Secondary battery and electronic device

Abstract

The invention relates to a secondary battery and electronic equipment, and the secondary battery comprises an electrode assembly, and the electrode assembly comprises a diaphragm, a first pole piece and a second pole piece opposite to the first pole piece in polarity. The first pole piece comprises a first double-sided pole piece arranged between two adjacent second pole pieces along a first direction, the first double-sided pole piece comprises a first current collector and a first active layer arranged on the surface of the first current collector, and the first current collector comprises a first polymer layer and first metal layers arranged on two opposite surfaces of the first polymer layer. The first pole piece comprises a first single-face pole piece, and the pole piece on the outermost layer of the electrode assembly is the first single-face pole piece in the first direction. The first single-sided pole piece comprises a second metal layer and a second active layer arranged on the surface, facing the second pole piece, of the second metal layer, the thickness of the second metal layer is 8-20 microns, and the first single-sided pole piece does not comprise a polymer layer. According to the secondary battery and the electronic equipment, the energy density of the secondary battery can be improved, and the bending and rolling possibility of the pole piece can be reduced.

Description

Technical Field

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

[0002] As the power source of electronic devices, secondary batteries are the key to ensure the normal use of electronic devices. The current collector is an important component of the secondary battery. It not only provides support for the active material layer, but also collects the current generated by the active material layer for external output. Therefore, the current collector has an important influence on the performance of the electrode plate and the electrochemical device. In order to reduce the metal burrs caused by mechanical damage to the current collector, the existing secondary batteries usually use a composite current collector (a composite structure of metal-polymer-metal). The composite current collector uses polymers as a mechanical support skeleton to thin the metal layer to reduce the metal burrs caused by mechanical damage to the battery cell. The two metal layers of the composite current collector need to be electrically connected, generally by roller welding, and protective glue needs to be applied to the weld mark. When the composite current collector is applied to the outermost pole piece of the electrode assembly of the secondary battery, the thickness of the pole piece will be increased due to roller welding and protective glue, thereby reducing the energy density of the secondary battery. In addition, when the outermost electrode sheet of the electrode assembly of the secondary battery is a single-sided composite current collector electrode sheet (only one surface of the current collector is coated with an active material layer), it is easy to bend and curl after cold pressing, resulting in the inability to smoothly carry out subsequent processes. Therefore, it is necessary to improve the above problems. Summary of the invention

[0003] The purpose of the present application is to provide a secondary battery and an electronic device, aiming to improve the energy density of the secondary battery while reducing the problem of bending and curling of the pole pieces of the secondary battery.

[0004] According to the first aspect of the present application, a secondary battery is provided, comprising an electrode assembly, the electrode assembly comprising a diaphragm and a plurality of first pole pieces and a plurality of second pole pieces, the first pole pieces and the second pole pieces are alternately stacked along a first direction, the second pole pieces have opposite polarities to the first pole pieces, and a diaphragm is arranged between adjacent first pole pieces and second pole pieces. The plurality of first pole pieces comprise a first double-sided pole piece, along the first direction, a first double-sided pole piece is arranged between two adjacent second pole pieces, the first double-sided pole piece comprises a first current collector and a first active layer arranged on the surface of the first current collector, the first current collector comprises a first metal layer and a first polymer layer, along the thickness direction of the first current collector, the first polymer layer is arranged on both opposite surfaces. The plurality of first pole pieces comprise a first single-sided pole piece, along the first direction, the outermost pole piece of the electrode assembly is the first single-sided pole piece. The first single-sided pole piece comprises a second metal layer and a second active layer, the second active layer is arranged on the surface of the second metal layer facing the second pole piece, the thickness of the second metal layer is D1, 8μm≤D1≤20μm, and the first single-sided pole piece does not include a polymer layer.

[0005] In the above technical solution, along the first direction, the outermost electrode of the electrode assembly is the first single-sided electrode. The first single-sided electrode is provided with a second active layer on the surface of the second metal layer facing the second electrode, so that the first single-sided electrode does not need to be roll-welded or affixed with protective glue, thereby reducing the thickness of the first single-sided electrode. In addition, by limiting that the first single-sided electrode does not include a polymer layer, it is different from the composite current collector, and the thickness of the second metal layer can be not less than 8 microns (μm) and not more than 20 microns, so that the second metal layer meets a certain strength and is not too thick, and while reducing the problem of bending and curling of the first single-sided electrode, the energy density of the secondary battery will not be greatly reduced.

[0006] In some preferred embodiments, the second metal layer includes aluminum, 9μm≤D1≤20μm; or, the second metal layer includes copper, 8μm≤D1≤20μm. When the second metal layer includes aluminum, its thickness is not less than 9 microns, so that it meets certain strength and has good anti-bending and curling performance. When the second metal layer includes copper, due to the higher strength of copper, the second metal layer can be made thinner, and its thickness can be not less than 8 microns, so that it meets certain strength and has good anti-bending and curling performance.

[0007] In some preferred embodiments, the second metal layer includes aluminum, 9μm≤D1≤15μm; or, the second metal layer includes copper, 8μm≤D1≤15μm. When the second metal layer includes aluminum, its thickness is not higher than 15 microns, so that it will not be too thick, and there can be extra space in the thickness direction, so that the second active layer on the surface of the second metal layer can be set thicker to improve the energy density of the secondary battery. When the second metal layer includes copper, its thickness is not higher than 15 microns, so that it will not be too thick, so that it will not be too thick, and there can be extra space in the thickness direction, so that the second active layer on the surface of the second metal layer can be set thicker to improve the energy density of the secondary battery.

