Positive electrode sheet, wound battery cell, and lithium-ion secondary battery

Abstract

The present application relates to the field of lithium-ion batteries, and discloses a positive electrode sheet, a wound battery cell and a lithium-ion secondary battery. The positive electrode sheet is provided, at one end thereof in the length direction, with an undercoat layer and a topcoat layer, both of which comprise inorganic particles having a particle size of not more than 500 nm; therefore, it can be ensured that, during a nail penetration test, the undercoat layer can prevent a short circuit caused by contact between a positive electrode current collector and a negative electrode active material layer, and during a hot-box test, the topcoat layer can isolate the positive electrode sheet on the outer side of the battery cell from contact with an electrolyte, thereby reducing heat generation caused by side reactions and further effectively improving the overall safety of the battery cell. Moreover, the use of inorganic particles having a relatively small particle size can further reduce the thickness of coating layers, shorten an ion transport path, and improve the rate performance of the battery cell. In addition, by providing the undercoat layer and the topcoat layer at only one end of the positive electrode sheet, loss of volumetric energy density caused by an excessively large thickness of the battery cell can be avoided, thereby ensuring the electrical performance of the battery.

Description

Positive electrode sheet, wound battery cell and lithium-ion secondary battery

[0001] This application claims priority to Chinese Patent Application No. 202510365773.8, filed on March 26, 2025, entitled "A Positive Electrode Sheet and a Lithium-ion Secondary Battery"; Chinese Patent Application No. 202510365753.0, filed on March 26, 2025, entitled "A Winded Cell and a Lithium-ion Secondary Battery"; and Chinese Patent Application No. 202520556482.2, filed on March 26, 2025, entitled "A Winded Cell and a Lithium-ion Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery technology, specifically to positive electrode sheets, wound cells, and lithium-ion secondary batteries. Background Technology

[0003] In recent years, lithium-ion rechargeable batteries have been widely used in daily life, appearing in electronic products such as smartphones and laptops, as well as in vehicles such as electric bicycles and electric cars. As the capacity of individual lithium-ion battery cells increases, higher requirements are being placed on the safety performance of these batteries.

[0004] Currently, safety performance tests for lithium-ion rechargeable batteries mainly include mechanical abuse tests, such as needle penetration tests, and thermal abuse tests, such as hot box tests. To improve the pass rate of batteries in these tests, related technologies disclose the application of a base coating on the positive / negative electrode current collectors to reduce the possibility of battery combustion, fire, and explosion, or the application of a surface coating on the surface of the positive / negative electrode active paste to improve the battery's thermal stability. However, since existing base or surface coatings are applied to the entire surface of the electrode, this affects the battery's volumetric energy density, which is detrimental to the battery's electrical performance. Furthermore, applying only a base or surface coating does not effectively improve the overall safety of the battery. Moreover, these thermally safe batteries typically use high-temperature resistant electrolyte systems, which leads to poor low-temperature performance and fails to meet the requirements for use in low-temperature environments. Summary of the Invention

[0005] In view of this, one objective of this application is to provide positive electrode plates, wound cells and lithium-ion secondary batteries, with the aim of effectively improving their safety performance without affecting the battery energy density.

[0006] Another objective of this application is to provide wound-type battery cells and lithium-ion secondary batteries, aiming to solve the problem of poor low-temperature performance of existing battery cells.

[0007] According to an embodiment of this application, in a first aspect, a positive electrode sheet is provided, including a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector, wherein a base coating layer and a top coating layer are provided at one end of the positive active layer along the length direction of the positive electrode sheet.

[0008] The base coating is located between the positive current collector and the positive active layer. The base coating includes first inorganic particles, a first binder and a first conductive agent. The median particle size Dv501 of the first inorganic particles is ≤500nm.

[0009] The surface coating is located on the surface of the positive electrode active layer opposite to the positive electrode current collector. The surface coating includes second inorganic particles and a second binder. The median particle size Dv502 of the second inorganic particles is ≤500nm.

[0010] In some optional embodiments, the positive electrode active layer includes a positive electrode active material, wherein the median particle size Dv503 of the positive electrode active material satisfies the following relationship with Dv501 and Dv502: Dv503≥100*(Dv501+Dv502).

[0011] Furthermore, 100nm ≥ Dv501 ≥ 10nm.

[0012] Furthermore, 100nm ≥ Dv502 ≥ 10nm.

[0013] Furthermore, 30μm≥Dv503≥5μm.

[0014] In some alternative embodiments, the first inorganic particle and / or the second inorganic particle includes at least one of silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

[0015] In some alternative embodiments, the projection of the top coating layer and the bottom coating layer in the thickness direction of the positive electrode sheet at least partially overlaps.

[0016] In some alternative embodiments, the top coating and / or the bottom coating are partially embedded in the positive electrode active layer.

[0017] Furthermore, the depth h1 into which the surface coating is embedded in the positive electrode active layer is 0.5μm-3μm.

[0018] Furthermore, the depth h2 of the base coating embedded in the positive electrode active layer is 0.5μm-4μm.

[0019] In some optional embodiments, the adhesion force between the positive current collector and the base coating is F1, the adhesion force between the positive current collector and the positive active layer is F2, and the adhesion force between the positive active layer and the top coating is F3, satisfying: F1 > F3 > F2.

[0020] Furthermore, F1 is 1gf / mm-10gf / mm.

[0021] Furthermore, F2 is 0.1gf / mm-5gf / mm.

[0022] Furthermore, the F3 is 0.5gf / mm-6gf / mm.

[0023] In some alternative embodiments, the base coating comprises 80%-99% of first inorganic particles, 0.5%-10% of first binder, and 0.5%-10% of first conductive agent, based on the mass of the base coating.

[0024] In some alternative embodiments, the surface coating comprises 90%-99% of second inorganic particles and 1%-10% of second binder, based on the mass of the surface coating.

[0025] In some alternative embodiments, the positive electrode active layer comprises 90%-99% positive electrode active material, 0.5%-5% second conductive agent, and 0.5%-5% third binder, based on the mass of the positive electrode active layer.

[0026] In some alternative embodiments, the content of the first binder in the base coating layer is greater than the content of the second binder in the top coating layer, and the content of the second binder in the top coating layer is greater than the content of the third binder in the positive electrode active layer.

[0027] In some alternative embodiments, on the same side of the positive electrode thickness direction, the length L1 of the bottom coating layer, the length L2 of the positive electrode active layer, and the length L3 of the top coating layer satisfy the following condition: L2 > L1 > L3.

[0028] Furthermore, L1 is 100mm-2000mm.

[0029] Furthermore, L2 is 500mm-3000mm.

[0030] Furthermore, L3 is 50mm-500mm.

[0031] In some alternative embodiments, at least one of the first adhesive, the second adhesive, and the third adhesive includes 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 derivatives thereof.

[0032] In some optional 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 nanotubes, metal powder, carbon fiber, and graphene.

[0033] Furthermore, 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.

[0034] In some optional embodiments, the positive electrode active material includes lithium cobalt oxide, which includes a first particle and a second particle, wherein the median particle size D1 of the first particle is 15 μm-30 μm and the median particle size D2 of the second particle is 1 μm-10 μm.

[0035] According to an embodiment of this application, in a second aspect, this application provides a lithium-ion secondary battery, including a wound cell. The wound cell includes a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, as described in the first aspect of this application. The positive electrode sheet, the negative electrode sheet, and the separator are stacked and wound together. The bottom coating layer and the top coating layer are both located at the winding tail end of the wound cell, and the top coating layer is located on the side of the positive electrode sheet facing the interior of the wound cell.

[0036] In some alternative embodiments, the resistivity of the negative electrode is ρ, 0.5Ω·cm≤ρ≤60Ω·cm.

[0037] In some alternative embodiments, the negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector, the negative active layer including a negative active material, the negative active material including graphite and silicon carbide materials; the negative active material includes 4%-30% silicon carbide material based on the mass of the negative active material.

[0038] In some alternative embodiments, the silicon-carbon material comprises a porous carbon framework and silicon particles deposited on the porous carbon framework.

[0039] In some alternative embodiments, along the winding direction of the positive electrode, the positive electrode sequentially includes a third electrode region, a second electrode region, and a first electrode region;

[0040] The first electrode region includes the positive current collector and the bottom coating, the positive active layer and the top coating disposed on the surface of the positive current collector facing the inside of the wound cell;

[0041] The second electrode region includes the positive current collector and the undercoating layer and the positive active layer disposed on both sides of the positive current collector;

[0042] The third electrode region includes the positive current collector and the positive active layer disposed on both sides of the positive current collector;

[0043] The surface resistance R1 of the first electrode region is 1Ω / cm-3Ω / cm, the surface resistance R2 of the second electrode region is 0.5Ω / cm-1.8Ω / cm, and the surface resistance R3 of the third electrode region is 0.3Ω / cm-1.0Ω / cm, satisfying the condition: R1>R2>R3.

[0044] In some optional embodiments, the thickness H1 of the first electrode region is 20μm-90μm, the thickness H2 of the second electrode region is 20μm-160μm, and the thickness H3 of the third electrode region is 20μm-160μm, satisfying: H1>0.25*(H2+H3).

[0045] Furthermore, the thickness H1 of the first electrode region, the thickness H2 of the second electrode region, and the thickness H3 of the third electrode region satisfy the following condition: H1 > 0.25*(H2+H3)+2.

[0046] The positive electrode sheet provided in this application has a base coating and a top coating at one end along its length. The base coating includes first inorganic particles, and the top coating includes second inorganic particles. The median particle size of both the first and second inorganic particles does not exceed 500 nm. By selecting smaller-sized first and second inorganic particles, more inorganic particles can be filled in the coating per unit thickness. This ensures that the base coating can prevent short circuits between the positive current collector and the negative active material layer during nail penetration testing, and that the top coating can isolate the positive electrode sheet on the outside of the cell from the electrolyte during hot box testing, reducing heat generation from side reactions and effectively improving the overall safety of the cell. Furthermore, using smaller-sized first and second inorganic particles can also reduce the coating thickness, shorten the ion transport path, and improve the rate performance of the cell. Simultaneously, by only setting the base coating and top coating at one end (i.e., locally) of the positive electrode sheet, the volumetric energy density loss caused by excessive cell thickness can be avoided, thus ensuring the battery's electrical performance.

[0047] According to an embodiment of this application, in a third aspect, a wound battery cell is provided, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the positive electrode sheet is stacked and wound with the separator and the negative electrode sheet; the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector, and a base coating layer and a surface coating layer are provided at the winding tail end of the positive electrode sheet;

[0048] The base coating is located between the positive current collector and the positive active layer, and the base coating includes first inorganic particles;

[0049] The surface coating is located on the surface of the positive electrode active layer opposite to the positive electrode current collector, and the surface coating includes second inorganic particles;

[0050] The electrolyte includes carboxylic acid ester solvents, and the content of the carboxylic acid ester solvents is A% based on the mass of the electrolyte, with 10 ≤ A ≤ 50%.

[0051] In some optional embodiments, the positive electrode includes a first electrode region, a second electrode region, and a third electrode region, wherein the third electrode region, the second electrode region, and the first electrode region are arranged sequentially along the winding direction of the positive electrode; the first electrode region includes the positive current collector and the bottom coating, the positive active layer, and the surface coating disposed on the surface of the positive current collector facing the inside of the wound cell; the resistivity of the first electrode region is R, where 200Ω·cm≤R≤1000Ω·cm.

[0052] In some alternative implementations, the following condition must be met: 500Ω·cm ≤ R ≤ 800Ω·cm.

[0053] In some alternative implementations, the following condition must be met: 8 ≤ R / A ≤ 80.

[0054] Furthermore, it satisfies: 10≤R / A≤80.

[0055] In some optional embodiments, the carboxylic acid ester solvent includes at least one of methyl formate, ethyl acetate, ethyl propionate, methyl acetate, methyl butyrate, methyl propyl carbonate, propyl propionate, ethyl butyrate, methyl fluoroformate, ethyl difluoroacetate, ethyl 2-fluoropropionate, methyl fluoroacetate, methyl 4-fluorobutyrate, propyl 2-fluoropropionate, and ethyl 4-fluorobutyrate.

[0056] In some optional embodiments, the electrolyte further includes lithium salt and nitrile additives, wherein the content of the nitrile additives is B% based on the mass of the electrolyte, and 2 ≤ B ≤ 10.

[0057] Furthermore, it satisfies 30≤R / B≤200.

[0058] In some alternative embodiments, the nitrile additive includes at least one selected from butadionitrile, glutaronitrile, adiponitrile, trans-butenedionitrile, trans-hexenedionitrile, glyceryl trionitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrionitrile, 1,2,3-tris(cyanoethoxy)propane, and 1,4-dicyano-2-butene.

[0059] In some optional embodiments, the second electrode region includes the positive current collector and the undercoating layer and the positive active layer disposed on both sides of the positive current collector;

[0060] The third electrode region includes the positive current collector and the positive active layer disposed on both sides of the positive current collector;

[0061] In the ARC self-heating test, the time required for the surface temperature of the first electrode region to reach 150°C is T1, and the time required for the surface temperature of the second electrode region and / or the third electrode region to reach 150°C is T2, where T1-T2 > 300 min.

[0062] In some optional embodiments, the coating porosity P1 of the first electrode region is 10%-14%, the coating porosity P2 of the second electrode region is 14%-18%, and the coating porosity P3 of the third electrode region is 18%-25%, satisfying P1 < P2 < P3.

[0063] In some alternative embodiments, on the same side in the thickness direction of the positive electrode sheet, the length L1 of the bottom coating layer, the length L2 of the positive electrode active layer, and the length L3 of the top coating layer satisfy: (L1+L3)≤1.5*L2.

[0064] Furthermore, L1 is 100mm-2000mm.

[0065] Furthermore, L2 is 500mm-3000mm.

[0066] Furthermore, L3 is 50mm-500mm.

[0067] In some alternative embodiments, the top coating and / or the bottom coating are partially embedded in the positive electrode active layer, wherein the depth h1 of the top coating embedded in the positive electrode active layer is less than the depth h2 of the bottom coating embedded in the positive electrode active layer.

[0068] Furthermore, the depth h1 into which the surface coating is embedded in the positive electrode active layer is 0.5μm-3μm.

[0069] Furthermore, the depth h2 of the base coating embedded in the positive electrode active layer is 1μm-3μm.

[0070] In some optional embodiments, in the thickness direction of the positive electrode sheet, the minimum thickness h3 of the top coating is 0.5μm-4μm, and the minimum thickness h4 of the bottom coating is 2μm-6μm, satisfying h3≤h4.

[0071] In some alternative embodiments, the base coating comprises 80%-99% first inorganic particles, 0.5%-10% first binder, and 0.5%-10% conductive agent, based on the mass of the base coating.

[0072] In some alternative embodiments, the surface coating comprises 90%-99% of second inorganic particles and 1%-10% of second binder, based on the mass of the surface coating.

[0073] In some alternative embodiments, the first inorganic particle and / or the second inorganic particle comprises at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide.

[0074] In some alternative embodiments, the first adhesive and / or the second adhesive comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, or derivatives thereof.

[0075] In some optional embodiments, the 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.

[0076] Furthermore, 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.

[0077] In some optional embodiments, the separator includes a substrate, a first adhesive layer, a ceramic layer, and a second adhesive layer. The first adhesive layer is disposed on one side surface of the substrate, the second adhesive layer is disposed on the surface of the substrate opposite to the first adhesive layer, and the ceramic layer is located between the substrate and the second adhesive layer and faces the positive electrode.

[0078] The ceramic layer comprises ceramic particles, and the median particle size Dv50 of the ceramic particles and the average thickness H of the surface coating satisfy the following: 0.5≤H / Dv50≤20, where H is 1μm-10μm and Dv50 is 0.5μm-2μm.

[0079] In some alternative embodiments, the ceramic particles include at least one of boehmite, alumina, zirconium oxide, silicon dioxide, silicon carbide, and silicon nitride.

[0080] The wound battery cell provided in this application has a base coating and a surface coating at the winding end of the positive electrode sheet. The base coating includes first inorganic particles, and the surface coating includes second inorganic particles. The base coating can prevent short circuits between the positive current collector and the negative electrode active material layer, while the surface coating can isolate the positive electrode sheet on the outside of the battery cell from the electrolyte, reducing heat generation from side reactions and thus effectively improving the overall safety of the battery cell. Simultaneously, adding an appropriate amount of carboxylic acid ester solvent to the electrolyte can improve the low-temperature performance of the battery cell, enabling the wound battery cell of this application to possess both good safety and low-temperature performance.

