Separator and method for manufacturing the same, and battery cell

Abstract

The application provides a diaphragm, and a composition of the diaphragm comprises a mixture of polyethylene, graphene oxide and an antioxidant. The combination of the graphene oxide and the polyethylene can make the diaphragm have good heat conduction capacity and heat dissipation performance while ensuring the basic function of the diaphragm in a battery, thereby effectively reducing the surface temperature of the battery cell.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, specifically to a separator and its preparation method, and a battery cell. Background Technology

[0002] With the rapid development of new energy technologies, lithium batteries have become the most widely used rechargeable batteries due to their advantages such as small size, high energy density, long cycle life, no memory effect, high average output voltage, and low self-discharge.

[0003] However, lithium batteries generate a large amount of heat during high-current discharge, which can damage the battery and even cause safety accidents. Therefore, improving the heat dissipation effect of lithium batteries is of great significance. Summary of the Invention

[0004] To address the aforementioned problems in the prior art, the purpose of this application is to provide a separator, its preparation method, and a battery cell. This separator exhibits excellent heat dissipation performance, effectively reducing the surface temperature of the battery cell.

[0005] To achieve the above objectives, this application adopts the following technical solution:

[0006] This application discloses a diaphragm whose material composition includes a mixture of polyethylene, graphene oxide, and antioxidants.

[0007] In one embodiment, the weight ratio of the polyethylene, graphene oxide, and antioxidant is (90-99)(0.5-9.5):0.5.

[0008] In one embodiment, the weight ratio of the polyethylene, graphene oxide, and antioxidant is (93.5–99)(0.5–6):0.5.

[0009] In one embodiment, the number of graphene oxide layers is 1 to 8.

[0010] In one embodiment, the thickness of the diaphragm is 5 μm to 30 μm.

[0011] In one embodiment, the porosity of the diaphragm is 20% to 60%.

[0012] This application also provides a method for preparing a diaphragm, comprising the following steps:

[0013] The polyethylene, the graphene oxide, and the antioxidant are mixed, a pore-forming agent is added, the mixture is plasticized, extruded, cooled, and subjected to a first stretching and a second stretching.

[0014] The pore-forming agent is extracted and subjected to a third stretching process, followed by heat setting to prepare the diaphragm.

[0015] In one embodiment, the method for preparing the diaphragm includes one or more of the following features:

[0016] (1) The stretching temperature for the first stretching, the second stretching and the third stretching is 115℃~135℃;

[0017] (2) The heat setting temperature is set to 130℃~140℃;

[0018] (3) The pore-forming agent is one or more of white oil, paraffin oil and polyethylene glycol, and the weight ratio of polyethylene to pore-forming agent is 1:(1-5).

[0019] This application also provides a battery cell, including the separator described in any of the above embodiments and a positive electrode and a negative electrode disposed on both sides of the separator.

[0020] This application also provides an electrical device, including the diaphragm or the battery cell described in any of the above embodiments.

[0021] Compared with existing technologies, this application has the following advantages:

[0022] This application provides a separator, the constituent materials of which include a mixture of polyethylene, graphene oxide, and antioxidants. The combined effect of graphene oxide and polyethylene ensures the basic function of the separator in the battery while enabling the separator to have good thermal conductivity and heat dissipation performance, thereby effectively reducing the surface temperature of the battery cell. Detailed Implementation

[0023] The following detailed description, in conjunction with specific embodiments, further illustrates the separator, the method for preparing the separator, and the battery cell of this application. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application. Of course, they are merely examples and are not intended to limit this application.

[0024] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein. When a numerical interval refers only to integers within that interval, it includes the two endpoints of the numerical range, as well as every integer between the two endpoints. In this context, this is equivalent to directly listing every integer; for example, if t is an integer selected from 1 to 10, it means t is any integer selected from the group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Furthermore, when multiple ranges are provided to describe a feature or characteristic, these ranges may be combined. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all subranges included therein.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0026] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0027] Lithium-ion batteries or lithium metal batteries generate a large amount of heat during high-current discharge, which can damage the battery and even cause safety accidents. This defect limits the application of batteries in high-power power consumption scenarios. Currently, lithium-ion batteries or lithium metal batteries have the following technologies to solve the problems of heat generation and heat dissipation: (1) Selecting positive and negative electrode active materials that can be charged and discharged at high current to reduce heat generation under high current conditions; (2) Using graphene as a conductive agent and heat dissipation agent in the positive and negative electrode formulation to improve heat dissipation under high current conditions; (3) Using thicker positive and negative electrode current collectors to reduce battery internal resistance, accelerate heat transfer, and improve heat dissipation under high current conditions; (4) Using a multi-tab structure to reduce battery internal resistance, accelerate heat transfer, and improve heat dissipation under high current conditions; (5) Using a lower positive and negative electrode coating density to reduce battery internal resistance, accelerate heat transfer, and improve heat dissipation under high current conditions. Based on rich experience and extensive research, the inventors of this application have found that the component with the worst heat dissipation performance in lithium-ion batteries or lithium metal batteries is the separator. However, there are currently few solutions for heat dissipation measures for the separator.

