Secondary battery and electric device

Abstract

The application discloses a secondary battery and an electric device. The secondary battery comprises an electrode assembly, the electrode assembly comprises a first pole piece, a second pole piece and a separator, the first pole piece and the second pole piece are opposite in polarity, and the first pole piece, the separator 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 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 a 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 beneficial to reducing the possibility that the first pole piece, the separator and the second pole piece are partially disengaged from the stacked state, and is beneficial to improving the cycle performance and safety of the secondary battery.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of energy storage, and particularly relates to a secondary battery and an electric device. BACKGROUND

[0002] The electrode assembly of the cylindrical secondary battery includes a positive electrode sheet, a negative electrode sheet and a separator, which are laminated and then wound into a cylindrical shape by a winding needle. During the winding process, the active material layer of the electrode sheet (the positive electrode sheet and the negative electrode sheet) facing the winding center is compressed due to the bending deformation of the electrode sheet, and the active material layer away from the winding center is stretched 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 the internal stress, making it difficult to maintain the partial laminated structure of the positive electrode sheet, the separator and the negative electrode sheet (as shown in FIG. 1). Figure 1 After long cycles, the inner circle of the electrode assembly is prone to have black spots and lithium precipitation, which adversely affects the service life of the secondary battery. SUMMARY

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

[0004] A first aspect of embodiments of the present application provides a secondary battery, comprising an electrode assembly, the electrode assembly comprising a tab and a separator, the tab comprising a first tab and a second tab, the first tab and the second tab being opposite in polarity, the first tab, the separator and the second tab being sequentially stacked and wound to form a cylindrical structure. The first tab comprises 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 being respectively arranged on two surfaces of the first current collector along a first direction, the first direction being a thickness direction of the first tab. Along a winding direction of the electrode assembly, a distance from the first active material layer to a winding start end of the first tab is not greater than a distance from the second active material layer to the winding start end of the first tab; the electrode assembly comprises a winding radius R1 observed along a winding central axis direction of the electrode assembly, the winding radius R1 being a difference between an outer diameter of the electrode assembly and an average thickness of the electrode assembly. The first tab has a loose region, the first active material layer has a first edge, the first edge being located at a winding start end of the first active material layer, the first edge being one edge line of the loose region, and a length of the loose region along the winding direction of the electrode assembly is 2πR1 from the first edge. The first tab comprises a first empty foil region, the first current collector of the first empty foil region not being covered by the first active material layer and the second active material layer, the first empty foil region being located at the winding start end of the first tab, and a length of the first empty foil region along the winding direction of the electrode assembly is less than or equal to 2πR1. The first tab has a first region, the first region being provided with a plurality of first recesses, at least part of the first region overlapping with the loose region, and a length of an overlapping part of the first region and the loose region along the winding direction of the electrode assembly is D, D≥4πR1 / 3.

[0005] In the secondary battery, by arranging the plurality of first recesses on at least part of the loose region of the first tab, the internal stress generated in the winding process of the first tab is reduced, thereby reducing the possibility of the part of the first tab, the part of the separator and the part of the second tab being out of the stacked state, and the cycle performance and the safety of the secondary battery are improved.

[0006] In an optional embodiment of the present application, along the winding direction of the electrode assembly, a length of the first tab is L, a 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, and thus the possibility of the first tab, the separator and the second tab located in the inner circle of the electrode assembly being out of the stacked state is reduced, thereby reducing the possibility of the secondary battery appearing black spots and lithium precipitation in long cycle; by setting L1≤3L / 5, the length of the first region along the winding direction of the electrode assembly is not too large, and the influence of the arrangement of the first region on the interface performance of the electrode assembly is reduced.

[0007] In an optional embodiment of the present application, the first electrode tab includes a first edge and a second edge oppositely arranged along a winding central 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. A distance of the first region to the first edge along the winding central axis direction of the electrode assembly is D1, and 1mm≤D1≤7mm. D1 is set to be greater than or equal to 1mm, so that the distance of the first region to the first edge is not too small, which is conducive to reducing the influence of the first recess on the interface performance of the electrode assembly; D1 is set to be less than or equal to 7mm, which is conducive to increasing the area of the first region, thereby improving the effect of the first region on reducing the internal stress of the first electrode tab.

[0008] In an optional embodiment of the present application, a distance of the first region to the second edge along the winding central axis direction of the electrode assembly is D2, and 1mm≤D2≤7mm. D2 is set to be greater than or equal to 1mm, so that the distance of the first region to the first edge is not too small, which is conducive to reducing the influence of the first recess on the interface performance of the electrode assembly; D2 is set to be less than or equal to 7mm, which is conducive to increasing the area of the first region, thereby improving the effect of the first region on reducing the internal stress of the first electrode tab.

[0009] In an optional embodiment of the present application, a part of the first electrode tab 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 surface of the first electrode tab which is not deformed by the recessing, and the first recesses and the first protrusions are correspondingly arranged along the first direction. The first protrusions are arranged, which is conducive to improving the adhesion between the first electrode tab and the separator and reducing the possibility that the first electrode tab and the separator are separated from the laminated structure due to internal stress.

[0010] In an optional embodiment of the present application, the first electrode tab includes a double-sided coating region, 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 region; the double-sided coating region includes a third edge and a fourth edge oppositely arranged along the winding direction of the first electrode tab. A distance of the first region to the third edge along the winding direction of the first electrode tab is L2, and L2≥1mm. In this way, the distance of the first region to the third edge is not too small, which is conducive to reducing the possibility that the active material of the edges of the first active material layer and the second active material layer falls off during the pressing process of the first electrode tab.

[0011] In an optional embodiment of the present application, a distance of the first region to the fourth edge along the winding direction of the first electrode tab is L3, and L3≥1mm. In this way, the distance of the first region to the fourth edge is not too small, which is conducive to reducing the possibility that the active material of the edges of the first active material layer and the second active material layer falls off during the pressing process of the first electrode tab.

[0012] In an optional embodiment of the present application, the second tab is recessed in the first direction to form a plurality of second protrusions and a plurality of second recesses, the second protrusions protrude from the surface of the second tab that is not deformed by the recessing, and the second recesses and the second protrusions are arranged correspondingly in the first direction; at least part of the plurality of first protrusions are accommodated in the second recesses. The accommodation of at least part of the first protrusions in the second recesses is conducive to reducing the total thickness of the first tab and the second tab after they are laminated, and improving the energy density of the secondary battery.

[0013] In an optional embodiment of the present application, in the flattened state of the first tab, the projected area of the first region in the first direction is S, the total projected area of the plurality of first recesses in the first direction is S1, and 0.06S≤S1≤0.5S. The setting of S1≥0.06S prevents the distribution of the first recesses in the first region from being too sparse, which is conducive to improving the effect of the first recesses in reducing the internal stress of the first tab; the setting of S1≤0.5S prevents the distribution of the first recesses in the first region from being too dense, which is conducive to reducing the influence of the setting of the first protrusions on the structural strength of the first current collector on the one hand, and reducing the influence of the setting of the first protrusions on the degree of impregnation of the first tab by the electrolyte on the other hand.

[0014] In an optional embodiment of the present application, the height of the first protrusion protruding from the surface of the first tab that is not deformed by the recessing in the first direction is T1, and 1μm≤T1≤10μm. The setting of T1≥1μm prevents the height of the first protrusion from being too small and the depth of the first recess from being too small, which is conducive to improving the effect of the first recesses in reducing the internal stress of the electrode assembly; the setting of T1≤10μm prevents the height of the first protrusion from being too large, which is conducive to reducing the influence of the setting of the first protrusions on the adhesion between the first tab and the separator, and reducing the possibility of damage to the first tab during the pressing of the first tab.

