Secondary battery and electronic device

Abstract

The present application discloses a secondary battery and an electronic device. A negative electrode active material layer is provided with a first groove, one part of a negative electrode tab is arranged in the first groove, and the other part of the negative electrode tab extends, along a first direction, out of the negative electrode active material layer. The secondary battery further comprises a first adhesive layer. The first adhesive layer is arranged on the surface of a positive electrode active material layer facing a negative electrode sheet, and along a third direction, the first adhesive layer covers the first groove. Along the first direction, the width of the first adhesive layer in a second direction increases. Along the second direction, the maximum length of the first adhesive layer is L, and the length of the first groove is L1, wherein 1.3≤L / L1≤3.5. The length of the first adhesive layer is set on the basis of the current density of the negative electrode active material layer at the first groove, thereby reducing the occurrence of lithium deposition, and reducing the impact of the first adhesive layer on the energy density of the secondary battery.

Description

Secondary batteries and electronic devices Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in electronic devices such as smartphones, laptops, wearable devices, and smart home devices due to their high energy density and environmental friendliness. Lithium-ion batteries consist of a positive electrode and a negative electrode, typically requiring positive and negative tabs to lead out the positive and negative terminals. For example, grooves are created in the active material layer of the negative electrode to connect to the negative tab and lead out the negative electrode. However, lithium ions that escape from the active material layer of the positive electrode at the corresponding grooves in the negative tab may have difficulty embedding into the negative electrode's active material layer, easily leading to lithium plating at the grooves in the negative tab.

[0003] Summary of the Invention

[0004] This application aims to provide a secondary battery and electronic device that reduces the risk of lithium plating in secondary batteries.

[0005] In order to solve its technical problems, the embodiments of this application adopt the following technical solutions:

[0006] In a first aspect, this application proposes a secondary battery, including a positive electrode sheet, a separator, and a negative electrode sheet stacked or stacked and wound together. The negative electrode sheet includes a negative active material layer facing the positive electrode sheet, and the positive electrode sheet includes a positive active material layer facing the negative active material layer. The secondary battery also includes a negative electrode tab, with a first groove formed in the negative active material layer. A portion of the negative electrode tab is disposed in the first groove, and another portion of the negative electrode tab extends out of the negative electrode sheet along a first direction. The secondary battery also includes a first adhesive layer, disposed on the surface of the positive active material layer facing the negative electrode sheet. Along a third direction, the projection of the first adhesive layer covers the first groove. Along the first direction, the width of the first adhesive layer increases in a second direction. Along the second direction, the maximum length of the first adhesive layer is L, and the length of the first groove is L1, where 1.3 ≤ L / L1 ≤ 3.5. The third direction refers to the thickness direction of the positive and / or negative electrode sheets, and the first, second, and third directions are mutually perpendicular.

[0007] In the above technical solution, the length of the first adhesive layer is set according to the current density of the negative electrode active material layer at the first groove. The current density is higher in the edge region in the first direction, resulting in a higher risk of lithium plating. Increasing the width of the first adhesive layer in the second direction isolates the portion of the negative electrode active material layer with higher current density at the edge, reducing lithium ion accumulation and thus reducing lithium plating. By setting the length gradient of the first adhesive layer according to the aforementioned current density, the distribution of the reaction zones between the positive and negative electrode active material layers can be optimized. This reduces lithium plating while improving the utilization rate of both layers, thereby increasing the energy density of the secondary battery.

[0008] Furthermore, limiting the ratio to 1.3 ≤ L / L1 ≤ 3.5 ensures that the portion with higher current density is isolated by the first adhesive layer, reducing lithium ion accumulation near the first groove and thus reducing lithium plating. This also reduces energy density loss in the secondary battery, facilitating a balance between minimizing energy density loss and reducing lithium plating. Simultaneously, by limiting the ratio to 1.3 ≤ L / L1 ≤ 3.5, local polarization of the negative electrode active material layer is reduced, decreasing electrolyte decomposition and thus reducing the formation of the solid electrolyte interphase (SEI) film, further reducing lithium plating. Additionally, it alleviates heat concentration at the first groove, reducing side reactions and improving the performance of the secondary battery.

[0009] In some embodiments, along the first direction, the maximum width of the first adhesive layer is W, and the width of the first groove is W1, where 1.1 ≤ W / W1 ≤ 2. This can reduce energy density loss while reducing lithium plating.

[0010] In some embodiments, 2 ≤ L / L1 ≤ 2.8; and / or, 1.33 ≤ W / W1 ≤ 1.87. This can further reduce lithium plating while reducing the impact of the first adhesive layer on the energy density of the secondary battery.

[0011] In some embodiments, along the second direction, the first adhesive layer includes a first portion and a second portion disposed sequentially, with the second portion connected to the first portion. Along the first direction, the width of the second portion is equal to the width of the first portion. Along the third direction, the first portion coincides with the first groove. Along the second direction, the maximum length of the second portion is L2, where 0.15 ≤ L2 / L1 ≤ 1.25. Setting the length of the second portion according to the aforementioned current density can reduce lithium plating on one side of the length direction of the first groove and reduce the loss of energy density in the secondary battery. Optionally, 1.5 mm ≤ L2 ≤ 12.5 mm.

[0012] In some embodiments, 0.6 ≤ L2 / L1 ≤ 0.9, and optionally, 6 mm ≤ L2 ≤ 9 mm, can reduce the impact on energy density while reducing lithium plating.

[0013] In some embodiments, 8mm ≤ L1 ≤ 20mm.

[0014] In some embodiments, a first through-hole is formed in the second portion along a third direction. The first through-hole exposes a portion of the positive electrode active material layer, which can undergo a lithium-ion intercalation / deintercalation reaction with the portion of the negative electrode active material layer that can exert capacity, thereby improving the utilization rate of both the positive and negative electrode active material layers and thus increasing the energy density of the secondary battery.

[0015] In some embodiments, when viewed along a third direction, the area of ​​the second portion is S1, and the area occupied by all the first through holes on the second portion is S2, where 40% ≤ S2 / S1 ≤ 60%. This can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer and the positive electrode active material layer, thereby increasing the energy density of the secondary battery.

[0016] In some embodiments, the radius of the first through hole is R1, where 0.1mm≤R1≤1mm. This can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer, thereby increasing the energy density of the secondary battery.

[0017] In some embodiments, the second portion includes a first region and a second region, with the first region located between the first portion and the second region along a second direction. Along the first direction, the maximum width of the first region is W2, and the maximum width of the second region is W3, where W3 < W2. In the second portion, the current density is higher closer to the first groove. By limiting W3 < W2, lithium plating can be further reduced, and the impact of the first adhesive layer on the energy density of the secondary battery can be minimized.

[0018] In some embodiments, along the second direction, the width of the first region is equal to the width of the second region.

[0019] In some embodiments, the second portion includes a first region and a second region, with the first region located between the first portion and the second region along a second direction. Viewed along a third direction, the area of ​​the first region is S. 11 The sum of the areas of all the first through holes in the first region is S. 21 Looking along the third direction, the area of ​​the second region is S. 12 The sum of the areas of all the first through holes in the second region is S. 22 S 21 / S 11 <S 22 / S 12 .

[0020] The first region, corresponding to the negative electrode active material layer, has a high current density, a high risk of lithium plating, and fewer negative electrode active materials capable of utilizing capacity. By setting a lower pore density in the first region, the amount of lithium delithiation in the first region can be reduced, thereby reducing lithium plating in the negative electrode active material layer. The second region, corresponding to the negative electrode active material layer, has a lower current density, a lower risk of lithium plating, and more negative electrode active materials capable of utilizing capacity. By setting a higher pore density in the second region, the amount of lithium delithiation in the second region can be increased, allowing more negative electrode active materials to participate in the electrochemical reaction and improving the energy density of the secondary battery. By rationally setting the distribution of the first through-holes in the second part, the energy density of the secondary battery can be increased while reducing lithium plating.

[0021] In some embodiments, on the side where the negative electrode tab extends, the positive electrode active material layer has an edge region, and a first adhesive layer covers a portion of the edge region. This can reduce lithium ion accumulation at the edge of the negative electrode active material layer, reduce lithium plating, and reduce side reactions.

[0022] In some embodiments, along the direction from the first portion to the second portion, the width of the second portion gradually decreases in the first direction, which is closer to the gradient design of the current density, which can further reduce lithium plating and further reduce the impact of the first adhesive layer on the energy density of the secondary battery.

