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 an electrolyte, the negative plate comprises a negative active layer, the negative active layer comprises a negative active material, and the negative active material comprises a silicon-based material; the silicon-based material comprises silicon-carbon particles and a coating layer located on the outer surfaces of the silicon-carbon particles; the coating layer comprises a first coating layer and a second coating layer; the first coating layer comprises amorphous carbon, and the second coating layer comprises lithium metaaluminate and further comprises at least one of aluminum oxide and aluminum fluoride; the electrolyte comprises a first solvent, and the first solvent comprises fluoro carboxylic ester. Volume expansion of the silicon carbon particles in the circulation process can be effectively buffered, the interface impedance is reduced, and the transmission rate of lithium ions is remarkably increased. The battery provided by the invention has excellent cycling stability and fast charging performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a lithium-ion secondary battery. Background Technology

[0002] With the rapid development of technology, the global energy landscape is undergoing profound changes, creating an increasingly urgent demand for high-performance batteries. However, the range and charging speed of existing batteries are insufficient to meet consumers' expectations for long-distance driving and convenient charging, necessitating batteries with high energy density and fast charging capabilities to enhance the user experience. One of the most common methods for increasing battery energy density is to incorporate silicon-based materials into the negative electrode active layer, utilizing silicon's high theoretical specific capacity (up to 4200 mAh / g at high temperatures and approximately 3580 mAh / g at room temperature) to increase the battery energy density to 300 Wh·kg. -1 above.

[0003] However, silicon-based materials undergo drastic volume expansion during cycling, leading to cracking of the negative electrode active layer and pulverization of the active material. This causes the solid electrolyte interphase (SEI) film to repeatedly rupture and regenerate, continuously consuming electrolyte and lithium source, thus deteriorating battery cycle performance. At the same time, the conductivity of silicon-based materials is much lower than that of graphite, resulting in a significant decrease in the ion or electron transport kinetics of the battery after adopting a silicon-based negative electrode. This manifests as an increased risk of lithium ion deposition (lithium plating), severely limiting the battery's fast charging capability.

[0004] Therefore, it is urgent to improve the problems of poor battery cycle stability and fast charging performance caused by the severe volume expansion of silicon-based materials and the low lithium-ion transport efficiency. Summary of the Invention

[0005] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery (hereinafter referred to as the battery). This invention, through synergistic improvement of the negative electrode and electrolyte, can effectively improve lithium-ion transport efficiency, significantly improve battery fast-charging performance, avoid severe lithium plating, and enhance battery cycle stability.

[0006] First, this invention improves the negative electrode sheet. The silicon-based material of this invention includes silicon-carbon particles and a coating layer on the outer surface of the silicon-carbon particles. The coating layer includes a first coating layer and a second coating layer. The synergistic effect of the two coating layers improves the structural stability of the negative electrode sheet and the cycle performance and rate performance of the battery. The first coating layer includes amorphous carbon, which has high electronic conductivity and elasticity, effectively buffering the volume expansion of silicon-carbon particles during lithium-ion intercalation / deintercalation, inhibiting particle agglomeration and negative electrode structure collapse, constructing a continuous electron transport channel, and improving electron conduction efficiency. The second coating layer is an aluminum-containing fast ion conductor coating layer (e.g., containing lithium aluminate, and at least one of aluminum oxide and aluminum fluoride). This coating layer, as a chemically stable protective layer, can isolate the silicon-based material from the electrolyte during the initial charge / discharge stage of the battery, before a dense and stable SEI film forms on the surface of the silicon-carbon particles, effectively suppressing the occurrence of side reactions. Electrolyte decomposition prevents the loss of active components in silicon-based materials due to interfacial reactions. Simultaneously, its excellent mechanical strength, combined with the elastic buffering effect of the first coating layer, further constrains the volume expansion of silicon-carbon particles during lithium-ion insertion / extraction, reduces stress concentration within the particles, minimizes the risk of silicon-carbon particle breakage and pulverization, and improves battery cycle stability. Furthermore, the aluminum-containing fast ion conductor itself possesses high ionic conductivity, and the inorganic SEI film it induces also exhibits high ionic conductivity, significantly enhancing the lithium-ion transport rate on and within the silicon-carbon particles, reducing interfacial impedance, and thus improving the battery's high-rate charge / discharge performance.

[0007] Secondly, this invention also improves the electrolyte by selecting a fluorocarboxylic acid ester with high oxidation resistance and low viscosity as the electrolyte solvent. This synergistic effect with the double-layer coating structure of the aforementioned silicon-based material significantly improves the battery's cycle performance and fast-charging performance. Specifically, the high oxidation resistance of the fluorocarboxylic acid ester can significantly improve the high-voltage stability of the electrolyte system, broaden the electrochemical window, and thus improve the high-voltage cycle stability of the battery. Furthermore, the local dipoles generated by the -CHF2 group in its molecule can enhance the Li... + Interaction with solvent molecules to optimize Li + The solvation structure enhances the migration efficiency of lithium ions in the electrolyte; on the other hand, the inherent low viscosity of the solvent allows it to quickly and fully wet the negative electrode, while the surface of the aluminum-containing fast ion conductor coating is rich in Lewis acid sites (such as Al). 3+It can adsorb fluorine atoms in fluorinated carboxylic acid esters and catalyze their preferential reduction on the surface of silicon carbon particles to form an SEI film containing more inorganic components (such as LiF). This not only inhibits negative electrode expansion and enhances interface stability, but also promotes faster transport of lithium ions to the surface of the bottom active material of the negative electrode, effectively alleviating the concentration polarization phenomenon of lithium ions, ensuring efficient insertion and extraction of lithium ions in the negative electrode material, and further improving the fast charging performance and cycle stability of the battery.

