A positive electrode sheet and a dry process for preparing the same

Abstract

The application provides a positive electrode sheet and a dry preparation process thereof. The dry preparation process of the positive electrode sheet comprises the following steps: S1. mixing and preparing a positive electrode active paste by using a positive electrode active material, PVDF and other auxiliary materials at 40-50 DEG C, wherein the mass percentage of the positive electrode active material in the positive electrode active paste is 96-98%, and the mass percentage of the PVDF is 0.5-1.0%; S2. spraying the positive electrode active paste on the surface of a positive electrode current collector by using a step-by-step spraying method at 45-55 DEG C, so that the positive electrode active paste forms at least two layers of positive electrode active coating on the surface of the positive electrode current collector, and a positive electrode sheet semi-finished product is prepared; and S3. performing a rolling operation on the positive electrode sheet semi-finished product, and a positive electrode sheet is prepared. The dry preparation process can avoid the problem of mutual diffusion between the positive electrode active materials in different positive electrode active coating layers after coating of the positive electrode active paste, and is beneficial to improving the uniformity of the positive electrode sheet.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery production, in particular to a positive electrode sheet and a dry preparation process thereof. BACKGROUND

[0002] With the rapid development of small electronic devices, new energy vehicles and other emerging industries, lithium ion batteries have become the mainstream of energy storage systems due to their high energy density and green, environmentally friendly and pollution-free advantages. Lithium ion batteries mainly include positive electrode sheets, negative electrode sheets, separators and electrolytes. Among them, the positive active material is the main component in the active coating of the positive electrode sheet, which determines the electrochemical performance of the positive electrode sheet. The traditional preparation method of the positive electrode sheet mainly mixes the positive active material, conductive agent, binder and other materials uniformly using a solvent to prepare a positive active slurry with a certain fluidity. Then, the positive active slurry is uniformly coated on the surface of the positive current collector aluminum foil through different coating methods such as rolling, spraying and doctor blade coating. After coating, the positive electrode sheet needs to be rolled to reduce the porosity of the positive active material in the positive active coating, so as to improve the capacity of the battery. The traditional preparation method of the positive electrode sheet has the advantages of convenient control, good consistency and simple operation. However, the positive active slurry after coating will have the problem of mutual diffusion between the positive active materials in different positive active coatings, which is not conducive to the uniformity of the positive electrode sheet, and further affects the comprehensive performance of the lithium ion battery. SUMMARY

[0003] In order to solve the problem of mutual diffusion between the positive active materials in different positive active coatings after coating of the positive active slurry and improve the uniformity of the positive electrode sheet, the present application provides a positive electrode sheet and a dry preparation process thereof.

[0004] According to a first aspect of the present application, a dry preparation process of a positive electrode sheet is provided, which comprises the following steps:

[0005] S1. Mixing and preparing a positive active slurry by using positive active material, PVDF and other auxiliary materials at 40-50℃. In the positive active slurry, the mass percentage of positive active material is 96-98%, and the mass percentage of PVDF is 0.5-1.0%;

[0006] S2. Spraying the positive active slurry on the surface of the positive current collector by using a step-by-step spraying method at 45-55℃, so that the positive active slurry forms at least two layers of positive active coating on the surface of the positive current collector, thereby preparing a positive electrode sheet semi-finished product;

[0007] S3. Rolling the positive electrode sheet semi-finished product to prepare a positive electrode sheet.

[0008] The dry preparation process provided by the present application directly mixes the positive electrode active material, the binder PVDF and other auxiliary materials at 40-50 DEG C to form a positive electrode active slurry in the preparation process of the positive electrode sheet, and no solvent is used in the slurry preparation process, which can effectively avoid the problem of mutual diffusion between the positive electrode active materials in different positive electrode active coatings after coating, and is conducive to improving the uniformity of the positive electrode sheet.

[0009] Preferably, in the positive electrode sheet, the layer of positive electrode active coating farthest from the positive electrode current collector is the top layer of positive electrode active coating, and the particle size of the positive electrode active material in the top layer of positive electrode active coating is larger than that of the positive electrode active material in other layers of positive electrode active coating.

[0010] The present application uses a positive electrode active material with a larger particle size to prepare a positive electrode active slurry and obtain a top layer of positive electrode active coating farthest from the positive electrode current collector through spraying, so that the top layer of positive electrode active coating is less likely to fall off, and the positive electrode active material with a larger particle size forms certain voids in the top layer of positive electrode active coating, which is conducive to improving the wettability of the electrolyte to the positive electrode active coating, stably forming an SEI film, and better exerting the specific capacity of the positive electrode active material, thereby giving the positive electrode sheet good electrochemical performance and cycle performance.

