Method for reverse analyzing particle size of lithium iron phosphate in positive electrode sheet

Abstract

The application discloses a method for reverse analysis of particle size of lithium iron phosphate of positive pole piece, and relates to the technical field of new energy batteries, and comprises the following steps: S1, the positive pole piece is soaked by a first organic solvent; S2, the positive pole piece is dried; S3, the positive pole piece is soaked in a second organic solvent to obtain positive pole powder; S4, a strong alkali solution is added for high-temperature stirring, dispersion and reflux; S5, stirring and dispersion are carried out based on a test type planetary vacuum stirrer; S6, titration and neutralization are carried out; S7, reaction byproducts, salt inorganic substances and PVDF are removed; S8, high-speed centrifugation is carried out; S9, lithium iron phosphate positive pole powder is obtained, and particle size testing is carried out. Through the multiple processes of organic solvent cleaning and strong alkali binder destruction, the PVDF, conductive agent and byproducts are completely removed without destroying the original crystal structure of lithium iron phosphate, so that high-purity and high-dispersity lithium iron phosphate positive pole powder is obtained.

Description

Technical Field

[0001] This invention relates to the field of new energy battery technology, specifically a method for reverse analysis of the particle size of lithium iron phosphate in positive electrode sheets. Background Technology

[0002] As renewable energy sources such as solar and wind power continue to increase their share 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 clean energy with unstable power generation. More and more countries have introduced relevant policies to support the development of energy storage batteries.

[0003] To make energy storage batteries more economical in various applications, it is necessary to continuously improve energy density, extend cycle performance, enhance charge and discharge efficiency, and reduce the cost per kilowatt-hour. As the electrode that provides energy to energy storage batteries, the particle size of the positive electrode material directly affects the lithium-ion diffusion rate, the uniformity of the electrode interface reaction, and the consistency of the rolling process. Reverse engineering 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 engineer the particle size of lithium iron phosphate, the following core issues need to be addressed: separation of the positive electrode current collector from the positive electrode material, interference from electrolyte and by-reaction residues, interference from conductive agents, and PVDF entanglement on the positive electrode material. Among these, the most difficult issue to resolve is the PVDF entanglement on the positive electrode material—that is, how to remove PVDF.

[0004] Existing technical pain points:

[0005] 1. Image analysis method for reverse analysis of lithium iron phosphate particle size:

[0006] Method: After disassembling the core, the positive electrode sheet was obtained. After cleaning and drying the electrode sheet, the electrode sheet was cut with an ion beam to prepare the electrode sheet cross section. The boundaries of lithium iron phosphate particles were manually identified using a scanning electron microscope, and their particle size was measured. Then, the particle size distribution was statistically analyzed using statistical tools.

[0007] Disadvantages: Statistical data rely too heavily on the number of fields of view and particles observed; strong interference from aggregates, conductive agents, etc., makes it difficult to distinguish particle boundaries, relies heavily on manual annotation, and has a large subjective error.

[0008] 2. Pyrometallurgical reverse analysis of lithium iron phosphate particle size:

[0009] Method: After disassembling the core, the positive electrode sheet was obtained. The electrode sheet was cleaned and scraped to obtain lithium iron phosphate powder. The powder was sent to a laboratory sintering furnace at 470℃ under a nitrogen atmosphere to remove PVDF. Then the powder was ground, ultrasonically dispersed, and particle size was tested using a particle size analyzer.

[0010] Disadvantages: The high calcination temperature can cause the crystal structure of the positive electrode active material to grow further, thereby changing the particle size distribution and leading to analysis failure.

[0011] For example, Chinese patent CN202010219542.3 discloses a lithium iron phosphate cathode sheet and its preparation method, as well as a lithium iron phosphate lithium-ion battery. The lithium iron phosphate cathode sheet contains lithium iron phosphate particles. Of these particles, 70-90% have a particle size in the range of 50-500 nm, 5-20% have a particle size greater than 500 nm and less than 1000 nm, and 2-10% have a particle size in the range of 1-10 μm. By compacting lithium iron phosphate particles within a certain particle size and proportion range, a lithium iron phosphate cathode sheet with ultra-high compaction density is prepared. However, the patent does not provide a method for measuring the particle size of the lithium iron phosphate particles, leading to a large subjective error in the labeled number of lithium iron phosphate particles. Summary of the Invention

[0012] To address the shortcomings of existing technologies, this invention provides a method for reverse analysis of the particle size of lithium iron phosphate cathode sheets, thereby solving the problems mentioned in the background section.

