Separator, electrochemical device and electronic device including the separator

Abstract

Provided are a separator capable of improving the dynamic characteristics, particularly the cycle characteristics under low temperature conditions, of a lithium ion battery, and an electrochemical device and an electronic device including the separator. [Problem solving] The present invention relates to a separator comprising a substrate and a first coating layer provided on at least one surface of the substrate, the first coating layer comprising a first polymer, a mass fraction x of the first polymer being 60% to 90% based on a total mass of the first coating layer, and a softening point of the first polymer being 90°C to 150°C.

Description

Technical Field

[0001] The present invention relates to the field of electrochemistry, and specifically to a separator, an electrochemical device including the separator, and an electronic device.

Background Art

[0002] Lithium-ion batteries have characteristics such as high specific energy, high operating voltage, low self-discharge rate, small volume, and light weight, and are widely applied in fields such as electrical energy storage, portable electronic devices, and electric vehicles.

[0003] With the rapid development of the application of lithium-ion batteries in fields such as consumer terminals, the requirements for the charge-discharge cycle characteristics of lithium-ion batteries are increasing. In particular, under low-temperature conditions, the actual charge-discharge characteristics of lithium-ion batteries are much smaller than the design values, and the further application of lithium-ion batteries is limited.

Summary of the Invention

[0004] An object of the present invention is to provide a separator, an electrochemical device including the separator, and an electronic device to improve the kinetic characteristics of lithium-ion batteries, especially the cycle characteristics under low-temperature conditions. The specific technical solutions are as follows.

[0005] In the content of the present invention, the present invention is described by taking a lithium-ion battery as an example of an electrochemical device, but the electrochemical device of the present invention is not limited to lithium-ion batteries.

[0006] A first aspect of the present invention provides a separator including a base material and a first coating layer provided on at least one surface of the base material, where the first coating layer includes a first polymer, and based on the total mass of the first coating layer, the mass fraction x of the first polymer is 60% to 90%, and the softening point of the first polymer is 90°C to 150°C.

[0007] The first coating layer of the present invention is provided on at least one surface of the substrate. For example, the first coating layer may be provided on one surface of the substrate, or the first coating layer may be provided on both surfaces of the substrate. Specifically, the positive electrode of the present invention may refer to the positive electrode sheet, and the negative electrode may specifically refer to the negative electrode sheet.

[0008] In the present invention, based on the total mass of the first coating layer, the mass fraction x of the first polymer is 60% to 90%, preferably 70% to 85%, and the softening point of the first polymer is 90°C to 150°C, preferably 135°C to 150°C. Without being limited by any theory, when the content of the first polymer is too high (for example, higher than 90%), the relative content of the auxiliary binder in the first coating layer decreases, the cohesive force of the first coating layer is low, and the adhesion between the first coating layer of the separator and the positive electrode decreases with the decrease in the cohesive force of the first coating layer. When the content of the first polymer is too low (for example, lower than 60%), the gaps formed by the first polymer decrease, which affects the transport between the interfaces of the first coating layer of the electrolyte solution and affects the cycle characteristics of the lithium-ion battery. When the softening point of the first polymer is too high (for example, higher than 150°C), the first polymer is difficult to soften during heating, the formed bonding area is small, and it affects the adhesion between the first coating layer and the electrode sheet. When the softening point of the first polymer is too low (for example, lower than 90°C), the first polymer is likely to soften and block the pores of the first coating layer or the separator, which affects the kinetic characteristics of the lithium-ion battery. The term "softening point" means the temperature at which a substance softens. By controlling the content and softening point of the first polymer in the first coating layer within the above ranges simultaneously, the present invention can obtain a first coating layer with excellent interfacial bonding performance and appropriate gaps between polymer particles, effectively improve the bonding performance and electrolyte wetting characteristics of the separator, and improve the kinetic characteristics of the lithium-ion battery, such as low-temperature cycle characteristics.

[0009] In one embodiment of the present invention, the monomer used in the first polymer includes at least one of vinylidene fluoride, hexafluoropropylene, ethylene, propylene, chloroethylene, chloropropene, acrylic acid, acrylate, styrene, butadiene, and acrylonitrile.

[0010] In one embodiment of the present invention, the thickness of the first coating layer is 3 μm to 40 μm. Without being limited by any theory, when the thickness of the first coating layer is too high (for example, higher than 40 μm), the ion transport distance becomes long, deteriorating the kinetic properties of the lithium-ion battery and simultaneously being disadvantageous for improving the energy density of the battery. When the thickness of the first coating layer is too low (for example, lower than 3 μm), it affects the wetting of the electrolyte between the coating layers and is disadvantageous for improving the kinetic properties of the lithium-ion battery. By controlling the thickness of the first coating layer of the present invention within the above range, the kinetic properties of the lithium-ion battery and the energy density of the battery can be further improved.

[0011] In one embodiment of the present invention, in any 250 μm × 200 μm region on the surface of the first coating layer, the number of particles of the first polymer with a maximum particle length of 10 μm to 30 μm is 10 to 30.

[0012] The separator of the present invention is observed at a magnification of 500 times using a SEM (scanning electron microscope). In any 250 μm × 200 μm region on the surface of the first coating layer, the number of particles of the first polymer with a maximum length of 10 μm to 30 μm is 10 to 30. Without being limited by any theory, on the one hand, by controlling the maximum length of the first polymer particles to 10 μm to 30 μm, aggregation due to the particle diameter of the particles being too small can be avoided, thereby improving the dispersibility of the first polymer particles. Also, it is possible to avoid the coating layer becoming thick due to the particle diameter of the particles being too large and affecting the energy density of the lithium-ion battery. However, when the number of particles with a maximum length of 10 μm to 30 μm is too large (for example, more than 30), the relative content of the first polymer is high, the content of the auxiliary binder is low, the cohesive force of the first coating layer is low, and the large-particle-shaped first polymer cannot exhibit the binding performance due to the low cohesive force of the first coating layer, causing a decrease in the binding force between the first coating layer and the positive electrode. By controlling the number of particles of the first polymer with a maximum length of 10 μm to 30 μm within the above range, the first polymer particles can exhibit a dot-like discrete distribution in the first coating layer, providing a passage for electrolyte transport and further improving the performance of the lithium-ion battery, especially the low-temperature characteristics.