[0011] Preferably, in addition to the positive electrode active material used to form the top layer of positive electrode active coating, the particle size of the positive electrode active material used to form each layer of positive electrode active coating gradually decreases in the direction away from the positive electrode current collector.

[0012] The dry preparation process of the positive electrode sheet provided by the present application uses a step-by-step spraying method, and the particle size of the positive electrode active material in the above-mentioned positive electrode active coating is set to make the positive electrode active material sprayed later more easily enter the gaps on the surface of the previous layer of positive electrode active coating, which is conducive to the compounding of adjacent positive electrode active coatings in a riveting structure, improves the adhesion between each positive electrode active coating, and further improves the structural stability of the positive electrode sheet. The positive electrode sheet is applied to a battery, which can improve the electrochemical performance and cycle performance of the battery.

[0013] Preferably, in S2, the spraying speed of the positive active paste is 5.5-8.0 m / min, for example, it can be 5.5 m / min, 6.5 m / min, 7.0 m / min, 7.5 m / min, 8.0 m / min, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0014] Controlling the speed of the positive active paste within the range of 5.5-8.0 m / min during spraying can improve the uniformity of the positive active paste on the surface of the positive current collector, making the thickness of the positive active coating more uniform. If the spraying speed is too slow, the production efficiency will be reduced; if the spraying speed is too fast, the positive active paste is prone to uneven distribution on the surface of the positive current collector, thereby affecting the electrochemical performance of the positive plate.

[0015] Preferably, in S3, the roller pressing operation conditions are as follows: temperature 100-120℃, pressure 0.7-1.2 MPa, and roller pressing speed 0.8-1.0 m / s, for example, the temperature can be 100℃, 105℃, 110℃, 115℃, 120℃, the pressure can be 0.7 MPa, 0.8 MPa, 0.9 MPa, 1.0 MPa, 1.1 MPa, 1.2 MPa, and the roller pressing speed can be 0.8 m / s, 0.85 m / s, 0.9 m / s, 0.95 m / s, 1.0 m / s, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0016] Controlling the roller pressing temperature, pressure, and roller pressing speed within the above ranges can improve the compaction density of each positive active coating, improve the bonding force between each positive active coating, and thereby improve the structural stability of the positive plate. If the roller pressing temperature, pressure, and roller pressing speed are too low, the plate is not compacted enough, and the positive active coatings are prone to peeling; if the roller pressing temperature, pressure, and roller pressing speed are too high, the positive active material particles are prone to being crushed or the positive current collector is prone to breaking, and both of the above conditions will result in a decrease in the electrochemical performance and cycle performance of the positive plate.

[0017] According to a second aspect of the present application, a positive plate is provided, which is prepared by the dry process described above.

[0018] The positive plate prepared by the dry process provided by the present application has good bonding performance between each positive active coating, making the structure of the positive plate stable, and when the positive plate is applied to a battery, the battery has good electrochemical performance and cycle performance.

[0019] Preferably, the types of positive active materials contained in two adjacent positive active coatings are different.

[0020] The different positive active materials in the two adjacent positive active coating layers of the positive plate can reduce the mutual diffusion of the positive active materials in each coating layer to a certain extent, avoid the mutual influence between the various positive active materials, and help to improve the consistency of the battery containing the positive plate.

[0021] Preferably, the positive active material contained in one of the two adjacent positive active coating layers includes a ternary positive active material, and the positive active material contained in the other positive active coating layer includes a lithium iron phosphate positive active material.

[0022] The use of ternary materials and lithium iron phosphate as the positive active materials in the two adjacent positive active coating layers of the positive plate can make each type of positive active material fully exert its own advantages and improve the overall electrochemical performance of the positive plate.

[0023] Preferably, the positive plate includes a positive current collector and a first positive active coating layer, a second positive active coating layer, and a third positive active coating layer arranged in sequence on the surface of the positive current collector; the positive active material contained in the first positive active coating layer is a ternary positive active material with a particle size D50 of 4.5-6.5 μm, the positive active material contained in the second positive active coating layer is a lithium iron phosphate positive active material with a particle size D50 of 1.5-3.5 μm, and the positive active material contained in the third positive active coating layer is a ternary positive active material with a particle size D50 of 9.0-12.0 μm, for example, the particle size D50 of the ternary positive active material contained in the first positive active coating layer can be 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, or 6.5 μm, the particle size D50 of the lithium iron phosphate positive active material contained in the second positive active coating layer can be 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, or 3.5 μm, and the particle size D50 of the ternary positive active material contained in the third positive active coating layer can be 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.5 μm, or 12.0 μm, but is not limited to the listed values, and other values not listed within the range are also applicable.