[0081] According to an embodiment of this application, in a fourth aspect, a wound battery cell is provided, comprising a first electrode, a separator, and a second electrode stacked and wound together, wherein the first electrode and the second electrode have opposite polarities, and the first electrode comprises:

[0082] First current collector, first active material layer, first coating layer and second coating layer;

[0083] Along the winding direction of the battery cell, the first current collector sequentially includes a first region, a second region, and a third region;

[0084] The first current collector is located on both sides of the first region, and a first active material layer is provided thereon.

[0085] The first current collector is provided with a first active material layer and a first coating layer on both sides of the second region;

[0086] The first current collector is located on one side of the third region and is sequentially provided with a first coating, a first active material layer and a second coating;

[0087] A first coating located in the second region and / or the third region is disposed between the first active material layer and the first current collector;

[0088] Along the length of the first electrode, the lengths of both the first coating and the second coating are less than the length of the first active material layer.

[0089] Beneficial Effects: The wound battery cell provided in the embodiments of this application, by setting a first coating and a second coating on the first electrode, can isolate the contact between the first current collector and the second electrode, thereby preventing short circuits and improving the safety performance of the wound battery cell, making it more conducive to passing the nail penetration test; and it can also isolate the contact between the electrolyte and the first active material layer of the positive electrode, reducing heat generation from side reactions, improving the thermal safety performance of the battery cell, and making it more conducive to passing the hot box test. It can pass the nail penetration test and hot box test with minimal energy density loss, thereby improving the safety performance of the battery cell, increasing the pass rate of the nail penetration test and hot box test for large-capacity battery cells, and the energy density loss is extremely low, having almost no impact on the electrical performance of the battery cell. By dividing the first current collector into regions, the first coating and the second coating are both set in local areas, which can minimize energy density loss while meeting safety performance requirements. Along the length direction of the first electrode, the length of both the first coating and the second coating is less than the length of the first active material layer; therefore, it is not necessary to set the first coating and the second coating in the entire area of ​​the first current collector.

[0090] In one alternative embodiment, the second coating is located on the side of the first current collector facing the inside of the wound cell; the second coating is located between the first active material layer of the first electrode and the separator.

[0091] Beneficial effects: By placing the second coating on the side of the first current collector facing the inside of the wound cell, the contact between the electrolyte and the first active material layer of the positive electrode can be separated, reducing heat generation from side reactions, improving the thermal safety performance of the cell, and making it easier to pass the hot box test; by placing the second coating between the first active material layer of the positive electrode and the separator, the second coating can be ensured to separate the contact between the electrolyte and the first active material layer of the positive electrode, reducing heat generation from side reactions, improving the thermal safety performance of the cell, and making it easier to pass the hot box test.

[0092] In one optional embodiment, the length of the first active material layer on the first side of the first current collector is L14, and the length of the first active material layer on the second side of the first current collector is L15, satisfying: L15 < L14.

[0093] The first coating on the first side near the tail end of the first electrode plate and the first active material layer on the first side near the tail end of the first electrode plate are at least partially overlapped in projection; the first coating on the second side near the tail end of the first electrode plate and the first active material layer on the second side near the tail end of the first electrode plate are at least partially overlapped in projection.

[0094] The projections of the first coating on the first side near the first electrode end and the first coating on the second side near the first electrode end at least partially overlap;

[0095] The length of the first coating on the first side of the first current collector is L11, the length of the first coating on the second side of the first current collector is L12, and the length of the second coating is L13, satisfying: L12 < L11 and L13 < L11;

[0096] And / or, the value of L11 ranges from 100mm to 2000mm;

[0097] And / or, the value of L12 ranges from 50mm to 1500mm;

[0098] And / or, the value of L13 ranges from 50mm to 500mm.

[0099] Beneficial effects: By adopting this configuration, energy density loss can be minimized while still achieving safety improvements.

[0100] In one optional embodiment, the end of the second coating near the tail end of the first electrode plate at least partially overlaps with the end of the first active material layer on the first side near the tail end of the first electrode plate; the end of the second coating near the head end of the first electrode plate at least partially overlaps with the end of the first active material layer on the second side near the tail end of the first electrode plate.

[0101] Beneficial effects: By ensuring that the projection of the end of the second coating 13 near the tail end of the first electrode plate at least partially overlaps with the projection of the end of the first active material layer 12 near the tail end of the first electrode plate on the first side; and the projection of the end of the second coating 13 near the head end of the first electrode plate at least partially overlaps with the projection of the end of the first active material layer 12 near the tail end of the first electrode plate on the second side, since the first active material layer 12 and the first coating 11 are only provided on one side in the third region 300, when the second coating 13 is further provided in this region, after the first electrode plate is wound, the thickness increase in this region along the thickness direction of the wound cell is not significant, thereby reducing energy density loss to a very low level and having almost no impact on the electrical performance of the cell.

[0102] In one optional embodiment, the thickness of the first coating is H11, the thickness of the second coating is H12, and the thickness of the first active material layer located in the first region is H13; satisfying: H11≤H13, and / or, H12≤H13; and / or, satisfying: H11≤H12.

[0103] Beneficial effects: The first coating prevents short circuits by isolating the first current collector from the second electrode. By making the thickness H11 of the first coating less than or equal to the thickness H13 of the first active material layer, the thinner first coating reduces energy density loss. The second coating isolates the electrolyte from the first active material layer of the positive electrode, reducing heat generation from side reactions. By making the thickness H12 of the second coating greater than or equal to the thickness H11 of the first coating, the larger second coating further reduces heat generation from side reactions, thus improving safety performance.

[0104] In one alternative embodiment, as shown in FIG5, the first coating is at least partially embedded in the first active material layer, and / or the second coating is at least partially embedded in the first active material layer; the depth to which the first coating is embedded in the first active material layer is not less than 0.5 μm; and / or the depth to which the second coating is embedded in the first active material layer is not less than 0.5 μm.

[0105] Beneficial effects: By embedding the first coating at least partially within the first active material layer, and / or, embedding the second coating at least partially within the first active material layer, it is beneficial to reduce the thickness of the wound battery cell and the contact resistance.

[0106] In one alternative implementation, the first electrode is a positive electrode and the second electrode is a negative electrode.

[0107] In one optional embodiment, the wound cell includes a flat region and an arc region. Both the first coating and the second coating are located at the tail end of the first electrode of the wound cell. The end of the first coating near the head end of the first electrode is located in the flat region of the wound cell. The end of the first coating near the tail end of the first electrode is located in the flat region of the wound cell. The end of the first coating near the head end of the first electrode is located at least beyond the center position of the wound cell along the width direction of the cell.

[0108] The second coating is located in the flat area of ​​the wound cell at the end closest to the first electrode; the second coating is located in the flat area of ​​the wound cell at the end closest to the first electrode.

[0109] Beneficial effects: Since the flat area is located on the large surface of the wound cell, it is more susceptible to damage from impacts or punctures. By setting the end of the first coating near the first electrode at least beyond the center of the wound cell, the first coating can cover a sufficient area of ​​the flat area of ​​the wound cell. This ensures that the first coating can better prevent the first current collector from contacting the second electrode when the wound cell is subjected to a puncture test, thus preventing short circuits and improving the safety performance of the wound cell, making it more likely to pass the puncture test.

[0110] In one alternative embodiment, the wound cell further includes a finishing tape, which is attached at least to a third region of the first current collector.

[0111] Beneficial effect: By attaching the finishing tape to at least the third region of the first current collector, the advantage of the smaller thickness of the first current collector in the third region can be taken advantage of, and the position of the finishing tape can be reasonably arranged to avoid excessively increasing the thickness of the wound battery cell.

[0112] In one optional embodiment, the material of the first coating includes first inorganic particles, a first adhesive, and a first conductive agent; the material of the second coating includes second inorganic particles and a second adhesive.

[0113] The first inorganic particles and / or the second inorganic particles include at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, titanium oxide, boehmite, and magnesium hydroxide.

[0114] The first adhesive and / or the second adhesive comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and derivatives thereof;

[0115] The first conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber.

[0116] According to an embodiment of this application, in a fifth aspect, a lithium-ion secondary battery is provided, including the wound cell described in the third or fourth aspect of this application. Since a lithium-ion secondary battery including a wound cell has the same effects as a wound cell, it will not be described again here.

[0117] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Attached Figure Description

[0118] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0119] Figure 1 is a schematic diagram of the cross-sectional structure of the positive electrode sheet along its length in one embodiment of this application.

[0120] Figure 2 is a cross-sectional SEM image of the surface coating in one embodiment of this application.

[0121] Figure 3 is a cross-sectional SEM image of the base coating in one embodiment of this application.

[0122] Figure 4 is a schematic cross-sectional view of a wound battery cell according to one embodiment of this application.

[0123] Figure 5 is a schematic diagram of the first electrode of this application.

[0124] Figure 6 is a schematic diagram of the region division of the first electrode in this application.

[0125] Figure 7 is a schematic diagram of the dimensions of the first electrode in this application.

[0126] Figure 8 is a cross-sectional schematic diagram of the wound battery cell of this application.

[0127] Figure 9 is a schematic diagram of another type of first electrode in this application.

[0128] The reference numerals in the attached drawings are explained as follows: 11, Positive current collector; 12, Positive active layer; 13, Undercoat; 14, Topcoat; 15, Positive electrode tab; 21, Negative current collector; 22, Negative active layer; 23, Negative electrode tab; 30, Separator; 31, Termination adhesive; I, First electrode region; II, Second electrode region; III, Third electrode region; L1 is the length of the undercoat; L2 is the length of the positive active layer; L3 is the length of the topcoat; 110, First current collector; 111, First coating; 112, First active material layer; 113, Second coating; 120, Second current collector; 121, Second active material layer; 130, Separator; 140, Termination adhesive tape; 100, First region; 200, Second region; 300, Third region; 400, Fourth region; L11 is the length of the first coating on the first side of the first current collector; L12 is the length of the first coating on the second side of the first current collector; L13 is the length of the second coating; L14 is the length of the first active material layer on the first side of the first current collector; L15 is the length of the first active material layer on the second side of the first current collector. Detailed Implementation

[0129] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0130] It should be noted in the description of this application that the terms "inner" and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying 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, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0131] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0132] In mechanical abuse tests such as needle penetration, the closer to the outer edge of the wound battery cell, the greater the deformation caused by abuse, and the easier it is to trigger an internal short circuit. There are four main modes of internal short circuits in batteries: 1) short circuit between the positive and negative electrode current collectors; 2) short circuit between the positive electrode current collector and the negative electrode active material; 3) short circuit between the positive electrode active material and the negative electrode current collector; and 4) short circuit between the positive and negative electrode active materials. Among these, the short circuit between the positive electrode current collector and the negative electrode active material is the most dangerous due to the highest heat generation. Additionally, in thermal abuse tests, the outer edge of the cell heats up the fastest and is most prone to thermal runaway.

[0133] Understandably, the outer side of the battery cell refers to the side away from the winding center, using the winding center of the wound battery cell as a reference. Correspondingly, the inner side of the battery cell refers to the side facing the winding center, using the winding center of the wound battery cell as a reference.

[0134] In order to improve the overall safety of the wound battery cell without affecting its electrical performance, according to the first aspect of this application, a positive electrode sheet is provided, as shown in FIG1, including a positive current collector 11 and a positive active layer 12 disposed on at least one side surface of the positive current collector 11. Along the length direction of the positive electrode sheet, a bottom coating layer 13 and a top coating layer 14 are provided at one end of the positive active layer.

[0135] The base coating 13 is located between the positive electrode current collector 11 and the positive electrode active layer 12. The base coating 13 includes first inorganic particles, a first binder and a first conductive agent. The median particle size Dv501 of the first inorganic particles is ≤500nm.

[0136] The surface coating 14 is located on the surface of the positive electrode active layer 12 opposite to the positive electrode current collector 11. The surface coating 14 includes second inorganic particles and a second binder. The median particle size Dv502 of the second inorganic particles is ≤500nm.

[0137] By selecting smaller-diameter first and second inorganic particles, more inorganic particles can be filled per unit thickness of the coating. This ensures that the undercoat prevents short circuits between the positive current collector and the negative active material layer during nail penetration testing, and that the topcoat isolates the positive electrode from the electrolyte during hot box testing, reducing heat generation from side reactions and effectively improving the overall safety of the cell. Furthermore, using smaller-diameter first and second inorganic particles also reduces the coating thickness, shortens the ion transport path, and improves the cell's rate performance. Simultaneously, by applying the undercoat and topcoat only to one end of the positive electrode (i.e., partially, not entirely), the overall safety of the cell is effectively enhanced. This avoids volumetric energy density loss due to excessive cell thickness, thus ensuring the battery's electrical performance.

[0138] For example, the median particle size Dv501 of the first inorganic particle can be 10nm, 40nm, 70nm, 100nm, 300nm, 500nm, etc., or fall within the range of any two of the above values. The first inorganic particle includes at least one of silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

[0139] For example, the median particle size Dv502 of the second inorganic particle can be 10nm, 40nm, 70nm, 100nm, 300nm, 500nm, etc., or fall within the range of any two of the above values. The second inorganic particle includes at least one of silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, and magnesium hydroxide.

[0140] The positive electrode active layer includes a positive electrode active material. In some embodiments, the median particle size Dv503 of the positive electrode active material satisfies the following relationship with the median particle size Dv501 of the first inorganic particles and the median particle size Dv502 of the second inorganic particles: Dv503 ≥ 100 * (Dv501 + Dv502). Wherein, the median particle size Dv503 of the positive electrode active material is 5 μm-30 μm. For example, Dv503 can be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, etc., or fall within the range of any two of the above values.

[0141] Because the particle size of the positive electrode active material is much larger than that of the first / second inorganic particles, the top and bottom coatings can be embedded in the positive electrode active layer to a certain extent, thus preventing the cell's volumetric energy density from being affected by excessive thickness. Simultaneously, using smaller-sized first / second inorganic particles, compared to larger-particle coatings, allows for a reduction in coating thickness while maintaining comparable safety performance, shortening the ion transport path and improving the cell's rate performance. Furthermore, it allows for more inorganic particles to be packed into the coating per unit thickness, thereby enhancing the bottom coating's resistance to nail penetration tests and the top coating's resistance to hot-box tests, ultimately improving the overall safety of the cell.

[0142] In this application, the median particle size Dv501 of the first inorganic particles, the median particle size Dv502 of the second inorganic particles, and the median particle size Dv503 of the positive electrode active material can all be obtained by laser particle size analyzer.

[0143] Please refer to Figures 2 and 3. In some embodiments, the thickness of the topcoat layer is 1.98 μm, 3.57 μm, or 3.97 μm, and the thickness of the bottomcoat layer is 2.86 μm, 3.37 μm, or 5.21 μm. As can be seen from Figures 2 and 3, both the topcoat and bottomcoat layers are partially embedded in the positive electrode active layer. The embedding depth h1 of the topcoat layer in the positive electrode active layer is 0.5 μm-3 μm, for example, it can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, or fall within any two of the above values. The embedding depth h2 of the bottomcoat layer in the positive electrode active layer is 0.5 μm-4 μm, for example, it can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, or 4 μm, or fall within any two of the above values.

[0144] It should be noted that, in this application, the term "embedding depth" refers to the degree to which the base coating / top coating is embedded into the positive electrode active layer, expressed in units of area (mm²). 2 The difference between the maximum thickness and the minimum thickness of the bottom / top coating in the cross-sectional SEM image at the interface between the inner bottom / top coating and the positive electrode active layer is used to represent this.

[0145] In some embodiments, the projections of the top coating and the bottom coating onto the thickness direction of the positive electrode sheet at least partially overlap. It is understood that this overlapping area is located at the winding tail end of the positive electrode sheet because: in needle penetration tests, the closer to the outer side of the wound cell, the greater the deformation caused by the needle penetration, and the more likely an internal short circuit will occur. Therefore, a bottom coating needs to be applied at the winding tail end of the positive electrode sheet (corresponding to the outer side of the wound cell); in hot box tests, the outer side of the wound cell heats up the fastest and is most prone to thermal runaway, thus requiring a top coating at the winding tail end of the positive electrode sheet. By simultaneously applying both the bottom coating and the top coating at the winding tail end of the positive electrode sheet, the risk to the cell during safety testing can be effectively reduced, improving the cell's safety performance. Furthermore, applying both the bottom coating and the top coating only at the winding tail end of the positive electrode sheet, rather than all of it, also prevents the cell from becoming too thick and affecting its volumetric energy density, thereby ensuring improved overall safety without affecting the cell's electrical performance.

