Composite diaphragm and secondary battery

Abstract

The composite diaphragm provided by the present invention comprises a diaphragm substrate and a coating layer provided on at least one surface of the diaphragm substrate, wherein the coating layer comprises polymer particles and a binder, wherein the polymer particles comprise a granular core polymer and a shell polymer coated on the core polymer, wherein the volume average particle size aD50 of the polymer particles and the thickness d of the coating layer satisfy 0.5≤(aD50-d)≤1, and the glass transition temperature Tg of the shell polymer ≥ 100°C. The binder composition of the present invention is used on the diaphragm, and the polymer particles can play a supporting / isolating function when the diaphragm is rolled up, effectively preventing the diaphragm from self-adhesion. When used in the preparation of secondary batteries, it can improve electrolyte infiltration, improve lithium deposition at the R corner of the battery, and improve the cycle capacity retention rate of the battery cell.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to a composite diaphragm and a secondary battery. Background Art

[0002] With the development of human society, problems such as energy shortage and environmental pollution are becoming increasingly serious. In recent years, in response to higher requirements in application fields such as the aerospace industry, long-range electric vehicles, and portable electronic products, high-performance, low-cost lithium-ion batteries have become a research and development hotspot.

[0003] Lithium-ion secondary batteries primarily consist of a positive electrode, a negative electrode, a separator, an electrolyte, and a battery casing. The separator is a key internal component of a lithium-ion secondary battery, separating the positive and negative electrodes to prevent contact and short circuits, while also providing a pathway for lithium ion transport. The performance of the separator determines the battery's interface structure, internal resistance, capacity, and cycle performance, and has a significant impact on battery safety.

[0004] At present, the separators used in lithium-ion batteries are generally polyolefin porous membranes. Since the melting point of this polyolefin porous membrane is lower than 200°C, when the battery temperature rises due to internal or external factors, the polyolefin porous membrane will shrink or melt, causing direct contact between the positive and negative electrodes, resulting in a battery short circuit, and then causing accidents such as battery combustion and explosion, which is not conducive to improving battery safety.

[0005] In order to improve this problem, researchers in recent years have begun to try to use binders to coat ceramic particles on the surface of the diaphragm substrate to make ceramic / polymer composite diaphragms. It is hoped that the heat resistance of ceramic particles can be used to reduce the thermal shrinkage of the diaphragm, and at the same time, polymers can be used to improve the adhesion between the composite diaphragm and the electrode sheet interface, thereby preventing the positive and negative electrodes of the battery from short-circuiting and improving the hardness of the battery cell.

[0006] To improve battery safety and electrochemical performance, various material modifications are performed on the surface of polyolefin separators through coating technology. Based on the pore-forming mechanism and solvent used in coating, polyolefin separators can be divided into water-based coating separators and oil-based coating separators.

[0007] Oil-based coating, currently the dominant coating method, involves dissolving a fluoropolymer such as PVdF in an organic solvent such as NMP to create a slurry, which is then applied to the separator substrate / ceramic membrane. The NMP and other organic solvents are then washed away with water or other solvents to form a porous coating. Compared to water-based coating, oil-based coating can improve the peel strength between the electrode and the cell, reaching up to 20 N / m, improving the long-term cycling life of the battery cell.

[0008] However, there are some problems with oily coating: First, the coated diaphragm will self-adhere during transportation in summer after being rolled up, causing the coating to fall off; second, since the oily coated PVdF is in a dissolved segment state, the gap between the electrode and the diaphragm at the R corner of the prepared wound battery cell (the R corner of the wound battery cell refers to the rounded or flat angle area formed at the corner of the positive electrode material, negative electrode material and diaphragm during the winding process of the battery cell) is small, affecting the wettability of the electrolyte. This poor electrolyte wettability will cause lithium plating in the battery, thereby affecting the cycle capacity retention rate of the battery cell.

[0009] Therefore, a new separator adhesive composition is needed to improve such problems, while maintaining the bonding performance, improving the separator self-adhesion and battery core performance. Summary of the Invention

[0010] In order to solve the problems of self-adhesion of the existing diaphragms during rolling and easy lithium deposition at the R corners of the battery cells, the present invention aims to provide a composite diaphragm and a secondary battery.

[0011] The present invention achieves the technical effects through the following technical solutions.