[0008] Based on this, the inventors of this invention propose the following solution: This invention provides a lithium-ion secondary battery, comprising a negative electrode and an electrolyte; the negative electrode includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-based material; the silicon-based material includes silicon-carbon particles and a coating layer located on the outer surface of the silicon-carbon particles; the coating layer includes a first coating layer and a second coating layer; the first coating layer includes amorphous carbon, the second coating layer includes lithium aluminate, and further includes at least one of aluminum oxide and aluminum fluoride; the electrolyte includes a first solvent, the first solvent including a fluorinated carboxylic acid ester.

[0009] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) By synergistically improving the negative electrode and electrolyte, the present invention can effectively buffer the volume expansion of silicon carbon particles during cycling, reduce the interface impedance, and significantly improve the lithium ion transport rate. (2) The battery of the present invention has excellent cycle stability and fast charging performance, and avoids the occurrence of serious lithium plating.

[0010] The endpoints and any values ​​of the ranges 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 endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Detailed Implementation

[0011] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0012] This invention provides a lithium-ion secondary battery, comprising a negative electrode and an electrolyte; the negative electrode includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-based material; the silicon-based material includes silicon-carbon particles and a coating layer located on the outer surface of the silicon-carbon particles; the coating layer includes a first coating layer and a second coating layer; the first coating layer includes amorphous carbon, the second coating layer includes lithium aluminate, and further includes at least one of aluminum oxide and aluminum fluoride.

[0013] Lithium aluminate, as an excellent lithium-ion conductor, can significantly improve lithium-ion transport efficiency and reduce interface impedance due to its high ionic conductivity. It can also promote the formation of a low-impedance SEI film, reducing the differences in lithium-ion transport kinetics on the surfaces of silicon-carbon particles of different sizes, resulting in a more uniform electrochemical reaction at the anode and reducing local polarization. When combined with alumina and / or aluminum fluoride, it can produce a synergistic effect, further optimizing the overall performance of the second coating layer, effectively improving the structural stability of the anode sheet, reducing the differences in lithium-ion transport kinetics on the surfaces of silicon-carbon particles, thereby suppressing local polarization and significantly improving the battery's cycle performance and fast-charging performance. Specifically: Alumina has good chemical stability, which can effectively block direct contact between the electrolyte and the negative electrode active material, inhibit the erosion of silicon carbon particles by the electrolyte, reduce side reactions, and enhance the mechanical strength of the second coating layer, thus alleviating the problem of coating layer cracking caused by volume expansion / contraction during the charging and discharging of silicon carbon negative electrodes; while aluminum fluoride can induce the formation of a stable solid electrolyte interface (SEI) film rich in lithium fluoride (LiF) at the negative electrode interface, optimize the composition and structure of the SEI film, improve the stability of the SEI film, further reduce the interfacial impedance, and improve the ionic conductivity.

[0014] In this invention, the thickness d1 of the second coating layer is 1nm-20nm (e.g., 1nm, 5nm, 10nm, 15nm or 20nm).

[0015] In one example, d1 is 2nm-10nm.

[0016] This invention, by controlling the thickness of the second coating layer within a specific range, can ensure the integrity and density of the coating layer, effectively isolating silicon-carbon particles from direct contact with the electrolyte, suppressing side reactions and stabilizing the SEI film, alleviating internal stress in the particles, and reducing the risk of cracking and pulverization. Simultaneously, it can ensure the continuity and unobstructedness of ion transport channels, maintain low interfacial impedance, and improve the battery's cycle stability and fast-charging performance. If the thickness of the second coating layer is too small (e.g., <1nm), it is difficult to form a complete and continuous dense coating layer, weakening its physical isolation and mechanical buffering effects, leading to continuous electrolyte decomposition, accelerated pulverization of active particles, and decreased interfacial stability. If the thickness of the second coating layer is too large (e.g., >20nm), although the protective effect is enhanced, the excessively thick dense layer will prolong the lithium-ion diffusion path, increase interfacial impedance, and may cause the coating layer itself to crack or peel off from the particle surface due to increased internal stress, damaging interfacial integrity and resulting in poor cycle performance and rate performance.

[0017] In this invention, the thickness of the second coating layer can be measured using conventional methods in the art. For example, the battery is discharged to 0% SOC (e.g., discharged to 2.7V), the negative electrode is disassembled and removed, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC solvent to remove lithium salts adhering to the negative electrode. After drying, the negative electrode active material powder is scraped off with a scraper. The powder is placed in ethanol solvent and ultrasonically dispersed. A microgrid copper mesh is then dipped into the dispersion, and after drying, an ultrathin sample is prepared. The thickness of the second coating layer is then obtained by observing the sample using a transmission electron microscope (TEM).

[0018] In this invention, the electrolyte includes a first solvent, which includes a fluorocarboxylic acid ester.

[0019] In one example, the fluorocarboxylic acid ester includes at least one of methyl difluoroacetate, ethyl difluoroacetate, 2,2-difluoroethyl acetate, propyl difluoroacetate, methyl trifluoroacetate, ethyl trifluoroacetate, and trifluoromethylbenzoate.

[0020] In one example, the fluorocarboxylic acid ester includes 2,2-difluoroethyl acetate.

[0021] In this invention, based on the total weight of the electrolyte, the mass content c1 of the fluorocarboxylic acid ester is 5%-60% (e.g., 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%).

[0022] In one instance, c1 is 10%-35%.

