Positive pole piece, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, particularly provides a positive pole piece and a preparation method thereof, and a lithium ion battery, and aims to solve the problem that the improvement effect of double-layer coating or thick coating on the cycle performance of the battery is not obvious in the prior art. Therefore, the positive pole piece comprises a positive current collector, a first positive coating and a second positive coating, and the first positive coating is arranged between the positive current collector and the second positive coating; one side, close to the second positive electrode coating, of the first positive electrode coating is provided with an infiltration structure extending from the surface to the positive electrode current collector; and the mass content of the conductive agent in the first positive electrode coating is higher than that of the conductive agent in the second positive electrode coating. According to the positive pole piece, the uniformity of dynamics of the whole positive pole piece can be ensured, and polarization is effectively reduced, so that the battery performance is improved.

Description

Technical Field

[0001] This application belongs to the technical field of lithium-ion batteries, and specifically provides a positive electrode sheet, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Currently, long-duration energy storage batteries need to cooperate with photovoltaic cells to achieve the same lifespan for energy storage and photovoltaic power generation. Therefore, there are relatively high requirements for the lifespan of long-duration energy storage batteries. However, as the rate of long-duration storage batteries decreases and the time increases, for long-duration energy storage batteries with a duration of 4 hours or more, compared with conventional energy storage batteries (with a rate of 0.5C or more), as the rate decreases and the time extends, more lithium ions are consumed, the charge-discharge depth deepens, which has a certain deteriorating effect on the battery lifespan.

[0003] Currently, the technologies for improving the lifespan of energy storage batteries in the market include using lithium supplementation technology, optimizing the formation process, improving the SEI film interface, double-layer coating technology, optimizing the electrode formula, etc., so as to reduce the side reactions at the battery interface, reduce the loss of active materials, increase and reduce the loss of active lithium, reduce the impedance, and stabilize the SEI film, etc., to extend the service life of the battery.

[0004] For the double-layer coating technology or thick coating technology, the wettability is insufficient in the later stage of battery cycling. At the end close to the current collector, due to low wettability and kinetics, the utilization of active lithium is insufficient and the active materials are lost, and the improvement effect on the cycling performance is not significant.

[0005] Correspondingly, there is a need in the art for a new technical solution to solve the above technical problems. Summary of the Invention

[0006] This application aims to solve the above technical problems, that is, to solve the problem that the double-layer coating or thick coating in the prior art has an insignificant improvement effect on the battery cycling performance.

[0007] In a first aspect, this application provides a positive electrode sheet, which includes a positive electrode current collector, a first positive electrode coating, and a second positive electrode coating. The first positive electrode coating is disposed between the positive electrode current collector and the second positive electrode coating; on the side of the first positive electrode coating close to the second positive electrode coating, there is a wetting structure extending from the surface towards the positive electrode current collector; the mass content of the conductive agent in the first positive electrode coating is higher than the mass content of the conductive agent in the second positive electrode coating.

[0008] In a preferred technical solution of the above positive electrode sheet, the wetting structure is a plurality of pores and / or a plurality of line gaps.

[0009] In the preferred technical solution of the above-mentioned positive electrode sheet, the depth of the infiltration structure is not less than 1 / 8 of the thickness of the first positive electrode coating, preferably the depth of the infiltration structure is not less than 1 / 5 of the thickness of the first positive electrode coating; and / or, the infiltration structure does not penetrate through the first positive electrode coating in the thickness direction of the first positive electrode coating.

[0010] In the preferred technical solution of the above-mentioned positive electrode sheet, the infiltration structure is a plurality of holes, and the plurality of holes are arranged in a matrix; or, the infiltration structure is a plurality of the line gaps, and the plurality of line gaps are spaced apart along the width direction of the first positive electrode coating.

[0011] In the preferred technical solution of the above-mentioned positive electrode sheet, in the first direction and / or the second direction, the distance between two adjacent holes is not less than 15 mm, where the first direction is perpendicular to the second direction.

[0012] In the preferred technical solution of the above-mentioned positive electrode sheet, the total cross-sectional area of the infiltration structure is not less than 40% - 80% of the area of the first positive electrode coating, preferably the total cross-sectional area of the infiltration structure is 60% - 80% of the area of the first positive electrode coating; and / or, the thickness of the first positive electrode coating is not less than the thickness of the second positive electrode coating; and / or, the shape of the holes includes at least one of a circle, a square, a rectangle, a triangle, a trapezoid or a rhombus.

[0013] In the preferred technical solution of the above-mentioned positive electrode sheet, the first positive electrode coating includes a positive electrode active material, a binder and a conductive agent. The mass percentage of the positive electrode active material is 92 wt% - 98 wt%, the mass percentage of the binder is 1.4 wt% - 5 wt%, and the mass percentage of the conductive agent is 0.6 wt% - 3 wt%; the second positive electrode coating includes a positive electrode active material, a binder and a conductive agent. The mass percentage of the positive electrode active material is 94 wt% - 98 wt%, the mass percentage of the binder is 1.5 wt% - 4 wt%, and the mass percentage of the conductive agent is 0.5 wt% - 2 wt%.

[0014] In the preferred technical solution of the above-mentioned positive electrode sheet, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating ≥ 1, preferably the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating ≥ 1.05; and / or, the binder is composed of a hydrophobic material and a binder.

[0015] In the preferred technical solution of the above-mentioned positive electrode sheet, in the binder, the mass proportion of the hydrophobic material is 10% - 90%, preferably the mass proportion of the hydrophobic material is 20% - 80%.

[0016] In the preferred technical solution of the above positive electrode sheet, the hydrophobic material includes one or more of tetrafluoroethylene, methyl methacrylate, perfluoroalkyl methacrylate, styrene, epoxy resin, 1,4-bis[(3,4-dioctyloxybenzene)-biamidino]benzene, polyethylene, polystyrene, polyvinylidene fluoride, octyl methacrylate, polyacrylonitrile, ethylene-acrylate copolymer, styrene-ethylene copolymer, vinylidene fluoride-styrene copolymer, organosilicon-modified polyurethane, perfluoroalkyl chain organic compounds; and / or, the binder includes one or more of polyvinylidene fluoride, polyacrylic acid, polyethersulfone; and / or, the positive electrode active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium ferrite, lithium nickelate; and / or, the conductive agent includes one or more of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes.

