Negative pole piece, preparation method of negative pole piece, secondary battery and power utilization device

Abstract

The invention discloses a negative pole piece and a preparation method thereof, a secondary battery and an electric device, the negative pole piece comprises a negative current collector, a first active material layer and a second active material layer, the negative current collector has a first surface and a second surface which are oppositely arranged along the thickness direction of the negative current collector, the first active material layer is arranged on the first surface, and the second active material layer is arranged on the second surface. The second active material layer is arranged on the second surface. In the length direction of the negative pole piece, one end of the first active material layer is a first end part, and the second surface is provided with a first empty current collector region; and the projection of the first end part is at least partially overlapped with the first empty current collector region when being observed along the thickness direction of the negative pole piece. The first end part corresponding to the second surface is free of the active material layer, so that the overall thickness of the negative pole piece at the first end part can be reduced, overpressure of the negative pole piece can be effectively reduced when the negative pole piece is subjected to cold pressing and rolling, and damage and fracture of the negative current collector are reduced.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly to a negative electrode sheet, a preparation method thereof, a secondary battery, and an electrical device. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, long cycle life, portability, etc., and are widely used in various energy storage fields. With the continuous iterative development of consumer lithium-ion batteries in recent years, the market has higher and higher requirements for the energy density of batteries.

[0003] Improving the energy density and enhancing the fast charging performance are one of the goals of the lithium-ion battery industry. Increasing the loading amount of the active material on the electrode sheet can not only improve the energy density but also achieve the purpose of reducing costs. For this reason, a double-layer coating technology has been developed. However, in the case of cold pressing and rolling, since there are active material layers on both sides, overpressure is very likely to occur, causing damage to the current collector of the electrode sheet, resulting in easy breakage of the tape during the subsequent processing of the electrode sheet. To ensure subsequent processing, in most cases, the compaction density is reduced, which leads to a loss of energy density and loses the significance of the double-layer coating for improving the energy density. Summary of the Invention

[0004] The purpose of the present application is to provide a negative electrode sheet, a preparation method thereof, a secondary battery, and an electrical device, aiming to improve the technical problem that the current collector is easily damaged by overpressure.

[0005] According to the first aspect of the present application, a negative electrode sheet is provided, which includes a negative electrode current collector, a first active material layer, and a second active material layer. Along the thickness direction of the negative electrode current collector, the negative electrode current collector has a first surface and a second surface that are oppositely arranged. The first active material layer is disposed on the first surface, and the second active material layer is disposed on the second surface. Along the length direction of the negative electrode sheet, one end of the first active material layer is a first end portion, and the second surface has a first empty current collector region. When observed along the thickness direction of the negative electrode sheet, the projection of the first end portion at least partially overlaps with the first empty current collector region. Along the length direction of the negative electrode sheet, the first active material layer further includes a second end portion that is oppositely arranged with respect to the first end portion, and the second surface further has a second empty current collector region. When observed along the thickness direction of the negative electrode sheet, the projection of the second end portion at least partially overlaps with the second empty current collector region.

[0006] In the above technical solution, the projection of the first end portion at least partially overlaps with the first empty current collector region. There is no active material layer at the location corresponding to the first end portion on the second surface, and there is no active material layer at the location corresponding to the second end portion on the second surface. That is, at the first end portion and the second end portion of the negative electrode tab, the negative electrode active material layers on both sides of the negative electrode current collector are misaligned. In the length direction of the negative electrode tab, the length of the first active material layer is longer than that of the second active material layer, and the projection length of the first active material layer on the first current collector completely covers the length of the second active material layer on the first current collector. In this way, the overall thickness of the negative electrode tab at the first end portion can be reduced. When cold-rolling the negative electrode tab by a cold-rolling roller, overpressure of the negative electrode tab can be effectively reduced, thereby reducing damage and fracture of the negative electrode current collector. Compared with the traditional solution, in the present application, there is no need to reduce the compaction density of the first active material layer and / or the second active material layer, and the high energy density of the negative electrode tab can be ensured. The empty current collector region described in the present application refers to a region on a certain surface of the current collector where no active material layer is provided.

[0007] In some preferred embodiments, when observing along the thickness direction of the negative electrode tab, along the length direction of the negative electrode tab, the overlapping length of the first end portion and the first empty foil region is L 1 mm, 3 mm ≤ L 1 ≤ 100 mm. By setting the misalignment length between the first end portion of the first active material layer and the second active material layer to be 3 mm or more, during the cold-rolling process of the negative electrode tab, the alignment of the end portions can be avoided, resulting in an increase in the local thickness of the end portions, thereby reducing the breakage of the negative electrode current collector due to excessive stress at the end portions of each active material layer, and at the same time reducing the process control difficulty; setting the misalignment length between the first end portion of the first active material layer and the second active material layer to be 100 mm or less can avoid excessive loss of the energy density of the secondary battery. It should be noted that the technical effects of the above range selection of L 1 are the preferred technical effects of the present application. As long as the first end portion of the first active material layer and the second active material layer are misaligned, the basic technical effect of reducing the overpressure of the negative electrode tab of the present application can be achieved. The same applies to the preferred embodiments hereinafter.

[0008] In some preferred embodiments, when observing along the thickness direction of the negative electrode tab, along the length direction of the negative electrode tab, the overlapping length of the second end portion and the second empty foil region is L 2 mm, 3 mm ≤ L 2≤100 mm. By setting the misalignment length between the second end of the first active material layer and the second active material layer to be 3 mm or more, it is possible to prevent the local thickness at the end from increasing due to the alignment of the ends during the cold rolling process of the negative electrode tab, thereby reducing the breakage of the negative electrode current collector at the ends of each active material layer due to excessive stress and simultaneously reducing the process control difficulty; setting the misalignment length between the second end of the first active material layer and the second active material layer to be 100 mm or less can avoid excessive loss of the energy density of the secondary battery.

