Battery diaphragm, battery cell, lithium ion battery and electronic device

Abstract

The invention discloses a battery diaphragm, a battery cell, a lithium ion battery and an electronic device. The battery diaphragm comprises a base membrane and a first coating layer, the base membrane comprises predetermined straight sections and predetermined corner sections which are sequentially and alternately arranged in the length direction, and the first coating layer is arranged on at least one side surface of the predetermined corner sections on the base membrane; the components of the first coating layer comprise carboxylated graphite and a hydroxyl-containing binder. The battery diaphragm can be suitable for configuring a winding type battery cell, and when the battery cell is assembled by matching with a positive plate and a negative plate, a first coating layer is arranged on one side, facing the negative plate, of the battery diaphragm corresponding to a preset corner section, and the first coating layer can absorb electrolyte to swell and store rich lithium sources; li < + > can be stored at the corner of a battery cell after long-time circulation, so that the lithium precipitation condition after long-time circulation of the battery is effectively improved, the long-circulation capacity retention ratio of the battery is improved, and meanwhile, the rate capability of the battery is improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a battery separator, a battery cell, a lithium-ion battery and an electronic device. Background Art

[0002] With the demand of consumer electronics and electric vehicle markets, the production and application of soft-pack batteries are growing rapidly, and the safety performance of batteries is becoming increasingly important. However, there is a problem of lithium deposition at the corners of the anode sheet of wound soft-pack batteries. Specifically, the positive and negative electrodes of the wound battery cells will expand and contract during use, and due to the space limitations at the corners, the electrodes cannot expand and contract freely. Therefore, during long-term use, the stress at the corners of the battery cells will gradually accumulate and cause some of the negative active materials on the surface of the negative electrode sheet to detach and crack. During multiple discharges, lithium ions do not have enough lithium insertion sites, and lithium dendrites are easily formed on the corner surface of the negative electrode sheet, which will reduce the discharge efficiency of the battery cell and affect the battery life.

[0003] Lithium deposition at corners is a common problem in the industry and a problem in high-rate charging of wound structures, which greatly limits the development of high-energy and high-rate wound cells. To this end, a method needs to be found to improve the problem of stress accumulation at the corners of the cell, which causes the negative electrode active material to break away and deposit lithium. Summary of the invention

[0004] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, the present application proposes a battery separator, a battery cell, a lithium-ion battery and an electronic device.

[0005] According to the first aspect of the present application, a battery separator is provided, comprising a base film and a first coating layer; the base film comprises predetermined straight sections and predetermined corner sections arranged alternately in sequence along the length direction, and the first coating layer is arranged on at least one side surface of the predetermined corner section on the base film; components of the first coating layer include a first binder and carboxylated graphite, and the first binder is selected from a hydroxyl-containing binder.

[0006] The battery separator can be used to configure a wound battery cell, and when the battery separator is configured to cooperate with the positive electrode sheet and the negative electrode sheet to assemble the battery cell, a first coating layer is provided on the side of the battery separator corresponding to the predetermined corner section facing the negative electrode sheet. When the battery cell is specifically assembled, the positive electrode sheet, the battery separator and the negative electrode sheet are sequentially stacked and wound to form a winding structure, and the winding structure includes a straight area and a corner area connecting the straight areas. The predetermined straight section of the base film in the battery separator is located in the straight area, and the predetermined corner section is located in the corner area. Then, in the corner area of ​​the battery cell, the surface of the battery separator at the corresponding position of the negative electrode sheet is provided with a first coating layer.

[0007] The battery separator according to the first aspect of the present application has the following beneficial effects: the battery separator can be used to configure a wound battery cell, wherein the base film includes predetermined straight sections and predetermined corner sections that are alternately arranged in sequence along the length direction, the predetermined corner sections connect adjacent predetermined straight sections, the predetermined straight sections correspond to the straight areas of the wound battery cell, and the predetermined corner sections correspond to the corner areas of the wound battery cell; at least one side surface of the predetermined corner section on the base film is provided with a first coating layer, and when the battery separator is configured to cooperate with the positive electrode sheet and the negative electrode sheet to assemble the wound battery cell, the predetermined corner section corresponds to the wound battery cell. The first coating layer is provided on the side of the corresponding battery separator facing the negative electrode sheet, that is, the first coating layer is located on the surface of the battery separator at the position corresponding to the negative electrode sheet in the corner area of ​​the battery cell, and the components of the first coating layer include carboxylated graphite and a hydroxyl-containing binder. The hydroxyl group in the binder can be bonded with the carboxylated graphite and stored in the graphite sheet layer, and then the first coating layer can absorb the electrolyte to swell and store rich lithium sources, so that it can replace the active material on the negative electrode sheet that falls off and fails due to cycle extrusion in the corner area of ​​the negative electrode sheet. The two work synergistically to store Li in the corner of the battery cell after long-term cycling. + , effectively improve the lithium plating of the battery after long-term cycling, improve the long-cycle capacity retention rate of the battery, and at the same time improve the battery's rate performance.

