Secondary battery, preparation method thereof and electronic equipment

Abstract

The invention discloses a secondary battery and a preparation method thereof, and electronic equipment, a part of a negative electrode lug extends out of a negative electrode piece along a first direction, and an isolating membrane is arranged on one side, facing a positive electrode piece, of a first groove. The isolating membrane comprises a first part and a first pore area, the first part coincides with the first groove, the first pore area is connected with the first part, and the first pore area is arranged around the first part. The average porosity of the first pore area is G1, and G1 is larger than or equal to 10% and smaller than or equal to 35%. The first pore area comprises a first sub-area and a third sub-area, the width of the first part is equal to the maximum width of the first sub-area in the first direction, the first sub-area is connected with the first part in the second direction, the maximum length of the first sub-area is Lm1, and Lm1 is larger than or equal to 1 mm and smaller than or equal to 12.5 mm. And in the first direction, the third sub-region is connected with the first part, the maximum width of the third sub-region is Wm3, and 1.5 mm < = Wm3 < = 15mm. The lithium precipitation can be reduced, and the energy density loss of the secondary battery can be reduced.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and a preparation method thereof, and an electronic device. Background Art

[0002] Secondary batteries such as lithium-ion batteries are widely used in electronic devices such as smartphones, portable notebooks, wearable devices, and smart homes due to their high energy density and environmental protection advantages. Lithium-ion batteries include positive and negative electrodes, and usually require positive and negative electrodes to lead out the positive and negative electrodes of the lithium-ion battery. For example, a groove is provided on the negative active material layer to connect the negative electrode tab to lead out the negative electrode. However, the lithium ions released from the corresponding negative electrode tab slots in the positive active material layer may be difficult to embed into the negative active material layer, which may easily lead to lithium precipitation. Summary of the invention

[0003] The present application aims to provide a secondary battery and a preparation method thereof, and an electronic device, aiming to reduce the risk of lithium plating of the secondary battery.

[0004] In order to solve the technical problem, the embodiment of the present application adopts the following technical solution:

[0005] In the first aspect, the present application proposes a secondary battery, including a positive electrode sheet, a separator, a negative electrode sheet and a negative electrode ear, wherein a separator is stacked between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet includes a negative electrode active material layer facing the positive electrode sheet, and the negative electrode active material layer is provided with a first groove. Part of the negative electrode ear is arranged in the first groove, and the other part of the negative electrode ear extends out of the negative electrode sheet along the first direction. The separator is arranged on the side of the first groove facing the positive electrode sheet, and the positive electrode sheet includes a positive electrode active material layer facing the negative electrode active material layer, and a first glue layer is arranged on the surface of the positive electrode active material layer facing the negative electrode active material layer, and along the third direction, the first glue layer covers the first groove. The separator includes a first part and a first pore area, and when viewed along the third direction, the first part overlaps with the first groove, the first pore area is connected to the first part, and the first pore area is arranged around the first part. The average porosity of the first pore area is G1, 10%≤G1≤35%. The first pore region includes a first sub-region. Along the first direction, the width of the first portion is equal to the maximum width of the first sub-region. Along the second direction, the first sub-region is connected to the first portion. The maximum length of the first sub-region is L m1 , 1mm≤L m1 ≤12.5mm. The first pore area includes a third sub-area, along the first direction, the third sub-area is connected to the first part, and the maximum width of the third sub-area is W m3 , 1.5mm≤W m3 ≤15mm. 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.

[0006] In the above technical solution, the pore area of ​​the first pore area is limited to 10%≤G1≤35%, so that part of the negative electrode active material layer with a larger current density near the first groove can be covered by the first pore area, reducing the aggregation of lithium ions near the first groove, thereby reducing the occurrence of lithium precipitation. Moreover, compared with extending the first glue layer, by setting the first pore area on the isolation membrane, the space occupied by the first glue layer can be effectively reduced, which is beneficial to improving the energy density of the secondary battery. At the same time, the first pore area can allow a small amount of lithium ions to be transmitted, which is beneficial to the full utilization of the active material layer, and can further improve the energy density of the secondary battery. In addition, the local polarization of the negative electrode active material layer is reduced, which can reduce the decomposition of the electrolyte, thereby reducing the generation of the solid electrolyte interface film, and further reducing lithium precipitation. Moreover, the heat concentration at the first groove can be alleviated, the occurrence of side reactions can be reduced, and the charge and discharge performance of the secondary battery can be improved.

[0007] In some embodiments, 10%≤G1≤30%, which can further reduce the occurrence of lithium plating and reduce the energy density loss of the secondary battery.

[0008] In some embodiments, the isolation membrane also includes a main body region, a first pore region is arranged between the first part and the main body region, the porosity of the main body region is G2, G1<G2, and the ion transmission speed is slowed down by reducing the porosity of the first pore region, so that the negative electrode can have sufficient margin to embed the lithium ions released from the positive electrode per unit time, thereby reducing the occurrence of lithium plating.

[0009] In some embodiments, along the second direction, the first pore region further includes a second sub-region, and the first portion is located between the first sub-region and the second sub-region. Along the first direction, the maximum width of the second sub-region is equal to the width of the first region. Along the second direction, the maximum length of the second sub-region is L m2 , the length of the first groove is L1, 12mm≤L m1 +L m2 +L1≤35mm, can reduce the occurrence of lithium plating. Further, 22mm≤L m1 +L m2 +L1≤28mm, which can reduce lithium plating and the energy density loss of secondary batteries.

[0010] In some embodiments, along the second direction, the first sub-region is divided into a first segment and a second segment of equal length, and the first segment is located between the second segment and the first portion. The average porosity of the first segment is G m11 The average porosity of the second section is G m12 , G m11 <G m12The second section is far away from the first groove, with a lower current density and a lower risk of lithium deposition. Therefore, a larger porosity is set in the second section to facilitate more lithium ions to participate in the electrochemical reaction, which is conducive to the full utilization of the active material layer and improves the energy density of the secondary battery. Among them, 10% ≤ G m11 ≤20%, 20%≤G m12 ≤30%.

[0011] In some embodiments, along the first direction, the maximum width of the first segment is W m11 , the maximum width of the second segment is W m12 , 0.95≤W m11 / W m12 ≤1.05; 1mm≤L m1 ≤10mm. Selecting the maximum length of the first sub-region according to the current density can reduce the occurrence of lithium plating on one side of the length direction of the first groove and reduce the loss of energy density of the secondary battery. Preferably, 5mm≤L m1 ≤9mm, which can further reduce the occurrence of lithium plating and reduce the energy density loss of secondary batteries.

[0012] In some embodiments, along the first direction, the maximum width of the first segment is W m11 , the maximum width of the second segment is W m12 , W m11 / W m12 >1.05; 1.5mm≤L m1 ≤12.5mm. By adjusting the shape of the first sub-region according to the current density and selecting the maximum length of the first sub-region, the occurrence of lithium deposition on one side of the length direction of the first groove can be reduced, and the loss of energy density of the secondary battery can be reduced. Preferably, 6mm≤L m1 ≤9mm, which can further reduce lithium plating and reduce the energy density loss of secondary batteries.

[0013] In some embodiments, along the first direction, the third sub-region is divided into a third segment and a fourth segment of equal width, and the third segment is located between the fourth segment and the first portion. The average porosity of the third segment is G m31 The average porosity of the fourth section is G m32 , G m31 <G m32 The fourth section is provided with a larger porosity, which can facilitate more lithium ions to participate in the electrochemical reaction, is conducive to the full utilization of the active material layer, and improves the energy density of the secondary battery.

[0014] In some embodiments, along the second direction, the maximum length of the third segment is L m31 , the maximum length of the fourth segment is L m32 , 0.95≤L m31 / L m32≤1.05; 2mm≤Wm3≤12mm. Selecting the maximum width of the third sub-region according to the current density can reduce the lithium deposition and the energy density loss of the secondary battery. Preferably, 4.5mm≤W m3 ≤12mm, which can further reduce the occurrence of lithium plating and further reduce the energy density loss of secondary batteries.

