Secondary battery and electric device

Abstract

The invention discloses a secondary battery and electric equipment, the secondary battery comprises an electrode assembly, the electrode assembly comprises a first pole piece, a second pole piece and an isolating membrane, the polarity of the first pole piece is opposite to that of the second pole piece, and the first pole piece, the isolating membrane and the second pole piece are sequentially stacked and then wound to form a cylindrical structure. The first pole piece comprises a first current collector, a first active substance layer and a second active substance layer, the first active substance layer and the second active substance layer are respectively arranged on two surfaces of the first current collector along a first direction, and the first direction is the thickness direction of the first pole piece. The first pole piece has a first region, the first region is provided with a plurality of first recesses, and the plurality of first recesses are formed in the first active material layer. The secondary battery is favorable for reducing the possibility of partially releasing the stacking state of the first pole piece, the isolating membrane and the second pole piece, and is favorable for improving the cycle performance and safety of the secondary battery.

Description

Technical Field

[0001] The present application belongs to the field of energy storage technology, and in particular relates to a secondary battery and electrical equipment. Background Art

[0002] The electrode assembly of a cylindrical secondary battery includes a positive electrode sheet, a negative electrode sheet and a separator. After the three are stacked, they are wound into a cylindrical shape by a winding needle. During the winding process, the active material layer of the electrode sheet (positive electrode sheet and negative electrode sheet) facing the winding center is compressed due to the bending deformation of the electrode sheet, and the active material layer facing away from the winding center is pulled due to the bending deformation of the electrode sheet, thereby generating internal stress in the electrode sheet. After the winding is completed, the winding needle is separated from the electrode assembly, and the part of the electrode sheet located in the inner circle of the electrode assembly is prone to move under the action of internal stress, making it difficult to maintain the partial stacking structure of the positive electrode sheet, separator and negative electrode sheet (such as Figure 1 After a long cycle, the inner ring of the electrode assembly is prone to black spots and lithium deposition, which has an adverse effect on the service life of the secondary battery. Summary of the invention

[0003] In view of the above situation, it is necessary to provide a secondary battery and an electric device that can reduce the possibility of partial release of the stacked structure of the positive electrode sheet, the separator and the negative electrode sheet.

[0004] The first aspect of the embodiment of the present application provides a secondary battery, including an electrode assembly, the electrode assembly includes a pole piece and a separator, the pole piece includes a first pole piece and a second pole piece, the first pole piece and the second pole piece have opposite polarities, the first pole piece, the separator and the second pole piece are sequentially stacked and wound to form a cylindrical structure. The first pole piece includes a first current collector, a first active material layer and a second active material layer, the first active material layer and the second active material layer are respectively arranged on two surfaces of the first current collector along a first direction, and the first direction is the thickness direction of the first pole piece. Along the winding direction of the electrode assembly, the distance from the first active material layer to the winding start end of the first pole piece is not greater than the distance from the second active material layer to the winding start end of the first pole piece; observed along the winding center axis direction of the electrode assembly, the electrode assembly includes a winding radius R1, and the winding radius R1 is the difference between the outer diameter of the electrode assembly and the average thickness of the electrode assembly. The first pole piece has an easy-to-loose area, the first active material layer has a first edge, the first edge is located at the winding start end of the first active material layer, the first edge is an edge line of the easy-to-loose area, and the length of the easy-to-loose area is 2πR1 from the first edge along the winding direction of the electrode assembly. The first pole piece includes a first empty foil area, the first current collector of the first empty foil area is not covered by the first active material layer and the second active material layer, the first empty foil area is located at the winding start end of the first pole piece, and the length of the first empty foil area is less than or equal to 2πR1 along the winding direction of the electrode assembly. The first pole piece has a first area, and the first area is provided with a plurality of first recesses, at least part of the first area overlaps with the easy-to-loose area, and the length of the overlapping part of the first area and the easy-to-loose area along the winding direction of the electrode assembly is D, and D≥4πR1 / 3.

[0005] In this secondary battery, by providing a plurality of first recesses in at least part of the easily loose area of ​​the first pole sheet, it is helpful to reduce the internal stress generated in the first pole sheet during the winding process, thereby reducing the possibility of part of the first pole sheet, part of the isolation membrane and part of the second pole sheet being released from the stacked state, which is beneficial to improving the cycle performance and safety of the secondary battery.

[0006] In an optional embodiment of the present application, along the winding direction of the electrode assembly, the length of the first pole piece is L, the length of the first region is L1, and 2L / 5≤L1≤3L / 5. By setting L1≥2L / 5, the length of the first region along the winding direction of the electrode assembly is not too small, and the reduced internal stress is not too small, which is conducive to reducing the possibility of the first pole piece, the isolation membrane, and the second pole piece located in the inner circle of the electrode assembly being released from the stacked state, thereby reducing the possibility of black spots and lithium precipitation in the secondary battery during long cycles; by setting L1≤3L / 5, the length of the first region along the winding direction of the electrode assembly is not too large, which is conducive to reducing the influence of the setting of the first region on the interface performance of the electrode assembly.

[0007] In an optional embodiment of the present application, the first pole piece includes a first edge and a second edge arranged relatively along the winding center axis direction of the electrode assembly, and the first active material layer and the second active material layer both extend to the first edge and the second edge. The distance from the first region to the first edge along the winding center axis direction of the electrode assembly is D1, 1mm≤D1≤7mm. Setting D1≥1mm, the distance from the first region to the first edge is not too small, which is conducive to reducing the influence of the setting of the first groove on the interface performance of the electrode assembly; setting D1≤7mm is conducive to increasing the area of ​​the first region, thereby enhancing the effect of the first region in reducing the internal stress of the first pole piece.

[0008] In an optional embodiment of the present application, the distance from the first region to the second edge along the winding center axis of the electrode assembly is D2, 1mm≤D2≤7mm. When D2≥1mm is set, the distance from the first region to the first edge is not too small, which is conducive to reducing the influence of the setting of the first groove on the interface performance of the electrode assembly; when D2≤7mm is set, it is conducive to increasing the area of ​​the first region, thereby enhancing the effect of the first region in reducing the internal stress of the first pole piece.

[0009] In an optional embodiment of the present application, a portion of the first pole piece is recessed along the first direction to form a plurality of first convex portions and a plurality of first concave portions, the first convex portions protrude from the non-recessed and deformed surface of the first pole piece, and the first concave portions and the first convex portions are correspondingly arranged along the first direction. The first convex portions are arranged to improve the adhesion between the first pole piece and the isolation film, and reduce the possibility of the stacking structure between the first pole piece and the isolation film being released due to internal stress.

[0010] In an optional embodiment of the present application, the first pole piece includes a double-sided coating area, in which the first current collector is covered by the first active material layer and the second active material layer at the same time, and the first area is located in the double-sided coating area; the double-sided coating area includes a third edge and a fourth edge that are relatively arranged along the winding direction of the first pole piece. The distance from the first area to the third edge along the winding direction of the first pole piece is L2, and L2 ≥ 1mm. In this way, the distance between the first area and the third edge is not too small, which is conducive to reducing the possibility of the active material at the edges of the first active material layer and the second active material layer falling off during the pressing process of the first pole piece.

[0011] In an optional embodiment of the present application, the distance from the first region to the fourth edge along the winding direction of the first pole piece is L3, and L3 ≥ 1 mm. In this way, the distance between the first region and the fourth edge is not too small, which is conducive to reducing the possibility of the active material at the edge of the first active material layer and the second active material layer falling off during the pressing process of the first pole piece.

[0012] In an optional embodiment of the present application, part of the second pole piece is recessed along the first direction to form a plurality of second convex parts and a plurality of second concave parts, the second convex parts protrude from the non-recessed and deformed surface of the second pole piece, and the second concave parts and the second convex parts are arranged correspondingly along the first direction; at least part of several of the plurality of first convex parts are accommodated in the second concave parts. At least part of the first convex part is accommodated in the second concave part, which is conducive to reducing the total thickness of the first pole piece and the second pole piece after being stacked, and improving the energy density of the secondary battery.

[0013] In an optional embodiment of the present application, when the first pole piece is flattened, the projected area of ​​the first region along the first direction is S, and the total projected area of ​​the plurality of first recesses along the first direction is S1, 0.06S≤S1≤0.5S. When S1≥0.06S is set, the first recesses are not too sparsely distributed in the first region, which is beneficial to improving the effect of the first recesses on reducing the internal stress of the first pole piece; when S1≤0.5S is set, the first recesses are not too densely distributed in the first region, which is beneficial to reducing the influence of the setting of the first recesses on the degree of electrolyte infiltration of the first pole piece on the one hand, and on the other hand, it is beneficial to reducing the influence of the setting of the first protrusions on the structural strength of the first current collector.