[0008] In some embodiments of the present application, the first adhesive is selected from at least one of polyethylene oxide (PEO), polyurethane adhesive, chloroprene rubber adhesive, polyvinyl butyral, phenolic resin, and hydroxy-terminated polybutadiene liquid rubber.

[0009] In some embodiments of the present application, the first binder is selected from polyethylene oxide with a molecular weight of 200,000 to 800,000.

[0010] In some embodiments of the present application, the components of the first coating layer include 5 wt% to 15 wt% of a first binder and 85 wt% to 95 wt% of carboxylated graphite;

[0011] And / or, the thickness of the first coating layer is 1 μm to 6 μm.

[0012] In some embodiments of the present application, the battery separator also includes a second coating layer, which is disposed on at least one side surface of the base film; the first coating layer is disposed on the surface of the second coating layer facing away from the base film and is located at a position corresponding to the predetermined corner section; the components of the second coating layer include a second binder and an inorganic filler.

[0013] In some embodiments of the present application, the second coating layer is disposed on both side surfaces of the base film, and the first coating layer is disposed on the surface of the second coating layer on at least one side of the base film.

[0014] In some embodiments of the present application, the second binder is selected from at least one of polyvinylidene fluoride, styrene-butadiene rubber, polytetrafluoroethylene, polyacrylonitrile, polyvinyl pyrrolidone, polyurethane, polyvinyl alcohol, and polyimide; and / or the inorganic filler is selected from at least one of aluminum oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, silicon dioxide, titanium dioxide, zirconium dioxide, magnesium hydroxide, aluminum hydroxide, and boehmite.

[0015] According to a second aspect of the present application, a battery cell is provided, which includes a positive electrode sheet, a battery separator and a negative electrode sheet; the positive electrode sheet, the battery separator and the negative electrode sheet are stacked in sequence and wound to form a winding structure, and the winding structure includes a straight area and a corner area connected to each other; the battery separator is selected from any one of the aforementioned battery separators, the predetermined straight section is located in the straight area, and the predetermined corner section is located in the corner area; the first coating layer is provided on the battery separator corresponding to the predetermined corner section, and the side facing the negative electrode sheet is provided with the first coating layer.

[0016] According to a third aspect of the present application, a lithium-ion battery is provided, wherein the lithium-ion battery comprises any one of the aforementioned battery cells.

[0017] According to a fourth aspect of the present application, an electronic device is provided, the electronic device comprising any one of the aforementioned lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a structural schematic diagram of an embodiment of a battery separator of the present application;

[0019] Figure 2 It is a structural schematic diagram of an embodiment of a battery cell of the present application;

[0020] Figure 3 This is a test result diagram of the powder loss of the negative electrode sheet in the corner area of ​​the cell of Example 7 after 600 cycles at a 1C rate. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the concept of the present application and the technical effects produced in combination with the embodiments, so as to fully understand the purpose, features and effects of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of them. Based on the embodiments of the present application, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present application.

[0022] The embodiments of the present application are described in detail below. The described embodiments are exemplary and are only used to explain the present application, and should not be understood as limiting the present application.

[0023] In the description of this application, "several" means more than one, "many" means more than two, "greater than", "less than", "exceed", etc. are understood to exclude the number itself, "above", "below", "within", etc. are understood to include the number itself, and "about" means within the range of ±20%, 10%, 8%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.2%, 0.1%, etc. of the number itself. If there is a description of "first" or "second", it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0024] In the description of the present application, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0025] This application proposes a battery separator, see Figure 1 , Figure 1 Schematic diagram of the structure of an embodiment of the battery separator of the present application. Figure 1 As shown, the battery separator 10 includes a base film 11 and a first coating layer 12; the base film 11 includes predetermined straight sections 111 and predetermined corner sections 112 which are alternately arranged in sequence along the length direction, and the first coating layer 12 is arranged on at least one side surface of the predetermined corner section 112 on the base film 11, and the components of the first coating layer 12 include a first binder and carboxylated graphite, and the first binder is selected from a hydroxyl-containing binder.

