Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. A secondary battery includes a case and an electrode assembly. The electrode assembly comprises a negative pole piece and a positive pole piece. The negative pole piece comprises a negative current collector and a negative active material layer. The positive pole piece comprises a positive current collector and a positive active material layer. Along the first direction, the number of the positive active material layers in the electrode assembly is N. The electrode assembly includes a first end and a second end opposite to each other in a second direction perpendicular to the first direction. At the first end, the edge of the positive active material layer is a first edge, and the edge of the negative active material layer is a second edge. The distance between the first edge of each positive electrode active material layer and the second edge with the shortest distance from the first edge in the first direction in the second direction is D. And along the second direction, the size of the positive electrode active material layer is D1, the size of the negative electrode active material layer is D2, and n layers in the N positive electrode active material layers meet the following condition: D is less than or equal to 0.38 (D2-D1). The electrode assembly can improve the reliability and the service life of the secondary battery.

Description

Secondary batteries and electronic devices Technical Field

[0001] The present application relates to the field of energy storage technology, and in particular to a secondary battery and an electronic device having the secondary battery. Background Art

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in electronic devices, power tools, electric vehicles, and other electronic products. After long-term charge and discharge cycles, lithium will precipitate on the surface of the negative electrode of the secondary battery, forming lithium dendrites. Lithium dendrites can easily pierce the separator and cause short-circuit failure, thus affecting the reliability and service life of the secondary battery.

[0003] Summary of the Invention

[0004] In view of this, it is necessary to provide a secondary battery with improved reliability and service life.

[0005] In addition, it is also necessary to provide an electronic device having the secondary battery.

[0006] In a first aspect, the present application provides a secondary battery comprising a shell and an electrode assembly. The shell accommodates the electrode assembly. The electrode assembly comprises a negative electrode plate, a positive electrode plate, and a separator disposed between the negative electrode plate and the positive electrode plate. The negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The positive electrode plate comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The thickness direction of the electrode assembly is defined as a first direction, and the number of positive electrode active material layers in the electrode assembly along the first direction is N. The electrode assembly comprises a first end and a second end opposite to each other along a second direction perpendicular to the first direction. At the first end, the edge of the positive electrode active material layer is a first edge, and the edge of the negative electrode active material layer is a second edge. The distance in the second direction between the first edge of each positive electrode active material layer and the second edge which is the shortest distance to the first edge along the first direction is D. Along the second direction, the positive electrode active material layer has a size D1, the negative electrode active material layer has a size D2, and n positive electrode active material layers in the N positive electrode active material layers satisfy the following condition: D ≤ 0.38 (D2 - D1). N and n are both positive integers, and n is less than N.

[0007] In the present application, a portion of the positive electrode active material layer is arranged in the N-layer positive electrode active material layer at the first end to meet the following conditions: D≤0.38(D2-D1). Therefore, during overcharging or cycling, even if the negative electrode active material layer fails to fully embed the lithium ions released by the corresponding positive electrode active material layer and lithium deposition occurs, the lithium deposition position of the negative electrode active material layer corresponding to the positive electrode active material layer that meets this condition and the lithium deposition position of other negative electrode active material layers can be misaligned with each other in the second direction. This is conducive to reducing the overall thickness of the electrode assembly and alleviating the extrusion between the pole pieces, thereby reducing the risk of lithium dendrites easily piercing the isolation membrane and causing short-circuit failure due to the large extrusion force between the pole pieces when the lithium dendrites generated by the negative electrode active material layer are at the same position in the second direction. Therefore, the reliability and service life of the secondary battery are improved.

[0008] In some possible implementations, 0.2 N ≤ n ≤ 0.8 N. Therefore, the risk of lithium dendrites generated in most of the negative electrode active material layers being located at the same position in the second direction, resulting in a large squeezing force between the electrode sheets and the risk of lithium dendrites easily piercing the separator and causing short circuit failure can be further reduced.

[0009] In some possible implementations, 0.4N≤n≤0.6N, so that the number of positive electrode active material layers meeting the above conditions is close to half of the total number of positive electrode active material layers. Therefore, even if lithium deposition occurs, the lithium deposition locations of the negative electrode active material layers close to half of the total number of layers are offset from the lithium deposition locations of the remaining negative electrode active material layers, which helps to further reduce the overall thickness of the electrode assembly and alleviate the compression between the electrode sheets.

[0010] In some possible implementations, the average value of the D values ​​of the n-layer positive electrode active material layer is greater than or equal to 0.1(D2-D1). Therefore, under the premise that the lithium deposition position of the negative electrode active material layer corresponding to the n-layer positive electrode active material layer is misaligned with the lithium deposition positions of other negative electrode active material layers in the second direction, the negative electrode active material layer can still extend beyond the positive electrode active material layer in the second direction to reduce the risk of lithium deposition in the negative electrode active material layer.

[0011] In some possible implementations, the values ​​of D of the n positive electrode active material layers are different, which can further increase the degree of dislocation of the lithium deposition position of the negative electrode active material layer, thereby further alleviating the compression between the electrode sheets.

[0012] In some possible implementations, the electrode assembly is a wound structure, and the second direction is the direction of the winding center axis of the electrode assembly. The wound structure is conducive to improving the production efficiency of the secondary battery.

[0013] In some possible implementations, the secondary battery further includes a tab, which is electrically connected to the electrode assembly and extends out of the electrode assembly from the first end, thereby facilitating the tab's use for electrical connection to an external device.

[0014] In some possible implementations, the electrode assembly has a laminated structure, which is beneficial for reducing the internal resistance of the secondary battery and improving high-rate charge and discharge performance.

[0015] In some possible implementations, the electrode assembly further includes a third end and a fourth end opposite to each other along a third direction perpendicular to both the first direction and the second direction. At the third end, the edge of the positive electrode active material layer is the third edge, and the edge of the negative electrode active material layer is the fourth edge. The distance in the third direction between the third edge of each positive electrode active material layer and the fourth edge that is the shortest distance from the third edge along the first direction is d. Along the third direction, the size of the positive electrode active material layer is D3, the size of the negative electrode active material layer is D4, and the m positive electrode active material layers among the N positive electrode active material layers meet the following conditions: d≤0.38(D4-D3), m is a positive integer, and m is less than N. Therefore, during overcharging or cycling, even if the negative electrode active material layer fails to fully intercalate the lithium ions released by the corresponding positive electrode active material layer and lithium plating occurs, the lithium plating position of the negative electrode active material layer corresponding to the positive electrode active material layer that meets this condition can be misaligned with the lithium plating positions of other negative electrode active material layers in the third direction. This helps to reduce the overall thickness of the electrode assembly and alleviate the squeezing between the pole pieces, thereby reducing the risk of lithium dendrites generated by the negative electrode active material layer easily piercing the isolation membrane and causing short circuit failure due to the large squeezing force between the pole pieces when the lithium dendrites are in the same position in the third direction.