[0146] In some embodiments, the base coating comprises, based on its mass, 80%-99% of first inorganic particles, 0.5%-10% of a first binder, and 0.5%-10% of a first conductive agent. The percentages above are by mass.

[0147] 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 derivatives thereof. 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 nanotubes, metal powder, carbon fiber, and graphene, wherein 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.

[0148] The base coating uses non-conductive or low-conductive inorganic particles to ensure good insulation when the base coating comes into contact with the negative electrode active material layer during the needle penetration test, thereby improving the needle penetration safety of the cell. At the same time, in order to ensure the electron conduction between the positive electrode active layer and the positive electrode current collector, the base coating also needs to have a certain degree of conductivity, so the base coating includes a small amount of conductive agent.

[0149] In some embodiments, the surface coating comprises 90%-99% of second inorganic particles and 1%-10% of a second binder, based on the mass of the surface coating. The percentages above are by mass.

[0150] 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 derivatives thereof.

[0151] The surface coating uses non-conductive or low-conductive inorganic particles, which makes the surface coating insulating or low-conductive. This can slow down the oxidation reaction of the electrolyte at high potentials, reduce the occurrence of side reactions, and increase the internal resistance of the cell when it runs away, thus improving the thermal safety of the cell.

[0152] In some embodiments, based on the mass of the positive electrode active layer, the positive electrode active layer comprises 90%-99% positive electrode active material, 0.5%-5% second conductive agent, and 0.5%-5% third binder. The percentages above refer to mass percentages.

[0153] As an example, the positive electrode active material includes lithium cobalt oxide; 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, and graphene, wherein 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; 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 derivatives thereof.

[0154] In some embodiments, the lithium cobalt oxide includes a first particle and a second particle. The median particle size D1 of the first particle is 15μm-30μm, for example, it can be 15μm, 18μm, 20μm, 25μm, 30μm, etc., or within the range of any two of the above values. The median particle size D2 of the second particle is 1μm-10μm, for example, it can be 1μm, 3μm, 5μm, 8μm, 10μm, etc., or within the range of any two of the above values.

[0155] Using small-particle lithium cobalt oxide can shorten the diffusion path of lithium ions, and its larger specific surface area is beneficial for improving rate performance. However, excessively small particles may cause agglomeration, which in turn affects lithium ion transport. On the other hand, large-particle lithium cobalt oxide can ensure the high-temperature performance of the battery cell. Therefore, by grading the particle size of lithium cobalt oxide, the battery cell can simultaneously possess good high-temperature and rate performance, and it is also beneficial for increasing the compaction density of the positive electrode, thereby improving the energy density of the battery cell.

[0156] In this application, the median particle size D1 of the first particle and the median particle size D2 of the second particle can both be obtained by laser particle size analyzer.

[0157] In some embodiments, the content of the first binder in the base coating is greater than the content of the second binder in the top coating, and the content of the second binder in the top coating is greater than the content of the third binder in the positive electrode active layer. Furthermore, the particle size of the inorganic particles in both the base coating and the top coating is smaller than the particle size of the positive electrode active material, thereby ensuring that the adhesion force F1 between the positive electrode current collector and the base coating, the adhesion force F3 between the positive electrode active layer and the top coating, and the adhesion force F2 between the positive electrode current collector and the positive electrode active layer satisfy the following order: F1 > F3 > F2.

[0158] Both F1 and F3 are greater than F2, indicating that the bottom coating layer adheres tightly to the positive electrode current collector, and the top coating layer adheres tightly to the positive electrode active layer. This means that both the bottom and top coating layers exhibit excellent adhesion, ensuring that they function effectively during nail penetration and hot box tests, thus improving the overall safety of the battery cell. Furthermore, both the bottom and top coating layers utilize small-diameter inorganic particles, which not only shortens the lithium-ion transport path and improves the cell's rate performance but also prevents volumetric energy density loss due to excessive cell thickness, thus guaranteeing the battery's electrical performance.

[0159] Specifically, in some embodiments, the adhesion force between the positive current collector and the base coating is F1, where F1 is 1gf / mm-10gf / mm. For example, F1 can be 1gf / mm, 3gf / mm, 5gf / mm, 7gf / mm, 10gf / mm, or within any two of the above values.

[0160] In some embodiments, the adhesion force between the positive current collector and the positive active layer is F2, where F2 is 0.1gf / mm-5gf / mm. For example, F2 can be 0.1gf / mm, 0.5gf / mm, 1gf / mm, 3gf / mm, 5gf / mm, or within any two of the above values.

[0161] In some embodiments, the adhesion force between the positive electrode active layer and the surface coating layer is F3, where F3 is 0.5gf / mm-6gf / mm. For example, F3 can be 0.5gf / mm, 1gf / mm, 2gf / mm, 4gf / mm, 6gf / mm, or within any two of the above values.

[0162] In some embodiments, referring to FIG1, on the same side in the thickness direction of the positive electrode sheet, the length L1 of the bottom coating layer 13, the length L2 of the positive electrode active layer 12, and the length L3 of the top coating layer 14 satisfy: L2 > L1 > L3.

[0163] By controlling the lengths of both the base coating and the top coating to be shorter than the length of the positive electrode active layer—that is, by only applying the base coating and top coating to specific areas of the positive electrode—it is possible to minimize energy density loss due to increased thickness while meeting safety performance requirements. Furthermore, compared to hot box testing, nail penetration testing has a wider range of testing locations for the cell, therefore it is also necessary to ensure that the length of the base coating on the positive electrode is greater than the length of the top coating.

[0164] It is understandable that the length direction of the positive electrode active layer, the length direction of the bottom coating layer, and the length direction of the top coating layer 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.

[0165] Specifically, in some embodiments, the length L1 of the base coating is 100mm-2000mm. For example, L1 can be 100mm, 300mm, 500mm, 800mm, 1000mm, 1300mm, 1500mm, 1800mm, 2000mm, etc., or within the range of any two of the above values.

[0166] In some embodiments, the length L2 of the positive electrode active layer is 500mm-3000mm. For example, L2 can be 500mm, 800mm, 1000mm, 1300mm, 1500mm, 1800mm, 2000mm, 2300mm, 2500mm, 2800mm, 3000mm, etc., or within any two of the above values.

[0167] In some embodiments, the length L3 of the surface coating is 50mm-500mm. For example, L3 can be 50mm, 80mm, 100mm, 200mm, 300mm, 400mm, 500mm, etc., or within the range of any two of the above values.

[0168] According to a second aspect of this application, a lithium-ion secondary battery is provided, including a wound cell, as shown in FIG4. The wound cell includes a positive electrode sheet, a negative electrode sheet, and a separator 30 disposed between the positive electrode sheet and the negative electrode sheet as described in the first aspect of this application. The positive electrode sheet, the negative electrode sheet, and the separator 30 are stacked and wound together. Referring also to FIG1, the bottom coating layer 13 and the top coating layer 14 in the positive electrode sheet are both located at the winding tail end of the wound cell, and the top coating layer 14 is located on the side of the positive electrode sheet facing the interior of the wound cell.

[0169] In some embodiments, referring again to FIG4, a plurality of positive electrode tabs 15 are provided on the positive electrode sheet located in the flat region of the wound cell, and a plurality of negative electrode tabs 23 are provided on the negative electrode sheet located in the flat region of the wound cell; at the same time, a terminating adhesive 31 is also provided at the winding end of the positive electrode sheet.

[0170] The negative electrode sheet includes a negative electrode current collector 21 and a negative electrode active layer 22 disposed on at least one side surface of the negative electrode current collector 21. The negative electrode active layer 22 includes a negative electrode active material. In some embodiments of this application, the negative electrode active material includes graphite and silicon carbide materials. Based on the mass of the negative electrode active material, the negative electrode active material includes 4%-30% silicon carbide material, so that the resistivity ρ of the negative electrode sheet is relatively large, satisfying 0.5Ω·cm≤ρ≤60Ω·cm. As an example, ρ can be 0.5Ω·cm, 2Ω·cm, 10Ω·cm, 20Ω·cm, 30Ω·cm, 40Ω·cm, 50Ω·cm, 60Ω·cm, etc., or within any two of the above values.

[0171] It is understandable that while the presence of silicon in silicon-carbon anode materials (i.e., silicon anodes) is beneficial for increasing the energy density of the battery cell, it also leads to poor stability of the solid electrolyte (SEI) film on the anode surface at high temperatures. Therefore, by combining the silicon anode with the positive electrode sheet described in the first aspect of this application, the poor high-temperature performance of the silicon anode can be compensated for, enabling the wound battery cell of this application to simultaneously possess the advantages of high energy density, overall safety, and good rate performance.

[0172] In some embodiments, the silicon-carbon material comprises a porous carbon framework and silicon particles deposited on the porous carbon framework. As an example, the content of silicon-carbon material in the negative electrode active material may be 4%, 7%, 10%, 15%, 20%, 25%, 30%, or within any range of two of the aforementioned values.

[0173] Referring again to Figure 1, the arrows indicate the winding direction (i.e., the length direction) of the positive electrode sheet. The positive electrode sheet, along its winding direction, sequentially includes a third electrode region III, a second electrode region II, and a first electrode region I. Specifically, the first electrode region I includes the positive current collector 11 and the base coating 13, the positive active layer 12, and the surface coating 14 disposed on the surface of the positive current collector 11 facing the inside of the wound cell. The second electrode region II includes the positive current collector 11 and the base coating 13 and the positive active layer 12 disposed on both sides of the positive current collector 11. The third electrode region III includes the positive current collector 11 and the positive active layer 12 disposed on both sides of the positive current collector 11. In addition, the winding tail end of the positive electrode sheet also includes an empty foil region, meaning that no coating is applied to the positive current collector in this region.

[0174] The presence of the undercoat layer results in a higher sheet resistance in the second electrode region compared to the third electrode region. This reduces short-circuit current during mechanical abuse such as needle penetration, decreases heat generation, and improves the needle penetration safety of the second electrode region. Compared to the second electrode region, the first electrode region, which also includes an insulating or low-conductivity surface coating, has the highest sheet resistance. This further reduces heat generation from side reactions of the electrolyte in this region while ensuring needle penetration safety, thus simultaneously improving both the thermal stability and needle penetration safety of the first electrode region. Therefore, in the positive electrode of this application, the sheet resistances of the three electrode regions satisfy R1 > R2 > R3, ensuring the overall safety of the cell. Furthermore, thanks to the localized placement of the undercoat and surface coating on the positive electrode, the cell thickness is reduced while meeting safety performance requirements, avoiding excessive energy density loss. In addition, the use of small-diameter inorganic particles in both the undercoat and surface coating not only shortens the lithium-ion transport path and improves the cell's rate performance but also prevents volumetric energy density loss due to excessive cell thickness, ensuring the battery's electrical performance.

[0175] In some embodiments, the surface resistance R1 of the first electrode region is 1Ω / cm-3Ω / cm. For example, R1 can be 1Ω / cm, 1.5Ω / cm, 2Ω / cm, 2.5Ω / cm, 3Ω / cm, etc., or within the range of any two of the above values.

[0176] In some embodiments, the surface resistance R2 of the second electrode region is 0.5Ω / cm-1.8Ω / cm. R2 can be 0.5Ω / cm, 0.8Ω / cm, 1Ω / cm, 1.3Ω / cm, 1.8Ω / cm, or within any two of the above values.

[0177] In some embodiments, the surface resistance R3 of the third electrode region is 0.3Ω / cm-1.0Ω / cm, and R3 can be 0.3Ω / cm, 0.5Ω / cm, 0.7Ω / cm, 0.9Ω / cm, 1Ω / cm, or within any two of the above values.

[0178] It should be noted that surface resistance refers to the resistance value of an electrode per unit area.

[0179] In some embodiments, the thickness H1 of the first electrode region is 20μm-90μm, the thickness H2 of the second electrode region is 20μm-160μm, and the thickness H3 of the third electrode region is 20μm-160μm, and satisfies: H1>0.25*(H2+H3).

[0180] It is understandable that for a positive electrode without a base coating and a top coating, the thickness of the single-sided region (i.e., the positive active layer is only provided on one side of the positive current collector, corresponding to the first electrode region in this application) is approximately half the thickness of the double-sided region (i.e., the positive active layer is provided on both sides of the positive current collector, corresponding to the second or third electrode region in this application), which is also 1 / 4 of the sum of the thicknesses of the second and third electrode regions. This application increases the thickness of the first electrode region by providing a base coating and a top coating, ensuring that the thicknesses of the three electrode regions satisfy H1 > 0.25*(H2+H3), preferably H1 > 0.25*(H2+H3)+2. However, to avoid affecting the cell's energy density and rate performance, the thickness of the first electrode region must not be too large, i.e., H1 should not exceed 90μm. This effectively improves the overall safety and rate performance of the cell without significantly reducing its energy density.

[0181] As an example, the thickness H1 of the first electrode region can be 20μm, 40μm, 60μm, 80μm, 90μm, etc., or within the range of any two of the above values; the thickness H2 of the second electrode region can be 20μm, 60μm, 80μm, 100μm, 120μm, 140μm, 160μm, etc., or within the range of any two of the above values; and the thickness H3 of the third electrode region can be 20μm, 60μm, 80μm, 100μm, 120μm, 140μm, 160μm, etc., or within the range of any two of the above values.

[0182] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Where the manufacturers of reagents or instruments are not specified, they are all commercially available conventional reagent products. In all embodiments and comparative examples of this application, the unit % represents mass percentage content.

[0183] Example 1

[0184] The preparation of the positive electrode sheet with the structure shown in Figure 1 includes the following steps:

[0185] Step 1: Preparing the base coating

[0186] Alumina (Dv501, 30nm), conductive carbon black, carbon nanotubes, and PVDF were taken in a mass ratio of 90:2:3:5. The alumina, carbon black, and carbon nanotubes were mixed thoroughly, and then PVDF and N-methylpyrrolidone (NMP) were added and stirred until homogeneous, resulting in a base coating slurry with a solid content of 30%. This base coating slurry was then applied to both sides of the aluminum foil in the thickness direction using a gravure coating method. After drying, a positive current collector with a localized base coating was obtained.

[0187] Step 2: Preparation of the positive electrode active layer

[0188] Conductive carbon black and carbon nanotubes were added to PVDF and stirred until homogeneous. Then, lithium cobalt oxide (Dv503, 15 μm) was added and stirred until homogeneous, resulting in a positive electrode active material slurry with a solid content of 75%. The positive electrode active material slurry was coated onto both sides of the positive electrode current collector with a local undercoat prepared in the first step. After drying, the positive electrode active layer was obtained. The positive electrode active layer contained 97.6% lithium cobalt oxide by mass, 1.05% PVDF by mass, and 1.35% conductive carbon black and carbon nanotubes by mass (carbon black to carbon nanotube mass ratio of 1:1). The lithium cobalt oxide consisted of a first particle with a particle size D1 of 20 μm and a second particle with a particle size D2 of 5 μm.

[0189] For ease of description, the two sides of the positive electrode are defined as surface A and surface C, respectively, where the length of the positive electrode active layer on surface A is greater than the length of the positive electrode active layer on surface C.

[0190] Step 3: Preparing the surface coating

[0191] Silica (Dv502 of 20nm) was added to PVDF and stirred until homogeneous. Then, NMP was added and stirred until homogeneous again to prepare a surface coating slurry with a solid content of 25%. This slurry was then coated onto the surface of the A-side positive electrode active layer prepared in the second step. After baking and rolling, the positive electrode sheet was obtained. The surface coating contained 90% silica and 10% PVDF by mass.

[0192] In this embodiment, the length of the bottom coating layer (L1) on surface A of the positive electrode is 200 mm, the length of the positive electrode active layer (L2) is 1500 mm, and the length of the top coating layer (L3) is 120 mm, satisfying L2 > L1 > L3. The thickness of the first electrode region (H1) is 55 μm, the thickness of the second electrode region (H2) is 96 μm, and the thickness of the third electrode region (H3) is 90 μm, satisfying H1 > 0.25*(H2+H3)+2.

[0193] The preparation of the negative electrode includes the following steps:

[0194] 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 were mixed evenly, and then an appropriate amount of deionized water was added to disperse the mixture evenly to prepare a negative electrode active material slurry. Based on the mass of the negative electrode active material, the negative electrode active material contained 10% silicon carbon and 90% graphite. The negative electrode active material slurry was coated on both sides of a carbon-coated copper foil, and after baking and rolling, a negative electrode sheet was obtained.

