Secondary battery and electric device

Abstract

The invention relates to a secondary battery and an electric device, and belongs to the technical field of batteries. The mass ratio of vinyl acetate in the ethylene-vinyl acetate copolymer in the first coating of the base membrane and the mass ratio of silicon element in the negative electrode material are controlled to meet a specific relationship, so that when the battery is in a thermal safety test high-temperature interval (110-130 DEG C), the battery can be molten to block holes of the base membrane, short-circuit current between the positive electrode and the negative electrode is blocked, and the service life of the battery is prolonged. And meanwhile, the holes of the diaphragm are ensured not to be blocked easily in the formation process, so that the internal resistance of the battery cell is relatively low, and the cycle performance is relatively good.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to secondary batteries and electrical devices. Background Technology

[0002] Lithium-ion batteries possess advantages such as high energy density, high charging efficiency, excellent cycle performance, and high output power, and are widely used in power, consumer, and energy storage applications. With continuous technological advancements, the application areas of lithium-ion batteries are constantly expanding, placing higher demands on their energy density (ED). However, the development of high-ED graphite anodes in the industry is currently nearing its limit, making it difficult to further improve the energy density of lithium-ion batteries. To address this, the industry has begun to explore the use of composite materials with higher theoretical energy densities, such as silicon, with graphite. However, silicon undergoes significant volume expansion after lithiation, which reduces the effective contact area between the positive and negative electrodes, leading to increased internal resistance and severely impacting safety performance. Especially under abuse scenarios (such as overcharging, over-discharging, short circuits, extrusion, nail penetration, and hot box testing), the battery is prone to fire and explosion risks. This issue has become a key bottleneck restricting the practical application of silicon-based composite materials. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the prior art and provide a secondary battery and power-consuming device.

[0004] To achieve the above objectives, in a first aspect, this application provides a secondary battery, including a separator and a negative electrode; The diaphragm includes a base membrane, the base membrane including a first surface and a second surface, wherein a first coating is provided on the first surface of the base membrane, the first coating comprising a first ethylene-vinyl acetate copolymer; The negative electrode sheet includes a negative electrode material, the negative electrode material includes a negative electrode active material, and the negative electrode active material includes silicon-carbon material; The secondary battery satisfies: 0.2 ≤ A / S ≤ 1. Wherein, S% is the mass percentage of silicon in the negative electrode material. A% represents the mass percentage of vinyl acetate in the first ethylene-vinyl acetate copolymer.

[0005] Secondly, this application provides an electrical device including the aforementioned secondary battery.

[0006] Compared with the prior art, the beneficial effects of this application are as follows: This application controls the mass ratio of vinyl acetate in the ethylene-vinyl acetate copolymer in the first coating of the base film and the mass ratio of silicon in the negative electrode material to meet a specific relationship, so that when the battery is in the high temperature range of thermal safety testing (110℃~130℃), it can melt and block the pores of the base film, hinder the short circuit current between the positive and negative electrodes, improve the safety performance of the battery, and at the same time ensure that the pores of the separator are not easily blocked during the formation process, resulting in lower internal resistance of the cell and better cycle performance. Attached Figure Description

[0007] Figure 1 This is a schematic diagram of the structure of the diaphragm obtained in Example 7, wherein 1-first coating, 2-second coating, 3-base film, 101-inorganic filler and binder in the first coating, 102-first EVA, 201-inorganic filler and binder in the second coating, and 202-second EVA. Detailed Implementation

[0008] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0009] In this application, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions composed of the listed features.

[0010] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0011] In this application, there are no particular restrictions on the specific dispersion and mixing methods.

[0012] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0013] According to a first aspect of this application, a secondary battery is provided, including a separator and a negative electrode. The diaphragm includes a base membrane, the base membrane including a first surface and a second surface, wherein a first coating is provided on the first surface of the base membrane, the first coating comprising a first ethylene-vinyl acetate copolymer (EVA); The negative electrode sheet includes a negative electrode material, the negative electrode material includes a negative electrode active material, and the negative electrode active material includes silicon-carbon material; The secondary battery satisfies: 0.2 ≤ A / S ≤ 1. Wherein, S% is the mass percentage of silicon in the negative electrode material.

[0014] A% represents the mass percentage of vinyl acetate (VA) in the first ethylene-vinyl acetate copolymer.

