Non-aqueous electrolyte and secondary battery

Abstract

A non-aqueous electrolyte and a secondary battery. The non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive. The additive comprises an imidazole compound and a cyclotriphosphazene compound; the ratio of the weight of the imidazole compound to the sum of the weights of the organic solvent and the additive in the non-aqueous electrolyte is 0.2% to 10%, and the ratio of the weight of the cyclotriphosphazene compound to the sum of the weights of the organic solvent and the additive in the non-aqueous electrolyte is 0.5% to 2%. The non-aqueous electrolyte can adapt to a high-voltage positive electrode material, and has good wettability for a separator, and a prepared lithium-ion secondary battery has good cycle performance, high-temperature storage performance, safety and stability in a high-voltage state.

Description

Non-aqueous electrolyte and secondary battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on August 25, 2023, with application number 2023110794783 and invention name “Non-aqueous Electrolyte and Secondary Battery”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of secondary batteries, and in particular to a non-aqueous electrolyte and a secondary battery. Background Art

[0003] The use of high-voltage positive electrode materials in traditional technology is one of the most effective ways to improve the energy density of lithium-ion batteries, but the operating voltage of the battery often leads to performance degradation. Because at high voltage, on the one hand, the crystal structure of the battery positive electrode has certain instability, and during the charge and discharge process, the structure will collapse, resulting in performance deterioration; on the other hand, at high voltage, the positive electrode surface is in a highly oxidized state and has high activity, which easily catalyzes the oxidative decomposition of the electrolyte. The decomposition products of the electrolyte are easily deposited on the positive electrode surface, blocking the lithium ion deintercalation channel, thereby deteriorating the battery performance. In addition to the structural stability of the electrode active material being challenged, the performance of the electrolyte not only greatly affects the performance of the battery, but also poses a safety hazard. Therefore, there is an urgent need to develop a secondary battery electrolyte with good cycle performance and high-temperature storage performance.

[0004] Summary of the Invention

[0005] Based on this, the objectives of this application include providing a non-aqueous electrolyte for a secondary battery that is compatible with high-voltage positive electrode materials, and the lithium-ion battery prepared therefrom has good medium-temperature cycling performance and high-temperature storage performance. In addition, this application also provides a secondary battery.

[0006] A first aspect of the present application provides a non-aqueous electrolyte for a secondary battery, the non-aqueous electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the additive comprises an imidazole compound and a cyclotriphosphazene compound;

[0007] The ratio of the weight of the imidazole compound to the sum of the weight of the organic solvent and the additive in the non-aqueous electrolyte is 0.2% to 10%;

[0008] The ratio of the weight of the cyclotriphosphazene compound to the sum of the weight of the organic solvent and the additive in the non-aqueous electrolyte is 0.5% to 2%.

[0009] In some embodiments, in the non-aqueous electrolyte, the imidazole compound has the following structure:

[0010] R1 to R4 in formula (I) are independently selected from halogen, C1 to C 20 Alkoxy, C1~C 20 Alkyl or halogen substituted C1~C 20 An alkyl group; X is selected from O, S or NH;

[0011] The cyclotriphosphazene compound has the following structure:

[0012] R5~R in formula (II) 10 Each independently selected from halogen, C1-C 20 Alkoxy, C1~C 20 Alkyl or halogen substituted C1~C 20 of alkyl.

[0013] In some embodiments, in the non-aqueous electrolyte, the weight ratio of the imidazole compound to the cyclotriphosphazene compound is (0.1-10):1.

[0014] In some embodiments, in the non-aqueous electrolyte, the concentration of the lithium salt in the non-aqueous electrolyte is 0.5 mol / L to 1.5 mol / L.

[0015] In some embodiments, in the non-aqueous electrolyte, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium trifluorosulfonyl, lithium difluoro(trifluoromethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methyl and lithium bis(trifluoroimidesulfonate).

[0016] In some embodiments, in the non-aqueous electrolyte, the organic solvent is selected from one or more of carbonate compounds and furan compounds.

[0017] In some embodiments, in the non-aqueous electrolyte, the additive further comprises one or more of a sulfonate compound, a fluorocarbonate, and a nitrile compound, wherein the nitrile compound contains a cyano group in its structure.

[0018] In some embodiments, the non-aqueous electrolyte comprises, by weight, 80 to 120 parts of carbonate compounds, 2 to 5 parts of sulfonate compounds, 2 to 5 parts of nitrile compounds, 6 to 9 parts of fluorocarbonate compounds, 0.1 to 10 parts of imidazole compounds, and 0.8 to 1.2 parts of cyclotriphosphazene compounds.

