Lithium manganese iron phosphate positive electrode slurry, pole piece, soft package battery cell and preparation method of lithium manganese iron phosphate positive electrode slurry

Abstract

The invention discloses lithium manganese iron phosphate positive electrode slurry, a pole piece, a soft package battery cell and a preparation method of the lithium manganese iron phosphate positive electrode slurry, the lithium manganese iron phosphate positive electrode slurry is characterized by comprising a lithium manganese iron phosphate material, a conductive agent, a binder, a dispersing agent, an additive, a solvent and a pH regulator; the lithium manganese iron phosphate battery comprises the following components in percentage by mass: more than or equal to 94.5% of a lithium manganese iron phosphate material, 0.1-0.5% of a dispersing agent, 1-2.5% of a binder, 1-2% of a conductive agent and 0.2-0.5% of an additive. The lithium manganese iron phosphate positive electrode plate is prepared from the lithium manganese iron phosphate positive electrode slurry. The lithium manganese iron phosphate soft package battery cell comprises the lithium manganese iron phosphate positive pole piece. According to the invention, the compaction density and peel strength of the lithium manganese iron phosphate pole piece are improved, and the processing properties of powder falling and the like are reduced; manganese dissolution is reduced, and the electrochemical performance of the battery cell is improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery cathode material technology, specifically relating to a lithium manganese iron phosphate cathode slurry, electrode sheet, soft-pack battery cell and its preparation method. Background Technology

[0002] Currently, in the new energy market, particularly in energy storage and power sectors, especially passenger vehicles, there is a broad market demand for high-energy-density battery cells. However, the energy density of mainstream lithium iron phosphate (LFP) cells is nearing its limit, leading to a focus on lithium manganese iron phosphate (LFP). LFP cells, with a platform voltage of 3.65V, offer approximately 14% higher energy density for the same capacity compared to LFP cells (3.2V), giving them greater competitive potential. Electrode fabrication is crucial for cell performance, especially for high-voltage LFP cells. Electrode characteristics are critical for maximizing cell capacity, improving range, and enhancing safety; therefore, fabricating superior electrodes is fundamental to improving cell performance.

[0003] Currently, there is limited research on improving the electrode process for pure lithium manganese iron phosphate (LMP) systems. Most LMP electrode process parameters are directly applied to LMP-ternary and LMP-ternary hybrid systems, with nearly identical material preparation, mixing, and coating parameters. However, LMP cathode materials are smaller and have a larger specific surface area than LMP particles, resulting in lower processability. Consequently, the prepared LMP electrodes often exhibit problems such as streaks, powder shedding, low peel strength, and low compaction density during coating and cold pressing. These issues are primarily caused by uneven slurry dispersion due to electrode formulation and preparation processes, leading to low adhesion. These processing problems can degrade cell performance. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a lithium manganese iron phosphate cathode slurry, electrode sheet, soft-pack battery cell and its preparation method, so as to solve the battery cell performance defects caused by the preparation defects of lithium iron phosphate electrode sheet.

[0005] Technical solution: The lithium manganese iron phosphate cathode slurry of the present invention includes lithium manganese iron phosphate material, conductive agent, binder, dispersant, additive, solvent, and pH adjuster; by mass content, lithium manganese iron phosphate material ≥94.5%, dispersant 0.1-0.5%, binder 1-2.5%, conductive agent 1-2%, additive 0.2-0.5%, and solvent and pH adjuster are not included in the content calculation.

[0006] Preferably, the manganese-iron ratio of the lithium manganese iron phosphate material is (6:4) to (8:2).

[0007] Further preferred options are lithium manganese iron phosphate materials with manganese iron ratios of 7:3, 6:4, and 8:2.

[0008] Preferably, the adhesive is polyvinylidene fluoride, including at least one of high molecular weight > 1 million and low molecular weight ≤ 1 million.

[0009] More preferably, the adhesive is at least one of PVDF5130 and PVDF900; Preferably, the pH adjuster includes at least one of oxalic acid and phosphoric acid.

