Secondary batteries and electronic devices

Abstract

This application discloses a secondary battery and electronic device, including a positive electrode, a separator, and a negative electrode, which are stacked or stacked and wound together. The positive electrode includes a positive active material layer, and the negative electrode includes a negative active material layer. Along a first direction, the negative active material layer includes a first portion and a second portion connected in sequence, the second portion being the portion of the negative active material layer extending beyond the positive active material layer on one side of the first direction. The first portion has a first groove, and the second portion has a second groove. Along a second direction, the width of the first groove is W1, and the width of the second groove is W2. Along a third direction, the depth of the first groove is D1, and the depth of the second groove is D2. The following condition is satisfied: W2 × D2 < W1 × D1. This reduces lithium plating, optimizes electrolyte distribution, reduces side reactions, enhances kinetics, and disperses stress, reducing cracks or fractures in the negative active material layer.

Description

Secondary battery and electronic device TECHNICAL FIELD

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

[0002] Lithium ion batteries are widely used in mobile phones, tablets, notebook computers and electric vehicles due to their high energy density, long cycle life and low pollution. With the development of electronic products, lithium ion batteries are increasingly pursuing high-rate charging and discharging. However, high-rate charging and discharging requires high lithium ion diffusion rate, which is prone to lithium precipitation.

[0003] SUMMARY

[0004] The present application aims to provide a secondary battery and an electronic device to reduce lithium precipitation in lithium ion batteries.

[0005] The embodiments of the present application adopt the following technical solutions:

[0006] In a first aspect, the present application provides a secondary battery, comprising a positive electrode sheet, a separator and a negative electrode sheet which are stacked or stacked and wound. The positive electrode sheet comprises a positive active material layer, and the negative electrode sheet comprises a negative active material layer. Along a first direction, the negative active material layer comprises a first part and a second part connected in sequence, and the second part is a part of the negative active material layer beyond the positive active material layer on one side of the negative active material layer in the first direction. The first part is provided with a first groove, and the second part is provided with a second groove. Along a second direction, the width of the first groove is W1, and the width of the second groove is W2. Along a third direction, the depth of the first groove is D1, and the depth of the second groove is D2. It is satisfied that W2xD2

[0007] In the above technical solution, the first part serves as the main reaction area with the positive active material layer. The first groove with large depth and width is arranged in the first part to improve the kinetics of the first part, increase the diffusion efficiency of lithium ions in the first part, reduce the accumulation of lithium ions, and further reduce lithium precipitation, thereby improving the charge-discharge rate of the secondary battery. The second groove with small depth and width is arranged in the second part to facilitate the negative active material layer to have sufficient excess capacity to embed the lithium ions extracted from the positive active material layer, which can further reduce lithium precipitation.

[0008] Compared with the first part, the second part is less involved in the electrochemical reaction, and the diffusion efficiency of the electrolyte is less required than the first part. By setting a first groove with a larger depth and a larger width in the first part and a second groove with a smaller depth and a smaller width in the second part, the distribution of the electrolyte can be optimized, the occurrence of side reactions can be reduced, and the charge and discharge performance of the secondary battery can be reduced.

[0009] At the same time, the second groove with a smaller width and a smaller depth can maintain the edge integrity of the negative active material layer to a certain extent. The second groove with a smaller width and a smaller depth will not cause excessive damage to the edge structure of the negative active material layer, which is conducive to the processing and assembly of the negative active material layer. At the same time, the second groove with a smaller width and a smaller depth can also reduce the risk of cracks and fractures in the edge area.

[0010] In addition, the second part is less involved in the electrochemical reaction, and the heat generation is relatively less. Such design can meet the heat dissipation demand to a certain extent, while reducing the energy density loss caused by excessive heat dissipation. By optimizing the heat distribution, the temperature gradient and thermal stress inside the secondary battery can be reduced, and the risk of thermal runaway can be reduced.

[0011] In some embodiments, the secondary battery includes a negative tab, the negative tab is electrically connected with the negative tab, and the negative tab extends from the second part to the negative tab along the first direction. The second groove with a smaller depth and a smaller width has less effect on the structure of the edge of the negative active material layer, which is conducive to the connection and extension of the negative tab. Moreover, the current density of the negative tab is large, and the second groove increases the contact area between the negative active material layer and the surrounding environment, so that the heat can be dissipated more quickly.

[0012] In some embodiments, along the first direction, one end of the second part is connected with the first part, and the other end of the second part includes a first edge, and the second groove extends to the first edge, so that the electrolyte can directly enter the second groove from the first edge, providing more transmission paths for lithium ions.

[0013] In some embodiments, along the first direction, the first groove and the second groove are communicated, forming a continuous channel system, providing more path options for the diffusion of lithium ions, so that lithium ions can migrate more quickly and uniformly inside the negative active material layer. At the same time, the communicated first groove and second groove can reduce the concentration gradient of lithium ions during transmission and reduce the probability of occurrence of local polarization.

[0014] In some embodiments, the first groove extends through the first portion along the first direction, which facilitates the electrolyte to enter the first groove and quickly soak the entire first portion. In other embodiments, the second groove extends through the second portion along the first direction. This facilitates the electrolyte to quickly enter the second groove and quickly soak the entire second portion.

[0015] In some embodiments, 70 pm≤W1≤100 pm and 50 pm≤W2≤80 pm, which improves the soaking effect of the electrolyte, thereby improving the charge-discharge rate of the secondary battery and reducing the risk of lithium precipitation.

[0016] In some embodiments, 10 pm≤D1≤30 pm and 5 pm≤D2≤15 pm, which improves the soaking effect of the electrolyte, thereby improving the charge-discharge rate of the secondary battery and reducing the risk of lithium precipitation.

[0017] In some embodiments, the first portion is provided with a plurality of first grooves arranged along the second direction, and the distance between two adjacent first grooves is H1, 1 mm≤H1≤3 mm, which improves the diffusion efficiency of lithium ions, adapts to the expansion of the negative active material layer, reduces the loss of energy density, and reduces lithium precipitation.

[0018] In some embodiments, the second portion is provided with a plurality of second grooves arranged along the second direction, and the distance between two adjacent second grooves is H2, 0.5 mm≤H2≤3.5 mm, which improves the diffusion efficiency of lithium ions, adapts to the expansion of the negative active material layer, reduces the loss of energy density, and reduces lithium precipitation.

[0019] In some embodiments, along the first direction, the length of the first groove is L1, 75 mm≤L1≤115 mm, which reduces the loss of energy density of the secondary battery, reduces the occurrence of lithium precipitation, and improves the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery.

[0020] In some embodiments, along the first direction, the length of the second groove is L2, 1.1 mm≤L2≤1.8 mm, which reduces the loss of energy density of the secondary battery, reduces the occurrence of lithium precipitation, and improves the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery.

[0021] In some embodiments, the positive electrode sheet, the separator film, and the negative electrode sheet are stacked and wound, and the separator film is arranged between the positive electrode sheet and the negative electrode sheet. Along the third direction, the thickness of the negative active material layer is T1, 35 pm≤T1≤75 pm, which facilitates the full reaction of the negative active material layer and improves the energy density of the secondary battery.

[0022] In some embodiments, the positive electrode tab and the negative electrode tab are alternately stacked along a third direction, and the adjacent positive electrode tab and the negative electrode tab are provided with the isolation film. The thickness of the negative electrode active material layer along the third direction is T1, and 45 pm≤T1≤85 pm. The negative electrode active material layer 22 can be fully reacted, and the energy density of the secondary battery can be improved.

