Negative electrode sheet and preparation method therefor, and secondary battery and electric device

Abstract

A negative electrode sheet and a preparation method therefor, and a secondary battery and an electric device. The negative electrode sheet comprises a negative electrode current collector, a first active material layer and a second active material layer. In the direction of thickness of the negative electrode current collector, the negative electrode current collector has a first surface and a second surface opposite each other, the first active material layer being arranged on the first surface, and the second active material layer being arranged on the second surface. In the direction of length of the negative electrode sheet, one end of the first active material layer is a first end, and the second surface is provided with a first bare current collector area. When viewed in the direction of thickness of the negative electrode sheet, the projection of the first end at least partially overlaps the first bare current collector area. There is no active material layer at the first end corresponding to the second surface. Therefore, the overall thickness of the negative electrode sheet at the first end can be reduced, such that, during cold rolling of the negative electrode sheet, over-compression in the negative electrode sheet can be effectively reduced, thereby reducing damage and breakage to the negative electrode current collector.

Description

Negative electrode sheet and preparation method thereof, secondary battery and electric device Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode plate and 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, and are widely used in various energy storage fields. With the continuous iterative development of consumer lithium-ion batteries in recent years, the market's requirements for battery energy density are getting higher and higher.

[0003] Improving energy density and enhancing fast-charging performance are among the goals of the lithium-ion battery industry. Increasing the active material loading on the electrode not only improves energy density but also reduces costs. Double-layer coating technology has been developed for this purpose. However, during cold roller pressing, double-layer coating is prone to overpressure due to the presence of active material layers on both sides, causing damage to the electrode collector and resulting in breakage during subsequent electrode processing. To ensure subsequent processing, the compaction density is often reduced, resulting in a loss of energy density and defeating the purpose of double-layer coating to increase energy density.

[0004] Summary of the Invention

[0005] The purpose of this application is to provide a negative electrode plate and 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.

[0006] According to the first aspect of the present application, a negative electrode plate is provided, comprising 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 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. Along the length direction of the negative electrode plate, one end of the first active material layer is a first end portion, and the second surface has a first empty current collector area. When observed along the thickness direction of the negative electrode plate, 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 plate, the first active material layer also includes a second end portion arranged opposite to the first end portion, and the second surface also has a second empty current collector area. When observed along the thickness direction of the negative electrode plate, the projection of the second end portion at least partially overlaps with the second empty current collector area.

[0007] In the above technical solution, the projection of the first end at least partially overlaps with the first empty current collector area, and there is no active material layer at the location of the second surface corresponding to the first end, and there is no active material layer at the location of the second surface corresponding to the second end. That is, at the first and second ends of the negative electrode sheet, the negative active material layers on both sides of the negative current collector are staggered. In the length direction of the negative electrode sheet, the length of the first active material layer is longer than 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 sheet at the first end can be reduced, and when the negative electrode sheet is cold-rolled, the overpressure on the negative electrode sheet can be effectively reduced, thereby reducing damage and fracture of the negative electrode collector. Compared with traditional solutions, in this application, there is no need to reduce the compaction density of the first active material layer and / or the second active material layer, which can ensure that the negative electrode sheet has a higher energy density. The empty current collector area described in this application refers to an area on a surface of the current collector where no active material layer is provided.

[0008] In some preferred embodiments, when observed along the thickness direction of the negative electrode sheet, the length of the overlap between the first end and the first empty foil area along the length direction of the negative electrode sheet is L1mm, and 3mm≤L1≤100mm. By setting the first end of the first active material layer and the second active material layer to be offset by 3mm or more, the negative electrode sheet can avoid end alignment during the cold rolling process, thereby preventing the negative electrode current collector from being damaged due to excessive force at the ends of each active material layer, while reducing the difficulty of process control; setting the first end of the first active material layer and the second active material layer to be offset by 100mm or less can avoid excessive loss of energy density of the secondary battery. It is worth noting that the technical effect of the above-mentioned L1 range selection is the preferred technical effect of this application. As long as the first end of the first active material layer and the second active material layer are offset, the basic technical effect of reducing the overvoltage of the negative electrode sheet of this application can be achieved. The same applies to the preferred embodiments below.

[0009] In some preferred embodiments, as viewed along the thickness direction of the negative electrode sheet, the length of overlap between the second end portion and the second hollow foil region along the length of the negative electrode sheet is L2 mm, with 3 mm ≤ L2 ≤ 100 mm. By setting the offset length of the second end portion of the first active material layer and the second active material layer to 3 mm or greater, the negative electrode sheet can be prevented from aligning its ends during cold rolling, thereby preventing damage to the negative electrode current collector due to excessive force at the ends of each active material layer and reducing the difficulty of process control. Setting the offset length of the second end portion of the first active material layer and the second active material layer to 100 mm or less can avoid excessive loss of energy density in the secondary battery.