[0008] In some preferred embodiments, the second active layer is a positive electrode active material layer, and the thickness of the second active layer is D2, 30μm≤D2≤70μm; or, the second active layer is a negative electrode active material layer, and the thickness of the second active layer is D2, 30μm≤D2≤50μm. When the second active layer is a positive electrode active material layer, its thickness is not less than 30 microns and not more than 70 microns, so that it can provide more energy while not generating large internal stress due to excessive thickness, thereby reducing the possibility of bending and curling of the first single-sided pole piece. When the second active layer is a negative electrode active material layer, its thickness is not less than 30 microns and not more than 50 microns, so that it can provide more energy while not generating large internal stress due to excessive thickness, thereby reducing the possibility of bending and curling of the first single-sided pole piece.

[0009] In some preferred embodiments, along the length or width direction of the second pole piece, the second pole piece includes a first part and a second part arranged in sequence. Along the first direction, the first part overlaps with the second active layer. The second part exceeds the part of the second active layer in the length or width direction of the second pole piece. The electrode assembly includes a first insulating layer, and the first insulating layer is arranged on the surface of the second metal layer facing the second pole piece, and along the first direction, the first insulating layer and the second part at least partially overlap. By arranging the second part of the second pole piece to exceed the second active layer in the length or width direction, the possibility of lithium precipitation caused by the second pole piece being unable to migrate the lithium ions released by the first single-sided pole piece when the secondary battery is charged can be reduced. By arranging the first insulating layer on the surface of the second metal layer facing the second pole piece, and the first insulating layer at least partially overlaps with the second part, the first insulating layer can shield the part of the second metal layer where the second active layer is not arranged, so as to reduce the possibility of the second metal layer where the second active layer is not arranged directly contacting the active layer of the second pole piece and short-circuiting.

[0010] In some preferred embodiments, along the width direction of the first current collector, the width of the first insulating layer is L1, 1.5mm≤L1≤6mm; and / or, along the thickness direction of the first current collector, the thickness of the first insulating layer is D3, 15μm≤D3≤30μm. The width of the first insulating layer is not less than 1.5 mm and not more than 6 mm, so that it can better insulate the second metal layer without the second active layer and the active layer of the second pole piece without excessively reducing the energy density of the secondary battery. The thickness of the first insulating layer is not less than 15 microns and not more than 30 microns, so that it can better prevent the burrs of the second metal layer from piercing without excessively reducing the energy density of the secondary battery.

[0011] In some preferred embodiments, a plurality of first pole pieces include a second single-sided pole piece, and along the second direction, the pole piece of the outermost layer of the electrode assembly is the second single-sided pole piece, and the second direction is the direction opposite to the first direction. The second single-sided pole piece includes a third metal layer and a third active layer, and the third active layer is arranged on the surface of the third metal layer facing the second pole piece, and the thickness of the third metal layer is D4, 8μm≤D4≤20μm, and the second single-sided pole piece does not include a polymer layer. By setting the pole piece of the outermost layer of the electrode assembly in the second direction as the second single-sided pole piece, the possibility of bending and curling of the pole piece of the outermost layer of the electrode assembly in the second direction is reduced. By limiting that the second single-sided pole piece does not include a polymer layer, it is different from the composite current collector, and the thickness of the third metal layer can be not less than 8 microns and not more than 20 microns, so that the third metal layer meets a certain strength and is not too thick, while reducing the problem of bending and curling of the second single-sided pole piece, the energy density of the secondary battery will not be reduced too much.

[0012] In some preferred embodiments, along the thickness direction of the first current collector, the first active layer is disposed on both opposite surfaces of the first current collector, and the thickness of the first current collector is D5, 5μm≤D5≤10μm. By disposing the first active layer on both opposite surfaces of the first current collector, the first current collector can carry more first active layers, thereby improving the energy density of the secondary battery. By limiting the thickness of the first current collector to not less than 5 microns and not more than 10 microns, the first current collector can carry more first active layers while not occupying too much space to reduce the energy density of the secondary battery.

[0013] In some preferred embodiments, the first metal layer includes aluminum, 7μm≤D5≤9μm; or, the first metal layer includes copper, 5.5μm≤D5≤7.5μm. When the second metal layer includes aluminum, the thickness of the first current collector is not less than 7 microns and not more than 9 microns, so that the first current collector can carry more first active layers while not occupying too much space and reducing the energy density of the secondary battery. When the second metal layer includes copper, due to the high strength of copper, the second metal layer can be made thinner, so that the first current collector can be made thinner, and the thickness of the first current collector is not less than 5.5 microns and not more than 7.5 microns, so that the first current collector can carry more first active layers while not occupying too much space and reducing the energy density of the secondary battery.

[0014] In some preferred embodiments, the second pole piece includes a fourth active layer and a second current collector, and the fourth active layer is disposed on both opposite surfaces of the second current collector along the thickness direction of the second current collector. The second current collector includes a fourth metal layer and a second polymer layer, and the fourth metal layer is disposed on both opposite surfaces of the second polymer layer along the thickness direction of the second current collector, and the thickness of the second current collector is D6, 5μm≤D6≤10μm. By arranging the fourth active layer on both opposite surfaces of the second current collector of the second pole piece, the second current collector can carry more fourth active layers, thereby improving the energy density of the secondary battery. By limiting the thickness of the second current collector to not less than 5 microns and not more than 10 microns, the second current collector can satisfy the requirement of being able to carry more fourth active layers without taking up too much space and reducing the energy density of the secondary battery.

[0015] In some preferred embodiments, the fourth metal layer includes aluminum, 7μm≤D6≤9μm; or, the fourth metal layer includes copper, 5.5μm≤D6≤7.5μm. When the fourth metal layer includes aluminum, the thickness of the second current collector is not less than 7 microns and not more than 9 microns, so that the second current collector can carry more fourth active layers while not occupying too much space and reducing the energy density of the secondary battery. When the fourth metal layer includes copper, due to the high strength of copper, the second metal layer can be made thinner, so that the second current collector can be made thinner, and the thickness of the second current collector is not less than 5.5 microns and not more than 7.5 microns, so that the second current collector can carry more fourth active layers while not occupying too much space and reducing the energy density of the secondary battery.