[0014] In an optional embodiment of the present application, the height of the first convex portion protruding from the non-depressed deformed surface of the first pole piece along the first direction is T1, 1μm≤T1≤10μm. When T1≥1μm is set, the height of the first convex portion is not too small, and the depth of the first concave portion is not too small, which is conducive to improving the effect of the first concave portion on reducing the internal stress of the electrode assembly; when T1≤10μm is set, the height of the first convex portion is not too large, which is conducive to reducing the effect of the setting of the first convex portion on the bonding force between the first pole piece and the isolation membrane, and is conducive to reducing the possibility of damage to the first pole piece during the pressing process.

[0015] In an optional embodiment of the present application, two adjacent first protrusions are arranged apart. The distance between any two adjacent first protrusions along the winding direction of the first pole piece is D3, 1.5mm≤D3≤3mm. When D3≥1.5mm is set, the distance between the two adjacent first protrusions is not too small, which is conducive to reducing the possibility of damage to the first pole piece during the pressing process; when D3≤3mm is set, the distance between the two adjacent first protrusions is not too large, which is conducive to increasing the number of first recesses, thereby improving the effect of the first recesses in reducing the internal stress of the electrode assembly.

[0016] In an optional embodiment of the present application, the distance between two adjacent first protrusions along the winding center axis direction of the electrode assembly is D4, 1.5mm≤D4≤3mm. When D4≥1.5mm is set, the distance between the two adjacent first protrusions is not too small, which is conducive to reducing the possibility of damage to the first pole piece during the pressing process; when D4≤3mm is set, the distance between the two adjacent first protrusions is not too large, which is conducive to increasing the number of first recesses, thereby improving the effect of the first recesses in reducing the internal stress of the electrode assembly.

[0017] In an optional embodiment of the present application, the first concave portion is formed by removing part of the material from the first active material layer. By setting the first concave portion in this way, on the one hand, the maximum static friction between the first pole piece and the isolation membrane can be increased, and the possibility of relative displacement between the first pole piece and the isolation membrane can be reduced; on the other hand, the electrolyte can be stored to improve the cycle performance of the secondary battery.

[0018] In an optional embodiment of the present application, in the first region, along the winding direction of the electrode assembly, in any 1mm length of the first pole sheet, the number of first recesses is N, 90≤N≤190. When N≥90, the distribution of the first recesses on the first pole sheet is not too sparse, which is conducive to improving the effect of the first recesses on releasing the stress on the first pole sheet; when N≤190, the distribution of the first recesses on the first pole sheet is not too dense, which is conducive to maintaining the CB of the electrode assembly when the first pole sheet is a negative pole sheet, reducing the possibility of lithium deposition in the electrode assembly during the cycle, and when the first pole sheet is a positive pole sheet, it is conducive to improving the capacity of the electrode assembly.

[0019] In an optional embodiment of the present application, when the first pole piece is flattened, the projected area of ​​the first region along the first direction is S, and the total projected area of ​​the plurality of first recesses along the first direction is S1, 0.04S≤S1≤0.5S. When S1≥0.04S is set, the distribution of the first recesses in the first region is not too sparse, which is beneficial to improving the effect of the first recesses in reducing the internal stress of the first pole piece; when S1≤0.5S is set, the distribution of the first recesses in the first region is not too dense, which is beneficial to maintaining the CB of the electrode assembly when the first pole piece is a negative pole piece, reducing the possibility of lithium deposition in the electrode assembly during the cycle, and when the first pole piece is a positive pole piece, it is beneficial to improve the capacity of the electrode assembly.

[0020] In an optional embodiment of the present application, 0.3S≤S1≤0.5S. When S1≥0.3S, the first recesses are not too sparsely distributed in the first region, which is beneficial to further improve the effect of the first recesses in reducing the internal stress of the first pole piece.

[0021] In an optional embodiment of the present application, the first recess extends along the winding center axis direction of the electrode assembly, and a plurality of first recesses are arranged at intervals along the winding direction of the electrode assembly, and the distance between any two adjacent first recesses along the winding direction of the first pole piece is D5, 0.2mm≤D5≤0.5mm. When D5≥0.2mm is set, the spacing between the first recesses is not too small, and the distribution of the first recesses in the first pole piece is not too dense. When the first pole piece is a negative pole piece, it is beneficial to maintain the CB of the electrode assembly and reduce the possibility of lithium deposition in the electrode assembly during the cycle. When the first pole piece is a positive pole piece, it is beneficial to increase the capacity of the electrode assembly; when D5≤0.5mm is set, the spacing between the first recesses is not too large, which is beneficial to improve the effect of reducing internal stress.

[0022] In an optional embodiment of the present application, the depth of the first recess along the first direction is H1, 1μm≤H1≤50μm. When H1≥1μm is set, the depth of the first recess is not too small, which is conducive to improving the effect of the first recess on reducing the internal stress of the first pole piece; when H1≤50μm is set, the depth of the first recess is not too large, and the active material removed is not too much. When the first pole piece is a negative pole piece, it is conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium deposition in the electrode assembly during the cycle. When the first pole piece is a positive pole piece, it is conducive to improving the capacity of the electrode assembly.

[0023] In an optional embodiment of the present application, the width of the first recess along the winding direction of the first pole piece is W1, 0.09mm≤W1≤0.2mm. When W1≥0.09mm is set, the width of the first recess is not too small, which is conducive to improving the effect of the first recess in reducing the internal stress of the first pole piece; when W1≤0.2mm is set, the width of the first recess is not too large, and the active material removed is not too much. When the first pole piece is a negative pole piece, it is conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium deposition in the electrode assembly during the cycle. When the first pole piece is a positive pole piece, it is conducive to increasing the capacity of the electrode assembly.

[0024] In an optional embodiment of the present application, the first pole piece is a negative pole piece, the second pole piece is a positive pole piece, and the first empty foil area is located in the innermost layer of the electrode assembly. The second pole piece includes a second current collector, a third active material layer and a fourth active material layer, and the third active material layer and the fourth active material layer are respectively arranged on the two surfaces of the second current collector along the first direction. Along the first direction, the third active material layer faces the first active material layer, and the projection of the third active material layer is located within the range of the first active material layer, and the fourth active material layer faces the second active material layer, and the projection of the fourth active material layer is located within the range of the second active material layer. In this way, it is beneficial to maintain the CB of the electrode assembly and reduce the possibility of lithium precipitation in the electrode assembly during the cycle; and since part of the first pole piece is located in the innermost layer of the electrode assembly, the first recess is arranged in the first pole piece, which is beneficial to improve the possibility that the first recess further reduces the part of the first pole piece, the part of the isolation membrane and the part of the second pole piece to release the stacking state.

[0025] A second aspect of the embodiments of the present application provides an electrical device, which includes a secondary battery as described in any of the above embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure when the positive electrode sheet, the isolation membrane and the negative electrode sheet of the inner circle of the electrode assembly are partially released from the stacked state.

[0027] Figure 2 It is a schematic structural diagram of a secondary battery in one embodiment of the present application.

[0028] Figure 3 yes Figure 2 Schematic diagram of the cross-sectional structure at III-III in the figure.

[0029] Figure 4 It is a schematic diagram of the structure of the first pole piece in a flattened state in one embodiment of the present application.

[0030] Figure 5 yes Figure 4 Schematic diagram of the cross-sectional structure at VV in the middle.

[0031] Figure 6 It is a schematic diagram of the structure of the first pole piece in a flattened state in one embodiment of the present application.

[0032] Figure 7 It is a schematic diagram of the structure of the first pole piece in a flattened state in one embodiment of the present application.

[0033] Figure 8 It is a schematic diagram of the structure of the first pole piece in a flattened state in one embodiment of the present application.

[0034] Fig. 9 yes Figure 8 Schematic diagram of the cross-sectional structure at IX-IX.

[0035] Fig.10 It is a schematic diagram of the structure of the second pole piece in a flattened state in one embodiment of the present application.

[0036] Fig.11 yes Fig.10 Schematic diagram of the cross-sectional structure at XI-XI.

[0037] Fig.12 It is a schematic diagram of the structure of an electrical device in an embodiment of the present application.

[0038] Main component symbols 1000, secondary battery; 100, electrode assembly; 200, housing; 201, main body; 202, pole; 10, first pole piece; 11, first current collector; 111, first empty foil area; 12, first active material layer; 121, first edge; 13, second active material layer; 14, first concave portion; 15, first convex portion; 16, third concave portion; 10a, single-sided coating area; 10b, double-sided coating area; 10c, first area; 10d, first edge; 10e, second edge; 10f, third edge; 10g, fourth edge; 20, second pole piece; 21, second current collector; 22, third active material layer; 23, fourth active material layer; 24, second concave portion; 25, second convex portion; 20a, second region; 30. Isolation film; 10000. Electrical equipment; X, first direction; Y, winding direction of the electrode assembly; Z, winding center axis direction. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments.

[0040] It should be noted that when an element is considered to be "connected" to another element, it may be directly connected to the other element or there may be a centrally disposed element at the same time. When an element is considered to be "disposed on" another element, it may be directly disposed on the other element or there may be a centrally disposed element at the same time.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0042] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0043] In the description of the embodiments of the present application, the term "perpendicular" is used to describe an ideal state between two components. In actual production or use, there may be a state that is approximately perpendicular between the two components. The two components described as "perpendicular" may not be absolutely straight lines or planes, but may be roughly straight lines or planes. From a macroscopic point of view, if the overall extension direction is a straight line or a plane, the components can be considered to be "straight lines" or "planes".

