Secondary battery and electronic device

Abstract

The invention relates to a secondary battery and electronic equipment, the secondary battery comprises an electrode assembly, the electrode assembly comprises a plurality of positive pole pieces and a plurality of negative pole pieces which are alternately laminated along a first direction, an isolating membrane is arranged between the adjacent positive pole piece and negative pole piece, the positive pole pieces comprise a first positive pole piece and a second positive pole piece, and the first positive pole piece and the second positive pole piece are arranged along the first direction. The first positive pole piece is the outermost pole piece of the electrode assembly, and the second positive pole piece is arranged between two adjacent negative pole pieces. The first positive pole piece comprises a first current collector and a first positive coating, the first current collector comprises a first surface and a second surface which are oppositely arranged along the first direction, the second surface faces the negative pole piece, and the first positive coating is arranged on the second surface. The first surface is provided with a plurality of grooves. The lithium precipitation risk of the secondary battery can be reduced, and the fast charging performance of the secondary battery is improved.

Description

Technical Field

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

[0002] The electrode assembly of a lithium-ion battery (secondary battery) is usually made by a lamination process or a winding process. The lithium-ion battery products made by the lamination process have the characteristics of high energy density, more stable internal structure and high safety.

[0003] Lithium-ion batteries usually include a single-sided positive electrode sheet with a coating on the outermost side and a double-sided positive electrode sheet with a coating on the inner side. However, the lithium removal rate of the single-sided positive electrode sheet may not match the lithium insertion rate of the corresponding negative electrode sheet, and some lithium ions may have difficulty in timely embedding into the negative electrode sheet corresponding to the single-sided positive electrode sheet, which may easily lead to lithium deposition. Summary of the invention

[0004] The embodiments of the present application aim to provide a secondary battery and an electronic device, which can reduce the technical problem that lithium plating is prone to occur in the secondary battery.

[0005] In order to solve the technical problem, the embodiment of the present application adopts the following technical solution:

[0006] In the first aspect, an embodiment of the present application proposes a secondary battery, including an electrode assembly, the electrode assembly including a plurality of positive electrode sheets and a plurality of negative electrode sheets alternately stacked along a first direction, and a separator is provided between adjacent positive electrode sheets and negative electrode sheets. The positive electrode sheets include a first positive electrode sheet and a second positive electrode sheet, and a second positive electrode sheet is provided between two adjacent negative electrode sheets. Along the first direction, the first positive electrode sheet is the outermost electrode sheet of the electrode assembly. The second positive electrode sheet is provided between two adjacent negative electrode sheets. The first positive electrode sheet includes a first current collector and a first positive electrode coating. Along the first direction, the first current collector includes a first surface and a second surface arranged opposite to each other, the second surface faces the negative electrode sheet, and the first positive electrode coating is provided on the second surface. The first surface is provided with a plurality of grooves.

[0007] In the above scheme, by providing a groove on the first current collector, the current-carrying area of ​​the first current collector can be reduced, that is, the resistance of the first positive electrode plate can be increased. When the secondary battery is charged and discharged, the current flowing through the first positive electrode plate can be reduced, thereby reducing the current density of the first positive electrode coating, reducing the deintercalation reaction of lithium ions, so that the negative electrode plate corresponding to the first positive electrode plate has enough margin per unit time to embed the lithium ions released from the first positive electrode coating, thereby reducing the occurrence of lithium plating. While improving the fast charging performance of the secondary battery, the risk of lithium plating of the secondary battery can be reduced.

[0008] In some embodiments, along the first direction, the thickness of the first current collector is D1, the depth of the groove is D2, and 10%≤D2 / D1≤70%. The risk of external lithium chromatography can be effectively reduced, and the first current collector still has a certain structural strength to reduce the occurrence of curling. Preferably, 33.33%≤D2 / D1≤50%. This can further reduce the risk of external lithium chromatography, improve the anti-curling and anti-tearing capabilities of the first current collector, and enhance the structural strength of the first current collector.

[0009] In some embodiments, the groove is arranged along the second direction, and along the third direction, the width of the first current collector is W1, the width of the groove is W2, and 0.5%≤W2 / W1≤4%. Among them, the first direction, the second direction and the third direction are perpendicular to each other. Not only can the current-carrying area of ​​the first current collector be effectively reduced, but also the phenomenon that the first current collector is prone to curling can be reduced. Preferably, 0.5%≤W2 / W1≤2%. In order to further optimize the width of the groove, the phenomenon that the first current collector is prone to curling can be improved.

[0010] In some embodiments, along the third direction, the sum of the widths of all the grooves is W3, 15% ≤ W3 / W1 ≤ 80%. The risk of external lithium chromatography can be reduced, the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography can be shortened, and the first current collector can still maintain a high structural strength and reduce the curling phenomenon. Preferably, 50% ≤ W3 / W1 ≤ 70%. This can further reduce the risk of external lithium chromatography and improve the structural strength of the first current collector.

[0011] In some embodiments, along the third direction, a plurality of grooves are arranged in sequence, and two adjacent grooves have a first interval L1, 50%≤L1 / W2≤200%. The spacing between the grooves is moderate, the stress is dispersed more evenly, and the curling of the first current collector is reduced. Preferably, 100%≤L1 / W2≤150%, while reducing the deformation of the first current collector, the short circuit risk of the secondary battery is reduced.

[0012] In some embodiments, along the second direction, the first current collector includes a first end face and a second end face that are relatively arranged, and the groove runs through the first end face and the second end face, which can reduce the current-carrying area of ​​the first current collector, thereby reducing the current density of the outer first positive electrode plate, and reducing the risk of lithium plating in the outer layer.

[0013] In some embodiments, the second positive electrode plate includes a second current collector and a second positive electrode coating disposed on two surfaces of the second current collector. The first current collector includes a first conductive layer, and the thickness of the first conductive layer is T1 along the first direction. The second current collector includes a second conductive layer, and the thickness of the second conductive layer is T2 along the first direction, and 67%≤T1 / T2≤150%. Preferably, 78%≤T1 / T2≤120%.

[0014] When the thickness T1 of the first conductive layer is less than or equal to the thickness T2 of the second conductive layer, the resistance of the first positive electrode sheet can be greater than or equal to the resistance of the second positive electrode sheet. When the secondary battery is charged and discharged, the current flowing through the first positive electrode sheet can be reduced, thereby reducing the current density of the first positive electrode coating, reducing the deintercalation reaction of lithium ions, so that the negative electrode sheet corresponding to the first positive electrode sheet has enough margin per unit time to embed the lithium ions released from the first positive electrode coating, which can further reduce the occurrence of lithium plating. It can enable the secondary battery to adapt to a larger charge and discharge rate, and can reduce the risk of lithium plating of the secondary battery while improving the fast charging performance of the secondary battery.

[0015] When the thickness T1 of the first conductive layer is greater than the thickness T2 of the second conductive layer, in the present application, a groove is provided on the first current collector, which can reduce the current-carrying area of ​​the first current collector. Therefore, appropriately increasing the thickness of the first conductive layer can enhance the anti-curling ability of the first current collector, which is beneficial to reduce deformation of the first positive electrode sheet while reducing lithium plating.

[0016] In a second aspect, an embodiment of the present application provides an electronic device, comprising a secondary battery as described in any embodiment of the first aspect above. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 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.