[0012] In a first aspect, the present invention provides a composite diaphragm, comprising a diaphragm substrate and a coating layer disposed on at least one surface of the diaphragm substrate, the coating layer comprising polymer particles and a binder, the polymer particles comprising a granular core polymer and a shell polymer coated on the core polymer, the volume average particle size aD50 of the polymer particles and the thickness d of the coating layer satisfy 0.5≤(aD50-d)≤1, and the glass transition temperature Tg of the shell polymer ≥ 100°C.

[0013] Furthermore, the mass ratio of the polymer particles to the binder is (15-35): (65-85). By adjusting the mass ratio of the polymer particles to the binder within an appropriate range, a binder composition that meets the requirements can be obtained.

[0014] Furthermore, the glass transition temperature (Tg) of the core polymer is ≤ 60°C. Preferably, the glass transition temperature (Tg) of the core polymer is between -50°C and 60°C, and the glass transition temperature (Tg) of the shell polymer is between 100°C and 150°C. In this way, the Tg of the shell polymer is higher than the atomization temperature (95°C) for powder making, which does not cause particle aggregation. Moreover, the Tg of the core polymer is lower than that of the shell polymer, and the Tg of the core polymer is lower than the hot pressing temperature (85°C). The core-shell structure can deform the particles during hot pressing, allowing the pole piece to contact the binder in the coating layer.

[0015] Furthermore, the mass ratio of the core polymer to the shell polymer in the polymer particles is (40-70):(30-60). This allows the shell polymer to effectively coat the core polymer, preventing particle aggregation during powder formation. The particles can also deform during hot pressing, allowing the electrode to effectively contact the binder coating in the coating layer.

[0016] Furthermore, the thickness d of the coating layer is 1-3 μm, the volume average particle size aD50 of the polymer particles is 1.5-4 μm, and the volume average particle size bD50 of the granular core polymer is 1.1-3.5 μm.

[0017] Furthermore, the polymerization monomer of the core polymer is selected from at least one of vinyl monomers, (meth)acrylic monomers, and (meth)acrylate monomers.

[0018] Furthermore, the core polymer may be composed of at least one monomer selected from styrene, methylstyrene, acrylic acid, methacrylic acid, ethyl acrylate, butyl acrylate, isooctyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and hydroxybutyl methacrylate. The selected monomers and their corresponding mass ratios must ensure that the core polymer has a glass transition temperature (Tg) of -50°C to 60°C.

[0019] Furthermore, the polymerization monomer of the shell polymer is at least one selected from vinyl monomers, amide monomers, (meth)acrylic monomers, and (meth)acrylate monomers.

[0020] Furthermore, the shell polymer may contain at least one monomer selected from the group consisting of styrene, methylstyrene, acrylic acid, methacrylic acid, acrylonitrile, acrylamide, dimethylacrylamide, hydroxymethylacrylamide, vinylpyrrolidone, methyl methacrylate, cyclohexyl methacrylate, ethyl acrylate, butyl acrylate, and isooctyl acrylate. The selected monomers and their corresponding mass ratios must ensure that the shell polymer has a glass transition temperature (Tg) of 100-150°C.

[0021] In a second aspect, the present invention provides a secondary battery comprising a negative electrode sheet, a positive electrode sheet, an electrolyte and the aforementioned composite separator.

[0022] Compared with the prior art, the present invention has the following beneficial effects.

[0023] The polymer particles of the present invention are spray-dried to form a powder, with a shell polymer coating the granular core polymer. Because its glass transition temperature (Tg) is significantly higher than the atomization temperature, the particles exhibit no significant adhesion during the powder-making process and can be redispersed to their original particle size in an organic solvent (such as NMP) while maintaining a spherical particle shape. Furthermore, the Tg of the core polymer is lower than that of the shell polymer, and the Tg of the core polymer is lower than the hot-pressing temperature. This allows the core-shell structure to deform during hot-pressing, allowing the electrode to contact the binder in the coating layer.

[0024] The polymer particles are mixed with a binder to create a new separator binder composition. This composition is then applied to the separator substrate, and then washed with water to remove the organic solvent, forming a porous coating. The spherical polymer particles act as a barrier and support when the separator is rolled up, effectively preventing the separator from self-adhesion.

[0025] In the prepared secondary battery cell, during hot pressing, the spherical particles at non-R corner positions will be deformed due to hot pressing, so that the binder in the coating layer and the electrode can maintain good adhesion. At the R corner, because the particles are not obviously hot pressed, the degree of particle deformation is small, and a certain gap can be maintained between the electrode and the diaphragm, thereby improving the electrolyte infiltration, thereby improving the battery lithium plating and improving the battery cell cycle capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the structure of the composite diaphragm in some embodiments of the present invention.

