Composite positive electrode sheet and preparation method therefor, and lithium ion battery comprising same

Abstract

A composite positive electrode sheet and a preparation method therefor, and a lithium ion battery comprising same. The positive electrode sheet comprises a current collector and active substance layers, wherein the active substance layers comprise a coating layer A, a coating layer B and a coating layer C; the coating layer A is arranged on the surface of the current collector, the coating layer B is arranged on the surface of the coating layer A, and the coating layer C is arranged on the surface of the coating layer B; the median particle size Dv50 of a positive active substance of the coating layer A ≤ the median particle size Dv50 of a positive active substance of the coating layer C < the median particle size Dv50 of a positive active substance of the coating layer B; and the active substances are selected from among one or a combination of lithium manganese iron phosphate, lithium iron phosphate, lithium vanadium phosphate and lithium cobalt phosphate. A positive electrode sheet of a structure containing three active substance layers is used, and the sizes of active substance particles of the active substance layers and the size relationship therebetween are adjusted and controlled, such that a positive electrode sheet with good electrochemical performance and capacity performance is prepared. Moreover, by means of optimizing a pulping process of a positive electrode material, the problems in the prior art of raw materials being wasted and the economic cost being high in the pulping process are solved.

Description

Composite positive electrode sheet, preparation method thereof, and lithium ion battery comprising the same Technical Field

[0001] The present application belongs to the field of energy storage technology, and specifically relates to a composite positive electrode sheet and a preparation method thereof, as well as a lithium-ion battery including the composite positive electrode sheet. Background Art

[0002] Lithium-ion batteries have a high safety factor, a long service life, and a wide range of application prospects. In addition, the performance and structure of the lithium-ion battery positive electrode have a significant impact on the overall performance of the lithium-ion battery.

[0003] In the preparation of lithium-ion battery positive electrode slurry, the optimization of the pulping process also plays a very important role. The pulping process of lithium battery positive electrode materials is closely related to their electrochemical performance. The common pulping process for lithium batteries is simply mixing, stirring and dispersing. The specific preparation method is as follows: ① Premixing: Add the main materials of the slurry to a blender and stir and premix to obtain a mixture; ② Wetting: Under vacuum conditions, stir and disperse the mixture with an organic solvent at a solid content of 65% to 70%; ③ High-speed dispersion: Add the organic solvent and stir and disperse at high speed under vacuum conditions to obtain a mixed slurry; ④ Sieving: The mixed slurry obtained in the high-speed dispersion step is sieved to remove large particles to obtain the positive electrode slurry. Because the common pulping process is difficult to process large particles of unbroken powder in the positive electrode slurry, they can only be removed through sieving. This results in a waste of raw materials and may cause uneven composition in fine areas of the electrode coating, resulting in a decrease in the electrochemical performance of the material.

[0004] Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present application provides a composite positive electrode sheet with excellent electrochemical and capacity performance and a preparation method thereof.

[0006] The present application provides a positive electrode sheet, characterized in that the positive electrode sheet includes a current collector and an active material layer; the active material layer includes a coating layer A, a coating layer B, and a coating layer C; the coating layer A is arranged on the surface of the current collector, the coating layer B is arranged on the surface of the coating layer A, and the coating layer C is arranged on the surface of the coating layer B;

[0007] The median particle size Dv50 of the positive electrode active material of the coating layer A is ≤ the median particle size Dv50 of the positive electrode active material of the coating layer C < the median particle size Dv50 of the positive electrode active material of the coating layer B, wherein the active material is a common positive electrode active material in the art, such as lithium manganese iron phosphate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, etc.

[0008] The present application adopts a positive electrode sheet with a three-layer structure, and controls the size relationship of the median particle sizes of the coating layer A, the coating layer B and the coating layer C, that is, the particle size of the coating layer A and the coating layer C is smaller than the particle size of the middle coating layer B; the coating layer A and the coating layer C use small particles, which can fully exert the material properties, such as shortening the lithium ion migration path, reducing electrochemical polarization, and improving the capacity performance of the lithium battery; the coating layer B uses large particles to increase the compaction density of the electrode sheet, and the gaps between large particles are larger than those between small particles, which is conducive to the infiltration of the electrolyte into the interior of the electrode sheet, and is beneficial to the electrochemical performance of the lithium battery.

[0009] Another object of the present application is to provide a method for preparing the positive electrode sheet of the present application, which specifically comprises the following steps:

[0010] (1) coating the positive electrode slurry 1 on the current collector, vacuum drying, and rolling to obtain a coating layer A / current collector composite structure;

[0011] (2) applying the positive electrode slurry 2 to the surface of the coating layer A of the coating layer A / current collector composite structure obtained in step (1), vacuum drying, and roller pressing to obtain a coating layer B / coating layer A / current collector composite structure;

[0012] (3) coating the positive electrode slurry 3 on the surface of the coating layer B of the coating layer B / coating layer A / current collector composite structure obtained in step (2), vacuum drying, and rolling to obtain the positive electrode sheet;

[0013] The median particle size Dv50 of the positive electrode slurry 1 is less than or equal to the median particle size Dv50 of the positive electrode slurry 3 and less than the median particle size Dv50 of the positive electrode slurry 2. Beneficial effects:

[0014] This application utilizes a three-layer active material structure for a positive electrode sheet and modulates the size and relationship of the active material particles to produce a positive electrode sheet with excellent electrochemical and capacity performance. Furthermore, by optimizing the slurrying process for the positive electrode material, the problems of raw material waste and high economic costs in the prior art slurrying process are resolved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG1 is a schematic diagram of the pulping process of lithium iron manganese phosphate of the present application;