[0024] The use of ternary materials with a particle size of 4.5-6.5 μm, lithium iron phosphate with a particle size of 1.5-3.5 μm, and ternary materials with a particle size of 9.0-12.0 μm as the positive active materials to form three positive active coating layers arranged in sequence on the surface of the positive current collector can improve the bonding force between the coating layers by using positive active materials with different particle sizes, thereby improving the structural stability and electrochemical performance of the positive plate, and the use of the three types of positive active materials can promote the capacity of the materials, thereby improving the energy density and cycle performance of the battery containing the positive plate.

[0025] Preferably, the thickness of the first positive electrode active coating is 120-130 μm, the thickness of the second positive electrode active coating is 55-65 μm, and the thickness of the third positive electrode active coating is 50-60 μm, for example, the thickness of the first positive electrode active coating can be 120 μm, 122 μm, 124 μm, 126 μm, 128 μm, 130 μm, the thickness of the second positive electrode active coating can be 55 μm, 57 μm, 59 μm, 61 μm, 63 μm, 65 μm, and the thickness of the third positive electrode active coating can be 50 μm, 52 μm, 54 μm, 56 μm, 58 μm, 60 μm, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0026] Preferably, the surface of the positive electrode current collector is provided with recesses, the depth of the recesses is H, and the distance between adjacent recesses is S, H and S satisfy 8≤H / S≤28.

[0027] Preferably, H=0.8-1.2 μm and S=0.3-0.7 mm, for example, H can be 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, and S can be 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0028] Preferably, H=1 μm and S=0.5 mm.

[0029] The positive electrode current collector with a surface having recesses can improve the surface tension of the positive electrode current collector, reduce the risk of belt breakage during the rolling process of the positive electrode sheet, improve the coating efficiency, and reduce the rate of rolling breakage. At the same time, the contact between the positive electrode active coating and the positive electrode current collector is more firm, further improving the adhesion therebetween and preventing peeling. DETAILED DESCRIPTION

[0030] The technical features in the technical solutions provided by the present application are described in further detail below in conjunction with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0031] Embodiment 1

[0032] A positive electrode sheet includes a positive electrode current collector and a first positive electrode active coating, a second positive electrode active coating, and a third positive electrode active coating arranged in sequence on the surface of the positive electrode current collector, and a preparation method thereof includes the following steps:

[0033] S1. A first positive electrode active slurry was prepared by uniformly mixing nickel cobalt manganese lithium oxide powder with a particle size of 5.0 μm, a binder polyvinylidene fluoride (PVDF), conductive carbon black (SP), and carbon nanotubes (CNT) in a mass ratio of 96:1:2:1 at 45°C; a second positive electrode active slurry was prepared by uniformly mixing lithium iron phosphate powder with a particle size of 2.0 μm, PVDF, SP, and CNT in a mass ratio of 97.2:0.5:1.6:0.5 at 45°C; and a third positive electrode active slurry was prepared by uniformly mixing nickel cobalt manganese lithium oxide with a particle size of 11.0 μm, PVDF, SP, and CNT in a mass ratio of 96:1:2:1 at 45°C.

[0034] S2. The first positive electrode active slurry was sprayed on the surface of the positive electrode current collector aluminum foil at 50°C using a step-by-step spraying method, so that the first positive electrode active slurry formed a first positive electrode active coating with a thickness of 125 μm on the surface of the positive electrode current collector, then the second positive electrode active slurry was sprayed on the surface of the first positive electrode active coating, so that the second positive electrode active slurry formed a second positive electrode active coating with a thickness of 60 μm on the surface of the first positive electrode active coating, and finally the third positive electrode active slurry was sprayed on the surface of the second positive electrode active coating, so that the third positive electrode active slurry formed a third positive electrode active coating with a thickness of 55 μm on the surface of the second positive electrode active coating, to obtain a positive electrode sheet semi-finished product.

[0035] In S2, the spraying speed of the first positive electrode active slurry, the second positive electrode active slurry, and the third positive electrode active slurry was 7.0 m / min.

[0036] S3. The positive electrode sheet semi-finished product was subjected to a rolling operation to obtain a positive electrode sheet.

[0037] In S3, the rolling operation conditions were as follows: temperature 110°C, pressure 1.0 MPa, and rolling speed 0.9 m / s.

