Secondary battery, electronic equipment and preparation method of secondary battery

Abstract

The invention relates to the technical field of energy storage, in particular to a secondary battery, electronic equipment and a preparation method of the secondary battery, the secondary battery comprises an electrode assembly and a tab, and a first pole piece, a first isolating membrane and a second pole piece of the electrode assembly are laminated to form a winding structure. The electrode assembly includes a first end face and a second end face. Part of the first pole pieces form the first side face and the second side face of the electrode assembly. The first isolating membrane comprises a first section and a second section located on the outermost circle of the winding structure. The second section comprises a first main body part and a first extension part, the first main body part covers the first side face and / or the second side face, and the first extension part is attached to the first end face. The part of the first section attached to the first end face is flattened along the first direction to form a first edge. The first extending part is provided with a second edge after being flattened, and the distance between the second edge and the first edge is W which is larger than or equal to 1 mm. And the second section restrains the winding structure, so that expansion is reduced, the deformation and warping risk of the outermost ring of the winding structure is reduced, and the lithium precipitation risk is reduced.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of energy storage technology, and in particular to a secondary battery, an electronic device, and a method for preparing the secondary battery. Background Art

[0002] A secondary battery is a device that converts external energy into electrical energy and stores it internally, allowing it to power external electronic devices (such as portable electronic devices) when needed. Secondary batteries are widely used in daily life, bringing great convenience and enrichment to people's daily lives.

[0003] As consumer electronics, powered transportation, energy storage devices, and basic electrical equipment place increasingly stringent demands on the battery life of secondary batteries, the volumetric energy density of secondary batteries is constantly being refined. Improving the volumetric energy density of secondary batteries can be achieved by increasing the coating weight, compaction density, and silicon content of the positive and negative electrodes. However, anodes with high compaction densities and high silicon contents experience large expansion rates, making the secondary battery susceptible to localized lithium deposition and risking poor appearance. Summary of the Invention

[0004] In view of the above problems, embodiments of the present application provide a secondary battery, an electronic device, and a method for preparing a secondary battery, which reduce the risk of lithium deposition in the secondary battery.

[0005] According to one aspect of an embodiment of the present application, a secondary battery is provided. The secondary battery includes an electrode assembly and a tab connected to the electrode assembly. The electrode assembly includes a first electrode sheet, a first separator, and a second electrode sheet, which are stacked to form a wound structure. The electrode assembly includes a first end face and a second end face disposed opposite each other along a first direction, where the first direction is the direction in which the tab extends from the electrode assembly. A portion of the first electrode sheet constitutes a first side face and a second side face disposed opposite each other in a second direction. The first side face and the second side face are the outermost electrode sheets of the wound structure in the second direction. The second direction is the thickness direction of the electrode assembly, and the first and second directions are perpendicular to each other. Along the winding direction of the electrode assembly, the first separator includes a first section and a second section, with the second section located at the outermost coil of the wound structure. The second section includes an integrally formed first main portion and a first extension portion. The first main portion covers at least a portion of the first side face and / or at least a portion of the second side face, and the first extension portion adheres to at least a portion of the first end face. The portion of the first section adheres to the first end face, when flattened along the first direction, has a first edge. When the second section is flattened along the first direction, the first extension portion extends beyond the first edge. The first extension portion, when flattened, has a second edge. The first edge and the second edge are located on the same side of the electrode assembly in the first direction. The second edge and the first edge are spaced apart in the first direction by a distance W, where W is ≥ 1 mm. By providing the second section of the first separator at the outermost circle of the wound structure, the second section constrains the edge of the outermost electrode piece of the wound structure, thereby reducing expansion of the electrode assembly, lowering the risk of deformation and warping of the edge of the outermost electrode piece of the wound structure, and reducing the risk of lithium deposition in the secondary battery.

[0006] In addition, the inventors of this application discovered that when the distance W between the second edge and the first edge in the first direction is less than 1mm, the first extension of the second section does not adequately restrain the edge of the outermost pole piece of the wound structure, resulting in a higher risk of warping and deformation at the edge of the outermost pole piece, and a higher risk of lithium deposition at the corresponding position. By limiting W to ≥ 1mm, the first extension of the second section exerts a certain restraining force on the edge of the outermost pole piece of the wound structure, reducing the risk of warping and deformation at the edge of the outermost pole piece, reducing the expansion rate of the electrode assembly, and reducing the risk of lithium deposition at the edge of the outermost pole piece.

[0007] In one or more of the above optional embodiments, the width of the first segment after being flattened along the first direction is W1, and the width of the second segment after being flattened along the first direction is W2, where W2 ≥ W1 + 1 mm. The inventors of this application have discovered that by limiting W2 ≥ W1 + 1 mm, the second segment of the first separator has a certain binding force on the wound structure, thereby reducing the risk of expansion of the electrode assembly, reducing the risk of deformation and warping of the outermost electrode of the wound structure, and reducing the degree of lithium deposition in the secondary battery.

[0008] In one or more optional embodiments above, W1+1mm≤W2≤W1+5mm. The inventors of the present application have found that when the difference between the width of the second section after being flattened along the first direction and the width of the first section after being flattened along the first direction is less than or equal to 5mm, the first isolation membrane has less influence on the infiltration and transmission of the electrolyte into the interior of the winding structure, and the risk of electrolyte bridge rupture during the circulation of the secondary battery is reduced, thereby reducing the impact on the cycle performance of the secondary battery. Limiting W1+1mm≤W2≤W1+5mm not only makes the second section of the first isolation membrane have a certain restraining effect on the winding structure, but also reduces the influence on the infiltration and transmission of the electrolyte into the interior of the winding structure, thereby reducing the impact on the cycle performance of the secondary battery.

[0009] In one or more optional embodiments above, along the second direction, the thickness of the electrode assembly is T1. Along the second direction, the size of the first extension portion is T2, 2.5mm≤T2≤T1. The inventors of the present application found that the thickness of a common winding structure is between 2.8mm and 8mm. When T2 is greater than or equal to 2.5mm, the second section of the first isolation membrane has a stronger binding effect on the edge of the outermost pole piece of the winding structure. When T2 is less than or equal to T1, the first isolation membrane has less effect on the infiltration and transmission of the electrolyte into the interior of the winding structure. Limiting 2.5mm≤T2≤T1 not only makes the second section of the first isolation membrane have a certain binding effect on the winding structure to a certain extent, but also reduces the influence of the infiltration and transmission of the electrolyte into the interior of the winding structure to a certain extent, thereby reducing the impact on the cycle performance of the secondary battery.

[0010] In one or more optional embodiments above, 3mm≤T2≤1 / 2T1. This limitation further improves the binding effect of the second section of the first separator on the outermost electrode of the wound structure, and further reduces the impact of the electrolyte infiltration and transmission into the interior of the wound structure.

[0011] In one or more of the above optional embodiments, the first pole piece includes a first current collector and a first active material layer. The first current collector includes a first surface and a second surface disposed opposite each other along the thickness direction of the first current collector, with the first surface facing away from the winding center of the wound structure. Along the winding direction, the first pole piece includes a first portion and a second portion. The first active material layer is applied to both the first and second surfaces of the first portion. The first active material layer is applied to the second surface of the second portion, while the first surface of the second portion is not applied to the active material layer. The second portion is located at the outermost portion of the wound structure. The first main portion covers at least a portion of the first portion. The inventors of this application have discovered that for a portion of the first pole piece coated on a single side with the first active material layer, the first and second surfaces experience uneven forces, making warping more likely. This is particularly true when the portion of the first pole piece coated on a single side with the first active material layer is located at the outermost portion of the wound structure due to the lack of winding restraint. By covering at least a portion of the second portion of the first pole piece (the outermost portion) with the second segment of the first separator, the risk of deformation and warping of the second portion (the outermost portion) of the first pole piece is reduced.

[0012] In one or more optional embodiments above, the first isolation membrane further includes a third section, and the first section, the third section, and the second section are connected in sequence along the winding direction. The first pole piece further includes a third portion, and the third portion is a pole piece located at the secondary outer ring of the winding structure, and the first portion, the third portion, and the second portion are connected in sequence along the winding direction. A first active material layer is provided on the second surface of the third portion, and no active material layer is provided on the first surface of the third portion. The third section includes an integrally arranged second main body portion and a second extension portion, the second main body portion covers at least a portion of the third portion, and the second extension portion is attached to at least a portion of the first end face. The width of the third section after flattening along the first direction is W3, and W3 ≥ W1 + 1 mm. The inventors of this application discovered that the third part (sub-outer circle) of the first electrode is coated with the first active material layer on one side, and the first surface and the second surface of the third part are unevenly stressed and prone to warping. By setting the third section of the first isolation membrane, the third section restrains the sub-outer circle of the winding structure, thereby improving the restraining effect of the first isolation membrane on the electrode assembly, reducing the risk of expansion of the electrode assembly, reducing the risk of deformation and warping of the edges of the outermost circle and sub-outer circle electrode sheets of the winding structure, and reducing the degree of lithium plating in the secondary battery.

[0013] In addition, when the difference between the width of the third section after being flattened along the first direction and the width of the first section after being flattened along the first direction is greater than or equal to 1mm, the third section has a strong binding force on the edge of the secondary outer ring pole piece of the wound structure, the risk of electrode assembly expansion is low, the risk of deformation and warping of the secondary outer ring of the wound structure is low, and the risk of lithium deposition in the secondary battery is low. By limiting W3 ≥ W1 + 1mm, the third section of the first separator has a certain binding force on the edge of the single-sided coated pole piece of the secondary outer ring of the wound structure, reducing the risk of electrode assembly expansion, reducing the risk of deformation and warping of the secondary outer ring of the wound structure, and reducing the degree of lithium deposition in the secondary battery.

[0014] In one or more of the above optional embodiments, the second and third sections together constitute the widened section of the first separator. Along the third direction, the width of the orthographic projection of the electrode assembly in the second direction is L, and the third direction, the first direction, and the second direction are mutually perpendicular. After the first separator is unrolled along the winding direction, the length of the widened section is L1, with 1 / 2L≤L1≤4L. The inventors of this application have discovered that when L1 is greater than or equal to 1 / 2L, the first separator has a better restraining effect on the winding structure; when L1 is less than or equal to 4L, the first separator does not absorb and consume a large amount of electrolyte, thereby affecting the infiltration and transmission of electrolyte into the winding structure, thereby having a smaller impact on the cycle life of the secondary battery and reducing the impact of the first separator on the volumetric energy density of the secondary battery. By limiting 1 / 2L≤L1≤4L, the widened section of the first separator has a certain restraining effect on the winding structure, reducing the risk of electrode assembly expansion and the risk of lithium plating in the secondary battery, while also reducing the impact of the first separator on the infiltration and transmission of electrolyte, thereby reducing the impact on the cycle life and volumetric energy density of the secondary battery.

[0015] In one or more of the above optional embodiments, L≤L1≤2L. This limitation further enhances the restraining effect of the widened section of the first separator on the wound structure, further reduces the risk of electrode assembly expansion, reduces the risk of lithium plating in the secondary battery, and further reduces the impact of the first separator on electrolyte infiltration and transmission, thereby reducing the impact on the cycle life and volume energy density of the secondary battery.

[0016] In one or more of the above optional embodiments, along the winding direction, the widened section includes a first starting edge, and the second pole piece includes a first ending edge. The first starting edge extends beyond the first ending edge in the winding direction. The spacing between the first starting edge and the first ending edge in the winding direction is L2, where 0≤L2≤10mm. The inventors of this application have discovered that defining the position of the first starting edge of the widened section with reference to the first ending edge of the second pole piece facilitates positioning and processing. Furthermore, when L2 is less than or equal to 10mm, the third section of the first separator has a better binding effect on the secondary outer ring (the third portion of the first pole piece) of the winding structure, thereby reducing the degree of lithium deposition in the second pole piece opposite the third portion of the first pole piece. By limiting 0≤L2≤10mm, the second and third sections of the first separator have a better binding effect on the outermost ring (second portion) and secondary outer ring (third portion) of the first pole piece, thereby reducing the degree of lithium deposition in the second pole piece opposite the third portion of the first pole piece.

