Secondary battery and electronic apparatus

Abstract

A secondary battery and an electronic apparatus. The secondary battery comprises a housing and an electrode assembly. The electrode assembly comprises a negative electrode sheet and a positive electrode sheet. The negative electrode sheet comprises a negative electrode current collector and negative electrode active material layers. The positive electrode sheet comprises a positive electrode current collector and positive electrode active material layers. In a first direction, there are N positive electrode active material layers in the electrode assembly. The electrode assembly comprises a first end and a second end which are opposite each other in a second direction perpendicular to the first direction. At the first end, an edge of each positive electrode active material layer is a first edge, and an edge of each 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 with the shortest distance from the first edge in the first direction is D. In the second direction, the dimension of each positive electrode active material layer is D1, the dimension of each negative electrode active material layer is D2, and n of the N positive electrode active material layers meet the following condition: D≤0.38(D2-D1). The electrode assembly can improve the reliability and service life of the secondary battery.

Description

Secondary battery and electronic device TECHNICAL FIELD

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

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

[0003] SUMMARY

[0004] Therefore, it is necessary to provide a secondary battery capable of improving reliability and service life.

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

[0006] The first aspect of 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 sheet, a positive electrode sheet, and a separator arranged 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 arranged on the negative electrode current collector. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector. The thickness direction of the electrode assembly is defined as the first direction. Along the first direction, the number of layers of the positive electrode active material layer 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 the first edge, and the edge of the negative electrode active material layer is the second edge. The distance between the first edge of each layer of the positive electrode active material layer and the second edge closest to the first edge along the first direction in the second direction is D. 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 of the N layers of the positive electrode active material layer satisfy the following condition: D≤0.38(D2-D1). N and n are positive integers, and n is less than N.

[0007] In the present application, part of the positive active material layers arranged at the first end meet the following condition: D≤0.38(D2-D1). Therefore, even if the negative active material layers fail to fully insert the lithium ions extracted by the corresponding positive active material layers and lithium precipitation occurs during overcharging or cycling, the lithium precipitation positions of the negative active material layers corresponding to the positive active material layers meeting the condition can be staggered with the lithium precipitation positions of the other negative active material layers in the second direction. This is beneficial to reduce the overall thickness of the electrode assembly and relieve the extrusion between the electrode sheets, thereby reducing the risk that the lithium dendrites generated by the negative active material layers are in the same position in the second direction, the extrusion force between the electrode sheets is large, and the lithium dendrites easily pierce the separator to cause short circuit failure. Therefore, the reliability and service life of the secondary battery are improved.

[0008] In some possible implementations, 0.2N≤n≤0.8N. Therefore, the risk that the lithium dendrites generated by most of the negative active material layers are in the same position in the second direction, the extrusion force between the electrode sheets is large, and the lithium dendrites easily pierce the separator to cause short circuit failure can be further reduced.

[0009] In some possible implementations, 0.4N≤n≤0.6N, so that the number of positive active material layers meeting the above condition is close to half of the total number of positive active material layers. Therefore, even if lithium precipitation occurs, the lithium precipitation positions of the negative active material layers close to half of the total number are staggered with the lithium precipitation positions of the remaining negative active material layers, which is beneficial to further reduce the overall thickness of the electrode assembly and relieve the extrusion between the electrode sheets.

[0010] In some possible implementations, the average value of the D values of the n layers of positive active material layers is greater than or equal to 0.1(D2-D1). Therefore, under the premise that the lithium precipitation positions of the negative active material layers corresponding to the n layers of positive active material layers are staggered with the lithium precipitation positions of the other negative active material layers in the second direction, the negative active material layers can still exceed the positive active material layers along the second direction to reduce the risk of lithium precipitation of the negative active material layers.

[0011] In some possible implementations, the D values of the n layers of positive active material layers are different. This can further improve the staggering degree of the lithium precipitation positions of the negative active material layers, thereby further relieving the extrusion between the electrode sheets.

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

[0013] In some possible implementations, the secondary battery further includes a tab, the tab is electrically connected to the electrode assembly, and the tab extends out of the electrode assembly from the first end. Therefore, the tab is convenient for electrically connecting external devices.

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

[0015] In some possible implementations, the electrode assembly further comprises 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 active material layer is a third edge, and the edge of the negative active material layer is a fourth edge. The distance between the third edge of each positive active material layer and the fourth edge closest to the third edge along the first direction in the third direction is d. Along the third direction, the size of the positive active material layer is D3, the size of the negative active material layer is D4, and m layers of the N layers of positive active material layers satisfy the following condition: d≤0.38(D4-D3), where m is a positive integer and m is less than N. Therefore, during overcharge or cycling, even if the negative active material layer fails to fully insert lithium ions extracted from the corresponding positive active material layer and lithium precipitation occurs, the lithium precipitation position of the negative active material layer corresponding to the positive active material layer satisfying the condition can be staggered with the lithium precipitation position of other negative active material layers in the third direction. This is advantageous for reducing the overall thickness of the electrode assembly and relieving the extrusion between the electrode sheets, thereby reducing the risk of lithium dendrites produced by the majority of negative active material layers being in the same position in the third direction, causing the extrusion force between the electrode sheets to be large, and the lithium dendrites easily piercing the separator to cause short circuit failure.