[0038] Example 2

[0039] A positive electrode sheet includes a positive electrode current collector and a first positive electrode active coating, a second positive electrode active coating, and a third positive electrode active coating sequentially arranged on the surface of the positive electrode current collector, and a preparation method thereof includes the following steps:

[0040] S1. A first positive electrode active slurry was prepared by uniformly mixing nickel cobalt manganese lithium manganate powder with a particle size of 4.5 μm, a binder polyvinylidene fluoride (PVDF), conductive carbon black (SP), and carbon nanotubes (CNT) in a mass ratio of 96:1:2:1 at 40°C; a second positive electrode active slurry was prepared by uniformly mixing lithium iron phosphate powder with a particle size of 1.5 μm, PVDF, SP, and CNT in a mass ratio of 97.2:0.5:1.6:0.5 at 40°C; and a third positive electrode active slurry was prepared by uniformly mixing nickel cobalt manganese lithium manganate with a particle size of 9.0 μm, PVDF, SP, and CNT in a mass ratio of 96:1:2:1 at 40°C;

[0041] S2. The first positive electrode active slurry was sprayed on the surface of the positive electrode current collector aluminum foil at 45°C using a step-by-step spraying method, so that the first positive electrode active slurry formed a first positive electrode active coating with a thickness of 120 μm on the surface of the positive electrode current collector; then the second positive electrode active slurry was sprayed on the surface of the first positive electrode active coating, so that the second positive electrode active slurry formed a second positive electrode active coating with a thickness of 55 μm on the surface of the first positive electrode active coating; and finally, the third positive electrode active slurry was sprayed on the surface of the second positive electrode active coating, so that the third positive electrode active slurry formed a third positive electrode active coating with a thickness of 50 μm on the surface of the second positive electrode active coating, thereby preparing a positive electrode sheet semi-product;

[0042] In S2, the spraying speed of the first positive electrode active slurry, the second positive electrode active slurry, and the third positive electrode active slurry was 5.5 m / min;

[0043] S3. The positive electrode sheet semi-product was subjected to a rolling operation, thereby preparing a positive electrode sheet;

[0044] In S3, the rolling operation conditions were as follows: temperature 100°C, pressure 0.7 MPa, and rolling speed 0.8 m / s.

[0045] Example 3

[0046] A positive electrode sheet includes a positive electrode current collector and a first positive electrode active coating, a second positive electrode active coating, and a third positive electrode active coating sequentially arranged on the surface of the positive electrode current collector, and a preparation method thereof includes the following steps:

[0047] S1. A lithium nickel cobalt manganese oxide powder with a particle size of 6.5 μm, a binder polyvinylidene fluoride (PVDF), conductive carbon black (SP) and carbon nanotubes (CNT) were mixed uniformly at a mass ratio of 96:1:2:1 at 50℃ to prepare a first positive electrode active slurry; a lithium iron phosphate powder with a particle size of 3.5 μm, PVDF, SP and CNT were mixed uniformly at a mass ratio of 97.2:0.5:1.6:0.5 at 50℃ to prepare a second positive electrode active slurry; a lithium nickel cobalt manganese oxide with a particle size of 12.0 μm, PVDF, SP and CNT were mixed uniformly at a mass ratio of 96:1:2:1 at 50℃ to prepare a third positive electrode active slurry;

[0048] S2. The first positive electrode active slurry was sprayed on the surface of the positive electrode current collector aluminum foil at 55℃ in a step-by-step spraying manner to form a first positive electrode active coating with a thickness of 130 μm on the surface of the positive electrode current collector, then the second positive electrode active slurry was sprayed on the surface of the first positive electrode active coating to form a second positive electrode active coating with a thickness of 65 μm on the surface of the first positive electrode active coating, and finally the third positive electrode active slurry was sprayed on the surface of the second positive electrode active coating to form a third positive electrode active coating with a thickness of 60 μm on the surface of the second positive electrode active coating, thereby preparing a positive electrode sheet semi-finished product;

[0049] In S2, the spraying speed of the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry was 8.0 m / min;

[0050] S3. The positive electrode sheet semi-finished product was subjected to a rolling operation to prepare a positive electrode sheet;

[0051] In S3, the rolling operation conditions were as follows: temperature 120℃, pressure 1.2 MPa, and rolling speed 1.0 m / s.

[0052] Example 4

[0053] This example provides a positive electrode sheet, which differs from Example 1 in that in the preparation step S1 of the positive electrode sheet, the particle size of the lithium nickel cobalt manganese oxide powder used to prepare the first positive electrode active slurry is 11.0 μm, and the particle size of the lithium nickel cobalt manganese oxide powder used to prepare the third positive electrode active slurry is 5.0 μm. Except for the above-mentioned differences, the materials, formula and preparation operation used in this example are strictly consistent with those of Example 1.

