Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. Comprising a positive electrode plate, a negative electrode plate and an electrolyte, the positive electrode plate comprises lithium cobalt oxide, the negative electrode plate comprises a silicon-based material, the electrolyte comprises a fluorosulfonamide compound, a nitrile compound and a first solvent, the first solvent comprises at least one of fluoroether, fluorobenzene and acetic acid 2, 2-difluoroethyl ester, and by taking the total mass of the electrolyte as a standard, the total mass of the lithium cobalt oxide and the total mass of the nitrile compound are equal to or smaller than 1. The content c1 of the fluorosulfonamide compound and the content c2 of the nitrile compound meet the following condition: 0.1 < = c1 / c2 < = 30; and the charging cut-off voltage of the lithium ion battery is greater than or equal to 4.48 V. The lithium ion secondary battery disclosed by the invention has relatively high energy density, relatively good cycle performance, excellent high-temperature storage performance and relatively good low-temperature performance.

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] As electronic products such as communication equipment and electric vehicles place increasingly higher demands on battery energy density, traditional graphite anode materials have been unable to meet market demand. Silicon anodes have a higher theoretical specific capacity (up to 4200mAh / g at high temperatures and approximately 3580mAh / g at room temperature), which is much higher than graphite anodes (theoretical specific capacity is 372mAh / g). Therefore, they are considered to be one of the key materials for improving the energy density of lithium-ion batteries. However, after lithiation, silicon anodes will form lithium-silicon alloys, leading to electrolyte consumption and loss of active lithium, which in turn causes problems with poor battery cycle performance and poor high-temperature storage performance.

[0003] Therefore, it is very important to invent a silicon-doped lithium-ion battery that can take into account high energy density as well as excellent cycle performance and high-temperature storage performance. Summary of the Invention

[0004] The present invention aims to overcome the aforementioned problems existing in the prior art and provides a lithium-ion secondary battery. The lithium-ion secondary battery of the present invention (hereinafter referred to as the battery) has highly compatible positive and negative electrodes, and an electrolyte, ensuring high energy density, good cycle performance, and high-temperature storage performance while also exhibiting excellent low-temperature performance.

[0005] In a high voltage system (e.g., charging cut-off voltage ≥ 4.48V), when the electrolyte contains fluorosulfonamide compounds, the low temperature performance of lithium cobalt oxide batteries will be reduced. The inventors have conducted a lot of research and found that the reasons for the above problems are: First, fluorosulfonamide compounds can reduce the side reactions between silicon-based materials and electrolytes, improve electrolyte stability and delay electrolyte consumption, thereby improving the cycle life of the battery and expansion during the cycle. However, in a high voltage lithium cobalt oxide system, the oxidation resistance of fluorosulfonamide compounds is general; Co on the positive electrode side 3+ Oxidized to Co 4+ ,Co 4+ The oxidation of fluorosulfonamide compounds will lead to defluorination reaction and free radical chain decomposition (generating HSO 3- NH 2-These side reaction products form a high-impedance CEI film on the cathode surface, which not only hinders lithium ion transport and seriously affects the battery's low-temperature performance, but also causes gassing, resulting in reduced safety during high-temperature storage and high-temperature cycling. Second, the strong molecular rigidity of fluorosulfonamide compounds may increase the viscosity of the electrolyte, affecting the diffusion rate of lithium ions (especially at low temperatures), thereby affecting the low-temperature performance of the battery.

[0006] Based on the above two reasons, the inventors conducted a lot of targeted research and found that:

[0007] First, adding at least one nitrile compound of formula II-IV to the electrolyte can not only improve the low-temperature performance of the battery, but also improve the battery gas production, and enhance the safety of the battery during high-temperature storage and high-temperature cycling. The reason is that: first, traditional nitrile additives (such as adiponitrile, 1,3,6-hexanetrinitrile, etc.) can complex the metal dissolved on the positive electrode side and protect the positive electrode, but they cannot repair the lithium ion conduction of the CEI membrane; while at least one nitrile compound of formula II-IV can not only complex with the metal dissolved on the positive electrode side, but its decomposition product can also achieve self-repair of the CEI membrane structure through dynamic bonds (such as hydrogen bonds, reversible PO bonds, etc.) during the battery cycle, thereby improving the ion conductivity of the CEI membrane. At the same time, it can also enhance the wettability of the electrolyte on the surface of the positive electrode sheet, and can also improve the ion conductivity of the positive electrode sheet to a certain extent. Secondly, nitrile compounds with specific structures have strong electron-withdrawing capabilities. By enhancing the electronic effect with fluorosulfonamide compounds, they improve the antioxidant capacity of fluorosulfonamide compounds and reduce their decomposition at high voltages. In addition, these nitrile compounds can form a film on the positive electrode side before fluorosulfonamide compounds, thereby indirectly stabilizing the fluorosulfonamide compounds. Therefore, the synergistic use of specific nitrile compounds and fluorosulfonamide compounds can expand the operating voltage window of the battery, reduce the growth of battery interface impedance, and reduce gas generation, making the battery more suitable for long-term storage at high voltage, safety of cycling at high temperature, and low-temperature performance.

[0008] Second, the improvement effect of adding only specific nitrile compounds to the electrolyte is limited, and the relationship between the content of nitrile compounds and fluorosulfonamide compounds in the electrolyte needs to be further defined. When the content ratio of fluorosulfonamide compounds to nitrile compounds is too small (for example, <0.1), the amount of fluorosulfonamide compounds added is less than that of nitrile compounds. At this time, the improvement of poor battery cycle performance and high-temperature storage performance is limited; and when the content ratio of fluorosulfonamide compounds to nitrile compounds is too large (for example, >30), the amount of fluorosulfonamide compounds added is more than that of nitrile compounds. At this time, nitrile compounds are not enough to improve the effect of negative sulfonamide compounds on the low-temperature performance and gas production of the battery. Only when the ratio of the two meets a specific range can the battery have both high energy density and excellent cycle performance, high-temperature storage performance and low-temperature performance.

[0009] Third, the first solvent not only does not participate in ion coordination but also has good storage stability and can reduce the viscosity of the electrolyte. Therefore, the addition of the first solvent to the above battery system can improve the overall lithium ion transport performance and high-temperature storage stability of the electrolyte, thereby further improving the high-temperature storage performance and low-temperature performance of the battery.

