Electrolyte and lithium ion battery

Abstract

The invention relates to the technical field of batteries, and provides an electrolyte and a lithium ion battery, the electrolyte comprises Awt% of ethyl difluoroacetate, Bwt% of ethylene carbonate, Cwt% of fluoroethylene carbonate and Dwt% of a first nitrile additive, C > B, A + B + C is more than or equal to 20 and less than or equal to 60, and (B + C) / D is more than or equal to 4 and less than or equal to 40; wherein the first nitrile additive comprises a compound as shown in a formula I which is described in the specification. The electrolyte can ensure that the battery adapts to a wide-temperature-range working environment, has good cycle life and safety performance, and can ensure the stability and safety of the high-voltage and high-energy-density battery.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in portable devices and electric vehicles due to their high energy density and long cycle life. However, the discharge performance of existing lithium-ion batteries in low-temperature environments faces many challenges. Low temperatures can cause the viscosity of the electrolyte to increase significantly, the ionic conductivity to decrease, and even the electrolyte to freeze, thereby hindering the transmission of lithium ions between the positive and negative electrodes. In addition, the solid electrolyte interface (SEI) film generated by the reaction between the electrolyte and the electrodes will become thicker at low temperatures. Lithium ions need to overcome a higher energy barrier when passing through the SEI film, resulting in a slower transmission speed. In high-temperature environments, the dissolution of the electrolyte interface film is accelerated, which can easily lead to thermal runaway. In addition, high-voltage and high-energy-density cathode materials are prone to structural instability, oxygen release, and side reactions under high voltage conditions, resulting in capacity fading and decreased cycle performance.

[0003] Therefore, developing an electrolyte that can balance low-temperature discharge performance and high-temperature stability to ensure the stability and safety of high-voltage and high-energy density batteries is a technical problem that needs to be solved urgently. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned problems existing in the prior art and provide an electrolyte and a lithium-ion battery containing the electrolyte, wherein the electrolyte can ensure that the battery can adapt to a wide temperature range working environment and has good cycle life and safety performance, while ensuring the stability and safety of high-voltage and high-energy density batteries.

[0005] To achieve the above object, the present invention provides an electrolyte in a first aspect, comprising Awt% of ethyl difluoroacetate, Bwt% of ethylene carbonate, Cwt% of fluoroethylene carbonate, and Dwt% of a first nitrile additive, wherein C>B, 20≤A+B+C≤60, and 4≤(B+C) / D≤40;

[0006] Wherein, the first nitrile additive comprises a compound represented by formula I,

[0007]

[0008] Wherein, R1, R2, and R3 are each independently selected from O, substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy, substituted or unsubstituted C2-C10 alkenyl, substituted or unsubstituted C2-C10 alkynyl, and the substituent is halogen.

[0009] A second aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte as described above.

[0010] The present invention adopts the above technical solution to achieve the following beneficial effects:

[0011] The electrolyte of the present invention can effectively balance the viscosity, conductivity and film-forming performance of the electrolyte at low temperatures by adjusting the content relationship of ethylene carbonate, fluoroethylene carbonate and ethyl difluoroacetate in the electrolyte, thereby improving the low-temperature discharge performance of the battery. At the same time, the first nitrile additive represented by Formula I is synergistically added, and its content is controlled to satisfy a corresponding relationship with that of ethylene carbonate and fluoroethylene carbonate, thereby improving the compatibility and stability of the first nitrile additive represented by Formula I in the electrolyte, thereby improving the stability of the interface film under high voltage and high temperature environments, significantly enhancing the safety of the battery, enabling the battery to adapt to a wide temperature range operating environment, and having good cycle life and safety performance, while ensuring the stability and safety of high-voltage and high-energy-density batteries.

[0012] 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 each range, the endpoint values of each range and the 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 as specifically disclosed in this article. Herein, unless otherwise specified, data ranges include endpoints. DETAILED DESCRIPTION

[0013] 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.

[0014] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meanings as commonly understood by one of ordinary skill in the art to which the present invention relates.