[0195] The preparation of lithium-ion secondary batteries includes the following steps:

[0196] The positive and negative electrode sheets are slit, fabricated, and wound with a separator to obtain a core. The core undergoes packaging, baking, electrolyte injection, formation, secondary sealing, sorting, and OCV to obtain a lithium-ion secondary battery. The A-side of the positive electrode sheet faces the winding center of the core, while the C-side faces away from the winding center. The electrolyte is a commercially available conventional electrolyte, and the lithium salt in the electrolyte is LiFP6.

[0197] Figure 2 is a cross-sectional SEM image of the surface coating obtained in this embodiment. As can be seen from the figure, the surface coating is partially embedded in the positive electrode active layer. The thickness of the surface coating at different locations can be 1.98 μm, 3.57 μm, and 3.97 μm. The unit area (mm²) of the surface coating... 2 The difference h1 between the maximum and minimum thickness of the inner surface coating is 1.99 μm.

[0198] Figure 3 is a cross-sectional SEM image of the base coating obtained in this embodiment. As can be seen from the figure, the base coating is partially embedded in the positive electrode active layer. The thickness of the base coating at different locations can be 2.86 μm, 3.37 μm, and 5.21 μm. The unit area of ​​the base coating is [missing information - likely a number in mm]. 2The difference h2 between the maximum and minimum thickness of the inner bottom coating is 2.35 μm.

[0199] Example 2

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

[0201] The base coating consists of 80% alumina, 10% PVDF, 4% conductive carbon black, and 6% carbon nanotubes.

[0202] Example 3

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

[0204] The base coating consists of 99% alumina, 0.5% PVDF, and 0.5% carbon nanotubes.

[0205] Example 4

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

[0207] The base coating consists of 95% magnesium oxide, 3% PP and 2% metallic magnesium powder.

[0208] Example 5

[0209] Except for the following, the rest of the content is the same as in Example 1.

[0210] The base coating consists of 85% silica, 5% PVA and 10% carbon fiber.

[0211] Example 6

[0212] Except for the following, the rest of the content is the same as in Example 1.

[0213] The base coating consists of 80% alumina, 12% PVDF, 4% conductive carbon black, and 4% carbon nanotubes.

[0214] Example 7

[0215] Except for the following, the rest of the content is the same as in Example 1.

[0216] The base coating consists of 89.6% alumina, 0.4% PVDF, 4% conductive carbon black, and 6% carbon nanotubes.

[0217] Example 8

[0218] Except for the following, the rest of the content is the same as in Example 1.

[0219] The surface coating consists of 99% silica and 1% PVDF.

[0220] Example 9

[0221] Except for the following, the rest of the content is the same as in Example 1.

[0222] The surface coating consists of 95% magnesium oxide and 5% PTFE.

[0223] Example 10

[0224] Except for the following, the rest of the content is the same as in Example 1.

[0225] The surface coating consists of 99.5% silica and 0.5% PVDF.

[0226] Example 11

[0227] Except for the following, the rest of the content is the same as in Example 1.

[0228] The surface coating consists of 89% silica and 11% PVDF.

[0229] Example 12

[0230] Except for the following, the rest of the content is the same as in Example 1.

[0231] The particle size of alumina in the base coating is 100 nm (Dv501), the particle size of lithium cobalt oxide in the positive electrode active layer is 15 μm (Dv503), and the particle size of silicon dioxide in the top coating is 50 nm (Dv502), satisfying Dv503≥100*(Dv501+Dv502).

[0232] The lithium cobalt oxide comprises a first particle and a second particle, wherein the median particle size D1 of the first particle is 15 μm and the median particle size D2 of the second particle is 10 μm.

[0233] Example 13

[0234] Except for the following, the rest of the content is the same as in Example 1.

[0235] The particle size of alumina in the base coating is 50 nm (Dv501), the particle size of lithium cobalt oxide in the positive electrode active layer is 30 μm (Dv503), and the particle size of silicon dioxide in the top coating is 100 nm (Dv502), satisfying Dv503≥100*(Dv501+Dv502).

[0236] The lithium cobalt oxide comprises a first particle and a second particle, wherein the median particle size D1 of the first particle is 30 μm and the median particle size D2 of the second particle is 1 μm.

[0237] Example 14

[0238] Except for the following, the rest of the content is the same as in Example 1.

[0239] The particle size of alumina in the base coating is 10 nm (Dv501), the particle size of lithium cobalt oxide in the positive electrode active layer is 5 μm (Dv503), and the particle size of silicon dioxide in the top coating is 70 nm (Dv502), which does not satisfy the condition Dv503≥100*(Dv501+Dv502).

[0240] Example 15

[0241] Except for the following, the rest of the content is the same as in Example 1.

[0242] The thickness H1 of the first electrode region is 90 μm, the thickness H2 of the second electrode region is 160 μm, and the thickness H3 of the third electrode region is 150 μm, satisfying: H1>0.25*(H2+H3)+2.

[0243] Example 16

[0244] Except for the following, the rest of the content is the same as in Example 1.

[0245] The thickness H1 of the first electrode region is 20 μm, the thickness H2 of the second electrode region is 40 μm, and the thickness H3 of the third electrode region is 30 μm, satisfying: H1>0.25*(H2+H3)+2.

[0246] Example 17

[0247] Except for the following, the rest of the content is the same as in Example 1.

[0248] The thickness H1 of the first electrode region is 50 μm, the thickness H2 of the second electrode region is 120 μm, and the thickness H3 of the third electrode region is 100 μm, which does not satisfy the condition: H1 > 0.25*(H2+H3)+2.

[0249] Example 18

[0250] Except for the following, the rest of the content is the same as in Example 1.

[0251] The negative electrode active material contains 4% silicon carbon and 96% graphite.

[0252] Example 19

[0253] Except for the following, the rest of the content is the same as in Example 1.

[0254] The negative electrode active material contains 20% silicon carbon and 80% graphite.

[0255] Example 20

[0256] Except for the following, the rest of the content is the same as in Example 1.

[0257] The negative electrode active material contains 30% silicon carbon and 70% graphite.

[0258] Example 21

[0259] Except for the following, the rest of the content is the same as in Example 1.

[0260] The negative electrode active material contains 50% silicon carbon and 50% graphite.

[0261] Example 22

[0262] Except for the following, the rest of the content is the same as in Example 1.

[0263] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 100mm, the length of the positive electrode active layer L2 is 500mm, and the length of the top coating layer L3 is 50mm, satisfying L2>L1>L3.

[0264] Example 23

[0265] Except for the following, the rest of the content is the same as in Example 1.

[0266] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 1000mm, the length of the positive electrode active layer L2 is 3000mm, and the length of the top coating layer L3 is 500mm, satisfying L2>L1>L3.

[0267] Example 24

[0268] Except for the following, the rest of the content is the same as in Example 1.

[0269] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 2000mm, the length of the positive electrode active layer L2 is 2200mm, and the length of the top coating layer L3 is 270mm, satisfying L2>L1>L3.

[0270] Example 25

[0271] On the surface of the positive electrode A, the length L3 of the top coating is equal to the length L1 of the bottom coating, both being 200 mm.

[0272] Comparative Example 1

[0273] Except for the following, the rest of the content is the same as in Example 1.

[0274] There is no base coating or surface coating on the positive electrode plate.

[0275] Comparative Example 2

[0276] Except for the following, the rest of the content is the same as in Example 1.

[0277] The median particle size Dv501 of alumina in the base coating is 550 nm.

[0278] Comparative Example 3

[0279] Except for the following, the rest of the content is the same as in Example 1.

[0280] The median particle size Dv502 of silica in the surface coating is 550 nm.

[0281] Test case

[0282] 1. Particle size test

[0283] Tests were performed using a laser particle size analyzer.

[0284] 2. Coating thickness and embedding depth test

[0285] This was derived from the SEM image of the coating cross-section.

[0286] 3. Adhesion test

[0287] Testing equipment: tensile testing machine.

[0288] The rolled positive electrode sheet is cut into 25mm widths. Along the boundary lines between the first electrode area I and the second electrode area II, and between the second electrode area II and the third electrode area III, as shown in Figure 1, the positive electrode sheet is sequentially divided into the first electrode area, the second electrode area, and the third electrode area.

[0289] Prepare a flat, thin steel plate and attach a strip of double-sided tape to its center. Adhere the third electrode area to the tape, ensuring a perfect fit between the electrode and the tape to avoid testing errors. Insert the steel plate with the adhered electrode into the lower clamp of the tensile testing machine and fix it vertically. Clamp the electrode without adhesive on the upper clamp, fixing it at a 90° angle to the electrode in the lower clamp. Set parameters such as test width, peel length, and peel speed on the tensile testing machine. Start the test program; the tensile testing machine will apply tensile force according to the set parameters and record the force changes during the peeling process. After the test, the peel strength curve and average value will be obtained, which represents the adhesion force F1 between the positive electrode current collector and the base coating.

[0290] Using the same method described above, the second electrode region and the third electrode region were tested respectively to obtain the adhesion force F2 between the positive current collector and the positive active layer, and the adhesion force F3 between the positive active layer and the surface coating.

[0291] 4. Surface resistance test

[0292] Discharge the battery to the lower limit voltage, then disassemble it in a drying room, take out the positive electrode and let it air dry naturally in the drying room, and then place the positive electrode on a two-probe surface resistance tester for testing.

[0293] 5. Resistivity test

[0294] Discharge the battery to the lower limit voltage, then disassemble it in a drying room, take out the negative electrode sheet and let it air dry naturally in the drying room, and then place the negative electrode sheet on a four-probe resistivity tester for testing.

[0295] 6. Energy density retention rate test

[0296] The prepared battery was charged to full capacity under constant current and constant voltage at 25℃, and then discharged to 3.0V at 0.5C. The discharged capacity was recorded as the battery capacity. The prepared battery was charged to 50% SOC at 25℃, and the battery thickness was tested using 600gPPG.

[0297] Calculate the volumetric energy density (ED) = battery capacity * platform voltage / battery length / width / thickness.

[0298] Calculate the ED loss rate as follows: (Energy density of Comparative Example 1 - Energy density of Example) / Energy density of Comparative Example 1.

[0299] 7. Capacity retention test

[0300] The prepared battery was charged to full capacity at constant current and constant voltage at 25℃, and then discharged to 3.0V at 0.2C. The discharged capacity was recorded as battery capacity C1. The battery was charged to full capacity again at constant current and constant voltage, and then discharged to 3.0V at 1C. The discharged capacity was recorded as battery capacity C2. 1C discharge capacity retention rate = C2 / C1*100%.

[0301] 8. Needle prick test

[0302] At room temperature, the battery cell is discharged at 1C to 3.0V, then charged at a constant current of 0.7C to 4.50V, with a cutoff current of 0.02C. This process of discharging at 1C to 3.0V is repeated five times. Then, it is charged again at a constant current of 0.7C to 4.50V, with a cutoff current of 0.02C. Within 48 hours of the test completion, a 2.5mm diameter steel needle is used to vertically penetrate the lithium-ion battery at three positions (left, center, and right) at a speed of 30mm / s. If the battery does not ignite or explode, it passes the test. Twenty samples are tested, and the pass rate of each sample is observed.

[0303] 9. Hot Box Test

[0304] The battery cell is fully charged to its maximum operating voltage of 4.5V. It is then placed in an oven and heated to the set target temperature (130℃, 132℃, 135℃, or 140℃) at a rate of 5±2℃ / min, and held for 60 minutes. The test is then complete. The cell is considered passed if it does not catch fire or explode; otherwise, it fails the test if the temperature continues to rise until it catches fire or explodes. Ten samples are tested, and each sample is observed to determine if it passes the test.

[0305] The test results are shown in Tables 1 and 2. In Table 1, " / " indicates that the item does not exist. F1 represents the adhesion between the positive current collector and the base coating, F2 represents the adhesion between the positive current collector and the positive active layer, F3 represents the adhesion between the positive active layer and the top coating, R1 is the sheet resistance of the first electrode area, R2 is the sheet resistance of the second electrode area, R3 is the sheet resistance of the third electrode area, and ρ is the resistivity of the negative electrode.

[0306] Table 1

[0307] Table 2

[0308] As can be seen from Tables 1 and 2, the positive electrode in Comparative Example 1 lacks both a base coating and a surface coating, resulting in poor safety for the battery, which fails the 2.5mm needle penetration test and the 140℃ furnace temperature test. Compared to Comparative Example 1, all embodiments show varying degrees of improved safety, lower ED loss rates, and better rate performance.

[0309] Compared to Example 1, the bottom coating in Comparative Example 2 uses inorganic particles with larger particle size, which is not conducive to improving the battery's needle penetration safety. In contrast, the top coating in Comparative Example 3 uses inorganic particles with larger particle size, which cannot effectively improve the battery's furnace temperature test safety. Furthermore, the rate performance of Comparative Examples 2-3 is significantly inferior to that of Example 1.

[0310] This demonstrates that by providing a base coating and a top coating at one end of the positive electrode along its length, and by ensuring that the inorganic particles in the base coating and top coating have a particle size of less than 500 nm, this application can improve the overall safety and rate performance of the battery cell with almost no loss in energy density.

[0311] To improve the low-temperature performance of the battery cell, according to a third aspect of this application, a wound battery cell is provided. Referring to Figure 4, the wound battery cell includes a positive electrode, a negative electrode, a separator 30, and an electrolyte. The positive electrode, the separator 30, and the negative electrode are stacked and wound together. A plurality of positive electrode tabs 15 are provided on the positive electrode located in the flat area of ​​the wound battery cell, and a plurality of negative electrode tabs 23 are provided on the negative electrode located in the flat area of ​​the wound battery cell. At the same time, a terminating adhesive 31 is provided at the winding end of the positive electrode.

[0312] Referring to Figure 1, the positive electrode sheet includes a positive current collector 11 and a positive active layer 12 disposed on at least one side surface of the positive current collector 11; the arrow indicates the winding direction of the positive electrode sheet, and a base coating 13 and a surface coating 14 are provided at the winding tail end of the positive electrode sheet. The base coating 13 is located between the positive current collector 11 and the positive active layer 12, and the base coating includes first inorganic particles; the surface coating 14 is located on the surface of the positive active layer 12 facing away from the positive current collector 11, and the surface coating includes second inorganic particles;

[0313] The electrolyte includes a carboxylic acid ester solvent, and the mass content of the carboxylic acid ester solvent is A% based on the mass of the electrolyte, with 10 ≤ A ≤ 50%.

[0314] In mechanical abuse tests such as nail penetration, the deformation caused by abuse is greater closer to the outer edge of the wound battery cell, making it more prone to triggering internal short circuits. This is especially true for contact short circuits between the positive current collector and the negative active material, which generate the most heat and are therefore the most dangerous. Therefore, this application provides an undercoating at the winding end of the positive electrode sheet (corresponding to the outer edge of the wound battery cell) to prevent contact short circuits between the positive current collector and the negative active material layer, thus improving the battery cell's pass rate in nail penetration tests.

[0315] In hot box tests, the outer side of the wound cell heats up the fastest and is most prone to thermal runaway. Therefore, this application provides a surface coating at the winding end of the positive electrode to isolate the positive electrode from the electrolyte on the outer side of the cell, reduce heat generation from side reactions, and thus improve the thermal safety of the cell.

[0316] This application only requires setting a base coating and a surface coating at the winding end of the positive electrode sheet to effectively reduce the risk of the cell in tests such as nail penetration and hot box, improve the overall safety performance of the cell, and at the same time prevent the cell from being too thick and affecting the volumetric energy density, thus ensuring that the overall safety of the cell is improved without affecting its electrical performance.

[0317] It is understandable that high-temperature resistant electrolyte systems are typically used to ensure battery safety. However, this results in poor low-temperature performance, failing to meet the requirements for use in low-temperature environments. Therefore, this application improves the low-temperature performance of the battery cell by adding an appropriate amount of carboxylic acid ester solvent to the electrolyte, enabling the wound battery cell of this application to possess both good safety and low-temperature performance.

[0318] As an example, the mass content A% of carboxylic acid ester solvent in the electrolyte can be 10%, 20%, 30%, 40%, 50%, or within any range of two of the above values. This application's research found that if the content of carboxylic acid ester solvent in the electrolyte is too low, the improvement effect on the low-temperature performance of the battery is not significant; conversely, if the content of carboxylic acid ester solvent in the electrolyte is too high, it will affect the thermal safety of the battery.

[0319] As an example, the carboxylic acid ester solvent includes at least one of methyl formate, ethyl acetate, ethyl propionate, methyl acetate, methyl butyrate, methyl propyl carbonate, propyl propionate, ethyl butyrate, methyl fluoroformate, ethyl difluoroacetate, ethyl 2-fluoropropionate, methyl fluoroacetate, methyl 4-fluorobutyrate, propyl 2-fluoropropionate, and ethyl 4-fluorobutyrate.