[0009] In some preferred embodiments, the first active material layer includes a first active material, the second active material layer includes a second active material, and the specific capacity of the second active material is greater than that of the first active material. Since there is no active material layer corresponding to the first end and the second end on the second surface, that is, the length of the second active material layer is shorter than that of the first active material layer, making the specific capacity of the second active material greater than that of the first active material can compensate for the capacity loss and improve the capacity of the secondary battery.

[0010] In some preferred embodiments, the specific capacity of the first active material is 345 mAh / g to 355 mAh / g, and the specific capacity of the second active material is 358 mAh / g to 365 mAh / g.

[0011] In a second aspect, the present application also provides a secondary battery, including a housing and an electrode assembly. The electrode assembly is received in the housing. The electrode assembly includes a positive electrode tab, a separator, and a negative electrode tab according to any one of the embodiments of the first aspect described above. The positive electrode tab, the separator, and the negative electrode tab are stacked and wound, with the first surface facing away from the winding center and the second surface facing the winding center.

[0012] In some preferred embodiments, the positive electrode tab includes a positive electrode current collector, a third active material layer, and a fourth active material layer. The third active material layer is disposed on the surface of the positive electrode current collector facing the second active material layer, and the fourth active material layer is disposed on the surface of the positive electrode current collector facing away from the second active material layer. The positive electrode current collector is electrically connected to a positive electrode tab. A first avoidance groove is formed in the second active material layer. At the position of the positive electrode current collector where the positive electrode tab is located, when viewed in the thickness direction of the positive electrode current collector, the projection of the positive electrode tab falls into the first avoidance groove. Since there is a first avoidance groove for avoiding the positive electrode tab at the corresponding negative electrode tab, the problem that the thickness of the positive electrode tab increases after connecting the positive electrode tab and affects the energy density of the battery can be reduced. Since the first avoidance groove is formed in the second active material of the negative electrode tab, in order to prevent lithium deposition, a protective adhesive layer can be attached in the area of the positive electrode tab, and the projection area of the protective adhesive layer completely covers the first avoidance groove.

[0013] In some preferred embodiments, the negative electrode current collector is electrically connected to a negative electrode tab, and a second avoidance groove is formed in the third active material. At the position of the negative electrode current collector where the negative electrode tab is located, when observed in the thickness direction of the negative electrode current collector, the projection of the negative electrode tab falls into the second avoidance groove, which can reduce the problem of the increase in the thickness of the positive electrode tab after connecting the negative electrode tab, and reduce the loss of the energy density of the battery. At the same time, a second avoidance groove is provided at the position of the positive electrode tab corresponding to the negative electrode tab, reducing the margin of the positive electrode active material layer, thereby ensuring that the negative electrode tab has enough margin to embed the lithium ions released from the positive electrode, so as to reduce the lithium deposition of the battery.

[0014] In some preferred embodiments, the first end portion and the first empty foil area are arranged close to the winding center. When observed in the thickness direction of the negative electrode tab, the overlapping length of the first end portion and the first empty foil area is L 1 mm, 60mm ≤ L 1 ≤ 100mm. The first active material layer further has a second end portion arranged away from the winding center, and the second surface has a second empty foil area away from the winding center. When observed in the thickness direction of the negative electrode tab, the overlapping length of the second end portion and the second empty foil area is L 2 mm, 3mm ≤ L 2 ≤ 8mm.

[0015] In a third aspect, the present application further provides an electrical device, including the secondary battery according to any one of the embodiments in the second aspect above.

[0016] In a fourth aspect, the present application further proposes a method for preparing a negative electrode tab according to any one of the embodiments in the first aspect above, including:

[0017] Providing a negative electrode current collector, which has a first surface and a second surface arranged opposite to each other in the thickness direction of the negative electrode current collector;

[0018] Providing a continuous first active material layer on the first surface;

[0019] Along the length direction of the negative electrode current collector, providing a plurality of second active material layers arranged at intervals on the second surface, and an interval area is formed between two adjacent second active material layers;

[0020] In the interval area, cutting the negative electrode current collector and the first active material layer to form a plurality of negative electrode tabs; wherein, in the interval area, at least one first empty current collector area of the negative electrode tab is formed on the first surface.

[0021] The additional aspects and advantages of the embodiments of the present application will be partially described, shown, or explained through the implementation of the embodiments of the present application in the following description. Description of the Drawings

[0022] One or more embodiments are illustrated by corresponding drawings, which do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements. Unless otherwise stated, the dimensions in the drawings do not constitute a proportional limitation.

[0023] Figure 1 The schematic structural diagram of the negative electrode sheet of some embodiments of the present application is shown;

[0024] Figure 2 The schematic winding structure diagram of the electrode assembly of some embodiments of the present application is shown;

[0025] Figure 3 The schematic structural diagram of the negative electrode sheet of some embodiments of the present application is shown;

[0026] Figure 4 The schematic structural diagram of the secondary battery of some embodiments of the present application is shown;

[0027] Figure 5 is Figure 2 The partial enlarged view of part A in

[0028] Figure 6 is Figure 2 The partial enlarged view of part B in

[0029] Figure 7 is Figure 2 The partial enlarged view of part C in

[0030] Figure 8 The schematic structural diagram of the negative electrode sheet roll;

[0031] Figure 9 The schematic diagram of the cutting preparation of the negative electrode sheet.