[0017] In one or more of the above optional embodiments, 0≤L2≤2mm. By further limiting 0≤L2≤2mm, the second and third segments of the first separator further enhance the restraining effect on the outermost ring (second portion) and the second outermost ring (third portion) of the first pole piece, thereby further reducing the risk of lithium deposition in the second pole piece opposite the third portion of the first pole piece.

[0018] In one or more optional embodiments above, the tensile strength of the second section at 25°C is F1, and the Brinell hardness of the second section at 25°C is H1. 10MPa≤F1≤30MPa, 20HB≤H1≤60HB. The inventors of the present application found that when F1 is greater than or equal to 10MPa and H1 is greater than or equal to 20HB, the first isolation film has a better effect on suppressing the expansion of the electrode assembly and restraining the edge of the outermost pole piece; when F1 is less than or equal to 30MPa and H1 is less than or equal to 60HB, the first extension portion is prone to thermal shrinkage. The first extension portion thermally shrinks under heating conditions and covers the first end face, which has a relatively good effect on suppressing the warping of the edge of the outermost first pole piece, thereby reducing the degree of lithium deposition at the edge of the pole piece. The conditions of 10MPa≤F1≤30MPa and 20HB≤H1≤60HB are limited, so that the first isolation film has a certain restraining effect on the expansion of the electrode assembly and the edge of the outermost electrode, and the first extension portion has good thermal shrinkage performance, thereby improving the first isolation film's effect of suppressing the warping of the edge of the outermost electrode and reducing the degree of lithium plating in the secondary battery.

[0019] In one or more of the above optional embodiments, 15MPa≤F1≤25MPa, and 30HB≤H1≤50HB. This limitation further enhances the first separator's ability to restrain the electrode assembly and the outermost electrode edge, further improves the first extension's ability to shrink, and further enhances the first separator's ability to suppress warping of the outermost electrode edge, thereby further reducing lithium deposition in the secondary battery.

[0020] In one or more optional embodiments above, the second section includes a first coating layer, and the material of the first coating layer includes at least one of an oxide, a nitride, and a carbide. The oxide includes at least one of aluminum oxide, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, chromium oxide, yttrium oxide, beryllium oxide, yttrium aluminum oxide, hafnium oxide, tantalum oxide, niobium oxide, tungsten oxide, cerium oxide, lanthanum oxide, neodymium oxide, samarium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, erbium oxide, thulium oxide, ytterbium oxide, lutetium oxide, manganese oxide, iron oxide, nickel oxide, cobalt oxide, zinc oxide, and tin oxide. The nitride includes at least one of boron nitride, silicon nitride, aluminum nitride, titanium nitride, zirconium nitride, chromium nitride, tantalum nitride, niobium nitride, tungsten nitride, vanadium nitride, molybdenum nitride, gallium nitride, indium nitride, aluminum gallium nitride, indium gallium nitride, manganese nitride, iron nitride, nickel nitride, copper nitride, cobalt nitride, zinc nitride, tin nitride, germanium nitride, hafnium nitride, hafnium titanium nitride, titanium aluminum nitride, titanium silicon nitride, titanium zirconium nitride, and titanium chromium nitride. The carbide includes at least one of tungsten carbide, silicon carbide, titanium carbide, zirconium carbide, hafnium carbide, tantalum carbide, niobium carbide, vanadium carbide, chromium carbide, molybdenum carbide, iron carbide, nickel carbide, cobalt carbide, manganese carbide, aluminum carbide, boron carbide, gallium carbide, indium carbide, germanium carbide, tin carbide, lead carbide, zinc carbide, copper carbide, silver carbide, gold carbide, platinum carbide, palladium carbide, cadmium carbide, and thallium carbide. Providing a first coating on the second segment increases the mechanical strength of the second segment, enhances the restraining effect of the first separator on the electrode assembly, and reduces expansion of the electrode assembly and warping of the outermost electrode. By limiting the material of the first coating, the first coating can improve the mechanical strength of the second segment to a certain extent.

[0021] In one or more of the above optional embodiments, the thermal shrinkage rate of the first segment when heated at 100°C for 10 seconds is S1, and the thermal shrinkage rate of the first extension portion when heated at 100°C for 10 seconds is S2, where S2>S1. The inventors of the present application have discovered that when the thermal shrinkage rate of the first segment is less than that of the first extension portion, the first segment is less likely to thermally shrink when the first extension portion is heated, thereby reducing the degradation of the secondary battery's performance during high-voltage testing due to thermal shrinkage of the first segment.

[0022] In one or more optional embodiments above, S1≤2.5%. 3%≤S2≤5%. The inventors of this application found that when S1≤2.5%, the thermal shrinkage rate of the first section is not too large, thereby reducing the risk of internal short circuit caused by thermal shrinkage in the first section and improving the quality rate of the secondary battery under high-voltage testing. When S2 is greater than or equal to 3%, the first extension portion may undergo thermal shrinkage under heating conditions, and the second section has a certain inhibitory effect on the warping of the edge of the outermost pole piece, and the degree of lithium plating is reduced. In addition, the thermal shrinkage rate and hardness of the material are mutually constrained. When S2 is less than or equal to 5%, the first extension portion has a certain hardness, and the restraining effect on the edge of the outermost pole piece is relatively good. By limiting 3%≤S2≤5%, the first extension portion has a certain thermal shrinkage performance and a certain hardness, so that the first extension portion has a certain restraining effect on the edge of the outermost pole piece of the winding structure, reducing the risk of warping and deformation of the edge of the outermost pole piece and reducing the degree of lithium plating.

[0023] In one or more optional embodiments above, the material of the first section includes a density of 0.94 g / cm at 25°C. 3 to 0.96 g / cm 3 The material of the first extension portion includes a polyethylene having a density of 0.91 g / cm at 25°C. 3 to 0.925 g / cm 3 This limitation ensures that the thermal shrinkage of the first section is smaller than that of the first extension, and that the first section and the first extension have a certain degree of hardness. This allows the first extension to exert a certain restraining effect on the edge of the outermost pole piece of the wound structure through thermal shrinkage, thereby reducing the risk of warping and deformation at the edge of the outermost pole piece and lowering the degree of lithium deposition. It also reduces the risk of internal short circuits caused by thermal shrinkage of the first section, thereby improving the quality rate of the secondary battery under high-voltage testing.

[0024] In one or more optional embodiments above, the secondary battery further includes a first adhesive member, which is bonded between the first end face and the first extension portion. The material of the first adhesive member includes at least one of hot melt adhesive, epoxy resin adhesive, light-curing adhesive, polyurethane adhesive, phenolic resin adhesive, acrylic adhesive, rubber adhesive, ethylene-vinyl acetate, polyamide, white latex, polyvinyl alcohol, cyanoacrylate adhesive, silicone adhesive, siloxane polymer, and acrylate. Through the provision of the first adhesive member, the first extension portion is bonded to the first end face through the first adhesive member, reducing the risk of rebound and warping of the first extension portion, improving the restraining effect of the first extension portion on the edge of the outermost pole piece of the winding structure, improving the overall structural stability of the secondary battery, and reducing the degree of lithium plating in the secondary battery.

[0025] In one or more of the above optional embodiments, the tab protrudes from the second end face along the first direction. Due to the provision of the tab, the thickness of the electrode assembly at one end where the tab is located is greater along the thickness direction of the electrode assembly. During formation, the interface adhesion of the electrode piece edge at the second end face is better, and the risk of deformation and warping of the outermost electrode piece edge at the second end face is lower. Therefore, it is preferred to provide the first extension of the second section of the first isolation membrane on the first end face opposite to the second end face.

[0026] In one or more of the above optional embodiments, the second section further includes a third extension integrally formed with the first main body, the third extension and the first extension being arranged opposite each other along the first direction, and the third extension being attached to the second end face. By providing the third extension of the second section, the third extension constrains the second end face, while the first extension constrains the first end face, thereby enhancing the constraining effect of the first separator on the wound structure, further reducing the risk of expansion of the electrode assembly, reducing the risk of deformation and warping at the edge of the outermost electrode of the wound structure, and reducing the degree of lithium deposition in the secondary battery.

[0027] In one or more optional embodiments above, the first pole piece is a positive pole piece, and the second pole piece is a negative pole piece. Along the first direction, the second pole piece is arranged beyond the first pole piece, the first section is arranged beyond the second pole piece, and the dimension of the first edge exceeding the edge of the second pole piece is W4, 0.05mm≤W4≤4mm. The inventors of this application found that when W4 is greater than or equal to 0.05mm, the risk of short circuit between the first pole piece and the second pole piece is low, and the high-voltage test quality rate of the secondary battery is improved; when W4 is less than or equal to 4mm, the influence of the first isolation membrane on the transmission of the electrolyte is reduced, and the influence on the cycle life of the secondary battery is reduced. By limiting 0.05mm≤W4≤4mm, the risk of short circuit between the first pole piece and the second pole piece can be reduced, and the influence of the first isolation membrane on the infiltration and transmission of the electrolyte can be reduced.

[0028] In one or more of the above optional embodiments, the secondary battery further includes a second separator, and the first separator and the second separator are respectively located on either side of the first electrode sheet along the thickness direction of the first electrode sheet. Within a winding, the first separator is closer to the first surface than the second separator. That is, within a winding, the first separator is closer to the outer side of the winding structure than the second separator. The second section of the first separator not only restrains the edge of the first electrode sheet, but also restrains the edge of the second electrode sheet, thereby improving the overall structural stability of the secondary battery.

[0029] In one or more optional embodiments above, the first isolation membrane includes a first substrate layer, and the material of the first substrate layer includes at least one of polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polyamide, fluorinated polypropylene, silicone rubber and polyester.

[0030] In one or more optional embodiments above, the thickness of the first isolation membrane is between 1μm and 20μm. The inventors of the present application found that when the thickness of the first isolation membrane is greater than or equal to 1μm, the first isolation membrane has a better restraining effect on the edge of the first pole piece, and the risk of warping and lithium deposition at the edge of the pole piece of the outermost circle of the winding structure is lower; when the thickness of the first isolation membrane is less than or equal to 20μm, when the second section of the first isolation membrane is folded to form the first end face, it is easier to fold, and the first isolation membrane has a better effect of suppressing the warping of the edge of the pole piece of the outermost circle of the winding structure, reducing the risk of lithium deposition and reducing the impact on the energy density of the secondary battery. By limiting the thickness of the first isolation membrane to between 1μm and 20μm, the first isolation membrane has a certain restraining effect on the edge of the first pole piece, and the impact of the first isolation membrane on the energy density of the secondary battery can be reduced.

[0031] According to one aspect of an embodiment of the present application, an electronic device is provided, comprising any one of the secondary batteries described above.

[0032] According to one aspect of an embodiment of the present application, a method for preparing any of the aforementioned secondary batteries is provided, comprising: a winding step: providing a first electrode sheet, a first separator, and a second electrode sheet, wherein the first separator comprises a first section and a second section arranged along the length of the first separator, with the second edge extending beyond the first edge along the first direction. The first electrode sheet, the first separator, and the second electrode sheet are sequentially stacked and wound to form a wound structure, wherein the second section is located at the outermost coil of the wound structure. A heat shrinking step: using a first sheet to push a portion of the second section extending beyond the first electrode sheet in the first direction toward the first end face, and simultaneously heating the extended section using a heating device to form a first extension portion, the heating temperature being 70°C to 150°C for a heating time of 1 second to 20 seconds. The inventors of the present application have discovered that low heating temperatures or short heating times impair the folding effect of the folded extended section and the coverage of the first end face by the formed first extension portion. Excessively high heating temperatures or long heating times result in significant heat shrinkage of the first section, increasing the risk of shorting between the first and second electrode sheets. By limiting the heating temperature and time, the extended portion can be folded over to constrain the edge of the electrode piece, allowing the first extension to cover the first end surface. This reduces the risk of electrode assembly expansion, the risk of deformation and warping of the outermost ring of the wound structure, and the risk of lithium deposition in the secondary battery. At the same time, the risk of shorting between the first and second electrode pieces is reduced, improving the high-voltage test quality rate.

