Positive plate and lithium ion secondary battery

Abstract

The invention relates to the field of lithium ion batteries, and discloses a positive plate and a lithium ion secondary battery. According to the positive plate, the bottom coating and the surface coating are arranged at one end of the length direction of the positive plate and both comprise inorganic particles with the particle size not exceeding 500nm, so that the bottom coating can be ensured to prevent a contact short circuit between a positive current collector and a negative active material layer in a needling test; in the hot box test, the surface coating can isolate the contact between the positive plate on the outer side of the battery cell and the electrolyte, so that the heat generated by side reaction is reduced, and the overall safety of the battery cell is effectively improved. Moreover, inorganic particles with relatively small particle sizes are adopted, so that the thickness of the coating can be reduced, the ion transmission path is shortened, and the rate capability of the battery cell is improved; meanwhile, only one end of the positive plate is provided with the bottom coating and the surface coating, so that the volume energy density loss caused by overlarge thickness of the battery cell can be avoided, and the electrical property of the battery is ensured.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a positive electrode sheet and a lithium-ion secondary battery. Background Art

[0002] In recent years, lithium-ion secondary batteries have been widely used in daily life, for example, in electronic products such as smart phones and laptops, as well as in transportation tools such as electric bicycles and electric cars. As the capacity of lithium-ion secondary battery cells increases, higher requirements are placed on the safety performance of lithium-ion secondary batteries.

[0003] At present, the safety performance tests for lithium-ion secondary batteries mainly include mechanical abuse represented by needle puncture and thermal abuse represented by hot box test. In order to improve the pass rate of the battery in the above tests, the relevant technology discloses setting a primer on the positive / negative current collector to reduce the possibility of battery combustion, fire and explosion, or setting a topcoat on the surface of the positive / negative active paste to improve the thermal stability of the battery. Since the existing primer or topcoat is coated on the entire surface of the pole piece, this will affect the volume energy density of the battery, which is not conducive to the performance of the battery's electrical performance, and only coating the primer or topcoat cannot effectively improve the overall safety of the battery. Summary of the invention

[0004] In view of this, the present application provides a positive electrode sheet and a lithium-ion secondary battery, aiming to effectively improve the safety performance of the battery without affecting the energy density of the battery.

[0005] According to an embodiment of the present application, in a first aspect, a positive electrode sheet is provided, comprising a positive electrode current collector and a positive electrode active layer disposed on at least one side surface of the positive electrode current collector, wherein a primer layer and a topcoat layer are disposed at one end of the positive electrode active layer along the length direction of the positive electrode sheet;

[0006] The undercoat layer is located between the positive electrode current collector and the positive electrode active layer, and the undercoat layer includes first inorganic particles, a first binder and a first conductive agent, and the median particle size of the first inorganic particles is Dv50 1 ≤500nm;

[0007] The topcoat layer is located on the surface of the positive electrode active layer away from the positive electrode current collector, and the topcoat layer includes second inorganic particles and a second binder, and the median particle size Dv50 of the second inorganic particles is 2 ≤500nm.

[0008] In some optional embodiments, the positive electrode active layer includes a positive electrode active material, and the median particle size Dv50 of the positive electrode active material is 3 With the Dv50 1and the Dv50 2 satisfies between: Dv50 3 ≥100 * (Dv50 1 + Dv50 2 ).

[0009] Furthermore, 100nm ≥ Dv50 1 ≥ 10nm.

[0010] Furthermore, 100nm ≥ Dv50 2 ≥ 10nm.

[0011] Furthermore, 30μm ≥ Dv50 3 ≥ 5μm.

[0012] In some alternative embodiments, the first inorganic particle and / or the second inorganic particle includes at least one of silica, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

[0013] In some alternative embodiments, the projection of the surface coating and the bottom coating in the thickness direction of the positive electrode sheet at least partially overlap.

[0014] In some alternative embodiments, the surface coating and / or the bottom coating are partially embedded in the positive electrode active layer.

[0015] Furthermore, the depth h 1 of the surface coating embedded in the positive electrode active layer is 0.5μm - 3μm.

[0016] Furthermore, the depth h 2 of the bottom coating embedded in the positive electrode active layer is 0.5μm - 4μm.

[0017] In some alternative embodiments, the adhesion between the positive electrode current collector and the bottom coating is F 1 , the adhesion between the positive electrode current collector and the positive electrode active layer is F 2 , and the adhesion between the positive electrode active layer and the surface coating is F 3 , satisfying: F 1 > F 3 > F 2 .

[0018] Furthermore, the F 1 is 1gf / mm - 10gf / mm.

[0019] Furthermore, the F 2 is 0.1gf / mm - 5gf / mm.

[0020] Furthermore, the F3 is 0.5 gf / mm - 6 gf / mm.

[0021] In some alternative embodiments, based on the mass of the bottom coating, the bottom coating comprises 80% - 99% of a first inorganic particle, 0.5% - 10% of a first binder, and 0.5% - 10% of a first conductive agent.

[0022] In some alternative embodiments, based on the mass of the surface coating, the surface coating comprises 90% - 99% of a second inorganic particle and 1% - 10% of a second binder.

[0023] In some alternative embodiments, based on the mass of the positive electrode active layer, the positive electrode active layer comprises 90% - 99% of a positive electrode active material, 0.5% - 5% of a second conductive agent, and 0.5% - 5% of a third binder.

[0024] In some alternative embodiments, the content of the first binder in the bottom coating is greater than the content of the second binder in the surface coating, and the content of the second binder in the surface coating is greater than the content of the third binder in the positive electrode active layer.

[0025] In some alternative embodiments, on the same side in the thickness direction of the positive electrode sheet, the length L of the bottom coating 1 and the length L of the positive electrode active layer 2 and the length L of the surface coating 3 satisfy: L 2 > L 1 > L 3 .

[0026] Further, the L 1 is 100 mm - 2000 mm.

[0027] Further, the L 2 is 500 mm - 3000 mm.

[0028] Further, the L 3 is 50 mm - 500 mm.

[0029] In some alternative embodiments, at least one of the first binder, the second binder, and the third binder comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - HFP), or their derivatives.

[0030] In some alternative embodiments, the first conductive agent and / or the second conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, metal powder, carbon fiber, and graphene.

[0031] Further, the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

[0032] In some alternative embodiments, the positive electrode active material includes lithium cobaltate, and the lithium cobaltate includes first particles and second particles. The median particle size D 1 of the first particles is 15 μm - 30 μm, and the median particle size D 2 of the second particles is 1 μm - 10 μm.

[0033] According to an embodiment of the present application, in a second aspect, the present application provides a lithium-ion secondary battery, including a wound cell. The wound cell includes the positive electrode sheet, the negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet according to the first aspect of the present application. The positive electrode sheet, the negative electrode sheet, and the separator are stacked and wound; both the bottom coating and the surface coating are located at the winding end of the wound cell, and the surface coating is located on the side of the positive electrode sheet facing the inside of the wound cell.

[0034] In some alternative embodiments, the resistivity of the negative electrode sheet is ρ, and 0.5 Ω·cm ≤ ρ ≤ 60 Ω·cm.