[0016] In some possible implementations, 0.2N≤m≤0.8N. Therefore, the risk of lithium dendrites produced by the majority of negative active material layers being in the same position in the third direction, causing the extrusion force between the electrode sheets to be large, and the lithium dendrites easily piercing the separator to cause short circuit failure can be further reduced.

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

[0018] In some possible implementations, at the first end, the edge of the separator is a fifth edge; along the second direction, the distance between the fifth edge and the second edge is a, and 0.3mm≤a≤1.5mm. Therefore, while enabling the separator to sufficiently reduce the risk of the first edge and the second edge being in contact, the impact of a being too large on the energy density of the secondary battery is also reduced.

[0019] In some possible implementations, the isolation film includes a substrate layer and a coating layer arranged in a stack, and the coating layer includes at least one of a ceramic material or a bonding material. The bonding material is used to improve the interfacial adhesion between the isolation film and the pole piece, reduce the swelling deformation of the electrode assembly during overcharging or cycling, and ensure the cycling capability of the secondary battery. The ceramic material is used to improve the heat resistance and puncture resistance of the isolation film.

[0020] In some possible implementations, the positive 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 active material layer includes one or more of a graphite-based material, an alloy-based material, lithium metal, a lithium metal alloy, a silicon material, a silicon-oxygen material, and a silicon-carbon material.

[0022] In some possible implementations, 0.3 mm≤D2-D1≤2 mm, so that the risk of lithium precipitation of the negative active material layer can be effectively reduced, and the waste of the negative active material can also be reduced when the above difference is large.

[0023] The second aspect of the present application also provides an electronic device including the secondary battery described above. The electronic device is powered by the secondary battery, and even if lithium precipitation occurs in the negative pole piece of the secondary battery, the risk of short circuit failure caused by lithium dendrites piercing the isolation film is reduced, thus being beneficial to improving the safety and service life of the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

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

[0025] FIG. 1 is a schematic structural view of a secondary battery from a first direction according to an embodiment of the present application.

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

[0027] FIG. 3 is a sectional view of the secondary battery shown in FIG. 1 along a section line III-III.

[0028] FIG. 4A is a schematic structural view of a secondary battery according to another embodiment of the present application.

[0029] FIG. 4B is a sectional view of the secondary battery shown in FIG. 4A along a section line IV-IV.

[0030] FIG. 5 is a sectional view of an isolation film of the secondary battery shown in FIG. 2, FIG. 3, or FIG. 4B.

[0031] FIG. 6 is a schematic structural view of a secondary battery from a first direction according to another embodiment of the present application.

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

[0033] Fig. 8 is a sectional view of the secondary battery shown in Fig. 6 along the section line VIII-VIII in another embodiment.

[0034] Fig. 9 is a sectional view of the secondary battery shown in Fig. 6 along the section line IX-IX in another embodiment.

[0035] Fig. 10 is a structural schematic diagram of an electronic device provided by an embodiment of the present application.

[0036] Main component symbol explanation electronic device 1 housing 10 electrode assembly 20 first end 20A second end 20B third end 20C fourth end 20D negative tab 21 positive tab 22 separator 23 fifth edge 23A negative tab 30 positive tab 40 secondary battery 100, 200 first section 201 second section 202 third section 203 fourth section 204 negative current collector 210 negative active material layer 211 second edge 211A fourth edge 211B positive current collector 220 positive 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 dimension D1, D2, D3, D4 distance D, d, a winding center axis O winding direction D first direction X second direction Y third direction Z dotted line A-A, B-B, L

[0037] The following detailed description will further explain the present application with reference to the above mentioned figures. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be described clearly and detailed below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Unless otherwise defined, all the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments and are not intended to limit the present application.

[0039] Hereinafter, the embodiments of the present application will be described in detail. However, the present application can be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and fully convey the scope of the application to those skilled in the art.

[0040] In addition, for the sake of brevity and clarity, in the drawings, the size or thickness of various components, layers, or regions can be exaggerated. Throughout the specification, like numbers refer to like elements throughout. As used herein, the terms "and / or" and "at least one of" include any and all combinations of one or more of the associated listed items. In addition, it should be understood that if an element A is said to be "connected to" element B, then element A can be directly connected to element B or an intervening element C can be present and element A and element B can be indirectly connected to each other through element C.

[0041] Further, use of "may" when describing embodiments of the present application means that one or more embodiments of the present application.

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

[0043] Spatially relative terms, such as "on", "above", and the like, can be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device or apparatus in use or operation in addition to the orientations depicted in the figures. For example, if a device or apparatus is turned over, then an element or feature that is described as being "above" or "on" another element or feature would then be oriented "below" or "on" the other element or feature. Thus, the exemplary term "above" can encompass both an orientation that is above and then below. It will be understood that, although the terms first, second, third, etc. can be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms are used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the exemplary embodiments.