[0015] The mass percentage of silicon in the negative electrode material can be obtained by ICP-OES (inductively coupled plasma optical emission spectrometry). The mass percentage of vinyl acetate (VA) in the first ethylene-vinyl acetate copolymer can be determined by thermogravimetric analysis (TGA). EVA decomposes and releases acetic acid above 250°C, and its weight loss rate is directly proportional to the VA content, which can be calculated using the formula: VA content in EVA (Wva) = weight loss rate (R) × 1.434; Wva represents the mass percentage of vinyl acetate (VA); R represents the mass percentage lost due to acetic acid decomposition in the temperature range of 250°C to 400°C, i.e., R = Δm / m 0 ×100% (△m is the mass lost, m) 0 The initial mass is the same for both (e.g., mg). 1.434 is a calculation coefficient, which is based on the molecular weight ratio of acetic acid (CH-COOH, molecular weight 60) to vinyl acetate (CHO-, molecular weight 86) (86 / 60≈1.433). This coefficient converts the measured mass loss of acetic acid back to the original mass of VA monomer.

[0016] By controlling A and S to satisfy the above relationship, the silicon content in the negative electrode material is well matched with the melting point of the first ethylene-vinyl acetate copolymer. As the silicon content in the negative electrode material increases, the corresponding separator has a lower thermal closure temperature. When the battery is in the high temperature range of thermal safety testing (110℃~130℃), it can melt and block the pores of the base film, hindering the short-circuit current between the positive and negative electrodes, improving the safety performance of the battery. At the same time, it ensures that the pores of the separator are not easily blocked during the formation process, resulting in lower internal resistance of the cell and better cycle performance.

[0017] For example, A / S is a range formed by any two values ​​of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or above.

[0018] In some implementations, S is between 5 and 50. Controlling the value of S within this range aims to reduce silicon-induced volume expansion while achieving higher energy density, thereby improving cycle performance and safety. For example, S is a range formed by any two of the following values: 5, 8, 10, 12, 15, 18, 20, 22, 25, 28, 30, 32, 35, 37, 40, 42, 45, 48, 50, or more.

[0019] In some embodiments, A is 5 to 20. In some embodiments, A is 5 to 10. Controlling the value of A within this range makes the melting point of the first ethylene-vinyl acetate copolymer more suitable, improving the safety performance of the battery while avoiding excessively low polymer melting points that could cause melting and pore blockage during formation, resulting in increased internal resistance and performance loss in the cell. For example, A is a range formed by any two of the values ​​5, 6, 7, 8, 9, 10, or more.

[0020] In some embodiments, the silicon-carbon material comprises at least one of elemental silicon, silicon-based alloys, and silicon oxides, along with carbon. In one embodiment, the silicon content in the silicon-carbon material is 40% to 60% by mass.

[0021] In some embodiments, the melt index of the first ethylene-vinyl acetate copolymer at a temperature of 190°C and a load of 2.16 kg is 5 to 20 g / 10 min, such as 5 g / 10 min, 8 g / 10 min, 10 g / 10 min, 12 g / 10 min, 15 g / 10 min, 18 g / 10 min, 20 g / 10 min or any two of the above ranges.

[0022] In some embodiments, the melting point of the first ethylene-vinyl acetate copolymer is 110–130°C.

[0023] For example, the melting point of the first ethylene-vinyl acetate copolymer is within the range formed by any two of the following values: 110°C, 115°C, 120°C, 125°C, 130°C, or above.

[0024] In some embodiments, the mass percentage of the first ethylene-vinyl acetate copolymer in the first coating is 5% to 45%. For example, the mass percentage of the first ethylene-vinyl acetate copolymer in the first coating is within the range formed by any two of the following values: 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or more. In a preferred embodiment, the mass percentage of the first ethylene-vinyl acetate copolymer in the first coating is 15% to 25%. Controlling the mass percentage of the first ethylene-vinyl acetate copolymer in the first coating within the range of 5% to 45%, particularly within the range of 15% to 25%, helps to improve battery safety performance at a lower cost, while also reducing internal resistance and improving cycle performance.

[0025] In some embodiments, the first coating further comprises inorganic fillers and binders, wherein the inorganic fillers account for 50% to 85% by mass and the binders account for 5% to 10% by mass.

[0026] For example, in the first coating, the mass percentage of inorganic filler is within the range formed by any two values ​​of 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or more.

