Lithium manganese iron phosphate slurry, pole piece and preparation method thereof

Abstract

The present application belongs to the technical field of lithium ion batteries, and particularly relates to a lithium manganese iron phosphate slurry, a pole piece and a preparation method thereof. In the preparation method, the lithium manganese iron phosphate material is added into a conductive glue solution in two steps, and then modified PVP is introduced as a dispersant. Under the dual action of steric hindrance and electrostatic repulsion, the viscosity of the slurry can be effectively reduced, and the slurry is allowed to change to a pseudoplastic fluid characteristic, so that the dispersibility and stability of the lithium manganese iron phosphate are significantly improved. In addition, the PVDF with different molecular weights is compounded, the stacking between the molecular weights is more compact, the intermolecular force between the PVDF and the powder is larger, the adhesion is better, and the problems such as cracking and powder falling in the coating process are reduced. Meanwhile, the complexing groups (polar groups such as carboxyl, hydroxyl, amide, imide, etc.) on the long chain of the macromolecular weight PVDF can form chemical bonds with the foil, further improving the adhesion, and improving the flexibility of the pole piece.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of lithium ion batteries, and particularly relates to a lithium iron manganese phosphate slurry, a pole piece and a preparation method thereof. BACKGROUND

[0002] The lithium iron manganese phosphate positive electrode material is widely concerned due to its high voltage platform and moderate theoretical specific capacity, but the electronic conductivity and ionic conductivity of the lithium iron manganese phosphate material are poor due to the presence of manganese elements, so the conductivity of the lithium iron manganese phosphate material is often improved by means of nano-particle, carbon coating and doping in commercial applications. However, the reduction of particle size and the increase of specific surface area caused by the above-mentioned means greatly affect the processability of the lithium iron manganese phosphate material. The existing homogenizing process, i.e. the traditional wet and dry homogenizing process, is difficult to completely disperse the lithium iron manganese phosphate material, and the particle dispersion is difficult, which leads to the difficulty in sieving the slurry and the coating of particles and scratches, and also causes the excessive absorption of glue, which reduces the bonding force between the particles and the foil and the flexibility of the pole piece, and increases the film resistance, which seriously affects the performance of the lithium iron manganese phosphate battery.

[0003] In view of this, the present application is proposed. SUMMARY

[0004] In order to solve the problems of the prior art, the present application provides a lithium iron manganese phosphate slurry, a pole piece and a preparation method thereof. The main material is added in steps during preparation, two kinds of polyvinylidene fluoride (PVDF) with different molecular weights are compounded as binders, and an appropriate amount of dispersant is added to obtain a lithium iron manganese phosphate slurry with high solid content, small fineness, good dispersity and stability. The lithium iron manganese phosphate pole piece with stable surface density, good flexibility, adhesion and conductivity is prepared by controlling the discharge viscosity. The problems of difficult homogenization of lithium iron manganese phosphate positive electrode material, low adhesion, poor flexibility of pole piece and large film resistance are solved.

[0005] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0006] In the first aspect, a preparation method of a lithium iron manganese phosphate slurry comprises the following preparation steps:

[0007] (1) mixing and configuring a glue solution by mixing a binder and part of a dispersant in a solvent, wherein the binder comprises PVDF-1 and PVDF-2, the PVDF-1 is polyvinylidene fluoride with a molecular weight of 50-90W, the PVDF-2 is modified polyvinylidene fluoride containing a carbonyl-based polar functional group with a molecular weight of 120-150W, and the dispersant is modified PVP;

[0008] (2) mixing graphene, carbon nanotubes and conductive carbon black as conductive agents in the glue solution obtained in step (1) in sequence to obtain a conductive glue solution;

[0009] (3) adding part of the lithium manganese iron phosphate into the conductive glue solution obtained in step (2) to mix, and then adding the remaining lithium manganese iron phosphate, dispersant and solvent into the slurry to mix, and then preparing the lithium manganese iron phosphate slurry.

[0010] Further, the carbonyl polar functional groups include one or more of carboxyl, hydroxyl, amide, imide;

[0011] And / or, the mass ratio of the PVDF-1 and PVDF-2 is 1:3-9;

[0012] And / or, the modified PVP is a fluorine-containing acrylate copolymer, a polyvinylpyrrolidone solution modified by an alcohol amine substance;

[0013] And / or, the mass ratio of the conductive carbon black, nanometer carbon tube and graphene is (1-1.5):(0.4-0.5):(0.1-0.5);

[0014] And / or, the amount of the carbon-coated lithium manganese iron phosphate accounts for 1.6-1.8wt% of the amount of the lithium manganese iron phosphate, the particle size D50 is 400-500nm, the particle size D90 is 0.8-1μm, and the specific surface area is 15-20m 2 / g;

[0015] And / or, the lithium manganese iron phosphate slurry includes a powder and a solvent, and the mass ratio of the powder and the solvent is (55:45)-(63:37).