[0016] In some possible implementations, 0.2 N ≤ m ≤ 0.8 N. Therefore, the risk of lithium dendrites generated in the majority of the negative electrode active material layer being located in the same position in the third direction, resulting in a large squeezing force between the electrode sheets and the risk of lithium dendrites easily piercing the separator and causing short-circuit failure can be further reduced.

[0017] In some possible implementations, the secondary battery further includes a tab, which is electrically connected to the electrode assembly and extends from the first end and / or the third end of the electrode assembly. The tab can be used to electrically connect to an external device.

[0018] In some possible implementations, the edge of the separator at the first end is the fifth edge; along the second direction, the distance between the fifth edge and the second edge is a, where 0.3 mm ≤ a ≤ 1.5 mm. Thus, while the separator can effectively reduce the risk of contact between the first and second edges, it also minimizes the impact of an excessively large a value on the secondary battery's energy density.

[0019] In some possible implementations, the separator includes a laminated substrate layer and a coating layer, wherein the coating layer comprises at least one of a ceramic material and an adhesive material. The adhesive material is used to improve the interfacial adhesion between the separator and the electrode, reduce expansion and deformation of the electrode assembly during overcharging or cycling, and ensure the cyclability of the secondary battery. The ceramic material is used to improve the separator's heat resistance and puncture resistance.

[0020] In some possible implementations, the positive electrode active material layer includes at least one of a lithium transition metal composite oxide or a lithium-containing transition metal phosphate compound.

[0021] In some possible implementations, the negative electrode active material layer includes one or more of graphite materials, alloy materials, lithium metal, lithium metal alloy, silicon materials, silicon-oxygen materials, and silicon-carbon materials.

[0022] In some possible implementations, 0.3 mm ≤ D2 - D1 ≤ 2 mm, thereby effectively reducing the risk of lithium plating in the negative electrode active material layer while also reducing the waste of negative electrode active material when the above difference is large.

[0023] A second aspect of the present application further provides an electronic device comprising the above-described secondary battery. The electronic device is powered by the above-described secondary battery, and even if lithium deposition occurs on the negative electrode of the secondary battery, the risk of lithium dendrites piercing the separator and causing short-circuit failure is reduced, thereby improving the safety and service life of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0025] FIG1 is a schematic structural diagram of a secondary battery provided in one embodiment of the present application when viewed from a first direction.

[0026] FIG. 2 is a cross-sectional view of the secondary battery shown in FIG. 1 along the cutting line II-II.

[0027] FIG3 is a cross-sectional view of the secondary battery shown in FIG1 along the cutting line III-III.

[0028] FIG4A is a schematic structural diagram of a secondary battery provided in another embodiment of the present application.

[0029] FIG4B is a cross-sectional view of the secondary battery shown in FIG4A along the cutting line IV-IV.

[0030] FIG5 is a cross-sectional view of the separator of the secondary battery shown in FIG2, FIG3 or FIG4B.

[0031] FIG6 is a schematic structural diagram of a secondary battery provided in another embodiment of the present application when viewed from a first direction.

[0032] FIG. 7 is a cross-sectional view of the secondary battery shown in FIG. 6 along the cutting line VII-VII in another embodiment.

[0033] FIG8 is a cross-sectional view of the secondary battery shown in FIG6 along the cutting line VIII-VIII in another embodiment.

[0034] FIG9 is a cross-sectional view of the secondary battery shown in FIG6 along the cutting line IX-IX in another embodiment.

[0035] FIG10 is a schematic structural diagram of an electronic device provided in one embodiment of the present application.

[0036] : Description of Main Component Symbols Electronic device 1 Housing 10 Electrode assembly 20 First end 20A Second end 20B Third end 20C Fourth end 20D Negative electrode sheet 21 Positive electrode sheet 22 Separator 23 Fifth edge 23A Negative electrode tab 30 Positive electrode tab 40 Secondary battery 100, 200 First section 201 Second section 202 Third section 203 Fourth section 204 Negative electrode current collector 210 Negative electrode active material layer 211 Second edge 211A Fourth edge 211B Positive electrode current collector 220 Positive electrode active material layer 221 First edge 221A Third edge 221B Substrate layer 231 Coating layer 232 First region 2100 First slot 2110 Second region 2200 Second slot 2210 Dimensions D1, D2, D3, D4 Distances D, d, a Winding center axis O Winding direction D First direction X Second direction Y Third direction Z Dashed lines AA, BB, L

[0037] The following specific implementation methods will further illustrate the present application in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application are described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0039] Below, embodiments of the present application will be described in detail. However, the present application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments illustrated herein. Rather, these exemplary embodiments are provided to make the present application thorough and detailed for those skilled in the art.

[0040] In addition, for the sake of brevity and clarity, the size or thickness of various components or layers may be exaggerated in the accompanying drawings. Throughout the text, the same numerical value refers to the same element. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more related enumerated items. In addition, it should be understood that when element A is referred to as "connecting" element B, element A can be directly connected to element B, or there may be an intermediate element C and element A and element B can be indirectly connected to each other.

[0041] Further, when describing embodiments of the present application, the use of “may” refers to “one or more embodiments of the present application.”

[0042] The technical terms used herein are for the purpose of describing specific embodiments and are not intended to limit this application. As used herein, the singular is intended to include the plural, unless the context clearly indicates otherwise. It should be further understood that the term "comprising", when used in this specification, refers to the presence of the described features, values, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components and / or combinations thereof.

[0043] Spatial related terms, such as "on" etc., can be used herein for convenient description, to describe the relationship between an element or feature and another element (multiple elements) or feature (multiple features) as illustrated in the figure. It should be understood that, in addition to the directions described in the figure, spatial related terms are intended to include different directions of equipment or devices in use or operation. For example, if the equipment in the figure is turned over, the elements described as "above" or "on" other elements or features will be oriented "below" or "below" other elements or features. Therefore, the exemplary term "on" can include the direction above and below. It should be understood that although the terms first, second, third, etc. can be used herein to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are used to distinguish an element, component, region, layer or part from another element, component, region, layer or part. Therefore, the first element, component, region, layer or part discussed below can be referred to as the second element, component, region, layer or part, without departing from the teachings of the exemplary embodiments.

[0044] As used herein, "parallel" and "perpendicular" are used to describe an ideal state between two components. In actual production or use, there may be a state between two components that is approximately parallel or perpendicular. For example, in combination with numerical descriptions, parallel can refer to the angle between two straight lines being between ±10°, parallel can also refer to the dihedral angle between two planes being between ±10°, and parallel can also refer to the angle between a straight line and a plane being between ±10°. Perpendicular can refer to the angle between two straight lines being between 90±10°, perpendicular can also refer to the dihedral angle between two planes being between 90±10°, and perpendicular can also refer to the angle between a straight line and a plane being between 90±10°. The two components described as "parallel" or "perpendicular" may not be absolute straight lines or planes, but may be roughly straight lines or planes. From a macroscopic perspective, a component can be considered a "straight line" or "plane" if its overall extension direction is a straight line or plane.