[0015] A first aspect of the present invention provides an electrolyte, comprising Awt% of ethyl difluoroacetate (DFEA), Bwt% of ethylene carbonate (EC), Cwt% of fluoroethylene carbonate (FEC), and Dwt% of a first nitrile additive, wherein C>B, 20≤A+B+C≤60 (for example, 20, 25, 30, 35, 40, 45, 50, 55, 60), and 4≤(B+C) / D≤40 (for example, 4, 6, 8, 10, 15, 20, 25, 30, 35, 40);

[0016] Wherein, the first nitrile additive comprises a compound represented by formula I,

[0017]

[0018] Wherein, R1, R2, and R3 are each independently selected from O, substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy, substituted or unsubstituted C2-C10 alkenyl, substituted or unsubstituted C2-C10 alkynyl, and the substituent is halogen.

[0019] Traditional solvents contain high levels of EC. To meet the requirements for low-temperature discharge performance, the present invention reduces the EC content, mitigating the adverse effects of EC's high viscosity on low-temperature performance. The fluorochemical compounds FEC and DFEA are then used in synergistic combination to improve low-temperature discharge performance, compensating for the insufficient negative electrode film formation caused by reduced EC content. However, further research revealed that the combination of ethyl difluoroacetate, ethylene carbonate, and fluoroethylene carbonate in the electrolyte presented a problem of difficulty balancing low-temperature discharge performance with performance at high temperatures and high voltages. To address this issue, the inventors, after extensive research and verification, further added a first nitrile additive represented by Formula I and adjusted the contents of these components in the electrolyte to simultaneously satisfy the conditions C>B, 20≤A+B+C≤60, and 4≤(B+C) / D≤40. This allows the battery to adapt to a wide temperature range operating environment, exhibit good cycling and safety performance, and ensure the stability and safety of high-voltage, high-energy-density batteries.

[0020] Specifically, in terms of low temperature, by increasing the FEC content so that C>B, the low temperature discharge can be improved, and at the same time, an appropriate amount of DFEA is used to satisfy 20≤A+B+C≤60, and the electrolyte is controlled to have low viscosity and high dielectric constant to maintain ionic conductivity; controlling 4≤(B+C) / D≤40 provides a sufficient and appropriate amount of the first nitrile additive, whose -CN group weakens the Li + The solvation shell reduces the desolvation energy. The phosphorus and cyanide groups in its molecules can form a polar structure, which combines with ethylene carbonate (EC) through hydrogen bonds or dipole-dipole interactions, which is more conducive to improving its compatibility and stability in the electrolyte.

[0021] This optimized electrolyte composition reduces the desolvation energy barrier of lithium ions, accelerates the transmission of lithium ions at low temperatures, and at the same time ensures that the electrolyte forms a stable and dense SEI film on the electrode surface. The first nitrile improves the compatibility and stability of various substances in the electrolyte, which can effectively improve the high-temperature cycle performance and safety performance of the battery. At high voltage, FEC can effectively avoid excessive oxidation of ethylene carbonate to produce gas (such as C2H4); and the first nitrile additive is oxidized to form a polymer containing P=O bonds and -CN, which fills the pores of the CEI film and can block the high-priced positive electrode active metals (such as Co) at high energy density. 4+ 、Ni 4+etc.) in contact with the electrolyte, synergistically enhancing the high-voltage interface stability of the positive electrode and improving the stability of the battery.

[0022] In the present invention, ethyl difluoroacetate refers to ethyl acetate with two fluorine atoms substituted at any position, and preferably includes at least one of 2,2-difluoroethyl acetate and ethyl 2,2-difluoroacetate.

[0023] In the present invention, "C1-C10 alkyl" refers to an alkyl group having 1 to 10 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and the like.

[0024] In the present invention, "C1-C10 alkoxy" refers to an alkoxy group having 1 to 10 carbon atoms, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, and the like.