[0015] In some embodiments, along the second direction, the maximum length of the third segment is L m31 , the maximum length of the fourth segment is L m32 , L m31 / L m32 >1.05; 3mm≤W m3 ≤15mm. Adjusting the shape of the third sub-region according to the current density and selecting the maximum width of the third sub-region can reduce the energy density loss of the secondary battery while reducing lithium plating. Preferably, 4.5mm≤W m3 ≤12mm, which can further reduce the occurrence of lithium plating and reduce the energy density loss of secondary batteries.

[0016] In some embodiments, along the second direction, the length of the first groove is L1, the length of the first adhesive layer is L2, and 1mm≤L2-L1≤10mm. The space occupied by the first adhesive layer can be reduced, and the energy density of the secondary battery can be improved. Preferably, 1mm≤L2-L1≤6mm, which can further reduce the impact of the first adhesive layer on the internal space of the secondary battery and further improve the energy density of the secondary battery.

[0017] In some embodiments, along the second direction, the first pore region further includes a second subregion, and the first portion is connected between the first subregion and the second subregion. Along the second direction, the negative electrode active material layer includes a first end and a second end that are oppositely disposed. Along the third direction, the projection of the first subregion on the negative electrode active material layer is located between the first groove and the first end, and the projection of the second subregion on the negative electrode active material layer is located between the first groove and the second end. Along the second direction, the distance from the first groove to the first end is L3, and the distance from the first groove to the second end is L4. Along the second direction, the maximum length of the first subregion is L m1 , the maximum length of the second sub-region is L m2 . L3>L4, L m1 >L m2 ; or, L3 <L4,L m1 <L m2 By reasonably setting the lengths of the first sub-region and the second sub-region, the risk of lithium plating can be reduced while reducing the size of the first glue layer and improving the energy density of the secondary battery.

[0018] In the second aspect, the present application also proposes a method for preparing a secondary battery, including: providing a positive electrode sheet, a separator, a negative electrode sheet and a negative electrode ear, the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer is provided with a first groove, a part of the negative electrode ear is arranged in the first groove, and another part of the negative electrode ear extends out of the negative electrode sheet along the first direction; the positive electrode sheet includes a positive electrode active material layer. Part of the separator is subjected to a heat treatment, the heated part includes a first pore region, and the average porosity of the first pore region is G1, 10%≤G1≤35%. A first glue layer is provided, and the first glue layer is bonded to the surface of the positive electrode active material layer and covers the first groove. The separator is arranged on the side of the first groove facing the positive electrode sheet, and the positive electrode active material layer is arranged facing the negative electrode active material layer. Wherein, along the third direction, the first glue layer covers the first groove, the separator includes a first part, the first part overlaps with the first groove, the first pore region is connected to the first part, and the first pore region is arranged around the first part.

[0019] In a third aspect, the present application further proposes an electronic device, comprising a secondary battery as in any embodiment of the first aspect above.

[0020] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] One or more embodiments are exemplarily described by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and the figures in the drawings do not constitute proportional limitations unless otherwise stated.

[0022] Figure 1 A schematic diagram of the structure of a secondary battery in some embodiments of the present application;

[0023] Figure 2 A schematic diagram of the winding structure of an electrode assembly in some embodiments of the present application;

[0024] Figure 3 A schematic diagram of the stacking structure of an electrode assembly in some embodiments of the present application;

[0025] Figure 4 A schematic diagram of the stacked structure of a positive electrode sheet, a separator and a negative electrode sheet in some embodiments of the present application;

[0026] Figure 5 A schematic diagram of the current density of the negative electrode active material layer at the first groove in some embodiments of the present application;

[0027] Figure 6Schematic diagram of the structure of the negative electrode active material layer, the first glue layer and the isolation film in some embodiments of the present application;

[0028] Figure 7 Schematic diagram of the structure of the negative electrode active material layer, the first glue layer and the isolation film in some embodiments of the present application;

[0029] Figure 8 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0030] Fig. 9 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0031] Fig.10 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0032] Fig.11 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0033] Fig.12 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0034] Fig.13 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0035] Fig.14 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0036] Fig.15 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0037] Fig.16 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0038] Fig.17 Schematic diagram of the structure of the negative electrode active material layer and the isolation membrane in some embodiments of the present application;

[0039] Fig.18 Schematic diagram of the structure of the negative electrode active material layer, the first glue layer and the isolation film in some embodiments of the present application;

[0040] Fig.19 Schematic diagram of the structure of the negative electrode active material layer, the first glue layer and the isolation film in some embodiments of the present application;

[0041] Fig. 20 This is a schematic diagram of the structure of the negative electrode sheet and the hot pressing part of the isolation film in some embodiments of the present application;

[0042] Fig.21 This is a schematic diagram of the structure of the negative electrode plate and the hot pressing part of the isolation membrane in some embodiments of the present application;.

[0043] Description of reference numerals:

[0044] 1000. Secondary battery;

[0045] 100. Electrode assembly;

[0046] 10. positive electrode sheet; 11. positive electrode current collector; 111. first surface; 112. second surface; 12. positive electrode active material layer;

[0047] 20, negative electrode plate; 20a, first end; 20b, second end; 21, negative electrode current collector; 211, third surface; 212, fourth surface; 22, negative electrode active material layer; 221, first groove; 222, first edge;

[0048] 30. isolation membrane; 31. main body region; 32. first pore region; 321. first sub-region; 3211. first section; 3212. second section; 322. second sub-region; 333. third sub-region; 3331. third section; 3332. fourth section; 33. first part;

[0049] 40. Negative electrode tab;

[0050] 50. First adhesive layer;

[0051] 60, second adhesive layer;

[0052] 200, housing;

[0053] X, first direction; Y, second direction; Z, third direction. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

[0055] Reference to "embodiment" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0056] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0057] 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, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0058] The technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0059] In the first aspect, the present application proposes a secondary battery 1000, please refer to Figure 1 and Figure 2 The secondary battery 1000 includes an electrode assembly 100 , a shell 200 and an electrolyte (not shown in the figure). The shell 200 can accommodate the electrode assembly 100 and the electrolyte. The electrolyte infiltrates the electrode assembly 100 in the shell 200 .

[0060] For the electrode assembly 100, please refer to Figure 2 and Figure 3 The electrode assembly 100 includes a positive electrode sheet 10, a negative electrode sheet 20 and a separator 30. The positive electrode sheet 10, the separator 30 and the negative electrode sheet 20 are stacked and wound, wherein Figure 2 The winding structure of the electrode assembly 100 is shown, for example, the positive electrode sheet 10 and / or the negative electrode sheet 20 are stacked in the thickness direction and wound in the length direction thereof to form a wound electrode assembly 100. In some other embodiments, the electrode assembly 100 may also adopt a laminated structure, wherein Figure 3 The stacking structure of the electrode assembly 100 is shown. Along the third direction Z (the thickness direction of the positive electrode sheets 10 and / or the negative electrode sheets 20 ), a plurality of positive electrode sheets 10 and a plurality of negative electrode sheets 20 are alternately stacked, and an isolation film 30 is arranged between adjacent positive electrode sheets 10 and negative electrode sheets 20 .

[0061] It should be noted that, in the embodiment of the present application, the first direction X is the width direction of the positive electrode sheet 10 and / or the negative electrode sheet 20, the second direction Y is the length direction of the positive electrode sheet 10 and / or the negative electrode sheet 20, and the third direction Z is the thickness direction of the positive electrode sheet 10 and / or the negative electrode sheet 20. The first direction X, the second direction Y, and the third direction Z are perpendicular to each other. In the laminated electrode assembly 100, the first direction X may also be the width direction of the positive electrode sheet 10 and / or the negative electrode sheet 20, and the second direction Y may also be the length direction of the positive electrode sheet 10 and / or the negative electrode sheet 20.

[0062] Please refer to Figure 4 The positive electrode sheet 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode current collector 11 may be an overall flat aluminum foil, titanium foil, nickel foil or stainless steel foil. The positive electrode active material layer 12 may be arranged 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 arranged opposite to each other. The positive electrode active material layer 12 may 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 an adhesive. The above-mentioned material components are mixed, stirred evenly and applied to the first surface 111 and / or the second surface 112, so as to obtain the positive electrode active material layer 12. Among them, 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 iron manganese phosphate.