[0019] In some embodiments, the non-aqueous electrolyte satisfies at least one of the following characteristics:

[0020] (1) The carbonate compound is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate and methyl butyrate;

[0021] (2) The furan compound is selected from one or more of tetrahydrofuran, furancarboxylic acid, nitrofurazone and furazolidinone;

[0022] (3) the fluorocarbonate compound is selected from one or more of fluoroethylene carbonate, fluoroethyl methyl carbonate and fluoropropylene carbonate;

[0023] (4) The sulfonate compound is selected from one or more of 1,3-propane sultone, 1,3-propylene sultone, and 3-fluoro-1,3-propane sultone;

[0024] (5) The nitrile compound is selected from one or more of succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile and glyceroltrinitrile.

[0025] A second aspect of the present application provides a secondary battery comprising the non-aqueous electrolyte described in the first aspect.

[0026] The non-aqueous electrolyte for a secondary battery provided in the present application uses a non-aqueous solution of an organic solvent and a lithium salt, including additives containing a specific content of imidazole compounds and cyclotriphosphazene compounds. When applied to lithium-ion secondary batteries, it can adapt to high-voltage positive electrode materials and increase the wettability of the electrolyte to the diaphragm. The imidazole compound can also remove water and hydrogen fluoride from the electrolyte, thereby reducing the production of byproducts on the surface of the SEI film during battery cycling and inhibiting the collapse of the electrode active material structure. The electron-donating group -P=N- of the cyclotriphosphazene compound can also prevent it from attacking organic solvents (such as fluoroethylene carbonate), thereby reducing the decomposition and gas production of the electrolyte. The prepared lithium-ion battery has good medium-temperature cycling performance (for example, 45°C) and high-temperature storage performance (for example, 85°C).

[0027] The preparation method of the non-aqueous electrolyte provided in the present application has simple steps, and the obtained non-aqueous electrolyte can be applied to batteries containing high-voltage positive electrode materials.

[0028] The secondary battery provided in the present application has a high capacity retention rate and a low thickness expansion rate when cycled at high voltage and high temperature, and the capacity characteristics of the secondary battery are still good after being stored under high temperature conditions. DETAILED DESCRIPTION

[0029] To facilitate understanding of the present application, the present application will be described in more detail below. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, the purpose of providing these embodiments is to provide a more thorough and comprehensive understanding of the disclosure of the present application.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0031] the term

[0032] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0033] Lithium-ion secondary batteries, also known as lithium-ion batteries or lithium batteries, generally have a negative electrode comprising non-lithium compounds such as hard carbon, carbon microspheres, natural graphite, artificial graphite, etc., and a positive electrode comprising a lithium-containing metal oxide.

[0034] "Alkyl" refers to a monovalent residue formed by the loss of a hydrogen atom from a saturated hydrocarbon containing a primary (normal) carbon atom, a secondary carbon atom, a tertiary carbon atom, a quaternary carbon atom, or a combination thereof. Phrases containing this term, for example, "C1-C6 alkyl" refers to an alkyl group containing 1 to 6 carbon atoms, each occurrence of which can be independently C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl or C6 alkyl. Suitable examples include, but are not limited to, ethyl (Et, -CH2CH3), 1-propyl (n-Pr, n-propyl, -CH2CH2CH3), 2-propyl (i-Pr, i-propyl, -CH(CH3)2), 1-butyl (n-Bu, n-butyl, -CH2CH2CH2CH3), 2-methyl-1-propyl (i-Bu, i-butyl, -CH2CH(CH3)2), 2-butyl (s-Bu, s-butyl, -CH(CH3)CH2C H3), 2-methyl-2-propyl (t-Bu, t-butyl, -C(CH3)3), 1-pentyl (n-pentyl, -CH2CH2CH2CH2CH3), 2-pentyl (-CH(CH3)CH2CH2CH3), 3-pentyl (-CH(CH2CH3)2), 2-methyl-2-butyl (-C(CH3)2CH2CH3), 3-methyl-2-butyl (-CH(CH3)CH(CH3)2), 3-methyl-1-butyl (-CH 2CH2CH(CH3)2), 2-methyl-1-butyl (-CH2CH(CH3)CH2CH3), 1-hexyl (-CH2CH2CH2CH2CH2CH3), 2-hexyl (-CH(CH3)CH2CH2CH2CH3), 3-hexyl (-CH(CH2CH3)(CH2CH2CH3)), 2-methyl-2-pentyl (-C(CH3)2CH2CH2CH3), 3-methyl-2-pentyl (-CH(CH3) CH(CH3)CH2CH3), 4-methyl-2-pentyl (-CH(CH3)CH2CH(CH3)2), 3-methyl-3-pentyl (-C(CH3)(CH2CH3)2), 2-methyl-3-pentyl (-CH(CH2CH3)CH(CH3)2), 2,3-dimethyl-2-butyl (-C(CH3)2CH(CH3)2) and 3,3-dimethyl-2-butyl (-CH(CH3)C(CH3)3. For example, "C1~C 20 "Alkyl" refers to an alkyl group containing 1 to 20 carbon atoms, each occurrence of which can be independently C1 alkyl to C 20 One of the alkyl groups.