[0027] Figure 2 This is a disassembly diagram of the battery cell prepared in Example 1 of the present application.

[0028] Figure 3 This is a disassembly diagram of the battery cell prepared in Comparative Example 1 of the present application.

[0029] In the figure: 1: diaphragm substrate; 2: coating layer; 21: polymer particles; d: thickness of the coating layer. DETAILED DESCRIPTION

[0030] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the embodiments described herein are part of the embodiments of the present invention, rather than all of the embodiments, and are only used to explain the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0031] It should be noted that the volume average particle size in the present invention is also called the median particle size, which refers to the particle size value corresponding to when the cumulative volume distribution percentage reaches 50%, and D50 is tested using a laser particle size analyzer.

[0032] The mass ratio of the core polymer to the shell polymer refers to the mass ratio of the polymerized monomers of the core polymer to the polymerized monomers of the shell polymer used in the polymerization reaction.

[0033] In a first aspect, the present invention provides a composite diaphragm, comprising a diaphragm substrate and a coating layer disposed on at least one surface of the diaphragm substrate, the coating layer comprising polymer particles and a binder, the polymer particles comprising a granular core polymer and a shell polymer coated on the core polymer, the volume average particle size aD50 of the polymer particles and the thickness d of the coating layer satisfy 0.5≤(aD50-d)≤1, and the glass transition temperature Tg of the shell polymer ≥100°C.

[0034] In some specific embodiments, the binder is selected from PVdF, polyacrylate, and polyimide.

[0035] In some specific embodiments, the mass ratio of the polymer particles to the binder is (15-35):(65-85). If the polymer particle content is too low, self-adhesion cannot be improved and lithium deposition at the rounded corners of the battery can occur. If the polymer particle content is too high, adhesion at non-rounded corners can be significantly reduced. By adjusting the mass ratio of the polymer particles to the binder within an appropriate range, a binder composition that meets the requirements can be obtained.

[0036] In some embodiments, the core polymer has a glass transition temperature Tg ≤ 60° C. Preferably, the core polymer has a glass transition temperature Tg of -50 to 60° C., and the shell polymer has a glass transition temperature Tg of 100 to 150° C.

[0037] Polymer particles are spray-dried to a powder. The temperature during the powdering process is maintained at 95°C, which facilitates the atomization and drying of water in the polymer particle reaction system. A shell polymer coats the core polymer particles. If the shell polymer's Tg is below 100°C, particle aggregation will occur. The shell polymer's Tg is significantly higher than the atomization temperature (95°C), resulting in no significant particle adhesion during the powdering process. The particles can be redispersed to their original size in organic solvents such as NMP, maintaining a spherical shape. Furthermore, the core polymer's Tg must be lower than that of the shell polymer and lower than the hot-pressing temperature (85°C). This allows the core-shell structure to deform during hot pressing, allowing contact between the electrode and the binder in the coating. Uniform particles with a Tg greater than 100°C are difficult to deform during hot pressing, and particles with non-rounded corners will prevent contact between the binder in the coating and the electrode, affecting adhesion.

[0038] In some specific embodiments, the mass ratio of the core polymer to the shell polymer in the polymer particles is (40-70): (30-60). If the core polymer accounts for too large a proportion, the shell polymer cannot effectively coat the core polymer, resulting in particle aggregation during powder formation; if the core polymer accounts for too small a proportion, deformation during hot pressing is not obvious, and the electrode cannot effectively contact the binder in the coating layer. By adjusting the mass ratio of the core polymer to the shell polymer within an appropriate range, the desired effect can be achieved: the shell polymer can effectively coat the core polymer, preventing particle aggregation during powder formation; the particles can deform during hot pressing, allowing effective contact with the binder in the coating layer.

[0039] In some specific embodiments, the coating layer has a thickness d of 1-3 μm, the polymer particles have a volume average particle size aD50 of 1.5-4 μm, and the granular core polymer has a volume average particle size bD50 of 1.1-3.5 μm. The coating layer thickness follows the current industry standard. If the polymer particle size is too small, it will not prevent adhesion and improve R-angle lithium precipitation. If the particle size is too large, it will affect the adhesion between the binder and the electrode after hot pressing. When the particle size of the polymer particles and the core polymer is within the appropriate range, adhesion can be prevented, R-angle lithium precipitation can be improved, and the adhesion between the binder and the electrode after hot pressing can be improved.