[0016] In a second aspect, the present application further proposes an electronic device, comprising a secondary battery as in any embodiment of the first aspect above.

[0017] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application.

[0018] The inventors of the present application have discovered that the energy density of a secondary battery can be improved by using a single-sided electrode for the outermost electrode sheet. Since the strength of conventional metal foil is higher than that of a composite current collector, using a metal foil of a certain thickness as the current collector for the outermost single-sided electrode sheet can reduce the possibility of bending or curling after cold pressing. The current collector of a single metal layer does not require roller welding, so there is no need to apply protective glue to protect the weld mark. The thickness of the outermost single-sided electrode sheet can be reduced, thereby improving the energy density of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] One or more embodiments are exemplarily described by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and unless otherwise stated, the dimensions in the drawings do not constitute proportional limitations.

[0020] Figure 1 A schematic diagram of the structure of a secondary battery in some embodiments of the present application;

[0021] Figure 2 This is a schematic diagram of the structure of the electrode assembly of some embodiments of the present application;

[0022] Figure 3 This is a schematic structural diagram of a first current collector in some embodiments of the present application;

[0023] Figure 4 This is a schematic structural diagram of a second current collector according to some embodiments of the present application;

[0024] Figure 5This is a schematic diagram of the structure of the electrode assembly of some embodiments of the present application.

[0025] Description of reference numerals:

[0026] 100. Secondary battery;

[0027] 10. Shell;

[0028] 20. Electrode assembly;

[0029] 21. first pole piece; 22. first double-sided pole piece; 221. first current collector; 2211. first polymer layer; 2212. first metal layer; 222. first active layer; 23. first single-sided pole piece; 231. second metal layer; 232. second active layer; 24. second single-sided pole piece; 241. third metal layer; 242. third active layer;

[0030] 25, second pole piece; 25a, first part; 25b, second part; 251, second current collector; 2511, second polymer layer; 2512, fourth metal layer; 252, fourth active layer;

[0031] 26. Diaphragm;

[0032] 27. a first insulating layer; 28. a second insulating layer;

[0033] X, first direction; Y, second direction; Z, third direction; M, fourth 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 conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

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

[0036] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0037] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0038] 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.

[0039] In the first aspect, the embodiment of the present application provides a secondary battery 100, please refer to Figure 1 The secondary battery 100 includes a shell 10 and an electrode assembly 20 . The shell 10 can accommodate the electrode assembly 20 and an electrolyte (not shown in the figure). The electrolyte infiltrates the electrode assembly 20 in the shell 10 .

[0040] For the electrode assembly 20, please refer to Figure 2 , Figure 2 The lamination structure of the electrode assembly 20 is shown. The electrode assembly 20 includes a diaphragm 26 and a plurality of first pole pieces 21 and a plurality of second pole pieces 25. The first pole pieces 21 and the second pole pieces 25 are alternately stacked along the first direction X. The second pole piece 25 has the opposite polarity to the first pole piece 21. For example, the first pole piece 21 may be a negative pole piece, and the second pole piece 25 may be a positive pole piece, or the first pole piece 21 may be a positive pole piece, and the second pole piece 25 may be a negative pole piece. A diaphragm 26 is provided between adjacent first pole pieces 21 and second pole pieces 25, and the diaphragm 26 is used to insulate and separate the first pole piece 21 and the second pole piece 25. In the embodiment of the present application, the electrode assembly 20 is described as a lamination structure as an example. In some other embodiments, the electrode assembly 20 may also be a winding structure. For example, the first pole piece 21, the diaphragm 26 and the second pole piece 25 are sequentially stacked and then wound to form a wound electrode assembly 20.

[0041] For the above-mentioned first pole piece 21, the first pole piece 21 includes a first double-sided pole piece 22, and along the first direction X, the first double-sided pole piece 22 is arranged between two adjacent second pole pieces 25, and the first double-sided pole piece 22 includes a first current collector 221 and a first active layer 222 arranged on the surface of the first current collector 221. In the embodiment of the present application, the first direction X and the second direction Y are two single directions in opposite directions.

[0042] In some embodiments, along the thickness direction (first direction X or second direction Y) of the first current collector 221, the first active layer 222 is disposed on both opposite surfaces of the first current collector 221. In this way, the first double-sided pole piece 22 can reduce the number of layers of the first current collector 221 under certain conditions of the first active layer 222, thereby improving the energy density of the secondary battery 100. The thickness of the first current collector 221 is D5, 5μm≤D5≤10μm, so that the first current collector 221 can carry more first active layers 222 without taking up too much space and reducing the energy density of the secondary battery 100.

[0043] As for the first active layer 222, the first active layer 222 is infiltrated by the electrolyte in the shell 10 to generate an electrochemical reaction. The first active layer 222 includes a first active material, a conductive agent, an adhesive, etc. The above materials are mixed and stirred evenly and coated on at least one surface of the first current collector 221 to obtain the first active layer 222. The first active material can be a positive electrode active material or a negative electrode active material. The positive electrode active material can include at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, lithium manganese oxide, and lithium manganese iron phosphate. The negative electrode active material can include at least one of graphite, silicon, hard carbon, and carbon fiber.

[0044] For the first current collector 221, please refer to Figure 2 and Figure 3 The first current collector 221 is a composite current collector (a composite structure of metal-polymer-metal). Specifically, the first current collector 221 includes a first metal layer 2212 and a first polymer layer 2211. Along the thickness direction of the first current collector 221, the first metal layer 2212 is provided on both opposite surfaces of the first polymer layer 2211. The first polymer layer 2211, as the main mechanical support layer of the first current collector 221, may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE). The first metal layer 2212 may include at least one of aluminum, copper, nickel, titanium, and silver. The first current collector 221 uses the first polymer layer 2211 as the main mechanical support layer, and the first metal layer 2212 can be thinned, thereby reducing the metal burrs caused by mechanical damage to the secondary battery 100, and reducing the mass of the first current collector 221, thereby improving the mass energy density of the secondary battery 100.