[0033] In one or more of the above optional embodiments, before the heat shrinking step, the method for preparing a secondary battery further includes applying an adhesive to the first end face or the side of the extension portion facing the winding center of the wound structure to bond the first extension portion to the first end face. The use of the adhesive bonds the extension portion to the first end face, thereby improving the bonding stability between the first extension portion and the first end face, reducing the risk of the first extension portion rebounding and warping, improving the overall structural stability of the secondary battery, and reducing the degree of lithium deposition in the secondary battery.

[0034] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the following description or through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] One or more embodiments are exemplarily illustrated by pictures in the corresponding drawings. These exemplifications do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements. Unless otherwise stated, the figures in the drawings do not constitute proportional limitations.

[0036] Figure 1 is a schematic diagram of a secondary battery provided in an embodiment of the present application; Figure 2 Schematic diagram of an explosion of a secondary battery provided in an embodiment of the present application; Figure 3 The electrode assembly provided in the embodiment of the present application is along Figure 2 Cross-sectional view of AA; Figure 4 is a schematic diagram of a first end surface observed along a first direction provided by an embodiment of the present application; Figure 5 is an expanded view of the first isolation membrane provided in an embodiment of the present application; Figure 6 is a cross-sectional view of a first isolation membrane provided in an embodiment of the present application; Figure 7 The embodiment of this application provides Figure 1 Cross-sectional view of the middle BB; Figure 8 is an expanded view of another implementation of the first isolation membrane provided in an embodiment of the present application; Figure 9 This is another implementation of the electrode assembly provided in the embodiment of the present application. Figure 2 Cross-sectional view of AA; Figure 10 is an expanded view of another implementation of the first isolation membrane provided in an embodiment of the present application; Figure 11 1 is a flow chart of a method for preparing a secondary battery provided in an embodiment of the present application; Figure 12 This is a schematic flow chart of another method for preparing a secondary battery provided in an embodiment of the present application.

[0037] The reference numerals in the accompanying drawings are as follows: Secondary battery 100; first direction D1, second direction D2, third direction D3; winding direction C1; Packaging bag 1, electrode assembly 2, tab 3, first adhesive 4; First pole piece 2001, second pole piece 2002, first isolation film 2003, second isolation film 2004; first surface 0211, second surface 0212; First end surface 21, second end surface 22, first side surface 23; second side surface 24; The first paragraph is 2031, the second paragraph is 2032, and the third paragraph is 2033; First edge 2031a; second edge 2032a; first coating layer 2032b; first substrate layer 2003a; First main portion 0321, first extension portion 0322, third extension portion 0323; A second main portion 0331 and a second extension portion 0332; Part 1 201, Part 202, Part 3 203; First current collector 021, first active material layer 022; first surface 0211, second surface 0212; A first starting edge 2a, a first ending edge 2b; Widened section 203'. DETAILED DESCRIPTION

[0038] In order to facilitate the understanding of the present application, the present application is described in more detail below with reference to the accompanying drawings and specific embodiments. It should be noted that when an element is described as being "fixed to" another element, it can be directly on the other element, or one or more centered elements can exist between them. When an element is described as being "connected to" another element, it can be directly connected to the other element, or one or more centered elements can exist between them. The terms "vertical", "horizontal", "left", "right", "inside", "outside" and similar expressions used in this specification are for illustrative purposes only.

[0039] Unless otherwise defined, all technical and scientific terms used in this specification have the same meanings as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" as used in this specification includes any and all combinations of one or more of the relevant listed items.

[0040] See also Figure 1 and Figure 2 , Figure 1 is a three-dimensional diagram of a secondary battery 100 provided in an embodiment of the present application. Figure 2 1 is an exploded view of a secondary battery 100 according to an embodiment of the present invention. The secondary battery 100 includes a packaging bag 1, an electrode assembly 2, and a tab 3. The electrode assembly 2 is contained in the packaging bag 1. The tab 3 is electrically connected to the electrode assembly 2.

[0041] In some embodiments, the secondary battery 100 is a soft-pack lithium-ion secondary battery 100 .

[0042] It is understandable that the secondary battery 100 may be other types besides the soft-pack lithium-ion secondary battery 100 , such as a sodium-ion secondary battery 100 , a solid-state soft-pack secondary battery 100 , or a semi-solid-state soft-pack secondary battery 100 .

[0043] In some embodiments, the secondary battery 100 includes an electrolyte, which is contained in the packaging bag 1 , and the electrode assembly 2 is immersed in the electrolyte.

[0044] Regarding the packaging bag 1, its shape can be adapted to the shape of the electrode assembly 2. In some embodiments, the packaging bag 1 is substantially in the shape of a flat rectangle.

[0045] It is worth noting that the embodiment of the present application does not impose any specific restrictions on the structure of the packaging bag 1, as long as it can meet the requirement of blocking the gas-liquid exchange between the electrode assembly 2 contained therein and the external environment.

[0046] Regarding the electrode assembly 2 , the electrode assembly 2 is a charge and discharge element in the secondary battery 100 .

[0047] For the convenience of description, the first direction D1 is defined as the length direction of the electrode assembly 2, and the first direction D1 is also the direction in which the tab 3 extends out of the electrode assembly 2. The second direction D2 is defined as the thickness direction D2 of the electrode assembly 2. The third direction D3 is defined as the width direction of the electrode assembly 2. Among them, the thickness direction of the electrode assembly 2, the length direction of the electrode assembly 2 and the width direction of the electrode assembly 2 are perpendicular to each other. The thickness, length and width of the electrode assembly 2 mentioned here are also the thickness, length and width of the secondary battery 100, and are all from Figure 1 Consider the secondary battery 100 shown in the figure. The smallest distance between points is the thickness, the largest distance between points is the length, and the other direction is the width. That is, the distance that is not greater than the height or is on the side of the height is the width, and the distance that is not greater than the width or is on both sides of the height and width is the thickness.

[0048] See also Figure 2 and Figure 3 , Figure 3This is an implementation of the electrode assembly 2 provided in the embodiment of the present application. Figure 2 The electrode assembly 2 includes a first electrode piece 2001, a second electrode piece 2002, and a first isolation film 2003 disposed between the first electrode piece 2001 and the second electrode piece 2002. The first electrode piece 2001, the first isolation film 2003, and the second electrode piece 2002 are wound to form a wound structure.

[0049] The polarity of the first electrode 2001 and the second electrode 2002 are opposite. If the first electrode 2001 is a positive electrode, the second electrode 2002 is a negative electrode. Conversely, if the first electrode 2001 is a negative electrode, the second electrode 2002 is a positive electrode.

[0050] Among them, the outermost pole piece of the electrode assembly 2 is the first pole piece 2001, and part of the first pole piece 2001 constitutes the first side surface 23 and the second side surface 24 of the electrode assembly 2 which are relatively arranged in the second direction D2. The first side surface 23 and the second side surface 24 are the two outermost pole pieces of the winding structure in the second direction D2, wherein the electrode assembly 2 includes a first end face 21 and a second end face 22 which are relatively arranged along the first direction D1.

[0051] The inventors of this application have discovered that as consumer electronics, power transportation facilities, energy storage devices and basic electrical equipment have put forward more stringent requirements on the battery life of the secondary battery 100, the volume energy density index of the secondary battery 100 has been continuously refreshed. The improvement of the volume energy density of the secondary battery 100 can be achieved by increasing the coating weight, compaction density and silicon content ratio of the positive and negative electrode sheets. However, the anode with a high compaction density and the anode with a high silicon content have a large expansion rate, and the secondary battery 100 is prone to lithium deposition locally, which poses a risk of poor appearance. In particular, the electrode sheet located at the outermost circle of the winding structure is prone to edge warping and deformation due to the expansion of the electrode assembly, resulting in a higher risk of lithium deposition at the position opposite to the edge of the outermost circle electrode sheet.

[0052] In the embodiments of this application, please refer to Figure 3 、 Figure 4 and Figure 5 , Figure 4 is a schematic diagram of observing the first end surface 21 along the first direction D1, Figure 5This is an expanded view of the first separator 2003. Along the winding direction C1 of the electrode assembly 2, the first separator 2003 comprises a first section 2031 and a second section 2032. The second section 2032 is located at the outermost coil of the winding structure. The second section 2032 comprises an integrally formed first main portion 0321 and a first extension portion 0322. The first main portion 0321 covers at least a portion of the first side surface 23 and / or at least a portion of the second side surface 24. The first extension portion 0322 adheres to at least a portion of the first end surface 21. When the portion of the first section 2031 adhering to the first end surface 21 is flattened along the first direction D1, it has a first edge 2031a. When the second section 2032 is flattened along the first direction D1, the first extension portion 0322 extends beyond the first edge 2031a. When flattened, the first extension portion 0322 has a second edge 2032a. The first edge 2031a and the second edge 2032a are located on the same side of the electrode assembly 2 in the first direction D1. The distance between the second edge 2032a and the first edge 2031a in the first direction D1 is W, where W is ≥ 1 mm. By providing the second segment 2032 of the first separator 2003 at the outermost circle of the wound structure, the second segment 2032 constrains the wound structure, reducing the risk of expansion of the electrode assembly 2, the risk of deformation and warping of the outermost electrode edge of the wound structure, and the risk of lithium deposition in the secondary battery 100.

[0053] In addition, the inventors of this application discovered that when the distance W between the second edge 2032a and the first edge 2031a in the first direction D1 is less than 1 mm, the first extension 0322 of the second segment 2032 insufficiently restrains the edge of the outermost electrode of the wound structure, resulting in a higher risk of warping and deformation at the edge of the outermost electrode, and a higher risk of lithium deposition at the corresponding location. In some embodiments of this application, by limiting W to ≥ 1 mm, the first extension 0322 of the second segment 2032 exerts a certain restraining force on the edge of the outermost electrode of the wound structure, reducing the risk of warping and deformation at the edge of the outermost electrode, reducing the expansion rate of the electrode assembly, and reducing the risk of lithium deposition at the edge of the outermost electrode.

[0054] The distance W between the second edge 2032a and the first edge 2031a in the first direction D1 may be tested using a vernier caliper. The specific method is as follows: Align the main scale and secondary scale of the vernier caliper with the starting point and end point of the object to be measured. Read the scale on the main scale and the scale on the secondary scale, add the two together, and record the readings. The inventors of the present application found that when the difference between the width W2 of the second section 2032 after being flattened along the first direction D1 and the width W1 of the first section 2031 after being flattened along the first direction D1 is less than 1 mm, the second section 2032 is relatively insufficient to restrain the winding structure, the risk of expansion of the electrode assembly 2 is relatively high, the risk of deformation and warping of the edge of the outermost pole piece of the winding structure is high, and the risk of lithium deposition in the secondary battery 100 is high. In some embodiments of the present application, W2≥W1+1mm is defined so that the second section 2032 of the first isolation membrane 2003 has a certain restraining force on the winding structure, reducing the risk of expansion of the electrode assembly 2, reducing the risk of deformation and warping of the outermost pole piece of the winding structure, and reducing the degree of lithium deposition in the secondary battery 100.

[0055] The method for testing the width W1 of the first section 2031 after being flattened along the first direction D1 may refer to the aforementioned method for testing the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0056] The method for testing the width W2 of the second segment 2032 after being flattened along the first direction D1 may refer to the aforementioned method for testing the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0057] The inventors of this application have discovered that when the difference between the width W2 of the second section 2032 when flattened along the first direction D1 and the width W1 of the first section 2031 when flattened along the first direction D1 is less than or equal to 5 mm, the first separator has a minimal impact on the infiltration and transmission of electrolyte into the interior of the wound structure, reducing the risk of electrolyte bridge failure during cycling of the secondary battery 100 and minimizing the impact on the cycling performance of the secondary battery 100. In some embodiments of this application, the limit of W1+1mm≤W2≤W1+5mm is set. This not only ensures that the second section 2032 of the first separator 2003 has a certain restraining effect on the wound structure, but also reduces the impact on the infiltration and transmission of electrolyte into the interior of the wound structure, thereby minimizing the impact on the cycling performance of the secondary battery 100.