[0018] Figure 1 is a schematic diagram of the structure of some secondary batteries provided in the embodiments of the present application;

[0019] Figure 2 is a schematic diagram of the structure of some electrode assemblies provided in the embodiments of the present application;

[0020] Figure 3 is a schematic diagram of the structure of some electrode assemblies provided in the embodiments of the present application;

[0021] Figure 4 It is a schematic diagram of the structure of some first positive electrode sheets provided in the embodiments of the present application;

[0022] Figure 5 It is a schematic diagram of the structure of some second positive electrode sheets provided in the embodiments of the present application;

[0023] Figure 6 It is a schematic diagram of the structure of some grooves provided in the embodiments of the present application;

[0024] Figure 7 It is a schematic diagram of the structure of some grooves provided in the embodiments of the present application;

[0025] Figure 8 is a schematic diagram of a structure in which some first pole tabs provided in the embodiments of the present application are regular pole tabs;

[0026] Fig. 9 is a schematic diagram of a structure in which some first pole tabs provided in the embodiments of the present application are irregular pole tabs;

[0027] Fig.10 is a schematic structural diagram of some first adapters provided in the embodiments of the present application;

[0028] Fig.11 is a schematic diagram of the structure of some second pole tabs provided in the embodiments of the present application as regular pole tabs;

[0029] Fig.12 is a schematic diagram of a structure in which some second pole tabs provided in the embodiments of the present application are irregular pole tabs;

[0030] Fig.13 is a schematic diagram of the structure of some first current collectors provided in the embodiments of the present application;

[0031] Fig.14 is a schematic diagram of the structure of some second current collectors provided in the embodiments of the present application;

[0032] Fig.15 is a schematic diagram of the structure of some first negative electrode sheets provided in the embodiments of the present application;

[0033] Fig.16 It is a schematic diagram of the structure of some first negative electrode plates provided in the embodiments of the present application.

[0034] Description of reference numerals:

[0035] 1000. Secondary battery;

[0036] 100. Electrode assembly;

[0037] 10. Positive electrode sheet; 11. First positive electrode sheet; 111. First current collector; 1111. First surface; 1112. Second surface; 1113. First end surface; 1114. First conductive layer; 111a. First sub-conductive layer; 111b. Second sub-conductive layer; 1115. First insulating polymer layer; 1116. Groove; 1117. Second end surface; 112. First positive electrode coating; 113. First pole ear; 12. Second positive electrode sheet; 121. Second current collector; 1211. Third surface; 1212. Fourth surface; 1213. Second conductive layer; 121a. Third sub-conductive layer; 121b. Fourth sub-conductive layer; 1214. Second insulating polymer layer; 122. Second positive electrode coating; 123. Second pole ear;

[0038] 20, negative electrode plate; 21, first negative electrode plate; 211, third current collector; 2111, fifth surface; 2112, sixth surface; 212, first negative electrode coating;

[0039] 30. Isolation film;

[0040] 40. A first adapter;

[0041] 200, housing;

[0042] X, first direction;

[0043] Y, second direction;

[0044] Z. Third direction. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0046] 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 "several" and "multiple" is more than two, unless otherwise clearly and specifically defined.

[0047] 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.

[0048] Reference to "embodiment" herein 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 locations 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. In addition, the technical features involved in the different embodiments of the present application described below may be combined with each other as long as they do not conflict with each other.

[0049] The laminated battery usually includes a positive electrode sheet with a coating on one side on the outermost layer (hereinafter referred to as a single-sided positive electrode sheet) and a positive electrode sheet with a coating on both sides on the inner layer (hereinafter referred to as a double-sided positive electrode sheet). The outermost side of the single-sided positive electrode sheet can be provided with a blank foil (without an active material layer), which can reduce the space occupied by the single-sided positive electrode sheet and improve the energy density of the secondary battery.

[0050] However, the single-sided positive electrode sheet is located at the outermost layer of the electrode assembly, with a small binding force. The current collector is only coated on one side, resulting in uneven stress distribution on both sides of the current collector, which can easily cause the current collector to curl toward the side away from the coating. The curling of the current collector can easily cause the coating to fall off, affecting the energy density of the secondary battery. In order to reduce the curling problem of the single-sided positive electrode sheet, the thickness of the current collector in the single-sided positive electrode sheet is usually increased to improve the deformation resistance of the single-sided positive electrode sheet.

[0051] The inventors of the present application have found that during the charging and discharging process of the secondary battery, the risk of lithium deposition (hereinafter referred to as external lithium deposition) on the negative electrode sheet corresponding to the single-sided positive electrode sheet is higher. When charging and discharging the secondary battery, without lithium deposition, the maximum charge and discharge rate that the double-sided positive electrode sheet can withstand is greater than the maximum charge and discharge rate that the single-sided positive electrode sheet can withstand. In order to improve the rate performance of the secondary battery, the maximum charge and discharge rate of the double-sided positive electrode sheet will be used as a benchmark, but this will exceed the tolerance range of the single-sided positive electrode sheet, making the risk of external lithium deposition higher than the risk of internal lithium deposition, which is easy to cause safety problems.

[0052] The inventors of this application have further discovered that lithium precipitation is related to the current density of the coating. Since the coating is only set on one side of the single-sided positive electrode sheet, the current is concentrated in the single-layer coating, and the reaction of lithium ion insertion and extraction is intense, which makes it difficult for the corresponding negative electrode sheet to insert the extracted lithium ions in time, thereby causing lithium precipitation. In addition, the increase in the thickness of the current collector of the outer single-sided positive electrode sheet means that the resistance of the current collector of the outer single-sided positive electrode sheet is smaller, the current density is greater, and the reaction of lithium ion insertion and extraction is more intense, resulting in a further increase in the risk of lithium precipitation.

[0053] In order to reduce the above problems, on the first aspect, the present application proposes a secondary battery 1000, which refers to a rechargeable and dischargeable soft-pack battery that can store and release electrical energy through reversible chemical reactions. The secondary battery 1000 can be a lithium-ion battery, a sodium-ion battery or a lithium polymer battery, etc.

[0054] Please refer to Figure 1 The secondary battery 1000 includes an electrode assembly 100 and a housing 200, wherein the electrode assembly 100 is accommodated in the housing 200. Figure 2 , Figure 2 The stacked structure of the electrode assembly 100 is shown, and the electrode assembly 100 includes a positive electrode sheet 10, a negative electrode sheet 20, and a separator 30. Along the first direction X (the thickness direction of the positive electrode sheet 10 and / or the negative electrode sheet 20), a plurality of positive electrode sheets 10 and a plurality of negative electrode sheets 20 are alternately stacked, and a separator 30 is arranged between adjacent positive electrode sheets 10 and negative electrode sheets 20, and the separator 30 is used to insulate and separate the positive electrode sheet 10 and the negative electrode sheet 20.

[0055] The positive electrode sheet 10 includes a first positive electrode sheet 11 and a second positive electrode sheet 12. Along the first direction X, the first positive electrode sheet 11 is the outermost positive electrode sheet 10 of the electrode assembly 100. The second positive electrode sheet 12 is the inner positive electrode sheet 10 of the electrode assembly 100, that is, the second positive electrode sheet 12 is arranged between two adjacent negative electrode sheets 20.

[0056] For the first positive electrode plate 11, please refer to Figure 3 and Figure 4 The first positive electrode plate 11 includes a first current collector 111 and a first positive electrode coating 112. The first current collector 111 is a conductive substrate, which can be an aluminum foil that is flat and has a strip structure. In some other embodiments, the material of the first current collector 111 can also be at least one of aluminum alloy, nickel, nickel alloy or stainless steel.

[0057] In the embodiment of the present application, the first positive electrode plate 11 adopts a single-sided coating structure, that is, along the first direction X, the first positive electrode coating 112 is only disposed on the surface of the first current collector 111 facing the negative electrode plate 20. For details, please refer to Figure 4 The first current collector 111 includes a first surface 1111 and a second surface 1112 that are arranged opposite to each other, the second surface 1112 is arranged facing the negative electrode plate 20, and the first positive electrode coating 112 is arranged on the second surface 1112. The first surface 1111 of the first positive electrode plate 11 has no corresponding negative electrode plate 20 and coating, which can reduce the space occupied by the first positive electrode plate 11 and improve the volume energy density of the secondary battery 1000.

[0058] The first positive electrode coating 112 includes a positive electrode active material, a positive electrode conductor, and a positive electrode adhesive, etc. The above materials are mixed and stirred evenly and applied to the surface of the first current collector 111 facing the negative electrode plate 20, thereby obtaining the first positive electrode coating 112. Among them, the positive electrode active material can be selected from at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron manganese phosphate or cobalt-free materials. There is no particular restriction on the positive electrode conductor and the positive electrode adhesive in the embodiment of the present application, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor can be selected from at least one of conductive carbon black, carbon nanotubes, carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The positive electrode adhesive can be selected from at least one of polyacrylate, polyimide, polyamide, polyamideimide, polyvinylidene fluoride, polyacrylonitrile, carboxymethyl cellulose, sodium carboxymethyl cellulose or potassium carboxymethyl cellulose.

