Battery cell and electrochemical device comprising same

Abstract

The invention relates to the field of battery materials, in particular to a battery cell and an electrochemical device comprising the battery cell, the battery cell comprises a positive pole piece, a negative pole piece, electrolyte and a diaphragm located between the positive pole piece and the negative pole piece; the diaphragm comprises a first coating and a second coating of a base membrane, the first coating is located on the surface of one side of the base membrane and faces the positive pole piece, the second coating is located on the surface of the other side of the base membrane and faces the negative pole piece, and the first coating and the second coating both comprise a first element and a second element; the difference between the mass content ratio b of the first element to the second element on the surface of the second coating and the mass content ratio a of the first element to the second element on the surface of the first coating is controlled to meet a certain range, so that the battery has long circulation and excellent thermal safety performance.

Description

Technical Field

[0001] This application relates to the field of battery materials, and particularly to an electrode core and an electrochemical device including the electrode core. Background Art

[0002] In the field of consumer 3C products, lithium-ion batteries have been widely used in products such as mobile phones, laptops, drones, and smart wearables due to their advantages of high voltage stability, high energy density, and long cycle life.

[0003] As an important component of lithium-ion batteries, the separator is placed between the positive and negative electrode plates to play a key role in isolating electrons and conducting ions. Currently, in order to meet the market demand for charging speed, double-sided oil-based coated separators are mainly used, but there are the following two problems: 1. The electrode core is prone to poor electrolyte infiltration, affecting the cycle life; 2. Too high interfacial adhesion is not conducive to heat dissipation during thermal reactions, affecting the thermal safety performance of the battery. Therefore, the prior art cannot effectively meet the requirements of long cycle life and excellent thermal safety performance. Summary of the Invention

[0004] This application provides an electrode core and an electrochemical device including the electrode core, aiming to solve to a certain extent the defect that existing lithium-ion batteries cannot take into account both long cycle performance and thermal safety performance.

[0005] In a first aspect, this application provides an electrode core, including a positive electrode plate, a negative electrode plate, an electrolyte, and a separator located between the positive electrode plate and the negative electrode plate; the separator includes a base film, a first coating located on one side of the base film and in contact with the positive electrode plate, and a second coating located on the other side of the base film and in contact with the negative electrode plate; the ratio a of the mass content of a first element to a second element on the surface of the first coating and the ratio b of the mass content of the first element to the second element on the surface of the second coating satisfy 5 ≤ b - a ≤ 18.5; the first element is C, and the second element is one or more of Al, Ba, Mg, Si, Sn, Ti, N.

[0006] In an optional embodiment, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, and the average particle size G μm of the positive electrode active material on the surface of the positive electrode plate satisfies the following relationship with a and b: 0.4 ≤ 0.5×(b - a) / G ≤ 3.

[0007] In an optional embodiment, the average particle size G μm of the positive electrode active material on the surface of the positive electrode plate satisfies: 2 ≤ G ≤ 10.

[0008] In an alternative embodiment, the average particle size G μm of the positive active material on the surface of the positive electrode sheet satisfies: 2 ≤ G ≤ 10.

[0009] In an alternative embodiment, the adhesive force L1 N / m between the first coating and the positive electrode sheet and the adhesive force L2 N / m between the second coating and the negative electrode sheet satisfy the following relationship: 2 ≤ L2 - L1 ≤ 12.

[0010] In an alternative embodiment, the adhesive force L1 N / m between the first coating and the positive electrode sheet satisfies: 3 ≤ L1 ≤ 12.

[0011] In an alternative embodiment, the adhesive force L2 N / m between the second coating and the negative electrode sheet satisfies: 5 ≤ L2 ≤ 28.

[0012] In an alternative embodiment, the electrolyte includes fluoroethylene carbonate. Based on the total mass of the electrolyte, the mass content of fluoroethylene carbonate is f, and 10% ≤ f ≤ 20%;

[0013] Under a nitrogen atmosphere, the diaphragm is subjected to thermogravimetric analysis by heating from 25°C to 800°C at a heating rate of 10 K / min, and the mass residual rate of thermal weight loss obtained is c, and 28% ≤ c ≤ 40%;

[0014] c and f satisfy the following relationship: 1.5 ≤ c / f ≤ 3.5.

[0015] In an alternative embodiment, based on the total mass of the electrolyte, the mass content of fluoroethylene carbonate contained in the electrolyte is 10% - 20%;

[0016] In an alternative embodiment, under a nitrogen atmosphere, the diaphragm is subjected to thermogravimetric analysis by heating from 25°C to 800°C at a heating rate of 10 K / min, and the mass residual rate of thermal weight loss obtained is 28% - 40%.

[0017] In an alternative embodiment, 1.5 ≤ a ≤ 4.5; and / or, 8 ≤ b ≤ 20; and / or, the thickness H2 of the second coating is 0.5 μm - 2 μm.

[0018] In an alternative embodiment, the first coating includes a first heat-resistant layer and an adhesive layer. The first heat-resistant layer is located between the base film and the adhesive layer. The adhesive layer includes a blank area and a coated area. The coated area of the adhesive layer includes first polymer particles, and the first heat-resistant layer includes first heat-resistant particles.

[0019] In an alternative embodiment, the second coating is a continuous coating layer including filler particles and a non-granular second polymer.

[0020] In an alternative embodiment, a second heat-resistant layer is further included between the second coating and the base film, and the second heat-resistant layer includes second heat-resistant particles.

[0021] In an alternative embodiment, the thickness of the base film is 3 μm - 12 μm.

[0022] In an alternative embodiment, the porosity of the base film is 25% - 55%.

[0023] In an alternative embodiment, the average pore diameter of the base film is 28 nm - 45 nm.

[0024] In an alternative embodiment, the composition of the base film includes one or more of polyolefin, non-woven fabric, and polyimide.

[0025] In an alternative embodiment, there are multiple holes on the surface of the second coating, and the number of holes with a pore diameter ≥ 0.3 μm is 5 - 55 in a unit area of 10 μm × 10 μm under a field of view magnified 10,000 times by a scanning electron microscope.

[0026] In an alternative embodiment, the thickness H1 of the adhesive layer in the first coating and the thickness H2 of the second coating satisfy the following relationship: 0.5 μm ≤ H1 - H2 ≤ 4 μm; and / or, the thickness H1 of the adhesive layer in the first coating is 1 μm - 5 μm.

[0027] In an alternative embodiment, the percentage of the projected area of the coating region on the first heat-resistant layer to the surface area on one side of the first heat-resistant layer is 15% - 60%.

[0028] In an alternative embodiment, the first polymer particles include primary particles, and the average particle size of the primary particles is 150 nm - 500 nm.

[0029] In an alternative embodiment, the first polymer particles include secondary particles, and the average particle size of the secondary particles is 3 μm - 12 μm;

[0030] In an alternative embodiment, the number of first polymer particles with a particle size ≥ 3 μm is 20 - 160 in a unit area of 100 μm × 100 μm under a field of view magnified 1000 times by a scanning electron microscope for the adhesive layer.

[0031] In an alternative embodiment, the high compression rate of the first polymer particles under the conditions of a temperature of 80 °C and a pressure of 1 MPa for 30 s is 20% - 80%.

[0032] In an alternative embodiment, based on the total mass of the coating area of the glue coating layer, the mass content of the first polymer particles is 95%-99%; the mass content of the first polymer binder is 1%-5%.

[0033] In an alternative embodiment, the average particle size of the first heat-resistant particles is 0.2 μm to 2 μm; and / or, the average particle size of the filler particles is 0.2 μm to 2 μm; and / or, the average particle size of the second heat-resistant particles is 0.2 μm to 2 μm.

[0034] In an alternative embodiment, the components of the filler particles, the first heat-resistant particles, and the second heat-resistant particles are independently selected from one or more of boehmite (γ-AlOOH), alumina (Al2O3), barium sulfate (BaSO4), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), silicon dioxide (SiO2), tin dioxide (SnO2), titanium dioxide (TiO2), barium titanate (BaTiO3), melamine cyanurate, 1,3,5-triazine-2,4,6-triamine, and melamine trithiocyanate.

