Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises a negative plate and electrolyte, the negative plate comprises a negative current collector and a negative active layer located on at least one side surface of the negative current collector, the negative active layer comprises a first coating and a second coating which are arranged in the thickness direction of the negative plate, and the first coating is located between the negative current collector and the second coating; the second coating comprises first silicon carbon, and the sphericity degree of the first silicon carbon is 0.8-1; the silicon content of the first silicon carbon is 30%-60%; the average particle size of the first silicon carbon is R1, and R1 is more than 6 microns and less than or equal to 15 microns; the electrolyte comprises a fluorine-containing solvent and a nitrile compound, the fluorine-containing solvent comprises fluoroethylene carbonate and / or ethyl difluoroacetate, based on the total weight of the electrolyte, the content of the fluorine-containing solvent is w1, and w1 is larger than or equal to 30% and smaller than or equal to 70%; the content of the nitrile compound is w2, and 0.4% < = w2 < = 7%. The lithium ion secondary battery provided by the invention has good cycling stability and dynamic performance at high voltage and high temperature.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a lithium-ion secondary battery. Background Art

[0002] With the rapid development of high-energy-density battery technology, the demand for lithium-ion batteries operating under high voltage (≥4.5V) and high temperature (≥45°C) conditions is becoming increasingly urgent. Silicon-carbon materials have the advantage of high gram capacity over graphite, significantly improving the high energy density of batteries and thus gaining widespread application. However, silicon-carbon materials exhibit large volume expansion during battery charge and discharge cycles, which limits the use and promotion of silicon-carbon anodes. To improve kinetic performance, silicon-carbon materials with high sphericity, characterized by fewer surface defects and uniform stress distribution, are often used. However, volume expansion issues still exist at high temperatures, and poor interfacial stability leads to insufficient stability. Therefore, further improving the use of silicon-carbon anode systems with high sphericity to improve the high-temperature stability and kinetic performance of lithium-ion secondary batteries under high-voltage systems is crucial for the promotion and use of silicon-carbon materials. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above-mentioned problems existing in the prior art and provide a lithium-ion secondary battery (hereinafter referred to as the battery). The negative electrode of the battery adopts a silicon-carbon material with a high sphericity, and is combined with an electrolyte including a fluorine-containing solvent and a low content of nitrile compounds to jointly improve the high-temperature stability and kinetic performance of the battery in a high-voltage system.

[0004] Silicon-carbon materials with high sphericity (e.g., sphericity ≥ 0.8) have smooth contours, good compressive resistance under high voltage systems, and are not easily broken under external pressure to avoid puncturing the diaphragm, thereby improving the K value yield of the battery. On the other hand, due to its few surface defects and uniform stress distribution in all directions, it not only alleviates volume expansion at room temperature, but also reduces the unevenness of the lithium ion diffusion path within the silicon-carbon material, significantly improving the kinetics of lithium ion insertion / extraction, thereby improving the kinetics of the battery. In addition, silicon-carbon materials with high sphericity have a small specific surface area and a small contact area with the electrolyte, which reduces the occurrence of side reactions. However, under high temperature conditions (e.g., ≥ 45°C), the volume expansion and interface side reactions of the silicon-carbon material will be further aggravated, causing the high-temperature cycle stability and rate performance of the battery to deteriorate. Based on this, the inventors of the present invention found that the first silicon carbon is placed in the second coating away from the negative electrode current collector. At this time, the second coating is closer to the positive electrode sheet, the electrolyte is fully infiltrated, the migration path of lithium ions is shorter, and the number of lithium ions that can be quickly embedded in lithium increases, which is beneficial to improving the kinetic performance and capacity of the negative electrode sheet and improving the rate performance. However, the second coating is more likely to contact the electrolyte, and the side reaction between the first silicon carbon and the electrolyte is aggravated at high temperatures, making it impossible to significantly improve the high-temperature cycle performance. Furthermore, the inventors of the present invention have found through experiments that the high-temperature cycle performance of the battery can be improved by improving the composition of the electrolyte. A fluorine-containing solvent system with high oxidation resistance and high thermal decomposition temperature is used to enhance the stability of the SEI film on the silicon carbon surface, inhibit the continuous decomposition of the electrolyte, protect the silicon carbon negative electrode, and avoid battery failure during cycling at high temperatures.

[0005] Based on this, the present invention provides the following technical solutions:

[0006] The present invention provides a lithium-ion secondary battery, comprising a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises a first coating layer and a second coating layer arranged along the thickness direction of the negative electrode sheet, wherein the first coating layer is located between the negative electrode current collector and the second coating layer; the second coating layer comprises a first silicon carbon, wherein the sphericity of the first silicon carbon is α1, α10.8-1; the mass content of silicon element in the first silicon carbon is 30%-60%; the average particle size of the first silicon carbon is R1, 6μm<R1≤15μm; the electrolyte comprises a fluorine-containing solvent and a nitrile compound, wherein the fluorine-containing solvent comprises fluoroethylene carbonate and / or ethyl difluoroacetate, and based on the total weight of the electrolyte, the content of the fluorine-containing solvent is w1, 30%≤w1≤70%; the content of the nitrile compound is w2, 0.4%≤w2≤7%.

[0007] Through the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0008] (1) The battery of the present invention has good high temperature cycle performance and good kinetic performance.

[0009] (2) The battery of the present invention has good cycle stability in a high voltage system.

[0010] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Shown is a cross-sectional SEM image of a negative electrode sheet in one embodiment of the present invention. DETAILED DESCRIPTION

[0012] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0013] The present invention provides a lithium-ion secondary battery, which includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector. The negative electrode active layer includes a first coating and a second coating arranged along the thickness direction of the negative electrode sheet, wherein the first coating is located between the negative electrode current collector and the second coating; the second coating includes a first silicon carbon, and the sphericity of the first silicon carbon is α1, and α1 is 0.8-1, for example, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98 or 1.

[0014] In the present invention, the mass content of silicon element in the first silicon carbon is 30%-60%, for example, 30%, 32%, 34%, 36%, 38%, 40%, 45%, 50%, 55% or 60%.

[0015] In the present invention, the average particle size of the first silicon carbon is R1, 6μm<R1≤15μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm or 15μm.

[0016] In one embodiment, the average particle size of the first silicon carbon is R1, 7 μm<R1≤13 μm.

[0017] Silicon-carbon materials with high sphericity have smooth surfaces and few defects. During the battery's charge and discharge process, the expansion stress is evenly dispersed, avoiding stress concentration. This improves the battery's cycle performance at room temperature and improves cyclic expansion to a certain extent. At the same time, it reduces the unevenness of the lithium ion diffusion path, lowers the local current density, and significantly improves the kinetics of lithium ion insertion / extraction, meeting the demand for high-rate charge and discharge without lithium precipitation. However, in high-temperature environments, the cyclic expansion of silicon-carbon materials is exacerbated, and side reactions with the electrolyte at the interface increase, resulting in insufficient interface stability, repeated rupture and formation of the SEI film, and consumption of active lithium and electrolyte, causing the battery's high-temperature cycle attenuation and reduced rate performance. To address the above-mentioned issues, the inventors of the present invention discovered that by placing a first silicon-carbon layer in a second coating layer facing away from the negative electrode current collector, the second coating layer is closer to the positive electrode sheet, allowing for more adequate electrolyte infiltration. This shortens the distance lithium ions migrate and allows them to quickly contact the first silicon-carbon layer. Consequently, the number of lithium ions that can rapidly intercalate lithium increases significantly, which is beneficial for enhancing the kinetic performance of the negative electrode sheet and improving the battery's rate performance. Furthermore, by placing the first silicon-carbon layer in the second coating layer away from the negative electrode current collector, the expansion of the negative electrode current collector caused by silicon-carbon material expansion can be reduced, thereby reducing the expansion stress on the negative electrode current collector. Furthermore, by placing the first silicon-carbon layer closer to the surface of the negative electrode active layer, there is more space for expansion, preventing the negative electrode active layer from falling off, reducing the relative displacement of particles in the negative electrode active layer, maintaining contact stability, and improving the battery's high-temperature cycling stability. Furthermore, by regulating the mass content and average particle size of silicon in the first silicon-carbon layer, it is possible to control the volume expansion rate while maintaining a high specific capacity, avoiding negative electrode structural collapse and improving cycling stability.

[0018] In the present invention, the average particle size of the first silicon carbon can be obtained by conventional testing methods in the field, for example, by the following method: discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly taking the negative electrode sheet, polishing the cross section of the negative electrode sheet with an argon ion grinder, and imaging the obtained cross section using a backscattering imaging mode in a scanning electron microscope (SEM); the silicon carbon particles in the image appear grayish white, and then the particle size measurement and distribution statistics are performed using electron microscope image analysis software such as ImageJ to obtain the average particle size of the first silicon carbon.