[0010] Based on this, the inventors of the present invention proposed the following solution:

[0011] The present invention provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte; the positive electrode comprises lithium cobalt oxide; the negative electrode comprises a silicon-based material; the electrolyte comprises a fluorosulfonamide compound, and the fluorosulfonamide compound comprises at least one of the substances represented by Formula I:

[0012]

[0013] wherein R1 and R2 each independently include at least one of the following groups which are substituted or unsubstituted with fluorine: a C1-C6 alkyl group, a C2-C6 alkenyl group, a C2-C6 alkynyl group, a C1-C6 cyano group, a phenyl group, a silane group, and a sulfone group;

[0014] The electrolyte further includes a nitrile compound, and the nitrile compound includes at least one of the substances represented by Formula II, Formula III, and Formula IV:

[0015]

[0016] wherein m1, m2 and m3 are each independently selected from an integer greater than or equal to 1, and R3, R4 and R5 each independently include a C1-C3 alkyl group or an O atom;

[0017]

[0018] wherein R6 comprises at least one of a vacant bond, a C1-C5 alkyl group, a C2-C5 alkenyl group, and a C2-C5 alkynyl group;

[0019]

[0020] Wherein, m4 is selected from an integer of 2-4, and R7 includes At least one of, R8 and R9 each independently include a C1-C3 alkyl group;

[0021] Based on the total mass of the electrolyte, the content c1 of the fluorosulfonamide compound and the content c2 of the nitrile compound satisfy the following conditions: c1 is 1%-30%, c2 is 0.3%-8%, and 0.1≤c1 / c2≤30; the electrolyte also includes a first solvent, the first solvent including at least one of fluoroether, fluorobenzene, and 2,2-difluoroethyl acetate; and the charging cut-off voltage of the lithium-ion battery is greater than or equal to 4.48V.

[0022] Through the above technical solution, the present invention has at least the following advantages compared with the prior art: the battery of the present invention can have both high energy density and excellent cycle performance, high temperature storage performance and low temperature performance.

[0023] 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. DETAILED DESCRIPTION

[0024] 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.

[0025] The present invention provides a lithium-ion secondary battery, which may include a positive electrode, a negative electrode, and an electrolyte; the positive electrode includes lithium cobalt oxide; the negative electrode includes a silicon-based material; the electrolyte includes a fluorosulfonamide compound, and the fluorosulfonamide compound includes at least one of the substances represented by Formula I:

[0026]

[0027] wherein R1 and R2 each independently include at least one of the following fluorine-substituted or unsubstituted groups: C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, C1-C6 cyano, phenyl, silyl, and sulfone groups, for example, C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, C2 alkenyl, C3 alkenyl, C4 alkenyl, C5 alkenyl, C6 alkenyl, C2 alkynyl, C3 alkynyl, C4 alkynyl, C5 alkynyl, C6 alkynyl, C2 cyano, C3 cyano, C4 cyano, C5 cyano, or C6 cyano.

[0028] In the present invention, the term "C1-C6 alkyl" refers to an alkyl group having 1 to 6 carbon atoms. "C2-C6 alkenyl" refers to an alkenyl group having 2 to 6 carbon atoms, and so on.

[0029] In one embodiment, the fluorosulfonamide compound includes

[0030] At least one of .

[0031] The electrolyte further includes a nitrile compound, and the nitrile compound includes at least one of the substances represented by Formula II, Formula III, and Formula IV:

[0032]

[0033] wherein m1, m2, and m3 are each independently selected from an integer greater than or equal to 1 (e.g., 1, 2, 3, 4, 5, or 6), and R3, R4, and R5 are each independently a C1-C3 alkyl group or -O-, such as a C1 alkyl group, a C2 alkyl group, or a C3 alkyl group;

[0034]

[0035] wherein R6 includes at least one of a blank bond, a C1-C5 alkyl group, a C2-C5 alkenyl group, and a C2-C5 alkynyl group, for example, a C1 alkyl group, a C2 alkyl group, a C3 alkyl group, a C4 alkyl group, a C5 alkyl group, a C2 alkenyl group, a C3 alkenyl group, a C4 alkenyl group, a C5 alkenyl group, a C2 alkynyl group, a C3 alkynyl group, a C4 alkynyl group, or a C5 alkynyl group;

[0036]

[0037] Wherein, m4 is selected from an integer of 2-4 (for example, 2, 3 or 4), and R7 includes At least one of the following, R8 and R9 each independently include a C1-C3 alkyl group, for example, a C1 alkyl group, a C2 alkyl group or a C3 alkyl group.

[0038] In one embodiment, the nitrile compound includes

[0039]

[0040]

[0041] At least one of .

[0042] In the present invention, based on the total mass of the electrolyte, the content c1 of the fluorosulfonamide compound is 1%-30%, for example, 1%, 3%, 5%, 7%, 9%, 11%, 13%, 15%, 17%, 20%, 23%, 26% or 30%.

[0043] In one example, c1 is 5%-15%.

[0044] In the present invention, based on the total mass of the electrolyte, the content c2 of the nitrile compound is 0.3%-8%, for example, 0.3%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5% or 8%.

[0045] In one example, c2 is 0.5%-4%.

[0046] In the present invention, c1 and c2 satisfy: 0.1≤c1 / c2≤30, for example, 0, 5, 10, 15, 20, 25 or 30.

[0047] In one example, 5≤c1 / c2≤20.

[0048] In the present invention, c1 and c2 also satisfy: 1%≤c1+c2≤35%, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30% or 35%.

[0049] In one example, 6%≤c1+c2≤17%.

[0050] In the present invention, the electrolyte further comprises a first solvent, wherein the first solvent comprises at least one of fluoroether, fluorobenzene and 2,2-difluoroethyl acetate; the structural formula of the 2,2-difluoroethyl acetate is

[0051]

[0052] In one embodiment, the fluoroether comprises

[0053] At least one of .

[0054] In one embodiment, the fluorobenzene comprises

[0055] At least one of .

[0056] In one example, the first solvent comprises

[0057] In the present invention, based on the total mass of the electrolyte, the content c3 of the first solvent is 3%-60%, for example, 3%, 10%, 20%, 30%, 40%, 50% or 60%.

[0058] In one example, c3 is 6%-55%.

[0059] In the present invention, the charging cut-off voltage of the lithium-ion battery is greater than or equal to 4.48V.

[0060] In one example, the charging cut-off voltage of the lithium-ion battery is greater than or equal to 4.55V.

[0061] In the present invention, the content c1 of the fluorosulfonamide compound, the content c2 of the nitrile compound, and the content c3 of the first solvent can be obtained by conventional methods in the art, such as by gas chromatography (GC) or gas chromatography-mass spectrometry (GCMS).

[0062] In the present invention, the electrolyte further comprises a boron-containing lithium salt, and the boron-containing lithium salt comprises at least one of lithium difluorooxalatoborate, lithium dioxalatoborate and lithium tetrafluoroborate.

[0063] In one example, the boron-containing lithium salt includes lithium difluorooxalatoborate.

[0064] In one example, based on the total mass of the electrolyte, the content c4 of the boron-containing lithium salt is 0.01%-2%, for example, 0.01%, 0.1%, 0.5%, 1%, 1.5% or 2%.

[0065] In one embodiment, c4 is 0.2%-1%.