[0035] In some alternative embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes graphite and a silicon-carbon material; based on the mass of the negative electrode active material, the negative electrode active material includes 4% - 30% of the silicon-carbon material.

[0036] In some alternative embodiments, the silicon-carbon material includes a porous carbon framework and silicon particles deposited on the porous carbon framework.

[0037] In some alternative embodiments, along the winding direction of the positive electrode sheet, the positive electrode sheet sequentially includes a third electrode region, a second electrode region, and a first electrode region;

[0038] The first electrode region includes the positive electrode current collector and the bottom coating, the positive electrode active layer, and the surface coating disposed on the surface of the positive electrode current collector facing the inside of the wound cell;

[0039] The second electrode sheet area includes the positive current collector and the bottom coating layer and the positive active material layer disposed on two side surfaces of the positive current collector;

[0040] The third electrode sheet area includes the positive current collector and the positive active material layer disposed on two side surfaces of the positive current collector;

[0041] The surface resistance R of the first electrode sheet area 1 is 1 Ω / cm - 3 Ω / cm, the surface resistance R of the second electrode sheet area 2 is 0.5 Ω / cm - 1.8 Ω / cm, the surface resistance R of the third electrode sheet area 3 is 0.3 Ω / cm - 1.0 Ω / cm, satisfying: R 1 > R 2 > R 3 .

[0042] In some alternative embodiments, the thickness H of the first electrode sheet area 1 is 20 μm - 90 μm, the thickness H of the second electrode sheet area 2 is 20 μm - 160 μm, the thickness H of the third electrode sheet area 3 is 20 μm - 160 μm, satisfying: H 1 > 0.25 * (H 2 + H 3 ).

[0043] Furthermore, the thickness H of the first electrode sheet area 1 and the thickness H of the second electrode sheet area 2 , the thickness H of the third electrode sheet area 3 satisfy: H 1 > 0.25 * (H 2 + H 3 ) + 2.

[0044] The technical solution of the present application has the following advantages:

[0045] The positive electrode sheet provided by the present application is provided with a bottom coating and a surface coating at one end in its length direction. The bottom coating includes first inorganic particles, and the surface coating includes second inorganic particles. The median particle sizes of the first inorganic particles and the second inorganic particles do not exceed 500 nm. By selecting first inorganic particles and second inorganic particles with smaller particle sizes, more inorganic particles can be filled in the coating per unit thickness, so as to ensure that the bottom coating can prevent contact short circuit between the positive electrode current collector and the negative electrode active material layer during the needle penetration test, and the surface coating can isolate the contact between the positive electrode sheet outside the battery cell and the electrolyte during the hot box test, reduce the heat generation of side reactions, and thus effectively improve the overall safety of the battery cell. Moreover, by using first inorganic particles and second inorganic particles with smaller particle sizes, the coating thickness can also be reduced, the ion transmission path can be shortened, and the rate performance of the battery cell can be improved; at the same time, only the bottom coating and the surface coating are provided at one end (i.e., locally) of the positive electrode sheet, which can avoid the loss of volume energy density caused by excessive thickness of the battery cell, thereby ensuring the electrical performance of the battery.

[0046] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the following description, or will be explained through the implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0048] Figure 1 It is a schematic cross-sectional structure diagram of the positive electrode sheet in its length direction in an embodiment of the present application.

[0049] Figure 2 It is a cross-sectional SEM diagram of the surface coating in an embodiment of the present application.

[0050] Figure 3 It is a cross-sectional SEM diagram of the bottom coating in an embodiment of the present application.

[0051] Figure 4 It is a schematic cross-sectional structure diagram of a wound battery cell in an embodiment of the present application.

[0052] Among them, the reference numerals are explained as follows:

[0053] 11. Positive electrode current collector; 12. Positive electrode active layer; 13. Bottom coating; 14. Surface coating; 15. Positive electrode tab; 21. Negative electrode current collector; 22. Negative electrode active layer; 23. Negative electrode tab; 30. Separator; 31. Termination glue;

[0054] I, first pole area; II, second pole area; III, third pole area;

[0055] L 1 is the length of the base coating; L 2 is the length of the positive electrode active layer; L 3 is the length of the surface coating. DETAILED DESCRIPTION

[0056] The following examples are provided for a better understanding of the present application, but are not limited to the best implementation mode described, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the protection scope of the present application.

[0057] It should be noted that in the description of this application, the terms "inside" and "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting this application. The terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0058] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0059] In mechanical abuse tests such as needle puncture, the closer to the outside of the wound battery cell, the greater the deformation caused by the abuse, and the easier it is to trigger an internal short circuit. There are four main modes of short circuits in the battery: one is the contact short circuit between the positive and negative current collectors, the second is the contact short circuit between the positive current collector and the negative active material, the third is the contact short circuit between the positive active material and the negative current collector, and the fourth is the contact short circuit between the positive and negative active materials. Among them, the contact short circuit between the positive current collector and the negative active material is the most dangerous because it generates the highest heat power. In addition, in thermal abuse tests, the outside of the battery cell is heated the fastest and is most likely to cause thermal runaway.

[0060] It is understood that the outer side of the battery cell refers to the side away from the winding center of the wound battery cell as a reference. Correspondingly, the inner side of the battery cell refers to the side toward the winding center of the wound battery cell as a reference.

[0061] In order to improve the overall safety of the wound battery without affecting the electrical performance, according to the first aspect of the present application, a positive electrode sheet is provided, such as Figure 1As shown, it includes a positive current collector 11 and a positive active layer 12 provided on at least one surface of the positive current collector 11. Along the length direction of the positive electrode sheet, a bottom coating 13 and a surface coating 14 are provided at one end of the positive active layer;

[0062] The bottom coating 13 is located between the positive current collector 11 and the positive active layer 12. The bottom coating 13 includes first inorganic particles, a first binder, and a first conductive agent. The median particle size Dv50 of the first inorganic particles 1 ≤500 nm;

[0063] The surface coating 14 is located on the surface of the positive active layer 12 facing away from the positive current collector 11. The surface coating 14 includes second inorganic particles and a second binder. The median particle size Dv50 of the second inorganic particles 2 ≤500 nm.

[0064] By selecting first inorganic particles and second inorganic particles with smaller particle sizes, more inorganic particles can be filled in the coating per unit thickness, so as to ensure that in the needle penetration test, the bottom coating can prevent the contact short circuit between the positive current collector and the negative active material layer, and in the hot box test, the surface coating can isolate the contact between the positive electrode sheet outside the battery cell and the electrolyte, reduce the heat generation of side reactions, and thus effectively improve the overall safety of the battery cell. Moreover, by using first inorganic particles and second inorganic particles with smaller particle sizes, the coating thickness can also be reduced, the ion transport path can be shortened, and the rate performance of the battery cell can be improved; at the same time, by only setting the bottom coating and the surface coating at one end (i.e., locally, not entirely) of the positive electrode sheet, the overall safety of the battery cell can be effectively improved, thereby avoiding the loss of volume energy density caused by excessive battery cell thickness and ensuring the electrical performance of the battery.