[0059] For the second positive electrode sheet 12, please refer to Figure 3 and Figure 5The second positive electrode plate 12 includes a second current collector 121 and a second coating 122 disposed on two surfaces of the second current collector 121 (two surfaces disposed opposite to each other along the thickness direction of the second current collector 121). The second current collector 121 has a similar structure to the first current collector 111, and may be made of an overall flat and strip-shaped aluminum foil. Alternatively, in some other embodiments, the material of the second current collector may also be at least one of aluminum alloy, nickel, nickel alloy or stainless steel.

[0060] In the embodiment of the present application, the second positive electrode plate 12 adopts a double-sided coating structure. Specifically, along the first direction X, the second current collector 121 has a third surface 1211 and a fourth surface 1212 that are arranged opposite to each other, and the third surface 1211 and the fourth surface 1212 are both provided with a second positive electrode coating 122. The second positive electrode coating 122 is provided on both sides of the inner layer, which can make full use of the space, thereby improving the energy density of the secondary battery 1000.

[0061] For the isolation film 30, please refer to Figure 2 and Figure 3 , the separator 30 is disposed between the positive electrode sheet 10 and the negative electrode sheet 20, and is used to insulate and separate the positive electrode sheet 10 and the negative electrode sheet 20. The separator 30 may be a PE separator 30 or a PP separator 30 containing ceramics. In some embodiments, the separator 30 facing the positive electrode sheet 10 and the surface facing the negative electrode sheet 20 may be provided with an adhesive coating. When the positive electrode sheet 10, the separator 30 and the negative electrode sheet 20 are stacked, the separator 30 may be directly bonded between the positive electrode sheet 10 and the negative electrode sheet 20, and is used to improve the bonding strength between the positive electrode sheet 10, the separator 30 and the negative electrode sheet 20, thereby improving the integrity of the electrode assembly 100.

[0062] Please refer to Figure 4 and Figure 6 , the first surface 1111 is provided with a plurality of grooves 1116. There is no specific limitation on the positions of the plurality of grooves 1116 on the first surface 1111. For example, the plurality of grooves 1116 may be arranged on the first surface 1111 along the second direction Y. The plurality of grooves 1116 may also be arranged on the first surface 1111 along the third direction Z, or may be staggered on the first surface 1111 along the second direction Y and the third direction Z. There is no specific limitation on the spacing between adjacent grooves 1116, and each spacing may be set equally or unequally. Among them, the second direction Y is the length direction of the first current collector 111, the third direction Z is the width direction of the first current collector 111, and the first direction X, the second direction Y and the third direction Z are perpendicular to each other.

[0063] In the embodiment of the present application, the first surface 1111 is provided with a plurality of grooves 1116, which can reduce the current-carrying area of ​​the first current collector 111, that is, increase the resistance of the first positive electrode plate 11. When the secondary battery 1000 is charged and discharged, the current flowing through the first positive electrode plate 11 can be reduced, thereby reducing the current density of the first positive electrode coating 112, reducing the deintercalation reaction of lithium ions, and making the negative electrode plate 20 corresponding to the first positive electrode plate 11 have sufficient margin per unit time to embed the lithium ions released from the first positive electrode coating 112, thereby reducing the occurrence of lithium plating. While improving the fast charging performance of the secondary battery 1000, the risk of lithium plating of the secondary battery 1000 can be reduced.

[0064] For the above groove 1116, please refer to Figure 6 and Figure 7 , along the first direction X, the thickness of the first current collector 111 is D1, the depth of the groove 1116 is D2, and 10%≤D2 / D1≤70%. This range not only reduces the risk of external lithium chromatography, but also the first current collector 111 has a certain structural strength, reducing the occurrence of curling. In some embodiments, the thickness range of the first current collector 111 is 5μm≤D1≤20μm. For example, when the thickness D1 of the first current collector 111 is 10μm, the depth D2 of the groove 1116 is 1μm to 7μm. When the thickness D1 of the first current collector 111 is 5μm, the depth D2 of the groove 1116 is 0.5μm to 3.5μm. It should be noted that when the depth of the groove 1116 is 3.5μm, the thickness of the connection between the groove 1116 and the groove 1116 is only 1.5μm. The thickness of the connection is too thin and it is easy to break. Therefore, in actual production, the thickness of the connection between the grooves 1116 and the grooves 1116 is as large as possible to be greater than 3 μm to reduce the risk of the connection between the grooves 1116 and the grooves 1116 being easily broken.

[0065] In some preferred embodiments, the ratio of the depth D2 of the groove 1116 to the thickness D1 of the first current collector 111 is in the range of 33.33%≤D2 / D1≤50%, so as to further reduce the risk of external lithium chromatography, improve the anti-curling and anti-tearing capabilities of the first current collector 111, and enhance the structural strength of the first current collector 111.

[0066] In some embodiments, please refer to Figure 6, the grooves 1116 are arranged along the second direction Y, and along the third direction Z, the width of the first current collector 111 is W1, the width of a single groove 1116 is W2, and 0.5%≤W2 / W1≤4%. In the embodiment of the present application, if a single groove 1116 is too wide, the structural strength inside the groove 1116 is weak, making it easy for the first current collector 111 to curl. If a single groove 1116 is too narrow, the current-carrying area of ​​the first current collector 111 is not significantly reduced. Controlling the width of a single groove 1116 to 0.5%≤W2 / W1≤4% not only effectively reduces the current-carrying area of ​​the first current collector 111, but also reduces the phenomenon that the first current collector 111 is prone to curling.

[0067] Furthermore, the ratio of the width W2 of a single groove 1116 to the width W1 of the first current collector 111 is 0.5%≤W2 / W1≤2%, so as to further optimize the width of the single groove 1116 and improve the phenomenon that the first current collector 111 is prone to curling.

[0068] In some embodiments, along the third direction Z, a plurality of grooves 1116 are arranged in sequence, and the plurality of grooves 1116 can be set to the same width or different widths. The sum of the widths of all grooves 1116 is W3, 15%≤W3 / W1≤80%. If the total width of all grooves 1116 accounts for too large a proportion of the width of the first current collector 111, the structural strength of the first current collector 111 will be weakened and prone to curling. If the total width of all grooves 1116 accounts for too small a proportion of the width of the first current collector 111, the current-carrying area of ​​the first current collector 111 is not significantly reduced, so that the risk of external chromatography lithium is not significantly reduced. When the sum of the widths W3 of all grooves 1116 is within the range of 15%≤W3 / W1≤80%, the risk of external chromatography lithium can be reduced, the difference between the risk of external chromatography lithium and internal chromatography lithium can be shortened, and the first current collector 111 can have a higher structural strength and reduce the phenomenon of curling.

[0069] In some preferred embodiments, the ratio of the sum of the widths W3 of all the grooves 1116 to the width W1 of the first current collector 111 is in the range of 50%≤W3 / W1≤70%, so as to further reduce the risk of external lithium plating and improve the structural strength of the first current collector 111.

[0070] In some embodiments, please refer to Figure 7 , along the third direction Z, a plurality of grooves 1116 are arranged in sequence, and two adjacent grooves 1116 have a first interval L1, 50%≤L1 / W2≤200%, so that the spacing between the grooves 1116 and the grooves 1116 is moderate, the stress is dispersed more evenly, and the curling of the first current collector 111 is reduced. Preferably, 100%≤L1 / W2≤150%, which can reduce the deformation of the first current collector 111 and reduce the short circuit risk of the secondary battery 1000.