[0063] Please refer to Figure 4 The negative electrode sheet 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. The negative electrode current collector 21 may be an integrally flat copper foil, titanium foil, nickel foil, stainless steel foil or silver foil. The negative electrode active material layer 22 may 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 may 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 an adhesive, etc. Such materials are mixed, stirred evenly, and coated on the third surface 211 and / or the fourth surface 212, thereby obtaining the negative electrode active material layer 22. Among them, the negative electrode active material includes one or more of graphite, soft carbon, hard carbon, elemental silicon, silicon oxide, silicon alloy, etc.

[0064] For the above-mentioned isolation film 30, in some embodiments, the isolation film 30 includes a substrate layer and a ceramic layer, the substrate layer includes at least one of polypropylene, polyethylene, polyvinylidene fluoride, polyimide or polyamide, etc., and the ceramic layer includes at least one of aluminum oxide, magnesium oxide or zirconium oxide. The isolation film 30 can be adapted to the electrolyte environment inside the secondary battery 100, thereby extending the service life of the isolation film 30.

[0065] The secondary battery 1000 further includes a negative electrode tab 40, see Figure 4 The negative electrode tab 40 is connected to the negative electrode sheet 20. For example, a first groove 221 is provided in the negative electrode active material layer 22, so that the negative electrode current collector 21 is exposed from the first groove 221. A part of the negative electrode tab 40 is provided 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. A second adhesive layer 60 can be provided to cover part of the negative electrode tab 40 to isolate the burrs on the negative electrode tab 40 and reduce the risk of the burrs piercing the separator 30. Another part of the negative electrode tab 40 extends out of the negative electrode active material layer 22 along the first direction X.

[0066] The secondary battery 1000 further includes a first adhesive layer 50, see Figure 4 , the first glue layer 50 is disposed on the surface of the positive electrode active material layer 12 facing the negative electrode active material layer 22. Along the third direction Z, the first glue layer 50 covers the first groove 221, so that the portion of the positive electrode active material layer 12 corresponding to the first groove 221 is covered by the first glue layer 50, which can reduce or prevent the release of lithium ions from the portion, thereby allowing the negative electrode active material layer 22 to have sufficient residual amount to embed lithium ions, thereby reducing the risk of lithium precipitation.

[0067] The inventors of this application have found that the negative electrode active material at the first groove 221 may form a solid electrolyte interface (SEI) film, which further hinders the insertion of lithium ions and aggravates the occurrence of lithium precipitation. The inventors of this application have concluded through analysis that there is a problem of uneven current density at the first groove 221 of the negative electrode active material layer 22. Please refer to Figure 5 , Figure 5 Schematic diagram showing the current density of the negative electrode active material layer 22 near the first groove 221, the darker the color, the greater the current density. Figure 5 It can be seen that the closer to the first groove 221, the greater the current density. When the current density of the negative electrode active material layer 22 is large, a large number of lithium ions reach the surface of the negative electrode active material layer 22 per unit time, which easily leads to the aggregation of lithium ions on the surface of the negative electrode active material layer 22, thereby causing lithium precipitation. In addition, high current density will cause the local polarization of the negative electrode active material layer 22 to increase (the potential in the local area deviates from the equilibrium potential), which may reduce the negative electrode potential. When the negative electrode potential is lower than the potential of lithium ions reduced to metallic lithium, it may cause lithium precipitation problems.

[0068] To reduce the above problems, in the embodiments of this application, please refer to Figure 6, the isolation membrane 30 includes a first pore region 32 and a first portion 33. When viewed along the third direction Z, the first portion 33 overlaps with the first groove 221, the first pore region 32 is connected to the first portion 33, and the first pore region 32 is arranged around the first portion 33, which may be around half a circle, 3 / 4 circle or one circle, etc. The average porosity of the first pore region 32 is G1, 10%≤G1≤35%. For example, by hot pressing the isolation membrane 30, the isolation membrane 30 forms a hot pressing region, and the hot pressing region includes the first portion 33 and the first pore region 32, which may close some pores in the hot pressing region and / or reduce the pore size, thereby reducing the porosity of the hot pressing region. The portion with a larger current density may be covered by the first pore region 32, thereby reducing the aggregation of lithium ions near the first groove 221, thereby reducing the occurrence of lithium precipitation.

[0069] Please refer to further Figures 10 to 11 as well as Figure 14 to Figure 15 The first pore region 32 includes a first sub-region 321. Along the second direction Y, the first sub-region 321 is connected to the first portion 33. The maximum length of the first sub-region 321 is L. m1 , 1mm≤L m1 ≤12.5mm. The first pore area 32 includes a third sub-area 333. Along the first direction X, the third sub-area 333 is connected to the first part 33. The maximum width of the third sub-area 333 is W. m3 , 1.5mm≤W m3 ≤15mm. Compared with extending the first adhesive layer 50, by setting a smaller porosity area on the isolation film 30, the space occupied by the first adhesive layer 50 can be effectively reduced, which is beneficial to improving the energy density of the secondary battery 1000. At the same time, the first pore area 32 can allow a small amount of lithium ions to be transmitted, which is beneficial to the full utilization of the active material layer and can further improve the energy density of the secondary battery 1000.

[0070] In addition, extending the first adhesive layer 50 increases the contact area between the first adhesive layer 50 and the electrolyte. After long-term use, the aging of the adhesive layer and the decrease in viscosity may cause it to move and break at the edge, which may cause the originally isolated positive electrode active material 12 to re-participate in the reaction and increase the risk of lithium precipitation. The isolation membrane 30 has a more stable property, which is beneficial to extending the service life of the secondary battery 100.

[0071] The inventor of the present application unexpectedly discovered that by limiting 10%≤G1≤35%, the local polarization of the negative electrode active material layer 22 is reduced, the decomposition of the electrolyte can be reduced, and the generation of the solid electrolyte interface film can be reduced, which can further reduce lithium precipitation. In addition, the heat concentration at the first groove 221 can be alleviated, the occurrence of side reactions can be reduced, and the charge and discharge performance of the secondary battery 1000 can be improved.

[0072] The inventors of the present application have found that the current density of the negative electrode active material layer 22 near the first groove 221 is relatively high, and the risk of lithium deposition is relatively high, but near the first groove 221, there is still a part of the negative electrode active material layer 22 that can undergo electrochemical reactions and exert capacity for the secondary battery 1000. Under the premise that lithium deposition does not occur under the same conditions, the part with a higher current density can receive fewer lithium ions than the part with a lower current density.

[0073] In the embodiment of the present application, 10%≤G1≤35% is defined, and there are relatively few channels through which lithium ions can pass in the first pore region 32, which slows down the ion transmission speed, and can facilitate the negative electrode active material layer 22 to have sufficient margin to embed the lithium ions released from the positive electrode per unit time, and can facilitate the use of part of the positive electrode active material layer 12 covered by the first pore region 32, which is beneficial to reduce the occurrence of lithium precipitation while improving the energy density of the secondary battery 1000. The porosity of the first pore region 32 can be regulated by parameters such as temperature, time and pressure during the hot pressing of the isolation membrane 30.

[0074] In some embodiments, 10%≤G1≤30%, which can further reduce the occurrence of lithium plating and reduce the energy density loss of the secondary battery 100 .

[0075] In some embodiments, the isolation membrane 30 also includes a main body region 31, for example, the portion of the isolation membrane 30 that is not hot-pressed forms the main body region 31, and a first pore region 32 is arranged between the first portion 33 and the main body region 31. The porosity of the main body region 31 is G2, G1<G2, and by reducing the porosity of the first pore region 32, the ion transmission speed is slowed down, which can facilitate the negative electrode to have sufficient margin per unit time to embed the lithium ions released from the positive electrode, thereby reducing the occurrence of lithium plating.

[0076] The porosity of the main region 31 of the isolation membrane 30 is generally 38%≤G2≤50%, that is, the porosity of the main region 31 is 38% to 50%, which can provide a smoother lithium ion transmission channel. Lithium ions can move relatively quickly in the main region 31, which is beneficial to the ion exchange of the secondary battery 1000 during normal charging and discharging.