[0010] 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 gram capacity of the second active material is greater than the gram capacity of the first active material. Because there is no active material layer at the second surface corresponding to the first end and the second end, that is, the second active material layer is longer than the first active material layer, the gram capacity of the second active material is greater than the gram capacity of the first active material, which can compensate for capacity loss and increase the capacity of the secondary battery.

[0011] In some preferred embodiments, 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.

[0012] In a second aspect, the present application further provides a secondary battery comprising a housing and an electrode assembly, the electrode assembly being housed within the housing and comprising a positive electrode sheet, a separator, and a negative electrode sheet as described in any of the embodiments of the first aspect. The positive electrode sheet, separator, and negative electrode sheet are stacked and wound, with a first surface facing away from the winding center and a second surface facing the winding center.

[0013] In some preferred embodiments, the positive electrode plate includes a positive electrode collector, a third active material layer and a fourth active material layer, the third active material layer is arranged on the surface of the positive electrode collector facing the second active material layer, and the fourth active material layer is arranged on the surface of the positive electrode collector away from the second active material layer. The positive electrode collector is electrically connected to the positive electrode tab, and a first avoidance groove is provided on the second active material layer. At the position of the positive electrode collector located at the positive electrode tab, when observed along the thickness direction of the positive electrode collector, the projection of the positive electrode tab falls into the first avoidance groove. Since the corresponding negative electrode plate has a first avoidance groove to avoid the positive electrode tab, the problem of increasing the thickness of the positive electrode plate after connecting the positive electrode tab and affecting the battery energy density can be reduced. Since the second active material of the negative electrode plate has a first avoidance groove, in order to prevent lithium plating, a protective adhesive layer can be affixed to the positive electrode tab area, and the projected area of ​​the protective adhesive layer completely covers the first avoidance groove.

[0014] In some preferred embodiments, 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. At the location of the negative electrode current collector at the negative electrode tab, the projection of the negative electrode tab, as viewed along the thickness of the negative electrode current collector, falls into the second avoidance groove. This can mitigate the problem of increased thickness of the positive electrode sheet after connection to the negative electrode tab, thereby reducing the loss of energy density in the battery. Furthermore, the second avoidance groove is provided at the location of the positive electrode tab corresponding to the negative electrode tab, reducing the excess volume of the positive electrode active material layer, thereby ensuring that the negative electrode tab has sufficient excess volume to absorb lithium ions released from the positive electrode, thereby reducing lithium deposition in the battery.

[0015] In some preferred embodiments, the first end portion and the first hollow foil region are positioned near the winding center. When viewed along the thickness direction of the negative electrode sheet, the first end portion and the first hollow foil region overlap by a length of L1 mm, with a range of 60 mm ≤ L1 ≤ 100 mm. The first active material layer further comprises a second end portion positioned away from the winding center, and the second surface comprises a second hollow foil region away from the winding center. When viewed along the thickness direction of the negative electrode sheet, the second end portion and the second hollow foil region overlap by a length of L2 mm, with a range of 3 mm ≤ L2 ≤ 8 mm.

[0016] In a third aspect, the present application further provides an electrical device comprising the secondary battery of any embodiment of the second aspect above.

[0017] In a fourth aspect, the present application further proposes a method for preparing a negative electrode sheet as in any embodiment of the first aspect, comprising:

[0018] Providing a negative electrode current collector, wherein the negative electrode current collector has a first surface and a second surface opposite to each other along a thickness direction of the negative electrode current collector;

[0019] providing a continuous first active material layer on the first surface;

[0020] 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, with a spacing region formed between two adjacent second active material layers;

[0021] 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.

[0022] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the following description through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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 are represented as similar elements, and unless otherwise stated, the dimensions in the drawings do not constitute proportional limitations.

[0024] FIG1 shows a schematic structural diagram of a negative electrode sheet in some embodiments of the present application;

[0025] FIG2 shows a schematic diagram of the winding structure of an electrode assembly according to some embodiments of the present application;

[0026] FIG3 shows a schematic structural diagram of a negative electrode sheet in some embodiments of the present application;

[0027] FIG4 shows a schematic structural diagram of a secondary battery in some embodiments of the present application;

[0028] FIG5 is a partial enlarged view of point A in FIG2;

[0029] FIG6 is a partial enlarged view of point B in FIG2;

[0030] FIG7 is a partial enlarged view of point C in FIG2;

[0031] FIG8 is a schematic diagram of the structure of a negative electrode sheet roll;

[0032] Figure 9 is a schematic diagram of the cutting and preparation of the negative electrode sheet.

