consistency. Reverse analysis of the particle size of lithium iron phosphate in retired batteries or competitor electrode sheets is crucial for optimizing battery design, process benchmarking and failure analysis. To reverse analyze the particle size of lithium iron phosphate, the following core problems need to be solved: positive electrode current collector and positive electrode material stripping, electrolyte and byproduct residue interference, conductive agent interference, and PVDF wrapping the positive electrode material, of which the most difficult to solve is the PVDF wrapping the positive electrode material, i.e. how to remove PVDF.

[0004] Existing technical pain points: 1. Image analysis method for reverse analysis of lithium iron phosphate particle size: Method: After disassembling the roll core, the positive electrode sheet is obtained. After cleaning and drying the electrode sheet, the electrode sheet cross section is prepared by ion beam cutting. Using a scanning electron microscope, the boundary of lithium iron phosphate particles is manually identified, and the particle size is measured. Then, using a statistical tool, the particle size distribution is calculated. Disadvantages: The statistical data is too dependent on the number of observed fields and the number of particles; agglomerates, conductive agents and other strong interference make it difficult to distinguish particle boundaries, and it is highly dependent on manual annotation with large subjective errors. 2. Fire method for reverse analysis of lithium iron phosphate particle size: Method: After disassembling the roll core, the positive electrode sheet is obtained. After cleaning the electrode sheet, the lithium iron phosphate powder is obtained by scraping. The powder is sent to a laboratory sintering furnace for calcination at 470℃ under a nitrogen atmosphere to remove PVDF. Then the powder is ground and dispersed by ultrasonic, and the particle size analyzer is used for particle size test. Disadvantages: The calcination temperature is relatively high, which can cause the crystal structure of the positive active material to grow further, thereby changing the particle size distribution and causing analysis failure.

[0005] For example, Chinese patent CN202010219542.3 discloses a lithium iron phosphate positive electrode sheet and a preparation method thereof, and a lithium iron phosphate lithium ion battery. The lithium iron phosphate positive electrode sheet contains lithium iron phosphate particles. In the lithium iron phosphate particles, the number of particles with a particle size in the range of 50-500 nm accounts for 70-90%, the number of particles with a particle size greater than 500 nm and less than 1000 nm accounts for 5-20%, and the number of particles with a particle size in the range of 1-10 μm accounts for 2-10%. By compacting the lithium iron phosphate particles within a certain particle size and proportion range, a lithium iron phosphate positive electrode sheet with ultra-high compacting density is prepared. However, the method for measuring the particle size of the lithium iron phosphate particles is not given, resulting in a large subjective error in the labeled number of lithium iron phosphate particles. SUMMARY

[0006] In view of the deficiencies of the prior art, the present application provides a method for reverse analysis of the particle size of lithium iron phosphate in a positive electrode sheet to solve the problems raised in the background art.

[0007] To achieve the above-mentioned purpose, the present application provides the following technical solutions: The present application provides a method for reverse analysis of the particle size of lithium iron phosphate in a positive electrode sheet, comprising the following steps: S1, soaking the positive electrode sheet in a first organic solvent to remove residual electrolyte and by-products on the surface of the positive electrode sheet; S2, placing the soaked positive electrode sheet in an oven to dry the first organic solvent, with an oven temperature of 70-90°C and a drying time of 0.5-2h; S3, soaking the dried positive electrode sheet in a second organic solvent, and scraping the positive electrode powder from the positive electrode foil with a ceramic knife under a wet state, and then drying the collected positive electrode powder; S4, weighing the dried positive electrode powder and adding it to a strong alkali solution for high-temperature stirring, dispersion and reflux to form a mixed solution; and using the strong alkali to destroy the bonding activity of the binder PVDF; S5, pouring the mixed solution into a test-type planetary vacuum stirrer for stirring and dispersion, with a dispersion speed of 600-1200 rpm and a dispersion time of 1-4h; and peeling off the inactivated binder PVDF; S6, adding a strong acid to the mixed solution for titration and neutralization until the mixed solution is neutral; S7, washing and filtering the neutralized mixed solution with deionized water for 1-5 times, placing it in an oven for baking, and then soaking the dried powder in a second organic solution, repeating ultrasonic treatment, washing and filtering for 1-5 times, and then sequentially soaking, ultrasonic treatment, washing and filtering with deionized water for 1-5 times to obtain the treated powder; and removing the reaction by-product salt inorganic matter and the inactivated binder PVDF; S8, deionized water is added to the treated powder, high-speed centrifugation is carried out by using a high-speed centrifuge, the upper liquid is removed by suction, the material adhered to the bottom of the centrifuge tube is collected, and the above operation is repeated 1-5 times; the conductive agent is removed by using the density difference between the conductive agent and the lithium iron phosphate; S9, the material adhered to the bottom of the centrifuge tube is placed into an oven and baked at a temperature of 70-90℃ for 4-12h to obtain lithium iron phosphate positive electrode powder, and the particle size is tested by using a particle size analyzer after manual grinding.