[0045] In this application, the relationship between parameter values ​​that is greater than, less than, or not equal to the design relationship needs to exclude the reasonable error of the measuring equipment.

[0046] Referring to Figures 1 to 3, one embodiment of the present application provides a secondary battery 100, comprising a shell 10, an electrode assembly 20, a negative electrode tab 30, a positive electrode tab 40 and an electrolyte (not shown). The electrode assembly 20 and the electrolyte are located inside the shell 10. The negative electrode tab 30 and the positive electrode tab 40 are both electrically connected to the electrode assembly 20, and extend from the inside of the shell 10 to the shell 10 to be electrically connected to an external device (not shown). In some embodiments, the shell 10 can be a packaging bag obtained by packaging with a packaging film (such as an aluminum-plastic film), that is, the secondary battery 100 can be a soft-pack battery. In other embodiments, the secondary battery 100 can also be a steel shell battery or an aluminum shell battery.

[0047] As shown in Figure 2, the electrode assembly 20 includes a negative electrode sheet 21, a positive electrode sheet 22 and a separator 23, and the separator 23 is arranged between the negative electrode sheet 21 and the positive electrode sheet 22. In some embodiments, the electrode assembly 20 is a wound structure, and the negative electrode sheet 21, the separator 23 and the positive electrode sheet 22 are stacked and wound in sequence to form the electrode assembly 20. The electrode assembly 20 has a winding center axis O perpendicular to the paper surface. The electrode assembly 20 has a winding direction D, which refers to the direction of moving from the inside to the outside around the winding center axis O along a certain point of the negative electrode sheet 21, the positive electrode sheet 22 or the separator 23 as shown in Figure 2. There are two types of winding directions D, namely, the direction of rotating clockwise or counterclockwise around the winding center axis O. In some embodiments, the winding direction D is the direction of rotating counterclockwise around the winding center axis O as shown in Figure 2.

[0048] In this application, the first direction X is the thickness direction of the electrode assembly 20. The second direction Y is the extension direction of the winding center axis O, and is also the direction in which the negative electrode tab 30 or the positive electrode tab 40 extends out of the electrode assembly 20. In some specific embodiments, when the electrode assembly 20 has a wound structure, the electrode assembly 20 includes a first segment 201, a second segment 202, a third segment 203, and a fourth segment 204, which are sequentially connected along the winding direction D. The first segment 201 and the third segment 203 are arranged opposite each other, and the second segment 202 and the fourth segment 204 are arranged opposite each other. In a cross-section perpendicular to the first direction X, the electrode assembly 20 can be flat or approximately circular. As shown in Figure 2, when the cross-section of the electrode assembly 20 is flat, the first segment 201 and the third segment 203 are respectively straight segments, the second segment 202 and the fourth segment 204 are respectively curved segments, and the thickness direction of the electrode assembly 20 is the direction from the first segment 201 to the third segment 203. In the present application, the innermost and left-side bent edge of the electrode assembly 10 extends in the first direction X to form a dotted line AA, and the innermost and right-side bent edge of the electrode assembly 10 extends in the first direction X to form a dotted line BB. When viewed from the second direction Y, the dotted line AA is the boundary between the second segment 202 and the first segment 201 or the third segment 203, and the dotted line BB is the boundary between the fourth segment 204 and the first segment 201 or the third segment 203. As shown in Figures 4A and 4B, in another embodiment, when the cross-section of the electrode assembly 20 is approximately circular (e.g., when the secondary battery 100 is a button battery), the thickness direction of the electrode assembly 20 can be considered as any direction perpendicular to the winding center axis O.

[0049] As shown in Figures 2 and 3, the negative electrode sheet 21 includes a negative electrode current collector 210 and a negative electrode active material layer 211 disposed on opposite surfaces of the negative electrode current collector 210, and the negative electrode tab 30 is connected to the negative electrode current collector 210. The positive electrode sheet 22 includes a positive electrode current collector 220 and a positive electrode active material layer 221 disposed on opposite surfaces of the positive electrode current collector 220, and the positive electrode tab 40 is connected to the positive electrode current collector 220.

[0050] In some embodiments, the positive electrode current collector 220 may use aluminum foil or nickel foil, and the negative electrode current collector 210 may use at least one of copper foil, nickel foil, or a carbon-based current collector.

[0051] The positive electrode active material layer 221 includes a positive electrode active material, which includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material may include at least one of a lithium transition metal composite oxide or a lithium-containing transition metal phosphate compound. In some embodiments, the lithium transition metal composite oxide is selected from at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), lithium manganese oxide, or lithium nickel manganese oxide. The lithium-containing transition metal phosphate compound is selected from at least one of lithium iron phosphate and lithium iron manganese phosphate.

[0052] The negative electrode active material layer 211 includes a negative electrode active material, which includes a negative electrode active material capable of reversible deintercalation of active ions. In some embodiments, the negative electrode active material may include one or more of a graphite material, an alloy material, lithium metal, a lithium metal alloy, a silicon material, a silicon-oxygen material, and a silicon-carbon material. For example, the graphite material may be selected from a combination of one or more of artificial graphite, natural graphite, and modified graphite; the alloy material may be selected from a combination of one or more of silicon, silicon oxide, tin, and titanium sulfide; and the lithium metal alloy may include lithium and at least one metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, or Sn.

[0053] The isolation film 23 includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Polyethylene and polypropylene have a good effect on improving short circuits.

[0054] For simplicity, the separator 23 in FIGS. 2, 3 and 4B is shown by a dashed line. FIG. 5 is a cross-sectional view of the separator 23 in some other embodiments. As shown in FIG. 5, the separator 23 may also be a multi-layer structure, which includes a substrate layer 231 and a coating layer 232 arranged in a stacked manner, and the coating layer 232 includes at least one of a ceramic material and an adhesive material. When the separator 23 includes both a ceramic material and an adhesive material at the same time, the ceramic material and the adhesive material may be separate coating layers or a mixed coating layer. Among them, the adhesive material is used to improve the interfacial adhesion force between the separator 23 and the electrode sheet, reduce the swelling deformation of the electrode assembly 20 during overcharging or cycling, and ensure the cycling ability of the secondary battery 100. The ceramic material is used to improve the heat resistance and puncture resistance of the separator 23. In some embodiments, the material of the adhesive material may be selected from at least one of a copolymer of difluoroethylene-hexafluoropropylene, a copolymer of vinylidene fluoride-trichloroethylene, polymethyl methacrylate, polyacrylic acid, polyacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, a copolymer of ethylene-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylated amylose, cyanoethylated polyvinyl alcohol, cyanoethylated cellulose, cyanoethylated sucrose, amylose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, a copolymer of acrylonitrile-styrene-butadiene, polyvinyl alcohol, polyvinyl ether, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of styrene-butadiene or polyvinylidene fluoride. The inorganic ceramic particles in the ceramic material may be selected from at least one of aluminum oxide, boehmite, barium sulfate, titanium dioxide and magnesium hydroxide.