[0054] Example 5

[0055] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that in the preparation step S1 of the positive plate, the particle size of the lithium nickel cobalt manganese oxide powder used for preparing the third positive active paste is 1.5 mu m. Except the above difference, the materials, the formula ratio and the preparation operation used in the embodiment are strictly the same as those in the embodiment 1.

[0056] Embodiment 6

[0057] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that in the preparation step S1 of the positive plate, the particle size of the lithium nickel cobalt manganese oxide powder used for preparing the third positive active paste is 1.5 mu m. Except the above difference, the materials, the formula ratio and the preparation operation used in the embodiment are strictly the same as those in the embodiment 1.

[0058] Embodiment 7

[0059] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that in the preparation step S1 of the positive plate, the particle size of the lithium nickel cobalt manganese oxide powder used for preparing the third positive active paste is 1.5 mu m. Except the above difference, the materials, the formula ratio and the preparation operation used in the embodiment are strictly the same as those in the embodiment 1.

[0060] Embodiment 8

[0061] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that in the preparation step S1 of the positive plate, the particle size of the lithium nickel cobalt manganese oxide powder used for preparing the third positive active paste is 1.5 mu m. Except the above difference, the materials, the formula ratio and the preparation operation used in the embodiment are strictly the same as those in the embodiment 1.

[0062] Embodiment 9

[0063] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that in the preparation step S1 of the positive plate, the particle size of the lithium nickel cobalt manganese oxide powder used for preparing the third positive active paste is 1.5 mu m. Except the above difference, the materials, the formula ratio and the preparation operation used in the embodiment are strictly the same as those in the embodiment 1.

[0064] Embodiment 10

[0065] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that: in the preparation step S1 of the positive plate, the lithium nickel cobalt manganese oxide powder used for preparing the first positive active paste and the third positive active paste is replaced into lithium iron phosphate powder. Except the above difference, the materials, the formula ratio and the preparation operation of the embodiment are strictly the same as those of the embodiment 1.

[0066] Embodiment 11

[0067] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that: in the preparation step S1 of the positive plate, the lithium nickel cobalt manganese oxide powder used for preparing the first positive active paste and the third positive active paste is replaced into lithium iron phosphate powder. Except the above difference, the materials, the formula ratio and the preparation operation of the embodiment are strictly the same as those of the embodiment 1.

[0068] Embodiment 12

[0069] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that: in the preparation step S2 of the positive plate, the thickness of the first positive active coating is 150 mu m, the thickness of the second positive active coating is 85 mu m, and the thickness of the third positive active coating is 80 mu m. Except the above difference, the materials, the formula ratio and the preparation operation of the embodiment are strictly the same as those of the embodiment 1.

[0070] Embodiment 13

[0071] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that: in the preparation step S2 of the positive plate, the spraying speed of the first positive active paste, the second positive active paste and the third positive active paste is 9.0 m / min. Except the above difference, the materials, the formula ratio and the preparation operation of the embodiment are strictly the same as those of the embodiment 1.

[0072] Embodiment 14

[0073] The embodiment provides a positive plate, and compared with the embodiment 1, the difference is that: in the preparation step S3 of the positive plate, the rolling operation condition is as follows, the temperature is 130 DEG C, the pressure is 1.3 MPa, and the rolling speed is 1.2 m / s. Except the above difference, the materials, the formula ratio and the preparation operation of the embodiment are strictly the same as those of the embodiment 1.

[0074] Embodiment 15

[0075] The present example provides a positive electrode sheet, compared with Example 1, the difference is that in the preparation step S3 of the positive electrode sheet, the rolling operation conditions are as follows: temperature 90℃, pressure 0.5MPa, rolling speed 0.6m / s. Except the above difference, the materials, formula ratio and preparation operation adopted in the present example are strictly consistent with Example 1.

[0076] Comparative Example 1

[0077] The present comparative example provides a positive electrode sheet, compared with Example 1, the difference is that in the preparation step S1 of the positive electrode sheet, the temperature for preparing the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry is 35℃. Except the above difference, the materials, formula ratio and preparation operation adopted in the present comparative example are strictly consistent with Example 1.

[0078] Comparative Example 2

[0079] The present comparative example provides a positive electrode sheet, compared with Example 1, the difference is that in the preparation step S1 of the positive electrode sheet, the temperature for preparing the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry is 55℃. Except the above difference, the materials, formula ratio and preparation operation adopted in the present comparative example are strictly consistent with Example 1.