[0044] The term "parallel" is used to describe the ideal state between two parts. In actual production or use, two parts may be approximately parallel. The two parts described as "parallel" may not be absolutely straight lines or planes, but may be roughly straight lines or planes. From a macroscopic point of view, if the overall extension direction is a straight line or a plane, the parts can be considered "straight" or "plane".

[0045] Reference to "embodiment" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. The various embodiments in the present application may be combined with each other in the absence of conflict.

[0046] The embodiment of the present application provides a secondary battery, including an electrode assembly, the electrode assembly includes a pole piece and a separator, the pole piece includes a first pole piece and a second pole piece, the first pole piece and the second pole piece have opposite polarities, and the first pole piece, the separator and the second pole piece are stacked and wound to form a cylindrical structure. The first pole piece includes a first current collector, a first active material layer and a second active material layer, and the first active material layer and the second active material layer are respectively arranged on two surfaces of the first current collector along a first direction, and the first direction is the thickness direction of the first pole piece. Along the winding direction of the electrode assembly, the distance from the first active material layer to the winding starting end of the first pole piece is not greater than the distance from the second active material layer to the winding starting end of the first pole piece; observed along the winding center axis direction of the electrode assembly, the electrode assembly includes a winding radius R1, and the winding radius R1 is the difference between the outer diameter of the electrode assembly and the average thickness of the electrode assembly. The first pole piece has an easy-to-loose area, the first active material layer has a first edge, the first edge is located at the winding start end of the first active material layer, the first edge is an edge line of the easy-to-loose area, and the length of the easy-to-loose area is 2πR1 from the first edge along the winding direction of the electrode assembly. The first pole piece includes a first empty foil area, the first current collector of the first empty foil area is not covered by the first active material layer and the second active material layer, the first empty foil area is located at the winding start end of the first pole piece, and the length of the first empty foil area is less than or equal to 2πR1 along the winding direction of the electrode assembly. The first pole piece has a first area, and the first area is provided with a plurality of first recesses, at least part of the first area overlaps with the easy-to-loose area, and the length of the overlapping part of the first area and the easy-to-loose area along the winding direction of the electrode assembly is D, and D≥4πR1 / 3.

[0047] In this secondary battery, by providing a plurality of first recesses in at least part of the easily loosened area of ​​the first pole sheet, it is helpful to reduce the internal stress generated in the first pole sheet during the winding process, thereby reducing the possibility of partial release of the stacking state of part of the first pole sheet, part of the isolation membrane and part of the second pole sheet, which is beneficial to improving the cycle performance and safety of the secondary battery.

[0048] The embodiments of the present application are further described below in conjunction with the accompanying drawings. In the accompanying drawings, the thickness direction of the first pole piece is the first direction X, the winding direction Y of the electrode assembly corresponds to the length direction of the first pole piece and the second pole piece in a flattened state, the winding direction of the first pole piece corresponds to the length direction of the first pole piece in a flattened state, and the winding center axis direction Z of the electrode assembly corresponds to the width direction of the first pole piece and the second pole piece in a flattened state.

[0049] like Figure 2 and Figure 3 As shown, an embodiment of the present application provides a secondary battery 1000 , including a housing 200 and an electrode assembly 100 , wherein the electrode assembly 100 is accommodated in the housing 200 .

[0050] In some embodiments, the housing 200 is a flexible packaging bag, such as an aluminum-plastic film. In other embodiments, the housing 200 is a hard shell, such as a plastic shell, or a metal shell including at least one of a steel alloy, an aluminum alloy, and a copper alloy.

[0051] In some embodiments, Figure 3 As shown, the electrode assembly 100 includes pole pieces and a separation membrane 30. The pole pieces include a first pole piece 10 and a second pole piece 20. The polarities of the first pole piece 10 and the second pole piece 20 are opposite. The first pole piece 10, the separation membrane 30 and the second pole piece 20 are stacked and wound into a cylindrical structure.

[0052] In some embodiments, Figure 2 As shown, the shell 200 is a hard shell, and the shell 200 includes a main body 201 and a pole 202. The pole 202 is connected to the main body 201 and insulated from the main body 201. One of the first pole piece 10 and the second pole piece 20 of the electrode assembly 100 is electrically connected to the main body 201, and the other is electrically connected to the pole 202.

[0053] In some embodiments, Figure 3 As shown, the first pole piece 10 includes a first current collector 11, a first active material layer 12 and a second active material layer 13. The first active material layer 12 and the second active material layer 13 are respectively arranged on two surfaces of the first current collector 11 along a first direction X. The first direction X is the thickness direction of the first pole piece 10.

[0054] In some embodiments, Figure 3 As shown, the second electrode sheet 20 includes a second current collector 21 , a third active material layer 22 and a fourth active material layer 23 , and the third active material layer 22 and the fourth active material layer 23 are respectively disposed on two surfaces of the second current collector 21 along the first direction X.

[0055] In some embodiments, the first electrode sheet 10 is a positive electrode sheet, the first current collector 11 is a positive electrode current collector, and the first active material layer 12 and the second active material layer 13 are positive electrode active material layers. The second electrode sheet 20 is a negative electrode sheet, the second current collector 21 is a negative electrode current collector, and the third active material layer 22 and the fourth active material layer 23 are negative electrode active material layers. In other embodiments, the first electrode sheet 10 is a negative electrode sheet, the first current collector 11 is a negative electrode current collector, and the first active material layer 12 and the second active material layer 13 are negative electrode active material layers. The second electrode sheet 20 is a positive electrode sheet, the second current collector 21 is a positive electrode current collector, and the third active material layer 22 and the fourth active material layer 23 are positive electrode active material layers.

[0056] In some embodiments, at least one of the positive current collector and the negative current collector is a metal layer. As an illustrative example, the positive current collector may be a metal layer including at least one of aluminum, nickel, tantalum, and titanium, such as aluminum foil. The negative current collector may be a metal layer including at least one of copper, nickel, tantalum, and titanium, such as copper foil. In other embodiments, at least one of the positive current collector and the negative current collector is a composite current collector.

[0057] In some embodiments, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganese iron phosphate, or lithium manganese oxide.

[0058] In some embodiments, the negative electrode active material includes at least one of graphite, hard carbon, soft carbon, silicon, silicon-oxygen material, and silicon-carbon material.

[0059] In some embodiments, the isolation film 30 is made of insulating film materials such as polyethylene film, polypropylene film, polyester film or polyimide film.

[0060] In some embodiments, the secondary battery 1000 further includes an electrolyte (not shown), which is contained in the housing 200 .

[0061] In some embodiments, the electrolyte includes an electrolyte salt. The electrolyte salt includes at least one of an organic lithium salt or an inorganic lithium salt.

[0062] In some embodiments, the electrolyte salt includes but is not limited to at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium hexafluorocesium oxide (LiCsF6), lithium perchlorate (LiClO4) or lithium trifluoromethanesulfonate (LiCF3SO3).

[0063] like Figure 4 and Figure 5 As shown, in some embodiments, along the winding direction Y of the electrode assembly, the distance from the first active material layer 12 to the winding start end of the first electrode sheet 10 is not greater than the distance from the second active material layer 13 to the winding start end of the first electrode sheet 10. In other words, along the winding direction Y of the electrode assembly, the edge of the first active material layer 12 facing the winding start end of the first electrode sheet 10 is flush with the edge of the second active material layer 13 facing the winding start end of the first electrode sheet 10; or, the edge of the first active material layer 12 facing the winding start end of the first electrode sheet 10 is closer to the winding start end of the first electrode sheet 10 than the edge of the second active material layer 13 facing the winding start end of the first electrode sheet 10.

[0064] In some embodiments, when observed along the winding center axis direction Z of the electrode assembly 100, the electrode assembly 100 includes a winding radius R1, which is the difference between the outer diameter of the electrode assembly 100 and the average thickness of the electrode assembly 100. The outer diameter of the electrode assembly 100 is the distance from the winding center of the electrode assembly to the outer surface of the electrode assembly; the average thickness of the electrode assembly 100 is calculated as follows: four orthogonal sampling points are taken at the tightly wound part of the electrode assembly 100, and the distances from the four sampling points along the radial direction of the electrode assembly 100 to the outer surface of the electrode assembly 100 are measured, and the average of the four distances is taken to obtain the average thickness of the electrode assembly 100. R represents the outer diameter of the electrode assembly 100, and R2 represents the average thickness of the electrode assembly 100, then R1=R-R2.