[0077] As for the material of the separator 30, the separator 30 includes a substrate layer and a ceramic layer. Along the thickness direction of the separator, the ceramic layer is arranged on the surface of the substrate layer. The substrate layer includes at least one of polyethylene, polypropylene, polytetrafluoroethylene, cellulose acetate or cellulose nanofibers. Each material has good chemical stability and can maintain the stability of its own structure and performance in the electrolyte environment, thereby effectively reducing the chemical reaction with the electrolyte and causing the separator 30 to fail. In addition, each material has good electrical insulation, which can effectively isolate the positive electrode sheet 10 from the negative electrode sheet 20 and reduce the occurrence of short circuits. At the same time, they all have certain flexibility and strength, which is conducive to reducing the damage of the separator 30, thereby effectively isolating the positive electrode sheet 10 from the negative electrode sheet 20. The ceramic layer includes at least one of boehmite, aluminum oxide, magnesium oxide, zirconium oxide or silicon dioxide. Each material has good insulation and thermal stability. When the internal temperature of the secondary battery 1000 increases, it can reduce heat diffusion, thereby reducing the thermal runaway of the secondary battery 1000, and effectively improving the safety performance of the secondary battery 1000.

[0078] It should be noted that the porosity of the isolation membrane is based on the smaller porosity of the substrate layer and the ceramic layer. For example, when the porosity of the substrate layer is less than that of the ceramic layer, the porosity of the substrate layer is the porosity of the isolation membrane. When the porosity of the ceramic layer is less than that of the substrate layer, the porosity of the ceramic layer is the porosity of the isolation membrane. When the isolation membrane is hot-pressed, the porosity of the substrate layer is usually changed, that is, the porosity of the substrate layer is the porosity G1 of the isolation membrane.

[0079] In some other embodiments, the isolation membrane may also include only one substrate layer, and the porosity of the substrate layer is the porosity G1 of the isolation membrane.

[0080] It should be noted that the following method is used to divide the main area 31 and the first pore area 32: the isolation film 30 arranged on the side of the first groove 221 facing the positive electrode plate is peeled off from the electrode assembly, the isolation film 30 near the position opposite to the first groove 221 is cut into an appropriate size, and then the peeled isolation film 30 is soaked in dimethyl carbonate (DMC) for 20 minutes to remove the electrolyte residue, and then the isolation film 30 is placed in an oven and dried at 60°C for 12 hours. The isolation film 30 treated by the above steps is placed in a container again, N-methylpyrrolidone (NMP) is added, and it is placed in an ultrasonic instrument with a heating function for ultrasonication, the temperature is controlled at 45°C, and the ultrasonication is performed for 3 hours. When the isolation film 30 becomes completely transparent, it is taken out to obtain the base film in the isolation film 30, and the base film test sample of the isolation film 30 is obtained. The test sample is gold-plated to improve the conductivity of the test sample, and then SEM (Scanning Electron Microscope, scanning electron microscope) to observe the surface and cross-section of the test sample at different positions, and then calculate the area and distribution of the pores and the porosity of the corresponding measurement area through image analysis software. Specifically, after the test sample is cut, a measurement area is selected at intervals of 0.5 mm along the second direction Y, and the area of ​​each measurement area is 0.5 mm 2 , the average porosity of each measurement area is obtained using SEM and image analysis software, and the midline of the area with an average porosity of 30%±1% is defined as the boundary between the main area 31 and the first pore area 32 in the second direction Y; similarly, a measurement area is selected at intervals of 0.5 mm along the third direction Z, and the area of ​​each measurement area is 0.5 mm 2 The average porosity of each measurement area is obtained using SEM and image analysis software, and the midline of the area with an average porosity of 30%±1% is defined as the boundary between the main area 31 and the first pore area 32 in the third direction Z.

[0081] Regarding the overall shape of the hot pressing area (including the first portion 33 and the first pore area 32), the overall shape of the hot pressing area can be set to a nearly square structure, such as Figure 6 As shown; it can also be set as a special-shaped structure according to the current density, such as Figure 7 As shown, along the first direction X, the length of the hot pressing area in the second direction Y is increased, that is, the length of the first pore area 32 in the second direction Y is increased. When the special-shaped structure is set according to the current density, it is beneficial to reduce lithium plating and improve the energy density of the secondary battery 1000.

[0082] In some embodiments, please refer to Figure 8 and Fig. 9, observed along the third direction Z, the first pore region 32 is arranged around the first portion 33. The current density around the first groove 221 is relatively large, and the lithium ion deintercalation reaction is intense. By arranging the first pore region 32 around the first portion 33, the amount of detachment around the first groove 221 can be effectively reduced, thereby reducing the occurrence of lithium precipitation. Among them, observed along the third direction Z, the first pore region 32 is also arranged around the first glue layer 50, and the first glue layer 50 can cover part of the first pore region 32.

[0083] Please refer to Figure 8 and Fig. 9 , along the first direction X, the negative electrode active material layer 22 has a first edge 222, and the first groove 221 may penetrate the first edge 222 or not penetrate the first edge 222. When not penetrating the first edge 222, the first pore area 32 may surround the first portion 33, that is, surround the first groove 221, which can effectively reduce the amount of separation around the first groove 221, thereby reducing the occurrence of lithium deposition. Please refer to Fig.10 and Fig.11 The first groove 221 may also penetrate the first edge 222 , that is, the first gap area 32 may surround a portion of the first groove 221 .

[0084] The inventor of the present application sets the maximum length of the first pore region 32 according to the current density of the negative electrode active material layer 22 at the first groove 221. In the embodiment of the present application, along the second direction Y, the maximum length of the first pore region 32 is L m , the length of the first groove 221 is L1, the first pore region 32 further includes a second sub-region 322, and along the second direction Y, the first portion 33 is located between the first sub-region 321 and the second sub-region 322, for example, L m =L m1 +L m2 +L1. In the embodiment of the present application, 12mm≤L m1 +L m2 +L1≤35mm, can reduce the occurrence of lithium plating. Further, 22mm≤L m1 +L m2 +L1≤28mm, which can reduce lithium plating and reduce the energy density loss of the secondary battery 100.

[0085] In some embodiments, please refer to Fig.12 and Fig.13 , along the second direction Y, the first sub-region 321 is divided into a first segment 3211 and a second segment 3212 of equal length. For example, the length of the first segment 3211 is L m11 , the length of the second segment 3212 is L m12 , L m11 =L m12The first section 3211 is located between the second section 3212 and the first portion 33. The average porosity of the first section 3211 is G m11 The average porosity of the second section 3212 is G m12 , G m11 <G m12 The closer to the first groove 221, the greater the current density, the more intense the lithium ion insertion and extraction reaction, and the higher the risk of lithium deposition. The first section 3211 is closer to the first groove 221, and its porosity is smaller, which can block more lithium ions and thus reduce the occurrence of lithium deposition. The second section 3212 is far away from the first groove 221, and the current density is small, and the risk of lithium deposition is low. Therefore, the second section 3212 is set with a larger porosity, which can facilitate more lithium ions to participate in the electrochemical reaction, which is conducive to the full utilization of the active material layer and improves the energy density of the secondary battery 1000. Among them, 10% ≤ G m11 ≤20%, 20%≤G m12 ≤30%. The boundary between the first section 3211 and the second section 3212 is determined by referring to the test method for the boundary between the main body area 31 and the first pore area 32. The center line of the test area with an average porosity of 20%±1% is regarded as the boundary between the first section 3211 and the second section 3212.

[0086] When the heat pressing area is a square structure, please refer to Fig.12 , along the first direction X, the maximum width of the first segment 3211 is W m11 , the maximum width of the second segment 3212 is W m12 , 0.95≤W m11 / W m12 ≤1.05; 1mm≤L m1 ≤10mm. Selecting the maximum length of the first sub-region 321 according to the current density can reduce the occurrence of lithium deposition on one side of the length direction of the first groove 221 and reduce the loss of energy density of the secondary battery 1000. Preferably, 5mm≤L m1 ≤9mm, which can further reduce the occurrence of lithium plating and reduce the energy density loss of the secondary battery 1000.

[0087] When the hot pressing area is a special-shaped structure, please refer to Fig.13 , along the first direction X, the maximum width of the first segment 3211 is W m11 , the maximum width of the second segment 3212 is W m12 , W m11 / W m12 >1.05; 1.5mm≤L m1≤12.5mm. By adjusting the shape of the first sub-region 321 according to the current density and selecting the maximum length of the first sub-region 321, the occurrence of lithium deposition on one side of the length direction of the first groove 221 can be reduced, and the loss of energy density of the secondary battery 1000 can be reduced. Preferably, 6mm≤L m1 ≤9mm, which can further reduce lithium plating and reduce the energy density loss of the secondary battery 1000.