[0058] In some embodiments of the present application, the thickness of the electrode assembly 2 along the second direction D2 is T1. The size of the first extension portion 0322 along the second direction D2 is (eg Figure 4 Middle T 21 +T 22) T2, 2.5mm≤T2≤T1. The inventors of this application have discovered that the thickness of a common wound structure is between 2.8mm and 8mm. When T2 is greater than or equal to 2.5mm, the second section 2032 of the first separator 2003 has a stronger binding effect on the edge of the outermost pole piece of the wound structure. When T2 is less than or equal to T1, the first separator has less influence on the infiltration and transmission of the electrolyte into the interior of the wound structure. In some embodiments of this application, the limit of 2.5mm≤T2≤T1 is set. This not only allows the second section 2032 of the first separator 2003 to have a certain binding effect on the wound structure to a certain extent, but also reduces the influence of the infiltration and transmission of the electrolyte into the interior of the wound structure to a certain extent, thereby reducing the impact on the cycle performance of the secondary battery 100.

[0059] The testing method for the thickness T1 of the electrode assembly 2 along the second direction D2 may refer to the testing method for the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0060] Among them, the above T 21 and T 22 The testing method of the distance W between the second edge 2032a and the first edge 2031a in the first direction D1 can be referred to the testing method of the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0061] In some embodiments of the present application, 3mm≤T2≤1 / 2T1 is defined. This definition further improves the binding effect of the second section 2032 of the first isolation film 2003 on the outermost electrode of the wound structure, further reducing the impact of the electrolyte infiltration and transmission into the interior of the wound structure.

[0062] In some embodiments of this application, please refer to Figure 3The first pole piece 2001 includes a first current collector 021 and a first active material layer 022. The first current collector 021 includes a first surface 0211 and a second surface 0212 disposed opposite each other along the thickness direction of the first current collector 021, with the first surface 0211 facing away from the winding center of the wound structure. Along the winding direction C1, the first pole piece 2001 includes a first portion 201 and a second portion 202. The first surface 0211 of the first portion 201 and the second surface 0212 of the second portion 201 are both provided with the first active material layer 022. The second surface 0212 of the second portion 202 is provided with the first active material layer 022, while the first surface 0211 of the second portion 202 is not provided with an active material layer. The second portion 202 is the pole piece located at the outermost ring of the wound structure. The first main portion 0321 covers at least a portion of the first portion 201. The second portion 202 of the first pole piece 2001 is coated on a single side with the first active material layer 022. The first and second surfaces 0211 and 0212 of the second portion 202 are subjected to uneven forces, making them susceptible to warping. This is particularly true when the second portion 202 is located at the outermost portion of the wound structure due to the lack of winding restraint. The second segment 2032 of the first isolation film 2003 covers at least a portion of the second portion 202 of the first pole piece 2001, reducing the risk of deformation and warping of the second portion 202 (the outermost portion) of the first pole piece 2001.

[0063] In some embodiments of the present application, the tensile strength of the second section 2032 at 25°C is F1, and the Brinell hardness of the second section 2032 at 25°C is H1. 10MPa≤F1≤30MPa. 20HB≤H1≤60HB. The inventors of the present application have found that when F1 is greater than or equal to 10MPa and H1 is greater than or equal to 20HB, the first isolation film has a better effect in suppressing the expansion of the electrode assembly and restraining the edge of the outermost electrode piece; when F1 is less than or equal to 30MPa and H1 is less than or equal to 60HB, the first extension portion 0322 can undergo thermal contraction under heating conditions, which has a relatively good effect in suppressing the warping of the edge of the outermost first electrode piece 2001, thereby reducing the degree of lithium deposition at the edge of the electrode piece. The conditions of 10MPa≤F1≤30MPa and 20HB≤H1≤60HB are limited, so that the first isolation film 2003 has a certain restraining effect on the expansion of the electrode assembly 2 and the edge of the outermost electrode, and the first extension portion 0322 has good thermal shrinkage performance, thereby improving the effect of the first isolation film 2003 in suppressing the warping of the edge of the outermost electrode and reducing the degree of lithium plating in the secondary battery 100.

[0064] In some embodiments of the present application, 15MPa≤F1≤25MPa and 30HB≤H1≤50HB are defined. This further enhances the first separator 2003's ability to restrain the electrode assembly 2 and the outermost electrode edge, further enabling the first extension 0322 to exhibit excellent thermal shrinkage, further enhancing the first separator 2003's ability to suppress warping of the outermost electrode edge, and thereby further reducing the degree of lithium deposition in the secondary battery 100.

[0065] The tensile strength F1 is measured by using a die cutter to punch out ten specimens, each 250 mm long and 15 mm wide. The first separator 2003 is then stretched in both the length and width directions using a high-speed rail tensile testing machine at a speed of 50 mm / min. The tensile displacement and force data are measured and recorded, and a tensile curve is plotted. The tensile strength of each specimen is calculated by dividing the tensile force by the tensile displacement. The average of the 10 specimens is taken as the tensile strength F1.

[0066] The Brinell hardness H1 measurement method is as follows: Use a die cutter to punch out the first isolation membrane 2003, cutting 10 specimens with a length of 250 mm and a width of 15 mm; Place the specimens stably on the hardness tester stage to avoid vibration; Apply a preload (approximately 10 kgf) at a speed of 5 mm / s, and after eliminating the gap, load the main load (500 kgf); During the load holding period, the indenter is absolutely stationary (vibration error ≤ ±1 HBW); After unloading, use a reading microscope to measure the indentation diameter (d) (accuracy ±0.01 mm), and take the average of the two diameters in the perpendicular direction. The Brinell hardness is calculated using the formula = P / S, where P is the applied load in kgf and S represents the surface area of ​​the indentation in mm. 2 The Brinell hardness of 10 specimens was tested and the average value was taken as the Brinell hardness H1.

[0067] The tensile strength F1 or Brinell hardness H1 of the first isolation film 2003 can be adjusted by adjusting the molecular weight of the polymer material used for the first isolation film 2003 or by providing a coating on the first isolation film 2003. The greater the molecular weight of the polymer material used, the greater the tensile strength F1 or Brinell hardness H1; assuming the same molecular weight of the polymer material, the greater the tensile strength F1 or Brinell hardness H1 of the first isolation film 2003 provided with the first coating 2032b described below.

[0068] In some embodiments of this application, please refer to Figure 6 , Figure 6This is a cross-sectional view of the first isolation film 2003 provided in an embodiment of the present application. The second section 2032 includes a first coating layer 2032b, which is made of at least one of an oxide, a nitride, and a carbide. The oxide includes at least one of aluminum oxide, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, chromium oxide, yttrium oxide, beryllium oxide, yttrium aluminum oxide, hafnium oxide, tantalum oxide, niobium oxide, tungsten oxide, cerium oxide, lanthanum oxide, neodymium oxide, samarium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, erbium oxide, thulium oxide, ytterbium oxide, lutetium oxide, manganese oxide, iron oxide, nickel oxide, cobalt oxide, zinc oxide, and tin oxide. The nitride includes at least one of boron nitride, silicon nitride, aluminum nitride, titanium nitride, zirconium nitride, chromium nitride, tantalum nitride, niobium nitride, tungsten nitride, vanadium nitride, molybdenum nitride, gallium nitride, indium nitride, aluminum gallium nitride, indium gallium nitride, manganese nitride, iron nitride, nickel nitride, copper nitride, cobalt nitride, zinc nitride, tin nitride, germanium nitride, hafnium nitride, hafnium titanium nitride, titanium aluminum nitride, titanium silicon nitride, titanium zirconium nitride, and titanium chromium nitride. The carbide includes at least one of tungsten carbide, silicon carbide, titanium carbide, zirconium carbide, hafnium carbide, tantalum carbide, niobium carbide, vanadium carbide, chromium carbide, molybdenum carbide, iron carbide, nickel carbide, cobalt carbide, manganese carbide, aluminum carbide, boron carbide, gallium carbide, indium carbide, germanium carbide, tin carbide, lead carbide, zinc carbide, copper carbide, silver carbide, gold carbide, platinum carbide, palladium carbide, cadmium carbide, and thallium carbide. Providing a first coating 2032b on the second segment 2032 increases the mechanical strength of the second segment 2032, enhances the restraining effect of the first separator 2003 on the electrode assembly 2, and reduces expansion of the electrode assembly 2 and warping of the outermost electrode. By limiting the material of the first coating 2032b, the first coating 2032b can improve the mechanical strength of the second segment 2032 to a certain extent.

[0069] In some embodiments of the present application, the thermal shrinkage rate of the first segment 2031 when heated at 100°C for 10 seconds is S1, and the thermal shrinkage rate of the first extension portion 0322 when heated at 100°C for 10 seconds is S2, where S2>S1. The inventors of the present application have discovered that when the thermal shrinkage rate of the first segment 2031 is less than that of the first extension portion 0322, the first segment 2031 is less likely to thermally shrink when the first extension portion 0322 is heated, thereby reducing the degradation of the performance of the secondary battery 100 during high-voltage testing caused by thermal shrinkage of the first segment 2031.

[0070] In some embodiments of the present application, S1≤2.5%. 3%≤S2≤5%. The inventors of the present application found that when S1≤2.5%, the thermal shrinkage rate of the first section 2031 is not too large, thereby reducing the risk of internal short circuit caused by thermal shrinkage of the first section 2031, and improving the quality rate of the secondary battery 100 under high-voltage testing. When S2 is greater than or equal to 3%, the first extension portion 0322 may shrink thermally under heating conditions, and the second section 2032 has a certain inhibitory effect on the warping of the edge of the outermost pole piece, and the degree of lithium precipitation is reduced. In addition, the thermal shrinkage rate and hardness of the material are mutually constrained. When S2 is less than or equal to 5%, the first extension portion 0322 has a certain hardness, and the restraining effect on the edge of the outermost pole piece is relatively good. By limiting S1≤2.5%. 3%≤S2≤5%, which not only makes the first extension portion 0322 have a certain thermal shrinkage performance, but also makes the first extension portion 0322 have a certain hardness, so that the first extension portion 0322 has a certain restraining effect on the edge of the outermost pole piece of the winding structure, reducing the risk of warping and deformation of the edge of the outermost pole piece and reducing the degree of lithium plating.

[0071] The S1 measurement method is as follows: Punch out the first section 2031 to obtain specimen 1. Use a vernier caliper to measure the initial length of specimen 1, recorded as l1. After heating specimen 1 at 100°C for 10 seconds, measure the final length of specimen 1 again, recorded as l2. Calculate the shrinkage ratio S1 as (l1-l2) / l1×100%. The S2 measurement method is similar and will not be repeated here.

[0072] The heat shrinkage rate of the first isolation film 2003 can be regulated by adjusting the density of the material used for the first isolation film 2003. The lower the density, the greater the heat shrinkage rate.

[0073] In some embodiments of the present application, the material of the first section 2031 includes a density of 0.94 g / cm at 25°C. 3 to 0.96 g / cm 3 The material of the first extension portion 0322 includes a density of 0.91 g / cm at 25°C. 3 to 0.925 g / cm 3 This limitation ensures that the thermal shrinkage rate of the first section 2031 is lower than that of the first extension 0322, and that the first section 2031 and the first extension 0322 have a certain degree of hardness. This allows the first extension 0322 to exert a certain restraining effect on the edge of the outermost electrode of the wound structure through thermal shrinkage, thereby reducing the risk of warping and deformation at the edge of the outermost electrode and lowering the degree of lithium deposition. It also reduces the risk of internal short circuits caused by thermal shrinkage of the first section, thereby improving the quality rate of the secondary battery under high-voltage testing.