[0032] Explanation of reference numerals:

[0033] 10. Negative electrode sheet; 11. Negative electrode current collector; 11a. First surface; 11b. Second surface; 111. First empty current collector area; 112. Second empty current collector area; 12. First active material layer; 121. First end; 122. Second end; 13. Second active material layer;

[0034] 1000. Secondary battery;

[0035] 100. Housing;

[0036] 200, Electrode assembly; 210, Positive electrode tab; 211, Positive current collector; 212, Third active material layer; 213, Fourth active material layer; 2121, Second avoidance groove; 220, Negative electrode tab roll; 11, Negative current collector; 12, First active material layer; 13, Second active material layer; 131, First avoidance groove; 230, Separator; 240, Positive electrode ear; 250, Negative electrode ear; 260, Spacing region;

[0037] X, Second direction; Y, Third direction; Z, First direction. Detailed implementation manners

[0038] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly described below with reference to the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application.

[0039] In this application, referring to "embodiment" means that the specific features, structures, or characteristics described in connection with the embodiment can be included in at least one embodiment of this application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments.

[0040] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0041] The technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0042] In a first aspect, this application provides a negative electrode tab. Please refer to Figure 1 , the negative electrode tab 10 includes a negative current collector 11, a first active material layer 12, and a second active material layer 13. Among them, the first active material layer 12 and the second active material layer 13 are respectively disposed on two surfaces in the thickness direction (first direction Z) of the current collector.

[0043] For the above-mentioned negative electrode current collector 11, the negative electrode current collector 11 is the conductive part in the negative electrode plate 10, which can transfer the electrons generated by the active material to the external circuit. The negative electrode current collector 11 usually has high mechanical strength, which can improve the overall strength of the negative electrode plate 10 and reduce the deformation or rupture of the negative electrode plate 10 during the charge and discharge process of the battery. For example, the negative electrode current collector 11 can be made of a metal material with high strength, specifically, copper foil, nickel foil or polymer copper foil (a polymer such as polyethylene, polypropylene or polyamide is provided on the surface of the copper foil) with an overall flat and strip-shaped structure can be used. The negative electrode current collector 11 provides a support and fixing platform for the active material, enabling the active material to be evenly distributed on the negative electrode current collector 11 and ensuring the structural stability of the negative electrode plate 10.

[0044] For the above-mentioned first active material layer 12 and second active material layer 13, please refer to Figure 1 , along the thickness direction (the first direction Z) of the negative electrode current collector 11, the negative electrode current collector 11 includes a relatively arranged first surface 11a and a second surface 11b. The first active material layer 12 can be disposed on the first surface 11a, and the second active material layer 13 can be disposed on the second surface 11b. The first active material layer 12 and the second active material layer 13 participate in a series of electrochemical reactions during the charge and discharge process of the battery, thereby realizing the conversion between electrical energy and chemical energy. Taking a lithium-ion battery as an example, during the charging process of the battery, lithium ions are removed from the positive electrode and migrate to the first active material layer 12 and / or the second active material layer 13 through the electrolyte. During the discharging process of the battery, lithium ions are removed from the first active material layer 12 and / or the second active material layer 13 and return to the positive electrode through the electrolyte, completing the charge and discharge cycle of the battery.

[0045] Among them, the first active material layer 12 includes a first active material, a conductive agent, an adhesive, etc. These materials are mixed and stirred evenly and then coated on the surface of the negative electrode current collector 11 to obtain the first active material layer 12. Among them, the first active material can be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, elemental silicon, silicon oxide, and silicon alloy; the same applies to the second active material layer 13.

[0046] In the embodiments of the present application, the negative electrode plate 10 with a double-layer active material layer can effectively improve the energy density of the battery. At the end of the first active material layer 12 and / or the second active material layer 13 (both ends in the length direction of the negative electrode plate 11, that is Figure 1At both ends of the second direction X, a convex structure is likely to appear. Especially when the ends of the first active material layer 12 overlap with those of the second active material layer 13, their convex structures may also overlap, resulting in an increase in the thickness of the negative electrode tab 10 at the ends of the active material layers. When cold-rolling the negative electrode tab 10, it is very easy to cause overpressure on the negative electrode current collector 11, damaging the negative electrode current collector 11, so that the negative electrode tab 10 breaks during subsequent processing.

[0047] To alleviate the above problems, along the length direction (the second direction X) of the negative electrode tab 10, one end of the first active material layer 12 is the first end 121, and the second surface 11b has the first empty current collector region 111. When observed along the thickness direction (the first direction Z) of the negative electrode tab 10, the projection of the first end 121 at least partially overlaps with the first empty current collector region 111. Along the length direction (the second direction X) of the negative electrode tab 10, the first active material layer 12 further includes a second end 122 opposite to the first end 121, and the second surface 11b further has a second empty current collector region 112 opposite to the first empty current collector region 111. When observed along the thickness direction (the first direction Z) of the negative electrode tab 10, the projection of the second end 122 at least partially overlaps with the second empty current collector region 112. There is no active material layer at the first end 121 corresponding to the second surface 11b, that is, at the first end 121 of the negative electrode tab 10, the active material layers on both sides of the negative electrode current collector 11 are misaligned. There is no active material layer at the second end 122 corresponding to the second surface 11b, that is, at the second end 122 of the negative electrode tab 10, the active material layers on both sides of the negative electrode current collector 11 are misaligned, which can reduce the overall thickness of the negative electrode tab 10 at the first end 121 and the second end 122. When cold-rolling the negative electrode tab 10, overpressure of the negative electrode tab 10 can be effectively reduced, thereby reducing damage and fracture of the negative electrode current collector 11. At the same time, it is not necessary to reduce the compaction density of the first active material layer 12 and / or the second active material layer 13 to ensure that the negative electrode tab 10 has a high energy density. Herein, the empty current collector region described in this application refers to a region on a certain surface of the current collector where no active material layer is provided.