[0066] In the lithium cobalt oxide system, lithium difluorooxalatoborate plays a unique role. The fluorine atoms and oxaloboric acid groups in its molecular structure have high electronegativity and reactivity, and can form a stable coordination structure with the cobalt atoms in the lithium cobalt oxide positive electrode material. The formation of this coordination structure can optimize the microstructure and chemical composition of the solid electrolyte interface (CEI) film on the positive electrode surface, reduce the impedance of the interface film, and thus reduce the transmission resistance of lithium ions in the interface film, improve the transmission efficiency of lithium ions, and further improve the battery's rate performance and low-temperature performance.

[0067] In the present invention, the electrolyte further comprises a second additive

[0068] In one example, based on the total mass of the electrolyte, the content c6 of the second additive is 0.2%-10%, for example, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0069] In one example, c6 is 0.5%-3%.

[0070] Under high temperature conditions, the structure and performance of the positive electrode active material are prone to change. The presence of P element in the positive electrode can enhance the structural stability of the positive electrode active material and slow down the structural decay caused by temperature rise. This is because: the P element has more binding sites with the positive electrode active material, can be more evenly distributed on the surface of the positive electrode active material, reacts with it to form more chemical bonds, enhances the overall stability and electrochemical activity of the positive electrode active material, and improves the high temperature performance of the battery. At the same time, It also contains the element F. During the battery's charge and discharge process, the F element can combine with lithium ions to form a stable LiF layer on the surface of the positive electrode, which can play a role in framework protection and provide a stable skeleton for the CEI membrane, making the CEI membrane less likely to break or fall off during the charge and discharge process. Moreover, because the LiF layer itself has a certain lithium ion conductivity, its stable skeleton structure provides more channels and space for the transmission of lithium ions, reducing the diffusion resistance of lithium ions in the CEI membrane, and further improving the battery's rate performance and low-temperature performance.

[0071] In the present invention, the content c4 of the boron-containing lithium salt in the electrolyte can be obtained by testing using conventional methods in the art, such as by ion chromatography (IC) testing.

[0072] In the present invention, c6 can be obtained by conventional methods in the art, such as IC or GCMS testing.

[0073] In the present invention, the silicon-based material includes secondary particles formed by primary particles, and the average particle size of the primary particles is 0.5μm-6μm, for example, 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm or 6μm; the average particle size of the secondary particles is 3μm-20μm, for example, 3μm, 5μm, 7μm, 9μm, 11μm, 13μm, 15μm, 17μm, 19μm or 20μm.

[0074] The smaller primary particles shorten the lithium-ion transmission distance, reduce diffusion resistance, and increase the lithium-ion transmission rate, thereby improving the battery's rate capability and low-temperature performance. They also effectively mitigate polarization and extend the battery's cycle life. Furthermore, the pores within the secondary particles absorb the volume expansion of silicon, inhibiting the expansion of silicon-based materials.

[0075] In the present invention, the average particle size of the silicon-based material primary and secondary particles can be measured using conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and removed, and the cross-section of the negative electrode sheet is polished using an argon ion milling apparatus. In a scanning electron microscope (SEM) image, at least 20 primary or secondary particles are selected, and the particle size of each particle is measured to obtain the average value. If the silicon-based material primary / secondary particles are regular circles in the image, the particle size of the particles is the diameter of the circle. If the silicon-based material primary / secondary particles are irregular circles in the image, the particle size of the particles is the diameter of an equivalent circle with the same area as the regular circle.

[0076] In the present invention, the silicon-based material includes at least one of elemental silicon, silicon carbon, silicon oxygen and silicon alloy; the silicon carbon refers to a material including elemental silicon and elemental carbon, and the silicon oxygen refers to a material including elemental silicon and elemental oxygen.

[0077] In one example, the silicon-based material includes silicon carbon.

[0078] In one example, the silicon carbon includes a porous carbon matrix and silicon particles located in pores of the porous carbon matrix.

[0079] In the present invention, the electrolyte further comprises a fluoroester compound. Based on the total mass of the electrolyte, the content c7 of the fluoroester compound is 2%-20%, for example, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, or 20%.

[0080] In one embodiment, the fluoroester compound includes at least one of fluoroethylene carbonate, fluoropropyl acetate, fluoromethyl propionate, and fluoroethyl propionate, wherein the fluoroester compound does not include 2,2-difluoroethyl acetate.

[0081] In one embodiment, the fluorinated ester compound includes fluorinated ethylene carbonate.

[0082] In one embodiment, c7 and c1 satisfy: 0.05≤c7 / c1≤10, for example, 0.05, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[0083] In one example, 0.3≤c7 / c1≤2.

[0084] Fluorinated ester compounds themselves generally have higher antioxidant properties than common carboxylic acid esters (such as ethyl propionate / propyl propionate). They can inhibit the reaction between the oxidant in the electrolyte and the battery's negative electrode, slowing the oxidative decomposition rate of the electrolyte. At the same time, they can also reduce the decomposition reaction of the electrolyte on the negative electrode surface, thereby reducing the occurrence of side reactions and maintaining the stability of the electrolyte's active ingredients and battery performance. Therefore, regulating the content of the fluorinated ester compound and the fluorosulfonamide compound to meet a specific relationship can further improve the battery's cycle stability.

[0085] In the present invention, the content c7 of the fluorinated ester compound can be obtained by conventional methods in the art, such as GC or GCMS testing.

[0086] In the present invention, 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 the silicon-based material, and the mass content w1 of the silicon element in the negative electrode active layer is 2%-30%, for example, 2%, 5%, 7%, 9%, 11%, 13%, 15%, 17%, 19%, 21%, 23%, 25%, 27% or 30%.

[0087] In one example, w1 is 5%-30%.

[0088] In one embodiment, w1 and c1 satisfy: 0.2≤w1 / c1≤10, for example, 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5 or 10.

[0089] In one example, 0.5≤w1 / c1≤2.

[0090] The use of silicon-based materials in negative electrode sheets can significantly improve the energy density of batteries. However, when the silicon content is high, the silicon-based materials will undergo significant volume expansion during the charge and discharge cycle, affecting the battery's rate charging performance, low-temperature discharge performance, and cycle stability. Fluorosulfonamide compounds have excellent film-forming properties. By adding a certain amount of fluorosulfonamide compounds, the SEI film of the silicon negative electrode can be continuously repaired, reducing the decomposition of the electrolyte. Therefore, the present invention can achieve effective protection of the negative electrode by regulating the content of silicon in the negative electrode active layer and the content of fluorosulfonamide compounds in the electrolyte to meet a specific relationship, effectively improve the battery's rate charging performance and low-temperature discharge performance, further enhance the battery's cycle stability, and extend the battery's service life.