[0027] For example, in the first coating, the mass percentage of the adhesive is within the range formed by any two values ​​of 5%, 6%, 7%, 8%, 9%, 10% or more.

[0028] In some embodiments, the inorganic filler in the first coating includes at least one of magnesium hydroxide, boehmite, and alumina.

[0029] In some embodiments, the binder in the first coating includes at least one selected from polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, cellulose, and polyvinylpyrrolidone.

[0030] In some embodiments, the areal density of the first coating is 1–5 g / cm³. 2 , such as 1g / cm 2 2g / cm 2 3g / cm 2 4g / cm 2 5g / cm 2 Or the range formed by any two of the above values.

[0031] In some embodiments, the first surface of the base film faces the positive electrode. When the battery is in the high-temperature range (110°C to 130°C) of thermal safety testing, heat generally accumulates first at the positive electrode. The first coating facing the positive electrode can better play its role in melting and blocking the pores of the base film, hindering the short-circuit current between the positive and negative electrodes, and improving the safety performance of the battery.

[0032] In some embodiments, the negative electrode active material further includes graphite, and the mass percentage of graphite in the negative electrode active material is 14% to 90%, such as a range formed by any two values ​​of 14%, 18%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or more.

[0033] In some embodiments, a second coating is provided on the second surface of the base film, the second coating comprising inorganic fillers and binders.

[0034] In some embodiments, the second coating further comprises a second ethylene-vinyl acetate copolymer, wherein the mass percentage of vinyl acetate in the second ethylene-vinyl acetate copolymer is 5% to 30%. In one embodiment, the mass percentage of vinyl acetate in the second ethylene-vinyl acetate copolymer is 10% to 30%, so that the melting point of the second ethylene-vinyl acetate copolymer is more suitable. This not only avoids melting and clogging of pores during the formation process, which would increase the internal resistance of the cell and cause performance loss, but also better improves the thermal shrinkage effect of the separator and enhances the safety performance of the battery. For example, the mass percentage of VA in the second ethylene-vinyl acetate copolymer is within the range formed by any two of the following values: 5%, 7%, 10%, 12%, 15%, 18%, 20%, 25%, 30%, or more.

[0035] In some embodiments, the melt index of the second ethylene-vinyl acetate copolymer at a temperature of 190°C and a load of 2.16 kg is 5 to 20 g / 10 min, such as 5 g / 10 min, 8 g / 10 min, 10 g / 10 min, 12 g / 10 min, 15 g / 10 min, 18 g / 10 min, 20 g / 10 min or any two of the above ranges.

[0036] In some embodiments, the melting point of the second ethylene-vinyl acetate copolymer is 100–110°C.

[0037] For example, the melting point of the second ethylene-vinyl acetate copolymer is within the range formed by any two of the following values: 100°C, 101°C, 102°C, 103°C, 104°C, 105°C, 106°C, 107°C, 108°C, 109°C, 110°C, or above.

[0038] In some embodiments, the mass percentage of the second ethylene-vinyl acetate copolymer in the second coating is 0% to 45%, such as within the range formed by any two values ​​of 0, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or more. In some embodiments, the mass percentage of the second ethylene-vinyl acetate copolymer in the second coating is 5% to 45% to better improve the thermal shrinkage of the separator and enhance the safety performance of the battery at a lower cost. When the mass percentage of the second ethylene-vinyl acetate copolymer in the second coating is 15% to 25%, it is more conducive to a balance between cost, safety performance, and cycle performance.

[0039] The melt index of both the first ethylene-vinyl acetate copolymer and the second ethylene-vinyl acetate copolymer is determined by passing the copolymer melt through a standard capillary die (typically 2.095 mm in diameter) for 10 minutes at a temperature of 190°C and a load of 2.16 kg. The mass extruded within this 10-minute period is measured (unit: g / 10 min). This value is the melt index.

[0040] The dispersibility index of the first ethylene-vinyl acetate copolymer and the second ethylene-vinyl acetate copolymer was tested by gel permeation chromatography (GPC), specifically as follows: the number-average molecular weight, weight-average molecular weight, and dispersibility index (PDI) were calculated based on the difference in retention time of polymers with different molecular weights in the chromatographic column.

[0041] The melting point of the first ethylene-vinyl acetate copolymer and the second ethylene-vinyl acetate copolymer was tested by differential scanning calorimetry (DSC), specifically as follows: the heat flow changes of the sample during the programmed temperature rise were monitored, and the temperature corresponding to the melting peak was the melting point.