[0025] In the present invention, "C2-C10 alkenyl" refers to an alkenyl group having 2 to 10 carbon atoms, such as ethenyl, propenyl, butenyl, pentenyl, hexenyl, and the like.

[0026] In the present invention, "C2-C10 alkynyl" refers to an alkynyl group having 2 to 10 carbon atoms, such as ethynyl, propynyl, butynyl, pentynyl, hexynyl, and the like.

[0027] In the present invention, "halogen" may be F, Cl, Br, etc.

[0028] In some embodiments, 10≤A≤50, for example, 10, 15, 20, 25, 30, 35, 40, 45, or 50.

[0029] In some embodiments, 1≤B≤10, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.

[0030] In some embodiments, 8≤C≤18, for example, 8, 9, 10, 11, 12, 14, 15, 16, 17, or 18.

[0031] In some embodiments, 0.1≤D≤5, for example, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5.

[0032] In some embodiments, A>C, increasing the content of ethyl difluoroacetate can significantly reduce the viscosity of the electrolyte and improve low-temperature discharge performance. A relatively high content of fluoroethylene carbonate can easily lead to high-temperature cycling and poor safety.

[0033] In some embodiments, 1.05≤A / C≤3.5, for example, 1.05, 1.2, 1.6, 1.8, 2.0, 2.4, 2.8, 3.0, 3.2, or 3.5. In this case, the film forming performance and the low-temperature discharge performance can be further improved.

[0034] In some embodiments, 0.8≤A / (B+C)≤5, for example, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5. When this relationship is satisfied, sufficient negative electrode film formation can be ensured. If A / (B+C)>5, the content of ethylene carbonate and fluoroethylene carbonate is too low, lithium salt dissociation is insufficient, negative electrode film formation quality is poor, and cycle stability is affected.

[0035] In some embodiments, 1.1≤C / B≤10, such as 1.1, 2, 3, 4, 5, 6, 7, 8, 9, and 10, has been found to improve low-temperature discharge while further ensuring high-temperature performance and safety. If only C>B but C / B>10 is satisfied, the fluoroethylene carbonate content is relatively high and the ethylene carbonate content is relatively low, resulting in a slight decrease in high-temperature performance compared to the case of 1.1≤C / B≤10. If C>B but C / B<1.1 is satisfied, the ethylene carbonate content is slightly high, the electrolyte viscosity increases, and the low-temperature performance is slightly reduced compared to the case of 1.1≤C / B≤10.

[0036] In some embodiments, R1, R2, and R3 are each independently selected from O, substituted or unsubstituted C1-C5 alkyl, substituted or unsubstituted C1-C5 alkoxy, substituted or unsubstituted C2-C4 alkenyl, or substituted or unsubstituted C2-C4 alkynyl, and the substituent is F.

[0037] In some embodiments, the first nitrile additive comprises at least one compound represented by Formula I-1 to Formula I-5:

[0038]

[0039]

[0040] In some embodiments, the electrolyte includes a second nitrile additive, and the second nitrile additive includes at least one of 1,3,6-hexane trinitrile, glycerol trinitrile, and compounds of Formula II-1 to Formula II-4:

[0041]

[0042] At least one of the cyano group, ether bond, and fluorine atom in the second nitrile additive molecule can form a polar structure by itself or with the phosphorus, cyano group, etc. in the first nitrile additive, and then combine with ethylene carbonate through hydrogen bonding or dipole-dipole interaction, further improving its compatibility and stability in the electrolyte.

[0043] In some embodiments, in the electrolyte, the content of the second nitrile additive is 0.6wt%-7wt%, for example, 0.6wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, and 7wt%.

[0044] In some embodiments, the electrolyte includes a functional additive, and the functional additive includes at least one of vinyl sulfate (DTD), 1,3-propane sultone (PS), 1,3-propylene sultone (PST), tetravinylsilane, tris(trimethylsilyl)borate, hexamethyldisilazane, fluorobenzene, triphenyl phosphite, and a compound represented by Formula III.