[0044] As used herein, the terms "parallel", "perpendicular" are used to describe an ideal state between two components. In a practical production or use state, there can exist a state close to parallel or perpendicular between two components. For example, in connection with numerical description, parallel can refer to an included angle between two straight lines within a range of ±10°, parallel can also refer to a dihedral angle between two planes within a range of ±10°, parallel can also refer to an included angle between a straight line and a plane within a range of ±10°. Perpendicular can refer to an included angle between two straight lines within a range of 90±10°, perpendicular can also refer to a dihedral angle between two planes within a range of 90±10°, perpendicular can also refer to an included angle between a straight line and a plane within a range of 90±10°. The two components described as "parallel", "perpendicular" can not be absolute straight lines, planes, but can be approximately straight lines or planes, as long as the overall extension direction is a straight line or a plane from a macroscopic point of view, the components can be considered as "straight lines" or "planes".

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

[0046] Referring to FIGS. 1 to 3, an embodiment of the present application provides a secondary battery 100 including a case 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 case 10. The negative electrode tab 30 and the positive electrode tab 40 are electrically connected to the electrode assembly 20 and extend from the inside of the case 10 to the outside of the case 10 to be electrically connected to an external device (not shown). In some embodiments, the case 10 can be a packaging bag obtained by packaging with a packaging film such as an aluminum laminate film, i.e., the secondary battery 100 can be a pouch battery. In other embodiments, the secondary battery 100 can also be a steel can battery or an aluminum can battery.

[0047] As shown in FIG. 2, the electrode assembly 20 includes a negative electrode tab 21, a positive electrode tab 22, and a separator 23 disposed between the negative electrode tab 21 and the positive electrode tab 22. In some embodiments, the electrode assembly 20 has a jelly-roll structure in which the negative electrode tab 21, the separator 23, and the positive electrode tab 22 are sequentially stacked and rolled to form the electrode assembly 20. The electrode assembly 20 has a winding central axis O perpendicular to the plane of the paper. The electrode assembly 20 has a winding direction D, which refers to a direction of movement from the inside to the outside along a point of the negative electrode tab 21, the positive electrode tab 22, or the separator 23 around the winding central axis O as shown in FIG. 2. The winding direction D can be two, i.e., a direction of clockwise or counterclockwise rotation around the winding central axis O. In some embodiments, the winding direction D is a direction of counterclockwise rotation around the winding central axis O as shown in FIG. 2.

[0048] In the present application, the first direction X is the thickness direction of the electrode assembly 20. The second direction Y is the extending direction of the winding central axis O, and is also the direction in which the negative tab 30 or the positive tab 40 extends out of the electrode assembly 20. In some embodiments, when the electrode assembly 20 is in 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 connected in sequence along the winding direction D, the first segment 201 and the third segment 203 are oppositely arranged, and the second segment 202 and the fourth segment 204 are oppositely arranged. In a cross section perpendicular to the first direction X, the electrode assembly 20 can be flat or approximately circular. As shown in FIG. 2, when the cross section of the electrode assembly 20 is flat, the first segment 201 and the third segment 203 are flat segments, and the second segment 202 and the fourth segment 204 are bent 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 bent edge at the innermost part of the electrode assembly 10 on the left side extends in the first direction X to form a dashed line A-A, and the bent edge at the innermost part of the electrode assembly 10 on the right side extends in the first direction X to form a dashed line B-B. As viewed from the second direction Y, the dashed line A-A is the boundary line between the second segment 202 and the first segment 201 or the third segment 203, and the dashed line B-B is the boundary line between the fourth segment 204 and the first segment 201 or the third segment 203. As shown in FIGS. 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 cell), the thickness direction of the electrode assembly 20 can be considered as any direction perpendicular to the winding central axis O.

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

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

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

[0052] The negative electrode active material layer 211 includes a negative electrode active material including a negative electrode active material capable of reversible deintercalation of active ions. In some embodiments, the negative electrode active material can include one or several of a graphite-based material, an alloy-based material, lithium metal, a lithium metal alloy, a silicon material, a silicon-oxygen material, and a silicon-carbon material. For example, the graphite-based material can be selected from a combination of one or more of artificial graphite, natural graphite, and modified graphite; the alloy-based material can be selected from a combination of one or more of silicon, silicon oxide, tin, and titanium sulfide, etc.; the lithium metal alloy includes 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 separator 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. Among the polyethylene and polypropylene, they have a good effect on improving short circuits.

[0054] For simplicity, the separator film 23 in FIGS. 2, 3 and 4B is shown by dashed lines. FIG. 5 is a cross-sectional view of the separator film 23 in some embodiments. As shown in FIG. 5, the separator film 23 can also be a multi-layer structure including a base layer 231 and a coating layer 232 arranged in layers, the coating layer 232 including at least one of a ceramic material or a binding material. When the separator film 23 includes both a ceramic material and a binding material, the ceramic material and the binding material can be separate coating layers or a mixed coating layer. The binding material is used to improve the interface adhesion between the separator film 23 and the electrode tab, reduce the swelling deformation of the electrode assembly 20 during overcharging or cycling, and ensure the cycling capability of the secondary battery 100. The ceramic material is used to improve the heat resistance and puncture resistance of the separator film 23. In some embodiments, the binding material can be selected from at least one of a copolymer of vinylidene fluoride-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, cyanoethyl amylopectin, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, amylopectin, 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 can be selected from at least one of aluminum oxide, boehmite, barium sulfate, titanium dioxide, and magnesium hydroxide.

