Secondary battery, method for manufacturing the same, and electronic device

Abstract

The application discloses a secondary battery and a preparation method thereof, and an electronic device. Part of the negative tab extends out of the negative tab sheet along a first direction, and a separator is arranged on one side of the first groove facing the positive tab. The separator comprises a first part and a first pore area. The first part is coincident 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 10%≤G1≤35%. The first pore area comprises a first sub-area and a third sub-area. Along the first direction, the width of the first part is equal to the maximum width of the first sub-area. Along the second direction, the first sub-area is connected with the first part, and the maximum length of the first sub-area is L m1 , 1mm≤L m1 ≤12.5mm. Along the first direction, the third sub-area is connected with the first part, and the maximum width of the third sub-area is W m3 , 1.5mm≤W m3 ≤15mm. The energy density loss of the secondary battery can be reduced while reducing lithium precipitation.

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 advantages. Lithium-ion batteries include positive and negative electrodes, and positive and negative tabs are usually required to lead out the positive and negative electrodes of the lithium-ion battery. For example, grooves are provided on the negative active material layer to connect the negative tabs and lead out the negative electrode. However, the lithium ions released from the corresponding negative tab slots in the positive active material layer may be difficult to embed into the negative active material layer, which can easily lead to lithium plating. 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 in the secondary battery.

[0004] In order to solve the technical problems, the embodiments of the present application adopt the following technical solutions:

[0005] In a first aspect, the present application provides a secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet, and a negative electrode tab, with the separator interposed between the positive and negative electrode sheets. The negative electrode sheet includes a negative active material layer facing the positive electrode sheet, the negative active material layer being provided with a first groove. A portion of the negative electrode tab is disposed within the first groove, with another portion of the negative electrode tab extending out of the negative electrode sheet in a first direction. The separator is disposed on the side of the first groove facing the positive electrode sheet. The positive electrode sheet includes a positive active material layer facing the negative active material layer. A first adhesive layer is disposed on the surface of the positive active material layer facing the negative active material layer, with the first adhesive layer covering the first groove in a third direction. The separator includes a first portion and a first pore region. When viewed in the third direction, the first portion overlaps with the first groove, the first pore region is in contact with the first portion, and the first pore region is disposed around the first portion. The average porosity of the first pore region is G1, with 10% ≤ G1 ≤ 35%. The first pore region includes a first subregion. Along the first direction, the width of the first portion is equal to the maximum width of the first subregion. Along the second direction, the first subregion is connected to the first portion. The maximum length of the first subregion 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. 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%, which can make the part of the negative electrode active material layer with a higher current density near the first groove covered by the first pore area, reducing the aggregation of lithium ions near the first groove, thereby reducing the occurrence of lithium plating. 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 plating. 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, and 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. By reducing the porosity of the first pore region, the ion transmission speed is slowed down, which makes it easier for the negative electrode to 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. Adjusting the shape of the first sub-region according to the current density and selecting the maximum length of the first sub-region can reduce the occurrence of lithium plating on one side of the length direction of the first groove and reduce the loss of secondary battery energy density. 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. By adjusting the maximum width of the third sub-region according to the current density, the energy density loss of the secondary battery can be reduced while reducing lithium precipitation. Preferably, 4.5mm≤Wm3≤12mm, which can further reduce lithium precipitation and further reduce the energy density loss of the secondary battery. m3 ≤12mm, which can further reduce lithium precipitation and further reduce the energy density loss of the secondary battery.

[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. By adjusting the shape of the third sub-region according to the current density, and selecting the maximum width of the third sub-region, the energy density loss of the secondary battery can be reduced while reducing lithium precipitation. Preferably, 4.5mm≤Wm3≤12mm, which can further reduce lithium precipitation and further reduce the energy density loss of the secondary battery. m3 ≤12mm, which can further reduce lithium precipitation and further reduce the energy density loss of the secondary battery.