[0065] Exemplarily, the median particle size Dv50 of the first inorganic particles 1 can be 10 nm, 40 nm, 70 nm, 100 nm, 300 nm, 500 nm, etc. or within the range composed of any two of the above values. The first inorganic particles include at least one of silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

[0066] Exemplarily, the median particle size Dv50 of the second inorganic particles 2 can be 10 nm, 40 nm, 70 nm, 100 nm, 300 nm, 500 nm, etc. or within the range composed of any two of the above values. The second inorganic particles include at least one of silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

[0067] The positive electrode active layer includes a positive electrode active material. In some embodiments, the median particle size Dv50 of the positive electrode active material is 3 The median particle size Dv50 of the first inorganic particles 1 and the median particle size Dv50 of the second inorganic particles 2 Between: Dv50 3 ≥100*(Dv50 1 +Dv50 2 ). Wherein, the median particle size Dv50 of the positive electrode active material 3 5μm-30μm, exemplary, Dv50 3 It can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, etc., or be within the range formed by any two of the above values.

[0068] Since the particle size of the positive electrode active material is much larger than that of the first / second inorganic particles, the topcoat layer and the bottomcoat layer can be embedded in the positive electrode active layer to a certain extent, thereby preventing the battery cell from affecting its volume energy density due to excessive thickness. At the same time, compared with the coating of large particles, the first / second inorganic particles with smaller particle sizes can reduce the coating thickness of the battery cell with comparable safety performance, shorten the ion transmission path, and improve the rate performance of the battery cell. On the other hand, more inorganic particles can be filled in the unit thickness coating, thereby improving the defense capability of the bottomcoat layer in the needle puncture test and the topcoat layer in the hot box test, thereby improving the overall safety of the battery cell.

[0069] In the present application, the median particle size Dv50 of the first inorganic particles is 1 , the median particle size Dv50 of the second inorganic particle 2 And the median particle size Dv50 of the positive electrode active material 3 Both can be obtained by laser particle size analyzer testing.

[0070] See also Figure 2 and Figure 3 In some embodiments, the thickness of the top layer is 1.98 μm, 3.57 μm, or 3.97 μm, and the thickness of the bottom layer is 2.86 μm, 3.37 μm, or 5.21 μm. Figure 2 and Figure 3 It can be seen that both the topcoat layer and the bottomcoat layer are partially embedded in the positive electrode active layer, and the depth of the topcoat layer embedded in the positive electrode active layer is h 1 is 0.5 μm-3 μm, for example, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, etc., or is within the range of any two of the above values; the depth h of the primer layer embedded in the positive electrode active layer 2is from 0.5 μm to 4 μm, and can be, for example, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, etc., or within the range composed of any two of the above values.

[0071] It should be noted that in this application, the term "embedding depth" represents the degree to which the bottom coating / surface coating is embedded in the positive electrode active layer, and is represented by the difference between the maximum thickness and the minimum thickness of the bottom coating / surface coating in the cross-sectional SEM image at the junction of the bottom coating / surface coating and the positive electrode active layer per unit area (mm 2 ).

[0072] In some embodiments, the projection of the surface coating and the bottom coating in the thickness direction of the positive electrode sheet at least partially overlap. It can be understood that this overlapping area is located at the winding end of the positive electrode sheet because: in the needle penetration test, the closer to the outer side of the wound battery cell, the greater the deformation amplitude caused by needle penetration, and the more likely an internal short circuit occurs. Therefore, a bottom coating needs to be provided at the winding end of the positive electrode sheet (corresponding to the outer side of the wound battery cell); in the hot box test, the outer side of the wound battery cell is heated fastest and is most likely to cause thermal runaway. Therefore, a surface coating needs to be provided at the winding end of the positive electrode sheet. By simultaneously providing a bottom coating and a surface coating at the winding end of the positive electrode sheet, the risk of the battery cell in the safety test can be effectively reduced, the safety performance of the battery cell can be improved, and only at the winding end of the positive electrode sheet, rather than the entire positive electrode sheet, the bottom coating and the surface coating are provided, which can also prevent the battery cell from being too thick and affecting the volume energy density, thereby ensuring the overall safety of the battery cell without affecting its electrical performance.

[0073] In some embodiments, based on the mass of the bottom coating, the bottom coating includes 80% - 99% of the first inorganic particles, 0.5% - 10% of the first binder, and 0.5% - 10% of the first conductive agent. The above % is the mass percentage content.

[0074] As an example, the first binder includes at least one of polyacrylic acid (PAA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyolefins (such as polypropylene PP, polyethylene PE, and other olefin copolymers), fluorinated rubber, polyimide (PI), polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - HFP), or their derivatives. As an example, the first conductive agent includes at least one of conductive carbon black (SP), acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, metal powder, carbon fiber, graphene, and the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

[0075] The bottom coating uses non-conductive or low-conductive first inorganic particles, which can ensure good insulation when the bottom coating contacts the negative electrode active material layer during the acupuncture test, thereby improving the acupuncture safety of the battery cell. At the same time, to ensure electron conduction between the positive electrode active layer and the positive electrode current collector, the bottom coating also needs to have a certain conductivity. Therefore, a small amount of conductive agent is included in the bottom coating.

[0076] In some embodiments, based on the mass of the surface coating, the surface coating includes 90%-99% of second inorganic particles and 1%-10% of a second binder. The above % is the mass percentage content.

[0077] As an example, the second binder includes at least one of polyacrylic acid (PAA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyolefins (such as polypropylene PP, polyethylene PE and other olefin copolymers), fluorinated rubber, polyimide (PI), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) or their derivatives.

[0078] The surface coating uses non-conductive or low-conductive second inorganic particles, making the surface coating have insulation or low conductivity, which can slow down the oxidation reaction of the electrolyte at high potential, reduce the occurrence of side reactions, and at the same time can increase the internal resistance of the battery cell when out of control, improving the thermal safety of the battery cell.

[0079] In some embodiments, based on the mass of the positive electrode active layer, the positive electrode active layer includes 90%-99% of positive electrode active material, 0.5%-5% of a second conductive agent, and 0.5%-5% of a third binder. The above % is the mass percentage content.

[0080] As an example, the positive electrode active material includes lithium cobaltate; the second conductive agent includes at least one of conductive carbon black (SP), acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, metal powder, carbon fiber, graphene, and the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, aluminum powder; the third binder includes at least one of polyacrylic acid (PAA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyolefins (such as polypropylene PP, polyethylene PE and other olefin copolymers), fluorinated rubber, polyimide (PI), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) or their derivatives.

[0081] In some embodiments, the lithium cobaltate includes a first particle and a second particle, and the median particle size D of the first particle 1 is 15 μm - 30 μm, which can be, for example, 15 μm, 18 μm, 20 μm, 25 μm, 30 μm, etc. or within the range composed of any two of the above values; the median particle size D of the second particle 2 is 1 μm - 10 μm, such as 1 μm, 3 μm, 5 μm, 8 μm, 10 μm, etc. or within the range composed of any two of the above values.

[0082] Using lithium cobaltate with small particles can shorten the diffusion path of lithium ions and, combined with its large specific surface area, is beneficial to improving the rate performance; however, if the particles are too small, it may cause agglomeration between the particles, which will instead affect the lithium ion transmission. And large-particle lithium cobaltate can ensure the high-temperature performance of the battery cell. Therefore, through the particle size grading of the lithium cobaltate particles, on the one hand, the battery cell can simultaneously have good high-temperature and rate performance, and on the other hand, it is beneficial to increase the compaction density of the positive electrode sheet, thereby improving the energy density of the battery cell.