[0035] In an alternative embodiment, the second polymer is selected from one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, copolymers of vinylidene fluoride and trichloroethylene, polymers of vinylidene fluoride and hexafluoropropylene, and copolymers of vinylidene fluoride and trichloroethylene.

[0036] In an alternative embodiment, the first polymer particles include a first polymer, and the first polymer includes a polymer copolymerized from one or more monomers selected from methyl methacrylate, acrylonitrile, butyl acrylate, isooctyl acrylate, octadecyl acrylate, ethyl acrylate, styrene, ethylene, and butadiene.

[0037] In an alternative embodiment, based on the total mass of the second coating, the mass content of the second polymer is 20%-70%.

[0038] In an alternative embodiment, based on the total mass of the second coating, the mass content of the filler particles is 30%-80%.

[0039] In an alternative embodiment, the percentage of the projected area of the coating area on the first heat-resistant layer to the surface area on one side of the first heat-resistant layer is 20%-40%.

[0040] In an alternative embodiment, the number of first polymer particles with a particle size ≥ 3 μm in a unit area of 100 μm × 100 μm in the field of view of a scanning electron microscope magnified 1000 times in the glue coating layer is 30-140.

[0041] In a second aspect, the present application provides an electrochemical device, including the battery cell according to any one of the first aspect.

[0042] The technical solution of the present application has the following advantages:

[0043] The battery cell provided by the present application includes a positive electrode plate, a negative electrode plate, an electrolyte, and a separator located between the positive electrode plate and the negative electrode plate; the separator includes a base film, a first coating, and a second coating, the first coating is located on one surface of the base film and faces the positive electrode plate, the second coating is located on the other surface of the base film and faces the negative electrode plate, and both the first coating and the second coating contain a first element and a second element; the first element is C, and the second element is one or more of Al, Ba, Mg, Si, Sn, Ti, N; the ratio a of the mass content of the first element to the second element on the surface of the first coating and the ratio b of the mass content of the first element to the second element on the surface of the second coating satisfy 5 ≤ b - a ≤ 18.5. By controlling the difference between the ratio b of the mass content of the first element to the second element on the surface of the second coating and the ratio a of the mass content of the first element to the second element on the surface of the first coating to satisfy the above range, on the one hand, the first coating and the second coating on both sides of the separator have different liquid absorption capabilities, improving the infiltration of the electrolyte in the thickness direction of the battery cell, so that it has good electrolyte wettability at room temperature, which is beneficial to improving the insertion and extraction of lithium ions between the positive and negative electrodes; on the other hand, it will also enhance the swelling property of the coating at high temperature, increase the battery impedance, reduce the lithium ion conduction rate, effectively slow down the further reaction heat generation of the positive electrode plate, the negative electrode plate and the electrolyte, thereby improving the furnace temperature performance and the thermal safety of the battery, enabling the battery cell to take into account long cycle performance and excellent thermal safety performance.

[0044] Additional aspects and advantages of the embodiments of the present application will be described and shown partially in the subsequent description, or will be explained through the implementation of the embodiments of the present application. Description of the Drawings

[0045] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0046] Figure 1 Schematic diagram of the positional relationship between the separator provided in Embodiment 1 and the positive electrode plate and the negative electrode plate;

[0047] Figure 2 Schematic diagram of the structure of the first coating provided in Embodiment 2;

[0048] Reference numerals: 110, porous base film, 120, glue layer, 130, second coating layer, 140, positive electrode sheet, 150, negative electrode sheet, 160, first heat-resistant layer, 121, blank area, 122, coating area. DETAILED DESCRIPTION

[0049] The following examples are provided for a better understanding of the present application, but are not limited to the best implementation mode described, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the protection scope of the present application.

[0050] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0051] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0052] The battery cells made with the existing diaphragms have the problem of poor electrolyte infiltration, which affects the cycle life, especially during the fast charging process, the cycle performance is seriously damaged. Improving the interface adhesion is conducive to improving the structural stability and cycle performance. However, too high interface adhesion is not conducive to the heat dissipation of thermal reaction, affecting the thermal safety performance of the battery. Therefore, the existing lithium-ion batteries cannot improve the long cycle while taking into account the excellent thermal safety performance. The technical solution adopted in this application is as follows.

[0053] In the first aspect, the present application provides a battery cell, comprising a positive electrode plate, a negative electrode plate, an electrolyte, and a separator located between the positive electrode plate and the negative electrode plate; the separator comprises a base film, a first coating and a second coating, the first coating is located on one side surface of the base film and faces the positive electrode plate, the second coating is located on the other side surface of the base film and faces the negative electrode plate, the first coating and the second coating both contain a first element and a second element; the first element is C, and the second element is one or more of Al, Ba, Mg, Si, Sn, Ti, and N; the ratio a of the mass content of the first element to the second element on the surface of the first coating and the ratio b of the mass content of the first element to the second element on the surface of the second coating satisfy 5≤ba≤18.5.

[0054] The ratio of C (carbon element) to the second element affects the swelling property of the coating surface and has a great influence on the wetting effect of the electrolyte. In this application, by controlling the difference within the above range, the first coating and the second coating on both sides of the separator have different liquid absorption capacities, have good electrolyte wettability at room temperature, improve the battery cycle performance, enhance the coating swelling property at high temperature, increase the battery impedance, reduce the lithium ion conduction rate, effectively slow down the further reaction heat generation of the positive electrode sheet, the negative electrode sheet and the electrolyte, can improve the furnace temperature performance, improve the thermal safety of the battery, and enable the battery cell to take into account long cycle performance and excellent thermal safety performance. In this application, the difference between the ratio b of the mass content of the first element to the mass content of the second element on the surface of the second coating and the ratio a of the mass content of the first element to the mass content of the second element on the surface of the first coating is strictly controlled. If b - a > 18.5, it will cause a large difference in the electrolyte infiltration speed between the first coating and the second coating, slow down the speed of lithium ions passing through the separator during charging, and easily form lithium deposition on the negative electrode surface, thus affecting the cycle performance; if b - a < 5, the speed of lithium ions passing through the separator during charging is relatively fast, and the impedance of the coating during thermal abuse is small, which is not conducive to suppressing the occurrence of side reactions between the positive electrode and the electrolyte, and the furnace temperature safety passing rate is relatively low. For example, b - a is 5, 7, 10, 12, 15, 18.5 or within the range composed of any two of the above values.

[0055] The term "mass content of the first element" refers to the percentage of the mass of the first element exposed on the coating surface in the total mass of all elements exposed on the coating surface.

[0056] The term "mass content of the second element" refers to the percentage of the mass of the second element exposed on the coating surface in the total mass of all elements exposed on the coating surface.

[0057] The term "ratio of the mass content of the first element to the mass content of the second element" refers to the ratio of the mass content of the first element exposed on the coating surface to the mass content of the second element exposed on the coating surface.

[0058] Specifically, when the coating contains more macropores, the elements exposed on the coating surface should also include the elements exposed through the coating pores, so that the element content exposed on the coating surface can reflect the overall comprehensive performance of the coating and the interfacial performance between the separator and the electrode sheet.

[0059] The test method for the mass content of the first element and the second element on the coating surface is as follows: EDS energy spectrum analysis by scanning electron microscope, surface scanning of the first coating or the second coating surface at a magnification of 500 times, repeating the above operation 5 times, statistically analyzing the element mass content, and taking the average value.

[0060] In an alternative embodiment, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the current collector. The positive electrode active material layer includes a positive electrode active material. The average particle size G μm of the positive electrode active material on the surface of the positive electrode plate, a, and b satisfy the following relational expression: 0.4 ≤ 0.5×(b - a) / G ≤ 3.

[0061] By controlling 0.5×(b - a) / G to be between 0.4 and 3, it helps to improve the stability of the CEI interface (solid electrolyte interface), significantly inhibits the transformation of the positive electrode crystal structure, and at the same time reduces the reaction rate between the positive electrode active material and the electrolyte, reducing the generation of heat and gas. Thus, while improving the cycle performance, the occurrence of thermal runaway is significantly reduced. When 0.5×(b - a) / G < 0.4, the stability of the separator-positive electrode interface is poor, the protection of the separator for the positive electrode structure decreases, and the effect of inhibiting side reactions between the positive electrode and the electrolyte is poor, reducing the furnace temperature passing rate; when 0.5×(b - a) / G > 3, the speed of lithium ions passing through the separator will decrease at this time, exacerbating the risk of insufficient lithium intercalation in the negative electrode to form black spot lithium deposition, which is not conducive to the cycle stability of the battery. For example, 0.5×(b - a) / G is 0.4, 0.8, 1.2, 2, 3, or within the range composed of any two of the above values.