[0048] Please refer to Figure 1 In some embodiments, when observed along the thickness direction (the first direction Z) of the negative electrode tab 10 and along the length direction (the second direction X) of the negative electrode tab 10, the overlapping length of the first end 121 and the first empty foil region is L 1 mm, 3mm ≤ L 1≤100 mm. By setting the misalignment length between the first end 121 of the first active material layer 12 and the second active material layer 13 to be 3 mm or more, it is possible to prevent the local thickness at the ends from increasing due to the alignment of the ends during the cold rolling process of the negative electrode sheet 10, thereby reducing the breakage of the negative electrode current collector 11 at the ends of each active material layer due to excessive stress. At the same time, the process control difficulty is reduced; setting the misalignment length between the first end 121 of the first active material layer 12 and the second active material layer 13 to be 100 mm or less can reduce the excessive loss of the energy density of the secondary battery. It should be noted that the technical effects of the above range selection of L 1 are the preferred technical effects of this application. As long as the first end 121 of the first active material layer 12 is misaligned with the second active material layer 13, the basic technical effect of reducing the overpressure of the negative electrode sheet 10 of this application can be achieved. The same applies to the preferred embodiments hereinafter.

[0049] Please refer to Figure 1 , when observing along the thickness direction (the first direction Z) of the negative electrode sheet 10, along the length direction (the second direction Y) of the negative electrode sheet 10, the overlapping length of the second end 122 and the second empty current collector region 112 is L 2 mm, 3 mm ≤ L 2 ≤ 100 mm. By setting the misalignment length between the second end 122 of the first active material layer 12 and the second active material layer 13 to be 3 mm or more, it is possible to prevent the local thickness at the ends from increasing due to the alignment of the ends during the cold rolling process of the negative electrode sheet 10, thereby reducing the breakage of the negative electrode current collector 11 at the ends of each active material layer due to excessive stress. At the same time, the process control difficulty is reduced; setting the misalignment length between the second end 122 of the first active material layer 12 and the second active material layer 13 to be 100 mm or less can avoid the excessive loss of the energy density of the secondary battery.

[0050] For example, when the above negative electrode sheet 10 is applied to the stacked electrode assembly 200, the values of L 1 and L 2 can be smaller, for example, 3 mm ≤ L 1 ≤ 8 mm, 3 mm ≤ L 2 ≤ 8 mm. Since the misalignment length between the first active material layer 12 and the second active material layer 13 (i.e., the overlapping length between the first end 121 and the first empty current collector region 111) can effectively improve the problem that the negative electrode current collector 11 is easily damaged by rolling when it is 3 mm, in order to reduce the processing difficulty, the overlapping length between the first end 121 and the first empty current collector region 111 can be appropriately increased. In order to ensure that the negative electrode sheet 10 can have a high energy density, the overlapping length should not be too long. Therefore, it can be set to be less than 8 mm, which can reduce the breakage of the negative electrode current collector 11 while ensuring that the negative electrode sheet 10 has a high energy density.

[0051] When the above-mentioned negative electrode tab 10 is applied in the wound electrode assembly 200, the length of the one of the first end portion 121 and the second end portion 122 that is closer to the winding center can take a larger value, while the length of the one that is farther from the winding center can take a smaller value. As an example, please refer to Figure 2 , the first end portion 121 is closer to the winding center, and its length L 1 satisfies 60 mm ≤ L 1 ≤ 100 mm. At the position close to the winding center of the electrode assembly 200, since there is no corresponding positive electrode tab 210 on the inner side of the negative electrode tab 10 facing the winding center, the active material on the inner side of the negative electrode tab 10 cannot participate in the electrochemical reaction. Therefore, in order to reduce the weight of the electrode assembly 200 and improve the energy density of the electrode assembly 200, only one side of the first winding turn of the negative electrode tab 10 is provided with an active material layer, and the inner side is usually set as an empty current collector area, and its length can be set to 60 mm to 100 mm.

[0052] At the position close to the winding center of the electrode assembly 200, there is a corresponding positive electrode tab 210 on the outer side of the negative electrode tab 10 facing away from the winding center. An active material layer provided on the outer side of the negative electrode tab 10 can participate in the electrochemical reaction with the active material layer on the positive electrode tab 210, that is, the first end portion 121 and the first empty current collector area 111 can at least partially overlap, so the overlapping length can be set to 60 mm to 100 mm.

[0053] The second end portion 122 is far from the winding center, and its length L 2 satisfies 3 mm ≤ L 2 ≤ 8 mm. At the position far from the winding center, there may be a corresponding positive electrode tab 210 on the inner side of the negative electrode tab 10 facing the winding center or on the outer side facing away from the winding center. Therefore, the empty current collector area at the end of the negative electrode tab 10 far from the winding center should not be too large. The 3 mm overlapping length between the second end portion 122 and the second empty current collector area 112 can effectively improve the problem that the negative current collector 11 is easily damaged by cold pressing. In order to reduce the processing difficulty, the overlapping length between the first end portion 121 and the first empty current collector area 111 can be appropriately increased. In order to ensure that the negative electrode tab 10 can have a high energy density, the overlapping length should not be too long, so it can be set to less than 8 mm, which can ensure that the negative electrode tab 10 has a high energy density while reducing the damage of the negative current collector 11 by pressing. Among them, when the second end portion 122 is close to the winding center, its length L 2 can also satisfy 60 mm ≤ L 2 ≤ 100 mm.