[0013] To achieve the above objectives, the present invention provides the following technical solution:

[0014] This invention provides a method for reverse analysis of the particle size of lithium iron phosphate in positive electrode sheets, comprising the following steps:

[0015] 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;

[0016] 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.

[0017] 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.

[0018] 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; use the strong alkali to destroy the binding activity of the PVDF binder;

[0019] 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.

[0020] S6. Add a strong acid to the mixed solution and titrate to neutralize it until the mixed solution is neutral;

[0021] 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.

[0022] 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.

[0023] 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.

[0024] To further optimize this technical solution, the positive electrode sheet is composed of lithium iron phosphate, conductive agent, PVDF binder, and aluminum foil.

[0025] To further optimize this technical solution, in step S1, the first organic solvent includes at least one organic solvent including 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.

[0026] To further optimize this technical solution, 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.

[0027] The drying temperature for the positive electrode powder is 70-90℃, and the drying time is 0.5-4h.

[0028] To further optimize this technical solution, in step S4, the strong alkaline solution includes at least one inorganic solution, including sodium hydroxide solution, potassium hydroxide solution, and calcium hydroxide solution, with a concentration of 1-5 mol / L and a weight ratio of inorganic solution to positive electrode powder of 100:1 to 10:1.

[0029] The reaction temperature for high-temperature stirring, dispersion, and reflux is 60-150℃, and the reaction time is 2-12h.

[0030] To further optimize this technical solution, 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.

[0031] To further optimize this technical solution, 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.

[0032] To further optimize this technical solution, after step S7 is completed, the slurry containing powder is further subjected to wet milling with 0.1-0.2mm zirconia beads 2-4 times using a wet nano-sand mill, with a circulation pressure of 5-15MPa 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℃, so as to further break down the residual agglomerates in the powder.

[0033] To further optimize this technical solution, in step S8, the speed of the high-speed centrifuge is 8000-12000 rpm, and the centrifugation time is 5 minutes each time.

[0034] To further optimize this technical solution, after step S8 is completed, the material adhering to the bottom of the centrifuge tube is first evacuated and then filled with argon under an inert argon atmosphere and the temperature is controlled at 80°C for 4-8 hours of rolling drying, and then dried for another hour under a vacuum of less than 50 Pa.

[0035] Compared with the prior art, the present invention provides a method for reverse analysis of the particle size of lithium iron phosphate in positive electrode sheets, which has the following beneficial effects:

[0036] This reverse-engineering method for analyzing the particle size of lithium iron phosphate (LFP) cathode sheets addresses problems in existing technologies, such as large image recognition errors, strong agglomeration interference, and material structure alterations caused by high-temperature calcination. Through multi-step organic solvent cleaning, strong alkali-based binder destruction, vacuum shearing and exfoliation, neutralization and impurity removal, centrifugal classification, and fine drying, it completely removes PVDF, conductive agents, and by-reaction products without damaging the original LFP crystal structure, thus obtaining high-purity, highly dispersed LFP cathode powder. This method significantly improves the accuracy and representativeness of particle size analysis results, providing reliable data support for the design optimization, performance traceability, and process benchmarking of energy storage battery material systems. Attached Figure Description

[0037] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic flowchart of a method for reverse analysis of the particle size of lithium iron phosphate in positive electrode sheets proposed in this invention.

[0039] Figure 2 This is a schematic diagram of the setting steps in step S7 of the method for reverse analysis of the particle size of lithium iron phosphate positive electrode sheet proposed in this invention.

[0040] Figure 3 This is a schematic diagram of the setting step S8 in the method for reverse analysis of the particle size of lithium iron phosphate positive electrode sheet proposed in this invention.

[0041] Figure 4 and Figure 5 These are all experimental scene illustrations from a method for reverse analysis of lithium iron phosphate particle size in positive electrode sheets proposed in this invention. Detailed Implementation

[0042] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0043] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0044] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single embodiment or an embodiment selectively excluded from other embodiments.

[0045] The specific applications of this method are as follows:

[0046] Reference Figure 1-5 This invention discloses a method for reverse analysis of the particle size of lithium iron phosphate (LFP) cathode sheets, wherein the cathode sheet is composed of LFP, a conductive agent, a PVDF binder, and aluminum foil. The working principle of the PVDF binder is that it forms hydrogen bonds with other component particles in the electrode through the CF bonds on its long chains. These hydrogen bonds firmly bind the particles together; removing PVDF involves breaking these CF bonds. Therefore, the wet process employed in this invention effectively removes by-reaction products, the conductive agent, and PVDF without damaging the LFP material structure, ultimately yielding LFP powder, thus enabling more accurate particle size analysis.