[0055] As shown in FIG. 3, to reduce the risk of lithium precipitation in the negative active material layer 211, the negative active material layer 211 and the positive active material layer 221 partially overlap when viewed from the first direction X, and the negative active material layer 211 extends beyond the positive active material layer 221 along the second direction Y from the overlapping portion with the positive active material layer 221. Along the second direction Y, the size of the positive active material layer 221 is D1, and the size of the negative active material layer 211 is D2. Since the negative active material layer 211 extends beyond the positive active material layer 221 along the second direction Y, D1 < D2. In some embodiments, 0.3 mm ≤ D2 - D1 ≤ 2 mm can be provided, so as to effectively reduce the risk of lithium precipitation in the negative active material layer 211, while also reducing the waste of negative active material when the above difference is large, i.e., the amount of negative active material in the negative active material layer 211 that extends beyond the positive active material layer 221 and does not contribute to capacity increases when the above difference is large.

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

[0057] At the first end 20A, the edge of the positive active material layer 221 is a first edge 221A. At the first end 20A, the edge of the negative active material layer 211 is a second edge 211A. Since the negative active material layer 211 extends beyond the positive active material layer 221 along the second direction Y, the second edge 211A extends beyond the first edge 221A along the second direction Y. The distance between the first edge 221A of each layer of the positive active material layer 221 and the second edge 211A closest to the first edge 221A along the first direction X in the second direction Y is D. It can be understood that the second edge 211A closest to the first edge 221A of a certain layer of the positive active material layer 221 along the first direction X refers to the second edge 211A of the negative active material layer 211 arranged opposite to the certain layer of the positive active material layer 221 through the separator 23.

[0058] The n layers of the n layers of positive electrode active material layers 221 satisfy the condition: D≤0.38(D2-D1), where N and n are positive integers, and n is less than N. That is, (N-n) layers of the n layers of positive electrode active material layers 221 satisfy the condition: D>0.38(D2-D1), and in some embodiments, the (N-n) layers of positive electrode active material layers 221 satisfy the condition: D=0.5(D2-D1). In the n layers of positive electrode active material layers 221 satisfying the condition: D≤0.38(D2-D1), the corresponding D values can be the same or different. For example, in the n layers of positive electrode active material layers 221, the D value of part of the positive electrode active material layers 221 is different from the D value of another part of the positive electrode active material layers 221.

[0059] In this application, the measurement steps of D, D2, and D1 can be: (1) using X-rays to perform two-dimensional projection and scanning test on the secondary battery 100 from the first direction X, and the instrument can use an instrument or device known to those skilled in the art (such as GE Phoenix vtomex S device), so as to obtain a CT image (such as the view angle shown in FIG. 3), from which the negative current collector 210 of the negative electrode tab 21 and the positive current collector 220 of the positive electrode tab 22 can be directly observed; (2) Since along the second direction Y, the edge of the negative current collector 210 is usually flush with the edge (i.e., the second edge 211A) of the negative electrode active material layer 211, and the edge of the positive current collector 220 is usually flush with the edge (i.e., the first edge 221A) of the positive electrode active material layer 221, therefore, 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 distance between the edge of the positive current collector 220 and the edge of the negative current collector 210 adjacent to the positive current collector 220 in the second direction Y, D2 is the size of the negative current collector 210 in the second direction Y, and D1 is the size of the positive current collector 220 in the second direction Y.

[0060] Along the second direction Y, the edge of the positive current collector 220 can also be misaligned with the edge of the positive electrode active material layer 221 (for example, in order to reduce the risk of the edge burr of the positive current collector 220 piercing the separator 23 and contacting the negative electrode tab 21, a ceramic layer can be coated at the edge of the positive current collector 220). In this case, in addition to calculating the distance between the edge of the positive current collector 220 and the edge of the negative current collector 210 adjacent to the positive current collector 220 in the second direction Y on the CT image, the shell 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 second direction Y in the electrode assembly 20 is measured. At this time, 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 can be: (1) using X-ray to perform two-dimensional projection and scanning test on the secondary battery 100 from the second direction Y, so as to obtain a CT image (as shown in the view angle of FIG. 2), from which the positive current collector 220 of the positive electrode sheet 22 can be directly observed, and the number of the positive current collector 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 opposite surfaces of each layer of the positive current collector 220 contain the positive active material layer 221, so as to obtain the number N of the positive active material layer 221. When determining the number n, the value of D measured above can be combined to determine. In other embodiments, the CT image of the secondary battery 100 can also be obtained from the first direction X using X-ray (as shown in the view angle of FIG. 3), and the number N can be determined by using similar steps.

[0062] When manufacturing, 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 active material layer 221 satisfies the above condition. In some embodiments, as shown in FIG. 3, the first edges 221A of the N layers of positive active material layers 221 can be located at approximately the same height in the second direction Y at the first end 20A of the electrode assembly 20, and the second edges 211A of the N layers of negative active material layers 211 are arranged staggered in the second direction Y, so that part of the N layers of positive active material layers 221 satisfies D≤0.38(D2-D1) and the other part of the N layers of positive active material layers 221 satisfies D>0.38(D2-D1). In other embodiments, the first edges 221A of the N layers of positive active material layers 221 can also be arranged staggered in the second direction Y at the first end 20A of the electrode assembly 20, and the second edges 211A of the N layers of negative active material layers 211 are located at approximately the same height in the second direction Y, which can also make part of the N layers of positive active material layers 221 satisfy D≤0.38(D2-D1) and the other part of the N layers of positive active material layers 221 satisfy D>0.38(D2-D1). In other embodiments, the first edges 221A of the N layers of positive active material layers 221 can be arranged staggered in the second direction Y at the first end 20A of the electrode assembly 20, and the second edges 211A of the N layers of negative active material layers 211 are also arranged staggered in the second direction Y, as long as part of the N layers of positive active material layers 221 satisfies D≤0.38(D2-D1) and the other part of the N layers of positive active material layers 221 satisfies D>0.38(D2-D1).