[0040] In some embodiments, the core polymer monomer is selected from at least one of a vinyl monomer, a (meth)acrylic monomer, and a (meth)acrylate monomer. In this application, a (meth)acrylic monomer refers to derivatives of methacrylic acid and acrylic acid, and a (meth)acrylate monomer refers to derivatives of methacrylate and acrylate.

[0041] In some specific embodiments, the polymerizable monomer of the shell polymer is selected from at least one of vinyl monomers, amide monomers, (meth)acrylic monomers, and (meth)acrylate monomers.

[0042] In some embodiments, the core polymer monomer can be selected from at least one of styrene, methylstyrene, acrylic acid, methacrylic acid, ethyl acrylate, butyl acrylate, isooctyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and hydroxybutyl methacrylate. The selected monomers and their corresponding mass ratios must ensure that the core polymer has a glass transition temperature (Tg) between -50°C and 60°C.

[0043] In some embodiments, the shell polymer may contain at least one monomer selected from styrene, methylstyrene, acrylic acid, methacrylic acid, acrylonitrile, acrylamide, dimethylacrylamide, hydroxymethylacrylamide, vinylpyrrolidone, methyl methacrylate, cyclohexyl methacrylate, ethyl acrylate, butyl acrylate, and isooctyl acrylate. The selected monomers and their corresponding mass ratios must ensure that the shell polymer has a glass transition temperature (Tg) of 100-150°C.

[0044] Preparation method of the binder composition of the present invention:

[0045] 1) Using water as the solvent, select the required core polymer monomer, add polyvinyl alcohol as a stabilizing dispersant, add an initiator, homogenize until the particle size is 1.1-3.5 μm, react at 85°C for 8 hours, and then cool to 40°C to obtain a core polymer dispersion;

[0046] 2) Using water as the solvent, select the monomers of the shell polymer that meet the requirements, add the emulsifier sodium dodecylbenzene sulfonate, pre-emulsify, add to the dispersion of the core polymer, swell for 24 hours, add the initiator, and react at 85°C for 8 hours to obtain a dispersion of the shell polymer coated on the core polymer;

[0047] 3) The dispersion obtained in step 2) is spray-dried to obtain polymer particles with a particle size of 1.5 to 4 μm.

[0048] 4) The polymer particles and the binder are first dispersed or dissolved in a solvent (such as NMP) in a mass ratio of (15-35): (65-85) to obtain a polymer particle dispersion and a binder solution, and then the two are mixed to obtain a binder composition.

[0049] In step 1) and step 2), the mass ratio of the polymerized monomers of the core polymer to the polymerized monomers of the shell polymer is the same as the mass ratio of the core polymer to the shell polymer, which is (40-70):(30-60); based on the total mass of the polymerized monomers of the core polymer and the polymerized monomers of the shell polymer, the amount of polyvinyl alcohol added is 0.5-2% of the total mass, the amount of sodium dodecylbenzenesulfonate added is 0.1-0.8% of the total mass, and the amount of initiator added twice is 0.2-0.4% of the total mass; the initiator is dibenzoyl peroxide, dodecyl peroxide or azobisisobutyronitrile.

[0050] The preparation methods, glass transition temperatures, and particle size control methods of the above-mentioned polymers are well known to those skilled in the art, or can be known to those skilled in the art based on existing technologies. For example, the glass transition temperature can be controlled by controlling the composition and ratio of the polymerized monomers, and the particle size of the polymer particles can be controlled by controlling conditions such as the solid content and the stirring speed during the synthesis process (for example, a speed of 3,000 to 20,000 rpm can be selected based on the different material compositions of the polymer particles). Generally, the higher the speed, the smaller the particle size of the polymer particles.

[0051] Figure 1 Schematic diagram of the structure of the diaphragm in some embodiments of the present invention. Figure 1 As shown, the composite diaphragm of the present invention includes a diaphragm substrate 1 and a coating layer 2 provided on at least one surface of the diaphragm substrate, wherein the coating layer 2 includes polymer particles 21 and a binder.

[0052] The volume average particle size aD50 of the polymer particles 21 and the thickness d of the coating layer satisfy 0.5≤(aD50−d)≤1.

[0053] In a second aspect, the present invention provides a secondary battery comprising a negative electrode sheet, a positive electrode sheet, an electrolyte and the aforementioned composite separator.

[0054] The specific embodiments of the present invention will be further explained below through examples and comparative examples.

[0055] Unless otherwise specified, the reagents, materials, and instruments used in the following description are all conventional reagents, conventional materials, and conventional instruments, all of which are commercially available. The reagents involved can also be synthesized by conventional synthetic methods. The methods in the examples, unless otherwise specified, are all conventional methods in the art.