[0047] The method includes the following steps:

[0048] S1. The positive electrode sheet is soaked in a first organic solvent, which 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 in 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.

[0049] This step is used to remove residual electrolyte and by-reaction products from the surface of the positive electrode.

[0050] 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.

[0051] S3. Immerse the dried positive electrode sheet in a second organic solvent, which is N-methylpyrrolidone or dimethylacetamide, for 5-10 minutes. The weight ratio of the second organic solvent to the positive electrode sheet is 50:1 to 2:1. While the electrode sheet is moist, scrape the positive electrode powder off the positive electrode foil with a ceramic knife. The collected positive electrode powder is then dried at 70-90℃ for 0.5-4 hours.

[0052] S4. Weigh the dried cathode powder and add it to a strong alkaline solution for high-temperature stirring, dispersion, and reflux. The reaction temperature is 60-150℃, and the reaction time is 2-12 hours to form a mixed solution. The strong alkaline solution includes at least one inorganic solution, such as sodium hydroxide solution, potassium hydroxide solution, or calcium hydroxide solution, with a concentration of 1-5 mol / L. The weight ratio of the inorganic solution to the cathode powder is 100:1 to 10:1.

[0053] This step utilizes a strong alkali to disrupt the adhesive activity of the PVDF binder.

[0054] S5. Pour the mixed solution into a test planetary vacuum mixer and stir to disperse it. The dispersion speed is 600-1200 rpm and the dispersion time is 1-4 hours.

[0055] This step involves shearing to break down the PVDF that has lost its adhesive activity and peeling it off from its encapsulated state.

[0056] S6. Add a strong acid to the mixed solution and titrate to neutralize it until the mixed solution is neutral. The strong acid solution includes at least one solution of hydrochloric acid, sulfuric acid, or nitric acid, and the concentration of the strong acid solution is 1-5 mol / L.

[0057] S7. Wash the neutralized mixed solution with deionized water and filter it 1-5 times. Place it in an oven to bake at a temperature of 70-90℃ for 0.5-4 hours. Soak the dried powder in a second organic solution, and repeat the process of sonication, washing, and filtration 1-5 times. The weight ratio of the second organic solvent to the treated powder is 100:1 to 10:1. Then, soak the powder in deionized water, sonicate, wash, and filter it 1-5 times to obtain the treated powder.

[0058] This step is used to remove reaction byproducts such as salts and inorganic substances, as well as the inactive PVDF binder.

[0059] S8. Add deionized water to the treated powder and centrifuge at high speed using a high-speed centrifuge at a speed of 8000-12000 rpm for 5 minutes each time. Remove the supernatant liquid and collect the material adhering to the bottom of the centrifuge tube. Repeat the above operation 1-5 times.

[0060] This step utilizes the density difference between the conductive agent and lithium iron phosphate to remove the conductive agent;

[0061] 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.

[0062] After step S7, a wet bead milling-ultrafine dispersion step is set up to further remove soft agglomerates formed due to incomplete destruction of the PVDF binder. The treated slurry is prepared as a suspension with a solid content of 10%, using deionized water as a solvent, and then fed into a wet sand mill lined with ceramic or polyurethane for circulating grinding. The grinding media used are zirconia beads with a particle size between 0.1 and 0.2 mm, and the filling rate is controlled within the range of 70% ± 5%. Throughout the grinding process, the circulating pressure is controlled between 5 and 15 MPa, the slurry flow rate is maintained between 0.5 and 1.0 liters per minute, and the number of grinding cycles is controlled between 2 and 4. To prevent the particles from overheating and causing structural changes, the slurry temperature at the grinding outlet is controlled to be no higher than 35°C through a cooling system. The ground slurry is then centrifuged at high speed to remove the grinding media before entering the subsequent centrifugation step. This effectively breaks down the agglomerated structure, fully dissociates the particles, and improves the representativeness and stability of the final particle size test.

[0063] After step S8, to prevent oxidation of the positive electrode powder during high-temperature drying, an inert atmosphere vacuum drying-passivation step is further implemented. Specifically, the material adhering to the bottom of the centrifuge tube is transferred to a vacuum drying oven equipped with a rolling support. First, a vacuum of no more than 100 Pa is applied, then high-purity argon gas (purity no less than 99.99%) is introduced to ensure that both oxygen and moisture content are below 1 ppm. Under the aforementioned inert atmosphere conditions, the drying oven temperature is maintained at 80 ± 2 °C, and the powder is continuously dried by rolling at a speed of 2 r / min for 4-8 hours. Subsequently, drying is maintained in a vacuum environment of no more than 50 Pa for 1 hour. This step not only effectively avoids oxidation of the powder due to oxygen in the air during heat treatment, but also ensures uniform particle dispersion and prevents agglomeration during rolling drying, helping subsequent particle size testing results to more accurately reflect the intrinsic particle size characteristics of the material.