[0320] It should be noted that the outer side of the cell refers to the side away from the winding center of the wound cell, using the winding center of the wound cell as a reference. Correspondingly, the inner side of the cell refers to the side facing the winding center, using the winding center of the wound cell as a reference.

[0321] Please refer to Figure 1. The arrows indicate the winding direction (i.e., the length direction) of the positive electrode sheet. The positive electrode sheet, along its winding direction, sequentially includes a third electrode region III, a second electrode region II, and a first electrode region I. Specifically, the first electrode region I includes the positive current collector 11 and the base coating 13, the positive active layer 12, and the surface coating 14 disposed on the surface of the positive current collector 11 facing the inside of the wound cell. The second electrode region II includes the positive current collector 11 and the base coating 13 and the positive active layer 12 disposed on both sides of the positive current collector 11. The third electrode region III includes the positive current collector 11 and the positive active layer 12 disposed on both sides of the positive current collector 11. In addition, the winding tail end of the positive electrode sheet also includes an empty foil region, meaning that no coating is applied to the positive current collector in this region.

[0322] In some embodiments, the resistivity R of the first electrode region is in the range of 200 Ω·cm to 1000 Ω·cm. For example, R can be 200 Ω·cm, 300 Ω·cm, 400 Ω·cm, 500 Ω·cm, 600 Ω·cm, 700 Ω·cm, 800 Ω·cm, 900 Ω·cm, 1000 Ω·cm, or any combination of the above values. Compared to the positive electrode active layer, both the undercoat and topcoat have lower conductivity, increasing the resistivity of the first electrode region. This reduces short-circuit current and heat generation during needle penetration abuse, and also reduces heat generation from side reactions between this region and the electrolyte during hot-box testing. Furthermore, because the undercoat and topcoat are only applied to the winding tail end (i.e., locally) of the positive electrode, excessive increases in cell thickness can be avoided, thereby improving overall safety with almost no loss in cell energy density. Using an electrolyte containing carboxylic acid ester solvents can further improve the low-temperature performance of the cell.

[0323] This application research found that when the resistivity R of the first electrode region and the mass content A% of the carboxylic acid ester solvent in the electrolyte satisfy 8≤R / A≤80, on the one hand, the battery cell can be guaranteed to have high energy density and overall safety, and on the other hand, the low-temperature performance of the battery cell can be further improved. This makes the wound battery cell of this application have good electrical performance and safety while also having good low-temperature performance.

[0324] As an example, the R / A value can be 8, 10, 15, 25, 40, 55, 70, 80, or within any range of two of the above values. If the R / A value is too small, it will affect the safety performance of the battery cell, while if the R / A value is too large, it will be detrimental to the low-temperature performance of the battery cell.

[0325] To further enhance the thermal safety of the battery cell, in some embodiments, the electrolyte further includes lithium salt and nitrile additives. Based on the mass of the electrolyte, the mass content (B%) of the nitrile additives is in the range of 2%-10%. For example, the mass content of the nitrile additives can be 2%, 4%, 6%, 8%, 10%, or any range consisting of two of the above values. If the content of the nitrile additives is too low, the desired effect will not be achieved; if the content of the nitrile additives is too high, it will increase the viscosity of the electrolyte, especially at low temperatures, thereby affecting the low-temperature performance of the battery cell.

[0326] As an example, the nitrile additive may be at least one of butadionitrile, glutaronitrile, adiponitrile, trans-butenedionitrile, trans-hexenedionitrile, glyceryl trionitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrionitrile, 1,2,3-tris(cyanoethoxy)propane, and 1,4-dicyano-2-butene.

[0327] As an example, the lithium salt may be one or a combination of several of the following: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluoroantimonyate (LiSbF6), lithium difluorophosphate (LiPF2O2), lithium 4,5-dicyano-2-trifluoromethylimidazolium (LiDTI), lithium dioxadienoate borate (LiBOB), lithium difluorooxalate borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium nitrate (LiNO3), LiN(SO2RF)2, and LiN(SO2F)(SO2RF), wherein RF = C n F 2n+1 n is an integer from 1 to 10.

[0328] This application research found that when the mass content B% of nitrile additives in the electrolyte and the resistivity R of the first electrode region satisfy 30≤R / B≤200, on the one hand, it can further ensure that the battery cell has a high energy density and overall safety, and on the other hand, it can also improve the low-temperature performance of the battery cell. This makes the wound battery cell of this application have good electrical performance and safety while also having good low-temperature performance.

[0329] As an example, the R / B value can be 30, 60, 90, 120, 150, 180, 200, or within any two of these values. If the R / B value is too small, the cell cannot simultaneously achieve thermal safety and low-temperature performance; if the R / B value is too large, it will affect the cell's electrical performance.

[0330] In some embodiments, the positive electrode is subjected to an ARC (Accelerating Rate Calorimetry) self-exothermic test, and the specific test method is as follows:

[0331] The wound battery cell of this application was fully charged with a 0.5C current (cutoff at 0.02C). The fully charged positive electrode was disassembled in a dissection room. The fully charged positive electrode and electrolyte (ethylene carbonate: diethyl carbonate = 3:7, volume ratio, where the lithium salt is 1.0 mol / L LiPF6) were placed in an aluminum-plastic film and sealed to prepare a dummy battery cell. The sample was tested using the heating-wait-heating mode of an ARC device until thermal runaway. The time required for the surface temperature of the first electrode region to reach 150°C was defined as T1, for example, 1500 min. The time required for the surface temperature of the second electrode region and / or the third electrode region to reach 150°C was defined as T2, for example, 1100 min, where T1 - T2 > 300 min. This demonstrates that adding a surface coating can significantly improve the thermal safety of the positive electrode.

[0332] In some embodiments, the coating porosity P1 of the first electrode region is 10%-14%. For example, P1 can be 10%, 11%, 12%, 13%, 14%, or any range of any two of the above values. The coating porosity P2 of the second electrode region is 14%-18%. For example, P2 can be 14%, 15%, 16%, 17%, 18%, or any range of any two of the above values. The coating porosity P3 of the third electrode region is 18%-25%. For example, P3 can be 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or any range of any two of the above values.

[0333] This study found that when the porosity P1 of the coating in the first electrode region, the porosity P2 of the coating in the second electrode region, and the porosity P3 of the coating in the third electrode region satisfy P1 < P2 < P3, it indicates that, compared to the third electrode region, the second electrode region has a lower porosity due to the presence of the undercoating layer, which increases the density of the coating in the second electrode region. This effectively prevents contact between the positive current collector and the negative active material layer during needle penetration abuse, improving the needle penetration safety of the battery cell. Similarly, compared to the second electrode region, the first electrode region includes both an undercoating layer and a topcoat layer, resulting in the lowest porosity and highest density of the coating in the first electrode region. This not only reduces the wetting ability of the electrolyte in this area and reduces heat generation from side reactions, but also better resists needle penetration tests, thereby improving the overall safety of the battery cell. Furthermore, compared to existing technologies that apply a base coating or surface coating to the entire positive electrode sheet, this application only requires applying the base coating and surface coating to the winding tail end of the positive electrode sheet. This effectively reduces the risks to the cell during tests such as nail penetration and hot box tests, while also preventing the cell from being too thick and affecting its volumetric energy density, thus ensuring improved overall safety without compromising the cell's electrical performance. Furthermore, using an electrolyte containing carboxylic acid ester solvents can further improve the cell's low-temperature performance.

[0334] Please continue referring to Figure 1. In some optional embodiments, on the same side of the positive electrode thickness direction, the length L1 of the bottom coating layer 13, the length L2 of the positive electrode active layer 12, and the length L3 of the top coating layer 14 satisfy: (L1+L3)≤1.5*L2. By controlling the lengths of the bottom coating layer and the top coating layer within the above range, it is possible to minimize the increase in cell thickness while ensuring good overall cell safety. Combined with the use of an electrolyte containing carboxylic acid ester solvents, the cell exhibits advantages such as high energy density, safety, and good low-temperature performance.

[0335] It is understandable that the length direction of the positive electrode active layer, the length direction of the bottom coating layer, and the length direction of the top coating layer 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.

[0336] Specifically, in some embodiments, the length L1 of the base coating is 100mm-2000mm. For example, L1 can be 100mm, 300mm, 500mm, 800mm, 1000mm, 1300mm, 1500mm, 1800mm, 2000mm, etc., or within the range of any two of the above values.

[0337] In some embodiments, the length L2 of the positive electrode active layer is 500mm-3000mm. For example, L2 can be 500mm, 800mm, 1000mm, 1300mm, 1500mm, 1800mm, 2000mm, 2300mm, 2500mm, 2800mm, 3000mm, etc., or within any two of the above values.

[0338] In some embodiments, the length L3 of the surface coating is 50mm-500mm. For example, L3 can be 50mm, 80mm, 100mm, 200mm, 300mm, 400mm, 500mm, etc., or within the range of any two of the above values.

[0339] Compared to hot box testing, nail penetration testing requires a wider range of testing locations within the cell, necessitating a longer undercoat layer than the topcoat layer. Furthermore, to prevent excessive cell thickness from affecting energy density, in some embodiments, the topcoat layer and / or the undercoat layer are partially embedded in the positive electrode active layer, with the embedding depth h1 of the topcoat layer being less than the embedding depth h2 of the undercoat layer. This reduces internal cell polarization, minimizes energy density loss, and improves overall safety without compromising cell electrical performance. Additionally, using an electrolyte containing carboxylic acid ester solvents further enhances the cell's low-temperature performance.

[0340] In some embodiments, in the thickness direction of the positive electrode sheet, the minimum thickness h3 of the top coating is 0.5μm-4μm. For example, h3 can be 0.5μm, 1μm, 2μm, 3μm, 4μm, etc., or within the range of any two of the above values; the minimum thickness h4 of the bottom coating is 2μm-6μm. For example, h4 can be 2μm, 3μm, 4μm, 5μm, 6μm, etc., or within the range of any two of the above values.

[0341] This application research found that when the minimum thickness h3 of the top coating is less than or equal to the minimum thickness h4 of the bottom coating, the internal polarization of the battery cell can be reduced, the energy density loss can be reduced, and the safety performance of the battery cell can be guaranteed. Furthermore, when combined with an electrolyte containing carboxylic acid ester solvents, the low-temperature performance of the battery cell can be further improved.

[0342] Please refer to Figures 2 and 3. In some embodiments, the thickness of the topcoat layer is, for example, 1.98 μm, 3.57 μm, or 3.97 μm, and the thickness of the bottomcoat layer is 2.86 μm, 3.37 μm, or 5.21 μm. As can be seen from Figures 2 and 3, the embedding depth h1 of the topcoat layer into the positive electrode active layer is 0.5 μm-3 μm, for example, it can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, or fall within any two of the above values; the embedding depth h2 of the bottomcoat layer into the positive electrode active layer is 1 μm-3 μm, for example, it can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, or fall within any two of the above values.

[0343] Understandably, "embedding depth" represents the degree to which the base coating / top coating is embedded into the positive electrode active layer, expressed per unit area (mm²). 2 The difference between the maximum thickness and the minimum thickness of the bottom / top coating in the cross-sectional SEM image at the interface between the inner bottom / top coating and the positive electrode active layer is used to represent this.

[0344] In some embodiments, the base coating comprises, based on its mass, 80%-90% of first inorganic particles, 0.5%-10% of a first binder, and 0.5%-10% of a conductive agent. All percentages are by mass.

[0345] As an example, the first inorganic particles include at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide; 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), or derivatives thereof; the conductive agent includes at least one of conductive carbon black (SP), acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, carbon fiber, and graphene, wherein 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.

[0346] The base coating uses non-conductive or low-conductive inorganic particles to ensure good insulation when the base coating comes into contact with the negative electrode active material layer during the needle penetration test, thereby improving the needle penetration safety of the cell. At the same time, in order to ensure the electron conduction between the positive electrode active layer and the positive electrode current collector, the base coating needs to have a certain degree of conductivity, so the base coating also includes a small amount of conductive agent.

[0347] In some embodiments, the surface coating comprises 90%-99% of second inorganic particles and 1%-10% of a second binder, based on the mass of the surface coating. The percentages above are by mass.

[0348] As an example, the second inorganic particle includes at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide; 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), or derivatives thereof.

[0349] The surface coating uses non-conductive or low-conductive inorganic particles, which makes the surface coating insulating or low-conductive. This can slow down the oxidation reaction of the electrolyte at high potentials, reduce the occurrence of side reactions, and increase the internal resistance of the cell when it runs away, thus improving the thermal safety of the cell.

[0350] In some embodiments, the separator includes a substrate, a first adhesive layer, a ceramic layer, and a second adhesive layer. The first adhesive layer is disposed on one side surface of the substrate, the second adhesive layer is disposed on the surface of the substrate opposite to the first adhesive layer, and the ceramic layer is located between the substrate and the second adhesive layer and faces the positive electrode.

[0351] The ceramic layer comprises ceramic particles. In some embodiments, the median particle size Dv50 of the ceramic particles is 0.5 μm-2 μm. For example, it can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, or within any two of the above values. In this application, the median particle size Dv50 of the ceramic particles is obtained by laser particle size analyzer.

[0352] Using smaller ceramic particles results in a thinner ceramic layer and better thermal shrinkage of the separator, preventing short circuits in the battery cell under overheating or abnormal conditions, thus ensuring battery safety. As an example, the ceramic particles include at least one of boehmite, alumina, zirconium oxide, silicon dioxide, silicon carbide, and silicon nitride.

[0353] This study found that when the median particle size Dv50 of the ceramic particles and the average thickness H of the surface coating satisfy 0.5 ≤ H / Dv50 ≤ 20, the thermal safety of the battery cell can be further improved. Furthermore, since the surface and base coatings are only applied at the winding end of the positive electrode, excessive increases in cell thickness can be avoided, thus improving overall safety with almost no loss in energy density. Using an electrolyte containing carboxylic acid ester solvents can further improve the low-temperature performance of the cell. If the H / Dv50 value is too small, it indicates that the ceramic particle size is too large or the surface coating thickness is too small, which is detrimental to the thermal safety of the cell. If the H / Dv50 value is too large, it indicates that the ceramic particle size is too small or the surface coating thickness is too large, leading to significant energy density loss and also hindering the improvement of low-temperature performance.

[0354] As an example, the value of H / Dv50 can be 0.5, 1, 3, 7, 10, 13, 17, 20, etc., or fall within the range of any two of the above values.

[0355] It should be noted that the average thickness of the surface coating is obtained by measuring the surface coating thickness at 10 locations in the cross-sectional SEM image of the positive electrode and calculating the average value.

[0356] Specifically, in some embodiments, the average thickness H of the surface coating is 1μm-10μm. For example, H can be 1μm, 3μm, 5μm, 7μm, 10μm, etc., or within the range of any two of the above values.

[0357] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Where the manufacturers of reagents or instruments are not specified, they are all commercially available conventional reagent products. In all embodiments and comparative examples of this application, the unit % represents mass percentage content.

[0358] Example 26

[0359] Step 1: Preparing the base coating

[0360] Alumina, conductive carbon black, carbon nanotubes, and PVDF were sequentially selected in a mass ratio of 90:2:3:5. The alumina, carbon black, and carbon nanotubes were mixed thoroughly, followed by the addition of PVDF and N-methylpyrrolidone (NMP) solvent, and stirred until homogeneous, yielding a base coating slurry with a solid content of 30%. This base coating slurry was then applied to both sides of the aluminum foil in the thickness direction using a gravure coating method. After drying, a positive current collector with a localized base coating was obtained.

[0361] Step 2: Preparation of the positive electrode active layer

[0362] Conductive carbon black and carbon nanotubes were added to PVDF and stirred until homogeneous. Then, lithium cobalt oxide was added and stirred until homogeneous again to prepare a positive electrode active material slurry with a solid content of 75%. The positive electrode active material slurry was coated on both sides of the positive electrode current collector with a local undercoat prepared in the first step. After drying, the positive electrode active layer was obtained. The mass fraction of lithium cobalt oxide in the positive electrode active layer was 97.6%, the mass fraction of PVDF was 1.05%, and the mass fraction of conductive carbon black and carbon nanotubes was 1.35% (the mass ratio of carbon black to carbon nanotubes was 1:1).

[0363] For ease of description, the two sides of the positive electrode are defined as surface A and surface C, respectively, where the length of the positive electrode active layer on surface A is greater than the length of the positive electrode active layer on surface C.