[0088] As for the second sub-region 322 , the second sub-region 322 may be configured similarly to the first sub-region 321 .

[0089] As for the width of the first pore region 32, along the first direction X, the closer the first groove 221 is to the center of the first groove 221, the greater the current density. The inventor of the present application sets the maximum width of the first pore region 32 according to the current density in the width direction. Please refer to Figures 8 to 11 , along the first direction X, the maximum width of the first pore region 32 is W m , the width of the first groove 221 is W1, for example, W m =W m3 +W1. In the embodiment of the present application, 16.5 mm ≤ W m3 +W1≤30mm, optionally, the maximum width of the third sub-region 333 is W m3 , 1.5mm≤W m3 ≤15mm, which can reduce the loss of energy density and the occurrence of lithium plating.

[0090] In some embodiments, please refer to Fig.16 and Fig.17 , along the first direction X, the third sub-region 333 is divided into a third segment 3331 and a fourth segment 3332 of equal width. For example, the width of the third segment 3331 is W m31 , the width of the fourth segment 3332 is W m32 , that is, W m31 =W m32 The third section 3331 is located between the fourth section 3332 and the first section 33. The average porosity of the third section 3331 is G m31 The average porosity of the fourth segment 3332 is G m32 , G m31 <G m32The closer to the first groove 221, the greater the current density, the more intense the lithium ion insertion and extraction reaction, and the higher the risk of lithium deposition. The third section 3331 is closer to the first groove 221, and its porosity is smaller, which can block more lithium ions and reduce the occurrence of lithium deposition. The fourth section 3332 is far away from the first groove 221, and the current density is small, and the risk of lithium deposition is low. Therefore, the fourth section 3332 is set with a larger porosity, which can facilitate more lithium ions to participate in the electrochemical reaction, which is conducive to the full utilization of the active material layer and improves the energy density of the secondary battery 1000. Among them, 10% ≤ G m31 ≤20%, 20%≤G m32 ≤30%. Similarly, the determination of the boundary line between the third section 3331 and the fourth section 3332 refers to the test method of the main body area 31 and the first pore area 32. The center line of the test area with an average porosity of 20%±1% is regarded as the boundary line between the third section 3331 and the fourth section 3332.

[0091] In some embodiments, please refer to Fig.16 When the hot pressing area is a square-like structure, along the second direction Y, the maximum length of the third section 3331 is L m31 , the maximum length of the fourth segment 3332 is L m32 , 0.95≤L m31 / L m32 ≤1.05; 2mm≤Wm3≤12mm. The maximum width of the third sub-region 333 is selected according to the current density, which can reduce the energy density loss of the secondary battery 1000 while reducing lithium plating. Preferably, 4.5mm≤Wm3≤12mm. m3 ≤12mm, which can further reduce the occurrence of lithium plating and further reduce the energy density loss of the secondary battery 1000.

[0092] In some embodiments, please refer to Fig.17 When the hot pressing area is a special-shaped structure, along the second direction Y, the maximum length of the third section 3331 is L m31 , the maximum length of the fourth segment 3332 is L m32 , L m31 / L m32 >1.05; 3mm≤W m3 ≤15mm. Adjusting the shape of the third sub-region 333 according to the current density and selecting the maximum width of the third sub-region 333 can reduce the energy density loss of the secondary battery 1000 while reducing lithium deposition. Preferably, 4.5mm≤W m3 ≤12mm, which can further reduce the occurrence of lithium plating and reduce the energy density loss of the secondary battery 1000.

[0093] Due to the closed-cell setting of the isolation film 30, the lithium ion transmission around the first groove 221 can be reduced, thereby reducing the occurrence of lithium plating. Therefore, the length of the first adhesive layer 50 can be adaptively reduced. For example, please refer to Fig.18 , along the second direction Y, the length of the first groove 221 is L1, the length of the first adhesive layer 50 is L2, 1mm≤L2-L1≤10mm. The space occupied by the first adhesive layer 50 can be reduced, the energy density of the secondary battery 1000 can be improved, and the adhesive bonding operation can be facilitated. Preferably, 1mm≤L2-L1≤6mm, which can further reduce the impact of the first adhesive layer 50 on the internal space of the secondary battery 1000, further improve the energy density of the secondary battery 1000, and facilitate the adhesive bonding operation.

[0094] In some embodiments, please refer to Figures 19 to 21 , along the second direction Y, the first pore region 32 includes a first sub-region 321 and a second sub-region 322, and the first portion 33 is connected between the first sub-region 321 and the second sub-region 322. Along the second direction Y, the negative electrode active material layer 22 includes a first end 20a and a second end 20b that are oppositely disposed. Along the third direction Z, the projection of the first sub-region 321 on the negative electrode active material layer 22 is located between the first groove 221 and the first end 20a, and the projection of the second sub-region 322 on the negative electrode active material layer 22 is located between the first groove 221 and the second end 20b. Along the second direction Y, the distance from the first groove 221 to the first end 20a is L3, and the distance from the first groove 221 to the second end 20b is L4.

[0095] The inventors of the present application have found that, for the negative electrode sheet 20, the larger L3 is, the larger the amount of the negative electrode active material layer 22 participating in the electrochemical reaction is, and the greater the current density between the first groove 221 and the first end 20a is. The inventors of the present application set the length difference between the first sub-region 321 and the second sub-region 322 based on the length L3 from the first groove 221 to the first end 20a and the length L4 from the first groove 221 to the second end 20b.

[0096] For example, along the second direction Y, the maximum length of the first sub-region 321 is L m1 , the maximum length of the second sub-region 322 is L m2 When L3>L4, it indicates that the current density between the first groove 221 and the first end 20a is relatively large, and L m1 >L m2 When L3<L4, it indicates that the current density between the first groove 221 and the second end 20b is relatively large, and L m1 <L m2By reasonably setting the lengths of the first sub-region 321 and the second sub-region 322, the risk of lithium deposition can be reduced while reducing the size of the first pore region 32, thereby increasing the energy density of the secondary battery 1000. When L3=L4, L m1 With L m2 Set to be approximately equal, for example |L m2 -L m1 |≤0.5mm.

[0097] On the second aspect, the present application also proposes a method for preparing a secondary battery 1000, including: providing a positive electrode sheet 10, a separator 30, a negative electrode sheet 20 and a negative electrode tab 40, the negative electrode sheet 20 including a negative electrode active material layer 22, the negative electrode active material layer 22 being provided with a first groove 221, a portion of the negative electrode tab 40 being arranged in the first groove 221, and another portion of the negative electrode tab 40 being extended out of the negative electrode sheet 20 along a first direction X; the positive electrode sheet 10 including a positive electrode active material layer 12.

[0098] A portion of the separator 30 is subjected to a heat treatment, wherein the heated portion includes the first pore region 32 , and the average porosity of the first pore region 32 is G1 , and 10%≤G1≤35%.

[0099] A first glue layer 50 is provided and adhered to the surface of the positive electrode active material layer 12 and covers the first groove 221 .

[0100] The separator 30 is disposed on the side of the first groove 221 facing the positive electrode sheet 10 , and the positive electrode active material layer 12 is disposed facing the negative electrode active material layer 22 .

[0101] In which, along the third direction Z, the first adhesive layer 50 covers the first groove 221 , the isolation film 30 includes a first portion 33 , the first portion 33 overlaps with the first groove 221 , the first pore area 32 is connected to the first portion 33 , and the first pore area 32 is arranged around the first portion 33 .

[0102] In the embodiment of the present application, by heating the isolation film 30, the isolation film 30 can form a first pore region 32 with a smaller porosity, and by limiting 10%≤G1≤35%, the portion with a larger current density can be covered by the first pore region 32, reducing the aggregation of lithium ions near the first groove 221, thereby reducing the occurrence of lithium precipitation. In addition, compared with extending the first adhesive layer 50, by setting different porosity regions for the isolation film 30, the space occupied by the first adhesive layer 50 can be effectively reduced, which is beneficial to improving the energy density of the secondary battery 1000. At the same time, the first pore region 32 can allow a small amount of lithium ions to be transmitted, which is beneficial to the full utilization of the active material layer, and can further improve the energy density of the secondary battery 1000.