[0023] In some embodiments, the first adhesive layer further includes a third portion. Along the second direction, the third portion, the first portion, and the second portion are sequentially arranged, with the third portion connected to the first portion. Along the first direction, the width of the third portion is equal to the width of the first portion. Along the second direction, the maximum length of the third portion is L3, where 0.15 ≤ L3 / L1 ≤ 1.25. Depending on the current density, the maximum length of the second portion can be set near the aforementioned edge region, i.e., in the second segment, which can reduce lithium plating on the other side of the length direction of the first groove and reduce the loss of energy density in the secondary battery. Optionally, 1.5 mm ≤ L3 ≤ 12.5 mm.

[0024] In some embodiments, 0.6 ≤ L3 / L1 ≤ 0.9, and optionally, 6 mm ≤ L3 ≤ 9 mm, can reduce the impact on energy density while reducing lithium plating.

[0025] In some embodiments, a second through hole is provided in the third part along the third direction, so that part of the positive electrode active material layer is exposed. The exposed positive electrode active material layer can also undergo lithium-ion intercalation / deintercalation reaction with the part of the negative electrode active material layer that can exert capacity, thereby further improving the utilization rate of the negative electrode active material layer and increasing the energy density of the secondary battery.

[0026] In some embodiments, the third portion includes a third region and a fourth region, with the fourth region located between the first portion and the third region along the second direction. Viewed along the third direction, the area of ​​the third region is S. 31 The sum of the areas of all the second through holes in the third region is S. 41 Looking along the third direction, the area of ​​the fourth region is S. 32 The sum of the areas of all the second through holes in the fourth region is S. 42 S 41 / S 31 >S 42 / S 32 .

[0027] The fourth region, corresponding to the negative electrode active material layer, has a high current density, a high risk of lithium plating, and fewer negative electrode active materials capable of utilizing capacity. By setting a lower pore density in the first region, the amount of lithium delithiation in the fourth region can be reduced, thereby reducing lithium plating in the negative electrode active material layer. The third region, corresponding to the negative electrode active material layer, has a lower current density, a lower risk of lithium plating, and more negative electrode active materials capable of utilizing capacity. By setting a higher pore density in the third region, the amount of lithium delithiation in the third region can be increased, allowing more negative electrode active materials to participate in the electrochemical reaction and improving the energy density of the secondary battery. By rationally setting the distribution of the first through-holes in the second part, lithium plating can be reduced while simultaneously increasing the energy density of the secondary battery.

[0028] In some embodiments, the first adhesive layer further includes a fourth portion, which is sequentially disposed with the first portion along the first direction and connected to the first portion. Along the third direction, the first portion overlaps with the first groove, and the fourth portion is disposed on the positive electrode active material layer. Along the first direction, the maximum width of the fourth portion is W4, where 1.5mm ≤ W4 ≤ 15mm, which can reduce energy density loss while reducing lithium plating. Further, 5mm ≤ W4 ≤ 13mm.

[0029] In some embodiments, the fourth portion includes a fifth region and a sixth region arranged sequentially, with the sixth region located between the first portion and the fifth region along a first direction. Along a second direction, the maximum length of the fifth region is L. 51 The maximum length of the sixth region is L. 61 L 51 <L 61 On the side of the first groove furthest from the edge of the negative electrode active material layer, the closer to the first groove, the greater the current density. Similarly, the length gradient of the fourth part can be set according to the current density to optimize the distribution of the fourth part, thereby reducing energy density loss and lithium plating.

[0030] In some embodiments, along the first direction, the width of the fifth region is equal to the width of the sixth region.

[0031] In some embodiments, a third through hole is provided in the fourth part along the third direction, so that part of the positive electrode active material layer is exposed. The exposed positive electrode active material layer can undergo lithium-ion intercalation / deintercalation reaction with the part of the negative electrode active material layer that can exert capacity, which can improve the utilization rate of the positive electrode active material layer and the negative electrode active material layer, thereby improving the energy density of the secondary battery.

[0032] In some embodiments, the fourth portion includes a fifth region and a sixth region, with the sixth region located between the first portion and the fifth region along a first direction. Viewed along a third direction, the area of ​​the fifth region is S. 51 The sum of the areas of all the third through holes in the fifth region is S. 61 Looking along the third direction, the area of ​​the sixth region is S. 52 The sum of the areas of all the third through holes in the sixth region is S. 62 S 61 / S 51 >S 62 / S 52 .

[0033] The sixth region, corresponding to the negative electrode active material layer, has a high current density, a high risk of lithium plating, and fewer negative electrode active materials capable of utilizing capacity. By setting a lower pore density in the sixth region, the amount of lithium delithiation in the sixth region can be reduced, thereby reducing lithium plating in the negative electrode active material layer. The fifth region, corresponding to the negative electrode active material layer, has a lower current density, a lower risk of lithium plating, and more negative electrode active materials capable of utilizing capacity. By setting a higher pore density in the fifth region, the amount of lithium delithiation in the fifth region can be increased, allowing more negative electrode active materials to participate in the electrochemical reaction and improving the energy density of the secondary battery. By rationally setting the distribution of the third through-hole in the fourth part, the energy density of the secondary battery can be increased while reducing lithium plating.

[0034] In some embodiments, along the second direction, the first adhesive layer includes a third portion, a first portion, and a second portion sequentially disposed, with the third portion connected to the first portion and the second portion connected to the first portion. Along the second direction, the negative electrode active material layer includes a first end and a second end disposed opposite to each other, with the second portion located between the first groove and the first end, and the third portion located between the first groove and the second end. Along the second direction, the length from the first groove to the first end is L4, and the length from the negative electrode tab to the second end is L5. Along the second direction, the length of the second portion is L2, and the length of the third portion is L3. L4 > L5, L2 > L3; or, L4 < L5, L2 < L3. By reasonably setting the lengths of the second and third portions, the size of the first adhesive layer can be reduced while lowering the risk of lithium plating, thereby increasing the energy density of the secondary battery.

[0035] In some embodiments, along a third direction, the thickness of the positive electrode active material layer covered by the first adhesive layer is H1, and the thickness of the positive electrode active material layer not covered by the first adhesive layer is H2, where H1 < H2, thereby reducing the amount of positive electrode active material layer covered by the first adhesive layer and reducing the impact of the first adhesive layer on the energy density of the secondary battery.

[0036] In some embodiments, the thickness of the first adhesive layer along the third direction is H3, where H1+H3≤H2, which allows the first adhesive layer to not occupy the thickness of the positive electrode active material layer, thus fully improving the energy density of the secondary battery and reducing lithium plating.

[0037] In some embodiments, the first adhesive layer includes a substrate layer and an adhesive layer, the adhesive layer being disposed on the surface of the substrate layer facing the positive electrode active material layer. The substrate layer is made of at least one of polyethylene terephthalate or polyimide, giving it good insulation properties, mechanical strength, and high-temperature resistance, providing structural support for the adhesive layer, and enabling the entire first adhesive layer to maintain a stable shape within the secondary battery. The adhesive layer is made of at least one of polyethylene, polypropylene, polyurethane, epoxy resin, styrene-butadiene rubber, polyvinylidene fluoride, or polyacrylic acid, and has good initial tack and holding power, allowing it to bond tightly to the positive electrode active material layer.

[0038] Secondly, this application also proposes an electronic device including a secondary battery as described in any of the embodiments of the first aspect above.

[0039] Additional aspects and advantages of the embodiments of this application will be described, shown, or illustrated in part by way of implementation of the embodiments of this application in the following description. Attached Figure Description

[0040] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0041] Figure 1 is a schematic diagram of the structure of a secondary battery according to some embodiments of this application;

[0042] Figure 2 is a schematic diagram of the winding structure of the electrode assembly in some embodiments of this application;

[0043] Figure 3 is a schematic diagram of the stacked structure of electrode assemblies in some embodiments of this application;

[0044] Figure 4 is a schematic diagram of the stacked structure of positive and negative electrode sheets in some embodiments of this application;

[0045] Figure 5 is a cross-sectional schematic diagram of the first adhesive layer in some embodiments of this application;

[0046] Figure 6 is a schematic diagram of the current density of a portion of the negative electrode active material layer at the first groove in some embodiments of this application.