[0023] When the content of fluorocarboxylic acid esters in the electrolyte is too low (e.g., less than 5%), their properties of optimizing solvation structure, improving oxidation resistance, and reducing viscosity cannot be fully utilized. The electrolyte exhibits insufficient stability under high voltage, inadequate electrode wetting, and deterioration of SEI film stability, making it difficult to exert a synergistic promoting effect on high-voltage fast charging and negative electrode interface stability. Conversely, when the content of fluorocarboxylic acid esters in the electrolyte is too high (e.g., greater than 60%), it easily leads to poor compatibility of the electrolyte system, reduced lithium salt dissociation, and decreased ionic conductivity. It may also cause excessive adsorption onto Al. 3+ Excessive growth of the SEI film due to site-specific instability increases interfacial impedance. Meanwhile, excessive fluorine content may deteriorate electrolyte fluidity, weaken lithium-ion transport efficiency, and ultimately affect the battery's fast-charging performance.

[0024] In this invention, the mass content of fluorocarboxylic acid esters in the electrolyte can be obtained by methods conventional in the art, such as gas chromatography (GC) or gas chromatography-mass spectrometry (GCMS).

[0025] In this invention, the sphericity s of the silicon-based material is 0.5-1 (e.g., 0.5, 0.6, 0.7, 0.8, 0.9 or 1).

[0026] In one example, the average sphericity s of the silicon-based material is 0.7-1.

[0027] Spherical silicon-based materials typically exhibit better packing density and higher specific surface area, which can improve battery energy density, effectively mitigate the volume expansion problem of silicon-based materials during charge and discharge, reduce internal stress during battery cycling, minimize electrode structure damage caused by volume changes, and improve battery cycle stability. Secondly, the more regular surface of spherical silicon-based materials facilitates the formation of a stable solid electrolyte interphase (SEI) film, reducing the occurrence of side reactions and further enhancing battery cycle performance. Simultaneously, it can reduce the differences in lithium ion diffusion paths in different directions, facilitating electron conduction and ion transport, and improving battery conductivity and rate performance.

[0028] In this invention, the silicon-carbon particles comprise a porous carbon matrix and silicon material located in the pores of the porous carbon matrix.

[0029] In this invention, the particle size Dv50 of the silicon-based material is 3μm-14μm (e.g., 3μm, 5μm, 7μm, 9μm, 11μm, or 14μm); the particle size Dv99 is 15μm-30μm (e.g., 15μm, 17μm, 19μm, 21μm, 23μm, 25μm, 27μm, or 30μm). The particle size Dv50 and particle size Dv99 of the silicon-based material can be obtained by conventional methods in the art, such as laser particle size analyzer testing.

[0030] The silicon-based material described in this invention has a wide range of particle size and sphericity distributions. While this allows for a more compact packing of silicon-carbon particles, increasing the contact area and thus improving the compaction density and structural stability of the negative electrode, the wide particle size distribution also easily leads to significant differences in lithium-ion transport kinetics between particles. Specifically, small-diameter silicon-carbon particles exhibit higher lithium-ion insertion / extraction efficiency, while large-diameter silicon-carbon particles have a relatively slow lithium-ion migration rate, ultimately resulting in insufficient utilization of the active material and affecting the high-rate charge / discharge performance of the battery. Introducing a second coating layer on the surface of the silicon-carbon material effectively solves these problems: it buffers the stress generated by volume changes during the expansion and contraction of silicon-carbon particles during charge / discharge, reducing microcracks within the material and improving battery cycle stability; it also leverages the excellent lithium-ion conductivity of the second coating layer to reduce the lithium-ion transport impedance on the surface of silicon-carbon particles of different sizes, narrowing the differences in lithium-ion insertion / extraction kinetics between particles, improving the overall utilization of the active material, and further enhancing the high-rate charge / discharge performance of the battery.

[0031] In this invention, the sphericity of the silicon-based material can be tested using conventional methods in the art. For example, after disassembling the battery, the negative electrode is removed, cleaned with dimethyl carbonate (DMC), and dried. The cross-section of the negative electrode is polished using an argon-ion polishing machine, and then observed using backscatter imaging mode in an SEM device. Silicon-based material particles with continuous and smooth contours are found. Any two points on the edge of the particle are connected to form a straight line segment inside the particle. The longest straight line segment inside the particle is selected, and its length is denoted as Z1. The midpoint of this longest straight line segment is taken, and a straight line is drawn through this midpoint to form a straight line segment with both ends at the edge of the particle. The shortest straight line segment is selected, and its length is denoted as Z2. The sphericity of the particle is then Z2 / Z1. At least 10 silicon-based material particles are selected, and the sphericity is measured and the average value is taken.

[0032] In this invention, the negative electrode active layer includes a first coating and a second coating disposed 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.

[0033] In one example, the first coating comprises a first carbon-based material and the silicon-based material.

[0034] In one example, the second coating comprises a second carbon-based material and the silicon-based material.

[0035] In one example, the mass content of silicon element a1 in the first coating and the mass content of silicon element a2 in the second coating satisfy: a1≤a2.

[0036] In this invention, the mass content a1 of silicon element in the first coating is 0.5%-50% (e.g., 0.5%, 1%, 5%, 10%, 20%, 30%, 40% or 50%).

[0037] In one instance, a1 ranges from 1% to 40%.

[0038] In this invention, the mass content a2 of silicon element in the second coating is 0.5%-50% (for example, 0.5%, 1%, 5%, 10%, 20%, 30%, 40% or 50%).

[0039] In one instance, a2 is 2.5%-45%.