[0008] Further optimization of the technical solution, the positive electrode sheet is composed of lithium iron phosphate, a conductive agent, a binder PVDF, and an aluminum foil.

[0009] Further optimization of the technical solution, in step S1, the first organic solvent includes at least one organic solvent selected from dimethyl carbonate, ethyl acetate, methyl acetate, methyl ethyl carbonate, ethanol, methanol, acetone, and isopropyl alcohol, the soaking time of the first organic solvent is 0.5-2h, and the weight ratio of the first organic solvent to the positive electrode sheet is 100:1 to 10:1.

[0010] Further optimization of the technical solution, in step S3, the second organic solvent is N-methylpyrrolidone or dimethylacetamide, the soaking time of the second organic solvent is 5-10min, and the weight ratio of the second organic solvent to the positive electrode sheet is 50:1 to 2:1. The drying temperature of the positive electrode powder is 70-90℃, and the time is 0.5-4h.

[0011] Further optimization of the technical solution, in step S4, the strong alkali solution includes at least one inorganic solution selected from sodium hydroxide solution, potassium hydroxide solution, and calcium hydroxide, the concentration of the inorganic solution is 1-5mol / L, and the weight ratio of the inorganic solution to the positive electrode powder is 100:1 to 10:1. The reaction temperature of high-temperature stirring dispersion reflux is 60-150℃, and the reaction time is 2-12h.

[0012] Further optimization of the technical solution, in step S6, the strong acid solution includes at least one solution selected from hydrochloric acid, sulfuric acid, and nitric acid, and the concentration of the strong acid solution is 1-5mol / L.

[0013] Further optimization of the technical solution, in step S7, the drying temperature of the oven is 70-90℃, the drying time is 0.5-4h, and the weight ratio of the second organic solvent to the treated powder is 100:1 to 10:1.

[0014] Further optimize the technical scheme, after the step S7 is completed, still include to the slurry with powder uses wet type nanometer sand mill to carry out 0.1-0.2mm zirconium oxide bead wet grinding 2-4 times, each cycle pressure is 5-15MPa, slurry flow is 0.5 to 1.0 liters per minute, and control outlet temperature is not higher than 35 DEG C, to further break the residual agglomerate in powder.

[0015] Further optimize the technical scheme, in the step S8, the speed of high-speed centrifuge is 8000-12000rpm, and each centrifugation time is 5min.

[0016] Further optimize the technical scheme, after the step S8 is completed, still include the substance adhered to the bottom of centrifugal tube is carried out 4-8h rolling drying under the inert atmosphere of argon, first vacuumizing, then filling argon and controlling temperature to be 80 DEG C, and then continues to dry 1h under the vacuum of less than 50Pa.