[0023] In some embodiments, the negative electrode active material layer comprises at least one of silicon, silicon oxide, silicon-carbon composite, and silicon alloy. The mass content of silicon in the negative electrode active material layer is G, and 3%≤G≤10%. The specific capacity of the silicon material (4200 mAh / g) is higher than that of the graphite material, which can improve the capacity of the secondary battery and further improve the charge-discharge rate of the secondary battery.

[0024] In some embodiments, the isolation film comprises a substrate layer, an adhesive layer, and a ceramic layer, the adhesive layer is arranged between the substrate layer and the ceramic layer, and the ceramic layer faces the negative electrode active material layer. The ceramic layer can improve the interface contact between the isolation film and the negative electrode active material layer, reduce the interface resistance, which helps to improve the transmission efficiency of lithium ions at the interface. The substrate layer comprises at least one of polyethylene, polypropylene, polytetrafluoroethylene, cellulose acetate, or cellulose nanofiber; and / or, the adhesive layer comprises at least one of polyvinylidene fluoride, polyvinyl butyral, or polyacrylate; and / or, the ceramic layer comprises at least one of boehmite, alumina, or silicon dioxide.

[0025] In some embodiments, the negative electrode active material layer further comprises a third portion, the first portion is connected between the second portion and the third portion along the first direction, and the third portion is a portion of the negative electrode active material layer beyond the positive electrode active material layer on the other side of the negative electrode active material layer in the first direction. The third portion is provided with a third groove, the width of the third groove along the second direction is W3, and the depth of the third groove along the third direction is D3, W3×D3

[0026] In some embodiments, the length of the third groove along the first direction is L3, and 0.5 mm≤L3≤1.0 mm. The energy density loss of the secondary battery can be reduced, the occurrence of lithium precipitation can be reduced, and the diffusion efficiency of lithium ions can be improved, thereby improving the charge-discharge rate of the secondary battery.

[0027] In some embodiments, the negative active material layer further comprises a fourth portion and a fifth portion, the first portion is connected between the fourth portion and the fifth portion along the second direction, the fourth portion is a portion of the negative active material layer beyond the positive active material layer on one side of the negative active material layer along the second direction, and the fifth portion is a portion of the negative active material layer beyond the positive active material layer on the other side of the negative active material layer along the second direction. The fourth portion is provided with a fourth groove, the width of the fourth groove along the second direction is W4, the depth of the fourth groove along the third direction is D4, and W4xD4W1xD1. The unit cross-sectional area of the fourth groove is less than the unit cross-sectional area of the first groove, which can reduce lithium precipitation, optimize electrolyte distribution, and reduce the occurrence of side reactions.

[0028] In other embodiments, the fifth portion is provided with a fifth groove, the width of the fifth groove along the second direction is W5, the depth of the fifth groove along the third direction is D5, and W5xD5W1xD1. The unit cross-sectional area of the fifth groove is less than the unit cross-sectional area of the first groove, which can reduce lithium precipitation, optimize electrolyte distribution, and reduce the occurrence of side reactions.

[0029] In some embodiments, the fourth portion is provided with a fourth groove, the length of the fourth groove along the first direction is L4, and 0.5mm≤L4≤1.0mm. This can reduce the loss of energy density of the secondary battery, reduce the occurrence of lithium precipitation, and improve the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery.

[0030] In other embodiments, the fifth portion is provided with a fifth groove, the length of the fifth groove along the first direction is L5, and 0.2mm≤L5≤0.7mm. This can reduce the loss of energy density of the secondary battery, reduce the occurrence of lithium precipitation, and improve the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery.

[0031] In a second aspect, the present application further provides an electronic device comprising the secondary battery according to any one of the embodiments of the first aspect.

[0032] Additional layers and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description, drawings, or by implementation of the embodiments of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0033] One or more embodiments are illustrated by way of example in the accompanying drawings that are not intended to be limiting of the embodiments so as to illustrate exemplary principles of the embodiments. The same reference numerals in different drawings represent the same element or components unless otherwise indicated. Drawings in which:

[0034] FIG. 1 is a schematic structural diagram of a secondary battery according to some embodiments of the present application;

[0035] FIG. 2 is a schematic diagram of a winding structure of an electrode assembly according to some embodiments of the present application;

[0036] Fig. 3 is a schematic view of a stack structure of an electrode assembly according to some embodiments of the present application;

[0037] Fig. 4 is a schematic view of a stack structure of a positive electrode sheet, a separator, and a negative electrode sheet according to some embodiments of the present application;

[0038] Fig. 5 is a schematic view of a structure of a negative electrode sheet according to some embodiments of the present application;

[0039] Fig. 6 is an enlarged view of a portion of Fig. 5;

[0040] Fig. 7 is a cross-sectional view taken along line B-B of Fig. 5;

[0041] Fig. 8 is a partial plan view of a negative electrode sheet according to some embodiments of the present application;

[0042] Fig. 9 is a schematic view of a structure of a separator according to some embodiments of the present application;

[0043] Fig. 10 is a plan view of a negative electrode sheet according to some embodiments of the present application;

[0044] Fig. 11 is a cross-sectional view taken along line C-C of Fig. 10.

[0045] Explanation of Reference Numerals:

[0046] 1000, secondary battery;

[0047] 100, electrode assembly;

[0048] 10, positive electrode sheet; 11, positive current collector; 111, first surface; 112, second surface; 12, positive active material layer;

[0049] 20, negative electrode sheet;

[0050] 21, negative current collector; 211, third surface; 212, fourth surface;

[0051] 22, negative active material layer; 221, first portion; 2211, first groove; 222, second portion; 2221, second groove; 223, third portion; 2231, third groove; 224, fourth portion; 2241, fourth groove; 225, fifth portion; 2251, fifth groove;

[0052] 30, separator; 31, base material layer; 32, adhesive layer; 33, ceramic layer; 40, negative tab;

[0053] 200, case;

[0054] X, first direction; Y, second direction; Z, third direction. DETAILED DESCRIPTION

[0055] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some but not all of the embodiments of the present application.

[0056] Reference to "embodiments" in this application means that the particular features, structures, or characteristics described with reference to the embodiments can be included in at least one embodiment of the present application. The phrase appears at various places in the specification does not necessarily all refer to the same embodiments, and is not necessarily mutually exclusive of other embodiments or alternative embodiments.

[0057] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0058] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents a "or" relationship between the associated objects before and after it.

[0059] The technical features involved in the different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0060] In a first aspect, the present application provides a secondary battery 1000, please refer to figure 1, the secondary battery 1000 package electrode assembly 100, shell 200 and electrolyte (not marked in the figure), the shell 200 can contain electrode assembly 100 and electrolyte, electrolyte in the shell 200 infiltrates electrode assembly 100.