[0103] In the third aspect, the present application also proposes an electronic device, including a secondary battery 1000 as in any embodiment of the first aspect described above. The electronic device of the embodiment of the present application is not particularly limited, and it 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, electric tools, battery cars, electric cars, ships, spacecraft, etc. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys, electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecrafts.

[0104] Example 1-1

[0105] Preparation of positive electrode:

[0106] The positive electrode active material is lithium cobalt oxide, 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 5 ) were mixed in a mass ratio of 94:3:3, N-methylpyrrolidone (NMP) was added as a solvent, and a positive electrode slurry with a solid content of 75wt% was prepared, and the mixture was stirred evenly in a vacuum mixer. An aluminum foil with a thickness of 8μm and a length of 1000mm was selected as the positive electrode current collector, and the positive electrode slurry was evenly coated on one surface of the positive electrode current collector aluminum foil, and dried at 110°C to obtain a positive electrode sheet with a single-sided positive electrode active material layer. Afterwards, the above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided positive electrode active material layer.

[0107] Preparation of negative electrode sheet:

[0108] The negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (Super P), and binder styrene-butadiene rubber (SBR) are mixed in a weight ratio of 87.5:10:1:1.5, and then deionized water is added as a solvent to prepare a negative electrode slurry with a solid content of 50wt%, and stirred evenly. A copper foil with a thickness of 5μm and a length of 1050mm is selected as the negative electrode current collector, and the negative electrode slurry is evenly coated on one surface of the negative electrode current collector copper foil, and an empty foil area without negative electrode slurry is reserved on the copper foil, and dried at 90°C to obtain a single-sided negative electrode sheet. After the above steps are completed, the single-sided coating of the negative electrode sheet has been completed. After that, the above steps are repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a double-sided coating of the negative electrode active material layer.

[0109] Preparation of isolation membrane:

[0110] Polyethylene is selected as a 7μm substrate layer, and polyvinylidene fluoride is selected as an adhesive layer. An alumina ceramic layer with a thickness of 2μm is set on the side of the adhesive layer away from the substrate layer to prepare a porous isolation membrane. Part of the isolation membrane is subjected to hot pressing treatment to form a square hot pressing area, which includes a first pore area, and the unhot pressed part forms a main body area. The porosity of the main body area is G2 is 50%, the porosity of the first pore area is G1 is 35%, and G1 < G2. The maximum length L of the first pore area m is 20mm, that is, L m1 +L m2 +L1=20mm, maximum width W m is 20mm.

[0111] Electrolyte preparation:

[0112] In a dry argon atmosphere, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain an organic solvent, and then lithium hexafluorophosphate was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0113] Preparation of lithium-ion secondary batteries:

[0114] By laser cleaning, a first groove of 10mm (L1) × 15mm (W1) is cleaned in the above-mentioned negative electrode active material layer, and a nickel sheet is selected as the negative electrode ear. The negative electrode ear is welded and connected to the negative electrode sheet in the first groove. After the positive electrode sheet is welded to the positive electrode ear, 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 length and width of 11mm (L2) × 16mm, and the first adhesive layer is bonded to the positive electrode active material layer. The isolation film, positive electrode sheet, isolation film, and negative electrode sheet prepared above are stacked in order and wound to obtain an electrode assembly. Observing along the thickness direction of the positive electrode sheet, the first adhesive layer is located in the hot pressing area and just covers the first groove. The overlapping part of the hot pressing area and the first adhesive layer forms the first part. The first part and the main area are the first pore area. The first sub-area and the second sub-area are on both sides of the first part, respectively. The width L of the first sub-area m1 and the width L of the second sub-region m2 Consistent or nearly consistent, L m1 5mm, L m2 The width of the isolation film is 5 mm. The hot pressing area exceeds the first adhesive layer, and the exceeding part forms a third sub-area with a width of W. m3 It is 4.5mm.

[0115] Different from Example 1-1, the relevant parameters in Examples 1-2 to 1-24 and Comparative Examples 1-1 to 1-9 are shown in Table 1 below, wherein in Comparative Example 1-1, the isolation film is not subjected to heat pressing. The first sub-region is divided into a first section and a second section of equal length, and the first section is located between the second section and the first part. The average porosity of the first section is G m11 The average porosity of the second section is G m12 .

[0116] Test method for G2 and G1: After dividing the main area and the first pore area, 5 measurement areas are selected on average in the main area, and the area of ​​each measurement area is 0.5mm 2 The porosity of the five measurement areas was obtained using SEM and image analysis software, and then the average porosity of the five measurement areas was calculated as the porosity G2 of the main area. Similarly, five measurement areas were selected on average in the first pore area, and the area of ​​each measurement area was 0.5 mm 2 The porosity of the above five measurement areas is obtained using SEM and image analysis software, and then the average porosity of the five measurement areas is calculated as the porosity G1 of the first pore area.

[0117] Lithium deposition test method:

[0118] Place the lithium-ion secondary battery at a test temperature of 25°C for 30 minutes and perform step-by-step charging according to the following charging steps:

[0119] (1) 5C constant current charging to 4.23V, constant voltage charging to 4C;

[0120] (2) 4C constant current charging to 4.3V, constant voltage charging to 3C;

[0121] (3) 3C constant current charging to 4.4V, constant voltage charging to 2C;

[0122] (4) 2C constant current charging to 4.5V, constant voltage charging to 0.05C;

[0123] After standing for 10 minutes, discharge according to the following steps:

[0124] 0.2C DC discharge to 3V.

[0125] The above charge and discharge process is one cycle. After repeating 100 cycles, when the lithium-ion secondary battery is in a fully charged state (the battery is designed to have a maximum voltage of 4.5V), the secondary battery is disassembled to obtain the positive electrode sheet, the separator and the negative electrode sheet. If the lithium deposition area near the negative electrode ear is greater than or equal to 2mm 2 , then it is judged as lithium deposition. Each group tests 20 lithium-ion batteries, the number of lithium deposition is X, and the lithium deposition rate is X / 20.

[0126] Table 1

[0127]

[0128]

[0129] According to the above Table 1, in combination with Examples 1-1 to 1-6 and Comparative Example 1-1, when the separator is subjected to hot pressing to form a square hot pressing area, and the separator forms a first pore area with a smaller porosity, the occurrence of lithium precipitation can be effectively reduced. This is because the portion with a larger current density is covered by the first pore area, which can reduce the aggregation of lithium ions near the first groove, thereby reducing the occurrence of lithium precipitation. In addition, the local polarization of the negative electrode active material layer is reduced, which can reduce the decomposition of the electrolyte, thereby reducing the generation of the solid electrolyte interface film, and further reducing lithium precipitation.

[0130] In Comparative Example 1-2, the porosity G1 of the first pore region is too large, which may cause too many lithium ions to participate in the electrochemical reaction, which may easily lead to the difficulty of timely embedding lithium ions in the negative electrode active material layer in a short time, and the risk of lithium precipitation is high. In Comparative Example 1-3, the lithium precipitation rate is similar to that of Example 1-6, but compared with Example 1-6, in Comparative Example 1-3, the porosity G1 of the first pore region is too small, and the diffusion and migration of lithium ions are hindered, which may cause the lithium ions that can normally participate in the electrochemical reaction to be blocked, resulting in the loss of energy density of the lithium ion battery. Therefore, in the embodiment of the present application, 10% ≤ G1 ≤ 35% can be selected, which can reduce lithium precipitation while allowing more lithium ions to normally participate in the electrochemical reaction and improve the energy density of the secondary battery. In Example 1-2 to Example 1-6, the lithium precipitation rate is further reduced. In the embodiment of the present application, 10% ≤ G1 ≤ 30% is preferred.