[0017] Compared with the prior art, the present application provides a method for reverse analysis of lithium iron phosphate particle size of positive electrode sheet, which has the following beneficial effects: The method for reverse analysis of lithium iron phosphate particle size of positive electrode sheet can significantly improve the accuracy and representativeness of particle size analysis results, and provide reliable data support for design optimization, performance tracing and process benchmarking of energy storage battery material system. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0019] Figure 1 A flowchart of a method for reverse analysis of lithium iron phosphate particle size of positive electrode sheet is provided. Figure 2 A setting step flowchart of step S7 in the method for reverse analysis of lithium iron phosphate particle size of positive electrode sheet is provided. Figure 3 A setting step flowchart of step S8 in the method for reverse analysis of lithium iron phosphate particle size of positive electrode sheet is provided. Figure 4 and Figure 5 Both are experimental scene display figures of a method for reverse analysis of particle size of positive plate lithium iron phosphate proposed in the application. DETAILED DESCRIPTION

[0020] In order to make the above-mentioned objectives, characteristics and advantages of the application more apparent and easy to understand, the specific embodiments of the application will be described in detail below with reference to the accompanying drawings.

[0021] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the application, but the application can also be implemented in other ways different from the description, and those skilled in the art can make similar generalizations without departing from the connotation of the application, so the application is not limited by the specific embodiments disclosed below.

[0022] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor does it mean an embodiment that is separate or selectively excluded from other embodiments.

[0023] The specific application of the method is as follows: Referring to Figures 1-5 A method for reverse analysis of particle size of positive plate lithium iron phosphate, the positive plate is composed of lithium iron phosphate, conductive agent, binder PVDF and aluminum foil. The working principle of the binder PVDF is to form hydrogen bonds between the C-F bonds on the PVDF long chain and the other component particles in the plate, and the action of the hydrogen bonds makes the component particles string together firmly. Therefore, the wet method adopted by the application effectively removes the by-product, conductive agent and PVDF without damaging the structure of the lithium iron phosphate material, and finally obtains lithium iron phosphate powder, so as to more accurately analyze the particle size.

[0024] The method comprises the following steps: S1, soaking the positive plate in a first organic solvent, the first organic solvent comprising at least one organic solvent selected from dimethyl carbonate, ethyl acetate, methyl acetate, methyl ethyl carbonate, ethanol, methanol, acetone and isopropyl alcohol, the soaking time of the first organic solvent being 0.5-2h, and the weight ratio of the first organic solvent to the positive plate being 100:1 to 10:1.

[0025] This step is used to remove the residual electrolyte and by-product on the surface of the positive plate.

[0026] S2, placing the soaked positive plate in an oven to dry the first organic solvent, the oven temperature being 70-90℃ and the time being 0.5-2h.

[0027] S3, soaking the dried positive electrode sheet in a second organic solvent, the second organic solvent being N-methyl pyrrolidone or dimethyl acetamide, the soaking time of the second organic solvent being 5-10 min, the weight ratio of the second organic solvent to the positive electrode sheet being 50:1 to 2:1. Under the wet state of the sheet, the positive electrode powder is scraped off from the positive electrode foil with a ceramic knife, and the collected positive electrode powder is dried again, the drying temperature being 70-90℃, and the time being 0.5-4h.

[0028] S4, weighing the dried positive electrode powder and adding it to a strong alkali solution for high-temperature stirring, dispersion and reflux, the reaction temperature being 60-150℃, and the reaction time being 2-12h, to form a mixed solution. The strong alkali solution includes at least one inorganic solution of sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide, and the concentration of the inorganic solution is 1-5mol / L, and the weight ratio of the inorganic solution to the positive electrode powder is 100:1 to 10:1.

[0029] This step uses a strong alkali to destroy the binding activity of the binder PVDF.

[0030] S5, stirring and dispersing the mixed solution in a test-type planetary vacuum stirrer, the dispersion rotation speed being 600-1200rpm, and the dispersion time being 1-4h.

[0031] This step breaks the PVDF that has lost the binding activity by shearing and peels it off from the coating.

[0032] S6, adding a strong acid to the mixed solution for titration and neutralization until the mixed solution is neutral. The strong acid solution includes at least one solution of hydrochloric acid, sulfuric acid, nitric acid, and the concentration of the strong acid solution is 1-5mol / L.