[0013] In one embodiment of the present invention, the ion impedance Z of the separator is 0.5 Ω to 1.2 Ω. Without being limited by any theory, when the ion impedance of the separator is too low (for example, lower than 0.5 Ω), the K value of the lithium-ion battery is too large, and there is a risk of self-discharge occurring easily. When the ion impedance of the separator is too high (for example, higher than 1.2 Ω), it affects the ion transport ability of the separator and deteriorates the kinetic characteristics of the lithium-ion battery. By controlling the ion impedance of the separator of the present invention within the above range, the ion transport ability of the separator can be improved, and at the same time, the kinetic characteristics of the lithium-ion battery can be improved, and the self-discharge phenomenon of the lithium-ion battery can be reduced. The term "K value" refers to the voltage drop of the battery per unit time.

[0014] In one embodiment of the present invention, the mass fraction x of the first polymer and the ionic impedance Z of the separator satisfy Z = x·b. Here, b represents the first ionic impedance coefficient and satisfies 1 ≤ b ≤ 1.2. When the content of the first polymer in the first coating layer and the ionic impedance of the separator satisfy the above relationship, the ionic transport ability of the separator can be further improved, the kinetic characteristics of the lithium-ion battery can be improved, and the self-discharge phenomenon of the lithium-ion battery can be reduced.

[0015] In one embodiment of the present invention, the adhesion force F between the first coating layer and the positive electrode sheet is 3 N / m to 35 N / m, preferably 15 N / m to 30 N / m. Without being limited to any theory, when the adhesion force between the first coating layer and the positive electrode sheet is too low (for example, lower than 3 N / m), it will affect the interfacial adhesion strength and the structural stability of the lithium-ion battery, and the lithium-ion battery is likely to expand after charge-discharge cycles. When the adhesion force between the first coating layer and the positive electrode sheet is too high (for example, higher than 35 N / m), more binder needs to be used, which is disadvantageous for improving the energy density of the lithium-ion battery. By controlling the adhesion force between the first coating layer and the positive electrode sheet of the present invention within the above range, the structural stability and energy density of the lithium-ion battery can be further improved.

[0016] In one embodiment of the present invention, the mass fraction x of the first polymer and the adhesion force F between the first coating layer and the positive electrode sheet satisfy F = x·a. Here, a represents the adhesion force coefficient and satisfies 5.0 ≤ a ≤ 30. When the content x of the first polymer in the first coating layer and the adhesion force F between the first coating layer and the positive electrode sheet satisfy the above relational expression, a first coating layer with excellent adhesion performance can be obtained, thereby further improving the interfacial adhesion performance of the separator.

[0017] In one embodiment of the present invention, the first coating layer further includes an auxiliary binder, and based on the total mass of the first coating layer, the mass fraction of the auxiliary binder is 10% to 40%. Without being limited by any theory, if the content of the auxiliary binder in the first coating layer is too high (for example, higher than 40%), the content of the first polymer decreases, the gaps formed by the particulate first polymer decrease, which affects the transport of the electrolyte at the interface of the first coating layer, and at the same time affects the adhesion between the first coating layer and the electrode sheet. If the content of the auxiliary binder in the first coating layer is too low (for example, lower than 10%), the cohesion of the first coating layer is low, and the binding performance of the first polymer decreases with the decrease in the cohesion of the first coating layer. By controlling the content of the auxiliary binder of the present invention within the above range, the adhesion between the first coating layer and the electrode sheet can be further improved.

[0018] The auxiliary binder is not particularly limited as long as it can meet the requirements of the present invention. For example, the auxiliary binder may be at least one of ethyl acrylate, butyl acrylate, ethyl methacrylate, styrene, chlorostyrene, fluorostyrene, methylstyrene, acrylic acid, methacrylic acid, maleic acid, acrylonitrile, and butadiene. Of homopolymers and copolymers In one embodiment of the present invention, one surface of the substrate includes a first coating layer, and the other surface of the substrate includes a second coating layer.

[0019] The thickness of the second coating layer of the present invention is 0.2 μm to 4 μm. The thickness of the second coating layer should neither be too low nor too high. If the thickness of the second coating layer is too low (for example, lower than 0.2 μm), the adhesion between interfaces is insufficient and the binding performance of the coating layer decreases. If the thickness of the second coating layer is too high (for example, higher than 4 μm), the transport distance of lithium ions in the separator becomes longer, which affects the rate performance of the lithium ion battery.

[0020] In one embodiment of the present invention, the second coating layer contains a second polymer, and the second polymer includes a polymer having a core-shell structure or a polymer without a core-shell structure. Based on the total mass of the second coating layer, the mass fraction of the second polymer is 78% to 87.5%. By controlling the content of the second polymer within the above range, a second coating layer with excellent interfacial binding performance can be obtained, thereby improving the kinetic characteristics of the entire lithium-ion battery, such as low-temperature cycle characteristics.

[0021] The present invention has no particular restrictions on polymers having a core-shell structure and polymers without a core-shell structure. For example, the main component of the core of a polymer having a core-shell structure may be a polymer, and the polymer may be a homopolymer formed by polymerizing one kind of polymerizable monomer, or a copolymer formed by polymerizing two or more kinds of polymerizable monomers. Specifically, the monomers used for the core of the polymer having a core-shell structure include at least one of ethyl acrylate, butyl acrylate, ethyl methacrylate, styrene, chlorostyrene, fluorostyrene, methylstyrene, acrylic acid, methacrylic acid, and maleic acid. The shell of the polymer having a core-shell structure may be a homopolymer of one kind of polymerizable monomer, or a copolymer of two or more kinds of polymerizable monomers, and the polymerizable monomers may include acrylates, aromatic monovinyl compounds, and vinyl cyanide compounds. Specifically, the monomers used for the shell of the polymer having a core-shell structure include at least one of methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylene, chlorostyrene, fluorostyrene, methylstyrene, acrylonitrile, and methacrylonitrile.

[0022] The monomers used for the polymer without a core-shell structure include at least one of acrylic acid, acrylate, butadiene, styrene, acrylonitrile, ethylene, chlorostyrene, fluorostyrene, and propylene.