[0040] Because the lithium-ion diffusion rate of silicon-based materials is much lower than that of graphite materials, and the volume change rate of silicon-based materials during charge and discharge is much greater than that of graphite materials, in order to reduce the degree of lithium-ion concentration polarization during charge and discharge and improve the SEI film rupture and negative electrode structure damage caused by negative electrode volume expansion, the negative electrode active layer of this invention includes a first coating near the current collector surface and a second coating away from the current collector surface. The mass content of silicon in the second coating is greater than or equal to the mass content of silicon in the first coating. This not only allows the lithium-ion migration rate in the first coating to be faster and reduces lithium-ion concentration polarization during charge and discharge, but also enables the formation of a more inorganic SEI film in the second coating, increasing the mechanical strength of the second SEI film, effectively suppressing the volume expansion of the negative electrode active material, and thus improving the battery cycle performance.

[0041] In this invention, the mass content a1 of silicon in the first coating and the mass content a2 of silicon in the second coating can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC (e.g., discharged to 2.7V), the negative electrode is disassembled and removed, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode. After drying, the negative electrode is cut into test samples with a specification of 2×2mm. The cross-section of the test sample is observed using a scanning electron microscope (SEM), and the interface between the first coating and the second coating can be clearly observed. Then, an energy dispersive spectroscopy (EDS) instrument is used to select multiple different test points in the first coating and the second coating area on both sides of the interface, and qualitative and quantitative analysis of silicon is performed in sequence. The average value of the mass content of silicon at the corresponding test points of each coating is taken to obtain the mass content a1 of silicon in the first coating and the mass content a2 of silicon in the second coating.

[0042] In this invention, the thickness d2 of the first coating is 10μm-30μm (e.g., 10μm, 15μm, 20μm, 25μm or 30μm).

[0043] In one instance, d2 is 12μm-25μm.

[0044] In this invention, the thickness d3 of the second coating is 5μm-25μm (e.g., 5μm, 10μm, 15μm, 20μm or 25μm).

[0045] In one instance, d3 is 8μm-20μm.

[0046] In this invention, the first carbon-based material and the second carbon-based material each independently include at least one of graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0047] In this invention, the average particle size of the first carbon-based material is 6μm-30μm (e.g., 6μm, 10μm, 15μm, 20μm, 25μm or 30μm); the average particle size of the second carbon-based material is 2μm-10μm (e.g., 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm).

[0048] The negative electrode employs a layered coating technology, with both the first and second coatings comprising carbon-based materials. The first carbon-based material has a larger particle size, resulting in greater pressure resistance and better chemical stability. During electrode rolling, the larger particles, under stress diffusion, create greater gaps and spaces between them, facilitating electrolyte filling and wetting, and improving electrolyte transport rate. The second carbon-based material, located further away from the negative electrode current collector and with a smaller particle size, can directly contact the electrolyte, resulting in a shorter ion transport distance. This significantly reduces ion diffusion resistance, accelerates electrolyte wetting, improves the charging window, and ultimately enhances the kinetics of the negative electrode side. Therefore, by controlling the particle sizes of the first and second carbon-based materials, the battery can achieve both good cycle stability and rate performance.

[0049] In this invention, the thickness of the first coating and the thickness of the second coating can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC (e.g., discharged to 2.7V), the negative electrode is disassembled and removed, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode. After drying, the negative electrode is cut into test samples with a specification of 2×2mm. The cross-section of the test sample is observed using SEM, and the interface between the first coating and the second coating can be clearly observed. Ten test sites are randomly selected on the surface of the first coating and the second coating, and the thickness of each site is measured. The average value is taken to obtain the thickness of the first coating and the second coating.

[0050] In this invention, the average particle size of the first carbon-based material and the average particle size of the second carbon-based material can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. Alternatively, the negative electrode sheet before soaking in electrolyte can be directly taken, and the negative electrode sheet can be cut using an argon ion milling machine with a CP laser. Then, SEM is used for observation. At least 20 particles of the first carbon-based material and the second carbon-based material are randomly selected, and the particle size of each carbon-based material is measured and the average value is taken. When the particles in the mirror image are regular circles, the particle size is the diameter of the regular circle. When the particles in the mirror image are not "regular circles", any two points on the edge of the particle are connected to form a straight line segment inside the particle, and the longest straight line segment inside the particle is selected as the particle size.

[0051] In this invention, the electrolyte further includes a first compound.

[0052] In one example, the first compound includes at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium difluorooxalate phosphate.

[0053] In one example, the mass content c2 of the first compound is 0.05%-1.8% (e.g., 0.05%, 0.1%, 0.3%, 0.5%, 0.7%, 1%, 1.2%, 1.4%, 1.6% or 1.8%) based on the total weight of the electrolyte.

[0054] In one instance, c2 is 0.2%–1.3%.

[0055] The electrolyte of this invention contains an appropriate amount of a first compound. This first compound can not only form a LiF-containing SEI film on the negative electrode surface, but also generate some organofluorine compounds with low surface energy. These organofluorine compounds possess both excellent elasticity and low surface energy characteristics, which can effectively buffer the huge volume expansion during the charging and discharging process of the negative electrode, promote the surface smoothing of the SEI film, reduce surface roughness, and reduce microscopic defects. At the same time, it can also interact with other components in the SEI film (such as lithium salt decomposition products) to form a more uniform and stable interface structure. This stable interface structure can further promote the formation of a dense SEI film, thereby reducing the battery interface impedance, improving the lithium-ion interface transport efficiency, and further improving the fast charging performance of the battery.

[0056] In this invention, the mass content of the first compound in the electrolyte can be obtained by methods conventional in the art, such as GC or GCMS testing.