[0042] In some embodiments, the inorganic filler accounts for 50% to 85% of the mass of the second coating, and the binder accounts for 5% to 10% of the mass of the coating.

[0043] For example, in the second coating, the mass percentage of inorganic filler is within the range formed by any two values ​​of 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or more.

[0044] For example, in the second coating, the mass percentage of the adhesive is within the range formed by any two values ​​of 5%, 6%, 7%, 8%, 9%, 10% or more.

[0045] In some embodiments, the inorganic filler in the second coating includes at least one of magnesium hydroxide, boehmite, and alumina.

[0046] In some embodiments, the binder in the second coating includes at least one of polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, cellulose, and polyvinylpyrrolidone.

[0047] In some embodiments, the areal density of the second coating is 1–5 g / cm³. 2 , such as 1g / cm 2 2g / cm 2 3g / cm 2 4g / cm 2 5g / cm 2 Or the range formed by any two of the above values.

[0048] In some embodiments, the second surface of the base film faces the negative electrode.

[0049] In some embodiments, the base film includes at least one of polyethylene, polypropylene, polyester, cellulose, polyimide, and aramid.

[0050] In some embodiments, the method for preparing the diaphragm includes the following steps: The first EVA, inorganic filler, binder and solvent are mixed to obtain the first slurry; Inorganic fillers, binders, and solvents are mixed to obtain a second slurry; The base film includes a first surface and a second surface. A first slurry is coated on the first surface and a second slurry is coated on the second surface. After drying, a diaphragm is obtained, wherein the first slurry forms a first coating of the diaphragm and the second slurry forms a second coating of the diaphragm.

[0051] In one embodiment, the solid content of the first slurry is 20% to 30% (by mass).

[0052] In one embodiment, the solvent in the first slurry is water.

[0053] In one embodiment, the solid content of the second slurry is 20% to 30% (by mass).

[0054] In one embodiment, the solvent in the second slurry is water.

[0055] In one embodiment, the preparation of the second slurry also uses a second EVA, that is, mixing the second EVA, inorganic filler, binder and deionized water to obtain the second slurry.

[0056] In some embodiments, the negative electrode material further includes a conductive agent, a binder, and a thickener. For example, the conductive agent in the negative electrode material includes at least one of acetylene black and carbon nanotubes; the binder includes at least one of styrene-butadiene rubber (SBR) and polyacrylic acid (PAA); and the thickener includes at least one of sodium carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA). In some embodiments, the mass percentage of the conductive agent in the negative electrode material is 0.5% to 5%; the mass percentage of the binder is 0.5% to 2%; and the mass percentage of the thickener is 0.5% to 2%. For example, the mass percentage of the conductive agent in the negative electrode material is within the range formed by any two values ​​of 0.5%, 1%, 2%, 3%, 4%, 5%, or more; the mass percentage of the binder is within the range formed by any two values ​​of 0.5%, 0.7%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or more; and the mass percentage of the thickener is within the range formed by any two values ​​of 0.5%, 0.7%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or more.

[0057] In some embodiments, the negative electrode sheet further includes a negative electrode current collector. Exemplarily, the negative electrode current collector includes at least one of copper foil and composite copper foil.

[0058] In one embodiment, the negative electrode material is located on at least one side of the negative electrode current collector.

[0059] In some embodiments, the positive electrode sheet includes a positive electrode material, which includes a positive electrode active material. For example, the positive electrode active material includes at least one of lithium cobalt oxide and lithium nickel cobalt manganese oxide. In one embodiment, the mass percentage of the positive electrode active material in the positive electrode material is 90% to 98%, such as within the range formed by any two of the following values: 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%.

[0060] In some embodiments, the positive electrode material further includes a conductive agent and a binder. For example, the conductive agent in the positive electrode material includes at least one of acetylene black and carbon nanotubes; the binder includes at least one of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In one embodiment, the mass percentage of the conductive agent in the positive electrode material is 0.5% to 5%; the mass percentage of the binder is 0.5% to 2%. For example, the mass percentage of the conductive agent in the positive electrode material is within the range formed by any two of the following values: 0.5%, 1%, 2%, 3%, 4%, 5%, or more; the mass percentage of the binder is within the range formed by any two of the following values: 0.5%, 0.7%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or more.