[0016] FIG2 is a schematic diagram of the structure of the composite positive electrode sheet of the present application;

[0017] FIG3 is an SEM image of the particle morphology of the lithium iron manganese phosphate powder in Example 1, wherein FIGa) shows a normal-sized particle powder, and FIGb) shows an unbroken large particle powder;

[0018] FIG4 is a physical picture of a portion of the large-particle powder slurry sieved in step 3 of Example 1;

[0019] FIG5 is a particle size distribution diagram of positive electrode slurries C and D prepared in Example 1;

[0020] Figure 6 is a diagram of the particle morphology and distribution of the cross section of the positive electrode sheet prepared in Example 1, wherein a) shows the cross section of coating layer A; b) shows the cross section of coating layer B. DETAILED DESCRIPTION

[0021] The technical solution of the present application is further described below with reference to specific embodiments. The protection scope of the present application is not limited to the following embodiments, which are listed for illustrative purposes only and do not limit the present application in any way.

[0022] In the context of this specification, except for matters explicitly stated, any matters or issues not mentioned are directly applicable to matters known in the art without any modification. Moreover, any embodiment described herein can be freely combined with one or more other embodiments described herein, and the technical solutions or technical ideas formed thereby are considered part of the original disclosure or original description of this application and should not be regarded as new content not disclosed or anticipated herein, unless a person skilled in the art considers that the combination is obviously unreasonable.

[0023] The present application provides a positive electrode sheet, characterized in that the positive electrode sheet includes a current collector and an active material layer; the active material layer includes a coating layer A, a coating layer B, and a coating layer C; the coating layer A is arranged on the surface of the current collector, the coating layer B is arranged on the surface of the coating layer A, and the coating layer C is arranged on the surface of the coating layer B;

[0024] The median particle size Dv50 of the positive electrode active material of the coating layer A is ≤ the median particle size Dv50 of the positive electrode active material of the coating layer C < the median particle size Dv50 of the positive electrode active material of the coating layer B, wherein the active material is a common positive electrode active material in the art, such as lithium manganese iron phosphate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, etc.

[0025] The present application adopts a positive electrode sheet with a three-layer structure, and controls the size relationship of the median particle sizes of the coating layer A, the coating layer B and the coating layer C, that is, the particle size of the coating layer A and the coating layer C is smaller than the particle size of the middle coating layer B; the coating layer A and the coating layer C use small particles, which can fully exert the material properties, such as shortening the lithium ion migration path, reducing electrochemical polarization, and improving the capacity performance of the lithium battery; the coating layer B uses large particles to increase the compaction density of the electrode sheet, and the gaps between large particles are larger than those between small particles, which is conducive to the infiltration of the electrolyte into the interior of the electrode sheet, and is beneficial to the electrochemical performance of the lithium battery.

[0026] According to a specific embodiment of the present application, the median particle size Dv50 of the positive electrode active material of the coating layer A is equal to the median particle size Dv50 of the positive electrode active material of the coating layer C.

[0027] According to a specific embodiment of the present application, the median particle size Dv50 of the positive electrode active material of the coating layer A is 0.4μm-0.6μm, the median particle size Dv50 of the positive electrode active material of the coating layer C is 0.4μm-0.6μm, and the median particle size Dv50 of the positive electrode active material of the coating layer B is 0.8μm-1μm.

[0028] In the technical solution of the present application, the three-layer structure of the positive electrode sheet adopts a smaller particle size, that is, all within 1 μm, which can significantly improve the capacity performance of the battery compared to a large-size structure.

[0029] According to a specific embodiment of the present application, the ratio of the compaction density of the coating layer A to the coating layer B is 1:(1-1.5); the ratio of the compaction density of the coating layer C to the coating layer B is 1:(1-1.5).

[0030] According to a specific embodiment of the present application, the ratio of the compaction density of the coating layer B to the coating layer A is 1.3:1.

[0031] According to a specific embodiment of the present application, the ratio of the compaction density of the coating layer C to the coating layer A is 1:1.

[0032] According to a specific embodiment of the present application, the thickness of the coating layer B is 20 μm-60 μm.

[0033] According to a specific embodiment of the present application, the thickness of the coating layer B is 50 μm.

[0034] In the technical solution of the present application, the capacity performance of the positive electrode sheet can be further improved by adjusting the thickness of the coating layer B.

[0035] According to a specific embodiment of the present application, the thickness of the coating layer A is 10 μm-30 μm, preferably 30 μm.

[0036] According to a specific embodiment of the present application, the thickness of the coating layer C is 10 μm-30 μm, preferably 30 μm.

[0037] According to a specific embodiment of the present application, the current collector is an aluminum foil having a thickness of 10-20 μm, preferably 16 μm.

[0038] Another object of the present application is to provide a method for preparing the positive electrode sheet of the present application, which specifically comprises the following steps:

[0039] (1) coating the positive electrode slurry 1 on the current collector, vacuum drying, and rolling to obtain a coating layer A / current collector composite structure;

[0040] (2) applying the positive electrode slurry 2 to the surface of the coating layer A of the coating layer A / current collector composite structure obtained in step (1), vacuum drying, and roller pressing to obtain a coating layer B / coating layer A / current collector composite structure;

[0041] (3) coating the positive electrode slurry 3 on the surface of the coating layer B of the coating layer B / coating layer A / current collector composite structure obtained in step (2), vacuum drying, and rolling to obtain the positive electrode sheet;

[0042] The median particle size Dv50 of the positive electrode slurry 1 is less than or equal to the median particle size Dv50 of the positive electrode slurry 3 and less than the median particle size Dv50 of the positive electrode slurry 2.