[0057] In this invention, the electrolyte further includes a second solvent; the second solvent includes at least one of the substances represented by Formula I: Formula I, R1 and R2 each independently include at least one of the following groups: C1-C10 alkyl, C2-C10 alkenyl, and C2-C10 alkynyl. For example, C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, C7 alkyl, C8 alkyl, C9 alkyl, C10 alkyl, C2 alkenyl, C3 alkenyl, C4 alkenyl, C5 alkenyl, C6 alkenyl, C7 alkenyl, C8 alkenyl, C9 alkenyl, C10 alkenyl, C2 alkynyl, C3 alkynyl, C4 alkynyl, C5 alkynyl, C6 alkynyl, C7 alkynyl, C8 alkynyl, C9 alkynyl, or C10 alkynyl.

[0058] In this invention, the term "C1-C10 alkyl" refers to an alkyl group having 1-10 carbon atoms; "C2-C10 alkenyl" refers to an alkenyl group having 2-10 carbon atoms; "C2-C10 alkynyl" refers to an alkynyl group having 2-10 carbon atoms, and so on.

[0059] In this invention, the second solvent includes , (Propyl propionate, PP) , , , , , , , , , , , , and At least one of them; In one example, the second solvent includes , , , and At least one of them.

[0060] In one example, the mass content c3 of the second solvent is 5%-50% (e.g., 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%) based on the total weight of the electrolyte.

[0061] In one instance, c3 is 10%-40%.

[0062] Compared to fluorocarboxylic acid esters, the second solvent has a lower viscosity, which can not only increase the migration rate of lithium ions in the electrolyte, but also wet the electrode more quickly, improve the utilization rate of electrode active materials, and further improve the cycle performance and fast charging performance of the battery.

[0063] In this invention, the mass content of the second solvent in the electrolyte can be obtained by methods conventional in the art, such as by GC or GCMS testing.

[0064] In this invention, the electrolyte further includes other organic solvents, electrolyte salts, and additives. The other organic solvents include carbonate solvents. The carbonate solvents include cyclic carbonate solvents and chain carbonate solvents. The cyclic carbonate solvents include at least one of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and butylene carbonate (BC). The chain carbonate solvents include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC). The electrolyte salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium di(trifluoromethanesulfonyl)imide, lithium di(pentafluoroethylsulfonyl)imide, lithium tri(trifluoromethanesulfonyl)methyl lithium, or lithium di(trifluoromethanesulfonyl)imide. The additives include at least one selected from vinylene carbonate, 1,3-propenesulfonyl lactone, vinyl ethylene carbonate, vinyl sulfate, succinate (SN), glutaronitrile, adiponitrile (ADN), heptonitrile, octanitrile, sebaconitrile, 1,3,6-hexanetrionitrile (HTCN), glycerol trionitrile, and 1,2-bis(2-cyanoethoxy)ethane.

[0065] In this invention, the negative electrode active layer further includes a negative electrode conductive agent and a negative electrode binder. The negative electrode conductive agent includes at least one of superconducting carbon black, acetylene black, carbon black, carbon dots, carbon nanotubes (including at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes), graphene, and carbon nanofibers; the negative electrode binder includes at least one of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylonitrile (PAN), polyacrylic acid, and polytetrafluoroethylene. Based on the total weight of the negative electrode active layer, the content of the negative electrode conductive agent is 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%), and the content of the negative electrode binder is 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%).

[0066] In this invention, the lithium-ion secondary battery further includes a positive electrode sheet, which comprises a positive current collector and a positive active layer located on at least one side of the surface of the positive current collector. The positive active layer comprises a positive electrode material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, and lithium-rich manganese-based materials. The positive electrode conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes (including at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon fibers. The positive electrode binder may include at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose, styrene-butadiene rubber, polytetrafluoroethylene, polyethylene oxide, polyacrylic acid, polyacrylonitrile (PAN), and derivatives of the above substances. Based on the total weight of the positive electrode active layer, the content of the positive electrode material can be 80%-98% (e.g., 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%), the content of the positive electrode conductive agent can be 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%), and the content of the positive electrode binder can be 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%).

[0067] In this invention, the battery may further include a separator, the separator comprising a substrate layer, a ceramic layer located on at least one side of the substrate layer, and adhesive layers located on both outer surfaces of the separator.

[0068] In one example, the substrate layer includes a matrix that may include at least one of polyethylene, polyvinyl chloride, polyoxyethylene, polypropylene, nylon, glass fiber, polyethylene phthalate (PET), polyimide (PI), aramid, cellulose, and nonwoven fabric.

[0069] In one example, the ceramic layer comprises a nitrogen-containing material and / or an inorganic material; the nitrogen-containing material includes melamine, melamine polyphosphate, melamine thiocyanate, melamine cyanurate, formaldehyde polymelamine hydrochloride, melamine polyphosphate, piperazine pyrophosphate, 1,3,5-triazine-2,4,6-triamine, symmetrical triaminotriazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazine-2-yl)guanidine, 2,4-diamino- The inorganic material comprises at least one of 6-dimethylamino-1,3,5-triazine, cyanuric chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-methoxy-1,3,5-triazine; the inorganic material comprises at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon dioxide, tin dioxide, titanium dioxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride.

[0070] In one example, the adhesive layer comprises at least one of polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), and polyvinylpyrrolidone (PVP).