[0091] In the present invention, the silicon content w1 in the negative electrode active layer can be measured by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed, and then 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 sheet. After drying, the negative electrode sheet is high-temperature treated at 400°C in an inert atmosphere for 2 hours (for example, in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can be peeled off from the negative electrode current collector, and the negative electrode active layer is collected as a test sample. Using a thermogravimetric analyzer (for example, a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg, and the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min in air or oxygen atmosphere, and then kept at 900°C for 40 minutes, so that the non-silicon components in the negative electrode active layer are volatilized and the silicon is fully oxidized to silicon dioxide. The remaining substance is the ash of the negative electrode active layer. The mass content of silicon in the negative electrode active layer can be calculated based on the mass of the ash. The calculation formula is as follows: mass content of silicon in the negative electrode active coating = 7×mass of ash / (15×mass of test sample).

[0092] In the present invention, the electrolyte further includes a nitrogen-containing lithium salt, and the nitrogen-containing lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide and lithium fluorosulfate imide. Based on the total mass of the electrolyte, the content c5 of the nitrogen-containing lithium salt is 0.5%-15%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0093] In one example, c5 is 1%-10%.

[0094] The addition of nitrogen-containing lithium salts to the electrolyte significantly increases the battery's decomposition voltage due to the introduction of nitrogen atoms, allowing it to remain stable at higher potentials and less susceptible to decomposition reactions. At the same time, the electronegativity of nitrogen makes the chemical bonds formed between the nitrogen-containing lithium salt and other components in the electrolyte more stable, maintaining the electrolyte's composition and performance in high-temperature environments, further enhancing the battery's high-temperature cycling stability and extending its service life.

[0095] In the present invention, the content c5 of the nitrogen-containing lithium salt in the electrolyte can be obtained by testing using conventional methods in the art, such as IC testing.

[0096] In the present invention, the electrolyte also includes ethylene carbonate. Based on the total mass of the electrolyte, the content c8 of the ethylene carbonate is 0%-4.95%, for example, 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 4.95%.

[0097] During battery cycling, ethylene carbonate forms a stable SEI film on the electrode surface, effectively preventing further decomposition of the electrolyte and reducing side reactions within the battery. Furthermore, ethylene carbonate has a high boiling point and maintains good chemical stability at high temperatures, thereby preserving the integrity of the battery's internal structure. Furthermore, it effectively improves the efficiency of lithium ion migration between electrodes, reducing the battery's internal resistance. This allows the battery to maintain a high capacity output after multiple charge and discharge cycles, thereby improving the battery's high-temperature capacity retention.

[0098] In the present invention, the ethylene carbonate content c8 can be measured by conventional methods in the art, such as by GC or GCMS testing.

[0099] In the present invention, the lithium cobalt oxide includes element Al. Based on the total mass of the lithium cobalt oxide, the content w2 of the Al element is 3000ppm-12000ppm, for example, 3000ppm, 4000ppm, 5000ppm, 6000ppm, 7000ppm, 8000ppm, 9000ppm, 10000ppm, 11000ppm or 12000ppm.

[0100] Further research has found that in lithium cobalt oxide battery systems, especially under high voltage conditions, the acidic substances produced by the decomposition of fluorosulfonamides will enter the lithium cobalt oxide body through surface oxygen vacancies, weakening the Co-O bond, triggering oxygen release, accelerating the dissolution of metallic cobalt, and deteriorating the structural stability of the lithium cobalt oxide, affecting the battery's cycle stability. When lithium cobalt oxide contains an appropriate amount of Al, it can reduce the reactivity of metallic cobalt, making it less likely to dissolve at high voltage, preventing adverse phase changes or lattice distortion in the structure of lithium cobalt oxide during charge and discharge, stabilizing the crystal structure of lithium cobalt oxide, and further improving the battery's cycle stability. Especially under high temperature conditions, this change helps stabilize the structure of the positive electrode material, reducing its gas generation caused by thermal decomposition or excessive redox reactions, and can effectively improve the battery's high-temperature storage gas generation problem. When the Al element doping level is too high (for example, ≥12000ppm), the battery capacity is lost; when the Al element doping level is too low (for example, ≤3000ppm), it cannot fully play its role in stabilizing the positive electrode structure and improving the battery's thermal stability.

[0101] In one example, the lithium cobalt oxide may further include other elements, such as at least one of Mg, Ti, Zr, Nb, and Mo.

[0102] In the present invention, the content w2 of the element Al in the lithium cobalt oxide can be measured by conventional methods in the art, such as inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0103] In the present invention, the particle size Dv50 of the lithium cobalt oxide is 5 μm-25 μm, for example, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, 17 μm, 19 μm, 21 μm, 23 μm or 25 μm.

[0104] In the present invention, the particle size Dv50 of the lithium cobalt oxide can be measured by conventional methods in the art, such as by a laser particle size analyzer.

[0105] In the present invention, the electrolyte further comprises a sulfur-containing additive, wherein the sulfur-containing additive comprises At least one of 1,3-propane sultone, 1,3-propylene sultone, 1,4-butane sultone, methylene methanedisulfonate, ethylene sulfate, and ethylene bissulfate.

[0106] In one example, the sulfur-containing additive includes 1,3-propane sultone and / or

[0107] In one example, based on the total mass of the electrolyte, the content of the sulfur-containing additive is 1%-10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0108] In one example, based on the total mass of the electrolyte, the content of the sulfur-containing additive is 2%-5%.

[0109] When sulfur-containing additives are added to the electrolyte, a stable CEI film can be formed on the surface of the positive electrode, effectively preventing direct contact between the electrolyte and the positive electrode material, reducing the occurrence of side reactions, and maintaining the structural stability of the positive electrode material. This allows the battery to maintain a high capacity and performance after multiple charge and discharge cycles, further improving the battery's cycle stability and extending its service life.

[0110] In the present invention, the content of the sulfur-containing additive can be measured by conventional methods in the art, such as by GC or GCMS testing.

[0111] In the present invention, the electrolyte further includes 1,4-dicyano-2-butene. Based on the total mass of the electrolyte, the content of the 1,4-dicyano-2-butene is 0.2%-5%, for example, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%.

[0112] In one example, the electrolyte further includes ethyl butyrate, and the content of the ethyl butyrate is 5%-20% based on the total mass of the electrolyte, for example, 5%, 8%, 11%, 14%, 17% or 20%.

[0113] Adding 1,4-dicyano-2-butene and ethyl butyrate to the electrolyte can produce a synergistic effect. Among them, 1,4-dicyano-2-butene has high electrochemical activity and can preferentially decompose on the surface of the positive electrode and participate in the film formation process, forming a thin and uniform initial interface film. It can not only effectively isolate the direct contact between the positive electrode material and the electrolyte, but also stabilize the interface environment of the positive electrode material through its special chemical structure, thereby protecting the positive electrode. At the same time, ethyl butyrate, as an auxiliary solvent, can adjust the viscosity and ion transport properties of the electrolyte, making the formed interface film denser and more stable, effectively protecting the positive electrode material from the erosion of the electrolyte and the influence of side reactions, and further enhancing the high-temperature cycle stability of the battery.

[0114] In the present invention, the contents of 1,4-dicyano-2-butene and ethyl butyrate can be measured by conventional methods in the art, such as GC or GCMS.