[0061] Referring to FIGS. 2 and 3, the electrode assembly 100 includes a positive electrode tab 10, a negative electrode tab 20, and a separator 30. The positive electrode tab 10, the separator 30, and the negative electrode tab 20 are stacked and wound, and FIG. 2 illustrates a wound structure of the electrode assembly 10, for example, stacked in a thickness direction of the positive electrode tab 10 and / or the negative electrode tab 20 and wound in a length direction thereof, to form a wound electrode assembly 100. In other embodiments, the electrode assembly 100 can also have a stacked structure, and FIG. 3 illustrates a stacked structure of the electrode assembly 10, in which a plurality of positive electrode tabs 10 and a plurality of negative electrode tabs 20 are alternately stacked in a third direction Z (a thickness direction of the positive electrode tab 10 and / or the negative electrode tab 20), and the separator 30 is disposed between adjacent positive electrode tabs 10 and negative electrode tabs 20.

[0062] It should be noted that, in the embodiments of the present application, the first direction X is a width direction of the positive electrode tab 10 and / or the negative electrode tab 20, the second direction Y is a length direction of the positive electrode tab 10 and / or the negative electrode tab 20, and the third direction Z is a thickness direction of the positive electrode tab 10 and / or the negative electrode tab 20. The first direction X, the second direction Y, and the third direction Z are perpendicular to each other. In the stacked electrode assembly, the first direction X can also be a width direction of the positive electrode tab 10 and / or the negative electrode tab 20, and the second direction Y can also be a length direction of the positive electrode tab 10 and / or the negative electrode tab 20.

[0063] Referring to FIG. 4, the positive electrode tab 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode current collector 11 serves as an electrically conductive base material of the positive electrode tab 10, and can be an integral flat aluminum foil. The aluminum foil has high electrical conductivity and small electrical resistance, which can improve the charge / discharge rate of the secondary battery 1000. In addition, the aluminum foil has certain strength and ductility, and is not prone to breakage or deformation during the production process such as winding or stacking, so as to ensure the structural integrity of the positive electrode tab 10. At the same time, the positive electrode of the secondary battery 1000 is at a high potential during the charge / discharge process, and the aluminum foil is relatively stable at this potential and is not prone to chemical reaction, thereby improving the charge / discharge stability of the secondary battery 1000. In other embodiments, the positive electrode current collector 11 can also be a titanium foil, a nickel foil, or a stainless steel foil, etc.

[0064] The positive electrode active material layer 12 can be arranged on at least one surface of the positive electrode current collector 11 in the thickness direction of the positive electrode current collector 11, for example, in the third direction Z. The positive electrode current collector 11 includes oppositely arranged first and second surfaces 111 and 112, and the positive electrode active material layer 12 can be arranged on the first surface 111 and / or the second surface 112. The positive electrode active material layer 12 includes a positive electrode active material, a conductive agent, and a binder, and the like. After the above-mentioned material components are mixed and uniformly stirred, the positive electrode active material layer 12 is obtained by coating the first surface 111 and / or the second surface 112. The positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, or lithium manganese iron phosphate.

[0065] Referring to FIG. 4, the negative electrode tab 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. The negative electrode current collector 21 serves as an electrically conductive base material of the negative electrode tab 20 and can be a whole flat copper foil. The copper foil has high electrical conductivity and small electrical resistance, which can improve the charge-discharge rate of the secondary battery 1000. In addition, the copper foil has certain strength and ductility, and is not prone to breakage or deformation in the production process such as winding or lamination, so as to ensure the structural integrity of the negative electrode tab 20. At the same time, the negative electrode of the lithium ion battery is at a relatively low potential during the charge-discharge process, and the copper foil is relatively stable at this potential and is not prone to chemical reaction, thereby improving the charge-discharge stability of the secondary battery 1000. In other embodiments, the negative electrode current collector 21 can also be a titanium foil, a nickel foil, a stainless steel foil, or a silver foil, etc.

[0066] The negative electrode active material layer 22 can be arranged on at least one surface of the negative electrode current collector 21 in the thickness direction of the negative electrode current collector 21, for example, in the third direction Z. The negative electrode current collector 21 includes oppositely arranged third and fourth surfaces 211 and 212, and the negative electrode active material layer 22 can be arranged on the third surface 211 and / or the fourth surface 212. The negative electrode active material layer 22 includes a negative electrode active material, a conductive agent, and a binder, and the like. After the above-mentioned material components are mixed and uniformly stirred, the negative electrode active material layer 22 is obtained by coating the third surface 211 and / or the fourth surface 212. The negative electrode active material includes one or more of graphite, soft carbon, hard carbon, carbon fiber, elemental silicon, silicon oxide compound, silicon alloy, etc.

[0067] In the embodiments of the present application, referring to FIGS. 4 and 5, in the first direction X, the negative electrode active material layer 22 includes a first portion 221 and a second portion 222 connected to each other. The second portion 222 is a portion of the negative electrode active material layer 22 on one side of the negative electrode active material layer 22 in the first direction X, which exceeds the positive electrode active material layer 12. This can make the negative electrode active material layer 22 on one side in the first direction X have sufficient excess amount to embed the lithium ions released from the positive electrode active material layer 12, so as to reduce the occurrence of lithium precipitation.

[0068] For the first portion 221, in some embodiments, the projection of the first portion 221 overlaps with the projection of the second portion 222 along the third direction Z, and the first portion 221 serves as a main reaction zone to electrochemically react with the positive active material layer 12 during charging and discharging of the secondary battery 1000.

[0069] In embodiments of the present application, referring to FIGS. 5 and 6, the first portion 221 is provided with a first groove 2211, and the second portion 222 is provided with a second groove 2221. The first groove 2211 and the second groove 2221 can be formed on the surface of the negative active material layer 22 by laser drilling or mechanical drilling. The first groove 2211 and the second groove 2221 can provide additional diffusion channels for lithium ions and serve as local rapid diffusion paths, so that lithium ions can migrate more rapidly within the negative active material layer 22, thereby improving the charging and discharging rate of the secondary battery 1000. For example, in the case of high-current charging and discharging, lithium ions can quickly reach the interior of the negative active material through the first groove 2211 and the second groove 2221, so that the secondary battery 1000 can respond more quickly to charging and discharging requirements.

[0070] However, the inventors of the present application have found that forming the first groove 2211 on the first portion 221 and the second groove 2221 on the second portion 222 can result in insufficient amount of the negative active material layer 22, making it difficult to timely embed lithium ions released from the positive active material layer 12, and causing lithium precipitation to occur. Meanwhile, during charging and discharging of the secondary battery 1000, the concentration of lithium ions within the negative active material layer 22 changes as the electrolyte continuously infiltrates the negative active material layer 22, and local excessive or insufficient concentration can cause side reactions, affecting the charging and discharging performance of the secondary battery 1000.

[0071] To reduce the above problems, in embodiments of the present application, referring to FIGS. 6 and 7, along the second direction Y, the width of the first groove 2211 is W1, and the width of the second groove 2221 is W2. Along the third direction Z, the depth of the first groove 2211 is D1, and the depth of the second groove 2221 is D2. W2xD2

[0072] The first part 221 is the main reaction area with the positive active material layer 12. The first groove 2211 with a large depth and a large width is arranged in the first part 221, which can improve the kinetics of the first part 221, improve the diffusion efficiency of lithium ions in the first part 221, reduce the accumulation of lithium ions, thereby reducing lithium precipitation, and improve the charge-discharge rate of the secondary battery 1000. The second groove 2221 with a small depth and a small width is arranged in the second part 222, which can facilitate the negative active material layer 22 to have sufficient excess capacity to embed the lithium ions released from the positive active material layer 12, and effectively reduce lithium precipitation.