[0016] Further, the powder consists of 94.6-97% of lithium manganese iron phosphate, 1-3% of conductive agent, 0.8-2% of binder, and 0.05-0.4% of dispersant by mass percentage;

[0017] And / or, the viscosity of the modified PVP is <200mPa·s, and the pH is between 10-12;

[0018] And / or, the solvent is N-methylpyrrolidone.

[0019] Further, in step (1), the solid content of the glue solution is 4-6%;

[0020] And / or, in step (3), the viscosity of the lithium manganese iron phosphate slurry is 3000-8000mPa·s.

[0021] Further, in steps (1) and (3), the mass ratio of the two additions of the dispersant is (1:3)-(1:1);

[0022] And / or, in step (3), the mass ratio of the two additions of the lithium manganese iron phosphate is (3:1)-(1:1).

[0023] Further, in step (1) and / or step (2) and / or step (3), the mixing each comprises stirring and scraping the material at a low speed first and then dispersing at a high speed.

[0024] Further, the revolution speed and the rotation speed of the stirring and scraping at a low speed are 10-15 rpm and 1000-1500 rpm respectively, and the time is 5-10 min.

[0025] And / or, the revolution speed and the rotation speed of the dispersing at a high speed are 40-55 rpm and 3500-5500 rpm respectively, and the time is 1-3 h.

[0026] In the second aspect, a lithium manganese iron phosphate slurry is prepared by the method in the first aspect.

[0027] In the third aspect, a lithium manganese iron phosphate pole piece is prepared by coating the lithium manganese iron phosphate slurry in the second aspect on a carbon-coated aluminum foil with a certain thickness, drying, and then rolling at a certain compaction density.

[0028] Further, the areal density is 350-380 g / m 2 ;

[0029] And / or, the thickness is 12-14 μm.

[0030] And / or, the compaction density is 2.2-2.4 g / cm 3

[0031] Compared with the prior art, the present application has at least the following beneficial effects:

[0032] In the preparation method, the lithium manganese iron phosphate material is added to the conductive glue solution in two steps, and the modified PVP is introduced as a dispersant. Under the dual action of steric hindrance and electrostatic repulsion, the viscosity of the slurry can be effectively reduced, and the slurry can be converted to a pseudoplastic fluid. The dispersibility and stability of the lithium manganese iron phosphate are significantly improved. In addition, the PVDF with different molecular weights is compounded, the stacking between the molecules with different molecular weights is more compact, the intermolecular force between the PVDF and the powder is larger, the adhesion is better, and the problems such as cracking and powder falling during the coating process are reduced. At the same time, the complexing groups (polar groups such as carboxyl, hydroxyl, amide, imide, etc.) on the long chain of the macromolecular PVDF can form chemical bonds with the foil, further improving the adhesion, and improving the flexibility of the pole piece. DETAILED DESCRIPTION

[0033] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. It should be appreciated by those skilled in the art that the embodiments are only used for understanding the present application, and should not be regarded as specific limitation on the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present application. The process parameters not specified in the following embodiments are usually according to the conventional conditions.

[0034] The endpoints of the ranges and any values disclosed in the present application are not limited to the precise values stated. The ranges or values should be interpreted as approximately between the stated values. For ranges, the endpoints are included between each respective range; the endpoints are included in the respective range or value; and each respective range or value includes single points between (and including) the range or value endpoints. The application encompasses one or more new ranges formed by combining the lower limit of one range or value with the upper limit of another range or value. Any numerical values include increments of one unit plus or minus 0.0001, 0.001, 0.01 or 0.1 % and this is true of all ranges and values.

[0035] According to a first aspect of the present application, a preparation method of lithium manganese iron phosphate slurry comprises the following preparation steps:

[0036] (1) a glue solution is prepared by mixing a binder and part of a dispersant in a solvent, wherein the binder comprises PVDF-1 and PVDF-2, the PVDF-1 is polyvinylidene fluoride with a molecular weight of 50-90W, the PVDF-2 is modified polyvinylidene fluoride containing a carbonyl-based polar functional group with a molecular weight of 120-150W, and the dispersant is modified PVP;

[0037] (2) graphene, nanometer carbon tube and conductive carbon black as conductive agents are sequentially added into the glue solution obtained in step (1) for mixing to obtain a conductive glue solution;

[0038] (3) part of lithium manganese iron phosphate is added into the conductive glue solution obtained in step (2) for mixing, and then the remaining lithium manganese iron phosphate, dispersant and solvent are added into the slurry for mixing, and the lithium manganese iron phosphate slurry is prepared after being uniformly mixed.