[0064] Based on the specific application of the above methods, the present invention will be described in detail with reference to the embodiments.

[0065] Control group: Particle size was directly tested using lithium iron phosphate powder of the same type and batch. The test results are shown in Table 1.

[0066] Example 1: The battery cell was discharged to 2.0V at 0.5C +0.02C, and the positive electrode was obtained by disassembly. The positive electrode was immersed in N-methylpyrrolidone (NMP solvent) for 5 minutes, and 5g of powder was collected by scraping with a ceramic knife. The powder was then dried in an oven at 85℃ for 12 hours to obtain lithium iron phosphate positive electrode powder.

[0067] The lithium iron phosphate powder obtained in this embodiment was manually ground and then subjected to particle size analysis using a particle size analyzer. The test results are shown in Table 1.

[0068] Example 2: The battery cell was discharged to 2.0V at 0.5C + 0.02C, and the positive electrode was obtained by disassembly. The positive electrode was soaked in 200mL of dimethyl carbonate (DMC solvent) for 1h, and then the electrode was dried in an 85℃ oven for 1h. After drying, the electrode was soaked in N-methylpyrrolidone (NMP solvent) for 5min, and 5g of powder was collected by scraping with a ceramic knife. The powder was then dried in an 85℃ oven for 12h to obtain positive lithium iron phosphate powder.

[0069] The lithium iron phosphate powder obtained in this embodiment was manually ground and then subjected to particle size analysis using a particle size analyzer. The test results are shown in Table 1.

[0070] Example 3: The battery cell was discharged to 2.0V at 0.5C +0.02C, and the positive electrode was obtained by disassembly. The positive electrode was soaked in 200mL of dimethyl carbonate (DMC solvent) for 1h, and then dried in an oven at 85℃ for 1h. After drying, the electrode was soaked in N-methylpyrrolidone (NMP solvent) for 5min. 5g of powder was collected by scraping with a ceramic knife and added to a conical flask containing 100mL of 1mol / L sodium hydroxide solution. The flask was magnetically stirred at 60℃ for 2h. Subsequently, the powder was ultrasonically washed and filtered with deionized water, and then dried in an oven at 85℃ for 12h to obtain lithium iron phosphate positive electrode powder.

[0071] The lithium iron phosphate powder obtained in this embodiment was manually ground and then subjected to particle size analysis using a particle size analyzer. The test results are shown in Table 1.

[0072] Example 4: The battery cell was discharged to 2.0V at 0.5C + 0.02C, and the positive electrode was obtained by disassembly. The positive electrode was soaked in 200mL of dimethyl carbonate (DMC solvent) for 2h, and then dried in an 85℃ oven for 2h. After drying, the electrode was soaked in N-methylpyrrolidone (NMP solvent) for 5min. 5g of powder was collected by scraping with a ceramic knife and added to a conical flask containing 100mL of 2mol / L sodium hydroxide solution. The mixture was magnetically stirred at 100℃ for 4h. Concentrated hydrochloric acid was added to neutralize the mixture until it was neutral. The mixture was then ultrasonically washed and filtered with deionized water, and then dried in an 85℃ oven for 12h to obtain lithium iron phosphate positive electrode powder.

[0073] The lithium iron phosphate powder obtained in this embodiment was manually ground and then subjected to particle size analysis using a particle size analyzer. The test results are shown in Table 1.

[0074] Example 5: The battery cell was discharged to 2.0V at 0.5C +0.02C, and the positive electrode was obtained by disassembly. The positive electrode was soaked in 200mL of dimethyl carbonate (DMC solvent) for 2h, and then dried in an 85℃ oven for 2h. After drying, the electrode was soaked in N-methylpyrrolidone (NMP solvent) for 5min. 5g of powder was collected by scraping with a ceramic knife and added to a conical flask containing 100mL of 2mol / L sodium hydroxide solution. The mixture was magnetically stirred at 120℃ for 8h. Subsequently, concentrated hydrochloric acid was added to neutralize the mixture until it was neutral. The neutralized solution was transferred to an experimental planetary vacuum stirrer for dispersion. The dispersion speed was adjusted to 800rpm and the dispersion time was 1h. The dispersed solution was ultrasonicated, washed, and filtered with 50mL of deionized water. The ultrasonication, washing, and filtration were performed once, and then the solution was dried in an 85℃ oven for 12h to obtain lithium iron phosphate positive electrode powder.