[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 influence 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 aperture 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 active material layer includes oppositely arranged first and second end portions. Along the third direction, the projection of the first sub-region on the negative active material layer is located between the first groove and the first end portion, and the projection of the second sub-region on the negative active material layer is located between the first groove and the second end portion. Along the second direction, the distance from the first groove to the first end portion is L3, and the distance from the first groove to the second end portion is L4. Along the second direction, the maximum length of the first sub-region is L m1 , and 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 and second sub-regions, the size of the first adhesive layer can be reduced while reducing the risk of lithium precipitation, and the energy density of the secondary battery can be improved.

[0018] In a second aspect, the application also provides a preparation method of the secondary battery, comprising: providing a positive electrode sheet, a separator, a negative electrode sheet, and a negative electrode tab, the negative electrode sheet comprising a negative electrode active material layer, the negative electrode active material layer being provided with a first groove, a part of the negative electrode tab being arranged in the first groove, and another part of the negative electrode tab extending out of the negative electrode sheet in a first direction; the positive electrode sheet comprising a positive electrode active material layer. A part of the separator is subjected to a heating treatment, and the heated part comprises a first porosity region, the average porosity of the first porosity region being G1, and 10%≤G1≤35%. A first adhesive layer is provided, and the first adhesive 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 faces the negative electrode active material layer. In a third direction, the first adhesive layer covers the first groove, the separator comprises a first part, the first part coincides with the first groove, the first porosity region is connected with the first part, and the first porosity region is arranged around the first part.

[0019] In a third aspect, the application also provides an electronic device comprising the secondary battery according to any one of the embodiments of the first aspect.

[0020] Additional layers and advantages of the embodiments of the application will be described, shown, or explained in part in the subsequent description, drawings, or by implementation of the embodiments of the application. BRIEF DESCRIPTION OF DRAWINGS

[0021] One or more embodiments are illustrated by way of example in the accompanying drawings, which are not intended to be limiting of the embodiments, and which do not constitute a definition of all possible embodiments, and wherein like numerals represent similar elements across several illustrations. Like numerals represent like elements.

[0022] Figure 1 Structure diagram of the secondary battery of some embodiments of the application;

[0023] Figure 2 Winding structure diagram of the electrode assembly of some embodiments of the application;

[0024] Figure 3 Lamination structure diagram of the electrode assembly of some embodiments of the application;

[0025] Figure 4 Lamination structure diagram of the positive electrode sheet, the separator, and the negative electrode sheet of some embodiments of the application;

[0026] Figure 5 Current density diagram of the negative electrode active material layer at the first groove of some embodiments of the 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 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] Figure 20 This is a schematic structural diagram of the hot pressing portion of the negative electrode sheet and the separator in some embodiments of the present application;

[0042] Figure 21 This is a schematic structural diagram of the hot-pressed portion of the negative electrode plate and 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 subregion; 3211. First section; 3212. Second section; 322. Second subregion; 333. Third subregion; 3331. Third section; 3332. Fourth section; 33. First portion;

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

[0056] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0057] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects 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). The shell 200 can accommodate the electrode assembly 100 and the electrolyte. The electrolyte in the shell 200 soaks the electrode assembly 100 .

[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 along 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 lamination structure of the electrode assembly 100 is shown. Along the third direction Z (the thickness direction of the positive electrode sheet 10 and / or the negative electrode sheet 20), multiple positive electrode sheets 10 and multiple negative electrode sheets 20 are alternately stacked, and an isolation film 30 is set between adjacent positive electrode sheets 10 and negative electrode sheets 20.

[0061] It should be noted that in the embodiments 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 can be made of an overall flat aluminum foil, titanium foil, nickel foil, or stainless steel foil. 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 each other. 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. 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 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 can be made of a flat copper foil, titanium foil, nickel foil, stainless steel foil, or silver foil. The negative electrode active material layer 22 can be provided 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 that are arranged opposite to each other. The negative electrode active material layer 22 can be provided 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. These materials are mixed, stirred, and evenly 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, elemental silicon, silicon oxides, silicon alloys, and the like.