[0061] In some embodiments, the positive electrode sheet further includes a positive current collector. For example, the positive current collector includes at least one of aluminum foil and composite aluminum foil.

[0062] In one embodiment, the positive electrode material is located on at least one side of the positive electrode current collector.

[0063] In some embodiments, the secondary battery further includes an electrolyte comprising a lithium salt and a solvent. Exemplarily, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium difluorobis(oxalate) phosphate, lithium tetrafluoroborate, lithium bis(oxalate) borate, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium bis(pentafluoroethylsulfonyl)imide, lithium trifluoromethylsulfinate, and lithium hexafluorozirconate (Li2ZrF6); the solvent includes at least one of ethylene carbonate, diethyl carbonate, propylene carbonate, vinylene carbonate, methyl ethyl carbonate, dimethyl carbonate, ethyl acetate, methyl acetate, n-propyl acetate, n-butyl acetate, isopropyl acetate, vinyl vinyl carbonate, propyl propionate, methyl butyrate, and ethyl butyrate. In one embodiment, the electrolyte contains 10% to 30% lithium salt by mass, and the solvent by mass is 60% to 90%. For example, the lithium salt in the electrolyte may be within the range formed by any two values ​​of 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, or more; the solvent by mass may be within the range formed by any two values ​​of 60%, 65%, 70%, 75%, 80%, 85%, 90%, or more.

[0064] Additives can be added to the electrolyte as needed. For example, in some embodiments, the secondary battery further includes at least one of the following: 1,3-propanesulfonate lactone, propylene sulfonate lactone, methyl vinyl sulfate, propylene sulfate, vinyl sulfite, butane sulfonate lactone, methane disulfonate, divinyl sulfate, tetraphenylphosphine, ethoxypentafluorocyclotriphosphazene, triargyl phosphate, tri(2,2,2-trifluoroethyl) phosphate, succinic acid, adiponitrile, 1,3,6-hexanetrionitrile, glutaronitrile, malononitrile, isobutyronitrile, butyronitrile, etc. In one embodiment, the mass percentage of the additive in the electrolyte is 0.1% to 5%, such as 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or any two of the above values ​​within a range.

[0065] According to a second aspect of this application, an electrical device is provided, including the secondary battery. The electrical device includes, but is not limited to, at least one of the following: mobile phone, laptop computer, drone, robot vacuum cleaner, e-cigarette, pure electric vehicle, hybrid electric vehicle, plug-in hybrid electric vehicle, electric bicycle, electric scooter, electric golf cart, electric truck, electric train, ship and satellite, energy storage system, etc.

[0066] The present application is further illustrated below with specific embodiments. The EVA used is prepared by high-pressure continuous bulk polymerization: ethylene monomer, vinyl acetate monomer, initiator di-tert-butyl peroxide, and molecular weight regulator propionaldehyde are mixed in proportion. Based on the total mass of ethylene monomer and vinyl acetate monomer, the mass ratio of ethylene is 67.5-97.5 wt%, the mass ratio of vinyl acetate is 2.5-32.5 wt%, and the mass percentage of initiator and regulator is 0.1 wt%. The mixture is fed into a reactor through a pressurization system and undergoes free radical copolymerization in a high-pressure reactor (stirred batch) at 200-220°C and 150-250 MPa. The VA content and melt index in the copolymer are adjusted by controlling the temperature, pressure, and material ratio. After the reaction, the unreacted monomers (ethylene and vinyl acetate) are separated by a depressurization system. The latter are recycled through a recycling system. The polymer melt is extruded and granulated to form the final product.

[0067] Example 1 This embodiment provides a lithium-ion secondary battery, the preparation method of which includes the following steps: (1) Preparation of the diaphragm The first EVA, inorganic filler alumina, binder polyvinylidene fluoride and deionized water are mixed to obtain a first slurry with a solid content of 25%. The second EVA, inorganic filler alumina, binder polyvinylidene fluoride and deionized water are mixed to obtain a second slurry with a solid content of 25%. A polyethylene-based membrane is used, which includes a first surface and a second surface. A first slurry is coated on the first surface and a second slurry is coated on the second surface. After drying, a diaphragm is obtained.

[0068] The VA content in the first and second EVA, the melt index at 190℃ and 2.16kg load, and the EVA content, inorganic filler content, binder content and areal density in the first and second coatings of the resulting diaphragm are shown in Table 1.