[0028] In lithium batteries, the separator is usually made of polyolefin material. Polyolefin separators are widely used because of their low raw material cost and easy processing. However, because polyethylene material is prone to thermal shrinkage in high-temperature environments, the separator's isolation effect is reduced, which can easily lead to the risk of short circuits. This property also greatly reduces the scope of application of polyolefin separators.

[0029] Embodiments of this application provide a diaphragm whose material composition includes a mixture of polyethylene, graphene oxide, and antioxidants.

[0030] In this application, the material composition of the separator includes a mixture of polyethylene, graphene oxide and antioxidant. The combined effect of graphene oxide and polyethylene ensures the basic function of the separator in the battery, while enabling the separator to have good thermal conductivity and heat dissipation performance, thereby effectively reducing the surface temperature of the battery cell.

[0031] Understandably, the basic functions of the separator in the battery include: (1) acting as a lithium-ion channel between the positive and negative electrodes. During charging, lithium ions flow from the positive electrode through the separator and are deposited on the lithium metal surface or embedded in graphite. During discharging, lithium ions flow from the negative electrode through the separator and are embedded in the positive electrode active material; (2) acting as an electronic insulating material for the positive and negative electrode materials, preventing electrons from being transferred between the positive and negative electrodes through the separator.

[0032] In one example, the molecular weight of the polyethylene is between 500,000 and 2,500,000. Specifically, the molecular weight of the polyethylene includes, but is not limited to, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,250,000, 1,300,000, 1,350,000, 1,400,000, 1,500,000, 2,000,000, 2,100,000, 2,200,000, 2,300,000, 2,400,000, or 2,500,000.

[0033] In one example, the weight ratio of the polyethylene, graphene oxide, and antioxidant is (90–99)(0.5–9.5):0.5.

[0034] More preferably, the weight ratio of polyethylene, graphene oxide, and antioxidant is (93.5–99)(0.5–6):0.5. Specifically, the weight ratio of polyethylene, graphene oxide, and antioxidant includes, but is not limited to, 93.5:6:0.5, 94.5:5:0.5, 95.5:4:0.5, 95:4.5:0.5, 96.5:3:0.5, 97.5:2:0.5, 98:1.5:0.5, 98:1.5:0.5, 98.5:1:0.5, or 99:0.5:0.5.

[0035] In one example, the graphene oxide has 1 to 8 layers. Specifically, the number of graphene oxide layers can be selected from 1, 2, 3 to 5, or 6 to 8 layers. More specifically, the number of graphene oxide layers includes, but is not limited to, 1, 2, 3, 4, 5, 6, 7, or 8 layers. Preferably, the graphene oxide has 1 layer.

[0036] In one example, the thickness of the diaphragm is 5 μm to 30 μm. It is understood that the thickness of the diaphragm can be any value between 5 μm and 30 μm. Specifically, the thickness of the diaphragm includes, but is not limited to, 5 μm, 6 μm, 7 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 11 μm, 12 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, or 30 μm.

[0037] In one example, the porosity of the membrane is 20% to 60%. It is understood that the porosity of the membrane can be any value between 20% and 60%. Specifically, the porosity of the membrane includes, but is not limited to, 20%, 21%, 25%, 30%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 50%, 55%, 56%, 57%, 58%, 59%, or 60%.

[0038] In one example, the antioxidants include phenols and amines.

[0039] In one example, the phenolic antioxidant includes one or more of monophenols, bisphenols, triphenols, polyphenols, hydroquinone, or thiobisphenols.

[0040] In one example, the amine antioxidant includes one or more of naphthylamine, diphenylamine, p-phenylenediamine, quinoline derivatives, phosphites, or thioesters.

[0041] This application also provides a method for preparing a diaphragm, comprising the following steps:

[0042] The polyethylene, the graphene oxide, and the antioxidant are mixed, a pore-forming agent is added, the mixture is plasticized, extruded, cooled, and subjected to a first stretching and a second stretching.

[0043] The pore-forming agent is extracted and subjected to a third stretching process, followed by heat setting to prepare the diaphragm.

[0044] In one example, the stretching temperature for the first, second, and third stretching is 115°C to 135°C. Understandably, the stretching temperature for the first, second, and third stretching can be selected from any value between 115°C and 135°C.