[0033] S7, washing and suction-filtering the neutralized mixed solution with deionized water for 1-5 times, and placing it in an oven for baking, the drying temperature of the oven being 70-90℃, and the drying time being 0.5-4h. The dried powder is soaked in a second organic solution, and is subjected to ultrasonic treatment, washing and suction-filtering repeatedly for 1-5 times, the weight ratio of the second organic solvent to the treated powder being 100:1 to 10:1; and then the treated powder is sequentially soaked in deionized water, subjected to ultrasonic treatment, washing and suction-filtering repeatedly for 1-5 times, to obtain a treated powder.

[0034] This step is used to remove the reaction byproduct salt inorganic matter and the binder PVDF that has lost the activity.

[0035] S8, deionized water is added to the treated powder, and high-speed centrifugation is performed using a high-speed centrifuge. The speed of the high-speed centrifuge is 8000-12000 rpm, and each time the centrifugation time is 5 min. The upper liquid is removed by suction, and the material adhered to the bottom of the centrifuge tube is collected. The above operation is repeated 1-5 times.

[0036] This step uses the difference in density between the conductive agent and lithium iron phosphate to remove the conductive agent. S9, the material adhered to the bottom of the centrifuge tube is placed in an oven and baked at a temperature of 70-90°C for 4-12h to obtain lithium iron phosphate positive electrode powder. After manual grinding, particle size testing is performed using a particle size analyzer.

[0037] After step S7 is completed, a wet bead milling-ultrafine dispersion step is set up to further remove soft agglomerates formed due to incomplete destruction of the binder PVDF. The treated slurry is prepared as a suspension with a solid content of 10%, with deionized water as the solvent, and is fed into a wet sanding device lined with ceramic or polyurethane for cyclic grinding. The grinding medium used is zirconium oxide beads with a particle size of 0.1 to 0.2 mm, and the filling rate is controlled within the range of 70%±5%. The circulating pressure during the entire grinding process is controlled within the range of 5 to 15 MPa, the slurry flow rate is maintained between 0.5 to 1.0 liters per minute, and the grinding times are controlled within the range of 2 to 4 times. In order to prevent overheating of the particles leading to structural changes, a cooling system is used to control the temperature of the slurry at the grinding outlet to not higher than 35°C. After grinding, the slurry is removed from the grinding medium by high-speed centrifugation and then enters the subsequent centrifugation step. This can effectively break the agglomerate structure, fully dissociate the particles, and improve the representativeness and stability of the final particle size test.

[0038] After step S8 is completed, an inert atmosphere vacuum drying-passivation step is further set up to prevent oxidation of the positive electrode powder during high-temperature drying. Specifically, the material adhered to the bottom of the centrifuge tube is transferred to a vacuum drying oven with a rolling support, first vacuumed to not higher than 100 Pa, then filled with high-purity argon (purity not less than 99.99%), to ensure that the oxygen and moisture content is less than 1 ppm. Under the above inert atmosphere conditions, the drying oven temperature is maintained at 80±2°C, and the rolling speed is maintained at 2r per minute for continuous rolling drying for 4-8h. Subsequently, the drying is maintained in a vacuum environment of not higher than 50 Pa for 1 hour. This step not only effectively avoids the oxidation reaction caused by oxygen in the air during the heat treatment process, but also makes the particles uniformly dispersed in the rolling state, preventing agglomeration, which helps the subsequent particle size test results more accurately reflect the intrinsic particle size characteristics of the material.

[0039] Based on the specific application of the above method, the invention is described in detail in combination with the examples.

[0040] Control group: the same model and batch of lithium iron phosphate powder is directly tested for particle size, and the test results are shown in Table 1.

[0041] Example 1: The battery cell was discharged to 2.0V at 0.5C+0.02C, and the positive electrode sheet was obtained by disassembly. The positive electrode sheet was soaked in N-methyl pyrrolidone (NMP solvent) for 5 min, and 5 g of powder was collected by scraping with a ceramic knife and dried in an 85°C oven for 12 h to obtain the positive electrode lithium iron phosphate powder.