[0047] Figure 7 is a schematic diagram of the structure of a portion of the negative electrode active material layer (including the first groove) and the first adhesive layer in some embodiments of this application;

[0048] Figure 8 is a schematic diagram of the structure of a portion of the negative electrode active material layer (including the first groove) and a portion of the positive electrode active material layer (including the first adhesive layer) in some embodiments of this application;

[0049] Figure 9 is a schematic diagram of the structure of a portion of the negative electrode active material layer (including the first groove) and a portion of the positive electrode active material layer (including the first adhesive layer) in some embodiments of this application;

[0050] Figure 10 is a schematic diagram of the structure of a portion of the negative electrode active material layer (including the first groove) and a portion of the positive electrode active material layer (including the first adhesive layer) in some embodiments of this application;

[0051] Figure 11 is a schematic diagram (top view) of the structure of the first adhesive layer according to an embodiment of this application;

[0052] Figure 12 is a schematic diagram of the negative electrode sheet and the first adhesive layer in some embodiments of this application;

[0053] Figure 13 is a schematic diagram of the structure of the positive electrode sheet in some embodiments of this application.

[0054] Explanation of reference numerals in the attached figures:

[0055] 1000, secondary battery;

[0056] 100. Electrode assembly;

[0057] 10. Positive electrode sheet; 11. Positive current collector; 111. First surface; 112. Second surface; 12. Positive active material layer; 121. Edge region;

[0058] 20. Negative electrode sheet; 20a. First end; 20b. Second end; 21. Negative current collector; 211. Third surface; 212. Fourth surface; 22. Negative active material layer; 221. First groove; 222. Edge;

[0059] 30. Separating membrane;

[0060] 40. Negative electrode;

[0061] 50. First adhesive layer; 50a. Substrate layer; 50b. Adhesive layer; 501. First segment; 502. Second segment; 51. First part; 52. Second part; 521. First region; 522. Second region; 523. First through hole; 53. Third part; 531. Third region; 532. Fourth region; 533. Second through hole;

[0062] 60. Second adhesive layer; 54. Fourth part; 541. Fifth region; 542. Sixth region; 543. Third through hole;

[0063] 200. Shell;

[0064] X, first direction; Y, second direction; Z, third direction. Detailed Implementation

[0065] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.

[0066] In this application, the reference to "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that is mutually exclusive with other embodiments.

[0067] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0068] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0069] The technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0070] In a first aspect, this application proposes a secondary battery 1000. Referring to Figure 1, the secondary battery 1000 includes an electrode assembly 100, a housing 200, and an electrolyte (not shown in the figure). The housing 200 can accommodate the electrode assembly 100 and the electrolyte, and the electrolyte wets the electrode assembly 100 within the housing 200.

[0071] Referring to Figures 2 and 3, the electrode assembly 100 includes a positive electrode 10, a negative electrode 20, and a separator 30. The positive electrode 10, separator 30, and negative electrode 20 are stacked and wound together. Figure 2 shows the winding structure of the electrode assembly 100, for example, stacked along the thickness direction of the positive electrode 10 and / or the negative electrode 20 and wound along their length direction to form a wound electrode assembly 100. In some other embodiments, the electrode assembly 100 may also adopt a stacked structure. Figure 3 shows the stacked structure of the electrode assembly 100, where multiple positive electrode 10s and multiple negative electrode 20 are alternately stacked along the third direction Z (the thickness direction of the positive electrode 10 and / or the negative electrode 20), and a separator 30 is disposed between adjacent positive electrode 10s and negative electrode 20s.

[0072] It should be noted that, in the embodiments of this application, the first direction X is the width direction of the positive electrode 10 and / or the negative electrode 20, the second direction Y is the length direction of the positive electrode 10 and / or the negative electrode 20, and the third direction Z is the thickness direction of the positive electrode 10 and / or the negative electrode 20. The first direction X, the second direction Y, and the third direction Z are all perpendicular to each other. In the stacked electrode assembly, the first direction X may also be the width direction of the positive electrode 10 and / or the negative electrode 20, and the second direction Y may also be the length direction of the positive electrode 10 and / or the negative electrode 20.

[0073] Referring to Figure 4, the positive electrode 10 includes a positive current collector 11 and a positive active material layer 12. The positive current collector 11 serves as the conductive substrate of the positive electrode 10 and can be made of flat aluminum foil. Aluminum foil has high conductivity and low resistance, which can improve the charge / discharge rate of the secondary battery 1000. Furthermore, aluminum foil has certain strength and ductility, making it less prone to breakage or deformation during winding or stacking processes, thus ensuring the structural integrity of the positive electrode 10. Simultaneously, the positive electrode of the secondary battery 1000 is at a high potential during charge / discharge, and the aluminum foil is relatively stable at this potential, making it less prone to chemical reactions, thereby improving the charge / discharge stability of the secondary battery 1000. In other embodiments, the positive current collector 11 can also be made of titanium foil, nickel foil, or stainless steel foil.

[0074] The positive electrode active material layer 12 can be disposed on at least one surface of the positive electrode current collector 11 in the thickness direction, for example, along the third direction Z. The positive electrode current collector 11 includes a first surface 111 and a second surface 112 disposed opposite to each other, and the positive electrode active material layer 12 can be disposed 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, etc. The above-mentioned material components are mixed, stirred evenly, and coated on the first surface 111 and / or the second surface 112 to obtain the positive electrode active material layer 12. 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.

[0075] Referring to Figure 4, the negative electrode 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. The negative electrode current collector 21 serves as the conductive substrate of the negative electrode 20 and can be made of flat copper foil. Copper foil has high conductivity and low resistance, which can improve the charge / discharge rate of the secondary battery 1000. Furthermore, copper foil has certain strength and ductility, making it less prone to breakage or deformation during winding or stacking processes, thus ensuring the structural integrity of the negative electrode 20. Simultaneously, the negative electrode of the secondary battery 1000 is at a low potential during charge / discharge, and the copper foil is relatively stable at this potential, making it less prone to chemical reactions, thereby improving the charge / discharge stability of the secondary battery 1000. In other embodiments, the negative electrode current collector 21 can also be made of titanium foil, nickel foil, stainless steel foil, or silver foil.

[0076] The negative electrode active material layer 22 can be disposed on at least one surface of the negative electrode current collector 21 in the thickness direction, for example, along the third direction Z. The negative electrode current collector 21 includes a third surface 211 and a fourth surface 212 disposed opposite to each other, and the negative electrode active material layer 22 can be disposed 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, etc. These materials are mixed, stirred evenly, and coated on the third surface 211 and / or the fourth surface 212 to obtain the negative electrode active material layer 22. The negative electrode active material includes one or more of graphite, soft carbon, hard carbon, carbon fiber, elemental silicon, silicon oxide, silicon alloy, etc.

[0077] The secondary battery 1000 also includes a negative electrode tab 40. Referring to Figure 4, the negative electrode tab 40 is connected to the aforementioned negative electrode sheet 20. For example, a first groove 221 is provided in the aforementioned negative electrode active material layer 22, so that the negative electrode current collector 21 is exposed from the first groove 221. A portion of the negative electrode tab 40 is disposed in the first groove 221 and connected to the negative electrode current collector 21. The connection method includes, but is not limited to, welding or conductive adhesive bonding. The other portion of the negative electrode tab 40 extends out of the negative electrode sheet 20 along the first direction X.

[0078] During charging, an external power source delivers current to the electrode assembly 100 through the tabs, causing lithium ions to escape from the positive electrode 10 and embed into the negative electrode 20. The inventors of this application have discovered that because the negative electrode active material layer 22 has a first groove 221, the remaining space of the negative electrode active material layer 22 is reduced. In the portion of the positive electrode active material layer 12 facing the first groove 221, the escaped lithium ions have difficulty embedding into the negative electrode active material layer 22, easily leading to lithium plating. Lithium plating forms lithium metal dendrites, which may pierce the separator, causing a short circuit between the positive and negative electrodes, resulting in safety issues. It also reduces the capacity and cycle life of the secondary battery 1000.

[0079] To mitigate the aforementioned problems, in the embodiments of this application, referring to FIG4, the secondary battery 1000 further includes a first adhesive layer 50, which is disposed on the surface of the positive electrode active material layer 12 facing the negative electrode plate 20. Along the third direction Z, the projection of the first adhesive layer 50 covers the first groove 221, ensuring that the portion of the positive electrode active material layer 12 corresponding to the first groove 221 is covered by the first adhesive layer 50. This reduces or prevents the extraction of lithium ions from this portion, thereby allowing the negative electrode active material layer 22 to have sufficient capacity to embed lithium ions, thus reducing the risk of lithium plating.