[0039] The preparation method of the present application adopts a complete wet process, the adding time of the dispersant and lithium manganese iron phosphate is adjusted, the amount is added multiple times, the conductive glue solution is prepared first, and then the lithium manganese iron phosphate is added twice, which can fully play the role of the PVDF binder and can also avoid the problem that the LMFP particles are easy to agglomerate. Under the dual action of steric hindrance and electrostatic repulsion, the modified PVP can effectively reduce the viscosity of the slurry, and at the same time, the slurry is changed to a pseudoplastic fluid characteristic, so that the dispersibility and standing stability of the lithium manganese iron phosphate are significantly improved.

[0040] Specifically, when the glue solution is prepared in advance, part of the dispersant is used to disperse the mixed binder, so that the binder is uniformly dispersed in the solvent, and the binding effect can be better. More specifically, the binder PVDF is a compound of 50-90W molecular weight polyvinylidene fluoride PVDF-1 and 120-150W molecular weight modified polyvinylidene fluoride PVDF-2 containing carbonyl-based polar functional groups. Not only has the binding effect of PVDF-1 and PVDF-2 respectively, but also the presence of carbonyl-based polar functional groups in PVDF-2 can effectively enhance the force between the positive active material and the current collector, form a chemical bond with the foil, further improve the adhesion, and improve the flexibility of the pole piece. Its unique structure can also effectively prevent the slurry from gelling; at the same time, it can also improve the adhesion and corrosion resistance of the pole piece in the electrolyte; and the PVDF with different molecular weights is compounded as PVDF-1 / PVDF-2, the stacking between the molecular weights is more compact, so that the intermolecular force between it and the powder is greater, the adhesion is better, and the problems such as cracking and powder falling during coating are reduced; and, can avoid the sedimentation caused by insufficient adhesion when using small molecular weight PVDF-1 alone and the gel phenomenon caused by agglomeration when using large molecular weight PVDF-2 alone, so as to ensure that the slurry has high solid content and high coating weight, and has good adhesion and flexibility of the pole piece, and improves the coating cracking. When the conductive glue solution is prepared, different conductive agents are added according to the difficulty of dispersion of the conductive agent from difficult to easy, and three kinds of conductive agents, graphene, nanometer carbon tube and conductive carbon black, are compounded to form an effective "point to line to surface" conductive network, so as to obtain a conductive glue solution in which the binder and the conductive agent are uniformly dispersed.

[0041] In the post-dispersed manganese iron phosphate lithium, the manganese iron phosphate lithium material (LMFP particles) is added into the conductive glue liquid in two steps to avoid the problem of easy agglomeration of the LMFP particles. Specifically, a part of the LMFP particles is added first, and the PVDF particles uniformly dispersed in the conductive glue liquid will be adsorbed onto the surface of the LMFP particles, and through the PVDF-1 adhesion and / or PVDF-2 adhesion and / or PVDF-1 / PVDF-2 adhesion, the contact area and contact points between particles are increased, and the adhesion performance is improved; at the same time, the multi-charge repulsion site effect of PVDF-1 and / or PVDF-2 and / or PVDF-1 / PVDF-2 and / or the unique structure effect of the polar functional groups of PVDF-2 can effectively prevent the slurry from gelling. Then the remaining LMFP particles and modified PVP are introduced, and the polar groups on the surface of the modified PVP are closely adsorbed on the surface of the LMFP particles, and the solvated chains at the other end together change the properties of the surface of the LMFP particles, thereby further reducing the agglomeration and settlement caused by the interaction between the LMFP particles, and finally forming a slurry with good dispersity and standing stability. In addition, the conductive network established by the three kinds of conductive agents is uniformly distributed around the LMFP particles, which significantly improves the poor conductivity of the LMFP caused by the existence of manganese elements, improves the conductivity of the pole piece, and further improves the inhibition of the battery polarization, reduces the internal resistance of the battery, and significantly improves the rate performance of the battery.

[0042] As an optional embodiment of the preparation method of the application, the carbonyl polar functional group includes one or more of carboxyl, hydroxyl, amide, and imide;

[0043] And / or, the mass ratio of the PVDF-1 and the PVDF-2 is 1:3-9 (such as 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9);

[0044] And / or, the modified PVP is a fluorine-containing acrylate copolymer or a polyvinylpyrrolidone solution modified by an alcohol amine substance;

[0045] And / or, the mass ratio of the conductive carbon black, the carbon nanotube, and the graphene is (1-1.5):(0.4-0.5):(0.1-0.5), specifically such as 1:0.4:0.5, 1.1:0.42:0.45, 1.2:0.44:0.35, 1.3:0.46:0.25, 1.4:0.48:0.15, 1.5:0.5:0.1.