[0064] In some embodiments, the separator 30 includes a substrate layer and a ceramic layer. The substrate layer includes at least one of polypropylene, polyethylene, polyvinylidene fluoride, polyimide, or polyamide, and the ceramic layer includes at least one of aluminum oxide, magnesium oxide, or zirconium oxide. This allows the separator 30 to adapt to the electrolyte environment within the secondary battery 100, extending the service life of the separator 30.

[0065] The secondary battery 1000 also includes a negative electrode tab 40, see Figure 4 The negative electrode tab 40 is connected to the negative electrode plate 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 portion of the negative electrode tab 40 is provided in the first groove 221 and connected to the negative electrode current collector 21. Connection methods include, but are not limited to, welding or conductive adhesive bonding. A second adhesive layer 60 can be provided to cover a portion of the negative electrode tab 40 to isolate burrs on the negative electrode tab 40 and reduce the risk of burrs piercing the separator 30. The other portion 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 adhesive 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 adhesive layer 50 covers the first groove 221. This allows the portion of the positive electrode active material layer 12 corresponding to the first groove 221 to be covered by the first adhesive layer 50. This reduces or prevents the release of lithium ions from this portion, thereby allowing the negative electrode active material layer 22 to have sufficient residual lithium ions to be embedded, thereby reducing the risk of lithium plating.

[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, further hindering the insertion of lithium ions and exacerbating 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 A schematic diagram showing the current density of the negative electrode active material layer 22 near the first groove 221 is shown. 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 causes lithium ions to accumulate 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 cause the negative electrode potential to decrease. When the negative electrode potential is lower than the potential at which lithium ions are reduced to metallic lithium, lithium precipitation may occur.

[0068] To reduce the above problems, in the embodiments of this application, please refer to Figure 6The 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, and the first pore region 32 is connected to the first portion 33. The first pore region 32 is arranged around the first portion 33, and can be around half a circle, 3 / 4 circle, or a full 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, which includes the first portion 33 and the first pore region 32. This can close some of the pores in the hot pressing region and / or reduce the pore diameter, thereby reducing the porosity of the hot pressing region. The portion with higher current density can be covered by the first pore region 32, reducing the accumulation of lithium ions near the first groove 221, thereby reducing the occurrence of lithium plating.

[0069] Please refer to further Figures 10 to 11 as well as Figures 14 and 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 portion 33. The maximum width of the third sub-area 333 is W m3 , 1.5mm≤W m3 ≤15mm. Compared to extending the first adhesive layer 50, providing a smaller porosity region in the separator 30 effectively reduces the space occupied by the first adhesive layer 50, thereby improving the energy density of the secondary battery 1000. Furthermore, the first porous region 32 allows for the transmission of a small amount of lithium ions, facilitating full utilization of the active material layer and further increasing 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, aging and decreased viscosity of the adhesive layer may cause it to shift 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 plating. The properties of the isolation membrane 30 are more stable, which is conducive to extending the service life of the secondary battery 100.

[0071] The inventors of this application unexpectedly discovered that by limiting the G1 ratio to 10% ≤ G1 ≤ 35%, local polarization of the negative electrode active material layer 22 is reduced, which can reduce electrolyte decomposition, thereby reducing the formation of solid electrolyte interface films, further reducing lithium deposition. Furthermore, heat concentration in the first groove 221 is alleviated, reducing the occurrence of side reactions and improving the charge and discharge performance of the secondary battery 1000.

[0072] 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 higher, resulting in a higher risk of lithium deposition. However, near the first groove 221, a portion of the negative electrode active material layer 22 is still capable of undergoing electrochemical reactions and contributing to the capacity of the secondary battery 1000. Under the same conditions, assuming no lithium deposition occurs, the portion with a higher current density can receive fewer lithium ions than the portion with a lower current density.