[0026] The battery separator 10 can be applied to a wound battery, that is, a battery separator for a wound battery, and thus corresponds to a wound battery structure. The base film 11 of the battery separator 10 includes predetermined straight sections 111 and predetermined corner sections 112 that are alternately arranged in sequence along the length direction, and the predetermined corner sections 112 connect adjacent predetermined straight sections 111. Along the arrangement direction of the predetermined straight sections 111 and the predetermined corner sections 112, the length of the predetermined corner sections 112 generally coincides with the corner arc length of the corresponding layer of the winding core when the core is wound, and thus the corner arc length of the winding core increases with the increase in the number of winding layers, and correspondingly, the length of the predetermined corner section 112 also increases accordingly. Therefore, it can be designed that along the arrangement direction of the predetermined straight section 111 and the predetermined corner section 112, the length of the predetermined corner section 112 is increasing or decreasing. Specifically, the length of the predetermined corner section 112 of the battery separator 10 corresponding to the corner area of ​​each layer of the winding core can be designed according to the combination of the pole pieces used in the core making and the number of winding layers. When winding the core, the side of the battery separator 10 with a shorter length of the predetermined corner section 112 is generally used as the starting end of the inner layer winding.

[0027] The base film 11 may be made of, but not limited to, one or more of a polyethylene (PE) film, a polypropylene (PP) film, a polyvinylidene fluoride (PVDF) film, a polyimide (PI) film, and an aramid film.

[0028] The first coating layer 12 is provided on at least one side surface of the predetermined corner section 112 on the base film 11. Specifically, in some embodiments, the first coating layer 12 can be provided on one side surface of the predetermined corner section 112 on the base film 11 (eg Figure 1 as shown); in other embodiments, the first coating layer 12 may be provided on both side surfaces of a predetermined corner section 112 on the base film 11. The thickness of the first coating layer 12 may be controlled at 1 μm to 6 μm, and further, the thickness of the first coating layer 12 may be controlled at 3 μm to 5 μm. For example, the thickness of the first coating layer 12 may be 1 μm, 2 μm, 2.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, and the like. If the thickness of the first coating layer is too large, the subsequent application of the battery separator in the preparation of the battery cell will increase the overall thickness of the battery cell and significantly reduce the volume energy density of the battery.

[0029] The components of the first coating layer 12 include a first binder and carboxylated graphite, and the first binder specifically uses a carboxyl-containing binder. Based on the position setting of the first coating layer 12 on the battery separator 10 and its component configuration, when the battery separator 10 is configured to cooperate with the positive electrode sheet and the negative electrode sheet to assemble a wound-type battery cell, a first coating layer 12 is provided on the side of the battery separator 10 corresponding to the predetermined corner section 112 facing the negative electrode sheet. The hydroxyl group of the binder in the first coating layer 12 can be bonded with the carboxylated graphite and stored in the graphite sheet layer, so that the first coating layer 12 can absorb the electrolyte and swell, and store a rich lithium source, which can replace the active material on the negative electrode sheet that falls off and fails due to cyclic extrusion in the corner area of ​​the negative electrode sheet in the wound-type battery cell, and store Li in the corner of the battery cell after long-term cycling. + It can effectively improve the lithium plating of the battery after long-term cycling, improve the capacity retention rate of the battery after long-term cycling, and at the same time improve the battery's rate performance.

[0030] The first binder can be at least one of polyethylene oxide (PEO), polyurethane adhesive, chloroprene rubber adhesive, polyvinyl butyral, phenolic resin, and hydroxy-terminated polybutadiene liquid rubber. PEO is preferably used, as PEO has good liquid absorption and swelling properties, and is bonded with carboxylated graphite and stored in graphite sheets. In the electrolyte, PEO absorbs the electrolyte and swells to form a gel state, which can store abundant Li + , which is the Li storage in the corner area of ​​the negative electrode after long cycles + Improve battery cycle performance.

[0031] Furthermore, in some embodiments, the first binder can be polyethylene oxide with a molecular weight of 200,000 to 800,000. The use of polyethylene oxide with a molecular weight of 200,000 to 800,000 can better bond with carboxylated graphite in the form of hydrogen bonds. If the molecular weight of PEO is too low, its viscosity decreases, which can easily cause the first coating layer 12 on the battery separator 10 to fall off and fail during long-term cycles, and cannot function; if the molecular weight of PEO is too high, its viscosity is too high, and it is too tightly bonded to graphite, which will cause Li + The difficulty in passing leads to a decrease in capacity retention.

[0032] In some embodiments, the components of the first coating layer 12 can be configured to include 5wt% to 15wt% of the first binder and 85wt% to 95wt% of the carboxylated graphite; further, the components of the first coating layer can be configured to include 8wt% to 12wt% of the first binder and 88wt% to 92wt% of the carboxylated graphite. For example, the content of the first binder in the first coating layer 12 can be controlled to be 5wt%, 7wt%, 8wt%, 9wt%, 10wt%, 12wt%, 14wt%, 15wt%, etc.; the content of the carboxylated graphite can be controlled to be 85wt%, 86wt%, 88wt%, 90wt%, 91wt%, 92wt%, 93wt%, 95wt%, etc.