[0042] After artificial grinding of the lithium iron phosphate powder obtained in this example, particle size testing was performed using a particle size analyzer, and the test results are shown in Table 1.

[0043] Example 2: The battery cell was discharged to 2.0V at 0.5C+0.02C, and the positive electrode sheet was obtained by disassembly. The positive electrode sheet was soaked in 200 mL of dimethyl carbonate (DMC solvent) for 1 h, and the sheet was taken out and dried in an 85°C oven for 1 h. The dried sheet was taken out and soaked in N-methyl pyrrolidone (NMP solvent) for 5 min, and 5 g of powder was collected by scraping with a ceramic knife and dried in an 85°C oven for 12 h to obtain the positive electrode lithium iron phosphate powder.

[0044] After artificial grinding of the lithium iron phosphate powder obtained in this example, particle size testing was performed using a particle size analyzer, and the test results are shown in Table 1.

[0045] Example 3: The battery cell was discharged to 2.0V at 0.5C+0.02C, and the positive electrode sheet was obtained by disassembly. The positive electrode sheet was soaked in 200 mL of dimethyl carbonate (DMC solvent) for 1 h, and the sheet was taken out and dried in an 85°C oven for 1 h. The dried sheet was taken out and soaked in N-methyl pyrrolidone (NMP solvent) for 5 min, and 5 g of powder was collected by scraping with a ceramic knife and added to a conical flask containing 100 mL of 1 mol / L sodium hydroxide solution. The temperature was 60°C, the stirring time was 2 h, and the subsequent ultrasonic, washing and suction filtration were performed using deionized water. The positive electrode lithium iron phosphate powder was obtained by drying in an 85°C oven for 12 h.

[0046] After artificial grinding of the lithium iron phosphate powder obtained in this example, particle size testing was performed using a particle size analyzer, and the test results are shown in Table 1.

[0047] Example 4: The battery cell is discharged to 2.0V at 0.5C+0.02C, the positive electrode sheet is obtained by disassembly, the positive electrode sheet is soaked in 200mL of dimethyl carbonate (DMC solvent) for 2h, the positive electrode sheet is taken out and dried in an 85°C oven for 2h, the dried positive electrode sheet is taken out and soaked in N-methyl pyrrolidone (NMP solvent) for 5min, 5g of powder is collected by scraping with a ceramic knife, and is added into a conical flask containing 100mL of 2mol / L sodium hydroxide solution, magnetic stirring is carried out at 100°C for 4h, concentrated hydrochloric acid is subsequently added for neutralization until the mixed solution is neutral, ultrasonic washing, washing and suction filtration are carried out with deionized water, and the positive electrode lithium iron phosphate powder is obtained by drying in an 85°C oven for 12h.

[0048] After the lithium iron phosphate powder obtained in this example is manually ground, particle size testing is carried out with a particle size analyzer, and the test results are shown in Table 1. Example 5: The battery cell is discharged to 2.0V at 0.5C+0.02C, the positive electrode sheet is obtained by disassembly, the positive electrode sheet is soaked in 200mL of dimethyl carbonate (DMC solvent) for 2h, the positive electrode sheet is taken out and dried in an 85°C oven for 2h, the dried positive electrode sheet is taken out and soaked in N-methyl pyrrolidone (NMP solvent) for 5min, 5g of powder is collected by scraping with a ceramic knife, and is added into a conical flask containing 100mL of 2mol / L sodium hydroxide solution, magnetic stirring is carried out at 120°C for 8h, concentrated hydrochloric acid is subsequently added for neutralization until the mixed solution is neutral, the neutralization reaction solution is transferred to a test planetary vacuum stirrer for stirring and dispersion, the dispersion speed is adjusted to 800rpm, and the dispersion time is 1h; the dispersed solution is ultrasonically washed, washed and suction filtered with 50mL of deionized water, the ultrasonic washing, washing and suction filtration are carried out once, and the positive electrode lithium iron phosphate powder is obtained by drying in an 85°C oven for 12h.

[0049] After the lithium iron phosphate powder obtained in this example is manually ground, particle size testing is carried out with a particle size analyzer, and the test results are shown in Table 1.