[0056] PVdF: from Arkema Kynar ® Flex LBG.

[0057] Polyvinyl alcohol: PVA1788.

[0058] Monomers according to the present invention are commercially available.

[0059] The glass transition temperature can be selected at the time of purchase or can be measured yourself, for example using a differential scanning calorimeter (DSC).

[0060] Example 1

[0061] This embodiment provides a binder composition comprising polymer particles and PVdF, wherein the polymer particles include a granular core polymer and a shell polymer coated outside the core polymer particles; the glass transition temperature Tg of the core polymer is 34.2°C; and the glass transition temperature Tg of the shell polymer is 113.2°C.

[0062] (1) Preparation of adhesive composition

[0063] Core polymer: 1 part of PVA1788, 40 parts of styrene, 2 parts of methacrylic acid, 18 parts of isooctyl acrylate, add 0.36 parts of dibenzoyl peroxide, the solvent is water, the solid content is controlled to 15%, homogenize to the volume average particle size bD50 of 1.2μm, the homogenization speed is 3000r / min, the time is 60min, react at 85℃ for 8 hours, cool to 40℃, and obtain a dispersion of the core polymer.

[0064] Shell polymer: 0.5 parts of sodium dodecylbenzenesulfonate, 38 parts of styrene, and 2 parts of methacrylic acid are pre-emulsified and added to the dispersion of the core polymer. After swelling for 24 hours, 0.24 parts of dibenzoyl peroxide are added. The solvent is water, and the solid content is controlled to be 15%. The reaction is carried out at 85°C for 8 hours to obtain a dispersion of the shell polymer coated on the outside of the core polymer.

[0065] After powdering by spray drying, polymer particles with a volume average particle size aD50 of 1.8 μm were obtained.

[0066] The polymer particles and PVdF are first dispersed or dissolved in NMP solvent in a mass ratio of 20:80, and the solid content is controlled to be 10% to obtain a polymer particle NMP dispersion and a PVdF NMP glue solution, and then the two are mixed to obtain a binder composition.

[0067] (2) Preparation of composite diaphragm

[0068] The prepared binder composition was coated on a separator substrate, and then NMP was washed away with water to form a porous coating layer. The thickness d of the coating layer was 1.2 μm.

[0069] (3) Preparation of positive electrode

[0070] N-methylpyrrolidone is used to disperse positive electrode active particles of lithium nickel cobalt manganese oxide NCM523, conductive carbon black Super P and binder composition polyvinylidene fluoride to form a positive electrode slurry; the positive electrode slurry is evenly coated on Al foil by coating, and after baking and rolling processes, a positive electrode including a positive electrode active layer is obtained.

[0071] The mass ratio of the positive electrode active particles, the conductive agent and the binder composition is 97.5:1.0:1.5.

[0072] (4) Preparation of negative electrode

[0073] A negative electrode slurry is formed by using water-dispersed negative electrode active particles of artificial graphite, conductive carbon black Super P, a binder composition of sodium carboxymethyl cellulose and styrene-butadiene latex; then a negative electrode active layer is formed on the surface of the negative electrode current collector through coating, baking and roller pressing processes to obtain a negative electrode.

[0074] The mass ratio of the negative electrode active particles, the conductive agent and the binder composition (the mass ratio of sodium carboxymethyl cellulose to styrene-butadiene latex is 1:1.5) is 96.5:1.0:2.5.

[0075] (5) Battery preparation

[0076] The prepared positive electrode, the composite separator of the present invention, and the negative electrode are stacked in sequence and then wound to obtain a battery cell. The battery cell is placed in an aluminum-plastic film, and the electrolyte is injected into the bare battery cell. After vacuum packaging, standing, formation, shaping, and capacity testing, a battery is obtained.

[0077] The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate, wherein the ratio of ethylene carbonate:ethyl methyl carbonate:diethyl carbonate is 3:2:5, and the concentration of lithium hexafluorophosphate is 1 mol / L.

[0078] Example 2

[0079] A binder composition, a separator, and a lithium-ion secondary battery were prepared according to the method of Example 1. The main difference between Example 2 and Example 1 is that the glass transition temperature Tg of the shell polymer is 148.6°C, and the glass transition temperature Tg of the core polymer is 58.7°C. The details are as follows.