[0063] Generally, the region of the negative active material layer near the second edge has a higher current density and a larger gap between the positive electrode sheet, which increases the impedance, compared to other regions of the negative active material layer. Moreover, the region of the negative active material layer near the second edge has faster heat dissipation and a lower temperature during charging and discharging of the secondary battery (reduced viscosity of the electrolyte and slower migration of lithium ions), compared to other regions of the negative active material layer. This can all lead to a decrease in kinetics of the region of the negative active material layer near the second edge, making the region prone to excessive accumulation of lithium ions. In the present application, the part of the n-layer positive active material layer 221 at the first end 20A meets the following condition: D≤0.38(D2-D1), so that even if the negative active material layer 211 at the first end 20A fails to fully intercalate the lithium ions extracted by the corresponding positive active material layer 221 and lithium precipitation occurs, the lithium precipitation position of the negative active material layer 211 corresponding to the positive active material layer 221 meeting the condition and the lithium precipitation position of the other negative active material layers 211 can be offset from each other in the second direction Y. This is beneficial to reduce the thickness of the electrode assembly 20 at the lithium precipitation position (i.e. reduce the overall thickness expansion of the electrode assembly 20), relieve the extrusion between the electrode sheets, and thus reduce the risk of lithium dendrites produced by the negative active material layer 211 at the first end 20A being at the same position in the second direction Y, resulting in a larger extrusion force between the electrode sheets, causing the lithium dendrites to easily pierce the separator 23 and trigger 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 of the n-layer positive active material layer 221 meeting the condition D≤0.38(D2-D1) are different, the degree of offset of the lithium precipitation position of the negative active material layer 211 can be further improved, thereby further relieving the extrusion between the electrode sheets.

[0064] Further, since the negative tab 30 is usually welded to the first region 2100 of the negative current collector 210 and the positive tab 40 is usually welded to the second region 2200 of the positive current collector 220, 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 when the positive tab 40 and the negative tab 30 extend out of the electrode assembly 20 from the first end 20A. The part of the n layers of positive active material layers 221 at the first end 20A in the application satisfies D≤0.38(D2-D1), so that the extrusion between the electrode sheets at the first end 20A after lithium precipitation can be alleviated, thereby reducing the risk that the electrode sheets at the first end 20A are more easily extruded and pierce the separator 23 due to the greater thickness of the first end 20A and lithium dendrites during overcharging or cycling. In addition, it can be understood that since the n layers of positive active material layers 221 at the first end 20A satisfy D≤0.38(D2-D1), correspondingly, the n layers of positive active material layers 221 at the second end 20B also satisfy D>0.38(D2-D1) at the same time, so that the application can also reduce the risk that the extrusion between the electrode sheets is greater when the lithium dendrites generated by the negative active material layers 211 at the second end 20B are at the same position in the second direction Y, causing the lithium dendrites to easily pierce the separator 23 and trigger short circuit failure.

[0065] In some embodiments, the average value of D in the n layers of positive active material layers 221 satisfying D≤0.38(D2-D1) is greater than or equal to 0.1(D2-D1). Specifically, if the n layers of positive active material layers 221 correspond to the same D value, the average value of D is equal to the D value corresponding to each of the n layers of positive active material layers 221. If the n layers of positive active material layers 221 correspond to different D values, the average value of D is the average value of the D values corresponding to the n layers of positive active material layers 221. Therefore, under the premise that the lithium precipitation positions of the n layers of positive active material layers 221 and the lithium precipitation positions of the other negative active material layers 211 are staggered in the second direction Y, the negative active material layers 211 can still exceed the positive active material layers 221 in the second direction Y to reduce the risk of lithium precipitation of the negative active material layers 211.

[0066] In some embodiments, 0.2N≤n≤0.8N. Therefore, the number of positive active material layers 221 in the N layers of positive active material layers 221 satisfying D≤0.38(D2-D1) increases. Even if lithium precipitation occurs, the risk that the lithium dendrites generated by most of the negative active material layers 211 are at the same position in the second direction Y, causing the extrusion between the electrode sheets to be greater and the lithium dendrites to easily pierce the separator 23 and trigger short circuit failure can be further reduced.