[0073] In addition, compared with the first part 221, the second part 222 participates in the electrochemical reaction less, and the diffusion efficiency of the electrolyte is less than that of the first part 221. By arranging the first groove 2211 with a large depth and a large width in the first part 221, and arranging the second groove 2221 with a small depth and a small width in the second part 222, the electrolyte distribution can be optimized, the side reaction can be reduced, and the charge-discharge performance of the secondary battery 1000 can be improved.

[0074] At the same time, the arrangement of the first groove 2211 and the second groove 2221 can relieve the volume change stress of the negative electrode plate 20. During the charge-discharge process of the secondary battery 1000, the negative active material will swell and shrink. The first groove 2211 and the second groove 2221 provide sufficient buffer space for the volume change of the negative active material. For example, when the negative active material swells, the first groove 2211 and the second groove 2221 can accommodate part of the swelling volume, reduce the extrusion stress between the negative active materials and between the negative active materials and the negative current collector 21, and reduce the risk of negative active material layer 22 powdering and structural damage caused by stress concentration, thereby prolonging the cycle life of the secondary battery 1000.

[0075] In addition, the second part 222 is arranged with the second groove 2221 with a small width and a small depth, which can maintain the edge integrity of the negative active material layer 22 to a certain extent. The second groove 2221 with a small width and a small depth will not cause excessive damage to the edge structure of the negative active material layer 22, which is beneficial to the processing and assembly of the negative active material layer 22. At the same time, the second groove 2221 with a small width and a small depth can also reduce the risk of cracks and fractures in the edge area.

[0076] The inventors of the present application have also found that the arrangement of the first groove 2211 and the second groove 2221 can improve the heat dissipation performance of the secondary battery 1000. The first groove 2211 and the second groove 2221 can serve as additional heat dissipation channels. During the charging and discharging process of the secondary battery 1000, especially during high-rate charging and discharging or long-time use, a large amount of heat will be generated inside the secondary battery 1000. The first groove 2211 and the second groove 2221 can increase the contact area between the negative active material layer 22 and the surrounding environment, so that the heat can be dissipated more quickly. For example, the heat can be exchanged with the electrolyte through the surface of the first groove 2211 and the second groove 2221, thereby reducing the temperature of the electrode.

[0077] In the embodiments of the present application, by arranging the first groove 2211 with a larger depth and a larger width in the first part 221, and arranging the second groove 2221 with a smaller depth and a smaller width in the second part 222, the heat distribution can be optimized.

[0078] The first part 221 serves as the main reaction area. By arranging the first groove 2211 with a larger depth and a larger width, a large amount of heat can be effectively dissipated, the temperature of the first part 221 can be maintained within a reasonable range, the heat concentration can be reduced, and the risk of active material aging, performance degradation, and thermal runaway caused by excessively high temperature can be reduced.

[0079] Although the second groove 2221 has a relatively weak heat dissipation capacity compared to the first groove 2211, the second part 222 generates relatively less heat because it participates less in the electrochemical reaction. This design can meet the heat dissipation needs of the second part 222 to some extent, while reducing the loss of energy density caused by excessive heat dissipation. Moreover, the second groove 2221 with a smaller depth and a smaller width has less impact on the structure of the negative active material layer 22, which can maintain the integrity and stability of the negative active material layer 22 in this part, thereby ensuring the normal intercalation and extraction of lithium ions in the edge region, reducing the side reactions and heat generation in the edge region.

[0080] By optimizing the heat distribution and improving the stability of the electrochemical reaction, the temperature gradient and thermal stress inside the secondary battery 1000 can be reduced, and the risk of thermal runaway can be reduced. For example, when the secondary battery 1000 encounters abnormal conditions such as overcharging, overdischarging, short circuit, etc., the generation and dissipation of heat can be better controlled, and the safety of the secondary battery 1000 can be improved.

[0081] In some embodiments, referring to FIG. 5, the secondary battery 1000 includes a negative tab 40 electrically connected to the negative tab 20, the negative tab 40 extending out of the negative tab 20 along the first direction X from the second portion 222. The second groove 2221 with a small depth and a small width has a small impact on the structure of the edge of the negative active material layer 22, which is conducive to the connection and extension of the negative tab 40. In addition, the negative tab 40 has a large current density, and the second groove 2221 increases the contact area of the negative active material layer 22 with the surrounding environment, so that heat can be dissipated more quickly.

[0082] In some embodiments, referring to FIG. 5 and FIG. 6, one end of the second portion 222 is connected to the first portion 221 along the first direction X, and the other end of the second portion 222 includes a first edge 2222, and the second groove 2221 extends to the first edge 2222. This can make the electrolyte directly enter the second groove 2221 from the first edge 2222, providing more transmission paths for lithium ions. For example, in the case of high-rate charging and discharging of the secondary battery 1000, the penetrating second groove 2221 reduces the resistance of lithium ions in the transmission process, so that the secondary battery 1000 can respond to the charging and discharging demand more quickly, and improve the rate performance of the secondary battery 1000.

[0083] Similarly, in some embodiments, the first groove 2211 penetrates the first portion 221 along the first direction X, which can facilitate the electrolyte to enter the first groove 2211 in a wide width and quickly soak the entire first portion 221. Based on the same inventive concept, the second groove 2221 penetrates the second portion 222 along the first direction X, which can facilitate the electrolyte to quickly enter the second groove 2221 and quickly soak the entire second portion 222.

[0084] In other embodiments, the first groove 2211 and the second groove 2221 are communicated along the first direction X, and the communicated first groove 2211 and the second groove 2221 form a continuous channel system in the entire negative active material layer 22. This provides more path options for the diffusion of lithium ions, so that lithium ions can migrate more quickly and uniformly inside the negative active material layer 22. At the same time, the communicated first groove 2211 and the second groove 2221 can reduce the concentration gradient of lithium ions in the transmission process and reduce the probability of occurrence of local polarization. In addition, it can promote the transfer and dissipation of heat in the entire negative active material layer 22, and reduce safety problems caused by local high temperature.

[0085] The inventors of the present application have found that if the width of the first groove 2211 and the second groove 2221 is too large, the overall mechanical strength of the negative active material layer 22 will be weakened, which may cause the negative active material layer 22 to fall off. Moreover, the wider groove will occupy more space, resulting in a decrease in the total amount of active material participating in the electrochemical reaction, thereby causing the energy density of the secondary battery 1000 to be lost, and also causing the negative electrode to be insufficient, thereby causing lithium precipitation. If the width of the first groove 2211 and the second groove 2221 is too small, the wettability of the electrolyte may be affected, and the diffusion speed of lithium ions will be limited, making it difficult to quickly reach the inside of the negative active material layer 22, thereby affecting the rate performance of the secondary battery 1000.

[0086] To reduce the above problems, in the embodiments of the present application, 70 pm≤W1≤100 pm and / or 50 pm≤W2≤80 pm are selected. The wettability of the electrolyte can be improved, thereby improving the charge-discharge rate of the secondary battery 1000 and reducing the risk of lithium precipitation.

[0087] Moreover, if the depth of the first groove 2211 and the second groove 2221 is too large, the overall mechanical strength of the negative active material layer 22 will be weakened, and the energy density of the secondary battery 1000 will be lost, and also causing the negative electrode to be insufficient, thereby causing lithium precipitation. If the depth is too small, the wettability of the electrolyte may be affected, thereby affecting the rate performance of the secondary battery 1000. In the embodiments of the present application, 10 pm≤D1≤30 pm and / or 5 pm≤D2≤15 pm are selected. The wettability of the electrolyte can be improved, thereby improving the charge-discharge rate of the secondary battery 1000 and reducing the risk of lithium precipitation.