[0045] In one example, the first stretch and the second stretch include lateral stretch and longitudinal stretch.

[0046] In one example, the temperatures for lateral stretching and longitudinal stretching are different.

[0047] In one example, the steps of the first stretching and the second stretching include: sequentially performing a first longitudinal stretching and a first transverse stretching on the cooled material, and then sequentially performing a second longitudinal stretching and a second transverse stretching.

[0048] In one example, the ratio of the first longitudinal stretch to the first transverse stretch is (12 to 17) times, and the temperatures of the first longitudinal stretch and the first transverse stretch are independently selected from 115°C to 135°C.

[0049] In one example, the ratio of the second longitudinal stretch and the second transverse stretch is (8 to 12) times, and the temperature of the second longitudinal stretch and the second transverse stretch is independently selected from 115°C to 135°C.

[0050] In one example, the step of extracting the pore-forming agent includes: fractional extraction of white oil using dichloromethane as an extractant.

[0051] In one example, the extraction time was 10 min to 40 min.

[0052] In one example, the third stretch includes a lateral stretch.

[0053] In one example, the third stretching step includes: performing a third lateral stretching on the diaphragm after the pore-forming agent has been extracted.

[0054] In one example, the stretching ratio of the third transverse stretch is (1 to 1.5) times, and the temperature of the third transverse stretch is independently selected from 115°C to 135°C.

[0055] In one example, the heat setting temperature was set to 130°C to 140°C.

[0056] In one example, the pore-forming agent is one or more of white oil, paraffin oil, and polyethylene glycol, and the weight ratio of polyethylene to the pore-forming agent is 1:(1-5). Specifically, the weight ratio of polyethylene to the pore-forming agent includes, but is not limited to, 1:1, 1:2, 1:2.5, 1:2.8, 1:2.9, 1:3, 1:3.1, 1:3.2, 1:3.5, 1:4, 1:4.5, 1:4.7, 1:4.8, 1:4.9, or 1:5.

[0057] In one example, the extrusion device is an extruder.

[0058] This application also provides a battery cell, including a separator as described in any of the above examples and a positive electrode and a negative electrode disposed on both sides of the separator.

[0059] In one example, the cell is a lithium-ion battery cell. Without limitation, the cell can be a lithium-ion cell or a lithium metal cell.

[0060] In one example, the thickness of the positive electrode is 70 μm to 100 μm. Specifically, the thickness of the positive electrode includes, but is not limited to, 70 μm, 71 μm, 75 μm, 79 μm, 80 μm, 85 μm, 86 μm, 87 μm, 88 μm, 89 μm, 90 μm, 91 μm, 92 μm, 95 μm, 96 μm, 97 μm, 98 μm, 99 μm, or 100 μm.

[0061] In one example, the parameters of the positive electrode include: a positive electrode width of 100mm to 200mm, a positive electrode height of 150mm to 300mm, a tab width of 40mm to 70mm, and a tab height of 15mm to 30mm.

[0062] In one example, the parameters of the negative electrode include: the width of the positive electrode is 100mm to 200mm, the height of the positive electrode is 150mm to 300mm, the width of the tab is 40mm to 70mm, and the height of the tab is 15mm to 30mm.

[0063] In one example, the positive electrode includes a positive current collector and an active slurry layer laminated on the surface of the positive current collector.

[0064] The active slurry layer includes a positive electrode active material, a conductive agent, and a binder.

[0065] In one example, the surface density of the active slurry layer is 200 g / m². 2 ~350g / m 2 Specifically, the surface density of the active slurry layer includes, but is not limited to, 200 g / m². 2 201g / m 2202g / m 2 250g / m 2 251g / m 2 290g / m 2 295g / m 2 296g / m 2 297g / m 2 298g / m 2 299g / m 2 300g / m 2 301g / m 2 302g / m 2 303g / m 2 304g / m 2 305g / m 2 310g / m 2 320g / m 2 330g / m 2 340g / m 2 Or 350g / m 2 .

[0066] In one example, the weight ratio of the positive electrode active material, conductive agent, and binder is (90-99):(0.5-5):(0.5-5). Specifically, the weight ratio of the positive electrode active material, conductive agent, and binder includes, but is not limited to, 90:5:5, 96:1:1, 97:1:1, 98:1:0.5, 98:1:1, 98:1:1.5, or 99:0.5:0.5.

[0067] In one example, the solid content of the slurry containing the positive electrode active material, conductive agent, and binder is 70% to 80%. Specifically, the solid content of the slurry containing the positive electrode active material, conductive agent, and binder includes, but is not limited to, 70%, 71%, 72%, 75%, 76%, 77%, 78%, 79%, or 80%.