[0043] According to a specific embodiment of the present application, the vacuum drying temperature in each of the above steps is independently 120° C. to 180° C., preferably 150° C.; the vacuum drying time is independently 8 h to 12 h, preferably 10 h.

[0044] According to a specific embodiment of the present application, the roller pressing device is a roller press.

[0045] According to a specific embodiment of the present application, the positive electrode slurry of the present application is prepared by a method comprising the following steps:

[0046] S1. Ball milling premixing: The positive electrode active material, the conductive agent, and the binder are placed in a ball mill and premixed by ball milling to obtain a mixed material, wherein the positive electrode active material accounts for 90-97 wt%, the conductive agent accounts for 1-5 wt%, and the binder accounts for 2-5 wt%;

[0047] S2, wetting and ball milling: adding an organic solvent to the mixed material obtained in step S1, and performing ball milling dispersion treatment to obtain a dispersed mixed material, wherein the organic solvent is N-methylpyrrolidone (NMP);

[0048] S3, sieving: filtering the dispersed mixed material obtained in step S2 through a 180-mesh sieve to remove the large-particle powder slurry, which is mixed slurry A; the remaining is mixed slurry B;

[0049] S4. Processing the mixed slurry A by low / high speed ball milling to further reduce the particle size of the particles in the slurry to obtain treated mixed slurry A, wherein the rotation speed of the low speed ball mill is 200 rpm to 400 rpm and the rotation speed of the high speed ball mill is 1200 rpm to 1800 rpm; dividing the mixed slurry B into two parts, namely 20% mixed slurry B and 80% mixed slurry B;

[0050] S5, high-speed dispersion: The treated mixed slurry A and 20% mixed slurry B are combined in a stirring tank and fully dispersed by high-speed stirring and dispersion; 80% mixed slurry B is also treated by the same high-speed stirring method, wherein the high-speed stirring conditions are: revolution speed 50 rpm, dispersion speed 25 m / s;

[0051] S6. Post-processing: The mixed slurry after high-speed stirring and dispersion is subjected to vacuum degassing, sieving and viscosity testing to obtain the positive electrode slurry.

[0052] In the method of the present application, the particle size of the obtained positive electrode slurry can be adjusted by adjusting the rotation speed and time of ball milling and the rotation speed and time of dispersion.

[0053] According to a specific embodiment of the present application, in step S1, the ball milling step is carried out in a ball mill, wherein the ball milling jar of the ball mill is one or more of a ceramic ball milling jar, an agate ball milling jar, and a polyurethane ball milling jar; and the ball milling beads are made of one or more of zirconia balls and polyurethane balls.

[0054] According to a specific embodiment of the present application, in step S1, the conductive agent is any one or more of conductive carbon black, graphene, carbon nanotubes, and Ketjen black; and the binder is polyvinylidene fluoride (PVDF).

[0055] Another object of the present application is to provide an energy storage device, which includes the above-mentioned positive electrode sheet or the positive electrode sheet prepared by the above-mentioned preparation method.

[0056] According to a specific embodiment of the present application, the energy storage device is a secondary battery.

[0057] A specific process flow for preparing the positive electrode slurry of the present application is shown in FIG1 , and includes the following steps:

[0058] 1) Ball milling premixing: The positive electrode active material, such as lithium manganese iron phosphate, which accounts for about 90-97%, the conductive agent, which accounts for about 1-5%, and the binder, which accounts for about 2-5%, are sequentially charged into a ball mill for ball milling premixing to obtain a mixed material, wherein the ball milling jar of the ball mill is one or more of a ceramic ball milling jar, an agate ball milling jar, and a polyurethane ball milling jar; the ball milling beads are made of one or more of zirconia balls and polyurethane balls; the conductive agent is any one or more of conductive carbon black, graphene, carbon nanotubes, and Ketjen black; and the binder is polyvinylidene fluoride (PVDF).

[0059] 2) Wetting and ball milling: A certain amount of N-methylpyrrolidone (NMP) is added to the mixture prepared in step 1), and the NMP is first fully dispersed and wetted by ball milling at a low speed (e.g., 150 rpm to 300 rpm). Then, the slurry main material is ground into one body by high-speed ball milling (e.g., 800 rpm to 1200 rpm), fully dispersed, and the degree of agglomeration of the material particles is reduced to obtain a dispersed mixture.

[0060] 3) Sieving: The dispersed mixed material obtained in step 2) is filtered through a 180-mesh sieve to remove the large-particle powder slurry, i.e., mixed slurry A. The remaining uniformly mixed slurry is mixed slurry B.

[0061] 4) Treatment of mixed slurry A: The large particles of powder were crushed into smaller pieces by low / high speed ball milling to obtain treated mixed slurry A, wherein the rotation speed of the low speed ball mill was 200 rpm to 400 rpm and the rotation speed of the high speed ball mill was 1200 rpm to 1800 rpm; the mixed slurry B was divided into two parts, namely 20% mixed slurry B and 80% mixed slurry B.