[0080] In some other embodiments, when the negative electrode tab 40 is welded to the negative electrode current collector 21, a second adhesive layer 60 can be used. The second adhesive layer 60 can be disposed on the negative electrode active material layer 22, and the second adhesive layer 60 covers the first groove 221, which can reduce welding burrs piercing the separator 30, thereby reducing the occurrence of short circuits. When the second adhesive layer 60 is provided, the length of the first adhesive layer 50 can be set to be greater than the length of the second adhesive layer 60, and the width of the first adhesive layer 50 can be set to be greater than the width of the second adhesive layer 60, so that the negative electrode active material layer 22 has sufficient margin to embed the lithium ions extracted from the positive electrode active material layer 12, thereby reducing lithium plating. For example, along the third direction Z, the projection of the second adhesive layer 60 is located within the projection of the first adhesive layer 50.

[0081] Regarding the material of the first adhesive layer 50, in some embodiments, referring to Figures 4 and 5, the first adhesive layer 50 includes a substrate layer 50a and an adhesive layer 50b, with the adhesive layer 50b disposed on the surface of the substrate layer 50a facing the positive electrode active material layer 12. The material of the substrate layer 50a includes at least one of polyethylene terephthalate or polyimide. The substrate layer 50a has good insulation properties, mechanical strength, and high-temperature resistance, providing structural support for the adhesive layer 50b, enabling the entire first adhesive layer 50 to maintain a stable shape within the secondary battery 1000. The material of the adhesive layer 50b includes at least one of polyethylene, polypropylene, polyurethane, epoxy resin, styrene-butadiene rubber, polyvinylidene fluoride, or polyacrylic acid. The adhesive layer 50b has good initial tack and holding power, allowing it to bond tightly to the positive electrode active material layer 12.

[0082] The inventors of this application have discovered that the negative electrode active material at the first groove 221 may form a solid electrolyte interphase (SEI) film, further hindering lithium ion insertion and exacerbating lithium plating. Through analysis, the inventors have determined that there is uneven current density at the first groove 221 of the negative electrode active material layer 22. Referring to Figure 6, which shows a schematic diagram of the current density of the negative electrode active material layer 22 near the first groove 221, darker colors represent higher current densities. As can be seen from Figure 6, the current density is higher closer to the first groove 221. When the current density of the negative electrode active material layer 22 is high, a large number of lithium ions reach the surface of the negative electrode active material layer 22 per unit time, easily leading to lithium ion accumulation on the surface of the negative electrode active material layer 22, thus causing lithium plating. Furthermore, high current density can lead to increased local polarization of the negative electrode active material layer 22 (the potential in a local area deviates from the equilibrium potential), which may lower the negative electrode potential. When the negative electrode potential is lower than the potential for lithium ions to be reduced to metallic lithium, lithium plating may occur.

[0083] The inventors of this application set the length of the first adhesive layer 50 according to the current density of the negative electrode active material layer 22 at the first groove 221. In the embodiments of this application, please refer to the figure. Along the second direction Y, the maximum length of the first adhesive layer 50 is L, and the length of the first groove 221 is L1, where 1.3 ≤ L / L1 ≤ 3.5. This allows the portion with a higher current density to be isolated by the first adhesive layer 50, reducing the accumulation of lithium ions near the first groove 221, thereby reducing lithium plating. Furthermore, limiting 1.3 ≤ L / L1 ≤ 3.5 can reduce the loss of 1000 mAh energy density in the secondary battery, making it easier to achieve a balance between reducing energy density loss and reducing lithium plating.

[0084] Meanwhile, the inventors of this application discovered that by limiting the polarization of the negative electrode active material layer 22 to 1.3 ≤ L / L1 ≤ 3.5, the local polarization is reduced, which can reduce electrolyte decomposition, thereby reducing the formation of the solid electrolyte interphase (SEI) film and further reducing lithium plating. Furthermore, it can alleviate heat concentration at the first groove 221, reduce side reactions, and thus improve the performance of the secondary battery 1000.

[0085] Referring further to Figures 6 to 8, the positive electrode active material layer 12 has an edge region 121. Near the edge 222 of the negative electrode active material layer 22, the current density is high, leading to a greater risk of lithium plating and higher heat generation, making side reactions more likely. To mitigate this problem, in the embodiments of this application, the first adhesive layer 50 covers a portion of the edge region 121, reducing lithium ion accumulation at the edge 222 of the negative electrode active material layer 22, thus reducing lithium plating and side reactions.

[0086] Because the current density increases closer to the first groove 221, and also increases closer to the edge 222 of the negative electrode active material layer 22, the first adhesive layer 50 can be configured with an irregular shape according to the current density. Referring to Figure 7, the width of the first adhesive layer 50 increases along the first direction X and in the second direction Y. For example, along the first direction X, the first adhesive layer 50 can be divided into a first segment 501 and a second segment 502 arranged sequentially, and along the second direction Y, the maximum length of the first segment 501 is L. a The maximum length of the second segment 502 is L. b L a <L b .

[0087] After the positive electrode 10 and the negative electrode 20 are stacked, the edge region 121 coincides with or nearly coincides with the edge 222 of the negative electrode active material layer 22, or the edge 222 extends beyond the first direction X. The first segment 501 is far from the edge region 121. The current density of the portion of the negative electrode active material layer 22 corresponding to the first segment 501 is relatively small, and the risk of lithium plating is low. Correspondingly, the first segment 501 with a shorter length can reduce lithium plating while allowing more of the positive electrode active material layer 12 and the negative electrode active material layer 22 to participate in the electrochemical reaction, thereby improving the energy density of the secondary battery 1000.

[0088] The second segment 502 is located near the edge region 121, where the current density is high and the risk of lithium plating is high. The longer second segment 502 isolates the portion with high current density at the edge 222 of the negative electrode active material layer 22, reducing lithium-ion accumulation and thus lithium plating. By setting the length gradient of the first adhesive layer 50 according to the aforementioned current density, the distribution of the reaction regions of the positive electrode active material layer 12 and the negative electrode active material layer 22 can be optimized. This reduces lithium plating while improving the utilization rate of both layers, thereby increasing the energy density of the secondary battery 1000.

[0089] Regarding the length of the first groove 221, in some embodiments, the length of the first groove 221 can be selected to be 8mm≤L1≤20mm. This can meet the manufacturing process requirements of the negative electrode tab 40 and balance the energy density.

[0090] Regarding the width of the first adhesive layer 50, please refer to Figures 6 and 7. Along the first direction X, the side of the first groove 221 furthest from the edge 222, closer to the center of the first groove 221, has a higher current density. The inventors of this application set the maximum width of the first adhesive layer 50 based on the current density in the width direction. Along the first direction X, the maximum width of the first adhesive layer 50 is W, and the width of the first groove 221 is W1, where 1.1 ≤ W / W1 ≤ 2. This reduces energy density loss while also reducing lithium plating. This reduces the impact on energy density while minimizing lithium plating.

[0091] In some instances, 2 ≤ L / L1 ≤ 2.8 can further reduce lithium plating while also reducing the impact of the first adhesive layer 50 on the energy density of the secondary battery 1000. Similarly, 1.33 ≤ W / W1 ≤ 1.87.

[0092] In some embodiments, referring to FIG9, along the second direction Y, the first adhesive layer 50 includes a first portion 51 and a second portion 52 sequentially disposed, the second portion 52 being connected to the first portion 51. Along the third direction Z, the first portion 51 coincides with the first groove 221, that is, along the second direction Y, the length of the first portion 51 can be set to L1. Along the second direction Y, the maximum length of the second portion 52 is L2, 0.15≤L2 / L1≤1.25. Based on the aforementioned current density, the maximum length of the second portion 52 can be set near the aforementioned edge region 121, that is, at the second segment 502, which can reduce lithium plating on one side of the length direction of the first groove 221 and reduce the loss of energy density of the secondary battery 1000.

[0093] Wherein, along the second direction Y, the length L2 of the second part 52 is selected as 1.5mm≤L2≤12.5mm, and can be any value from 1.5mm to 12.5mm. For example, L2 can be selected from 1.5mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, or 12.5mm, etc. Preferably, 6mm≤L2≤9mm can reduce the impact on energy density while reducing lithium plating.

[0094] In some embodiments, along the first direction, the width of the second portion 52 is equal to the width of the first portion 51. For example, when the size of the first groove 221 is small, referring to Figure 10, the width of the first portion 51 is W. a The width of the first part 51 is the same as the width of the first groove 221, that is, W a =W1, the width of the second part 52 is W b W a =W1=W b .