[0068] In one example, the viscosity of the slurry containing the positive electrode active material, conductive agent, and binder is 7000 mPa·s to 8500 mPa·s. Specifically, the viscosity of the slurry containing the positive electrode active material, conductive agent, and binder includes, but is not limited to, 7000 mPa·s, 7100 mPa·s, 7200 mPa·s, 7500 mPa·s, 7600 mPa·s, 7700 mPa·s, 7799 mPa·s, 7800 mPa·s, 7801 mPa·s, 7810 mPa·s, 7900 mPa·s, 8000 mPa·s, 8100 mPa·s, 8200 mPa·s, 8300 mPa·s, 8400 mPa·s, or 8500 mPa·s.

[0069] In one example, the positive current collector is aluminum foil.

[0070] In one example, the thickness of the positive current collector is 12 μm.

[0071] In one example, the positive electrode active material includes LiCoO2 and LiNi. 0.8 Co 0.1 Mn 0.1 One or more of O2 and NCA.

[0072] In one example, the conductive agent includes one or more of graphene, conductive carbon black, and conductive graphite.

[0073] In one example, the negative electrode includes a negative current collector and a lithium metal layer stacked on the surface of the negative current collector.

[0074] In one example, the negative current collector is a copper foil, a microporous copper foil, a thin steel sheet, or a thin steel mesh.

[0075] In one example, the areal density of the lithium metal layer on one side is 2 g / m². 2 ~4g / m 2 Specifically, the areal density of the lithium metal layer on one side is 2 g / m³. 2 2.1g / m 2 2.2g / m 2 2.3g / m 2 2.4g / m 2 2.5g / m 2 2.6g / m 2 2.65g / m 2 2.66g / m 2 2.67g / m 2 2.68g / m 2 2.69g / m 2 2.7g / m 2 2.8g / m 2 2.9g / m 2 3g / m 2 3.1g / m 2 3.2g / m 2 3.3g / m 2 3.4g / m 2 3.5g / m 2 3.6g / m 2 3.7g / m 2 3.8g / m 2 3.9g / m 2or 4g / m 2 .

[0076] In one example, the thickness of the lithium metal layer on one side is 3 μm to 8 μm. Specifically, the thickness of the lithium metal layer on one side is 3 μm, 3.5 μm, 4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm, 5 μm, 5.1 μm, 5.2 μm, 5.3 μm, 5.4 μm, 5.5 μm, 6 μm, 7 μm, 7.5 μm, or 8 μm.

[0077] This application also provides an electrical device, including the diaphragm or the battery cell described in any of the above examples.

[0078] The following are specific embodiments. Unless otherwise specified, the raw materials used in the embodiments are all commercially available.

[0079] In this example, the electrolyte was purchased from Hunan Dajing New Materials Co., Ltd., specification: DJ6056YI.

[0080] In the example, the aluminum-plastic film was purchased from Mingguan New Materials Co., Ltd., with a thickness of 153μm.

[0081] In the examples, the graphene oxide was purchased from Shenzhen Guosen Leading Technology Co., Ltd., and came in three types: single layer, 3-5 layers, and 6-8 layers.

[0082] In the example, the lithium metal anode sheet was purchased from Tianjin Zhongneng Lithium Industry Co., Ltd., with a double-sided lithium layer thickness of 10μm.

[0083] Example 1

[0084] Example 1 provides a method for preparing a battery cell including a separator, the specific preparation method is as follows:

[0085] Preparation of the positive electrode: NCM(811), graphene, and PVDF were mixed in a weight ratio of 98:1:1 to prepare a slurry with a solid content of 76% and a viscosity of 7800 mPa·s. The slurry was then uniformly coated onto an aluminum foil with a thickness of 12 μm and a coating surface density of 300 g / m². 2 The aluminum foil coated with slurry is rolled to a thickness of 90 μm to prepare a positive electrode sheet. After preparation, the positive electrode sheet is cut to a width of 161.9 mm, a height of 209 mm, a tab width of 55 mm, and a tab height of 23.5 mm.

[0086] Preparation of the negative electrode: The lithium metal negative electrode was placed in an argon-drying atmosphere with a moisture content of less than 1 ppm and an oxygen content of less than 1 ppm. The lithium metal foil of the negative electrode was then bonded to a copper foil using a roll forming process. The areal density of the double-sided lithium metal layer was 5.34 g / m². 2 A negative electrode sheet with a thickness of 10 μm was prepared. After preparation, the negative electrode sheet was cut to a width of 161.9 mm, a height of 213 mm, a tab width of 55 mm, and a tab height of 22.5 mm.