[0010] Preferably, the dispersant includes at least one of PVP (polyvinylpyrrolidone), CGP (carboxymethyl cellulose), polyphosphate, SDS (sodium dodecyl sulfate), fatty alcohol polyoxyethylene ether, PAM (polyacrylamide), and NVP (N-vinylpyrrolidone); the additive includes at least one of acrylate polymers, unsaturated polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty amine polyoxyethylene ether, polyoxyethylene ether phosphate, fatty alcohol polyoxyethylene ether phosphate, polyetheramine and fatty amine polyoxyethylene ether phosphate, and polyimide (PI); the conductive agent includes at least one of carbon black SP, carbon nanotubes, graphene, and vapor-grown carbon fibers; and the solvent is N-methylpyrrolidone.

[0011] The lithium manganese iron phosphate cathode sheet of the present invention is made from the lithium manganese iron phosphate cathode slurry.

[0012] The lithium manganese iron phosphate soft-pack battery cell of the present invention includes the lithium manganese iron phosphate positive electrode sheet.

[0013] The preparation method of the lithium manganese iron phosphate cathode slurry of the present invention includes the following steps: S1. Prepare the adhesive solution, which is made of binder and solvent, wherein the total amount of binder added is 1-2.5% of the total solids by mass ratio, the concentration of the adhesive solution is 3-8%, and the solution is stirred and dispersed under vacuum conditions; S2. Then add the first conductive agent, calculated by mass percentage, the amount added is 0.5-1% of the total solids, and stir and disperse under vacuum to obtain the first dispersion; S3. The second conductive agent, calculated by mass ratio, is added to the first dispersion at an amount of 0.5-1% of the total solids to obtain the second dispersion; S4. Add lithium manganese iron phosphate material to the second dispersion. The amount added is 95-97% of the total solids according to the mass ratio. Divide the lithium manganese iron phosphate material into two equal parts and add them in sequence. Stir and disperse under vacuum to obtain the third dispersion. S5. Add dispersant and additive to the third dispersion, and disperse under vacuum to obtain the fourth dispersion with a particle size <20um; S6. Add solvent to the fourth dispersion to adjust the slurry viscosity to 6000-8000 mPa.s, add pH adjuster to adjust the slurry pH value to 9-11, and then stir and disperse under vacuum conditions; S7. Defoam and disperse by stirring; S8. The well-dispersed slurry is sieved through a screen to obtain the slurry after impurities are removed.

[0014] Preferably, the preparation method of the lithium manganese iron phosphate cathode slurry of the present invention includes the following steps: S1. Prepare the adhesive solution, which is prepared from binder and solvent, wherein the total amount of PVDF binder added is 1-2.5% of the total solids by mass ratio, the adhesive solution concentration is 3-8%, and it is dispersed under vacuum conditions with a revolution speed of 10-30 rpm, a rotation speed of 800-1200 rpm, for 4-6 hours; S2. Then add the first conductive agent, calculated by mass ratio, the amount added is 0.5-1% of the total solids, disperse for 10-20 min under the conditions of revolution speed of 8-12 rpm and rotation speed of 400-600 rpm, then adjust the revolution speed to 40-50 rpm and disperse under vacuum for 1-3 h to obtain the first dispersion; S3. The second conductive agent, calculated by mass ratio, is added to the first dispersion at an amount of 0.5-1% of the total solids and dispersed under vacuum conditions at a revolution speed of 40-50 rpm and a rotation speed of 1500-2500 rpm for 0.5-1.5 hours to obtain the second dispersion. S4. Add lithium manganese iron phosphate material to the second dispersion. The amount added should be 95-97% of the total solids by mass. Divide the lithium manganese iron phosphate material into two equal parts and add them. After adding the first part, disperse it for 20-40 minutes at a revolution speed of 8-12 rpm and a rotation speed of 400-600 rpm. Then add the second part of lithium manganese iron phosphate and disperse it for 20-40 minutes at a revolution speed of 8-12 rpm and a rotation speed of 400-600 rpm. Then adjust the revolution speed to 40-50 rpm, the rotation speed to 1500-2500 rpm, and disperse it under vacuum for 1-3 hours. Control the dispersion temperature at 35-45℃ to obtain the third dispersion. S5. Add a dispersant to the third dispersion, the amount of which accounts for 0.1-0.5% of the total solids; add an additive, the amount of which accounts for 0.2-0.5% of the total solids. Then, adjust the revolution speed to 40-50 rpm, the rotation speed to 1500-2500 rpm, and the dispersion under vacuum conditions of -80 kPa to -98 kPa for 0.5-1.5 h, and control the dispersion temperature at 35-45℃ to obtain the fourth dispersion with a particle size <20 μm. S6. Add NMP to the fourth dispersion to adjust the slurry viscosity to 6000-8000 mPa·s, add at least one of oxalic acid and phosphoric acid to adjust the slurry pH to 9-11, and then adjust the revolution speed to 20-30 rpm, the rotation speed to 800-1200 rpm, and disperse under vacuum for 20-40 min. S7. Degassing: Set the rotation speed to 0 and the revolution speed to 10-20 rpm, and stir for 20-40 minutes. S8. The dispersed slurry is sieved through a 100-200 mesh sieve to obtain the slurry after impurities are removed.