[0075] The lithium iron phosphate powder obtained in this embodiment was manually ground and then subjected to particle size analysis using a particle size analyzer. The test results are shown in Table 1.

[0076] Example 6: The battery cell was discharged to 2.0V at 0.5C +0.02C, and the positive electrode was obtained by disassembly. The positive electrode was soaked in 200mL of dimethyl carbonate (DMC solvent) for 2h, and then dried in an 85℃ oven for 2h. After drying, the electrode was soaked in N-methylpyrrolidone (NMP solvent) for 5min. 5g of powder was collected by scraping with a ceramic knife and added to a conical flask containing 100mL of 3mol / L sodium hydroxide solution. The mixture was magnetically stirred at 150℃ for 12h. Subsequently, 1mol / L concentrated hydrochloric acid was added to neutralize the mixture until it was neutral. The neutralized solution was transferred to an experimental planetary vacuum stirrer for dispersion. The dispersion speed was adjusted to 1200rpm and the dispersion time was 2h. The dispersed solution was washed and filtered 5 times with deionized water.

[0077] Then, the material was soaked, sonicated, washed, and filtered in 100 mL of N-methylpyrrolidone (NMP solvent), and the process was repeated 5 times. Subsequently, the material was soaked, sonicated, washed, and filtered in 100 mL of deionized water, and the process was repeated 5 times. The filtered material was dissolved in 100 mL of deionized water and centrifuged at 10,000 rpm. The supernatant was carefully removed, and the material adhering to the bottom of the centrifuge tube was collected. The above steps were repeated 5 times. The centrifuged material was dried in an oven at 85°C for 12 hours to obtain lithium iron phosphate positive electrode powder.

[0078] The lithium iron phosphate powder obtained in this embodiment was manually ground and then subjected to particle size analysis using a particle size analyzer. The test results are shown in Table 1.

[0079]

[0080] Note: D10 / 50 / 90 / 100 represents the particle size corresponding to a sample when the cumulative particle size distribution percentage reaches 10 / 50 / 90 / 100%.

[0081] The lithium iron phosphate powder in the control group above was from the same batch as the lithium iron phosphate in the electrodes of Examples 1-6. The cells in Examples 1-6 were all fresh batteries within 6 months (no tests other than capacity calibration were performed on the batteries).

[0082] In Table 1, Examples 1 and 2 did not effectively address the PVDF encapsulation problem. Compared with the control group, the particle size of lithium iron phosphate powder was directly measured, and the particle size of D10, especially D50, D90, and D100, increased by more than double. This was mainly due to the influence of the binder PVDF, which caused the particles to adhere tightly and become clustered together.

[0083] In Examples 3 and 4 in Table 1, the bonding performance of PVDF was reduced through a strong alkali reaction, and the particle size of large particles was significantly reduced.

[0084] In Examples 5 and 6 of Table 1, the powder obtained through strong alkali reaction, stirring and shearing, and continuous washing and impurity removal is basically close to the true particle size distribution. SEM and EDS analysis of the treated powder showed no F component, indicating that the operation of Examples 5 and 6 can effectively remove electrolyte and by-reaction residues, conductive agents, and PVDF, and can be used as a means of reverse analysis to test particle size of competitors.

[0085] In summary, this method achieves the complete removal of PVDF, conductive agents, and by-reaction products without damaging the original crystal structure of lithium iron phosphate, thereby obtaining high-purity and highly dispersible lithium iron phosphate cathode powder.

[0086] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

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; 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 is 100:1 to 10:

1. 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. The second organic solvent is N-methylpyrrolidone or dimethylacetamide. The soaking time of the second organic solvent is 5-10 min. 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 0.5-4 h. 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; The strong alkaline solution includes at least one inorganic solution, such as sodium hydroxide solution, potassium hydroxide solution, or calcium hydroxide solution, with a concentration of 1-5 mol / L and a weight ratio of inorganic solution to cathode powder of 100:1 to 10:1; the reaction temperature for high-temperature stirring, dispersion, and reflux is 60-150℃, and the reaction time is 2-12 h. 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 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.

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 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.

5. 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.

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 S8, the high-speed centrifuge rotates at 8000-12000 rpm, and each centrifugation takes 5 minutes.

7. 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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