[0050] Example 6: The battery cell is discharged to 2.0V at 0.5C+0.02C, the positive electrode sheet is obtained by disassembly, the positive electrode sheet is soaked in 200mL of dimethyl carbonate (DMC solvent) for 2h, the positive electrode sheet is taken out and dried in an 85°C oven for 2h, the dried positive electrode sheet is taken out and soaked in N-methyl pyrrolidone (NMP solvent) for 5min, 5g of powder is collected by scraping with a ceramic knife, and is added into a conical flask containing 100mL of 3mol / L sodium hydroxide solution, magnetic stirring is carried out at 150°C for 12h, 1mol / L concentrated hydrochloric acid is subsequently added for neutralization until the mixed solution is neutral, the neutralization reaction solution is transferred to a test planetary vacuum stirrer for stirring and dispersion, the dispersion speed is adjusted to 1200rpm, and the dispersion time is 2h; the dispersed solution is washed and suction filtered with deionized water for 5 times.

[0051] Then soak, ultrasonic, washing and suction filtration with 100 mL of N-methyl pyrrolidone (NMP solvent) for 5 times; then soak, ultrasonic, washing and suction filtration with 100 mL of deionized water for 5 times; centrifuge the material dissolved in 100 mL of deionized water at a speed of 10000 rpm, carefully remove the upper liquid, collect the material adhered to the bottom of the centrifuge tube, and repeat the above operation for 5 times; dry the centrifuged material in an oven at 85℃ for 12h to obtain the positive electrode lithium iron phosphate powder.

[0052] After artificial grinding of the lithium iron phosphate powder obtained in the embodiment, particle size analysis was performed to test the particle size, and the test results are shown in Table 1.

[0053] Note: D10 / 50 / 90 / 100 indicates the particle size corresponding to the cumulative particle size distribution percentage of 10 / 50 / 90 / 100% of a sample.

[0054] The lithium iron phosphate powder of the above-mentioned control group and the lithium iron phosphate of the electrode plates of Examples 1-6 are of the same batch, and the cells in Examples 1-6 are analyzed using fresh batteries (batteries are not tested except for capacity calibration) within 6 months; Examples 1 and 2 in Table 1 are not effectively treated PVDF wrapped, and the control group directly measures the particle size of the lithium iron phosphate powder, which increases by more than several times at D10, especially D50, D90 and D100, which is mainly affected by the binder PVDF, resulting in tight bonding and wrapping between particles; Examples 3 and 4 in Table 1 reduce the bonding performance of PVDF through strong alkali reaction, and the particle size value of the large particles is significantly reduced; Examples 5 and 6 in Table 1, through strong alkali reaction, stirring and shearing and continuous washing and impurity removal, the obtained powder is basically close to the true particle size distribution, through SEM and EDS analysis of the treated powder, no F component is detected, which shows that the operation of Examples 5 and 6 can effectively remove the electrolyte and byproduct residues, conductive agent and PVDF, which can be used as a means of reverse analysis test particle size.

[0055] In summary, the method realizes the complete removal of PVDF, conductive agent and byproduct without destroying the original crystal structure of lithium iron phosphate, thereby obtaining high-purity and high-dispersion lithium iron phosphate positive electrode powder.

[0056] It should be noted that the above examples are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalently replaced, without departing from the spirit and scope of the technical solutions of the present application, which should be covered in the scope of the claims of the present application.