[0364] Step 3: Preparing the surface coating

[0365] After adding silica to PVDF and stirring until homogeneous, NMP is added and stirred until homogeneous again, resulting in a surface coating slurry with a solid content of 25%. This slurry is then coated onto the surface of the A-side positive electrode active layer prepared in the second step. After baking and rolling, the positive electrode sheet is obtained. The surface coating contains 90% silica and 10% PVDF by mass.

[0366] In this embodiment, the length of the bottom coating layer L1 on the A side of the positive electrode sheet is 200 mm, the length of the positive electrode active layer L2 is 1500 mm, and the length of the top coating layer L3 is 120 mm, satisfying (L1+L3)≤1.5*L2.

[0367] Step 4: Preparation of the negative electrode sheet

[0368] A negative electrode active material slurry was prepared by uniformly mixing 97.3% graphite mixed with silicon and carbon (containing 5% silicon and carbon), 0.5% conductive carbon black, 1.3% PVDF and 0.9% CMC, and then adding an appropriate amount of deionized water to disperse it evenly. The negative electrode active material slurry was coated on both sides of carbon-coated copper foil, and the negative electrode sheet was obtained after baking and rolling.

[0369] Step 5: Fabrication of lithium-ion secondary batteries

[0370] The positive and negative electrode sheets are slit, fabricated, and then wound with a separator to obtain a core. The core undergoes encapsulation, baking, electrolyte injection, formation, secondary sealing, sorting, and OCV (Optical Characteristic Cell) processing to obtain a lithium-ion secondary battery.

[0371] The A-side of the positive electrode plate faces the winding center of the core, while the C-side faces away from the winding center of the core.

[0372] The electrolyte is a commercially available conventional electrolyte, with ethyl propionate content of 30%, adiponitrile content of 6%, and lithium salt of LiFP6 in the solvent;

[0373] The separator comprises a first adhesive layer, a substrate layer, a ceramic layer, and a second adhesive layer stacked sequentially. The first adhesive layer is bonded to the negative electrode, and the second adhesive layer is bonded to the positive electrode. The ceramic particles in the ceramic layer have a particle size Dv50 of 0.7 μm.

[0374] Figure 2 is a cross-sectional SEM image of the surface coating obtained in this embodiment. As can be seen from the figure, the surface coating is partially embedded in the positive electrode active layer. The thickness of the surface coating at different locations can be 1.98 μm, 3.57 μm, and 3.97 μm. The unit area (mm²) of the surface coating... 2 The difference between the maximum and minimum thickness of the inner surface coating (i.e., the depth to which the surface coating is embedded in the positive electrode active layer), h1, is 1.99 μm. The average thickness H of the surface coating is 5 μm.

[0375] Figure 3 is a cross-sectional SEM image of the base coating obtained in this embodiment. As can be seen from the figure, the base coating is partially embedded in the positive electrode active layer. The thickness of the base coating at different locations can be 2.86 μm, 3.37 μm, and 5.21 μm. The unit area of ​​the base coating is [missing information - likely a number in mm]. 2 The difference between the maximum and minimum thickness of the inner bottom coating (i.e., the depth to which the bottom coating is embedded in the positive electrode active layer) h2 is 2.35 μm.

[0376] Example 27

[0377] Except for the following, the rest of the content is the same as in Example 1.

[0378] The electrolyte contains 10% ethyl propionate, a carboxylic acid ester solvent.

[0379] Example 28

[0380] Except for the following, the rest of the content is the same as in Example 1.

[0381] The electrolyte contains 50% ethyl propionate, a carboxylic acid ester solvent.

[0382] Example 29

[0383] Except for the following, the rest of the content is the same as in Example 1.

[0384] Replace 30% of ethyl propionate in the electrolyte with 20% propyl propionate.

[0385] Example 30

[0386] Except for the following, the rest of the content is the same as in Example 1.

[0387] Replace 30% of ethyl propionate in the electrolyte with 15% ethyl propionate and 15% propyl propionate.

[0388] Example 31

[0389] Except for the following, the rest of the content is the same as in Example 1.

[0390] Replace 30% of ethyl propionate in the electrolyte with 25% methyl formate;

[0391] The base coating consists of 80% alumina, 12% PVDF, 4% carbon black, and 4% carbon nanotubes.

[0392] Example 32

[0393] Except for the following, the rest of the content is the same as in Example 1.

[0394] Replace 30% of ethyl propionate in the electrolyte with 12.5% ​​methyl chloroacetate;

[0395] The base coating consists of 99% alumina, 0.5% PVDF, and 0.5% carbon nanotubes.

[0396] Example 33

[0397] Except for the following, the rest of the content is the same as in Example 1.

[0398] The electrolyte contains 10% adiponitrile, a nitrile additive.

[0399] Example 34

[0400] Except for the following, the rest of the content is the same as in Example 1.

[0401] The content of adiponitrile, a nitrile additive, in the electrolyte is 3%.

[0402] Example 35

[0403] Except for the following, the rest of the content is the same as in Example 1.

[0404] Replace 6% adiponitrile in the electrolyte with 4% triglyceride.

[0405] Example 36

[0406] Except for the following, the rest of the content is the same as in Example 1.

[0407] Replace 6% adiponitrile in the electrolyte with 5% succinate.

[0408] Example 37

[0409] Except for the following, the rest of the content is the same as in Example 1.

[0410] Replace 6% adiponitrile in the electrolyte with 8% 1,3,6-hexanetrionitrile;

[0411] The base coating consists of 80% alumina, 12% PVDF, 4% carbon black, and 4% carbon nanotubes.

[0412] Example 38

[0413] Except for the following, the rest of the content is the same as in Example 1.

[0414] The content of adiponitrile, a nitrile additive, in the electrolyte is 1.8%.

[0415] Example 39

[0416] Except for the following, the rest of the content is the same as in Example 1.

[0417] The content of adiponitrile, a nitrile additive, in the electrolyte is 21%.

[0418] Example 40

[0419] Except for the following, the rest of the content is the same as in Example 1.

[0420] The base coating consists of 80% alumina, 10% PVDF, 4% carbon black, and 6% carbon nanotubes.

[0421] Example 41

[0422] Except for the following, the rest of the content is the same as in Example 1.

[0423] The base coating consists of 99% alumina, 0.5% PVDF, and 0.5% carbon nanotubes.

[0424] Example 42

[0425] Except for the following, the rest of the content is the same as in Example 1.

[0426] The base coating consists of 95% magnesium oxide, 3% PP and 2% metallic magnesium powder.

[0427] Example 43

[0428] Except for the following, the rest of the content is the same as in Example 1.

[0429] The base coating consists of 85% silica, 5% PVA and 10% carbon fiber.

[0430] Example 44

[0431] Except for the following, the rest of the content is the same as in Example 1.

[0432] The base coating consists of 80% alumina, 12% PVDF, 4% carbon black, and 4% carbon nanotubes.

[0433] Example 45

[0434] Except for the following, the rest of the content is the same as in Example 1.

[0435] The base coating consists of 89.6% alumina, 0.4% PVDF, 4% carbon black, and 6% carbon nanotubes.

[0436] Example 46

[0437] Except for the following, the rest of the content is the same as in Example 1.

[0438] The surface coating consists of 99% silica and 1% PVDF.

[0439] Example 47

[0440] Except for the following, the rest of the content is the same as in Example 1.

[0441] The surface coating consists of 95% magnesium oxide and 5% PTFE.

[0442] Example 48

[0443] Except for the following, the rest of the content is the same as in Example 1.

[0444] The surface coating consists of 99.5% silica and 0.5% PVDF.

[0445] Example 49

[0446] Except for the following, the rest of the content is the same as in Example 1.

[0447] The surface coating consists of 89% silica and 11% PVDF.

[0448] Example 50

[0449] Except for the following, the rest of the content is the same as in Example 1.

[0450] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 100mm, the length of the positive electrode active layer L2 is 500mm, and the length of the top coating layer L3 is 50mm, satisfying (L1+L3)≤1.5*L2.

[0451] Example 51

[0452] Except for the following, the rest of the content is the same as in Example 1.

[0453] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 1000mm, the length of the positive electrode active layer L2 is 3000mm, and the length of the top coating layer L3 is 500mm, satisfying (L1+L3)≤1.5*L2.

[0454] Example 52

[0455] Except for the following, the rest of the content is the same as in Example 1.

[0456] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 2000mm, the length of the positive electrode active layer L2 is 2200mm, and the length of the top coating layer L3 is 270mm, satisfying (L1+L3)≤1.5*L2.

[0457] Example 53

[0458] On the surface of the positive electrode A, the length of the bottom coating layer L1 is 1460mm, the length of the positive electrode active layer L2 is 1500mm, and the length of the top coating layer L3 is 800mm, which does not satisfy (L1+L3)≤1.5*L2.

[0459] Example 54

[0460] Except for the following, the rest of the content is the same as in Example 1.

[0461] The ceramic particles in the diaphragm have a Dv50 of 1.2 μm, and the average thickness H of the surface coating is 3 μm.

[0462] Example 55

[0463] Except for the following, the rest of the content is the same as in Example 1.

[0464] The ceramic particles in the diaphragm have a Dv50 of 0.5 μm, and the average thickness H of the surface coating is 10 μm.

[0465] Example 56

[0466] Except for the following, the rest of the content is the same as in Example 1.

[0467] The ceramic particles in the diaphragm have a Dv50 of 2 μm, and the average thickness H of the surface coating is 1 μm.

[0468] Example 57

[0469] Except for the following, the rest of the content is the same as in Example 1.

[0470] The ceramic particles in the diaphragm have a Dv50 of 2.5 μm, and the average thickness H of the surface coating is 1 μm.

[0471] Example 58

[0472] Except for the following, the rest of the content is the same as in Example 1.

[0473] The ceramic particles in the diaphragm have a Dv50 of 0.5 μm, and the average thickness H of the surface coating is 11 μm.

[0474] Comparative Example 4

[0475] Except for the following, the rest of the content is the same as in Example 1.

[0476] There is no base coating or surface coating on the positive electrode plate.

[0477] Comparative Example 5

[0478] Except for the following, the rest of the content is the same as in Example 1.

[0479] The electrolyte contains 55% ethyl propionate, a carboxylic acid ester solvent.

[0480] Comparative Example 6

[0481] Except for the following, the rest of the content is the same as in Example 1.

[0482] The electrolyte contains 7% ethyl propionate, a carboxylic acid ester solvent.

[0483] Test case

[0484] 1. Particle size test

[0485] Tests were performed using a laser particle size analyzer.

[0486] 2. Coating thickness and embedding depth test

[0487] This was derived from the SEM image of the coating cross-section.

[0488] 3. Porosity test

[0489] Along the boundary lines between the first electrode region I and the second electrode region II, and between the second electrode region II and the third electrode region III, as shown in Figure 1, the positive electrode is sequentially divided into the first electrode region, the second electrode region, and the third electrode region.

[0490] The first electrode region is placed in a true density tester, using helium as the medium. The pressure in the measuring chamber is gradually increased to a specified value, and then the helium expands into the expansion chamber. The equilibrium pressure of the two processes is automatically recorded by the instrument. According to the law of conservation of mass, after calibrating the volumes of the measuring chamber and the expansion chamber using a standard sphere, the difference between the two is the true volume of the electrode. Then, using the formula porosity = (electrode volume - true electrode volume) / electrode volume, the porosity of the coating in the first electrode region is calculated.

[0491] Using the same method described above, the coating porosity of the second and third electrode regions was tested and calculated.

[0492] 4. ARC self-heating test

[0493] The battery cell was fully charged with a current of 0.5C (cut off at 0.02C), and the fully charged positive electrode was disassembled in the dissection room. The fully charged positive electrode and the electrolyte (ethylene carbonate: diethyl carbonate = 3:7, volume ratio, of which the lithium salt is 1.0 mol / L LiPF6) were placed in an aluminum-plastic film and sealed to prepare a dummy battery cell. The sample was tested in the heating-waiting-heating mode of the ARC equipment until thermal runaway.

[0494] 5. Resistivity test

[0495] Discharge the battery to the lower limit voltage, then disassemble it in a drying room, take out the positive electrode and let it air dry naturally in the drying room, and then place the positive / negative electrode on a four-probe resistivity tester for testing.

[0496] 6. Energy density retention rate test

[0497] The prepared battery was charged to full capacity under constant current and constant voltage at 25℃, and then discharged to 3.0V at 0.5C. The discharged capacity was recorded as the battery capacity.

[0498] The prepared finished battery was charged to 50% SOC at 25°C, and the battery thickness was tested using 600g PPG.

[0499] Calculate the volumetric energy density (ED) = battery capacity * platform voltage / battery length / width / thickness.

[0500] Calculate the ED loss rate as (Energy density of Comparative Example 4 - Energy density of Example) / Energy density of Comparative Example 4.

[0501] 7. Needle prick test

[0502] At room temperature, the battery cell is discharged at 1C to 3.0V, then charged at a constant current of 0.7C to 4.50V, with a cutoff current of 0.02C. This process of discharging at 1C to 3.0V is repeated five times. Then, it is charged again at a constant current of 0.7C to 4.50V, with a cutoff current of 0.02C. Within 48 hours of the test completion, a 2.5mm diameter steel needle is used to vertically penetrate the lithium-ion battery at three positions (left, center, and right) at a speed of 30mm / s. If the battery does not ignite or explode, it passes the test. Twenty samples are tested, and the pass rate of each sample is observed.

[0503] 8. Hot Box Test

[0504] The battery cell is fully charged to its maximum operating voltage of 4.5V. It is then placed in an oven and heated to the set target temperature (130℃, 132℃, 135℃, or 140℃) at a rate of 5±2℃ / min, and held for 60 minutes. The test is then complete. The cell is considered passed if it does not catch fire or explode; otherwise, it fails the test if the temperature continues to rise until it catches fire or explodes. Ten samples are tested, and each sample is observed to determine if it passes the test.

[0505] 9. Low-temperature discharge performance test

[0506] Fully charge the battery cell to the upper limit of the operating voltage of 4.5V, then discharge it at 0.2C to 3.0V. The capacity discharged is recorded as the battery capacity C1. Transfer it to a constant temperature chamber at -20℃, let it stand for 4 hours, and then discharge it at 0.2C to 3.0V. The capacity discharged is recorded as the battery capacity C2. Low temperature discharge capacity retention rate = C2 / C1*100%.

[0507] The test results are shown in Tables 3 and 4. In Table 3, " / " indicates that the item does not exist. P1 represents the coating porosity of the first electrode region, P2 represents the coating porosity of the second electrode region, P3 represents the coating porosity of the third electrode region, T1 is the time required for the surface temperature of the first electrode region to reach 150℃, T2 is the time required for the surface temperature of the second electrode region to reach 150℃, R is the resistivity of the first electrode region, A% is the content of carboxylic acid ester solvent in the electrolyte, and B% is the content of nitrile additives in the electrolyte.

[0508] Table 3

[0509] Table 4

[0510] As can be seen from Tables 3 and 4, the positive electrode in Comparative Example 4 does not have a base coating and a surface coating. Therefore, the safety of Comparative Example 4 battery is poor and it cannot pass the 2.5mm needle penetration test and the 140℃ furnace temperature test.

[0511] Compared to Comparative Example 4, the battery safety performance of all embodiments was improved to varying degrees, and the ED loss rate was generally lower, with better low-temperature discharge performance. Compared to Example 1, the positive electrode sheets of Comparative Examples 5-6 also have a base coating and a surface coating, which can improve the battery safety performance. However, the content of carboxylic acid ester solvents in the electrolyte of Comparative Example 6 is too low, which is not conducive to improving the battery's low-temperature performance. Conversely, the content of carboxylic acid ester solvents in the electrolyte of Comparative Example 5 is too high. Although it can improve the low-temperature discharge performance to a certain extent, a higher content of carboxylic acid ester solvents does not necessarily mean better low-temperature performance, and it can also affect the battery's thermal safety.

[0512] This demonstrates that by setting a base coating and a surface coating at the winding end of the positive electrode sheet, the overall safety of the battery can be improved with minimal loss of cell energy density. At the same time, using an electrolyte containing an appropriate amount of carboxylic acid ester solvent can improve the battery's low-temperature performance, thus enabling the battery of this application to possess both good overall safety and low-temperature performance.

[0513] The embodiments of this application are described below with reference to Figures 5 to 9.