[0073] In the embodiment of the present application, the limit of 10% ≤ G1 ≤ 35% is set. The number of channels through which lithium ions can pass in the first pore region 32 is relatively small, which slows down the ion transmission rate. This allows the negative electrode active material layer 22 to have sufficient margin to embed lithium ions released from the positive electrode per unit time, and allows the portion of the positive electrode active material layer 12 covered by the first pore region 32 to be utilized, which is beneficial for increasing the energy density of the secondary battery 1000 while reducing the occurrence of lithium plating. The porosity of the first pore region 32 can be controlled by parameters such as temperature, time, and pressure during the hot pressing process of the separator 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. A first pore region 32 is provided between the first portion 33 and the main body region 31. The porosity of the main body region 31 is G2, G1<G2. By reducing the porosity of the first pore region 32, the ion transmission speed is slowed down, which makes it easier for the negative electrode to have sufficient margin to embed the lithium ions released from the positive electrode per unit time, 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 relatively smooth 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] Regarding the material of the separator 30, the separator 30 comprises a substrate layer and a ceramic layer. The ceramic layer is disposed on the surface of the substrate layer along the thickness of the separator. The substrate layer comprises at least one of polyethylene, polypropylene, polytetrafluoroethylene, cellulose acetate, or cellulose nanofibers. Each material has excellent chemical stability, maintaining its structural and performance stability in the electrolyte environment, thereby effectively reducing chemical reactions with the electrolyte that could lead to separator 30 failure. Each material also has excellent electrical insulation properties, effectively isolating the positive electrode 10 from the negative electrode 20 and reducing the risk of short circuits. Furthermore, each material possesses a certain degree of flexibility and strength, which helps to reduce damage to the separator 30 and effectively isolate the positive electrode 10 from the negative electrode 20. The ceramic layer comprises at least one of boehmite, aluminum oxide, magnesium oxide, zirconium oxide, or silicon dioxide. Each material has excellent insulation and thermal stability. When the internal temperature of the secondary battery 1000 rises, it can reduce heat diffusion, thereby reducing the risk of thermal runaway in 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 separator is determined by the smaller porosity of the substrate layer or the ceramic layer. For example, if 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 separator. If 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 separator. When the separator 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 separator.

[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 ultrasonicated 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) is used to measure the conductivity of the test sample. Microscope (scanning electron microscope) was used to observe the surface and cross-section of the test sample at different locations. The area and distribution of the pores were then calculated using image analysis software, and the porosity of the corresponding measurement area was calculated. Specifically, after the test sample was cut, a measurement area was selected at intervals of 0.5 mm along the second direction Y. The area of ​​each measurement area was 0.5 mm. 2 The average porosity of each measurement area was obtained using SEM and image analysis software. The midline of the area with an average porosity of 30% ± 1% was defined as the boundary between the main area 31 and the first pore area 32 in the second direction Y. Similarly, a measurement area was selected at intervals of 0.5 mm along the third direction Z, and the area of ​​each measurement area was 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 dividing line 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 an approximately square structure, such as Figure 6 As shown; it can also be set to 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 while improving the energy density of the secondary battery 1000.

[0082] In some embodiments, please refer to Figure 8 and Figure 9Observed along the third direction Z, the first pore region 32 is disposed around the first portion 33. The current density near the first groove 221 is high, and the lithium ion intercalation and deintercalation reaction is intense. By arranging the first pore region 32 around the first portion 33, the amount of lithium ion deintercalation around the first groove 221 can be effectively reduced, thereby reducing the occurrence of lithium deposition. Observed along the third direction Z, the first pore region 32 is also disposed around the first adhesive layer 50, and the first adhesive layer 50 can partially cover the first pore region 32.

[0083] Please refer to Figure 8 and Figure 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 it does not penetrate 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 plating. Please refer to Figure 10 and Figure 11 The first groove 221 may also pass through the first edge 222 , that is, the first aperture area 32 may surround a portion of the first groove 221 .