[0015] In an optional embodiment of the present application, the distance between any two adjacent first protrusions is D3, and 1.5mm≤D3≤3mm. The setting of D3≥1.5mm prevents the distance between the two adjacent first protrusions from being too small, which is conducive to reducing the possibility of damage to the first tab during the pressing of the first tab; the setting of D3≤3mm prevents the distance between the two adjacent first protrusions from being too large, which is conducive to increasing the number of first recesses and thus 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 central axis direction of the electrode assembly is D4, and 1.5 mm≤D4≤3 mm. By setting D4≥1.5 mm, the distance between two adjacent first protrusions is not too small, which is conducive to reducing the possibility of damage to the first electrode sheet during pressing of the first electrode sheet. By setting D4≤3 mm, the distance between two adjacent first protrusions is not too large, which is conducive to increasing the number of first recesses and thus improving the effect of the first recesses on reducing the internal stress of the electrode assembly.

[0017] In an optional embodiment of the present application, the first recess is formed by removing part of the material of the first active material layer. By this way of arranging the first recess, on the one hand, the maximum static friction between the first electrode sheet and the separator can be increased, and the possibility of relative displacement between the first electrode sheet and the separator can be reduced. On the other hand, the electrolyte can be stored, and the cycle performance of the secondary battery can be improved.

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

[0019] In an optional embodiment of the present application, in the flattened state of the first electrode sheet, the projected area of the first region along the first direction is S, the total projected area of the plurality of first recesses along the first direction is S1, and 0.04S≤S1≤0.5S. By setting S1≥0.04S, the distribution of the first recesses in the first region is not too sparse, which is conducive to improving the effect of the first recesses on reducing the internal stress of the first electrode sheet. By setting S1≤0.5S, the distribution of the first recesses in the first region is not too dense, which is conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium precipitation in the electrode assembly during the cycle process when the first electrode sheet is a negative electrode sheet. When the first electrode sheet is a positive electrode sheet, the capacity of the electrode assembly can be improved.

[0020] In an optional embodiment of the present application, 0.3S≤S1≤0.5S. By setting S1≥0.3S, the distribution of the first recesses in the first region is not too sparse, which is conducive to further improving the effect of the first recesses on reducing the internal stress of the first electrode sheet.

[0021] In an optional embodiment of the present application, the first recesses extend along the winding central axis direction of the electrode assembly, and the plurality of first recesses are arranged at intervals along the winding direction of the electrode assembly. The distance between any two adjacent first recesses along the winding direction of the first electrode tab is D5, and 0.2 mm≤D5≤0.5 mm. By setting D5≥0.2 mm, the interval between the first recesses is not too small, and the distribution of the first recesses on the first electrode tab is not too dense. When the first electrode tab is a negative electrode tab, this is conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium precipitation of the electrode assembly during the cycle process. When the first electrode tab is a positive electrode tab, this is conducive to improving the capacity of the electrode assembly. By setting D5≤0.5 mm, the interval between the first recesses is not too large, which is conducive to improving 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, and 1 μm≤H1≤50 μm. By setting H1≥1 μm, the depth 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 electrode tab. By setting H1≤50 μm, the depth of the first recess is not too large, and the removed active material is not too much. When the first electrode tab is a negative electrode tab, this is conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium precipitation of the electrode assembly during the cycle process. When the first electrode tab is a positive electrode tab, this 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 electrode tab is W1, and 0.09 mm≤W1≤0.2 mm. By setting W1≥0.09 mm, 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 electrode tab. By setting W1≤0.2 mm, the width of the first recess is not too large, and the removed active material is not too much. When the first electrode tab is a negative electrode tab, this is conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium precipitation of the electrode assembly during the cycle process. When the first electrode tab is a positive electrode tab, this is conducive to improving the capacity of the electrode assembly.

[0024] In an optional embodiment of the present application, the first electrode tab is a negative electrode tab, and the second electrode tab is a positive electrode tab. The first hollow foil region is located at the innermost layer of the electrode assembly. The second electrode tab comprises a second current collector, a third active material layer and a fourth active material layer. 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 is towards the first active material layer, and the projection of the third active material layer is within the range of the first active material layer. The fourth active material layer is towards the second active material layer, and the projection of the fourth active material layer is within the range of the second active material layer. In this way, the CB of the electrode assembly can be maintained, and the possibility of lithium precipitation of the electrode assembly during the cycle process can be reduced. In addition, since part of the first electrode tab is located at the innermost layer of the electrode assembly, and the first recess is arranged on the first electrode tab, the possibility of the first recess further reducing the delamination of part of the first electrode tab, part of the separator and part of the second electrode tab can be improved.

[0025] A second aspect of the embodiments of the present application provides a consumer device comprising the secondary battery according to any one of the preceding embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a structure schematic diagram of the partial delamination state of the positive electrode tab, the separator and the negative electrode tab of the inner ring of the electrode assembly.

[0027] Figure 2 is a structure schematic diagram of the secondary battery in an embodiment of the present application.

[0028] Figure 3 is Figure 2 is a cross-sectional structure schematic diagram at III-III in FIG. 1.

[0029] Figure 4 is a structure schematic diagram of the first electrode tab in the flattened state in an embodiment of the present application.

[0030] Figure 5 is Figure 4 is a cross-sectional structure schematic diagram at V-V in FIG. 2.

[0031] Figure 6 is a structure schematic diagram of the first electrode tab in the flattened state in an embodiment of the present application.

[0032] Figure 7 is a structure schematic diagram of the first electrode tab in the flattened state in an embodiment of the present application.

[0033] Figure 8 is a structure schematic diagram of the first electrode tab in the flattened state in an embodiment of the present application.

[0034] Figure 9 is Figure 8 is a cross-sectional structural schematic view at IX-IX in FIG.

[0035] Figure 10 is a structural schematic view of the second tab in a flattened state in an embodiment of the present application.

[0036] Figure 11 is Figure 10 is a cross-sectional structural schematic view at XI-XI in FIG.

[0037] Figure 12 is a structural schematic view of an electrical device in an embodiment of the present application.

[0038] Main element symbol explanation

[0039] 1000, secondary battery; 100, electrode assembly; 200, case; 201, main body portion; 202, pole;

[0040] 10, first tab; 11, first current collector; 111, first empty-foil region; 12, first active material layer; 121, first edge; 13, second active material layer; 14, first recessed portion; 15, first protruding portion; 16, third recessed portion; 10a, single-sided coating region; 10b, double-sided coating region; 10c, first region; 10d, first edge; 10e, second edge; 10f, third edge; 10g, fourth edge;

[0041] 20, second tab; 21, second current collector; 22, third active material layer; 23, fourth active material layer; 24, second recessed portion; 25, second protruding portion; 20a, second region;

[0042] 30, separator film;

[0043] 10000, electrical device;

[0044] X, first direction; Y, winding direction of the electrode assembly; Z, winding central axis direction. DETAILED DESCRIPTION

[0045] The technical solutions in the embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments.

[0046] It should be noted that when an element is considered to be “connected” to another element, it can be directly connected to the other element or a middle element can exist at the same time. When an element is considered to be “provided” on another element, it can be directly provided on the other element or a middle element can exist at the same time.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application.

[0048] In the description of the embodiments of the present application, the technical terms "first", "second", and the like 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 "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0049] In the description of the embodiments of the present application, the term "vertical" is used to describe the ideal state between two components. In the actual production or use state, there can be an approximate vertical state between the two components. The two components described as "vertical" can not be an absolute straight line, plane, but can be approximately straight or planar, and the overall extension direction can be considered as a straight line or a plane.

[0050] The term "parallel" is used to describe the ideal state between two components. In the actual production or use state, there can be an approximate parallel state between the two components. The two components described as "parallel" can not be an absolute straight line, plane, but can be approximately straight or planar, and the overall extension direction can be considered as a straight line or a plane.

[0051] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily mutually exclusive of one another. Various features that are described in the specification can be combined with each other without limitation.