[0083] In this application, the median particle size D of the first particle 1 and the median particle size D of the second particle 2 can both be measured by a laser particle size analyzer.

[0084] In some embodiments, the content of the first binder in the bottom coating is greater than the content of the second binder in the surface coating, and the content of the second binder in the surface coating is greater than the content of the third binder in the positive electrode active layer. Also, the particle sizes of the inorganic particles in the bottom coating and the surface coating are both smaller than the particle size of the positive electrode active material, so that the adhesion force F 1 between the positive electrode current collector and the bottom coating, the adhesion force F 3 between the positive electrode active layer and the surface coating, and the adhesion force F 2 between the positive electrode current collector and the positive electrode active layer 1 satisfy: F 3 >F 2 .

[0085] F 1 and F 3 are both greater than F 2 , indicating that the adhesion between the bottom coating and the positive electrode current collector is tight, and the adhesion between the surface coating and the positive electrode active layer is tight, that is, both the bottom coating and the surface coating show good adhesion, thereby ensuring that the bottom coating and the surface coating can play their respective roles in the acupuncture and hot box tests and improving the overall safety of the battery cell. At the same time, both the bottom coating and the surface coating use inorganic particles with small particle sizes, which can not only shorten the lithium ion transmission path and improve the rate performance of the battery cell, but also avoid the loss of volume energy density caused by excessive battery cell thickness and ensure the electrical performance of the battery.

[0086] Specifically, in some embodiments, the adhesion between the positive current collector and the bottom coating is F 1 , and the F 1 is 1 gf / mm - 10 gf / mm. As an example, the F 1 can be 1 gf / mm, 3 gf / mm, 5 gf / mm, 7 gf / mm, 10 gf / mm, etc., or within the range formed by any two of the above values.

[0087] In some embodiments, the adhesion between the positive current collector and the positive active layer is F 2 , and the F 2 is 0.1 gf / mm - 5 gf / mm. As an example, the F 2 can be 0.1 gf / mm, 0.5 gf / mm, 1 gf / mm, 3 gf / mm, 5 gf / mm, etc., or within the range formed by any two of the above values.

[0088] In some embodiments, the adhesion between the positive active layer and the surface coating is F 3 , and the F 3 is 0.5 gf / mm - 6 gf / mm. As an example, the F 3 can be 0.5 gf / mm, 1 gf / mm, 2 gf / mm, 4 gf / mm, 6 gf / mm, etc., or within the range formed by any two of the above values.

[0089] In some embodiments, referring to Figure 1 , on the same side in the thickness direction of the positive electrode sheet, the length L 1 of the bottom coating 13, the length L 2 of the positive active layer 12, and the length L 3 of the surface coating 14 satisfy: L 2 > L 1 > L 3 .

[0090] By controlling that the lengths of both the bottom coating and the surface coating are less than the length of the positive active layer, that is, by only providing the bottom coating and the surface coating locally on the positive electrode sheet, it is possible to minimize the loss of energy density caused by the increase in thickness while meeting the safety performance requirements. Moreover, compared with the hot box test, the needle penetration test has a greater requirement for the range of the test position of the battery cell. Therefore, it is also necessary to ensure that the length of the bottom coating on the positive electrode sheet is greater than the length of the surface coating.

[0091] It can be understood that the length directions of the positive active layer, the bottom coating, and the surface coating are all consistent with the length direction of the positive electrode sheet, and the length direction of the positive electrode sheet is perpendicular to the thickness direction of the positive electrode sheet.

[0092] Specifically, in some embodiments, the length L of the bottom coating 1 is 100 mm - 2000 mm. As an example, L 1 can be, for example, 100 mm, 300 mm, 500 mm, 800 mm, 1000 mm, 1300 mm, 1500 mm, 1800 mm, 2000 mm, etc., or within the range formed by any two of the above values.

[0093] In some embodiments, the length L of the positive active layer 2 is 500 mm - 3000 mm. As an example, L 2 can be, for example, 500 mm, 800 mm, 1000 mm, 1300 mm, 1500 mm, 1800 mm, 2000 mm, 2300 mm, 2500 mm, 2800 mm, 3000 mm, etc., or within the range formed by any two of the above values.

[0094] In some embodiments, the length L of the surface coating 3 is 50 mm - 500 mm. As an example, L 3 can be, for example, 50 mm, 80 mm, 100 mm, 200 mm, 300 mm, 400 mm, 500 mm, etc., or within the range formed by any two of the above values.

[0095] According to the second aspect of the present application, a lithium-ion secondary battery is provided, including a wound core, as Figure 4 shown. The wound core includes the positive electrode sheet, negative electrode sheet, and separator 30 provided between the positive electrode sheet and the negative electrode sheet according to the first aspect of the present application. The positive electrode sheet, negative electrode sheet, and separator 30 are stacked and wound; also refer to Figure 1 , the bottom coating 13 and the surface coating 14 in the positive electrode sheet are both located at the winding end of the wound core, and the surface coating 13 is located on the side of the positive electrode sheet facing the inside of the wound core.

[0096] In some embodiments, also refer to Figure 4 , a plurality of positive electrode tabs 15 are provided on the positive electrode sheet located in the flat area of the wound core, and a plurality of negative electrode tabs 23 are provided on the negative electrode sheet located in the flat area of the wound core; meanwhile, a termination adhesive 31 is also provided at the winding end of the positive electrode sheet.

[0097] The negative electrode sheet includes a negative electrode current collector 21 and a negative electrode active layer 22 provided on at least one surface of the negative electrode current collector 21. The negative electrode active layer 22 includes a negative electrode active material. In some embodiments of the present application, the negative electrode active material includes graphite and a silicon-carbon material. Based on the mass of the negative electrode active material, the negative electrode active material includes 4%-30% of the silicon-carbon material, such that the resistivity ρ of the negative electrode sheet is relatively large, satisfying 0.5 Ω·cm ≤ ρ ≤ 60 Ω·cm. As an example, ρ can be, for example, 0.5 Ω·cm, 2 Ω·cm, 10 Ω·cm, 20 Ω·cm, 30 Ω·cm, 40 Ω·cm, 50 Ω·cm, 60 Ω·cm, etc. or within the range composed of any two of the above values.

[0098] It can be understood that for a negative electrode sheet containing a silicon-carbon material (i.e., a silicon negative electrode), due to the presence of silicon elements, although it is beneficial to improve the energy density of the battery cell, it will also lead to poor stability of the solid electrolyte interface (SEI) film on the surface of the negative electrode under high-temperature conditions. Therefore, by combining the silicon negative electrode with the positive electrode sheet described in the first aspect of the present application, the defect of poor high-temperature performance of the silicon negative electrode can be compensated, so that the wound battery cell of the present application has the advantages of high energy density, good overall safety, and good rate performance.

[0099] In some embodiments, the silicon-carbon material includes a porous carbon skeleton and silicon particles deposited on the porous carbon skeleton. As an example, the content of the silicon-carbon material in the negative electrode active material can be 4%, 7%, 10%, 15%, 20%, 25%, 30%, etc. or within the range composed of any two of the above values.