[0046] and / or, the amount of carbon coating on the lithium manganese iron phosphate is 1.6-1.8wt% (such as 1.63wt%, 1.65wt%, 1.69wt%, 1.73wt%, 1.75wt%, 1.78wt%) of the lithium manganese iron phosphate, the particle size D50 is 400-500nm (such as 420nm, 440nm, 460nm, 480nm), the particle size D90 is 0.8-1μm (such as 0.83μm, 0.85μm, 0.87μm, 0.89μm, 0.93μm, 0.95μm, 0.98μm), the specific surface area is 15-20m 2 / g (such as 16m 2 / g, 17m 2 / g, 18m 2 / g, 19m 2 / g);

[0047] and / or, the lithium manganese iron phosphate slurry includes a powder and a solvent, and the mass ratio of the powder and the solvent is (55:45)-(63:37), specifically such as 56:44, 57:43, 58:42, 59:41, 60:40, 61:39, 62:38.

[0048] As an optional embodiment of the preparation method of the present application, the powder consists of 94.6-97% lithium manganese iron phosphate, 1-3% conductive agent, 0.8-2% binder, and 0.05-0.4% dispersant by mass percentage;

[0049] and / or, the viscosity of the modified PVP is <200mPa·s, and the pH is between 10-12;

[0050] and / or, the solvent is N-methyl pyrrolidone.

[0051] As an optional embodiment of the preparation method of the present application, in step (1), the solid content of the glue solution is 4-6% (such as 4.3%, 4.6%, 4.9%, 5.2%, 5.5%, 5.7%);

[0052] and / or, in step (3), the viscosity of the lithium manganese iron phosphate slurry is 3000-8000mPa·s (such as 3500mPa·s, 4000mPa·s, 4500mPa·s, 5000mPa·s, 5500mPa·s, 6000mPa·s, 6500mPa·s, 7000mPa·s, 7500mPa·s).

[0053] As an optional embodiment of the preparation method of the application, in steps (1) and (3), the mass ratio of the two additions of the dispersant is (1:3) to (1:1), specifically 1:2.7, 1:2.5, 1:2.2, 1:2, 1:1.7, 1:1.5, 1:1.2;

[0054] And / or, in step (3), the mass ratio of the two additions of the lithium iron manganese phosphate is (3:1) to (1:1), specifically 2.7:1, 2.5:1, 2.2:1, 2:1, 1.7:1, 1.5:1, 1.2:1.

[0055] As an optional embodiment of the preparation method of the application, in steps (1) and / or step (2) and / or step (3), the mixing each includes first slow stirring and scraping and then high-speed dispersion.

[0056] As an optional embodiment of the preparation method of the application, the revolution speed and the rotation speed of the slow stirring and scraping are 10-15 rpm and 1000-1500 rpm respectively, and the time is 5-10 min.

[0057] And / or, the revolution speed and the rotation speed of the high-speed dispersion are 40-55 rpm and 3500-5500 rpm respectively, and the time is 1-3 h.

[0058] According to a second aspect of the application, a lithium iron manganese phosphate slurry is prepared by the preparation method of the first aspect.

[0059] According to a third aspect of the application, a lithium iron manganese phosphate pole piece is obtained by coating the lithium iron manganese phosphate slurry of the second aspect on a carbon-coated aluminum foil with a certain thickness according to a certain surface density, drying, and then rolling according to a certain compacting density.

[0060] Further, the surface density is 350-380 g / m 2 ;

[0061] And / or, the thickness is 12-14 μm;

[0062] And / or, the compacting density is 2.2-2.4 g / cm 3 .

[0063] The preparation of the slurry and the pole piece of the application can be realized by using the existing technical equipment, and the equipment is not listed one by one. The application provides a lithium iron manganese phosphate slurry, a pole piece and a preparation method thereof, and the preparation method of the lithium iron manganese phosphate slurry and the pole piece is as follows:

[0064] S1, first, the binder, 50% of dispersant and solvent are added into the double planetary mixer in proportion, mixed uniformly, stirred slowly at 10-15 rpm and 1000-1500 rpm for 5-10 min, scraped, then dispersed at 40 rpm and 3500 rpm, vacuum degree is -90 KPa, high speed dispersion for 1-3 h, to obtain the glue liquid with solid content of 4-6%;

[0065] S2, then, the graphene is added into the glue liquid in proportion, stirred slowly at 10-15 rpm and 1000-1500 rpm for 10 min, scraped, then dispersed at 45 rpm and 4500 rpm, vacuum degree is -90 KPa, high speed dispersion for 1 h, then the carbon nanotube is added into the glue liquid in proportion, stirred slowly at 10-15 rpm and 1000-1500 rpm for 10 min, scraped, then dispersed at 45 rpm and 4500 rpm, vacuum degree is -90 KPa, high speed dispersion for 1 h, finally, the conductive carbon black is added into the glue liquid in proportion, stirred slowly at 10-15 rpm and 1000-1500 rpm for 10 min, scraped, then dispersed at 45 rpm and 4500 rpm, vacuum degree is -90 KPa, high speed dispersion for 1.5 h, to obtain the conductive glue liquid;