[0019] In the present invention, the sphericity of the first silicon carbon can be obtained by conventional testing methods in the art, for example, by the following method: discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly taking the negative electrode sheet, polishing the cross section of the negative electrode sheet with an argon ion grinder, and imaging the obtained cross section using a backscattering imaging mode in a scanning electron microscope (SEM) device; analyzing the image of the first silicon carbon located in the first coating layer in the SEM photograph (backscattering mode) of the negative electrode active layer at a certain magnification (such as 2500 times) using image processing software such as (Image Pro Plus), first measuring the average particle size to determine the first silicon carbon, obtaining the perimeter and area of ​​each particle, and calculating the perimeter equivalent radius r1 and area equivalent radius r2 of each first silicon carbon particle respectively, then the sphericity S of each particle = r2 / r1, and then performing a quantity-weighted average of the sphericity of each particle to obtain the sphericity of the first silicon carbon.

[0020] In the present invention, the electrolyte includes a fluorinated solvent and a nitrile compound, the fluorinated solvent includes fluoroethylene carbonate (FEC) and / or ethyl difluoroacetate, and based on the total weight of the electrolyte, the content of the fluorinated solvent is w1, 30%≤w1≤70% (for example, 30%, 32%, 34%, 36%, 38%, 40%, 45%, 50%, 55%, 60%, 65% or 70%); the content of the nitrile compound is w2, 0.4%≤w2≤7% (for example, 0.4%, 0.45%, 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, 6% or 7%).

[0021] In one embodiment, 40%≤w1≤62%; 0.5%≤w2≤6%.

[0022] In one embodiment, 40%≤w1≤62%; 1%≤w2≤6%.

[0023] In the present invention, the nitrile compound includes at least one of succinonitrile (SN), adiponitrile (ADN), glycerol trinitrile, 1,4-dicyano-2-butene, and 1,3,6-hexane trinitrile (HTCN).

[0024] In the present invention, the ethyl difluoroacetate refers to ethyl acetate having two fluorine atoms substituted at any position.

[0025] In one embodiment, the ethyl difluoroacetate includes 2,2-difluoroethyl acetate (DFEA) and / or ethyl 2,2-difluoroacetate.

[0026] In yet another embodiment, the ethyl difluoroacetate comprises DFEA.

[0027] In the present invention, based on the total weight of the electrolyte, the content of the fluoroethylene carbonate is w3, 10%≤w3≤20%, for example, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%.

[0028] In one embodiment, 14%≤w3≤17%.

[0029] In the present invention, based on the total weight of the electrolyte, the content of ethyl difluoroacetate is w4, 20%≤w4≤60%, for example, 20%, 22%, 24%, 26%, 28%, 30%, 35%, 40%, 45%, 50%, 55% or 60%.

[0030] In one embodiment, 25%≤w4≤50%.

[0031] In one embodiment, the electrolyte does not include ethylene carbonate.

[0032] Although the use of a first silicon carbon with a higher sphericity in the negative electrode and its placement in a second coating away from the negative electrode current collector can improve the stability of the battery at high temperatures, it still cannot meet the use requirements. The inventors of the present invention further conducted experimental studies and found that by regulating the composition of the electrolyte and adopting a fluorinated solvent system (for example, including FEC and / or ethyl difluoroacetate), it is possible to cooperate with the negative electrode system and structural design to further improve the high temperature cycle performance of the battery. On the one hand, a high content of fluorinated solvents can increase the thermal decomposition temperature and oxidation resistance with the help of the fluorinated groups therein, so that the electrolyte can remain stable under high temperature or high pressure systems, inhibit the volatilization and oxidative decomposition of the electrolyte, reduce the risk of battery gas production, and avoid battery bloating; on the other hand, it can preferentially decompose on the surface of the negative electrode to form a composite SEI film rich in LiF and organic fluoride, thereby improving the generation efficiency of the SEI film on the surface of the negative electrode and solving the problem of poor mechanical strength of the SEI film formed by EC. Silicon-carbon materials with higher sphericity make the formation of SEI film more uniform by reducing the surface curvature radius, while fluorine-containing solvents have dynamic repair capabilities. The two work together to achieve high-temperature cycling performance under a high voltage of 4.5V, with both good capacity retention and low expansion rate.

[0033] When the content of fluorinated solvent is too low (for example, w1 < 30%), there are too few fluorinated groups in the electrolyte, which will not improve the antioxidant properties of the electrolyte enough. The solvent in the electrolyte is easy to decompose and produce gas, which will have little effect on the high-voltage performance of the battery. In addition, the film-forming stability is poor, and a dense and stable interface film cannot be formed. It is impossible to inhibit the continuous decomposition of the electrolyte and the silicon-carbon powdering on the surface of the negative electrode, resulting in degradation of the battery's high-temperature storage and high-temperature cycle performance. When the content of fluorinated solvent is too high (for example, w1 > 70%), there are too many fluorinated groups in the electrolyte, which will make it difficult for lithium ions in the electrolyte to desolvate, increase the viscosity of the electrolyte, deteriorate the kinetic performance, reduce the overall ionic conductivity, and make the migration of lithium ions more difficult at low temperatures, resulting in a decrease in low-temperature performance.

[0034] In addition, it is also necessary to regulate the FEC content w3 and the ethyl difluoroacetate content w4 in the electrolyte and keep the fluorine-containing solvent content w1 within an appropriate range: when w3 is less (for example, less than 10%), the negative electrode SEI film is not formed completely, and the side reactions during charging and discharging are aggravated; when w3 is more (for example, greater than 20%), the battery aeration is aggravated, the interface film thickness increases, active lithium is lost, and the cycle performance deteriorates; when w4 is less (for example, less than 20%), its antioxidant ability is not fully exerted, and the high voltage performance of the battery cannot be improved; and when the amount of w4 is more (for example, greater than 60%), the overall viscosity of the electrolyte increases, the conductivity decreases, and the battery cycle performance decreases.

[0035] In the present invention, when ethyl difluoroacetate satisfies 20%≤w4≤60%, EC may not be added to the electrolyte, which can solve the problems of poor cycle stability and battery gas production at high voltage caused by unstable EC structure, enhance the stability of the interface film, and improve high-temperature performance.

[0036] Regulating the content of nitrile compounds (w2) can form complex protection on the surface of the positive electrode material, preventing the dissolution of metal ions, improving the stability of the interface, and thus improving the cycle life at high temperatures; regulating the content of nitrile compounds within an appropriate range will participate in the solvation structure in the electrolyte, making it easier for lithium ions to escape, and improving the rate performance. Nitrile additives combined with fluorinated solvent systems can synergistically improve the stability of the electrolyte under high voltage, inhibit the oxidative decomposition of the electrolyte, and reduce gas production during high-temperature storage of the battery; fluorinated solvents can form a LiF-rich SEI film on the surface of the negative electrode, enhancing the toughness of the SEI film, improving the ionic conductivity and mechanical strength of the SEI film, inhibiting the rupture caused by the volume expansion of the silicon-based negative electrode, and improving the cycle stability of the battery.

[0037] In the present invention, the content w1 of the fluorine-containing solvent, the content w2 of the nitrile compound, the content w3 of the fluoroethylene carbonate and the content w4 of the ethyl difluoroacetate can be obtained by conventional testing methods in the art, for example, by gas chromatography or gas chromatography-mass spectrometry.

[0038] In the present invention, the electrolyte includes ethylene carbonate (EC), and the content of ethylene carbonate is w5 based on the total weight of the electrolyte, w5≤5%, for example, 0.1%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%.

[0039] At high charge cutoff voltages (e.g., ≥4.5V), EC is unstable and easily reacts with the cathode material, accelerating the material's phase transition and releasing singlet oxygen, leading to cathode structural collapse and electrolyte oxidative decomposition. At high temperatures, gas production (such as CO and H2) occurs, which, combined with the cathode oxygen evolution reaction, produces H2O2, exacerbating the risk of thermal runaway. Reducing or eliminating EC usage can improve the cathode's cycling stability at high voltages and enhance high-temperature storage and furnace temperature safety. For the anode, the SEI film formed in the EC system lacks mechanical strength at high voltages and is insufficient to protect the anode. Therefore, the use of fluorinated solvents to replace or partially replace EC is conducive to the formation of a denser and more stable SEI film. The use of fluorinated solvents can preferentially reduce the surface of silicon-carbon materials to form an organic-inorganic composite SEI film rich in LiF (high elastic modulus), thereby improving the mechanical strength of the SEI film. Since the SEI film has a three-dimensional cross-linked structure, it can adapt to the volume expansion of silicon-carbon particles. Especially for the negative electrode system using silicon-carbon materials with higher sphericity under high voltage and high temperature conditions, it can inhibit the rupture of the SEI film and the continuous decomposition of the electrolyte caused by expansion during the high-temperature cycle of the battery, thereby improving the stability of the electrolyte and avoiding high-temperature cycle failure.