[0054] In some embodiments, the first active material layer 12 includes a first active material, and the second active material layer 13 includes a second active material, and the gram capacity of the second active material is greater than the gram capacity of the first active material. The second active material layer 13 uses a second active material with a larger gram capacity to improve the energy density of the battery. For example, the gram capacity of the first active material is 345mAh / g to 355mAh / g, and the gram capacity of the second active material is 358mAh / g to 365mAh / g, wherein both the first active material and the second active material can use the above-mentioned graphite, soft carbon, hard carbon or carbon fiber, etc. Taking graphite as an example, the gram capacity of different graphites may be different. For example, graphite with high crystallinity usually has a higher gram capacity because its crystal structure is more ordered, which is conducive to the insertion and extraction of lithium ions. The smaller grain size can also provide more active surface area, promote electrochemical reactions, and thus increase the gram capacity; in addition, the purity and surface morphology of graphite will also affect the gram capacity. Graphite with high purity usually has better electrochemical properties, and graphite that has been surface treated or modified can improve its wettability and reactivity with the electrolyte, thereby increasing the gram capacity.

[0055] In some embodiments, the first active material layer 12 may be a single-layer coating structure, or a double-layer or multi-layer coating structure. Figure 3 , the first active material layer 12 is a double-layer coating structure, the first active material layer 12 includes a first coating layer 12a and a second coating layer 12b, and the second coating layer 12b is arranged between the first coating layer 12a and the negative electrode current collector 11. In the present application, by setting the structure of the first coating layer 12a and the second coating layer 12b with different dynamic properties, the dynamic properties of the first active material layer 12 can be improved, and the first active material layer 12 can also have a higher energy density. Optionally, the first coating layer 12a and the second coating layer 12b can also be staggered and stacked, which can effectively reduce the problem of excessive thickness at the end of the first active material layer 12 and reduce the risk of fracture failure of the negative electrode current collector 11 during the cold rolling process. The second active material layer 13 can also be arranged similarly to the first active material layer 12.

[0056] In the second aspect, the present application also proposes a secondary battery 1000, please refer to Figure 2 and Figure 4 The secondary battery 1000 includes a housing 100 and an electrode assembly 200, and the electrode assembly 200 is accommodated in the housing 100. The electrode assembly 200 includes a positive electrode sheet 210, a separator 230 and a negative electrode sheet 10, and the positive electrode sheet 210, the separator 230 and the negative electrode sheet 10 are stacked and wound.

[0057] Among them, the first surface 11a of the negative electrode current collector 11 is arranged away from the winding center, and the second surface 11b is arranged facing the winding center. After the negative electrode sheet 10 is wound, the first surface 11a is away from the winding center and is the long surface of the negative electrode, so the first active material layer 12 with a larger length dimension can be arranged; the second surface 11b faces the winding center and is the short surface of the negative electrode, so the second active material layer 13 with a smaller length dimension is adopted.

[0058] Please refer to Figure 2 , Figure 4 and Figure 5 , the positive electrode sheet 210 includes a positive electrode current collector 211, a third active material layer 212 and a fourth active material layer 213. The positive electrode current collector 211 serves as the conductive base material of the positive electrode sheet 210, and it can adopt an aluminum foil with an overall flat and strip-shaped structure. The third active material layer 212 and the fourth active material layer 213 are respectively arranged on two surfaces in the thickness direction of the positive electrode current collector 211. For example, the third active material layer 212 is arranged on the surface of the positive electrode current collector 211 facing the second active material layer 13, and the fourth active material layer 213 is arranged on the surface of the positive electrode current collector 211 away from the second active material layer 13. The positive electrode current collector 211 is electrically connected to a positive electrode tab 240, and the electrically connected manner can be welding, clamping or bonding with a conductive adhesive, etc.

[0059] Among them, the third active material layer 212 and the fourth active material layer 213 include a positive electrode active material, a conductive agent, an adhesive, etc. After the above-mentioned various material components are mixed and stirred evenly, they are coated on two surfaces in the thickness direction of the positive electrode current collector 211, so as to obtain the third active material layer 212 and the fourth active material layer 213. The positive electrode active material can be selected from one or more of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium manganese iron phosphate, and cobalt-free materials.

[0060] Please refer to Figure 6 , a first avoidance groove 131 is formed on the second active material layer 13. At the position of the positive electrode tab 240 on the positive electrode current collector 211, when observed along the thickness direction (the first direction Z) of the positive electrode current collector 211, the projection of the positive electrode tab 240 falls into the first avoidance groove 131, that is, the first avoidance groove 131 completely covers the positive electrode tab 240. For example, a first tab groove (not marked in the figure) is formed in the third active material layer 212, and the positive electrode tab 240 is electrically connected to the positive electrode current collector 211 in the first tab groove. When the positive electrode sheet 210, the separator 230 and the negative electrode sheet 10 are stacked and wound, even if the positive electrode tab 240 protrudes from the first tab groove, since there is a first avoidance groove 131 for avoiding the positive electrode tab 240 at the corresponding position of the negative electrode sheet 10, the problem that the thickness of the positive electrode sheet 210 increases after connecting the positive electrode tab 240 and affects the battery energy density can be reduced.

[0061] Please refer toFigure 7 The negative current collector 11 is electrically connected to a negative tab 250. The negative electrode plate 10 can also be directly electrically connected to the negative current collector 11 by means such as welding, clamping, or bonding with a conductive adhesive. A second avoidance groove 2121 is formed in the third active material of the positive electrode plate 210. At the position of the negative tab 250 on the negative current collector 11, when observed along the thickness direction (the first direction Z) of the negative current collector 11, the projection of the negative tab 250 falls into the second avoidance groove 2121, that is, the second avoidance groove 2121 completely covers the negative tab 250, which can reduce the problem of the increase in the thickness of the positive electrode plate 210 caused by connecting the negative electrode plate 10 and reduce the energy density loss of the battery. At the same time, by providing the second avoidance groove 2121 at the position of the positive electrode plate 210 corresponding to the negative electrode plate 10, the margin of the positive active material layer is reduced, so as to ensure that the negative electrode plate 10 has enough margin to embed the lithium ions released from the positive electrode, thereby reducing lithium deposition in the battery.