[0080] Comparative Example 3

[0081] The present comparative example provides a positive electrode sheet, compared with Example 1, the difference is that in the preparation step S2 of the positive electrode sheet, the spraying temperature is 40℃. Except the above difference, the materials, formula ratio and preparation operation adopted in the present comparative example are strictly consistent with Example 1.

[0082] Comparative Example 4

[0083] The present comparative example provides a positive electrode sheet, compared with Example 1, the difference is that in the preparation step S2 of the positive electrode sheet, the spraying temperature is 60℃. Except the above difference, the materials, formula ratio and preparation operation adopted in the present comparative example are strictly consistent with Example 1.

[0084] Test Example

[0085] 1. Test subjects

[0086] The positive electrode sheet prepared in Examples 1-15 and Comparative Examples 1-4 was used as a test object, and the positive electrode sheet was applied to a lithium ion battery, and the lithium ion battery was subjected to related performance tests. The specific preparation method of the lithium ion battery for testing is as follows: the negative electrode material graphite, the conductive agent acetylene black, and the binder CMC, SBR were prepared into a slurry in a mass percentage of 94:1:2:3, and coated on a copper foil current collector, vacuum dried, and a negative electrode sheet was prepared; ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L; the positive electrode sheet, the separator film (polyethylene film), and the negative electrode sheet prepared in Examples 1-15 and Comparative Examples 1-4 were stacked in order, with the separator film between the positive and negative electrode sheets to play a role of isolation, and then wound to obtain a bare cell, and the bare cell was placed in an outer packaging shell, dried, and then injected with the above electrolyte, and after vacuum packaging, standing, formation, shaping, and other processes, a lithium ion battery was obtained for related performance tests.

[0087] 2. Test content

[0088] (1) Direct current impedance (DCR)

[0089] 1) The calibrated battery was charged at a current of 1 / 3I(A) of the battery capacity and a voltage of 3.65V constant current and constant voltage to 3.65V, and stood for 30 min;

[0090] 2) The battery was discharged at a current of 1 / 3I(A) of the battery capacity to 50% SOC of the cell capacity, and stood for 30 min; the standing termination voltage was V1;

[0091] 3) The battery was discharged at a current of 1I(A) of the battery capacity for a certain time t, and the discharge termination voltage V2 was calculated, and DCR=(V1-V2) / I.

[0092] (2) Cycle performance

[0093] The calibrated battery was charged at a current of 1I(A) of the battery capacity and discharged at 1I(A) for 500 cycles, and the capacity retention rate was calculated.

[0094] (3) Energy density

[0095] 1) The battery was discharged at a current of 1 / 3I(A) of the battery capacity to 2.5V, and stood for 30 min;

[0096] 2) The battery was charged at a current of 1 / 3I(A) of the battery capacity and a voltage of 3.65V constant current and constant voltage to 3.65V, and stood for 30 min;

[0097] 3) Step 1) was repeated, and the discharge energy E (in Wh) was calculated;

[0098] 4) Repeat steps 2)~3) 2 times, and take the average value Ea of the discharge energy E of 3 times.

[0099] 5) Measure the weight M of the battery cell (in kg);

[0100] 6) Calculate the discharge energy density PED of the test object (in Wh / kg), according to the following formula: PED = Ea / M.

[0101] 3. Experimental results

[0102] Table 1. Results of related performance tests of lithium ion batteries

[0103]

[0104]

[0105] The positive electrode sheets prepared in Examples 1~15 and Comparative Examples 1~4 were applied to lithium ion batteries, and the results of related performance tests of the lithium ion batteries are shown in Table 1.