[0033] Explanation of reference numerals: 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 portion; 122, second end portion; 13, second active material layer; 1000, secondary battery; 100, housing; 200, electrode assembly; 210, positive electrode sheet; 211, positive electrode current collector; 212, third active material layer; 213, fourth active material layer; 2121, second avoidance groove; 220, negative electrode sheet roll; 11, negative electrode current collector; 12, first active material layer; 13, second active material layer; 131, first avoidance groove; 230, separator; 240, positive electrode tab; 250, negative electrode tab; 260, spacing region; X, second direction; Y, third direction; Z, first direction. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly described below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

[0035] References to "embodiments" in this application mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments.

[0036] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0037] The technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0038] On the first aspect, the present application proposes a negative electrode plate. Please refer to Figure 1. The negative electrode plate 10 includes a negative electrode current collector 11, a first active material layer 12 and a second active material layer 13, wherein the first active material layer 12 and the second active material layer 13 are respectively arranged on the two surfaces of the current collector in the thickness direction (first direction Z).

[0039] As 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 a 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 battery charging and discharging process. For example, the negative electrode current collector 11 can be made of a metal material with high strength, and specifically can be made of copper foil, nickel foil or polymer copper foil (the surface of the copper foil is provided with a high molecular polymer such as polyethylene, polypropylene or polyamide) that is flat and strip-shaped. The negative electrode current collector 11 provides a support and fixed platform for the active material, so that the active material can be evenly distributed on the negative electrode current collector 11, ensuring the structural stability of the negative electrode plate 10.

[0040] Regarding the above-mentioned first active material layer 12 and second active material layer 13, please refer to Figure 1. Along the thickness direction (first direction Z) of the negative electrode current collector 11, the negative electrode current collector 11 includes a first surface 11a and a second surface 11b arranged opposite each other. The first active material layer 12 can be arranged on the first surface 11a, and the second active material layer 13 can be arranged 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 battery charging and discharging process, thereby realizing the conversion between electrical energy and chemical energy. Taking lithium-ion batteries as an example, during the battery charging process, lithium ions are released 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 battery discharging process, lithium ions are released 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 battery's charge and discharge cycle.

[0041] The first active material layer 12 includes a first active material, a conductive agent, and a binder. These materials are mixed, stirred, and evenly coated on the surface of the negative electrode current collector 11 to form the first active material layer 12. 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.

[0042] In the embodiment of the present application, the negative electrode sheet 10 using a double-layer active material layer can effectively improve the energy density of the battery. At the ends of the first active material layer 12 and / or the second active material layer 13 (the two ends in the length direction of the negative electrode sheet 11, that is, the two ends in the second direction X in Figure 1), a protrusion structure is likely to appear. In particular, when the ends of the first active material layer 12 and the second active material layer 13 overlap, their protrusion structures may also overlap, resulting in an increase in the thickness of the negative electrode sheet 10 at each end of the active material layer. When the negative electrode sheet 10 is cold-rolled, it is easy to cause overpressure on the negative electrode collector 11, causing damage to the negative electrode collector 11, so that the negative electrode sheet 10 breaks during subsequent processing.

[0043] To alleviate the above-mentioned problem, along the length direction (second direction X) of the negative electrode sheet 10, one end of the first active material layer 12 is a first end portion 121, and the second surface 11b has a first empty current collector area 111. When viewed along the thickness direction (first direction Z) of the negative electrode sheet 10, the projection of the first end portion 121 at least partially overlaps with the first empty current collector area 111. Along the length direction (second direction X) of the negative electrode sheet 10, the first active material layer 12 also includes a second end portion 122 disposed opposite the first end portion 121, and the second surface 11b also has a second empty current collector area 112 disposed opposite the first empty current collector area 111. When viewed along the thickness direction (first direction Z) of the negative electrode sheet 10, the projection of the second end portion 122 at least partially overlaps with the second empty current collector area 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 sheet 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 sheet 10, the active material layers on both sides of the negative electrode current collector 11 are misaligned. This can reduce the overall thickness of the negative electrode sheet 10 at the first end 121 and the second end 122. When the negative electrode sheet 10 is cold-rolled, it can effectively reduce overpressure on the negative electrode sheet 10, thereby reducing damage and fracture of the negative electrode current collector 11. At the same time, there is no need to reduce the compaction density of the first active material layer 12 and / or the second active material layer 13, so as to ensure that the negative electrode sheet 10 has a high energy density. Among them, the empty current collector area mentioned in this application refers to an area on a surface of the current collector where no active material layer is provided.