[0088] For the measurement of the depth and width of the first groove 2211 and the second groove 2221, the width and depth can be measured by an optical microscope. The sample is wiped with alcohol to clean the surface of the pores and remove any residual materials or contaminants. The sample is placed on the stage of the microscope, and the focus of the microscope is adjusted until the first groove 2211 and the second groove 2221 are clearly visible (a high magnification objective lens is used). The width of the first groove 2211 and the second groove 2221 is measured using the measurement function of the microscope. A ruler can be placed in the field of view of the microscope, and then the distance between the edges of the first groove 2211 and the second groove 2221 is read. The depth of the first groove 2211 and the second groove 2221 is measured using a three-dimensional profilometer with a depth measurement function.

[0089] For the distribution of the first groove 2211 on the negative active material layer 22, in some embodiments, the first portion 221 is provided with a plurality of first grooves 2211 arranged along the second direction Y, which can improve the kinetics of the negative electrode tab 20 and improve the wettability of the electrolyte.

[0090] The inventors of the present application have found that when the distance between two adjacent first grooves 2211 is too large, the diffusion path of lithium ions in the negative electrode active material layer 22 will be too long. This makes lithium ions need to overcome greater resistance during transmission, and the diffusion speed is slowed down. Moreover, during the charging and discharging process, the negative electrode active material will change in volume. If the distance between two adjacent first grooves 2211 is too large, the negative electrode active material in the region without the buffering of the first grooves 2211 may bear greater stress, and is prone to phenomena such as pulverization and cracking. If the distance is too small, it means that there are more first grooves 2211, which will cause the energy density of the secondary battery 1000 to be lost, and will also cause the negative electrode to be insufficient, so that lithium is precipitated.

[0091] To reduce the above problems, in the embodiments of the present application, please refer to FIG. 8, the distance between two adjacent first grooves 2211 is H1, 1mm≤H1≤3mm, which can improve the diffusion efficiency of lithium ions, and can adapt to the expansion of the negative electrode active material layer 22, reduce the loss of energy density, and reduce lithium precipitation.

[0092] As for the distribution of the second grooves 2221, based on the same inventive concept, the second part 222 is provided with a plurality of second grooves 2221, the plurality of second grooves 2221 are arranged along the second direction Y, the distance between two adjacent second grooves 2221 is H2, 0.5mm≤H2≤3.5mm, which can improve the diffusion efficiency of lithium ions, and can adapt to the expansion of the negative electrode active material layer 22, reduce the loss of energy density, and reduce lithium precipitation. It should be noted that when the second grooves 2221 are in communication with the first grooves 2211, H2>H1 can also be used, for example, 1.5mm≤H2≤3.5mm, and a continuous punching process can be used to directly form continuous first grooves 2211 and second grooves 2221 in the negative electrode active material layer 22.

[0093] As for the length of the first grooves 2211, too large length will also cause the loss of energy density of the secondary battery 1000, and will also cause the negative electrode to be insufficient, so that lithium is precipitated. If the length is too small, it may be difficult to provide sufficient transmission channels for lithium ions, and the diffusion speed of lithium ions will be limited, which will affect the rate performance of the secondary battery 1000.

[0094] In the embodiments of the present application, along the first direction X, the length of the first grooves 2211 is L1, 75mm≤L1≤115mm, which can reduce the loss of energy density of the secondary battery 1000, reduce the occurrence of lithium precipitation, and can improve the diffusion efficiency of lithium ions, thereby improving the charging and discharging rate of the secondary battery 1000.

[0095] For the length of the second groove 2221, based on the same inventive concept, in embodiments of the application, the length of the second groove 2221 in the first direction X is L2, 1.1 mm≤L2≤1.8 mm, which can reduce the loss of energy density of the secondary battery 1000, reduce the occurrence of lithium precipitation, and can improve the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery 1000.

[0096] It should be noted that when the first groove 2211 penetrates the first part 221, the length of the first groove 2211 is the width of the first part 221 in the width direction of the negative electrode tab 20 (the first direction X). Similarly, when the second groove 2221 penetrates the second part 222, the length of the second groove 2221 is the width of the second part 222 in the width direction of the negative electrode tab 20 (the first direction X).

[0097] For the thickness of the negative electrode active material layer 22, due to the arrangement of the first groove 2211 and the second groove 2221 described above, the kinetics of the negative electrode tab 20 can be improved, and the ion diffusion efficiency can be improved. In combination with the size design of the first groove 2211 and the second groove 2221, the kinetic performance of the negative electrode tab 20 can be better. In embodiments of the application, the negative electrode active material layer 22 with a larger thickness can be adapted, and the energy density of the secondary battery 1000 can be improved.

[0098] For example, when the jelly-roll type electrode assembly 100 is used, the positive electrode tab 10, the separator 30 and the negative electrode tab 20 are stacked and wound, and the separator 30 is arranged between the positive electrode tab 10 and the negative electrode tab 20. In the third direction Z, the thickness of the negative electrode active material layer 22 is T1, 35 μm≤T1≤75 μm. The negative electrode active material layer 22 can be fully reacted, and the energy density of the secondary battery 1000 can be improved.

[0099] When the jelly-roll type battery and assembly are used, the positive electrode tab 10 and the negative electrode tab 20 are alternately stacked, and the separator 30 is arranged between adjacent positive electrode tab 10 and negative electrode tab 20. The jelly-roll type electrode assembly 100 has better kinetic performance, and in the third direction Z, the thickness of the negative electrode active material layer 22 is T1, 45 μm≤T1≤85 μm. The negative electrode active material layer 22 can be fully reacted, and the energy density of the secondary battery 1000 can be improved.

[0100] For the measurement of the thickness of the negative electrode active material layer 22, the secondary battery 1000 can be discharged to 0 SOC% (State Of Charge, State Of Charge), disassembled, cleaned and dried; the thickness of the tab is measured using a micrometer. After subtracting the thickness of the negative electrode current collector 21, the thickness of the single-layer or double-layer negative electrode active material layer is obtained. When a double-layer negative electrode active material layer is used, the thickness of the single-layer negative electrode active material layer 22 can be obtained by dividing by 2.

[0101] In some other embodiments, the first groove 2211 and the second groove 2221 can improve the kinetics of the negative electrode sheet 20 and increase the ion diffusion efficiency. In combination with the size design of the first groove 2211 and the second groove 2221, the kinetics of the negative electrode sheet 20 can be further improved. In the embodiments of the present application, the negative electrode active material with a large thickness and specific capacity can be adapted.

[0102] For example, the negative electrode active material layer 22 includes at least one of silicon, silicon oxide, silicon-carbon composite, and silicon alloy. Compared with the conventional graphite material (372 mAh / g of specific capacity), the specific capacity (4200 mAh / g of specific capacity) of the silicon material is higher than that of the graphite material, which can improve the capacity of the secondary battery 1000 and further improve the charge-discharge rate of the secondary battery 1000. In the negative electrode active material layer 22, the mass content of silicon is G, and 3%≤G≤10%. Optionally, when the electrode assembly 100 is in a stacked shape, 6%≤G≤10%; and when the electrode assembly 100 is in a rolled shape, 3%≤G≤8%, which can improve the kinetics of the negative electrode sheet 20 and further improve the capacity of the secondary battery 1000.