[0017] In a second aspect, the present application provides a method for preparing a positive electrode sheet for preparing the above positive electrode sheet. The method for preparing the positive electrode sheet includes the following steps: (1) Coating and drying a coating of a first positive electrode coating on a positive electrode current collector to form the first positive electrode coating on the positive electrode current collector to obtain an intermediate positive electrode sheet; (2) Using a laser to punch holes and / or draw lines on the surface of the first positive electrode coating of the intermediate positive electrode sheet to obtain a wetting structure on the surface of the first positive electrode coating away from the positive electrode current collector; (3) Coating and drying a coating of a second positive electrode coating on the first positive electrode coating to form the second positive electrode coating on the first positive electrode coating to obtain a positive electrode sheet.

[0018] In a third aspect, the present application provides a lithium-ion battery, and the lithium-ion battery includes the above positive electrode sheet.

[0019] Compared with the positive electrode sheet of the prior art, the positive electrode sheet of the present application has at least the following technical effects:

[0020] The positive electrode sheet of the present application includes a positive electrode current collector, a first positive electrode coating, and a second positive electrode coating stacked in sequence. By providing a wetting structure on the first positive electrode coating and making the amount of the conductive agent in the first positive electrode coating higher than that in the second positive electrode coating, the wetting effect of the electrolyte on the first positive electrode coating can be improved, the wettability of the first positive electrode coating in the electrode sheet can be increased, the ion / electron transport path of the first positive electrode coating can be shortened, the charge transfer resistance can be reduced, the longitudinal lithium intercalation uniformity of the electrode sheet can be optimized, the kinetics can be increased, and at the same time, the kinetics of the second positive electrode coating can be relatively reduced, and side reactions can be reduced, which can ensure the uniformity of the kinetics of the entire positive electrode sheet, effectively reduce polarization, and thus improve the battery performance.

[0021] In addition, both the first positive electrode coating and the second positive electrode coating include a hydrophobic material. On the one hand, it can shorten the time required for baking the electrode sheet or the battery cell, reducing costs. On the other hand, it can reduce the moisture content in the positive electrode sheet, reducing the side reactions of the battery cell caused by moisture, and further improving the cycle performance and storage life of the battery cell. Description of the Drawings

[0022] The preferred embodiments of the present application will be described below with reference to the accompanying drawings, in which:

[0023] Figure 1 is a schematic structural diagram of the positive electrode sheet of the present application;

[0024] Figure 2 is a top view of the first embodiment of the first positive electrode coating of the present application;

[0025] Figure 3 is a top view of the second embodiment of the first positive electrode coating of the present application;

[0026] Figure 4 is a top view of the third embodiment of the first positive electrode coating of the present application.

[0027] List of reference numerals:

[0028] 1. Positive electrode current collector; 2. First positive electrode coating; 3. Second positive electrode coating; 4. Infiltration structure; 41. Hole; 42. Line gap. Detailed Embodiments

[0029] The preferred embodiments of the present application will be described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present application and are not intended to limit the protection scope of the present application.

[0030] In the present application, the term "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B may be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after.

[0031] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or similar expressions thereof refer to any combination of these items, including any combination of single items (items) or plural items (items). For example, "at least one (item) of a, b, or c", or, "at least one (item) of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0032] It should be understood that in various embodiments of the present application, the magnitudes of the serial numbers of the above processes do not mean the sequence of execution. Some or all of the steps can be executed in parallel or sequentially. The execution sequence of each process should be determined according to its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0033] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms of "a", "the", and "said" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0034] The weights of the relevant components mentioned in the specification of the embodiments of the present application can not only refer to the specific contents of each component, but also represent the proportional relationship of the weights between the components. Therefore, as long as the contents of the relevant components in the specification of the embodiments of the present application are enlarged or reduced in proportion, they are within the scope disclosed in the specification of the embodiments of the present application. Specifically, the mass described in the specification of the embodiments of the present application can be mass units well known in the chemical industry such as μg, mg, g, kg, etc.

[0035] The terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be called the second XX, and similarly, the second XX can also be called the first XX. Thus, the features defined with "first" and "second" can explicitly or implicitly include one or more of such features.

[0036] The experimental methods in the following embodiments are all conventional methods unless otherwise specified, and the materials, reagents, etc. used in the following embodiments can all be obtained from commercial channels unless otherwise specified.

[0037] Based on the problem pointed out in the background art that the improvement effect of double-layer coating or thick coating on the battery cycle performance in the prior art is not significant.

[0038] The present application provides a positive electrode plate. By providing a wetting structure on the first positive electrode coating and at the same time making the amount of the conductive agent in the first positive electrode coating higher than that in the second positive electrode coating, the wetting effect of the electrolyte on the first positive electrode coating can be improved, the wettability of the first positive electrode coating in the electrode plate can be increased, the ion / electron transmission path of the first positive electrode coating can be shortened, the charge transfer resistance can be reduced, the longitudinal lithium intercalation uniformity of the electrode plate can be optimized, the kinetics can be increased, and at the same time the kinetics of the second positive electrode coating can be relatively reduced, and side reactions can be reduced, so as to ensure the uniformity of the kinetics of the entire positive electrode plate, effectively reduce polarization, and thus improve the battery performance.

[0039] Specifically, please refer to Figure 1 , the positive electrode plate of the present application includes a positive electrode current collector 1, a first positive electrode coating 2 and a second positive electrode coating 3, and the first positive electrode coating 2 is disposed between the positive electrode current collector 1 and the second positive electrode coating 3. Among them, the first positive electrode coating 2 is provided with an infiltration structure 4 extending from the surface toward the positive electrode current collector 1 on the side close to the second positive electrode coating 3. The setting of the infiltration structure 4 can accelerate the infiltration speed of the electrolyte into the first positive electrode coating 2; the mass content of the conductive agent in the first positive electrode coating 2 is higher than that in the second positive electrode coating 3.

[0040] By making the amount of the conductive agent in the first positive electrode coating 2 higher than that in the second positive electrode coating 3 and simultaneously providing an infiltration structure 4 on the side of the first positive electrode coating 2 close to the second positive electrode coating 3, the positive electrode plate of the present application can effectively improve the infiltration effect of the electrolyte in the first positive electrode coating 2, thereby increasing the wettability of the first positive electrode coating 2, which is also beneficial to the insertion and extraction of lithium ions during the charge and discharge process, thereby increasing the kinetics of the first positive electrode coating 2, enabling the kinetics of the entire positive electrode plate to be uniform, effectively reducing polarization, reducing side reactions, and thus improving the battery performance.