[0071] In some embodiments, please refer to Figure 6 , along the second direction Y, the first current collector 111 includes a first end face 1113 and a second end face 1117 that are arranged opposite to each other, and the groove 1116 runs through the first end face 1113 and the second end face 1117. Among them, the groove 1116 can completely run through the first end face 1113 and the second end face 1117, or the groove 1116 partially runs through the first end face 1113 and the second end face 1117, which can reduce the current-carrying area of ​​the first current collector 111, thereby reducing the current density of the outer first positive electrode plate 11, and reducing the risk of external lithium chromatography. In particular, when the groove 1116 completely runs through the first end face 1113 and the second end face 1117, the current-carrying area of ​​the first current collector 111 is smaller, the resistance of the first positive electrode plate 11 is larger, and the risk of external lithium chromatography is reduced more significantly.

[0072] In some embodiments, please refer to Figure 3 The first positive electrode sheet 11 further includes a first electrode tab 113. The first electrode tab 113 is connected to the first current collector 111, and the first electrode tab 113 extends out of the first current collector 111 along the second direction Y. Figure 8 , along the third direction Z, the minimum width of the first pole ear 113 is W4. The minimum width of the first pole ear 113 refers to the width of the position where the current carrying area is the smallest when the current flows into or out of the first pole ear 113. The first pole ear 113 can be a regular shape, such as a rectangle or a square, and the minimum width of the first pole ear 113 is the width of the first pole ear 113 itself. The first pole ear 113 can also be an irregular shape, such as a wave shape, a vase shape, or a sawtooth shape. In some embodiments, please refer to Fig. 9 Along the second direction Y, the width of the first pole ear 113 in the third direction Z is reduced, and the minimum width of the first pole ear 113 is the width of the position where the intercepting area is the smallest when current flows in or out.

[0073] Please refer to Fig.10 One end of the first pole tab 113 can be connected to the first current collector 111, and the other end can be connected to the first adapter 40, and the first adapter 40 directly extends out of the housing 200 to lead out the polarity. The material of the first pole tab 113 can be the same as that of the first current collector 111, for example, the first pole tab 113 can be made of at least one of aluminum alloy, nickel, nickel alloy, stainless steel, etc.

[0074] When the first current collector 111 is a metal foil current collector (the first current collector 111 is made of metal as a whole), the first pole tab 113 can be die-cut on the first current collector 111. The first pole tab 113 is connected to the first current collector 111 by welding or conductive adhesive bonding. When the first current collector 111 is a composite current collector, the first pole tab 113 can also be die-cut on the first current collector 111.

[0075] Please refer to Figure 3 The second positive electrode sheet 12 includes a second electrode tab 123, which is connected to the second current collector 121 and extends out of the second current collector 121 along the second direction Y. Fig.11 , along the third direction Z, the minimum width of the second pole ear 123 is W5. The minimum width of the second pole ear 123 refers to the width of the position where the current carrying area is the smallest when the current flows into or out of the second pole ear 123. The second pole ear 123 can be a regular shape, such as a rectangle or a square, and the minimum width of the second pole ear 123 is the width of the second pole ear 123 itself. The second pole ear 123 can also be an irregular shape, such as a wave shape, a vase shape, or a sawtooth shape. In some embodiments, please refer to Fig.12 Along the second direction Y, the width of the second pole ear 123 in the third direction Z is reduced, and the minimum width of the second pole ear 123 is the width of the position where the intercepting area is the smallest when current flows in or out.

[0076] Please refer to Fig.10 One end of the second pole tab 123 can be connected to the second current collector 121, and the other end can be connected to the first adapter 40, and the first adapter 40 directly extends out of the housing 200 to lead out the polarity. The material of the second pole tab 123 can be the same as that of the second current collector 121, for example, the second pole tab 123 can be made of at least one of aluminum alloy, nickel, nickel alloy, stainless steel, etc.

[0077] When the second current collector 121 is a metal foil current collector (the second current collector 121 is made of metal as a whole), the second pole tab 123 can be die-cut on the second current collector 121. The second pole tab 123 is connected to the second current collector 121 by welding or conductive adhesive bonding. When the second current collector 121 is a composite current collector, the second pole tab 123 can also be die-cut on the second current collector 121.

[0078] In the embodiment of the present application, along the third direction Z, the minimum width W4 of the first pole ear 113 is less than the minimum width W5 of the second pole ear 123, then the current-carrying area of ​​the first pole ear 113 is less than the current-carrying area of ​​the second pole ear 123, so that the resistance of the first positive electrode plate 11 is greater than the resistance of the second positive electrode plate 12. When the secondary battery 1000 is charged and discharged, the current flowing through the first positive electrode plate 11 can be reduced, thereby reducing the current density of the first positive electrode coating 112, reducing the deintercalation reaction of lithium ions, so that the negative electrode plate 20 corresponding to the first positive electrode plate 11 has sufficient margin per unit time to embed the lithium ions released from the first positive electrode coating 112, thereby reducing the occurrence of lithium plating. The secondary battery 1000 can be adapted to a larger charge and discharge rate, and the risk of lithium plating of the secondary battery 1000 can be reduced while improving the fast charging performance of the secondary battery 1000.

[0079] In some embodiments, 40%≤W4 / W5≤90%, which can reduce the impact on the strength of the first pole ear 113 while reducing the external lithium chromatography, and reduce the impact on the current carrying capacity of the first pole ear 113, and the width range of the second pole ear 123 is 3mm≤W5≤10mm. In the embodiment of the present application, the width W4 of the first pole ear 113 ranges from 1.2mm≤W4≤9mm. The specific width of the first pole ear 113 can be set according to the corresponding relationship and the width of the second pole ear 123. For example, W4 can be 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm or 9mm, etc.

[0080] In some other embodiments, when the secondary battery 1000 is a power battery, the width of the first pole ear 113 can also be selected as 1mm≤W4≤90mm. In the embodiment of the present application, the secondary battery 1000 is preferably a secondary battery 1000 of a consumer electronic device, and the range can be selected as 1.2mm≤W4≤9mm.

[0081] In some preferred embodiments, the ratio of W4 to W5 is in the range of 50%≤W4 / W5≤70%. The minimum width W5 of the second pole tab 123 can be set to 3mm to 10mm. According to the above correspondence, the minimum width W4 of the first pole tab 113 can be in the range of 1.5mm≤W1≤7mm. This can further reduce the occurrence of lithium plating and enable the first pole tab 113 to have higher strength and higher current carrying capacity.

[0082] In some embodiments, please refer to Figure 3 , along the first direction X, the projection of the first pole ear 113 and the projection of the second pole ear 123 at least partially overlap. Fig.10 The secondary battery 1000 further includes a first adapter 40, the first pole ear 113 and the second pole ear 123 are stacked along the first direction X, and the first adapter 40 is connected to the first pole 113 and the second pole ear 123, so as to facilitate stacking the first pole ear 113 and the second pole ear 123 into a whole. The first adapter 40 is used for the overall connection, which is conducive to simplifying the polarity lead-out operation.

[0083] In some embodiments, please refer to Figure 4 and Fig.12 The first current collector 111 includes a first conductive layer 1114, which is a metal layer that can conduct current, and the current can flow smoothly in the first conductive layer 1114. The first conductive layer 1114 includes at least one of conductive metals such as aluminum, aluminum alloy, nickel, nickel alloy or stainless steel. Each material has high conductivity, which is conducive to improving the charge and discharge rate of the secondary battery 1000, and has high mechanical strength, which can reduce the curling of the first conductive layer 1114.

[0084] When the first current collector 111 is a metal foil current collector (the first current collector 111 is entirely made of metal), the first conductive layer 1114 is the first current collector 111 itself.

[0085] When the first current collector 111 is a composite current collector, the first conductive layer 1114 is a part of the first current collector 111. For example, please refer to Fig.13 The first current collector 111 includes a first conductive layer 1114 and a first insulating polymer layer 1115. Specifically, the first conductive layer 1114 includes a first sub-conductive layer 111a and a second sub-conductive layer 111b, and the first insulating polymer layer 1115 is disposed between the first sub-conductive layer 111a and the second sub-conductive layer 111b.