[0033] In order to improve the performance of the battery separator 10, the battery separator 10 may further include a second coating layer 13, the second coating layer 13 is disposed on at least one side surface of the base film 11, the first coating layer 12 is disposed on the surface of the second coating layer 13 that is away from the base film 11, and is located at a position corresponding to the predetermined corner section 112. Specifically, in some embodiments, the second coating layer 13 may be disposed only on one side surface of the base film 11, and the first coating layer 12 is disposed on the second coating layer 13 at a position corresponding to the predetermined corner section 112 on the surface away from the base film 11. In other embodiments, the second coating layer 13 is disposed on both side surfaces of the base film 11, the first coating layer 12 may be disposed on the surface of the second coating layer 13 on at least one side of the base film 11, and is located at a position corresponding to the predetermined corner section 112; that is, in the case where the second coating layer 13 is disposed on both side surfaces of the base film 11, the first coating layer 12 may be disposed on the surface of the second coating layer 13 on one side of the base film 11 that is away from the base film 11 (such as Figure 1 As shown), the second coating layer 13 on both sides of the base film 11 may also be disposed away from the surface of the base film 11.

[0034] The second coating layer 13 can be set according to the performance requirements of the battery separator 10. For example, the second coating layer 13 can be designed as a heat-resistant layer to improve the heat resistance of the battery separator 10. Further, the components of the second coating layer 13 may include a second binder and an inorganic filler. Among them, the second binder can be but not limited to polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyacrylonitrile, polyvinyl pyrrolidone (PVP), polyurethane (PU), polyvinyl alcohol, polyimide (PI), etc.; the inorganic filler can be but not limited to aluminum oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, silicon dioxide, titanium dioxide, zirconium dioxide, magnesium hydroxide, aluminum hydroxide, boehmite, etc.

[0035] This application also proposes a battery cell, see Figure 2 , Figure 2 A schematic diagram of the structure of an embodiment of the battery cell of the present application is shown. Figure 2As shown, the battery cell includes a positive electrode sheet 20, a battery separator 10 and a negative electrode sheet 30, which are stacked in sequence and wound to form a winding structure, and the winding structure includes a straight area 41 and a corner area 42 connected to each other. Figure 1 The battery separator 10 of the embodiment shown has a structure as described above and will not be described in detail. The predetermined straight section 111 of the base film 11 in the battery separator 10 is located in the straight area 41, the predetermined corner section 112 is located in the corner area 42, and the first coating layer 12 is provided on the side of the battery separator 10 corresponding to the predetermined corner section 112 facing the negative electrode sheet 30.

[0036] Through the above structural arrangement, on the corner area 42 of the battery cell, the surface of the battery separator 10 at the corresponding position of the negative electrode sheet 30 is provided with a first coating layer 12. Combined with the arrangement of the battery separator 10 and the first coating layer 12 thereon, the hydroxyl group of the binder in the first coating layer 12 can be bonded with the carboxylated graphite and stored in the graphite sheet, and then the first coating layer 12 can absorb the electrolyte to swell and store rich lithium sources, which can replace the active material on the negative electrode sheet 30 that has fallen off and failed due to cyclic extrusion in the corner area, and store Li in the corner after long-term cycling. + It can effectively improve the lithium plating of the battery after long-term cycling, improve the capacity retention rate of the battery after long-term cycling, and at the same time improve the battery's rate performance.

[0037] This embodiment adopts Figure 1 The battery separator 10 of the illustrated embodiment is assembled into a battery cell in cooperation with the positive electrode sheet 20 and the negative electrode sheet 30. Of course, in battery cells of other embodiments, the battery separator configurations of other embodiments described above may also be adopted. Since the battery cell adopts the battery separator of the above-mentioned embodiment, it at least has the technical effects corresponding to the technical solutions of the adopted embodiments, and will not be described one by one here.

[0038] The above battery cell can be further applied to lithium-ion batteries, and the present application further proposes a lithium-ion battery, which includes the above battery cell, and further includes a shell and an electrolyte, wherein the battery cell and the electrolyte are encapsulated in the shell. Since the lithium-ion battery adopts the battery cell of the above embodiment, the lithium-ion battery has a high long-cycle capacity retention rate, excellent rate performance, and excellent long-cycle performance.

[0039] The above lithium-ion battery can be further applied to electronic devices to power the load in the electronic device, and the present application also proposes an electronic device, which includes the above lithium-ion battery. The electronic device includes but is not limited to a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a transceiver, an electronic notepad, a calculator, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a camera, etc. Since the electronic device adopts the lithium-ion battery of the above embodiment, it at least has the technical effect corresponding to the technical solution of the adopted embodiment, and will not be repeated here.