[0067] Further, in some embodiments, 0.4N≤n≤0.6N. At this time, the number of the N layers of the positive active material layer 221 satisfying D≤0.38(D2-D1) is close to the number of the N layers of the positive active material layer 221 satisfying D>0.38(D2-D1), i.e., the number of the N layers of the positive active material layer 221 satisfying D≤0.38(D2-D1) is close to half of the total number of the N layers of the positive active material layer 221. Therefore, even if lithium precipitation occurs, the lithium precipitation positions of the negative active material layer 211 close to half of the total number of the layers and the lithium precipitation positions of the remaining negative active material layer 211 are staggered with each other, which is conducive 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. 6-9, 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 jelly-roll structure in which a plurality of positive electrode sheets 22 and a plurality of negative electrode sheets 21 are alternately stacked in sequence, one negative electrode sheet 21 is arranged between every two adjacent positive electrode sheets 22, and one positive electrode sheet 22 is arranged between every two adjacent negative electrode sheets 21. The separator 23 is arranged between the adjacent positive electrode sheet 22 and the negative electrode sheet 21. A three-dimensional coordinate system is established according to a first direction X, a second direction Y and a third direction Z perpendicular to each other, the first direction X is the stacking direction of the positive electrode sheet 22 and the negative electrode sheet 21 in the jelly-roll structure, and the second direction Y is the direction in which the negative tab 30 or the positive tab 40 extends out of the electrode assembly 20.

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

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

[0071] At the first end 20A, the edge of the positive active material layer 221 is a first edge 221A. At the first end 20A, the edge of the negative active material layer 211 is a second edge 211A. The distance between the first edge 221A of each layer of the positive active material layer 221 and the second edge 211A closest to the first edge 221A along the first direction X in the second direction Y is D. The n layers of the positive active material layers 221 among the N layers of the positive active material layers 221 satisfy the condition: D≤0.38(D2-D1). Therefore, even if the negative active material layer 211 at the first end 20A fails to fully intercalate the lithium ions extracted by the corresponding positive active material layer 221 to cause lithium precipitation during overcharge or cycling, the lithium precipitation position of the negative active material layer 211 corresponding to the positive active material layer 221 satisfying the condition can be staggered with the lithium precipitation position of the other negative active material layers 211 in the second direction Y. This is beneficial to reduce the overall thickness of the electrode assembly 20 and relieve the extrusion between the electrode sheets at the lithium precipitation position, thereby reducing the risk of lithium dendrites at the same position in the second direction Y due to the extrusion between the electrode sheets being too large, causing the lithium dendrites to easily pierce the separator 23 and trigger short circuit failure.

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

[0073] Further, as shown in FIG. 7, in some embodiments, at the third end 20C, the edge of the positive active material layer 221 is a third edge 221B. At the third end 20C, the edge of the negative active material layer 211 is a fourth edge 211B. Since the negative active material layer 211 extends beyond the positive active material layer 221 in 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 of the N positive active material layers 221 and the fourth edge 211B closest to the third edge 221B in the first direction X in the third direction Z is d. The m positive active material layers 221 of the N positive active material layers 221 satisfy the condition that d≤0.38(D4-D3), where m is a positive integer and m is less than N. By setting part of the N positive active material layers 221 at the third end 20C to satisfy the condition that d≤0.38(D4-D3), even if lithium precipitation occurs at the third end 20C due to the failure of the negative active material layer 211 to fully intercalate lithium ions extracted from the corresponding positive active material layer 221 during overcharging or cycling, the lithium precipitation position of the negative active material layer 211 corresponding to the positive active material layer 221 satisfying the condition can be staggered with the lithium precipitation positions of other negative 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 pressure between the electrode sheets, thereby reducing the risk of lithium dendrites generated by the negative active material layer 211 at the third end 20C being in the same position in the third direction Z, causing the electrode sheets to be pressed together, and leading to the lithium dendrites easily piercing the separator 23 and causing short circuit failure. In some embodiments, 0.2N≤m≤0.8N.

[0074] As shown in FIGS. 3, 8, and 9, in some embodiments, at the first end 20A, the edge of the separator 23 is a fifth edge 23A. In order to sufficiently reduce the risk of the first edge 221A and the second edge 211A coming into contact and causing short circuit failure, the negative active material layer 211 and the separator 23 are partially overlapped when viewed in the first direction X, and the separator 23 extends beyond the negative active material layer 211 in the second direction Y. Since the separator 23 extends beyond the negative active material layer 211 in the second direction Y, the fifth edge 23A extends beyond the second edge 211A in the second direction Y. The distance between the fifth edge 23A and the second edge 211A in the second direction Y is a, and 0.3mm≤a≤1.5mm, thereby sufficiently reducing the risk of the first edge 221A and the second edge 211A coming into contact while also reducing the impact on the energy density of the secondary battery 100 or 200 when a is too large.

[0075] The secondary battery 100, 200 of the present application includes all kinds of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors (e.g., supercapacitors). Alternatively, the secondary battery 100, 200 can be a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, and a lithium ion polymer secondary battery.

[0076] Referring to FIG. 10, an electronic device 1 according to an embodiment of the present application includes the secondary battery 100 (or the secondary battery 200) described above. The electronic device 1 is powered by the secondary battery 100 described above, and the secondary battery 100 satisfies the following condition by disposing part of the N layers of positive electrode active material layers 221: D≤0.38(D2-D1), thereby reducing the risk of short circuit failure caused by lithium dendrites easily piercing the separator 23 due to a large extrusion force between the electrode sheets when the lithium precipitation position of the negative electrode active material layer 211 is at the same height, and thus maintaining a high reliability and service life of the secondary battery 100. In an embodiment, the electronic device 1 of the present application can be, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable phone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0077] The performance of the secondary battery provided by the present application is described below through specific examples and comparative examples. The present application is described by taking a lithium ion soft pack battery as an example and combining the specific preparation process and test method, and those skilled in the art should understand that the preparation method described in the present application is only an example, and any other suitable preparation method is within the scope of the present application.