[0115] In the present invention, the positive electrode sheet may include a positive electrode current collector and a positive electrode active coating located on at least one surface of the positive electrode current collector. The positive electrode active coating may include a positive electrode active material. The positive electrode active material includes the lithium cobalt oxide. The positive electrode active material may also include active materials commonly used in the art, such as at least one of lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. The positive electrode active coating may also include a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent may include, for example, at least one of superconducting carbon, acetylene black, carbon black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The positive electrode binder may include a binder commonly used in the art, such as at least one of polyvinylidene fluoride, styrene-butadiene rubber, and an aluminate coupling agent.

[0116] In the present invention, the negative electrode sheet may include a negative electrode current collector and a negative electrode active coating located on at least one surface of the negative electrode current collector. The negative electrode active coating may include a negative electrode active material. The negative electrode active material may include the silicon-based material. The negative electrode active material may also include a carbon-based material, such as at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, and hard carbon. The negative electrode active coating may also include at least one of a lithium-aluminum alloy, a tin-based negative electrode material, and a lithium transition metal oxide. The negative electrode active coating may also include a negative electrode conductive agent and a negative electrode binder. The negative electrode conductive agent may include a conductive agent commonly used in the art, such as at least one of superconducting carbon, acetylene black, carbon black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The negative electrode binder may include a binder commonly used in the art, such as at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and an aluminate coupling agent.

[0117] In the present invention, the battery may further include a separator, which may include separators commonly used in the art, such as at least one of polypropylene, polyethylene, and polyvinylidene fluoride.

[0118] In the present invention, the electrolyte also includes a second solvent and an electrolyte salt, and the second solvent includes a carbonate solvent and / or a carboxylate solvent. The carbonate solvent may include a cyclic carbonate solvent and a chain carbonate solvent. The cyclic carbonate solvent may include a cyclic carbonate conventionally used in the art, such as including at least one of propylene carbonate (PC) and butylene carbonate (BC). The chain carbonate solvent may include a chain carbonate conventionally used in the art, such as including at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC). The carboxylate solvent may include a carboxylate conventionally used in the art, such as including at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP) and methyl butyrate (MB). The electrolyte salt includes at least one of lithium hexafluorophosphate (LiPF6), gold hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, tris(trifluoromethylsulfonyl)methyllithium, and lithium bis(trifluoromethylsulfonyl)imide.

[0119] In the present invention, the electrolyte may further include other additives, such as at least one of vinylene carbonate, vinyl sulfate, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, 1,3,6-hexanetrinitrile (HTCN), glycerol trinitrile and 1,2-bis(2-trioxyethyl)ethane.

[0120] It should be noted that the numerical expressions such as "first" and "second" in the present invention are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0121] The present invention will be described in detail below through examples. The examples described in the present invention are only some examples of the present invention, not all examples. All other examples obtained by persons of ordinary skill in the art based on the examples of the present invention without creative work are within the scope of protection of the present invention.

[0122] In the following examples, unless otherwise specified, all materials used were commercially available analytical grade.

[0123] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0124] Example 1

[0125] Prepare the battery as follows:

[0126] (1) Preparation of electrolyte

[0127] In a glove box filled with argon (H2O < 0.1ppm, O2 < 0.1ppm), 10% 2,2-difluoroethyl acetate and the second solvent (PC and PP are added in a mass ratio of 1:5) are mixed evenly and then added in sequence.

[0128] (c1 is 5%), (c2 = 1%), fluoroethylene carbonate (c7 = 10%), ethylene carbonate (c8 = 3%), and 2% HTCN were mixed evenly, and 15% lithium hexafluorophosphate (LiPF6) was added. After passing the moisture and free acid tests, an electrolyte was obtained. The c1 / c2 ratio was 5; the c1+c2 ratio was 6%; and the c7 / c1 ratio was 2.

[0129] (2) Preparation of positive electrode sheet

[0130] Lithium cobalt oxide (Dv50 is 13μm, w2 is 5512ppm), polyvinylidene fluoride, conductive carbon black (SP) and single-walled carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5, and N-methylpyrrolidone (NMP) is added with a solid content of 75wt%. The mixture is stirred under the action of a vacuum mixer until the mixed system becomes a positive electrode active slurry with uniform fluidity; the positive electrode active slurry is evenly coated on both surfaces of an aluminum foil; the coated aluminum foil is dried, and then rolled and slit to obtain the desired positive electrode sheet.

[0131] (3) Preparation of negative electrode sheet

[0132] The negative electrode active material (a combination of artificial graphite and silicon-based material, wherein the average particle size of the primary particles of the silicon-based material is 3 μm, the average particle size of the secondary particles is 12 μm, w1 is 10%, and w1 / c1 is 2), sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black (SP) and single-walled carbon nanotubes are mixed in a mass ratio of 94.5:2.5:1.5:1:0.5, and deionized water is added with a solid content of 65 wt%. A negative electrode active slurry is obtained under the action of a vacuum mixer; the negative electrode active slurry is evenly coated on both surfaces of a copper foil; the coated copper foil is dried at room temperature, then transferred to an 80°C oven for drying for 10 hours, and then cold pressed and slit to obtain a negative electrode sheet.

[0133] (4) Preparation of batteries

[0134] The positive electrode sheet prepared in step (2), the negative electrode sheet prepared in step (3) and the isolation film (polyethylene film) are stacked in the order of positive electrode sheet, isolation film and negative electrode sheet, and then wound to obtain a battery cell; the battery cell is placed in an outer packaging aluminum foil, and the electrolyte prepared in step (1) is injected into the outer packaging. After vacuum packaging, standing, forming, shaping, sorting and other processes, a lithium-ion battery is obtained.

[0135] Example 2

[0136] Prepare the battery as follows:

[0137] (1) Preparation of electrolyte

[0138] In a glove box filled with argon (H2O < 0.1ppm, O2 < 0.1ppm), 15% 2,2-difluoroethyl acetate and the second solvent (PC and PP are added in a mass ratio of 1:5) are mixed evenly and then added in sequence.

[0139] (c1 is 10%) (c2 is 0.5%), fluoroethylene carbonate (c7 is 12%), and 2% HTCN are mixed evenly, and 15% lithium hexafluorophosphate (LiPF6) is added. After passing the moisture and free acid tests, the electrolyte is obtained. Here, c1 / c2 is 20; c1+c2 is 10.5%; and c7 / c1 is 1.2.

[0140] (2) Preparation of positive electrode sheet

[0141] Lithium cobalt oxide (Dv50 is 5μm, w2 is 5512ppm), polyvinylidene fluoride, conductive carbon black (SP) and single-walled carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5, and N-methylpyrrolidone (NMP) is added with a solid content of 75wt%. The mixture is stirred under the action of a vacuum mixer until the mixed system becomes a positive electrode active slurry with uniform fluidity; the positive electrode active slurry is evenly coated on both surfaces of an aluminum foil; the coated aluminum foil is dried, and then rolled and slit to obtain the desired positive electrode sheet.