Claims

1. A method for reverse analysis of the particle size of lithium iron phosphate in positive electrode sheets, characterized in that, Includes the following steps: S1. The positive electrode sheet is soaked in the first organic solvent to remove the residual electrolyte and by-reaction products on the surface of the positive electrode sheet; S2. Place the soaked positive electrode sheet in an oven and dry it with the first organic solvent. The oven temperature is 70-90℃ and the time is 0.5-2h. S3. Immerse the dried positive electrode sheet in the second organic solvent. While the electrode sheet is wet, use a ceramic knife to scrape the positive electrode powder off the positive electrode foil. The collected positive electrode powder is then dried. S4. Weigh the dried positive electrode powder and add it to a strong alkaline solution for high-temperature stirring, dispersion and reflux to form a mixed solution; Strong alkalis are used to destroy the bonding activity of PVDF adhesive; S5. Pour the mixed solution into a test planetary vacuum mixer and stir to disperse it at a speed of 600-1200 rpm for 1-4 hours; then peel off the deactivated PVDF binder. S6. Add a strong acid to the mixed solution and titrate to neutralize it until the mixed solution is neutral; S7. Wash the neutralized mixed solution with deionized water and filter it 1-5 times. Place it in an oven to bake. Soak the dried powder in a second organic solution and repeat the process of sonication, washing and filtration 1-5 times. Then soak it in deionized water, sonicate, wash and filter it 1-5 times to obtain the treated powder. Remove the reaction byproducts, inorganic salts and the inactive binder PVDF. S8. Add deionized water to the treated powder, centrifuge at high speed using a high-speed centrifuge, remove the upper liquid, collect the material adhering to the bottom of the centrifuge tube, and repeat the above operation 1-5 times; remove the conductive agent by utilizing the density difference between the conductive agent and lithium iron phosphate. S9. Place the material adhering to the bottom of the centrifuge tube into an oven and bake at 70-90℃ for 4-12 hours to obtain lithium iron phosphate cathode powder. After manual grinding, the particle size is tested using a particle size analyzer.

2. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, The positive electrode sheet is composed of lithium iron phosphate, conductive agent, PVDF binder, and aluminum foil.

3. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, In step S1, the first organic solvent includes at least one organic solvent, such as dimethyl carbonate, ethyl acetate, methyl acetate, ethyl methyl carbonate, ethanol, methanol, acetone, and isopropanol. The soaking time of the first organic solvent is 0.5-2 hours, and the weight ratio of the first organic solvent to the positive electrode sheet is 100:1 to 10:

1.

4. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, In step S3, the second organic solvent is N-methylpyrrolidone or dimethylacetamide, the soaking time of the second organic solvent is 5-10 min, and the weight ratio of the second organic solvent to the positive electrode sheet is 50:1 to 2:

1. The drying temperature for the positive electrode powder is 70-90℃, and the drying time is 0.5-4h.

5. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, In step S4, the strong alkaline solution includes at least one inorganic solution, including sodium hydroxide solution, potassium hydroxide solution, and calcium hydroxide solution. The concentration of the inorganic solution is 1-5 mol / L, and the weight ratio of the inorganic solution to the positive electrode powder is 100:1 to 10:

1. The reaction temperature for high-temperature stirring, dispersion, and reflux is 60-150℃, and the reaction time is 2-12h.

6. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, In step S6, the strong acid solution includes at least one solution including hydrochloric acid, sulfuric acid, and nitric acid, and the concentration of the strong acid solution is 1-5 mol / L.

7. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, In step S7, the drying temperature of the oven is 70-90℃, and the drying time is 0.5-4h; the weight ratio of the second organic solvent to the treated powder is 100:1 to 10:

1.

8. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, After step S7 is completed, the slurry containing powder is further subjected to wet milling with 0.1-0.2 mm zirconia beads 2-4 times using a wet nano-sand mill, with a circulation pressure of 5-15 MPa each time, a slurry flow rate of 0.5 to 1.0 liters per minute, and the outlet temperature is controlled not to exceed 35°C, in order to further break down the residual agglomerates in the powder.

9. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, In step S8, the high-speed centrifuge rotates at 8000-12000 rpm, and each centrifugation takes 5 minutes.

10. The method for reverse analysis of lithium iron phosphate particle size in a positive electrode sheet according to claim 1, characterized in that, After step S8 is completed, the process further includes drying the material adhering to the bottom of the centrifuge tube under an inert argon atmosphere by first evacuating the vacuum and then filling it with argon while controlling the temperature at 80°C for 4-8 hours, and then continuing to dry it under a vacuum of less than 50 Pa for 1 hour.

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