[0052] Embodiments of this application provide a secondary battery, including an electrode assembly. The electrode assembly includes an electrode sheet and a separator. The electrode sheet includes a first electrode sheet and a second electrode sheet, which have opposite polarities. The first electrode sheet, the separator, and the second electrode sheet are stacked and wound to form a cylindrical structure. The first electrode sheet includes a first current collector, a first active material layer, and a second active material layer. The first and second active material layers are respectively disposed on two surfaces of the first current collector along a first direction, which is the thickness direction of the first electrode sheet. Along the winding direction of the electrode assembly, the distance from the first active material layer to the winding start end of the first electrode sheet is not greater than the distance from the second active material layer to the winding start end of the first electrode sheet. Viewed along the winding center axis of the electrode assembly, the electrode assembly includes a winding radius R1, which is the difference between the outer diameter of the electrode assembly and the average thickness of the electrode assembly. The first electrode has a loose region, and the first active material layer has a first edge located at the starting end of the winding of the first active material layer. The first edge is a side line of the loose region, and the length of the loose region along the winding direction of the electrode assembly is 2πR1. The first electrode includes a first empty foil region, in which a first current collector is not covered by the first active material layer and the second active material layer. The first empty foil region is located at the starting end of the winding of the first electrode, and the length of the first empty foil region along the winding direction of the electrode assembly is less than or equal to 2πR1. The first electrode has a first region, which has a plurality of first recesses. At least a portion of the first region overlaps with the loose region, and the length of the overlapping portion of the first region and the loose region along the winding direction of the electrode assembly is D, where D ≥ 4πR1 / 3.

[0053] In this secondary battery, by providing at least a plurality of first recesses in the easily loosened area of ​​the first electrode, it is beneficial to reduce the internal stress generated by the first electrode during the winding process, thereby reducing the possibility of partial delamination of the first electrode, the separator and the second electrode, and improving the cycle performance and safety of the secondary battery.

[0054] The embodiments of this application will be further described below with reference to the accompanying drawings. In the drawings, the thickness direction of the first electrode is taken as the first direction X, the winding direction Y of the electrode assembly corresponds to the length direction of the first and second electrodes in the flattened state, the winding direction of the first electrode corresponds to the length direction of the first electrode in the flattened state, and the winding center axis direction Z of the electrode assembly corresponds to the width direction of the first and second electrodes in the flattened state.

[0055] like Figure 2 and Figure 3 As shown, an embodiment of this application provides a secondary battery 1000, including a housing 200 and an electrode assembly 100, the electrode assembly 100 being housed within the housing 200.

[0056] 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, and in yet other embodiments, a metal shell including at least one of a steel alloy, an aluminum alloy, and a copper alloy.

[0057] In some embodiments, as shown in FIG. 1, the electrode assembly 100 includes a first electrode tab 10 and a second electrode tab 20, the first electrode tab 10 and the second electrode tab 20 being opposite in polarity, the first electrode tab 10, the separator 30, and the second electrode tab 20 being stacked and wound into a cylindrical structure. Figure 3

[0058] In some embodiments, as shown in FIG. 2, the housing 200 is a hard shell, the housing 200 including a main body 201 and a pole 202, the pole 202 being connected to the main body 201 and insulated from the main body 201, one of the first electrode tab 10 and the second electrode tab 20 of the electrode assembly 100 being electrically connected to the main body 201, and the other being electrically connected to the pole 202. Figure 2

[0059] In some embodiments, as shown in FIG. 3, the first electrode tab 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 being respectively disposed on two surfaces of the first current collector 11 along a first direction X, the first direction X being a thickness direction of the first electrode tab 10. Figure 3

[0060] In some embodiments, as shown in FIG. 4, the second electrode tab 20 includes a second current collector 21, a third active material layer 22, and a fourth active material layer 23, the third active material layer 22 and the fourth active material layer 23 being respectively disposed on two surfaces of the second current collector 21 along the first direction X. Figure 3

[0061] In some embodiments, the first electrode tab 10 is a positive electrode tab, the first current collector 11 is a positive current collector, and the first active material layer 12 and the second active material layer 13 are positive active material layers. The second electrode tab 20 is a negative electrode tab, the second current collector 21 is a negative current collector, and the third active material layer 22 and the fourth active material layer 23 are negative active material layers. In other embodiments, the first electrode tab 10 is a negative electrode tab, the first current collector 11 is a negative current collector, and the first active material layer 12 and the second active material layer 13 are negative active material layers. The second electrode tab 20 is a positive electrode tab, the second current collector 21 is a positive current collector, and the third active material layer 22 and the fourth active material layer 23 are positive active material layers.

[0062] ​​​​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 can be a metal layer including at least one of aluminum, nickel, tantalum, titanium, such as an aluminum foil. The negative current collector can be a metal layer including at least one of copper, nickel, tantalum, titanium, such as a copper foil. In other embodiments, at least one of the positive current collector and the negative current collector is a composite current collector.

[0063] In some embodiments, the positive active material includes at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium iron phosphate, lithium manganese iron phosphate, or lithium manganate.

[0064] In some embodiments, the negative active material includes at least one of graphite, hard carbon, soft carbon, silicon, silicon-oxygen material, silicon-carbon material.

[0065] In some embodiments, the separator 30 is an insulating film material such as a polyethylene film, a polypropylene film, a polyester film, or a polyimide film.

[0066] In some embodiments, the secondary battery 1000 further includes an electrolyte (not shown) that is housed in the case 200.

[0067] 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.

[0068] In some embodiments, the electrolyte salt includes, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium hexafluorocerate (LiCsF6), lithium perchlorate (LiClO4), or lithium trifluoromethanesulfonate (LiCF3SO3).

[0069] As shown in FIGS. 1A and 1B, in some embodiments, the first active material layer 12 is disposed between the first electrode tab 10 and the second electrode tab 20. In some embodiments, the second active material layer 13 is disposed between the first electrode tab 10 and the second electrode tab 20. Figure 4 and Figure 5 As shown in FIGS. 1A and 1B, 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 tab 10 is not greater than the distance from the second active material layer 13 to the winding start end of the first electrode tab 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 tab 10 is flush with the edge of the second active material layer 13 facing the winding start end of the first electrode tab 10; or, the edge of the first active material layer 12 facing the winding start end of the first electrode tab 10 is closer to the winding start end of the first electrode tab 10 than the edge of the second active material layer 13 facing the winding start end of the first electrode tab 10.

[0070] In some embodiments, the electrode assembly 100 comprises a winding radius R1 in the direction of the winding central axis Z of the electrode assembly 100, the winding radius R1 being 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 by taking four orthogonal sampling points on the tightly wound part of the electrode assembly 100, measuring the distance from the four sampling points to the outer surface of the electrode assembly 100 in the radial direction of the electrode assembly 100, and then taking the average of the four distances to obtain the average thickness of the electrode assembly 100. Let R represent the outer diameter of the electrode assembly 100, and R2 represent the average thickness of the electrode assembly 100, then R1 = R - R2.

[0071] The first electrode sheet 10 has a loose region, and the first active material layer 12 has a first edge 121 (as shown) at the winding start end of the first active material layer, the first edge 121 being one edge line of the loose region, and the length of the loose region being 2πR1 in the winding direction of the electrode assembly from the first edge 121. Figure 5 The first electrode sheet comprises a first empty foil area 111, the first current collector of the first empty foil area 111 not being covered by the first active material layer 12 and the second active material layer 13, the first empty foil area 111 being located at the winding start end of the first electrode sheet 10, and the length of the first empty foil area 111 being less than or equal to 2πR1 in the winding direction Y of the electrode assembly.