[0095] The inventors of this application have discovered that the current density of the negative electrode active material layer 22 near the first groove 221 is relatively high, resulting in a higher risk of lithium plating. However, near the first groove 221, a portion of the negative electrode active material layer 22 can still undergo electrochemical reactions and contribute to the capacity of the secondary battery 1000. Under the same conditions without lithium plating, the portion with higher current density can accept fewer lithium ions than the portion with lower current density, rather than being unable to accept any. Both portions with higher and lower current densities can contribute a certain capacity. The inventors of this application have addressed this by creating a through-hole in the second portion 52. For example, referring to Figure 11, along the third direction Z, the second portion 52 has a first through-hole 523. The first through-hole 523 exposes a portion of the positive electrode active material layer 12. This exposed portion of the positive electrode active material layer 12 can undergo lithium-ion intercalation / deintercalation reactions with the portion of the negative electrode active material layer 22 that contributes to the capacity, thereby improving the utilization rate of both the positive and negative electrode active material layers 12 and ultimately increasing the energy density of the secondary battery 1000.

[0096] Regarding the area ratio of the first through-hole 523 on the second part 52, the inventors of this application have found that if the area ratio of the first through-hole 523 on the second part 52 is too large, it may cause more lithium ions to be released from the positive electrode active material layer 12, which may easily lead to insufficient remaining material in the negative electrode active material layer 22, thereby causing lithium plating problems. On the other hand, if the area ratio of the first through-hole 523 on the second part 52 is too small, it is difficult for the portion of the negative electrode active material layer 22 that can exert its capacity to fully exert its capacity, resulting in a loss of energy density in the secondary battery 1000.

[0097] In the embodiments of this application, when viewed along the third direction Z, the area of ​​the second part 52 is S1, and the sum of the areas occupied by all the first through holes 523 on the second part 52 is S2. 40%≤S2 / S1≤60%, which can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer 22 and the positive electrode active material layer 12, thereby improving the energy density of the secondary battery 1000.

[0098] To measure the area ratio of the first through-hole 523 on the second part 52, an image analysis method can be used. First, the second part 52 is separated from the first adhesive layer 50. After scanning or photographing the image of the first adhesive layer 50, it is imported into image analysis software such as Adobe Photoshop. The contrast and brightness of the image are adjusted to make the boundary between the second part 52 and the first through-hole 523 clearer. Using a threshold adjustment tool, the image is converted into a binary image, i.e., the adhesive part is white and the through-hole part is black. In the image analysis software, the measurement tool can be used to measure the total area of ​​the second part 52 and the total area of ​​the first through-hole 523.

[0099] According to Figure 6, since the current density is greater closer to the first groove 221, the distribution density gradient of the first through-hole 523 in the first adhesive layer 50 can be set according to the magnitude of the current density. For example, referring to Figure 11, the second part 52 includes a first region 521 and a second region 522. Along the second direction Y, the first region 521 is located between the first part 51 and the second region 522. Observed along the third direction Z, the area of ​​the first region 521 is S. 11 The sum of the areas of all the first through holes 523 in the first region 521 is S. 21 Looking along the third direction Z, the area of ​​the second region 522 is S. 12 The sum of the areas of all the first through holes 523 in the second region 522 is S. 22 S 21 / S 11 <S 22 / S 12 .

[0100] The negative electrode active material layer 22 corresponding to the first region 521 has a large current density, which increases the risk of lithium plating. Furthermore, there are few negative electrode active materials that can perform their capacity. By setting the pore density of the first region 521 to be smaller, the amount of lithium de-lithiation in the first region 521 can be reduced, thereby reducing the occurrence of lithium plating in the negative electrode active material layer 22.

[0101] The negative electrode active material layer 22 corresponding to the second region 522 has a lower current density, resulting in a lower risk of lithium plating. It also allows for a greater amount of negative electrode active material to utilize its capacity. By increasing the pore density of the second region 522, the amount of lithium removal can be improved, facilitating the participation of more negative electrode active material in the electrochemical reaction and increasing the energy density of the secondary battery 1000. By rationally distributing the first through-holes 523 in the second part 52, the energy density of the secondary battery 1000 can be increased while reducing lithium plating.

[0102] In some embodiments, along the second direction Y, the width of the first region 521 is equal to the width of the second region 522.

[0103] Regarding the radius of the first through-hole 523, the inventors of this application have discovered that if the radius of the first through-hole 523 is too large, it may cause more lithium ions to be extracted per unit area of ​​the positive electrode active material layer 12, easily leading to insufficient remaining area on one side of the negative electrode active material layer 22, thus causing lithium plating problems. Conversely, if the radius of the first through-hole 523 is too small, fewer lithium ions are extracted per unit area, making it difficult for the portion of the negative electrode active material layer 22 that can utilize its capacity to fully exert its capacity, resulting in a loss of energy density in the secondary battery 1000. In the embodiments of this application, the radius of the first through-hole 523 is R1, where 0.1mm ≤ R1 ≤ 1mm. This can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer 22, thereby increasing the energy density of the secondary battery 1000. The radius of the first through-hole 523 can be the radius of the first through-hole 523 itself, or the radius of the fitted circle containing the first through-hole 523.

[0104] Regarding the width of the second portion 52, please refer to Figure 11. In some embodiments, along the first direction X, the maximum width of the first region 521 is W2, and the maximum width of the second region 522 is W3, where W3 < W2. In the second portion 52, the closer to the first groove 221, the greater the current density. The second region 522 is closer to the first groove 221. By limiting W3 < W2, lithium plating can be further reduced, as can the impact of the first adhesive layer 50 on the energy density of the secondary battery 1000.

[0105] In some embodiments, along the direction from the first portion 51 to the second portion 52, the width of the second portion 52 gradually decreases in the first direction X. As can be seen from Figure 6, the current density gradually decreases in the direction from the first groove 221 to both sides. By limiting the width of the second portion 52 to gradually decrease in the first direction X, a gradient design closer to the current density can be achieved, which can further reduce lithium plating and further reduce the impact of the first adhesive layer 50 on the energy density of the secondary battery 1000.

[0106] In some embodiments, the first adhesive layer 50 further includes a third portion 53, which may be configured similarly to the first portion 51 described above. Along the second direction Y, the maximum length of the third portion 53 is L3, where 0.15 ≤ L3 / L1 ≤ 1.25. Based on the aforementioned current density, the maximum length of the second portion 52 can be positioned near the edge region 121, i.e., in the second segment 502. This can reduce lithium plating on the other side of the length direction of the first groove 221 and reduce the loss of energy density in the secondary battery 1000.

[0107] Wherein, along the second direction Y, the length L3 of the second portion 52 can be selected as 1.5mm≤L3≤12.5mm, and can be any value from 1.5mm to 12.5mm. For example, L3 can be selected from 1.5mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, or 12.5mm, etc. Preferably, 6mm≤L3≤9mm can reduce the impact on energy density while reducing lithium plating.

[0108] In some embodiments, along the first direction X, the width of the third portion 53 is equal to the width of the first portion 51. For example, referring to Figure 11, the width of the third portion 53 is W. c W c =W a =W1.

[0109] Referring to Figures 9 and 11, along the third direction Z, the third part 53 has a second through hole 533. The second through hole 533 exposes part of the positive electrode active material layer 12. The exposed positive electrode active material layer 12 can also undergo lithium-ion intercalation / deintercalation reaction with the above-mentioned part of the negative electrode active material layer 22 that can exert capacity, further improving the utilization rate of the negative electrode active material layer 22 and increasing the energy density of the secondary battery 1000.

[0110] Regarding the area ratio of the second through hole 533 on the third part 53, in the embodiments of this application, when viewed along the third direction Z, the area of ​​the third part 53 is S3, and the sum of the areas occupied by all the second through holes 533 on the third part 53 is S4. 40%≤S4 / S3≤60%, which can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer 22, thereby improving the energy density of the secondary battery 1000.

[0111] In some embodiments, referring to FIG11, the third portion 53 includes a third region 531 and a fourth region 532. Along the second direction Y, the fourth region 532 is located between the first portion 51 and the third region 531. Viewed along the third direction Z, the area of ​​the third region 531 is S. 31 The sum of the areas of all the second through holes 533 in the third region 531 is S. 41 Looking along the third direction Z, the fourth region is 5. 32 The area is S 32 The sum of the areas of all the second through holes 533 in the fourth region 532 is S. 42 S 41 / S 31 >S 42 / S 32 .