[0074] In some embodiments of this application, please refer to Figure 7 , Figure 7 The embodiment of this application provides Figure 1 The cross-sectional view of BB. Figure 7 As shown, the secondary battery 100 also includes a first adhesive member 4, which is bonded between the first end face 21 and the first extension portion 0322. The material of the first adhesive member 4 includes at least one of hot melt adhesive, epoxy resin adhesive, light-curing adhesive, polyurethane adhesive, phenolic resin adhesive, acrylic adhesive, rubber adhesive, ethylene vinyl acetate, polyamide, white latex, polyvinyl alcohol, cyanoacrylate adhesive, silicone adhesive, siloxane polymer, and acrylate. Through the provision of the first adhesive member 4, the first extension portion 0322 is bonded to the first end face 21 via the first adhesive member 4, reducing the risk of the first extension portion 0322 rebounding and warping, improving the first extension portion 0322's restraining effect on the edge of the outermost electrode of the wound structure, improving the overall structural stability of the secondary battery 100, and reducing the degree of lithium deposition in the secondary battery 100.

[0075] In some embodiments of this application, please refer to Figure 2 The tab 3 protrudes from the second end face 22 along the first direction D1. Due to the provision of the tab 3, the thickness of the electrode assembly 2 at one end where the tab 3 is located is greater along the thickness direction of the electrode assembly 2. This improves interfacial adhesion between the electrode piece edges at the second end face 22 during formation, and reduces the risk of deformation and warping of the outermost electrode piece edges at the second end face 22. Therefore, it is preferred to provide the first extension portion 0322 of the second section 2032 of the first isolation film 2003 on the first end face 21 opposite the second end face 22.

[0076] In some embodiments of this application, please refer to Figure 2 、 Figure 7 and Figure 8 , Figure 8 This is an expanded view of another implementation of the first isolation membrane 2003 provided in an embodiment of the present application. The second section 2032 also includes a third extension portion 0323 integrally provided with the first main body portion 0321. The third extension portion 0323 and the first extension portion 0322 are arranged relative to each other along the first direction D1, and the third extension portion 0323 is affixed to the second end face 22. By providing the third extension portion 0323 of the second section 2032, the third extension portion 0323 constrains the second end face 22, and the first extension portion 0322 constrains the first end face 21, thereby enhancing the constraining effect of the first isolation membrane 2003 on the wound structure, further reducing the risk of expansion of the electrode assembly 2, reducing the risk of deformation and warping of the edge of the outermost pole piece of the wound structure, and reducing the degree of lithium deposition in the secondary battery 100.

[0077] In some embodiments of this application, please refer to Figure 7, first electrode 2001 is the positive electrode, and second electrode 2002 is the negative electrode. Along first direction D1, second electrode 2002 is positioned beyond first electrode 2001, and first segment 2031 is positioned beyond second electrode 2002. The dimension W4 by which first edge 2031a extends beyond the edge of second electrode 2002 is 0.05 mm ≤ W4 ≤ 4 mm. The inventors of this application have discovered that by limiting the dimension 0.05 mm ≤ W4 ≤ 4 mm, the risk of shorting between first electrode 2001 and second electrode 2002 can be reduced, while also minimizing the impact of first isolation film 2003 on electrolyte wetting and transmission.

[0078] The test method for determining the distance W4 between the first edge 2031a and the first edge 2031a in the first direction D1 can refer to the test method for determining the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0079] In some embodiments of this application, please refer to Figure 3 Secondary battery 100 also includes a second separator 2004. Along the thickness direction of first electrode sheet 2001, first separator 2003 and second separator 2004 are located on either side of first electrode sheet 2001. Within a winding, first separator 2003 is closer to first surface 0211 than second separator 2004. That is, within a winding, first separator 2003 is closer to the outside of the winding structure than second separator 2004. The second section 2032 of first separator 2003 not only constrains the edge of first electrode sheet 2001 but also the edge of second electrode sheet 2002, thereby improving the overall structural stability of secondary battery 100.

[0080] In some embodiments of this application, please refer to Figure 6 The first isolation film 2003 includes a first substrate layer 2003a, and the material of the first substrate layer 2003a includes at least one of polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polyamide, fluorinated polypropylene, silicone rubber and polyester.

[0081] In some embodiments of this application, please refer to Figure 6The thickness T of the first separator 2003 is between 1 μm and 20 μm. The inventors of this application have discovered that when the thickness of the first separator 2003 is greater than or equal to 1 μm, the first separator 2003 effectively restrains the edge of the first electrode 2001, and the risk of warping and lithium deposition at the outermost electrode edge of the wound structure is reduced. When the thickness of the first separator 2003 is less than or equal to 20 μm, the second section 2032 of the first separator 2003 is more easily folded when it is folded to form the first end surface 21. The first separator 2003 effectively suppresses warping at the outermost electrode edge of the wound structure, reducing the risk of lithium deposition and minimizing the impact on the energy density of the secondary battery 100. By limiting the thickness of the first separator 2003 to between 1 μm and 20 μm, the first separator 2003 has a certain restraining effect on the edge of the first electrode 2001 and reduces the impact of the first separator 2003 on the energy density of the secondary battery 100.

[0082] The method for testing the thickness T of the first isolation film 2003 may refer to the aforementioned method for testing the distance W between the second edge 2032 a and the first edge 2031 a in the first direction D1 .

[0083] See also Figure 9 and Figure 10 , Figure 9 This is another implementation of the electrode assembly 2 provided in the embodiment of the present application. Figure 2 Cross-sectional view of AA in the figure. Figure 10This is an expanded view of another implementation of the first isolation membrane 2003 provided in an embodiment of the present application. In this embodiment, the first isolation membrane 2003 further includes a third segment 2033, wherein the first segment 2031, the third segment 2033, and the second segment 2032 are sequentially connected along the winding direction C1. The first pole piece 2001 further includes a third portion 203, which is located at the secondary outer ring of the winding structure. The first portion 201, the third portion 203, and the second portion 202 are sequentially connected along the winding direction C1. The second surface 0212 of the third portion 203 is provided with a first active material layer 022, while the first surface 0211 of the third portion 203 is not provided with an active material layer. The third segment 2033 includes an integrally arranged second main portion 0331 and a second extension portion 0332. The second main portion 0331 covers at least a portion of the third portion 203, and the second extension portion 0332 adheres to at least a portion of the first end surface 21. The width of the third section 2033 after flattening along the first direction D1 is W3, where W3 ≥ W1 + 1 mm. In some embodiments of the present application, the third section 203 (secondary outer ring) of the first electrode sheet 2001 is coated on a single side with the first active material layer 022. The first surface 0211 and the second surface 0212 of the third section 203 are subjected to uneven force, making them susceptible to warping. The third section 2033 of the first separator 2003 constrains the secondary outer ring of the wound structure, enhancing the first separator 2003's restraining effect on the electrode assembly 2, reducing the risk of expansion of the electrode assembly 2, and lowering the risk of deformation and warping at the edges of the outermost and secondary outer ring electrode sheets of the wound structure, thereby reducing the extent of lithium deposition in the secondary battery 100. In addition, the inventors of this application discovered that when the difference between the width of the third section 2033 after being flattened along the first direction D1 and the width of the first section 2031 after being flattened along the first direction D1 is greater than or equal to 1 mm, the third section 2033 has a strong binding force on the edge of the secondary outer ring electrode piece of the wound structure, the risk of electrode assembly 2 expansion is reduced, the risk of deformation and warping of the secondary outer ring of the wound structure is reduced, and the risk of lithium deposition in the secondary battery 100 is reduced. By limiting W3 ≥ W1 + 1 mm, the third section 2033 of the first separator 2003 has a certain binding force on the edge of the single-sided coated electrode piece of the secondary outer ring of the wound structure, reducing the risk of expansion of the electrode assembly 2, reducing the risk of deformation and warping of the secondary outer ring of the wound structure, and reducing the degree of lithium deposition in the secondary battery 100.

[0084] The testing method for the width W3 of the third section 2033 after being flattened along the first direction D1 may refer to the aforementioned testing method for the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0085] Among them, the second section 2032 and the third section 2033 together constitute the widened section 203' of the first isolation membrane 2003. Along the third direction D3, the width of the orthographic projection area of ​​the electrode assembly 2 on the second direction D2 is L, and the third direction D3, the first direction D1 and the second direction D2 are perpendicular to each other. After the first isolation membrane 2003 is unfolded along the winding direction C1, the length of the widened section 203' is L1, and 1 / 2L≤L1≤4L. The inventors of the present application found that when L1 is greater than or equal to 1 / 2L, the first isolation membrane 2003 has a better restraining effect on the winding structure; when L1 is less than or equal to 4L, the first isolation membrane 2003 will not affect the infiltration and transmission of the electrolyte into the winding structure due to the absorption and consumption of more electrolyte, and has little impact on the cycle life of the secondary battery 100. In some embodiments of the present application, by limiting 1 / 2L≤L1≤4L, the widened section 203' of the first isolation membrane 2003 has a certain restraining effect on the winding structure, reducing the risk of expansion of the electrode assembly 2 and the risk of lithium plating in the secondary battery 100, and reducing the influence of the first isolation membrane 2003 on the infiltration and transmission of the electrolyte, thereby reducing the influence on the cycle life of the secondary battery 100.

[0086] The testing method for the width L of the orthographic projection area of ​​the electrode assembly 2 in the second direction D2 along the third direction D3 may refer to the testing method for the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0087] The method for testing the length L1 of the widened section 203 ′ after the first isolation film 2003 is unfolded along the winding direction C1 may refer to the aforementioned method for testing the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0088] In some embodiments of the present application, L≤L1≤2L is defined. This definition further enhances the restraining effect of the widened section 203' of the first separator 2003 on the wound structure, further reduces the risk of expansion of the electrode assembly 2, reduces the risk of lithium deposition in the secondary battery 100, and further reduces the impact of the first separator 2003 on electrolyte infiltration and transmission, thereby reducing the impact on the cycle life of the secondary battery 100.

[0089] In some embodiments of the present application, along the winding direction C1, the widened section 203' includes a first starting edge 2a, and the second pole piece 2002 includes a first ending edge 2b. The first starting edge 2a extends beyond the first ending edge 2b along the winding direction C1. The spacing between the first starting edge 2a and the first ending edge 2b along the winding direction C1 is L2, where 0≤L2≤10mm. The inventors of the present application have discovered that defining the position of the first starting edge 2a of the widened section 203' by reference to the first ending edge 2b of the second pole piece 2002 facilitates positioning and processing. Furthermore, when L2 is less than or equal to 10mm, the third section 2033 of the first isolation membrane 2003 effectively constrains the secondary outer ring of the winding structure (the third portion 203 of the first pole piece 2001), thereby reducing the degree of lithium deposition in the second pole piece 2002, which is opposite the third portion 203 of the first pole piece 2001. By limiting 0≤L2≤10mm, the second section 2032 and the third section 2033 of the first isolation membrane 2003 have a better restraining effect on the outermost circle (second part 202) and the second outer circle (third part 203) of the first pole piece 2001, thereby reducing the degree of lithium deposition in the second pole piece 2002 opposite to the third part 203 of the first pole piece 2001.

[0090] The method for testing the distance L2 between the first starting edge 2a and the first ending edge 2b along the winding direction C1 may refer to the method for testing the distance W between the second edge 2032a and the first edge 2031a in the first direction D1.

[0091] In some embodiments of the present application, 0≤L2≤2mm is defined. By further defining 0≤L2≤2mm, the second segment 2032 and the third segment 2033 of the first isolation film 2003 further enhance the binding effect on the outermost ring (second portion 202) and the second outermost ring (third portion 203) of the first pole piece 2001, thereby further reducing the risk of lithium deposition in the second pole piece 2002 opposite the third portion 203 of the first pole piece 2001.