[0045] In some embodiments, the first metal layer 2212 includes aluminum, 7μm≤D5≤9μm; or, the first metal layer 2212 includes copper, 5.5μm≤D5≤7.5μm. Due to the higher strength of copper, the first metal layer 2212 can be made thinner, so that the first current collector 221 can be made thinner. By limiting 5.5μm≤D5≤7.5μm, the first current collector 221 can support a larger first active layer 222 without taking up too much space and reducing the energy density of the secondary battery 100.

[0046] The two metal layers of the composite current collector need to be electrically connected, which is generally done by roller welding, and protective glue needs to be applied to the weld marks. When the composite current collector is applied to the outermost electrode sheet of the secondary battery 100, the roller welding and protective glue will increase the thickness of the electrode sheet, thereby reducing the energy density of the secondary battery 100. In addition, the outermost electrode sheet of the electrode assembly 20 is generally a single-sided electrode sheet (only one surface of the current collector is coated with an active material layer). This is because if the outermost electrode sheet of the electrode assembly 20 is a double-sided electrode sheet (both surfaces of the current collector are coated with active material layers), there will always be one surface of the active material layer that cannot participate in the charge and discharge cycle, which will increase the cost and reduce the energy density of the secondary battery 100. In the electrode assembly 20 with a stacked structure, the electrode sheets need to be cold pressed and cut before stacking. When the outermost electrode sheet of the electrode assembly 20 is a single-sided electrode sheet, it is easy to bend and curl after cold pressing, resulting in the subsequent process cannot proceed smoothly. For example, due to the different elongation rates of the active material layer and the current collector of the single-sided electrode sheet, as the internal stress in the active material layer is released, the active material layer will bend and curl toward the current collector, resulting in the subsequent electrode sheet cutting process cannot proceed smoothly, thereby affecting the production efficiency of the battery and reducing production capacity. The prior art provides hard ceramics on the surface of the current collector of the single-sided electrode sheet facing away from the active material layer to reduce the possibility of bending and curling of the single-sided electrode sheet. However, the hard ceramics will increase the thickness of the single-sided electrode sheet, thereby reducing the energy density of the secondary battery 100.

[0047] To improve the above problems, in the embodiments of this application, please refer to Figure 2, a plurality of first pole pieces 21 include a first single-sided pole piece 23. Along the first direction X, the outermost pole piece of the electrode assembly 20 is the first single-sided pole piece 23. The first single-sided pole piece 23 includes a second metal layer 231 and a second active layer 232. The second active layer 232 is arranged on the surface of the second metal layer 231 facing the second pole piece 25. Since the second active layer 232 is not arranged on the surface of the first single-sided pole piece 23 facing away from the second pole piece 25, the cost is reduced and the energy density of the secondary battery 100 is improved. The first single-sided pole piece 23 does not include a polymer layer. For a single-layer metal layer and a composite current collector of the same thickness, since the thickness of the single-layer metal layer is greater than the thickness of the metal layer of the composite current collector, the strength of the single-layer metal layer is higher than that of the composite current collector, and it can have better anti-bending and curling performance. In addition, the single-layer metal layer does not need to be rolled and protected by adhesive, thereby reducing the thickness of the pole piece and improving the energy density of the secondary battery 100. Preferably, the thickness of the second metal layer 231 is D1, 8μm≤D1≤20μm, so that the second metal layer 231 meets a certain strength and is not too thick, while reducing the bending and curling problems of the first single-sided electrode sheet 23 without significantly reducing the energy density of the secondary battery 100.

[0048] As for the second active layer 232, the second active layer 232 is infiltrated by the electrolyte in the housing 10 to generate an electrochemical reaction. The second active layer 232 includes a second active material, a conductive agent, an adhesive, etc. The above materials are mixed and stirred evenly and coated on the surface of the second metal layer 231 facing the second pole piece 25, thereby obtaining the second active layer 232. The second active material can be a positive electrode active material or a negative electrode active material. The positive electrode active material can include at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, lithium manganese oxide, and lithium manganese iron phosphate. The negative electrode active material can include at least one of graphite, silicon, hard carbon, and carbon fiber.

[0049] In some embodiments, the second metal layer 231 includes aluminum, 9μm≤D1≤20μm; or, the second metal layer 231 includes copper, 8μm≤D1≤20μm. Since copper has a high strength, the second metal layer 231 can be made thinner. By limiting 8μm≤D1≤20μm, the second metal layer 231 meets a certain strength and has better anti-bending and curling performance. In some other embodiments, the second metal layer 231 may include at least one of nickel, titanium, and silver.

[0050] In some embodiments, the second metal layer 231 includes aluminum, 9μm≤D1≤15μm; or, the second metal layer 231 includes copper, 8μm≤D1≤15μm, so that the second metal layer 231 is not too thick, and there can be extra space in the thickness direction of the second metal layer 231 (first direction X or second direction Y), so that the second active layer 232 on the surface of the second metal layer 231 can be set thicker to improve the energy density of the secondary battery 100.

[0051] In some embodiments, the second active layer 232 is a positive electrode active material layer, and the thickness of the second active layer 232 is D2, 30μm≤D2≤70μm; or, the second active layer 232 is a negative electrode active material layer, and the thickness of the second active layer 232 is D2, 30μm≤D2≤50μm, so that the second active layer 232 can provide more energy while not generating greater internal stress due to excessive thickness, thereby reducing the possibility of bending and curling of the first single-sided pole piece 23.