[0055] As shown in FIG. 3, in order to reduce the risk of lithium deposition in the negative electrode active material layer 211, when observed from the first direction X, the negative electrode active material layer 211 and the positive electrode active material layer 221 are arranged to partially overlap, and the negative electrode active material layer 211 extends from the overlapping portion with the positive electrode active material layer 221 along the second direction Y to extend beyond the positive electrode active material layer 221. Along the second direction Y, the size of the positive electrode active material layer 221 is D1, and the size of the negative electrode active material layer 211 is D2. Since the negative electrode active material layer 211 extends beyond the positive electrode active material layer 221 along the second direction Y, D1 < D2. In some embodiments, 0.3 mm ≤ D2 - D1 ≤ 2 mm may be set, so as to effectively reduce the risk of lithium deposition in the negative electrode active material layer 211 while also reducing the waste of the negative electrode active material when the above difference is large, that is, when the above difference is large, the amount of negative electrode material that does not play a capacity role in the negative electrode active material layer 211 extending beyond the positive electrode active material layer 221 increases.

[0056] The electrode assembly 20 includes a first end 20A and a second end 20B that are opposite each other along the second direction Y. In some embodiments, the first end 20A is the head of the electrode assembly 20, and the second end 20B is the tail of the electrode assembly 20. Both the negative electrode tab 30 and the positive electrode tab 40 extend from the first end 20A. In some embodiments, one of the negative active material layers 211 of the negative electrode sheet 21 is provided with a first slot 2110, a first region 2100 of the negative current collector 210 is exposed in the first slot 2110, and the negative electrode tab 30 is welded to the first region 2100. One of the positive active material layers 221 of the positive electrode sheet 22 is provided with a second slot 2210, a second region 2200 of the positive current collector 220 is exposed in the second slot 2210, and the positive electrode tab 40 is welded to the second region 2200. Along the first direction X, the number of positive active material layers 221 in the electrode assembly 20 is N. It can be understood that along the first direction X, the electrode assembly 20 also has N layers of negative active material layers 211 disposed opposite N layers of positive active material layers 221. In some embodiments, a dotted line L can be drawn along the first direction X through the electrode assembly 20 (as shown in FIG. 2 ). This dotted line L can pass through the winding center axis O of the electrode assembly 20. When viewed from the second direction Y, the dotted line L will intersect each positive active material layer 221 located in the first section 201. The dotted line L will also intersect each positive active material layer 221 located in the third section 203. The total number of intersections is N. When determining the number n, it can be determined based on the number of positive active material layers 221 that intersect the same dotted line L and satisfy the condition D ≤ 0.38 (D2 - D1).

[0057] At the first end 20A, the edge of the positive electrode active material layer 221 is a first edge 221A. At the first end 20A, the edge of the negative electrode active material layer 211 is a second edge 211A. Because the negative electrode active material layer 211 extends beyond the positive electrode active material layer 221 in the second direction Y, the second edge 211A extends beyond the first edge 221A in the second direction Y. The distance D in the second direction Y between the first edge 221A of each positive electrode active material layer 221 and the second edge 211A that is shortest from the first edge 221A in the first direction X is understood to be the second edge 211A of the negative electrode active material layer 211 that is located opposite the positive electrode active material layer 221 via the separator 23.

[0058] The n-layer positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 satisfies the following condition: D ≤ 0.38 (D2-D1), where N and n are both positive integers, and n is less than N. That is, the (Nn)-layer positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 satisfies the following condition: D > 0.38 (D2-D1), and in some embodiments, the (Nn)-layer positive electrode active material layer 221 satisfies the following condition: D = 0.5 (D2-D1). Among the n-layer positive electrode active material layers 221 that satisfy the condition D ≤ 0.38 (D2-D1), the corresponding D values ​​can be the same or different. For example, among the n-layer positive electrode active material layers 221, the D values ​​of some positive electrode active material layers 221 are different from the D values ​​of another portion of the positive electrode active material layers 221.

[0059] In the present application, the measurement steps of D, D2, and D1 can be: (1) using X-rays to perform two-dimensional projection and scanning tests on the secondary battery 100 from a first direction X, and the instrument can be an instrument or equipment known to those skilled in the art (such as GE Phoenix vtomex S equipment), so as to obtain a CT image (as shown in the viewing angle of FIG3), from which the negative electrode collector 210 of the negative electrode plate 21 and the positive electrode collector 220 of the positive electrode plate 22 can be directly observed; (2) since along the second direction Y, the edge of the negative electrode collector 210 is usually flush with the edge of the negative electrode active material layer 211 (i.e., the second edge 211A), and the edge of the positive electrode collector 220 is usually flush with the edge of the positive electrode active material layer 221 (i.e., the first edge 221A), a caliper or other suitable measuring tool can be used to directly measure the values ​​of D, D2, and D1 on the CT image, where D is the value of the positive electrode collector 220. The distance between the edge of the positive electrode collector 220 and the edge of the negative electrode collector 210 adjacent to the positive electrode collector 220 in the second direction Y, D2 is the size of the negative electrode collector 210 in the second direction Y, and D1 is the size of the positive electrode collector 220 in the second direction Y.

[0060] Along the second direction Y, the edge of the positive electrode current collector 220 may also be misaligned with the edge of the positive electrode active material layer 221 (for example, to reduce the risk of burrs on the edge of the positive electrode current collector 220 piercing the separator 23 and contacting the negative electrode tab 21, a ceramic layer may be coated on the edge of the positive electrode current collector 220). In this case, in addition to calculating the distance between the edge of the positive electrode current collector 220 and the edge of the negative electrode current collector 210 adjacent to the positive electrode current collector 220 in the second direction Y on the CT map, the casing 10 of the secondary battery 100 can be further disassembled to obtain the electrode assembly 20, and then the size of the ceramic layer in the electrode assembly 20 in the second direction Y can be measured. In this case, D is the sum of the above distance and the size of the ceramic layer in the second direction Y.

[0061] When determining the number N, the steps may be: (1) using X-rays to perform two-dimensional projection and scanning tests on the secondary battery 100 from the second direction Y to obtain a CT image (viewing angle shown in FIG2 ), from which the positive electrode current collector 220 of the positive electrode sheet 22 can be directly observed, and the number of positive electrode current collectors 220 on the CT image is calculated; (2) disassembling the shell 10 of the secondary battery 100 to obtain the electrode assembly 20, and then comparing the electrode assembly 20 with the CT image to determine whether the two opposite surfaces of each layer of the positive electrode current collector 220 contain a positive electrode active material layer 221, thereby obtaining the number N of positive electrode active material layers 221. When determining the number n, it can be determined in combination with the value of D measured above. In other embodiments, X-rays can also be used to obtain a CT image of the secondary battery 100 from the first direction X (viewing angle shown in FIG3 ), and similar steps can be used to determine the number N.