[0092] The present application also provides an electronic device comprising a secondary battery 100 according to any of the aforementioned embodiments. The electronic device may be an energy storage product, a mobile phone, a tablet, a drone, a single-wheeled or two-wheeled electric vehicle, or an electric cleaning tool. For example, in the case of the aforementioned drone, the secondary battery 100 is mounted on the drone and is used to power loads on the drone, including the flight system, control system, and camera system.

[0093] The present application provides a method for preparing the secondary battery 100 according to any of the above embodiments. Figure 2 、 Figure 3 、 Figure 5 and Figure 11 , Figure 11is a flow chart of a method for preparing a secondary battery 100 provided in an embodiment of the present application, as shown in FIG. Figure 11 , a method for preparing a secondary battery 100, comprising: Winding step: Provide a first pole piece 2001, a first isolation film 2003, and a second pole piece 2002, wherein the first isolation film 2003 includes a first section 2031 and a second section 2032 arranged along the length direction of the first isolation film 2003, and along the first direction D1, the second edge 2032a extends beyond the first edge 2031a. The first pole piece 2001, the first isolation film 2003, and the second pole piece 2002 are stacked and wound in sequence to form a wound structure, wherein the second section 2032 is located at the outermost circle of the wound structure.

[0094] The first section 2031 and the second section 2032 of the first isolation membrane 2003 may be integrally formed, or the first section 2031 and the second section 2032 of the first isolation membrane 2003 may not be integrally formed.

[0095] Heat shrinking step: Use the first plate to push the second section 2032 along the first direction D1 beyond the extension of the first pole piece 2001 toward the first end surface 21. At the same time, use a heating device to heat the extension part to form a first extension part 0322. The heating temperature is 70℃~150℃ and the heating time is 1s~20s.

[0096] The inventors of this application have discovered that when the heating temperature is low or the heating time is short, the folding effect of the folded extension portion is affected, affecting the coverage of the first extension portion 0322 on the first end face 21; when the heating temperature is too high or the heating time is too long, the first section 2031 shrinks to a high degree, and the risk of a short circuit between the first electrode 2001 and the second electrode 2002 is high. By limiting the heating temperature and heating time, the extension portion can be folded to form a constraint on the edge of the electrode, so that the first extension portion 0322 can cover the first end face 21, reducing the risk of expansion of the electrode assembly 2, reducing the risk of deformation and warping of the edge of the outermost electrode of the wound structure, and reducing the risk of lithium deposition in the secondary battery 100. At the same time, the risk of a short circuit between the first electrode 2001 and the second electrode 2002 is reduced, and the quality rate of the high-voltage test of the secondary battery 100 is improved.

[0097] In some embodiments of this application, please refer to Figure 12Before the heat shrinking step, the method for preparing secondary battery 100 further includes applying an adhesive to first end surface 21 or to the side of the extension portion facing the winding center of the wound structure to bond the first extension portion 0322 to first end surface 21. The adhesive bonds the extension portion to first end surface 21, improving the bonding stability between first extension portion 0322 and first end surface 21 and reducing the risk of rebound and warping of first extension portion 0322. This improves the overall structural stability of secondary battery 100 and reduces the degree of lithium deposition in secondary battery 100.

[0098] After the secondary battery 100 is finally formed, the applied adhesive forms a first adhesive member 4 (eg Figure 7 shown).

[0099] In order to evaluate the beneficial effects of the secondary battery 100 of the present application, tests were performed on it. In the following embodiments and comparative examples, all reagents, materials, and instruments used are commercially available unless otherwise specified.

[0100] Example 1 Preparation of the negative electrode sheet: Mix the negative electrode active materials artificial graphite, conductive carbon black, and styrene-butadiene rubber in a weight ratio of 96:1.5:2.5, add deionized water as a solvent, and prepare a 50wt% slurry. Stir evenly. Apply the slurry to one surface of a 6μm thick negative electrode current collector copper foil, then dry it at 110°C to obtain a negative electrode sheet coated on one side with a negative electrode active material layer. When preparing a double-sided coated negative electrode sheet, repeat the above steps on the other surface of the negative electrode sheet to obtain a negative electrode sheet coated on both sides with a negative electrode active material layer. The coated sheet is then cold-pressed to a thickness of 105μm, and a negative electrode tab 3 made of nickel is welded to the exposed copper foil.

[0101] Preparation of the positive electrode sheet: The positive electrode active material lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride are mixed in a weight ratio of 97.5:1.0:1.5, and N-methylpyrrolidone is added as a solvent to prepare a slurry with a solid content of 75wt%, and stirred evenly. The slurry is evenly coated on one surface of the aluminum foil and then dried at 90°C to obtain a positive electrode sheet coated with the positive electrode active material on one side. When preparing a double-sided coated positive electrode sheet, repeat the above coating steps on the other surface of the aluminum foil. The coated electrode sheet is then cold pressed to a thickness of 95μm, and the positive electrode tab 3 is welded to the exposed aluminum foil. The positive electrode tab 3 is made of aluminum.

[0102] Preparation of the electrolyte: In a dry argon atmosphere, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were first mixed in a mass ratio of 30:50:20 to form a basic organic solvent. Then, lithium hexafluorophosphate was added to the basic organic solvent and dissolved and mixed evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0103] Preparation of the first isolation film 2003: A 7 μm thick polyethylene porous polymer film was used as the first isolation film 2003 .

[0104] Preparation of electrode assembly 2: The positive electrode sheet, the first isolation membrane 2003 and the negative electrode sheet are stacked, and the first isolation membrane 2003 includes a first section 2031 and a second section 2032 arranged along the length direction of the first isolation membrane 2003, and the second edge 2032a exceeds the first edge 2031a along the first direction D1; the first electrode sheet 2001, the first isolation membrane 2003 and the second electrode sheet 2002 are stacked in sequence and wound to form a winding structure, wherein the second section 2032 is located at the outermost circle of the winding structure; the first plate is used to push the extension part of the second section 2032 that exceeds the first electrode sheet 2001 along the first direction D1 to fold toward the first end face 21, and at the same time, the extension part is hot-pressed using a heating device, wherein the heating temperature is 100°C and the heating time is 15s. After the hot pressing is completed, the extension part forms a first extension part 0322 covering the first end face 21.

[0105] Electrode Assembly 2 Assembly: Prepare a punched aluminum-plastic film with the pit facing upwards. Place the electrode assembly 2 in the pit and apply external pressure to tighten. Then, place another punched aluminum-plastic film with the pit facing downwards over the electrode assembly 2. Heat-seal the two films using hot pressing to form three sealed edges, completing the assembled electrode assembly 2.

[0106] Liquid injection packaging: electrolyte is injected into the assembled electrode assembly 2 on the side of the aluminum-plastic film that has not been heat-sealed. After vacuum packaging, standing, hot pressing, shaping and other processes, the secondary battery 100 is manufactured.

[0107] The secondary batteries 100 of the embodiments and comparative examples were subjected to cycle performance tests, including a cycle capacity retention test, a thickness expansion rate test, and a lithium plating degree test. The test results are recorded in a table.

[0108] The test steps for the cycle capacity retention rate are as follows: 1) at a test temperature of 25°C, the secondary battery 100 is allowed to stand for 50 min and charged according to the following charging steps: (a) 1.1C constant current charging to 4.1 V, with the cutoff capacity of this step set to C1; (b) 0.5C constant current charging to 4.4 V, constant voltage charging to 0.05C, with the cutoff capacity of this step set to C2; (c) standing for 5 min, 1.1C constant current discharge to 3.5 V, with the cutoff capacity of this step set to C3; (d) 0.5C constant current discharge to 3 V, with the cutoff capacity of this step set to C4; (e) standing for 5 min; (f) if (C1+C2) / (C3+C4)>1.02, suspend charging; (g) if (C1+C2) / (C3+C4)≤1.02, standing for 5 min, and then repeating steps (a) to (g) until 400 cycles are completed. 2) The discharge capacity at the third cycle is taken as the baseline capacity. The ratio of the discharge capacity at the 400th cycle to the baseline capacity, multiplied by 100%, is the capacity retention rate after 400 cycles. For each set of examples or comparative examples, 20 secondary batteries 100 were tested. The capacity retention rates of the 20 samples were averaged and recorded in a table.

[0109] The thickness expansion rate test procedure is as follows: record the thickness h0 of the secondary battery before cycling and the thickness h after the 400th cycle, and then calculate the thickness expansion rate of the secondary battery = (h / h0-1) × 100%. For each set of examples or comparative examples, average the thickness expansion rates of the 20 samples and record them in a table.

[0110] Lithium Deposition Rate Test: After 400 cycles, the secondary battery 100 was discharged to 0% SOC (State of Charge). The electrodes were disassembled and lithium deposition observed in the negative electrode region opposite the second segment 2032 and the third segment 2033. The number of negative electrode plates opposite the second segment 2032 or the third segment 2033 exhibiting lithium deposition was recorded as X1. The lithium deposition rate for each example or comparative example was X1 / 20, and the lithium deposition rate was recorded in a table. The volumetric energy density test steps were as follows: 1) At 25°C, the secondary battery was allowed to rest for 10 minutes, then charged at a constant current of 0.2C to 4.5V, then charged at a constant voltage of 0.02C, and allowed to rest for 5 minutes. The battery was then discharged at a constant current of 0.2C to 3V, allowed to rest for 5 minutes, and the discharge capacity, C0, was recorded. 2) The length, width, and thickness of the secondary battery were measured using a PPG battery thickness gauge. Volumetric energy density was calculated using the following formula: Plateau voltage × C0 / (length × width × thickness). The volume energy densities of the 20 secondary batteries 100 in each group of examples or comparative examples were averaged and recorded in a table.

[0111] The Hi-pot (high voltage resistance test) test steps are as follows: detect the leakage current generated by the lithium-ion battery prepared in the embodiment or comparative example under the test voltage of 100V output by the high-voltage machine, and then calculate the resistance value = test voltage / leakage current, and compare the calculated resistance value with the set judgment resistance. In this application, the preset value of the judgment resistance is 5mΩ.

[0112] If the detected resistance value is greater than or equal to the preset value of 5mΩ, the product under test is judged to have passed the test (OK); if the detected resistance value is less than the preset value of 5mΩ, the test voltage is instantly cut off and the product under test is judged to have failed the test (NG).

[0113] In each group of examples or comparative examples, 100 secondary batteries 100 were tested. The number of secondary batteries 100 that passed the test was X2, and the test pass rate was X2 / 100×100%, i.e., the Hi-pot rate, which was recorded in the table.

[0114] The differences between Examples 1 to 20 and Comparative Examples 1 to 3 are related parameters, which are shown in Table 1. The thickness T1 of the electrode assembly 2 in all comparative examples and examples is 8 mm.

[0115] Table 1

[0116] Note: “ / ” in the table means there is no data for this item.

[0117] As shown in Table 1, Comparative Examples 1, 2, and 3, as well as Examples 1 through 20, when the second edge 2032a of the second segment 2032 extends beyond the first edge 2031a of the first segment 2031, that is, when the second segment 2032 has the first extension 0322 covering the first end face 21, the cycle capacity retention rate is improved, the thickness expansion rate is reduced, and the lithium deposition rate is reduced. This is because when the first extension 0322 is provided and covers at least the first end face 21, the second segment 2032 constrains the edge of the outermost electrode of the wound structure, reducing expansion of the electrode assembly 22, lowering the risk of deformation and warping at the edge of the outermost electrode of the wound structure, and reducing the risk of lithium deposition in the secondary battery 100.

[0118] As shown in Table 1, Comparative Examples 2 and 3, and Example 6, when W ≥ 1 mm, the cycle capacity retention rate improves, the thickness expansion rate decreases, and the lithium deposition rate decreases. This is because when W ≥ 1 mm, the first extension 0322 of the second segment 2032 exerts a certain binding force on the edge of the outermost electrode of the wound structure, reducing the risk of warping and deformation at the edge of the outermost electrode, and reducing the expansion rate of the electrode assembly 22 and the risk of lithium deposition at the edge of the outermost electrode.