[0023] In one embodiment of the present invention, the second coating layer may further contain a thickener, an auxiliary binder, and a wetting agent. The role of the thickener is to increase the slurry stability and prevent the sedimentation of the slurry. The thickener is not particularly limited as long as the object of the present invention can be achieved. For example, it may be sodium carboxymethyl cellulose. The auxiliary binder plays an auxiliary binding role and further improves the binding performance of the second coating layer. The present invention has no particular limitation on the auxiliary binder, and it is sufficient as long as the object of the present invention can be achieved. For example, the auxiliary binder may include at least one of homopolymers and copolymers of ethyl acrylate, butyl acrylate, ethyl methacrylate, styrene, chlorostyrene, fluorostyrene, methylstyrene, acrylic acid, methacrylic acid, maleic acid, acrylonitrile, and butadiene. The role of the wetting agent is to reduce the surface energy of the slurry and prevent coating leakage. The wetting agent is not particularly limited as long as the object of the present invention can be achieved. For example, the wetting agent may include at least one of dimethyl siloxane, polyethylene oxide, oxyethylene alkylphenol ether, polyoxyethylene aliphatic alcohol ether, polyoxyethylene polyoxypropylene block copolymer, and sodium dioctyl sulfosuccinate.

[0024] In one embodiment of the present invention, based on the total mass of the second coating layer, the mass fraction of the second polymer is 78% - 87.5%, the mass fraction of the auxiliary binder is 5% - 10%, the mass fraction of the thickener is 0.5% - 2%, and the mass fraction of the wetting agent is 7% - 10%, and a second coating layer with excellent binding performance can be obtained.

[0025] In one embodiment of the present invention, an inorganic coating layer is further provided between the first coating layer and the substrate and / or between the second coating layer and the substrate, and the thickness of the inorganic coating layer is 0.5 μm - 6 μm.

[0026] Exemplarily, an inorganic coating layer is provided between the first coating layer and the substrate, or an inorganic coating layer is provided between the second coating layer and the substrate, or inorganic coating layers are provided between the first coating layer and the substrate and between the second coating layer and the substrate. All of the above installation methods can further improve the strength of the separator.

[0027] Without being limited to any theory, if the thickness of the inorganic coating layer is too low (e.g., lower than 0.5 μm), the strength of the separator will decrease, which is disadvantageous for improving the cycle characteristics of the lithium-ion battery. If the thickness of the inorganic coating layer is too high (e.g., higher than 6 μm), the overall thickness of the separator will increase, which is disadvantageous for improving the energy density of the lithium-ion battery. By controlling the thickness of the inorganic coating layer within the above range, both the cycle characteristics and the energy density of the lithium-ion battery can be improved.

[0028] In one embodiment of the present invention, the inorganic coating layer contains inorganic particles, and the Dv50 of the inorganic particles is 0.1 μm to 3 μm. The particle size of the inorganic particles should neither be too low nor too high. If the particle size of the inorganic particles is too large (e.g., larger than 3 μm), it is difficult to achieve a thin coating design for the inorganic coating layer, which will affect the energy density of the lithium-ion battery. If the particle size of the inorganic particles is too small (e.g., smaller than 0.1 μm), the deposition pores of the inorganic particles will become smaller, which will affect ion transport and thus affect the kinetic characteristics of the lithium-ion battery.

[0029] The inorganic particles are not particularly limited as long as the object of the present invention can be achieved. However, the inorganic particles may contain at least one of boehmite, magnesium hydroxide, alumina, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, aluminum nitride, and silicon nitride.

[0030] In one embodiment of the present invention, the inorganic particles may be boehmite, the Dv50 of the boehmite is 0.1 μm to 3 μm, and the aspect ratio is 1 to 3. By controlling the particle size of the boehmite within the above range, the strength of the inorganic coating layer can be further improved, thereby improving the performance of the separator.

[0031] In the present invention, the surface of the separator having the first coating layer can be brought into contact with the positive electrode sheet, and the surface of the separator having the second coating layer can be brought into contact with the negative electrode sheet. A good binding effect can be obtained between the separator and the positive and negative electrode sheets, and better wetting characteristics of the electrolyte can be obtained between the separator and the positive electrode sheet, thereby improving the low-temperature characteristics and rapid charge cycle characteristics of the lithium-ion battery. The separator of the present invention has lithium-ion permeability and electron blocking properties.

[0032] In one embodiment of the present invention, the first polymer is secondary particles, and the average particle size D50 of the secondary particles is 10 μm to 30 μm. The secondary particles may be formed by aggregation of the primary particles of the first polymer. For example, the secondary particles are formed by aggregation of the primary particles of PVDF. As a result, the interior of the secondary particles has more voids, and the electrolyte can easily penetrate through these voids, which is advantageous for improving the wetting characteristics of the electrolyte of the separator.

[0033] The method for preparing the first polymer of the present invention is not particularly limited, and a preparation method known to those skilled in the art can be used. For example, it can be produced by the following preparation method.

[0034] The reaction kettle is evacuated and the oxygen gas is replaced with nitrogen gas. Then, deionized water, vinylidene fluoride (VDF), perfluoroalkyl carboxylate as an emulsifier, and isopropanol as a chain transfer agent are added into the reaction kettle with a stirrer until the pressure of the reaction kettle reaches about 3.5 MPa. Then, the temperature is raised to 50 °C to 70 °C, the rotation speed of the stirrer is 70 r / min to 100 r / min, the polymerization reaction is started, and at the same time, the vinylidene fluoride monomer is continuously replenished to maintain the pressure of the reaction kettle at 3.5 MPa. The reaction is stopped until the solid content of the emulsion in the reactor reaches 25% to 30%. The unreacted monomer is recovered, the polymer emulsion is discharged, and after centrifugation, washing, and drying, the first polymer particles are obtained.

[0035] The present invention does not particularly limit the initiator, as long as it can initiate the polymerization of the monomer. For example, diisopropylbenzene hydroperoxide may be used. The present invention does not particularly limit the addition amounts of the monomer, deionized water, initiator, and chain transfer agent, as long as it can ensure that the polymerization reaction of the added monomer occurs. For example, the deionized water is 5 to 10 times the mass of the monomer, the initiator accounts for 0.05% to 0.5% of the mass of the monomer, the emulsifier accounts for 0.1% to 1% of the mass of the monomer, and the chain transfer agent accounts for 3% to 7% of the mass of the monomer.

[0036] The positive electrode sheet of the present invention is not particularly limited as long as the object of the present invention can be achieved. For example, the positive electrode sheet usually includes a positive electrode current collector and a positive electrode active material layer. Here, the positive electrode current collector may be an aluminum foil, an aluminum alloy foil, a composite current collector, or the like. The positive electrode active material layer contains a positive electrode active material and a conductive agent, and the positive electrode active material may contain at least one of lithium nickel cobalt manganate (811, 622, 523, 111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganate, lithium iron manganese phosphate, and lithium titanate. The conductive agent may contain at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon nanofibers, flake graphite, acetylene black, carbon black, ketjen black, carbon dots, and graphene.