[0066] S3, then, 75% of the lithium manganese iron phosphate is added into the conductive glue liquid, stirred slowly at 10-15 rpm and 1000-1500 rpm for 5-10 min, scraped, then dispersed at 50 rpm and 5500 rpm, vacuum degree is -90 KPa, high speed dispersion for 1-2 h, mixed uniformly; finally, the remaining 25% of the lithium manganese iron phosphate, 50% of the dispersant and solvent are added into the slurry in turn, stirred slowly at 10-15 rpm and 1000-1500 rpm for 5-10 min, scraped, then dispersed at 50 rpm and 5500 rpm, vacuum degree is -90 KPa, high speed dispersion for 2-3 h, mixed uniformly, to obtain the lithium manganese iron phosphate slurry with viscosity of 3000-8000 mPa·s.

[0067] S4, the slurry is coated on the carbon-coated aluminum foil with thickness of 12-14 μm according to the surface density of 350-380 g / m 2 , dried, then rolled according to the compacted density of 2.2-2.4 g / cm 3 , to obtain a lithium manganese iron phosphate pole piece.

[0068] The manganese iron phosphate slurry comprises a powder and a solvent in a mass ratio of (55:45) to (63:37); the powder consists of 94.6 to 97% manganese iron phosphate, 1 to 3% conductive agent, 0.8 to 2% binder, and 0.05 to 0.4% dispersant, according to the total mass of the powder; the conductive agent is a combination of conductive carbon black, nanometer carbon tube, and graphene in a mass ratio of (1 to 1.5):(0.4 to 0.5):(0.1 to 0.5); the binder is a combination of PVDF-1 and PVDF-2, the PVDF-1 is polyvinylidene fluoride with a molecular weight of 50 to 90W, the PVDF-2 is modified polyvinylidene fluoride containing a carbonyl-based polar functional group, the molecular weight of the PVDF-2 is 120 to 150W, the carbonyl-based polar functional group includes one or more of carboxyl, hydroxyl, amide, and imide, and the mass ratio of the PVDF-1 to the PVDF-2 is 1:3 to 9; the dispersant is modified PVP, the modified PVP is a solution of polyvinylpyrrolidone (PVP) modified by fluorine-containing acrylate copolymer and alcohol amine, the viscosity of the solution is less than 200mPa·s, the effective ingredient is 20%, the pH is 10 to 12, and the moisture is less than 1000ppm; the amount of carbon-coated manganese iron phosphate is 1.6 to 1.8wt% of the amount of manganese iron phosphate, the particle size D50 is 400 to 500nm, the particle size D90 is 0.8 to 1μm, and the specific surface area is 15 to 20m 2 / g; and the solvent is N-methylpyrrolidone (NMP).

[0069] The embodiments of the present application will be described in detail below with reference to specific examples and comparative examples, and the following examples are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. The PVDF-1 used in the examples and comparative examples is synthesized by emulsion method, the PVDF-2 is synthesized by suspension method, the dispersant is CGI-D37P product of Shenzhen Yinxin New Material Co., Ltd., the viscosity of the product is less than 200mPa·s, the effective ingredient is 20%, the pH is 10 to 12, and the moisture is less than 1000ppm. If no specific conditions are specified, the operations will be performed under normal conditions. If no manufacturer of reagents or instruments is specified, the reagents and instruments are normal products that can be purchased through normal channels.

[0070] Example 1

[0071] The present embodiment provides a manganese iron phosphate slurry and a pole piece, and a preparation method thereof, which comprises the following steps:

[0072] S1, first, PVDF-1, PVDF-2, 50% of dispersant and NMP are added into a double planetary disperser in proportion, mixed uniformly, stirred at 10 rpm and 1000 rpm for 10 min, scraped, then dispersed at 40 rpm and 3500 rpm for 2 h at a vacuum degree of -90 KPa, to obtain a glue solution with a solid content of 6%;

[0073] S2, graphene is added into the glue solution in proportion, stirred at 10 rpm and 1000 rpm for 10 min, scraped, then dispersed at 45 rpm and 4500 rpm for 1 h at a vacuum degree of -90 KPa, then nanometer carbon tube is added into the glue solution in proportion, stirred at 10 rpm and 1000 rpm for 10 min, scraped, then dispersed at 45 rpm and 4500 rpm for 1 h at a vacuum degree of -90 KPa, finally, conductive carbon black is added into the glue solution in proportion, stirred at 10 rpm and 1000 rpm for 10 min, scraped, then dispersed at 45 rpm and 4500 rpm for 1.5 h at a vacuum degree of -90 KPa, to obtain a conductive glue solution;

[0074] S3, then 75% of lithium manganese iron phosphate is added into the conductive glue solution, stirred at 10 rpm and 1000 rpm for 10 min, scraped, then dispersed at 50 rpm and 5500 rpm for 2 h at a vacuum degree of -90 KPa, mixed uniformly; finally, the remaining 25% of lithium manganese iron phosphate and 50% of dispersant and NMP are added into the slurry, stirred at 10 rpm and 1000 rpm for 10 min, scraped, then dispersed at 50 rpm and 5500 rpm for 3 h at a vacuum degree of -90 KPa, mixed uniformly, to obtain a lithium manganese iron phosphate slurry with a viscosity of 3000-8000 mPa·s.