[0044] Referring to Figure 1 , in some embodiments, when viewed along the thickness direction (first direction Z) of the negative electrode sheet 10 , the overlap length of the first end portion 121 and the first hollow foil area along the length direction (second direction X) of the negative electrode sheet 10 is L1 mm, with 3 mm ≤ L1 ≤ 100 mm. By setting the offset length of the first end portion 121 of the first active material layer 12 and the second active material layer 13 to 3 mm or greater, the negative electrode sheet 10 can be prevented from aligning its ends during the cold rolling process, thereby preventing damage to the negative electrode current collector 11 due to excessive force at the ends of each active material layer. This also reduces the difficulty of process control; setting the offset length of the first end portion 121 of the first active material layer 12 and the second active material layer 13 to 100 mm or less can reduce excessive loss of secondary battery energy density. It is worth noting that the technical effect of the above-mentioned L1 range selection is the preferred technical effect of this application. As long as the first end 121 of the first active material layer 12 and the second active material layer 13 are staggered, the basic technical effect of reducing the overvoltage of the negative electrode plate 10 of this application can be achieved. The same applies to the preferred implementation methods below.

[0045] Referring to Figure 1 , when viewed along the thickness direction (first direction Z) of the negative electrode sheet 10, along the length direction (second direction Y) of the negative electrode sheet 10, the overlap length between the second end portion 122 and the second empty current collector area 112 is L2 mm, with 3 mm ≤ L2 ≤ 100 mm. By setting the offset length of the second end portion 122 of the first active material layer 12 and the second active material layer 13 to 3 mm or greater, the negative electrode sheet 10 can be prevented from aligning its ends during the cold rolling process, thereby preventing damage to the negative electrode current collector 11 due to excessive force at the ends of each active material layer. This also reduces the difficulty of process control; setting the offset length of the second end portion 122 of the first active material layer 12 and the second active material layer 13 to 100 mm or less can avoid excessive loss of secondary battery energy density.

[0046] For example, when the above-mentioned negative electrode plate 10 is used in a laminated electrode assembly 200, the values ​​of L1 and L2 can be relatively small, for example, 3mm≤L1≤8mm, 3mm≤L2≤8mm. Since the offset length of the first active material layer 12 and the second active material layer 13 (that is, the overlapping length of the first end 121 and the first empty current collector area 111) is 3mm, it can effectively improve the problem that the negative electrode current collector 11 is easily crushed during rolling. In order to reduce the difficulty of processing, the overlapping length of the first end 121 and the first empty current collector area 111 can be appropriately increased. In order to ensure that the negative electrode plate 10 can have a high energy density, the overlapping length should not be too long, so it can be set to less than 8mm, which can reduce the crushing of the negative electrode current collector 11 while ensuring that the negative electrode plate 10 has a high energy density.

[0047] When the above-described negative electrode sheet 10 is used in a wound electrode assembly 200, the length of the first end 121 or the second end 122 closer to the winding center can be greater, while the length of the one farther from the winding center can be smaller. As an example, referring to FIG. 2 , the first end 121 is closer to the winding center, and its length L1 satisfies 60 mm ≤ L1 ≤ 100 mm. Near the winding center of the electrode assembly 200, because there is no corresponding positive electrode sheet 210 on the inner side of the negative electrode sheet 10 facing the winding center, the active material on the inner side of the negative electrode sheet 10 cannot participate in the electrochemical reaction. Therefore, to reduce the weight of the electrode assembly 200 and increase its energy density, the active material layer is provided on only one side of the first winding of the negative electrode sheet 10, and the inner side is generally provided as an empty current collector area, the length of which can be set to 60 mm to 100 mm.

[0048] Near the winding center of the electrode assembly 200, there is a corresponding positive electrode sheet 210 on the outside of the negative electrode sheet 10 away from the winding center. An active material layer is provided on the outside of the negative electrode sheet 10, which can participate in electrochemical reactions with the active material layer on the positive electrode sheet 210, that is, the first end 121 and the first empty current collector area 111 can at least partially overlap, so the overlapping length can be set to 60mm to 100mm.