[0037] The negative electrode sheet of the present invention is not particularly limited as long as the object of the present invention can be achieved. For example, the negative electrode sheet usually includes a negative electrode current collector and a negative electrode active material layer. Here, the negative electrode current collector may be a copper foil, an aluminum foil, an aluminum alloy foil, a composite current collector, or the like. The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material may contain at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, hard carbon, silicon, silicon carbon, and lithium titanate.

[0038] The base material of the present invention includes, but is not limited to, at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, and aramid. For example, polyethylene contains at least one component selected from high-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene have excellent short-circuit prevention effects and can improve the stability of lithium-ion batteries due to the shut-down effect.

[0039] The lithium-ion battery of the present invention further includes an electrolyte, which may be one or more of a gel electrolyte, a solid electrolyte, and an electrolytic solution. The electrolytic solution contains a lithium salt and a non-aqueous solvent. In some embodiments of the present invention, the lithium salt is one or more selected from the group consisting of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, and lithium difluoroborate. For example, LiPF6 is used as the lithium salt because it can provide high ionic conductivity and improve cycle characteristics. The non-aqueous solvent may be a carbonic ester, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof. The above carbonic ester compound may be a chain carbonic ester compound, a cyclic carbonic ester compound, a fluorocarbonic ester compound, or a combination thereof.

[0040] Examples of the above-mentioned chain carbonate compounds are dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof. Examples of the cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), and combinations thereof. Examples of the fluorocarbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethyl ethylene carbonate, and combinations thereof. Examples of the above-mentioned carboxylic acid ester compounds are methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone, mevalonolactone, caprolactone, and combinations thereof. Examples of the above-mentioned ether compounds are dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof. Examples of the above-mentioned other organic solvents are dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, phosphate esters, and combinations thereof.

[0041] The second aspect of the present invention provides an electrochemical device including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolytic solution, where the separator is positioned between the positive electrode sheet and the negative electrode sheet, and the separator is the separator described in any one of the above embodiments. The third aspect of the present invention provides an electronic device including the electrochemical device described in the embodiments of the present invention and having good low-temperature cycle characteristics.

[0042] The electronic device of the present invention is not particularly limited and may be any known electronic device used in the prior art. In some embodiments, the electronic device is a notebook computer, a pen-input computer, a mobile computer, an e-book player, a mobile phone, a portable facsimile, a portable copier, a portable printer, a stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disk, a transceiver, an electronic notebook, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, an assist bicycle, a bicycle, a lighting fixture, a toy, a game machine, a clock, a power tool, a flash, a camera, a large household storage battery, and a lithium-ion capacitor, etc., but is not limited thereto.

[0043] The preparation process of the electrochemical device is well known to those skilled in the art and is not particularly limited to the present invention. For example, a lithium-ion battery can be manufactured by superposing a positive electrode and a negative electrode with a separator in between, and then winding, folding, etc. this as necessary and putting it into a case, and injecting an electrolytic solution into the case and sealing it. Among them, the separator used is the separator provided by the present invention. Also, as necessary, an overcurrent prevention element, a lead plate, etc. may be provided in the case to prevent an increase in the internal pressure and overcharge / discharge of the lithium-ion battery.

[0044] In the present invention, the term "Dv50" indicates the particle diameter at which the cumulative distribution of particles is 50%, that is, the volume content of particles smaller than this particle diameter accounts for 50% of the total particles.

[0045] The present invention provides a separator, an electrochemical device including the separator, and an electronic device. Here, the separator includes a base material and a first coating layer disposed on at least one surface of the base material. The first coating layer includes a first polymer. By simultaneously controlling the content and softening point of the first polymer in the first coating layer within the above range, a first coating layer with excellent interfacial binding performance and appropriate gaps between polymer particles can be obtained, effectively improving the binding performance of the separator and the wetting characteristics of the electrolyte, and improving the kinetic characteristics of the lithium-ion battery, such as low-temperature cycle characteristics.

Brief Description of the Drawings

[0046] To more clearly illustrate the technical solutions of the present invention and the prior art, the drawings used in the examples and the prior art will be briefly described below. It is obvious that the following drawings are only some examples of the present invention.

[0047]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0048] To better clarify the objectives, technical solutions, and advantages of the present invention, the present invention will be described in more detail below with reference to the drawings and examples. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other technical solutions obtained by those skilled in the art are within the protection scope of the present invention.

[0049] As shown in FIG. 1, the separator of the present invention includes a base material 1 and a first coating layer 2 provided on one surface of the base material 1.

[0050] As shown in FIG. 2, the separator of the present invention includes a base material 1 and a first coating layer 2 and a second coating layer 3 respectively provided on two surfaces of the base material 1.

[0051] In one embodiment of the present invention, as shown in FIG. 3, an inorganic coating layer 4 is provided between the first coating layer 2 and the base material 1.

[0052] In one embodiment of the present invention, as shown in FIG. 4, an inorganic coating layer 4 is provided between the base material 1 and the second coating layer 3.

[0053] In one embodiment of the present invention, as shown in FIG. 5, an inorganic coating layer 4 is provided between the base material 1 and the first coating layer 2, and an inorganic coating layer 4 is also provided between the base material 1 and the second coating layer 3.

[0054] Examples Hereinafter, the embodiments of the present invention will be described in more detail by giving examples and comparative examples. Various tests and evaluations are carried out as follows. Unless otherwise specified, "parts" and "%" are based on mass.

[0055] Measurement method and apparatus: Measurement of the adhesion between the first coating layer of the separator and the positive electrode sheet: Using Chinese national standard GB / T 2790-1995, that is, using the 180° peel measurement standard to measure the adhesion between the first coating layer of the separator and the positive electrode sheet. Cut the separator with the first coating layer and the positive electrode sheet into samples of 54.2 mm × 72.5 mm. Combine the surface of the separator with the first coating layer with the positive electrode sheet, and perform hot pressing using a hot press machine. The hot pressing conditions are: temperature is 85 °C, pressure is 1 Mpa, and hot pressing time is 85 s (seconds). Cut the combined sample into strips of 15 mm × 54.2 mm, and measure the adhesion between the first coating layer of the separator and the positive electrode sheet according to the 180° peel measurement standard.