[0062] In an alternative embodiment, the average particle size G μm of the positive electrode active material on the surface of the positive electrode plate satisfies: 2 ≤ G ≤ 10. For example, G can be 2, 3, 5, 8, 10, or within the range composed of any two of the above values.

[0063] In this application, the test method for the average particle size G of the positive electrode active material on the surface of the positive electrode plate is as follows: Place the positive electrode plate under the field of view of a scanning electron microscope magnified 1000 times, and use the smallest square or rectangle to completely enclose 1 positive electrode active material. At this time, the side length of the square or the length of the long side of the rectangle is the particle size of a single particle. To improve the accuracy of the test results, in the scanned image, arbitrarily select an area of 100 μm × 100 μm, measure the particle sizes of 200 positive electrode active material particles, and take their average value as the average particle size of the material. It should be noted that if the number of particles in the captured image is less than 200, then capture multiple images, and set the average value of the particle sizes of a total of 200 positive electrode active material particles as the average particle size.

[0064] In an alternative embodiment, the adhesion force L1 N / m between the first coating and the positive electrode plate and the adhesion force L2 N / m between the second coating and the negative electrode plate satisfy the following relational expression: 2 ≤ L2 - L1 ≤ 12. By controlling L2 - L1 to be between 2 and 12, the difference between the adhesion forces L2 and L1 is controlled within the above range, and an appropriate coating gap can be created. During the cyclic charge and discharge process, the electrolyte can wet back and forth through multiple paths. On the one hand, it reduces the deterioration of lithium deposition caused by uneven surface stress of the negative electrode due to the expansion of negative active particles. At the same time, the separator and the electrode plate are not prone to adverse phenomena such as peeling off or stripping. On the other hand, it can reduce the difference in cycle failure of the positive electrode material between the arc region and the planar region during the cycle, enabling the positive electrode materials in the planar region and the arc region to maintain uniform stability during the cycle, avoiding changes in the crystal structure of the positive electrode material in local areas, which may lead to a decrease in the furnace temperature performance of the battery. Thus, while improving the cycle performance, the occurrence of thermal runaway is significantly reduced. If L2 - L1 is too small, the adhesion forces of the two coatings to the electrode plate are relatively close at this time, which can improve the overall hardness of the battery and is beneficial to reducing local deformation problems caused by negative electrode expansion during the cycle. However, it will cause the separator to be difficult to separate from the electrode plate during battery assembly or cycling, hindering the flow and penetration of the electrolyte, resulting in insufficient wetting in some areas, such as the arc position. On the other hand, heat is easily accumulated under high-temperature conditions, which may exacerbate the side reactions between the positive electrode or the negative electrode and the electrolyte, deteriorating the furnace temperature passing rate; if L2 - L1 is too large, the adhesion force on the negative electrode side is much higher than that on the positive electrode side at this time, making the electrolyte more likely to penetrate to places where it should not be, which may cause other problems, such as leakage or uneven wetting. At the same time, the separator is easily detached from the electrode plate. On the other hand, the battery will also experience volume changes during the charge and discharge process. A too large difference in adhesion force is likely to cause irregular deformation of the battery during cycling, and black spot lithium deposition is more likely to form on the surface of the negative electrode. Exemplarily, L2 - L1 is 2, 4, 6, 8, 10, 12, or within the range composed of any two of the above values. The test methods for the adhesion force L1 between the first coating and the positive electrode plate and the adhesion force L2 between the second coating and the negative electrode plate are as follows: Taking L1 as an example, disassemble the battery cell to obtain a sample in which the positive electrode sheet and the separator are bonded together, and cut a sample with a length of 100 mm and a width of 15 mm from the above sample; fix the separator and the positive electrode sheet in the above sample on the upper fixture and the lower fixture of a universal tensile testing machine respectively; then perform a 180° peel on the above sample, with a test speed of 100 mm / min and a test displacement distance of 100 mm, and record the adhesion force value.

[0065] In an alternative embodiment, the electrolyte includes fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate is f based on the total mass of the electrolyte. The diaphragm is subjected to thermogravimetric analysis by heating from 25°C to 800°C at a heating rate of 10 K / min in a nitrogen atmosphere, and the mass residual rate of the thermal weight loss obtained is c. c and f satisfy the following relational expression: 1.5 ≤ c / f ≤ 3.5. By controlling c / f to be between 1.5 and 3.5, the initial efficiency performance and high-temperature performance of the battery can be improved simultaneously. During the first formation process of the battery, the interfacial performance between the diaphragm and the electrode can be improved, so that the electrolyte content between the diaphragm and the electrode is appropriate, thereby ensuring that while FEC acts on the negative electrode to form a dense and uniform SEI film, the interfacial impedance of the negative electrode can be improved to a certain extent, so that the impedance of the system is the lowest and the initial efficiency of the battery is improved. When the battery temperature rises or thermal runaway occurs, it can reduce the side reaction between the impurities in the diaphragm system and FEC at high temperature, or prevent the by-products in the electrolyte from corroding the impurities of the diaphragm to generate heat, thereby exacerbating the thermal runaway of the battery. If the c / f ratio is too small, the interfacial impedance is small, a dense and uniform SEI film can be formed, and the first formation efficiency can be improved, but at high temperature, the side reaction between the electrolyte and the positive / negative electrode will be exacerbated, and the heat generation acceleration will cause a significant decrease in the furnace temperature passing rate. If the c / f ratio is too large, the interfacial impedance is large, the first formation reaction is incomplete, the stability of the SEI is poor, the SEI film is easily damaged during the cycling process, and the lithium insertion resistance is too high, resulting in an increased risk of lithium precipitation. For example, c / f is 1.5, 1.8, 2, 2.5, 3, 3.5 or within the range composed of any two of the above values.

[0066] In an alternative embodiment, 1.5 ≤ a ≤ 4.5; and / or, 8 ≤ b ≤ 20. As an example, a can be 1.5, 2, 3, 4.5 or within the range composed of any two of the above values. As an example, b can be 8, 10, 15, 20, 20 or within the range composed of any two of the above values. a can be achieved by adjusting one or more of the factors such as the particle size of the first heat-resistant particles, the content of the first heat-resistant particles, the percentage of the projected area of the coating area in the coating layer on the first heat-resistant layer to the surface area of one side of the first heat-resistant layer, the particle size of the first polymer particles, the molecular weight of the first polymer, and the proportion of the monomers containing ester groups in the first polymer. b can be achieved by adjusting one or more of the factors such as the particle size of the filler particles, the filler particles, and the content of the number of pores in the coating layer.

[0067] In an alternative embodiment, the adhesion force L1 N / m between the first coating and the positive electrode sheet satisfies: 3 ≤ L1 ≤ 12; and / or, the adhesion force L2 N / m between the second coating and the negative electrode sheet satisfies: 5 ≤ L2 ≤ 28. The adhesion force between the separator and the electrode sheet has a significant impact on the wettability of the electrolyte during the cycling test, and the electrolyte needs to uniformly wet the entire cell structure. Excessive adhesion force will cause the separator to be difficult to separate from the electrode sheet during battery assembly or cycling, hindering the flow and penetration of the electrolyte, resulting in insufficient wetting in some areas, such as the arc position. Insufficient adhesion force makes the electrolyte more likely to penetrate to places where it should not be, which may cause other problems, such as leakage or uneven wetting, and at the same time, the separator is easily detached from the electrode sheet. On the other hand, the battery will also experience volume changes during charge and discharge, resulting in certain stresses between the electrode sheet and the separator. If the adhesion force is not appropriate, these stresses may exacerbate the structural problems. For example, excessive adhesion force may lead to delamination or fracture, while insufficient adhesion force may accelerate material wear. And in this application, by controlling the adhesion force L1 between the first coating and the positive electrode sheet and the adhesion force L2 between the second coating and the negative electrode sheet within the above ranges, it is possible to ensure sufficient and uniform wetting while improving the structural stability of the cell, thereby further improving the thermal safety performance and cycling stability of the cell. As an example, L1 can be 3, 4, 5, 6, 7, 10, 12 or within the range composed of any two of the above values. As an example, L2 can be 5, 6, 7, 8, 9, 10, 11, 12, 15, 20, 28 or within the range composed of any two of the above values.