[0065] The first electrode sheet 10 has a loose area, and the first active material layer 12 has a first edge 121 (such as Figure 5 As shown in FIG. 1 ), the first side 121 is located at the winding start end of the first active material layer, and the first side 121 is an edge line of the easy loose area. From the first side 121, along the winding direction of the electrode assembly, the length of the easy loose area is 2πR1. The first pole sheet includes a first empty foil area 111, and the first current collector of the first empty foil area 111 is not covered by the first active material layer 12 and the second active material layer 13. The first empty foil area 111 is located at the winding start end of the first pole sheet 10, and along the winding direction Y of the electrode assembly, the length of the first empty foil area 111 is less than or equal to 2πR1.

[0066] In the above content, when determining the tightly wound portion of the electrode assembly 100, the following method can be used: observe the electrode assembly 100 using a computed tomography (CT) scanner, and the observation angle is facing the winding center axis direction Z, observe the winding structure of the first pole piece 10 and the second pole piece 20, and determine one of the innermost first pole piece 10 and the second pole piece 20 that is closer to the winding of the electrode assembly. For the convenience of explanation, it is assumed to be the first pole piece 10, and a moving point is assumed. The moving point starts from the winding starting end of the first pole piece 10, and moves along the winding direction of the electrode assembly 100 on the surface of the first pole piece 10 facing the winding center of the electrode assembly 100. When the external position where the moving point is located is radially outward of the cylindrical structure, three consecutive layers of pole pieces and the isolation film sandwiched by the three layers of pole pieces are sequentially fitted along the radial direction of the cylindrical structure, it is considered that the tightly wound portion of the electrode assembly 100 starts from the position of this point. Among them, still taking the first pole piece 10 as the inner pole piece of the electrode assembly 100 as an example, the continuous three-layer pole piece refers to the part of the first pole piece 10 demarcated along the winding direction of the electrode assembly from the first position point and the part of the second pole piece 20 sandwiched in the first pole piece 10 of this circle. The standard for judging whether the continuous three-layer pole piece and the isolation film 30 sandwiched by the three-layer pole piece are in a bonding state is: measure the spacing between the first current collector 11 and the second current collector 21 in the CT image, and then compare this spacing with the sum of the thickness of the active material layer located between the first current collector 11 and the second current collector 12 and the thickness of the isolation film 30. When the difference does not exceed 5% of the spacing, it is considered to be in a bonding state. For the continuous three-layer pole piece, a measurement point can be taken every 10° central angle. When these measurement points meet the above requirements, it is considered that the three-layer pole piece and the isolation film 30 sandwiched therein are in a bonding state.

[0067] It should be noted that the above method is a strict method for determining the tightly wound portion of the electrode assembly 100. In operation, since the tightly wound portion of the electrode assembly 100 is relative to the loose portion, and the loose portion of the electrode assembly 100 does not have a continuous spiral structure, while the tightly wound portion has a continuous spiral structure, when observing the CT image, it is often possible to intuitively distinguish the tightly wound portion from the loose portion.

[0068] It should be further explained that, in some embodiments of the present application, the electrode assembly 100 does not have a loose part.

[0069] In some embodiments, Figure 4 As shown, the first pole piece 10 has a first region 10c, and the first region 10c is provided with a plurality of first recesses 14. At least part of the first region 10c overlaps with the easy loose region. Along the winding direction Y of the electrode assembly, the length of the overlapping portion of the first region 10c and the easy loose region is D, and D≥4πR1 / 3.

[0070] In this secondary battery 1000, by providing a plurality of first recesses 14 in at least part of the easily loosened area of ​​the first pole sheet 10, it is helpful to reduce the internal stress generated in the first pole sheet 10 during the winding process, thereby reducing the possibility of releasing the stacking state of part of the first pole sheet 10, part of the isolation membrane 30 and part of the second pole sheet 20, which is helpful to improve the cycle performance and safety of the secondary battery 1000.

[0071] In the embodiment of the present application, the range of the first region 10c is defined by the outermost edges of the plurality of first grooves 14. Specifically, when the first pole piece 10 is flattened, the first region 10c is rectangular when viewed along the first direction X, and the side lines of the first region 10c are tangent to or overlap with the edges of the first grooves 14 or pass through the vertices of the edges of the first grooves 14, and the four side lines of the first region 10c are parallel to the four edges of the surface of the first pole piece 10 along the first direction X. Figure 4 , Figure 6 , Figure 7 and Figure 8 The range of the first region 10c is shown by a dotted line.

[0072] In some embodiments, Figure 4 As shown, along the winding direction Y of the electrode assembly, the length of the first pole piece 10 is L, the length of the first region 10c is L1, and 2L / 5≤L1≤3L / 5. When L1≥2L / 5 is set, the length of the first region 10c along the winding direction of the electrode assembly 100 is not too small, and the reduced internal stress is not too small. In this way, it is beneficial to reduce the possibility of the first pole piece 10, the separator 30 and the second pole piece 20 located in the inner circle of the electrode assembly 100 being released from the stacking state, thereby reducing the possibility of black spots and lithium deposition in the secondary battery 1000 during long cycles; when L1≤3L / 5 is set, the length of the first region 10c along the winding direction of the electrode assembly 100 is not too large, which is beneficial to reduce the influence of the setting of the first region 10c on the interface performance of the electrode assembly 100. Among them, the values ​​of L and L1 can be measured with a ruler.

[0073] In some embodiments, at least some of the openings of the first recesses 14 are oriented toward the winding center of the electrode assembly 100 along the first direction X. This helps to reduce the internal stress generated by the compressed one of the first active material layer 12 and the second active material layer 13 after the first electrode sheet 10 is wound.

[0074] In some embodiments, at least some of the openings of the first recesses 14 face away from the winding center of the electrode assembly 100 along the first direction X. This helps reduce the internal stress generated by the tensioned one of the first active material layer 12 and the second active material layer 13 after the first electrode sheet 10 is wound.

[0075] In some embodiments, Figure 5 As shown, the first pole piece 10 includes a single-sided coating area 10a and a double-sided coating area 10b. In the single-sided coating area 10a, a portion of the surface of the first current collector 11 along the first direction X is only covered by the first active material layer 12, and in the double-sided coating area 10b, the first current collector 11 is covered by both the first active material layer 12 and the second active material layer 13.

[0076] In some embodiments, at least a portion of the first region 10c is located in the single-sided coating area 10a.

[0077] In some embodiments, the first region 10c is entirely located in the double-sided coating area 10b.

[0078] In some embodiments, Figure 5 As shown, part of the first pole piece 10 is recessed along the first direction X to form a plurality of first convex portions 15 and a plurality of first concave portions 14, the first convex portions 15 protrude from the non-recessed and deformed surface of the first pole piece 10, and the first concave portions 14 and the first convex portions 15 are correspondingly arranged along the first direction X. The first convex portions 15 are arranged to improve the adhesion between the first pole piece 10 and the isolation film 30, and reduce the possibility of releasing the stacked structure between the first pole piece 10 and the isolation film 30 due to internal stress.

[0079] In some embodiments, the first concave portion 14 and the first convex portion 15 are formed by pressing with an embossing roller.

[0080] In some embodiments, Figure 4 and Figure 5 As shown, the first protrusion 15 is hemispherical in shape.

[0081] In some embodiments, Figure 6 As shown, the first protrusion 15 is in a strip shape, and a plurality of first protrusions 15 are arranged at intervals.

[0082] In some embodiments, Figure 7 As shown, the first protrusion 15 is in a strip shape, and a plurality of first protrusions 15 intersect to form a grid shape.

[0083] In some embodiments, Figure 4 As shown, the first region 10c is located in the double-sided coating region 10b, and the double-sided coating region 10b includes a third edge 10f and a fourth edge 10g that are relatively arranged along the winding direction of the first pole piece 10. The distance from the first region 10c to the third edge 10f along the winding direction of the first pole piece 10 is L2, and L2 ≥ 1mm. In this way, the distance between the first region 10c and the third edge 10f is not too small, which is conducive to reducing the possibility of the active material at the edges of the first active material layer 12 and the second active material layer 13 falling off during the pressing process of the first pole piece 10. Among them, the value of L2 can be measured with a ruler.

[0084] In some embodiments, Figure 4 As shown, the distance from the first region 10c to the fourth edge 10g along the winding direction of the first pole piece 10 is L3, and L3 ≥ 1 mm. In this way, the distance between the first region 10c and the fourth edge 10g is not too small, which is conducive to reducing the possibility of the active material at the edge of the first active material layer 12 and the second active material layer 13 falling off during the pressing process of the first pole piece 10. The value of L3 can be measured with a ruler.

[0085] In some embodiments, Fig.10 and Fig.11 As shown, part of the second pole piece 20 is recessed along the first direction X to form a plurality of second convex portions 25 and a plurality of second concave portions 24, the second convex portions 25 protrude from the non-recessed and deformed surface of the second pole piece 20, and the second concave portions 24 and the second convex portions 25 are correspondingly arranged along the first direction X. At least part of several of the plurality of first convex portions 15 are accommodated in the second concave portions 24. At least part of the first convex portions 15 is accommodated in the second concave portions 24, which is conducive to reducing the total thickness of the first pole piece 10 and the second pole piece 20 after being stacked, and improving the energy density of the secondary battery 1000.