[0062] 5) High-speed dispersion: The treated mixed slurry A and 20% mixed slurry B are combined in a double planetary stirring tank and fully dispersed by high-speed stirring and dispersion. 80% mixed slurry B is also treated by the same high-speed stirring method, wherein the high-speed stirring conditions are: revolution speed 50 rpm, dispersion speed 25 m / s.

[0063] 6) Post-processing: The mixed slurry after high-speed stirring and dispersion is subjected to vacuum degassing, sieving and viscosity testing to obtain a positive electrode slurry.

[0064] The particle size of the obtained positive electrode slurry can be adjusted by adjusting the rotation speed and time of ball milling and the rotation speed and time of dispersion.

[0065] A specific process flow of the composite positive electrode sheet of the present application includes the following steps:

[0066] 1) Preparation of coating layer A: First, the positive electrode slurry 1 was coated on a 16 μm aluminum foil, placed in a vacuum oven, dried at 150 ° C for 10 h, and then rolled on a roller press. The thickness of the coating layer A was about 30 μm and the compaction density was about 1.8 g / cm 3 , to obtain a coating layer A / current collector composite structure;

[0067] 2) Preparation of coating layer B: First, the positive electrode slurry 2 was coated on the coating layer A of the coating layer A / current collector composite structure prepared in step 1, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, the coating layer B was rolled on a roller press. The thickness of the coating layer B was about 20 μm, and the compaction density was 1:1 with that of the coating layer A, that is, 1.8 g / cm 3, to obtain a coating layer B / coating layer A / current collector composite structure;

[0068] 3) Preparation of coating layer C: First, the positive electrode slurry 3 was coated on the coating layer B of the coating layer B / coating layer A / current collector composite structure prepared in step 2, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, the coating layer C was rolled on a roller press. The thickness of the coating layer C was about 30 μm, and the compaction density was 1:1 with that of the coating layer A, that is, 1.8 g / cm 3 , a positive electrode sheet having coating layers A, B, and C is prepared,

[0069] The median particle size Dv50 of the positive electrode slurry 1 is less than or equal to the median particle size Dv50 of the positive electrode slurry 3 and less than the median particle size Dv50 of the positive electrode slurry 2.

[0070] The positive electrode slurries 1, 2, and 3 can all be prepared using the preparation method described above, wherein the parameters in the preparation process, such as the speed and time of ball milling, and the speed and time of dispersion, can be adjusted according to the desired particle size.

[0071] Since the particle size of the positive electrode slurry changes very little after vacuum drying, the difference between the particle size of the particles in the positive electrode slurry and the particle size of the corresponding particles in the coating layer can be ignored.

[0072] In the technical solution of the present application, the slurries of coating layer A and coating layer C can be the same or different, as long as the following conditions are met: the median particle size Dv50 of the positive electrode active material of coating layer A ≤ the median particle size Dv50 of the positive electrode active material of coating layer C < the median particle size Dv50 of the positive electrode active material of coating layer B.

[0073] The specific preparation process of the lithium-ion battery of the present application may include the following steps:

[0074] 1) Artificial graphite, conductive carbon black, and sodium carboxymethyl cellulose were weighed in a mass ratio of 95%:2.5%:2.5% in a stirring tank. An appropriate amount of deionized water was added and stirred for 5 hours to obtain a uniform slurry with appropriate viscosity. The slurry was then coated on a 10 μm thick copper foil and dried in a vacuum oven at 150°C for 15 hours to obtain a negative electrode sheet.

[0075] 2) Place the positive and negative electrode sheets in a press for pressing, and then use a punch to cut out Φ15mm positive electrode discs and Φ18mm negative electrode discs respectively.

[0076] 3) The positive and negative electrode discs are placed in a glove box filled with an argon protective atmosphere for battery assembly, wherein a solution obtained by dissolving 1 mol / L lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate with a molar ratio of 1:1 is used as the electrolyte; the positive electrode disc, the negative electrode disc, the polyethylene separator and other components are assembled together, and then the electrolyte is injected to finally produce a lithium-ion battery.

[0077] Example

[0078] Preparation of positive electrode slurry and positive electrode sheet

[0079] Example 1

[0080] (1) Preparation of positive electrode slurry:

[0081] 1) Ball milling premixing: lithium manganese iron phosphate, a conductive agent, and a binder were weighed in a mass ratio of 95%, 2%, and 3%, respectively, and sequentially loaded into a ball mill for ball milling premixing. The ball milling method was: a speed of 350 rpm, a ball-to-material ratio of 3:1, and a ball milling time of 1 h to obtain a mixed material, wherein the ball milling jar of the ball mill was a ceramic ball milling jar, the ball milling beads were made of zirconia balls, the conductive agent was conductive carbon black, and the binder was polyvinylidene fluoride (PVDF).

[0082] The particle morphology of the lithium iron manganese phosphate powder is shown in FIG3 . As can be seen from FIG3 , in addition to the particles of normal size, there are also some large particles that have not been crushed in the lithium iron manganese phosphate powder raw material.

[0083] 2) Wetting and ball milling: Add a certain amount of N-methylpyrrolidone (NMP), first perform low-speed ball milling (speed 200 rpm, time 1 hour) to fully disperse the NMP and wet the powder, then use high-speed ball milling (speed 1000 rpm, time 2 hours) to grind the main material of the slurry into one, fully disperse it, and reduce the degree of agglomeration of the material particles.

[0084] 3) Sieving: Filter out the large-particle powder slurry through a 180-mesh sieve as shown in FIG4 , namely, mixed slurry A, and the remaining uniformly mixed slurry is mixed slurry B.