[0041] In practical applications, the first positive electrode coating 2 and the second positive electrode coating 3 can be provided only on one side of the positive electrode current collector 1, or the first positive electrode coating 2 and the second positive electrode coating 3 can be provided on both sides of the positive electrode current collector 1, as long as the first positive electrode coating 2 is located between the positive electrode current collector 1 and the second positive electrode coating 3. Those skilled in the art can set the coating positions and numbers of the first positive electrode coating 2 and the second positive electrode coating 3 according to actual needs.

[0042] It should be noted that the present application does not impose any restrictions on the specific structure of the infiltration structure 4, as long as the infiltration structure 4 can accelerate the infiltration speed of the electrolyte. In practical applications, those skilled in the art can set the specific structure of the infiltration structure 4 according to actual needs. For example, the infiltration structure 4 can be set as a plurality of holes, or the infiltration structure 4 can be set as a plurality of grooves, or the infiltration structure can be set as a plurality of linear gaps, or the infiltration structure can also be set as a structure combining holes and linear gaps. Adjustments and changes to the specific structure of the infiltration structure 4 do not deviate from the basic principle of the present application and should all be limited within the protection scope of the present application.

[0043] Preferably, the infiltration structure 4 is a plurality of holes 41 and / or a plurality of linear gaps 42.

[0044] In some embodiments, the infiltration structure 4 does not penetrate the first positive electrode coating 2 in the thickness direction of the first positive electrode coating 2.

[0045] In some embodiments, the depth of the infiltration structure 4 is not less than 1 / 8 of the thickness of the first positive electrode coating 2, and further preferably, the depth of the infiltration structure 4 is not less than 1 / 5 of the thickness of the first positive electrode coating 2.

[0046] The depth of the infiltration structure 4 is controlled to be not less than 1 / 8 of the thickness of the first positive electrode coating 2, and preferably, the depth of the infiltration structure 4 is not less than 1 / 5 of the thickness of the first positive electrode coating 2, so as to improve the infiltration effect after the electrolyte infiltrates into the first positive electrode coating 2, reduce the infiltration difficulty of the electrolyte in the first positive electrode coating 2, help improve the overall infiltration performance of the positive electrode sheet, and ensure the uniformity of the dynamics of the entire positive electrode sheet.

[0047] It should be noted that the depth of the infiltration structure 4 is the length that the infiltration structure 4 extends along the thickness direction of the first positive electrode coating 2.

[0048] In some embodiments, reference may be made to Figure 2 and Figure 3 , the infiltration structure 4 is a plurality of holes 41, and the plurality of holes 41 are arranged in a matrix. Arranging the plurality of holes 41 in a matrix facilitates preparation and makes the plurality of holes 41 evenly distributed, so that the wettability of each region of the first positive electrode coating 2 remains consistent, and can effectively improve the battery performance.

[0049] In some embodiments, the total cross-sectional area of the infiltration structure 4 is 40% - 80% of the area of the first positive electrode coating 2, and further preferably, the total cross-sectional area of the infiltration structure 4 is 60% - 80% of the area of the first positive electrode coating 2.

[0050] The total cross-sectional area of the infiltration structure 4 is controlled to be 40% - 80% of the area of the first positive electrode coating 2, preferably controlled to be 60% - 80%, so as to provide sufficient infiltration space on the surface of the first positive electrode coating 2, effectively ensure the infiltration effect after the electrolyte infiltrates into the first positive electrode coating 2, reduce the infiltration difficulty of the electrolyte in the first positive electrode coating 2, help improve the overall infiltration performance of the positive electrode sheet, and ensure the uniformity of the dynamics of the entire positive electrode sheet.

[0051] It should be noted that the cross-sectional area of the infiltration structure 4 is the area of the projection of the infiltration structure 4 in the direction perpendicular to the first positive electrode coating 2, and the total cross-sectional area of the infiltration structure 4 is the sum of the cross-sectional areas of all the infiltration structures 4.

[0052] In some embodiments, in the first direction and / or the second direction, the distance between two adjacent holes 41 is not less than 15 mm, wherein the first direction is perpendicular to the second direction. The first direction and the second direction are the transverse direction and the longitudinal direction of the matrix respectively.

[0053] Making the distance between two adjacent holes 41 not less than 15 mm facilitates production and manufacturing.

[0054] It should be noted that the distance between two adjacent holes 41 is the straight-line distance between the two closest points among the edges of the two adjacent holes 41 that are close to each other. Refer to the attached Figure 2 and Figure 3 , where d1 is the distance between two adjacent holes 41 in the first direction, and d2 is the distance between two adjacent holes 41 in the second direction.

[0055] In some embodiments, the shape of the hole 41 includes at least one of a circle, a square, a rectangle, a triangle, a trapezoid, or a rhombus.

[0056] It should be noted that this application does not impose any restrictions on the shape of the hole 41. In practical applications, those skilled in the art can set the specific shape of the hole 41 according to actual needs. Adjustments and changes to the specific shape of the hole 41 do not deviate from the basic principle of this application and should all be within the protection scope of this application.

[0057] In some embodiments, when the hole 41 is a circular hole, its diameter is not less than 10 mm. Making the diameter of the hole 41 not less than 10 mm facilitates production and manufacturing.

[0058] In some embodiments, when the hole 41 is a rectangular hole, its width is not less than 10 mm.

[0059] In practical applications, the total cross-sectional area ratio of the infiltration structure 4 to the area of the first positive electrode coating 2 can be adjusted by controlling the diameter or width of the hole 41 and the distance between two adjacent holes 41, so as to obtain a better infiltration structure 4.

[0060] In some embodiments, refer to Figure 4 , the infiltration structure 4 is a plurality of line gaps 42, and the plurality of line gaps 42 are spaced apart along the width direction of the first positive electrode coating 2.

[0061] In some embodiments, the thickness of the first positive electrode coating 2 is not less than the thickness of the second positive electrode coating 3.

[0062] In some embodiments, the first positive electrode coating 2 includes a positive electrode active material, a binder, and a conductive agent. The mass percentage of the positive electrode active material is 92 wt% - 98 wt%, the mass percentage of the binder is 1.4 wt% - 5 wt%, and the mass percentage of the conductive agent is 0.6 wt% - 3 wt%. The second positive electrode coating 3 includes a positive electrode active material, a binder, and a conductive agent. The mass percentage of the positive electrode active material is 94 wt% - 98 wt%, the mass percentage of the binder is 1.5 wt% - 4 wt%, and the mass percentage of the conductive agent is 0.5 wt% - 2 wt%.