[0040] In the present invention, the electrolyte includes a non-fluorocarbonate solvent, and the content of the non-fluorocarbonate solvent is w6, 5%≤w6≤60%, based on the total weight of the electrolyte, for example, 5%, 6%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%.

[0041] In one embodiment, 5%≤w6≤20%.

[0042] In the present invention, w1 and w6 satisfy: 0.05≤w6 / w1≤1, for example, 0.05, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1, 1.2 or 1.4.

[0043] In one embodiment, w1 and w6 satisfy: 0.05≤w6 / w1≤0.5.

[0044] In the present invention, the non-fluorocarbonate solvent further includes ethyl methyl carbonate (EMC) and / or diethyl carbonate (DEC).

[0045] Although the fluorine-containing solvent system can improve the antioxidant properties of the overall battery electrolyte, it will cause the viscosity of the electrolyte to increase, and the dissociation ability of the lithium salt is poor, which is not conducive to the kinetic performance. Adding non-fluorocarbonate solvents to compound with fluorine-containing solvents and regulating w6 / w1 within a certain range can not only avoid the reduction of the dissociation degree of lithium salts in the electrolyte, but also maintain high antioxidant properties, so that the battery has both good kinetic performance and cycle stability.

[0046] In the present invention, the electrolyte further includes a sulfur-containing additive, and the content of the sulfur-containing additive is w7, 2%≤w7≤8%, based on the total weight of the electrolyte, for example, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 6%, 7% or 8%.

[0047] In one embodiment, 2%≤w7≤5%.

[0048] In the present invention, the sulfur-containing additive includes a first sulfur-containing additive and / or a second sulfur-containing additive, wherein the first sulfur-containing additive includes

[0049] and At least one of .

[0050] In the present invention, the second sulfur-containing additive comprises At least one of .

[0051] In one embodiment, the second sulfur-containing additive comprises

[0052] In one embodiment, the sulfur-containing additive includes (Ⅰ-9).

[0053] In the present invention, based on the total weight of the electrolyte, the content of the first sulfur-containing additive is 1%-2%, for example, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or 2%.

[0054] In the present invention, based on the total weight of the electrolyte, the content of the second sulfur-containing additive is 2%-6%, for example, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5% or 6%.

[0055] The silicon-carbon material with higher sphericity in the negative electrode can evenly disperse the expansion stress in all directions when it expands, alleviating the stress concentration of the negative electrode sheet during high-temperature cycling. At the same time, less EC can alleviate the damage of HF to the interface, inhibit the generation of cracks and the shedding of negative electrode active materials, and maintain the integrity of the electrode structure. The addition of sulfur-containing additives can further improve the structural stability of the battery at high temperatures.

[0056] By regulating the content of the first sulfur-containing additive in the electrolyte, it can be preferentially reduced in the formation stage to form a SEI film, thereby increasing the lithium ion transmission rate of the SEI film, stabilizing the structure of the SEI film, and improving the high-temperature stability of the battery's SEI film, thereby improving the battery's high-temperature storage performance.

[0057] A second sulfur-containing additive is added to the electrolyte. It oxidizes and decomposes on the cathode surface to form an inorganic-organic composite CEI film rich in sulfides (e.g., Li2SO3, Li2S) and LiF. This film has high mechanical strength, inhibits the release of lattice oxygen from the cathode material and the dissolution of transition metals (e.g., Mn, Co) from the cathode, and maintains stability at high temperatures (e.g., 60°C) and high voltages (e.g., a battery charge cutoff voltage of 4.5V or above), effectively preventing high-temperature gassing. The second sulfur-containing additive has multifunctional functional groups, combining good film-forming ability and ionic conductivity, which helps enhance ion transport at high temperatures and improves the high-temperature cycling stability of the battery. When combined with FEC and ethyl difluoroacetate, on the one hand, it can make up for the insufficient dielectric constant of DFEA, increase the lithium ion migration rate, and improve the low-temperature performance of the battery; on the other hand, the reduction products of the fluorinated solvent FEC and ethyl difluoroacetate (such as organic matter containing CF groups and LiF) and the decomposition products of the second sulfur-containing additive compound (such as polysulfides) together form a flexible organic outer layer to buffer the volume change stress of the silicon-based or lithium metal negative electrode. For example, the organic layer generated by the CF groups derived from the ethyl difluoroacetate solvent and the sulfide can enhance the mechanical stability of the SEI film. The introduction of the second sulfur-containing additive can further optimize the interfacial chemistry through the sulfur-fluorine exchange reaction (SF exchange), forming a more stable sulfur-fluorine composite interface SEI layer, and improving high-temperature storage stability. Fluorinated solvents change the solvation structure of the electrolyte through strong Li+ coordination ability, reducing the corrosion of free solvent molecules on the silicon-based negative electrode; sulfur compounds participate in the interfacial reaction through anions to form an interfacial phase with high ionic conductivity, synergistically improving the lithium ion transmission efficiency and improving low-temperature discharge performance.

[0058] In the present invention, the electrolyte further includes lithium bis(trifluoromethanesulfonimide) (LiTFSI), and the content of the lithium bis(trifluoromethanesulfonimide) is w8, 2%≤w8≤10%, based on the total weight of the electrolyte, for example, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 6%, 7%, 8%, 9% or 10%.

[0059] In one embodiment, 5%≤w8≤10%.

[0060] LiTFSI's molecular structure contains highly electronegative fluorine atoms and stable sulfonyl groups (-SO2-). Its thermal decomposition temperature (>350°C) is significantly higher than that of traditional lithium salts (such as LiPF6, which has a decomposition temperature of approximately 200°C). It is not easily decomposed during high-temperature storage and charge-discharge processes, reducing gas production (such as CO, CO2) and side reactions. The electrolyte system, which uses a highly spherical silicon-carbon material with LiTFSI and a fluorinated solvent (such as FEC and / or ethyl difluoroacetate), can utilize the electrolyte system's low viscosity (<10mPa·s) and high dissociation degree (>90%) of the solvation structure to reduce the desolvation energy barrier of lithium ions at high temperatures (<40kJ / mol), alleviating concentration polarization during the lithium insertion / delithiation process. At the same time, LiTFSI's high lithium ion transference number (>0.6) can reduce the electrolyte viscosity and increase the lithium ion transfer rate in silicon-carbon, improving high-temperature performance and charge-discharge rate performance (for example, 2C capacity retention rate >80%). High-silicon materials with higher sphericity have more uniform stress distribution, and their isotropic expansion can reduce the generation of local cracks, inhibit the peeling of negative electrode active materials and negative electrode current collectors, reduce the risk of local heat generation, and improve the high-temperature performance of the battery.

[0061] In the present invention, the content w5 of the ethylene carbonate, the content w6 of the non-fluorocarbonate solvent, the content w7 of the sulfur-containing additive and the content w8 of the lithium bis(trifluoromethanesulfonimide) can be obtained by conventional testing methods in the art, for example, by gas chromatography or gas chromatography-mass spectrometry.

[0062] In the present invention, the second coating further includes second silicon carbon, the average particle size of the second silicon carbon is R2, 2μm≤R2≤6μm (for example, 2μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm or 6μm).

[0063] In one embodiment, the average particle size of the second silicon carbon is R2, 3 μm≤R2≤5 μm.

[0064] In the present invention, the sphericity of the second silicon carbon is α2, 0.8≤α2≤1, for example, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98 or 1.

[0065] In one embodiment, 0.8≤α1≤1.

[0066] In one embodiment, 0.9≤α2≤1.

[0067] In the present invention, the mass content of silicon in the second silicon-carbon is 20%-45%, for example, 20%, 22%, 24%, 26%, 28%, 30%, 35%, 40% or 45%.

[0068] like Figure 1 The figure shows a cross-sectional SEM image of the negative electrode sheet in one embodiment of the present invention. It can be seen from the figure that the second coating layer away from the negative electrode current collector has silicon carbon particles (grayish white) with high sphericity. The larger particle size is the first silicon carbon, and the smaller particle size is the second silicon carbon.

[0069] A first silicon carbon with a higher sphericity is set in the second coating, and a second silicon carbon can be further set. When the volume expands, the expansion stress is evenly dispersed, reducing particle breakage and cracks. The second carbon-based material can serve as a buffer matrix to buffer the stress generated by the expansion of silicon carbon, improve structural stability, and thus optimize the cycle stability of the battery; the silicon-carbon material has high sphericity, a smooth surface, and low friction resistance. The first silicon carbon with a larger particle size is embedded in the second carbon-based material to form a skeleton structure, and the second silicon carbon with a smaller particle size is filled in the gaps in the skeleton, thereby improving the packing density of the particles in the second coating, increasing the compaction density, making the pores in the second coating evenly distributed, accelerating lithium ion deintercalation, and improving rate performance.