[0049] The second end 122 is distal from the winding center, and its length L2 satisfies 3mm≤L2≤8mm. Away from the winding center, a corresponding positive electrode sheet 210 may exist on either the inner side of the negative electrode sheet 10 facing the winding center or the outer side facing away from the winding center. Therefore, the empty current collector area of ​​the negative electrode sheet 10 at the end distal from the winding center should not be too large. A 3mm overlap between the second end 122 and the second empty current collector area 112 effectively mitigates the problem of the negative electrode collector 11 being easily crushed during cold rolling. To reduce processing difficulty, the overlap between the first end 121 and the first empty current collector area 111 can be appropriately increased. To ensure that the negative electrode sheet 10 has a high energy density, the overlap length should not be too long and can be set to less than 8mm. This reduces the risk of crushing the negative electrode collector 11 while ensuring a high energy density. When the second end 122 is close to the winding center, its length L2 can also satisfy 60mm≤L2≤100mm.

[0050] 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, wherein the gram capacity of the second active material is greater than the gram capacity of the first active material. Using the second active material with a larger gram capacity in the second active material layer 13 can increase the energy density of the battery. For example, 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. Both the first and second active materials can be graphite, soft carbon, hard carbon, or carbon fiber, as described above. Taking graphite as an example, the gram capacity of different graphites may vary. For example, graphite with high crystallinity generally has a higher gram capacity because its crystal structure is more ordered, which facilitates the insertion and extraction of lithium ions. Smaller grain size can also provide more active surface area, promoting electrochemical reactions and thus increasing gram capacity. Furthermore, the purity and surface morphology of the graphite also affect the gram capacity. Graphite with high purity generally has better electrochemical performance, while graphite that has been surface treated or modified can improve its wettability and reactivity with the electrolyte, thereby increasing gram capacity.

[0051] In some embodiments, the first active material layer 12 can be a single-layer coating structure, or a double-layer or multi-layer coating structure. As an example, referring to Figure 3, the first active material layer 12 is a double-layer coating structure, and 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 a structure of a first coating layer 12a and a second coating layer 12b with different kinetic properties, the kinetic 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 and 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.

[0052] Secondly, the present application also proposes a secondary battery 1000. Referring to Figures 2 and 4, the secondary battery 1000 includes a housing 100 and an electrode assembly 200, which is housed within the housing 100. The electrode assembly 200 includes a positive electrode sheet 210, a separator 230, and a negative electrode sheet 10, which are stacked and wound.

[0053] The first surface 11a of the negative electrode current collector 11 is positioned away from the winding center, while the second surface 11b faces the winding center. After the negative electrode sheet 10 is wound, the first surface 11a faces away from the winding center and represents the long side of the negative electrode, thus allowing for the use of the larger first active material layer 12. The second surface 11b faces the winding center and represents the short side of the negative electrode, thus allowing for the use of the smaller second active material layer 13.

[0054] Referring to Figures 2, 4, and 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 substrate of the positive electrode sheet 210 and can be an overall flat, strip-shaped aluminum foil. The third active material layer 212 and the fourth active material layer 213 are respectively disposed on two surfaces of the positive electrode current collector 211 in the thickness direction. For example, the third active material layer 212 is disposed 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 disposed on the surface of the positive electrode current collector 211 facing away from the second active material layer 13. The positive electrode current collector 211 is electrically connected to the positive electrode tab 240, and the electrical connection method can be welding, clamping, or conductive adhesive bonding.

[0055] The third active material layer 212 and the fourth active material layer 213 include a positive electrode active material, a conductive agent, and a binder. These components are mixed, stirred, and evenly coated on both surfaces of the positive electrode current collector 211 in the thickness direction, thereby forming 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 iron manganese phosphate, and cobalt-free materials.

[0056] Referring to FIG6 , a first avoidance groove 131 is defined in the second active material layer 13. At the location of the positive electrode tab 240 on the positive current collector 211, when viewed along the thickness direction (first direction Z) of the positive current collector 211, the projection of the positive electrode tab 240 falls into the first avoidance groove 131, i.e., the first avoidance groove 131 completely covers the positive electrode tab 240. For example, a first tab groove (not shown) is defined in the third active material layer 212. The positive electrode tab 240 is electrically connected to the positive current collector 211 within the first tab groove. When the positive electrode sheet 210, separator 230, and negative electrode sheet 10 are stacked and wound, even if the positive electrode tab 240 protrudes from the first tab groove, the first avoidance groove 131 provided on the corresponding negative electrode sheet 10 to avoid the positive electrode tab 240 can mitigate the problem of increased thickness of the positive electrode sheet 210 after connecting the positive tab 240, thereby reducing the battery's energy density.