[0056] Measurement of the thickness of the first coating layer in the separator: 1) Measurement of the thickness of the base material: Under normal temperature environment, take the base material, cut it into strips with a width of 50 mm along the TD direction, prepare 3 samples for parallel testing, and use a length measuring instrument with an accuracy of 0.0001 mm (Mitutoyo Litematic VL-50B, the diameter of the measuring head is 5 mm, and the measuring pressure is 0.01 N) to uniformly measure 10 data points along the central position in the TD direction. After measuring the 3 parallel samples, take the average value d1 of the 30 measurement data as the thickness of the base material.

[0057] 2) Measurement of the total thickness of the coating film: Apply the first coating layer to the above base material and dry it at 80 °C to obtain a separator with the first coating layer. Measure it according to the measurement method of the thickness of the base material. When measuring, face the surface without the coating layer downward. Obtain the total thickness d2 of the separator.

[0058] 3) Calculation of the thickness of the first coating layer: d = d2 - d1.

[0059] Measurement of the low-temperature characteristics of lithium-ion batteries: 1): In an environment of 25 °C, perform the first charge and discharge on the lithium-ion battery after formation: Perform constant current and constant voltage charging with a charging current of 0.1 C until the upper limit voltage reaches 4.45 V, and let the fully charged lithium-ion battery stand for 5 minutes.

[0060] 2) Discharge to 3 V at a rate of 0.2 C, record the discharge capacity of the first cycle, and let it stand for another 5 minutes.

[0061] 3) First, charge at a constant current to 4.45 V at a charging rate of 1.5 C, then charge at a constant voltage to 0.02 C, and then let it stand for 5 minutes.

[0062] 4) Adjust the furnace temperature to {25, 10, 0, -10, -20, 45, 60} °C, let it stand for another 5 minutes, then discharge to 3 V at a rate of 0.2 C, and let it stand for another 5 minutes.

[0063] 5) Adjust the furnace temperature to 25 °C and let the lithium-ion battery stand for 60 minutes.

[0064] 6) Repeat the above steps 4) to 5). After measuring each temperature condition in order according to the temperature conditions in step 4), record the final discharge capacity of the lithium-ion battery under each temperature condition in order. Then, select the final discharge capacity of the lithium-ion battery recorded under the -20 °C condition, and calculate the low-temperature capacity retention rate of the lithium-ion battery under the -20 °C condition according to the following formula.

[0065] Low-temperature capacity retention rate = (Final discharge capacity of the lithium-ion battery under the -20 °C condition / Discharge capacity of the first cycle of the lithium-ion battery under the 25 °C condition) × 100%.

[0066] Measurement of the softening point of the first polymer: Using a general-purpose differential scanning calorimeter (DSC) method, take 5 mg of the first polymer sample prepared in each example and comparative example, heat it to 150 °C at a heating rate of 5 °C / min, collect the DSC curve, and determine the softening point, i.e., the softening temperature, of the first polymer from the obtained DSC curve.

[0067] Measurement of the number of particles of the first polymer within an area of 250 μm × 200 μm on the surface of the first coating layer: After cutting the separator coated with the first coating layer into 10 mm × 10 mm samples, the samples were placed under SEM and observed at a magnification of 500 times. Further, five arbitrary regions of 250 μm × 200 μm were selected from the field of view, and the number of particles of the first polymer with a maximum length of 10 μm to 30 μm in the selected regions was recorded. Next, the average value was taken, that is, the number of particles of the first polymer with a maximum length of 10 μm to 30 μm per unit area of the first coating layer.

[0068] Measurement of the ionic impedance of the separator: 1. Preparation of symmetric cells: The separator and the electrode sheets were combined into one symmetric cell: The number of separator layers in the symmetric cell was designed by stacking them step by step. The number of layers was 2, 4, 6, 8, 10, and 12 separator layers respectively. Both of the two electrode sheets of the symmetric cell were negative electrode sheets that had not undergone charge-discharge cycles (i.e., new negative electrodes).

[0069] 2. Measurement of the ionic impedance of the separator sample: First, the impedance values of the separators of the symmetric cells with the above different numbers of separator layer designs were measured using a chemical workstation (EIS). Next, fitting of the gradient data values was performed on the measured ionic impedance values of the symmetric cells with different numbers of separator layer designs, and based on the fitting results, the ionic impedance of the single-layer separator was calculated, with the unit being Ω.

[0070] Measurement of the 1.5C discharge rate characteristics: At 25°C, the lithium-ion battery after formation was charged at a constant current to 4.45 V at a rate of 0.2C, and then charged at a constant voltage until the current was below 0.05C. After that, it was left standing for 30 minutes, and then discharged at a constant current to 3.0 V at a rate of 0.2C. The 0.2C rate discharge capacity of the lithium-ion battery was measured.

[0071] At 25°C, the lithium-ion battery was charged at a constant current to 4.45 V at a rate of 0.2C, and then charged at a constant voltage until the current was below 0.05C. After that, it was left standing for 30 minutes, and then discharged at a constant current to 3.0 V at a rate of 1.5C, and measured to obtain the 1.5C rate discharge capacity of the lithium-ion battery.

[0072] The maintenance rate (%) of the 1.5C rate discharge capacity of the lithium-ion secondary battery = 1.5C rate discharge capacity / 0.2C rate discharge capacity × 100%.

[0073] Example 1 <1-1. Preparation of the first polymer particles> The reaction kettle was evacuated and the oxygen gas was replaced with nitrogen gas. Then, deionized water, vinylidene fluoride (VDF), diisopropylbenzene hydroperoxide as the initiator, perfluoroalkyl carboxylate as the emulsifier, and isopropanol as the chain transfer agent were added into the reaction kettle with a stirrer until the pressure of the reaction kettle reached 3.5 MPa. The deionized water was 7 times the mass of the vinylidene fluoride monomer, the initiator accounted for 0.2% of the mass of the vinylidene fluoride monomer, the emulsifier accounted for 0.5% of the mass of the vinylidene fluoride monomer, and the chain transfer agent accounted for 5% of the mass of the vinylidene fluoride monomer. Then, the temperature was raised to 60°C, the rotation speed of the stirrer was 80 r / min, the polymerization reaction was started, and at the same time, the vinylidene fluoride monomer was continuously replenished to maintain the pressure of the reaction kettle at 3.5 MPa. The reaction was stopped until the solid content of the emulsion in the reactor reached 25%. The unreacted monomer was recovered, the polymer emulsion was discharged, and after centrifugation, washing, and drying, the first polymer particles were obtained. The softening point of the first polymer particles was 120°C, and the Dv50 was 20 μm.