[0103] For the measurement of the mass content of silicon, the content of silicon can be indirectly calculated by measuring the curve of the mass change of the sample with temperature under the control of the atmosphere. In some embodiments, referring to FIGS. 4 and 9, the separator 30 includes a substrate layer 31, an adhesive layer 32, and a ceramic layer 33. The adhesive layer 32 is arranged between the substrate layer 31 and the ceramic layer 33, and the ceramic layer 33 faces the negative electrode active material layer 22. The ceramic layer 33 usually has a certain pore structure, which can adjust the transmission of lithium ions in the separator 30 to a certain extent, so that the lithium ions are more uniformly distributed on the surface of the negative electrode active material layer 22, reducing the local high or low concentration of lithium ions, thereby improving the charge-discharge performance and cycle life of the secondary battery 1000. In addition, the ceramic layer 33 can improve the interface contact between the separator 30 and the negative electrode active material layer 22, and reduce the interface resistance, which is helpful to improve the transmission efficiency of lithium ions at the interface.

[0104] The substrate layer 31 includes at least one of polyethylene, polypropylene, polytetrafluoroethylene, cellulose acetate, or cellulose nanofiber. The adhesive layer 32 includes at least one of polyvinylidene fluoride, polyvinyl butyral, or polyacrylate. The ceramic layer 33 includes at least one of boehmite, alumina, or silicon dioxide, which has better high-temperature resistance and better mechanical properties, and can prolong the service life of the separator 30.

[0105] In the process of stacking or stacking and winding the positive electrode tab 10 and the negative electrode tab 20, the negative active material layer 22 can extend beyond the positive active material layer 12 on both sides in the width direction (the first direction X) or on both sides in the length direction (the second direction Y) of the negative active material layer 22. To reduce the occurrence of lithium precipitation, the negative active material layer 22 is usually provided with multiple portions extending beyond the positive active material layer 12 in the length direction and the width direction.

[0106] For example, referring to FIG. 8, the negative active material layer 22 further includes a third portion 223, and the first portion 221 is connected between the second portion 222 and the third portion 223 in the first direction X. The third portion 223 is a portion of the negative active material layer 22 extending beyond the positive active material layer 12 on the other side in the first direction X. Similar to the second portion 222, the third portion 223 is provided with a third groove 2231, and the width of the third groove 2231 is W3 in the second direction Y, and the depth of the third groove 2231 is D3 in the third direction Z. W3 x D3 < W1 x D1. This can reduce lithium precipitation, optimize electrolyte distribution, and reduce the occurrence of side reactions.

[0107] Since the above-mentioned negative tab 40 does not extend from the third portion 223, the current density of the third portion 223 is smaller than that of the second portion 222. The width of the third portion 223 in the width direction (the first direction X) of the electrode tab can be set to be smaller than the width of the second portion 222, and the third groove 2231 is similar. For example, the length of the third groove 2231 is L3 in the first direction X, and 0.5 mm ≤ L3 ≤ 1.0 mm. This can reduce the loss of energy density of the secondary battery 1000, reduce the occurrence of lithium precipitation, and improve the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery 1000. The third groove 2231 can also penetrate the third portion 223, which can improve the kinetics of the negative electrode tab 20. The third groove 2231 can also be in communication with the first groove 2211 to further improve the kinetics of the negative electrode tab 20.

[0108] In other embodiments, the negative active material layer 22 further includes a fourth portion 224 and a fifth portion 225, and the first portion 221 is connected between the fourth portion 224 and the fifth portion 225 in the second direction Y. The fourth portion 224 is a portion of the negative active material layer 22 extending beyond the positive active material layer 12 on one side in the second direction Y, and the fifth portion 225 is a portion of the negative active material layer 22 extending beyond the positive active material layer 12 on the other side in the second direction Y.

[0109] The fourth portion 224 is provided with a fourth groove 2241, which can be arranged along the second direction Y. Please refer to FIG. 10 and FIG. 11. Along the first direction X, the width of the fourth groove 2241 is W4. Along the third direction Z, the depth of the fourth groove 2241 is D4. W4 x D4 < W1 x D1. For example, 50 μm ≤ W4 ≤ 80 μm, and 5 μm ≤ D4 ≤ 15 μm. The unit cross-sectional area of the fourth groove 2241 is less than that of the first groove 2211, which can reduce lithium precipitation, optimize electrolyte distribution, and reduce the occurrence of side reactions.

[0110] As for the length of the fourth groove 2241 along the first direction X, in some embodiments, the length of the fourth groove 2241 is L4, and 0.5 mm ≤ L4 ≤ 1.0 mm. This can reduce the loss of energy density of the secondary battery 1000, reduce the occurrence of lithium precipitation, and improve the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery 1000. The fourth groove 2241 can also extend through the fourth portion 224 along the second direction Y, which can further improve the kinetics of the negative electrode sheet 20.

[0111] Based on the same inventive concept, the fifth portion 225 is provided with a fifth groove 2251, which can be arranged along the second direction Y. Please refer to FIG. 10 and FIG. 11. Along the first direction X, the width of the fifth groove 2251 is W5. Along the third direction Z, the depth of the fifth groove 2251 is D5. W5 x D5 < W1 x D1. For example, 50 μm ≤ W5 ≤ 80 μm, and 5 μm ≤ D5 ≤ 15 μm. The unit cross-sectional area of the fifth groove 2251 is less than that of the first groove 2211, which can reduce lithium precipitation, optimize electrolyte distribution, and reduce the occurrence of side reactions.

[0112] As for the length of the fifth groove 2251 along the first direction X, in some embodiments, the length of the fifth groove 2251 is L5, and 0.5 mm ≤ L5 ≤ 1.0 mm. This can reduce the loss of energy density of the secondary battery 1000, reduce the occurrence of lithium precipitation, and improve the diffusion efficiency of lithium ions, thereby improving the charge-discharge rate of the secondary battery 1000. The fifth groove 2251 can also extend through the fifth portion along the second direction Y, which can further improve the kinetics of the negative electrode sheet 20.

[0113] In a second aspect, the present application also provides an electronic device comprising the secondary battery 1000 according to any one of the embodiments of the first aspect. The electronic device according to the embodiments of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device includes, but is not limited to, a Bluetooth headset, a mobile phone, a tablet, a notebook computer, an electric toy, an electric tool, an electric vehicle, an electric car, a ship, a spacecraft, and the like. Among them, the electric toy can include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, an electric plane toy, and the like, and the spacecraft can include an airplane, a rocket, a space shuttle, a spacecraft, and the like.

[0114] Embodiment 1

[0115] Preparation of the positive electrode tab

[0116] The positive electrode active material lithium iron phosphate, the positive electrode conductive agent acetylene black, and the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight 5 x 10 5 ) in a mass ratio of 94:3:3 were mixed, N-methyl pyrrolidone (NMP) was added as a solvent, and a positive electrode slurry with a solid content of 75 wt% was prepared and stirred uniformly in a vacuum stirrer. An aluminum foil with a thickness of 8 μm and a length of 1000 mm was selected as the positive electrode current collector, and the positive electrode slurry was uniformly coated on one surface of the positive electrode current collector aluminum foil, and dried at 110°C to obtain a positive electrode tab with a single-sided positive electrode active material layer. Then, the above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode tab with a double-sided positive electrode active material layer.