[0119] As shown in Table 1, Examples 1 to 5, when W2 ≥ W1 + 1 mm, the cycle capacity retention rate improves, the thickness expansion rate decreases, and the lithium deposition rate decreases. This is because when W2 ≥ W1 + 1 mm, the second section 2032 of the first separator 2003 exerts a certain restraining force on the wound structure, reducing the risk of expansion of the electrode assembly 22, the risk of deformation and warping of the outermost electrode of the wound structure, and the degree of lithium deposition in the secondary battery 100. When W2 ≤ W1 + 5 mm, the first separator 2003 has a smaller impact on the infiltration and transmission of the electrolyte into the wound structure, reducing the risk of electrolyte bridge failure during cycling of the secondary battery 100 and minimizing the impact on the cycling performance of the secondary battery 100. Limiting W1 + 1 mm ≤ W2 ≤ W1 + 5 mm ensures that the second section 2032 of the first separator 2003 exerts a certain restraining effect on the wound structure while minimizing the impact on the infiltration and transmission of the electrolyte into the wound structure, thereby minimizing the impact on the cycling performance of the secondary battery 100.

[0120] As shown in Table 1, Comparative Examples 2 and 3, as well as Examples 1 through 8, when T2 is greater than or equal to 2.5 mm, the second section 2032 of the first separator 2003 has a stronger restraining effect on the edge of the outermost electrode of the wound structure. When T2 is less than or equal to T1, the first separator 2003 has a smaller impact on the infiltration and transmission of electrolyte into the interior of the wound structure. The restriction of 2.5 mm ≤ T2 ≤ T1 not only ensures a certain restraining effect of the second section 2032 of the first separator 2003 on the wound structure, but also reduces the impact of electrolyte infiltration and transmission into the interior of the wound structure, thereby reducing the impact on the cycle performance of the secondary battery 100. Furthermore, the restriction of 3 mm ≤ T2 ≤ 1 / 2 T1 further enhances the restraining effect of the second section 2032 of the first separator 2003 on the outermost electrode of the wound structure, further reducing the impact of electrolyte infiltration and transmission into the interior of the wound structure.

[0121] As shown in Table 1, Examples 1 and 9 to 13, when W4 is greater than or equal to 0.05 mm, the risk of shorting between the first and second pole pieces 2001 and 2002 is low, and the high-voltage test quality rate of the secondary battery 100 is improved. When W4 is less than or equal to 4 mm, the effect of the first separator 2003 on electrolyte transmission is reduced, thereby minimizing the impact on the cycle life of the secondary battery 100. By limiting 0.05 mm ≤ W4 ≤ 4 mm, the risk of shorting between the first and second pole pieces 2001 and 2002 is reduced, and the effect of the first separator 2003 on electrolyte infiltration and transmission is also reduced.

[0122] It can be seen from Example 1 and Example 14 to Example 17 in Table 1 that when the thickness of the first isolation film 2003 is greater than or equal to 1 μm, the first isolation film 2003 has a better restraining effect on the edge of the first electrode 2001, and the risk of warping and lithium deposition at the edge of the electrode of the outermost circle of the wound structure is low, and it is beneficial to reduce the risk of short circuit between the first electrode 2001 and the second electrode 2002, thereby improving the high-voltage test quality rate of the secondary battery 100; when the thickness of the first isolation film 2003 is less than or equal to 20 μm, when the second section 2032 of the first isolation film 2003 is folded to form the first end surface 21, it is easier to fold, and the first isolation film 2003 has a better effect of suppressing the warping of the edge of the electrode of the outermost circle of the wound structure, the risk of lithium deposition is reduced, and the impact on the energy density of the secondary battery 100 is reduced. By limiting the thickness of the first isolation film 2003 to between 1 μm and 20 μm, the first isolation film 2003 can have a certain restraining effect on the edge of the first pole piece 2001, thereby improving the high-voltage test quality of the secondary battery 100, and reducing the impact of the first isolation film 2003 on the energy density of the secondary battery 100.

[0123] As shown in Table 1, Example 1 and Example 18, the provision of the first adhesive 4 improves the cycle capacity retention rate, reduces the thickness expansion rate, and reduces the lithium deposition rate. This is because the provision of the first adhesive 4 allows the first extension 0322 to be bonded to the first end face 21 via the first adhesive 4, reducing the risk of rebound and warping of the first extension 0322 and improving the restraint effect of the first extension 0322 on the edge of the outermost electrode of the wound structure. This improves the overall structural stability of the secondary battery 100 and reduces the degree of lithium deposition in the secondary battery 100.

[0124] As can be seen from Table 1, Example 1 and Example 19, when the first extension portion 0322 covers the end face of the side where the tab 3 extends, the improvement effect on the cycle capacity retention rate, thickness expansion rate, and lithium deposition rate of the secondary battery 100 is slightly less than when the first extension portion 0322 covers the opposite side where the tab 3 extends. This is because, along the thickness direction of the electrode assembly 22, the thickness of the end of the electrode assembly 22 where the tab 3 is located is greater, resulting in better interfacial adhesion at the edge of the electrode piece at the second end face 22 during formation, and a lower risk of deformation and warping of the outermost electrode piece edge at the second end face 22. In other words, when the first extension portion 0322 covers the opposite side where the tab 3 extends, the improvement in the cycle capacity retention rate, thickness expansion rate, and lithium deposition rate of the secondary battery 100 is greater. Therefore, it is preferred to provide the first extension portion 0322 of the second section 2032 of the first separator 2003 on the first end face 21 opposite the second end face 22.

[0125] As shown in Table 1 for Examples 1 and 20, the provision of the third extension 0323 improves the cycle capacity retention rate, reduces the thickness expansion rate, and decreases the lithium deposition rate. This is because the provision of the third extension 0323 in the second section 2032 constrains the second end face 22, while the first extension 0322 constrains the first end face 21. This enhances the restraining effect of the first separator 2003 on the wound structure, further reducing the risk of expansion of the electrode assembly 22, the risk of deformation and warping at the edge of the outermost electrode of the wound structure, and the degree of lithium deposition in the secondary battery 100.

[0126] The difference between Examples 21 to 38 and Example 1 lies in the different related parameters, which are shown in Table 2 below.

[0127] Table 2

[0128] As shown in Table 2, Examples 1 and 21 to 26, when F1 is greater than or equal to 10 MPa, the first separator 2003 effectively suppresses the expansion of the electrode assembly 22 and effectively restrains the edge of the outermost electrode piece. When F1 is less than or equal to 30 MPa, the first extension 0322 is susceptible to thermal contraction. Under heating conditions, the first extension 0322 thermally contracts and covers the first end surface 21, effectively suppressing warping at the edge of the outermost first electrode piece 2001, thereby reducing the degree of lithium deposition at the electrode piece edge. The limitation of 10 MPa ≤ F1 ≤ 30 MPa ensures that the first separator 2003 has a certain restraining effect on the expansion of the electrode assembly 22 and the edge of the outermost electrode piece, while also ensuring that the first extension 0322 has good thermal contraction performance, thereby improving the first separator 2003's ability to suppress warping at the edge of the outermost electrode piece and reducing the degree of lithium deposition in the secondary battery 100.

[0129] As can be seen from Table 2, Examples 1 and 27 to 31, when H1 is greater than or equal to 20HB, the first separator 2003 effectively suppresses the expansion of the electrode assembly 22 and effectively restrains the edge of the outermost electrode piece. When H1 is less than or equal to 60HB, the first extension 0322 is susceptible to thermal contraction. Under heating conditions, the first extension 0322 thermally contracts and covers the first end surface 21, effectively suppressing warping at the edge of the outermost first electrode piece 2001, thereby reducing the degree of lithium deposition at the electrode piece edge. The limitation of 20HB≤H1≤60HB ensures that the first separator 2003 has a certain restraining effect on the expansion of the electrode assembly 22 and the edge of the outermost electrode piece, while also ensuring that the first extension 0322 has good thermal contraction performance, thereby improving the first separator 2003's ability to suppress warping at the edge of the outermost electrode piece and reducing the degree of lithium deposition in the secondary battery 100.

[0130] As shown in Table 2 for Examples 1 and 32, when the thermal shrinkage rate of the first segment 2031 is less than that of the first extension 0322, the performance of the secondary battery 100 during the high-voltage test is improved. This is because when the thermal shrinkage rate of the first segment 2031 is less than that of the first extension 0322, the first segment 2031 is less likely to shrink when the first extension 0322 is heated, thus reducing the potential for a decrease in the performance of the secondary battery 100 during the high-voltage test due to thermal shrinkage of the first segment 2031.

[0131] As shown in Table 2 for Examples 1, 33, and 34, when S1 ≤ 2.5%, the quality rate of the secondary battery 100 under high-voltage testing improves. This is because when S1 ≤ 2.5%, the thermal shrinkage of the first section 2031 is not excessive, thereby reducing the risk of internal short circuits caused by thermal shrinkage of the first section 2031 and improving the quality rate of the secondary battery 100 under high-voltage testing.

[0132] It can be seen from Example 1 and Examples 35 to 38 in Table 2 that when 3%≤S2≤5%, the cycle capacity retention rate is improved, the thickness expansion rate is reduced, and the lithium plating rate is reduced. This is because when S2 is less than or equal to 5%, the first extension portion 0322 has a certain hardness, and the restraining effect on the edge of the outermost pole piece is relatively good. By limiting 3%≤S2≤5%, the first extension portion 0322 has a certain thermal shrinkage performance and a certain hardness, so that the first extension portion 0322 has a certain restraining effect on the edge of the outermost pole piece of the wound structure, reducing the risk of warping and deformation of the edge of the outermost pole piece and reducing the degree of lithium plating.

[0133] In addition to the first segment 2031-2031 and the second segment 2032-2032, the first isolation film 2003-2003 in Examples 39-50 also includes a third segment 2033-2032. The difference between Examples 39-50 lies in the different parameters, as shown in Table 3 below.

[0134] Table 3

[0135] As shown in Table 3, Examples 1, 39, and 40, when W3 is greater than or equal to 1 mm, the third segment 2033 exerts a strong restraint on the edge of the secondary outer ring electrode piece of the wound structure, reducing the risk of electrode assembly 22 expansion, the risk of deformation and warping of the secondary outer ring of the wound structure, and the risk of lithium deposition in the secondary battery 100. By limiting W3 to W1+1 mm, the third segment 2033 of the first separator 2003 exerts a certain restraint on the edge of the single-sided coated electrode piece of the secondary outer ring of the wound structure, reducing the risk of electrode assembly 22 expansion, reducing the risk of deformation and warping of the secondary outer ring of the wound structure, and reducing the degree of lithium deposition in the secondary battery 100.

[0136] As shown in Table 3, Example 1 and Examples 41 to 46, when 1 / 2L ≤ L1 ≤ 4L, the cycle capacity retention rate improves, the thickness expansion rate decreases, the lithium plating rate decreases, and the volumetric energy density of the secondary battery 100 is higher. This is because when L1 is greater than or equal to 1 / 2L, the first separator 2003 effectively constrains the wound structure. When L1 is less than or equal to 4L, the first separator 2003 does not absorb and consume a large amount of electrolyte, which would affect the electrolyte infiltration and transport into the wound structure, thus having a smaller impact on the cycle life of the secondary battery 100 and reducing the impact of the first separator 2003 on the volumetric energy density of the secondary battery 100. By limiting 1 / 2L ≤ L1 ≤ 4L, the widened section 203' of the first separator 2003 has a certain constraining effect on the wound structure, reducing the risk of electrode assembly 22 expansion and the risk of lithium plating in the secondary battery 100, while also reducing the impact of the first separator 2003 on electrolyte infiltration and transport, thereby reducing the impact on the cycle life and volumetric energy density of the secondary battery 100. Furthermore, it is preferred that L≤L1≤2L, so as to further enhance the restraining effect of the widened section 203' of the first isolation membrane 2003 on the winding structure, further reduce the risk of expansion of the electrode assembly 22, reduce the risk of lithium plating in the secondary battery 100, and further reduce the influence of the first isolation membrane 2003 on the infiltration and transmission of the electrolyte, thereby reducing the influence on the cycle life and volume energy density of the secondary battery 100.