[0072] In the above, 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, with the observation angle facing the winding center axis direction Z. Observe the winding structure of the first electrode 10 and the second electrode 20, and determine which of the innermost first electrode 10 and the second electrode 20 is closer to the electrode assembly. For the sake of explanation, assume it is the first electrode 10. Assume a moving point that starts from the winding start end of the first electrode 10 and moves along the winding direction of the electrode assembly 100 on the surface of the first electrode 10 facing the winding center of the electrode assembly 100. When the moving point is located at the outer position, three consecutive layers of electrodes and the separator sandwiched between these three layers of electrodes are successively attached to the cylindrical structure radially outward, it is considered that the position from that point is the tightly wound portion of the electrode assembly 100. Taking the first electrode 10 as the inner layer electrode of the electrode assembly 100 as an example, the three consecutive electrode layers refer to the portion of the first electrode 10 defined in one circle along the winding direction of the electrode assembly, starting from the first position point, and the portion of the second electrode 20 sandwiched within this circle of the first electrode 10. The standard for determining whether the three consecutive electrode layers and the separator 30 sandwiched between them are in a bonded state is as follows: measure the distance between the first current collector 11 and the second current collector 21 in the CT image, and then compare this distance with the sum of the thickness of the active material layer between the first current collector 11 and the second current collector 12 and the thickness of the separator 30. When the difference does not exceed 5% of the distance, it is considered to be in a bonded state. For the three consecutive electrode layers, a measurement point can be taken every 10° central angle. When all these measurement points meet the aforementioned requirements, it is considered that the three electrode layers and the separator 30 sandwiched within them are in a bonded state.

[0073] 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, it is often possible to visually distinguish the tightly wound portion from the loose portion when observing CT images.

[0074] It should be further noted that in some embodiments of this application, the electrode assembly 100 does not have a loose portion.

[0075] In some embodiments, such as Figure 4 As shown, the first electrode 10 has a first region 10c, the first region 10c is provided with a plurality of first recesses 14, at least a portion of the first region 10c overlaps with the easily loosened region, along the winding direction Y of the electrode assembly, the length of the overlapping portion of the first region 10c and the easily loosened region is D, D≥4πR1 / 3.

[0076] In this secondary battery 1000, by providing at least a plurality of first recesses 14 in the easily loosened area of ​​the first electrode 10, it is beneficial to reduce the internal stress generated by the first electrode 10 during the winding process, thereby reducing the possibility of the first electrode 10, the separator 30 and the second electrode 20 being delaminated, which is beneficial to improving the cycle performance and safety of the secondary battery 1000.

[0077] In the embodiments of this application, the range of the first region 10c is defined by the outermost edges of the plurality of first recesses 14. Specifically, when the first electrode 10 is flattened and viewed along the first direction X, the shape of the first region 10c is rectangular. The edges of the first region 10c are tangent to or overlap with the edges of the first recesses 14 or pass through the vertices of the edges of the first recesses 14, and the four edges of the first region 10c are parallel to the four edges of the surface of the first electrode 10 along the first direction X. Figure 4 , Figure 6 , Figure 7 and Figure 8 The first region 10c is shown in dashed line.

[0078] In some embodiments, such as Figure 4 As shown, along the winding direction Y of the electrode assembly, the length of the first electrode 10 is L, and the length of the first region 10c is L1, where 2L / 5 ≤ L1 ≤ 3L / 5. Setting L1 ≥ 2L / 5 ensures that the length of the first region 10c along the winding direction of the electrode assembly 100 is not too small, and the reduction in internal stress is not too small. This helps reduce the possibility of the first electrode 10, the separator 30, and the second electrode 20 located in the inner ring of the electrode assembly 100 delaminating, thereby reducing the possibility of black spots and lithium plating appearing in the secondary battery 1000 during long cycles. Setting L1 ≤ 3L / 5 ensures that the length of the first region 10c along the winding direction of the electrode assembly 100 is not too large, which helps reduce the impact of the setting of the first region 10c on the interface performance of the electrode assembly 100. The values ​​of L and L1 can be measured using a ruler.

[0079] In some embodiments, at least a portion of the openings of the first recesses 14 are oriented along the first direction X toward the winding center of the electrode assembly 100. This helps to reduce the internal stress generated by the compression of one of the first active material layer 12 and the second active material layer 13 after the first electrode 10 is wound.

[0080] In some embodiments, at least a portion 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 to reduce the internal stress generated in the first active material layer 12 and the second active material layer 13 under tension after the first electrode 10 is wound.

[0081] In some embodiments, such asFigure 5 As shown, the first tab 10 includes a single-coated region 10a and a double-coated region 10b. In the single-coated region 10a, only a part of the surface of the first current collector 11 along the first direction X is covered by the first active material layer 12, and in the double-coated region 10b, the first current collector 11 is covered by both the first active material layer 12 and the second active material layer 13.

[0082] In some embodiments, at least a part of the first region 10c is located in the single-coated region 10a.

[0083] In some embodiments, the first region 10c is entirely located in the double-coated region 10b.

[0084] In some embodiments, as shown, Figure 5 The part of the first tab 10 is recessed along the first direction X to form a plurality of first protrusions 15 and a plurality of first recesses 14, the first protrusions 15 protrude from the surface of the first tab 10 that is not deformed by recessing, and the first recesses 14 and the first protrusions 15 are correspondingly arranged along the first direction X. The first protrusions 15 are arranged to facilitate improving the adhesion between the first tab 10 and the isolation film 30 and reducing the possibility of the first tab 10 and the isolation film 30 being separated from the laminated structure due to internal stress.

[0085] In some embodiments, the first recesses 14 and the first protrusions 15 are formed by pressing with an embossing roller.

[0086] In some embodiments, as shown, Figure 4 and Figure 5 The first protrusions 15 are semispherical in shape.

[0087] In some embodiments, as shown, Figure 6 The first protrusions 15 are strips, and the plurality of first protrusions 15 are arranged in an interval.

[0088] In some embodiments, as shown, Figure 7 The first protrusions 15 are strips, and the plurality of first protrusions 15 intersect to form a grid shape.

[0089] In some embodiments, as shown, Figure 4 The first region 10c is located in the double-coated region 10b, and the double-coated region 10b includes a third edge 10f and a fourth edge 10g that are oppositely arranged along the winding direction of the first tab 10. The distance from the first region 10c to the third edge 10f along the winding direction of the first tab 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 facilitates reducing the possibility of the active material of 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 tab 10. The value of L2 can be measured with a ruler.

[0090] In some embodiments, as shown in FIG. 1A, the first region 10c is located at the center of the first tab 10, and the second region 10d is located at the edge of the first tab 10. Figure 4 In some embodiments, as shown in FIG. 1A, the first region 10c is located at the center of the first tab 10, and the second region 10d is located at the edge of the first tab 10.

[0091] In some embodiments, as shown in FIG. 1A, the first region 10c is located at the center of the first tab 10, and the second region 10d is located at the edge of the first tab 10. Figure 10 and Figure 11 In some embodiments, as shown in FIG. 1A, the first region 10c is located at the center of the first tab 10, and the second region 10d is located at the edge of the first tab 10.

[0092] In some embodiments, the first protrusions 15 are semispherical, and the second protrusions 25 are strip-shaped, and the plurality of second protrusions 25 intersect to form a grid shape. In this way, as many first protrusions 15 as possible are accommodated in the second recesses 24 to improve the energy density of the secondary battery 1000.

[0093] In some embodiments, in the flattened state of the first tab 10, the projected area of the first region 10c along the first direction X is S, the total projected area of the plurality of first recesses 14 along the first direction X is S1, and 0.06S≤S1≤0.5S. Setting S1≥0.06S, the distribution of the first recesses 14 in the first region 10c is not too sparse, which is conducive to improving the effect of the first recesses 14 in reducing the internal stress of the first tab 10. The setting of the first protrusions 15 increases the adhesion between the first tab 10 and the separator 30, affecting the infiltration of the electrode liquid. Setting S1≤0.5S, the distribution of the first recesses 14 in the first region 10c is not too dense, which is conducive to reducing the influence of the setting of the first recesses 14 on the degree of infiltration of the first tab 10 by the electrolyte, and on the other hand, it is conducive to reducing the influence of the setting of the first protrusions 15 on the structural strength of the first current collector 11.

[0094] In the embodiments of the present application, the values of S and S1 can be observed and measured by a CCD camera.