[0062] During fabrication, the relative positions of the positive electrode sheet 22 and the negative electrode sheet 21 in the second direction Y can be set so that the positive electrode active material layer 221 meets the above conditions. In some embodiments, as shown in FIG3 , the first edges 221A of the N-layer positive electrode active material layer 221 can be positioned at approximately the same height in the second direction Y at the first end 20A of the electrode assembly 20, while the second edges 211A of the N-layer negative electrode active material layer 211 are offset relative to each other in the second direction Y, such that a portion of the N-layer positive electrode active material layer 221 satisfies D ≤ 0.38 (D2 - D1) while another portion of the positive electrode active material layer 221 satisfies D > 0.38 (D2 - D1). In other embodiments, it may also be arranged that at the first end 20A of the electrode assembly 20, the first edges 221A of the N-layer positive active material layer 221 are staggered with each other in the second direction Y, and the second edges 211A of the N-layer negative active material layer 211 are approximately at the same height in the second direction Y. Similarly, part of the positive active material layer 221 in the N-layer positive active material layer 221 satisfies D≤0.38(D2-D1) while another part of the positive active material layer 221 satisfies D>0.38(D2-D1). In other embodiments, it can also be arranged at the first end 20A of the electrode assembly 20, the first edges 221A of the N-layer positive active material layer 221 are staggered in the second direction Y, and the second edges 211A of the N-layer negative active material layer 211 are also staggered in the second direction Y, as long as part of the positive active material layer 221 in the N-layer positive active material layer 221 satisfies D≤0.38(D2-D1) and the other part of the positive active material layer 221 satisfies D>0.38(D2-D1).

[0063] Generally speaking, the area near the second edge of the negative electrode active material layer has a higher current density than other areas of the negative electrode active material layer, and the gap between it and the positive electrode plate is larger, which increases the impedance. In addition, the area near the second edge of the negative electrode active material layer dissipates heat faster than other areas of the negative electrode active material layer, and the temperature is lower during the charge and discharge of the secondary battery (the electrolyte viscosity is reduced, and the lithium ion migration speed is slowed). This may lead to reduced dynamics in the area near the second edge of the negative electrode active material layer, making this area prone to excessive lithium ion accumulation. In the present application, a portion of the positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 at the first end 20A satisfies the following condition: D≤0.38(D2-D1). Therefore, during overcharging or cycling, even if the negative electrode active material layer 211 at the first end 20A fails to fully embed the lithium ions released by the corresponding positive electrode active material layer 221 and lithium deposition occurs, the lithium deposition position of the negative electrode active material layer 211 corresponding to the positive electrode active material layer 221 that meets this condition and the lithium deposition position of other negative electrode active material layers 211 can be offset from each other in the second direction Y. This is conducive to reducing the thickness of the electrode assembly 20 at the lithium deposition position (i.e., reducing the overall thickness expansion of the electrode assembly 20), alleviating the squeezing between the pole pieces, thereby reducing the risk of lithium dendrites generated by the negative electrode active material layer 211 at the first end 20A being in the same position in the second direction Y, which is large, causing the lithium dendrites to easily pierce the separator 23 and cause short circuit failure. Therefore, the reliability and service life of the secondary battery 100 are improved. It can be understood that when the corresponding D values ​​in the n-layer positive electrode active material layer 221 that meet the condition of D≤0.38(D2-D1) are different, the degree of dislocation of the lithium deposition position of the negative electrode active material layer 211 can be further increased, thereby further alleviating the extrusion between the pole pieces.

[0064] Furthermore, since the negative electrode tab 30 is typically welded to the first region 2100 of the negative electrode current collector 210 and the positive electrode tab 40 is welded to the second region 2200 of the positive electrode current collector 220, when the positive electrode tab 40 and the negative electrode tab 30 extend from the first end 20A of the electrode assembly 20, the thickness of the electrode assembly 20 at the first end 20A is greater than the thickness of the electrode assembly 20 at the second end 20B. In the present application, a portion of the positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 at the first end 20A satisfies D≤0.38(D2-D1), so the compression between the electrode pieces at the first end 20A after lithium deposition can be alleviated, thereby reducing the risk that the first end 20A, due to its larger thickness, is more likely to form lithium dendrites during overcharging or cycling, resulting in a greater compression force on the electrode pieces at the first end 20A and puncturing the separator 23. In addition, it can be understood that since the n-layer positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 at the first end 20A satisfies D≤0.38(D2-D1), correspondingly, the n-layer positive electrode active material layer 221 at the second end 20B also satisfies D>0.38(D2-D1). Therefore, the present application can also reduce the risk of lithium dendrites generated in the negative electrode active material layer 211 at the second end 20B being in the same position in the second direction Y, where the squeezing force between the pole pieces is large, causing the lithium dendrites to easily pierce the isolation membrane 23 and cause short circuit failure.

[0065] In some embodiments, in the n-layer positive electrode active material layer 221 that satisfies the condition D≤0.38(D2-D1), the average value of D is greater than or equal to 0.1(D2-D1). Specifically, if the D values ​​corresponding to the n-layer positive electrode active material layer 221 are the same, the average value of D is equal to the D value corresponding to each positive electrode active material layer 221 in the n-layer positive electrode active material layer 221. If the D values ​​corresponding to the n-layer positive electrode active material layer 221 are different, the average value of D is the average value of the D values ​​corresponding to the n-layer positive electrode active material layer 221. Therefore, while the lithium deposition position of the negative electrode active material layer 211 corresponding to the n-layer positive electrode active material layer 221 is offset from the lithium deposition positions of other negative electrode active material layers 211 in the second direction Y, the negative electrode active material layer 211 can still extend beyond the positive electrode active material layer 221 along the second direction Y to reduce the risk of lithium deposition in the negative electrode active material layer 211.

[0066] In some embodiments, 0.2N≤n≤0.8N. Therefore, the number of positive electrode active material layers 221 satisfying D≤0.38(D2-D1) in the N-layer positive electrode active material layer 221 increases. Even if lithium deposition occurs, the risk of lithium dendrites generated in the majority of the negative electrode active material layer 211 being located in the same position in the second direction Y, resulting in a greater squeezing force between the electrode sheets and the risk of lithium dendrites easily piercing the separator 23 and causing a short-circuit failure, can be further reduced.

[0067] Further, in some embodiments, 0.4N ≤ n ≤ 0.6N. At this time, the number of the positive electrode active material layers 221 in the N-layer positive electrode active material layer 221 that satisfy D ≤ 0.38(D2 - D1) is close to the number of the positive electrode active material layers 221 that satisfy D > 0.38(D2 - D1), that is, the number of the positive electrode active material layers 221 in the N-layer positive electrode active material layer 221 that satisfy D ≤ 0.38(D2 - D1) is close to half of the total number of the positive electrode active material layers 221. Therefore, even if lithium plating occurs, the lithium plating positions of the negative electrode active material layers 211 that are close to half of the total number of layers are misaligned with the lithium plating positions of the remaining negative electrode active material layers 211, which is beneficial to further reducing the overall expansion of the electrode assembly 20 as a whole and relieving the extrusion between the electrode sheets.