[0068] In an alternative embodiment, 28% ≤ c ≤ 40%. The separator set in this way can enhance the stability of the separator at high temperatures, reduce the side reactions between the impurities in the separator system and the components in the electrolyte at high temperatures, thereby avoiding thermal runaway. For example, c is 28%, 30%, 35%, 40% or within the range composed of any two of the above values.

[0069] In an alternative embodiment, 10% ≤ f ≤ 20%. The electrolyte set in this way makes the content of fluorinated carbonate in the electrolyte between the separator and the electrode sheet appropriate, so as to ensure that the fluorinated carbonate acts on the negative electrode to form a dense and uniform SEI film while improving the interfacial impedance of the positive electrode to a certain extent, so that the impedance of the system is the lowest and the first efficiency of the battery is improved. f is 10%, 12%, 15%, 20% or within the range composed of any two of the above values.

[0070] In an alternative embodiment, the thickness H1 of the first coating and the thickness H2 of the second coating satisfy the following relationship: 0.5 μm ≤ H1 - H2 ≤ 4 μm; with such a setting, the effect of increasing the impedance of the separator at high temperatures is better, and it has good ion conductivity at the operating temperature. For example, H1 - H2 is 0.5, 1, 2, 4 μm or within the range formed by any two of the above values.

[0071] In an alternative embodiment, the thickness H1 of the first coating is 1 μm - 5 μm; and / or, the thickness H2 of the second coating is 0.5 μm - 2 μm. The first coating and the second coating with such thickness ranges can well improve the adhesion between the separator and the electrode sheet, avoid short circuits caused by the contact of the positive and negative electrodes, and improve the battery safety. For example, H1 is 1 μm, 2 μm, 5 μm or within the range formed by any two of the above values. H2 is 0.5 μm, 1 μm, 2 μm or within the range formed by any two of the above values.

[0072] In an alternative embodiment, the first coating includes a first heat-resistant layer and an adhesive layer. The first heat-resistant layer is located between the base film and the adhesive layer. The adhesive layer includes a blank area and a coated area. The coated area of the adhesive layer includes first polymer particles, and the first heat-resistant layer includes first heat-resistant particles. For example, the first polymer particles include a first polymer, and the first polymer includes a polymer copolymerized from one or more monomers of methyl methacrylate, acrylonitrile, butyl acrylate, isooctyl acrylate, octadecyl acrylate, ethyl acrylate, styrene, ethylene, and butadiene.

[0073] In an alternative embodiment, the molecular weight of the first polymer is 15,000 - 200,000.

[0074] In an alternative embodiment, based on the total mass of the first polymer, the mass percentage of the monomer containing an ester group in the first polymer is 50% - 100%.

[0075] In an alternative embodiment, the first polymer particles include primary particles with an average particle size of 150 nm - 500 nm; and / or, the first polymer includes secondary particles with an average particle size of 3 μm - 12 μm. Herein, the primary particle refers to a single particle, and the secondary particle refers to a large particle formed by agglomeration and combination of two or more primary particles. When the particle size of the secondary particle is too large, the wettability effect of the electrolyte is better, but the thickness is relatively large, resulting in more energy density loss. When the particle size is too small, the wetting improvement effect is poor, and there is also a risk of adhesion and pore blockage. In this application, by controlling the particle size of the secondary particle within the above range, while ensuring good wettability of the electrolyte, the loss of energy density is avoided. And controlling the primary particle within the above range is beneficial to forming secondary particle aggregates with uniform particle size during centrifugal drying. For example, the average particle size of the primary particles of the first polymer particles is 150 nm, 200 nm, 300 nm, 500 nm or within the range composed of any two of the above values. The average particle size of the secondary particles of the first polymer particles is 3 μm, 5 μm, 10 μm, 12 μm or within the range composed of any two of the above values.

[0076] In an alternative embodiment, the second coating is a continuous coating layer including filler particles and non-granular second polymer. For example, the second polymer is selected from one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, copolymers of vinylidene fluoride - trichloroethylene, vinylidene fluoride - hexafluoropropylene polymers, copolymers of vinylidene fluoride - trichloroethylene.

[0077] In an alternative embodiment, a second heat-resistant layer is further included between the second coating and the base film, and the second heat-resistant layer includes second heat-resistant particles. For example, the components of the filler particles, the first heat-resistant particles, and the second heat-resistant particles are independently selected from one or more of boehmite (γ - AlOOH), alumina (Al2O3), barium sulfate (BaSO4), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), silica (SiO2), tin dioxide (SnO2), titanium dioxide (TiO2), barium titanate (BaTiO3), melamine cyanurate, 1,3,5 - triazine - 2,4,6 - triamine, melamine trithiocyanate.

[0078] In an alternative embodiment, the coated area of the glue layer further includes a first polymer binder. For example, the first polymer binder is selected from one or more of polyacrylate, polyvinyl alcohol, styrene - butadiene rubber.

[0079] In an alternative embodiment, the first heat-resistant layer further comprises a second polymer binder, a first thickener, and a first wetting agent. For example, the second polymer binder is selected from one or more of polyacrylate, polyvinyl alcohol, and styrene-butadiene rubber; the first thickener is selected from one or more of sodium carboxymethyl cellulose, alginic acid, and gelatin; the first wetting agent is selected from one or more of alkyl naphthalene sulfonates, alkyl benzene sulfonates, and polyoxyethylene fatty alcohol ethers; wherein, the alkyl naphthalene sulfonates are selected from one or more of sodium butyl naphthalene sulfonate, sodium dodecyl naphthalene sulfonate, potassium butyl naphthalene sulfonate, and potassium dodecyl naphthalene sulfonate; the alkyl benzene sulfonates are selected from one or more of sodium hexadecyl benzene sulfonate, sodium dodecyl benzene sulfonate, potassium hexadecyl benzene sulfonate, and potassium dodecyl benzene sulfonate.

[0080] In an alternative embodiment, the second heat-resistant layer further comprises a third binder, a second thickener, and a second wetting agent. For example, the third binder is selected from one or more of polyacrylate, polyvinyl alcohol, and styrene-butadiene rubber; the second thickener is selected from one or more of sodium carboxymethyl cellulose, alginic acid, and gelatin; the second wetting agent is selected from one or more of alkyl naphthalene sulfonates, alkyl benzene sulfonates, and polyoxyethylene fatty alcohol ethers.

[0081] In an alternative embodiment, the thickness of the base film is 3 μm - 12 μm; and / or, the porosity of the base film is 25% - 55%; and / or, the average pore size of the base film is 28 nm - 45 nm; and / or, the composition of the base film includes one or more of polyolefins, non-woven fabrics, and polyimides. By using a separator with such thickness, porosity, average pore size, or composition, it not only helps to maintain the tensile strength of the separator and improve the safety of the battery cell, but also is beneficial to obtaining good liquid retention capacity, thereby delaying the attenuation of battery capacity. For example, the thickness of the base film can be 3 μm, 4 μm, 5 μm, 8 μm, 12 μm or within the range composed of any two of the above values, the porosity can be 25%, 30%, 40%, 50%, 55% or within the range composed of any two of the above values, and the average pore size can be 28 nm, 35 nm, 40 nm, 45 nm or within the range composed of any two of the above values. The base film is selected from polypropylene microporous membranes and / or polyethylene microporous membranes.