[0069] (2) Preparation of positive electrode The positive electrode active material lithium cobalt oxide, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in an N-methylpyrrolidone solvent system at a mass ratio of 98:1.2:0.8. The mixture was then coated onto an Al foil, dried, rolled, and slit to obtain the positive electrode sheet.

[0070] (3) Preparation of negative electrode The negative electrode active material, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were mixed in a deionized water solvent system at a mass ratio of 98.1:0.5:0.7:0.7. The mixture was then coated onto Cu foil, dried, rolled, and slit to obtain the negative electrode sheet. The negative electrode active material is a composite material formed from graphite and silicon-carbon. The silicon content in the silicon-carbon material is 60% by mass, and the silicon exists in the form of elemental silicon. The silicon content in the negative electrode material is shown in Table 1.

[0071] (4) Preparation of electrolyte A solution prepared by mixing lithium salt LiPF6 with a non-aqueous organic solvent (ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): propyl propionate (PP): ethylene carbonate (VC)) in a mass ratio of 25:25:15:31:4, with a mass ratio of 8:92) is used as the electrolyte for lithium batteries.

[0072] (5) Preparation of secondary batteries The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide safety isolation (the first surface of the separator base film faces the positive electrode). The electrode assembly is then wound to form the electrode assembly. The electrode assembly is placed in a packaging shell, electrolyte is injected, and the package is sealed to obtain a secondary battery.

[0073] Examples 2-18 and Comparative Examples 1-3 These embodiments and comparative examples all provide a lithium-ion secondary battery, which differs from Embodiment 1 in that: The VA content in the first and second EVA, the melt index at 190℃ and 2.16kg load, the EVA content, inorganic filler content, binder content and areal density in the first and second coatings, and the silicon content in the negative electrode material are shown in Table 1 (only the ratio of graphite to silicon carbon is changed, while the ratios of negative electrode active material, conductive agent, binder and thickener remain unchanged).

[0074] Table 1 The above embodiments and comparative examples were subjected to hot box testing, and the specific steps are as follows: Step 1: Charge the prepared battery at a constant current of 0.5C to the cutoff voltage of 4.5V, then charge it at a constant voltage to the cutoff current of 200mA, and then let it stand for 5 minutes. Step 2: Attach the temperature sensing wire to the fully charged battery between the two tabs, connect the two tabs to monitor the voltage, and suspend the battery cell vertically inside the box. Step 3: Heat the box to 130°C at a rate of 5°C and maintain the temperature for 60 minutes.

[0075] Evaluation criteria: The battery passes if it does not catch fire or explode; otherwise, it fails.

[0076] The above embodiments and comparative examples were subjected to room temperature cycling performance tests, and the specific steps are as follows: Step 1: At 25℃, charge the lithium-ion battery at a constant current of 0.7C to 4.5V, and then charge it at a constant voltage until the current is 0.5C. At this time, the lithium-ion battery is fully charged. Record the charging capacity at this time, which is the first charging capacity. Step 2: After letting the lithium-ion battery rest for 5 minutes, discharge it to 3V at a constant current of 0.5C, and let it rest for 5 minutes. This completes one charge-discharge cycle. Record the discharge capacity at this point, which is the first cycle discharge capacity. Cycle 300 times under these conditions and calculate the capacity retention rate on the 300th cycle. The capacity retention rate % is calculated as: discharge capacity of the current cycle / discharge capacity of the first cycle.

[0077] The test results are shown in Table 2.

[0078] Table 2 Hot box throughput 25℃ 300-cycle volume retention Example 1 8 / 10 93.23％ Example 2 9 / 10 93.11％ Example 3 9 / 10 90.85％ Example 4 7 / 10 90.25％ Example 5 7 / 10 90.19％ Example 6 9 / 10 90.12％ Example 7 9 / 10 89.13％ Example 8 10 / 10 88.77％ Example 9 9 / 10 92.01％ Example 10 10 / 10 91.89％ Example 11 9 / 10 90.13％ Example 12 10 / 10 87.31％ Example 13 10 / 10 96.01％ Example 14 9 / 10 88.34％ Example 15 9 / 10 89.52％ Example 16 9 / 10 89.17％ Example 17 8 / 10 93.17％ Example 18 8 / 10 93.25％ Comparative Example 1 4 / 10 89.78％ Comparative Example 2 2 / 10 87.92％ Comparative Example 3 7 / 10 80.01％ Results Analysis: By comparing Examples 1-18 and Comparative Examples 1-3, it can be seen that adding EVA to the first coating is beneficial to improving safety performance. Controlling the mass ratio of vinyl acetate A in the EVA and the mass ratio of silicon in the negative electrode material to satisfy 0.2≤A / S≤1 is beneficial to the balance between safety and cycle performance. However, when A / S does not satisfy the above relationship, it will lead to poor safety or poor cycle performance.