[0035] "Alkoxy" refers to a group having an -O-alkyl group, i.e., an alkyl group as defined above connected to a parent core structure via an oxygen atom. Phrases containing this term, for example, "C1-C6 alkoxy" means that the alkyl portion contains 1 to 6 carbon atoms, and each occurrence can be independently C1 alkoxy, C2 alkoxy, C3 alkoxy, C4 alkoxy, C5 alkoxy or C6 alkoxy. Suitable examples include, but are not limited to, methoxy (-O-CH3 or -OMe), ethoxy (-O-CH2CH3 or -OEt), propoxy (-O-(CH2)2CH3 or -O-CH(CH3)2), tert-butoxy (-OC(CH3)3 or -OtBu), pentoxy (-O-(CH2)4CH3 or -O-CH(CH3)-(CH2)2CH3), propoxy (-O-(CH2)5CH3). For example, "C1-C 20 "Alkoxy" means that the alkyl portion contains 1 to 6 carbon atoms, each occurrence can be independently C1 alkoxy to C 20 One of the alkoxy groups.

[0036] "Halogen" or "halo" refers to -F, -Cl, -Br or -I.

[0037] Herein, the terms "preferred," "better," "more preferred," and "suitable" are used solely to describe preferred implementations or examples and should not be construed as limiting the scope of protection of this application. If multiple "preferred" terms appear in a technical solution, each "preferred" term is considered independent unless otherwise specified and there are no contradictions or mutual constraints.

[0038] In this application, "further", "further", "particularly" and the like are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of this application.

[0039] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions of the listed features.

[0040] In this application, when it comes to a numerical interval (i.e., a numerical range), unless otherwise specified, the distribution of the optional numerical values ​​in the numerical interval is considered continuous and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical interval, and each numerical value between the two numerical endpoints. Unless otherwise specified, when a numerical interval only refers to an integer in the numerical interval, including the two endpoint integers of the numerical range and each integer between the two endpoints is equivalent to directly enumerating each integer. When multiple numerical ranges are provided to describe a feature or characteristic, these numerical ranges can be merged. In other words, unless otherwise specified, the numerical range disclosed herein should be understood to include any and all subranges included therein. The "numerical value" in the numerical interval can be any quantitative value, such as a number, a percentage, a ratio, etc. "Numerical interval" allows for broadly including quantitative intervals such as percentage intervals, ratio intervals, and ratio intervals.

[0041] Unless otherwise specified, the temperature parameters in this application allow for both constant temperature treatment and temperature fluctuations within a certain temperature range. It should be understood that the constant temperature treatment allows for temperature fluctuations within the accuracy range of instrument control. Fluctuations within ranges such as ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are permitted.

[0042] Lithium-ion batteries, due to their advantages such as rechargeability, light weight, compact size, and long cycle life, are widely used in consumer electronics such as mobile phones and computers. They are also an excellent choice for electric vehicles and large-scale energy storage devices. However, with technological advancements, market demands for higher energy density in batteries are increasing, making it difficult for current commercial lithium-ion batteries to meet these requirements. Using high-voltage cathode materials is one of the most effective ways to increase the energy density of lithium-ion batteries, but increasing the operating voltage of lithium-ion batteries often results in performance degradation.

[0043] Under high voltage, the battery's electrode materials and electrolyte materials are subject to greater conditions. On the one hand, the crystal structure of the battery's positive electrode is somewhat unstable, and during the high-voltage charge and discharge process, structural collapse may occur, leading to performance deterioration; on the other hand, under high voltage, the positive electrode surface is in a highly oxidized state, and its high activity easily catalyzes the oxidative decomposition of the electrolyte. The decomposition products are deposited on the surface of the positive electrode material, which may cause blockage of the lithium ion deintercalation channel, and thus deteriorate the battery performance.

[0044] The purpose of the present invention is to provide a non-aqueous electrolyte for lithium-ion secondary batteries with good cycle performance and high-temperature storage performance, aiming to solve the problems of insufficient cycle performance and high-temperature storage performance and safety and stability of existing lithium-ion battery electrolytes under high-voltage conditions.