[0063] Control the mass percentage of the conductive agent in the first positive electrode coating 2 to be 0.6 wt% to 3 wt%, control the mass percentage of the conductive agent in the second positive electrode coating 3 to be 0.5 wt% to 2 wt%, and at the same time make the mass percentage of the conductive agent in the first positive electrode coating 2 greater than that in the second positive electrode coating 3. In such a setting, the kinetics of the first positive electrode coating 2 and the second positive electrode coating 3 can be kept uniform, and when assembled into a battery for use, the cycle performance of the battery can be effectively improved.

[0064] Preferably, the ratio of the mass percentage of the conductive agent in the first positive electrode coating 2 to the mass percentage of the conductive agent in the second positive electrode coating 3 ≥ 1, and preferably the ratio of the mass percentage of the conductive agent in the first positive electrode coating 2 to the mass percentage of the conductive agent in the second positive electrode coating 3 ≥ 1.05.

[0065] Preferably, the binder material is composed of a hydrophobic material and a binder.

[0066] Preferably, in the binder material, the mass proportion of the hydrophobic material is 10% to 90%, and preferably the mass proportion of the hydrophobic material is 20% to 80%.

[0067] Preferably, the hydrophobic material includes one or more of tetrafluoroethylene, methyl methacrylate, perfluoroalkyl methacrylate, styrene, epoxy resin, 1,4-bis[(3,4-dioctyloxybenzene)-biamide group] benzene, polyethylene, polystyrene, polyvinylidene fluoride, polyoctyl methacrylate, polyacrylonitrile, ethylene-acrylate copolymer, styrene-ethylene copolymer, vinylidene fluoride-styrene copolymer, silicone-modified polyurethane, perfluoroalkyl chain organic matter, etc.

[0068] Preferably, the binder includes one or more of polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and polyethersulfone (PES).

[0069] Preferably, the positive electrode active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium ferrite, and lithium nickelate.

[0070] Preferably, the conductive agent includes one or more of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0071] In the second aspect, the present application provides a method for preparing a positive electrode plate for preparing the positive electrode plate provided in the first aspect. The preparation method includes the following steps:

[0072] (1) Coat the coating of the first positive electrode coating 2 on the positive electrode current collector 1 and dry it to form the first positive electrode coating 2 on the positive electrode current collector 1, and obtain an intermediate positive electrode plate.

[0073] (2) Use a laser to punch holes and / or draw lines on the surface of the first positive electrode coating 2 of the middle positive electrode sheet, so as to obtain a wetting structure 4 on the surface of the first positive electrode coating 2 away from the positive electrode current collector 1.

[0074] (3) Coat the paint of the second positive electrode coating 3 on the first positive electrode coating 2 and dry it to form the second positive electrode coating 3 on the first positive electrode coating 2, thereby obtaining a positive electrode sheet.

[0075] In the third aspect of the present application, a lithium-ion battery is provided. The lithium-ion battery includes the positive electrode sheet provided in the first aspect.

[0076] Next, the positive electrode sheet of the present application will be described in detail through several specific embodiments.

[0077] Example 1

[0078] The positive electrode sheet of this embodiment is prepared through the following steps:

[0079] (1) Prepare the slurry of the first positive electrode coating

[0080] Mix 97.4 wt% of lithium iron phosphate as the positive electrode active material, 1.8 wt% of the binder material, and 0.8 wt% of conductive carbon black evenly, and use N-methylpyrrolidone as the solvent to make a first slurry with a solid content of 61 wt%. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene], and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene]=7:3.

[0081] (2) Prepare the slurry of the second positive electrode coating

[0082] Mix 97.7 wt% of lithium iron phosphate, 1.8 wt% of the binder material, and 0.5 wt% of conductive carbon black evenly, and use N-methylpyrrolidone as the solvent to make a second slurry with a solid content of 61 wt%. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene], and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene]=8:2.

[0083] In this embodiment, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is 1.6.

[0084] (3) Prepare the positive electrode sheet

[0085] The prepared first slurry is evenly coated on both sides of the aluminum foil (the positive electrode current collector). After drying, a first positive electrode coating is formed on the surface of the aluminum foil to obtain an intermediate positive electrode sheet. The thickness of the single-layer first positive electrode coating of the intermediate positive electrode sheet is 40 μm. After being compacted by a roll press, a laser is used to uniformly score the surface of the first positive electrode coating of the intermediate positive electrode sheet, and the scoring depth is 20 μm, so as to obtain a plurality of line gaps (i.e., infiltration structures) on the surface of the first positive electrode coating away from the positive electrode current collector. The plurality of line gaps are spaced along the width direction of the first positive electrode coating, and the total cross-sectional area of the infiltration structure is 60% of the area of the first positive electrode coating. The second slurry is evenly coated on the surface of the first positive electrode coating. After drying, a second positive electrode coating is formed on the surface of the first positive electrode coating. Among them, the thickness of the second positive electrode coating is 40 μm, and a positive electrode sheet is obtained after being compacted by a roll press.

[0086] Example 2

[0087] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the structure of the infiltration structure is different.

[0088] Specifically, the infiltration structure in this embodiment is a round hole. A plurality of round holes are arranged in an array, and the diameter of the round hole is 15 mm. The total cross-sectional area of the infiltration structure is 60% of the area of the first positive electrode coating.

[0089] Example 3

[0090] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the structure of the infiltration structure is different.

[0091] Specifically, the infiltration structure in this embodiment is a rectangular hole. A plurality of rectangular holes are arranged in an array, and the width of the rectangular hole is 15 mm and the length is 25 mm. The total cross-sectional area of the infiltration structure is 60% of the area of the first positive electrode coating.

[0092] Example 4

[0093] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the depth of the infiltration structure is different.

[0094] Specifically, the depth of the infiltration structure in this embodiment is 8 μm.

[0095] Example 5

[0096] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the depth of the infiltration structure is different.

[0097] Specifically, the depth of the infiltration structure in this embodiment is 5 μm.

[0098] Example 6

[0099] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the depth of the infiltration structure is different.

[0100] Specifically, the depth of the infiltration structure in this embodiment is 45 μm.

[0101] Example 7

[0102] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 2. The difference between this embodiment and Embodiment 2 is only that: the ratio of the total cross-sectional area of the infiltration structure to the area of the first positive electrode coating is different.