[0086] The provision of the first insulating polymer layer 1115 can reduce the thickness of the first conductive layer 1114, reduce the weight of the first positive electrode plate 11, and improve the weight energy density of the secondary battery 1000. In particular, when the thickness of the first conductive layer 1114 is relatively thin, the first insulating polymer layer 1115 is added to improve the structural strength of the first current collector 111 and reduce the curling phenomenon of the first positive electrode plate 11. Since the first insulating polymer layer 1115 has electrical insulation, it is difficult for charges to pass through the first insulating polymer layer 1115. Therefore, the first insulating polymer layer 1115 will not change the thickness and conductive properties of the conductive part in the first current collector 111.

[0087] In some embodiments, the first insulating polymer layer 1115 includes at least one of polyester, polyamide, modified polyolefin, olefin copolymer or unsaturated olefin copolymer. Polyester includes but is not limited to polybutylene terephthalate (PBT), polycarbonate (PC), and polyether polyester. Polyamide includes but is not limited to polyamide 6 (PA6) and polyamide 66 (PA66). Modified polyolefin includes but is not limited to polymaleic anhydride, polymethyl methacrylate, and glycidyl methacrylate grafted polyethylene. Olefin copolymers include but are not limited to polypropylene and ethylene-propylene copolymers. Unsaturated olefin copolymers include but are not limited to [methyl or ethylene-(methyl)] acrylate copolymers. The above materials are selected as the material of the first insulating polymer layer 1115 to ensure that the strength, toughness, insulation, and formability of the first insulating polymer layer 1115 meet the requirements of the composite current collector and provide good support for the first conductive layer 1114.

[0088] In some embodiments, when the first current collector 111 is a composite current collector, a groove 1116 may also be provided in the first conductive layer 1114. For example, a groove 1116 is provided on the surface of the first sub-conductive layer 111a away from the first insulating polymer layer 1115 to reduce the risk of external lithium plating.

[0089] In some embodiments, the second current collector 121 includes a second conductive layer 1213. Similar to the first conductive layer 1114, the second conductive layer 1213 can conduct current, and the current can flow smoothly in the second conductive layer 1213. The second conductive layer 1213 includes at least one of aluminum, aluminum alloy, nickel, nickel alloy or stainless steel. Each material has high conductivity, which is beneficial to improve the charge and discharge rate of the secondary battery 1000, and has high mechanical strength, which can reduce the curling of the second conductive layer 1213. Among them, the materials of the first conductive layer 1114 and the second conductive layer 1213 need to be consistent when they are set. For example, the first conductive layer 1111 and the second conductive layer 1213 are both aluminum foil.

[0090] When the second current collector 121 is a metal foil current collector (the second current collector 121 is entirely made of metal), the second conductive layer 1213 is the second current collector 121 itself.

[0091] When the second current collector 121 is a composite current collector, the second conductive layer 1213 is a part of the second current collector 121. Fig.14 The second current collector includes a second conductive layer 1213 and a second insulating polymer layer 1214. Specifically, the second conductive layer 1213 includes a third sub-conductive layer 121a and a fourth sub-conductive layer 121b, and the second insulating polymer layer 1214 is disposed between the third sub-conductive layer 121a and the fourth sub-conductive layer 121b.

[0092] The provision of the second insulating polymer layer 1214 can make the second conductive layer 1213 thinner, reduce the weight of the second positive electrode plate, and improve the weight energy density of the secondary battery 1000. In particular, when the thickness of the second conductive layer 1213 is relatively thin, the second insulating polymer layer 1214 is added to improve the structural strength of the second current collector 121, thereby improving the curling phenomenon of the second positive electrode plate 12. Since the second insulating polymer layer 1214 has electrical insulation, it is difficult for charges to pass through the second insulating polymer layer 1214. Therefore, the second insulating polymer layer 1214 will not change the thickness and conductive properties of the conductive part in the second current collector 121. In the embodiment of the present application, the second insulating polymer layer 1214 includes at least one material of the above-mentioned first insulating polymer layer 1115.

[0093] In some embodiments, please refer to Figure 4 and Figure 5, along the first direction X, the thickness of the first conductive layer 1114 is T1. The thickness of the second conductive layer 1213 is T2. The ratio of the thickness T1 of the first conductive layer 1114 to the thickness T2 of the second conductive layer 1213 is 67%≤T1 / T2≤150%. The thickness range of the second conductive layer 1213 is 1μm≤T2≤30μm. In the embodiment of the present application, the thickness T1 of the first conductive layer 1114 ranges from 0.67μm≤T1≤45μm. It should be noted that in the embodiment of the present application, the thickness of the first conductive layer 1114 is the thickness of the conductive layer of the first current collector 111 where no groove is provided. When the first current collector 111 is made of metal as a whole, the thickness T1 of the first conductive layer 1114 is equal to the thickness D1 of the first current collector 111.

[0094] When the thickness T1 of the first conductive layer 1114 is less than or equal to the thickness T2 of the second conductive layer 1213, the resistance of the first positive electrode plate 11 can be greater than or equal to the resistance of the second positive electrode plate 12. When the secondary battery 1000 is charged and discharged, the current flowing through the first positive electrode plate 11 can be reduced, thereby reducing the current density of the first positive electrode coating 112, reducing the deintercalation reaction of lithium ions, and making the negative electrode plate 20 corresponding to the first positive electrode plate 11 have enough surplus per unit time to embed the lithium ions released from the first positive electrode coating 112, which can further reduce the occurrence of lithium plating. The secondary battery 1000 can be adapted to a larger charge and discharge rate, and the risk of lithium plating of the secondary battery 1000 can be reduced while improving the fast charging performance of the secondary battery 1000.

[0095] When the thickness T1 of the first conductive layer 1114 is greater than the thickness T2 of the second conductive layer 1213, in the present application, a groove 1116 is provided on the first current collector 111, so that the current-carrying area of ​​the first current collector 111 can be reduced. Therefore, appropriately increasing the thickness of the first conductive layer 1114 can enhance the anti-curling ability of the first current collector 111, which is beneficial to reduce deformation of the first positive electrode plate 11 while reducing lithium plating.

[0096] In some preferred embodiments, the thickness T1 of the first conductive layer 1114 is less than the thickness T2 of the second conductive layer 1213, and the ratio of T1 to T2 is in the range of 78%≤T1 / T2≤120%, so as to further reduce the risk of external lithium plating and reduce the curling of the first conductive layer 1114.

[0097] In some preferred embodiments, the thickness T2 of the second conductive layer 1213 can be set to 6 μm to 15 μm, and according to the relationship between the thickness of the first conductive layer 1114 and the thickness of the second conductive layer 1213 78≤T1 / T2≤120%, the thickness T1 of the first conductive layer 1114 can be in the range of 4.68 μm≤T1≤18 μm. For example, T1 can be 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, etc.

[0098] In some embodiments, please refer to Figure 2 The negative electrode sheet 20 includes a first negative electrode sheet 21, and the first negative electrode sheet 21 is disposed between two adjacent positive electrode sheets 10. Figure 3 , the first negative electrode plate 21 includes a third current collector 211. The third current collector 211 may be made of copper foil, nickel foil or titanium foil that is flat and strip-shaped. In some other embodiments, the third current collector 211 may also be made of copper alloy foil, stainless steel foil, nickel foam, copper foam or a composite current collector (such as a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, a titanium copper composite current collector, etc.).

[0099] In the embodiment of the present application, the first negative electrode plate 21 adopts a double-sided coating structure. Fig.15 The first negative electrode sheet 21 further includes a first negative electrode coating 212 disposed on two surfaces of the third current collector 211. Along the first direction X, the third current collector 211 has a fifth surface 2111 and a sixth surface 2112 disposed opposite to each other, and both the fifth surface 2111 and the sixth surface 2112 are provided with the first negative electrode coating 212.