[0074] <1-2. Preparation of the positive electrode sheet> Lithium cobaltate as the positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 94:3:3. Then, N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75% and stirred uniformly. The slurry was uniformly coated on one surface of an aluminum foil with a thickness of 12 μm, dried under the condition of 90 °C, and cold-rolled to obtain a positive electrode sheet with a thickness of 100 μm for the positive electrode active material layer. Then, the above steps were repeated on the other surface of the positive electrode sheet to obtain a positive electrode sheet with positive electrode active material layers coated on both sides. The positive electrode sheet was cut into a size of 74 mm × 867 mm, tabs were welded, and then it was prepared for the next step.

[0075] <1-3. Preparation of negative electrode sheet> Artificial graphite as the negative electrode active material, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed at a mass ratio of 96:1:1.5:1.5. Then, deionized water was added as a solvent to prepare a slurry with a solid content of 70% and stirred uniformly. The slurry was uniformly coated on one surface of a copper foil with a thickness of 8 μm, dried under the condition of 110 °C, and cold-rolled to obtain a negative electrode sheet with a negative electrode active material layer coated on one side with a thickness of 150 μm. Then, the above coating steps were repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet with negative electrode active material layers coated on both sides. The negative electrode sheet was cut into a size of 74 mm × 867 mm, tabs were welded, and then it was prepared for the next step.

[0076] <1-4. Preparation of separator> The prepared first polymer particles and acrylonitrile as the auxiliary binder were added to a stirrer at a mass ratio of 90:10, stirred and mixed uniformly. Then, deionized water was added and stirred to adjust the slurry viscosity to 100 mPa·s and the solid content to 12% to obtain Slurry A. Slurry A was uniformly coated on one surface of a PE substrate with a thickness of 5 μm to obtain a first coating layer with a thickness of 20 μm, and drying was completed in an oven to obtain a separator.

[0077] When the obtained separator was observed by SEM at a magnification of 500 times, the number of particles of the first polymer with a maximum length of 10 μm to 30 μm was 30 in any 250 μm × 200 μm region (per unit area) on the surface of the first coating layer.

[0078] <1-5. Preparation of Electrolyte> In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC), which are non-aqueous organic solvents, were mixed at a mass ratio of 20:30:20:28:2. Then, lithium hexafluorophosphate (LiPF6) was dissolved in the non-aqueous organic solvent and uniformly mixed to obtain an electrolyte. Here, the mass ratio of LiPF6 to the non-aqueous organic solvent was 8:92.

[0079] <1-6. Preparation of Lithium-Ion Battery> The prepared positive electrode sheet, separator, and negative electrode sheet were laminated in order, with the surface of the separator having the first coating layer in contact with the positive electrode sheet, and then wound to obtain an electrode assembly. The electrode assembly was placed in an aluminum plastic film outer packaging bag, the moisture was removed at 80 °C, the prepared electrolyte was poured in, and a lithium-ion battery was obtained through processes such as vacuum packaging, standing, formation, and shaping.

[0080] Example 2 In <Preparation of Separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 85:15 (i.e., the content of the first polymer was 85%).

[0081] Example 3 In <Preparation of Separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 80:20 (i.e., the content of the first polymer was 80%).

[0082] Example 4 In <Preparation of separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 75:25 (i.e., the content of the first polymer was 75%).

[0083] Example 5 In <Preparation of separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 70:30 (i.e., the content of the first polymer was 70%).

[0084] Example 6 In <Preparation of separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 65:35 (i.e., the content of the first polymer was 65%).

[0085] Example 7 In <Preparation of separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 60:40 (i.e., the content of the first polymer was 60%).

[0086] Example 8 In <Preparation of the first polymer particles>, it was carried out in the same manner as in Example 4, except that the softening point of the first polymer was adjusted to 90 °C.

[0087] Example 9 In <Preparation of the first polymer particles>, it was carried out in the same manner as in Example 4, except that the softening point of the first polymer was adjusted to 100 °C.

[0088] Example 10 In <Preparation of the first polymer particles>, it was carried out in the same manner as in Example 4, except that the softening point of the first polymer was adjusted to 110 °C.

[0089] Example 11 In <Preparation of the first polymer particles>, it was carried out in the same manner as in Example 4, except that the softening point of the first polymer was adjusted to 130 °C.

[0090] Example 12 In <Preparation of the First Polymer Particles>, the procedure was the same as in Example 4, except that the softening point of the first polymer was adjusted to 140 °C.

[0091] Example 13 In <Preparation of the First Polymer Particles>, the procedure was the same as in Example 4, except that the softening point of the first polymer was adjusted to 150 °C.

[0092] Example 14 In Example 4, the procedure was the same as in Example 4, except that the polyvinylidene fluoride particles in the first coating layer were changed to a copolymer formed of 75% vinylidene fluoride and 25% hexafluoropropylene on a mass fraction basis.

[0093] Example 15 In Example 4, the procedure was the same as in Example 4, except that the polyvinylidene fluoride particles in the first coating layer were changed to a copolymer formed of 60% styrene, 25% butadiene, and 15% acrylic acid on a mass fraction basis.

[0094] Example 16 In Example 4, the procedure was the same as in Example 4, except that the polyvinylidene fluoride particles in the first coating layer were changed to a copolymer formed of 70% styrene and 30% acrylate on a mass fraction basis.

[0095] Example 17 In Example 4, the procedure was the same as in Example 4, except that the polyvinylidene fluoride particles in the first coating layer were changed to a copolymer formed of 30% acrylic acid, 35% acrylonitrile, and 35% styrene on a mass fraction basis.

[0096] Example 18 In <Preparation of the Separator>, the procedure was the same as in Example 4, except that the thickness of the first coating layer was 3 μm.

[0097] Example 19 In <Preparation of the Separator>, the procedure was the same as in Example 4, except that the thickness of the first coating layer was 5 μm.

[0098] Example 20 In <Preparation of Separator>, it was carried out in the same manner as in Example 4, except that the thickness of the first coating layer was 8 μm.

[0099] Example 21 In <Preparation of Separator>, it was carried out in the same manner as in Example 4, except that the thickness of the first coating layer was 50 μm.

[0100] Example 22 In <Preparation of Separator>, it was carried out in the same manner as in Example 4, except that the thickness of the first coating layer was 10 μm.

[0101] Example 23 In <Preparation of Separator>, it was carried out in the same manner as in Example 4, except that the thickness of the first coating layer was 40 μm.