[0514] According to a fourth aspect of this application, a wound battery cell is provided, comprising a first electrode, a separator, and a second electrode stacked and wound together, wherein the first electrode and the second electrode have opposite polarities, and the first electrode comprises:

[0515] First current collector 110, first active material layer 112, first coating 111 and second coating 113;

[0516] Along the cell winding direction, the first current collector 110 sequentially includes a first region 100, a second region 200 and a third region 300;

[0517] The first current collector 110 is provided with a first active material layer 112 on both sides of the first region 100;

[0518] The first current collector 110 is provided with a first active material layer 112 and a first coating layer 111 on both sides of the second region 200;

[0519] The first current collector 110 is located on one side of the third region 300 and is provided with a first coating 111, a first active material layer 112 and a second coating 113 in sequence;

[0520] A first coating 111 located in the second region 200 and / or the third region 300 is disposed between the first active material layer 112 and the first current collector 110 on at least one side;

[0521] Along the length of the first electrode, the lengths of the first coating 111 and the second coating 113 are both less than the length of the first active material layer 112.

[0522] In this embodiment, the first active material layer 112 can be specifically disposed on both sides of the first current collector 110 along the thickness direction.

[0523] In this embodiment, the first electrode can serve as the positive electrode in a wound battery cell, and correspondingly, the first current collector 110 can be made of aluminum. The following description uses the first electrode as the positive electrode as an example to illustrate this solution.

[0524] The function of the first coating 111 is to prevent short circuits. The first coating 111 is disposed between the first active material layer 112 and the first current collector 110 on at least one side, which can isolate the contact between the first current collector and the second electrode and play a role in preventing short circuits.

[0525] In some embodiments, the second coating 113 is disposed on the side of the first active material layer 112 away from the first current collector 110, and located on the side of the first current collector 110 facing the interior of the wound cell. The function of the second coating 113 is to isolate the electrolyte from the contact between the electrolyte and the first active material layer 112 of the positive electrode, thereby reducing heat generation from side reactions. It should be noted that the side facing the interior of the wound cell refers to the side of the first current collector 110 near the winding center, with the winding center of the wound cell as a reference.

[0526] In this embodiment, both the first coating 111 and the second coating 113 are located at the winding end of the first electrode sheet.

[0527] For the needle penetration test, the closer to the outside of the wound cell, the greater the deformation caused by the needle penetration test, and the more likely it is to cause an internal short circuit. In this embodiment, a first coating layer 111 is provided between the first active material layer 112 and the first current collector 110. When facing the needle penetration test, it can isolate the contact between the first current collector and the second electrode, thereby preventing short circuits and improving the safety performance of the wound cell, making it more conducive to passing the needle penetration test.

[0528] For hot box testing, the outermost part of the wound cell heats up the fastest. In this embodiment, a second coating 113 is provided at the winding end of the first electrode. The second coating 113 is provided on the side of the first active material layer 112 away from the first current collector 110 and on the side of the first current collector 110 facing the inside of the wound cell. This can isolate the electrolyte from the contact between the electrolyte and the first active material layer 112 of the positive electrode, reduce the heat generated by side reactions, improve the thermal safety performance of the cell, and make it more conducive to passing the hot box test.

[0529] The wound battery cell provided in the embodiments of this application, by providing a first coating 111 and a second coating 113 at the winding end of the first electrode sheet, especially at the winding end of the positive electrode sheet, can isolate the contact between the first current collector and the second electrode sheet, thereby preventing short circuits and improving the safety performance of the wound battery cell, making it more conducive to passing the needle penetration test; and can also isolate the contact between the electrolyte and the first active material layer 112 of the positive electrode, reducing heat generation from side reactions, improving the thermal safety performance of the battery cell, and making it more conducive to passing the hot box test.

[0530] Although applying the first coating 111 and the second coating 113 to the entire area of ​​the first current collector 110 can also improve the safety performance of the wound battery cell, compared to full coating, the first electrode provided in the embodiments of this application only applies the first coating 111 and the second coating 113 to a local area, which can minimize energy density loss while meeting safety performance requirements.

[0531] The wound battery cell provided in the embodiments of this application has a length of the first coating 111 and the second coating 113 that are both less than the length of the first active material layer 112 along the length direction of the first electrode sheet. Therefore, it is not necessary to provide the first coating 111 and the second coating 113 in the entire area of ​​the first current collector 110. With minimal energy density loss, the battery cell can pass the needle penetration test and hot box test, thereby improving the safety performance of the battery cell, increasing the pass rate of the needle penetration test and hot box test of large-capacity battery cells, and the energy density loss is extremely low, with almost no impact on the electrical performance of the battery cell.

[0532] Additionally, both the first coating 111 and the second coating 113 are located at the winding end of the first electrode, further reducing energy density loss.

[0533] By dividing the first current collector 110 into regions, both the first coating 111 and the second coating 113 are placed in local areas, which can minimize energy density loss while meeting safety performance requirements.

[0534] Furthermore, the second coating 113 is applied only on one side of the third region 300, eliminating the need for a double-sided application, which helps reduce energy density loss and provides excellent safety improvement.

[0535] Additionally, the first current collector 110 also includes a fourth region 400, on both sides of the first current collector 110 located in the fourth region 400, where no other material is disposed, thus forming a blank current collector.

[0536] In some embodiments, as shown in FIG7, the length of the first active material layer 112 on the first side of the first current collector 110 is L14, and the length of the first active material layer 112 on the second side of the first current collector 110 is L15, satisfying: L15 < L14.

[0537] The projection of the first coating 111 on the first side near the tail end of the first electrode plate at least partially overlaps with the projection of the first active material layer 112 on the first side near the tail end of the first electrode plate; the projection of the first coating 111 on the second side near the tail end of the first electrode plate at least partially overlaps with the projection of the first active material layer 112 on the second side near the tail end of the first electrode plate.

[0538] The projection of the first coating 111 on the first side near the first electrode head end and the first coating 111 on the second side near the first electrode head end at least partially overlap;

[0539] The length of the first coating 111 on the first side of the first current collector 110 is L11, the length of the first coating 111 on the second side of the first current collector 110 is L12, and the length of the second coating 113 is L13, satisfying: L12 < L11 and L13 < L11.

[0540] In some embodiments, the value of L11 ranges from 100mm to 2000mm;

[0541] In some embodiments, the value of L12 ranges from 50mm to 1500mm;

[0542] In some embodiments, the value of L13 ranges from 50mm to 500mm.

[0543] As an example, L11 can be 100mm, 300mm, 500mm, 800mm, 1000mm, 1300mm, 1500mm, 1800mm, 2000mm, etc., or within the range of any two of the above values.

[0544] As an example, L12 can be 50mm, 80mm, 100mm, 200mm, 300mm, 450mm, 500mm, 800mm, 1000mm, 1300mm, 1500mm, etc., or within the range of any two of the above values.

[0545] As an example, L13 can be 50mm, 80mm, 100mm, 200mm, 300mm, 400mm, 500mm, etc., or within the range of any two of the above values.

[0546] By adopting this configuration, energy density loss can be minimized while still achieving safety improvements.

[0547] As a variation, the end of the first coating 111 on the first side near the tail end of the first electrode can also be shorter than the end of the first active material layer 112 on the first side near the tail end of the first electrode.

[0548] Similarly, as a variation, the end of the first coating 111 on the second side near the tail end of the first electrode can also be shorter than the end of the first active material layer 112 on the second side near the tail end of the first electrode.

[0549] In some embodiments, as shown in FIG7, the projection of the end of the second coating 113 near the tail end of the first electrode plate at least partially overlaps with the projection of the end of the first active material layer 112 near the tail end of the first electrode plate on the first side; the projection of the end of the second coating 113 near the head end of the first electrode plate at least partially overlaps with the projection of the end of the first active material layer 112 near the tail end of the first electrode plate on the second side.

[0550] By ensuring that the projection of the end of the second coating 113 near the tail end of the first electrode plate at least partially overlaps with the projection of the end of the first active material layer 112 near the tail end of the first electrode plate on the first side; and the projection of the end of the second coating 113 near the head end of the first electrode plate at least partially overlaps with the projection of the end of the first active material layer 112 near the tail end of the first electrode plate on the second side, and in conjunction with Figure 6, since the first active material layer 112 and the first coating 111 are only provided on one side in the third region 300, when the second coating 113 is further provided in this region, after the first electrode plate is wound, the thickness increase in this region along the thickness direction of the wound cell is not significant, thereby reducing energy density loss to a very low degree and having almost no impact on the electrical performance of the cell.

[0551] As a variation, the end of the second coating 113 near the tail end of the first electrode can also be shorter than the end of the first active material layer 112 near the tail end of the first electrode on the first side.

[0552] In some embodiments, as shown in FIG7, the thickness of the first coating 111 is H11, the thickness of the second coating 113 is H12, and the thickness of the first active material layer 112 located in the first region 100 is H13; satisfying: H11≤H13, and / or, H12≤H13.

[0553] In some embodiments, as shown in FIG7, the following condition is satisfied: H11≤H12.

[0554] In this embodiment, the first coating 111 is disposed between the first active material layer 112 and the first current collector 110 on at least one side. Since the function of the first coating 111 is to prevent short circuits and isolate the contact between the first current collector and the second electrode, it plays a role in preventing short circuits. By making the thickness H11 of the first coating 111 less than or equal to the thickness H13 of the first active material layer 112, the thickness of the first coating 111 is thinner, which can help reduce energy density loss.

[0555] The second coating 113 is disposed on the side of the first active material layer 112 away from the first current collector 110 and on the side of the first current collector 110 facing the inside of the wound cell. Since the function of the second coating 113 is to separate the electrolyte from the contact between the first active material layer 112 of the positive electrode, the heat generated by the side reaction is reduced. By making the thickness H12 of the second coating 113 greater than or equal to the thickness H11 of the first coating 111, the thickness of the second coating 113 is larger, which can better reduce the heat generated by the side reaction, thereby improving the safety performance.

[0556] In some embodiments, referring to FIG9, the first coating 111 is at least partially embedded in the first active material layer 112, and / or the second coating 113 is at least partially embedded in the first active material layer 112.

[0557] In some embodiments, the first coating is embedded in the first active material layer to a depth of not less than 0.5 μm; and / or, the second coating is embedded in the first active material layer to a depth of not less than 0.5 μm.

[0558] By embedding the first coating 111 at least partially within the first active material layer 112, and / or embedding the second coating 113 at least partially within the first active material layer 112, it is beneficial to reduce the thickness of the wound cell and the contact resistance.

[0559] In some embodiments, one of the first electrode and the second electrode is a positive electrode and the other is a negative electrode.

[0560] Furthermore, in this embodiment, the first electrode is a positive electrode and the second electrode is a negative electrode.

[0561] At least one of the positive electrode and the negative electrode includes the first electrode as described above. The negative electrode includes a second current collector 120 and a second active material layer 121 disposed on at least one side of the second current collector 120.

[0562] In some embodiments, the wound cell includes a flat region and an arc region, and both the first coating 111 and the second coating 113 are located at the tail end of the first electrode; the end of the first coating 111 near the head end of the first electrode is located in the flat region of the wound cell; the end of the first coating 111 near the tail end of the first electrode is located in the flat region of the wound cell; the end of the first coating 111 near the head end of the first electrode is located at least beyond the center position of the wound cell along the width direction of the cell.

[0563] The end of the second coating 113 near the first electrode is located in the flat area of ​​the wound cell; the end of the second coating 113 near the tail of the first electrode is located in the flat area of ​​the wound cell.

[0564] The cell width direction specifically refers to the direction shown in Figure 8.

[0565] Since the flat area is located on the large surface of the wound cell, it is more susceptible to damage from impacts or punctures. By setting the end of the first coating 111 near the first electrode at least beyond the center of the wound cell, the first coating 111 can cover a sufficient area of ​​the flat area of ​​the wound cell. This ensures that when the wound cell is subjected to a puncture test, the first coating 111 can isolate the contact between the first current collector and the second electrode, preventing short circuits and improving the safety performance of the wound cell, making it more likely to pass the puncture test.

[0566] In some embodiments, the second coating 113 is located between the first active material layer 112 of the positive electrode and the separator 130.

[0567] By placing the second coating 113 between the first active material layer 112 of the positive electrode and the separator 130, the second coating 113 can ensure that the electrolyte is separated from the contact between the first active material layer 112 of the positive electrode, reduce the heat generated by side reactions, improve the thermal safety performance of the battery cell, and make it more conducive to passing the hot box test.

[0568] In some embodiments, the wound cell further includes a finishing tape 140, which is attached to at least the third region 300 of the first current collector 110.

[0569] By attaching the finishing tape 140 to at least the third region 300 of the first current collector 110, the advantage of the smaller thickness of the first current collector 110 in the third region 300 can be taken advantage of, and the position of the finishing tape 140 can be reasonably arranged to avoid excessively increasing the thickness of the wound battery cell.

[0570] In some embodiments, the material of the first coating 111 includes first inorganic particles, a first adhesive, and a first conductive agent; the material of the second coating 113 includes second inorganic particles and a second adhesive.

[0571] The first inorganic particles and / or the second inorganic particles include at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, titanium oxide, boehmite, and magnesium hydroxide.

[0572] The first adhesive and / or the second adhesive comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and derivatives thereof;

[0573] The first conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber.

[0574] The lithium-ion secondary battery includes tabs. In this embodiment, the tabs adopt a CTP (center-mounted tab technology) structure. The first active material layer 112 has a groove, and a tab groove is provided in the groove. The tab is located in the tab groove and is electrically connected to the first current collector 110.

[0575] The tab groove is located on the side of the first current collector 110 that is away from the second coating 113.

[0576] In this embodiment, the tab of the CTP structure is prepared as follows: the active layer on the first electrode sheet at the location for mounting the tab is cleaned away to obtain a groove. At this time, part of the active layer on one side of the first electrode sheet is cleaned away, exposing the current collector for mounting the tab. The tab is placed in the groove and electrically connected to the current collector. In this embodiment, the tab groove is located at the middle position along the length direction of the first electrode sheet.

[0577] According to a fifth aspect of this application, a lithium-ion secondary battery is also provided, comprising: a wound cell as described above.

[0578] The specific preparation process of the lithium-ion secondary battery provided in this embodiment is as follows:

[0579] Step 1: Prepare the positive electrode first coating 111 slurry. Apply the slurry to aluminum foil using a gravure coating or skip coating method. After drying, a positive electrode current collector partially coated with the first coating 111 is obtained. The preparation method of the partially undercoated positive electrode first coating 111 provided in this embodiment is as follows: Nano-alumina is mixed evenly with conductive agents carbon black and carbon nanotubes. Then, a certain amount of binder polyvinylidene fluoride adhesive and NMP are added and stirred evenly to obtain the positive electrode first coating 111 slurry.

[0580] Step 2: Prepare the positive electrode active material layer slurry. Add conductive carbon black and carbon nanotubes to the PVDF adhesive and stir until homogeneous. Then add lithium cobalt oxide and stir until homogeneous again to prepare the positive electrode active material layer slurry. Coat the positive electrode active material layer slurry onto the surface of the first positive electrode coating 111 to obtain the positive electrode underlayer. The mass fraction of lithium cobalt oxide in the positive electrode active material 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% (where the mass ratio of carbon black to carbon nanotubes is 1:1).

[0581] Step 3: Prepare the second positive electrode coating 113 slurry. Add nano-silica to the PVDF adhesive and stir until homogeneous. Then add NMP and stir until homogeneous again to obtain the second positive electrode coating 113 slurry. Coat the surface of the positive electrode single-sided sheet with the second positive electrode coating 113 slurry. After baking and rolling, the positive electrode single-sided sheet is obtained. The mass fraction of nano-silica in the second positive electrode coating 113 is 90%, and the mass fraction of PVDF is 10%.

[0582] Step 4: Prepare the negative electrode active layer slurry. Mix 97.3% graphite, 0.5% conductive carbon black, 1.3% binder and 0.9% dispersant evenly, then add an appropriate amount of deionized water and disperse evenly to prepare the negative electrode active layer slurry. Coat the negative electrode active layer slurry onto carbon-coated copper foil, and obtain the negative electrode sheet after baking and rolling.

[0583] Step 5: After the positive and negative electrode sheets are slit, made into sheets, and wound with a separator, a wound cell is obtained. The wound cell is then packaged, baked, injected with electrolyte, formed, resealed, sorted, and subjected to OCV to obtain a lithium-ion secondary battery.

[0584] The electrolyte is a commercially available conventional electrolyte, and the lithium salt in it is LiFP6.

[0585] Safety performance testing may include furnace temperature testing.

[0586] As shown in Figure 7, the length of the first coating 111 on the first side of the first current collector 110 is L11, the length of the second coating 113 is L13, and the length of the first active material layer 112 on the first side of the first current collector 110 is L14.

[0587] Referring to Table 1 below, the safety performance and volumetric energy density of lithium-ion secondary batteries are verified through several examples and comparative examples.