[0015] More preferably, the preparation method of the lithium manganese iron phosphate cathode slurry of the present invention includes the following steps: S1. Prepare the adhesive solution, which is prepared from PVDF5130, PVDF900, and NMP solvent, wherein, according to the mass ratio, PVDF5130:PVDF900 = (1:3) to (3:1), the total amount of PVDF added is 1-2.5% of the total solids, the adhesive solution concentration is 3-8%, and it is dispersed for 5 hours under vacuum conditions with a revolution speed of 20 rpm and a rotation speed of 1000 rpm; more preferably, PVDF5130:PVDF900 = (1:3) or (1:2) or (1:1) or (2:1) or (3:1); S2. Then add the first conductive agent. According to the mass ratio, the amount added is 0.5-1% of the total solids. Disperse for 15 minutes at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then adjust the revolution speed to 45 rpm and disperse under vacuum for 2 hours to obtain the first dispersion. S3. The second conductive agent, calculated by mass ratio, is added to the first dispersion at an amount of 0.5-1% of the total solids and dispersed under vacuum conditions at a revolution speed of 45 rpm and a rotation speed of 2000 rpm for 1 hour to obtain the second dispersion. S4. Add lithium manganese iron phosphate material to the second dispersion. According to the mass ratio, the amount added is 95-97% of the total solids. Divide the lithium manganese iron phosphate material into two equal parts and add them. After adding the first part, disperse it for 30 minutes at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then add the second part of lithium manganese iron phosphate and disperse it for 30 minutes at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then adjust the revolution speed to 45 rpm, the rotation speed to 2000 rpm, and disperse it under vacuum for 2 hours. Control the dispersion temperature at 40℃ to obtain the third dispersion. S5. Add at least one of the following dispersants to the third dispersion: PVP (polyvinylpyrrolidone), CGP, polyphosphate, fatty alcohol polyoxyethylene ether, polyacrylamide (PAM), and NVP (N-vinylpyrrolidone), with the addition amount accounting for 0.1-0.5% of the total solids; add at least one of the following additives: acrylate polymer, unsaturated polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty amine polyoxyethylene ether, polyoxyethylene ether phosphate, fatty alcohol polyoxyethylene ether phosphate, polyetheramine and fatty amine polyoxyethylene ether phosphate, and polyimide (PI), with the addition amount accounting for 0.2-0.5% of the total solids. Then, adjust the revolution speed to 45 rpm, the rotation speed to 2000 rpm, and the vacuum conditions from -80 kPa to -98 kPa for 1 h, and control the dispersion temperature at 40℃ to obtain the fourth dispersion with a particle fineness <20 μm. S6. Add NMP to the fourth dispersion to adjust the slurry viscosity to 6000-8000 mPa·s, add at least one of oxalic acid and phosphoric acid to adjust the slurry pH to 9-11, and then adjust the revolution speed to 25 rpm, the rotation speed to 1000 rpm, and disperse under vacuum for 30 min. S7. Degassing: Set the rotation speed to 0 and the revolution speed to 15 rpm and stir for 30 minutes. S8. The dispersed slurry is sieved through a 150-mesh sieve to obtain the slurry after removing impurities. The sieving time is <20 min.