[0086] In some embodiments, the first protrusion 15 is hemispherical, the second protrusion 25 is strip-shaped, and a plurality of second protrusions 25 intersect to form a grid, so as to accommodate as many first protrusions 15 as possible in the second recess 24 to improve the energy density of the secondary battery 1000.

[0087] In some embodiments, when the first pole piece 10 is flattened, the projection area of ​​the first region 10c along the first direction X is S, and the total projection area of ​​the plurality of first recesses 14 along the first direction X is S1, 0.06S≤S1≤0.5S. When S1≥0.06S is set, the first recesses 14 are not too sparsely distributed in the first region 10c, which is beneficial to improving the effect of the first recesses 14 on reducing the internal stress of the first pole piece 10; the setting of the first protrusion 15 will increase the adhesion between the first pole piece 10 and the isolation film 30, affecting the infiltration of the electrode liquid, and when S1≤0.5S is set, the first recesses 14 are not too densely distributed in the first region 10c, which is beneficial to reducing the influence of the setting of the first recesses 14 on the infiltration degree of the first pole piece 10 by the electrolyte, and on the other hand, it is beneficial to reduce the influence of the setting of the first protrusion 15 on the structural strength of the first current collector 11.

[0088] In the embodiments of the present application, the values ​​of S and S1 can be observed and measured using a CCD camera.

[0089] In some embodiments, Figure 5As shown, the height of the first convex portion 15 protruding from the surface of the first pole piece 10 along the first direction X is T1; 1μm≤T1≤10μm. When T1≥1μm is set, the height of the first convex portion 15 is not too small, and the depth of the first concave portion 14 is not too small, which is conducive to improving the effect of the first concave portion 14 on reducing the internal stress of the electrode assembly 100; as the height of the first convex portion 15 increases, the contact area between the isolation film 30 and the first pole piece 10 decreases, and the bonding force becomes weaker. When T1≤10μm is set, the height of the first convex portion 15 is not too large, which is conducive to reducing the influence of the setting of the first convex portion 15 on the bonding force between the first pole piece 10 and the isolation film 30, and is conducive to reducing the possibility of damage to the first pole piece 10 during the pressing process of the first pole piece 10.

[0090] In the embodiment of the present application, T1 can be observed and measured using a 3D camera.

[0091] In some embodiments, Figure 4 As shown, two adjacent first protrusions 15 are set apart. The distance between any two adjacent first protrusions 15 along the winding direction of the first pole piece 10 is D3, 1.5mm≤D3≤3mm. When D3≥1.5mm is set, the distance between two adjacent first protrusions 15 is not too small, which is beneficial to reduce the possibility of damage to the first pole piece 10 during the pressing process of the first pole piece 10; when D3≤3mm is set, the distance between two adjacent first protrusions 15 is not too large, which is beneficial to increase the number of first recesses 14, thereby improving the effect of the first recesses 14 in reducing the internal stress of the electrode assembly 100.

[0092] In some embodiments, Figure 4 As shown, the distance between two adjacent first protrusions 15 along the winding center axis direction Z of the electrode assembly 100 is D4, 1.5mm≤D4≤3mm. When D4≥1.5mm is set, the distance between two adjacent first protrusions 15 is not too small, which is conducive to reducing the possibility of damage to the first pole sheet 10 during the pressing process of the first pole sheet 10; when D4≤3mm is set, the distance between two adjacent first protrusions 15 is not too large, which is conducive to increasing the number of first recesses 14, thereby improving the effect of the first recesses 14 on reducing the internal stress of the electrode assembly 100.

[0093] In the embodiments of the present application, the values ​​of D3 and D4 can be measured using a ruler.

[0094] In some embodiments, Figure 8 and Fig. 9As shown, the first concave portion 14 is formed by removing part of the material from the first active material layer 12. By providing the first groove in this way, on the one hand, the maximum static friction between the first pole piece 10 and the isolation membrane 30 can be increased, and the possibility of relative displacement between the first pole piece 10 and the isolation membrane 30 can be reduced; on the other hand, the electrolyte can be stored to improve the cycle performance of the secondary battery 1000.

[0095] In some embodiments, the method of removing part of the material of the first recess 14 is laser etching.

[0096] In some embodiments, when the first pole piece 10 is flattened, the projection area of ​​the first region 10c along the first direction X is S, and the total projection area of ​​the plurality of first recesses 14 along the first direction X is S1, 0.04S≤S1≤0.5S. When S1≥0.04S is set, the distribution of the first recesses 14 in the first region 10c is not too sparse, which is beneficial to improving the effect of the first recesses 14 on reducing the internal stress of the first pole piece 10; when S1≤0.5S is set, the distribution of the first recesses 14 in the first region 10c is not too dense, which is beneficial to maintaining the CB (Cell Balance, the ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area) of the electrode assembly 100 when the first pole piece 10 is a negative pole piece, reducing the possibility of lithium deposition in the electrode assembly 100 during the cycle, and when the first pole piece 10 is a positive pole piece, it is beneficial to improve the capacity of the electrode assembly 100.

[0097] In some embodiments, 0.3S≤S1≤0.5S. When S1≥0.3S, the first recesses 14 are not too sparsely distributed in the first region 10 c, which is beneficial to further improve the effect of the first recesses 10 c in reducing the internal stress of the first pole piece 10 .

[0098] In some embodiments, in the first region 10c, along the winding direction Y of the electrode assembly, in any 1 mm length of the first pole sheet 10, the number of the first recesses 14 is N, 90≤N≤190. When N≥90, the distribution of the first recesses 14 on the first pole sheet 10 is not too sparse, which is conducive to improving the effect of the first recesses 14 releasing the stress on the first pole sheet 10; when N≤190, the distribution of the first recesses 14 on the first pole sheet 10 is not too dense, which is conducive to maintaining the CB of the electrode assembly 100 when the first pole sheet 10 is a negative pole sheet, reducing the possibility of lithium deposition in the electrode assembly 100 during the cycle, and when the first pole sheet 10 is a positive pole sheet, it is conducive to improving the capacity of the electrode assembly 100.

[0099] In some embodiments, Figure 8As shown, the first recess 14 extends along the winding center axis direction Z of the electrode assembly 100, and a plurality of first recesses 14 are arranged at intervals along the winding direction of the electrode assembly 100. The distance between any two adjacent first recesses 14 along the winding direction of the first pole piece 10 is D5, and 0.2mm≤D5≤0.5mm. When D5≥0.2mm is set, the spacing between the first recesses 14 is not too small, and the distribution of the first recesses 14 in the first pole piece 10 is not too dense. When the first pole piece 10 is a negative pole piece, it is beneficial to maintain the CB of the electrode assembly 100 and reduce the possibility of lithium deposition in the electrode assembly 100 during the cycle. When the first pole piece 10 is a positive pole piece, it is beneficial to increase the capacity of the electrode assembly 100; when D5≤0.5mm is set, the spacing between the first recesses 14 is not too large, which is beneficial to improve the effect of reducing internal stress.

[0100] In the embodiment of the present application, the value of D5 can be observed and measured using a CCD camera.

[0101] In some embodiments, Fig. 9 As shown, the depth of the first recess 14 along the first direction X is H1, 1μm≤H1≤50μm. When H1≥1μm is set, the depth of the first recess 14 is not too small, which is conducive to improving the effect of the first recess 14 on reducing the internal stress of the first pole piece 10; when H1≤50μm is set, the depth of the first recess 14 is not too large, and the active material removed is not too much. When the first pole piece 10 is a negative pole piece, it is conducive to maintaining the CB of the electrode assembly 100 and reducing the possibility of lithium deposition in the electrode assembly 100 during the cycle. When the first pole piece 10 is a positive pole piece, it is conducive to improving the capacity of the electrode assembly 100.

[0102] In the embodiment of the present application, the value of H1 can be observed and measured using a 3D camera.

[0103] In some embodiments, Figure 8 As shown, the width of the first recess 14 along the winding direction of the first pole piece 10 is W1, 0.09mm≤W1≤0.2mm. When W1≥0.09mm is set, the width of the first recess 14 is not too small, which is conducive to improving the effect of the first recess 14 on reducing the internal stress of the first pole piece 10; when W1≤0.2mm is set, the width of the first recess 14 is not too large, and the active material removed is not too much. When the first pole piece 10 is a negative pole piece, it is conducive to maintaining the CB of the electrode assembly 100 and reducing the possibility of lithium deposition in the electrode assembly 100 during the cycle. When the first pole piece 10 is a positive pole piece, it is conducive to improving the capacity of the electrode assembly 100.

[0104] In the embodiment of the present application, the value of W1 can be observed and measured using a CCD camera.

[0105] In some embodiments, the first recess 14 extends along the winding central axis direction Z of the electrode assembly 100, and the plurality of first recesses 14 are distributed at equal intervals along the winding direction of the first pole piece 10. This is conducive to improving the uniformity of the internal stress distribution on the first pole piece 10, thereby improving the stability of the stacked structure of the first pole piece 10, the isolation film 30 and the second pole piece 20.