[0040] The inventors conducted a large number of research experiments during the research and experiment process, including designing and preparing different battery separators and lithium-ion batteries, and testing battery performance. Some of the experimental cases and effect test results are listed below to illustrate this application:

[0041] Example 1

[0042] This embodiment prepares a battery separator, and the specific steps include:

[0043] S1. Aluminum oxide, PVDF and a dispersing agent are taken in a mass ratio of 80:20:5.5, and mixed with a solvent N-methyl-2-pyrrolidone (NMP) to prepare an aluminum oxide / PVDF slurry with a solid content of 35%. The dispersing agent in the aluminum oxide / PVDF slurry is 1wt% carboxymethyl cellulose (CMC), 0.5wt% polyvinyl alcohol (PVA) and 4wt% polyacrylic acid; then a PE base film is taken, and the aluminum oxide / PVDF slurry is coated on both sides of the PE base film. The single-layer coating thickness is 1.5μm, and then dried and divided into strips. The strip width is based on the actual negative electrode sheet width of the battery cell + 2.2mm. In this embodiment, the width is controlled to be 82.2mm to obtain a composite film;

[0044] S2. Carboxylated graphite and PEO with a molecular weight of 200,000 to 800,000 are taken at a mass ratio of 90:10, and mixed with water to prepare a coating slurry. The mass proportion of water in the coating slurry is 49.5%, the concentration of carboxylated graphite is 48 mg / mL, and the concentration of PEO is 20 mg / mL;

[0045] S3. According to the structure and winding parameters of the target winding core, the composite film including the base film is divided into predetermined straight sections corresponding to the straight areas of each layer of the target winding core and predetermined corner sections corresponding to the corner areas of each layer of the target winding core; the mixed slurry obtained in step S2 is coated on one side of the surface of the predetermined corner section of the composite film with a coating thickness of 3 μm and a coating width of 2 mm; and then dried at 60°C to form a first coating layer containing carboxylated graphite and PEO on one side of the surface of the predetermined corner section of the composite film to obtain a battery separator.

[0046] Example 2

[0047] A battery separator is prepared in this embodiment. The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, the mass ratio of carboxylated graphite to PEO is 95:5, and other operations are the same as those in embodiment 1.

[0048] Example 3

[0049] A battery separator is prepared in this embodiment. The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, the mass ratio of carboxylated graphite to PEO is 85:15, and other operations are the same as those in embodiment 1.

[0050] Example 4

[0051] A battery separator is prepared in this embodiment. The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, PEO with a molecular weight of 20,000 to 200,000 is used to replace PEO with a molecular weight of 200,000 to 800,000 in embodiment 1, and other operations are the same as those in embodiment 1.

[0052] Example 5

[0053] A battery separator is prepared in this embodiment. The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, PEO with a molecular weight of 800,000 to 1.5 million is used to replace PEO with a molecular weight of 200,000 to 800,000 in embodiment 1, and other operations are the same as those in embodiment 1.

[0054] Comparative Example 1

[0055] A battery separator is prepared in this comparative example. The difference between this comparative example and Example 1 is that this comparative example does not provide a first coating layer in the predetermined corner section, that is, the operations of step S2 and step S3 in Example 1 are cancelled, and the composite film prepared according to step S1 in Example 1 is directly used as the battery separator.

[0056] Comparative Example 2

[0057] A battery separator is prepared in this comparative example. The difference between this comparative example and Example 1 is that: in step S2 of this comparative example, the addition of PEO is cancelled, and an equal amount of carboxylated graphite is used to replace the PEO in step S2 of Example 1, and the carboxylated graphite is mixed with a solvent to prepare a slurry; other operations are the same as Example 1.

[0058] Comparative Example 3

[0059] A battery separator was prepared in this comparative example. The difference between this comparative example and Example 1 is that in step S2 of this comparative example, graphite (i.e., non-grafted carboxylated graphite) was used to replace the carboxylated graphite used in Example 1; other operations were the same as in Example 1.

[0060] Embodiments 6 to 10

[0061] Examples 6 to 10 respectively use the battery separators prepared in Examples 1 to 5 to prepare lithium-ion batteries, and the specific steps include:

[0062] S1. Preparation of negative electrode sheet: Carboxylated graphite, carboxymethyl cellulose (CMC) and a binder are taken in a mass ratio of 97.7:1.1:1.2, and mixed evenly with solvent water to prepare a negative electrode slurry, in which the mass proportion of solvent water in the negative electrode slurry is 49.5%; the negative electrode slurry is coated on the current collector copper foil, dried at 85°C, and then cold pressed and slit to obtain a negative electrode sheet.