[0079] A comparison of Examples 1 to 3 shows that when A is in the range of 5 to 10, it is more conducive to balancing safety and cycle performance.

[0080] A comparison of Examples 4-8 shows that when the mass percentage of vinyl acetate in the second EVA is in the range of 10% to 30%, it is more conducive to balancing safety and cycle performance.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this article and are not intended to limit the scope of protection of this article. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this article without departing from the essence and scope of the technical solutions of this article.

Claims

1. A secondary battery characterized by comprising: The secondary battery comprises a separator, a positive electrode sheet, and a negative electrode sheet. The separator comprises a base film, the base film comprising a first surface and a second surface, wherein a first coating layer is provided on the first surface of the base film, and the first coating layer comprises a first ethylene-vinyl acetate copolymer; The negative electrode sheet comprises a negative electrode material, and the negative electrode material comprises a negative electrode active material, and the negative electrode active material comprises a silicon-carbon material; The secondary battery satisfies 0.2≤A / S≤1, wherein S% is the mass percentage of silicon in the negative electrode material, A% is the mass percentage of vinyl acetate in the first ethylene-vinyl acetate copolymer.

2. The secondary battery according to claim 1, wherein The S is 5-50.

3. The secondary battery according to claim 1, wherein The A is 5-10.

4. The secondary battery according to claim 1, wherein At least one of the following conditions is satisfied: A1. The silicon-carbon material comprises at least one of elemental silicon, silicon-based alloy, silicon oxide and carbon, and the mass percentage of silicon in the silicon-carbon material is 40%-60%; A2. The mass percentage of the first ethylene-vinyl acetate copolymer in the first coating layer is 5%-45%; A3. The melting point of the first ethylene-vinyl acetate copolymer is 110-130℃, and the melt index under the temperature of 190℃ and the load of 2.16kg is 5-20g / 10min; A4. The first coating has an area density of 1 to 5 g / cm 2 ; A5. The first surface of the base film faces the positive electrode sheet; A6. The negative electrode active material further comprises graphite, and the mass percentage of graphite in the negative electrode active material is 14%-90%.

5. The secondary battery according to claim 1, wherein A second coating layer is provided on the second surface of the base film, and the second coating layer comprises inorganic filler and binder.

6. The secondary battery according to claim 5, wherein The second coating layer further comprises a second ethylene-vinyl acetate copolymer, and the mass percentage of vinyl acetate in the second ethylene-vinyl acetate copolymer is 10%-30%.

7. The secondary battery according to claim 5, wherein the negative electrode is a lithium metal electrode. At least one of the following conditions is satisfied: B1. The mass percentage of the second ethylene-vinyl acetate copolymer in the second coating layer is 5%-45%; B2. The melting point of the second ethylene-vinyl acetate copolymer is 100-110℃, and the melt index under the temperature of 190℃ and the load of 2.16kg is 5-20g / 10min; B3. the second coating has an areal density of 1 to 5 g / cm 2 ; B4. The second surface of the base film faces the negative electrode sheet.

8. The secondary battery according to claim 5, wherein The first coating layer further comprises inorganic filler and binder.

9. The secondary battery according to claim 8, wherein At least one of the following conditions is satisfied: C1. In the first coating layer, the mass percentage of inorganic filler is 50%-85%, and the mass percentage of binder is 5%-10%; C2. In the second coating layer, the mass percentage of inorganic filler is 50%-95%, and the mass percentage of binder is 5%-10%; C3. The inorganic filler in the first coating layer and the second coating layer respectively independently comprises at least one of magnesium hydroxide, boehmite, and aluminum oxide; C4. The binder in the first coating layer and the second coating layer respectively independently comprises at least one of polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, cellulose, and polyvinylpyrrolidone; C5. The base film comprises at least one of polyethylene, polypropylene, polyester, cellulose, polyimide, and aramid.

10. An electrical device, characterized by The secondary battery comprises a separator, a positive electrode sheet, and a negative electrode sheet.

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