[0106] It should be noted that in the embodiments of the present application, "equal spacing" does not mean that the two distances are equal in an absolute sense. For three adjacent first concave portions 14, the distance between two adjacent first concave portions 14 is represented by a and b, respectively, and a≤b. When (ba) / a≤10%, the three first concave portions 14 are considered to be equally spaced.

[0107] In some embodiments, Fig. 9 As shown, after part of the material of the second active material layer 13 is removed, a plurality of third recesses 16 are formed to further reduce the internal stress on the first pole piece 10 .

[0108] In some embodiments, Figure 4 and Figure 8 As shown, the first electrode sheet 10 includes a first edge 10d and a second edge 10e that are arranged opposite to each other along the winding center axis direction Z of the electrode assembly 100, and the first active material layer 12 and the second active material layer 13 both extend to the first edge 10d and the second edge 10e. The distance from the first region 10c to the first edge 10d along the winding center axis direction Z of the electrode assembly 100 is D1, 1mm≤D1≤7mm. Setting D1≥1mm, the distance from the first region 10c to the first edge 10d is not too small, which is conducive to reducing the influence of the setting of the first recess 14 on the interface performance of the electrode assembly 100; specifically, for the scheme of pressing out the first recess 14, the first region 10c maintains a certain distance from the first edge 10d, which is conducive to reducing the possibility of the active material at the edge of the first active material layer 12 and the second active material layer 13 falling off during the pressing process. For the scheme of removing part of the material of the first active material layer 12 to obtain the first recess 14, the first region 10c maintains a certain distance from the first edge 10d, which is conducive to reducing the possibility of the electrolyte enriching at the end of the electrode assembly 100 on the same side as the first edge 10d and affecting the cycle performance of the circulating secondary battery 1000. Setting D1≤7mm is conducive to increasing the area of ​​the first region 10c, thereby enhancing the effect of the first region 10c on reducing the internal stress of the first electrode sheet 10. Among them, the value of D1 can be measured with a ruler.

[0109] In some embodiments, Figure 4 and Figure 8As shown, the distance from the first region 10c to the second edge 10e along the winding center axis direction Z of the electrode assembly 100 is D2, 1mm≤D2≤7mm. When D2≥1mm is set, the distance from the first region 10c to the first edge 10d is not too small, which is conducive to reducing the influence of the setting of the first groove on the interface performance of the electrode assembly 100; when D2≤7mm is set, it is conducive to increasing the area of ​​the first region 10c, thereby enhancing the effect of the first region 10c in reducing the internal stress of the first pole piece 10. Among them, the value of D2 can be measured with a ruler.

[0110] In some embodiments, Fig.10 and Fig.11 As shown, the second electrode sheet 20 includes a second current collector 21 , a third active material layer 22 and a fourth active material layer 23 , and the third active material layer 22 and the fourth active material layer 23 are respectively disposed on two surfaces of the second current collector 21 along the first direction X.

[0111] In some embodiments, the first electrode sheet 10 is a negative electrode sheet, the second electrode sheet 20 is a positive electrode sheet, and the first empty foil area 111 is located in the innermost layer of the electrode assembly 100; along the first direction X, the third active material layer 22 faces the first active material layer 12, and the projection of the third active material layer 22 is located within the range of the first active material layer 12, and the fourth active material layer 23 faces the second active material layer 13, and the projection of the fourth active material layer is located within the range of the second active material layer. In this way, it is beneficial to maintain the CB of the electrode assembly and reduce the possibility of lithium deposition in the electrode assembly during the cycle; and since part of the first electrode sheet 10 is located in the innermost layer of the electrode assembly 100, the first recess 14 is set in the first electrode sheet 10, which is beneficial to improve the possibility that the first recess 14 further reduces the release of the stacking state of part of the first electrode sheet 10, part of the isolation film 30 and part of the second electrode sheet 20.

[0112] In some embodiments, the second pole piece 20 has a second region 20a, the second region 20a is provided with a plurality of second recesses 24, the depth direction of the plurality of second recesses 24 is along the first direction X, and the plurality of second recesses 24 are formed in the third active material layer 22. By providing a plurality of second recesses 24 in the second pole piece 20, it is beneficial to further reduce the internal stress generated in the second pole piece 20 during the winding process, thereby reducing the possibility of black spots and lithium deposition in the secondary battery 1000 during long cycles.

[0113] In some embodiments, the second recess 24 is formed by pressing the second electrode sheet 20 , or is obtained by removing part of the material from the third active material layer 22 .

[0114] like Fig.12As shown, an embodiment of the present application further provides an electric device 10000, and the electric device 10000 includes the secondary battery 1000 involved in any of the aforementioned embodiments.

[0115] In some embodiments, the types of electrical devices 10000 include but are not limited to mobile phones, laptop computers, power tools, electric toys, and electronic cigarettes.

[0116] In order to verify the effect of the solution provided by the embodiment of the present application on reducing the internal stress of the electrode assembly 100 , the inventor of the present application conducted the following experiment.

[0117] The experiment includes 3 groups of comparative examples and 5 groups of embodiments, and each group of comparative examples and each group of embodiments includes 100 secondary batteries 1000. In this experiment, the first electrode sheet 10 is set as a positive electrode sheet.

[0118] In Example 1, the preparation process of the secondary battery 1000 includes the following steps: (1) Preparation of positive electrode sheet: Active materials lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), CNT (carbon nanotube), polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97.5:0.5:0.5:1.5, N-methylpyrrolidone (NMP) is added as a solvent, and a positive electrode active material with a solid content of 75wt% is prepared, and stirred evenly for use. Aluminum foil is used as a positive electrode current collector. The above active material is evenly coated on one surface of the positive electrode current collector along the thickness direction using a slit coater, and then dried at 90°C to obtain a positive electrode sheet coated with a single-sided positive electrode active material. The above coating steps are then repeated on the other side of the positive electrode current collector along the thickness direction. The coated positive electrode sheet is then cold pressed. After cold pressing, a first convex portion 15 and a first concave portion 14 are pressed on the positive electrode sheet using an embossing roller, and the area where the first concave portion 14 is located is the first area 10c. The positive electrode sheet includes an empty foil area, which is not covered by the positive electrode active material layer, and the empty foil area is located at the winding start end of the positive electrode sheet. The positive electrode ear is welded to the empty foil area. The length L of the prepared positive electrode sheet is 120 mm and the width is 36 mm.

[0119] In the prepared positive electrode sheet, along the winding direction of the electrode assembly 100, the length of the empty foil area is equal to 2πR1', where R1' is the radius of the winding needle used in the step of winding the electrode assembly. The one of the two positive electrode active material layers closer to the winding start end of the positive electrode sheet is defined as the first active material layer (if the distances between the two and the winding start end of the positive electrode sheet are the same, either one is selected), the first active material layer 12 has a first edge 121, and the first edge 121 is located at the winding start end of the first active material layer 12. The positive electrode sheet is defined as having an easy loose area, and the first edge 121 is a side line of the easy loose area. From the first edge 121, along the winding direction of the electrode assembly, the length of the easy loose area is 2πR1', and the length of the overlapping part of the first area and the easy loose area is equal to 4πR1' / 3.

[0120] (2) Preparation of negative electrode sheet: Active materials artificial graphite, conductive carbon black (Super P), styrene-butadiene rubber (SBR), CMC (sodium carboxymethyl cellulose) are mixed in a weight ratio of 97:0.5:1.3:1.2, deionized water is added as a solvent, and a weight percentage of 50wt% of negative electrode active material is prepared, and the mixture is stirred evenly for use. A copper foil with a thickness of 100mm is used as the negative electrode current collector. The above negative electrode active material is evenly coated on one surface of the negative electrode current collector along its thickness direction using a slit coater, and then dried at 110°C to obtain a negative electrode sheet coated with a negative electrode active material layer on one side. Then, the above steps are repeated on the other side of the negative electrode current collector along its thickness direction to obtain a negative electrode sheet coated with a negative electrode active material layer on both sides. Then, the coated negative electrode sheet is cold pressed. The negative electrode sheet includes an empty foil area, which is not covered by the negative electrode active material layer. The empty foil area is located at the starting end of the winding of the negative electrode sheet, and the negative electrode ear is welded to the empty foil area. The length of the prepared negative electrode sheet is 115 mm and the width is 37.2 mm.

[0121] (3) Preparation of electrolyte: In a dry argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were first mixed in a mass ratio of EC:EMC:DEC = 30:50:20 to form a basic organic solvent. Then, lithium salt lithium hexafluorophosphate (LiPF6) was added to the basic organic solvent to dissolve and mix well to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0122] (4) Preparation of the isolation membrane 30: A 7 μm thick polyethylene porous polymer film was used as the isolation membrane 30.

[0123] (5) Preparation of the electrode assembly 100: The positive electrode sheet, the separator 30 and the negative electrode sheet are stacked along the first direction X and then wound using a winding needle to obtain the electrode assembly 100.