[0106] The positive electrode sheet provided in Examples 1-3 is prepared by directly mixing the positive electrode active material with the binder PVDF and other auxiliary materials at 40-50°C to form a positive electrode active slurry, then spraying the positive electrode active slurry at a speed of 5.5-8.0 m / min on the surface of the positive electrode current collector at 45-55°C and roll-pressing at 110°C, 1.0 MPa, and a roll-pressing speed of 0.9 m / s, wherein the positive electrode sheet comprises a positive electrode current collector and a first positive electrode active coating layer, a second positive electrode active coating layer, and a third positive electrode active coating layer arranged in sequence on the surface of the positive electrode current collector, the first positive electrode active coating layer contains the positive electrode active material nickel cobalt lithium manganate with a particle size D50 = 4.5-6.5 μm, the second positive electrode active coating layer contains the positive electrode active material lithium iron phosphate with a particle size D50 = 1.5-3.5 μm, and the third positive electrode active coating layer contains the positive electrode active material nickel cobalt lithium manganate with a particle size D50 = 9.0-12.0 μm. The positive electrode sheet provided in Examples 1-3 is applied to a lithium ion battery, and the resulting lithium ion battery has good electrochemical performance and cycle performance, with a DCR value of 0.613-0.625 mΩ, a capacity retention rate of up to 96.26-96.65%, and an energy density of up to 237-248 Wh / kg. This is mainly because in the preparation process of the positive electrode sheet provided in Examples 1-3, the positive electrode active material is directly mixed with the binder PVDF and other auxiliary materials at 40-50°C to form a positive electrode active slurry, and no solvent is used in the slurry preparation process, which effectively avoids the problem of mutual diffusion between the positive electrode active materials in different positive electrode active coating layers after coating of the positive electrode active slurry, and is conducive to improving the uniformity of the positive electrode sheet. At the same time, the slurry preparation temperature, spraying temperature, and roll-pressing conditions of the positive electrode active slurry are adjusted to make the PVDF uniformly dispersed among the positive electrode active materials, the positive electrode active slurry formed is uniform and does not cake, and the PVDF in the positive electrode active slurry can exhibit sufficient adhesion at 45-55°C after being sprayed, so that the positive electrode active slurry can quickly form a positive electrode active coating layer after being sprayed, which is conducive to forming a positive electrode active coating layer with distinct layers and stable structure. The application of the positive electrode sheet provided in Examples 1-3 to a lithium ion battery makes the lithium ion battery have good electrochemical performance and cycle performance.

[0107] Compared with Example 1, the positive electrode sheet provided in Comparative Examples 1-2 has a slurry preparation temperature of 35°C and 55°C, respectively, in the preparation process, and the positive electrode sheet provided in Comparative Examples 3-4 has a spraying temperature of 40°C and 60°C, respectively, in the preparation process. The DCR value of the lithium ion battery to which the positive electrode sheet provided in Comparative Examples 1-4 is applied is higher than that of Example 1, and the capacity retention rate and energy density are lower than those of Example 1.

[0108] Compared with Example 1, the first positive electrode active coating in the positive electrode sheet provided by Example 4 contains lithium nickel cobalt manganese oxide with a particle size D50 = 11.0 μm, the third positive electrode active coating contains lithium nickel cobalt manganese oxide with a particle size D50 = 5.0 μm, the third positive electrode active coating in the positive electrode sheet provided by Example 5 contains lithium nickel cobalt manganese oxide with a particle size D50 = 1.5 μm, the second positive electrode active coating in the positive electrode sheet provided by Example 6 contains lithium iron phosphate with a particle size D50 = 8.0 μm, and the positive electrode sheets provided by Examples 4-6 are applied to lithium ion batteries, the DCR value of the lithium ion batteries is higher than that of Example 1, and the capacity retention rate is lower than that of Example 1.

[0109] Compared with Example 1, the first positive electrode active coating and the third positive electrode active coating in the positive electrode sheet provided by Example 7 both contain lithium iron phosphate, the second positive electrode active coating contains lithium nickel cobalt manganese oxide, the first positive electrode active coating in the positive electrode sheet provided by Example 8 contains lithium iron phosphate, the second positive electrode active coating in the positive electrode sheet provided by Example 9 contains lithium nickel cobalt manganese oxide, and the third positive electrode active coating contains lithium iron phosphate, the positive electrode sheets provided by Examples 7-9 are applied to lithium ion batteries, the DCR value of the lithium ion batteries is higher than that of Example 1, and the capacity retention rate and the energy density are both lower than those of Example 1; the first positive electrode active coating and the third positive electrode active coating in the positive electrode sheet provided by Example 10 both contain lithium iron phosphate, the second positive electrode active coating in the positive electrode sheet provided by Example 11 contains lithium nickel cobalt manganese oxide, and the positive electrode sheets provided by Examples 10-11 are applied to lithium ion batteries, the DCR value of the lithium ion batteries is significantly higher than that of Example 1, and the capacity retention rate is significantly lower than that of Example 1.

[0110] Compared with Example 1, the thicknesses of the first positive electrode active coating, the second positive electrode active coating, and the third positive electrode active coating in the positive electrode sheet provided by Example 12 are 150 μm, 85 μm, and 80 μm, respectively, and the positive electrode sheet provided by Example 12 is applied to a lithium ion battery, the DCR value of the lithium ion battery is higher than that of Example 1, and the capacity retention rate is significantly lower than that of Example 1.