[0052] In some embodiments, the plurality of first pole pieces 21 include a second single-sided pole piece 24. Along the second direction Y, the outermost pole piece of the electrode assembly 20 is the second single-sided pole piece 24. The second direction Y is the opposite direction to the first direction X. The second single-sided pole piece 24 includes a third metal layer 241 and a third active layer 242. The third active layer 242 is disposed on the surface of the third metal layer 241 facing the second pole piece 25. Since the third active layer 242 is not disposed on the surface of the second single-sided pole piece 24 facing away from the second pole piece 25, the cost is reduced and the energy density of the secondary battery 100 is improved. Preferably, the thickness of the third metal layer 241 is D4, 8μm≤D4≤20μm. The second single-sided pole piece 24 does not include a polymer layer. Different from the composite current collector, the third metal layer 241 can meet a certain strength and will not be too thick. While reducing the bending and curling problems of the second single-sided pole piece 24, the energy density of the secondary battery 100 will not be reduced too much.

[0053] In some embodiments, please refer to Figure 2 and Figure 4The second pole piece 25 includes a fourth active layer 252 and a second current collector 251. Along the thickness direction of the second current collector 251, the second current collector 251 has two opposite surfaces provided with the fourth active layer 252. In this way, the second pole piece 25 can reduce the number of layers of the second current collector 251 under certain conditions of the fourth active layer 252, thereby improving the energy density of the secondary battery 100. The second current collector 251 is a composite current collector. Specifically, the second current collector 251 includes a fourth metal layer 2512 and a second polymer layer 2511. Along the thickness direction of the second current collector 251, the second polymer layer 2511 has two opposite surfaces provided with the fourth metal layer 2512. The second current collector 251 uses the second polymer layer 2511 as the main mechanical support layer, and can reduce the fourth metal layer 2512. This reduces the metal burrs caused by mechanical damage to the secondary battery 100, and reduces the mass of the second current collector 251, thereby improving the mass energy density of the secondary battery 100. The first polymer layer 2211, as the main mechanical support layer of the first current collector 221, may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE). The thickness of the second current collector 251 is D6, 5μm≤D6≤10μm, so that the second current collector 251 can support more fourth active layers 252 without taking up too much space to reduce the energy density of the secondary battery 100.

[0054] In some embodiments, the fourth metal layer 2512 includes aluminum, 7μm≤D6≤9μm; or, the fourth metal layer 2512 includes copper, 5.5μm≤D6≤7.5μm. Since copper has a high strength, the fourth metal layer 2512 can be made thinner, so that the second current collector 251 can be made thinner. By limiting 5.5μm≤D6≤7.5μm, the second current collector 251 can be able to carry more fourth active layers 252 while not occupying too much space and reducing the energy density of the secondary battery 100. In other embodiments, the fourth metal layer 2512 can include at least one of nickel, titanium, and silver.

[0055] As for the fourth active layer 252, the fourth active layer 252 is infiltrated by the electrolyte in the shell 10 to generate an electrochemical reaction. The fourth active layer 252 includes a fourth active material, a conductive agent, an adhesive, etc. The above materials are mixed and stirred evenly and coated on the two opposite surfaces of the second current collector 251, thereby obtaining the fourth active layer 252. The fourth active material can be a positive electrode active material or a negative electrode active material. The positive electrode active material can include at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, lithium manganese oxide, and lithium manganese iron phosphate. The negative electrode active material can include at least one of graphite, silicon, hard carbon, and carbon fiber.

[0056] In some embodiments, please refer to Figure 5 , the first electrode sheet 21 is a positive electrode sheet, the second electrode sheet 25 is a negative electrode sheet, and the second active layer 232 is a positive electrode active material layer. Along the length or width direction of the second electrode sheet 25 (the third direction Z or the fourth direction M), the second electrode sheet 25 includes a first part 25a and a second part 25b arranged in sequence. Along the first direction X, the first part 25a overlaps with the second active layer 232. The second part 25b exceeds the part of the second active layer 232 in the length or width direction of the second electrode sheet 25, so that the positive electrode active material of the second active layer 232 is less than the negative electrode active material of the second electrode sheet 25. During the charging process of the secondary battery 100, the rate at which the positive electrode active material of the second active layer 232 releases lithium ions is lower than the rate at which the negative electrode active material of the second electrode sheet 25 absorbs lithium ions, thereby reducing the possibility of lithium precipitation on the surface of the fourth active layer 252 of the second electrode sheet 25. The electrode assembly 20 includes a first insulating layer 27, which is arranged on the surface of the second metal layer 231 facing the second electrode plate 25. Along the first direction X, the first insulating layer 27 at least partially overlaps with the second portion 25b, so that the first insulating layer 27 can block the portion of the second metal layer 231 where the second active layer 232 is not set, so as to reduce the possibility of the second metal layer 231 where the second active layer 232 is not set directly contacting the fourth active layer 252 of the second electrode plate 25 and causing a short circuit.

[0057] In some embodiments, along the width direction of the first current collector 221 (the third direction Z), the width of the first insulating layer 27 is L1, 1.5mm≤L1≤6mm, so that the first insulating layer 27 can better insulate the second metal layer 231 without the second active layer 232 and the fourth active layer 252 of the second pole piece 25 without significantly reducing the energy density of the secondary battery 100; and / or, along the thickness direction of the first current collector 221, the thickness of the first insulating layer 27 is D3, 15μm≤D3≤30μm, so that the first insulating layer 27 can better prevent the burrs of the second metal layer 231 from piercing, while not significantly reducing the energy density of the secondary battery 100.

[0058] In some embodiments, the electrode assembly 20 includes a second insulating layer 28, which is disposed on the surface of the third metal layer 241 facing the second electrode plate 25. Along the first direction X, the second insulating layer 28 at least partially overlaps with the second portion 25b, so that the second insulating layer 28 can block a portion of the third metal layer 241 where the third active layer 242 is not disposed, so as to reduce the possibility of the third metal layer 241 where the third active layer 242 is not disposed directly contacting the fourth active layer 252 of the second electrode plate 25 and causing a short circuit.