[0124] (6) Assembly of secondary battery 1000: The electrode assembly 100 is installed in the shell 200. The shell 200 includes a main body 201 and a pole 202. The pole 202 is connected to the main body 201 and is insulated from the main body 201. The electrode assembly 100 is accommodated in the shell 200. The negative electrode sheet of the electrode assembly 100 is electrically connected to the main body 201 of the shell 200, and the positive electrode sheet is electrically connected to the pole 202. After the processes of liquid injection, packaging, static placement, hot pressing, shaping, etc., the secondary battery 1000 is manufactured.

[0125] The secondary battery 1000 in Comparative Example 1 is substantially the same as that in Example 1, except that the first protrusion 15 is not formed by pressing the first electrode sheet 10 in Comparative Example 1.

[0126] The preparation process of the secondary battery 1000 in Comparative Example 2 is substantially the same as that in Example 1, except that the length of the overlapping portion between the first region and the easy loose region of the secondary battery 1000 in Comparative Example 2 is less than 4πR1′ / 3.

[0127] The preparation process of the secondary battery 1000 in Comparative Example 3 is substantially the same as that in Example 1, except that the length of the empty foil area of ​​the positive electrode sheet of the secondary battery 1000 in Comparative Example 3 is greater than 2πR1′.

[0128] The preparation process of the secondary battery 1000 in Example 2 is substantially the same as that in Example 1, except that the length of the overlapping portion between the first region and the easily loose region in Example 2 is greater than 4πR1′ / 3.

[0129] The preparation process of the secondary battery 1000 in Example 3 is substantially the same as that in Example 1, except that the length of the empty foil area of ​​the positive electrode sheet of the secondary battery 1000 in Example 2 is less than 2πR1′.

[0130] The preparation process of the secondary battery 1000 in Examples 4-5 is substantially the same as that in Example 1, except that the winding needle radius R1 ′ used is different.

[0131] After the secondary batteries 1000 in comparative examples 1-3 and embodiments 1-5 are prepared, CT is used to scan the electrode assemblies 100 of the secondary batteries 1000 in each group, and R1 is calculated. The number of secondary batteries 1000 in each group that meet the relationship R1=R1'±5%R1' is counted as a percentage of the total number of secondary batteries 1000 in each group, represented by Q, and the experimental results are recorded in the following Table 1.

[0132] Table 1 It can be seen from Table 1 that in Examples 1-5, the secondary battery 1000 satisfies that the length of the empty foil area of ​​the positive electrode sheet is less than or equal to 2πR1', and the length of the overlapping part of the first area and the easy-to-loose area is greater than or equal to 4πR1' / 3. Based on this, the number of secondary batteries 10000 in Examples 1-5 that satisfy the relationship of R1=R1'±5%R1' is significantly higher than that in Comparative Examples 1-3. For the secondary battery 1000 that satisfies this relationship, there is no significant difference between its winding radius and the winding needle radius, and it can be considered that the secondary battery is basically not loose. It can be seen that the use of the secondary battery provided by the embodiment of the present application is conducive to reducing the possibility of part of the first electrode sheet, part of the isolation membrane and part of the second electrode sheet being released from the stacked state, and is conducive to improving the cycle performance and safety of the secondary battery.

[0133] It should be noted that, based on the aforementioned experiments, the winding radius R1 of the secondary battery 1000 designed by the inventive concept of the present application is substantially consistent with the winding needle radius R1'. Therefore, in the embodiments of the present application, the winding radius R1 is used as the basis for measuring the length of the first empty foil area, the length of the easily loose area, and the length of the overlapping portion between the first area and the easily loose area.

[0134] On the basis of the above experiments, the inventors of the present application further explored the effect of the parameters related to the first recess 14 on reducing the internal stress of the electrode assembly 100. This part of the experiment includes 30 groups of embodiments, which are marked as Embodiment 6 to Embodiment 35, and each group of embodiments includes 20 secondary batteries 1000. Among them, Embodiment 6 is composed of 20 secondary batteries 1000 randomly selected from Embodiment 1.

[0135] The preparation process of the secondary battery 1000 in Examples 7-19 is basically the same as that in Example 1, except that the relevant parameters of the first recess 14 are different. The relevant parameters of the first recess 14 in Examples 6-19 are recorded in Table 2.

[0136] The preparation process of the secondary battery 1000 in Examples 20-35 is basically the same as that in Example 1, the difference is that the positive electrode sheet in the secondary battery 1000 in Examples 20-35 is not pressed to form the first convex portion 15 and the first concave portion 14, but a plurality of first concave portions 14 are etched on the first active material layer 12 using a laser. The relevant parameters of the first concave portion 14 in the secondary battery 1000 in Examples 20-35 are recorded in Table 2.

[0137] After the secondary batteries 1000 in Examples 6-35 were prepared, each group of 20 secondary batteries 1000 was subjected to a long cycle test. The test process was as follows: 1) Maintain the test temperature at 25°C; 2) Let the secondary battery 1000 stand for 30 minutes; 3) 5C constant current charging to 4.25V, then constant voltage charging to 3C; 4) 3C constant current charging to 4.35V, then constant voltage charging to 1.5C; 5) 1.5C constant current charge to 4.45V, then constant voltage charge to 0.05C; 6) Let stand for 5 minutes; 7) 0.7C constant current discharge to 3V, record the first discharge cycle capacity; 8) Let stand for 5 minutes; 9) Repeat steps 3 to 8 500 times; 10) The electrode assembly 100 was observed using a computed tomography (CT) scanner, and the winding radius R1 of the electrode assembly 100 and the outer diameter R of the electrode assembly 100 were measured, and the value of R1 / R was calculated and recorded in Table 2. The value of R1 / R is used to characterize the looseness of the electrode assembly 100. The larger the value of R1 / R, the higher the looseness of the electrode assembly 100, and the smaller the value of R1 / R, the smaller the looseness of the electrode assembly 100.

[0138] Table 2 Note: In Table 2, “ / ” indicates that there is no such data. The units of L1, W1 and D5 are mm, and the units of S1 and S are mm. 2 , the unit of T1 is μm.

[0139] In Examples 7-10 and 21-24, the secondary battery 1000 satisfies 0.4≤L1 / L. Compared with Example 6, the value of R1 / R of the secondary battery 1000 in Examples 7-10 is smaller; compared with Example 20, the value of R1 / R of the secondary battery 1000 in Examples 21-24 is smaller. It can be seen that by setting L1≥2L / 5, the length of the first region 10c along the winding direction of the electrode assembly 100 is not too small, and the reduced internal stress is not too small, which is conducive to reducing the possibility of the first pole piece 10, the separator 30 and the second pole piece 20 located in the inner circle of the electrode assembly 100 being released from the stacked state, thereby reducing the possibility of black spots and lithium precipitation in the secondary battery 1000 in a long cycle. Compared with Example 23, the value of R1 / R of the secondary battery 1000 in Example 24 is increased. The reason is that for the solution of removing part of the active material to obtain the first recess 14, the removal of the active material has a certain influence on the friction between the first pole piece 10 and the isolation film 30. Specifically, as the length of the first region 10c increases, the number of the first recesses 14 increases, the friction between the first pole piece 10 and the isolation film 30 decreases, and the looseness of the electrode assembly 100 increases. Therefore, by setting L1≤3L / 5, the length of the first region 10c along the winding direction of the electrode assembly 100 is not too large, which is conducive to reducing the influence of the setting of the first region 10c on the interface performance of the electrode assembly 100; and, for the solution of removing part of the active material to obtain the first recess 14, setting L1≤3L / 5 can also maintain the friction between the first pole piece 10 and the isolation film 30 within an appropriate range, reducing the degree and possibility of looseness of the electrode assembly 100.

[0140] In Example 8 and Examples 12-15, the secondary battery 1000 satisfies 0.06S≤S1. Compared with Example 11, the value of R1 / R of the secondary battery 1000 in Example 8 and Examples 12-15 is smaller. It can be seen that for the scheme of pressing out the first recess 14, setting S1≥0.06S, the distribution of the first recess 14 in the first area 10c is not too sparse, which is conducive to improving the effect of the first recess 14 on reducing the internal stress of the first pole piece 10; on this basis, setting S1≤0.5S, the distribution of the first recess 14 in the first area 10c is not too dense, which is conducive to reducing the influence of the setting of the first recess 14 on the degree of electrolyte infiltration of the first pole piece 10.

[0141] In Example 8 and Example 17-18, the secondary battery 1000 satisfies 1μm≤T1≤10μm. Compared with Example 16 and Example 19, the value of R1 / R of the secondary battery 1000 in Example 8 and Example 17-18 is smaller. It can be seen that by setting T1≥1μm, the height of the first convex portion 15 is not too small, and the depth of the first concave portion 14 is not too small, which is conducive to improving the effect of the first concave portion 14 on reducing the internal stress of the electrode assembly 100; by setting T1≤10μm, the height of the first convex portion 15 is not too large, and the concave portion is conducive to reducing the effect of the setting of the first convex portion 15 on the bonding force between the first pole piece 10 and the isolation film 30.