[0112] The negative electrode active material layer 22 corresponding to the fourth region 532 has a large current density, which increases the risk of lithium plating. Furthermore, there are few negative electrode active materials that can perform their capacity. By setting the pore density of the first region 521 to be smaller, the amount of lithium delithiation in the fourth region 532 can be reduced, thereby reducing the occurrence of lithium plating in the negative electrode active material layer 22.

[0113] The negative electrode active material layer 22 corresponding to the third region 531 has a lower current density, resulting in a lower risk of lithium plating. It also allows for a greater amount of negative electrode active material to utilize its capacity. By increasing the pore density of the third region 531, the amount of lithium removal in the third region 531 can be improved, facilitating the participation of more negative electrode active material in the electrochemical reaction and increasing the energy density of the secondary battery 1000. By rationally distributing the first through-hole 523 in the second part 52, the energy density of the secondary battery 1000 can be increased while reducing lithium plating.

[0114] Similar to the first through hole 523, the radius of the second through hole 533 in this embodiment is R2, where 0.1mm≤R2≤1mm. This can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer 22, thereby increasing the energy density of the secondary battery 1000.

[0115] The first through hole 523 and the second through hole 533 can be formed on the first adhesive layer 50 through processes such as mechanical punching and laser drilling. For example, by using the pin roller punching method in mechanical punching, the radii of each first through hole 523 and each second through hole 533 can be made to be approximately the same, which makes it easier to set the hole density according to the current density.

[0116] In some embodiments, referring to FIG6, a relatively large current density still exists on the side of the first groove 221 away from the edge 222, which poses a risk of lithium plating. To reduce this problem, in embodiments of this application, referring to FIG11, the first adhesive layer 50 further includes a fourth portion 54. Along the first direction X, the fourth portion 54 is sequentially disposed with the first portion 51 and connected to the first portion 51. Along the third direction Z, the first portion 51 coincides with the first groove 221, and the fourth portion 54 is disposed on the positive electrode active material layer 12. Along the first direction X, the maximum width of the fourth portion 54 is W4, 1.5mm≤W4≤15mm, which can reduce energy density loss while reducing lithium plating. Further, 5mm≤W4≤13mm.

[0117] In some other embodiments, the fourth portion 54 includes a fifth region 541 and a sixth region 542, with the sixth region 542 located between the first portion 51 and the fifth region 541 along a first direction X. The fifth region 541 has a maximum length of L along a second direction Y. 51The maximum length of region 542 in the sixth region is L. 61 L 51 <L 61 On the side of the first groove 221 away from the edge 222 of the negative electrode active material layer 22, the closer to the first groove 221, the greater the current density. Similarly, the length gradient of the fourth part 54 can be set according to the current density to optimize the distribution of the fourth part 54, thereby reducing energy density loss and lithium plating.

[0118] In some embodiments, along the first direction X, the width of the fifth region 541 is equal to the width of the sixth region 542.

[0119] In some embodiments, referring to FIG11, a third through hole 543 is provided in the fourth part 54 along the third direction Z. The third through hole 543 exposes part of the positive electrode active material layer 12. The exposed positive electrode active material layer 12 can undergo lithium-ion intercalation / deintercalation reaction with the above-mentioned part of the negative electrode active material layer 22 that can exert capacity, which can improve the utilization rate of the positive electrode active material layer 12 and the negative electrode active material layer 22, thereby improving the energy density of the secondary battery 1000.

[0120] In some other embodiments, when viewed along the third direction Z, the area of ​​the fourth portion 54 is S5, and the sum of the areas occupied by all the third through holes 543 on the fourth portion 54 is S6. 40% ≤ S6 / S5 ≤ 60%, which can reduce the risk of lithium plating while improving the utilization rate of the negative electrode active material layer 22, thereby increasing the energy density of the secondary battery 1000.

[0121] The closer to the first groove 221, the greater the current density. Similar to the distribution of the first through-hole 523, the distribution density gradient of the third through-hole 543 in the fourth part 54 can be set according to the magnitude of the current density. For example, the fourth part 54 includes a fifth region 541 and a sixth region 542. Along the first direction X, the sixth region 542 is located between the first part 51 and the fifth region 541. Observed along the third direction Z, the area of ​​the fifth region 541 is S. 51 The sum of the areas of all the third through holes 543 in the fifth region 541 is S. 61 Looking along the third direction Z, the area of ​​the sixth region 542 is S. 52 The sum of the areas of all the third through holes 543 in the sixth region 542 is S. 62 S 61 / S 51 >S 62 / S 52 .

[0122] The negative electrode active material layer 22 corresponding to the sixth region 542 has a large current density, which increases the risk of lithium plating. Furthermore, there are few negative electrode active materials that can perform their capacity. By setting the pore density of the sixth region 542 to be smaller, the amount of lithium de-lithiation in the sixth region 542 can be reduced, thereby reducing the occurrence of lithium plating in the negative electrode active material layer 22.

[0123] The negative electrode active material layer 22 corresponding to the fifth region 541 has a lower current density, resulting in a lower risk of lithium plating. It also allows for a greater amount of negative electrode active material to utilize its capacity. By increasing the pore density of the fifth region 541, the amount of lithium removal in the fifth region 541 can be improved, facilitating the participation of more negative electrode active material in the electrochemical reaction and increasing the energy density of the secondary battery 1000. By rationally distributing the third through-hole 543 in the fourth part 54, the energy density of the secondary battery 1000 can be increased while reducing lithium plating.

[0124] Similar to the first through-hole 523, the radius of the third through-hole 543 is R3, where 0.1mm ≤ R3 ≤ 1mm. This reduces the risk of lithium plating while improving the utilization rate of the negative electrode active material layer 22, thereby increasing the energy density of the secondary battery 1000. The radius of the third through-hole 543 can be either the radius of the through-hole itself or the radius of the fitted circle containing the through-hole.

[0125] In some embodiments, along the second direction Y, the first adhesive layer 50 includes a third portion 53, a first portion 51, and a second portion 52 sequentially disposed, with the third portion 53 connected to the first portion 51 and the second portion 52 connected to the first portion 51. Referring to FIG12, along the second direction Y, the negative electrode active material layer 22 includes a first end 20a and a second end 20b disposed opposite to each other, with the second portion 52 located between the first groove 221 and the first end 20a, and the third portion 53 located between the first groove 221 and the second end 20b. Along the second direction Y, the length from the first groove 221 to the first end 20a is L4, and the length from the first groove 221 to the second end 20b is L5.

[0126] The inventors of this application have discovered that for the negative electrode 20, the larger L4 is, the greater the amount of negative electrode active material layer 22 participating in the electrochemical reaction, and the greater the current density between the first groove 221 and the first end 20a. Based on the length L4 from the first groove 221 to the first end 20a and the length L5 from the first groove 221 to the second end 20b, the inventors of this application set the length difference between the second part 52 and the third part 53.

[0127] For example, along the second direction Y, the length of the second part 52 is L2, and the length of the third part 53 is L3. When L4 > L5, it indicates that the current density between the first groove 221 and the first end 20a is relatively large, so L2 > L3 can be set. When L4 < L5, it indicates that the current density between the first groove 221 and the second end 20b is relatively large, so L2 < L3 can be set. By reasonably setting the lengths of the second part 52 and the third part 53, the size of the first adhesive layer 50 can be reduced while lowering the risk of lithium plating, thereby increasing the energy density of the secondary battery 1000. When L4 = L5, L3 and L2 can be set to be approximately equal, for example, |L3 - L2| ≤ 0.5 mm.

[0128] In some embodiments, referring to FIG13, along the third direction Z, the thickness of the positive electrode active material layer 12 covered by the first adhesive layer 50 is H1, and the thickness of the positive electrode active material layer 12 not covered by the first adhesive layer 50 is H2, where H1 < H2. This reduces the amount of positive electrode active material layer 12 covered by the first adhesive layer 50, thereby reducing the impact of the first adhesive layer 50 on the energy density of the secondary battery 1000.

[0129] When the second part 52 has a first through hole 523, the third part 53 has a second through hole 533, and the fourth part 54 has a third through hole 543, the amount of positive electrode active material layer 12 covered can be reduced, thereby reducing the extraction of lithium ions and thus reducing lithium plating.