[0016] The preparation method of the lithium manganese iron phosphate positive electrode sheet of the present invention includes the following steps: coating the sieved slurry onto the surface of the current collector, coating both sides, and baking; compacting the prepared electrode sheet to obtain the lithium manganese iron phosphate positive electrode sheet.

[0017] Preferably, the sieved slurry is coated onto the surface of the current collector, with double-sided coating, and baked at 90℃-120℃; the prepared electrode is then compacted to a compaction density of 2.0-2.3 g / cm³. 3 .

[0018] The preparation method of the lithium manganese iron phosphate soft-pack battery cell of the present invention includes the following steps: using the lithium manganese iron phosphate positive electrode sheet as the positive electrode of the soft-pack battery, the graphite negative electrode sheet as the negative electrode of the soft-pack battery, the N / P ratio is 1.1-1.2, the electrolyte is a special electrolyte for lithium manganese iron phosphate, and the soft-pack battery is prepared by stacking process to obtain the lithium manganese iron phosphate soft-pack battery cell.

[0019] Preferably, lithium manganese iron phosphate positive electrode is used as the positive electrode of the soft-pack battery, graphite negative electrode is used as the negative electrode of the soft-pack battery, the N / P ratio is 1.1-1.2, the electrolyte is a lithium manganese iron phosphate special electrolyte, the separator is Celgard2500, and then the soft-pack battery is prepared by stacking process.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. It increases the compaction density of lithium manganese iron phosphate electrode by about 1%, increases the peel strength by about 288%, and reduces processing performance such as powder shedding; 2. It reduces manganese leaching by about 85%, improves the electrochemical performance of the battery cell, and increases the capacity by about 6%. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the embodiments.

[0022] Example 1

[0023] (a) Raw materials for preparing lithium manganese iron phosphate cathode slurry Table 1 Raw materials used in Example 1

[0024] (II) Preparation of lithium manganese iron phosphate cathode slurry S1. Prepare the adhesive solution, which is prepared from PVDF5130, PVDF900 and NMP solvent, wherein the mass ratio of PVDF5130:PVDF900 is 1:3, the total amount of PVDF added is 2% of the total solids, the adhesive solution concentration is 5%, and it is dispersed for 5 hours under the conditions of revolution speed of 20 rpm, rotation speed of 1000 rpm and vacuum of -85 kPa; S2. Then add the first conductive agent, which is 0.5% of the total solids according to the mass ratio. Disperse for 15 min at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then adjust the revolution speed to 45 rpm and disperse for 2 h under vacuum of -85 kPa to obtain the first dispersion. S3. The second conductive agent, calculated by mass ratio, is added to the first dispersion at 1% of the total solid amount and dispersed under the following conditions: revolution speed of 45 rpm, rotation speed of 2000 rpm, and vacuum of -85 kPa for 1 hour to obtain the second dispersion. S4. Add lithium manganese iron phosphate cathode material to the second dispersion. According to the mass ratio, the amount added is 96% of the total. Divide the lithium manganese iron phosphate cathode material into two equal parts and add them. After adding the first part, disperse it for 30 min at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then add the second part of lithium manganese iron phosphate and disperse it for 30 min at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then adjust the revolution speed to 45 rpm, the rotation speed to 2000 rpm, and disperse it under vacuum of -85 kPa for 2 h. Control the dispersion temperature at 40℃ to obtain the third dispersion. S5. Add dispersant PVP (polyvinylpyrrolidone) to the third dispersion at a rate of 0.1% of the total solids; add additive polyimide (PI) at a rate of 0.4% of the total solids. Then, adjust the revolution speed to 45 rpm, the rotation speed to 2000 rpm, and the dispersion under vacuum of -85 kPa for 1 h, and control the dispersion temperature at 40℃ to obtain the fourth dispersion with a particle size of <20 μm. S6. Add NMP to the fourth dispersion to adjust the slurry viscosity to 6000-8000 mPa.s, add oxalic acid to adjust the slurry pH to 9-11, and then adjust the revolution speed to 25 rpm, the rotation speed to 1000 rpm, and disperse for 30 min under vacuum of -85 kPa. S7. Degassing: Set the rotation speed to 0 and the revolution speed to 15 rpm and stir for 30 minutes. S8. The dispersed slurry is sieved through a 150-mesh sieve to obtain the slurry after removing impurities. The sieving time is <20 min.