[0075] S4, the slurry is coated on a 12-14 μm thick carbon-coated aluminum foil at a surface density of 350-380 g / m 2 , dried, then rolled at a compacted density of 2.2-2.4 g / mm 3 , to obtain a lithium manganese iron phosphate pole piece.

[0076] The mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry in this embodiment is 58:42. According to the total mass of the powder, the powder is composed of 96.2% lithium manganese iron phosphate (the amount of carbon coating accounts for 1.61% of the amount of lithium manganese iron phosphate, the particle size D50 is 407 nm, the particle size D90 is 0.848 μm, the specific surface area is 16.506 m 2 / g), 1.2% conductive carbon black, 0.4% carbon nanotubes, 0.4% graphene, 0.2% PVDF-1 with a molecular weight of 85.6W, 1.4% PVDF-2 with an amide and imide group modified with a molecular weight of 130.1W, and 0.2% dispersant (Shenzhen Yinxin New Material Co., Ltd., CGI-D37P);

[0077] Example 2:

[0078] The preparation method is consistent with that of Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42. According to the total mass of the powder, the powder is composed of 96.2% lithium manganese iron phosphate, 1.2% conductive carbon black, 0.4% carbon nanotubes, 0.4% graphene, 0.3% PVDF-1, 1.3% PVDF-2, and 0.2% dispersant.

[0079] Example 3:

[0080] The preparation method is consistent with that of Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42. According to the total mass of the powder, the powder is composed of 96.2% lithium manganese iron phosphate, 1.2% conductive carbon black, 0.4% carbon nanotubes, 0.4% graphene, 0.4% PVDF-1, 1.2% PVDF-2, and 0.2% dispersant.

[0081] Example 4:

[0082] The preparation method is consistent with that of Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry in this embodiment is 58:42. According to the total mass of the powder, the powder is composed of 96.2% lithium manganese iron phosphate, 1.3% conductive carbon black, 0.5% carbon nanotubes, 0.2% graphene, 0.4% PVDF-1, 1.2% PVDF-2, and 0.2% dispersant.

[0083] Comparative Example 1:

[0084] The difference from Example 1 is that the two-step addition of lithium manganese iron phosphate is changed to one-step addition. The specific process is as follows:

[0085] S3, then 100wt% of the lithium manganese iron phosphate is added into the conductive glue solution, stirring at 10rpm and 1000rpm for 10min, scraping, then high speed dispersion at 50rpm and 5500rpm for 2h with a vacuum degree of -90KPa, mixing uniformly; finally, 50% of the dispersant and NMP are added into the slurry, stirring at 10rpm and 1000rpm for 10min, scraping, then high speed dispersion at 50rpm and 5500rpm for 3h with a vacuum degree of -90KPa, mixing uniformly, to obtain the lithium manganese iron phosphate slurry with a viscosity of 3000-8000mPa·s.

[0086] Comparative Example 2:

[0087] Difference from Example 1: the dispersant is added at one time, only in step (1).

[0088] Comparative Example 3:

[0089] Difference from Example 1: the dispersant is added at one time, only in step (3).

[0090] Comparative Example 4:

[0091] Difference from Example 1: the dispersant is changed to PVP solution without any modification.

[0092] Comparative Example 5:

[0093] The preparation method is consistent with Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and according to the total mass of the powder, the powder is composed of 96.2% of lithium manganese iron phosphate, 1.5% of conductive carbon black, 0.5% of carbon nanotubes, 0.4% of PVDF-1, 1.2% of PVDF-2, and 0.2% of dispersant.

[0094] Comparative Example 6:

[0095] The preparation method is consistent with Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and according to the total mass of the powder, the powder is composed of 96.2% of lithium manganese iron phosphate, 1.5% of conductive carbon black, 0.5% of graphene, 0.4% of PVDF-1, 1.2% of PVDF-2, and 0.2% of dispersant.

[0096] Comparative Example 7:

[0097] The preparation method is consistent with Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and the powder consists of 95.95% lithium manganese iron phosphate, 1.6% conductive carbon black, 0.3% carbon nanotubes, 1.3% PVDF-1, 0.7% PVDF-2, and 0.15% dispersant, according to the total mass of the powder.