[0142] In Example 22 and Examples 26-29, the secondary battery 1000 satisfies 0.04S≤S1. Compared with Example 25, the value of R1 / R of the secondary battery 1000 in Example 22 and Examples 26-29 is smaller. It can be seen that for the scheme of etching the first recess 14, setting S1≥0.04S, the distribution of the first recess 14 in the first area 10c is not too sparse, which is conducive to improving the effect of the first recess 14 on reducing the internal stress of the first pole piece 10; on this basis, setting S1≤0.5S, the distribution of the first recess 14 in the first area 10c is not too dense, which is conducive to reducing the influence of the setting of the first recess 14 on the degree of electrolyte infiltration of the first pole piece 10.

[0143] In Examples 32 to 35, the secondary battery 1000 satisfies 0.2mm≤D5≤0.5mm. Compared with Examples 31 and 32, the value of R1 / R of the secondary battery 1000 in Examples 32-35 is smaller. It can be seen that by setting D5≤0.5mm, the spacing between the first recesses 14 is not too small, which is beneficial to improve the effect of reducing internal stress. On this basis, by setting D5≥0.2mm, the spacing between the first recesses 14 is not too small, and the distribution of the first recesses 14 in the first pole piece 10 is not too dense, which is beneficial to improve the capacity of the electrode assembly 100.

[0144] In Examples 32 to 35, the secondary battery 1000 satisfies 90≤N≤190. Compared with Examples 30 and 31, the value of R1 / R of the secondary battery 1000 in Examples 32-35 is smaller. It can be seen that when N≥90 is set, the distribution of the first recess 14 on the first pole piece 10 is not too sparse, which is conducive to improving the effect of the first recess 14 releasing the stress on the first pole piece 10; when N≤190 is set, the distribution of the first recess 14 on the first pole piece 10 is not too dense, which is conducive to improving the capacity of the electrode assembly 100.

[0145] In Examples 32 to 35, the secondary battery 1000 satisfies 0.09mm≤W1≤0.2mm. Compared with Examples 30 and 31, the value of R1 / R of the secondary battery 1000 in Examples 32-35 is smaller. It can be seen that by setting W1≥0.09mm, the width of the first recess 14 is not too small, which is conducive to improving the effect of the first recess 14 on reducing the internal stress of the first pole piece; by setting W1≤0.2mm, the width of the first recess 14 is not too large, and the active material removed is not too much. When the first pole piece 10 is a negative pole piece, it is conducive to maintaining the CB of the electrode assembly 100 and reducing the possibility of lithium deposition in the electrode assembly 100 during the cycle. When the first pole piece 10 is a positive pole piece, it is conducive to improving the capacity of the electrode assembly 100.

[0146] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present application and are not intended to be limiting of the present application. As long as they are within the spirit and scope of the present application, appropriate changes and modifications to the above embodiments are within the scope of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly comprising a pole piece and a separator, the pole piece comprising a first pole piece and a second pole piece, the first pole piece and the second pole piece having opposite polarities, the first pole piece, the separator and the second pole piece being stacked and wound into a cylindrical structure, characterized in that: The first pole piece includes a first current collector, a first active material layer and a second active material layer, wherein the first active material layer and the second active material layer are respectively arranged on two surfaces of the first current collector along a first direction, and the first direction is a thickness direction of the first pole piece; Along the winding direction of the electrode assembly, the distance from the first active material layer to the winding starting end of the first electrode sheet is not greater than the distance from the second active material layer to the winding starting end of the first electrode sheet; Observed along the winding center axis direction of the electrode assembly, the winding radius of the electrode assembly is R1, where R1 is the difference between the outer diameter of the electrode assembly and the average thickness of the electrode assembly along the radial direction; The first electrode sheet has an easy-to-loose area, the first active material layer has a first edge, the first edge is located at the winding start end of the first active material layer, the first edge is an edge line of the easy-to-loose area, and the length of the easy-to-loose area from the first edge along the winding direction of the electrode assembly is 2πR1; The first pole sheet includes a first empty foil area, the first current collector of the first empty foil area is not covered by the first active material layer and the second active material layer, the first empty foil area is located at the winding start end of the first pole sheet, and along the winding direction of the electrode assembly, the length of the first empty foil area is less than or equal to 2πR1; The first active material layer has a first region, the first region is provided with a plurality of first recesses, at least part of the first region overlaps with the easy-to-loose region, and along the winding direction of the electrode assembly, the length of the overlapping portion between the first region and the easy-to-loose region is D, and D≥4πR1 / 3.

2. The secondary battery according to claim 1, wherein: Along the winding direction of the electrode assembly, the length of the first pole sheet is L, the length of the first region is L1, and 2L / 5≤L1≤3L / 5.

3. The secondary battery according to claim 1, wherein: The first electrode sheet includes a first edge and a second edge which are arranged opposite to each other along the winding center axis direction of the electrode assembly; the first active material layer and the second active material layer both extend to the first edge and the second edge; A distance from the first region to the first edge along the winding center axis of the electrode assembly is D1, 1 mm ≤ D1 ≤ 7 mm; and / or A distance from the first region to the second edge along the winding center axis of the electrode assembly is D2, and 1 mm ≤ D2 ≤ 7 mm.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: Part of the first pole piece is recessed along the first direction to form a plurality of first protrusions and a plurality of first recesses, the first protrusions protrude from the non-recessed and deformed surface of the first pole piece, and the first recesses and the first protrusions are correspondingly arranged along the first direction.

5. The secondary battery according to claim 4, characterized in that: The first pole piece includes a double-sided coating area, in which the first current collector is covered by the first active material layer and the second active material layer at the same time, and the first region is located in the double-sided coating area; the double-sided coating area includes a third edge and a fourth edge that are arranged opposite to each other along the winding direction of the first pole piece; The distance from the first region to the third edge along the winding direction of the first pole piece is L2, L2 ≥ 1 mm; and / or A distance from the first region to the fourth edge along the winding direction of the first pole piece is L3, and L3≥1 mm.

6. The secondary battery according to claim 4, characterized in that: Part of the second pole piece is recessed along the first direction to form a plurality of second convex portions and a plurality of second concave portions, the second convex portions protrude from the non-recessed and deformed surface of the second pole piece, and the second concave portions and the second convex portions are correspondingly arranged along the first direction; At least a portion of some of the plurality of first protrusions is received in the second recess.

7. The secondary battery according to claim 4, characterized in that: When the first pole piece is flattened, a projection area of ​​the first region along the first direction is S, a total projection area of ​​the plurality of first recesses along the first direction is S1, and 0.06S≤S1≤0.5S.

8. The secondary battery according to claim 4, characterized in that: A height of the first protrusion protruding from the surface of the first pole piece along the first direction is T1, and 1 μm≤T1≤10 μm.

9. The secondary battery according to claim 4, characterized in that: Two adjacent first protrusions are spaced apart from each other; The distance between two adjacent first protrusions along the winding direction of the first pole piece is D3, 1.5 mm ≤ D3 ≤ 3 mm; The distance between two adjacent first protrusions along the winding center axis direction of the electrode assembly is D4, and 1.5 mm≤D4≤3 mm.

10. The secondary battery according to any one of claims 1 to 3, characterized in that: The first recessed portion is formed by removing a portion of material from the first active material layer.

11. The secondary battery according to claim 10, characterized in that: In the first region, along the winding direction of the electrode assembly, in the first pole sheet with a length of 1 mm, the number of the first recesses is N, and 90≤N≤190.

12. The secondary battery according to claim 10, characterized in that When the first pole piece is flattened, a projection area of ​​the first region along the first direction is S, a total projection area of ​​the plurality of first recesses along the first direction is S1, and 0.04S≤S1≤0.5S.

13. The secondary battery according to claim 12, characterized in that: 0.3S≤S1≤0.5S.

14. The secondary battery according to claim 10, characterized in that: The first recess extends along the winding center axis direction of the electrode assembly, and the multiple first recesses are arranged at intervals along the winding direction of the electrode assembly. The distance between two adjacent first recesses along the winding direction of the first pole piece is D5, and 0.2mm≤D5≤0.5mm.

15. The secondary battery according to claim 10, wherein: A depth of the first recess along the first direction is H1, 1 μm≤H1≤50 μm.

16. The secondary battery according to claim 10, characterized in that: The width of the first recess along the winding direction of the first pole piece is W1, 0.09 mm≤W1≤0.2 mm.

17. The secondary battery according to claim 1, wherein: The first electrode sheet is a negative electrode sheet, the second electrode sheet is a positive electrode sheet, and the first empty foil area is located at the innermost layer of the electrode assembly; The second pole piece includes a second current collector, a third active material layer and a fourth active material layer, wherein the third active material layer and the fourth active material layer are respectively arranged on two surfaces of the second current collector along the first direction; Along the first direction, the third active material layer faces the first active material layer, and the projection of the third active material layer is located within the range of the first active material layer. The fourth active material layer faces the second active material layer, and the projection of the fourth active material layer is located within the range of the second active material layer.

18. An electrical equipment, characterized in that: Includes the secondary battery according to any one of claims 1 to 17.

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