[0095] In some embodiments, as shown in FIG. 1A, the first region 10c is located at the center of the first tab 10, and the second region 10d is located at the edge of the first tab 10. Figure 5As shown, the height of the first protrusion 15 protruding from the surface of the first pole piece 10 in the first direction X is T1; 1 pm ≤ T1 ≤ 10 pm. T1 is set to be ≥ 1 pm, so that the height of the first protrusion 15 is not too small and the depth of the first recess 14 is not too small, which is conducive to improving the effect of the first recess 14 in reducing the internal stress of the electrode assembly 100; as the height of the first protrusion 15 increases, the contact area between the isolation film 30 and the first pole piece 10 decreases and the adhesion becomes weaker, and T1 is set to be ≤ 10 pm, so that the height of the first protrusion 15 is not too large, which is conducive to reducing the influence of the setting of the first protrusion 15 on the adhesion between the first pole piece 10 and the isolation film 30, and reducing the possibility of damage to the first pole piece 10 in the process of pressing the first pole piece 10.

[0096] In the embodiments of the present application, T1 can be observed and measured by a 3D camera.

[0097] In some embodiments, as shown in Figure 4 The distance between any two adjacent first protrusions 15 in the winding direction of the first pole piece 10 is D3, and 1.5 mm ≤ D3 ≤ 3 mm. D3 is set to be ≥ 1.5 mm, so that the distance between the two adjacent first protrusions 15 is not too small, which is conducive to reducing the possibility of damage to the first pole piece 10 in the process of pressing the first pole piece 10; and D3 is set to be ≤ 3 mm, so that the distance between the two adjacent first protrusions 15 is not too large, which is conducive to increasing the number of first recesses 14 and thus improving the effect of the first recess 14 in reducing the internal stress of the electrode assembly 100.

[0098] In some embodiments, as shown in Figure 4 The distance between the two adjacent first protrusions 15 in the winding central axis direction Z of the electrode assembly 100 is D4, and 1.5 mm ≤ D4 ≤ 3 mm. D4 is set to be ≥ 1.5 mm, so that the distance between the two adjacent first protrusions 15 is not too small, which is conducive to reducing the possibility of damage to the first pole piece 10 in the process of pressing the first pole piece 10; and D4 is set to be ≤ 3 mm, so that the distance between the two adjacent first protrusions 15 is not too large, which is conducive to increasing the number of first recesses 14 and thus improving the effect of the first recess 14 in reducing the internal stress of the electrode assembly 100.

[0099] In the embodiments of the present application, the values of D3 and D4 can be measured by a ruler.

[0100] In some embodiments, as shown in Figure 8 and Figure 9As shown, the first recesses 14 are formed by removing part of the material of the first active material layer 12. By this way of arranging the first recesses 14, on the one hand, the maximum static friction between the first tab 10 and the separator 30 can be increased, and the possibility of relative displacement between the first tab 10 and the separator 30 can be reduced; on the other hand, the electrolyte can be stored, and the cycle performance of the secondary battery 1000 can be improved.

[0101] In some embodiments, the first recesses 14 are formed by laser etching.

[0102] In some embodiments, in the flattened state of the first tab 10, the projected area of the first region 10c along the first direction X is S, the total projected area of the plurality of first recesses 14 along the first direction X is S1, and 0.04S≤S1≤0.5S. By setting S1≥0.04S, the distribution of the first recesses 14 in the first region 10c is not too sparse, which is conducive to improving the effect of the first recesses 14 in reducing the internal stress of the first tab 10; by setting S1≤0.5S, the distribution of the first recesses 14 in the first region 10c is not too dense, which is conducive to maintaining the CB (Cell Balance, the ratio of the capacity per unit area of the negative electrode to the capacity per unit area of the positive electrode) of the electrode assembly 100 when the first tab 10 is a negative tab, and reducing the possibility of lithium precipitation of the electrode assembly 100 during the cycle process, and is conducive to improving the capacity of the electrode assembly 100 when the first tab 10 is a positive tab.

[0103] In some embodiments, 0.3S≤S1≤0.5S. By setting S1≥0.3S, the distribution of the first recesses 14 in the first region 10c is not too sparse, which is conducive to further improving the effect of the first recesses 14 in reducing the internal stress of the first tab 10.

[0104] In some embodiments, in the first region 10c, along the winding direction Y of the electrode assembly, in any 1mm length of the first tab 10, the number of the first recesses 14 is N, and 90≤N≤190. By setting N≥90, the distribution of the first recesses 14 on the first tab 10 is not too sparse, which is conducive to improving the effect of the first recesses 14 in releasing the stress on the first tab 10; by setting N≤190, the distribution of the first recesses 14 on the first tab 10 is not too dense, which is conducive to maintaining the CB of the electrode assembly 100 when the first tab 10 is a negative tab, and reducing the possibility of lithium precipitation of the electrode assembly 100 during the cycle process, and is conducive to improving the capacity of the electrode assembly 100 when the first tab 10 is a positive tab.

[0105] In some embodiments, as shown in FIG. 1, the first recesses 14 are arranged in the first region 10c in a staggered manner. Figure 8As shown, the first recesses 14 extend along the winding central 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 electrode tab 10 is D5, and 0.2 mm≤D5≤0.5 mm. By setting D5≥0.2 mm, the spacing between the first recesses 14 is not too small, and the distribution of the first recesses 14 on the first electrode tab 10 is not too dense. When the first electrode tab 10 is a negative electrode tab, it is beneficial to maintain the CB of the electrode assembly 100 and reduce the possibility of lithium precipitation of the electrode assembly 100 during the cycle process. When the first electrode tab 10 is a positive electrode tab, it is beneficial to improve the capacity of the electrode assembly 100. By setting D5≤0.5 mm, the spacing between the first recesses 14 is not too large, which is beneficial to improve the effect of reducing the internal stress.

[0106] In the embodiments of the present application, the value of D5 can be observed and measured by using a CCD camera.

[0107] In some embodiments, as shown in Figure 9 the depth of the first recess 14 along the first direction X is H1, and 1 μm≤H1≤50 μm. By setting H1≥1 μm, the depth of the first recess 14 is not too small, which is beneficial to improve the effect of the first recess 14 in reducing the internal stress of the first electrode tab 10. By setting H1≤50 μm, the depth of the first recess 14 is not too large, and the removed active material is not too much. When the first electrode tab 10 is a negative electrode tab, it is beneficial to maintain the CB of the electrode assembly 100 and reduce the possibility of lithium precipitation of the electrode assembly 100 during the cycle process. When the first electrode tab 10 is a positive electrode tab, it is beneficial to improve the capacity of the electrode assembly 100.

[0108] In the embodiments of the present application, the value of H1 can be observed and measured by using a 3D camera.

[0109] In some embodiments, as shown in Figure 8 the width of the first recess 14 along the winding direction of the first electrode tab 10 is W1, and 0.09 mm≤W1≤0.2 mm. By setting W1≥0.09 mm, the width of the first recess 14 is not too small, which is beneficial to improve the effect of the first recess 14 in reducing the internal stress of the first electrode tab 10. By setting W1≤0.2 mm, the width of the first recess 14 is not too large, and the removed active material is not too much. When the first electrode tab 10 is a negative electrode tab, it is beneficial to maintain the CB of the electrode assembly 100 and reduce the possibility of lithium precipitation of the electrode assembly 100 during the cycle process. When the first electrode tab 10 is a positive electrode tab, it is beneficial to improve the capacity of the electrode assembly 100.

[0110] In the embodiments of the present application, the value of W1 can be observed and measured by using a CCD camera.

[0111] In some embodiments, the first recesses 14 extend 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 electrode tab 10. In this way, it is beneficial to improve the uniformity of the internal stress distribution on the first electrode tab 10, thereby improving the stability of the laminated structure of the first electrode tab 10, the separator 30, and the second electrode tab 20.

[0112] It should be noted that in the embodiments of the present application, "equal intervals" does not mean that the two distances are equal in the absolute sense. For three adjacent first recesses 14, the distances between the adjacent two first recesses 14 are represented by a and b respectively, a≤b, and when (b-a) / a≤10%, it is considered that the three first recesses 14 are at equal intervals.

[0113] In some embodiments, as shown in Figure 9 , the second active material layer 13 removes part of the material to form a plurality of third recesses 16, so as to further reduce the internal stress on the first electrode tab 10.