[0062] The first end portion 121 and the first empty current collector area 111 are arranged close to the winding center. When observed along the thickness direction (the first direction Z) of the negative electrode plate 10, the overlapping length of the first end portion 121 and the first empty current collector area 111 is L 1 mm, 60 mm ≤ L 1 ≤ 100 mm; at the winding center of the electrode assembly 200, since there is no corresponding positive electrode plate 210 inside the negative electrode plate 10, the active material on the inner side of the negative electrode plate 10 cannot participate in the electrochemical reaction. Therefore, in order to reduce the weight of the electrode assembly 200 and improve the energy density of the electrode assembly 200, the inner side of the end portion of the negative electrode plate 10 close to the winding center is usually set as an empty current collector area, that is, no active material layer is coated. This empty current collector area winds around the inner circle of the electrode assembly 200, and its length can be 60 mm to 100 mm. At the position close to the winding center of the electrode assembly 200, there is a corresponding positive electrode plate 210 on the outer side of the negative electrode plate 10 away from the winding center. Therefore, an active material layer is provided on the outer side of the negative electrode plate 10 to participate in the electrochemical reaction with the active material layer on the positive electrode plate 210, that is, a dislocation arrangement of the active material layers on the two surfaces of the negative current collector 11 is formed at the first end portion 121.

[0063] The first active material layer 12 further has a second end portion 122 arranged away from the winding center. The second surface 11b has a second empty current collector area 112 away from the winding center. When observed along the thickness direction (the first direction Z) of the negative electrode plate 10, the overlapping length of the second end portion 122 and the second empty foil area is L 2 mm, 3 mm ≤ L 2≤8 mm. When the dislocation length between the first active material layer 12 and the second active material layer 13 is 3 mm, the problem that the negative electrode current collector 11 is easily damaged by rolling can be effectively improved. To reduce the processing difficulty, the dislocation length between the first active material layer 12 and the second active material layer 13 can be appropriately increased. In order to ensure that the negative electrode plate 10 can have a high energy density, the dislocation length between the first active material layer 12 and the second active material layer 13 should not be set too long, so it can be set to be less than 8 mm.

[0064] Thirdly, the present application provides an electrical device, including the secondary battery 1000 according to any one of the embodiments of the first aspect above.

[0065] Fourthly, the present application also provides a method for preparing the negative electrode plate 10. Please refer to Figure 8 and Figure 9 wherein Figure 8 shows the structure of the negative electrode plate roll 220, Figure 9 shows the structure of cutting the negative electrode plate roll 220 to form the negative electrode plate 10. The method includes:

[0066] Providing the negative electrode current collector 11, along the thickness direction of the negative electrode current collector 11, the negative electrode current collector 11 has a first surface 11a and a second surface 11b which are oppositely arranged;

[0067] Setting a continuous first active material layer 12 on the first surface 11a;

[0068] Along the length direction (the second direction X) of the negative electrode current collector 11, arranging a plurality of second active material layers 13 at intervals on the second surface 11b, and an interval region 260 is formed between two adjacent second active material layers 13;

[0069] In the interval region 260, cutting the negative electrode current collector 11 and the first active material layer 12 to form a plurality of negative electrode plates 10; wherein, in the interval region 260, at least one first empty current collector region 111 of the negative electrode plate 10 is formed on the first surface 11a. It can be understood that in the interval region 260, a second empty current collector region 112 of another negative electrode plate 10 may also be formed on the first surface 11a.

[0070] When cold-rolling the negative electrode plate 10 prepared by the above method, overpressure of the negative electrode plate 10 can be effectively reduced, thereby reducing damage and fracture of the negative electrode current collector 11; compared with the traditional scheme, there is no need to reduce the compaction density of the first active material layer 12 and / or the second active material layer 13, and the negative electrode plate 10 can be ensured to have a high energy density.

[0071] In the embodiments of the present application, a cold-rolling test is carried out on the negative electrode plate:

[0072] Example 1

[0073] For the convenience of understanding the technical concept and technical effect of the present application, the following takes a lithium-ion battery as an example for experimental illustration.

[0074] Experiment 1: [Preparation of large wound negative electrode sheets]

[0075] Example 1

[0076] Artificial graphite (specific capacity = 350 mAh / g), carboxymethyl cellulose (CMC, weight average molecular weight 9.0×10 5 ), binder styrene-butadiene rubber (SBR, weight average molecular weight 5×10 6 ) are mixed in a mass ratio of 97.8:1.2:1, and then deionized water is added as a solvent, and stirred in a vacuum mixer until a first slurry with a solid content of 50 wt% and a uniform system is obtained.

[0077] Artificial graphite (specific capacity = 362 Ah / g, temperature coefficient = 97.5%), carboxymethyl cellulose (CMC, weight average molecular weight 9.0×10 5 ), binder styrene-butadiene rubber (SBR, weight average molecular weight 5×10 6 ) are mixed in a mass ratio of 97.5:1.2:1.3, and then deionized water is added as a solvent, and stirred in a vacuum mixer until a second slurry with a solid content of 50 wt% and a uniform system is obtained.

[0078] The first slurry is uniformly coated on one surface in the thickness direction of a negative electrode current collector copper foil with a thickness of 8 μm (tensile strength = 500 MPa) through a single-chamber extrusion coating die head, and dried at 90 °C to obtain a large wound negative electrode sheet coated with a first active material layer; the second slurry is uniformly coated on the other surface in the thickness direction of the negative electrode current collector copper foil, and dried at 90 °C to obtain a large wound negative electrode sheet coated with a first active material layer and a second active material layer. The thickness of the first active material layer and the second active material layer before cold pressing is 110 μm. After baking the negative electrode sheet at 290 °C for 10 h, it is cooled to room temperature in an environment with humidity <5%.