[0588] The specific methods for safety performance testing are as follows:

[0589] ① Needle prick test:

[0590] At room temperature, the battery cell is discharged at 1C to 3.0V, then charged at a constant current of 0.7C to 4.50V, with a cutoff current of 0.02C. This process of discharging at 1C to 3.0V is repeated five times. Then, it is charged again at a constant current of 0.7C to 4.50V, with a cutoff current of 0.02C. Within 48 hours of the test completion, a 2.5mm diameter steel needle is used to vertically penetrate the lithium-ion secondary battery at three positions (left, center, and right) at a speed of 30mm / s. If no fire or explosion occurs, the test is considered passed. Twenty samples are tested, and the pass rate of each sample is observed.

[0591] ②Hotbox test:

[0592] The battery cell is fully charged to its maximum operating voltage of 4.5V. It is then placed in an oven and heated to the set target temperature (130℃, 132℃, 135℃, or 140℃) at a rate of 5±2℃ / min, and held for 60 minutes. The test is then complete. The cell is considered passed if it does not catch fire or explode; otherwise, it fails the test if the temperature continues to rise until it catches fire or explodes. Ten samples are tested, and each sample is observed to determine if it passes the test.

[0593] The specific method for measuring volumetric energy density is as follows:

[0594] The prepared battery was charged to full capacity under constant current and constant voltage at 25℃, and then discharged to 3.0V at 0.5C. The discharged capacity was recorded as the battery capacity.

[0595] The prepared finished battery was charged to 50% SOC at 25°C, and the battery thickness was tested using 600g PPG.

[0596] Calculate the volumetric energy density (ED) = battery capacity * platform voltage / battery length / width / thickness.

[0597] Calculate the ED loss rate as (Energy density of Comparative Example 7 - Energy density of Example) / Energy density of Comparative Example 7.

[0598] Table 5

[0599] As can be seen from the table above, in Comparative Example 7, the first current collector 110 does not have a first coating 111 and a second coating 113 on both sides. As a result, it failed the hot box test and the needle penetration test, indicating poor safety.

[0600] The volumetric energy density of Comparative Example 7 was used as the basis for comparison. In Example 59, the first side of the first current collector 110 was provided with both a first coating 111 and a second coating 113. It was able to pass the hot box test and the needle penetration test smoothly, and the loss of volumetric energy density was well controlled, thus balancing energy density and safety.

[0601] In Example 60, the length L13 of the second coating 113 is the same as that in Example 1, while the length L11 of the first coating 111 on the first side of the first current collector 110 is longer than that in Example 59. Although it still meets the requirements of safety performance testing, the volumetric energy density loss is greater.

[0602] In Example 61, the length L11 of the first coating 111 on the first side of the first current collector 110 is the same as in Example 59, but the length L13 of the second coating 113 is shorter than that in Example 59. Although the volumetric energy density loss is well controlled, the pass rate of the hot box test is slightly lower.

[0603] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer provided on at least one side surface of the positive electrode current collector; characterized in that, Along the length of the positive electrode sheet, a base coating and a top coating are provided at one end of the positive electrode active layer; The base coating is located between the positive current collector and the positive active layer. The base coating includes first inorganic particles, a first binder and a first conductive agent. The median particle size Dv501 of the first inorganic particles is ≤500nm. The surface coating is located on the surface of the positive electrode active layer opposite to the positive electrode current collector. The surface coating includes second inorganic particles and a second binder. The median particle size Dv502 of the second inorganic particles is ≤500nm.

2. The positive electrode sheet according to claim 1, characterized by 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, 30μm ≥ Dv503 ≥ 5μm; And / or, the first inorganic particle and / or the second inorganic particle includes 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 by The projections of the surface coating and the base coating in the thickness direction of the positive electrode sheet at least partially overlap; And / or, the top coating and / or the bottom coating are partially embedded in the positive electrode active layer; Preferably, the depth h1 of the surface coating embedded in the positive electrode active layer is 0.5 μm-3 μm; Preferably, the depth h2 of the base coating layer embedded in the positive electrode active layer is 0.5μm-4μm.

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

5. The positive electrode sheet according to any one of claims 1 to 4, characterized by, Based on the mass of the base coating, the base coating comprises 80%-99% of first inorganic particles, 0.5%-10% of first binder, and 0.5%-10% of first conductive agent; And / or, based on the mass of the surface coating, the surface coating comprises 90%-99% of second inorganic particles and 1%-10% of second binder; And / or, based on the mass of the positive electrode active layer, the positive electrode active layer comprises 90%-99% positive electrode active material, 0.5%-5% second conductive agent and 0.5%-5% third binder; Preferably, the content of the first binder in the base coating layer is greater than the content of the second binder in the top coating layer, and the content of the second binder in the top coating layer is greater than the content of the third binder in the positive electrode active layer; Preferably, on the same side of the thickness direction of the positive electrode sheet, the length L1 of the bottom coating layer, the length L2 of the positive electrode active layer, and the length L3 of the top coating layer satisfy the following relationship: 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 adhesive, the second adhesive, and the third adhesive 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 by The positive electrode active material includes lithium cobalt oxide, which includes a first particle and a second particle. The median particle size D1 of the first particle is 15μm-30μm, and the median particle size D2 of the second particle is 1μm-10μm.

7. A lithium-ion secondary battery comprising a jelly-roll type electrode core, the jelly-roll type electrode core comprising a positive electrode sheet, a separator, and a negative electrode sheet, the positive electrode sheet being provided in a laminated and wound state with the separator and the negative electrode sheet; characterized by, The positive electrode is the positive electrode as described in any one of claims 1-6, and both the bottom coating and the top coating are located at the winding tail end of the wound cell, with the top coating located on the side of the positive electrode facing the interior of the wound cell.

8. The lithium-ion secondary battery according to claim 7, characterized by The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, which includes graphite and silicon-carbon materials. Based on the mass of the negative electrode active material, the negative electrode active material includes 4%-30% silicon-carbon materials. Preferably, the silicon-carbon material comprises a porous carbon framework and silicon particles deposited on the porous carbon framework; Preferably, the resistivity of the negative electrode is ρ, where 0.5Ω·cm≤ρ≤60Ω·cm.

9. The lithium-ion secondary battery according to claim 7, characterized by Along the winding direction of the positive electrode, the positive electrode sequentially includes a third electrode region, a second electrode region, and a first electrode region; The first electrode region includes the positive current collector and the bottom coating, the positive active layer and the top coating disposed on the surface of the positive current collector facing the inside of the wound cell; The second electrode region includes the positive current collector and the undercoating layer and the positive active layer disposed on both sides of the positive current collector; The third electrode region includes the positive current collector and the positive active layer disposed on both sides of the positive current collector; The surface resistance R1 of the first electrode region is 1Ω / cm-3Ω / cm, the surface resistance R2 of the second electrode region is 0.5Ω / cm-1.8Ω / cm, and the surface resistance R3 of the third electrode region is 0.3Ω / cm-1.0Ω / cm, satisfying the condition: R1>R2>R3.

10. The lithium-ion secondary battery according to claim 9, characterized by The thickness H1 of the first electrode region is 20μm-90μm, the thickness H2 of the second electrode region is 20μm-160μm, and the thickness H3 of the third electrode region is 20μm-160μm, satisfying: H1>0.25*(H2+H3), preferably, H1>0.25*(H2+H3)+2.

11. A wound battery cell, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode, the separator, and the negative electrode are stacked and wound together; characterized in that, The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. A base coating and a top coating are provided at the winding tail end of the positive electrode sheet. The base coating is located between the positive current collector and the positive active layer, and the base coating includes first inorganic particles; The surface coating is located on the surface of the positive electrode active layer opposite to the positive electrode current collector, and the surface coating includes second inorganic particles; The electrolyte includes a carboxylic acid ester solvent, and the mass content of the carboxylic acid ester solvent is A% based on the mass of the electrolyte, with 10 ≤ A ≤ 50%.

12. The wound cell according to claim 11, wherein, The positive electrode plate includes a first electrode region, a second electrode region, and a third electrode region; The third electrode region, the second electrode region, and the first electrode region are arranged sequentially along the winding direction of the positive electrode. The first electrode region includes the positive current collector and the bottom coating, the positive active layer and the top coating disposed on the surface of the positive current collector facing the inside of the wound cell; The resistivity of the first electrode region is R, 200Ω·cm≤R≤1000Ω·cm; Preferably, the condition 8 ≤ R / A ≤ 80 is met; Preferably, the carboxylic acid ester solvent includes at least one of methyl formate, ethyl acetate, ethyl propionate, methyl acetate, methyl butyrate, methyl propyl carbonate, propyl propionate, ethyl butyrate, methyl fluoroformate, ethyl difluoroacetate, ethyl 2-fluoropropionate, methyl fluoroacetate, methyl 4-fluorobutyrate, propyl 2-fluoropropionate, and ethyl 4-fluorobutyrate.

13. The wound cell according to claim 12, wherein, Satisfy the following conditions: 500Ω·cm≤R≤800Ω·cm, and / or, 10≤R / A≤80; And / or, the electrolyte further includes lithium salt and nitrile additives, wherein the mass content of the nitrile additives is B% based on the mass of the electrolyte, and 2≤B≤10; Preferably, the condition 30 ≤ R / B ≤ 200 is met; Preferably, the nitrile additive includes at least one selected from butadionitrile, glutaronitrile, adiponitrile, trans-butenedionitrile, trans-hexenedionitrile, glyceryl trionitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrionitrile, 1,2,3-tris(cyanoethoxy)propane, and 1,4-dicyano-2-butene.

14. The wound cell according to claim 12, wherein, The second electrode region includes the positive current collector and the undercoating layer and the positive active layer disposed on both sides of the positive current collector; The third electrode region includes the positive electrode current collector and the positive electrode active layer disposed on both sides of the positive electrode current collector; In the ARC self-heating test, the time required for the surface temperature of the first electrode area to reach 150°C is T1, and the time required for the surface temperature of the second electrode area and / or the third electrode area to reach 150°C is T2, where T1-T2>300min; And / or, the coating porosity P1 of the first electrode region is 10%-14%, the coating porosity P2 of the second electrode region is 14%-18%, and the coating porosity P3 of the third electrode region is 18%-25%, satisfying P1 < P2 < P3.

15. The wound cell according to claim 11, wherein, On the same side of the positive electrode sheet in the thickness direction, the length L1 of the bottom coating layer, the length L2 of the positive electrode active layer, and the length L3 of the top coating layer satisfy: (L1+L3)≤1.5*L2; Preferably, L1 is 100mm-2000mm; Preferably, L2 is 500mm-3000mm; Preferably, L3 is 50mm-500mm.

16. The wound cell according to claim 11, wherein, The top coating and / or the bottom coating are partially embedded in the positive electrode active layer, and the depth h1 of the top coating embedded in the positive electrode active layer is less than the depth h2 of the bottom coating embedded in the positive electrode active layer. Preferably, the depth h1 of the surface coating embedded in the positive electrode active layer is 0.5 μm-3 μm; Preferably, the depth h2 of the base coating layer embedded in the positive electrode active layer is 1μm-3μm.

17. The wound cell of claim 11, wherein, In the thickness direction of the positive electrode sheet, the minimum thickness h3 of the top coating is 0.5μm-4μm, and the minimum thickness h4 of the bottom coating is 2μm-6μm, satisfying h3≤h4.

18. The wound cell according to any one of claims 11-17, wherein, Based on the mass of the base coating, the base coating comprises 80%-99% of first inorganic particles, 0.5%-10% of first binder, and 0.5%-10% of conductive agent; And / or, based on the mass of the surface coating, the surface coating comprises 90%-99% of second inorganic particles and 1%-10% of second binder; Preferably, the first inorganic particles and / or the second inorganic particles comprise at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide. Preferably, the first adhesive and / or the second adhesive comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, or derivatives thereof. Preferably, the 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.

19. The wound cell of claim 11, wherein, The separator includes a substrate, a first adhesive layer, a ceramic layer, and a second adhesive layer. The first adhesive layer is disposed on one side surface of the substrate, and the second adhesive layer is disposed on the surface of the substrate opposite to the first adhesive layer. The ceramic layer is located between the substrate and the second adhesive layer and faces the positive electrode. The ceramic layer comprises ceramic particles, and the median particle size Dv50 of the ceramic particles and the average thickness H of the surface coating satisfy the following: 0.5≤H / Dv50≤20, where H is 1μm-10μm and Dv50 is 0.5μm-2μm. Preferably, the ceramic particles include at least one of boehmite, alumina, zirconium oxide, silicon dioxide, silicon carbide, and silicon nitride.

20. A jelly-roll type battery cell comprising a first electrode sheet, a separator, and a second electrode sheet laminated and wound, the first electrode sheet and the second electrode sheet being opposite in polarity, characterized by The first electrode includes a first current collector, a first active material layer, a first coating layer, and a second coating layer; Along the cell winding direction, the first current collector sequentially includes a first region, a second region, and a third region; The first current collector is provided with the first active material layer on both sides of the first region; The first current collector is provided with the first active material layer and the first coating layer on both sides of the second region; The first current collector is located on one side of the third region and is sequentially provided with the first coating, the first active material layer and the second coating; The first coating located in the second region and / or the third region is disposed between the first active material layer and the first current collector; Along the length of the first electrode, the lengths of both the first coating and the second coating are less than the length of the first active material layer.

21. The wound cell according to claim 20, wherein, The second coating is located on the side of the first current collector facing the inside of the wound cell; the second coating is located between the first active material layer of the first electrode and the separator.

22. The wound cell of claim 20, wherein, The length of the first active material layer on the first side of the first current collector is L14, and the length of the first active material layer on the second side of the first current collector is L15, satisfying: L15 < L14. The first coating on the first side near the tail end of the first electrode plate has at least partially overlapped with the first active material layer on the first side near the tail end of the first electrode plate in its projection; the first coating on the second side near the tail end of the first electrode plate has at least partially overlapped with the first active material layer on the second side near the tail end of the first electrode plate in its projection. The first coating on the first side near the first electrode head end and the first coating on the second side near the first electrode head end project at least partially overlap; The length of the first coating on the first side of the first current collector is L11, the length of the first coating on the second side of the first current collector (10) is L12, and the length of the second coating is L13, satisfying: L12 < L11, and satisfying: L13 < L11; And / or, the value of L11 ranges from 100mm to 2000mm; And / or, the value of L12 is in the range of 50mm-1500mm; And / or, the value of L13 is in the range of 50mm-500mm.

23. The wound cell according to claim 22, wherein, The second coating at one end near the tail end of the first electrode plate at least partially overlaps with the projection of the first active material layer on the first side near the tail end of the first electrode plate; the second coating at one end near the head end of the first electrode plate at least partially overlaps with the projection of the first active material layer on the second side near the tail end of the first electrode plate.

24. The wound cell of claim 20, wherein, The thickness of the first coating is H11, the thickness of the second coating is H12, and the thickness of the first active material layer located in the first region is H13; satisfying: H11≤H13, and / or, H12≤H13; And / or, satisfying: H11≤H12.

25. The wound cell according to any one of claims 20-24, wherein, The first coating is at least partially embedded within the first active material layer, and / or the second coating is at least partially embedded within the first active material layer; The depth to which the first coating is embedded in the first active material layer is not less than 0.5 μm; And / or, the second coating is embedded in the first active material layer to a depth of not less than 0.5 μm.

26. The wound cell of claim 20, wherein, The first electrode is the positive electrode, and the second electrode is the negative electrode.

27. The wound cell of claim 20, wherein, The wound battery cell includes a flat region and an arc region. Both the first coating and the second coating are located at the tail end of the first electrode. The end of the first coating near the head end of the first electrode is located in the flat region of the wound battery cell. The end of the first coating near the tail end of the first electrode is located in the flat region of the wound battery cell. The end of the first coating near the head end of the first electrode is located at least beyond the center position of the wound battery cell along the width direction. The second coating is located in the flat region of the wound cell at one end near the first electrode; the second coating is located in the flat region of the wound cell at one end near the tail end of the first electrode.

28. The wound cell of claim 20, wherein, The wound cell also includes a finishing adhesive tape, which is attached to at least the third region of the first current collector.

29. The wound cell of claim 20, wherein, The material of the first coating includes first inorganic particles, a first binder, and a first conductive agent; the material of the second coating includes second inorganic particles and a second binder. The first inorganic particle and / or the second inorganic particle includes at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, titanium oxide, boehmite, and magnesium hydroxide; The first adhesive and / or the second adhesive comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, and derivatives thereof; The first conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber.

30. A lithium-ion secondary battery, characterized by comprising: Includes the wound battery cell as described in any one of claims 11 to 29.

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