[0142] (3) Preparation of negative electrode sheet

[0143] The negative electrode active material (a combination of artificial graphite and silicon-based material, wherein the average particle size of the primary particles of the silicon-based material is 1 μm, the average particle size of the secondary particles is 3 μm, w1 is 10%, and w1 / c1 is 1) is mixed with sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black (SP) and single-walled carbon nanotubes in a mass ratio of 94.5:2.5:1.5:1:0.5, and deionized water is added with a solid content of 65 wt% to obtain a negative electrode active slurry under the action of a vacuum mixer; the negative electrode active slurry is evenly coated on both surfaces of a copper foil; the coated copper foil is dried at room temperature, then transferred to an 80°C oven for drying for 10 hours, and then cold pressed and slit to obtain a negative electrode sheet.

[0144] (4) Preparation of batteries

[0145] The positive electrode sheet prepared in step (2), the negative electrode sheet prepared in step (3) and the isolation film (polyethylene film) are stacked in the order of positive electrode sheet, isolation film and negative electrode sheet, and then wound to obtain a battery cell; the battery cell is placed in an outer packaging aluminum foil, and the electrolyte prepared in step (1) is injected into the outer packaging. After vacuum packaging, standing, forming, shaping, sorting and other processes, a lithium-ion battery is obtained.

[0146] Example 3

[0147] Prepare the battery as follows:

[0148] (1) Preparation of electrolyte

[0149] In a glove box filled with argon (H2O < 0.1ppm, O2 < 0.1ppm), 20% 2,2-difluoroethyl acetate and the second solvent (PC and PP are added in a mass ratio of 1:5) are mixed evenly and then added in sequence.

[0150] (c1 is 15%) (c2 = 2%), fluoroethylene carbonate (c7 = 5%), ethylene carbonate (c8 = 4.95%), and 2% HTCN were mixed evenly, and 15% lithium hexafluorophosphate (LiPF6) was added. After passing the moisture and free acid tests, the electrolyte was obtained. The c1 / c2 ratio was 7.5; the c1+c2 ratio was 17%; and the c7 / c1 ratio was 0.33.

[0151] (2) Preparation of positive electrode sheet

[0152] Lithium cobalt oxide (Dv50 is 25μm, w2 is 5512ppm), polyvinylidene fluoride, conductive carbon black (SP) and single-walled carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5, and N-methylpyrrolidone (NMP) is added with a solid content of 75wt%. The mixture is stirred under the action of a vacuum mixer until the mixed system becomes a positive electrode active slurry with uniform fluidity; the positive electrode active slurry is evenly coated on both surfaces of an aluminum foil; the coated aluminum foil is dried, and then rolled and slit to obtain the desired positive electrode sheet.

[0153] (3) Preparation of negative electrode sheet

[0154] The negative electrode active material (a combination of artificial graphite and silicon-based material, wherein the average particle size of the primary particles of the silicon-based material is 6 μm, the average particle size of the secondary particles is 20 μm, w1 is 10%, and w1 / c1 is 0.67), sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black (SP) and single-walled carbon nanotubes are mixed in a mass ratio of 94.5:2.5:1.5:1:0.5, and deionized water is added with a solid content of 65 wt%. A negative electrode active slurry is obtained under the action of a vacuum mixer; the negative electrode active slurry is evenly coated on both surfaces of a copper foil; the coated copper foil is dried at room temperature, then transferred to an 80°C oven for drying for 10 hours, and then cold pressed and slit to obtain a negative electrode sheet.

[0155] (4) Preparation of batteries

[0156] The positive electrode sheet prepared in step (2), the negative electrode sheet prepared in step (3) and the isolation film (polyethylene film) are stacked in the order of positive electrode sheet, isolation film and negative electrode sheet, and then wound to obtain a battery cell; the battery cell is placed in an outer packaging aluminum foil, and the electrolyte prepared in step (1) is injected into the outer packaging. After vacuum packaging, standing, forming, shaping, sorting and other processes, a lithium-ion battery is obtained.

[0157] Example 4

[0158] This set of examples is used to verify the impact of changes in the "content c1 of the fluorosulfonamide compound".

[0159] This group of examples was carried out with reference to Example 1, except that the content of the fluorosulfonamide compound was changed, as follows:

[0160] In Example 4a, c1 is 1%; wherein c1 / c2 is 1; c1+c2 is 2%;

[0161] In Example 4b, c1 is 30%, wherein c1 / c2 is 30; c1+c2 is 32%; and c3 is 31%.

[0162] Example 5

[0163] This group of examples is used to verify the impact of changes in the "type of nitrile compounds".

[0164] This group of examples is carried out with reference to Example 1, except that the nitrile compound is

[0165] Example 6

[0166] This group of examples is used to verify the impact of changes in the "content c2 of nitrile compounds".

[0167] This group of examples is carried out with reference to Example 1, except that the content of the nitrile compound is changed, as follows:

[0168] In Example 6a, c2 is 0.3%; wherein c1 / c2 is 16.67; c1+c2 is 5.3%;

[0169] In Example 6b, c2 is 8%, wherein c1 / c2 is 0.625; c1+c2 is 13%.

[0170] Example 7

[0171] This set of embodiments is used to verify the impact of the change of "c1 / c2".

[0172] This group of examples was carried out with reference to Example 1, except that c1 / c2 was regulated by changing the content c1 of the fluorosulfonamide compound and the content c2 of the nitrile compound, as follows:

[0173] In Example 7a, c1 is 5%, c2 is 2%, wherein c1 / c2 is 2.5; c1+c2 is 7%;

[0174] In Example 7b, c1 is 15%, c2 is 0.5%, wherein c1 / c2 is 30; c1+c2 is 15.5%.

[0175] Example 8

[0176] This set of examples is used to verify the impact of changing the “first solvent type”.

[0177] This group of examples was carried out with reference to Example 1, except that the type of the first solvent was changed, as follows:

[0178] In embodiment 8a, the first solvent comprises

[0179] In embodiment 8b, the first solvent comprises and 2,2-difluoroethyl acetate;

[0180] In embodiment 8c, the first solvent comprises and 2,2-difluoroethyl acetate.

[0181] Example 9

[0182] This group of examples is used to verify the impact of changes in the "content w2 of the Al element in lithium cobalt oxide".

[0183] This group of examples is carried out with reference to Example 1, except that the content of Al element in lithium cobalt oxide is changed, specifically as follows:

[0184] Example 9a, w2 is 0 ppm;

[0185] Example 9b, w2 is 12015 ppm.

[0186] Example 10

[0187] This set of examples is used to verify the impact of changes in the "content c3 of the first solvent".

[0188] This group of examples was carried out with reference to Example 1, except that the content of the first solvent was changed, as follows:

[0189] Example 10a, c3 is 5%;

[0190] Example 10b, c3 is 55%.