[0080] Core polymer: 1 part of PVA1788, 47 parts of styrene, 2 parts of acrylic acid, 11 parts of isooctyl acrylate, 0.36 parts of dibenzoyl peroxide, the solvent is water, the solid content is controlled at 15%, and the mixture is homogenized to a volume average particle size bD50 of 1.2 μm. The homogenization speed is 3000 r / min and the time is 60 min.

[0081] Shell polymer: 0.5 parts of sodium dodecylbenzenesulfonate, 15 parts of styrene, 21 parts of methylstyrene, 2 parts of acrylic acid, 0.24 parts of dibenzoyl peroxide, the solvent is water, and the solid content is controlled to be 15%.

[0082] After powdering by spray drying, polymer particles with a volume average particle size aD50 of 1.8 μm were obtained.

[0083] The mass ratio of the binder composition: polymer particles and PVdF was 20:80, and the thickness d of the coating layer was 1.2 μm.

[0084] Example 3

[0085] A binder composition, a separator, and a lithium-ion secondary battery were prepared according to the method of Example 1. The main difference between Example 3 and Example 1 is that the glass transition temperature Tg of the shell polymer is 99.0°C, and the glass transition temperature Tg of the core polymer is -49.0°C. The details are as follows.

[0086] Core polymer: 1 part of PVA1788, 7 parts of styrene, 2 parts of methacrylic acid, 51 parts of isooctyl acrylate, 0.36 parts of dibenzoyl peroxide, the solvent is water, the solid content is controlled at 15%, and it is homogenized to a volume average particle size bD50 of 1.2 μm. The homogenization speed is 3000 r / min and the time is 60 min.

[0087] Shell polymer: 0.5 parts of sodium dodecylbenzenesulfonate, 36 parts of styrene, 2 parts of butyl acrylate, 2 parts of methacrylic acid, 0.24 parts of dibenzoyl peroxide, the solvent is water, and the solid content is controlled to be 15%.

[0088] After powdering by spray drying, polymer particles with a volume average particle size aD50 of 1.8 μm were obtained.

[0089] The binder composition includes polymer particles and PVdF in a mass ratio of 20:80, and the thickness d of the coating layer is 1.2 μm.

[0090] Example 4

[0091] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The main differences between Example 4 and Example 1 are: the mass ratio of polymer particles to PVdF is 35:65, and the mass ratio of core polymer to shell polymer in the polymer particles is 40:60.

[0092] Example 5

[0093] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The main difference between Example 5 and Example 1 is that the mass ratio of the polymer particles and PVdF is 15:85, and the mass ratio of the core polymer to the shell polymer in the polymer particles is 70:30.

[0094] Example 6

[0095] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The main difference between Example 6 and Example 1 is that the mass ratio of polymer particles and PVdF is 30:70, and the mass ratio of core polymer to shell polymer in the polymer particles is 50:50.

[0096] Example 7

[0097] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The main differences between Example 7 and Example 1 are: the thickness d of the coating layer is 1 μm, the volume average particle size aD50 of the polymer particles is 1.5 μm, the volume average particle size bD50 of the granular core polymer is 1.1 μm, the homogenization speed is 3000 r / min, and the time is 90 min.

[0098] Example 8

[0099] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The difference between Example 8 and Example 1 is that the thickness d of the coating layer is 3 μm, the volume average particle size aD50 of the polymer particles is 4 μm, the volume average particle size bD50 of the granular core polymer is 3.5 μm, the homogenization speed is 3000 r / min, and the time is 35 min.

[0100] Comparative Example 1

[0101] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The difference between Comparative Example 1 and Example 1 is that conventional PVdF was selected as the binder composition and no polymer particles were added.

[0102] Comparative Example 2

[0103] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The difference between Comparative Example 2 and Example 1 is that the binder composition contains only polymer particles but no PVdF.

[0104] Comparative Example 3

[0105] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The difference between Comparative Example 3 and Example 1 is that the binder composition includes polymer particles and PVdF, the glass transition temperature Tg of the polymer is 120°C, but the polymer particles are not core-shell structures, but polyacrylamide particle powder.

[0106] Comparative Example 4

[0107] A binder composition, a separator and a lithium ion secondary battery were prepared according to the method of Example 1. The difference between Comparative Example 6 and Example 1 is that in the polymer particles, the glass transition temperature Tg of the core polymer is 99.0°C, and the glass transition temperature Tg of the shell polymer is -49.0°C.

[0108] Core polymer: 1 part of PVA1788, 36 parts of styrene, 2 parts of butyl acrylate, 2 parts of methacrylic acid, 0.36 parts of dibenzoyl peroxide, the solvent is water, the solid content is controlled at 15%, and the homogenization is performed to a volume average particle size bD50 of 1.2 μm at a homogenization speed of 3000 r / min for 60 min.