[0082] In an alternative embodiment, the surface of the second coating has a plurality of pores. Under a field of view magnified 10,000 times by a scanning electron microscope, the number of pores with a pore diameter ≥ 0.3 μm in a unit area of 10 μm × 10 μm is 5 - 55. In the present application, the number of pores on the surface of the second coating within the above range is beneficial to the sufficient infiltration of the electrolyte into the coating, taking into account the interfacial adhesion force, promoting the lithium intercalation efficiency of the negative electrode, and thus improving the cycle stability of the battery cell. For example, under a field of view magnified 10,000 times by a scanning electron microscope, the number of pores with a pore diameter ≥ 0.3 μm in a unit area of 10 μm × 10 μm is 5, 10, 15, 20, 30, 50, 55, or within the range formed by any two of the above values.

[0083] In an alternative embodiment, the percentage of the projected area of the coated area on the first heat-resistant layer to the surface area of one side of the first heat-resistant layer is 15% - 60%. When the projected area of the coated area is too large, the wetting effect of the separator on the electrolyte is poor. When the projected area of the coated area is too small, the adhesion force between the separator and the electrode sheet is insufficient, and coating peeling and falling off are likely to occur during the cycling process. In the present application, controlling the percentage of the projected area of the coated area within the above range can take into account good electrolyte infiltration and good interfacial bonding effect, thereby improving the cycle performance and thermal safety performance of the battery cell. In the present invention, the percentage of the projected area of the coated area on the first heat-resistant layer to the surface area of one side of the first heat-resistant layer (referred to as the projected area percentage or coverage rate of the coated area) can be calculated from SEM (scanning electron microscope) images. Specifically, the following steps can be included: Under a field of view magnified 1000 times by SEM on the surface of the separator, arbitrarily take an area of 100 μm × 100 μm, divide this area into 400 uniform squares of 5 μm × 5 μm, and calculate the total number X of squares occupied by the first polymer particles (note: when the first polymer particle occupies an area of the square ≥ 50%, it is regarded as occupied; otherwise, it is regarded as unoccupied). Then the coverage rate = X / 400 × 100%. In order to increase the accuracy of the data, randomly select 5 points for area division calculation, repeat the above operation, and take the average value of 5 times. For example, the percentage of the projected area of the coated area on the first heat-resistant layer to the surface area of one side of the first heat-resistant layer is 15%, 20%, 30%, 50%, 60%, or within the range formed by any two of the above values.

[0084] In an alternative embodiment, the number of first polymer particles with a particle size ≥ 3 μm in a unit area of 100 μm × 100 μm in the view field of a scanning electron microscope magnified 1000 times in the glue coating layer is 20 - 160; for example, the number of first polymer particles with a particle size ≥ 3 μm in a unit area of 100 μm × 100 μm in the view field of a scanning electron microscope magnified 1000 times in the glue coating layer can be 20, 30, 50, 80, 150, 160, or within the range composed of any two of the above values. In the present invention, the number of first polymer particles with a particle size ≥ 3 μm on the surface of the glue coating layer can be calculated through SEM (scanning electron microscope) pictures, which specifically includes the following steps: on the scanning image of the surface of the glue coating layer of the separator obtained by using SEM, draw the smallest rectangle that completely encloses the area of 1 first polymer particle, that is, draw a rectangle where the edges of the first polymer particle are in contact with the four sides of the rectangle, and the length of the long side of the rectangle is the particle size of the first polymer particle. In an arbitrarily selected area of 100 μm × 100 μm on the surface of the glue coating layer, measure the particle size of the first polymer particle and calculate the number of first polymer particles with a particle size ≥ 3 μm; repeat the above operation 5 times, and take the average value as the number of first polymer particles with a particle size ≥ 3 μm. It should be noted that the scanning image can be obtained by observing the surface of the functional coating with a magnification of 1000 times using an electron emission type scanning electron microscope (S-3400N manufactured by Hitachi, Ltd.).

[0085] In an alternative embodiment, the high compression ratio of the first polymer particles under the conditions of a temperature of 80 °C and a pressure of 1 MPa for hot pressing for 30 s is 20% - 80%; controlling the high compression ratio of the first polymer particles measured under the above conditions within the above range can balance the interfacial adhesion force and the energy density, and the cycle stability is better. When the compression ratio is too small, the thickness loss is large, resulting in a decrease in the energy density. When the compression ratio is too large, the material strength is insufficient, and it is prone to being extruded by the negative electrode expansion force in the later stage of the cycle, resulting in the collapse of the material structure. For example, the high compression ratio of the secondary particles of the first polymer particles under the conditions of a temperature of 80 °C and a pressure of 1 MPa for hot pressing for 30 s is 20%, 30%, 50%, 80%, or within the range composed of any two of the above values.

[0086] In an alternative embodiment, based on the total mass of the coating area of the glue coating layer, the mass content of the first polymer particles is 95% - 99%; the mass content of the first polymer binder is 1% - 5%; by controlling the mass contents of the first polymer and the first polymer binder within the above range, the adhesion force between the first polymer and the heat-resistant layer can be improved without affecting the lithium ion transmission.

[0087] In an alternative embodiment, the average particle size of the first heat-resistant particles is 0.2 μm to 2 μm; and / or, the average particle size of the filler particles is 0.2 μm to 2 μm; and / or, the average particle size of the second heat-resistant particles is 0.2 μm to 2 μm; controlling the average particle size of the first heat-resistant particles and / or the second heat-resistant particles and / or the filler particles within the above range is not only beneficial to controlling the diaphragm with a suitable porosity range, thereby improving the migration efficiency of lithium ions, but also beneficial to improving the heat resistance of the diaphragm, thereby improving the thermal safety of the battery.

[0088] In an alternative embodiment, based on the total mass of the first heat-resistant layer, the heat-resistant layer comprises 92 wt% to 95 wt% of the first heat-resistant particles, 4 wt% to 7 wt% of the second polymer binder, 0.3 wt% to 0.5 wt% of the first thickener, and 0.1 wt% to 0.5 wt% of the first wetting agent.

[0089] In an alternative embodiment, based on the total mass of the second heat-resistant layer, the heat-resistant layer comprises 92 wt% to 95 wt% of the second heat-resistant particles, 4 wt% to 7 wt% of the third binder, 0.3 wt% to 0.5 wt% of the second thickener, and 0.1 wt% to 0.5 wt% of the second wetting agent.

[0090] In an alternative embodiment, based on the total mass of the second coating, the mass content of the second polymer is 20% - 70%. Exemplarily, the mass content of the second polymer is 20%, 30%, 40%, 50%, 60%, 70% or within the range composed of any two of the above values.

[0091] In an alternative embodiment, based on the total mass of the second coating, the mass content of the filler particles is 30% - 80%. Exemplarily, the mass content of the filler particles is 30%, 40%, 50%, 60%, 70%, 80% or within the range composed of any two of the above values.

[0092] In an alternative embodiment, the electrolyte contains a carbonate solvent. Based on the total mass of the electrolyte, the mass content of the carbonate solvent is 15% - 75%. Exemplarily, the mass content of the carbonate solvent is 15%, 20%, 35%, 45%, 65%, 75% or within the range composed of any two of the above values.

[0093] In an alternative embodiment, the carbonate solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Exemplarily, the carbonate solvent includes, but is not limited to, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 1-3:1-3:0.5-2:3-8.

[0094] In an alternative embodiment, the electrolyte contains a lithium salt. Based on the total mass of the electrolyte, the mass percentage of the lithium salt is 8%-20%. Exemplarily, the mass percentage of the lithium salt is 8%, 10%, 15%, or within the range formed by any two of the above values.

[0095] In an alternative embodiment, the lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), and lithium bis(oxalato)borate (LiBOB).

[0096] In an alternative embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material. The negative electrode active material can be a negative electrode active material known in the art for batteries, such as one or more of graphite and silicon-based materials.

[0097] In an alternative embodiment, the silicon-based material is selected from one or more of elemental silicon, silicon oxide compounds (such as SiOx / C), silicon-carbon composites (such as Si / C), silicon-nitrogen composites, silicon alloys, elemental tin, tin oxide compounds, tin alloys, elemental titanium, titanium oxide compounds, and titanium alloys.

[0098] Based on the mass of the negative electrode active material layer, the content of the negative electrode active material is ≥96%. Exemplarily, the content of the negative electrode active material is 96%, 97%, 98%, 99%, or within the range formed by any two of the above values.

[0099] In an alternative embodiment, the negative electrode active material layer contains a silicon-based material, and based on the total amount of the negative electrode active material layer, the content of the silicon-based material is 0-30 wt%.