[0087] Preparation of the diaphragm: Polyethylene with an average molecular weight of 1.3 million was selected. Polyethylene, single-layer graphene oxide, and antioxidant were mixed in a weight ratio of 98.5:1:0.5 and poured into the extruder hopper. Simultaneously, 300 parts of white oil were added. The screw and extrusion rate were adjusted to ensure thorough mixing and plasticization of the high-molecular-weight polyethylene and white oil. The mixture was extruded through a die and cooled on cooling rollers to form a cast sheet. The cast sheet was then subjected to MD stretching and TD stretching, both with a stretching ratio of 10 times. The MD stretching temperature was 120℃, and the TD stretching temperature was 125℃. The stretched diaphragm was then subjected to MD stretching again, with a stretching ratio of 10 times and a stretching temperature of 130℃, followed by TD stretching, with a stretching ratio of 10 times and a stretching temperature of 130℃. Dichloromethane was used as the extractant for fractional extraction of the white oil for 30 minutes. The extracted diaphragm was then subjected to TD stretching, with a stretching ratio of 1.2 times and a stretching temperature of 130℃. The TD-stretched diaphragm was heat-set at 135℃ to prepare a diaphragm with a thickness of 9μm. The porosity of the diaphragm was tested to be 41.3%. After the diaphragm was prepared, it was cut to a width of 221mm.

[0088] The above-mentioned positive electrode sheets (10 pieces), negative electrode sheets (11 pieces), and separator sheets are stacked and assembled into a core. Then, aluminum-plastic film is used to package it to complete the top and side sealing. Then, it is baked and injected with electrolyte. During the electrolyte injection process, the amount of electrolyte injected is 80g. After aging and formation, it is made into a soft-pack battery cell of model 16167227. The final dimensions of the battery cell are thickness × width × height (MAX): 1.6 × 167 × 227 mm.

[0089] Example 2

[0090] Example 2 is basically the same as Example 1, the main difference being that: during the preparation of the diaphragm, polyethylene, single-layer graphene oxide and antioxidant were mixed in a weight ratio of 97.5:2:0.5, and the porosity of the diaphragm was tested to be 40.3%.

[0091] Example 3

[0092] Example 3 is basically the same as Example 1, the main difference being that: during the preparation of the diaphragm, polyethylene, single-layer graphene oxide and antioxidant were mixed in a weight ratio of 96.5:3:0.5, and the porosity of the diaphragm was tested to be 39.4%.

[0093] Example 4

[0094] Example 4 is basically the same as Example 1, the main difference being that: during the preparation of the diaphragm, polyethylene, single-layer graphene oxide and antioxidant were mixed in a weight ratio of 95.5:4:0.5, and the porosity of the diaphragm was tested to be 38.6%.

[0095] Example 5

[0096] Example 5 is basically the same as Example 1, the main difference being that: during the preparation of the diaphragm, polyethylene, single-layer graphene oxide and antioxidant were mixed in a weight ratio of 94.5:5:0.5, and the porosity of the diaphragm was tested to be 37.5%.

[0097] Example 6

[0098] Example 6 is basically the same as Example 1, the main difference being that: during the preparation of the diaphragm, polyethylene, single-layer graphene oxide and antioxidant were mixed in a weight ratio of 93.5:6:0.5, and the porosity of the diaphragm was tested to be 36.6%.

[0099] Example 7

[0100] Example 7 is basically the same as Example 4, the main difference being that in the preparation of the membrane, single-layer graphene oxide is replaced with 3-5 layers of graphene oxide. Specifically, polyethylene, 3-5 layers of graphene oxide, and antioxidant are mixed in a weight ratio of 95.5:4:0.5, and the membrane porosity is tested to be 39.2%. The specific preparation method is as follows:

[0101] Preparation of the positive electrode: NCM(811), graphene, and PVDF were mixed in a weight ratio of 98:1:1 to prepare a slurry with a solid content of 76% and a viscosity of 7800 mPa·s. The slurry was then uniformly coated onto an aluminum foil with a thickness of 12 μm and a coating surface density of 300 g / m². 2 The aluminum foil coated with slurry is rolled to a thickness of 90 μm to prepare a positive electrode sheet. After preparation, the positive electrode sheet is cut to a width of 161.9 mm, a height of 209 mm, a tab width of 55 mm, and a tab height of 23.5 mm.

[0102] Preparation of the negative electrode: The lithium metal negative electrode was placed in an argon-drying atmosphere with a moisture content of less than 1 ppm and an oxygen content of less than 1 ppm. The lithium metal foil of the negative electrode was then bonded to a copper foil using a roll forming process. The areal density of the double-sided lithium metal layer was 5.34 g / m². 2 A negative electrode sheet with a thickness of 10 μm was prepared. After preparation, the negative electrode sheet was cut to a width of 161.9 mm, a height of 213 mm, a tab width of 55 mm, and a tab height of 22.5 mm.