[0068] Please refer to FIGS. 6 to 9. Another embodiment of the present application further provides a secondary battery 200, which is different from the above-mentioned secondary battery 100 in that the electrode assembly 20 is a stacked structure, and a plurality of positive electrode sheets 22 and a plurality of negative electrode sheets 21 in the stacked structure are alternately stacked in sequence. A negative electrode sheet 21 is provided between every two adjacent positive electrode sheets 22, and a positive electrode sheet 22 is provided between every two adjacent negative electrode sheets 21. The separator 23 is disposed between the adjacent positive electrode sheet 22 and negative electrode sheet 21. A three-dimensional coordinate system is established according to the first direction X, the second direction Y, and the third direction Z that are perpendicular to each other in pairs. The first direction X is the stacking direction of the positive electrode sheet 22 and the negative electrode sheet 21 in the stacked structure, and the second direction Y is the direction in which the negative electrode tab 30 or the positive electrode tab 40 extends out of the electrode assembly 20.

[0069] In order to reduce the risk of lithium plating in the negative electrode active material layer 211, when observed from the first direction X, the negative electrode active material layer 211 and the positive electrode active material layer 221 are partially overlapped. The negative electrode active material layer 211 extends from the overlapping portion with the positive electrode active material layer 221 along the second direction Y to exceed the positive electrode active material layer 221, and the negative electrode active material layer 211 also extends from the overlapping portion with the positive electrode active material layer 221 along the third direction Z to exceed the positive electrode active material layer 221. Along the second direction Y, the size of the positive electrode active material layer 221 is D1, and the size of the negative electrode active material layer 211 is D2. Since the negative electrode active material layer 211 extends beyond the positive electrode active material layer 221 along the second direction Y, D1 < D2. Along the third direction Z, the size of the positive electrode active material layer 221 is D3, and the size of the negative electrode active material layer 211 is D4. Since the negative electrode active material layer 211 extends beyond the positive electrode active material layer 221 along the third direction Z, D3 < D4. In some embodiments, 0.3 mm ≤ D2 - D1 ≤ 2 mm and 0.3 mm ≤ D4 - D3 ≤ 2 mm can be set.

[0070] Along the first direction X, the number of positive electrode active material layers 221 in the electrode assembly 20 is N. As shown in Figures 8 and 9, the electrode assembly 20 includes a first end 20A and a second end 20B that are opposite to each other along the second direction Y. As shown in Figure 7, the electrode assembly 20 also includes a third end 20C and a fourth end 20D that are opposite to each other along the third direction Z. The first end 20A, the third end 20C, the second end 20B, and the fourth end 20D of the electrode assembly 20 are connected in sequence. The negative electrode tab 30 and the positive electrode tab 40 extend from the first end 20A of the electrode assembly 20, respectively. In other embodiments, the negative electrode tab 30 may extend from the second end 20B, the third end 20C, or the fourth end 20D of the electrode assembly 20, and the positive electrode tab 40 may extend from the second end 20B, the third end 20C, or the fourth end 20D of the electrode assembly 20, and the positive electrode tab 40 may extend from the second end 20B, the third end 20C, or the fourth end 20D of the electrode assembly 20.

[0071] At the first end 20A, the edge of the positive electrode active material layer 221 is a first edge 221A. At the first end 20A, the edge of the negative electrode active material layer 211 is a second edge 211A. The distance D in the second direction Y between the first edge 221A of each positive electrode active material layer 221 and the second edge 211A that is shortest to the first edge 221 along the first direction X is . The n-layer positive electrode active material layer 221 in the n-layer positive electrode active material layer 221 satisfies the following condition: D ≤ 0.38(D2 - D1). Therefore, during overcharging or cycling, even if the negative electrode active material layer 211 at the first end 20A fails to fully incorporate lithium ions released from the corresponding positive electrode active material layer 221 and lithium deposition occurs, the lithium deposition location of the negative electrode active material layer 211 corresponding to the positive electrode active material layer 221 that meets this condition can be offset from the lithium deposition locations of other negative electrode active material layers 211 in the second direction Y. This is beneficial to reducing the overall thickness of the electrode assembly 20, easing the squeezing between the pole pieces at the lithium decomposition position, thereby reducing the risk of lithium dendrites generated by the negative electrode active material layer 211 at the first end 20A being in the same position in the second direction Y, resulting in a large squeezing force between the pole pieces, which may cause the lithium dendrites to easily pierce the isolation membrane 23 and cause short circuit failure.

[0072] It is understood that the negative electrode tab 30 and the positive electrode tab 40 can be formed separately by die-cutting processes. In this case, the edge of the negative electrode active material layer 211 may be partially located on the negative electrode current collector 210 and partially located on the negative electrode tab 20, and the edge of the positive electrode active material layer 221 may be partially located on the positive electrode current collector 220 and partially located on the positive electrode tab 40. In this application, the first edge 221A of the positive electrode active material layer 221 refers to the edge of the positive electrode active material layer 221 located on the positive electrode current collector 220, and the second edge 211A of the negative electrode active material layer 211 refers to the edge of the negative electrode active material layer 211 located on the negative electrode current collector 210.

[0073] Furthermore, as shown in FIG7 , in some embodiments, at the third end 20C, the edge of the positive electrode active material layer 221 is a third edge 221B. At the third end 20C, the edge of the negative electrode active material layer 211 is a fourth edge 211B. Because the negative electrode active material layer 211 extends beyond the positive electrode active material layer 221 along the third direction Z, the fourth edge 211B extends beyond the third edge 221B in the third direction Z. The distance between the third edge 221B of each positive electrode active material layer 221 and the fourth edge 211B that is shortest in the first direction X from the third edge 221B is d in the third direction Z. Among the N positive electrode active material layers 221, m positive electrode active material layers 221 satisfy the following condition: d ≤ 0.38(D4 - D3), where m is a positive integer and m is less than N. By setting a portion of the positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 at the third end 20C to meet the following condition: d≤0.38(D4-D3), during overcharging or cycling, even if the negative electrode active material layer 211 at the third end 20C fails to fully embed the lithium ions released by the corresponding positive electrode active material layer 221 and lithium deposition occurs, the lithium deposition position of the negative electrode active material layer 211 corresponding to the positive electrode active material layer 221 that meets this condition can be offset from the lithium deposition position of other negative electrode active material layers 211 in the third direction Z. This is beneficial for reducing the overall thickness of the electrode assembly 20 and alleviating the extrusion between the pole pieces, thereby reducing the risk of lithium dendrites generated by the negative electrode active material layer 211 at the third end 20C being in the same position in the third direction Z. The extrusion force between the pole pieces is large, thereby reducing the risk of lithium dendrites easily piercing the separator 23 and causing short circuit failure. In some embodiments, 0.2N≤m≤0.8N.