[0117] Preparation of the negative electrode tab

[0118] The negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (Super P), and binder styrene butadiene rubber (SD-3) were mixed in a weight ratio of 89.5:8:1:1.5, and then deionized water was added as a solvent to prepare a negative electrode slurry with a solid content of 50 wt% and stirred uniformly. A copper foil with a thickness of 5 μm and a length of 1050 mm was selected as the negative electrode current collector, and the negative electrode slurry was uniformly coated on one surface of the negative electrode current collector copper foil, and a copper foil area without negative electrode slurry was reserved, and dried at 90°C to obtain a single-sided negative electrode tab. After the above steps are completed, the single-sided coating of the negative electrode tab is completed. Then, the above steps are repeated on the other surface of the negative electrode tab to obtain a negative electrode tab with a double-sided negative electrode active material layer. Among them, the negative electrode active material layer includes a first part and a second part, and when the positive electrode tab and the negative electrode tab are stacked, the first part overlaps the positive electrode active material layer along the thickness direction of the negative electrode tab, and the second part exceeds the positive electrode active material layer in the width direction of the negative electrode tab.

[0119] The first groove is opened in the first part and the second groove is opened in the second part by a laser drilling process. The first groove has a width W1 of 65 μm, a depth D1 of 13 μm, and a spacing H1 of 2 mm between adjacent two first grooves. The second groove has a width W2 of 70 μm, a depth D2 of 10 μm, and a spacing H2 of 1.5 mm between adjacent two second grooves.

[0120] <Preparation of the isolation film>

[0121] A porous isolation film is prepared by selecting polyethylene as a 7 μm base material layer, selecting polyvinylidene fluoride as a bonding layer, and disposing an aluminum oxide ceramic layer with a thickness of 2 μm on the side of the bonding layer away from the base material layer.

[0122] <Preparation of the electrolyte>

[0123] In a dry argon atmosphere, ethylene carbonate, methyl ethyl carbonate and diethyl carbonate are mixed in a mass ratio of 30:50:20 to obtain an organic solution, then lithium salt lithium hexafluorophosphate is added to the organic solvent to dissolve and mix uniformly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0124] <Preparation of the lithium ion battery>

[0125] The above-prepared isolation film, positive electrode sheet, isolation film, negative electrode sheet are stacked in order to obtain an electrode assembly, and the tab is welded; the punched aluminum plastic film is placed in the assembly clamp with the pit surface facing up, and the electrode assembly is placed in the pit and pressed tightly. Then another punched aluminum plastic film is covered on the electrode assembly with the pit surface facing down, and after packaging, liquid injection, hot pressing, shaping and other processes, a lithium ion secondary battery is obtained.

[0126] Lithium precipitation / purple stain test method:

[0127] The secondary battery is placed in an environment with a test temperature of 25℃ for 30 min, and is subjected to step charging to 4.5V according to the following charging steps:

[0128] (1) 5C constant current charging to 4.23V;

[0129] (2) 4C constant current charging to 4.3V;

[0130] (3) 3C constant current charging to 4.5V;

[0131] (4) 2C constant current charging to 4.5V, constant voltage charging cutoff current to 0.05C;

[0132] After standing for 10 min, discharge according to the following steps:

[0133] 0.2C constant current discharge to 3V.

[0134] The above charge and discharge process is a cycle, after repeating 100 cycles, when the lithium ion battery is in full charge state (battery design maximum voltage 4.5V), the lithium ion battery is disassembled, the negative electrode sheet is obtained, the surface of the negative electrode sheet is golden yellow, the purple spot area is dark purple, and the lithium deposition area is gray white, if the lithium deposition area of the negative electrode sheet is greater than or equal to 1mm 2 , then it is determined that lithium deposition occurs, and the test fails. Each group is tested for 20 times, the number of failures is N, and the lithium deposition rate is N / 20; if the purple spot area of the negative electrode sheet is greater than or equal to 1mm 2 , then it is determined that purple spot occurs, and the test fails. Each group is tested for 20 times, the number of failures is N, and the purple spot rate is N / 20.

[0135] Low temperature capacity retention test method:

[0136] 1) The secondary battery is placed at a test temperature of 25℃ for 5 minutes, and discharged at 0.2C constant current to 3.0V, and placed for 5 minutes, and the discharge capacity is taken as the first discharge capacity;

[0137] 2) Charge to 4.50V at 0.2C constant current, and then charge to 0.02C at 4.50V constant voltage, and place for 5 minutes; 3) adjust the temperature in the furnace to-20℃, and place for 60 minutes, and discharge at 0.2C constant current to 3.0V;

[0138] Place for 5 minutes, and the ratio of the discharge capacity of the secondary battery after low temperature discharge to the first capacity multiplied by 100% is the low temperature capacity retention.

[0139] Different from example 1, the related parameters in examples 2 to 36, and comparative examples 1 to 3 are shown in table 1.

[0140] Table 1

[0141] According to Table 1, in the comparative example 3, the width and depth of the first groove and the second groove are both large, the cycle capacity retention rate is large, the electrode kinetics is good, however, the lithium precipitation rate and the purple stain rate are both high. In combination with examples 1 to 36 and comparative examples 1 to 3, when W2x D2 < W1x D1 is adopted, the lithium precipitation risk can be effectively reduced. This is because the second groove with small depth and small width in the second part can facilitate the negative active material layer to have sufficient excess lithium ions embedded in the positive active material layer, further reducing lithium precipitation. Moreover, the cycle capacity retention rate is good, because the first part serves as the main reaction area with the positive active material layer, the first groove with large depth and large width in the first part can improve the kinetics of the first part, improve the diffusion efficiency of lithium ions in the first part, and thus improve the charge-discharge rate of the lithium ion battery. Moreover, the purple stain rate is also low, because the first groove with large depth and large width in the first part and the second groove with small depth and small width in the second part can optimize the electrolyte distribution and reduce the occurrence of side reactions.

[0142] In combination with examples 1 to 6, in example 5, the purple stain rate is lower than that of example 1, and the low-temperature capacity retention rate is higher than that of example 1, which indicates that in example 5, the kinetics is better and the electrolyte distribution is more optimal. In combination with examples 2 to 5, in the examples of the present application, 70 pm ≤ W1 ≤ 100 pm can be selected, which can reduce lithium precipitation, optimize electrolyte distribution, and improve the kinetics of the negative electrode sheet.

[0143] In combination with examples 7 to 11 and example 4, in example 10, the low-temperature capacity retention rate is significantly higher than that of example 7, which indicates that the charge-discharge performance of the lithium ion battery in example 10 is higher than that of example 7, and the lithium precipitation rate and the purple stain rate are also closer to those of example 7. In the examples of the present application, in combination with examples 8 to 10 and example 4, 50 pm ≤ W2 ≤ 80 pm can be selected, which can reduce lithium precipitation, optimize electrolyte distribution, and improve the kinetics of the negative electrode sheet.