[0078] Example 1

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

[0080] (2) Preparation of the positive electrode sheet: the positive electrode active material lithium cobaltate (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97.5:1.0:1.5, N-methyl pyrrolidone (NMP) was added as a solvent, a slurry with a solid content of 75wt% was prepared and stirred uniformly. The slurry was uniformly coated on one surface of a positive current collector aluminum foil with a thickness of 12μm, and then dried at 90°C to obtain a positive electrode active material layer with a thickness of 100μm. The slurry was uniformly coated on the surface, and then dried at 90°C to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides; then the above obtained negative electrode sheet was cold-pressed, slitted, and cut to obtain a positive electrode sheet. Among them, along the second direction, the size D1 of the positive electrode active material layer is 78.0mm.

[0081] (3) Preparation of the electrolyte: in a dry argon atmosphere, first, the organic solvents ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in 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 uniformly, to obtain an electrolyte with a lithium salt concentration of 1.15mol / L.

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

[0083] (5) Preparation of the battery: the above positive electrode sheet was removed from part of the positive electrode active material layer to form a first slot, the first area of the aluminum foil was exposed to the first slot, and one end of the positive electrode tab was placed in the first slot and welded to the first area of the aluminum foil.

[0084] The negative electrode tab is removed from part of the negative active material layer to form a second slot, a second region of the copper foil is exposed to the second slot, one end of the negative tab is placed in the second slot and the negative tab is welded to the second region of the copper foil.

[0085] The positive electrode tab, the separator and the negative electrode tab prepared above are stacked to form a laminated electrode assembly, and the separator is arranged between the positive electrode tab and the negative electrode tab to play a role of separation. The number of layers N of the positive active material layer is 20, the first edges of the N layers of the positive active material layer are at the same height, and the second edges of the N layers of the negative active material layer are arranged staggered to each other, so that there are n layers of the positive active material layer in the N layers of the positive active material layer, and the n layers of the positive active material layer satisfy 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 of the n layers of the positive active material layer are the same. Then, the electrode assembly is placed in an aluminum plastic film packaging bag, the positive tab and the negative tab both extend out of the packaging bag, electrolyte is injected after drying, and a secondary battery is obtained after processes such as vacuum packaging, standing, formation, degassing and edge cutting.

[0086] Examples 2-7

[0087] The difference from Example 1 is that the value of n.

[0088] Comparative Example 1

[0089] The difference from Example 1 is that none of the N layers of the positive active material layer satisfies the condition: D≤0.38(D2-D1), i.e. n=0, and the corresponding D value of the N layers of the positive active material layer is 0.45(D2-D1).

[0090] Comparative Example 2

[0091] The difference from Example 1 is that all the N layers of the positive active material layer satisfy the condition: D=0.38(D2-D1), i.e. n=N.

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

[0093] The test steps of the overcharge test include: 1) the battery is charged at 0.2C constant current to the cut-off voltage at 25℃, and then charged at constant voltage to 0.05C; 2) after the battery is wrapped with white foam cotton with a thickness of 10mm (the foam cotton needs to wrap the whole surface of the battery), the battery is placed into an overcharge and overdischarge tester (manufacturer: Arbin, model: BT-ML-30V15A), and then the battery is charged at 1C constant current to 18.5V, and charged at constant voltage for 2h; 3) the changes of open circuit voltage and temperature of the battery during overcharge and overdischarge are monitored, and whether the battery smokes or catches fire is observed, and the results are recorded in Table 1.

[0094] The cycle test procedure is as follows: 1) place the battery in a 25℃ constant temperature box, stand for 30 minutes, so that the battery reaches constant temperature; 2) charge the battery, specifically: charge at 2.45C constant current to 4.18V, discharge at 2.06C constant voltage, charge at 2.01C constant current to 4.28V, discharge at 1.76C constant voltage, charge at 1.65C constant current to 4.38V, discharge at 1.2C constant voltage, charge at 1.2C constant current to 4.5V, discharge at 1C constant voltage, charge at 1C constant current to 4.53V, discharge at 0.228C constant voltage; (3) stand for 5min, discharge the battery, specifically: discharge at 0.7C constant current to 3.0V, stand for 5min, this is one charge-discharge cycle, the first discharge capacity is 100%; 3) repeat 1000 times of charge-discharge cycle; 4) record the thickness h0 of the battery before cycle and the thickness h after cycle, calculate the thickness expansion rate of the battery = (h / h0-1) x 100%; 5) observe whether the battery smokes, catches fire and other phenomena, the results are recorded in Table 1.

[0095] Table 1

[0096] From the test results in Table 1, compared with Comparative Example 1-2, Example 1 meets the following conditions by setting part of the positive active material layer: D≤0.38(D2-D1), the lithium precipitation positions of the negative active material layer are staggered, so the risk of lithium dendrite piercing the separator film to cause short circuit and fire is reduced after overcharge test and cycle test, and the thickness expansion rate of the overall electrode assembly is also reduced.