[0074] As shown in Figures 3, 8, and 9, in some embodiments, the edge of the separator 23 at the first end 20A is a fifth edge 23A. To minimize the risk of short-circuit failure caused by contact between the first edge 221A and the second edge 211A, the negative electrode active material layer 211 and the separator 23 partially overlap, as viewed in the first direction X. The separator 23 extends from the overlap with the negative electrode active material layer 211 along the second direction Y to extend beyond the negative electrode active material layer 211. Because the separator 23 extends beyond the negative electrode active material layer 211 along the second direction Y, the fifth edge 23A extends beyond the second edge 211A in the second direction Y. Along the second direction Y, the distance between the fifth edge 23A and the second edge 211A is a, with a range of 0.3 mm ≤ a ≤ 1.5 mm. This minimizes the risk of contact between the first edge 221A and the second edge 211A while also reducing the impact of an excessively large value of a on the energy density of the secondary battery 100 or 200.

[0075] The secondary batteries 100 and 200 of the present application include all types of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors (e.g., supercapacitors). Optionally, the secondary batteries 100 and 200 may be lithium secondary batteries, including lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries.

[0076] Referring to FIG. 10 , one embodiment of the present application further provides an electronic device 1 comprising the aforementioned secondary battery 100 (or secondary battery 200 ). The electronic device 1 is powered by the aforementioned secondary battery 100 , and the secondary battery 100 satisfies the following condition by providing a portion of the positive electrode active material layer 221 in the N-layer positive electrode active material layer 221 : D ≤ 0.38 (D2 - D1). This reduces the risk of lithium dendrites easily piercing the separator 23 and causing short-circuit failure due to the large squeezing force between the electrodes when the lithium deposition positions of the negative electrode active material layer 211 are at the same height, thereby maintaining high reliability and service life of the secondary battery 100 . In one embodiment, the electronic device 1 of the present application can be, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium-ion capacitor, etc.

[0077] The performance of the secondary battery provided in this application is described below through specific examples and comparative examples. The present application is described using a lithium-ion soft-pack secondary battery as an example, with specific preparation processes and testing methods. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0078] Example 1

[0079] (1) Preparation of negative electrode sheet: The negative electrode active materials artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) are mixed in a weight ratio of 96:1.5:2.5, deionized water is added as a solvent, and a slurry with a weight percentage of 70 wt% is prepared and stirred evenly. The slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 10 μm, and dried at 110°C to obtain a negative electrode sheet with a coating thickness of 150 μm and a negative electrode active material layer coated on one side. The above steps are repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a negative electrode active material layer coated on both sides; then the positive electrode sheet obtained above is cold pressed, slit, and cut to obtain a negative electrode sheet. In the second direction, the dimension D2 of the negative electrode active material layer is 79.5 mm.

[0080] (2) Preparation of positive electrode sheet: The positive electrode active material lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97.5:1.0:1.5, and N-methylpyrrolidone (NMP) 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 a positive electrode current collector aluminum foil with a thickness of 12μm, and then dried at 90°C to obtain a positive electrode sheet with a positive electrode active material layer thickness of 100μm. The slurry is evenly coated on the surface and then dried at 90°C to obtain a positive electrode sheet with a positive electrode active material layer coated on both sides; the negative electrode sheet obtained above is then cold pressed, slit, and cut to obtain a positive electrode sheet. Wherein, the size D1 of the positive electrode active material layer along the second direction is 78.0mm.

[0081] (3) Preparation of electrolyte: In a dry argon atmosphere, organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were first mixed at a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0082] (4) Preparation of isolation film: A polyethylene (PE) film with a thickness of 15 μm was selected.

[0083] (5) Preparation of a battery: A portion of the positive electrode active material layer is removed from the positive electrode sheet to form a first slot, with the first area of ​​the aluminum foil exposed in the first slot. One end of the positive electrode tab is placed in the first slot and the positive electrode tab is welded to the first area of ​​the aluminum foil.

[0084] A portion of the negative electrode active material layer is removed from the negative electrode plate to form a second slot, and the second area of ​​the copper foil is exposed in the second slot. One end of the negative electrode tab is placed in the second slot and the negative electrode tab is welded to the second area of ​​the copper foil.

[0085] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked to form a laminated electrode assembly, with the separator positioned between the positive and negative electrode sheets to act as an insulator. The number of positive active material layers, N, is 20. The first edges of the N positive active material layers are located at the same height, while the second edges of the N negative active material layers are staggered, so that n positive active material layers exist in the N positive active material layers, satisfying D = 0.38 (D2 - D1) (i.e., D = 0.57 mm), and n = 0.5N (i.e., n = 10). That is, the corresponding D values ​​in the n positive active material layers are the same. The electrode assembly is then placed in an aluminum-plastic film packaging bag, with both the positive and negative electrode tabs extending outside the bag. After drying, the electrolyte is injected, and a secondary battery is obtained through vacuum packaging, standing, formation, degassing, and trimming.

[0086] Examples 2-7

[0087] The difference from Example 1 lies in the value of n.

[0088] Comparative Example 1

[0089] The difference from Example 1 is that the N-layer positive electrode active material layer does not meet the following condition: D≤0.38(D2-D1), that is, n=0, and the D value corresponding to the N-layer positive electrode active material layer is 0.45(D2-D1).

[0090] Comparative Example 2

[0091] The difference from Example 1 is that the N-layer positive electrode active material layer meets the following condition: D=0.38(D2-D1), that is, n=N.

[0092] The batteries of each embodiment and comparative example were subjected to overcharge test and cycle test, and the corresponding test results are recorded in Table 1.

[0093] The overcharge test procedure includes the following: 1) charging the battery at a constant current of 0.2C to the cutoff voltage at 25°C, and then charging it at a constant voltage to 0.05C; 2) wrapping the battery with 10mm thick white foam (the foam must cover the entire surface of the battery), placing the battery in an overcharge and overdischarge tester (manufacturer: Arbin, model: BT-ML-30V15A), and then charging the battery at a constant current of 1C to 18.5V and constant voltage for 2 hours; 3) monitoring the battery's open circuit voltage and temperature during the overcharge and overdischarge test, and observing whether the battery emits smoke or catches fire. The results are recorded in Table 1.

[0094] The cycle test steps are as follows: 1) Place the battery in a 25°C constant temperature box and let it stand for 30 minutes to allow the battery to reach a constant temperature; 2) Charge the battery, specifically: charge to 4.18V at 2.45C constant current, discharge to 2.06C at constant voltage, charge to 4.28V at 2.01C constant current, discharge to 1.76C at constant voltage, charge to 4.38V at 1.65C constant current, discharge to 1.2C at constant voltage, charge to 4.5V at 1.2C constant current, discharge to 1C at constant voltage, and charge to 4.53V at 1C constant current. , constant voltage discharge to 0.228C; (3) let it stand for 5 minutes, discharge the battery, specifically discharge it at a constant current of 0.7C to 3.0V, let it stand for 5 minutes, this is one charge and discharge cycle, and the capacity of the first discharge is 100%; 3) repeat the charge and discharge cycle 1000 times; 4) record the thickness h0 of the battery before the cycle and the thickness h after the cycle, and calculate the thickness expansion rate of the battery = (h / h0-1)×100%; 5) observe whether the battery has smoke, fire, etc., and record the results in Table 1.