[0111] Compared with Example 1, the spraying speed of the positive electrode active paste during the preparation of the positive electrode sheet provided by Example 13 is 9.0 m / min, and the positive electrode sheet provided by Example 13 is applied to a lithium ion battery, the DCR value of the lithium ion battery is higher than that of Example 1, and the capacity retention rate is lower than that of Example 1. The reason for the above results is that the spraying speed is too fast, which causes the positive electrode active paste to be unevenly distributed on the surface of the positive electrode current collector, thereby affecting the electrochemical performance and cycle performance of the positive electrode sheet.

[0112] Compared with Example 1, the positive electrode sheet provided in Example 14 is prepared at a temperature of 130℃, a pressure of 1.3 MPa and a rolling speed of 1.2 m / s in the rolling operation, and the positive electrode sheet provided in Example 15 is prepared at a temperature of 90℃, a pressure of 0.5 MPa and a rolling speed of 0.6 m / s in the rolling operation. When the positive electrode sheets provided in Examples 14-15 are applied to lithium ion batteries, the DCR values of the lithium ion batteries are significantly higher than that of Example 1, and the capacity retention rate is lower than that of Example 1. The reason for the above results is that: if the rolling temperature, pressure and rolling speed are too low, the positive electrode sheet is not compacted enough, and the positive electrode active coatings are prone to peeling; if the rolling temperature, pressure and rolling speed are too high, the positive electrode active material particles are prone to be crushed or the positive electrode current collector is prone to be broken, that is, too low or too high rolling temperature, pressure and rolling speed will result in the decrease of the electrochemical performance and cycle performance of the positive electrode sheet.

[0113] The above examples are only used to illustrate the technical solutions of the present application, but not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the above examples, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently, and these modifications or replacements are within the protection scope of the present application.

Claims

1. A dry process for producing a positive electrode sheet, characterized by, The method comprises the following steps: S1. mixing a positive electrode active material, PVDF and other auxiliary materials at 40-50 DEG C to prepare a positive electrode active paste, wherein no solvent is used in the paste preparation process, the mass percentage of the positive electrode active material in the positive electrode active paste is 96-98%, and the mass percentage of the PVDF is 0.5-1.0%; S2. spraying the positive electrode active paste on the surface of a positive electrode current collector in a step-by-step manner at 45-55 DEG C to form a laminated positive electrode active coating on the surface of the positive electrode current collector, thereby obtaining a positive electrode sheet semi-product; S3. performing a rolling operation on the positive electrode sheet semi-product to obtain the positive electrode sheet, wherein the rolling operation is performed at a temperature of 100-120 DEG C, a pressure of 0.7-1.2 MPa, and a rolling speed of 0.8-1.0 m / s; in the positive electrode sheet, the positive electrode active coating layer farthest from the positive electrode current collector is a top positive electrode active coating layer, and the particle size of the positive electrode active material in the top positive electrode active coating layer is larger than that of the positive electrode active material in other positive electrode active coating layers; in two adjacent positive electrode active coating layers, one of the positive electrode active coating layers contains a ternary positive electrode active material, and the other of the positive electrode active coating layers contains a lithium iron phosphate positive electrode active material; the positive electrode sheet comprises the positive electrode current collector and first, second and third positive electrode active coating layers arranged in sequence on the surface of the positive electrode current collector; the thickness of the first positive electrode active coating layer is 120-130 mu m, the thickness of the second positive electrode active coating layer is 55-65 mu m, and the thickness of the third positive electrode active coating layer is 50-60 mu m.

2. The dry process for preparing the positive electrode sheet according to claim 1, wherein: In addition to the positive electrode active material used to form the top positive electrode active coating layer, the particle size of the positive electrode active material used to form each positive electrode active coating layer gradually decreases in the direction away from the positive electrode current collector.

3. The dry process for preparing the positive electrode sheet according to claim 1, wherein: In the S2, the spraying speed of the positive electrode active paste is 5.5-8.0 m / min.

4. A positive electrode sheet characterized by comprising: Prepared by the dry process according to any one of claims 1-3.

5. The positive electrode sheet according to claim 4, wherein: The first positive electrode active coating layer contains the ternary positive electrode active material with a particle size D50 = 4.5-6.5 mu m, the second positive electrode active coating layer contains the lithium iron phosphate positive electrode active material with a particle size D50 = 1.5-3.5 mu m, and the third positive electrode active coating layer contains the ternary positive electrode active material with a particle size D50 = 9.0-12.0 mu m.

6. The positive electrode sheet according to claim 5, wherein: The surface of the positive electrode current collector is provided with recessed portions, the depth of the recessed portions is H, and the distance between adjacent recessed portions is S, wherein 8<=H / S<=28.

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