[0098] Comparative Example 8:

[0099] The preparation method is consistent with Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and the powder consists of 95.9% lithium manganese iron phosphate, 2.0% conductive carbon black, 1.5% PVDF-1, 0.5% PVDF-2, and 0.1% dispersant, according to the total mass of the powder.

[0100] Comparative Example 9:

[0101] The preparation method is consistent with Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and the powder consists of 95.9% lithium manganese iron phosphate, 1.2% conductive carbon black, 0.4% carbon nanotubes, 0.4% graphene, 1.6% PVDF-1, and 0.2% dispersant, according to the total mass of the powder.

[0102] Comparative Example 10:

[0103] The preparation method is consistent with Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and the powder consists of 95.95% lithium manganese iron phosphate, 1.2% conductive carbon black, 0.4% carbon nanotubes, 0.4% graphene, 1.6% PVDF-2, and 0.2% dispersant, according to the total mass of the powder.

[0104] Test and Results

[0105] The viscosity, solid content, fineness of the lithium manganese iron phosphate slurry prepared in the examples and comparative examples, the peeling strength, film resistance, and flexibility of the lithium manganese iron phosphate electrode are tested, and the method is as follows:

[0106] Peeling strength test method: a universal testing machine is used for testing. That is, the pole piece is cut into 15*200mm size, then the transparent tape is transversely pasted at the bottom of the dried non-scale steel plate ruler, the end surface is flush; then, the double-sided tape is pasted on the transparent tape, the length is the same as the width of the transparent tape, and the position is in the middle; finally, the test sample is pasted on the double-sided tape, the end surface is flush, the pressure roller is rolled back and forth on the surface of the pole piece, then the pole piece is folded 180° and clamped on the clamping device of the universal testing machine, the stretching is carried out at a speed of 50mm / min at room temperature, the average value of 30-100mm of stretching is selected, the positive pole piece is peeled, when the pole piece current collector and coating are completely separated, the test result of the peeling strength of the pole piece coating is read.

[0107] Membrane resistance test method: a direct current four-probe tester is used to measure the resistance of the pole piece, the pole piece is placed on the platform below the tester, the tester support stud is adjusted to make all the needle tips on the four probes in good contact with the pole piece, 15 points of each sample are tested, the interval between each point is 2cm, then the test is started, and the resistance value displayed after the current value is stable is recorded as the surface resistance of the pole piece.

[0108] The test results are as follows:

[0109]

[0110]

[0111] As shown in the above table, the conductive glue solution obtained by the method of the application and the way of adding the lithium manganese iron phosphate material into the glue solution in two steps make the slurry obtained in examples 1-4 have a viscosity of 6000-7000mPa·s, a fineness of 8-13μm, a solid content of 57-60%, a lower membrane resistance of the pole piece, lower than 220mΩ, a higher peeling strength, not less than 50N / m, and no light transmission after two folds, so that the pole piece prepared by the method has good flexibility, conductivity and bonding performance; it can be seen from examples 1-4 that the fineness of the slurry is below 15μm, which indicates that the dispersion of the slurry by the preparation method is more uniform, and there is no particle agglomeration.

[0112] Further, it can be seen from the peel strength of Examples 1-3 that the peel strength of the electrode sheet gradually decreases as the content of PVDF-2 decreases, which indicates that the complex groups (polar groups such as carboxyl, hydroxyl, amide, imide, etc.) on the long chain of PVDF-2 can form chemical bonds with the foil and effectively enhance the force between the positive active material and the current collector, further improving the adhesion, and also improving the flexibility of the electrode sheet. Further, as can be seen from Examples 1-3, when the conductive agent is added in a mass ratio of 3:1:1 of conductive carbon black, nanometer carbon tube and graphene, the sheet resistance of the electrode sheet gradually decreases as the content of the macromolecular PVDF-2 decreases, indicating that the conductive performance of the electrode sheet is improved, which indicates that PVDF-2 has the effect of improving the sheet resistance.

[0113] As can be seen from the comparison of Example 1 and Comparative Example 1, when the main material is added in one step, the fineness of the slurry is large, there are obvious particle protrusions on the surface of the electrode sheet, and the dispersibility is poor, which has adverse effects on the capacity, cycle and safety of the subsequent battery.

[0114] As can be seen from the comparison of Example 1 and Comparative Examples 2-4, not adding the dispersant in steps or not using the modified PVP has a significant adverse effect on the viscosity, fineness and solid content of the slurry, the dispersing effect of the slurry is poor, and even the sheet resistance and peel strength of the electrode sheet are affected, the flexibility of the electrode sheet becomes very poor, and the electrode sheet is not transparent after one fold, the surface of the electrode sheet has a large number of pits, which has adverse effects on the capacity and cycle performance of the subsequent battery.