[0100] Continue to refer to Figure 1 , the direction indicated by the arrow is the winding direction (i.e., the length direction) of the positive electrode sheet. The positive electrode sheet sequentially includes a third electrode region III, a second electrode region II, and a first electrode region I along its winding direction. Among them, the first electrode region I includes the positive electrode current collector 11 and the bottom coating 13, the positive electrode active layer 12, and the surface coating 14 provided on the surface of the positive electrode current collector 11 facing the inside of the wound battery cell. The second electrode region II includes the positive electrode current collector 11 and the bottom coating 13 and the positive electrode active layer 12 provided on both surfaces of the positive electrode current collector 11. The third electrode region III includes the positive electrode current collector 11 and the positive electrode active layer 12 provided on both surfaces of the positive electrode current collector 11. In addition, an empty foil region is included at the winding end of the positive electrode sheet, that is, no coating is provided on the positive electrode current collector in this region.

[0101] Due to the presence of the bottom coating, the surface resistance of the second electrode region is greater than that of the third electrode region. Thus, the short-circuit current during mechanical abuse such as pinprick can be reduced, heat generation can be decreased, and the pinprick safety of the second electrode region can be improved. Compared with the second electrode region, since the first electrode region further includes an insulating or low-conductive surface coating, its surface resistance is the largest. On the basis of ensuring pinprick safety, the heat generation from side reactions of the electrolyte in this region can be further reduced, that is, the thermal stability and pinprick safety of the first electrode region can be improved simultaneously. Therefore, in the positive electrode sheet of the present application, the surface resistances of the three electrode regions satisfy R 1 >R 2 >R 3 , which can ensure the overall safety of the battery cell. Moreover, thanks to the local setting of the bottom coating and the surface coating on the positive electrode sheet, while the battery cell meets the safety performance, its thickness is reduced, avoiding excessive loss of energy density. In addition, since both the bottom coating and the surface coating adopt inorganic particles with small particle sizes, it can not only shorten the lithium-ion transmission path, improve the rate performance of the battery cell, but also avoid the loss of volumetric energy density caused by excessive thickness of the battery cell, ensuring the electrical performance of the battery.

[0102] In some embodiments, the surface resistance R 1 of the first electrode region is 1 Ω / cm - 3 Ω / cm. As an example, R 1 can be 1 Ω / cm, 1.5 Ω / cm, 2 Ω / cm, 2.5 Ω / cm, 3 Ω / cm, etc. or within the range composed of any two of the above values.

[0103] In some embodiments, the surface resistance R 2 of the second electrode region is 0.5 Ω / cm - 1.8 Ω / cm, and R 2 can be 0.5 Ω / cm, 0.8 Ω / cm, 1 Ω / cm, 1.3 Ω / cm, 1.8 Ω / cm, etc. or within the range composed of any two of the above values.

[0104] In some embodiments, the surface resistance R 3 of the third electrode region is 0.3 Ω / cm - 1.0 Ω / cm, and R 3 can be 0.3 Ω / cm, 0.5 Ω / cm, 0.7 Ω / cm, 0.9 Ω / cm, 1 Ω / cm, etc. or within the range composed of any two of the above values.

[0105] It should be noted that the surface resistance refers to the resistance value of the electrode sheet per unit area.

[0106] In some embodiments, the thickness H 1 of the first electrode region is 20 μm - 90 μm, and the thickness H 2is 20μm - 160μm, the thickness H of the third electrode region 3 is 20μm - 160μm, and satisfies: H 1 > 0.25 * (H 2 + H 3 ).

[0107] It can be understood that for the positive electrode sheet without a bottom coating and a surface coating, the thickness of the single-sided region (i.e., the positive electrode active layer is provided only on one surface of the positive electrode current collector, corresponding to the first electrode region in the present application) is basically equivalent to 1 / 2 of the thickness of the double-sided region (i.e., the positive electrode active layer is provided on both surfaces of the positive electrode current collector, corresponding to the second electrode region or the third electrode region in the present application), that is, 1 / 4 of the sum of the thicknesses of the second electrode region and the third electrode region. By providing a bottom coating and a surface coating in the first electrode region in the present application, the thickness of the first electrode region will be increased, so that the thicknesses of the three electrode regions satisfy H 1 > 0.25 * (H 2 + H 3 ), preferably, satisfies H 1 > 0.25 * (H 2 + H 3 ) + 2. However, in order not to affect the energy density and rate performance of the battery cell, it is necessary to control the thickness of the first electrode region not to be too large, that is, H 1 does not exceed 90μm, so that the overall safety and rate performance of the battery cell can be effectively improved without almost losing the energy density of the battery cell.

[0108] As an example, the thickness H of the first electrode region 1 can be 20μm, 40μm, 60μm, 80μm, 90μm, etc. or within the range composed of any two of the above values, and the thickness H of the second electrode region 2 can be 20μm, 60μm, 80μm, 100μm, 120μm, 140μm, 160μm, etc. or within the range composed of any two of the above values, and the thickness H of the third electrode region 3 can be 20μm, 60μm, 80μm, 100μm, 120μm, 140μm, 160μm, etc. or within the range composed of any two of the above values.

[0109] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed in the present application. For those not specifying specific experimental steps or conditions in the embodiments and comparative examples, the operations or conditions of the conventional experimental steps described in the literature in the art can be followed. For the reagents or instruments without indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase. In all embodiments and comparative examples of the present application, the unit % represents mass percentage content.

[0110] Example 1

[0111] To prepare a positive electrode sheet with the structure as Figure 1 shown, the following steps are included:

[0112] The first step: Prepare the bottom coating

[0113] Take alumina (Dv50 1 is 30 nm), conductive agent carbon black, carbon nanotubes and PVDF in the mass ratio of 90:2:3:5 in sequence. After mixing alumina with carbon black and carbon nanotubes evenly, then add PVDF and solvent N-methylpyrrolidone (NMP) and stir evenly to obtain a bottom coating slurry with a solid content of 30%. Coat the bottom coating slurry on both surfaces of the aluminum foil in the thickness direction by gravure coating, and obtain a positive electrode current collector with a partial bottom coating after drying.

[0114] The second step: Prepare the positive electrode active layer

[0115] Add conductive carbon black and carbon nanotubes into PVDF and stir evenly, then add lithium cobalt oxide (Dv50 3 is 15 μm) and stir evenly to prepare a positive electrode active material slurry with a solid content of 75%. Coat the positive electrode active material slurry on both surfaces of the positive electrode current collector with a partial bottom coating prepared in the first step, and obtain the positive electrode active layer after drying. The mass fraction of lithium cobalt oxide in the positive electrode active layer is 97.6%, the mass fraction of PVDF is 1.05%, and the mass fraction of conductive carbon black and carbon nanotubes is 1.35% (the mass ratio of carbon black to carbon nanotubes is 1:1); among them, lithium cobalt oxide includes a first particle with a particle size D 1 of 20 μm and a second particle with a particle size D 2 of 5 μm.