[0059] In some embodiments, along the width direction of the first current collector 221 (the third direction Z), the width of the second insulating layer 28 is L2, 1.5mm≤L2≤6mm, so that the second insulating layer 28 can better insulate the third metal layer 241 without the third active layer 242 and the fourth active layer 252 of the second pole piece 25 without significantly reducing the energy density of the secondary battery 100; and / or, along the thickness direction of the first current collector 221, the thickness of the second insulating layer 28 is D7, 15μm≤D7≤30μm, so that the second insulating layer 28 can better prevent the burrs of the third metal layer 241 from piercing, while not significantly reducing the energy density of the secondary battery 100.

[0060] In the second aspect of the present application, an electronic device is also proposed, including a secondary battery 100 as in any embodiment of the first aspect described above. The electronic device of the embodiment of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, electronic devices include but are not limited to Bluetooth headsets, mobile phones, tablets, laptops, electric toys, electric tools, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys, electric airplane toys, and the like, and spacecraft can include airplanes, rockets, space shuttles, spacecraft, and the like.

[0061] Test part:

[0062] 1. Impact test of lithium-ion batteries:

[0063] Test process: In a test environment of 20±5℃, place the sample on the test table with the coding side facing up, and place a round rod with a diameter of 15.8mm at the center of the wide side of the sample, with the round rod perpendicular to the long axis of the sample. Use a 9.1±0.1kg weight to drop it vertically from a height of 610±25mm and drop it at the intersection of the round rod and the sample. Judgment criteria: no fire or explosion.

[0064] 2.Energy density test:

[0065] Taking the cell thickness of 2500um as the calculation unit, the electrode thickness reduction value is divided by 3000 to obtain the energy density gain value;

[0066] 3. Single-sided pole curling leads to pole scrapping test:

[0067] The single-sided pole piece curling is easy to break under a certain tension and the pole piece warps and delaminates after lamination, resulting in a reduced cell quality rate. For example, a 1000m long pole piece can produce 20,000 cells, but only 17,000 are actually produced, and the corresponding pole piece scrap rate is 17,000 / 20,000 = 85%;

[0068] Example 1

[0069] <Preparation of positive electrode sheet>:

[0070] The positive electrode active materials lithium cobalt oxide (LiCoO2), carbon black (Super P), and polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97.5:1.0:1.5, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 75wt%, which is then stirred evenly.

[0071] Polyethylene terephthalate is selected as the polymer layer, and two metal layers of aluminum are evaporated on the two surfaces of the polymer layer respectively. The above slurry is coated on the surface of one metal layer and a positive electrode empty foil section is reserved. The slurry is dried to obtain a single-sided composite positive electrode sheet coated with a positive electrode active material layer on one side. On the other metal layer, the above steps are repeated to obtain a double-sided composite positive electrode sheet coated with a positive electrode active material layer on both sides.

[0072] Aluminum foil is selected as the positive electrode current collector, the above slurry is evenly coated on one surface of the aluminum foil, and a positive electrode empty foil section is reserved on the surface. The slurry is dried to obtain a single-sided positive electrode sheet coated with a positive electrode active material layer on one side.

[0073] <Preparation of negative electrode sheet>:

[0074] Using graphite as the negative electrode active material, the negative electrode active material graphite, the binder styrene butadiene rubber (SBR) and the thickener sodium carboxymethyl cellulose (CMC) are mixed in a weight ratio of 96:2:2, deionized water is added as a solvent, a slurry with a solid content of 70wt% is prepared, and the slurry is stirred evenly.

[0075] Polyethylene terephthalate is selected as the polymer layer, and two metal layers of copper are respectively evaporated on the two surfaces of the polymer layer. The above slurry is coated on the surface of one metal layer and a negative electrode empty foil section is reserved. The slurry is dried to obtain a single-sided composite negative electrode sheet coated with a negative electrode active material layer on one side. On the other metal layer, the above steps are repeated to obtain a double-sided composite negative electrode sheet coated with a negative electrode active material layer on both sides.

[0076] <Preparation of diaphragm>:

[0077] A polyethylene porous membrane is used as a substrate layer, and a ceramic layer containing alumina ceramics and a PVDF binder is coated on one surface of the substrate layer as a diaphragm (CCS), wherein the mass percentage of the alumina ceramics in the ceramic layer is 95%.

[0078] <Electrolyte Preparation>:

[0079] In a dry argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are first mixed in a mass ratio of EC:EMC:DEC=30:50:20 to form a basic organic solvent, and then lithium salt lithium hexafluorophosphate (LiPF6) is added to the basic organic solvent to dissolve and mix evenly to obtain an electrolyte with a LiPF6 mass concentration of 12.5%.

[0080] <Preparation of lithium-ion batteries>:

[0081] The positive electrode sheet, separator and negative electrode sheet prepared above are stacked in sequence to form an electrode assembly, and the electrode assembly is hot pressed. The electrode assembly is placed in an aluminum-plastic film packaging bag, and an electrolyte is injected and packaged to obtain a lithium-ion battery.

[0082] The relevant parameters in Comparative Examples 1-8 and Examples 2 to 5 are shown in Table 1 below, where:

[0083] The outermost electrode sheets of the electrode assemblies in Comparative Examples 1-8 and Examples 2 to 5 along the first direction and the second direction are all single-sided positive electrode sheets.

[0084] The single-sided positive electrode sheet in Comparative Examples 1-3 is coated with a hard ceramic layer on the surface of the positive current collector away from the positive active material layer. The material and preparation of the hard ceramic: First, prepare the adhesive glue. Use an ordinary stirring tank to first inject a certain amount of solvent NMP, add the binder polyacrylic acid powder according to the designed solid content, and stir for 4 to 6 hours to obtain the polyacrylic acid glue. The polyacrylic acid glue is a colorless and transparent liquid with a certain viscosity. The solid content can be controlled between 5% and 10% as needed. The prepared glue generally needs to be vacuumed and left to stand for more than 12 hours in order to eliminate the bubbles generated during the stirring process. Then a certain amount is transported to the slurry preparation mixer through a sealed pipeline through a metering pump, and the dye graphite is added. At the same time, the mixer is started to revolve and rotate. The revolution speed is set to (25±5) r / min, the rotation speed is set to (500±50) r / min, and NMP spraying is assisted to allow the extremely light graphite to be fully mixed into the polyacrylic acid glue. The stirring time is 1 hour.