[0070] In the present invention, the average particle size of the second silicon carbon can be obtained by conventional testing methods in the field, for example, by the following method: discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly taking the negative electrode sheet, polishing the cross section of the negative electrode sheet with an argon ion grinder, and imaging the obtained cross section using a backscattering imaging mode in a scanning electron microscope (SEM); the silicon carbon particles in the image appear grayish white, and then the particle size measurement and distribution statistics are performed using electron microscope image analysis software such as ImageJ to obtain the average particle size of the second silicon carbon.

[0071] In the present invention, the sphericity of the second silicon carbon can be obtained by conventional testing methods in the art, for example, by the following method: discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly taking the negative electrode sheet, polishing the cross section of the negative electrode sheet using an argon ion grinder, and imaging the obtained cross section using a scanning electron microscope (SEM) device using a backscattering imaging mode; analyzing the image of the second silicon carbon located in the first coating layer in the SEM photograph (backscattering mode) of the negative electrode active layer at a certain magnification (such as 2500 times) using image processing software such as (Image Pro Plus), first measuring the average particle size to determine the second silicon carbon, obtaining the perimeter and area of ​​each particle, and calculating the perimeter equivalent radius r1' and area equivalent radius r2' of each second silicon carbon particle respectively, then the sphericity S' of each particle = r2' / r1', and then performing a quantity-weighted average of the sphericity of each particle to obtain the sphericity of the second silicon carbon.

[0072] In the present invention, the mass content of silicon in the first silicon-carbon and the mass content of silicon in the second silicon-carbon can be obtained by conventional testing methods in the art, for example, by the following method: using scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) analysis; the battery is discharged at a low rate to empty, the battery is disassembled, the negative electrode sheet is removed, and after argon ion polishing technology, a cross-sectional sample of the negative electrode sheet can be obtained. The cross-section is then tested in a backscattered mode of a scanning electron microscope at a magnification of 10K. The middle area of ​​a single silicon-carbon particle is selected for EDS analysis, and only the carbon and silicon contents are tested, wherein the weight percentage of silicon in the silicon-carbon is γ. The first silicon-carbon and the second silicon-carbon are distinguished by their average particle size, and the silicon content is calculated separately. The weighted average of 10 first silicon-carbon particles and second silicon-carbon particles is taken to obtain the obtained value.

[0073] In the present invention, the first coating also includes a first carbon-based material, and the average particle size of the first carbon-based material is 6μm-30μm (for example, 6μm, 7μm, 8μm, 9μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 22μm, 24μm, 26μm, 28μm or 30μm).

[0074] In the present invention, the second coating also includes a second carbon-based material, and the average particle size of the second carbon-based material is 2μm-10μm (for example, 2μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm).

[0075] In one embodiment, the average particle size of the first carbon-based material is 15 μm-25 μm, and the average particle size of the second carbon-based material is 5 μm-15 μm.

[0076] In the present invention, the first carbon-based material and the second carbon-based material each independently include artificial graphite.

[0077] The negative electrode active layer includes a first coating layer and a second coating layer. The first coating layer is close to the negative electrode current collector, while the second coating layer is away from the negative electrode current collector. The second carbon-based material located in the second coating layer has a smaller particle size, preferentially contacts the electrolyte, shortens the diffusion path of lithium ions, has better kinetics, and meets the needs of fast charging. The first carbon-based material located in the first coating layer has a large average particle size and does not contain silicon-carbon materials. The number of lithium ions migrating over long distances is reduced. In addition, the large-particle carbon-based material has larger inter-particle gaps, which is conducive to the infiltration of the electrolyte and reduces the capacity decay during the cycle.

[0078] In the present invention, the average particle size of the first carbon-based material, the average particle size of the second carbon-based material, the average particle size of the second carbon-based material and the average particle size of the second silicon-carbon can be obtained by conventional testing methods in the field, for example, by the following method: discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet, or directly take the negative electrode sheet, polish the cross section of the negative electrode sheet with an argon ion grinder, and use the backscattering imaging mode to image the obtained cross section in a scanning electron microscope (SEM) device; the first coating and the second coating are distinguished in the obtained cross-sectional SEM, wherein the silicon-carbon material appears off-white, and then the particle size is measured and the distribution statistics are performed using electron microscope image analysis software such as ImageJ, and the first silicon-carbon and the second silicon-carbon are distinguished and the average particle size is calculated; the first carbon-based material and the second carbon-based material appear off-black, and then the particle size of the first carbon-based material located in the first coating and the second carbon-based material located in the second coating are measured and the distribution statistics are performed using electron microscope image analysis software such as ImageJ to obtain their respective average particle sizes.

[0079] In the present invention, the compaction density of the negative electrode sheet is 1.6 g / cm 3 -1.9g / cm 3 , for example 1.6 g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 , 1.75g / cm 3 , 1.8g / cm 3 , 1.85g / cm 3 or 1.9 g / cm 3 .

[0080] In the present invention, the compacted density of the first coating is 1.65 g / cm 3 -1.7g / cm 3 , for example 1.65 g / cm 3 , 1.66g / cm 3 , 1.67g / cm 3 , 1.68g / cm 3 , 1.69g / cm 3 or 1.7 g / cm 3 .

[0081] In the present invention, the compacted density of the second coating is 1.7 g / cm 3 -1.75g / cm 3 , for example 1.7 g / cm 3 , 1.71g / cm 3 , 1.72g / cm 3 , 1.73g / cm 3 , 1.74g / cm3 or 1.75g / cm 3 .

[0082] The compacted density of the negative electrode sheet refers to the actual compacted density measured after the battery is disassembled and removed. This is different from the compacted density obtained by roller-pressing the negative electrode sheet during battery production. This is because after the negative electrode sheet is assembled into a battery, there will be physical and chemical rebound, resulting in compaction rebound, making the actual compacted density of the negative electrode sheet measured after battery production lower than the compacted density obtained after roller pressing.

[0083] In the present invention, the compaction density of the negative electrode sheet can be tested by conventional methods in the field. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and taken out, and after soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet, and then the residue on the surface of the negative electrode sheet is washed with deionized water. After drying, at least 20 sites are selected on the negative electrode sheet, and the thickness of the negative electrode sheet at each site is measured using a micrometer, and the average value h (unit is μm) is taken; the negative electrode sheet is punched into a disc with a diameter of 44.3 mm using a punching mold, 10 discs are taken, the mass of each disc is weighed, and the average value m (unit is mg) is taken to calculate the surface density M1 = (m-m1) × 100 / 1540.25, where m1 is the mass of the negative electrode collector in the disc, unit is mg, and the unit of surface density is mg / cm 2 The compaction density is calculated using the surface density M1 and the thickness average h, using the following formula: compaction density = M1 × 20 / (h-thickness of the negative electrode current collector), where the unit of the thickness of the negative electrode current collector is μm.

[0084] In the present invention, the ratio of the thickness of the first coating layer to the second coating layer is (0.5-4):1, for example, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.5:1, 3:1, 3.5:1 or 4:1.

[0085] In the present invention, the thickness of the first coating layer is 10 μm-40 μm, for example, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm.

[0086] In the present invention, the thickness of the second coating layer is 5 μm-40 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 24 μm, 28 μm, 30 μm, 35 μm or 40 μm.

[0087] It can be understood that the thickness of the first coating layer and the thickness of the second coating layer are both the thickness of a single side of the negative electrode current collector.

[0088] The first coating has a low overall compaction density and a large average particle size, which improves the overall energy density of the negative electrode. The second coating has a high compaction density and a small average particle size, which provides more complete contact with the electrolyte and better dynamics. Adjusting the thickness ratio of the first and second coatings can achieve a balance between high energy density, good dynamics, and interfacial stability.

[0089] In the present invention, the thickness of the first coating and the thickness of the second coating can be obtained by conventional testing methods in the field, for example, by the following method: discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, soaking it in DMC solvent for 12 hours, and then rinsing it with DMC solvent to remove the lithium salt attached to the negative electrode sheet, drying the negative electrode sheet and polishing its cross section with an argon ion grinder, imaging the obtained cross section using a backscattering imaging mode in a scanning electron microscope (SEM) device, randomly selecting at least 10 different silicon-carbon particles closest to the negative electrode current collector along the thickness direction, measuring the distance from the lowest point of each particle to the negative electrode current collector as the thickness of the second coating, and measuring the distance from the lowest point of each particle to the surface of the negative electrode active layer as the thickness of the first coating, calculating the average value respectively, and obtaining the thickness of the first coating and the thickness of the second coating.