[0103] Specifically, the total cross-sectional area of the infiltration structure in this embodiment is 40% of the area of the first positive electrode coating.

[0104] Example 8

[0105] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 2. The difference between this embodiment and Embodiment 2 is only that: the ratio of the total cross-sectional area of the infiltration structure to the area of the first positive electrode coating is different.

[0106] Specifically, the total cross-sectional area of the infiltration structure in this embodiment is 50% of the area of the first positive electrode coating.

[0107] Example 9

[0108] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 2. The difference between this embodiment and Embodiment 2 is only that: the ratio of the total cross-sectional area of the infiltration structure to the area of the first positive electrode coating is different.

[0109] Specifically, the total cross-sectional area of the infiltration structure in this embodiment is 70% of the area of the first positive electrode coating.

[0110] Example 10

[0111] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 2. The difference between this embodiment and Embodiment 2 is only that: the ratio of the total cross-sectional area of the infiltration structure to the area of the first positive electrode coating is different.

[0112] Specifically, the total cross-sectional area of the infiltration structure in this embodiment is 80% of the area of the first positive electrode coating.

[0113] Example 11

[0114] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the component ratios in the first positive electrode coating and the second positive electrode coating are different.

[0115] Specifically, the components and ratios of the first positive electrode coating in this embodiment are as follows: 98 wt% of lithium iron phosphate, 1.4 wt% of binder material, and 0.6 wt% of conductive carbon black. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene, and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 7:3.

[0116] The components and ratios of the second positive electrode coating in this embodiment are as follows: 98 wt% of lithium iron phosphate, 1.5 wt% of binder material, and 0.5 wt% of conductive carbon black. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene, and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 8:2.

[0117] Among them, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is 1.2.

[0118] Example 12

[0119] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the component ratios in the first positive electrode coating and the second positive electrode coating are different.

[0120] Specifically, the components and ratios of the first positive electrode coating in this embodiment are as follows: 92 wt% of lithium iron phosphate, 5 wt% of binder material, and 3 wt% of conductive carbon black. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene, and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 7:3.

[0121] The components and ratios of the second positive electrode coating in this embodiment are as follows: 94 wt% of lithium iron phosphate, 4 wt% of binder material, and 2 wt% of conductive carbon black. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene, and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene = 8:2.

[0122] Among them, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is 1.5.

[0123] Example 13

[0124] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the component ratios in the first positive electrode coating and the second positive electrode coating are different.

[0125] Specifically, the components and ratios of the first positive electrode coating in this embodiment are as follows: 94 wt% of lithium iron phosphate, 5 wt% of binder material, and 1 wt% of conductive carbon black. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene, and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene = 7:3.

[0126] The components and ratios of the second positive electrode coating in this embodiment are as follows: 95 wt% of lithium iron phosphate, 4 wt% of binder material, and 1 wt% of conductive carbon black. Among them, the binder material is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene, and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biacylamino]benzene = 8:2.

[0127] Among them, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is 1.0.

[0128] Example 14

[0129] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the component ratios in the first positive electrode coating and the second positive electrode coating are different.

[0130] Specifically, the components and ratios of the first positive electrode coating in this embodiment are as follows: lithium iron phosphate 94.9 wt%, binder 3 wt%, conductive carbon black 2.1 wt%. Among them, the binder is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene], and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] = 7:3.

[0131] The components and ratios of the second positive electrode coating in this embodiment are as follows: lithium iron phosphate 95.5 wt%, binder 2.5 wt%, conductive carbon black 2 wt%. Among them, the binder is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene], and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] = 8:2.

[0132] Among them, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is 1.05.

[0133] Example 15

[0134] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that: the component ratios in the first positive electrode coating and the component ratios in the second positive electrode coating are different.

[0135] Specifically, the components and ratios of the first positive electrode coating in this embodiment are as follows: lithium iron phosphate 95 wt%, binder 4.4 wt%, conductive carbon black 0.6 wt%. Among them, the binder is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene], and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] = 7:3.

[0136] The components and ratios of the second positive electrode coating in this embodiment are as follows: lithium iron phosphate 96 wt%, binder 3.5 wt%, conductive carbon black 0.5 wt%. Among them, the binder is PVDF and 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene], and the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] = 8:2.

[0137] Among them, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is 1.2.

[0138] Example 16

[0139] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the first positive electrode coating is different.

[0140] Specifically, in the binder material of the first positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene]=8:2.

[0141] Example 17

[0142] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the first positive electrode coating is different.

[0143] Specifically, in the binder material of the first positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene]=5:5.

[0144] Example 18

[0145] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the first positive electrode coating is different.

[0146] Specifically, in the binder material of the first positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene]=2:8.

[0147] Example 19

[0148] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the first positive electrode coating is different.

[0149] Specifically, in the binder material of the first positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene] is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamidobenzene]=1:9.

[0150] Example 20

[0151] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the first positive electrode coating is different.

[0152] Specifically, in the binder material of the first positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 9:1.

[0153] Example 21

[0154] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the second positive electrode coating is different.

[0155] Specifically, in the binder material of the second positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 5:5.

[0156] Example 22

[0157] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the second positive electrode coating is different.

[0158] Specifically, in the binder material of the second positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 2:8.

[0159] Example 23

[0160] The preparation method of the positive electrode sheet in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the second positive electrode coating is different.

[0161] Specifically, in the binder material of the second positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 9:1.

[0162] Example 24

[0163] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the proportion of the hydrophobic material in the binder material in the second positive electrode coating is different.

[0164] Specifically, in the binder material of the second positive electrode coating in this embodiment, the mass ratio of PVDF to 1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene is PVDF:1,4-bis[(3,4-dioctyloxybenzene)-biamide]benzene = 1:9.

[0165] Example 25

[0166] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the components of the first positive electrode coating and the second positive electrode coating are different.

[0167] Specifically, in the first positive electrode coating and the second positive electrode coating in this embodiment, the hydrophobic material is styrene, the binder is polyacrylic acid (PAA), the positive electrode active material is lithium iron manganese phosphate, and the conductive agent is multi-walled carbon nanotubes.

[0168] Example 26

[0169] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the components of the first positive electrode coating and the second positive electrode coating are different.