[0063] S2. Preparation of positive electrode sheets: lithium iron phosphate, conductive agent superconducting carbon (Super-P), binder polyvinylidene fluoride (PVDF) are taken in a mass ratio of 97:1.5:1.5, and mixed evenly with solvent N-methyl-2-pyrrolidone (NMP) to obtain positive electrode slurry, in which the mass proportion of NMP in the positive electrode slurry is 75%; the positive electrode slurry is coated on the current collector aluminum foil, dried at 85°C, and then cold pressed and slit to obtain positive electrode sheets.

[0064] S3. Preparation of electrolyte: Lithium hexafluorophosphate (LiPF6) is dissolved in a mixed solvent consisting of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a mass ratio of 1:2:1 to obtain an electrolyte with a concentration of 1 mol / L.

[0065] S3. Battery assembly: stack the positive electrode sheet, battery separator and negative electrode sheet in sequence and wind them in the same direction, with the side of the battery separator provided with the first coating layer facing the negative electrode sheet, and the predetermined corner section of the base film in the battery separator serving as the winding corner, and wind to obtain a battery cell; then use aluminum-plastic film for packaging, inject electrolyte, let stand at room temperature for 24 hours, let stand at 45°C for 16 hours, and then go through the processes of formation, secondary sealing, capacity division, etc. to obtain a lithium-ion battery.

[0066] Comparative Examples 4 to 6

[0067] Comparative Examples 4 to 6 each prepare a lithium ion battery, specifically using the battery separators prepared in Comparative Examples 1 to 3, and preparing the lithium ion battery in the same preparation method as Examples 6 to 10.

[0068] Comparative Example 7

[0069] This comparative example prepares a lithium-ion battery, and the difference between this comparative example and comparative example 4 is that this comparative example sets a first coating layer on the negative electrode sheet in the corner area of ​​the battery cell. Specifically, after the negative electrode sheet is prepared according to the method of step S1 in comparative example 4, this comparative example further refers to the method of step S2 and step S3 in embodiment 1 to set a coating layer on the negative electrode sheet; specifically, after the mixed slurry is prepared according to step S2 in embodiment 1, refer to step S3 in embodiment 1, and divide the negative electrode sheet according to the structure and winding parameters of the target winding core, and divide it into a predetermined straight section corresponding to the straight area of ​​each layer of the target winding core and a predetermined corner section corresponding to the corner area of ​​each layer of the target winding core; the mixed slurry prepared in step S2 is used to coat the surface of the predetermined corner section on the negative electrode sheet on one side, and the coating thickness is 3μm and the coating width is 2mm; then it is dried at 60℃ to obtain a negative electrode sheet for battery assembly, and a first coating layer containing carboxylated graphite and PEO is formed on one side of the surface of the predetermined corner section on the negative electrode sheet. During the battery assembly process, the side of the negative electrode sheet provided with the first coating layer is arranged toward the battery separator. Other operations are the same as those in Comparative Example 4.

[0070] Comparative Example 8:

[0071] A battery separator was prepared in this comparative example. The difference between this comparative example and Example 1 is that in step S2 of this comparative example, the mass ratio of carboxylated graphite to PEO is 98:2, and other operations are the same as those in Example 1.

[0072] Comparative Example 9:

[0073] A battery separator was prepared in this comparative example. The difference between this comparative example and Example 1 is that in step S2 of this comparative example, the mass ratio of carboxylated graphite to PEO is 83:17, and other operations are the same as those in Example 1.

[0074] Comparative Example 10

[0075] In this comparative example, a battery separator is prepared. The difference between this comparative example and Example 1 is that the thickness of the first coating layer in step S3 of this comparative example is adjusted from 3 μm in Example 1 to 0.5 μm.

[0076] Comparative Example 11

[0077] In this comparative example, a battery separator is prepared. The difference between this comparative example and Example 1 is that the thickness of the first coating layer in step S3 of this comparative example is adjusted from 3 μm in Example 1 to 9 μm.

[0078] Performance Testing

[0079] At a voltage range of 2.5 to 4.2 V, the battery test system of Newwell Electronics Co., Ltd. was used to perform charge and discharge tests at 0.2C, 0.5C, 1C, and 2C rates, and 600 times of charge and discharge at a constant current density of 1C. According to the above method, the capacity retention rate and lithium deposition at the corner of the negative electrode sheet of the lithium ion batteries prepared in Examples 6 to 10 and Comparative Examples 4 to 10 were tested respectively, and the cycle specific capacity of the lithium ion batteries prepared in Example 6, Comparative Example 4 and Comparative Example 6 at different rates was tested, and the results are shown in Tables 1 and 2.