[0071] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0072] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0073] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0074] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0075] Example 1 (1) Preparation of positive electrode sheet Lithium cobalt oxide, PVDF, conductive carbon black and carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil and then baked, rolled and cut to obtain the positive electrode sheet. (2) Preparation of negative electrode sheet Artificial graphite (average particle size 18.6 μm), silicon-based materials, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes were mixed in a mass ratio of 74.5:20:2.5:1.5:1:0.5, and deionized water was added. The mixture was stirred under vacuum to prepare the first negative electrode slurry. Then, artificial graphite (average particle size 6.5 μm), silicon-based materials, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes were mixed in a mass ratio of 74.5:20:2.5:1.5:1:0.5. Mix the ingredients in a mass ratio of 70:24.5:2.5:1.5:1:0.5, add deionized water, and stir under vacuum to prepare a second negative electrode slurry. Coat the first negative electrode slurry onto both sides of a 6μm thick carbon-coated copper foil and dry it. Then coat the second negative electrode slurry onto the dried surfaces of the first negative electrode slurry and bake it in an oven at 80℃ for 10 hours. Finally, roll and slit the foil to obtain the negative electrode sheet. The silicon-based material comprises silicon-carbon particles and a coating layer on the outer surface of the silicon-carbon particles. The coating layer includes a first coating layer and a second coating layer. The first coating layer comprises amorphous carbon, and the second coating layer comprises lithium aluminate and aluminum oxide. The thickness d1 of the second coating layer is 6.5 nm. The sphericity s of the silicon-based material is 0.85, and the particle size Dv50 of the silicon-based material is 9.6 μm and Dv99 is 22.1 μm. The mass content of Si element a1 in the first coating layer is 20%, the mass content of Si element a2 in the second coating layer is 25%, the thickness d2 of the first coating layer is 18.5 μm, and the thickness d3 of the second coating layer is 14.7 μm. (3) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), EC and PC were mixed evenly at a mass ratio of 1:1. Then, fully dried LiPF6 was quickly added and dissolved. After dissolving, 2,2-difluoroethyl acetate, lithium difluorophosphate, propyl propionate, and additives (ADN:SN:HTCN = 0.5:1:2.5) were added in sequence and stirred evenly. After passing the tests for moisture and free acid, the desired electrolyte was obtained. Among them, C1 is 20%, C2 is 0.8%, C3 is 25%, and the mass content of the additive in the electrolyte is 4%; (4) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a polyethylene film with a thickness of 8 μm, coated with a boehmite ceramic layer with a thickness of 2 μm on one side of the polyethylene film, and then coated with a polyvinylidene fluoride adhesive layer with a thickness of 1 μm on both sides) are stacked in the order of positive electrode sheet, separator and negative electrode sheet to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte prepared in step (3) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained.

[0076] Example 2 (1) Preparation of positive electrode sheet Lithium cobalt oxide, PVDF, conductive carbon black and carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil and then baked, rolled and cut to obtain the positive electrode sheet. (2) Preparation of negative electrode sheet Artificial graphite (average particle size 8.3 μm), silicon-based materials, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes were mixed in a mass ratio of 74.5:20:2.5:1.5:1:0.5, and deionized water was added. The mixture was stirred under vacuum to prepare the first negative electrode slurry. Then, artificial graphite (average particle size 3.5 μm), silicon-based materials, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes were mixed in a mass ratio of 74.5:20:2.5:1.5:1:0.5. The first negative electrode slurry was prepared by mixing ingredients in a mass ratio of 70:24.5:2.5:1.5:1:0.5, adding deionized water, and stirring under vacuum. The mixture was coated onto both sides of a 6μm thick carbon-coated copper foil and dried. The second negative electrode slurry was then coated onto the dried surfaces of the first negative electrode slurry and baked in an oven at 80℃ for 10 hours. After rolling and slitting, the negative electrode sheet was obtained. The silicon-based material comprises silicon-carbon particles and a coating layer on the outer surface of the silicon-carbon particles. The coating layer includes a first coating layer and a second coating layer. The first coating layer comprises amorphous carbon, and the second coating layer comprises lithium aluminate and aluminum oxide. The thickness d1 of the second coating layer is 2.1 nm. The sphericity s of the silicon-based material is 0.7, and the particle size Dv50 of the silicon-based material is 3.3 μm and Dv99 is 15.2 μm. The mass content of Si element a1 in the first coating layer is 1%, the mass content of Si element a2 in the second coating layer is 2.5%, the thickness d2 of the first coating layer is 12.3 μm, and the thickness d3 of the second coating layer is 8.2 μm. (3) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), EC and PC were mixed evenly at a mass ratio of 1:1. Then, fully dried LiPF6 was quickly added and dissolved. After dissolving, 2,2-difluoroethyl acetate, lithium difluorooxalate borate, propyl propionate, and additives (ADN:SN:HTCN = 0.5:1:2.5) were added in sequence and stirred evenly. After passing the tests for moisture and free acid, the desired electrolyte was obtained. Among them, c1 is 10%, c2 is 0.2%, c3 is 10%, and the mass content of the additive in the electrolyte is 4%; (4) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a polyethylene film with a thickness of 8 μm, coated with a boehmite ceramic layer with a thickness of 2 μm on one side of the polyethylene film, and then coated with a polyvinylidene fluoride adhesive layer with a thickness of 1 μm on both sides) are stacked in the order of positive electrode sheet, separator and negative electrode sheet to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte prepared in step (3) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained.