[0084] The inventors of the present application set 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 aperture 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 can reduce lithium plating while reducing the energy density loss of the secondary battery 100.

[0085] In some embodiments, please refer to Figure 12 and Figure 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, thereby reducing the occurrence of lithium deposition. The second section 3212 is farther away from the first groove 221, and the current density is smaller, and the risk of lithium deposition is lower. 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 beneficial 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 region 31 and the first pore region 32. The centerline of the test area with an average porosity of 20%±1% is considered the boundary between the first section 3211 and the second section 3212.

[0086] When the hot pressing area is a square structure, please refer to Figure 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. The maximum length of the first sub-region 321 is selected according to the current density, which can reduce the occurrence of 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. 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 Figure 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. 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 can reduce the occurrence of 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. 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 this 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, 16.5mm≤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 energy density loss and lithium plating.

[0090] In some embodiments, please refer to Figure 16 and Figure 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 section 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 has a smaller porosity, which can block more lithium ions and thus reduce the occurrence of lithium deposition. The fourth section 3332 is far away from the first groove 221, and has a smaller current density and a lower risk of lithium deposition. 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 boundary between the third segment 3331 and the fourth segment 3332 is determined by referring to the test method for the main body region 31 and the first pore region 32. The center line of the test area with an average porosity of 20%±1% is considered the boundary between the third segment 3331 and the fourth segment 3332.

[0091] In some embodiments, please refer to Figure 16 When the hot pressing area is a square 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 lithium plating while reducing the energy density loss of the secondary battery 1000. Preferably, 4.5mm≤W 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 Figure 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 plating. 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] And due to the closed pore structure of the isolation film 30, the transmission of lithium ions around the first groove 221 can be reduced, thereby reducing the occurrence of lithium precipitation, and thus the length of the first adhesive layer 50 can be adaptively reduced. For example, please refer to Figure 18 In the second direction Y, the length of the first groove 221 is L1, and 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 operation can be facilitated. Preferably, 1mm≤L2-L1≤6mm, which can further reduce the influence 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 operation.

[0094] In some embodiments, please refer to Figures 19 to 21 In the second direction Y, the first pore region 32 includes a first sub-region 321 and a second sub-region 322, and the first part 33 is connected between the first sub-region 321 and the second sub-region 322. In the second direction Y, the negative electrode active material layer 22 includes a first end portion 20a and a second end portion 20b arranged opposite to each other, and in 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 portion 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 portion 20b. In the second direction Y, the distance from the first groove 221 to the first end portion 20a is L3, and the distance from the first groove 221 to the second end portion 20b is L4.

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

[0096] For example, in the second direction Y, the maximum length of the first sub-region 321 is L m1 , and 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 portion 20a is larger, and L m1 >L m2 When L3 m1 <L m2By reasonably setting the lengths of the first sub-region 321 and the second sub-region 322, the size of the first pore region 32 can be reduced while reducing the risk of lithium precipitation, thereby improving the energy density of the secondary battery 1000. When L3 = L4, L m1 is approximately equal to L m2 , for example, |L m2 -L m1 |≤0.5mm.

[0097] In a second aspect, the application further provides a preparation method of a secondary battery 1000, comprising: 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 comprising a negative electrode active material layer 22, the negative electrode active material layer 22 being provided with a first groove 221, a part of the negative electrode tab 40 being arranged in the first groove 221, and another part of the negative electrode tab 40 extending out of the negative electrode sheet 20 along a first direction X; the positive electrode sheet 10 comprising a positive electrode active material layer 12.

[0098] A part of the separator 30 is subjected to a heating treatment, and the heated part comprises the first pore region 32, the average porosity of the first pore region 32 being G1, 10%≤G1≤35%.

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

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

[0101] Wherein, along the third direction Z, the first adhesive layer 50 covers the first groove 221, the separator 30 comprises a first part 33, the first part 33 coincides with the first groove 221, the first pore region 32 is connected with the first part 33, and the first pore region 32 is arranged around the first part 33.