[0085] 4) Treatment of mixed slurry A: first, use low-speed ball milling (speed 300 rpm, time 1 h) to disperse the particles, and then use high-speed ball milling (speed 1500 rpm, time 1 h) to break up the large particles into smaller particles to form a slurry with smaller particle size; Treatment of mixed slurry B: divide the mixed slurry into two parts, namely 20% mixed slurry B and 80% mixed slurry B.

[0086] 5) High-speed dispersion: The treated mixed slurry A and 20% mixed slurry B were combined in a double planetary stirring tank and subjected to high-speed stirring and dispersion (revolution speed of 50 rpm, dispersion speed of 25 m / s, time of 2 h) to fully disperse the slurry and achieve appropriate viscosity; 80% mixed slurry B was also treated in the same high-speed stirring manner.

[0087] 6) Post-processing: The mixed slurry after high-speed stirring and dispersion was subjected to vacuum degassing, sieving and viscosity testing to prepare positive electrode slurries C and D.

[0088] The positive electrode slurries C and D were sampled for particle size distribution testing. The results are shown in FIG5 . The particle size Dv50 of the positive electrode slurry C is approximately 0.8 μm-1 μm, and the particle size Dv50 of the positive electrode slurry D is approximately 0.4 μm-0.6 μm.

[0089] (2) Preparation of composite positive electrode sheet:

[0090] 1) Coating layer A and roller pressing: First, the positive electrode slurry D was coated on a 16 μm aluminum foil and placed in a vacuum oven and dried at 150 ° C for 10 hours. Then, it was rolled on a roller press. The thickness of the coating layer A was about 30 μm and the compaction density was about 1.8 g / cm 3 , to obtain a coating layer A / current collector composite structure.

[0091] 2) Coating layer B and roller pressing: First, the positive electrode slurry C is coated on the coating layer A of the coating layer A / current collector composite structure prepared in step 1, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is rolled on a roller press. The thickness of the coating layer B is about 20 μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8 g / cm 3 , to obtain a coating layer B / coating layer A / current collector composite structure.

[0092] 3) Coating layer C and roller pressing: First, the positive electrode slurry D is coated on the coating layer B of the coating layer B / coating layer A / current collector composite structure prepared in step 2, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is rolled on a roller press. The thickness of the coating layer C is about 30μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8g / cm 3 , a positive electrode sheet having coating layers A, B, and C was obtained.

[0093] Since the particle size of the positive electrode slurry changes very little after vacuum drying, the difference between the particle size of the particles in the positive electrode slurry and the particle size of the corresponding coating layer can be ignored. Therefore, the particle size distributions of the coating layers A, B, and C in the composite positive electrode sheet prepared in this example are 0.4μm-0.6μm, 0.8μm-1μm, and 0.4μm-0.6μm, respectively.

[0094] Observing the cross-sectional morphology of the composite positive electrode sheet, as shown in FIG6 , the particles of coating layer A are small and evenly distributed, while the particles of coating layer B are large, which is beneficial to improving the compaction density and electrolyte infiltration effect.

[0095] Example 2

[0096] Other conditions remain unchanged. The compaction density of coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted. The ratio of the compaction density of coating layer B to coating layer A is changed from 1:1 to 1.1:1, which is 1.98 g / cm 3 , the compaction density of coating layer A and coating layer C remains unchanged.

[0097] Example 3

[0098] Other conditions remain unchanged. The compaction density of coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted. The ratio of the compaction density of coating layer B to coating layer A is changed from 1:1 to 1.2:1, which is 2.16 g / cm 3 , the compaction density of coating layer A and coating layer C remains unchanged.

[0099] Example 4

[0100] Other conditions remain unchanged. The compaction density of coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted. The ratio of the compaction density of coating layer B to coating layer A is changed from 1:1 to 1.3:1, which is 2.34 g / cm 3 , the compaction density of coating layer A and coating layer C remains unchanged.

[0101] Example 5

[0102] Other conditions remain unchanged. The compaction density of coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted. The ratio of the compaction density of coating layer B to coating layer A is changed from 1:1 to 1.4:1, which is 2.52 g / cm 3 , the compaction density of coating layer A and coating layer C remains unchanged.

[0103] Example 6

[0104] Other conditions remain unchanged. The compaction density of coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted. The ratio of the compaction density of coating layer B to coating layer A is changed from 1:1 to 1.5:1, which is 2.70 g / cm 3 , the compaction density of coating layer A and coating layer C remains unchanged.

[0105] Example 7

[0106] While other conditions remain unchanged, the thickness of the coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted, and the thickness of the coating layer B is increased from 20 μm to 30 μm.

[0107] Example 8

[0108] While other conditions remain unchanged, the thickness of the coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted, and the thickness of the coating layer B is increased from 20 μm to 40 μm.

[0109] Example 9

[0110] While other conditions remain unchanged, the thickness of the coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted, and the thickness of the coating layer B is increased from 20 μm to 50 μm.

[0111] Example 10

[0112] While other conditions remain unchanged, the thickness of the coating layer B in step 2 of the preparation of the composite positive electrode sheet is adjusted, and the thickness of the coating layer B is increased from 20 μm to 60 μm.