[0100] In some embodiments, the positive electrode active material layer or the negative electrode active layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), styrene-butadiene rubber, polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. In some embodiments, optionally, the binder accounts for 0.5-3% of the total weight of the positive electrode active material layer. Optionally, the binder accounts for 0.5-3% of the total weight of the negative electrode active material layer.

[0101] In some embodiments, the positive electrode active material layer or the negative electrode active layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of carbon nanotubes, acetylene black, carbon black, Ketjen black, graphene, and carbon nanofibers.

[0102] In some embodiments, optionally, the conductive agent accounts for 0.5-5% of the total weight of the positive electrode active material layer.

[0103] In some embodiments, the positive electrode active material layer or the negative electrode active layer may further optionally include a thickening agent. As an example, the conductive agent may include sodium carboxymethyl cellulose.

[0104] In some embodiments, optionally, the thickening agent accounts for 0.1-2% of the total weight of the positive electrode active material layer.

[0105] In a second aspect, the present application provides an electrochemical device including the battery cell according to any one of the first aspect.

[0106] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application. In all the embodiments and comparative examples of the present application, the unit wt% represents mass percentage.

[0107] Example 1

[0108] The present example provides a method for preparing a battery cell, including the following steps

[0109] (1) Preparation of the positive electrode sheet

[0110] Lithium nickel cobalt manganate (molecular formula: LiNi 0.6 Co 0.1 Mn 0.3O2), binder polyvinylidene fluoride (PVDF 500), and conductive agent (conductive carbon black Super P: carbon nanotube mass ratio of 2:1) are mixed in N-methylpyrrolidone (NMP) solvent and continuously stirred by a blender to form a uniform and flowing positive electrode slurry. Subsequently, the positive electrode slurry is coated on an aluminum foil with a thickness of 9 μm, sent into a vacuum oven at 120 °C for drying for 6 h, and then rolled and slit to obtain the required positive electrode sheet.

[0111] (2) Preparation of negative electrode sheet

[0112] Graphite, thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber are mixed in an aqueous solvent according to a weight ratio of 98.2:0.6:1.2 and continuously stirred by a blender to form a uniform and flowing negative electrode slurry. Subsequently, the slurry is coated on the surface of a current collector copper foil with a thickness of 6 μm, sent into a vacuum oven at 120 °C for drying for 6 h, and then rolled and slit to obtain the required negative electrode sheet.

[0113] (3) Preparation of electrolyte

[0114] In a glove box filled with argon (moisture < 1 ppm, oxygen content < 1 ppm), ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and diethyl carbonate solvents are mixed in a mass ratio of 2:2:1:5 to form a uniform solvent, obtaining a mixed solvent. 15% of LiPF6 and 15% of fluoroethylene carbonate based on the total mass of the electrolyte are slowly added to the mixed solvent. After stirring evenly, the required basic lithium-ion battery electrolyte is obtained.

[0115] (4) Preparation of separator

[0116] 1. Preparation of the second coating: The second polymer PVDF and the organic solvent DMAC (dimethylacetamide) are blended, fully stirred and dissolved, and then filler particles are added. After stirring and dispersing evenly, a mixed slurry with a solid content of 8% is obtained. Based on the 100% solid mass ratio of the slurry, the mass ratio of PVDF to the mass of the filler particles is 4:6. The mixed slurry is coated on the first surface of a porous base film (specific type: PE, thickness of 5 μm, porosity of 40%, average pore diameter of 41 nm) through a gravure roll, and the organic solvent is extracted through a water bath for pore formation, and then dried in a multi-stage oven at 60 °C to form a second coating with a thickness of 1.5 μm. The second polymer is PVDF, LBG from Arkema is selected, the filler particles are conventional alumina, the average particle size of alumina is 0.8 μm, the mass content of PVDF in the second coating is 40 wt%, and the mass content of the filler is 60 wt%.

[0117] 2. Preparation of the first coating:

[0118] 1) Preparation of the first heat-resistant layer: The first heat-resistant particles, the second polymer binder, the first thickener, the first wetting agent and deionized water are blended to obtain a ceramic slurry with a solid content of 35%, wherein the mass ratio of the first heat-resistant particles: the second polymer binder: the first thickener: the first wetting agent is 94.5:5:0.4:0.1 calculated based on 100% solid mass, and the slurry is fully stirred and dispersed, and then coated on the second surface of the porous base film by a gravure roller, and dried in a multi-section oven at 60°C to form a first heat-resistant layer with a thickness of 1 μm. The first heat-resistant particles use aluminum oxide with an average particle size of 0.8 μm, the second polymer binder is polyacrylate, the first thickener is CMC, and the first wetting agent is sodium dodecylbenzene sulfonate.

[0119] 2) Preparation of the coating layer: The first polymer, the first polymer binder and deionized water are blended to obtain a mixed slurry with a solid content of 5%, wherein the mass ratio of the first polymer: the first polymer binder is 95:5 calculated based on 100% solid mass, and the slurry is fully stirred and dispersed, and then coated on the surface of the first heat-resistant layer by a gravure roller, and dried in a multi-section oven at 60°C to form a coating layer with a thickness of 3μm, thereby obtaining the diaphragm described in the invention application. The coating layer includes a blank area and a coating area, and the projection area of the coating area of the coating layer accounts for 27.2% of the total area of the coating layer (referred to as "the area ratio of the coating area") The first polymer binder is polyacrylate, and the first polymer particles (purchased from Shenzhen Bairou New Materials Technology Co., Ltd., model DWP4201A) have an average primary particle size of 300nm and an average secondary particle size of 6.5μm. The polymer monomers include styrene, isooctyl acrylate and methyl methacrylate, and the copolymerization ratio is 64:8:28. The height compression rate of the first polymer after hot pressing for 30 seconds at a temperature of 80° C. and a pressure of 1 MPa is 65%.

[0120] The prepared diaphragm, see Figure 1 As shown, it includes a porous base film 110, a second coating 130 located on the first surface of the porous base film, and a first coating located on the second surface of the porous base film, wherein the first coating includes a first heat-resistant layer 160 and a glue layer 120, wherein the first heat-resistant layer 160 is located between the porous base film 110 and the glue layer 120. The second coating 130 is in contact with the negative electrode plate 150, and the glue layer 120 of the first coating is in contact with the positive electrode plate 140. Figure 2 As shown, the adhesive layer 120 includes a blank area 121 and a coating area 122 .

[0121] (5) Preparation of lithium-ion batteries

[0122] Prepare a bare battery cell by winding the above-prepared positive electrode sheet, separator, and negative electrode sheet; then place the bare battery cell in an aluminum-plastic film, inject the above-prepared electrolyte into the dried bare battery cell, and obtain the required lithium-ion battery through processes such as vacuum packaging, normal-temperature standing, and high-temperature formation.

[0123] The preparation methods of the battery cells in Examples 2 to 7 are basically the same as those in Example 1, and the same batches of positive electrode sheets and negative electrode sheets as those in Example 1 are used. The difference lies only in that during the preparation process of the glue-coated layer of the separator, the area ratio of the coated area of the glue-coated layer is adjusted, so that the ratio a of the mass content of C element to Al element on the surface of the first coating layer and the adhesion force L1 between the first coating layer and the positive electrode sheet are different, as shown in Tables 1 and 2 specifically.

[0124] The preparation methods of the battery cells in Examples 8 to 10 are basically the same as those in Example 1, and the same batches of positive electrode sheets and negative electrode sheets as those in Example 1 are used. The difference lies only in that during the preparation process of the separator, the mass ratio of PVDF to filler particles in the second coating layer is adjusted, so that the mass content of PVDF and the mass content of filler particles in the second coating layer are different. Specifically, in Example 8, the mass ratio of PVDF to filler particles is adjusted to 59:41, so that the mass content of PVDF in the second coating layer is 59 wt%, and the mass content of the filler is 41 wt%; in Example 9, the mass ratio of PVDF to filler particles is adjusted to 32.5:67.5, so that the mass content of PVDF in the second coating layer is 32.5 wt%, and the mass content of the filler is 67.5 wt%; in Example 10, the mass ratio of PVDF to filler particles is adjusted to 31.2:68.8, so that the mass content of PVDF in the second coating layer is 31.2 wt%, and the mass content of the filler is 68.8 wt%; thereby making the ratio b of the mass content of the first element to the second element on the surface of the second coating layer, the adhesion force L2 between the second coating layer and the negative electrode sheet, and the thermal weight loss mass residue rate c in Examples 8 to 10 different, as shown in Tables 1 and 2 specifically.