[0095] Table 1

[0096] It can be seen from the test results in Table 1 that compared with Comparative Examples 1-2, Example 1 satisfies the following conditions by setting a portion of the positive electrode active material layer: D≤0.38(D2-D1), and the lithium deposition positions of the negative electrode active material layer are staggered with each other. Therefore, after the overcharge test and the cycle test, the risk of lithium dendrites piercing the isolation membrane and causing short circuit fire is reduced, and the overall thickness expansion rate of the electrode assembly is also reduced.

[0097] Compared to Examples 6-7, the number n of positive electrode active material layers meeting the aforementioned conditions in Examples 1-5 satisfies the following conditions: 0.2N ≤ n ≤ 0.8N, thus further reducing the overall thickness expansion rate of the electrode assembly. Specifically, Examples 1, 4-5 satisfy the following conditions: 0.4N ≤ n ≤ 0.6N, resulting in the lowest thickness expansion rate of the electrode assembly after cycle testing.

[0098] Example 8

[0099] The difference from Example 1 is that the D value corresponding to the n-layer positive electrode active material layer is different.

[0100] Examples 9-13

[0101] The difference from Example 8 lies in the value of n.

[0102] The batteries of each embodiment were subjected to an overcharge test and a cycle test, and the corresponding test results are recorded in Table 2.

[0103] Table 2

[0104] It can be seen from the test results in Table 2 that, compared with Example 1, Example 8 satisfies the following conditions by setting a portion of the positive electrode active material layer: D≤0.38(D2-D1), and setting the D value corresponding to the portion of the positive electrode active material layer to be different, which is beneficial to improving the degree of dislocation of the lithium deposition position of the negative electrode active material layer. Therefore, after the overcharge test and the cycle test, the risk of lithium dendrites piercing the isolation membrane and causing a short circuit and fire is further reduced, and the overall thickness expansion rate of the electrode assembly is also further reduced.

[0105] Compared to Example 13, the number n of positive electrode active material layers meeting the aforementioned conditions in Examples 8-12 satisfies: 0.2N ≤ n ≤ 0.8N, thus further reducing the overall thickness expansion rate of the electrode assembly. Specifically, Examples 8, 11-12 satisfy: 0.4N ≤ n ≤ 0.6N, resulting in the lowest thickness expansion rate of the electrode assembly after cycle testing.

[0106] Examples 14-18

[0107] The corresponding D values ​​in the n positive electrode active material layers are different, and the average value of the D values ​​is different from that in Example 1.

[0108] The batteries of each embodiment were subjected to an overcharge test and a cycle test, and the corresponding test results are recorded in Table 3.

[0109] Table 3

[0110] It can be seen from the test results in Table 3 that, compared with Example 18, Examples 1, 14-17 meet the following conditions by setting a partial positive electrode active material layer: D≤0.38(D2-D1), and the average value of D is greater than or equal to 0.1(D2-D1), thereby ensuring that the negative electrode active material layer exceeds the positive electrode active material layer, reducing the risk of lithium plating, and reducing the overall thickness expansion rate of the electrode assembly.

[0111] The above disclosure is only a preferred embodiment of the present application, and certainly cannot be used to limit the present application. Therefore, equivalent changes made based on the present application are still within the scope covered by the present application.

Claims

1. A secondary battery, comprising a housing and an electrode assembly, wherein the housing accommodates the electrode assembly, the electrode assembly comprises a negative electrode sheet, a positive electrode sheet, and a separator disposed between the negative electrode sheet and the positive electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein: The thickness direction of the electrode assembly is defined as a first direction, and along the first direction, the number of positive electrode active material layers in the electrode assembly is N; The electrode assembly comprises a first end and a second end opposite to each other along a second direction perpendicular to the first direction; at the first end, the edge of the positive electrode active material layer is a first edge, and the edge of the negative electrode active material layer is a second edge; the first edge of each positive electrode active material layer and the second edge with the shortest distance to the first edge along the first direction are at a distance D in the second direction; Along the second direction, the size of the positive electrode active material layer is D1, the size of the negative electrode active material layer is D2, and the n-layer positive electrode active material layer in the N-layer positive electrode active material layer satisfies the following condition: D≤0.38(D2-D1), N and n are both positive integers, and n is less than N.

2. The secondary battery according to claim 1, wherein 0.2N≤n≤0.8N.

3. The secondary battery according to claim 2, wherein: 0.4N≤n≤0.6N.

4. The secondary battery according to claim 1, wherein The average value of the D values ​​of the n positive electrode active material layers is greater than or equal to 0.1 (D2-D1).

5. The secondary battery according to claim 1, wherein The values ​​of D of the n positive electrode active material layers are different.

6. The secondary battery according to claim 1, wherein The electrode assembly is a winding structure, and the second direction is the direction of the winding center axis of the electrode assembly.

7. The secondary battery according to claim 6, wherein: The secondary battery further includes a tab, the tab being electrically connected to the electrode assembly, and the tab extending out of the electrode assembly from the first end.

8. The secondary battery according to claim 1, wherein The electrode assembly is a laminate structure.

9. The secondary battery according to claim 8, wherein: The electrode assembly further includes a third end and a fourth end opposite to each other along a third direction perpendicular to both the first direction and the second direction; At the third end, the edge of the positive electrode active material layer is the third edge, and the edge of the negative electrode active material layer is the fourth edge; the distance between the third edge of each positive electrode active material layer and the fourth edge that is the shortest distance from the third edge along the first direction in the third direction is d; Along the third direction, the size of the positive electrode active material layer is D3, the size of the negative electrode active material layer is D4, and the m positive electrode active material layers among the N positive electrode active material layers satisfy the following condition: d≤0.38(D4-D3), m is a positive integer, and m is less than N.

10. The secondary battery according to claim 9, wherein 0.2N≤m≤0.8N.

11. The secondary battery according to claim 9, wherein The secondary battery further includes a tab, which is electrically connected to the electrode assembly and extends out of the electrode assembly from the first end and / or the third end.

12. The secondary battery according to claim 1, wherein At the first end, the edge of the isolation film is a fifth edge; along the second direction, the distance between the fifth edge and the second edge is a, 0.3 mm≤a≤1.5 mm.

13. The secondary battery according to claim 1, wherein The isolation film includes a substrate layer and a coating layer which are stacked, and the coating layer includes at least one of a ceramic material and an adhesive material.

14. The secondary battery according to claim 1, wherein The positive electrode active material layer includes at least one of a lithium transition metal composite oxide or a lithium transition metal phosphate compound.

15. The secondary battery according to claim 1, wherein The negative electrode active material layer includes one or more of graphite materials, alloy materials, lithium metal, lithium metal alloy, silicon material, silicon-oxygen material and silicon-carbon material.

16. The secondary battery according to claim 1, wherein 0.3mm≤D2-D1≤2mm. 17 . An electronic device comprising the secondary battery according to claim 1 .

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