[0045]

[0046] In some embodiments, the content of the functional additive in the electrolyte is less than 6wt%, such as 0.1wt%, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, and 6wt%.

[0047] In some embodiments, the electrolyte further includes a lithium salt, which can be selected from electrolyte lithium salts conventionally used in the art, including but not limited to at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium tetrafluoroborate, lithium difluorooxalatoborate (LiODFB), lithium bis(oxalatoborate), lithium bis(fluorosulfonyl)imide, 4,5-dicyano-2-trifluoromethylimidazole, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium difluorobis(oxalatoborate), lithium hexafluorozirconate and lithium trifluoromethylsulfinate.

[0048] In some embodiments, the content of the lithium salt in the electrolyte is 10wt%-25wt%, such as 10wt%, 12wt%, 14wt%, 16wt%, 18wt%, 20wt%, 22wt%, 24wt%, and 25wt%.

[0049] In some embodiments, the electrolyte further includes an organic solvent, and the organic solvent includes propylene carbonate (PC), ethyl butyrate (EB), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate (EA), propyl acetate (PA), n-butyl acetate, isobutyl acetate, methyl propionate, propyl propionate (PP), ethyl propionate (EP), methyl butyrate, ethyl butyrate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TFEPE) and at least one of the fluorinated compounds of the above organic solvents.

[0050] A second aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte as described above.

[0051] In some embodiments, the lithium-ion battery is a lithium-ion secondary battery.

[0052] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on one or both sides of the negative electrode current collector surface, wherein the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material. Research has found that fluoroethylene carbonate can be reduced on the silicon surface in the silicon-based negative electrode to form nano-LiF, providing a dense SEI film, while the reduction product of the first nitrile additive represented by Formula I (such as Li3P, RP=O) can co-deposit with LiF to form a "LiF-Li3P-organophosphorus" composite SEI film, which effectively improves interfacial ion conduction, enhances the ductility of the SEI film, reduces electrolyte side reactions, ensures negative electrode interface stability, and thereby inhibits silicon particle breakage and expansion of the silicon-based negative electrode.

[0053] In some embodiments, the silicon-based material includes a silicon-carbon material and / or a silicon-oxygen material.

[0054] In some embodiments, the silicon content in the negative electrode active layer is 1.5wt%-30wt%, for example, 1.5wt%, 2wt%, 8wt%, 10wt%, 12wt%, 16wt%, 20wt%, 25wt%, or 30wt%.

[0055] In some embodiments, the negative electrode active layer includes a negative electrode conductor and a negative electrode binder.

[0056] In some embodiments, the negative electrode active layer includes 0.1 wt%-10 wt% of the negative electrode conductive agent, for example, 0.1 wt%, 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 7 wt%, 9 wt%, or 10 wt%.

[0057] In some embodiments, the negative electrode active layer includes 0.1 wt%-10 wt% of the negative electrode binder, for example, 0.1 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, or 10 wt%.

[0058] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector, wherein the positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium cobalt oxide. Studies have found that when the positive electrode material is lithium cobalt oxide (referred to as LCO), the LiF-rich CEI film generated by fluoroethylene carbonate in the electrolyte can inhibit the release of lattice oxygen in the lithium cobalt oxide in the positive electrode material. At the same time, the first nitrile additive shown in Formula I is oxidized to form a polymer containing P=O and cyano groups, which can fill the pores of the CEI film, thereby blocking the high-valent Co 4+ Contact with the electrolyte further improves the stability and safety of the battery at high voltage and high energy density.

[0059] In some embodiments, the positive active layer includes 80 wt%-99.8 wt% of the positive active material, for example, 80 wt%, 84 wt%, 86 wt%, 88 wt%, 90 wt%, 94 wt%, 98 wt%, or 99.8 wt%.

[0060] In some embodiments, the positive electrode active layer includes a positive electrode conductor and a positive electrode binder.

[0061] In some embodiments, the positive electrode active layer includes 0.1 wt%-10 wt% of the positive electrode conductive agent, for example, 0.1 wt%, 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 7 wt%, 9 wt%, or 10 wt%.