[0077] Example 3 (1) Preparation of positive electrode sheet Lithium cobalt oxide, PVDF, conductive carbon black and carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil and then baked, rolled and cut to obtain the positive electrode sheet. (2) Preparation of negative electrode sheet Artificial graphite (average particle size 25.7 μm), silicon-based materials, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes were mixed in a mass ratio of 74.5:20:2.5:1.5:1:0.5, and deionized water was added. The mixture was stirred under vacuum to prepare the first negative electrode slurry. Then, artificial graphite (average particle size 7.8 μm), silicon-based materials, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes were mixed in a mass ratio of 74.5:20:2.5:1.5:1:0.5. Mix the ingredients in a mass ratio of 70:24.5:2.5:1.5:1:0.5, add deionized water, and stir under vacuum to prepare a second negative electrode slurry. Coat the first negative electrode slurry onto both sides of a 6μm thick carbon-coated copper foil and dry it. Then coat the second negative electrode slurry onto the dried surfaces of the first negative electrode slurry and bake it in an oven at 80℃ for 10 hours. Finally, roll and slit the foil to obtain the negative electrode sheet. The silicon-based material comprises silicon-carbon particles and a coating layer on the outer surface of the silicon-carbon particles. The coating layer includes a first coating layer and a second coating layer. The first coating layer comprises amorphous carbon, and the second coating layer comprises lithium aluminate and aluminum oxide. The thickness d1 of the second coating layer is 9.6 nm. The sphericity s of the silicon-based material is 0.95, and the particle size Dv50 of the silicon-based material is 13.8 μm and Dv99 is 29.5 μm. The mass content of Si element a1 in the first coating layer is 40%, the mass content of Si element a2 in the second coating layer is 45%, the thickness d2 of the first coating layer is 24.6 μm, and the thickness d3 of the second coating layer is 19.6 μm. (3) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), EC and PC were mixed evenly at a mass ratio of 1:1. Then, fully dried LiPF6 was quickly added and dissolved. After dissolving, 2,2-difluoroethyl acetate, lithium tetrafluoroborate, propyl propionate, and additives (ADN:SN:HTCN = 0.5:1:2.5) were added in sequence and stirred evenly. After passing the tests for moisture and free acid, the desired electrolyte was obtained. Of these, C1 is 35%, C2 is 1.3%, C3 is 40%, and the mass content of the additive in the electrolyte is 4%. (4) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a polyethylene film with a thickness of 8 μm, coated with a boehmite ceramic layer with a thickness of 2 μm on one side of the polyethylene film, and then coated with a polyvinylidene fluoride adhesive layer with a thickness of 1 μm on both sides) are stacked in the order of positive electrode sheet, separator and negative electrode sheet to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte prepared in step (3) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained.

[0078] Example 4 group This set of embodiments is based on Embodiment 1, except that the composition of the second coating layer and the thickness d1 of the second coating layer are changed, as follows: Example 4a, the second coating layer includes lithium aluminate and aluminum fluoride; d1 is 6.1 nm; Example 4b, the second coating layer includes lithium aluminate, aluminum oxide and aluminum fluoride; d1 is 5.4 nm.

[0079] Example 5 group This set of embodiments is based on Embodiment 1, except that the thickness d1 of the second coating layer is changed, as follows: Example 5a, d1 is 1.2 nm; Example 5b, d1 is 19.8 nm.

[0080] Example 6 group This set of embodiments is based on Example 1, except that the mass content of 2,2-difluoroethyl acetate in the electrolyte is changed to alter c1, as detailed below: Example 6a, c1 is 5%; Example 6b, c1 is 60%.

[0081] Example 7 group This set of embodiments is based on Example 1, except that the mass content of the first compound, lithium difluorophosphate, in the electrolyte is changed to alter c2, as detailed below: Example 7a, c2 is 0.05%; Example 7b, c2 is 1.8%.

[0082] Example 8 group This set of embodiments is based on Example 1, except that the mass content of the second solvent, propyl propionate, in the electrolyte is changed to alter c3, as detailed below: Example 8a, c3 is 5%; Example 8b, c3 is 50%.

[0083] Example 9 group This set of embodiments follows the same procedure as Embodiment 1, except that the sphericity of the silicon-based material is changed, as follows: In Example 9a, the sphericity s of the silicon-based material is 0.55; In Example 9b, the sphericity s of the silicon-based material is 0.65.

[0084] Example 10 group This set of embodiments is based on Embodiment 1, except that the mass content a1 of silicon in the first coating and the mass content a2 of silicon in the second coating are changed, as follows: Example 10a, a1 is 0.8%, a2 is 0.5%; Example 10b, a1 is 50%, a2 is 48%.

[0085] Example 11 group This set of embodiments is based on Embodiment 1, except that the thickness d2 of the first coating, the thickness d3 of the second coating, and the average particle size of the first carbon-based material and the second carbon-based material are changed, as follows: In Example 11a, d2 is 10.1 μm, d3 is 5.4 μm, the average particle size of the first carbon-based material is 6.8 μm, and the average particle size of the second carbon-based material is 2.2 μm; In Example 11b, d2 is 29.9 μm, d3 is 24.8 μm, the average particle size of the first carbon-based material is 29.8 μm, and the average particle size of the second carbon-based material is 9.8 μm.

[0086] Example 12 This embodiment is based on Embodiment 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 9.8 μm.

[0087] Example 13 group This set of examples follows the same procedure as Example 1, except that the composition of the fluorocarboxylic acid ester in the electrolyte is changed, as follows: Example 13a: Fluorocarboxylic acid esters include ethyl difluoroacetate; Example 13b: Fluorocarboxylic acid esters include propyl difluoroacetate; Example 13c: Fluorocarboxylic acid esters include methyl difluoroacetate; Example 13d: Fluorocarboxylic acid esters include ethyl trifluoroacetate.

[0088] Comparative Example 1 This comparative example is based on Example 1, except that the silicon-based material contains only the first coating layer.

[0089] Comparative Example 2 This comparative example is based on Example 1, except that the electrolyte does not contain fluorinated carboxylic acid esters.

[0090] Comparative Example 3 Groups This comparative study was conducted with reference to Example 1, except that the composition of the second coating layer and the thickness d1 of the second coating layer were changed, as follows: Comparative Example 3a, the second coating layer includes aluminum fluoride; d1 is 7.1 nm; Comparative Example 3b, the second coating layer includes aluminum oxide; d1 is 7.2 nm; Comparative Example 3c, the second coating layer includes lithium aluminate; d1 is 7.6 nm.