[0191] Example 11

[0192] This group of examples is used to verify the impact of changes in the "type of boron-containing lithium salt and its content c4 in the electrolyte".

[0193] This group of examples is carried out with reference to Example 1, except that a boron-containing lithium salt is added and the type of the boron-containing lithium salt and the content in the electrolyte are changed, as follows:

[0194] Example 11a, the boron-containing lithium salt includes lithium bis(oxalatoborate), and c4 is 0.1%;

[0195] Example 11b, the boron-containing lithium salt includes lithium tetrafluoroborate, and c4 is 0.6%;

[0196] Example 11c, the boron-containing lithium salt includes lithium difluorooxalatoborate, and c4 is 1%.

[0197] Example 12

[0198] This group of examples is used to verify the effects of changes in the type of nitrogen-containing lithium salt and its content c5 in the electrolyte.

[0199] This group of examples was carried out with reference to Example 1, except that nitrogen-containing lithium salt was added and the type of nitrogen-containing lithium salt and the content in the electrolyte were changed, as follows:

[0200] Example 12a, the nitrogen-containing lithium salt includes lithium fluorosulfate imide, and c5 is 0.5%;

[0201] Example 12b, the nitrogen-containing lithium salt includes lithium bis(trifluoromethanesulfonyl)imide, and c5 is 5%;

[0202] Example 12c, the nitrogen-containing lithium salt includes lithium bis(trifluoromethanesulfonyl)imide, and c5 is 15%.

[0203] Example 13

[0204] This set of examples is used to verify the impact of changing the "content c6 of the second additive".

[0205] This group of examples is carried out with reference to Example 1, except that a second additive is added and the content of the second additive in the electrolyte is changed as follows:

[0206] Example 13a, c6 is 0.2%;

[0207] Example 13b, c6 is 3%.

[0208] Example 14

[0209] This group of examples is used to verify the impact of changes in the "content c7 of fluoroester compounds".

[0210] This group of examples was carried out with reference to Example 1, except that c7 was 20%.

[0211] Example 15

[0212] This set of examples is used to verify the effects of changes in the type of sulfur-containing additives and their content in the electrolyte.

[0213] This group of examples was carried out with reference to Example 1, except that a sulfur-containing additive was added and the type of the sulfur-containing additive and the content in the electrolyte were changed, as follows:

[0214] Example 15a, the sulfur-containing additive comprises The content in the electrolyte is 1%;

[0215] In Example 15b, the sulfur-containing additive includes 1,3-propane sultone, and the content of the sulfur-containing additive in the electrolyte is 5%.

[0216] Example 16

[0217] This group of examples is used to verify the effects of changes in “components in the electrolyte”.

[0218] This group of examples was carried out with reference to Example 1, except that 1% of lithium difluorooxalatoborate, 15% of lithium bis(trifluoromethanesulfonylimide), 3% of the second additive, and 5% of 1,3-propanesultone were added to the electrolyte.

[0219] Example 17

[0220] This group of examples is used to verify the effects of changes in “components in the electrolyte”.

[0221] This group of examples was carried out with reference to Example 16, except that 3% of 1,4-dicyano-2-butene was added to the electrolyte.

[0222] Example 18

[0223] This group of examples is used to verify the effects of changes in “components in the electrolyte”.

[0224] This group of examples was carried out with reference to Example 17, except that 15% ethyl butyrate was added to the electrolyte.

[0225] Example 19

[0226] This group of examples is used to verify the impact of changes in "silicon-based materials".

[0227] This group of examples was carried out with reference to Example 1, except that the silicon-based material only included primary particles, and the average particle size of the primary particles was 3 μm.

[0228] Example 20

[0229] This group of examples is used to verify the impact of changes in the "mass content w1 of silicon element in silicon-based materials".

[0230] This group of examples was carried out with reference to Example 1, except that the mass content of silicon in the silicon-based material was changed, as follows:

[0231] Example 20a, w1 is 2%;

[0232] Example 20b, w1 is 5%;

[0233] In Example 20c, w1 is 30%.

[0234] Comparative Example 1

[0235] The process is carried out in accordance with Example 1, except that the electrolyte does not include the nitrile compound.

[0236] Comparative Example 2

[0237] The same procedure was followed with reference to Example 1, except that the type of nitrile compound was changed, as follows:

[0238] In Comparative Example 2a, the nitrile compound is adiponitrile;

[0239] In Comparative Example 2b, the nitrile compound is 1,3,6-hexanetrinitrile.

[0240] Comparative Example 3

[0241] The same procedure was carried out as in Example 3, except that the content of the fluorosulfonamide compound c1 and the content of the nitrile compound c2 were changed to adjust c1 / c2, as follows:

[0242] In comparative example 3a, c1 is 0.1% and c2 is 8%; wherein c1 / c2 is 0.0125; c1+c2 is 8.1%;

[0243] In Comparative Example 3b, c1 is 30%, c2 is 0.3%, wherein c1 / c2 is 100; c1+c2 is 30.3%, and c3 is 45%.

[0244] Comparative Example 4

[0245] The process is carried out in accordance with Example 1, except that the electrolyte does not include the first solvent.

[0246] Test Case

[0247] (1) Cycle test

[0248] The lithium ion batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows:

[0249] At 45°C, the battery was discharged at 1C to 3.0V, then allowed to stand for 10 minutes, then charged at 1C to 4.55V with a cutoff current of 0.025C and allowed to stand for 10 minutes. The thickness of the battery at this time was recorded as the initial thickness; then discharged at a rate of 1.2C to a cutoff voltage of 3.0V, allowed to stand for 10 minutes, and the discharge capacity of the battery at this time was recorded as the initial capacity; the cycle process was as follows: charged at 1C to 4.55V with a cutoff current of 0.025C and allowed to stand for 10 minutes, then discharged at a rate of 1.2C to a cutoff voltage of 3.0V and allowed to stand for 10 minutes. The above cycle process was repeated until the 600th cycle. After the 600th cycle charging process was completed and allowed to stand for 10 minutes, the thickness of the battery was measured and recorded as the thickness after the cycle. Then, it was discharged at 1.2C to 3.0V and allowed to stand for 10 minutes. The discharge capacity of the battery at this time was recorded as the capacity after the cycle. The cycle capacity retention rate = capacity after cycle × 100% / initial capacity, thickness expansion rate = (thickness after cycle - initial thickness) × 100% / initial thickness. The cycle capacity retention rate and thickness expansion rate are recorded in Table 1.

[0250] (2) High temperature storage test

[0251] The lithium-ion batteries prepared in the examples and comparative examples were subjected to high-temperature storage tests. The specific test methods are as follows:

[0252] At 25℃±3℃, discharge the fully charged battery at 0.5C to the lower limit voltage of 3.0V, let it stand for 10 minutes, then fully charge it at 0.5C constant current with a cutoff current of 0.02C, let it stand for 10 minutes, and test the thickness of the battery in the fully charged state at 25℃±3℃; then place the battery in an 85℃ environment for 6 hours, take it out, test the battery thickness, and record the thickness expansion rate of the battery.