[0062] In some embodiments, the positive electrode active layer includes 0.1 wt%-10 wt% of the binder, for example, 0.1 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, or 10 wt%.

[0063] In some embodiments, the diaphragm includes a base film, the base film includes at least one of polyethylene and polypropylene, the diaphragm may also include a ceramic coating, the ceramic coating may include ceramic particles and a binder, the ceramic particles include at least one of aluminum oxide, boehmite, silicon dioxide, zirconium dioxide, barium sulfate, magnesium hydroxide, fluorapatite, fluorphlogopite, mullite, aluminum titanate, copper oxide, titanium dioxide, and zinc oxide, and the binder includes but is not limited to at least one of polyvinylidene fluoride compounds and polyacrylate compounds.

[0064] Unless otherwise specified, the raw materials and methods for preparing the lithium-ion battery described in the present invention are conventional choices in the art.

[0065] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0066] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0067] The present invention will be described in detail below with reference to specific embodiments. These embodiments are intended to help you understand the present invention but are not intended to limit it.

[0068] Example 1

[0069] (1) Preparation of positive electrode sheet: The positive electrode active material LiCoO2, the conductive agent carbon black, and the binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:1:1, and fully stirred to form a uniform positive electrode slurry. The positive electrode slurry is coated on the safety primer, dried, rolled, and cut to obtain the positive electrode sheet.

[0070] (2) Preparation of Negative Electrode Sheet: Artificial graphite, silicon-carbon material, conductive agent carbon black, binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose were weighed in a weight ratio of 80.7:17:0.6:1:0.7, dispersed in an appropriate amount of deionized water, and thoroughly stirred to form a uniform negative electrode slurry. The negative electrode slurry was then coated onto a negative electrode current collector copper foil, dried, rolled, and cut to obtain a negative electrode sheet containing a negative electrode active layer. The negative electrode active layer contained 9 wt% silicon.

[0071] (3) Preparation of electrolyte: In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate / ethyl difluoroacetate / EP / PP were mixed uniformly, with the EP / PP mass ratio being 15 / 27, the ethylene carbonate content being 4.9 wt% (i.e., B being 4.9), and the ethyl difluoroacetate content being 24.4 wt% (i.e., A being 24.4), and then 14 wt% of fully dried lithium hexafluorophosphate (LiPF6) based on the total mass of the electrolyte was added thereto. and 5wt% LiTFSI, and after dissolving, 13wt% of fluoroethylene carbonate (i.e., C is 13), 2wt% of 1,3,6-hexanetrinitrile, 1wt% of tris(2-cyanoethyl)phosphate (i.e., the compound represented by formula I-1, D is 1), 2wt% of the compound represented by formula III, 1wt% PS, 0.2wt% PST, 0.5wt% DTD, and 0.3wt% LiODFB were added based on the total mass of the electrolyte and stirred evenly. After passing the moisture and free acid tests, the electrolyte was obtained.

[0072] (4) Diaphragm: A gravure coating process is used to coat a coating layer slurry with ceramic particles as aluminum oxide (Al2O3) and a binder as PVDF on both sides of a PE diaphragm substrate, and the diaphragm is obtained after drying.

[0073] (5) Assembly of lithium-ion batteries: The positive electrode, negative electrode and separator are placed in order, with the separator located between the positive and negative electrodes. The electrodes are then welded and wound to obtain a core. The core is then placed in an aluminum-plastic film packaging bag. The process includes liquid injection, formation, secondary sealing, and sorting to prepare a lithium-ion battery.

[0074] Comparative Example 1

[0075] The operation was carried out according to the method described in Example 1, except that the electrolyte did not contain ethyl difluoroacetate, ethylene carbonate, fluoroethylene carbonate and one of the first nitrile additives, and their contents were shown in Table 1.