[0080] Table 1 Capacity retention rate of each lithium-ion battery and lithium deposition at the corner of the negative electrode

[0081]

[0082] Table 2 Cycle capacity of lithium-ion batteries at different rates (unit: mAh / g)

[0083]

[0084] By comparing Example 6 and Comparative Example 4 in Table 1, it can be seen that in the lithium-ion battery of Example 6, a coating layer composed of carboxylated graphite and hydroxyl-containing binder PEO is arranged on the surface of the battery diaphragm corresponding to the negative electrode sheet in the corner area of ​​the battery cell, which is beneficial to improving the capacity retention rate of the battery after long cycles and significantly improving the lithium deposition at the corner of the negative electrode sheet. The content retention rate of Example 6 (93.4%) is 9.7% higher than that of the lithium-ion battery in Comparative Example 4. This is mainly because the first coating layer coated on the surface of the battery diaphragm in the corner area can accommodate Li + , lithium can be reused in the long cycle process, thereby improving the capacity retention rate of the long cycle. However, the battery separator at the corresponding position of the negative electrode sheet in the corner area of ​​the lithium-ion battery in Comparative Example 4 is not provided with the above coating layer, and lithium deposition and capacity loss will be obvious at the rear corner after long-term circulation.

[0085] Compared with the lithium-ion battery of comparative example 7 (wherein a coating layer is provided on the negative electrode sheet in the corner area of ​​the battery cell), the first coating layer containing carboxylated graphite and a hydroxyl-containing binder is provided on the battery separator at the position corresponding to the negative electrode sheet in the corner area of ​​the battery cell, and thus, the long cycle retention rate can be higher, and the lithium precipitation at the corner of the negative electrode sheet can be improved. In addition, it was found during the test that, as in the lithium-ion battery of comparative example 7, a first coating layer containing carboxylated graphite and PEO is provided on the negative electrode sheet in the corner area of ​​the battery cell. When the electrode sheet is wound into a battery cell, the bare battery cell will still crack and fall off in a small part at the head corner position due to stress extrusion after hot pressing, and there is a situation of powder falling off at the corner (such as Figure 3As shown in the figure, it is not conducive to the adhesion of the coating layer, resulting in insufficient lithium insertion sites for lithium ions at the corners. After long-term cycling, there is slight lithium deposition at the corners of the negative electrode head.

[0086] Comparing the performance test results of lithium-ion batteries in Examples 6 to 8 and Comparative Examples 5, 6, 8, and 9 in Table 1, it can be seen that the use of a hydroxyl-containing binder PEO in combination with carboxylated graphite can improve the long cycle retention rate of the battery. When the mass ratio of the components of the first coating layer is not within the range of 5wt% to 15wt% carboxylated graphite and 85wt% to 95wt% PEO, its capacity retention rate will decrease, and there will be varying degrees of lithium precipitation. This is mainly because the hydroxyl groups of the binder PEO can be bonded with the carboxylated graphite and stored in the carboxylated graphite flakes. Within a specific range, the two work synergistically to absorb electrolyte swelling and store abundant lithium sources, which is a good way to store Li at the corners after long cycles. + , improve the battery cycle performance; from the above, when the mass ratio of carboxylated graphite to hydroxyl-containing binder PEO is controlled to 9:1, the effect is better. In Comparative Example 5, a first coating layer containing only carboxylated graphite but no hydroxyl-containing binder PEO is set on the battery separator at the corresponding position of the negative electrode sheet in the corner area of ​​the battery cell. Due to the lack of hydroxyl-containing binder PEO, the first coating layer is easy to fall off and fail. In Comparative Example 6, the first coating layer of the battery separator is made of non-grafted carboxylated graphite. After a long period of battery circulation, lithium is slightly deposited, but the capacity retention rate decreases. This is mainly because there is no bonding between graphite and PEO, which leads to the electrolyte (Li + ) is insufficiently absorbed, resulting in a decrease in capacity retention rate.

[0087] From the performance test results of the lithium-ion batteries of Example 6 and Comparative Example 10, it can be seen that when the thickness of the first coating layer is only 0.5 μm, the corresponding battery capacity retention rate is as low as 8.3%, and there is slight lithium precipitation. This is mainly because the coating layer thickness is too thin, resulting in insufficient lithium insertion sites, which leads to lithium precipitation and decreased capacity retention rate. In addition, it was found through testing that in Comparative Example 11, when the thickness of the first coating layer is as high as 9 μm, the battery capacity under the same volume will decrease, reducing the overall volume energy density of the battery.