[0102] Example 24 In <Preparation of Separator>, a second coating layer was added, and it was carried out in the same manner as in Example 4, except that <Preparation of Lithium-Ion Battery> was different from Example 4.

[0103] <Preparation of the Second Coating Layer> A polymer binder having no core-shell structure (a copolymer obtained by polymerizing 80% styrene, 10% isobutyl acrylate, and 10% acrylonitrile based on mass fraction, and Dv50 is 0.3 μm), carboxymethyl cellulose sodium as a thickener, and dimethyl siloxane as a wetting agent were added to a stirrer in a mass ratio of 85:14:1 and stirred uniformly, and then deionized water was added and stirred to adjust the viscosity of the slurry to 40 mPa·s, and the solid content was 5% to obtain Slurry B. The above Slurry B was uniformly coated on the other surface of the PE substrate to obtain a second coating layer with a thickness of 2 μm and dried in an oven.

[0104] <Preparation of Lithium-Ion Battery> Next, the above-prepared positive electrode sheet, separator, and negative electrode sheet were laminated. The surface of the separator having the first coating layer was brought into contact with the positive electrode sheet, and the surface of the separator having the second coating layer was brought into contact with the negative electrode sheet, followed by winding to obtain an electrode assembly. The electrode assembly was placed in an aluminum-plastic film outer packaging bag, and moisture was removed at 80°C. Then, the prepared electrolytic solution was poured in, and through processes such as vacuum packaging, standing, formation, and shaping, a lithium-ion battery was obtained.

[0105] Example 25 In <Preparation of Separator>, it was carried out in the same manner as in Example 24, except that the thickness of the second coating layer was set to 0.2 μm.

[0106] Example 26 In <Preparation of Separator>, it was carried out in the same manner as in Example 24, except that the thickness of the second coating layer was set to 4 μm.

[0107] Example 27 In <Preparation of Separator>, as shown in Figure 3, it was carried out in the same manner as in Example 24, except that an inorganic coating layer was provided between the first coating layer and the base material. The thickness of the inorganic coating layer was 3 μm, the inorganic particles in the inorganic coating layer were boehmite, Dv50 was 0.75 μm, and the aspect ratio was 1.

[0108] Example 28 In <Preparation of Separator>, it was carried out in the same manner as in Example 27, except that the thickness of the inorganic coating layer was set to 0.5 μm and the Dv50 of boehmite was set to 0.85 μm.

[0109] Example 29 In <Preparation of Separator>, it was carried out in the same manner as in Example 27, except that the thickness of the inorganic coating layer was set to 6 μm and Dv50 was set to 0.99 μm.

[0110] Example 30 In <Preparation of Separator>, as shown in FIG. 4, an inorganic coating layer was provided between the second coating layer and the base material, and alumina with a Dv50 of 0.99 μm was used as the inorganic particles in the inorganic coating layer. Otherwise, it was carried out in the same manner as in Example 24. The thickness of the inorganic coating layer was 3 μm.

[0111] Example 31 In <Preparation of Separator>, as shown in FIG. 5, inorganic coating layers were provided both between the first coating layer and the base material and between the second coating layer and the base material. Alumina with a Dv50 of 0.99 μm was used as the inorganic particles in the inorganic coating layer. Otherwise, it was carried out in the same manner as in Example 24. The thickness of a single layer of the inorganic coating layer was 2 μm.

[0112] Example 32 In <Preparation of the First Polymer Particles>, it was carried out in the same manner as in Example 4, except that the primary particles of the obtained first polymer were aggregated to form secondary particles.

[0113] <Preparation of Secondary Particles> The primary particles of the first polymer were dispersed in deionized water, and using an MSK-SFM-10 vacuum stirrer, stirring was carried out for 120 minutes under the conditions that the revolution rotation speed was 40 rpm and the rotation rotation speed was 1500 rpm to obtain a primary particle slurry with a solid content of 10%.

[0114] The primary particle slurry was transferred to the nozzle of the centrifugal rotating disk of a spray drying granulator. The centrifugal rotation speed was 2000 rpm to form minute droplets. The inlet temperature of the spray drying granulator was 110 °C, the outlet temperature was 100 °C, and after cooling, the powder was collected to obtain secondary particles of PVDF.

[0115] Comparative Example 1 In <Preparation of Separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 95:5 (i.e., the content of the first polymer was 95%).

[0116] Comparative Example 2 In <Preparation of Separator>, it was carried out in the same manner as in Example 1, except that the mass ratio of the first polymer particles to the auxiliary binder was 55:45 (i.e., the content of the first polymer was 55%).

[0117] Comparative Example 3 In <Preparation of the First Polymer Particles>, it was carried out in the same manner as in Example 1, except that the softening point of the first polymer was adjusted to 165 °C.

[0118] Comparative Example 4 In <Preparation of the First Polymer Particles>, it was carried out in the same manner as in Example 1, except that the softening point of the first polymer was adjusted to 70 °C.

[0119] Comparative Example 5 In <Preparation of the First Polymer Particles>, it was carried out in the same manner as in Example 27, except that the mass ratio of the first polymer particles to the auxiliary binder was 55:45 (i.e., the content of the first polymer was 55%) and the softening point of the first polymer was 70 °C.

[0120] The preparation parameters and measurement results of each example and comparative example are shown in Tables 1 to 5.

[0121]

Table 1

[0122]

Table 2

[0123]

Table 3

[0124]

Table 4

[0125]

Table 5

[0126] As can be seen from Examples 1 to 7 and Comparative Examples 1 to 2, as the content of the first polymer in the first coating layer increases, the number of particles of the first polymer per unit area of the surface of the separator under SEM increases, and the adhesion force F between the first coating layer of the separator and the positive electrode plate gradually decreases. However, when the content of the first polymer is too high (for example, Comparative Example 1), it affects the adhesion force between the first coating layer and the positive electrode. When the content of the first polymer is too low (for example, Comparative Example 2), it affects the low-temperature capacity retention rate of the lithium-ion battery. As can be seen from Examples 4, 8 to 13 and Comparative Examples 3 to 4, as the softening point of the first polymer increases, the adhesion force F between the first coating layer and the positive electrode plate tends to decrease, and the low-temperature capacity retention rate of the lithium-ion battery generally shows an upward trend. However, when the softening point of the first polymer is too high (for example, Comparative Example 3), it affects the adhesion force between the first coating layer and the electrode plate, and when the softening point of the first polymer is too low (for example, Comparative Example 4), it affects the low-temperature capacity retention rate of the lithium-ion battery. As can be seen from the above, by controlling the content and softening point of the first polymer within the scope of the present invention at the same time, a lithium-ion battery with excellent comprehensive performance such as interfacial adhesion performance, low-temperature cycle characteristics, and rate characteristics can be obtained.