[0116] For the convenience of description, the two surfaces of the positive electrode sheet are respectively defined as the A surface and the C surface, and the length of the positive electrode active layer on the A surface is greater than that on the C surface.

[0117] The third step: Prepare the surface coating

[0118] Add silicon dioxide (Dv50 2 is 20 nm) into PVDF and stir evenly, then add NMP and stir evenly to prepare a surface coating slurry with a solid content of 25%. Coat the surface coating slurry on the surface of the positive electrode active layer on the A surface prepared in the second step, and obtain the positive electrode sheet after baking and rolling. The mass fraction of silicon dioxide in the surface coating is 90%, and the mass fraction of PVDF is 10%.

[0119] On the A surface of the positive electrode sheet prepared in this example, the length L 1 of the bottom coating is 200 mm, and the length L 2is 1500 mm, and the length L of the surface coating 3 is 120 mm, satisfying L 2 >L 1 >L 3 。The thickness H of the first electrode sheet area 1 is 55 μm, and the thickness H of the second electrode sheet area 2 is 96 μm, and the thickness H of the third electrode sheet area 3 is 90 μm, satisfying: H 1 >0.25 * (H 2 +H 3 ) + 2.

[0120] Prepare a negative electrode sheet, including the following steps:

[0121] Mix 97.3% of the negative electrode active material, 0.5% of the conductive carbon black, 1.3% of the PVDF, and 0.9% of the CMC evenly, and then add an appropriate amount of deionized water to disperse evenly to prepare a negative electrode active material slurry. Based on the mass of the negative electrode active material, the negative electrode active material contains 10% of silicon-carbon and 90% of graphite; coat the negative electrode active material slurry on both side surfaces of the carbon-coated copper foil, and obtain the negative electrode sheet after baking and rolling.

[0122] Prepare a lithium-ion secondary battery, including the following steps:

[0123] Wind the positive and negative electrode sheets after slitting and tabbing with a separator to obtain a wound core, and obtain a lithium-ion secondary battery through encapsulation, baking, liquid injection, formation, secondary encapsulation, sorting, and OCV. Among them, the A side of the positive electrode sheet faces the winding center of the wound core, and the C side faces away from the winding center of the wound core; the electrolyte is a commercially available conventional electrolyte, and the lithium salt in the electrolyte is LiFP 6 。

[0124] Figure 2 is the cross-sectional SEM image of the surface coating prepared in this example. It can be seen from the figure that the surface coating is partially embedded in the positive electrode active layer, and the thickness of the surface coating at different positions can be 1.98 μm, 3.57 μm, 3.97 μm. The difference h 2 between the maximum value and the minimum value of the surface coating thickness per unit area mm 1 of the surface coating is 1.99 μm.

[0125] Figure 3 is the cross-sectional SEM image of the bottom coating prepared in this example. It can be seen from the figure that the bottom coating is partially embedded in the positive electrode active layer, and the thickness of the bottom coating at different positions can be 2.86 μm, 3.37 μm, 5.21 μm. The difference h 2 between the maximum value and the minimum value of the bottom coating thickness per unit area mm 2It is 2.35 μm.

[0126] Example 2

[0127] Except for the following content, the rest is the same as Example 1.

[0128] The bottom coating comprises 80% alumina, 10% PVDF, 4% conductive carbon black, and 6% carbon nanotubes.

[0129] Example 3

[0130] Except for the following content, the rest is the same as Example 1.

[0131] The bottom coating comprises 99% alumina, 0.5% PVDF, and 0.5% carbon nanotubes.

[0132] Example 4

[0133] Except for the following content, the rest is the same as Example 1.

[0134] The bottom coating comprises 95% magnesium oxide, 3% PP, and 2% magnesium metal powder.

[0135] Example 5

[0136] Except for the following content, the rest is the same as Example 1.

[0137] The bottom coating comprises 85% silica, 5% PVA, and 10% carbon fiber.

[0138] Example 6

[0139] Except for the following content, the rest is the same as Example 1.

[0140] The bottom coating comprises 80% alumina, 12% PVDF, 4% conductive carbon black, and 4% carbon nanotubes.

[0141] Example 7

[0142] Except for the following content, the rest is the same as Example 1.

[0143] The bottom coating comprises 89.6% alumina, 0.4% PVDF, 4% conductive carbon black, and 6% carbon nanotubes.

[0144] Example 8

[0145] Except for the following content, the rest is the same as Example 1.

[0146] The top coating comprises 99% silica and 1% PVDF.

[0147] Example 9

[0148] The rest is the same as that of Example 1 except for the following content.

[0149] The surface coating comprises 95% magnesium oxide and 5% PTFE.

[0150] Example 10

[0151] The rest is the same as that of Example 1 except for the following content.

[0152] The surface coating comprises 99.5% silica and 0.5% PVDF.

[0153] Example 11

[0154] The rest is the same as that of Example 1 except for the following content.

[0155] The surface coating comprises 89% silica and 11% PVDF.

[0156] Example 12

[0157] The rest is the same as that of Example 1 except for the following content.

[0158] The particle size Dv50 of aluminum oxide in the bottom coating 1 is 100 nm, the particle size Dv50 of lithium cobalt oxide in the positive electrode active layer 3 is 15 μm, and the particle size Dv50 of silica in the surface coating 2 is 50 nm, satisfying Dv50 3 ≥100*(Dv50 1 +Dv50 2 ).

[0159] Among them, the lithium cobalt oxide comprises a first particle and a second particle, and the median particle size D 1 of the first particle is 15 μm, and the median particle size D 2 of the second particle is 10 μm.

[0160] Example 13

[0161] The rest is the same as that of Example 1 except for the following content.

[0162] The particle size Dv50 of aluminum oxide in the bottom coating 1 is 50 nm, the particle size Dv50 of lithium cobalt oxide in the positive electrode active layer 3 is 30 μm, and the particle size Dv50 of silica in the surface coating 2 is 100 nm, satisfying Dv50 3 ≥100*(Dv50 1 +Dv50 2 ).

[0163] Among them, lithium cobaltate includes a first particle and a second particle, and the median particle size D of the first particle 1 is 30 μm, and the median particle size D of the second particle 2 is 1 μm.

[0164] Example 14

[0165] Except for the following content, the rest is the same as that of Example 1.

[0166] The particle size Dv50 of alumina in the bottom coating 1 is 10 nm, the particle size Dv50 of lithium cobaltate in the positive electrode active layer 3 is 5 μm, and the particle size Dv50 of silicon dioxide in the surface coating 2 is 70 nm, not meeting Dv50 3 ≥100 * (Dv50 1 + Dv50 2 ).

[0167] Example 15

[0168] Except for the following content, the rest is the same as that of Example 1.

[0169] The thickness H of the first electrode region 1 is 90 μm, the thickness H of the second electrode region 2 is 160 μm, and the thickness H of the third electrode region 3 is 150 μm, meeting: H 1 >0.25 * (H 2 + H 3 ). + 2.

[0170] Example 16

[0171] Except for the following content, the rest is the same as that of Example 1.

[0172] The thickness H of the first electrode region 1 is 20 μm, the thickness H of the second electrode region 2 is 40 μm, and the thickness H of the third electrode region 3 is 30 μm, meeting: H 1 >0.25 * (H 2 + H 3 ). + 2.