[0100] The first negative electrode coating 212 includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode adhesive, etc. The above-mentioned material components are mixed and stirred evenly and applied to the fifth surface 2111 and the sixth surface 2112 of the third current collector 211, thereby obtaining the first negative electrode coating 212 and the second negative electrode coating 213. Among them, the negative electrode active material can be selected from at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, silicon-carbon composite, tin, stannous oxide, tin oxide, lithium-aluminum alloy, metallic lithium, etc. In the embodiment of the present application, there is no particular limitation on the negative electrode conductive agent and the negative electrode adhesive, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent may include at least one of the above-mentioned positive electrode conductive agents, and the negative electrode adhesive may include at least one of the above-mentioned positive electrode adhesives.

[0101] In some embodiments, please refer to Figure 4 and Figure 5, the thickness of the first positive electrode coating 112 is T3, the thickness of the second positive electrode coating 122 is T4, and 50%≤T3 / T4<100%. Since the thickness of the first current collector 111 is thinned, and the first positive electrode coating 112 is only set on one surface, if the first positive electrode coating 112 is too thick, the first positive electrode plate 11 is prone to stress imbalance, which is easy to cause curling. If the first positive electrode coating 112 is too thin, there is less positive electrode active material, and the capacity of the secondary battery 1000 is reduced. The thickness of the first positive electrode coating 112 is reduced to 50%≤T3 / T4<100%, so that the stress between the first current collector 111 and the first positive electrode coating 112 in the first positive electrode plate 11 is balanced, and the curling phenomenon of the first positive electrode plate 11 is reduced. In addition, the thinning of the thickness of the first positive electrode coating 112 can reduce the amount of lithium desorption of the first positive electrode coating 112 and reduce the risk of external lithium chromatography. Preferably, 50%≤T3 / T4<90%, which can reduce the curling of the first positive electrode sheet 11 and reduce the short circuit risk of the secondary battery 1000.

[0102] In some embodiments, please refer to Figure 5 The thickness T4 of the second positive electrode coating 122 set on the third surface 1211 and the thickness T5 of the second positive electrode coating 122 set on the second surface 1212 are approximately equal within an error range of -0.5μm≤T5-T4≤0.5μm, so that the performance of the secondary battery 1000 is more balanced and stable, and the local performance imbalance caused by the excessive difference in thickness of the second positive electrode coating 122 on the two surfaces of the second current collector 121 is reduced.

[0103] In some embodiments, please refer to Fig.16 , the thickness T6 of the first negative electrode coating 212 set on the fifth surface 2111 and the thickness T7 of the first negative electrode coating 212 set on the sixth surface 2112 are approximately equal within the error range of -0.5μm≤T7-T6≤0.5μm, so that the performance of the secondary battery 1000 is more balanced and stable, and the local performance imbalance caused by the excessive difference in thickness of the first negative electrode coating 212 on the two surfaces of the third current collector 211 is reduced.

[0104] In some embodiments, please refer to Figure 3 Along the first direction X, the projection of the first positive electrode coating 112 falls within the projection range of the first negative electrode coating 212, so that the first negative electrode plate 21 has sufficient margin to embed the lithium ions released from the first positive electrode plate 11, which can reduce the lithium deposition phenomenon of the first negative electrode plate 21.

[0105] For further information, please refer to Figure 4 and Fig.16, along the length direction of the first positive electrode sheet 11 (the second direction Y), the length of the first positive electrode coating 112 is L2. Along the length direction of the second positive electrode sheet 12 (the second direction Y), the length of the first negative electrode coating 212 is L3, then 0.6mm≤L3-L2≤4mm. Along the width direction of the first positive electrode sheet 11 (the third direction Z), the width of the first positive electrode coating 112 is W6. Along the width direction of the second positive electrode sheet 12 (the third direction Z), the width of the first negative electrode coating 212 is W7, then 0.6mm≤W7-W6≤4mm. The negative electrode sheet 20 has enough margin to accommodate the lithium ions released from the positive electrode sheet 10, which can further reduce the lithium precipitation phenomenon of the first negative electrode sheet 21, and limit 0.6mm≤L3-L2≤4mm and 0.6mm≤W7-W6≤4mm, so that the secondary battery 1000 has a higher energy density.

[0106] In the second aspect, the embodiment of 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 used for any electronic device known in the prior art. For example, the electronic device includes but is not limited to a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0107] In the embodiments of the present application, a lithium ion secondary battery is taken as an example to perform a lithium deposition test on it.

[0108] The test is as follows:

[0109] During the battery assembly preparation stage, a reference electrode 1 is placed between the negative electrode sheet and the separator corresponding to the first positive electrode sheet, and a reference electrode 2 is placed between the center negative electrode sheet and the separator (the center negative electrode sheet refers to the negative electrode sheet in the middle or near the middle of the total negative electrode sheets in the secondary battery. For example, if the total number of negative electrode sheets is 14, the center negative electrode sheet is the 7th layer of negative electrode sheets, and both sides can be used). The production process of the reference electrode is as follows:

[0110] (1) Copper wire pretreatment: Select a copper wire with a diameter of 30 μm and a length of 100 mm. Soak one end of the copper wire (about 30 mm) in concentrated sulfuric acid for 2 h to remove the surface insulating paint. Then use alcohol ultrasonic cleaning for 10 to 15 minutes, repeat 2 to 3 times, and finally dry it in an oven for use.

[0111] (2) Insert copper wire into the battery: In the designated insertion layer, place the processed end of the copper wire at the center of the separator corresponding to the negative electrode surface, and fix the copper wire with adhesive tape at the edge of the separator. Cover the copper wire with a separator about 10 mm wide and the same length as the width of the negative electrode sheet to separate the copper wire from the negative electrode sheet. Finally, continue the electrode assembly and liquid injection packaging process according to the normal stacking assembly sequence. The copper wire is led out from the side aluminum-plastic film packaging edge. Because the copper wire is thin enough, no additional packaging treatment is required.

[0112] (3) Copper wire transfer welding: Weld the copper wire to the nickel plate tab and fix it to the aluminum plastic film shell with adhesive tape. Use the positive and negative poles of the multimeter to clamp the reference electrode and the positive or negative pole of the battery. If there is voltage, it means that the three-electrode battery is manufactured normally.

[0113] (4) Lithium plating: Using a charge and discharge instrument, the positive electrode is clamped to the positive electrode of the battery, and the negative electrode is clamped to the reference electrode, and charged at a current of 20μA for 10h; then the positive electrode is clamped to the negative electrode of the battery, and the negative electrode is clamped to the reference electrode, and charged at a current of 20μA for 10h.

[0114] The test process is as follows:

[0115] (1) Preparation: Use an electrochemical workstation as the test equipment to provide charging current and monitor the battery voltage, including the voltage between the positive and negative electrodes of the battery and the voltage V between the negative electrode tab and the reference electrode 1. t1 , the voltage V between the negative electrode tab and the reference electrode 2 t2 .

[0116] (2) Charging: Charge the battery according to the normal charging process of the battery. The present embodiment and the comparative example both adopt the following process: first apply 3C constant current charging, charge to 4.5V and then switch to constant voltage charging mode, and stop charging after the current drops to 0.05C. Obtain V changing with time t1 , extract the minimum value as V1, and obtain the V that changes with time t2 , extract the minimum value as V2.

[0117] (3) Calculate the voltage difference Δ=V1-V2.

[0118] Evaluation principle: The lower the negative electrode potential, the higher the risk of lithium deposition.

[0119] Evaluation criteria: Δ>0, the risk of lithium in the inner layer is higher than that of lithium in the outer layer; Δ<0, the risk of lithium in the outer layer is higher than that of lithium in the inner layer; Δ=0, the risk of lithium in the outer layer is equal to that of lithium in the inner layer. The larger the absolute value of Δ, the greater the difference between lithium in the inner and outer layers.

[0120] Example 1

[0121] Preparation of lithium-ion secondary batteries

[0122] (1) Preparation of the first positive electrode sheet: Lithium cobalt oxide, a positive electrode active material, acetylene black, a positive electrode conductive agent, and polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10 5 ) were mixed in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone (NMP) was added as a solvent, a positive electrode slurry with a solid content of 75 wt% was prepared, and the slurry was stirred evenly under a vacuum mixer.