[0076] Example 2 and Comparative Example 2

[0077] The operation was carried out according to the method described in Example 1, except that the contents of ethyl difluoroacetate, ethylene carbonate and fluoroethylene carbonate in the electrolyte were different, as shown in Table 1.

[0078] Example 3 and Comparative Example 3

[0079] The operation was carried out according to the method described in Example 1, except that the type and content of the first nitrile additive were different, as shown in Table 1.

[0080] Table 1

[0081] serial number A B C The first nitrile additive D C / B A+B+C (B+C) / D A / (B+C) A / C Example 1 24.4 4.9 13 Compound represented by formula I-1 1 √ 42.3 17.9 1.36 1.88 Comparative Example 1-1 / * * * * / / / / / Comparative Example 1-2 * / * * * / / / / / Comparative Examples 1-3 * * / * * / / / / / Comparative Examples 1-4 * * * * / / / / / / Comparative Example 2-1 24.4 10 8 * * 0.8 42.4 18 1.4 3.1 Comparative Example 2-2 8 1 8 * * 8.0 17 9 0.9 1.0 Comparative Examples 2-3 47 9 15 * * 1.7 71 24 2.0 3.1 Comparative Examples 2-4 24.4 12.5 18 * * 1.4 54.9 30.5 0.8 1.4 Comparative Examples 2-5 24.4 1 2 * * 2.0 27.4 3 8.1 12.2 Example 2-1 24.4 1.5 16.5 * * 11.0 42.4 18 1.4 1.5 Example 2-2 34 7 16 * * 2.3 57 23 1.5 2.1 Example 2-3 12 3 9 * * 3.0 24 12 1.0 1.3 Examples 2-4 11 7 8 * * 1.1 26 15 0.7 1.4 Examples 2-5 50 1 8 * * 8.0 59 9 5.6 6.3 Examples 2-6 10 10 11 * * 1.1 31 21 0.5 0.9 Example 3-1 * * * Compound represented by formula I-2 * * * * * * Example 3-2 * * * Compound represented by formula I-3 * * * * * * Example 3-3 * * * Compound represented by formula I-4 * * * * * * Examples 3-4 * * * Compound represented by formula I-5 * * * * * * Examples 3-5 * * * * 0.6 * * 29.8 * * Examples 3-6 * * * * 4 * * 4.5 * * Comparative Example 3-1 * * * * 0.3 * * 59.7 * * Comparative Example 3-2 * * * * 5.5 * * 3.3 * *

[0082] Note: “*” indicates the same as Example 1, and “ / ” indicates not added or not calculated.

[0083] Example 4 Group

[0084] The operation was carried out according to the method described in Example 1, except that the type and content of the second nitrile additive were different, as shown in Table 2.

[0085] Table 2

[0086] serial number Type and content of the second nitrile additive Example 1 2 wt% 1,3,6-hexanetrinitrile Example 4-1 2wt% of the compound represented by formula II-1 Example 4-2 2wt% of the compound represented by formula II-3 Example 4-3 4.5 wt% 1,3,6-hexanetrinitrile Example 4-4 1 wt% 1,3,6-hexanetrinitrile Examples 4-5 7.5 wt% 1,3,6-hexanetrinitrile Examples 4-6 0.5 wt% 1,3,6-hexanetrinitrile

[0087] Example 5 Group

[0088] The operation was carried out according to the method described in Example 1, except that the types and contents of the functional additives were different, as shown in Table 3.

[0089] Table 3

[0090] serial number Type and content of functional additives / wt% Example 1 2wt% compound represented by formula III, 1wt% PS, 0.2wt% PST, 0.5wt% DTD Example 5-1 2wt% compound represented by formula III, 1wt% PS, 0.2wt% PST Example 5-2 2wt% compound represented by formula III, 1wt% PS Example 5-3 2 wt% of the compound represented by formula III Example 5-4 1wt%PS, 0.2wt%PST, 0.5wt%DTD

[0091] Test Case

[0092] The performance of the batteries prepared in the examples and comparative examples was tested. The specific testing methods are as follows. The test results are shown in Table 4.