[0125] The preparation methods of the battery cells in Examples 11 - 13 are basically the same as those in Example 1. The difference lies only in that the average particle size of lithium nickel cobalt manganese oxide is adjusted, so that the numerical value of the average particle size G μm of the positive electrode active material on the surface of the positive electrode sheet is different, as shown in Table 1 specifically.

[0126] The preparation methods of the battery cells in Examples 14 - 15 are basically the same as those in Example 1, and the same batches of positive electrode sheets and negative electrode sheets as those in Example 1 are used. The difference lies only in that the mass content of fluoroethylene carbonate added to the electrolyte is adjusted (the mass content of the lithium salt remains unchanged), as shown in Table 2.

[0127] The preparation method of the battery cell in Example 16 is basically the same as that in Example 1, and the same batch of positive and negative electrode sheets as those in Example 1 are used. The difference lies only in the preparation process of the glue coating layer of the separator. During this process, the mass ratio of PVDF to filler particles in the second coating is adjusted (specifically, in this example, the mass ratio of PVDF to filler particles is adjusted to 74.4:25.6, so that the mass content of PVDF in the second coating is 74.4 wt%, and the mass content of the filler is 25.6 wt%), and the mass content of fluoroethylene carbonate added to the electrolyte, as shown in Tables 1 and 2.

[0128] The preparation methods of the battery cells in Examples 17 to 21 are basically the same as that in Example 1. The difference lies only in that during the preparation process of the first coating and / or the second coating, the thickness H1 of the glue coating layer in the first coating and / or the thickness H2 of the second coating are adjusted differently, as shown in Table 3.

[0129] The preparation method of the battery cell in Example 22 is basically the same as that in Example 1. The difference lies only in the preparation method of the separator. In the preparation process of the first heat-resistant layer in this example, the slurry is simultaneously coated on the second surface of the porous base film and the surface of the second coating through a gravure roll, and dried in a multi-section oven at 60 °C to form a first heat-resistant layer with a thickness of 1 μm and a second heat-resistant layer with a thickness of 1 μm.

[0130] The preparation method of the battery cell in Example 23 is basically the same as that in Example 1. The difference lies only in the preparation method of the separator. In the preparation process of the second coating in this example, the same mass of boehmite (γ-AlOOH) is used to replace alumina as the filler particles. In the preparation process of the first heat-resistant layer, the same mass of boehmite (γ-AlOOH) is used to replace alumina as the heat-resistant particles. The average particle size of boehmite (γ-AlOOH) is 0.8 μm.

[0131] The preparation method of the battery cell in Example 24 is basically the same as that in Example 1. The difference lies only in the preparation method of the separator. In the preparation process of the second coating in this example, the same mass of melamine cyanurate is used to replace alumina as the filler particles. In the preparation process of the first heat-resistant layer, the same mass of melamine cyanurate is used to replace alumina as the heat-resistant particles. The average particle size of melamine cyanurate is 0.8 μm.

[0132] The preparation method of the battery cell in Example 25 is basically the same as that in Example 9. The difference lies only in adjusting the average particle size of lithium nickel cobalt manganese oxide, so that the numerical value of the average particle size G μm of the positive active material on the surface of the positive electrode sheet is different, as shown in Table 1 specifically.

[0133] The preparation method of the battery cell in Example 26 is basically the same as that in Example 8, except that the average particle size of lithium nickel cobalt manganate is adjusted, so that the numerical value of the average particle size Gμm of the positive active material on the surface of the positive electrode sheet is different, as shown in Table 1 specifically.

[0134] The preparation method of the battery cell in Example 27 is basically the same as that in Example 8, and the same batch of positive electrode sheets and negative electrode sheets as those in Example 8 are used. The difference is only that the mass content of fluoroethylene carbonate added to the electrolyte is adjusted (the mass content of the lithium salt remains unchanged), as shown in Table 2.

[0135] The preparation method of the battery cell in Example 28 is basically the same as that in Example 9, and the same batch of positive electrode sheets and negative electrode sheets as those in Example 8 are used. The difference is only that the mass content of fluoroethylene carbonate added to the electrolyte is adjusted (the mass content of the lithium salt remains unchanged), as shown in Table 2.

[0136] The preparation method of the battery cell in Comparative Example 1 is basically the same as that in Example 1, and the same batch of positive electrode sheets and negative electrode sheets as those in Example 1 are used. The difference is only that the diaphragm preparation method is different. In the diaphragm preparation process of this comparative example, the preparation of the first heat-resistant layer and the coating layer is omitted, and in the preparation process of the second coating layer, "coating the mixed slurry on the first surface of the porous base film through a gravure roll" is replaced by "coating the mixed slurry on the first surface and the second surface of the porous base film through a gravure roll". Other process conditions and raw materials are the same as those in Example 1, forming a second coating layer with a thickness of 1.5 μm on both sides.

[0137] The differences in Comparative Examples 2-3 are only in the diaphragm preparation process, and the same batch of positive electrode sheets and negative electrode sheets as those in Example 1 are used. The thickness of the first heat-resistant layer is adjusted to 2 μm, and the mass ratio of PVDF to filler particles in the second coating layer is adjusted (specifically, in Comparative Example 2, the mass ratio of PVDF to filler particles is adjusted to 79.9:20.1, so that the mass content of PVDF in the second coating layer is 79.9 wt%, and the mass content of the filler is 20.1 wt%; in Comparative Example 3, the mass ratio of PVDF to filler particles is adjusted to 24.6:75.4, so that the mass content of PVDF in the second coating layer is 24.6 wt%, and the mass content of the filler is 75.4 wt%), so that the ratio b of the mass content of the first element to the second element on the surface of the second coating layer, the adhesion force L2 between the second coating layer and the negative electrode sheet, and the thermal weight loss mass residue rate c are different, as shown in Table 1 and Table 2 specifically.

[0138] The preparation method of the battery cell in Comparative Example 4 is basically the same as that in Example 9, and the positive and negative electrode sheets of the same batch as those in Example 1 are used. The difference lies only in that during the preparation process of the glue coating layer of the separator, the area ratio of the coating area of the glue coating layer is adjusted, and the mass ratio of PVDF to filler particles in the second coating layer is adjusted (specifically, the mass ratio of PVDF to filler particles is adjusted to 33:67, so that the mass content of PVDF in the second coating layer is 33 wt%, and the mass content of the filler is 67 wt%), resulting in different ratios a of the mass content of C element to Al element on the surface of the first coating layer, the adhesion force L1 between the first coating layer and the positive electrode sheet, the ratio b of the mass content of the first element to the second element on the surface of the second coating layer, the adhesion force L2 between the second coating layer and the negative electrode sheet, and the thermal weight loss mass residue rate c. The specific values are shown in Tables 1 and 2.

[0139] Table 1 Physical parameters of the separator and the positive electrode sheet

[0140]

[0141]

[0142] Table 2 Physical parameters of the separator, the electrode sheet and the electrolyte

[0143]

[0144]

[0145] Table 3 Thickness and thickness difference of each layer in the separator

[0146] H1 (μm) H2 (μm) H1 - H2 (μm) Example 1 3 1.5 1.5 Example 17 5 1.5 3.5 Example 18 3 2 1 Example 19 1 0.5 0.5 Example 20 0.5 0.5 0 Example 21 6 4.5 1.5

[0147] Test example

[0148] The lithium-ion batteries prepared in each example and comparative example were taken for the following tests:

[0149] 1. Cycling performance: In an environment of 25°C ± 2°C, constant current and constant voltage charging was carried out at 0.7C to 4.45V, and cut off at 0.05C. The initial thickness P0 was recorded. Then, constant current discharge was carried out at 0.2C to 3.0V, and the initial discharge capacity was recorded as C0. After standing for 10 min, the cycling mode was: constant current and constant voltage charging at 1.2C to 4.2V, then switching to constant current and constant voltage charging at 0.7C to 4.45V, cut off at 0.05C, standing for 5 min, and discharging at 0.7C to 3.0V. After 1000 cycles, constant current and constant voltage charging was carried out at 0.7C to 4.45V, cut off at 0.05C, and the final thickness P1 was recorded. Then, constant current discharge was carried out at 0.2C to 3.0V, and the discharge capacity after 1000 cycles was recorded as C1. After 1000 cycles, the battery was disassembled to confirm the lithium deposition situation on the surface of the negative electrode sheet of the battery.