[0097] Compared with Example 6-7, the number n of positive active material layers meeting the above conditions in Examples 1-5 meets: 0.2N≤n≤0.8N, so the thickness expansion rate of the overall electrode assembly is further reduced. Among them, Examples 1, 4-5 meet: 0.4N≤n≤0.6N, so the thickness expansion rate of the electrode assembly after cycle test is the lowest.

[0098] Example 8

[0099] The difference from Example 1 is that the D values corresponding to the n layers of positive active material layers are different.

[0100] Examples 9-13

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

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

[0103] Table 2

[0104] From the test results in Table 2, compared with Example 1, Example 8 meets the following condition by setting part of the positive active material layer: D≤0.38(D2-D1), and the D value corresponding to the part of the positive active material layer is different, thereby facilitating the degree of misplacement of the lithium precipitation position of the negative active material layer, and thus the risk of lithium dendrite puncturing the separator film to cause short circuit and fire is further reduced after the overcharge test and the cycle test, and the thickness expansion rate of the overall electrode assembly is further reduced.

[0105] Compared with Example 13, the number n of the positive active material layers meeting the above condition in Examples 8-12 satisfies: 0.2N≤n≤0.8N, and thus the thickness expansion rate of the overall electrode assembly is further reduced. Among them, Examples 8, 11-12 satisfy: 0.4N≤n≤0.6N, and thus the thickness expansion rate of the electrode assembly after the cycle test is the lowest.

[0106] Examples 14-18

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

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

[0109] Table 3

[0110] From the test results in Table 3, compared with Example 18, Examples 1, 14-17 meet the following condition by setting part of the positive 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 active material layer is greater than the positive active material layer, reducing the risk of lithium precipitation, and reducing the thickness expansion rate of the overall electrode assembly.

[0111] The above disclosure is only the preferred embodiments of the present application, and of course cannot be used to limit the present application, and thus equivalent changes made by the present application still fall within the scope of the present application.

Claims

A secondary battery includes a housing and an electrode assembly, the housing accommodating the electrode assembly, the electrode assembly including a negative electrode tab, a positive electrode tab, and a separator disposed between the negative electrode tab and the positive electrode tab, the negative electrode tab including a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the positive electrode tab including a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein, a thickness direction of the electrode assembly is defined as a first direction, and along the first direction, a number of layers of the positive electrode active material layer in the electrode assembly is N; the electrode assembly includes a first end and a second end opposite to each other along a second direction perpendicular to the first direction; at the first end, an edge of the positive electrode active material layer is a first edge, and an edge of the negative electrode active material layer is a second edge; a distance between the first edge of each layer of the positive electrode active material layer and the second edge closest to the first edge along the first direction in the second direction is D; along the second direction, a size of the positive electrode active material layer is D1, a size of the negative electrode active material layer is D2, and n layers of the positive electrode active material layer among the N layers of the positive electrode active material layer satisfy the following condition: D≤0.38(D2-D1), N and n are positive integers, and n is less than N. The secondary battery according to claim 1, wherein 0.2N≤n≤0.8N. The secondary battery according to claim 2, wherein 0.4N≤n≤0.6N. The secondary battery according to claim 1, wherein an average value of the D values of the n layers of the positive electrode active material layer is greater than or equal to 0.1(D2-D1). The secondary battery according to claim 1, wherein the D values of the n layers of the positive electrode active material layer are different. The secondary battery according to claim 1, wherein the electrode assembly is in a wound structure, and the second direction is a direction of a winding center axis of the electrode assembly. The secondary battery according to claim 6, wherein the secondary battery further includes a tab electrically connected to the electrode assembly, and the tab extends out of the electrode assembly from the first end. The secondary battery according to claim 1, wherein the electrode assembly is in a stacked structure. 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, an edge of the positive electrode active material layer is a third edge, and an edge of the negative electrode active material layer is a fourth edge; a distance between the third edge of each layer of the positive electrode active material layer and the fourth edge closest to the third edge along the first direction in the third direction is d; along the third direction, a size of the positive electrode active material layer is D3, a size of the negative electrode active material layer is D4, and m layers of the positive electrode active material layer among the N layers of the positive electrode active material layer satisfy the following condition: d≤0.38(D4-D3), m is a positive integer, and m is less than N. The secondary battery according to claim 9, wherein 0.2N≤m≤0.8N. The secondary battery according to claim 9, wherein the secondary battery further includes a tab electrically connected to the electrode assembly, and the tab extends out of the electrode assembly from the first end and / or the third end. The secondary battery according to claim 1, wherein at the first end, an edge of the separator is a fifth edge; along the second direction, a distance between the fifth edge and the second edge is a, and 0.3mm≤a≤1.5mm. The secondary battery according to claim 1, wherein The separator film includes a base material layer and a coating layer stacked, and the coating layer includes at least one of a ceramic material or a binding material. The secondary battery according to claim 1, wherein The positive electrode active material layer includes at least one of a lithium transition metal complex oxide or a lithium-containing transition metal phosphoric compound. The secondary battery according to claim 1, wherein The negative electrode active material layer includes one or several of a graphite-based material, an alloy-based material, lithium metal, a lithium metal alloy, a silicon material, a silicon-oxygen material, and a silicon-carbon material. The secondary battery according to claim 1, wherein 0.3 mm ≤ D2 - D1 ≤ 2 mm. An electronic device including the secondary battery of any one of claims 1 to 16.

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