[0090] In the present invention, the first silicon-carbon and the second silicon-carbon each independently include a porous carbon matrix and silicon materials located in the pores of the porous carbon matrix.

[0091] In the present invention, the first coating layer includes a first negative electrode active material including a first carbon-based material; the second coating layer includes a second negative electrode active material including a second carbon-based material, a first silicon-carbon, and a second silicon-carbon.

[0092] By regulating the mass content of silicon elements in the first silicon-carbon and the second silicon-carbon, it is possible to control the volume expansion rate while maintaining a high specific capacity, avoid the collapse of the negative electrode structure, embed the silicon material between the layers of the carbon-based material or in the porous carbon matrix, and use graphite to buffer the expansion stress of silicon. At the same time, the lithium embedding properties of graphite provide stable capacity.

[0093] In the present invention, the lithium-ion secondary battery further comprises a diaphragm, which comprises a base film and a nitrogen-containing coating located on at least one side of the base film opposite to each other in the thickness direction. The diaphragm further comprises a glue layer located on at least one outer surface of the diaphragm.

[0094] In the present invention, the nitrogen-containing coating comprises at least one of melamine and its derivatives, polyacrylonitrile and nitrile rubber.

[0095] In one embodiment, the melamine and its derivatives include at least one of melamine, melamine cyanurate, thiocyanate and its derivatives.

[0096] In one embodiment, the melamine and its derivatives include at least one of melamine and melamine cyanurate.

[0097] It can be understood that the derivative refers to a group introduced into the above-mentioned substance, and the beneficial effects of the present invention can be achieved as long as the derivative has a cyano group.

[0098] In the present invention, the lithium-ion secondary battery further includes a positive electrode sheet, and the nitrogen-containing coating is disposed opposite to the positive electrode sheet.

[0099] In one embodiment, the glue layer is located on a surface of the nitrogen-containing coating layer facing the positive electrode sheet and facing away from the base film, and on a surface of the base film facing the negative electrode sheet.

[0100] In one embodiment, the glue layer is located on a surface of the nitrogen-containing coating layer facing the positive electrode sheet and away from the base film, and on a surface of the nitrogen-containing coating layer facing the negative electrode sheet and away from the base film.

[0101] The nitrogen-containing coating has strong lithium affinity and can regulate Li by coordinating with lithium ions. + The flux distribution of melamine can suppress the phase change and transition metal dissolution problems caused by excessive local current density at the positive electrode interface, especially the triazine ring structure in melamine and its derivatives. Its lithiophilic surface can preferentially adsorb lithium ions in the electrolyte, reduce the desolvation energy barrier of lithium ions, and alleviate the interface kinetic hysteresis. The triazine group combines with free PF6, HF and other substances in the electrolyte to reduce the chemical corrosion of the positive electrode active material, alleviate interface degradation, improve the positive electrode interface and enhance high-temperature cycle stability. Melamine and its derivatives also have a high decomposition temperature and a rigid molecular skeleton, which can improve the stability of the diaphragm under high temperature and high pressure, inhibit the oxidation and decomposition of the electrolyte to produce gas during high-temperature cycles, and reduce the risk of thermal runaway at the interface.

[0102] Placing the nitrogen-containing coating opposite the positive electrode sheet is beneficial for capturing HF and H2O residues in the electrolyte, reducing corrosion to the positive electrode active material, and improving the stability of the positive electrode structure.

[0103] In the present invention, the charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V.

[0104] In the present invention, the battery further comprises a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector, the positive electrode active layer comprising a positive electrode active material, the positive electrode active material comprising lithium cobalt oxide and / or a Lia Ni x Co y M k Substances of O2, where 0.85 ≤ a ≤ 1.1, 0.3 < x ≤ 0.9, 0 ≤ y ≤ 0.5, 0.1 ≤ z ≤ 0.5, 0 ≤ k ≤ 0.15; M is at least one of Mg, Zn, Ga, Ba, Al, Fe, Cr, Sn, V, Mn, Sc, Ti, Nb, Mo, and Zr.

[0105] In one embodiment, the positive electrode active material includes lithium cobalt oxide.

[0106] In the present invention, the electrolyte further includes propylene carbonate (PC) and / or non-fluorinated carboxylic acid ester solvents.

[0107] Preferably, the non-fluorinated carboxylic acid ester solvent is selected from one or more of the following solvents: propyl acetate (PA), n-butyl acetate (n-BA), isobutyl acetate, n-pentyl acetate, isoamyl acetate, ethyl propionate (EP), propyl propionate (PP), methyl butyrate, ethyl butyrate, and n-ethyl butyrate.

[0108] In the present invention, the term "0% SOC" refers to the battery being discharged to 2.7V at 0.1C.

[0109] It should be noted that in the present invention, the numerical representations such as "first", "second", etc. are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0110] The present invention will be described in detail below through examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all the examples. All other examples obtained by those of ordinary skill in the art based on the examples in the present invention without creative efforts fall within the scope of protection of the present invention.

[0111] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0112] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0113] Example 1:

[0114] Prepare a lithium-ion secondary battery according to the following method:

[0115] (1) Preparation of the electrolyte:

[0116] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), based on the total weight of the electrolyte, 54% of a fluorinated solvent (w1 is 54%), 0% of EC (w5 is 0%), and 13% of DEC (w6 is 13%) were mixed, and then added to PC and mixed uniformly to form an organic solvent, wherein the fluorinated additive consisted of 15% of FEC (w3 is 14%) and 40% of DFEA (w4 is 40%);

[0117] Based on the total weight of the electrolyte, 5.2% of a nitrile compound (w2 is 5.2%, composed of SN, ADN and HTCN in a mass ratio of 2:1:2), 4.2% of a sulfur-containing additive (w7 is 4.2%, including sulfur-containing additives represented by formula I-1 and formula I-9, the content of formula I-1 is 1%, and the content of formula I-9 is 3.2%), and 7.1% of LiTFSI (w8 is 7.1%) are added to the organic solvent to form a mixed solvent;

[0118] 12% of lithium hexafluorophosphate, a lithium salt, based on the total weight of the electrolyte, was added to the above mixed solvent and stirred evenly. After passing the moisture and free acid tests, an electrolyte was obtained, and the sum of the weight contents of the components in the electrolyte was 100%.

[0119] (2) Preparation of negative electrode sheet:

[0120] A first carbon-based material (artificial graphite, with an average particle size of 21.5 μm) was used as a first negative electrode active material. The first negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), SuperP, and single-walled carbon nanotubes were mixed in a mass ratio of 95:2:2:0.75:0.25. An appropriate amount of water was added as a solvent, and the mixture was stirred uniformly in a vacuum mixer to obtain a first negative electrode slurry.

[0121] The second carbon-based material (artificial graphite, average particle size of 5.4 μm), the first silicon carbon (sphericity of 0.85, R1 of 7.8 μm, silicon content of 42%) and the second silicon carbon (sphericity of 0.93, R2 of 4.3 μm, silicon content of 30%) were mechanically mixed for 5 minutes at a mass ratio of 0.7:0.15:0.15, and mixed evenly as the second negative electrode active material. The second negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), SuperP and single-walled carbon nanotubes were mixed at a mass ratio of 95:2:2:0.75:0.25, and an appropriate amount of water was added as a solvent. After stirring evenly with a vacuum mixer, a second negative electrode slurry was obtained;

[0122] The first negative electrode slurry was evenly coated on both sides of the high-strength carbon-coated copper foil to form a first coating. The second negative electrode slurry was then evenly coated on the surface of the first coating to form a second coating. After coating, the film was transferred to a vacuum drying oven and baked at 85°C for 12 hours. After roller pressing and die cutting, the negative electrode sheet was obtained. The compaction density of the negative electrode sheet was 1.75g / cm 3 .

[0123] (3) Preparation of positive electrode sheet:

[0124] Lithium cobalt oxide (LCO), polyvinylidene fluoride (PVDF), acetylene black, and single-walled carbon nanotubes (CNTs) were mixed in a mass ratio of 97:1.5:1.25:0.25. N-methylpyrrolidone (NMP) was added as a solvent and stirred uniformly in a vacuum mixer to produce a positive electrode slurry. The slurry was evenly coated on both sides of a 10μm-thick high-strength aluminum foil. After coating, the foil was dried in a vacuum oven at 95°C for 12 hours. The positive electrode sheets were then rolled and die-cut.

[0125] (4) Preparation of diaphragm:

[0126] Polyacrylonitrile, melamine cyanurate, and sodium polyacrylate are mixed in a mass ratio of 72:24:6 to obtain a nitrogen-containing coating slurry; polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), and polyvinyl pyrrolidone (PVP) are mixed in a mass ratio of 40:44:15:1 to obtain a glue layer slurry. A 4mm base film is prepared and rolled up using a wet biaxial stretching process. The rolled base film is unwound into a coating area, and the nitrogen-containing coating slurry is applied to one side of the base film to obtain a nitrogen-containing coating. The water is dried in an oven, and the glue layer slurry is then sprayed onto the surface of the nitrogen-containing coating and the other surface of the base film to obtain a glue layer. Finally, the water is dried in an oven and rolled up to obtain a diaphragm.