[0093] (1) Low temperature cycle performance test: Place the lithium-ion battery in a 0℃ environment and let it stand for 4 hours. When the battery temperature is 0±2℃, discharge it at 0.2C to 3.0V; let it stand for 10 minutes; charge it at a constant current of 0.34C to the upper limit voltage (4.53V), then charge it at a constant voltage of 4.53V to 0.05C, then discharge it at a constant current of 0.5C to 3.0V, and let it stand for 10 minutes; this is one charge and discharge cycle, and the charge and discharge cycles are 150 times. The highest discharge capacity of the first three cycles is recorded as the initial capacity Q1, and the discharge capacity of the 150th cycle is Q2. Calculate the battery capacity retention rate = Q2 / Q1×100%.

[0094] (2) High temperature cycle performance test: Place the lithium-ion battery in a 45℃ environment and let it stand for 2 hours. When the battery temperature is 45±2℃, discharge it to 3.0V at 0.2C; let it stand for 10 minutes; charge it to 4.25V at 2C (current cutoff 1.5C) and then transfer it to 1.5C (cutoff 0.05C) to the upper limit voltage (4.53V), then charge it to 0.05C at a constant voltage of 4.53V, then discharge it to 3.5V at 1C and then discharge it to 3V at 0.7C, let it stand for 5 minutes. This is one charge and discharge cycle. The full charge thickness is recorded as T1, and the charge and discharge cycle is repeated 500 times. The highest discharge capacity of the first three cycles is recorded as the initial capacity Q3, the discharge capacity of the 500th cycle is Q4, and the final 500T full charge thickness is T2. Calculate the battery capacity retention rate = Q4 / Q3×100%, and the thickness expansion rate = (T2 / T1-1)×100%.

[0095] (3) 128℃ thermal shock test: The above lithium-ion battery is subjected to a 128℃ thermal shock test. The test process is as follows: First, the battery is charged to the upper limit voltage at a constant current and constant voltage of 0.7C, and the cut-off current is 0.05C. The initial state of the battery, including voltage, internal resistance, thickness, etc., is tested; then the above battery is placed in a convection or circulating hot air box, heated at an initial temperature of 25±3℃, with a heating rate of 5±2℃, and heated to 128±2℃ for 60 minutes. The test ends after the test. The passing standard of the 128℃ thermal shock test is: the battery cell does not catch fire or explode. The results are shown in the form of pass number / test number, such as 10 / 15, which means that 10 out of 15 test batteries passed.

[0096] Table 4

[0097]

[0098]

[0099] It can be seen from the data of Comparative Example 1 that when the electrolyte lacks one of ethyl difluoroacetate, ethylene carbonate, fluoroethylene carbonate and the first nitrile additive, the prepared battery cannot have good low-temperature and high-temperature performance. For example, although the batteries prepared in Comparative Examples 1-4 have good capacity retention under low-temperature conditions, their cycle performance under high-temperature conditions is poor and their thermal shock resistance is poor.

[0100] Comparison of Example 1 with Comparative Example 2 and Example 2 shows that when the battery does not satisfy at least one of C>B, 20≤A+B+C≤60, and 4≤(B+C) / D≤40, the resulting battery cannot adapt to a wide temperature range operating environment. When A+B+C is too large or too small, the battery's high-temperature performance and thermal safety performance deteriorate significantly. When B and (B+C) / D are too large, the high-temperature performance and thermal safety performance deteriorate significantly. When C and (B+C) / D are too small, the low-temperature performance deteriorates significantly. Comparison of Example 1 with Example 2 shows that, while satisfying C>B, 20≤A+B+C≤60, and 4≤(B+C) / D≤40, further satisfying 0.8≤A / (B+C)≤5, 1.05≤A / C≤3.5, and 1.1≤C / B≤10 can further improve the battery's performance in a wide temperature range operating environment.