[0113] Comparative Example 1

[0114] (1) Preparation of positive electrode slurry (materials are exactly the same as in Example 1):

[0115] 1) Ball milling premixing: lithium manganese iron phosphate, a conductive agent, and a binder were weighed in a mass ratio of 95%, 2%, and 3%, respectively, and sequentially loaded into a ball mill for ball milling premixing. The ball milling method was: a speed of 350 rpm, a ball-to-material ratio of 3:1, and a ball milling time of 1 h to obtain a mixed material, wherein the ball milling jar of the ball mill was a ceramic ball milling jar, the ball milling beads were made of zirconia balls, the conductive agent was conductive carbon black, and the binder was polyvinylidene fluoride (PVDF).

[0116] 2) Wetting and ball milling: Add a certain amount of N-methylpyrrolidone (NMP), first perform low-speed ball milling (speed 200 rpm, time 1 hour) to fully disperse the NMP and wet the powder, then use high-speed ball milling (speed 1000 rpm, time 2 hours) to grind the main material of the slurry into one, fully disperse it, and reduce the degree of agglomeration of the material particles.

[0117] 3) High-speed dispersion: Add about 0.1% of the dispersant (polyacrylic acid) by mass ratio of the main material of the slurry into a double planetary stirring tank, and then add a certain amount of NMP. Use high-speed stirring and dispersion method (revolution speed of 50 rpm, dispersion speed of 25 m / s, time for 2 hours) to make the slurry fully dispersed and have appropriate viscosity.

[0118] 4) Post-processing: The mixed slurry after high-speed stirring and dispersion is subjected to vacuum degassing treatment (vacuum degree is -100 kPa), sieving and viscosity testing to finally obtain the positive electrode slurry.

[0119] The particle size Dv50 of the positive electrode slurry in Comparative Example 1 is 0.7-0.9 μm.

[0120] (2) Preparation of composite positive electrode sheet:

[0121] 1) Coating layer A and roller pressing: First, the positive electrode slurry was coated on a 16 μm aluminum foil and placed in a vacuum oven and dried at 150 ° C for 10 hours. Then, it was rolled on a roller press. The thickness of the coating layer A was about 30 μm and the compaction density was about 1.8 g / cm 3 , to obtain a coating layer A / current collector composite structure.

[0122] 2) Coating layer B and roller pressing: First, the positive electrode slurry is coated on the coating layer A of the coating layer A / current collector composite structure prepared in step 1, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is roller pressed on a roller press. The thickness of the coating layer B is about 20 μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8 g / cm 3 , to obtain a coating layer B / coating layer A / current collector composite structure.

[0123] 3) Coating layer C and roller pressing: First, the positive electrode slurry is coated on the coating layer B of the coating layer B / coating layer A / current collector composite structure prepared in step 2, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is rolled on a roller press. The thickness of the coating layer B is about 30μm, and the compaction density ratio with the coating layer A is 1:1, that is, 1.8g / cm 3 , a positive electrode sheet having coating layers A, B, and C was obtained.

[0124] Comparative Example 2

[0125] (1) Preparation of positive electrode slurry (materials are exactly the same as in Example 1) (wet slurrying process):

[0126] 1) Gluing: Lithium manganese iron phosphate (the number of particles with a particle size >10 μm accounts for approximately 0.5%), a conductive agent, and a binder are weighed in a mass ratio of 95%, 2%, and 3%, respectively. The conductive agent and binder are first loaded into a double planetary stirring tank, and then a certain amount of NMP is added. The speed is set to 800 rpm and the stirring time is 2 h to prepare a PVDF glue solution, wherein the conductive agent is conductive carbon black and the binder is polyvinylidene fluoride (PVDF).

[0127] 2) High-speed dispersion: The weighed lithium iron phosphate powder and a dispersant (polyacrylic acid) accounting for about 0.1% of the mass ratio of the main material of the slurry are added to a double planetary stirring tank, and then a certain amount of NMP is added. The slurry is fully dispersed by high-speed stirring and dispersion (revolution speed of 50 rpm, dispersion speed of 25 m / s, time for 2 h).

[0128] 3) Post-processing: The mixed slurry after high-speed stirring and dispersion is subjected to vacuum degassing treatment (vacuum degree is -100 kPa), sieving and viscosity testing to finally obtain the positive electrode slurry.

[0129] The particle size Dv50 of the positive electrode slurry in Comparative Example 2 is 1-1.2 μm.

[0130] (2) Preparation of composite positive electrode sheets

[0131] 1) Coating layer A and roller pressing: First, the positive electrode slurry was coated on a 16 μm aluminum foil and placed in a vacuum oven and dried at 150 ° C for 10 hours. Then, it was rolled on a roller press. The thickness of the coating layer A was about 30 μm and the compaction density was about 1.8 g / cm 3 , to obtain a coating layer A / current collector composite structure.

[0132] 2) Coating layer B and roller pressing: First, the positive electrode slurry is coated on the coating layer A of the coating layer A / current collector composite structure prepared in step 1, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is roller pressed on a roller press. The thickness of the coating layer B is about 20 μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8 g / cm 3 , to obtain a coating layer B / coating layer A / current collector composite structure.

[0133] 3) Coating layer C and roller pressing: First, the positive electrode slurry is coated on the coating layer B of the coating layer B / coating layer A / current collector composite structure prepared in step 2, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is rolled on a roller press. The thickness of the coating layer C is about 30μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8g / cm 3 , a positive electrode sheet having coating layers A, B, and C was obtained.