[0102] In the embodiments of the application, by heating the separator 30, the separator 30 can form the first pore region 32 with a smaller porosity, and the range of 10%≤G1≤35% is limited, so that the part with a larger current density is covered by the first pore region 32, the aggregation of lithium ions near the first groove 221 is reduced, and the occurrence of lithium precipitation is reduced. Moreover, compared with extending the first adhesive layer 50, by arranging different porosity regions on the separator 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 enable 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 a third aspect, the present application further proposes an electronic device, comprising a secondary battery 1000 according to any embodiment of the first aspect. The electronic device of the embodiments of the present 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, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0104] Example 1-1

[0105] Preparation of positive electrode:

[0106] The positive electrode active material lithium cobalt oxide, the positive electrode conductive agent acetylene black, the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight 5×10 5 ) were mixed in a mass ratio of 94:3:3, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a positive electrode slurry with a solid content of 75wt%. The mixture was then 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. The positive electrode slurry was evenly coated on one surface of the aluminum foil and dried at 110°C to obtain a positive electrode sheet coated on one side with a positive electrode active material layer. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated on both sides with a positive electrode active material layer.

[0107] Preparation of negative electrode sheet:

[0108] The negative electrode active materials, graphite powder, silicon powder, conductive carbon black (Super P), and binder styrene-butadiene rubber (SBR), are mixed in a weight ratio of 87.5:10:1:1.5. Deionized water is then added as a solvent to prepare a negative electrode slurry with a solid content of 50 wt%, which is then stirred evenly. A copper foil with a thickness of 5 μm and a length of 1050 mm is selected as the negative electrode current collector. The negative electrode slurry is evenly coated on one surface of the negative electrode current collector copper foil, and an empty foil area is reserved on the copper foil for uncoated negative electrode slurry. The copper foil is then dried at 90°C to obtain a single-sided negative electrode sheet. After completing the above steps, the negative electrode sheet is coated on one side. The above steps are then repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet coated on both sides with a negative electrode active material layer.

[0109] Preparation of isolation membrane:

[0110] A porous separator was prepared by using polyethylene as a 7μm substrate layer and polyvinylidene fluoride as an adhesive layer. A 2μm thick alumina ceramic layer was placed on the side of the adhesive layer facing away from the substrate layer. A portion of the separator was hot-pressed to form a square hot-pressed area. The hot-pressed area included a first pore area, and the unhot-pressed portion formed a main body area. The porosity of the main body area was G2, which was 50%, and the porosity of the first pore area was G1, which was 35%, with G1 < G2. The maximum length of the first pore area was L. 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 uniformly 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 active material layer, and a nickel sheet is selected as the negative electrode tab, and the negative electrode tab is welded to the negative electrode sheet in the first groove. After the positive electrode sheet is welded to the positive electrode tab, polyethylene terephthalate is selected as the base material 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, which is bonded to the positive active material layer. The isolation membrane, positive electrode sheet, isolation membrane, 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 pore area is between the first part and the main area. The first sub-area and the second sub-area are on both sides of the first part. The width L of the first sub-area is 2.5mm. 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 5mm. 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] Unlike Example 1-1, the relevant parameters of Examples 1-2 to 1-24 and Comparative Examples 1-1 to 1-9 are shown in Table 1 below. In Comparative Example 1-1, the isolation film was not subjected to hot pressing. The first sub-region was divided into a first section and a second section of equal length, with the first section located between the second section and the first portion. The average porosity of the first section was G m11 The average porosity of the second section is G m12 .

[0116] G2 and G1 test method: After dividing the main area and the first pore area, an average of 5 measurement areas are selected 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 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 within the first pore area, and the area of ​​each measurement area was 0.5 mm. 2 The porosity of the above 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 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 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, separator and negative electrode sheet. If the lithium deposition area near the negative electrode tab is greater than or equal to 2mm 2 , it is determined to be lithium deposition. Each group tests 20 lithium-ion batteries. The number of lithium depositions is X, and the lithium deposition rate is X / 20.