[0131] Combined with Examples 1-4 and Examples 1-7 to 1-14 and Comparative Examples 1-1 and Comparative Examples 1-4 to 1-5, in Comparative Example 1-4, L m1 +L m2 +L1 is small, it is difficult to effectively cover the area with high current density, and the risk of lithium deposition is high. m1 +L m2 +L1 is large, the diffusion and migration of lithium ions are seriously hindered, and some lithium ions are difficult or even impossible to transmit, resulting in lithium ion accumulation. There is also the possibility of lithium precipitation, and it will seriously cause the energy density loss of the secondary battery. Refer to Comparative Examples 1-8 to 1-9. In Comparative Example 1-8, the length and width are further reduced, and the risk of lithium precipitation is further increased. In Comparative Example 1-9, it will cause serious loss of energy density. Therefore, in the embodiments of the present application, combined with Examples 1-4 and Examples 1-7 to 1-14, 12mm≤L m1 +L m2In Examples 1-4 and 1-10 to 1-12, the lithium plating rate is small, and the lengths of the first sub-region and the second sub-region are smaller, which can further reduce the energy density loss of the secondary battery. Considering the reduction of energy density loss and lithium plating, 20 mm ≤ L m1 +L m2 +L1≤28mm.

[0132] As for the length of the first sub-region, in combination with Embodiments 1-4 and 1-7 to 1-14, 1 mm ≤ L may be selected. m1 ≤12.5mm. In Example 1-14, the lithium deposition rate is similar to that in Example 1-13, but compared with Example 1-13, in Example 1-14, the length of the first sub-region is too large, resulting in a more serious loss of energy density of the secondary battery. Therefore, in the embodiment of the present application, 1mm≤L m1 In Examples 1-4 and 1-10 to 1-12, the lithium plating rate is small and the length of the first sub-region is smaller, which can further reduce the energy density loss of the secondary battery. Preferably, 5 mm ≤ L m1 ≤9mm.

[0133] In combination with Example 1-4, Example 1-15 to Example 1-20 and Comparative Example 1-1, Comparative Example 1-6 to 1-7, the risk of lithium deposition is relatively low in Example 1-4, Example 1-15 to Example 1-20. In Comparative Example 1-6, the width of the third sub-region is too small to effectively block lithium ions, and the risk of lithium deposition is relatively high. In Comparative Example 1-7, the width of the third sub-region is too large, which may block too many lithium ions, causing lithium ion accumulation and deteriorating the electrode interface, and there is also the possibility of lithium deposition. Therefore, in the embodiments of the present application, 1.5mm≤W can be selected m3 ≤15mm.

[0134] Similarly, in Examples 1-4 and Examples 1-16 to 1-20, the lithium precipitation rate is further reduced than that in Example 1-15. The lithium precipitation rate in Example 1-20 is similar to that in Example 1-19. In Example 1-19, the width of the third sub-region is smaller, which can reduce the energy density loss of the secondary battery. Therefore, in the embodiments of the present application, 2mm≤W can be selected. m3 In Examples 1-4 and 1-18 to 1-19, the lithium deposition rate is further reduced, and the width of the third sub-region is further reduced compared with that of Example 1-20. In the embodiments of the present application, preferably 4.5 mm ≤ W m3 ≤12mm.

[0135] In combination with Examples 1-4, 1-21 to 1-24, in Example 1-24, G m11 >Gm12 , and the current density is greater closer to the first groove, and the first section is closer to the first groove, so in Examples 1-24, the risk of lithium deposition is higher. m11 <G m12 .

[0136] In Example 1-21, the porosity of the first section is too small, which may cause energy density loss, and the porosity of the second section is too large, and the risk of lithium precipitation is higher than that of Examples 1-22 to 1-23 and Example 1-4. Therefore, in the examples of the present application, 10% ≤ G m11 ≤20%, and 20%≤G m12 ≤30%.

[0137] Different from Example 1-1, in Examples 2-1 to 2-20, along the width direction of the positive electrode plate, the length of the first pore area in the length direction of the positive electrode plate is increased to form a special-shaped structure similar to the current density concentration area. The relevant parameters are shown in Table 2 below.

[0138] Table 2

[0139] <![CDATA[G2]]> <![CDATA[G1]]> <![CDATA[G m11 ]]> <![CDATA[G m12 ]]> <![CDATA[L m1 +L m2 +L1(mm)]]> <![CDATA[L m1 (mm)]]> <![CDATA[W m3 (mm)]]> Lithium deposition rate Example 2-1 50％ 35％ 35％ 35％ 20 5 4.5 9 / 20 Example 2-2 50％ 30％ 30％ 30％ 20 5 4.5 6 / 20 Example 2-3 50％ 25％ 25％ 25％ 20 5 4.5 5 / 20 Embodiment 2-4 50％ 20％ 20％ 20％ 20 5 4.5 5 / 20 Embodiment 2-5 50％ 15％ 15％ 15％ 20 5 4.5 3 / 20 Embodiment 2-6 50％ 10％ 10％ 10％ 20 5 4.5 1 / 20 Embodiment 2-7 50％ 20％ 20％ 20％ 12 1 4.5 9 / 20 Embodiment 2-8 50％ 20％ 20％ 20％ 13 1.5 4.5 7 / 20 Embodiment 2-9 50％ 20％ 20％ 20％ 15 2.5 4.5 6 / 20 Embodiment 2-10 50％ 20％ 20％ 20％ 22 6 4.5 4 / 20 Example 2-11 50％ 20％ 20％ 20％ 26 8 4.5 3 / 20 Example 2-12 50％ 20％ 20％ 20％ 28 9 4.5 2 / 20 Example 2-13 50％ 20％ 20％ 20％ 30 10 4.5 1 / 20 Embodiment 2-14 50％ 20％ 20％ 20％ 35 12.5 4.5 0 / 20 Example 2-15 50％ 20％ 20％ 20％ 20 5 1.5 9 / 20 Example 2-16 50％ 20％ 20％ 20％ 20 5 2 7 / 20 Example 2-17 50％ 20％ 20％ 20％ 20 5 3 5 / 20 Example 2-18 50％ 20％ 20％ 20％ 20 5 9 3 / 20 Example 2-19 50％ 20％ 20％ 20％ 20 5 12 1 / 20 Example 2-20 50％ 20％ 20％ 20％ 20 5 15 1 / 20

[0140] According to the above Table 2, combined with Examples 2-1 to 2-6 and Comparative Example 1-1, when the separator is subjected to hot pressing to form a special-shaped hot pressing area, and the separator forms a first pore area with a smaller porosity, the occurrence of lithium precipitation can be effectively reduced. This is because the portion with a larger current density is covered by the first pore area, which can reduce the aggregation of lithium ions near the first groove, thereby reducing the occurrence of lithium precipitation. In addition, the local polarization of the negative electrode active material layer is reduced, which can reduce the decomposition of the electrolyte, thereby reducing the generation of the solid electrolyte interface film, and further reducing lithium precipitation.

[0141] Compared with the above-mentioned comparative example 1, in Examples 2-1 to 2-6, the risk of lithium plating is lower, and 10%≤G1≤35% can be selected, which can reduce lithium plating while allowing more lithium ions to participate in the electrochemical reaction normally, thereby improving the energy density of the secondary battery. In Examples 2-2 to 2-6, the lithium plating rate is further reduced, and in the embodiments of the present application, 10%≤G1≤30% is preferred.

[0142] In combination with Embodiment 2-4 and Embodiments 2-7 to 2-14, 12 mm ≤ L m1 +L m2 In Examples 2-4 and 2-10 to 2-12, the lithium plating rate is small, and the lengths of the first sub-region and the second sub-region are smaller, which can further reduce the energy density loss of the secondary battery. Considering the reduction of energy density loss and lithium plating, 20 mm ≤ Lm1 +L m2 +L1≤28mm.

[0143] As for the length of the first sub-region, in combination with Embodiment 2-4 and Embodiments 2-7 to 2-14, 1 mm ≤ L m1 ≤12.5mm. In Example 2-7, the risk of lithium deposition is still high. For the special-shaped hot pressing area, 1.5mm≤L m1 In Examples 2-10 to 2-12, the lithium plating rate is small, and the length of the first sub-region is smaller, which can further reduce the energy density loss of the secondary battery. Preferably, 6 mm ≤ L m1 ≤9mm.

[0144] The risk of lithium deposition in Examples 2-15 to 2-20 is relatively low. In the examples of the present application, 1.5 mm ≤ W m3 ≤15mm. In Examples 2-17 to 2-20, the risk of lithium deposition is further reduced, and 3mm≤W can be further selected. m3 In Examples 2-4 and 2-18 to 2-19, the lithium deposition rate is further reduced, and the width of the third sub-region is further reduced compared with that of Example 2-20. In the embodiments of the present application, preferably 4.5 mm ≤ W m3 ≤12mm.