[0127] (5) Preparation of batteries:

[0128] The positive electrode sheet, separator and negative electrode sheet are stacked in order, ensuring that the separator is located between the positive and negative electrode sheets to play an isolating role and the side of the separator with the nitrogen-containing coating is opposite to the positive electrode sheet, and then a bare cell without liquid injection is obtained by winding; the bare cell is placed in an outer packaging foil, and the electrolyte prepared in (1) is injected into the dried bare cell. After vacuum packaging, standing, forming, shaping, sorting and other processes, the desired lithium-ion battery is obtained.

[0129] Among them, w6 / w1 is 0.241, the thickness of the first coating is 24 μm, the thickness of the second coating is 8.2 μm, and the ratio of the thickness of the first coating to the second coating is 2.93:1.

[0130] Example 2:

[0131] Prepare lithium-ion secondary batteries according to the following method:

[0132] (1) Preparation of electrolyte:

[0133] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), based on the total weight of the electrolyte, 42% of a fluorinated solvent (w1 is 42%), 0% of EC (w5 is 0%), and 20% of DEC (w6 is 20%) were mixed, and then added to PC and mixed uniformly to form an organic solvent, wherein the fluorinated additive consisted of 17% of FEC (w3 is 17%) and 25% of DFEA (w4 is 25%);

[0134] Based on the total weight of the electrolyte, 6% of a nitrile compound (w2 is 6%, composed of SN, ADN and HTCN in a mass ratio of 1:1:3), 5% of a sulfur-containing additive (w7 is 5%, including sulfur-containing additives represented by formula I-1 and formula I-9, the content of formula I-1 is 1%, and the content of formula I-9 is 4%), and 9.8% of LiTFSI (w8 is 9.8%) are added to the organic solvent to form a mixed solvent;

[0135] 12% of lithium hexafluorophosphate, a lithium salt, based on the total weight of the electrolyte, was added to the above mixed solvent and stirred evenly. After passing the moisture and free acid tests, an electrolyte was obtained, and the sum of the weight contents of the components in the electrolyte was 100%.

[0136] (2) Preparation of negative electrode sheet:

[0137] A first carbon-based material (artificial graphite, with an average particle size of 15.2 μm) was used as a first negative electrode active material. The first negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), SuperP, and single-walled carbon nanotubes were mixed in a mass ratio of 95:2:2:0.75:0.25. An appropriate amount of water was added as a solvent, and the mixture was stirred uniformly in a vacuum mixer to obtain a first negative electrode slurry.

[0138] The second carbon-based material (artificial graphite, average particle size of 8.6 μm), the first silicon carbon (sphericity of 0.89, R1 of 7 μm, silicon content of 35%) and the second silicon carbon (sphericity of 0.9, R2 of 3.2 μm, silicon content of 25%) were mechanically mixed for 5 minutes at a mass ratio of 0.7:0.15:0.15, and mixed evenly as the second negative electrode active material. The second negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), SuperP and single-walled carbon nanotubes were mixed at a mass ratio of 95:2:2:0.75:0.25, and an appropriate amount of water was added as a solvent. After stirring evenly with a vacuum mixer, the second negative electrode slurry was obtained;

[0139] The first negative electrode slurry was evenly coated on both sides of the high-strength carbon-coated copper foil to form a first coating. The second negative electrode slurry was then evenly coated on the surface of the first coating to form a second coating. After coating, the film was transferred to a vacuum drying oven and baked at 85°C for 12 hours. After roller pressing and die cutting, the negative electrode sheet was obtained. The compaction density of the negative electrode sheet was 1.62g / cm 3 .

[0140] (3) Preparation of positive electrode sheet:

[0141] Lithium cobalt oxide (LCO), polyvinylidene fluoride (PVDF), acetylene black, and single-walled carbon nanotubes (CNTs) were mixed in a mass ratio of 97:1.5:1.25:0.25. N-methylpyrrolidone (NMP) was added as a solvent and stirred uniformly in a vacuum mixer to produce a positive electrode slurry. The slurry was evenly coated on both sides of a 10μm-thick high-strength aluminum foil. After coating, the foil was dried in a vacuum oven at 95°C for 12 hours. The positive electrode sheets were then rolled and die-cut.

[0142] (4) Preparation of diaphragm:

[0143] Polyacrylonitrile, melamine cyanurate, and sodium polyacrylate are mixed in a mass ratio of 72:24:6 to obtain a nitrogen-containing coating slurry; polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), and polyvinyl pyrrolidone (PVP) are mixed in a mass ratio of 40:44:15:1 to obtain a glue layer slurry. A 4mm base film is prepared and rolled up using a wet biaxial stretching process. The rolled base film is unwound into a coating area, and the nitrogen-containing coating slurry is applied to one side of the base film to obtain a nitrogen-containing coating. The water is dried in an oven, and the glue layer slurry is then sprayed onto the surface of the nitrogen-containing coating and the other surface of the base film to obtain a glue layer. Finally, the water is dried in an oven and rolled up to obtain a diaphragm.

[0144] (5) Preparation of batteries:

[0145] The positive electrode sheet, separator and negative electrode sheet are stacked in order, ensuring that the separator is located between the positive and negative electrode sheets to play an isolating role and the side of the separator with the nitrogen-containing coating is opposite to the positive electrode sheet, and then a bare cell without liquid injection is obtained by winding; the bare cell is placed in an outer packaging foil, and the electrolyte prepared in (1) is injected into the dried bare cell. After vacuum packaging, standing, forming, shaping, sorting and other processes, the desired lithium-ion battery is obtained.

[0146] Among them, w6 / w1 is 0.476, the thickness of the first coating is 19.6 μm, the thickness of the second coating is 10.4 μm, and the ratio of the thickness of the first coating to the second coating is 1.88:1.

[0147] Example 3:

[0148] Prepare lithium-ion secondary batteries according to the following method:

[0149] (1) Preparation of electrolyte:

[0150] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), based on the total weight of the electrolyte, 62% of a fluorinated solvent (w1 is 62%), 2% of EC (w5 is 2%), and 10% of DEC (w6 is 10%) were mixed, and then added to PC and mixed uniformly to form an organic solvent, wherein the fluorinated additive consisted of 15% of FEC (w3 is 15%) and 47% of DFEA (w4 is 47%);

[0151] Based on the total weight of the electrolyte, 3% of a nitrile compound (w2 is 3%, composed of SN, ADN and HTCN in a mass ratio of 1:2:2), 3% of a sulfur-containing additive (w7 is 3%, including sulfur-containing additives represented by formula I-1 and formula I-9, the content of formula I-1 is 1%, and the content of formula I-9 is 2%), and 5% of LiTFSI (w8 is 5%) are added to the organic solvent to form a mixed solvent;

[0152] 12% of lithium hexafluorophosphate, a lithium salt, based on the total weight of the electrolyte, was added to the above mixed solvent and stirred evenly. After passing the moisture and free acid tests, an electrolyte was obtained, and the sum of the weight contents of the components in the electrolyte was 100%.

[0153] (2) Preparation of negative electrode sheet:

[0154] A first carbon-based material (artificial graphite, with an average particle size of 24.7 μm) was used as a first negative electrode active material. The first negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), SuperP, and single-walled carbon nanotubes were mixed in a mass ratio of 95:2:2:0.75:0.25. An appropriate amount of water was added as a solvent, and the mixture was stirred uniformly in a vacuum mixer to obtain a first negative electrode slurry.

[0155] The second carbon-based material (artificial graphite, average particle size of 14.8 μm), the first silicon carbon (sphericity of 0.82, R1 of 12.7 μm, silicon content of 50%) and the second silicon carbon (sphericity of 0.95, R2 of 5 μm, silicon content of 30%) were mechanically mixed for 5 minutes at a mass ratio of 0.7:0.15:0.15, and mixed evenly as the second negative electrode active material. The second negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), SuperP and single-walled carbon nanotubes were mixed at a mass ratio of 95:2:2:0.75:0.25, and an appropriate amount of water was added as a solvent. After stirring evenly with a vacuum mixer, a second negative electrode slurry was obtained;

[0156] The first negative electrode slurry was evenly coated on both sides of the high-strength carbon-coated copper foil to form a first coating. The second negative electrode slurry was then evenly coated on the surface of the first coating to form a second coating. After coating, the foil was transferred to a vacuum drying oven and baked at 85°C for 12 hours. After roller pressing and die cutting, the negative electrode sheet was obtained. The compaction density of the negative electrode sheet was 1.79g / cm 3 .