[0126] Table 1

[0127]

[0128]

[0129] According to Table 1 above, in combination with Examples 1-1 to 1-6 and Comparative Example 1-1, it can be seen that when the separator is hot-pressed to form a square hot-pressed area, and the separator is formed into a first pore area with a smaller porosity, the occurrence of lithium plating can be effectively reduced. This is because the portion with a higher current density is covered by the first pore area, which can reduce the accumulation of lithium ions near the first groove, thereby reducing the occurrence of lithium plating. 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 formation of a solid electrolyte interface film, and further reducing lithium plating.

[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 make it difficult for the negative electrode active material layer to be timely embedded with lithium ions in a short period of time, and the risk of lithium plating is high. In Comparative Example 1-3, the lithium plating 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 easily cause lithium ions that can normally participate in the electrochemical reaction to be blocked, resulting in a 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 plating while allowing more lithium ions to normally participate in the electrochemical reaction and improve the energy density of the secondary battery. In Examples 1-2 to 1-6, the lithium plating 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 too small 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 plating, 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 plating is further increased. In Comparative Example 1-9, it will lead to serious loss of energy density. Therefore, in the embodiments of this 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, 20mm≤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 of Example 1-13, but compared with Example 1-13, the length of the first sub-region in Example 1-14 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, 5mm≤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 plating in Example 1-4, Example 1-15 to Example 1-20 is relatively low. In Comparative Example 1-6, the width of the third sub-region is too small, making it difficult to effectively block lithium ions, and the risk of lithium plating 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 worsening the electrode interface, and there is also the possibility of lithium plating. Therefore, in the embodiments of the present application, 1.5mm≤W can be selected m3 ≤15mm.

[0134] Similarly, in Examples 1-4 and 1-16 to 1-20, the lithium deposition rate is further reduced than that in Example 1-15. The lithium deposition 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 embodiment of the present application, 2mm≤W 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 closer to the first groove, the greater the current density is. The first section is closer to the first groove, so in Examples 1-24, the risk of lithium deposition is higher. In the embodiment of this application, G is selected 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 deposition is higher than that of Examples 1-22 to 1-23 and Example 1-4. Therefore, in the examples of this application, 10% ≤ G m11 ≤20%, and 20%≤G m12 ≤30%.

[0137] Different from Example 1-1, in Examples 2-1 to 2-20, the length of the first pore area in the length direction of the positive electrode sheet is increased along the width direction of the positive electrode sheet 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]]> G m11 ]]> <![CDATA[G m12 ]]> <![CDATA[L m1 +L m2 +L1(mm)]]> 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 Examples 2-4 50％ 20％ 20％ 20％ 20 5 4.5 5 / 20 Examples 2-5 50％ 15％ 15％ 15％ 20 5 4.5 3 / 20 Examples 2-6 50％ 10％ 10％ 10％ 20 5 4.5 1 / 20 Examples 2-7 50％ 20％ 20％ 20％ 12 1 4.5 9 / 20 Examples 2-8 50％ 20％ 20％ 20％ 13 1.5 4.5 7 / 20 Examples 2-9 50％ 20％ 20％ 20％ 15 2.5 4.5 6 / 20 Example 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 Examples 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 Table 2 above, combined with Examples 2-1 to 2-6 and Comparative Example 1-1, when the separator is hot-pressed to form a special-shaped hot-pressed region and a first pore region with a smaller porosity is formed in the separator, the occurrence of lithium plating can be effectively reduced. This is because the portion with a higher current density is covered by the first pore region, which can reduce the accumulation of lithium ions near the first groove, thereby reducing the occurrence of lithium plating. 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 formation of a solid electrolyte interface film, further reducing lithium plating.