[0025] (III) Preparation of lithium manganese iron phosphate cathode sheet The sieved slurry is coated onto the surface of carbon-coated aluminum foil, with double-sided coating, and baked at 90℃-120℃; the prepared electrode sheet is compacted to a compaction density of 2.3g / cm3.

[0026] (iv) Preparation of lithium iron phosphate pouch cells The prepared lithium manganese iron phosphate positive electrode sheet was used as the positive electrode of the soft-pack battery, and the graphite negative electrode sheet was used as the negative electrode of the soft-pack battery. The N / P ratio was 1.15, the electrolyte was a lithium manganese iron phosphate special electrolyte, and the separator was Celgard2320. The soft-pack battery was then prepared by stacking process.

[0027] Example 2 (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 1:2, and the other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0028] Example 3

[0029] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130 to PVDF900 is 1:1, and the other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0030] Example 4

[0031] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1, and the other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0032] Example 5

[0033] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 3:1, and the other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0034] Example 6

[0035] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1; The amount of the first conductive agent added in step S2 is 0.6%; The amount of the second conductive agent added in step S3 is 0.9%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0036] Example 7

[0037] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1; The amount of the first conductive agent added in step S2 is 0.7%; The amount of the second conductive agent added in step S3 is 0.8%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0038] Example 8

[0039] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1; The amount of the first conductive agent added in step S2 is 0.8%; The amount of the second conductive agent added in step S3 is 0.7%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0040] Example 9

[0041] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1; The amount of the first conductive agent added in step S2 is 0.9%; The amount of the second conductive agent added in step S3 is 0.6%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0042] Example 10

[0043] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1; The amount of the first conductive agent added in step S2 is 0.8%; The amount of the second conductive agent added in step S3 is 0.7%; The amount of dispersant added in step S5 is 0.2%; The amount of additive added in step S5 is 0.3%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0044] Example 11 (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) In step S1, the ratio of PVDF5130:PVDF900 is 2:1; The amount of the first conductive agent added in step S2 is 0.8%; The amount of the second conductive agent added in step S3 is 0.7%; The amount of dispersant added in step S5 is 0.3%; The amount of additive added in step S5 is 0.2%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0045] Example 12 (a) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; the difference is that only PVDF5130 is used as the binder. (ii) The total amount of PVDF5130 added in step S1 is 2% of the total solids, and the other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0046] Example 13

[0047] (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) The total amount of PVDF added in step S1 is 1% of the total solids; The amount of the first conductive agent added in step S2 is 1%; The amount of the second conductive agent added in step S3 is 1%; The amount of dispersant added in step S5 is 0.5%; The amount of additive added in step S5 is 0.5%; The other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0048] Comparative Example 1 (a) The other raw materials used in the preparation of lithium manganese iron phosphate cathode sheets are the same as those in Example 1; the difference is that they do not contain dispersants and additives. (ii) The total amount of PVDF added in step S1 is 2.5% of the total solids; The amount of the first conductive agent added in step S2 is 1%; The amount of the second conductive agent added in step S3 is 1%; Without adding dispersants and additives, the other steps are the same as in Example 1; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0049] Comparative Example 2 (a) Raw materials for preparing lithium manganese iron phosphate cathode sheets Table 2 Raw materials used in Comparative Example 2