[0057] Referring to Figure 7 , the negative electrode current collector 11 is electrically connected to the negative electrode tab 250. The negative electrode plate 10 can also be directly electrically connected to the negative electrode current collector 11 by welding, clamping, or conductive adhesive bonding. A second escapement groove 2121 is provided on the third active material layer of the positive electrode plate 210. At the location of the negative electrode tab 250 on the negative electrode current collector 11, when viewed along the thickness direction (first direction Z) of the negative electrode current collector 11, the projection of the negative electrode tab 250 falls into the second escapement groove 2121. In other words, the second escapement groove 2121 completely covers the negative electrode tab 250. This reduces the increase in thickness of the positive electrode plate 210 caused by connecting the negative electrode plate 10, thereby reducing the loss of battery energy density. Furthermore, the provision of the second escapement groove 2121 at the location of the positive electrode plate 210 corresponding to the negative electrode plate 10 reduces the excess positive electrode active material layer, thereby ensuring that the negative electrode plate 10 has sufficient excess to absorb lithium ions released from the positive electrode, thereby reducing lithium deposition in the battery.

[0058] The first end 121 and the first empty current collector area 111 are arranged near the winding center. When observed along the thickness direction (first direction Z) of the negative electrode plate 10, the overlapping length of the first end 121 and the first empty current collector area 111 is L1mm, 60mm≤L1≤100mm; at the winding center of the electrode assembly 200, since there is no corresponding positive electrode plate 210 on the inner side of the negative electrode plate 10, the active material located 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 increase the energy density of the electrode assembly 200, the inner side of the end of the negative electrode plate 10 near the winding center is usually set as an empty current collector area, that is, no active material layer is coated. The empty current collector area is wound on the inner circle of the electrode assembly 200, and its length can be 60mm to 100mm. Near the winding center of the electrode assembly 200, there is a corresponding positive electrode sheet 210 on the outside of the negative electrode sheet 10 away from the winding center. Therefore, an active material layer is arranged on the outside of the negative electrode sheet 10, which can participate in electrochemical reactions with the active material layer on the positive electrode sheet 210, that is, the active material layers on the two surfaces of the negative electrode current collector 11 are staggered at the first end 121.

[0059] The first active material layer 12 also has a second end portion 122 disposed away from the winding center, and the second surface 11b has a second empty current collector region 112 disposed away from the winding center. When viewed along the thickness direction (first direction Z) of the negative electrode sheet 10, the length of overlap between the second end portion 122 and the second empty foil region is L2 mm, and 3 mm ≤ L2 ≤ 8 mm. A 3 mm offset length between the first active material layer 12 and the second active material layer 13 can effectively alleviate the problem of the negative electrode current collector 11 being easily crushed during rolling. To reduce processing difficulty, the offset length of 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 sheet 10 has a high energy density, the offset length between the first active material layer 12 and the second active material layer 13 should not be too long, and therefore can be set to less than 8 mm.

[0060] In a third aspect, the present application proposes an electrical device, comprising the secondary battery 1000 according to any one of the embodiments of the first aspect.

[0061] Fourthly, the present application further proposes a method for preparing a negative electrode sheet 10. Please refer to Figures 8 and 9, where Figure 8 shows the structure of a negative electrode sheet roll 220, and Figure 9 shows the structure of cutting the negative electrode sheet roll 220 to form a negative electrode sheet 10. The method includes:

[0062] Providing a negative electrode current collector 11, wherein the negative electrode current collector 11 has a first surface 11a and a second surface 11b opposite to each other along a thickness direction of the negative electrode current collector 11;

[0063] A continuous first active material layer 12 is provided on the first surface 11 a ;

[0064] Along the length direction (second direction X) of the negative electrode current collector 11, a plurality of second active material layers 13 are arranged at intervals on the second surface 11b, and a spacing region 260 is formed between two adjacent second active material layers 13;

[0065] In the spacing region 260, the negative electrode current collector 11 and the first active material layer 12 are cut to form a plurality of negative electrode sheets 10. In the spacing region 260, the first surface 11a forms at least one first empty current collector region 111 of the negative electrode sheet 10. It is understood that in the spacing region 260, the first surface 11a may also form a second empty current collector region 112 of another negative electrode sheet 10.

[0066] The negative electrode sheet 10 prepared by the above method can effectively reduce overpressure on the negative electrode sheet 10 when the negative electrode sheet 10 is cold-rolled, thereby reducing damage and fracture of the negative electrode current collector 11; compared with traditional solutions, there is no need to reduce the compaction density of the first active material layer 12 and / or the second active material layer 13, which can ensure that the negative electrode sheet 10 has a higher energy density.