[0157] (3) Preparation of positive electrode sheet:

[0158] Lithium cobalt oxide (LCO), polyvinylidene fluoride (PVDF), acetylene black, and single-walled carbon nanotubes (CNTs) were mixed in a mass ratio of 97:1.5:1.25:0.25. N-methylpyrrolidone (NMP) was added as a solvent and stirred uniformly in a vacuum mixer to produce a positive electrode slurry. The slurry was evenly coated on both sides of a 10μm-thick high-strength aluminum foil. After coating, the foil was dried in a vacuum oven at 95°C for 12 hours. The positive electrode sheets were then rolled and die-cut.

[0159] (4) Preparation of diaphragm:

[0160] Polyacrylonitrile, melamine cyanurate, and sodium polyacrylate are mixed in a mass ratio of 72:24:6 to obtain a nitrogen-containing coating slurry; polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), and polyvinyl pyrrolidone (PVP) are mixed in a mass ratio of 40:44:15:1 to obtain a glue layer slurry. A 4mm base film is prepared and rolled up using a wet biaxial stretching process. The rolled base film is unwound into a coating area, and the nitrogen-containing coating slurry is applied to one side of the base film to obtain a nitrogen-containing coating. The water is dried in an oven, and the glue layer slurry is then sprayed onto the surface of the nitrogen-containing coating and the other surface of the base film to obtain a glue layer. Finally, the water is dried in an oven and rolled up to obtain a diaphragm.

[0161] (5) Preparation of batteries:

[0162] The positive electrode sheet, separator and negative electrode sheet are stacked in order, ensuring that the separator is located between the positive and negative electrode sheets to play an isolating role and the side of the separator with the nitrogen-containing coating is opposite to the positive electrode sheet, and then a bare cell without liquid injection is obtained by winding; the bare cell is placed in an outer packaging foil, and the electrolyte prepared in (1) is injected into the dried bare cell. After vacuum packaging, standing, forming, shaping, sorting and other processes, the desired lithium-ion battery is obtained.

[0163] Among them, w6 / w1 is 0.161, the thickness of the first coating is 28 μm, the thickness of the second coating is 16.3 μm, and the ratio of the thickness of the first coating to the second coating is 1.72:1.

[0164] Example 4 group:

[0165] This set of examples is used to verify the impact of the change of "w1" by changing the content of FEC (w3) and the content of DFEA (w4), as follows:

[0166] Example 4a is based on Example 3, except that w1 is 64%, w3 is 10%, and w4 is 54%; in this case, w6 / w1 is 0.156;

[0167] Example 4b is based on Example 1, except that w1 is 31%, w3 is 10%, and w4 is 21%; in this case, w6 / w1 is 0.419;

[0168] Example 4c is based on Example 1, except that w1 is 44%, w3 is 19%, and w4 is 25%; in this case, w6 / w1 is 0.295.

[0169] Example 5 group:

[0170] This group of examples is used to verify the effect of the change of "w2", which is achieved by changing the content of nitrile compounds, as follows:

[0171] Example 5a, based on Example 1, except that w2 is 0.4%;

[0172] Example 5b is based on Example 1, except that w2 is 6.5%.

[0173] Example 6:

[0174] Based on Example 1, except that w5 is 3.5%.

[0175] Example 7:

[0176] This set of examples is used to verify the effect of the change of "w6 / w1", which is achieved by changing the content of DEC and / or the content of fluorinated solvent, as follows:

[0177] Based on Example 3, the difference is that w1 is 69%, w6 is 5%, w3 is 15%, and w4 is 54%; at this time, w6 / w1 is 0.072.

[0178] Example 8 group:

[0179] This set of examples is used to verify the impact of the change of "w7" by changing the content and / or type of sulfur-containing additives, as follows:

[0180] Example 8a is based on Example 1, except that w7 is 2.2%, wherein the sulfur-containing additive includes the sulfur-containing additives represented by Formula I-1 and Formula I-9, the content of Formula I-1 is 1%, and the content of Formula I-9 is 1.2%;

[0181] Example 8b is based on Example 1, except that w7 is 7%, w4 is 38.5%, and w1 is 52.5%; wherein the sulfur-containing additive includes the sulfur-containing additives represented by Formula I-1 and Formula I-9, the content of Formula I-1 is 1%, and the content of Formula I-9 is 6%;

[0182] Example 8c is based on Example 1, except that w7 is 4%; wherein the sulfur-containing additive includes the sulfur-containing additive represented by Formula I-9.

[0183] Example 9 group:

[0184] This group of examples is used to verify the impact of the change of "w8", which is achieved by changing the content of LiTFSI, as follows:

[0185] Example 9a is based on Example 1, except that w8 is 2%;

[0186] Example 9b is based on Example 1, except that w8 is 8.4%.

[0187] Example 10 group:

[0188] This set of examples is used to verify the impact of changes in "R1 and R2", as follows:

[0189] Example 10a is based on Example 1, except that R1 is 6.2 μm and R2 is 2.3 μm;

[0190] Example 10b is based on Example 1, except that R1 is 14.9 μm and R2 is 5.9 μm.

[0191] Example 11 group:

[0192] This set of examples is used to verify the impact of changes in the "average particle size of the first carbon-based material and the average particle size of the second carbon-based material", as follows:

[0193] Example 11a is based on Example 1, except that the average particle size of the first carbon-based material is 29.5 μm, and the average particle size of the second carbon-based material is 9.8 μm; at this time, the thickness of the first coating is 30 μm, the thickness of the second coating is 15 μm, and the ratio of the thickness of the first coating to the second coating is 2:1.

[0194] Example 11b is based on Example 1, except that the average particle size of the first carbon-based material is 6.3 μm, and the average particle size of the second carbon-based material is 2 μm. At this time, the thickness of the first coating is 30 μm, the thickness of the second coating is 15 μm, and the ratio of the thickness of the first coating to the second coating is 2:1.

[0195] Example 12 group:

[0196] This set of examples is used to verify the effect of changing the "ratio of the thickness of the second coating layer to the first coating layer" by changing the thickness of the second coating layer, as follows:

[0197] Example 12a is based on Example 1, except that the average particle size of the first carbon-based material is 6.3 μm, and the average particle size of the second carbon-based material is 2 μm. At this time, the thickness of the first coating is 12 μm, the thickness of the second coating is 24 μm, and the ratio of the thickness of the first coating to the second coating is 0.5:1.

[0198] Example 12b is based on Example 1, except that the thickness of the second coating layer is 6 μm, and the ratio of the thickness of the first coating layer to the second coating layer is 4:1.

[0199] Example 13 group:

[0200] This set of examples is used to verify the effects of changes in "the silicon content of the first silicon-carbon, the silicon content of the second silicon-carbon, and / or the blending amount of the first silicon-carbon and the second silicon-carbon", as follows:

[0201] Example 13a is based on Example 1, except that the first negative electrode active material is composed of a first carbon-based material, a first silicon-carbon (silicon content of 60%), and a second silicon-carbon (silicon content of 20%) in a mass ratio of 0.7:0.1:0.2;

[0202] Example 13b is based on Example 1, except that the first negative electrode active material is composed of a first carbon-based material, a first silicon-carbon (silicon content is 30%) and a second silicon-carbon (silicon content is 45%) in a mass ratio of 0.7:0.2:0.1.

[0203] Example 14:

[0204] Based on Example 1, the difference is that the nitrogen-containing coating of the separator is arranged back to back with the positive electrode sheet.

[0205] Example 15:

[0206] Based on Example 1, the difference is that the first negative electrode active material consists of a first carbon-based material and a first silicon-carbon in a mass ratio of 0.7:0.3.

[0207] In the lithium-ion secondary batteries prepared in Examples 1 to 15, the compaction density of the first coating layer was 1.65 g / cm 3 -1.7g / cm 3 In the range of 1.7 g / cm 3 -1.75g / cm 3 within the range.

[0208] Comparative Example 1:

[0209] Based on Example 1, the difference is that the first negative electrode active material is composed of the first carbon-based material and the second silicon-carbon in a mass ratio of 0.7:0.3.

[0210] Comparative Example 2:

[0211] Based on Example 1, the difference is that the first coating layer is away from the negative electrode current collector, and the second coating layer is close to the negative electrode current collector.

[0212] Comparative Example 3 groups:

[0213] This comparative example is used to verify the effect of the change in "w1" by changing the content of FEC and DEFA, as follows:

[0214] Comparative Example 3a is based on Example 3, except that w1 is 72%, w3 is 17%, w4 is 55%, and w6 is 2%, and w6 / w1 is 0.03;

[0215] Example 3b is based on Example 1, except that w1 is 26%, w3 is 12%, w4 is 14%, and w6 is 42%. In this case, w6 / w1 is 1.62.