[0141] Compared to Comparative Example 1, Examples 2-1 to 2-6 have a lower risk of lithium plating, and a range of 10% ≤ G1 ≤ 35% can be selected. This can reduce lithium plating while allowing more lithium ions to participate in the electrochemical reaction normally, thereby increasing the energy density of the secondary battery. In Examples 2-2 to 2-6, the lithium plating rate is further reduced. In the examples of the present application, a range of 10% ≤ G1 ≤ 30% is preferred.

[0142] In combination with Examples 2-4 and 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, 20mm≤Lm1 +L m2 +L1≤28mm.

[0143] As for the length of the first sub-region, in combination with Embodiments 2-4 and 2-7 to 2-14, 1 mm ≤ L m1 ≤12.5mm. In Examples 2-7, the risk of lithium plating is still high. For special-shaped hot pressing areas, 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. It is preferred that 6mm≤L m1 ≤9mm.

[0144] The risk of lithium deposition in Examples 2-15 to 2-20 is relatively low. In the examples of this 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 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 Example 2-20. In the embodiments of the present application, preferably 4.5 mm ≤ W m3 ≤12mm.

[0145] Different from Examples 1-4, the relevant parameters in Examples 3-1 to 3-7 are shown in Table 3. Different from Examples 2-4, the relevant parameters in Examples 3-8 to 3-14 are shown in Table 3. The length of the first adhesive 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 adhesive layer is too small, and the current density near the first groove is large, which may make it difficult for the first adhesive layer to effectively cover this part, the lithium ion deintercalation reaction is still intense, and the risk of lithium plating 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 adhesive layer is too small, and the current density near the first groove is large, which may also make it difficult for the first adhesive layer to effectively cover this part, the lithium ion deintercalation reaction is still intense, and the risk of lithium plating 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 deposition 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 loss of secondary battery energy density is smaller. When a special-shaped hot pressing area is used, the lithium deposition 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 loss of secondary battery energy density 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. Based on the concept of the present application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes in different aspects of the present application as described above. For the sake of simplicity, they are not provided in detail. Although the present application has been described in detail with reference to the aforementioned embodiments, ordinary technicians in this field should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. 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 provided 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 provided 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 provided 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 membrane includes a first portion and a first pore area. 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 subregion. Along the first direction, the width of the first portion is equal to the maximum width of the first subregion. Along the second direction, the first subregion is connected to the first portion. The maximum length of the first subregion is L m1 , 1 mm≤L m1 ≤12.5mm; The first pore region includes a third subregion. Along the first direction, the third subregion is connected to the first portion. The maximum width of the third subregion 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, wherein 10%≤G1≤30%。 3. The secondary battery according to claim 1, wherein 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, where G1<G2.

4. The secondary battery according to claim 1, wherein Along the second direction, the first pore region further includes a second subregion, and the first portion is located between the first subregion and the second subregion; along the first direction, a maximum width of the second subregion is equal to a width of the first portion; 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.

5. The secondary battery according to claim 4, wherein 22mm≤L m1 +L m2 +L1≤28mm。 6. The secondary battery according to claim 1, wherein 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 section 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, wherein 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, wherein 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, wherein 6mm≤L m1 ≤9mm。 12. The secondary battery according to claim 1, wherein 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, wherein: 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, wherein: 4.5mm≤W m3 ≤12mm。 15. The secondary battery according to claim 12, wherein: 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, wherein 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, wherein: 1mm≤L2-L1≤6mm.

19. The secondary battery according to claim 1, wherein 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 opposite to each other; along the third direction, a projection of the first subregion on the negative electrode active material layer is located between the first groove and the first end, and a 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 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: Providing a positive electrode sheet, a separator, a negative electrode sheet, and a negative electrode tab, wherein the negative electrode sheet includes 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 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 positive electrode sheet includes 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, and 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 coincides 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 secondary battery according to any one of claims 1 to 19 is included.

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