[0134] Comparative Example 3

[0135] (1) Preparation of positive electrode slurry (materials are exactly the same as in Example 1) (dry slurrying process):

[0136] 1) Premixing: Lithium manganese iron phosphate (the number of particles with a particle size greater than 10 μm accounts for approximately 0.5%), a conductive agent, and a binder are weighed in a mass ratio of 95%, 2%, and 3%, respectively, and sequentially loaded into a double planetary stirring tank for premixing. The speed is set to 350 rpm and the ball milling time is 1 hour to prepare a mixed material, wherein the conductive agent is conductive carbon black and the binder is polyvinylidene fluoride (PVDF).

[0137] 2) High-speed dispersion: Add about 0.1% of the dispersant (polyacrylic acid) by mass ratio of the main material of the slurry into a double planetary stirring tank, and then add a certain amount of NMP. Use high-speed stirring and dispersion method (revolution speed of 50 rpm, dispersion speed of 25 m / s, time for 2 hours) to fully disperse the slurry.

[0138] 3) Post-processing: The mixed slurry after high-speed stirring and dispersion is subjected to vacuum degassing treatment (vacuum degree is -100 kPa), sieving and viscosity testing to finally obtain the positive electrode slurry.

[0139] The particle size of the positive electrode slurry in Comparative Example 3 is 1.5-2 μm.

[0140] (2) Preparation of composite positive electrode sheets

[0141] 1) Coating layer A and roller pressing: First, the positive electrode slurry was coated on a 16 μm aluminum foil and placed in a vacuum oven and dried at 150 ° C for 10 hours. Then, it was rolled on a roller press. The thickness of the coating layer A was about 30 μm and the compaction density was about 1.8 g / cm 3 , to obtain a coating layer A / current collector composite structure.

[0142] 2) Coating layer B and roller pressing: First, the positive electrode slurry is coated on the coating layer A of the coating layer A / current collector composite structure prepared in step 1, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is roller pressed on a roller press. The thickness of the coating layer B is about 20 μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8 g / cm 3 , to obtain a composite structure of coating layer B / coating layer A / current collector.

[0143] 3) Coating layer C and roller pressing: First, the positive electrode slurry is coated on the coating layer B of the coating layer B / coating layer A / current collector composite structure prepared in step 1, and placed in a vacuum oven and dried at 150°C for 10 hours. Then, it is rolled on a roller press. The thickness of the coating layer C is about 30μm, and the compaction density is 1:1 with the coating layer A, that is, 1.8g / cm 3 , a positive electrode sheet having coating layers A, B, and C was obtained.

[0144] Battery preparation and testing

[0145] 1) Artificial graphite, conductive carbon black, and sodium carboxymethyl cellulose were weighed in a mass ratio of 95%:2.5%:2.5% in a stirring tank. An appropriate amount of deionized water was added and stirred for 5 hours to obtain a uniform slurry with appropriate viscosity. The slurry was then coated on a 10 μm thick copper foil and dried in a vacuum oven at 150°C for 15 hours to obtain a negative electrode sheet.

[0146] 2) The positive electrode sheets prepared in Examples 1 to 10 and Comparative Examples 1 to 3 and the negative electrode sheet obtained in step 1) were placed in a press and pressed, and then a punch was used to cut out Φ15 mm positive electrode discs and Φ18 mm negative electrode discs, respectively;

[0147] 3) The positive and negative electrode discs are placed in a glove box filled with an argon protective atmosphere for battery assembly, wherein a solution obtained by dissolving 1 mol / L lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate with a molar ratio of 1:1 is used as the electrolyte; the positive electrode disc, the negative electrode disc, the polyethylene separator and other components are assembled together, and then the electrolyte is injected to finally produce a lithium-ion battery.

[0148] The electrochemical performance of the lithium-ion button-type half-cells obtained above was tested using a battery tester (Neware CT4000, Neware Electronics Co., Ltd.). The test results are shown in Table 1. Capacity retention was calculated as follows: In a 1C charge-discharge cycle test, the capacity at cycle 1 was defined as the initial capacity, and the capacity at cycle 150 was divided by the initial capacity to obtain the retention value.

[0149] Table 1 Electrochemical performance test results of lithium-ion button half-cell

[0150] From Example 1 and Comparative Example 1 (conventional method), Comparative Example 2 (wet slurrying process), and Comparative Example 3 (dry slurrying process), the slurry of the method of the present application has good fluidity and appropriate viscosity, and the size of large particles becomes smaller, the overall particle size distribution is more appropriate, and it shows higher discharge capacity and cycle performance.

[0151] According to Examples 1, 2, 3, 4, 5, 6, and 7 in Table 1, the thickness of the coating layer A of the composite positive electrode sheet of the present application is about 30 μm, and the compaction density is about 1.8 g / cm 3 , and the compaction density ratio of the coating layer A and coating layer B gradually increases from 1:1 to 1:1.5. The electrochemical performance of the lithium battery first increases and then decreases. The reason is that the compaction density of the coating layer B is too high, which will deteriorate the electrolyte infiltration effect and make it difficult to exert the material properties. Therefore, the optimal compaction density ratio of the coating layers A and B of the composite positive electrode sheet is 1:1.3.