[0079] Among them, the active material layers on both sides of the negative electrode current collector are arranged in a structure with both ends misaligned in the length direction when the negative electrode current collector is unfolded. Specifically, the length of the first end is 60 mm (the length L 1 of the first end overlapping with the first empty current collector area = 60 mm), and the length of the second end is 3 mm (the length L 2 of the second end overlapping with the second empty current collector area = 3 mm).

[0080] Examples 2 to 13: Except for the length L 1and / or the length L of the second single-sided coating portion 2 Except for the different lengths, the other parameters are the same as those in Embodiment 1. For details, please refer to Table 1 below.

[0081] In Comparative Example 1, the active material layers on both sides of the current collector were aligned, that is, L 1 = 0 mm, L 2 = 0 mm.

[0082] Belt breakage rate test method: Take about 1000 meters of the wound and coated negative electrode sheet, perform over-roll cold pressing in a cold press, and set the compaction density of the electrode sheet to 1.7 g / cm 3 , the total number of meters Y before cold pressing, and the number of meters lost due to belt breakage during cold pressing is Y'. Then the cold pressing belt breakage rate = Y' / Y × 100%.

[0083] Table 1

[0084]

[0085] According to Table 1 above, in combination with Comparative Example 1 and Embodiments 1 to 8, when the two ends of the active material layer on the negative electrode sheet are arranged in a staggered manner, the belt breakage rate of the negative electrode sheet during the production of large rolls can be effectively reduced, and the continuity of large roll production and product quality can be improved. When L 1 is the same, the belt breakage rates of Embodiments 1, 3, and 4 are significantly lower than that of Embodiment 2. It can be seen that setting the staggered length to more than 3 mm can further reduce the belt breakage rate. The belt breakage rates of Embodiments 5-8 are not further improved compared with Embodiments 1, 3, and 4. However, in Embodiments 1, 3, and 4, the staggered length is shorter and the impact on the energy density is smaller. Therefore, in this application, the preferred staggered length is 3 mm to 8 mm, which can further reduce the belt breakage rate of the negative electrode sheet during production and take into account the energy density. It should be noted that for the belt breakage during the production of the negative electrode sheet, the influence laws of L 1 and L 2 on the belt breakage rate are the same.

[0086] Experiment 2: [Thermal Pressing Pass Rate Test of Lithium-Ion Batteries]

[0087] Embodiment 14

[0088] <Preparation of Negative Electrode Sheet>

[0089] By cutting the large roll of the negative electrode sheet in Embodiment 1, a negative electrode sheet with a single-sided coating thickness of 90 μm and a width × length of 76 mm × 860 mm was obtained. A negative electrode tab slot with a size of 12 mm × 35 mm was opened on the negative electrode sheet by laser, and a negative electrode tab with a thickness of 110 μm and a width × length of 8 mm × 55 mm was welded in the negative electrode tab slot.

[0090] <Preparation of the positive electrode plate>

[0091] Mix the positive electrode active material lithium iron phosphate, the positive electrode conductive agent acetylene black, and the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight of 5×10 5 ) in a mass ratio of 94:3:3, add N-methylpyrrolidone (NMP) as a solvent, and stir in a vacuum mixer until a positive electrode slurry with a solid content of 75 wt% and a homogeneous system is obtained. Coat the positive electrode slurry evenly on one surface of a positive electrode current collector aluminum foil with a thickness of 6 μm, and dry it at 90°C to obtain a positive electrode plate with a single-sided coated positive electrode active material layer (thickness 80 μm). Then, repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode plate with a double-sided coated positive electrode active material layer. After cold pressing and slitting, a 10 mm×35 mm positive electrode tab slot is opened on the positive electrode plate by laser, and a positive electrode tab with a thickness of 116 μm and a width×length of 5 mm×55 mm is welded in the positive electrode tab slot to obtain a positive electrode plate with a specification of 74 mm×851 mm for standby.

[0092] <Separator> Use a polyethylene (PE) porous film with a thickness of 8 μm as the separator.

[0093] <Preparation of the electrolyte>

[0094] In a dry argon atmosphere, mix ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 30:50:20 to obtain an organic solution, and then add lithium hexafluorophosphate as a lithium salt to the organic solvent, dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0095] <Preparation of the lithium-ion battery>

[0096] Stack the above-prepared separator, positive electrode plate, separator, and negative electrode plate in sequence, and wind them to obtain an electrode assembly. In the electrode assembly, the first active material layer of the negative electrode plate faces away from the winding center plane, the second active material layer faces the winding center plane, and the first end is arranged close to the winding center with the first empty foil area (i.e., the starting end of the negative electrode plate winding). Apply a thermal pressure of 5 MPa, a temperature of 65°C, and a pressure holding time of 10 s to the electrode assembly. Place the electrode assembly in an outer packaging aluminum-plastic film, remove moisture at 80°C, inject the electrolyte and seal it, and obtain a lithium-ion battery through processes such as formation, degassing, and shaping.

[0097] Example 15: On the basis of Example 14, use a laser to open a 14 mm×40 mm first avoidance groove on the second active material layer of the negative electrode plate, and stick a 20 mm×50 mm green glue in the positive electrode tab area.

[0098] Example 15: On the basis of Example 14, a first avoidance groove with a size of 14 mm × 40 mm is formed on the second active material layer of the negative electrode tab by laser, and a green adhesive tape with a size of 20 mm × 50 mm is pasted on the positive electrode tab area.