[0114] In some embodiments, as shown in Figure 4 and Figure 8 , the first electrode tab 10 includes a first edge 10d and a second edge 10e oppositely arranged along the winding central 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 central axis direction Z of the electrode assembly 100 is D1, and 1mm≤D1≤7mm. By setting D1≥1mm, the distance from the first region 10c to the first edge 10d is not too small, which is beneficial to reduce the influence of the arrangement of the first recess 14 on the interface performance of the electrode assembly 100. Specifically, for the scheme of pressing the first recess 14, the first region 10c is kept a certain distance from the first edge 10d, which is beneficial to reduce the possibility of the active material of 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 is kept a certain distance from the first edge 10d, which is beneficial to reduce the possibility that the electrolyte is enriched on the end of the electrode assembly 100 on the same side as the first edge 10d, thereby affecting the cycle performance of the cycle secondary battery 1000. By setting D1≤7mm, it is beneficial to increase the area of the first region 10c, thereby improving the effect of the first region 10c on reducing the internal stress of the first electrode tab 10. The value of D1 can be measured by a ruler.

[0115] In some embodiments, as shown in Figure 4 and Figure 8As shown, the distance from the first region 10c to the second edge 10e along the winding central axis direction Z of the electrode assembly 100 is D2, 1 mm≤D2≤7 mm. D2 is set to be≥1 mm so that 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; D2 is set to be≤7 mm, which is conducive to increasing the area of the first region 10c, thereby improving the effect of the first region 10c on reducing the internal stress of the first tab 10. The value of D2 can be measured by a ruler.

[0116] In some embodiments, as shown in Figure 10 and Figure 11 As shown, the second tab 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 arranged on the two surfaces of the second current collector 21 along the first direction X.

[0117] In some embodiments, the first tab 10 is a negative tab, the second tab 20 is a positive tab, the first empty foil area 111 is located at 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 conducive to maintaining the CB of the electrode assembly and reducing the possibility of lithium precipitation of the electrode assembly during the cycle process; and since part of the first tab 10 is located at the innermost layer of the electrode assembly 100, the first recess 14 is arranged on the first tab 10, which is conducive to improving the possibility of the first recess 14 further reducing the delamination state of part of the first tab 10, part of the separator 30, and part of the second tab 20.

[0118] In some embodiments, the second tab 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 arranging a plurality of second recesses 24 on the second tab 20, it is conducive to further reducing the internal stress generated by the second tab 20 during winding, thereby reducing the possibility of black spots and lithium precipitation of the secondary battery 1000 during long cycle.

[0119] In some embodiments, the second recess 24 is formed by pressing the second tab 20, or obtained by removing part of the material of the third active material layer 22.

[0120] As shown in Figure 12As shown, the embodiments of the present application also provide a power consuming device 10000, which includes the secondary battery 1000 involved in any of the foregoing embodiments.

[0121] In some embodiments, the types of the power consuming device 10000 include, but are not limited to, mobile phones, notebook computers, power tools, electric toys, and electronic cigarettes.

[0122] To verify the effect of the scheme provided by the embodiments of the present application on reducing the internal stress of the electrode assembly 100, the inventors of the present application conducted the following experiment.

[0123] 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.

[0124] In Example 1, the preparation process of the secondary battery 1000 includes the following steps:

[0125] (1) Preparation of the positive electrode sheet: mix active material lithium cobaltate (LiCoO2), conductive carbon black (Super P), CNT (carbon nanotube), and polyvinylidene fluoride (PVDF) according to a weight ratio of 97.5:0.5:0.5:1.5, add N-methyl pyrrolidone (NMP) as a solvent, adjust to a solid content of 75wt% of the positive active material, and stir uniformly for standby use. Use aluminum foil as the positive current collector. Use a slot coater to uniformly coat the above active material on one surface of the positive current collector in the thickness direction, and then dry at 90°C to obtain a positive electrode sheet coated with positive active material on one surface. Then repeat the above coating step on the other surface of the positive current collector in the thickness direction. Then cold-press the coated positive electrode sheet. After cold-pressing, use an embossing roller to press the first convex portion 15 and the first concave portion 14 on the positive electrode sheet, 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 active material layer, and the empty foil area is located at the winding starting end of the positive electrode sheet. The positive tab is welded to the empty foil area. The length L of the prepared positive electrode sheet is 120mm, and the width is 36mm.

[0126] In the prepared positive electrode sheet, the length of the empty foil region along the winding direction of the electrode assembly 100 is equal to 2πR1', where R1' is the radius of the winding needle used in the step of winding the electrode assembly. One of the two positive electrode active material layers closer to the winding starting end of the positive electrode sheet is defined as the first active material layer (if both are the same distance from the winding starting end of the positive electrode sheet, either one is selected), and the first active material layer 12 has a first edge 121 at the winding starting end of the first active material layer 12, defining an easy-to-loosen region of the positive electrode sheet, with the first edge 121 being one side of the easy-to-loosen region. From the first edge 121, the length of the easy-to-loosen region along the winding direction of the electrode assembly is 2πR1', and the length of the overlapping portion of the first region and the easy-to-loosen region is equal to 4πR1' / 3.

[0127] (2) Preparation of the negative electrode sheet: The active material artificial graphite, conductive carbon black (Super P), styrene-butadiene rubber (SBR), and CMC (sodium carboxymethyl cellulose) were mixed in a weight ratio of 97:0.5:1.3:1.2, deionized water was added as a solvent, and a negative electrode active material with a weight percentage of 50wt% was prepared and stirred uniformly for use. A copper foil with a thickness of 10 μm was used as the negative current collector. The above-mentioned negative electrode active material was uniformly coated on one surface of the negative current collector in the thickness direction using a slot coater, and then dried at 110°C to obtain a negative electrode sheet with a negative electrode active material layer coated on one surface. Then the above steps were repeated on the other surface in the thickness direction to obtain a negative electrode sheet with a negative electrode active material layer coated on both surfaces. The coated negative electrode sheet was then cold-pressed. The negative electrode sheet included an empty foil region that was not covered by the negative electrode active material layer, and the empty foil region was located at the winding starting end of the negative electrode sheet. The negative tab was welded to the empty foil region. The length of the prepared negative electrode sheet was 115 mm, and the width was 37.2 mm.

[0128] (3) Preparation of the electrolyte: In a dry argon atmosphere, first, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC: EMC: DEC = 30:50:20 to form a base organic solvent, and then lithium salt lithium hexafluorophosphate (LiPF6) was added to the base organic solvent to dissolve and mix uniformly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0129] (4) Preparation of the separator film 30: A 7 μm thick polyethylene porous polymer film was used as the separator film 30.

[0130] (5) Preparation of the electrode assembly 100: The positive electrode sheet, the separator film 30, and the negative electrode sheet were stacked in the first direction X and then wound using a winding needle to obtain the electrode assembly 100.

[0131] (6) Assembly of the secondary battery 1000: The electrode assembly 100 is loaded into the case 200, which includes the main body part 201 and the pole 202, the pole 202 being connected to the main body part 201 and being insulated from the main body part 201, the electrode assembly 100 being accommodated in the case 200, the negative electrode tab of the electrode assembly 100 being electrically connected to the main body part 201 of the case 200, and the positive electrode tab being electrically connected to the pole 202. Through the processes of injection, packaging, standing, heat pressing, formation, and shaping, the secondary battery 1000 is obtained.

[0132] The secondary battery 1000 in Comparative Example 1 is basically the same as that in Example 1, except that the first protruding part 15 is not pressed in the first electrode tab 10 in Comparative Example 1.

[0133] The secondary battery 1000 in Comparative Example 2 is basically the same as that in Example 1, except that the length of the overlapping part of the first region and the easily loosened region in the secondary battery 1000 in Comparative Example 2 is less than 4πR1’ / 3.

[0134] The secondary battery 1000 in Comparative Example 3 is basically the same as that in Example 1, except that the length of the hollow foil region of the positive electrode tab in the secondary battery 1000 in Comparative Example 3 is greater than 2πR1’.