[0170] Specifically, in the first positive electrode coating and the second positive electrode coating in this embodiment, the hydrophobic material is tetrafluoroethylene, the binder is polyethersulfone (PES), the positive electrode active material is lithium iron phosphate, and the conductive agent is conductive carbon black.

[0171] Example 27

[0172] The preparation method of the positive electrode plate in this embodiment is the same as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is only that the components of the first positive electrode coating and the second positive electrode coating are different.

[0173] Specifically, in the first positive electrode coating and the second positive electrode coating in this embodiment, the hydrophobic material is perfluoroalkyl methacrylate, the binder is polyvinylidene fluoride (PVDF), the positive electrode active material is lithium nickelate, and the conductive agent is single-walled carbon nanotubes.

[0174] Comparative Example 1

[0175] The preparation method of the positive electrode plate in this comparative example is the same as that in Embodiment 1. The difference between this comparative example and Embodiment 1 is only that this comparative example does not have an infiltration structure, that is, wire laying is not carried out in step (3).

[0176] Comparative Example 2

[0177] The preparation method of the positive electrode plate of this comparative example is the same as that of Example 1. The difference between this comparative example and Example 1 is only that: this comparative example is not provided with an infiltration structure and does not include a second positive electrode coating, and the thickness of the first positive electrode coating is 80 μm.

[0178] Comparative Example 3

[0179] The preparation method of the positive electrode plate of this comparative example is the same as that of Example 1. The difference between this comparative example and Example 1 is only that: the binder material of this comparative example is only PVDF and does not include a hydrophobic material.

[0180] The positive electrode plates, negative electrode plates and separators of the above-mentioned examples and comparative examples are respectively used to form a bare battery cell by winding, and aluminum pole ears and nickel-plated copper pole ears are respectively led out. The bare battery cell is clamped with a glass clamp, and the force of the glass clamp is about 100 MPa / m 2 , and vacuum baked at 85 °C for 24 hours, and then encapsulated with an aluminum-plastic film. After encapsulation, the battery is subjected to processes such as formation, aging, and grading to make a lithium-ion battery.

[0181] Among them, the negative electrode plate is prepared by the following steps: 96.4 wt% of negative electrode active material graphite, 1.2 wt% of styrene-butadiene rubber, 0.6 wt% of sodium carboxymethylcellulose, 0.8 wt% of sodium polyacrylate and 1 wt% of conductive carbon black are mixed evenly, and deionized water is used as a solvent to make a negative electrode slurry with a solid content of 55 wt%. The negative electrode slurry is evenly coated on both sides of the copper foil, and after drying and compaction by a roll press, a negative electrode plate is obtained. The single-layer coating thickness of the dried negative electrode slurry is 75 μm.

[0182] The electrolyte uses a lithium hexafluorophosphate electrolyte containing 1 M, and the solvent is a mixed solvent of ethylene carbonate / dimethyl carbonate / 1,2-propylene glycol carbonate mixed in a volume ratio of 1:1:1.

[0183] Comparative Example 4

[0184] The preparation method of the positive electrode plate of this comparative example is the same as that of Example 1. The difference between this comparative example and Example 1 is only that: this comparative example does not include a second positive electrode coating, the thickness of the first positive electrode coating is 80 μm, and the depth of the infiltration structure of this comparative example is 70 μm.

[0185] Test Example 1

[0186] The positive electrode sheets prepared in the examples and comparative examples were die-cut, and the moisture content of the electrode sheets was measured and recorded as the moisture before baking. Then, the battery cells before encapsulation and before electrolyte injection were placed in a contact oven and baked at 90 °C for a specified time (the specified time is shown in Table 1). After baking, the moisture content of the positive electrode sheets of the battery cells was measured and recorded as the moisture after baking. The test results are shown in Table 1.

[0187] Table 1 Moisture content of the positive electrode sheets of the examples and comparative examples before and after baking

[0188] It can be seen from the data in Table 1 that:

[0189] By comparing Examples 1 to 27 with Comparative Examples 1 to 4, the moisture content of the positive electrode sheets of Examples 1 to 27 and Comparative Examples 1, 2, and 4 before baking was much lower than that of the positive electrode sheet of Comparative Example 3 before baking. Moreover, the moisture content of the positive electrode sheets of Examples 1 to 27 and Comparative Examples 1, 2, and 4 after baking for 4 hours was much lower than that of the positive electrode sheet of Comparative Example 3 after baking for 12 hours. From this, it can be known that adding a hydrophobic material to the positive electrode coating can effectively reduce the moisture in the positive electrode sheet and can effectively shorten the time required for baking the electrode sheet and the battery cell.

[0190] Test Example 2

[0191] The lithium-ion batteries prepared in the examples and comparative examples were tested for their electrochemical performance. The test results are shown in Table 2.

[0192] Among them, the detection items include:

[0193] (1) 25 °C normal temperature cycling experiment: The lithium-ion batteries prepared in Examples 1 to 27 and Comparative Examples 1 to 4 were placed in an environment of (25 ± 2) °C and left to stand for 2 to 3 hours. When the battery body reached (25 ± 2) °C, the battery was charged at a constant power of 0.15P to the upper limit voltage of 3.65V. After the battery was fully charged, it was left to stand for 10 min, and then discharged at a constant power of 0.15P to the cut-off voltage of 2.5V. The highest discharge energy of the first three cycles was recorded as the initial energy Q1. When the cycling reached the required number of times, the last discharge energy Q2 of the battery was recorded; the recording results are shown in Table 2. The energy retention rate was calculated as follows: Energy retention rate (%) = Q2 ÷ Q1 * 100%.

[0194] (2) 45°C High-Temperature Cycling Experiment: The lithium-ion batteries prepared in Examples 1 to 27 and Comparative Examples 1 to 4 were placed in an environment of (45 ± 2)°C and left standing for 2 to 3 hours. When the battery body reached (45 ± 2)°C, the battery was charged at a constant power of 0.15P to the upper limit voltage of 3.65V. After the battery was fully charged, it was left standing for 10 minutes, and then discharged at a constant power of 0.15P to the cut-off voltage of 2.5V. The highest discharge energy of the first three cycles was recorded as the initial energy Q1. When the cycle reached the required number of times, the last discharge energy Q2 of the battery was recorded; the recording results are shown in Table 2. The energy retention rate was calculated as follows: Energy retention rate (%) = Q2 ÷ Q1 * 100%.