[0067] In the embodiment of the present application, the negative electrode is subjected to a cold pressure test:

[0068] Example 1

[0069] To facilitate understanding of the technical concept and technical effects of the present application, the following experimental description is given using a lithium-ion battery as an example.

[0070] Experiment 1: [Preparation of large negative electrode roll]

[0071] Example 1

[0072] Artificial graphite (gram capacity = 350 mAh / g), carboxymethyl cellulose (CMC, weight average molecular weight 9.0×10 5 ), binder styrene-butadiene rubber (SBR, weight average molecular weight of 5×10 6 ), mixed according to a mass ratio of 97.8:1.2:1, and then added with deionized water as a solvent, and stirred under the action of a vacuum stirrer to form a first slurry with a solid content of 50wt% and a uniform system.

[0073] Artificial graphite (gram 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 of 5×10 6 ), mixed according to a mass ratio of 97.5:1.2:1.3, and then added with deionized water as a solvent, and stirred under the action of a vacuum stirrer to obtain a second slurry with a solid content of 50 wt% and a uniform system.

[0074] The first slurry was evenly coated onto one surface of an 8μm-thick negative electrode current collector copper foil (tensile strength = 500MPa) in the thickness direction using a single-cavity extrusion coating die. The foil was then dried at 90°C to produce a large roll of negative electrode sheets coated with the first active material layer. The second slurry was evenly coated onto the other surface of the negative electrode current collector copper foil in the thickness direction and dried at 90°C to produce a large roll of negative electrode sheets coated with the first and second active material layers. The thickness of the first and second active material layers before cold pressing was 110μm. The negative electrode sheets were baked at 290°C for 10 hours and then cooled to room temperature in an environment with a humidity of <5%.

[0075] Among them, the active material layers on both sides of the negative electrode current collector are staggered at both ends in the length direction when the negative electrode current collector is unfolded. Specifically, the length of the first end is 60 mm (the length of the first end overlapping with the first empty current collector area L1 = 60 mm), and the length of the second end is 3 mm (the length of the second end overlapping with the second empty current collector area L2 = 3 mm).

[0076] Examples 2 to 13: Except for the length L1 of the first single-sided coating portion and / or the length L2 of the second single-sided coating portion, the other parameters are the same as those of Example 1. Please refer to Table 1 below for details.

[0077] In Comparative Example 1, the active material layers on both sides of the current collector are aligned, that is, L1 = 0 mm, L2 = 0 mm.

[0078] Belt breakage rate test method: Take about 1000 meters of the negative electrode sheet after winding and coating, and perform roller cold pressing in a cold press. The compaction density of the electrode sheet is set to 1.7g / cm 3 The total number of meters before cold pressing is Y, and the number of meters lost due to belt breakage during cold pressing is Y', so the cold pressing belt breakage rate = Y' / Y×100%.

[0079] Table 1

[0080] According to Table 1 above, combined with Comparative Example 1 and Examples 1 to 8, it can be seen that when the two ends of the active material layer on the negative electrode sheet are staggered, the breaking rate of the negative electrode sheet in the process of producing large rolls can be effectively reduced, and the continuity and product quality of large roll production can be improved. Under the same L1, the breaking rate of Example 1, Example 3, and Example 4 is significantly lower than that of Example 2. It can be seen that setting the staggered length to more than 3mm can further reduce the breaking rate. The breaking rate of Examples 5-8 is not further improved compared with Example 1, Example 3, and Example 4. However, in Example 1, Example 3, and Example 4, the staggered length is shorter and has less impact on energy density. Therefore, in this application, the preferred staggered length is 3mm to 8mm, which can further reduce the breaking rate of the negative electrode sheet in production and take into account energy density. It is worth noting that for the breaking of the negative electrode sheet in the production process, the influence of L1 and L2 on the breaking rate is the same.

[0081] Experiment 2: [Hot-pressure pass rate test of lithium-ion batteries]

[0082] Example 14

[0083] <Preparation of negative electrode sheet>

[0084] By cutting the large roll of negative electrode sheet in Example 1, a negative electrode sheet with a single-sided coating thickness of 90 μm and a width × length of 76 mm × 860 mm is obtained, a 12 mm × 35 mm negative electrode tab slot is 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 is welded in the negative electrode tab slot.