[0088] By comparing Examples 6, 9, and 10 in Table 1, it can be seen that in the lithium-ion battery of Example 6, the molecular weight of PEO used in the first coating layer of the battery separator is 200,000 to 800,000, and the battery capacity retention rate is high, at 93.4%, which is 2.4% and 2.1% higher than that of the lithium-ion batteries of Examples 9 and 10, respectively. This is mainly because when the molecular weight of PEO is too low, its viscosity decreases, which can easily cause the first coating layer coated on the battery separator to fall off and fail to function during long cycles; and when the molecular weight of PEO is too large, its viscosity is too large and it is too close to graphite, resulting in Li +The difficulty in passing leads to a decrease in capacity retention.

[0089] By comparing Examples 6 to 8 and Comparative Examples 4 to 6 in Table 1, it can be seen that the coating layer containing carboxylated graphite is provided at the battery separator in the corner area of ​​the battery cell, which is beneficial to improve the corner lithium precipitation after long-term battery cycling. When the graphite is not carboxylated (such as Comparative Example 6) and the PEO content is 0 (such as Comparative Example 5), its long-cycle capacity retention rate is only about 85%, which is 8.4% lower than that of Example 6. This is because the carboxylated graphite and PEO have a bonding effect, swell after absorbing the electrolyte, store rich lithium sources, and provide sufficient lithium ions for long cycles.

[0090] In addition, it can be seen from Example 6 and Comparative Example 4 in Table 2 that the carboxylated graphite and the hydroxyl-containing binder PEO can work together to improve the rate performance of the battery. Compared with the lithium ion battery in Comparative Example 4, the lithium ion battery in Example 6 has a cycle capacity of 18.4 mAh / g greater at a 1C rate. It can be seen from Example 6 and Comparative Example 6 in Table 2 that the lithium ion battery in Example 6 has a cycle capacity of 12.2 mAh / g greater at a 1C rate than the lithium ion battery in Comparative Example 4. It can be seen that when graphite is not carboxylated, its rate performance is not significantly improved. Therefore, in Example 6, carboxylated graphite and the hydroxyl-containing binder PEO are used, and the two work together to improve the rate performance of the battery.

[0091] The present application is described in detail above in conjunction with the embodiments, but the present application is not limited to the above embodiments, and various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present application. In addition, the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

Claims

1. A battery separator, characterized in that: It includes a base film and a first coating layer; the base film includes predetermined straight sections and predetermined corner sections which are alternately arranged in sequence along the length direction, and the first coating layer is arranged on at least one side surface of the predetermined corner section on the base film; the components of the first coating layer include a first binder and carboxylated graphite, and the first binder is selected from a hydroxyl-containing binder.

2. The battery separator according to claim 1, characterized in that: The first adhesive is selected from at least one of polyethylene oxide, polyurethane adhesive, chloroprene rubber adhesive, polyvinyl butyral, phenolic resin, and hydroxyl-terminated polybutadiene liquid rubber.

3. The battery separator according to claim 2, characterized in that: The first binder is selected from polyethylene oxide with a molecular weight of 200,000 to 800,000.

4. The battery separator according to claim 1, characterized in that: The first coating layer comprises 5wt% to 15wt% of a first binder and 85wt% to 95wt% of carboxylated graphite; And / or, the thickness of the first coating layer is 1 μm to 6 μm.

5. The battery separator according to any one of claims 1 to 4, characterized in that: It also includes a second coating layer, which is arranged on at least one side surface of the base film; the first coating layer is arranged on the surface of the second coating layer away from the base film and is located at a position corresponding to the predetermined corner section; the components of the second coating layer include a second binder and an inorganic filler.

6. The battery separator according to claim 5, characterized in that: The second coating layer is disposed on both side surfaces of the base film, and the first coating layer is disposed on a surface of the second coating layer on at least one side of the base film.

7. The battery separator according to claim 5, characterized in that: The second binder is selected from at least one of polyvinylidene fluoride, styrene-butadiene rubber, polytetrafluoroethylene, polyacrylonitrile, polyvinyl pyrrolidone, polyurethane, polyvinyl alcohol, and polyimide; and / or the inorganic filler is selected from at least one of aluminum oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, silicon dioxide, titanium dioxide, zirconium dioxide, magnesium hydroxide, aluminum hydroxide, and boehmite.

8. A battery cell, characterized in that: It comprises a positive electrode sheet, a battery separator and a negative electrode sheet; the positive electrode sheet, the battery separator and the negative electrode sheet are sequentially stacked and wound to form a winding structure, and the winding structure comprises a straight area and a corner area connected to each other; the battery separator is selected from the battery separator according to any one of claims 1 to 7, the predetermined straight section is located in the straight area, and the predetermined corner section is located in the corner area; the first coating layer is provided on the battery separator corresponding to the predetermined corner section, and the side facing the negative electrode sheet.

9. A lithium ion battery, characterized in that: Comprising the battery cell as claimed in claim 8.

10. An electronic device, characterized in that: Includes the lithium ion battery as claimed in claim 9.

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