[0123] Aluminum foil is selected as the first current collector (first conductive layer), and the first current collector and the first conductive layer have the same structure. The room temperature conductivity of the aluminum foil is 3.774e7 S / m, the thickness D1 of the first current collector is 15 μm, the width W1 is 69 mm, and the length is 69 mm. The positive electrode slurry is evenly coated on the second surface of the first current collector, dried, and a first positive electrode coating with a thickness of 34 μm is formed on the second surface to obtain a first positive electrode sheet with a single-sided coating of positive electrode active material.

[0124] The first surface of the first current collector is grooved by laser drilling, the depth D2 of the groove is 1 μm, the width W2 of the groove is 1.04 mm, and the sum of the widths W3 of the grooves is 52 mm.

[0125] (2) Preparation of the second positive electrode sheet: Lithium cobalt oxide, a positive electrode active material, acetylene black, a positive electrode conductive agent, and polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10 5 ) were mixed in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone (NMP) was added as a solvent, a positive electrode slurry with a solid content of 75 wt% was prepared, and the slurry was stirred evenly under a vacuum mixer.

[0126] Aluminum foil is selected as the second current collector (second conductive layer), and the second current collector and the second conductive layer have the same structure. The room temperature conductivity of the aluminum foil is 3.774e7 S / m, and the thickness, width and length of the second current collector are 9 μm, 69 mm and 69 mm, respectively. The positive electrode slurry is evenly coated on the third and fourth surfaces of the second current collector, and dried to form a second positive electrode coating with a thickness of 34 μm on both the third and fourth surfaces, thereby obtaining a second positive electrode sheet coated with positive electrode active materials on both sides.

[0127] (3) Preparation of negative electrode sheet: The negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (SuperP), and binder styrene acrylic rubber (SD-3) were mixed in a weight ratio of 89.5:8:1:1.5, and then deionized water was added as a solvent to prepare a negative electrode slurry with a solid content of 50wt%, and stirred evenly.

[0128] A copper foil with a room temperature conductivity of 5.998e7 S / m and a thickness of 6 μm was selected as the third current collector. The negative electrode slurry was evenly coated on both surfaces of the third current collector and dried to form a first negative electrode coating with a thickness of 42 μm on both surfaces of the third current collector, thereby obtaining a first negative electrode sheet with a double-sided negative electrode coating. The length of the first negative electrode sheet was 70 mm and the width was 69.7 mm.

[0129] (4) Preparation of electrolyte: In a dry argon atmosphere, ethylene carbonate (EC),

[0130] Ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=30:50:20 to form a basic organic solvent, and then lithium salt lithium hexafluorophosphate (LiPF6) is added to the basic organic solvent to dissolve and mix evenly to obtain an electrolyte with a LiPF6 mass concentration of 12.5%.

[0131] (5) Preparation of isolation membrane: A polyethylene porous membrane is used as a substrate layer, and a ceramic layer containing alumina ceramic and polyvinylidene fluoride binder is coated on one surface of the substrate layer as an isolation membrane, wherein the mass percentage of alumina ceramic in the ceramic layer is 95%, the thickness of the substrate layer is 5 μm, and the thickness of the ceramic layer is 1.8 μm.

[0132] (6) Preparation of electrode assembly: The above-mentioned positive electrode sheet, separator and negative electrode sheet are stacked, and nickel sheet and aluminum sheet with specifications of 12 mm×6 mm are selected as tabs. The aluminum tab is welded to the positive electrode sheet (aluminum foil of the positive electrode sheet), and the nickel tab is welded to the negative electrode sheet to form an electrode assembly for use. Among them, the first positive electrode sheet has 2 layers, the second positive electrode sheet has 11 layers, the first negative electrode sheet has 12 layers, and the separator has 24 layers.

[0133] (7) Electrode assembly assembly: Place the aluminum-plastic film formed by punching holes in an assembly fixture, with the pit side facing upward, place the electrode assembly in the pit, and set a seal at the two transfer tabs after integral welding, and apply external force to press tightly. Then, cover the electrode assembly with another aluminum-plastic film formed by punching holes with the pit side facing downward, and heat-seal the four sides of the two aluminum-plastic films by hot pressing to obtain an assembled electrode assembly.

[0134] (8) Liquid injection packaging: The assembled electrode assembly is injected with electrolyte, and after vacuum packaging, static standing, hot pressing, shaping and other processes, a lithium-ion secondary battery is produced.

[0135] The relevant parameters of Examples 2 to 16 can be referred to in Table 1 below.

[0136] Different from Example 1, in Example 9, the depth D2 of the groove is 5 μm, the ratio of the depth D2 of the groove to the thickness D1 of the first current collector is 33.33%, and the sum of the widths W3 of the grooves is 6.9 mm.

[0137] The difference between Comparative Example 1 and Example 1 is that no groove is provided in Comparative Example 1.

[0138] Table 1

[0139]

[0140] According to Table 1 above, in combination with Examples 1 to 8, and Comparative Example 1, it can be seen that when the sum of the widths of the grooves W3 is a constant, as the groove depth D2 increases, the value of Δ gradually increases, and the absolute value of Δ gradually decreases, indicating that the provision of grooves can reduce the risk of external lithium chromatography, reduce the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography, and improve the fast charging performance of the secondary battery. However, as the depth of the groove increases, the thickness of the first positive electrode sheet decreases, and the first positive electrode sheet is prone to curling and tearing. Therefore, in combination with Examples 2 to 7, when the groove depth is 10% ≤ D2 / D1 ≤ 70%, the absolute value of Δ is relatively small, and the first positive electrode sheet does not curl or only curls slightly.

[0141] Compared with Example 5 and Example 6, the voltage difference Δ is closer, but the groove depth in Example 6 is deeper, and the first positive electrode plate is prone to curling. Therefore, in the embodiments of the present application, it is further preferred that 33.33%≤D2 / D1≤50%.

[0142] According to Table 1 above, in combination with Examples 9 to 16, and Comparative Example 1, it can be seen that when the groove depth D2 is a constant, as the sum of the groove widths W3 increases, the value of Δ gradually increases, the absolute value of Δ gradually decreases, the risk of external lithium chromatography gradually decreases, and the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography gradually decreases. In Example 9, when W3 / W1 is equal to 10%, the voltage difference Δ is less than 10mV, indicating that the risk of external lithium chromatography and the risk of internal lithium chromatography are quite different, and the risk of external lithium chromatography is not significantly reduced. In Example 16, when W3 / W1 is equal to 85%, the total width of the groove accounts for a large proportion of the width of the first current collector, and the first current collector is prone to stress imbalance, resulting in curling of the first positive electrode sheet. Therefore, in the embodiments of the present application, in combination with Examples 10 to 15, 15%≤W3 / W1≤80% can be selected. In Example 12 to Example 13, when 50%≤W3 / W1≤70%, Δ is relatively large and the absolute value of Δ is relatively small, indicating that the difference between the outer and inner layers of lithium is small. In addition, the total width of the groove accounts for a moderate width of the first current collector, reducing the curling of the first positive electrode sheet. In the embodiment of the present application, 50%≤W3 / W1≤70% may be preferred.

[0143] Different from Example 1, in Example 17, the depth D2 of the groove is 5 μm, the ratio of the depth D2 of the groove to the thickness D1 of the first current collector is 33.33%, the width W2 of a single groove is 0.21 mm, and the sum of the widths W3 of the grooves is 6.9 mm. The relevant parameters of Examples 18 to 24 can be referred to in Table 2 below.

[0144] Table 2

[0145]

[0146] According to Table 2 above, in combination with Examples 17 to 24, and Comparative Example 1, it can be seen that when the groove depth D2 is a constant, as the single groove width W2 increases, the value of Δ gradually increases, and the absolute value of Δ gradually decreases, indicating that the risk of external lithium chromatography gradually decreases, and the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography gradually decreases, thereby improving the fast charging performance of the secondary battery.