[0135] The secondary battery 1000 in Example 2 is basically the same as that in Example 1, except that the length of the overlapping part of the first region and the easily loosened region in the secondary battery 1000 in Example 2 is greater than 4πR1’ / 3.

[0136] The secondary battery 1000 in Example 3 is basically the same as that in Example 1, except that the length of the hollow foil region of the positive electrode tab in the secondary battery 1000 in Example 2 is less than 2πR1’.

[0137] The secondary battery 1000 in Examples 4-5 is basically the same as that in Example 1, except that the winding needle radius R1’ used is different.

[0138] After the secondary batteries 1000 in Comparative Examples 1-3 and Examples 1-5 are prepared, the electrode assembly 100 of each group of the secondary batteries 1000 is scanned by CT, and R1 is calculated, and the percentage of the number of the secondary batteries 1000 in each group that satisfy the relationship R1=R1’±5%R1’ to the total number of the secondary batteries 1000 in each group is calculated, denoted as Q, and the experimental results are recorded in Table 1 below.

[0139] Table 1

[0140]

[0141] As can be seen from Table 1, in Examples 1-5, the secondary battery 1000 satisfies that the length of the hollow-foil region of the positive electrode sheet is less than or equal to 2πR1', and the length of the overlapping part of the first region and the easy-loosen region is greater than or equal to 4πR1' / 3. On this basis, the number of secondary batteries 1000 in Examples 1-5 that satisfy the relationship R1=R1'±5%R1' is significantly higher than that in Comparative Examples 1-3. For the secondary batteries 1000 that satisfy this relationship, there is no significant difference between the winding radius and the winding needle radius, and it can be considered that the secondary battery has not loosened substantially. It can be seen that the secondary battery provided by the embodiments of the present application is beneficial to reduce the possibility of the first part of the first electrode sheet, the part of the separator film and the second part of the electrode sheet out of the stacked state, and is beneficial to improve the cycle performance and safety of the secondary battery.

[0142] It should be noted that based on the foregoing experiments, the winding radius R1 of the secondary battery 1000 designed based on the inventive concept of the present application is substantially consistent with the winding needle radius R1', so in the embodiments of the present application, the winding radius R1 is used as the basis for measuring the length of the first hollow-foil region, the length of the easy-loosen region, and the length of the overlapping part of the first region and the easy-loosen region.

[0143] Based on the foregoing experiments, the inventors of the present application further explored the effect of the first recess 14 related parameters on reducing the internal stress of the electrode assembly 100. This part of the experiment includes 30 groups of examples, which are labeled as Examples 6 to 35 in turn, and each group of examples includes 20 secondary batteries 1000. Among them, Example 6 is composed of 20 secondary batteries 1000 randomly selected from Example 1.

[0144] The preparation process of the secondary battery 1000 in Examples 7-19 is basically the same as that in Example 1, and the difference lies in that the related parameters of the first recess 14 are different. The related parameters of the first recess 14 in Examples 6-19 are recorded in Table 2.

[0145] The preparation process of the secondary battery 1000 in Examples 20-35 is basically the same as that in Example 1, and the difference lies in that the positive electrode sheet in the secondary battery 1000 in Examples 20-35 does not press the first protrusion 15 and the first recess 14, but uses laser to etch a plurality of first recesses 14 on the first active material layer 12. The related parameters of the first recess 14 in the secondary battery 1000 in Examples 20-35 are recorded in Table 2.

[0146] After the preparation of the secondary battery 1000 in Examples 6-35, the 20 secondary batteries 1000 in each group are subjected to long cycle tests, and the test process is as follows:

[0147] 1) Maintain the test temperature at 25°C;

[0148] 2) Let the secondary battery 1000 stand for 30 min;

[0149] 3) 5C constant current charging to 4.25V, then constant voltage charging to 3C;

[0150] 4) 3C constant current charging to 4.35V, then constant voltage charging to 1.5C;

[0151] 5) 1.5C constant current charging to 4.45V, then constant voltage charging to 0.05C;

[0152] 6) Let stand for 5 min;

[0153] 7) 0.7C constant current discharging to 3V, record the first discharge cycle capacity;

[0154] 8) Let stand for 5 min;

[0155] 9) Cycle the steps 3 to 8 for 500 times;

[0156] 10) Observe the electrode assembly 100 by using a computed tomography (CT) scanner, and measure the winding radius R1 of the electrode assembly 100, and the outer diameter R of the electrode assembly 100, calculate the value of R1 / R and record in Table 2. Wherein, the value of R1 / R is used to represent the looseness of the electrode assembly 100, the greater the value of R1 / R, the higher the looseness of the electrode assembly 100, the smaller the value of R1 / R, the smaller the looseness of the electrode assembly 100.

[0157] Table 2

[0158]

[0159]

[0160] Note: In Table 2, " / " means no data, the units of L1, W1 and D5 are mm, the units of S1 and S are mm 2 , and the unit of T1 is μm.

[0161] In Embodiments 7-10 and Embodiments 21-24, the secondary battery 1000 satisfies 0.4≤L1 / L, and the value of R1 / R of the secondary battery 1000 in Embodiments 7-10 is smaller than that in Embodiment 6; the value of R1 / R of the secondary battery 1000 in Embodiments 21-24 is smaller than that in Embodiment 20. It can be seen that, by setting L1≥2L / 5, the length of the first region 10c in the winding direction of the electrode assembly 100 is not too small, and the reduced internal stress is not too small, so as to facilitate reducing the possibility of the first electrode plate 10, the separator 30 and the second electrode plate 20 located in the inner circle of the electrode assembly 100 to be delaminated, thereby reducing the possibility of the secondary battery 1000 to appear black spots and lithium precipitation in long cycle. Compared with Embodiment 23, the value of R1 / R of the secondary battery 1000 in Embodiment 24 is increased, which is because, for the scheme 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 electrode plate 10 and the separator 30, specifically, as the length of the first region 10c increases, the number of the first recess 14 increases, the friction between the first electrode plate 10 and the separator 30 decreases, and the looseness of the electrode assembly 100 increases. Therefore, by setting L1≤3L / 5, the length of the first region 10c in 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 interfacial properties of the electrode assembly 100; and for the scheme 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 electrode plate 10 and the separator 30 within an appropriate range, and reduce the degree and possibility of the electrode assembly 100 to be loose.

[0162] In Embodiments 8 and Embodiments 12-15, the secondary battery 1000 satisfies 0.06S≤S1. Compared with Embodiment 11, the value of R1 / R of the secondary battery 1000 in Embodiments 8 and Embodiments 12-15 is smaller. It can be seen that, for the scheme of pressing the first recess 14, by setting S1≥0.06S, the distribution of the first recess 14 in the first region 10c is not too sparse, which is beneficial to improve the effect of the first recess 14 to reduce the internal stress of the first electrode plate 10; on this basis, by setting S1≤0.5S, the distribution of the first recess 14 in the first region 10c is not too dense, which is beneficial to reduce the influence of the setting of the first recess 14 on the degree of the first electrode plate 10 to be impregnated with electrolyte.

[0163] In Embodiment 8 and Embodiments 17-18, the secondary battery 1000 satisfies 1 μm ≤ T1 ≤ 10 μm. Compared with Embodiment 16 and Embodiment 19, the value of R1 / R of the secondary battery 1000 in Embodiments 8 and Embodiments 17-18 is smaller. It can be seen that, by setting T1 ≥ 1 μm, the height of the first protrusion 15 is not too small, and the depth of the first recess 14 is not too small, which is beneficial to improve the effect of the first recess 14 on reducing the stress in the electrode assembly 100; by setting T1 ≤ 10 μm, the height of the first protrusion 15 is not too large, and the recess is beneficial to reduce the influence of the setting of the first protrusion 15 on the adhesion between the first electrode plate 10 and the separator 30.