[0173] Example 17

[0174] Except for the following content, the rest is the same as that of Example 1.

[0175] The thickness H of the first electrode region 1 is 50 μm, the thickness H of the second electrode region 2 is 120 μm, and the thickness H of the third electrode region3 is 100 μm, not meeting the requirement: H 1 > 0.25 * (H 2 + H 3 ) + 2.

[0176] Example 18

[0177] Except for the following content, the rest is the same as in Example 1.

[0178] The negative electrode active material contains 4% silicon carbide and 96% graphite.

[0179] Example 19

[0180] Except for the following content, the rest is the same as in Example 1.

[0181] The negative electrode active material contains 20% silicon carbide and 80% graphite.

[0182] Example 20

[0183] Except for the following content, the rest is the same as in Example 1.

[0184] The negative electrode active material contains 30% silicon carbide and 70% graphite.

[0185] Example 21

[0186] Except for the following content, the rest is the same as in Example 1.

[0187] The negative electrode active material contains 50% silicon carbide and 50% graphite.

[0188] Example 22

[0189] Except for the following content, the rest is the same as in Example 1.

[0190] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 100 mm, the length L of the positive electrode active layer 2 is 500 mm, the length L of the surface coating 3 is 50 mm, meeting the requirement L 2 > L 1 > L 3 .

[0191] Example 23

[0192] Except for the following content, the rest is the same as in Example 1.

[0193] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 1000 mm, the length L of the positive electrode active layer 2 is 3000 mm, the length L of the surface coating 3is 500 mm, satisfying L 2 > L 1 > L 3 .

[0194] Example 24

[0195] Except for the following content, the rest is the same as in Example 1.

[0196] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 2000 mm, the length L of the positive active layer 2 is 2200 mm, the length L of the surface coating 3 is 270 mm, satisfying L 2 > L 1 > L 3 .

[0197] Example 25

[0198] On the A side of the positive electrode sheet, the length L of the surface coating 3 is equal to the length L of the bottom coating 1 and both are 200 mm.

[0199] Comparative Example 1

[0200] Except for the following content, the rest is the same as in Example 1.

[0201] No bottom coating and surface coating are provided on the positive electrode sheet.

[0202] Comparative Example 2

[0203] Except for the following content, the rest is the same as in Example 1.

[0204] The median particle size Dv50 of alumina in the bottom coating 1 is 550 nm.

[0205] Comparative Example 3

[0206] Except for the following content, the rest is the same as in Example 1.

[0207] The median particle size Dv50 of silica in the surface coating 2 is 550 nm.

[0208] Test Example

[0209] 1. Particle size test

[0210] Test is carried out using a laser particle size analyzer.

[0211] 2. Coating thickness and embedding depth test

[0212] Obtained from the SEM image of the coating cross-section.

[0213] 3. Cohesion Test

[0214] Testing Equipment: Tensile Testing Machine

[0215] Cut the rolled positive electrode sheet into a width of 25 mm, and along the Figure 1 demarcation line between the first electrode area I and the second electrode area II and the demarcation line between the second electrode area II and the third electrode area III shown in

[0216] Prepare a flat thin steel plate, stick a piece of double-sided tape at its center position, paste the third electrode area on the double-sided tape, ensure that the electrode sheet is completely matched and adhered to the adhesive tape to avoid test errors, insert the steel plate with the adhered and fixed electrode sheet into the lower clamp of the tensile testing machine, fix it vertically, clamp the non-taped electrode sheet on the upper clamp, fix it at 90° to the electrode sheet of the lower clamp, and set parameters such as test width, peeling length, and peeling speed on the tensile testing machine. Start the test program, and the tensile testing machine will apply a tensile force according to the set parameters and record the change of force value during the peeling process. After the test, data such as the peeling strength curve and the average value will be obtained, which is the cohesion force F between the positive current collector and the bottom coating 1 .

[0217] Using the same method as above, test the second electrode area and the third electrode area respectively to obtain the cohesion force F between the positive current collector and the positive active layer 2 , and the cohesion force F between the positive active layer and the surface coating 3 .

[0218] 4. Surface Resistance Test

[0219] Discharge the battery to the lower limit voltage, then disassemble it in a drying room, take the positive electrode sheet and dry it in the drying room in a natural state, and then place the positive electrode sheet on a two-probe surface resistance tester for testing

[0220] 5. Resistivity Test

[0221] Discharge the battery to the lower limit voltage, then disassemble it in a drying room, take the negative electrode sheet and dry it in the drying room in a natural state, and then place the negative electrode sheet on a four-probe resistivity tester for testing

[0222] 6. Energy Density Retention Rate Test

[0223] At 25°C, charge the prepared finished battery at constant current and constant voltage until it is fully charged, and then discharge it at 0.5C to 3.0V. The discharged capacity is recorded as the battery capacity

[0224] At 25°C, charge the prepared finished battery to 50% SOC, and use 600 g PPG to test the battery thickness

[0225] Calculate volume energy density (ED) = battery capacity * platform voltage / battery length / width / thickness.

[0226] Calculate the ED loss rate = (energy density of comparative example 1 - energy density of embodiment) / energy density of comparative example 1.

[0227] 7. Capacity retention rate test

[0228] At 25°C, the prepared finished battery is charged to full charge at constant current and constant voltage, then discharged to 3.0V at 0.2C, and the released capacity is recorded as battery capacity C1; it is charged to full charge at constant current and constant voltage again, then discharged to 3.0V at 1C, and the released capacity is recorded as battery capacity C2; 1C discharge capacity retention rate = C2 / C1*100%.

[0229] 8. Acupuncture test

[0230] At room temperature, discharge the battery to 3.0V at 1C, charge it to 4.50V at 0.7C constant current, and the cut-off current is 0.02C, discharge it to 3.0V at 1C, cycle it 5 times, and charge it to 4.50V at 0.7C constant current, and the cut-off current is 0.02C. Within 48 hours after the test is completed, use a 2.5mm diameter steel needle to vertically penetrate the left, middle, and right positions of the lithium-ion battery at a speed of 30mm / s. If there is no fire or explosion, it passes. Test 20 samples and observe whether each sample passes the test.

[0231] 9. Hot box test

[0232] Fully charge the battery to the upper limit voltage of 4.5V, then place the battery in an oven, heat it to the set target temperature (130℃, 132℃, 135℃ or 140℃) at a temperature rise rate of 5±2℃ / min, and keep it for 60 minutes, then the test ends; the battery is considered to have passed if it does not catch fire or explode; if the battery temperature continues to rise until it catches fire or explodes, it has failed the test. Test 10 samples and observe whether each sample passes the test.

[0233] The above test results are shown in Table 1-Table 2. In Table 1, “ / ” means that the content does not exist. 1 represents the bonding force between the positive electrode current collector and the bottom coating, F 2 Represents the bonding force between the positive electrode current collector and the positive electrode active layer, F 3 Represents the bonding force between the positive electrode active layer and the surface coating, R 1 is the surface resistance of the first electrode area, R 2 is the surface resistance of the second electrode area, R 3 is the surface resistance of the third electrode area, and ρ is the resistivity of the negative electrode.