[0099] Example 16: On the basis of Example 15, a second avoidance groove with a size of 14 mm × 40 mm is formed on the active material layer of the positive electrode tab opposite to the negative electrode tab by laser.

[0100] Comparative Example 2: The large roll of the negative electrode tab in Comparative Example 1 is cut to obtain a negative electrode tab suitable for manufacturing a lithium-ion battery. The remaining parameters are the same as those in Example 14 and will not be described herein again.

[0101] Select 10 batteries prepared in Examples 14 - 16 and Comparative Example 2 respectively. Measure the maximum thickness of each battery with a micrometer and calculate the average value. Then conduct a hot pressing test experiment (hot pressing conditions: hot pressing pressure 0.25 MPa, temperature 65 °C, time 4 s). After the experiment, disassemble the lithium-ion batteries and record the number of damaged negative current collectors as shown in Table 2 below.

[0102] Table 2

[0103]

[0104] It can be seen from Comparative Example 2 and Examples 14 - 16 that using a negative electrode tab with both ends of the active material layer misaligned in a lithium-ion battery can effectively improve the passing rate of the lithium-ion battery in the hot pressing experiment, thereby improving the safety performance of the lithium-ion battery. Forming the first avoidance groove and the second avoidance groove can reduce the maximum thickness of the battery to a certain extent and improve the energy density.

[0105] The above are only the embodiments of the present application, and thus do not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present application, or directly or indirectly applied in other related technical fields, shall be equally included in the patent protection scope of the present application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector, a first active material layer and a second active material layer, wherein along the thickness direction of the negative electrode current collector, the negative electrode current collector has a first surface and a second surface arranged opposite to each other, the first active material layer is arranged on the first surface, and the second active material layer is arranged on the second surface, characterized in that: Along the length direction of the negative electrode sheet, one end of the first active material layer is a first end portion, and the second surface has a first empty current collector area; Observed along the thickness direction of the negative electrode sheet, the projection of the first end portion at least partially overlaps with the first empty current collector area; Along the length direction of the negative electrode sheet, the first active material layer further includes a second end portion arranged opposite to the first end portion, and the second surface further includes a second empty current collector region; Observed along the thickness direction of the negative electrode sheet, the projection of the second end portion at least partially overlaps with the second empty current collector area.

2. The negative electrode sheet according to claim 1, characterized in that: Observed along the thickness direction of the negative electrode sheet and along the length direction of the negative electrode sheet, the overlapping length of the first end portion and the first empty foil area is L1 mm, and 3 mm ≤ L1 ≤ 100 mm.

3. The negative electrode sheet according to claim 1, characterized in that: Observed along the thickness direction of the negative electrode sheet and along the length direction of the negative electrode sheet, the overlapping length of the second end portion and the second empty foil area is L2 mm, and 3 mm ≤ L2 ≤ 100 mm.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that: The first active material layer includes a first active material, and the second active material layer includes a second active material, wherein the gram capacity of the second active material is greater than the gram capacity of the first active material.

5. The negative electrode sheet according to claim 4, characterized in that: The gram capacity of the first active material is 345 mAh / g to 355 mAh / g, and the gram capacity of the second active material is 358 mAh / g to 365 mAh / g.

6. A secondary battery, comprising a housing and an electrode assembly, wherein the electrode assembly is accommodated in the housing, characterized in that: The electrode assembly comprises a positive electrode sheet, a separator and a negative electrode sheet as claimed in any one of claims 1 to 5; The positive electrode sheet, the separator and the negative electrode sheet are stacked and wound, the first surface is arranged away from the winding center, and the second surface is arranged facing the winding center.

7. The secondary battery according to claim 6, characterized in that: The positive electrode sheet comprises a positive electrode current collector, a third active material layer and a fourth active material layer, wherein the third active material layer is arranged on a surface of the positive electrode current collector facing the second active material layer, and the fourth active material layer is arranged on a surface of the positive electrode current collector facing away from the second active material layer; The positive electrode current collector is electrically connected to the positive electrode tab, and a first avoidance groove is provided on the second active material layer; Observing along the thickness direction of the positive electrode current collector, the projection of the positive electrode tab falls into the first avoidance groove.

8. The secondary battery according to claim 7, characterized in that: The negative electrode current collector is electrically connected to the negative electrode tab, and a second avoidance groove is provided on the third active material; Observing along the thickness direction of the negative electrode current collector, the projection of the negative electrode tab falls into the second avoidance groove.

9. The secondary battery according to any one of claims 6 to 8, characterized in that: The first end portion and the first empty foil area are arranged close to the winding center, and when viewed along the thickness direction of the negative electrode sheet, the first end portion and the first empty foil area overlap by a length of L1 mm, 60 mm ≤ L1 ≤ 100 mm; The first active material layer also has a second end portion arranged away from the winding center, and the second surface has a second empty foil area away from the winding center. When observed along the thickness direction of the negative electrode sheet, the overlapping length of the second end portion and the second empty foil area is L2mm, 3mm≤L2≤8mm.

10. An electrical device, characterized in that: A secondary battery comprising any one of claims 6 to 9.

11. A method for preparing a negative electrode sheet according to any one of claims 1 to 5, characterized in that: include: Providing a negative electrode current collector, wherein along a thickness direction of the negative electrode current collector, the negative electrode current collector has a first surface and a second surface that are arranged opposite to each other; Disposing a continuous first active material layer on the first surface; A plurality of second active material layers are arranged at intervals on the second surface along the length direction of the negative electrode current collector, and a spacing region is formed between two adjacent second active material layers; In the spacing area, the negative electrode current collector and the first active material layer are cut to form a plurality of negative electrode sheets; wherein, in the spacing area, at least one first empty current collector area of ​​the negative electrode sheet is formed on the first surface.

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