[0147] In Example 17 and Example 18, the voltage difference Δ is relatively close, but the width of a single groove in Example 18 is relatively large, which can reduce the electrons gathering on the surface of the first current collector and make the current distribution more uniform. In Example 23 and Example 24, Δ is relatively close, but the width of a single groove in Example 24 is wider, which reduces the mechanical strength of the first current collector and makes it easy to curl. Therefore, in the embodiments of the present application, in combination with Examples 18 to 23, 0.5%≤W2 / W1≤4% can be selected.

[0148] In Examples 18 to 21, as the width of a single groove gradually increases, Δ increases relatively quickly, the risk of external lithium chromatography is significantly reduced, and the width of a single groove accounts for a moderate proportion of the width of the first current collector, and the mechanical strength of the first positive electrode sheet is better. In Examples 21 to 23, Δ increases slowly, and the width of a single groove accounts for a relatively large proportion of the width of the first current collector, and the first positive electrode sheet is prone to curling. Therefore, in the embodiments of the present application, 0.5%≤W2 / W1≤2% may be preferred.

[0149] Different from Example 1, in Example 25, the depth D2 of the groove is 5 μm, the ratio of the depth D2 of the groove to the thickness D1 of the first current collector is 33.33%, the width W2 of a single groove is 1.04 mm, and the first spacing L1 between two adjacent grooves is 0.3 mm. The relevant parameters of Examples 26 to 32 can be referred to in Table 3 below.

[0150] Table 3

[0151]

[0152] According to the above Table 3, combined with Examples 25 to 32, when the groove depth D2 is a constant value, as the width of the first interval L1 between two adjacent grooves increases, the curling phenomenon of the first positive electrode sheet is significantly improved.

[0153] In Example 25, although Δ is -6.1mV, the first positive electrode plate has moderate curling. In Example 32, the absolute value of Δ is large, indicating that the risk of external lithium chromatography and the risk of internal lithium chromatography are quite different. In the embodiments of the present application, in combination with Examples 26 to 31, the first positive electrode plate did not curl or only curled slightly, and Δ was relatively small, indicating that the risk of external lithium chromatography was relatively low, so 50% ≤ L1 / W2 ≤ 200% can be selected.

[0154] In Examples 27 to 31, the absolute value of the voltage difference Δ is relatively small, indicating that the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography is relatively low, and the first positive electrode plate does not curl, indicating that the first positive electrode plate has better structural strength. In Examples 29 and 31, Δ differs by only 0.3mV, but in Example 31, the width of the first spacing L1 is too large, and electrons are easily gathered between two adjacent grooves, making the local current density too high, affecting the performance of the secondary battery. Therefore, the embodiment of the present application can preferably be 100%≤L1 / W2≤150%.

[0155] Different from Example 1, in Example 33, the thickness T1 of the first conductive layer is 4 μm, that is, the thickness D1 of the first current collector is 4 μm, the thickness T2 of the second conductive layer is 9 μm, and the ratio of the depth D2 of the groove to the thickness D1 of the first current collector is 33.33%. The relevant parameters in Examples 34 to 42 can be referred to in Table 4 below.

[0156] Table 4

[0157] <![CDATA[T1(μm)]]> <![CDATA[T2(μm)]]> <![CDATA[T1 / T2(%)]]> Voltage difference Δ(mV) Embodiment 33 4 9 44％ 3.9 Embodiment 34 5 9 56％ 2.4 Embodiment 35 6 9 67％ -0.4 Embodiment 36 7 9 78％ -1.5 Embodiment 37 8 9 89％ -2.8 Embodiment 38 9 9 100％ -3.7 Embodiment 39 10 10 100％ -4.0 Embodiment 40 10.8 9 120％ -4.8 Embodiment 41 13.5 9 150％ -6.1 Embodiment 42 15.3 9 170％ -8.4

[0158] According to Table 4 above, in combination with Examples 33 to 42, when the ratio of the depth D2 of the groove to the thickness D1 of the first current collector is a constant, as the ratio of T1 to T2 decreases, the value of Δ gradually increases, indicating that the risk of external lithium chromatography gradually decreases, and when 44% ≤ T1 / T2 < 67%, the risk of external lithium chromatography is even lower than the risk of internal lithium chromatography. However, in Examples 33 to 34, since the thickness T1 of the first conductive layer itself is relatively thin, and a groove is provided on the first conductive layer, it is very easy to cause the first positive electrode plate to curl. When 67% ≤ T1 / T2 < 160%, as the ratio of T1 to T2 decreases, the absolute value of Δ gradually decreases, indicating that the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography gradually decreases. However, in Example 42, the value of Δ is relatively small, and the absolute value of Δ is relatively large, indicating that the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography is large. Therefore, in the embodiments of the present application, in combination with Examples 35 to 42, 67% ≤ T1 / T2 ≤ 150% can be selected.

[0159] In Example 35 and Example 36, Δ is close to 0, indicating that the risk of external lithium chromatography in both is low, but the thickness of the first conductive layer in Example 36 is relatively thick, which makes the structural strength of the first positive electrode plate better and reduces curling. In Example 36 to Example 40, the voltage difference Δ is relatively large, the risk of external lithium chromatography is reduced, and the absolute value of Δ is relatively small, indicating that the difference between the external lithium chromatography and the internal lithium chromatography is small. The discharge rate without lithium chromatography in the outer layer and the fast charging performance of the secondary battery are improved. Therefore, the embodiment of the present application may preferably be 78%≤T1 / T2<120%.

[0160] 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 an electrode assembly, wherein the electrode assembly comprises a plurality of positive electrode sheets and a plurality of negative electrode sheets alternately stacked along a first direction, and a separator is disposed between adjacent positive electrode sheets and negative electrode sheets, characterized in that: The positive electrode sheet comprises a first positive electrode sheet and a second positive electrode sheet. Along the first direction, the first positive electrode sheet is the outermost electrode sheet of the electrode assembly, and the second positive electrode sheet is arranged between two adjacent negative electrode sheets. The first positive electrode plate comprises a first current collector and a first positive electrode coating. Along the first direction, the first current collector comprises a first surface and a second surface arranged opposite to each other, the second surface faces the negative electrode plate, and the first positive electrode coating is arranged on the second surface; The first surface is provided with a plurality of grooves.

2. The secondary battery according to claim 1, characterized in that: Along the first direction, the thickness of the first current collector is D1, the depth of the groove is D2, and 10%≤D2 / D1≤70%.

3. The secondary battery according to claim 2, characterized in that: 33.33%≤D2 / D1≤50%.

4. The secondary battery according to claim 1, characterized in that: The groove is arranged along the second direction, and along the third direction, the width of the first current collector is W1, the width of the groove is W2, and 0.5%≤W2 / W1≤4%; The first direction, the second direction and the third direction are perpendicular to each other.

5. The secondary battery according to claim 4, characterized in that: 0.5%≤W2 / W1≤2%.

6. The secondary battery according to claim 4, characterized in that: Along the third direction, the sum of the widths of all the grooves is W3, and 15%≤W3 / W1≤80%.

7. The secondary battery according to claim 6, characterized in that: 50%≤W3 / W1≤70%.

8. The secondary battery according to claim 4, characterized in that: Along the third direction, a plurality of the grooves are arranged in sequence, and a first interval L1 is provided between two adjacent grooves, and 50%≤L1 / W2≤200%.

9. The secondary battery according to claim 8, characterized in that: 100%≤L1 / W2≤150%.

10. The secondary battery according to claim 4, characterized in that: Along the second direction, the first current collector includes a first end surface and a second end surface that are opposite to each other, and the groove runs through the first end surface and the second end surface.

11. The secondary battery according to claim 1, characterized in that: The second positive electrode plate includes a second current collector and a second positive electrode coating disposed on two surfaces of the second current collector; The first current collector includes a first conductive layer, and along the first direction, the thickness of the first conductive layer is T1; The second current collector includes a second conductive layer. Along the first direction, the thickness of the second conductive layer is T2, and 67%≤T1 / T2≤150%.

12. The secondary battery according to claim 11, characterized in that: 78%≤T1 / T2≤120%.

13. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 12.

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