[0253] (3) Low temperature discharge test

[0254] The lithium-ion batteries prepared in the examples and comparative examples were subjected to low-temperature discharge tests. The specific test methods are as follows:

[0255] At 25°C ± 3°C, discharge a fully charged battery at 0.5C to a lower voltage of 3.0V. Let it rest for 10 minutes, and record the initial discharge capacity, Q0. Then, place the battery in a 0°C constant temperature chamber and fully charge it at a constant current of 0.5C, with a cutoff current of 0.02C. After 6 hours of storage, discharge the battery at a constant current of 0.5C, and record the discharge capacity, Q1. The low-temperature discharge capacity retention rate is calculated as Q1 / Q0 × 100%.

[0256] Table 1

[0257]

[0258]

[0259] As can be seen from Table 1, the battery of the present invention has significantly improved 45°C cycle performance, 85°C high-temperature storage performance, and low-temperature performance compared to the comparative example.

[0260] 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: Including positive electrode sheet, negative electrode sheet and electrolyte; The positive electrode sheet includes lithium cobalt oxide; The negative electrode sheet comprises a silicon-based material; The electrolyte includes a fluorosulfonamide compound, and the fluorosulfonamide compound includes at least one of the substances represented by Formula I: wherein R1 and R2 each independently include at least one of the following groups which are substituted or unsubstituted with fluorine: a C1-C6 alkyl group, a C2-C6 alkenyl group, a C2-C6 alkynyl group, a C1-C6 cyano group, a phenyl group, a silane group, and a sulfone group; The electrolyte further includes a nitrile compound, and the nitrile compound includes at least one of the substances represented by Formula II, Formula III, and Formula IV: wherein m1, m2 and m3 are each independently selected from an integer greater than or equal to 1, and R3, R4 and R5 each independently include a C1-C3 alkyl group or an O atom; wherein R6 comprises at least one of a vacant bond, a C1-C5 alkyl group, a C2-C5 alkenyl group, and a C2-C5 alkynyl group; Wherein, m4 is selected from an integer of 2-4, and R7 includes At least one of, R8 and R9 each independently include a C1-C3 alkyl group; Based on the total mass of the electrolyte, the content c1 of the fluorosulfonamide compound and the content c2 of the nitrile compound satisfy the following conditions: c1 is 1%-30%, c2 is 0.3%-8%, and 0.1≤c1 / c2≤30; The electrolyte further includes a first solvent, wherein the first solvent includes at least one of fluoroether, fluorobenzene and 2,2-difluoroethyl acetate; The charging cut-off voltage of the lithium-ion battery is greater than or equal to 4.48V.

2. The lithium ion secondary battery according to claim 1, wherein 5≤c1 / c2≤20; and / or, c1 is 5%-15%; and / or, c2 is 0.5%-4%; and / or, 1%≤c1+c2≤35%; preferably, 6%≤c1+c2≤17%; And / or, based on the total mass of the electrolyte, the first solvent content c3 is 3%-60%, preferably 6%-55%.

3. The lithium ion secondary battery according to claim 1 or 2, wherein the fluorosulfonamide compound comprises At least one of; And / or, the nitrile compound includes At least one of; And / or, the fluoroether comprises At least one of; And / or, the fluorobenzene includes At least one of .

4. The lithium ion secondary battery according to claim 1 or 2, wherein The electrolyte further includes a boron-containing lithium salt, wherein the boron-containing lithium salt includes at least one of lithium difluorooxalatoborate, lithium bisoxalatoborate, and lithium tetrafluoroborate; preferably, based on the total mass of the electrolyte, the content c4 of the boron-containing lithium salt is 0.01%-2%; And / or, the electrolyte further comprises a nitrogen-containing lithium salt, wherein the nitrogen-containing lithium salt comprises at least one of lithium bis(trifluoromethanesulfonyl)imide and lithium fluorosulfuric acid imide; preferably, based on the total mass of the electrolyte, the content c5 of the nitrogen-containing lithium salt is 0.5%-15%; And / or, the electrolyte further comprises a second additive Preferably, based on the total mass of the electrolyte, the content c6 of the second additive is 0.2%-10%.

5. The lithium ion secondary battery according to claim 1 or 2, wherein The silicon-based material includes secondary particles formed from primary particles; the average particle size of the primary particles is 0.5 μm-6 μm, and the average particle size of the secondary particles is 3 μm-20 μm; And / or, the silicon-based material includes at least one of elemental silicon, silicon carbon, silicon oxygen and silicon alloy; preferably, the silicon-based material includes the silicon carbon; Preferably, the silicon carbon comprises a porous carbon matrix and silicon particles located in pores inside the porous carbon matrix.

6. The lithium ion secondary battery according to claim 1 or 2, wherein 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 the silicon-based material, and the mass content w1 of silicon element in the negative electrode active layer is 2%-30%; preferably 5%-30%; Preferably, w1 and c1 satisfy: 0.2≤w1 / c1≤10.

7. The lithium ion secondary battery according to claim 1 or 2, wherein The electrolyte further comprises a fluorinated ester compound; preferably, based on the total mass of the electrolyte, the content c7 of the fluorinated ester compound is 2%-20%; And / or, the electrolyte further comprises ethylene carbonate, and the content c8 of the ethylene carbonate is 0%-4.95% based on the total mass of the electrolyte; Preferably, c7 and c1 satisfy: 0.05≤c7 / c1≤10.

8. The lithium ion secondary battery according to claim 1 or 2, wherein The lithium cobalt oxide includes the element Al; Preferably, based on the total mass of the lithium cobalt oxide, the content w2 of the Al element is 3000ppm-12000ppm; Preferably, the particle size Dv50 of the lithium cobalt oxide is 5 μm-25 μm.

9. The lithium ion secondary battery according to claim 1 or 2, wherein The electrolyte further comprises a sulfur-containing additive, wherein the sulfur-containing additive comprises At least one of 1,3-propane sultone, 1,3-propylene sultone, 1,4-butane sultone, methylene methanedisulfonate, ethylene sulfate, and ethylene bissulfate; Preferably, the sulfur-containing additive includes 1,3-propane sultone and / or Preferably, based on the total mass of the electrolyte, the content of the sulfur-containing additive is 1%-10%.

10. The lithium ion secondary battery according to claim 1 or 2, wherein The electrolyte further comprises 1,4-dicyano-2-butene; preferably, the content of the 1,4-dicyano-2-butene is 0.2%-5% based on the total mass of the electrolyte; And / or, the electrolyte further comprises ethyl butyrate; preferably, based on the total mass of the electrolyte, the content of ethyl butyrate is 5%-20%.

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