[0195] (3) 45°C High-Temperature Storage Experiment: The lithium-ion batteries prepared in Examples 1 to 27 and Comparative Examples 1 to 4 were placed in an environment of (25 ± 2)°C and left standing for 2 to 3 hours. When the battery body reached (25 ± 2)°C, the battery was charged at a constant power of 0.15P to the upper limit voltage of 3.65V. After the battery was fully charged, it was left standing for 10 minutes, and then discharged at a constant power of 0.15P to the cut-off voltage of 2.5V. The discharge energy at this time was recorded as the initial energy Q1. Subsequently, the battery was charged at a constant power of 0.15P to the upper limit voltage of 3.65V; the charging energy at this time was recorded as the initial charging energy Q4. Then, the lithium-ion batteries prepared in Examples 1 to 27 and Comparative Examples 1 to 4 were placed in an environment of (45 ± 2)°C and stored for 280 days. Among them, they were taken out once every 30 days. After being taken out, they were left standing for 2h until the battery temperature reached room temperature, and then discharged at a constant power of 0.15P to 2.5V, and the energy was recorded. Then, it was left standing for 10 minutes, and then charged at a constant power of 0.15P to the upper limit voltage of 3.65V, left standing for 10 minutes, and then discharged at a constant power of 0.15P to the lower limit voltage of 2.5V, and the energy was recorded. After leaving standing for 10 minutes, it was charged at a constant power of 0.15P to the upper limit voltage of 3.65V, and then placed back in an environment of (45 ± 2)°C for continued storage; the total storage time was 270 days. The lithium-ion batteries of Examples 1 to 27 and Comparative Examples 1 to 3 after 270 days of storage were placed in an environment of (25 ± 2)°C and left standing for 2 to 3 hours. When the battery body reached (25 ± 2)°C, it was discharged at a constant power of 0.15P to the cut-off voltage of 2.5V. The discharge energy at this time was recorded as the final discharge energy Q2. The battery was charged at a constant power of 0.15P to the upper limit voltage of 3.65V, and the charging energy Q3 at this time was recorded. The recording results are shown in Table 2. The energy retention rate was calculated as follows: Energy retention rate (%) = Q2 ÷ Q1 * 100%. Energy recovery rate (%) = Q3 ÷ Q4 * 100%.

[0196] Table 2 Electrochemical Performance Test Data of Examples and Comparative Examples

[0197] It can be seen from the data in Table 2 that:

[0198] 1. Comparing Examples 1 - 27 with Comparative Example 1, Comparative Example 2, and Comparative Example 4, the data of the energy retention rate after 500 cycles at 25°C, the energy retention rate after 500 cycles at 45°C, the energy retention rate after 270 - day high - temperature storage at 45°C, and the energy recovery rate of Examples 1 - 27 are all better than those of Comparative Example 1, Comparative Example 2, and Comparative Example 4. Thus, it can be known that by providing a wetting structure on the first positive electrode coating and making the mass content of the conductive agent in the first positive electrode coating higher than that in the second positive electrode coating, the positive electrode sheet of the present application can effectively improve the battery performance.

[0199] 2. Comparing Examples 1 - 27 with Comparative Example 3, the data of the energy retention rate after 500 cycles at 25°C, the energy retention rate after 500 cycles at 45°C, the energy retention rate after 270 - day high - temperature storage at 45°C, and the energy recovery rate of Examples 1 - 27 are all better than those of Comparative Example 3. Thus, it can be known that adding a hydrophobic material to the positive electrode sheet of the present application can significantly improve the cycling performance of the battery cell and also has a certain improvement on the high - temperature storage performance.

[0200] 3. Comparing Example 1 with Examples 4 - 6, the data of the energy retention rate after 500 cycles at 25°C, the energy retention rate after 500 cycles at 45°C, the energy retention rate after 270 - day high - temperature storage at 45°C, and the energy recovery rate of Example 5 are worse than those of Example 1 and Example 4. The energy retention rates after 500 cycles at 25°C and 45°C of Example 6 are better than those of Example 1 and Example 4, while the data of the energy retention rate after 270 - day high - temperature storage at 45°C and the energy recovery rate of Example 6 are worse than those of Example 1 and Example 4. Thus, it can be known that when punching and / or wiring, if the depth is too small, it will affect both the cycling performance and the high - temperature storage performance, and if the depth is too large, although the cycling performance is improved, the high - temperature storage performance will decrease instead. Therefore, the depth of the wetting structure needs to be controlled within a certain range. Preferably, the depth of the wetting structure is not less than 1 / 5 of the thickness of the first positive electrode coating, and the wetting structure does not penetrate the first positive electrode coating.

[0201] 4. Comparing Example 1 with Examples 7 - 10, the data of Example 7 are worse than those of Example 1, Examples 8 - 10, and the data of Example 8 are worse than those of Example 1, Examples 9 - 10. Thus, it can be known that in practical applications, to ensure good cycling performance and high - temperature storage performance, the area ratio of the wetting structure needs to be controlled. Preferably, the total cross - sectional area of the wetting structure is 40% - 80% of the area of the first positive electrode coating, and more preferably 60% - 80%.

[0202] 5. Comparing Example 1 with Examples 11 - 15, the data of Example 13 is worse than that of Example 1, Examples 11 - 12, and Examples 14 - 15. The data of Example 14 is worse than that of Example 1, Examples 11 - 12, and Example 15. Thus, it can be seen that in practical applications, to ensure good cycle performance and high-temperature storage performance, it is necessary to control the dosages of the conductive agent in the first positive electrode coating and the conductive agent in the second positive electrode coating. Preferably, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is ≥1, and more preferably, the ratio of the mass percentage of the conductive agent in the first positive electrode coating to the mass percentage of the conductive agent in the second positive electrode coating is ≥1.05.

[0203] 6. Comparing Example 1 with Examples 16 - 20, the data of Examples 19 and 20 is worse than that of Example 1 and Examples 16 - 18. At the same time, comparing Example 1 with Examples 21 - 24, the data of Examples 23 and 24 is worse than that of Example 1 and Examples 21 - 22. Thus, it can be seen that in practical applications, to ensure good cycle performance and high-temperature storage performance, it is necessary to control the proportion of the hydrophobic material in the first positive electrode coating and the second positive electrode coating. Preferably, the mass percentage of the hydrophobic material in the binder is 10% - 90%, and more preferably 20% - 80%.