[0145] Different from Example 1-4, the relevant parameters in Examples 3-1 to 3-7 are shown in Table 3, and different from Example 2-4, the relevant parameters in Examples 3-8 to 3-14 are shown in Table 3. Among them, the length of the first glue layer is L2, and the length of the first groove is L1.

[0146] Table 3

[0147]

[0148]

[0149] In combination with Examples 1-4 and Examples 3-1 to 3-7, when a square hot pressing area is used, in Example 3-1, the length of the first glue layer is too small, and the current density is large near the first groove, which may cause the first glue layer to be difficult to effectively cover the part, the lithium ion deintercalation reaction is still violent, and the risk of lithium precipitation is still high. In combination with Examples 2-4 and Examples 3-8 to 3-14, when a special-shaped hot pressing area is used, in Example 3-8, the length of the first glue layer is too small, and the current density is large near the first groove, which may also cause the first glue layer to be difficult to effectively cover the part, the lithium ion deintercalation reaction is still violent, and the risk of lithium precipitation is still high. Therefore, in the embodiments of the present application, 1mm≤L2-L1≤10mm can be selected.

[0150] When a square hot pressing area is used, the lithium precipitation rate of Example 3-5 is similar to that of Examples 3-6 to 3-7, but in Example 3-5, the length of the first adhesive layer is smaller, and the energy density loss of the secondary battery is smaller. When a special-shaped hot pressing area is used, the lithium precipitation rate of Example 3-12 is similar to that of Examples 3-13 to 3-14, but in Example 3-12, the length of the first adhesive layer is smaller, and the energy density loss of the secondary battery is smaller. Therefore, in the embodiments of the present application, it is preferred that 1mm≤L2-L1≤6mm.

[0151] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Under the concept of the present application, the technical features in the above embodiments or different embodiments may also be combined, the steps may be implemented in any order, and there are many other changes in 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 aforementioned embodiments, a person of ordinary skill in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions 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, a negative electrode sheet and a negative electrode tab, wherein the separator is stacked between the positive electrode sheet and the negative electrode sheet; the negative electrode sheet comprises a negative electrode active material layer facing the positive electrode sheet, and the negative electrode active material layer is provided with a first groove; a portion of the negative electrode tab is arranged in the first groove, and another portion of the negative electrode tab extends out of the negative electrode sheet along a first direction; the separator is arranged on a side of the first groove facing the positive electrode sheet, the positive electrode sheet comprises a positive electrode active material layer facing the negative electrode active material layer, a first glue layer is arranged on a surface of the positive electrode active material layer facing the negative electrode active material layer, and along a third direction, the first glue layer covers the first groove, characterized in that The isolation film includes a first portion and a first pore area, and when viewed along the third direction, the first portion overlaps with the first groove, the first pore area is connected to the first portion, and the first pore area is arranged around the first portion; The average porosity of the first pore region is G1, 10%≤G1≤35%; The first pore region includes a first sub-region. Along the first direction, the width of the first portion is equal to the maximum width of the first sub-region. Along the second direction, the first sub-region is connected to the first portion. The maximum length of the first sub-region is L m1 , 1mm≤L m1 ≤12.5mm; The first pore region includes a third sub-region. Along the first direction, the third sub-region is connected to the first portion. The maximum width of the third sub-region is W. m3 , 1.5mm≤W m3 ≤15mm; 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 in that: 10％≤G1≤30％。 3. The secondary battery according to claim 1, characterized in that: The isolation membrane further includes a main body region, the first pore region is disposed between the first portion and the main body region, the porosity of the main body region is G2, and G1<G2.

4. The secondary battery according to claim 1, characterized in that: Along the second direction, the first pore region further includes a second sub-region, and the first portion is located between the first sub-region and the second sub-region; along the first direction, the maximum width of the second sub-region is equal to the width of the first region; Along the second direction, the maximum length of the second sub-area is L m2 , the length of the first groove is L1, 12mm≤L m1 +L m2 +L1≤35mm.

5. The secondary battery according to claim 4, characterized in that: 22mm≤L m1 +L m2 +L1≤28mm。 6. The secondary battery according to claim 1, characterized in that: Along the second direction, the first sub-region is divided into a first segment and a second segment of equal length, and the first segment is located between the second segment and the first portion; The average porosity of the first section is G m11 The average porosity of the second segment is G m12 , G m11 <G m12 .

7. The secondary battery according to claim 6, characterized in that: 10%≤G m11 ≤20%,20%≤G m12 ≤30%。 8. The secondary battery according to claim 6, characterized in that: Along the first direction, the maximum width of the first segment is W m11 , the maximum width of the second segment is W m12 , 0.95≤W m11 / W m12 ≤1.05; 1mm≤L m1 ≤10mm.

9. The secondary battery according to claim 8, characterized in that: 5mm≤L m1 ≤9mm。 10. The secondary battery according to claim 6, characterized in that: Along the first direction, the maximum width of the first segment is W m11 , the maximum width of the second segment is W m12 , W m11 / W m12 >1.05; 1.5mm≤L m1 ≤12.5mm.

11. The secondary battery according to claim 10, characterized in that: 6mm≤L m1 ≤9mm。 12. The secondary battery according to claim 1, characterized in that: Along the first direction, the third sub-region is divided into a third segment and a fourth segment of equal width, and the third segment is located between the fourth segment and the first portion; The average porosity of the third section is G m31 The average porosity of the fourth section is G m32 , G m31 <G m32 .

13. The secondary battery according to claim 12, characterized in that: Along the second direction, the maximum length of the third segment is L m31 , the maximum length of the fourth segment is L m32 , 0.95≤L m31 / L m32 ≤1.05;2mm≤W m3 ≤12mm.

14. The secondary battery according to claim 13, characterized in that: 4.5mm≤W m3 ≤12mm。 15. The secondary battery according to claim 12, characterized in that: Along the second direction, the maximum length of the third segment is L m31 , the maximum length of the fourth segment is L m32 , L m31 / L m32 >1.05; 3mm≤W m3 ≤15mm.

16. The secondary battery according to claim 15, characterized in that: 4.5mm≤W m3 ≤12mm。 17. The secondary battery according to claim 1, characterized in that: Along the second direction, the length of the first groove is L1, the length of the first adhesive layer is L2, and 1mm≤L2-L1≤10mm.

18. The secondary battery according to claim 17, characterized in that: 1mm≤L2-L1≤6mm.

19. The secondary battery according to claim 1, characterized in that: Along the second direction, the first pore region further includes a second sub-region, and the first portion is connected between the first sub-region and the second sub-region; Along the second direction, the negative electrode active material layer includes a first end and a second end that are oppositely arranged, and along the third direction, a projection of the first sub-region on the negative electrode active material layer is located between the first groove and the first end, and a projection of the second sub-region on the negative electrode active material layer is located between the first groove and the second end; Along the second direction, the distance from the first groove to the first end is L3, and the distance from the first groove to the second end is L4; Along the second direction, the maximum length of the first sub-region is L m1 , the maximum length of the second sub-region is L m2 ; L3>L4, L m1 >L m2 ; or, L3 <L4,L m1 <L m2 .

20. A method for preparing a secondary battery according to any one of claims 1 to 19, characterized in that: include: A positive electrode sheet, a separator, a negative electrode sheet and a negative electrode tab are provided, wherein the negative electrode sheet comprises a negative electrode active material layer, the negative electrode active material layer is provided with a first groove, a portion of the negative electrode tab is provided in the first groove, and another portion of the negative electrode tab is extended out of the negative electrode sheet along a first direction; the positive electrode sheet comprises a positive electrode active material layer; Heating a portion of the isolation film, wherein the heated portion includes a first pore region, wherein an average porosity of the first pore region is G1, and 10%≤G1≤35%; Providing a first adhesive layer, bonding the first adhesive layer to the surface of the positive electrode active material layer and covering the first groove; The isolation membrane is arranged on the side of the first groove facing the positive electrode plate, and the positive electrode active material layer is arranged facing the negative electrode active material layer; wherein, along the third direction, the first glue layer covers the first groove, the isolation membrane includes a first part, the first part overlaps with the first groove, the first pore area is connected to the first part, and the first pore area is arranged around the first part.

21. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 19.

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