[0050] (II) Preparation of lithium manganese iron phosphate cathode sheet S1. Prepare the adhesive solution, which is prepared by PVDF5130 and NMP solvent, with the total amount of PVDF added being 2% of the total solids and the adhesive solution concentration being 5%. Disperse the solution for 5 hours under the conditions of a revolution speed of 20 rpm, a rotation speed of 1000 rpm, and a vacuum of -85 kPa. S2. Then add the first conductive agent. According to the mass ratio, the amount added is 0.8% of the total solids. Disperse for 15 minutes at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then adjust the revolution speed to 45 rpm and disperse for 2 hours under vacuum of -85 kPa to obtain the first dispersion. S3. The second conductive agent, calculated by mass ratio, is added to the first dispersion at 0.7% of the total solids and dispersed under the following conditions: revolution speed of 45 rpm, rotation speed of 2000 rpm, and vacuum of -85 kPa for 1 hour to obtain the second dispersion. S4. Add lithium manganese iron phosphate cathode material to the second dispersion. According to the mass ratio, the amount added is 96% of the total. Divide the lithium manganese iron phosphate cathode material into two equal parts and add them. After adding the first part, disperse it for 30 min at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then add the second part of lithium manganese iron phosphate and disperse it for 30 min at a revolution speed of 10 rpm and a rotation speed of 500 rpm. Then adjust the revolution speed to 45 rpm, the rotation speed to 2000 rpm, and disperse it under vacuum of -85 kPa for 3 h. Control the dispersion temperature at 40℃ to obtain the third dispersion. S5. Add NMP to the dispersion obtained in the previous step to adjust the slurry viscosity to 6000-8000 mPa.s, and then adjust the revolution speed to 25 rpm, the rotation speed to 1000 rpm, and disperse for 30 min under vacuum of -85 kPa. S6. Degassing: Set the rotation speed to 0 and the revolution speed to 15 rpm and stir for 30 minutes. S7. The dispersed slurry is sieved through a 150-mesh sieve to obtain the slurry after removing impurities; S8. Apply the sieved slurry to the surface of the carbon-coated aluminum foil, double-sided coating, and bake at 90℃-120℃. S9. Compact the prepared electrode sheet to a compaction density of 2.3 g / cm3; (III) Preparation of lithium iron phosphate soft-pack cells.

[0051] Comparative Example 3 (i) The raw materials used to prepare the lithium manganese iron phosphate positive electrode sheet are all the materials in Example 1; (ii) No oxalic acid is added to adjust NMP in step S6; The other steps are the same as in Example 10; (III) The manufacturing process of lithium manganese iron phosphate soft-pack battery cells is the same as in Example 1.

[0052] Tests and Results The viscosity, fineness, solids content, peel strength, and ultimate compaction test results of the lithium manganese iron phosphate slurry prepared in the experimental example and comparative example are as follows: Table 3 Processing performance test data

[0053] The results of the first-efficiency, 1C specific capacity, resistance, and rate capability tests of the lithium manganese iron phosphate pouch cells prepared in the examples and comparative examples are as follows: Table 4 Electrochemical Performance Tests

[0054] The data tables of processing performance of lithium manganese iron phosphate electrodes and lithium manganese iron phosphate pouch cells obtained from Examples 1-13 and Comparative Examples 1-3 show that the compaction density, electrode peel strength, electrode resistance, cell internal resistance, first efficiency, rate capability, and cycle performance of the lithium manganese iron phosphate electrode sheets obtained using the scheme of the present invention are all superior to the test data in the comparative examples. Among them, the lithium manganese iron phosphate electrode sheets and pouch cells obtained in Example 10 have the best processing performance and electrochemical performance. By controlling and optimizing the proportion of different mass fractions of binders, the proportion of different types of conductive agents, the proportion of additives and dispersants, and the addition of oxalic acid, the solid content, electrode peel strength, and electrode compaction density of the lithium manganese iron phosphate slurry are improved, while the slurry viscosity and membrane resistance are reduced. This improves the first efficiency, specific capacity, rate capability, and cycle performance of the lithium manganese iron phosphate pouch cells. The addition of oxalic acid can reduce manganese dissolution.

Claims

1. A lithium iron phosphate cathode slurry, characterized in that, Includes lithium manganese iron phosphate material, conductive agent, binder, dispersant, additive, solvent, and pH adjuster; by mass content (total solids), lithium manganese iron phosphate material ≥94.5%, dispersant 0.1-0.5%, binder 1-2.5%, conductive agent 1-2%, additive 0.2-0.5%, and solvent and pH adjuster are not included in the content calculation.