[0204] The positive electrode tab of the present application includes a positive electrode current collector, a first positive electrode coating, and a second positive electrode coating that are sequentially stacked. By providing a wetting structure on the first positive electrode coating and making the dosage of the conductive agent in the first positive electrode coating higher than that in the second positive electrode coating, the wetting effect of the electrolyte on the first positive electrode coating can be improved, the wettability of the first positive electrode coating in the tab can be increased, the kinetics can be increased, and the insufficient utilization rate of active lithium and the loss of active material caused by low wetting and kinetics of the coating near the current collector side in the later stage of cycling can be reduced; at the same time, the kinetics of the second positive electrode coating is relatively reduced, and the problem of increased side reactions and increased loss of active lithium caused by high activity of the coating on the side far from the current collector is reduced, side reactions are reduced, the uniformity of the kinetics of the entire positive electrode tab can be ensured, polarization can be effectively reduced, and thus the battery performance can be improved.

[0205] In addition, both the first positive electrode coating and the second positive electrode coating include a hydrophobic material. On the one hand, it can shorten the time required for baking the tab or the battery cell, reduce costs. On the other hand, it can reduce the moisture content in the positive electrode tab, reduce the side reactions of the battery cell caused by moisture, and further improve the cycle life of the battery cell.

[0206] So far, the technical solution of the present application has been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present application is obviously not limited to these specific embodiments. Without departing from the principle of the present application, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode plate comprises a positive electrode current collector (1), a first positive electrode coating (2) and a second positive electrode coating (3), wherein the first positive electrode coating (2) is arranged between the positive electrode current collector (1) and the second positive electrode coating (3); The first positive electrode coating (2) is provided with a wetting structure (4) extending from the surface toward the positive electrode current collector (1) on a side close to the second positive electrode coating (3); The mass content of the conductive agent in the first positive electrode coating (2) is higher than the mass content of the conductive agent in the second positive electrode coating (3).

2. The positive electrode sheet according to claim 1, characterized in that: The wetting structure (4) is a plurality of holes (41) and / or a plurality of linear gaps (42).

3. The positive electrode sheet according to claim 2, characterized in that: The depth of the wetting structure (4) is not less than 1 / 8 of the thickness of the first positive electrode coating (2), preferably the depth of the wetting structure (4) is not less than 1 / 5 of the thickness of the first positive electrode coating (2); And / or, the wetting structure (4) does not penetrate the first positive electrode coating (2) in the thickness direction of the first positive electrode coating (2).

4. The positive electrode sheet according to claim 2, characterized in that: The infiltration structure (4) is a plurality of holes (41), and the plurality of holes (41) are arranged in a matrix; Alternatively, the wetting structure (4) is a plurality of line gaps (42), and the plurality of line gaps (42) are distributed at intervals along the width direction of the first positive electrode coating (2).

5. The positive electrode sheet according to claim 4, characterized in that: In the first direction and / or the second direction, the distance between two adjacent holes (41) is not less than 15 mm, wherein the first direction is perpendicular to the second direction.

6. The positive electrode sheet according to claim 2, characterized in that: The total cross-sectional area of ​​the wetting structure (4) is 40% to 80% of the area of ​​the first positive electrode coating (2), preferably the total cross-sectional area of ​​the wetting structure (4) is 60% to 80% of the area of ​​the first positive electrode coating (2); And / or, the shape of the hole (41) includes at least one of a circle, a square, a rectangle, a triangle, a trapezoid or a rhombus.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The first positive electrode coating (2) comprises a positive electrode active material, a binding material and a conductive agent, wherein the mass percentage of the positive electrode active material is 92wt% to 98wt%, the mass percentage of the binding material is 1.4wt% to 5wt%, and the mass percentage of the conductive agent is 0.6wt% to 3wt%; The second positive electrode coating (3) comprises a positive electrode active material, a binding material and a conductive agent, wherein the mass percentage of the positive electrode active material is 94wt% to 98wt%, the mass percentage of the binding material is 1.5wt% to 4wt%, and the mass percentage of the conductive agent is 0.5wt% to 2wt%.

8. The positive electrode sheet according to claim 7, characterized in that: The ratio of the mass percentage of the conductive agent in the first positive electrode coating (1) to the mass percentage of the conductive agent in the second positive electrode coating (2) is ≥1, preferably the ratio of the mass percentage of the conductive agent in the first positive electrode coating (1) to the mass percentage of the conductive agent in the second positive electrode coating (2) is ≥1.05; And / or, the bonding material consists of a hydrophobic material and a bonding agent.

9. The positive electrode sheet according to claim 8, characterized in that: In the bonding material, the mass proportion of the hydrophobic material is 10% to 90%, and preferably the mass proportion of the hydrophobic material is 20% to 80%.

10. The positive electrode sheet according to claim 8, characterized in that: The hydrophobic material includes one or more of tetrafluoroethylene, methyl methacrylate, perfluoroalkyl methacrylate, styrene, epoxy resin, 1,4-bis[(3,4-dioctyloxybenzene)-biphenylamide]benzene, polyethylene, polystyrene, polyvinylidene fluoride, polyoctyl methacrylate, polyacrylonitrile, ethylene-acrylate copolymer, styrene-ethylene copolymer, vinylidene fluoride-styrene copolymer, silicone-modified polyurethane, and perfluoroalkyl chain organic matter; And / or, the binder includes one or more of polyvinylidene fluoride, polyacrylic acid, and polyether sulfone; And / or, the positive electrode active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium ferrite, and lithium nickelate; And / or, the conductive agent includes one or more of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

11. A method for preparing a positive electrode sheet, used for preparing the positive electrode sheet according to any one of claims 1 to 10, characterized in that: The following steps are involved: (1) coating a first positive electrode coating (2) on a positive electrode current collector (1) and drying the coating to form the first positive electrode coating (2) on the positive electrode current collector (1) to obtain an intermediate positive electrode sheet; (2) using a laser to punch holes and / or lines on the surface of the first positive electrode coating (2) of the intermediate positive electrode sheet, so as to obtain a wetting structure (4) on the surface of the first positive electrode coating (2) away from the positive electrode current collector (1); (3) coating the second positive electrode coating (3) on the first positive electrode coating (2) and drying the coating to form the second positive electrode coating (3) on the first positive electrode coating (2) to obtain a positive electrode sheet.

12. A lithium ion battery, characterized in that: A positive electrode sheet comprising the positive electrode sheet according to any one of claims 1 to 10 or a positive electrode sheet prepared by the method for preparing the positive electrode sheet according to claim 11.

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