[0103] Preparation of the diaphragm: Polyethylene with an average molecular weight of 1.3 million was selected. Polyethylene, 3-5 layers of graphene oxide, and antioxidant were mixed in a weight ratio of 98.5:1:0.5 and poured into the extruder hopper. Simultaneously, 300 parts of white oil were added. The screw and extrusion rate were adjusted to ensure thorough mixing and plasticization of the high-molecular-weight polyethylene and white oil. The mixture was extruded through a die and cooled on cooling rollers to form a cast sheet. The cast sheet was then subjected to MD stretching and TD stretching, both with a stretching ratio of 10 times. The MD stretching temperature was 120℃, and the TD stretching temperature was 125℃. The stretched diaphragm was then subjected to MD stretching again, with a stretching ratio of 10 times and a stretching temperature of 130℃, followed by TD stretching, with a stretching ratio of 10 times and a stretching temperature of 130℃. Dichloromethane was used as the extractant for fractional extraction of the white oil for 30 minutes. The extracted diaphragm was then subjected to TD stretching, with a stretching ratio of 1.2 times and a stretching temperature of 130℃. The TD-stretched diaphragm was heat-set at 135℃ to prepare a diaphragm with a thickness of 9μm. The porosity of the diaphragm was tested to be 39.2%. After the diaphragm was prepared, it was cut to a width of 221mm.

[0104] The above-mentioned positive electrode sheets (10 pieces), negative electrode sheets (11 pieces), and separator sheets are stacked and assembled into a core. Then, aluminum-plastic film is used to package it to complete the top and side sealing. Then, baking and liquid injection are carried out. During the liquid injection process, the liquid injection amount is 80g. After aging and formation, a soft-pack battery cell of model 16167227 is produced. The final dimensions of the battery cell are thickness × width × height (MAX): 1.6 × 167 × 227 mm.

[0105] Example 8

[0106] Example 8 is basically the same as Example 4, the main difference being that in the preparation of the membrane, single-layer graphene oxide is replaced with 6-8 layers of graphene oxide. Specifically, polyethylene, 6-8 layers of graphene oxide, and antioxidant are mixed in a weight ratio of 95.5:4:0.5, and the membrane porosity is tested to be 39.7%. The specific preparation method is as follows:

[0107] Preparation of the positive electrode: NCM(811), graphene, and PVDF were mixed in a weight ratio of 98:1:1 to prepare a slurry with a solid content of 76% and a viscosity of 7800 mPa·s. The slurry was then uniformly coated onto an aluminum foil with a thickness of 12 μm and a coating surface density of 300 g / m². 2 The aluminum foil coated with slurry is rolled to a thickness of 90 μm to prepare a positive electrode sheet. After preparation, the positive electrode sheet is cut to a width of 161.9 mm, a height of 209 mm, a tab width of 55 mm, and a tab height of 23.5 mm.

[0108] Preparation of the negative electrode: The lithium metal negative electrode was placed in an argon-drying atmosphere with a moisture content of less than 1 ppm and an oxygen content of less than 1 ppm. The lithium metal foil of the negative electrode was then bonded to a copper foil using a roll forming process. The areal density of the double-sided lithium metal layer was 5.34 g / m². 2 A negative electrode sheet with a thickness of 10 μm was prepared. After preparation, the negative electrode sheet was cut to a width of 161.9 mm, a height of 213 mm, a tab width of 55 mm, and a tab height of 22.5 mm.

[0109] Preparation of the diaphragm: Polyethylene with an average molecular weight of 1.3 million was selected. Polyethylene, 6-8 layers of graphene oxide, and antioxidant were mixed in a weight ratio of 98.5:1:0.5 and poured into the extruder hopper. Simultaneously, 300 parts of white oil were added. The screw and extrusion rate were adjusted to ensure thorough mixing and plasticization of the high-molecular-weight polyethylene and white oil. The mixture was extruded through a die and cooled on cooling rollers to form a cast sheet. The cast sheet was then subjected to MD stretching and TD stretching, both with a stretching ratio of 10 times. The MD stretching temperature was 120℃, and the TD stretching temperature was 125℃. The stretched diaphragm was then subjected to MD stretching again, with a stretching ratio of 10 times and a stretching temperature of 130℃, followed by TD stretching, with a stretching ratio of 10 times and a stretching temperature of 130℃. Dichloromethane was used as the extractant for fractional extraction of the white oil for 30 minutes. The extracted diaphragm was then subjected to TD stretching, with a stretching ratio of 1.2 times and a stretching temperature of 130℃. The TD-stretched diaphragm was heat-set at 135℃ to prepare a diaphragm with a thickness of 9μm. The porosity of the diaphragm was tested to be 39.7%. After the diaphragm was prepared, it was cut to a width of 221mm.