[0216] Comparative Example 4:

[0217] This set of comparison ratios is used to verify the impact of the change in "w2", as follows:

[0218] Comparative Example 4a is based on Example 1, except that w2 is 0%;

[0219] Comparative Example 4b is based on Example 2, except that w2 is 11%, w3 is 15%, w4 is 23%, and w1 is 38%.

[0220] It can be understood that the “silicon content” mentioned in the above embodiments and comparative examples refers to the “mass content of silicon element in the corresponding silicon carbon”.

[0221] Test example:

[0222] (1) 45℃ high temperature cycle test:

[0223] The lithium-ion secondary batteries prepared in the Examples and Comparative Examples were subjected to a cycling test. The specific testing method is as follows: At 45°C, the battery was discharged at a rate of 1C to 3.0V, then allowed to stand for 10 minutes. It was then charged at a rate of 1C to 4.55V with a cutoff current of 0.025C and allowed to stand for 10 minutes. The battery was then discharged at a rate of 1.2C to a cutoff voltage of 3.0V and allowed to stand for 10 minutes. The discharge capacity at this point was recorded as the initial capacity. This charge and discharge process was repeated until the 600th cycle of constant voltage charging was completed and allowed to stand for 10 minutes. The battery was then discharged at a rate of 1.2C to 3.0V and allowed to stand for 10 minutes. The discharge capacity at this point was recorded as the final capacity. The cycle capacity retention ratio is calculated as follows: final capacity × 100% / initial capacity. The 45°C high-temperature cycle capacity retention ratio is reported in Table 1.

[0224] (2) 85℃ high temperature storage test:

[0225] The lithium-ion secondary batteries prepared in the examples and comparative examples were subjected to a high-temperature storage test. The specific test method is as follows: at 25°C ± 3°C, the fully charged battery was discharged at 0.5C to a lower limit voltage of 3.0V, allowed to stand for 10 minutes, and then fully charged at 0.5C constant current with a cutoff current of 0.02C. The battery was allowed to stand for 10 minutes. The thickness of the fully charged battery at 25°C ± 3°C was measured, and the initial thickness was recorded as h1. The battery was then placed in an environment of 85°C for 6 hours and then taken out. The thickness of the battery was measured and recorded as h2. The thickness expansion rate of the battery was recorded as (h2-h1) / h1×100% in Table 1.

[0226] (3) 0℃ low temperature discharge test:

[0227] The lithium-ion secondary batteries prepared in the Examples and Comparative Examples were subjected to a low-temperature discharge test. The specific test method is as follows: At 25°C ± 3°C, a fully charged battery was discharged at 0.5C to a lower voltage of 3.0V. The battery was allowed to rest for 10 minutes, and the initial discharge capacity, Q0, was recorded. The battery was then placed in a 0°C incubator and fully charged at a constant current of 0.5C, with a cutoff current of 0.02C. After 6 hours of storage, the battery was discharged at a constant current of 0.5C, and the discharge capacity, Q1, was recorded. The low-temperature discharge capacity retention ratio is calculated as Q1 / Q0 × 100%. This is reported in Table 1.

[0228] Table 1:

[0229]

[0230]

[0231] It can be seen from Table 1 that the lithium ion battery prepared by the present invention has better high-temperature cycle performance and high-temperature storage performance than the comparative example, and its low-temperature discharge is improved.

[0232] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that: The lithium-ion secondary battery includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector. The negative electrode active layer includes a first coating layer and a second coating layer provided along the thickness direction of the negative electrode sheet, wherein the first coating layer is located between the negative electrode current collector and the second coating layer. The second coating layer includes a first silicon carbon. The first silicon carbon has a sphericity of α1, and α1 is 0.8-1. The mass content of silicon element in the first silicon carbon is 30%-60%. The average particle size of the first silicon carbon is R1, and 6μm<R1≤15μm. The electrolyte includes a fluorine-containing solvent and a nitrile compound, wherein the fluorine-containing solvent includes fluoroethylene carbonate and / or ethyl difluoroacetate. Based on the total weight of the electrolyte, the content of the fluorine-containing solvent is w1, 30%≤w1≤70%; the content of the nitrile compound is w2, 0.4%≤w2≤7%.

2. The lithium ion secondary battery according to claim 1, wherein The nitrile compound includes at least one of succinonitrile, adiponitrile, glycerol trinitrile, 1,4-dicyano-2-butene and 1,3,6-hexane trinitrile; And / or, the ethyl difluoroacetate includes 2,2-difluoroethyl acetate and / or ethyl 2,2-difluoroacetate; and / or, based on the total weight of the electrolyte, the content of the fluoroethylene carbonate is w3, 10%≤w3≤20%; and / or, based on the total weight of the electrolyte, the content of ethyl difluoroacetate is w4, 20%≤w4≤60%; Preferably, based on the total weight of the electrolyte, the content of ethyl difluoroacetate is w4, 20%≤w4≤60%, and the electrolyte does not include ethylene carbonate.

3. The lithium ion secondary battery according to claim 2, wherein 40％≤w1≤62％； and / or, 0.5% ≤ w2 ≤ 6%; and / or, 14% ≤ w3 ≤ 17%; and / or, 25%≤w4≤50%.

4. The lithium ion secondary battery according to claim 1, wherein The electrolyte comprises ethylene carbonate, and the content of the ethylene carbonate is w5, w5≤5%, based on the total weight of the electrolyte; and / or, the electrolyte comprises a non-fluorocarbonate solvent, and the content of the non-fluorocarbonate solvent is w6, 5%≤w6≤60% based on the total weight of the electrolyte; Preferably, 5%≤w6≤20%; Preferably, w1 and w6 satisfy: 0.05≤w6 / w1<1; More preferably, w1 and w6 satisfy: 0.05≤w6 / w1≤0.

5.

5. The lithium ion secondary battery according to claim 1, wherein The electrolyte further comprises a sulfur-containing additive, wherein the content of the sulfur-containing additive is w7, 2%≤w7≤8% based on the total weight of the electrolyte; And / or, the electrolyte further comprises lithium bis(trifluoromethanesulfonyl)imide, and the content of the lithium bis(trifluoromethanesulfonyl)imide is w8, 2%≤w8≤10% based on the total weight of the electrolyte; Preferably, 2%≤w7≤5%; Preferably, 5%≤w8≤10%.

6. The lithium ion secondary battery according to claim 1, wherein The electrolyte further comprises a sulfur-containing additive, wherein the sulfur-containing additive comprises a first sulfur-containing additive and / or a second sulfur-containing additive, wherein the first sulfur-containing additive comprises At least one of; And / or, the second sulfur-containing additive comprises At least one of; Preferably, the second sulfur-containing additive comprises and / or, based on the total weight of the electrolyte, the content of the first sulfur-containing additive is 1%-2%; And / or, based on the total weight of the electrolyte, the content of the second sulfur-containing additive is 2%-6%.

7. The lithium ion secondary battery according to claim 1, wherein The first coating layer includes a first carbon-based material, and the average particle size of the first carbon-based material is 6 μm-30 μm; And / or, the second coating further comprises a second carbon-based material, and the average particle size of the second carbon-based material is 2 μm-10 μm; And / or, the first carbon-based material includes artificial graphite; And / or, the second carbon-based material includes artificial graphite.

8. The lithium ion secondary battery according to claim 1, wherein The second coating further includes a second silicon carbon, wherein the average particle size of the second silicon carbon is R2, 2 μm≤R2≤6 μm; And / or, the mass content of silicon in the second silicon-carbon is 20%-50%; and / or, the sphericity of the second silicon carbon is α2, 0.8≤α2≤1; Preferably, 0.8≤α1≤1, 0.9≤α2≤1; Preferably, the first silicon carbon comprises a porous carbon matrix and silicon material located in the pores inside the porous carbon matrix; Preferably, the second silicon carbon comprises a porous carbon matrix and carbon materials located in pores inside the porous carbon matrix.

9. The lithium ion secondary battery according to claim 1, wherein The lithium-ion secondary battery further includes a diaphragm, the diaphragm including a base film and a nitrogen-containing coating located on at least one surface of the base film opposite to each other in the thickness direction, and the diaphragm further includes a glue layer located on at least one outer surface of the diaphragm; and / or, the nitrogen-containing coating comprises at least one of melamine and its derivatives, polyacrylonitrile and nitrile rubber; Preferably, the melamine and its derivatives include at least one of melamine, melamine cyanurate and thiocyanate and its derivatives; Preferably, the lithium-ion secondary battery further comprises a positive electrode sheet, and the nitrogen-containing coating is arranged opposite to the positive electrode sheet.

10. The lithium ion secondary battery according to claim 1, wherein The charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V.

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