[0085] Secondly, add the main ceramic material. In order to ensure that the main material and the glue can be effectively and fully dispersed, it is generally added in steps, that is, 50% of the main material (such as alumina or boehmite, etc.) is added first, and the orbital speed and the rotation speed are set to (30±5) r / min and (300±50) r / min, respectively. After stirring for 5 minutes, the remaining 50% of the main material is added, assisted by appropriate NMP spraying, and the above parameters are maintained for more than 1.5 hours. During the process, the stirring paddle is scraped as needed to prevent the powder adhering to the stirring paddle from being unable to be fully wetted due to the large wetting angle of the solid-liquid interface.

[0086] Finally, spray all the remaining NMP solvent into the mixer, increase the stirring speed, the rotation speed is (35±5) r / min, the revolution speed is (800±50) r / min, stir for 10-30 minutes, then increase the revolution speed to (1300±50) r / min and stir for 1.5 hours to complete the slurry preparation. The prepared slurry needs to be transferred to a transfer tank for vacuum defoaming treatment before use.

[0087] The ceramic slurry is evenly coated on the single-sided electrode area using extrusion coating equipment, and a single-sided cathode electrode with a ceramic layer is obtained after drying.

[0088] Table 1

[0089]

[0090] According to the above Table 1, combined with Comparative Examples 1 to 3 and Examples 1 to 5, it can be known that if the outermost electrode sheets of the electrode assembly along the first direction and the second direction are all single-sided positive electrode sheets, when the single-sided positive electrode sheet adopts a composite current collector, the single-sided positive electrode sheet is easy to curl and cause the electrode sheet to be scrapped. Although the possibility of curling of the single-sided positive electrode sheet can be reduced by coating a ceramic layer on the surface of the composite current collector of the single-sided positive electrode sheet away from the positive active layer, a thicker ceramic layer needs to be coated, which makes the single-sided positive electrode sheet too thick and will lose a large amount of energy density. If the single-sided positive electrode sheet adopts a single-layer aluminum foil as a current collector, under the condition of the same thickness, the possibility of curling of the single-sided positive electrode sheet can be reduced, and a good impact test pass rate can also be guaranteed, that is, the single-sided positive electrode sheet adopts a single-layer aluminum foil as a current collector. While ensuring a good impact test pass rate, it can have better energy density and anti-curling performance.

[0091] Combining Comparative Examples 7 to 8 and Examples 1 to 5, it can be seen that if a single-sided positive electrode sheet uses a single-layer aluminum foil as a current collector, when the thickness of the aluminum foil is greater than 9 μm, the probability of the single-sided positive electrode sheet being scrapped due to curling is reduced. When the thickness of the aluminum foil is greater than 20 μm, it is impossible to obtain a good energy density and impact test pass rate. Therefore, the thickness of the aluminum foil is preferably 9 μm to 20 μm.

[0092] When aluminum foil is used as the current collector, the thickness of the aluminum foil is in the range of 9μm to 15μm, which can ensure that the energy density of the battery cell, the impact test pass rate, and the scrap rate of the electrode curling can all be controlled.

[0093] Combining Comparative Examples 4 to 6 and Example 2, Example 4 and Example 5, it can be seen that if the double-sided positive electrode sheet in the middle layer of the electrode assembly uses a composite current collector as the positive electrode current collector, compared with the double-sided positive electrode sheet in the middle layer using aluminum foil as the positive electrode current collector, the lithium-ion battery can obtain a good impact test pass rate without reducing the energy density. Among them, for the laminated structure electrode assembly, if the double-sided positive electrode sheet in the middle layer uses aluminum foil as the positive electrode current collector, the aluminum foil is easy to break during cold pressing when the thickness is 8μm, and the lithium-ion battery cannot be successfully prepared. Therefore, the aluminum foil of the double-sided positive electrode sheet in Comparative Examples 4 to 6 is designed to be 10μm to ensure that the lithium-ion battery sample can be successfully prepared.

[0094] The relevant parameters of Comparative Examples 9 to 16 and Examples 6 to 10 are shown in Table 2 below, where:

[0095] The outermost electrode sheets of the electrode assemblies in Comparative Examples 9-16 and Examples 6 to 10 along the first direction and the second direction are all single-sided negative electrode sheets.

[0096] The single-sided negative electrode sheets in Comparative Examples 9-11 are coated with hard ceramic on the surface of the negative electrode current collector facing away from the negative electrode active material layer.

[0097] Table 2

[0098]

[0099] According to Table 2 above, combined with Comparative Examples 9 to 11 and Examples 6 to 10, it can be seen that if the outermost electrode sheets of the electrode assembly along the first direction and the second direction are all single-sided negative electrode sheets, when the single-sided negative electrode sheet adopts a composite current collector, the single-sided negative electrode sheet is prone to curling and the electrode sheet is scrapped. Although the possibility of curling of the single-sided negative electrode sheet can be reduced by coating a ceramic layer on the surface of the composite current collector of the single-sided negative electrode sheet away from the negative electrode active layer, a thicker ceramic layer needs to be coated, which makes the single-sided negative electrode sheet too thick and will lose a large amount of energy density. If the single-sided negative electrode sheet adopts a single-layer copper foil as the current collector, under the condition of the same thickness, the possibility of curling of the single-sided negative electrode sheet can be reduced, that is, the single-sided negative electrode sheet adopts a single-layer copper foil as the current collector to have better energy density and anti-curling performance.