[0164] In Embodiment 22 and Embodiments 26-29, the secondary battery 1000 satisfies 0.04S ≤ S1. Compared with Embodiment 25, the value of R1 / R of the secondary battery 1000 in Embodiments 22 and Embodiments 26-29 is smaller. It can be seen that, for the scheme of etching the first recess 14, by setting S1 ≥ 0.04S, the distribution of the first recess 14 in the first area 10c is not too sparse, which is beneficial to improve the effect of the first recess 14 on reducing the stress in the first electrode plate 10; on this basis, by setting S1 ≤ 0.5S, the distribution of the first recess 14 in the first area 10c is not too dense, which is beneficial to reduce the influence of the setting of the first recess 14 on the degree of impregnation of the first electrode plate 10 by the electrolyte.

[0165] In Embodiments 32 to 35, the secondary battery 1000 satisfies 0.2 mm ≤ D5 ≤ 0.5 mm. Compared with Embodiment 31 and Embodiment 32, the value of R1 / R of the secondary battery 1000 in Embodiments 32-35 is smaller. It can be seen that, by setting D5 ≤ 0.5 mm, the spacing between the first recesses 14 is not too large, which is beneficial to improve the effect of reducing the stress. On this basis, by setting D5 ≥ 0.2 mm, the spacing between the first recesses 14 is not too small, and the distribution of the first recesses 14 in the first electrode plate 10 is not too dense, which is beneficial to improve the capacity of the electrode assembly 100.

[0166] In Embodiments 32 to 35, the secondary battery 1000 satisfies 90 ≤ N ≤ 190. Compared with Embodiment 30 and Embodiment 31, the value of R1 / R of the secondary battery 1000 in Embodiments 32-35 is smaller. It can be seen that, by setting N ≥ 90, the distribution of the first recesses 14 on the first electrode plate 10 is not too sparse, which is beneficial to improve the effect of the first recess 14 on releasing the stress on the first electrode plate 10; by setting N ≤ 190, the distribution of the first recesses 14 on the first electrode plate 10 is not too dense, which is beneficial to improve the capacity of the electrode assembly 100.

[0167] In Embodiments 32 to 35, the secondary battery 1000 satisfies 0.09 mm≤W1≤0.2 mm. Compared with Embodiments 30 and 31, the value of R1 / R of the secondary battery 1000 in Embodiments 32-35 is smaller. It can be seen that, by setting W1≥0.09 mm, the width of the first recess 14 is not too small, which is conducive to improving the effect of the first recess 14 in reducing the internal stress of the first electrode tab; by setting W1≤0.2 mm, the width of the first recess 14 is not too large, and the removed active material is not too much, when the first electrode tab 10 is a negative electrode tab, it is conducive to maintaining the CB of the electrode assembly 100 and reducing the possibility of lithium precipitation of the electrode assembly 100 during the cycle process, and when the first electrode tab 10 is a positive electrode tab, it is conducive to improving the capacity of the electrode assembly 100.

[0168] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present application, and are not used as limitations to the present application, and as long as the above embodiments are within the spirit and scope of the present application, any suitable changes and modifications made to the above embodiments are within the disclosure range of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly including electrode sheets and a separator, the electrode sheets including a first electrode sheet and a second electrode sheet, the first electrode sheet and the second electrode sheet having opposite polarities, the first electrode sheet, the separator sheet and the second electrode sheet being stacked and wound into a cylindrical structure, characterized in that, The first electrode 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 disposed on two surfaces of the first current collector along a first direction, where the first direction is the thickness direction of the first electrode. Along the winding direction of the electrode assembly, the distance from the first active material layer to the winding start end of the first electrode sheet is not greater than the distance from the second active material layer to the winding start end of the first electrode sheet. Viewed along the winding center axis 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 in the radial direction. The first electrode has a loose region, the first active material layer has a first side, the first side is located at the starting end of the winding of the first active material layer, the first side is a side line of the loose region, and the length of the loose region is 2πR1 from the first side along the winding direction of the electrode assembly. The first electrode includes a first empty foil region. The first current collector in the first empty foil region is not covered by the first active material layer and the second active material layer. The first empty foil region is located at the winding start end of the first electrode. Along the winding direction of the electrode assembly, the length of the first empty foil region is less than or equal to 2πR1. The first active material layer has a first region, the first region has a plurality of first recesses, at least a portion of the first region overlaps with the loose region, and along the winding direction of the electrode assembly, the length of the overlapping portion of the first region and the loose region is D, where D≥4πR1 / 3.

2. The secondary battery as described in claim 1, characterized in that, Along the winding direction of the electrode assembly, the length of the first electrode is L, the length of the first region is L1, and 2L / 5≤L1≤3L / 5.

3. The secondary battery as described in claim 1, characterized in that, The first electrode includes a first edge and a second edge disposed opposite to each other along the winding central axis of the electrode assembly; both the first active material layer and the second active material layer extend to the first edge and the second edge; The distance from the first region to the first edge along the winding center axis of the electrode assembly is D1, where 1mm ≤ D1 ≤ 7mm; and / or The distance from the first region to the second edge along the winding center axis of the electrode assembly is D2, where 1mm ≤ D2 ≤ 7mm.

4. The secondary battery as described in any one of claims 1-3, characterized in that, A portion of the first electrode sheet 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 unrecessed surface of the first electrode sheet, and the first recesses and the first protrusions are correspondingly arranged along the first direction.

5. The secondary battery as described in claim 4, characterized in that, The first electrode includes a double-sided coated area, in which the first current collector is simultaneously covered by the first active material layer and the second active material layer, and the first region is located in the double-sided coated area; the double-sided coated area includes a third edge and a fourth edge disposed opposite to each other along the winding direction of the first electrode. The distance from the first region along the winding direction of the first electrode to the third edge is L2, where L2 ≥ 1 mm; and / or The distance from the first region to the fourth edge along the winding direction of the first electrode is L3, where L3 ≥ 1 mm.

6. The secondary battery as described in claim 4, characterized in that, A portion of the second electrode sheet is recessed along the first direction to form a plurality of second protrusions and a plurality of second recesses. The second protrusions protrude from the unrecessed surface of the second electrode sheet, and the second recesses and the second protrusions are correspondingly arranged along the first direction. At least a portion of some of the plurality of first protrusions is received within the second recess.

7. The secondary battery as described in claim 4, characterized in that, With the first electrode 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, where 0.06S≤S1≤0.5S.

8. The secondary battery as described in claim 4, characterized in that, The height of the first protrusion protruding from the surface of the first electrode along the first direction is T1, where 1μm≤T1≤10μm.

9. The secondary battery as described in claim 4, characterized in that, The two adjacent first protrusions are positioned apart from each other; The distance between two adjacent first protrusions along the winding direction of the first electrode sheet is D3, where 1.5mm≤D3≤3mm; The distance between two adjacent first protrusions along the winding center axis of the electrode assembly is D4, where 1.5mm≤D4≤3mm.

10. The secondary battery as described in any one of claims 1-3, characterized in that, The first recess is formed by removing part of the material from the first active material layer.

11. The secondary battery as described in claim 10, characterized in that, Within the first region, along the winding direction of the electrode assembly, the number of the first recesses in the first electrode sheet with a length of 1 mm is N, where 90 ≤ N ≤ 190.

12. The secondary battery as described in claim 10, characterized in that, With the first electrode 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, where 0.04S≤S1≤0.5S.

13. The secondary battery as described in claim 12, characterized in that, 0.3S≤S1≤0.5S.

14. The secondary battery as described in claim 10, characterized in that, The first recess extends along the winding center axis of the electrode assembly, and the plurality of 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 electrode sheet is D5, where 0.2mm≤D5≤0.5mm.

15. The secondary battery as described in claim 10, characterized in that, The depth of the first recess along the first direction is H1, where 1μm≤H1≤50μm.

16. The secondary battery as described in claim 10, characterized in that, The width of the first recess along the winding direction of the first electrode sheet is W1, 0.09mm≤W1≤0.2mm.

17. The secondary battery as described in claim 1, characterized in that, The first electrode is a negative electrode, the second electrode is a positive electrode, and the first empty foil area is located in the innermost layer of the electrode assembly; The second electrode 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 disposed 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; 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 appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 17.

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