[0130] When the second part 52 does not have a first through hole 523, the third part 53 has a second through hole 533, and the fourth part 54 has a third through hole 543, the covered positive electrode active material layer 12 is difficult or even impossible to participate in the electrochemical reaction. The first adhesive layer 50 can be set to not exceed the above-mentioned uncovered positive electrode active material layer 12. For example, along the third direction Z, the thickness of the first adhesive layer 50 is H3, H1+H3≤H2, so that the first adhesive layer 50 does not occupy the thickness of the positive electrode active material layer 12, which can fully improve the energy density of the secondary battery 1000 and reduce lithium plating.

[0131] Secondly, this application also proposes an electronic device, including a secondary battery 1000 as described in any embodiment of the first aspect above. The electronic device in this application is not particularly limited and can be any electronic device known in the prior art. For example, electronic devices include, but are not limited to, Bluetooth headsets, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., while spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0132] Experiment 1: Lithium Plating Test of Lithium-ion Batteries

[0133] Example A1:

[0134] <Preparation of the positive electrode>

[0135] The positive electrode active material is lithium iron phosphate, the positive electrode conductive agent is acetylene black, and the positive electrode binder is polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10⁻⁶). 5 The materials were mixed at a mass ratio of 94:3:3, with N-methylpyrrolidone (NMP) added as a solvent to prepare a positive electrode slurry with a solid content of 75 wt%, and stirred evenly under vacuum. An aluminum foil with a thickness of 8 μm and a length of 1000 mm was selected as the positive electrode current collector. The positive electrode slurry was uniformly coated on one surface of the aluminum foil and dried at 110°C to obtain a positive electrode sheet with a single-sided coating of positive active material. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive active material.

[0136] <Preparation of Negative Electrode Sheets>

[0137] A negative electrode active material (graphite powder), silicon powder, conductive carbon black (Super P), and binder (styrene-butadiene rubber (SBR)) were mixed in a weight ratio of 87.5:10:1:1.5. Deionized water was then added as a solvent to prepare a negative electrode slurry with a solid content of 50 wt%, and the mixture was stirred thoroughly. A copper foil with a thickness of 5 μm and a length of 1050 mm was selected as the negative electrode current collector. The negative electrode slurry was uniformly coated onto one surface of the copper foil, leaving an uncoated area. The foil was then dried at 90°C to obtain a single-sided negative electrode sheet. This process completes the single-sided coating of the negative electrode sheet. The same steps were then repeated on the other surface of the negative electrode sheet to obtain a double-sided coated negative electrode sheet.

[0138] <Preparation of the separating membrane>

[0139] A porous polyethylene (PE) film with a thickness of 7 μm was used as the separator.

[0140] <Electrolyte Preparation>

[0141] 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 solvent. Then, lithium hexafluorophosphate is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0142] <Preparation of Lithium-ion Batteries>

[0143] A 10mm (L1) × 15mm (W1) groove is created in the negative electrode active material layer using laser cleaning. A nickel sheet is selected as the negative electrode tab, which is welded to the negative electrode sheet in the first groove. After welding the positive electrode tab to the positive electrode sheet, polyethylene terephthalate is selected as the substrate layer, and epoxy resin is used as the adhesive layer to prepare a first adhesive layer with a maximum length L of 13mm and a maximum width W of 20mm. The first adhesive layer is placed on the positive electrode active material layer. The separator, positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in sequence and wound to obtain the electrode assembly. The first adhesive layer corresponds to the first groove. The first adhesive layer extends 1.5mm beyond the length of the positive electrode sheet on one side (second part L2) and 1.5mm beyond the length of the positive electrode sheet on the other side (third part L3). The width of the positive electrode sheet is divided into a first segment and a second segment, with the maximum length of the first segment being L. a The length is 11mm, and the second segment is near the edge, with a maximum length of L. b The diameter is 13mm. The electrode assembly is placed in the housing, the positive and negative tabs are led out, and then it is encapsulated and injected with electrolyte.

[0144] Lithium plating test

[0145] Place the secondary battery in an environment with a test temperature of 25℃ for 30 minutes, then charge it to 4.5V in stages according to the following charging steps:

[0146] (1) Charge at 5C constant current to 4.23V;

[0147] (2) Charge at 4C constant current to 4.3V;

[0148] (3) Charge to 4.5V using 3C constant current;

[0149] (4) Charge at 2C constant current to 4.5V, then charge at constant voltage to 0.05C;

[0150] After letting it stand for 10 minutes, proceed with the following steps to discharge:

[0151] Discharge to 3V using 0.2C DC.

[0152] The above charging and discharging process constitutes one cycle. After repeating 100 cycles, when the battery is fully charged (maximum battery design voltage 4.5V), the secondary battery is disassembled to obtain the negative electrode. If the lithium deposition area on the surface of the negative electrode active material layer near the negative electrode tab is found to be greater than or equal to 2mm, then... 2 If the number of cells with lithium plating is X, then the lithium plating rate is X / 20. Each group of 20 batteries is tested, and the number of cells with lithium plating is X.

[0153] Unlike Example A1, the relevant parameters in Examples A2 to A28 and Comparative Examples A1 to A12 are shown in Table 2 below. The maximum length of the second segment is L. b It is consistent with the maximum length L of the first adhesive layer.

[0154] Table 2

[0155] According to Table 1 above, the lithium plating rate in Comparative Example A5 is similar to that in Example A1. In Example A1, L... a <L b This allows more active material layers to be exposed, meaning more active material layers participate in the electrochemical reaction, which can improve the energy density of lithium-ion batteries. Therefore, when L / L1 ≥ 1.3, L can be selected. a <L b .

[0156] In Comparative Example A6, the first adhesive layer is too long, resulting in a significant loss of energy density. Furthermore, an excessively long first adhesive layer covers too much active material, potentially hindering ion transport, affecting electrolyte distribution, and causing uneven pressure distribution between electrodes. This can easily lead to electrode interface deterioration, affecting lithium-ion transport and intercalation, and also carries the risk of lithium plating. In Example A11, the risk of lithium plating is lower than in Comparative Example A6. Moreover, Example A11 uses L... a <L b The first adhesive layer is shorter and has less impact on energy density. Therefore, in the embodiments of this application, when 1.3 ≤ L / L1 ≤ 3.5, L is selected. a <L b That is, by increasing the width of the first adhesive layer (the width is greater on the side closer to the positive electrode active material layer along the tab extension direction), the energy density can be improved and the impact on lithium plating is small.

[0157] As can be seen from Examples A1 to A28 and Comparative Examples A1 to A12, setting the first adhesive layer according to the current density gradient, and when 1.3 ≤ L / L1 ≤ 3.5, can effectively reduce lithium plating. Furthermore, the closer the negative electrode active material layer is to the edge, the greater the current density; the closer it is to the first groove, the greater the current density. (Limited L) a <L b This method has a relatively small impact on the risk of lithium plating. Furthermore, by setting the length gradient of the first adhesive layer according to the current density, the distribution of the reaction regions of the positive and negative active material layers can be optimized. This can reduce lithium plating while improving the utilization rate of the positive and negative active material layers, thereby increasing the energy density of the secondary battery. Similarly, the optional values ​​are 0.15≤L2 / L1≤1.25 and 0.15≤L3 / L1≤1.25.

[0158] In Examples A5 to A8, 2 ≤ L / L1 ≤ 2.8 is satisfied, and in Examples A13 to A14, Examples 19 to 20, and Examples 25 to 26, 2 ≤ L / L1 ≤ 2.8 is also approximately satisfied, further reducing the risk of lithium plating. In the embodiments of this application, 2 ≤ L / L1 ≤ 2.8 is preferred, which can reduce the impact on energy density while reducing lithium plating. Similarly, it is preferred that 0.6 ≤ L2 / L1 ≤ 0.9 and 0.6 ≤ L3 / L1 ≤ 0.9.

[0159] Unlike Example A7, the relevant parameters in Examples B1 to B18 are shown in Table 2 below. The first adhesive layer extends beyond the first groove along the width direction of the positive electrode sheet, with a maximum extension width of W4 (the fourth part). The parameters of W4 are as follows. Along the width direction of the positive electrode sheet, the fourth part includes a fifth region and a sixth region. The sixth region is close to the first groove. The length relationship between the fifth region (L4) and the sixth region (L5) is shown in Table 2 below.