[0100] Combining Comparative Examples 15 to 16 and Examples 6 to 10, it can be seen that if a single-sided negative electrode sheet uses a single-layer copper foil as a current collector, when the thickness of the copper foil is less than 8 μm, the single-sided negative electrode sheet is prone to curling and the electrode sheet is scrapped. When the thickness of the copper foil is greater than 20 μm, a good energy density cannot be obtained. Therefore, the thickness of the copper foil needs to be limited to 8 μm to 20 μm.

[0101] Combining Comparative Examples 12 to 14 and Examples 7, 9 and 10, it can be seen that if the double-sided negative electrode sheet in the middle layer of the electrode assembly uses a composite current collector as the negative electrode current collector, compared with the double-sided negative electrode sheet in the middle layer using copper foil as the negative electrode current collector, the lithium-ion battery can improve the energy density without increasing the probability of the single-sided electrode sheet being scrapped due to the curling of the electrode sheet. Among them, for the laminated structure electrode assembly, if the double-sided negative electrode sheet in the middle layer uses copper foil as the negative electrode current collector, the copper foil thickness is often 5μm, 6μm, 8μm, 10μm and the like, and generally does not use 6.5μm thickness. When the copper foil is too thin, it is easy to break during cold pressing and cannot be successfully prepared into a lithium-ion battery. Therefore, the copper foil of the double-sided negative electrode sheet in Comparative Examples 12 to 14 is designed to be 10μm to ensure that the lithium-ion battery sample can be successfully prepared.

[0102] The above description is only 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 secondary battery, comprising an electrode assembly, the electrode assembly comprising a diaphragm and a plurality of first pole sheets and a plurality of second pole sheets, the first pole sheets and the second pole sheets are alternately stacked along a first direction, the second pole sheets have opposite polarities to the first pole sheets, and the diaphragm is disposed between adjacent first pole sheets and second pole sheets; The plurality of first pole pieces include a first double-sided pole piece, wherein the first double-sided pole piece is disposed between two adjacent second pole pieces along the first direction, the first double-sided pole piece includes a first current collector and a first active layer disposed on the surface of the first current collector, the first current collector includes a first metal layer and a first polymer layer, and the first metal layer is disposed on both opposite surfaces of the first polymer layer along the thickness direction of the first current collector; It is characterized in that The plurality of first pole pieces further include a first single-sided pole piece, and along the first direction, the outermost pole piece of the electrode assembly is the first single-sided pole piece; The first single-sided pole piece includes a second metal layer and a second active layer. The second active layer is arranged on the surface of the second metal layer facing the second pole piece. The thickness of the second metal layer is D1, 8μm≤D1≤20μm. The first single-sided pole piece does not include a polymer layer.

2. The secondary battery according to claim 1, characterized in that: The second metal layer includes aluminum, 9 μm≤D1≤20 μm; or, the second metal layer includes copper, 8 μm≤D1≤20 μm.

3. The secondary battery according to claim 2, characterized in that: The second metal layer includes aluminum, 9 μm≤D1≤15 μm; or, the second metal layer includes copper, 8 μm≤D1≤15 μm.

4. The secondary battery according to claim 1, characterized in that: The second active layer is a positive electrode active material layer, and the thickness of the second active layer is D2, 30 μm≤D2≤70 μm; or, the second active layer is a negative electrode active material layer, and the thickness of the second active layer is D2, 30 μm≤D2≤50 μm.

5. The secondary battery according to claim 1, characterized in that: Along the length or width direction of the second pole piece, the second pole piece includes a first portion and a second portion arranged in sequence; along the first direction, the first portion overlaps with the second active layer; and the second portion exceeds the portion of the second active layer in the length or width direction of the second pole piece; The electrode assembly includes a first insulating layer, which is disposed on a surface of the second metal layer facing the second pole piece, and along the first direction, the first insulating layer at least partially overlaps with the second portion.

6. The secondary battery according to claim 5, characterized in that: Along the width direction of the first current collector, the width of the first insulating layer is L1, 1.5 mm ≤ L1 ≤ 6 mm; and / or, along the thickness direction of the first current collector, the thickness of the first insulating layer is D3, 15 μm ≤ D3 ≤ 30 μm.

7. The secondary battery according to claim 1, characterized in that: The plurality of first pole pieces further include a second single-sided pole piece, and along a second direction, the outermost pole piece of the electrode assembly is the second single-sided pole piece, and the second direction is a direction opposite to the first direction; The second single-sided pole piece includes a third metal layer and a third active layer. The third active layer is arranged on the surface of the third metal layer facing the second pole piece. The thickness of the third metal layer is D4, 8μm≤D4≤20μm. The second single-sided pole piece does not include a polymer layer.

8. The secondary battery according to claim 1, characterized in that: The first active layer is disposed on two opposite surfaces of the first current collector along a thickness direction of the first current collector, and the thickness of the first current collector is D5, 5 μm≤D5≤10 μm.

9. The secondary battery according to claim 8, characterized in that: The first metal layer includes aluminum, 7 μm≤D5≤9 μm; or, the first metal layer includes copper, 5.5 μm≤D5≤7.5 μm.

10. The secondary battery according to claim 1, characterized in that: The second pole piece comprises a fourth active layer and a second current collector, and along the thickness direction of the second current collector, the fourth active layer is disposed on two opposite surfaces of the second current collector; The second current collector includes a fourth metal layer and a second polymer layer. Along the thickness direction of the second current collector, the fourth metal layer is disposed on two opposite surfaces of the second polymer layer. The thickness of the second current collector is D6, 5μm≤D6≤10μm.

11. The secondary battery according to claim 10, characterized in that: The fourth metal layer includes aluminum, 7 μm≤D6≤9 μm; or, the fourth metal layer includes copper, 5.5 μm≤D6≤7.5 μm.

12. An electronic device, characterized in that: Comprising the secondary battery as claimed in any one of claims 1 to 11.

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