[0234] Table 1

[0235]

[0236]

[0237] Table 2

[0238]

[0239]

[0240] It can be seen from Table 1 - Table 2 that the positive electrode sheet in Comparative Example 1 is not provided with a bottom coating and a surface coating. Therefore, the safety of the battery in Comparative Example 1 is poor and it cannot pass the 2.5 - mm acupuncture test and the 140°C furnace temperature test. Compared with Comparative Example 1, the safety of all the examples has been improved to varying degrees, and the ED loss rate is relatively low and the rate performance is good.

[0241] Compared with Example 1, in Comparative Example 2, inorganic particles with a larger particle size are used in the bottom coating, which is not conducive to improving the acupuncture safety of the battery. In Comparative Example 3, inorganic particles with a larger particle size are used in the surface coating, and the acupuncture safety of the battery cannot be effectively improved. Moreover, the rate performance of Comparative Examples 2 - 3 is significantly inferior to that of Example 1.

[0242] This shows that by providing a bottom coating and a surface coating at one end in the length direction of the positive electrode sheet in this application, and the particle size of the inorganic particles in the bottom coating and the surface coating is less than 500 nm, the overall safety and rate performance of the battery cell can be improved with almost no loss of the energy density of the battery cell.

[0243] Obviously, the above - mentioned examples are only for clearly giving examples and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or alterations can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or alterations derived therefrom are still within the protection scope of the present invention.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer disposed on at least one side surface of the positive electrode current collector; characterized in that: A primer layer and a topcoat layer are provided at one end of the positive electrode active layer along the length direction of the positive electrode sheet; The undercoat layer is located between the positive electrode current collector and the positive electrode active layer, the undercoat layer comprises first inorganic particles, a first binder and a first conductive agent, and the median particle size Dv501 of the first inorganic particles is ≤500 nm; The surface coating layer is located on a surface of the positive electrode active layer away from the positive electrode current collector, and the surface coating layer includes second inorganic particles and a second binder, wherein the median particle size Dv502 of the second inorganic particles is ≤500 nm.

2. The positive electrode sheet according to claim 1, characterized in that: The positive electrode active layer includes a positive electrode active material, and the median particle size Dv503 of the positive electrode active material satisfies the following relationship with Dv501 and Dv502: Dv503≥100*(Dv501+Dv502); Preferably, 100nm≥Dv501≥10nm; Preferably, 100nm≥Dv502≥10nm; Preferably, 30 μm ≥ Dv503 ≥ 5 μm; And / or, the first inorganic particles and / or the second inorganic particles include at least one of silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

3. The positive electrode sheet according to claim 1, characterized in that: The projections of the top coating layer and the bottom coating layer in the thickness direction of the positive electrode sheet at least partially overlap; and / or, the topcoat layer and / or the bottomcoat layer are partially embedded in the positive electrode active layer; Preferably, the depth h1 of the surface coating layer embedded in the positive electrode active layer is 0.5 μm-3 μm; Preferably, the depth h2 of the primer layer embedded in the positive electrode active layer is 0.5 μm-4 μm.

4. The positive electrode sheet according to claim 1, characterized in that: The bonding force between the positive electrode current collector and the bottom coating is F1, the bonding force between the positive electrode current collector and the positive electrode active layer is F2, and the bonding force between the positive electrode active layer and the top coating is F3, satisfying: F1>F3>F2; Preferably, F1 is 1gf / mm-10gf / mm; Preferably, F2 is 0.1 gf / mm-5 gf / mm; Preferably, the F3 is 0.5 gf / mm-6 gf / mm.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: Based on the mass of the primer layer, the primer layer comprises 80%-99% of first inorganic particles, 0.5%-10% of a first binder, and 0.5%-10% of a first conductive agent; and / or, based on the mass of the topcoat layer, the topcoat layer comprises 90%-99% of the second inorganic particles and 1%-10% of the second binder; And / or, based on the mass of the positive electrode active layer, the positive electrode active layer comprises 90%-99% of the positive electrode active material, 0.5%-5% of the second conductive agent and 0.5%-5% of the third binder; Preferably, the content of the first binder in the undercoat layer is greater than the content of the second binder in the topcoat layer, and the content of the second binder in the topcoat layer is greater than the content of the third binder in the positive electrode active layer; Preferably, on the same side in the thickness direction of the positive electrode sheet, the length L1 of the undercoat layer, the length L2 of the positive electrode active layer and the length L3 of the surface coating layer satisfy: L2>L1>L3; more preferably, L1 is 100mm-2000mm, and / or, L2 is 500mm-3000mm; and / or, L3 is 50mm-500mm; Preferably, at least one of the first binder, the second binder, and the third binder comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, polyvinylidene fluoride-hexafluoropropylene copolymer, or derivatives thereof; Preferably, the first conductive agent and / or the second conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, carbon fiber, and graphene; more preferably, the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

6. The positive electrode sheet according to claim 2, characterized in that: The positive electrode active material includes lithium cobalt oxide, and the lithium cobalt oxide includes first particles and second particles. The median particle size D1 of the first particles is 15 μm-30 μm, and the median particle size D2 of the second particles is 1 μm-10 μm.

7. A lithium-ion secondary battery, comprising a wound cell, wherein the wound cell comprises a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound; characterized in that: The positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 6, the base coating layer and the surface coating layer are both located at the winding tail end of the wound battery cell, and the surface coating layer is located on the side of the positive electrode sheet facing the inside of the wound battery cell.

8. The lithium ion secondary battery according to claim 7, characterized in that: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises a negative electrode active material, wherein the negative electrode active material comprises graphite and silicon-carbon material; based on the mass of the negative electrode active material, the negative electrode active material comprises 4%-30% of the silicon-carbon material; Preferably, the silicon-carbon material comprises a porous carbon skeleton and silicon particles deposited on the porous carbon skeleton; Preferably, the resistivity of the negative electrode sheet is ρ, 0.5Ω·cm≤ρ≤60Ω·cm.

9. The lithium ion secondary battery according to claim 7, characterized in that: Along the winding direction of the positive electrode sheet, the positive electrode sheet sequentially includes a third electrode sheet area, a second electrode sheet area and a first electrode sheet area; The first pole piece region includes the positive electrode current collector and the base coating layer, the positive electrode active layer and the surface coating layer arranged on the surface of the positive electrode current collector facing the inner side of the wound battery core; The second electrode area includes the positive electrode current collector and the primer layer and the positive electrode active layer disposed on both side surfaces of the positive electrode current collector; The third electrode region includes the positive electrode current collector and the positive electrode active layer disposed on both side surfaces of the positive electrode current collector; The surface resistance R1 of the first pole piece region is 1Ω / cm-3Ω / cm, the surface resistance R2 of the second pole piece region is 0.5Ω / cm-1.8Ω / cm, and the surface resistance R3 of the third pole piece region is 0.3Ω / cm-1.0Ω / cm, satisfying: R1>R2>R3.

10. The lithium ion secondary battery according to claim 9, characterized in that: The thickness H1 of the first pole piece region is 20 μm-90 μm, the thickness H2 of the second pole piece region is 20 μm-160 μm, and the thickness H3 of the third pole piece region is 20 μm-160 μm, satisfying: H1>0.25*(H2+H3).

Images