2. The lithium iron phosphate cathode slurry according to claim 1, characterized in that, The manganese-iron ratio of the lithium manganese iron phosphate material is (6:4) to (8:2).

3. The lithium iron phosphate cathode slurry according to claim 1, characterized in that, The adhesive is polyvinylidene fluoride, including at least one of high molecular weight > 1 million and low molecular weight ≤ 1 million.

4. The lithium iron phosphate cathode slurry according to claim 1, characterized in that, The pH adjuster includes at least one of oxalic acid and phosphoric acid.

5. The lithium iron phosphate cathode slurry according to claim 1, characterized in that, The dispersant includes at least one of PVP (polyvinylpyrrolidone), CGP (carboxymethyl cellulose), polyphosphate, SDS (sodium dodecyl sulfate), fatty alcohol polyoxyethylene ether, PAM (polyacrylamide), and NVP (N-vinylpyrrolidone); the additive includes at least one of acrylate polymers, unsaturated polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty amine polyoxyethylene ether, polyoxyethylene ether phosphate, fatty alcohol polyoxyethylene ether phosphate, polyetheramine and fatty amine polyoxyethylene ether phosphate, and polyimide (PI); the conductive agent includes at least one of carbon black SP, carbon nanotubes, graphene, and vapor-grown carbon fibers; and the solvent is N-methylpyrrolidone.

6. A lithium manganese iron phosphate positive electrode sheet, characterized in that, It is made from the lithium manganese iron phosphate cathode slurry as described in claim 1.

7. A lithium iron phosphate (LFP) soft-pack battery cell, characterized in that, Including the lithium manganese iron phosphate positive electrode sheet as described in claim 6.

8. A method for preparing the lithium manganese iron phosphate cathode slurry according to claim 1, characterized in that, Includes the following steps: S1. Prepare the adhesive solution, which is made of binder and solvent, wherein the total amount of binder added is 1-2.5% of the total solids by mass ratio, the concentration of the adhesive solution is 3-8%, and the solution is stirred and dispersed under vacuum conditions; S2. Then add the first conductive agent, calculated by mass percentage, the amount added is 0.5-1% of the total solids, and stir and disperse under vacuum to obtain the first dispersion; S3. The second conductive agent, calculated by mass ratio, is added to the first dispersion at an amount of 0.5-1% of the total solids to obtain the second dispersion; S4. Add lithium manganese iron phosphate material to the second dispersion. The amount added is 95-97% of the total solids according to the mass ratio. Divide the lithium manganese iron phosphate material into two equal parts and add them in sequence. Stir and disperse under vacuum to obtain the third dispersion. S5. Add dispersant and additive to the third dispersion, and disperse under vacuum to obtain the fourth dispersion with a particle size <20um; S6. Add solvent to the fourth dispersion to adjust the slurry viscosity to 6000-8000 mPa.s, add pH adjuster to adjust the slurry pH value to 9-11, and then stir and disperse under vacuum conditions; S7. Defoam and disperse by stirring; S8. The well-dispersed slurry is sieved through a screen to obtain the slurry after impurities are removed.

9. A method for preparing the lithium manganese iron phosphate positive electrode sheet according to claim 6, characterized in that, Includes the following steps: The sieved slurry is coated onto the surface of the current collector, with double-sided coating, and then baked. The prepared electrode is then compacted to obtain the lithium manganese iron phosphate positive electrode.

10. A method for preparing the lithium manganese iron phosphate soft-pack battery cell according to claim 7, characterized in that, The process includes the following steps: using the lithium manganese iron phosphate positive electrode sheet as described in claim 6 as the positive electrode of the soft-pack battery, using the graphite negative electrode sheet as the negative electrode of the soft-pack battery, having an N / P ratio of 1.1-1.2, using a lithium manganese iron phosphate-specific electrolyte, and preparing the soft-pack battery using a stacking process to obtain a lithium manganese iron phosphate soft-pack battery cell.

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