[0160] Table 2

[0161] According to Table 2 above, and in conjunction with Examples B1 to B18, the risk of lithium plating is relatively low in Examples B4 to B16. In this application, a value of 1.1 ≤ W / W1 ≤ 2 can be selected to minimize the risk of lithium plating. Furthermore, limiting L4 to L5 corresponds to a smaller impact on the risk of lithium plating. On the side of the first groove away from the edge of the negative electrode active material layer, the closer to the first groove, the higher the current density. Similarly, the length gradient of the fourth part can be set according to the current density to optimize the distribution of the fourth part, thereby reducing energy density loss and lithium plating. In conjunction with Examples B7 to B14, a value of 1.33 ≤ W / W1 ≤ 1.87 is preferred, which can reduce lithium plating while reducing the impact of the first adhesive layer on energy density.

[0162] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; under the concept of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of this application as described above, which are not provided in detail for the sake of brevity; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this 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 negative electrode sheet comprising a negative electrode active material layer facing the positive electrode sheet, the positive electrode sheet comprising a positive electrode active material layer facing the negative electrode active material layer, the secondary battery further comprising a negative electrode tab, the negative electrode active material layer being provided with a first recess, a portion of the negative electrode tab being disposed in the first recess, another portion of the negative electrode tab protruding from the negative electrode sheet in a first direction, characterized in that, The secondary battery also includes a first adhesive layer; The first adhesive layer is disposed on the surface of the positive electrode active material layer facing the negative electrode sheet, and in a third direction, the first adhesive layer covers the first groove; Along the first direction, the width of the first adhesive layer is increased in the second direction; Along the second direction, the maximum length of the first adhesive layer is L, and the length of the first groove is L1, where 1.3 ≤ L / L1 ≤ 3.5; Wherein, the third direction is the thickness direction of the positive electrode sheet, and the first direction, the second direction, and the third direction are perpendicular to each other.

2. The secondary battery according to claim 1, characterized by Along the first direction, the maximum width of the first adhesive layer is W, and the width of the first groove is W1, where 1.1 ≤ W / W1 ≤ 2.

3. The secondary battery according to claim 2, characterized by 2≤L / L1≤2.8; and / or, 1.33≤W / W1≤1.

87.

4. The secondary battery according to claim 1 or 2, characterized by Along the second direction, the first adhesive layer includes a first portion and a second portion, the second portion being connected to the first portion; along the first direction, the width of the second portion is equal to the width of the first portion; Along the third direction, the first portion coincides with the first groove; Along the second direction, the maximum length of the second part is L2, 0.15≤L2 / L1≤1.

25.

5. The secondary battery according to claim 4, characterized by 0.6≤L2 / L1≤0.

9.

6. The secondary battery according to claim 4, characterized by 1.5mm≤L2≤12.5mm.

7. The secondary battery according to claim 6, characterized by 6mm≤L2≤9mm.

8. The secondary battery according to any one of claims 1 to 7, characterized by, 8mm≤L1≤20mm.

9. The secondary battery according to any one of claims 4 to 8, characterized by Along the third direction, the second part has a first through hole.

10. The secondary battery according to claim 9, characterized by Viewed along the third direction, the area of ​​the second portion is S1, the area of ​​all the first through holes is S2, and 40% ≤ S2 / S1 ≤ 60%.

11. The secondary battery according to claim 9 or 10, characterized by The radius of the first through hole is R1, where 0.1mm ≤ R1 ≤ 1mm.

12. The secondary battery according to any one of claims 4 to 11, characterized by The second part includes a first region and a second region, and along the second direction, the first region is located between the first part and the second region; Along the first direction, the maximum width of the first region is W2, and the maximum width of the second region is W3, where W3 < W2.

13. The secondary battery according to claim 12, characterized by Along the second direction, the width of the first region is equal to the width of the second region.

14. The secondary battery according to any one of claims 10 to 13, characterized by The second part includes a first region and a second region, and along the second direction, the first region is located between the first part and the second region; S is the area of the first region as viewed in the third direction 11 S is the sum of the areas of all the first through holes of the first region as viewed in the third direction 21 ; When viewed along the third direction, the area of ​​the second region is S. 12 The sum of the areas of all the first through holes in the second region is S. 22 S 21 / S 11 <S 22 / S 12 .

15. The secondary battery according to any one of claims 1 to 14, characterized by On one side of the negative electrode tab extending direction, the positive electrode active material layer has an edge region, and the first adhesive layer covers a portion of the edge region.

16. The secondary battery according to any one of claims 4 to 13, characterized by Along the direction from the first part to the second part, the width of the second part gradually decreases in the first direction.

17. The secondary battery according to any one of claims 4 to 13, characterized by The first adhesive layer further includes a third portion, which, along the second direction, is sequentially arranged with the third portion, the first portion, and the second portion connected to the first portion; Along the first direction, the width of the third portion is equal to the width of the first portion; Along the second direction, the maximum length of the third portion is L3, where 0.15 ≤ L3 / L1 ≤ 1.

25.

18. The secondary battery according to claim 17, characterized by 0.6≤L3 / L1≤0.

9.

19. The secondary battery according to claim 17 or 18, characterized by 1.5mm≤L3≤12.5mm.

20. The secondary battery according to claim 19, characterized by 6mm≤L3≤9mm.

21. The secondary battery according to any one of claims 17 to 20, characterized by, Along the third direction, the third part has a second through hole.

22. The secondary battery according to claim 21, characterized by The third part includes a third region and a fourth region, and along the second direction, the fourth region is located between the first part and the third region; S is the area of the third region as viewed in the third direction 31 S is the sum of the areas of all the second through holes in the third region as viewed in the third direction 41 ; S 32 , the sum of the areas of all the second through holes in the fourth region is S 42 , S 41 / S 31 >S 42 / S 32 .

23. The secondary battery according to any one of claims 1 to 22, characterized by The first adhesive layer further includes a fourth part, which is arranged sequentially with the first part along the first direction, and the fourth part is connected to the first part; Along the third direction, the first part coincides with the first groove, and the fourth part is disposed on the positive electrode active material layer; Along the first direction, the maximum width of the fourth part is W4, where 1.5mm ≤ W4 ≤ 15mm.

24. The secondary battery according to claim 23, characterized by 5mm≤W4≤13mm.

25. The secondary battery according to claim 23 or 24, characterized by The fourth part includes a fifth region and a sixth region arranged sequentially, and along the first direction, the sixth region is located between the first part and the fifth region; In the second direction, the maximum length of the fifth region is L 51 , the maximum length of the sixth region is L 61 , L 51 < L 61 .

26. The secondary battery according to claim 25, characterized by Along the first direction, the width of the fifth region is equal to the width of the sixth region.

27. The secondary battery according to any one of claims 24 to 26, characterized by, Along the third direction, the fourth part has a third through hole.

28. The secondary battery according to claim 27, characterized by The fourth part includes a fifth region and a sixth region, and along the first direction, the sixth region is located between the first part and the fifth region; S, as viewed in the third direction 51 , the total of the fifth regions The sum of the areas of the third through holes is S 61 ; S 52 , the sum of areas of all the third through holes of the sixth region is S 62 , S 61 / S 51 >S 62 / S 52 .

29. The secondary battery according to any one of claims 1 to 28, characterized by Along the second direction, the first adhesive layer includes a third part, a first part, and a second part arranged sequentially, wherein the third part is connected to the first part, and the second part is connected to the first part; Along the second direction, the negative electrode active material layer includes a first end and a second end disposed opposite to each other, the second portion being located between the first groove and the first end, and the third portion being located between the first groove and the second end; Along the second direction, the length from the first groove to the first end is L4, and the length from the negative electrode tab to the second end is L5; Along the second direction, the length of the second part is L2, and the length of the third part is L3; L4 > L5, L2 > L3; or, L4 < L5, L2 < L3.

30. The secondary battery according to any one of claims 1 to 29, characterized by Along the third direction, the thickness of the positive electrode active material layer covered by the first adhesive layer is H1, and the thickness of the positive electrode active material layer not covered by the first adhesive layer is H2, where H1 < H2.

31. The secondary battery according to claim 30, characterized by Along the third direction, the thickness of the first adhesive layer is H3, where H1+H3≤H2.

32. The secondary battery according to any one of claims 1 to 31, characterized in that, The first adhesive layer includes a substrate layer and an adhesive layer, wherein the adhesive layer is disposed on the surface of the substrate layer facing the positive electrode active material layer; The material of the substrate layer includes at least one of polyethylene terephthalate and polyimide; The adhesive layer is made of at least one of polyethylene, polypropylene, polyurethane, epoxy resin, styrene-butadiene rubber, polyvinylidene fluoride, and polyacrylic acid.

33. An electronic device, comprising: Includes the secondary battery as described in any one of claims 1 to 32.

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