[0150] Capacity retention rate: C = C1 / C0×100%, thickness swelling rate: P = (P1 - P0) / P0×100%.

[0151] The level of lithium plating is represented by 0, 1, 2, 3, 4, 5. 0 represents no lithium plating, 5 represents severe lithium plating, and 1, 2, 3, 4 represent different degrees of lithium plating. The larger the number, the more severe the lithium plating degree.

[0152] 2. Thermal stability and safety test

[0153] The lithium-ion battery is discharged at 0.5C and charged to full at 1C standard. The lithium-ion battery is placed in a thermal chamber and heated to 130°C at a rate of (5°C ± 2°C) / min and held for 60 min to end the experiment. Record the state of the lithium-ion battery before and after the test. If the lithium-ion battery catches fire, the furnace temperature test fails. Twenty cells are tested at a time, and the furnace temperature pass rate is expressed as "the number of lithium-ion batteries passing the furnace temperature test / the total number of lithium-ion batteries".

[0154] Please refer to Table 4 for the test results of the above items.

[0155] Table 4 Performance test results

[0156]

[0157]

[0158] It can be seen from the results of the above table that compared with Comparative Examples 1-4, each embodiment of the present application can improve the furnace temperature performance and the thermal safety of the battery, enabling the cells to have both long cycle life and excellent thermal safety performance.

[0159] Obviously, the above embodiments are only examples for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A battery cell, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator located between the positive electrode sheet and the negative electrode sheet; the separator includes a base film, a first coating, and a second coating. The first coating is located on one surface of the base film and faces the positive electrode sheet, and the second coating is located on the other surface of the base film and faces the negative electrode sheet. Both the first coating and the second coating contain a first element and a second element; the first element is C, and the second element is one or more of Al, Ba, Mg, Si, Sn, Ti, N; the ratio a of the mass content of the first element to the second element on the surface of the first coating and the ratio b of the mass content of the first element to the second element on the surface of the second coating satisfy 5 ≤ b - a ≤ 18.

5.

2. The battery cell according to claim 1, wherein, The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The average particle size G μm of the positive electrode active material on the surface of the positive electrode sheet, a, and b satisfy the following relational expression: 0.4 ≤ 0.5×(b - a) / G ≤ 3; and / or, the average particle size G μm of the positive electrode active material on the surface of the positive electrode sheet satisfies: 2 ≤ G ≤ 10.

3. The battery cell according to claim 1, characterized in that, The adhesion force L1 N / m between the first coating and the positive electrode sheet and the adhesion force L2 N / m between the second coating and the negative electrode sheet satisfy the following relational expression: 2 ≤ L2 - L1 ≤ 12; and / or, the adhesion force L1 N / m between the first coating and the positive electrode sheet satisfies: 3 ≤ L1 ≤ 12; and / or, the adhesion force L2 N / m between the second coating and the negative electrode sheet satisfies: 5 ≤ L2 ≤ 28.

4. The battery cell according to claim 1, wherein the electrolyte includes fluoroethylene carbonate. Based on the total mass of the electrolyte, the mass content of fluoroethylene carbonate is f, and 10% ≤ f ≤ 20%; under a nitrogen atmosphere, the separator is subjected to thermogravimetric analysis by heating from 25°C to 800°C at a heating rate of 10 K / min, and the thermogravimetric mass residual rate obtained is c, and 28% ≤ c ≤ 40%; c and f satisfy the following relational expression: 1.5 ≤ c / f ≤ 3.

5.

5. The battery cell according to any one of claims 1-4, characterized in that, 1.5 ≤ a ≤ 4.5; and / or, 8 ≤ b ≤ 20; and / or, the thickness H2 of the second coating is 0.5 μm - 2 μm.

6. The cell according to any one of claims 1-4, characterized in that, The separator satisfies one or more of the following conditions: A. The first coating includes a first heat-resistant layer and an adhesive layer. The first heat-resistant layer is located between the base film and the adhesive layer. The adhesive layer includes a blank area and a coated area. The coated area of the adhesive layer includes first polymer particles, and the first heat-resistant layer includes first heat-resistant particles; B. The second coating is a continuous coating layer including filler particles and a non-granular second polymer; C. A second heat-resistant layer including second heat-resistant particles is further included between the second coating and the base film; D. The thickness of the base film is 3 μm - 12 μm; E. The porosity of the base film is 25% - 55%; F. The average pore diameter of the base film is 28 nm - 45 nm; G. The composition of the base film includes one or more of polyolefin, non-woven fabric, and polyimide; H. The surface of the second coating has multiple pores. Under a field of view magnified 10,000 times by a scanning electron microscope, the number of pores with a pore diameter ≥ 0.3 μm in a unit area of 10 μm × 10 μm is 5 - 55.

7. The battery cell according to claim 6, characterized in that, The thickness H1 of the adhesive layer and the thickness H2 of the second coating satisfy the following relationship: 0.5 μm ≤ H1 - H2 ≤ 4 μm; and / or, the thickness H1 of the adhesive layer is 1 μm - 5 μm.

8. The battery cell according to claim 6, wherein, The battery cell satisfies one or more of the following conditions: A. The percentage of the projected area of the coated area on the first heat-resistant layer to the surface area of one side of the first heat-resistant layer is 15% - 60%; B. The first polymer particles include primary particles, and the average particle diameter of the primary particles is 150 nm - 500 nm; C. The first polymer particles include secondary particles, and the average particle diameter of the secondary particles is 3 μm - 12 μm; D. Under a field of view magnified 1,000 times by a scanning electron microscope, the number of first polymer particles with a particle diameter ≥ 3 μm in a unit area of 100 μm × 100 μm of the adhesive layer is 20 - 160; E. The high compression rate of the first polymer particles under the conditions of a temperature of 80 °C and a pressure of 1 MPa for 30 s is 20% - 80%; F. The average particle diameter of the first heat-resistant particles is 0.2 μm - 2 μm; and / or, the average particle diameter of the filler particles is 0.2 μm - 2 μm; and / or, the average particle diameter of the second heat-resistant particles is 0.2 μm - 2 μm; G. The compositions of the filler particles, the first heat-resistant particles, and the second heat-resistant particles are independently selected from one or more of boehmite, alumina, barium sulfate, magnesia, magnesium hydroxide, silica, stannic oxide, titanium oxide, barium titanate, melamine cyanurate, 1,3,5-triazine-2,4,6-triamine, and melamine trithiocyanate; H. The second polymer is selected from one or more of polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, copolymers of vinylidene fluoride and trichloroethylene, polymers of vinylidene fluoride and hexafluoropropylene, and copolymers of vinylidene fluoride and trichloroethylene; I. The first polymer particles include a first polymer, and the first polymer includes a polymer copolymerized from one or more monomers of methyl methacrylate, acrylonitrile, butyl acrylate, isooctyl acrylate, octadecyl acrylate, ethyl acrylate, styrene, ethylene, and butadiene; J. Based on the total mass of the second coating, the mass content of the second polymer is 20% - 70%; K. Based on the total mass of the second coating, the mass content of the filler particles is 30% - 80%.

9. The battery cell according to claim 8, wherein, The percentage of the projected area of the coated area on the first heat-resistant layer to the surface area of one side of the first heat-resistant layer is 20% - 40%; and / or, under a field of view magnified 1,000 times by a scanning electron microscope, the number of first polymer particles with a particle diameter ≥ 3 μm in a unit area of 100 μm × 100 μm of the adhesive layer is 30 - 140.

10. An electrochemical device, characterized in that, Comprising the battery cell according to any one of claims 1-9.

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