[0144] In combination with examples 12 to 18 and example 4, in example 13, the lithium precipitation rate is significantly lower than that of example 18, and the purple stain rate and the low-temperature capacity retention rate are closer to those of example 18. In terms of comprehensive performance, examples 13 to 17 and example 4 are better than examples 12 and 18. In the examples of the present application, 10 pm ≤ D1 ≤ 30 pm can be selected, which can reduce lithium precipitation, optimize electrolyte distribution, and improve the kinetics of the negative electrode sheet.

[0145] In combination with Embodiments 19 to 22 and Embodiment 4, in Embodiment 20, the lithium precipitation rate is lower than that in Embodiment 22, and the low-temperature capacity retention rate is also closer to that in Embodiment 22. In combination with Embodiments 20 to 21 and Embodiment 4, in the embodiments of the present application, 5 μm≤D2≤15 μm can be selected, which can reduce lithium precipitation, optimize electrolyte distribution, and improve the kinetics of the negative electrode sheet.

[0146] In combination with Embodiments 23 to 28 and Embodiment 4, in Embodiments 24 to 27 and Embodiment 4, the lithium precipitation rate, purple stain rate, and low-temperature capacity retention rate are all better than those in Embodiments 23 and 28. In the embodiments of the present application, in combination with Embodiments 24 to 27, 1 mm≤H1≤3 mm can be selected, which can reduce lithium precipitation, optimize electrolyte distribution, and improve the kinetics of the negative electrode sheet.

[0147] In combination with Embodiments 29 to 36 and Embodiment 4, in Embodiment 30, the low-temperature capacity retention rate is higher than that in Embodiment 36, Embodiment 30 has better kinetics, and the charge-discharge rate performance of the lithium ion battery is better than that in Embodiment 36. In combination with Embodiments 30 to 35, in the embodiments of the present application, 0.5 mm≤H2≤3.5 mm can be selected, which can reduce lithium precipitation, optimize electrolyte distribution, and improve the kinetics of the negative electrode sheet.

[0148] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; under the idea of the present application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above, which are not provided in detail for the sake of simplicity; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, a separator, and a negative electrode sheet which are stacked or stacked and wound, the positive electrode sheet comprising a positive electrode active material layer, the negative electrode sheet comprising a negative electrode active material layer, the negative electrode active material layer comprising a first portion and a second portion which are connected in this order in a first direction, the second portion being a portion of the negative electrode active material layer which protrudes beyond the positive electrode active material layer on one side of the negative electrode active material layer in the first direction, characterized in that, The first part is provided with a first groove, and the second part is provided with a second groove; In the second direction, the width of the first groove is W1, and the width of the second groove is W2, In the third direction, the depth of the first groove is D1, and the depth of the second groove is D2; W2×D2 The first direction, the second direction, and the third direction are perpendicular to each other, and the third direction is the thickness direction of the negative electrode tab.

2. The secondary battery according to claim 1, characterized by The secondary battery comprises a negative electrode tab, which is electrically connected to the negative electrode tab, and in the first direction, the negative electrode tab extends from the second part to the negative electrode tab.

3. The secondary battery according to claim 1 or 2, characterized by In the first direction, one end of the second part is connected to the first part, and the other end of the second part comprises a first edge, and the second groove extends to the first edge.

4. The secondary battery according to any one of claims 1 to 3, characterized by In the first direction, the first groove is communicated with the second groove.

5. The secondary battery according to any one of claims 1 to 4, characterized by The first groove penetrates the first part in the first direction, and / or the second groove penetrates the second part in the first direction.

6. The secondary battery according to any one of claims 1 to 5, characterized by 70μm≤W1≤100μm, 50μm≤W2≤80μm.

7. The secondary battery according to any one of claims 1 to 6, characterized by 10μm≤D1≤30μm, 5μm≤D2≤15μm.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The first part is provided with a plurality of first grooves, and the plurality of first grooves are arranged in the second direction, and the distance between adjacent two first grooves is H1, 1mm≤H1≤3mm; and / or, The second part is provided with a plurality of second grooves, and the plurality of second grooves are arranged in the second direction, and the distance between adjacent two second grooves is H2, 0.5mm≤H2≤3.5mm.

9. The secondary battery according to any one of claims 1 to 8, characterized by, In the first direction, the length of the first groove is L1, 75mm≤L1≤115mm; and / or, In the first direction, the length of the second groove is L2, 1.1mm≤L2≤1.8mm.

10. The secondary battery according to any one of claims 1 to 9, characterized by The positive electrode tab, the isolation film and the negative electrode tab are laminated and wound, and the isolation film is arranged between the positive electrode tab and the negative electrode tab; In the third direction, the thickness of the negative electrode active material layer is T1, 35μm≤T1≤75μm.

11. The secondary battery according to any one of claims 1 to 9, characterized by In the third direction, the positive electrode tab and the negative electrode tab are alternately laminated, and the isolation film is arranged between adjacent positive electrode tab and negative electrode tab; In the third direction, the thickness of the negative electrode active material layer is T1, 45μm≤T1≤85μm.

12. The secondary battery according to any one of claims 1 to 9, characterized by The negative electrode active material layer comprises at least one of silicon, silicon oxide, silicon-carbon composite and silicon alloy; In the negative electrode active material layer, the mass content of silicon element is G, 3%≤G≤10%.

13. The secondary battery according to any one of claims 1 to 12, characterized by The isolation film comprises a substrate layer, an adhesive layer and a ceramic layer, the adhesive layer is arranged between the substrate layer and the ceramic layer, and the ceramic layer faces the negative electrode active material layer. The base material layer includes at least one of polyethylene, polypropylene, polytetrafluoroethylene, cellulose acetate, or cellulose nanofiber; and / or, the adhesive layer includes at least one of polyvinylidene fluoride, polyvinyl butyral, or polyacrylate; and / or, the ceramic layer includes at least one of boehmite, alumina, or silica.

14. The secondary battery according to any one of claims 1 to 13, characterized by The negative electrode active material layer further includes a third portion, along the first direction, the first portion is connected between the second portion and the third portion, the third portion is a portion of the negative electrode active material layer beyond the positive electrode active material layer on the other side in the first direction; The third portion is provided with a third groove, along the second direction, the width of the third groove is W3, along the third direction, the depth of the third groove is D3, W3×D3 15. The secondary battery according to claim 14, characterized by Along the first direction, the length of the third groove is L3, 0.5mm≤L3≤1.0mm.

16. The secondary battery according to any one of claims 1 to 15, characterized by The negative electrode active material layer further includes a fourth portion and a fifth portion, along the second direction, the first portion is connected between the fourth portion and the fifth portion, the fourth portion is a portion of the negative electrode active material layer beyond the positive electrode active material layer on one side in the second direction, and the fifth portion is a portion of the negative electrode active material layer beyond the positive electrode active material layer on the other side in the second direction; The fourth portion is provided with a fourth groove, along the second direction, the width of the fourth groove is W4, along the third direction, the depth of the fourth groove is D4, W4×D4 The fifth portion is provided with a fifth groove, along the second direction, the width of the fifth groove is W5, along the third direction, the depth of the fifth groove is D5, W5×D5 17. The secondary battery according to claim 16, characterized by The fourth portion is provided with a fourth groove, along the first direction, the length of the fourth groove is L4, 0.5mm≤L4≤1.0mm; and / or, The fifth portion is provided with a fifth groove, along the first direction, the length of the fifth groove is L5, 0.2mm≤L5≤0.7mm.

18. An electronic device, comprising: A secondary battery including the secondary battery as claimed in any one of claims 1 to 17.

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