[0085] <Preparation of positive electrode sheet>

[0086] The positive electrode active material lithium iron phosphate, the positive electrode conductive agent acetylene black, the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight 5×10 5 ) were mixed in a mass ratio of 94:3:3, and N-methylpyrrolidone (NMP) was added as a solvent. The mixture was stirred in a vacuum mixer until a solid content of 75 wt% and a uniform system was obtained. The positive electrode slurry was evenly coated on one surface of a 6 μm thick positive electrode current collector aluminum foil and dried at 90°C to obtain a positive electrode sheet coated with a positive electrode active material layer (80 μm thick) on one side. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. After cold pressing and slitting, a 10 mm × 35 mm positive electrode tab slot was laser-cut on the positive electrode tab slot. A positive electrode tab with a thickness of 116 μm and a width × length of 5 mm × 55 mm was welded into the positive electrode tab slot to obtain a positive electrode sheet with a specification of 74 mm × 851 mm for future use.

[0087] <Separator> A polyethylene (PE) porous film with a thickness of 8 μm was used as the separator.

[0088] <Preparation of Electrolyte>

[0089] In a dry argon atmosphere, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain an organic solution, and then lithium hexafluorophosphate was added to the organic solvent to dissolve and mix uniformly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0090] <Preparation of lithium-ion batteries>

[0091] The separator, positive electrode sheet, separator, and negative electrode sheet prepared above were stacked in order and wound to form an electrode assembly. The first active material layer of the negative electrode sheet in the electrode assembly was facing away from the winding center plane, and the second active material layer was facing the winding center plane. The first end and the first hollow foil area were positioned near the winding center (i.e., the starting point of the negative electrode sheet winding). The electrode assembly was hot-pressed at a pressure of 5 MPa, a temperature of 65°C, and a holding time of 10 seconds. The electrode assembly was placed in an outer packaging aluminum-plastic film, dehydrated at 80°C, injected with electrolyte, and packaged. After the formation, degassing, and shaping process, a lithium-ion battery was obtained.

[0092] Example 15: Based on Example 14, a first avoidance groove of 14 mm × 40 mm was opened on the second active material layer of the negative electrode plate by laser, and a green glue of 20 mm × 50 mm was affixed to the positive electrode tab area.

[0093] Example 15: Based on Example 14, a first avoidance groove of 14 mm × 40 mm was opened on the second active material layer of the negative electrode plate by laser, and a green glue of 20 mm × 50 mm was affixed to the positive electrode tab area.

[0094] Example 16: Based on Example 15, a second avoidance groove of 14 mm × 40 mm is opened on the active material layer of the positive electrode sheet opposite to the negative electrode tab using a laser.

[0095] Comparative Example 2: A negative electrode sheet suitable for making a lithium-ion battery was obtained by cutting the large roll of negative electrode sheet in Comparative Example 1. The remaining parameters were the same as those in Example 14 and are not repeated here.

[0096] Ten batteries prepared in Examples 14-16 and Comparative Example 2 were selected, and the maximum thickness of each battery was measured using a micrometer, and the average value was calculated. Then, a hot pressing test experiment was performed (hot pressing conditions: hot pressing pressure 0.25 MPa, temperature 65°C, time 4 s). After the experiment was completed, the lithium-ion batteries were disassembled and the number of damaged negative electrode current collectors was recorded, as shown in Table 2 below.

[0097] Table 2

[0098] It can be seen from Comparative Example 2 and Examples 14-16 that the negative electrode sheets with staggered ends of the active material layer in lithium-ion batteries can effectively improve the pass rate of lithium-ion batteries in hot pressing tests, thereby improving the safety performance of lithium-ion batteries. The opening of 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.

[0099] The above description is merely an embodiment of the present application and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also 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. The negative electrode current collector has a first surface and a second surface disposed opposite each other along the thickness direction of the negative electrode current collector. The first active material layer is disposed on the first surface, and the second active material layer is disposed on the second surface. The negative electrode sheet is 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; When viewed 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 disposed opposite to the first end portion, and the second surface further includes a second empty current collector area; When viewed 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. 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 housed in the housing, wherein: The electrode assembly comprises a positive electrode sheet, a separator and a negative electrode sheet according to 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 includes 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 the surface of the positive electrode current collector facing the second active material layer, and the fourth active material layer is arranged 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 the positive electrode tab, and a first avoidance groove is opened on the second active material layer; When viewed 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, wherein: The negative electrode current collector is electrically connected to the negative electrode tab, and a second avoidance groove is opened on the third active material; When viewed 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. 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 L1 mm, where 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 length of overlap between the second end portion and the second empty foil area is L2 mm, 3 mm ≤ L2 ≤ 8 mm.

10. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to 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 the negative electrode current collector has a first surface and a second surface opposite to each other along a thickness direction of the negative electrode current collector; providing 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, with a spacing region 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.

Images