[0127] As can be seen from Example 27 and Comparative Example 5, by controlling the content and softening point of the first polymer within the scope of the present invention at the same time, the interfacial adhesion performance of the first coating layer and the low-temperature cycle characteristics of the lithium-ion battery can be improved.

[0128] The components of the first polymer also generally affect the performance of the first coating layer. As can be seen from Examples 1 to 17, the first coating layer containing the first polymer component of the present invention can endow the lithium-ion battery with excellent low-temperature cycle characteristics and rate characteristics.

[0129] As can be seen from Examples 4 and 18 to 23, as the thickness of the first coating layer increases, the adhesion force F between the first coating layer and the positive electrode sheet increases, and the low-temperature capacity retention rate of the lithium-ion battery generally shows an upward trend. As can be seen from Examples 4, 18 to 20, 22 to 23 and Example 21, by controlling the thickness of the first coating layer within the scope of the present invention, the ion transport ability, the binding performance between interfaces, the low-temperature cycle characteristics and the rate characteristics of the lithium-ion battery can be further improved.

[0130] The thickness of the second coating layer, the thickness of the inorganic coating layer, the composition and particle size of the inorganic particles, and the installation method of the second coating layer and the inorganic coating layer in the separator also generally affect the performance of the separator, thereby affecting the kinetic characteristics of the lithium-ion battery. As can be seen from Examples 24 to 31, if the thickness of the second coating layer and the thickness of the inorganic coating layer are within the scope of the present invention, a lithium-ion battery with excellent low-temperature cycle characteristics can be obtained.

[0131] As can be seen from Example 4 and Example 32, the separator containing the first polymer secondary particles has a lower ion impedance, which is considered to be because the secondary particles have more voids and the electrolyte penetrates more easily through these voids to improve the wetting characteristics of the electrolyte of the separator of the present invention.

[0132] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Modifications, equivalent substitutions, improvements, etc. made within the scope of the gist and principles of the present invention are all included in the protection scope of the present invention.

Claims

1. A separator, comprising: a base material and a first coating layer provided on at least one surface of the base material, wherein the first coating layer contains a first polymer, and based on the total mass of the first coating layer, the mass fraction x of the first polymer is 60% to 90%, and the softening point of the first polymer is 90°C to 150°C, a separator, wherein the first coating layer further contains an auxiliary binder, and based on the total mass of the first coating layer, the mass fraction of the auxiliary binder is 10% to 40%, wherein the first polymer is secondary particles, and the particle size Dv50 of the secondary particles is 10 μm to 30 μm, a separator.

2. The separator according to claim 1, wherein the thickness of the first coating layer is 3 μm to 40 μm.

3. In any 250 μm × 200 μm region on the surface of the first coating layer, the number of particles with the maximum length of the particles of the first polymer being 10 μm to 30 μm is 10 to 30, the separator according to claim 1.

4. The separator according to claim 1, wherein the ionic impedance Z of the separator is 0.5 Ω to 1.2 Ω.

5. The mass fraction x of the first polymer and the ionic impedance Z of the separator satisfy Z = x · b, where b represents the first ionic impedance coefficient and satisfies 1 ≤ b ≤ 1.2, the separator according to claim 1.

6. The separator according to claim 1, wherein the adhesion force F between the first coating layer and the positive electrode sheet is 3 N / m to 35 N / m.

7. The mass fraction x of the first polymer and the adhesion force F between the first coating layer and the positive electrode sheet are: satisfy F = x · a, where a represents the adhesion force coefficient and satisfies 5.0 ≤ a ≤ 30, the separator according to claim 1.

8. The auxiliary binder contains at least one of homopolymers and copolymers of ethyl acrylate, butyl acrylate, ethyl methacrylate, styrene, chlorostyrene, fluorostyrene, methylstyrene, acrylic acid, methacrylic acid, maleic acid, acrylonitrile, and butadiene, the separator according to claim 1.

9. The monomer used in the first polymer includes at least one of vinylidene fluoride, hexafluoropropylene, ethylene, propylene, chloroethylene, chloropropene, acrylic acid, acrylate, styrene, butadiene, and acrylonitrile. The separator according to claim 1.

10. A first coating layer is provided on one surface of the base material, a second coating layer is provided on the other surface of the base material, and the thickness of the second coating layer is 0.2 μm to 4 μm. The separator according to claim 1.

11. The second coating layer contains a second polymer, and based on the total mass of the second coating layer, the mass fraction of the second polymer is 78% to 87.5%. The separator according to claim 10.

12. The second coating layer contains a second polymer. The second polymer includes a polymer having a core-shell structure or a polymer having no core-shell structure. The monomers used in the core of the polymer having a core-shell structure include at least one of ethyl acrylate, butyl acrylate, ethyl methacrylate, styrene, chlorostyrene, fluorostyrene, methylstyrene, acrylic acid, methacrylic acid, and maleic acid. The monomers used in the shell of the polymer having a core-shell structure include at least one of methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylene, chlorostyrene, fluorostyrene, methylstyrene, acrylonitrile, and methacrylonitrile. The monomers used in the polymer having no core-shell structure are selected from at least one of acrylic acid, acrylate, butadiene, styrene, acrylonitrile, ethylene, chlorostyrene, fluorostyrene, and propylene. The separator according to claim 10.

13. An inorganic coating layer is further provided between the first coating layer and the base material and / or between the second coating layer and the base material, and the thickness of the inorganic coating layer is 0.5 μm to 6 μm. The separator according to claim 10.

14. The inorganic coating layer contains inorganic particles, the inorganic particles contain at least one of boehmite, magnesium hydroxide, alumina, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, aluminum nitride, and silicon nitride, and the Dv50 of the inorganic particles is 0.1 μm to 3 μm. The separator according to claim 13.

15. The inorganic particles are boehmite, the Dv50 of the boehmite is 0.1 μm to 3 μm, and the aspect ratio is 1 to 3. The separator according to claim 14.

16. An electrochemical device including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolytic solution, the separator being located between the positive electrode sheet and the negative electrode sheet, and the separator being the separator according to any one of claims 1 to 15. An electrochemical device.

17. An electronic device including the electrochemical device according to claim 16.

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