[0109] Shell polymer: 0.5 parts of sodium dodecylbenzenesulfonate, 7 parts of styrene, 2 parts of methacrylic acid, 51 parts of isooctyl acrylate, 0.24 parts of dibenzoyl peroxide, solvent is water, solid content is controlled to 15%;

[0110] After powdering by spray drying, polymer particles with a particle size of 1.8 μm were obtained.

[0111] The binder composition includes polymer particles and PVdF in a mass ratio of 20:80, and the thickness d of the coating layer is 1.2 μm.

[0112] Comparative Example 5

[0113] A binder composition, a separator and a lithium-ion secondary battery were prepared according to the method of Example 1. The difference between Comparative Example 5 and Example 1 is that in the binder composition, the mass ratio of the polymer particles to PVdF is 55:45.

[0114] Comparative Example 6

[0115] A binder composition, a diaphragm and a lithium-ion secondary battery were prepared according to the method of Example 1. The difference between Comparative Example 6 and Example 1 is that the binder composition is coated on the diaphragm substrate, the volume average particle size bD50 of the granular core polymer is 1.5 μm, the homogenization speed is 3000 r / min, the time is 45 min, the volume average particle size aD50 of the polymer particles is 2 μm, and the thickness d of the coating layer is 3 μm.

[0116] Performance Testing

[0117] The performance test of the diaphragms and secondary batteries prepared in the above examples and comparative examples is shown in Table 1. The disassembly diagrams of the battery cells prepared in Example 1 and Comparative Example 1 are shown in Table 1. Figure 2 、 3 shown.

[0118] [Separator Self-Adhesion Strength] The bond strength between separators was tested using a computerized puncture and peel tester under heat and pressure at 40°C / 1 MPa / 60 seconds. If the bond strength is greater than 1 N / m, the separator is prone to self-adhesion, which can cause the coating to fall off.

[0119] [Bond Strength between Diaphragm and Positive Electrode] Cut the diaphragm and positive electrode into 4*14 and 3*12 strips respectively. Hot press the diaphragm and electrode at 85℃ / 1.5MPa / 60s, and test the bond strength using a computerized puncture and peel tester.

[0120] [Battery 1C / 1C cycle 800 cycle capacity retention rate] The lithium ion secondary battery was charged at 25°C at a constant current of 1C to 4.50V, then charged at a constant voltage of 0.05C to 4.50V, and then discharged at a discharge rate of 1C to 3.0V. This charge and discharge cycle was repeated 800 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q at the 800th cycle were measured. 800 ; Calculate the capacity retention rate Q after 800 cycles according to the following formula: Capacity retention rate Q = Q 800 / Q1*100%.

[0121] [Lithium deposition at the R corner of the battery cell] Disassemble the battery cell and observe whether there is lithium deposition at the R corner.

[0122] [Hipot test] The battery cell is tested with an internal resistance meter under 100kgf, 85℃, and 30s hot pressing. The voltage is set to 250V and the resistance is set to 100MΩ. If the test resistance is lower than 100MΩ, it will be judged as defective and the battery cell is broken down.

[0123] Table 1

[0124]

[0125] From the test results of Examples 1 to 8 and Comparative Example 1, it can be seen that the conventional PVdF without the addition of polymer particles as the binder composition can improve the peeling bond strength between the positive electrode sheet, but because the self-adhesion bond strength of the separator (the bond strength between the separators) is greater than 1N / m, the separator is prone to self-adhesion after winding, which will cause the coating to fall off. The gap between the electrode sheet and the separator in the R corner of the prepared wound battery cell is small, the electrolyte infiltration is poor, and the battery lithium deposition (such as Figure 3 As shown in the figure), Hipot defects occur in some locations, which affects the cycle capacity retention rate of the battery cell.

[0126] From the test results of Examples 1 to 8 and Comparative Example 2, it can be seen that when PVdF-free polymer particles are used as the binder composition, the self-adhesion strength of the diaphragm is low and there is no lithium deposition in the battery, but the adhesion between the diaphragm and the electrode is very weak, resulting in a low cycle capacity retention rate of the battery cell.

[0127] It can be seen from the test results of Examples 1 to 8 and Comparative Example 3 that the selected polymer particles are not core-shell structures and are polyacrylamide particle powder as the binder composition. The diaphragm has no self-adhesion and no lithium deposition in the battery. However, because the deformation of the particles at non-R corners is small, the electrode and the PVdF coating cannot effectively contact each other, resulting in low adhesion between the electrode and the diaphragm and low cycle capacity retention of the battery cell.