[0101] A comparison of Example 1 and Example 3 shows that different first nitrile compounds can improve the battery's cycling performance under low- and high-temperature conditions to a certain extent and reduce the thickness expansion rate under high-temperature conditions. Furthermore, a comparison of Example 1 with Examples 3-5 and 3-6, as well as Comparative Examples 3-1 and 3-2, shows that as the content of the first nitrile additive increases, the battery's low- and high-temperature performance, as well as its thermal shock resistance, first increases and then decreases. Furthermore, when 4 ≤ (B + C) / D ≤ 40 is satisfied, the overall battery performance is significantly improved.

[0102] From the comparison between Example 1 and Example 4, it can be seen that when the second nitrile additive contained in the electrolyte is 1,3,6-hexanetrinitrile, it is more conducive to improving the overall performance of the battery, and when the content of the second nitrile additive gradually increases, the performance of the battery at low and high temperatures shows a trend of first increasing and then decreasing. When the content of the second nitrile additive is 1-4wt%, the effect is relatively better.

[0103] From the comparison between Example 1 and Example 5, it can be seen that when the functional additive contains the compound represented by Formula III, it is beneficial to improve the high-temperature performance of the battery; when the functional additive contains the compound represented by Formula III, PS, PST and DTD at the same time, the overall performance of the prepared battery is better.

[0104] It should be noted that, in this article, the terms "comprise", "include" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present invention is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in the opposite order according to the functions involved. For example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises Awt% of ethyl difluoroacetate, Bwt% of ethylene carbonate, Cwt% of fluoroethylene carbonate, and Dwt% of a first nitrile additive, wherein C>B, 20≤A+B+C≤60, and 4≤(B+C) / D≤40; Wherein, the first nitrile additive comprises a compound represented by formula I, Wherein, R1, R2, and R3 are each independently selected from O, substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy, substituted or unsubstituted C2-C10 alkenyl, substituted or unsubstituted C2-C10 alkynyl, and the substituent is halogen.

2. The electrolyte according to claim 1, characterized in that 10≤A≤50； and / or, 1≤B≤10; and / or, 8≤C≤18; and / or, 0.1≤D≤5.

3. The electrolyte according to claim 1 or 2, characterized in that A>C; And / or, 0.8≤A / (B+C)≤5.

4. The electrolyte according to any one of claims 1 to 3, characterized in that 1.1≤C / B≤10; And / or, 1.05≤A / C≤3.

5.

5. The electrolyte according to any one of claims 1 to 4, characterized in that R1, R2, and R3 are each independently selected from O, substituted or unsubstituted C1-C5 alkyl, substituted or unsubstituted C1-C5 alkoxy, substituted or unsubstituted C2-C4 alkenyl, or substituted or unsubstituted C2-C4 alkynyl, and the substituent is F; Preferably, the first nitrile additive comprises at least one of the compounds represented by Formula I-1 to Formula I-5:

6. The electrolyte according to any one of claims 1 to 5, characterized in that The electrolyte includes a second nitrile additive, and the second nitrile additive includes at least one of 1,3,6-hexane trinitrile, glycerol trinitrile, and compounds represented by formula II-1 to formula II-4: Preferably, in the electrolyte, the content of the second nitrile additive is 0.6 wt%-7 wt%.

7. The electrolyte according to any one of claims 1 to 6, characterized in that The electrolyte includes a functional additive, wherein the functional additive includes at least one of vinyl sulfate, 1,3-propane sultone, 1,3-propylene sultone, tetravinylsilane, tris(trimethylsilyl)borate, hexamethyldisilazane, fluorobenzene, triphenyl phosphite and a compound represented by formula III. Preferably, the content of the functional additive in the electrolyte is less than 6 wt %.

8. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte according to any one of claims 1 to 7.

9. The lithium-ion battery according to claim 8, characterized in that The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on one side or both sides of the surface of the negative electrode current collector, the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material; Preferably, the silicon content in the negative electrode active layer is 1.5 wt%-30 wt%.

10. The lithium-ion battery according to claim 8 or 9, characterized in that The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium cobalt oxide.

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