[0091] Test case (1) Loop test The lithium-ion batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows: At 25℃, the battery was charged at 5C to 4.55V, with a cutoff current of 0.05C, and then left to stand for 5 minutes. The thickness of the battery at this time was recorded as the initial thickness a mm. Then, it was discharged at 5C to the cutoff voltage of 3.0V and left to stand for 5 minutes. This is one charge-discharge cycle. The discharge capacity of the battery at this time was recorded as the initial capacity C0. The discharge capacity of the Nth cycle was recorded as C1. The discharge capacity of the Nth cycle C1 was divided by the initial capacity C0 to obtain the cycle capacity retention rate R = C1 / C0. The above charge-discharge cycle process was repeated until the battery cycle capacity retention rate decreased to 80%. The number of repeated cycles is the cycle number. The thickness of the battery after the Nth cycle charging process was completed and left to stand for 5 minutes was recorded as the cycle thickness b mm. The thickness expansion rate = 100% × (ba) / b. The cycle performance of the battery was evaluated by the cycle number and the thickness expansion rate, and the results are recorded in Table 1.

[0092] (2) Lithium plating test The lithium-ion batteries prepared in the examples and comparative examples were subjected to lithium plating tests. The specific test methods are as follows: The resulting batteries were charged at 25°C at a rate of 5C to a cutoff voltage of 4.55V and a cutoff current of 0.05C. After resting for 5 minutes, they were discharged at a rate of 5C to a cutoff voltage of 3V. This constitutes one charge-discharge cycle. After 300 cycles, the batteries were disassembled, and the lithium plating state on the surface of the negative electrode was observed. The criteria for judging the lithium plating phenomenon on the negative electrode were: 1) no lithium plating; 2) lithium plating on the top, bottom, and creases was recorded as slight lithium plating; 3) lithium plating on the entire surface was recorded as severe lithium plating. The results are shown in Table 1.

[0093] Table 1 As can be seen from Table 1, compared with the comparative example, the battery of the present invention can avoid severe lithium plating under high-rate charge and discharge conditions and has excellent cycle performance.

[0094] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes a negative electrode and an electrolyte; The negative electrode sheet includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material; the silicon-based material includes silicon carbon particles and a coating layer located on the outer surface of the silicon carbon particles; the coating layer includes a first coating layer and a second coating layer; the first coating layer includes amorphous carbon, the second coating layer includes lithium aluminate, and further includes at least one of aluminum oxide and aluminum fluoride; The electrolyte includes a first solvent, which includes a fluorocarboxylic acid ester.

2. The lithium-ion secondary battery according to claim 1, wherein, The fluorocarboxylic acid esters include at least one selected from methyl difluoroacetate, ethyl difluoroacetate, 2,2-difluoroethyl acetate, propyl difluoroacetate, methyl trifluoroacetate, ethyl trifluoroacetate, and trifluoromethylbenzoate. Preferably, the fluorocarboxylic acid ester includes 2,2-difluoroethyl acetate.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The thickness d1 of the second coating layer is 1nm-20nm; preferably 2nm-10nm. And / or, based on the total weight of the electrolyte, the mass content of the fluorocarboxylic acid ester c1 is 5%-60%; preferably 10%-35%.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The sphericity s of the silicon-based material is 0.5-1; And / or, the silicon-carbon particles comprise a porous carbon matrix and silicon material located in the pores of the porous carbon matrix; And / or, the particle size Dv50 of the silicon-based material is 3μm-14μm, and the particle size Dv99 is 15μm-30μm.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein, The negative electrode active layer includes a first coating and a second coating disposed 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; Preferably, the first coating comprises a first carbon-based material and the silicon-based material; Preferably, the second coating comprises a second carbon-based material and the silicon-based material; More preferably, the mass content a1 of silicon in the first coating and the mass content a2 of silicon in the second coating satisfy: a1≤a2.

6. The lithium-ion secondary battery according to claim 5, wherein, The thickness d2 of the first coating is 10μm-30μm; preferably 12μm-25μm; And / or, the thickness d3 of the second coating is 5μm-25μm; preferably 8μm-20μm; And / or, the mass content a1 of silicon in the first coating is 0.5%-50%; preferably 1%-40%; And / or, the mass content a2 of silicon element in the second coating is 0.5%-50%; preferably 2.5%-45%.

7. The lithium-ion secondary battery according to claim 5, wherein, The first carbon-based material and the second carbon-based material each independently include at least one of graphite, mesophase carbon microspheres, hard carbon, and soft carbon; And / or, the average particle size of the first carbon-based material is 6μm-30μm, and the average particle size of the second carbon-based material is 2μm-10μm.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein, The electrolyte also includes a first compound; Preferably, the first compound includes at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium difluorooxalate phosphate. Preferably, based on the total weight of the electrolyte, the mass content c2 of the first compound is 0.05%-1.8%; more preferably 0.2%-1.3%.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, The electrolyte further includes a second solvent; the second solvent includes at least one of the substances represented by Formula I: Equation I, R1 and R2 each independently include at least one of the following groups: C1-C10 alkyl, C2-C10 alkenyl and C2-C10 alkynyl.

10. The lithium-ion secondary battery according to claim 9, wherein, The second solvent includes , , , , , , , , , , , , , , and At least one of them; Preferably, the second solvent includes , , , and At least one of them; Preferably, based on the total weight of the electrolyte, the mass content of the second solvent c3 is 5%-50%; more preferably 10%-40%.

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