[0128] It can be seen from the test results of Examples 1 to 3 and Comparative Example 4 that in the polymer particles, when the Tg of the core polymer is -50 to 60°C and the Tg of the shell polymer is 100 to 150°C, the performance is better; if the Tg of the core polymer is greater than the Tg of the shell polymer, and the Tg of the shell polymer is too low (<100°C), the particles will have obvious adhesion during the powder making process, resulting in particle aggregation and inability to redisperse, the particles at some positions will be larger, the Tg of the core polymer will be too high, and the particles will be difficult to deform during hot pressing, resulting in the diaphragm and the electrode being unable to effectively contact after hot pressing at the non-R corner, affecting the bonding strength between the diaphragm and the electrode, lithium deposition in the battery, and poor Hipot in some positions, resulting in a low battery cell cycle capacity retention rate.

[0129] From the test results of Examples 1, 4 to 6 and Comparative Example 5, it can be seen that in the binder composition, when the mass ratio of polymer particles to PVdF is (15 to 35): (65 to 85), the performance is better; if the mass of the polymer particles is greater than the mass of PVdF, the coverage of PVdF is low, affecting the effective contact area between the diaphragm and the electrode, resulting in a decrease in the bonding strength between the diaphragm and the electrode, thereby resulting in a low cycle capacity retention rate of the battery cell.

[0130] It can be seen from the test results of Examples 1, 7, 8 and Comparative Example 6 that the binder composition is coated on the diaphragm substrate. When the volume average particle size aD50 of the polymer particles and the thickness d of the coating layer satisfy 0.5≤(aD50-d)≤1, the performance is better; if the particle size of the polymer particles is smaller than the thickness of the coating layer, the polymer particles cannot play an isolation / support role and cannot effectively prevent the diaphragm from self-adhesion; the bonding force of the diaphragm self-adhesion (the bonding strength between the diaphragms) is greater than 1N / m, and the diaphragm self-adhesion is prone to occur after the coated diaphragm is rolled up, which will cause the coating to fall off, lithium deposition in the battery, and poor Hipot in some positions, resulting in a low cycle capacity retention rate of the battery cell.

[0131] The present invention is further described above with the aid of specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in this field after reading this specification are all within the scope of protection of the present invention.

Claims

1. A composite diaphragm, characterized in that: The composite diaphragm includes a diaphragm substrate and a coating layer disposed on at least one surface of the diaphragm substrate, the coating layer including polymer particles and a binder, the polymer particles including a granular core polymer and a shell polymer coated outside the core polymer, the mass ratio of the core polymer to the shell polymer in the polymer particles being (40-70):(30-60); the volume average particle size aD50 of the polymer particles and the thickness d of the coating layer satisfy 0.5≤(aD50-d)≤1, and the glass transition temperature Tg of the shell polymer is 100-150°C; The polymer particles are obtained by spray drying, the glass transition temperature Tg of the shell polymer is higher than the spray drying temperature, the glass transition temperature Tg of the core polymer is lower than the Tg of the shell polymer, and the glass transition temperature Tg of the core polymer is lower than the hot pressing temperature.

2. The composite diaphragm according to claim 1, characterized in that The mass ratio of the polymer particles to the binder is (15-35): (65-85).

3. The composite diaphragm according to claim 1, characterized in that The core polymer has a glass transition temperature Tg≤60°C.

4. The composite diaphragm according to claim 3, characterized in that The glass transition temperature Tg of the core polymer is -50 ~ 60 ° C.

5. The composite diaphragm according to claim 1, characterized in that The thickness d of the coating layer is 1-3 μm, the volume average particle size aD50 of the polymer particles is 1.5-4 μm, and the volume average particle size bD50 of the granular core polymer is 1.1-3.5 μm.

6. The composite diaphragm according to claim 4, characterized in that The polymerization monomer of the core polymer is selected from at least one of vinyl monomers, (meth)acrylic monomers, and (meth)acrylate monomers.

7. The composite diaphragm according to claim 1, characterized in that The polymerization monomer of the shell polymer is selected from at least one of vinyl monomers, amide monomers, (meth)acrylic monomers, and (meth)acrylate monomers.

8. A secondary battery, characterized in that: The invention comprises a negative electrode sheet, a positive electrode sheet, an electrolyte and the composite separator according to any one of claims 1 to 7.

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