[0110] The above-mentioned positive electrode sheets (10 pieces), negative electrode sheets (11 pieces), and separator sheets are stacked and assembled into a core. Then, aluminum-plastic film is used to package it to complete the top and side sealing. Then, baking and liquid injection are carried out. During the liquid injection process, the liquid injection amount is 80g. After aging and formation, a soft-pack battery cell of model 16167227 is produced. The final dimensions of the battery cell are thickness × width × height (MAX): 1.6 × 167 × 227 mm.

[0111] Comparative Example 1

[0112] Comparative Example 1 is basically the same as Example 1, the main difference being that in the preparation of the diaphragm, polyethylene, monolayer graphene oxide, and antioxidant were mixed in a weight ratio of 99.5:0:0.5, and the diaphragm porosity was tested to be 42.1%. The specific preparation method is as follows:

[0113] Preparation of the positive electrode: NCM(811), graphene, and PVDF were mixed in a weight ratio of 98:1:1 to prepare a slurry with a solid content of 76% and a viscosity of 7800 mPa·s. The slurry was then uniformly coated onto an aluminum foil with a thickness of 12 μm and a coating surface density of 300 g / m². 2 The aluminum foil coated with slurry is rolled to a thickness of 90 μm to prepare a positive electrode sheet. After preparation, the positive electrode sheet is cut to a width of 161.9 mm, a height of 209 mm, a tab width of 55 mm, and a tab height of 23.5 mm.

[0114] Preparation of the negative electrode: The lithium metal negative electrode was placed in an argon-drying atmosphere with a moisture content of less than 1 ppm and an oxygen content of less than 1 ppm. The lithium metal foil of the negative electrode was then bonded to a copper foil using a roll forming process. The areal density of the double-sided lithium metal layer was 5.34 g / m². 2 A negative electrode sheet with a thickness of 10 μm was prepared. After preparation, the negative electrode sheet was cut to a width of 161.9 mm, a height of 213 mm, a tab width of 55 mm, and a tab height of 22.5 mm.

[0115] Preparation of the diaphragm: Polyethylene with an average molecular weight of 1.3 million was selected. Polyethylene, monolayer graphene oxide, and antioxidant were mixed in a weight ratio of 99.5:0:0.5 and poured into the extruder hopper. Simultaneously, 300 parts of white oil were added. The screw and extrusion rate were adjusted to ensure thorough mixing and plasticization of the high-molecular-weight polyethylene and white oil. The mixture was extruded through a die and cooled on cooling rollers to form a cast sheet. The cast sheet was then subjected to MD stretching and TD stretching, both with a stretching ratio of 10 times. The MD stretching temperature was 120℃, and the TD stretching temperature was 125℃. The stretched diaphragm was then subjected to MD stretching again, with a stretching ratio of 10 times and a stretching temperature of 130℃, followed by TD stretching, with a stretching ratio of 10 times and a stretching temperature of 130℃. Dichloromethane was used as the extractant for fractional extraction of the white oil for 30 minutes. The extracted diaphragm was then subjected to TD stretching, with a stretching ratio of 1.2 times and a stretching temperature of 130℃. The TD-stretched diaphragm was heat-set at 135℃ to prepare a diaphragm with a thickness of 9μm. The porosity of the diaphragm was tested to be 42.1%. After the diaphragm was prepared, it was cut to a width of 221mm.

[0116] The above-mentioned positive electrode sheets (10 pieces), negative electrode sheets (11 pieces), and separator sheets are stacked and assembled into a core. Then, aluminum-plastic film is used to package it to complete the top and side sealing. Then, baking and liquid injection are carried out. During the liquid injection process, the liquid injection amount is 80g. After aging and formation, a soft-pack battery cell of model 16167227 is produced. The final dimensions of the battery cell are thickness × width × height (MAX): 1.6 × 167 × 227 mm.

[0117] Electrochemical property testing

[0118] The cells were charged to 4.2V using a 1C constant current and constant voltage method, with a cutoff current of 0.05C. They were then discharged using a 20C constant current method, with a discharge cutoff voltage of 3.0V. The highest surface temperature of the cells during the first 20C discharge was measured using thermocouples in both the example and comparative examples. The ratio of the cell's first 20C discharge capacity to its 1C charging capacity was measured, and the capacity retention rate after 50 cycles is shown in Table 1.