[0115] As can be seen from the comparison of Example 4 and Comparative Examples 5-8, only when the three kinds of conductive agents are used in combination, the sheet resistance of the electrode sheet is low and the conductivity is good. Furthermore, as can be seen from the comparison of Example 4 and Comparative Examples 5 and 6, when the other components are the same and only the conductive agent composition is different, the use of the three kinds of conductive agents not only improves the conductivity, but also improves the peel strength of the obtained electrode sheet.

[0116] As can be seen from the comparison of Examples 1-3 and Comparative Examples 9 and 10, when the other components are the same and only the binder composition is different, only when the small molecular weight polyvinylidene fluoride and the modified polyvinylidene fluoride containing carbonyl polar functional groups are used in combination, the obtained slurry has small fineness and high solid content, the obtained electrode sheet has good conductivity, high peel strength and good flexibility. In particular, as can be seen from the comparison of Examples 1-3 and Comparative Example 9, the use of the modified polyvinylidene fluoride containing carbonyl polar functional groups can significantly improve the peel strength of the electrode sheet.

[0117] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application; although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions recorded in the above embodiments can be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a lithium iron manganese phosphate slurry, characterized by, The preparation method comprises the following steps: (1) mixing and configuring a glue solution by mixing a binder and part of a dispersant in a solvent, wherein the binder comprises PVDF-1 and PVDF-2, the PVDF-1 is polyvinylidene fluoride with a molecular weight of 50-90W, the PVDF-2 is modified polyvinylidene fluoride containing a carbonyl-based polar functional group with a molecular weight of 120-150W, and the dispersant is modified PVP; the carbonyl-based polar functional group comprises one or more of a hydroxyl group, an amide group and an imide group; the modified PVP is a fluorine-containing acrylic ester copolymer or a polyvinylpyrrolidone solution modified by an alcohol amine substance; the viscosity of the modified PVP is less than 200mPa·s, and the pH is between 10 and 12; (2) sequentially adding graphene, nanometer carbon tubes and conductive carbon black as conductive agents into the glue solution obtained in step (1) to mix, to obtain a conductive glue solution; the mass ratio of the conductive carbon black, the nanometer carbon tubes and the graphene is (1-1.5):(0.4-0.5):(0.2-0.4); (3) adding part of lithium manganese iron phosphate into the conductive glue solution obtained in step (2) to mix, uniformly adding the remaining lithium manganese iron phosphate and the dispersant and the solvent into the slurry to mix, and uniformly preparing a lithium manganese iron phosphate slurry; the mass ratio of the two-time addition of the lithium manganese iron phosphate is (3:1)-(1.7:1).

2. The preparation method according to claim 1, characterized in that, the mass ratio of the PVDF-1 and the PVDF-2 is 1:3-9; And / or, the amount of the carbon-coated manganese iron lithium phosphate is 1.6-1.8wt% of the amount of the manganese iron lithium phosphate, the particle size D50 is 400-500nm, the particle size D90 is 0.8-1μm, and the specific surface area is 15-20m 2 / g. and / or, the lithium manganese iron phosphate slurry comprises a powder and a solvent, and the mass ratio of the powder and the solvent is (55:45)-(63:37).

3. The preparation method according to claim 2, characterized in that, the powder is composed of 94.6-97% of lithium manganese iron phosphate, 1-3% of a conductive agent, 0.8-2% of a binder and 0.05-0.4% of a dispersant by mass percentage; and / or, the solvent is N-methyl pyrrolidone.

4. The preparation method according to claim 1, characterized in that, in step (1), the solid content of the glue solution is 4-6%; and / or, in step (3), the viscosity of the lithium manganese iron phosphate slurry is 3000-8000mPa·s.

5. The preparation method according to claim 1, characterized in that, in steps (1) and (3), the mass ratio of the two-time addition of the dispersant is (1:3)-(1:1).

6. The preparation method according to claim 1, characterized in that, in step (1) and / or step (2) and / or step (3), the mixing each comprises first slowly stirring and scraping the material and then high-speed dispersing.

7. The preparation method according to claim 6, characterized in that, the revolution speed and the rotation speed of the slow stirring and scraping are 10-15rpm and 1000-1500rpm respectively, and the time is 5-10min; and / or, the revolution speed and the rotation speed of the high-speed dispersing are 40-55rpm and 3500-5500rpm respectively, and the time is 1-3h.

8. A lithium iron manganese phosphate slurry characterized by: prepared by any one of the preparation methods in claims 1-7.

9. A lithium iron manganese phosphate pole piece characterized by: the lithium manganese iron phosphate slurry in claim 8 is coated on a carbon-coated aluminum foil with a certain thickness at a certain surface density, dried, and then rolled at a certain compacting density to obtain the lithium manganese iron phosphate pole piece.

10. The lithium iron manganese phosphate pole piece of claim 9, wherein: The areal density is 350-380 g / m 2 ; and / or, the thickness is 12-14μm. and / or the compacted density is 2.2 to 2.4 g / cm3 3 .

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