[0152] According to Examples 1, 7, 8, 9, and 10 in Table 1, the thickness of the coating layer A of the composite positive electrode sheet of the present application is about 30 μm, and the thickness of the coating layer B increases from 20 μm to 60 μm. The electrochemical performance of the lithium battery first increases and then decreases. The reason is that when the thickness of the coating layer B is too thick, it will make it difficult for the electrolyte to penetrate into the interior and the material properties cannot be exerted. Therefore, the optimal thickness of the coating layer B of the composite positive electrode sheet is 50 μm.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a current collector and an active material layer; the active material layer includes a coating layer A, a coating layer B and a coating layer C; the coating layer A is arranged on the surface of the current collector, the coating layer B is arranged on the surface of the coating layer A, and the coating layer C is arranged on the surface of the coating layer B; The median particle size Dv50 of the positive electrode active material of the coating layer A≤the median particle size Dv50 of the positive electrode active material of the coating layer C<the median particle size Dv50 of the positive electrode active material of the coating layer B. Wherein, the active material is selected from one of lithium iron manganese phosphate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate or a combination thereof.

2. The positive electrode sheet according to claim 1, characterized in that: The median particle size Dv50 of the positive electrode active material of the coating layer A is 0.4 μm-0.6 μm, the median particle size Dv50 of the positive electrode active material of the coating layer C is 0.4 μm-0.6 μm, and the median particle size Dv50 of the positive electrode active material of the coating layer B is 0.8 μm-1 μm.

3. The positive electrode sheet according to claim 1, characterized in that: The ratio of the compaction density of the coating layer A to the coating layer B is 1:(1-1.5), preferably 1:1.3; the ratio of the compaction density of the coating layer C to the coating layer B is 1:(1-1.5).

4. The positive electrode sheet according to claim 1, characterized in that: The ratio of the compaction density of the coating layer A to the coating layer B is 1:1.

3.

5. The positive electrode sheet according to claim 1, characterized in that: The ratio of the compaction density of the coating layer C to the coating layer A is 1:

1.

6. The positive electrode sheet according to claim 1, characterized in that: The coating layer B has a thickness of 20 μm-60 μm.

7. The positive electrode sheet according to claim 1, characterized in that: The coating layer B has a thickness of 50 μm.

8. The method for preparing the positive electrode sheet according to any one of claims 1 to 7, characterized in that: The specific steps include: (1) coating the positive electrode slurry 1 on the current collector, vacuum drying, and rolling to obtain a coating layer A / current collector composite structure; (2) coating the positive electrode slurry 2 on the surface of the coating layer A of the coating layer A / current collector composite structure obtained in step (1), vacuum drying, and rolling to obtain a coating layer B / coating layer A / current collector composite structure; (3) coating the positive electrode slurry 3 on the surface of the coating layer B of the coating layer B / coating layer A / current collector composite structure obtained in step (2), vacuum drying, and rolling to obtain the positive electrode sheet; The median particle size Dv50 of the positive electrode slurry 1 is less than or equal to the median particle size Dv50 of the positive electrode slurry 3 and less than the median particle size Dv50 of the positive electrode slurry 2.

9. The method according to claim 8, characterized in that The vacuum drying temperature in each step is independently 120° C. to 180° C., preferably 150° C.; the vacuum drying time is independently 8 h to 12 h, preferably 10 h.

10. The method according to claim 8, characterized in that A positive electrode slurry is prepared by a method comprising the following steps: S1. Ball milling premixing: The positive electrode active material, the conductive agent and the binder are loaded into a ball mill for ball milling premixing to obtain a mixed material, wherein the positive electrode active material accounts for 90-97wt%, the conductive agent accounts for 1-5wt%, and the binder accounts for 2-5wt%; S2, wetting and ball milling: adding an organic solvent to the mixed material obtained in step S1, and performing ball milling dispersion treatment to obtain a dispersed mixed material, wherein the organic solvent is N-methylpyrrolidone (NMP); S3, sieving: filtering the dispersed mixed material obtained in step S2 through a 180-mesh sieve to remove the large-particle powder slurry, which is mixed slurry A; the rest is mixed slurry B; S4, treating the mixed slurry A by low / high speed ball milling to further reduce the particle size of the particles in the slurry to obtain treated mixed slurry A, wherein the rotation speed of the low speed ball mill is 200 rpm to 400 rpm, and the rotation speed of the high speed ball mill is 1200 rpm to 1800 rpm; dividing the mixed slurry B into two parts, i.e., 20% mixed slurry B and 80% mixed slurry B; S5, high-speed dispersion: the treated mixed slurry A and 20% mixed slurry B are combined in a stirring tank, and the slurry is fully dispersed by high-speed stirring and dispersion; 80% mixed slurry B is also treated in the same high-speed stirring manner, wherein the conditions of the high-speed stirring are: revolution speed 50 rpm, dispersion speed 25 m / s; S6. Post-processing: vacuum degassing, sieving and viscosity testing are performed on the mixed slurry after high-speed stirring and dispersion to obtain the positive electrode slurry.

11. The method according to claim 10, characterized in that In step S1, the conductive agent is any one or more of conductive carbon black, graphene, carbon nanotubes, and Ketjen black; and the binder is polyvinylidene fluoride (PVDF).

12. The method according to claim 10, characterized in that In step S2, ball milling is performed as follows: first, the NMP is fully dispersed and wetted by a low-speed ball mill with a rotation speed of 150 rpm to 300 rpm, and then the main material of the slurry is ground into one by a high-speed ball mill with a rotation speed of 800 rpm to 1200 rpm.

13. An energy storage device, characterized in that: The positive electrode sheet comprises any one of claims 1 to 7.

14. The energy storage device according to claim 13, characterized in that: The energy storage device is a secondary battery.

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