[0119] Table 1

[0120]

[0121] The test results of 20C discharge capacity / 1C charge capacity show that in Examples 1-8, the separator can achieve the basic functions in the battery cell, maintaining more than 98% of the 1C charge capacity at a 20C discharge capacity. Furthermore, the capacity retention rate test results after 50 cycles show that the capacity retention rate of the battery cells in Examples 1-8 after 50 cycles is better than that of Comparative Example 1. In Comparative Example 1, without the addition of graphene oxide, the tested surface temperature of the battery cell was 75.8℃. In Examples 1-8, with the addition of graphene oxide to the separator, the tested surface temperatures of the battery cells ranged from 45.6℃ to 71.3℃, all lower than the 75.8℃ in Comparative Example 1. This indicates that adding graphene oxide to the separator improves the heat dissipation performance of the separator and reduces the surface temperature of the battery cell. Examples 1-4 show that the cell surface temperature decreases with increasing graphene oxide weight percentage. In Examples 4-6, the cell surface temperature increases with further increases in graphene oxide weight percentage. This is because the membrane porosity decreases with increasing graphene oxide weight percentage, resulting in a balance between heat dissipation from graphene oxide and heat generation from decreased porosity. A comparison of Examples 4 with Examples 7 and 8 reveals that the cell surface temperature is lower in Example 4, indicating that when the graphene oxide weight percentage is consistent, the heat dissipation effect of the membrane prepared with single-layer graphene oxide is better than that of the membrane prepared with multi-layer graphene oxide.

[0122] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0123] The embodiments described above are merely illustrative of several implementation methods of this application, intended to facilitate a detailed understanding of the technical solutions of this application, but should not be construed as limiting the scope of protection of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the scope of protection of this application. It should be understood that technical solutions obtained by those skilled in the art based on the technical solutions provided in this application through logical analysis, reasoning, or limited experimentation are all within the scope of protection of the appended claims. Therefore, the scope of protection of this patent application should be determined by the content of the appended claims, and the specification can be used to interpret the content of the claims.

Claims

1. A diaphragm, characterized in that, The membrane is composed of a mixture of polyethylene, graphene oxide, and antioxidants; wherein the weight ratio of the polyethylene, graphene oxide, and antioxidants is (90~99):(0.5~9.5):0.

5. The method for preparing the diaphragm includes the following steps: mixing the polyethylene, the graphene oxide, and the antioxidant; adding a pore-forming agent and mixing and plasticizing; extruding; cooling; performing a first stretching and a second stretching; extracting the pore-forming agent; performing a third stretching; and heat-setting to prepare the diaphragm. The third stretching step includes: performing a third transverse stretching on the diaphragm after extracting the pore-forming agent; The stretching temperature for the third stretch is 115℃~135℃; The heat setting temperature is set to 130℃~140℃.

2. The diaphragm according to claim 1, characterized in that, The weight ratio of the polyethylene, graphene oxide and antioxidant is (93.5~99):(0.5~6):0.

5.

3. The diaphragm according to claim 1, characterized in that, The graphene oxide has 1 to 8 layers.

4. The diaphragm according to any one of claims 1 to 3, characterized in that, The thickness of the diaphragm is 5 μm to 30 μm.

5. The diaphragm according to any one of claims 1 to 3, characterized in that, The porosity of the diaphragm is 20% to 60%.

6. The diaphragm according to any one of claims 1 to 3, characterized in that, The antioxidants include one or more of phenolic antioxidants and amine antioxidants; The phenolic antioxidants include one or more of monophenols, bisphenols, triphenols, polyphenols, hydroquinones, and thiobisphenols. The amine antioxidants include one or more of naphthylamine, diphenylamine, p-phenylenediamine, and quinoline derivatives.

7. The diaphragm according to any one of claims 1 to 3, characterized in that, The step of extracting the pore-forming agent includes: using dichloromethane as an extractant to perform segmented extraction of the pore-forming agent.

8. The diaphragm according to claim 1, characterized in that, The method for preparing the diaphragm includes one or more of the following features: (1) The stretching temperature for the first, second and third stretching is 115℃~135℃; (2) The heat setting temperature is set to 130℃~140℃; (3) The pore-forming agent is one or more of white oil, paraffin oil, and polyethylene glycol, and the weight ratio of polyethylene to pore-forming agent is 1: (1~5).

9. A battery cell, characterized in that, It includes the diaphragm as described in any one of claims 1 to 8, and the positive electrode and negative electrode disposed on both sides of the diaphragm.

10. An electrical appliance, characterized in that, It includes the diaphragm as described in any one of claims 1 to 8 or the battery cell as described in claim 9.

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