[0137] As shown in Table 3 for Examples 1 and 47 to 50, when 0 ≤ L2 ≤ 10 mm, the cycle capacity retention rate improves, the thickness expansion rate decreases, and the lithium deposition rate decreases. This is because when L2 is less than or equal to 10 mm, the third section 2033 of the first separator 2003 effectively restrains the secondary outer ring (the third portion 203 of the first pole piece 2001) of the wound structure, thereby reducing the degree of lithium deposition in the second pole piece 2002, which is opposite the third portion 203 of the first pole piece 2001. By limiting 0 ≤ L2 ≤ 10 mm, the second section 2032 and third section 2033 of the first separator 2003 effectively restrain the outermost ring (the second portion 202) and the secondary outer ring (the third portion 203) of the first pole piece 2001, thereby reducing the degree of lithium deposition in the second pole piece 2002, which is opposite the third portion 203 of the first pole piece 2001. Furthermore, it is preferred that 0≤L2≤2mm, which further improves the binding effect of the second section 2032 and the third section 2033 of the first isolation membrane 2003 on the outermost circle (second part 202) and the second outer circle (third part 203) of the first pole piece 2001, and further reduces the risk of lithium deposition in the second pole piece 2002 opposite to the third part 203 of the first pole piece 2001.

[0138] It should be noted that the preferred embodiments of the present application are given in the specification and drawings of this application. However, the present application can be implemented in many different forms and is not limited to the embodiments described in this specification. These embodiments are not intended to be additional limitations on the content of this application. The purpose of providing these embodiments is to make the understanding of the disclosure of this application more thorough and comprehensive. In addition, the above-mentioned technical features can be combined with each other to form various embodiments not listed above, which are all considered to be within the scope of the description of this application; further, it is obvious to those skilled in the art that improvements or changes can be made based on the above description, and all such improvements and changes should fall within the scope of protection of the claims attached to this application.

Claims

1. A secondary battery comprising an electrode assembly and a tab connected to the electrode assembly, wherein the electrode assembly comprises a first electrode sheet, a first separator, and a second electrode sheet, wherein the first electrode sheet, the first separator, and the second electrode sheet are stacked to form a wound structure; The electrode assembly includes a first end surface and a second end surface arranged opposite to each other along a first direction, wherein the first direction is a direction in which the electrode tab extends out of the electrode assembly; A portion of the first electrode sheet constitutes a first side surface and a second side surface of the electrode assembly that are opposite to each other in a second direction. The first side surface and the second side surface are the outermost electrode sheets of the wound structure in the second direction. The second direction is the thickness direction of the electrode assembly. The first direction and the second direction are perpendicular to each other. Along the winding direction of the electrode assembly, the first separator includes a first section and a second section, and the second section is located at the outermost circle of the winding structure; The second section includes a first main portion and a first extension portion that are integrally provided, the first main portion covers at least a portion of the first side surface and / or at least a portion of the second side surface, and the first extension portion is attached to at least a portion of the first end surface; The portion of the first section attached to the first end surface has a first edge after being flattened along the first direction, and the first extension portion extends beyond the first edge after the second section is flattened along the first direction; and The first extension portion has a second edge after being flattened. The first edge and the second edge are located on the same side of the electrode assembly in the first direction. The distance between the second edge and the first edge in the first direction is W, and W is ≥ 1 mm.

2. The secondary battery according to claim 1, wherein The width of the first section after being flattened along the first direction is W1, and the width of the second section after being flattened along the first direction is W2, where W2 ≥ W1 + 1 mm.

3. The secondary battery according to claim 2, wherein W1+1mm≤W2≤W1+5mm.

4. The secondary battery according to claim 1, wherein Along the second direction, the thickness of the electrode assembly is T1; Along the second direction, a dimension of the first extension portion is T2, 2.5 mm ≤ T2 ≤ T1.

5. The secondary battery according to claim 4, wherein 3mm≤T2≤1 / 2T1.

6. The secondary battery according to claim 1, wherein The first pole piece includes a first current collector and a first active material layer, the first current collector includes a first surface and a second surface disposed opposite to each other along a thickness direction of the first current collector, and the first surface faces away from the winding center of the winding structure; Along the winding direction, the first pole piece includes a first portion and a second portion, and the first surface of the first portion and the second surface of the first portion are both provided with the first active material layer; The second surface of the second portion is provided with the first active material layer, and the first surface of the second portion is not provided with an active material layer; The second part is a pole piece located at the outermost circle of the winding structure; The first body portion covers at least a portion of the first portion.

7. The secondary battery according to claim 6, characterized in that The first isolation film further includes a third section, and the first section, the third section and the second section are sequentially connected along the winding direction; The first pole piece further includes a third portion, the third portion being a pole piece located at the secondary outer ring of the winding structure, and the first portion, the third portion and the second portion are sequentially connected along the winding direction; The second surface of the third portion is provided with the first active material layer, and the first surface of the third portion is not provided with an active material layer; The third section includes a second main body portion and a second extension portion that are integrally provided, the second main body portion covers at least a portion of the third section, and the second extension portion is attached to at least a portion of the first end surface; The width of the third section after being flattened along the first direction is W3, where W3≥W1+1mm.

8. The secondary battery according to claim 7, wherein: The second section and the third section together constitute a widened section of the first isolation membrane; Along the third direction, the width of the orthographic projection area of ​​the electrode assembly in the second direction is L, and the third direction, the first direction and the second direction are perpendicular to each other; After the first isolation film is unfolded along the winding direction, the length of the widened section is L1, and 1 / 2L≤L1≤4L.

9. The secondary battery according to claim 8, characterized in that L≤L1≤2L.

10. The secondary battery according to claim 8, wherein Along the winding direction, the widened section includes a first starting edge, and the second pole piece includes a first ending edge; the first starting edge extends beyond the first ending edge along the winding direction; A distance between the first starting edge and the first ending edge along the winding direction is L2, 0≤L2≤10mm.

11. The secondary battery according to claim 10, wherein 0≤L2≤2mm.

12. The secondary battery according to claim 1, wherein The tensile strength of the second segment at 25°C is F1, and the Brinell hardness of the second segment at 25°C is H1; 10MPa≤F1≤30MPa; 20HB≤H1≤60HB.

13. The secondary battery according to claim 12, characterized in that 15MPa≤F1≤25MPa; 30HB≤H1≤50HB.

14. The secondary battery according to claim 12, wherein: The second section includes a first coating layer, wherein the material of the first coating layer includes at least one of oxide, nitride, and carbide; The oxide includes at least one of aluminum oxide, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, chromium oxide, yttrium oxide, beryllium oxide, yttrium aluminum oxide, hafnium oxide, tantalum oxide, niobium oxide, tungsten oxide, cerium oxide, lanthanum oxide, neodymium oxide, samarium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, erbium oxide, thulium oxide, ytterbium oxide, lutetium oxide, manganese oxide, iron oxide, nickel oxide, cobalt oxide, zinc oxide, and tin oxide; The nitride includes at least one of boron nitride, silicon nitride, aluminum nitride, titanium nitride, zirconium nitride, chromium nitride, tantalum nitride, niobium nitride, tungsten nitride, vanadium nitride, molybdenum nitride, gallium nitride, indium nitride, aluminum gallium nitride, indium gallium nitride, manganese nitride, iron nitride, nickel nitride, copper nitride, cobalt nitride, zinc nitride, tin nitride, germanium nitride, hafnium nitride, hafnium titanium nitride, titanium aluminum nitride, titanium silicon nitride, titanium zirconium nitride, and titanium chromium nitride; The carbide includes at least one of tungsten carbide, silicon carbide, titanium carbide, zirconium carbide, hafnium carbide, tantalum carbide, niobium carbide, vanadium carbide, chromium carbide, molybdenum carbide, iron carbide, nickel carbide, cobalt carbide, manganese carbide, aluminum carbide, boron carbide, gallium carbide, indium carbide, germanium carbide, tin carbide, lead carbide, zinc carbide, copper carbide, silver carbide, gold carbide, platinum carbide, palladium carbide, cadmium carbide, and thallium carbide.

15. The secondary battery according to claim 1, wherein The thermal shrinkage rate of the first section when heated at 100° C. for 10 seconds is S1, and the thermal shrinkage rate of the first extension portion when heated at 100° C. for 10 seconds is S2, where S2>S1.

16. The secondary battery according to claim 15, characterized in that S1≤2.5%； 3%≤S2≤5%。 17. The secondary battery according to claim 15, characterized in that The material of the first section includes a density of 0.94 g / cm at 25°C. 3 to 0.96 g / cm 3 The material of the first extension portion includes a polyethylene having a density of 0.91 g / cm at 25°C. 3 to 0.925 g / cm 3 of polyethylene.

18. The secondary battery according to claim 1, wherein The secondary battery further includes a first adhesive member, the first adhesive member being bonded between the first end surface and the first extension portion; The material of the first adhesive includes at least one of hot melt adhesive, epoxy resin adhesive, light curing adhesive, polyurethane adhesive, phenolic resin adhesive, acrylic adhesive, rubber adhesive, ethylene vinyl acetate, polyamide, white latex, polyvinyl alcohol, cyanoacrylate adhesive, silicone adhesive, siloxane polymer, and acrylate.

19. The secondary battery according to claim 1, wherein The tab protrudes from the second end surface along the first direction.

20. The secondary battery according to claim 1, wherein The second section further includes a third extension portion integrally provided with the first main body portion. The third extension portion and the first extension portion are arranged opposite to each other along the first direction, and the third extension portion is attached to the second end surface.

21. The secondary battery according to claim 1, wherein The first pole piece is a positive pole piece, and the second pole piece is a negative pole piece; Along the first direction, the second pole piece is arranged beyond the first pole piece, the first section is arranged beyond the second pole piece, and the dimension of the first edge exceeding the edge of the second pole piece is W4, 0.05mm≤W4≤4mm.

22. The secondary battery according to claim 6, characterized in that The secondary battery further includes a second isolation film, and along the thickness direction of the first electrode sheet, the first isolation film and the second isolation film are respectively located on both sides of the first electrode sheet; In one winding turn, the first isolation film is closer to the first surface than the second isolation film.

23. The secondary battery according to claim 1, wherein The first isolation film includes a first substrate layer, and a material of the first substrate layer includes at least one of polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polyamide, fluorinated polypropylene, silicone rubber and polyester.

24. The secondary battery according to claim 1, wherein The thickness of the first isolation film is between 1 μm and 20 μm.

25. An electronic device, characterized in that: The secondary battery comprises the secondary battery according to any one of claims 1 to 24.

26. A method for preparing a secondary battery according to any one of claims 1 to 24, characterized in that: include: Winding step: providing the first pole piece, the first isolation film, and the second pole piece, wherein the first isolation film includes a first section and a second section arranged along a length direction of the first isolation film, and along the first direction, the second edge exceeds the first edge; The first pole piece, the first isolation film, and the second pole piece are sequentially stacked and wound to form the wound structure, wherein the second section is located at the outermost circle of the wound structure; Heat shrinking step: Use the first plate to push the second section along the first direction beyond the extension part of the first pole piece to fold toward the first end face. At the same time, use a heating device to heat the extension part to form the first extension part. The heating temperature is 70℃~150℃ and the heating time is 1s~20s.

27. The method for preparing a secondary battery according to claim 26, wherein: Before the heat shrinking step, the method further includes coating an adhesive on the first end surface or the side of the extension portion facing the winding center of the winding structure, wherein the adhesive is used to bond the first extension portion and the first end surface.

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