[0045] A first aspect of the present application is a non-aqueous electrolyte for a secondary battery. The non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive. The additive includes an imidazole compound and a cyclotriphosphazene compound.

[0046] In some embodiments, in the non-aqueous electrolyte, the ratio of the weight of the imidazole compound to the sum of the weight of the organic solvent and the additive in the non-aqueous electrolyte is 0.2% to 10%, and can also be selected from any one of the following ratios or an interval consisting of any two ratios: 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 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%, 10%, etc.

[0047] In some embodiments, in the non-aqueous electrolyte, the ratio of the weight of the cyclotriphosphazene compound to the sum of the weights of the organic solvent and the additive in the non-aqueous electrolyte is 0.5% to 2%, and can also be selected from any one of the following ratios or an interval consisting of any two ratios: 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, 2%, etc.

[0048] The non-aqueous electrolyte for secondary batteries provided in the present application adopts a non-aqueous solution and a lithium salt containing additives including specific contents of imidazole compounds and cyclotriphosphazene compounds and an organic solvent. Appropriate contents of imidazole compounds and cyclotriphosphazene compounds can act synergistically. Its application in lithium-ion secondary batteries can adapt to high-voltage positive electrode materials, increase the wettability of the electrolyte on the diaphragm, reduce byproducts on the surface of the SEI film, inhibit the structural collapse of the electrode active material, and reduce the decomposition and gas production of the electrolyte. It can improve the chemical stability of the electrolyte components during battery cycling at high voltage, thereby improving the battery's cycling performance and safety, as manifested in the lithium-ion battery having good medium-temperature cycling performance and high-temperature storage performance.

[0049] Specifically, the imidazole compounds can also remove water and hydrogen fluoride from the electrolyte, thereby reducing the production of by-products on the surface of the SEI film during battery cycling, inhibiting the dissolution of cobalt ions in electrode materials such as lithium cobalt oxide, thereby avoiding the collapse of the electrode material structure; the electron-donating group -P=N- in the cyclotriphosphazene compounds can also capture the Lewis acid PF5 in the electrolyte and the steric hindrance of the benzene-like structure to prevent it from attacking organic solvents (such as fluoroethylene carbonate), thereby reducing the gas production of the electrolyte and avoiding excessive expansion of the battery.

[0050] In some embodiments, in the non-aqueous electrolyte, the weight ratio of the imidazole compound to the non-aqueous electrolyte is (0.002-0.1):1, further can be (0.0028-0.085):1, and can also be selected from any one of the following weight ratios or any two weight ratios: 0.002, 0.0028, 0.003, 0.004, 0.0046, 0.005, 0. 0.06, 0.007, 0.008, 0.009, 0.0092, 0.01, 0.015, 0.018, 0.02, 0.025, 0.03, 0.035, 0.04, 0.044, 0.045, 0.05, 0.055, 0.06, 0.065, 0.07, 0.075, 0.08, 0.085, 0.09, 0.095, 0.1, etc. A more suitable weight ratio of the imidazole compound to the non-aqueous electrolyte is beneficial to further improve the chemical stability of the electrolyte components during battery cycling at high voltage, thereby improving the cycle performance and safety of the battery. If the weight ratio of the imidazole compound to the non-aqueous electrolyte is too high, it may lead to poor compatibility with the positive and negative electrode interfaces, thereby causing cycle degradation; if the weight ratio of the imidazole compound to the non-aqueous electrolyte is too low, it may cause insufficient protection of the interface, thereby failing to achieve the above-mentioned improvement effect.

[0051] In some embodiments, in the non-aqueous electrolyte, the weight ratio of the cyclotriphosphazene compound to the non-aqueous electrolyte is (0.005-0.05):1, further can be (0.0085-0.009):1, and can also be selected from any one of the following weight ratios or a range consisting of any two weight ratios: 0.005, 0.006, 0.007, 0.008, 0.0085, 0.0088, 0.009, 0.0091, 0.0092, 0.01, 0.015, 0.018, 0.02, 0.025, 0.03, 0.035, 0.04, 0.044, 0.045, 0.05, etc. A more appropriate weight ratio of the cyclotriphosphazene compound to the non-aqueous electrolyte is beneficial for further improving electrolyte gassing and preventing excessive battery expansion. If the weight ratio of the cyclotriphosphazene compound to the non-aqueous electrolyte is too high, it may lead to poor interface compatibility with the negative electrode side, and thus poor cycle stability; if the weight ratio of the cyclotriphosphazene compound to the non-aqueous electrolyte is too low, it may cause insufficient interface protection effect on the positive electrode, which is insufficient to achieve the above-mentioned effect.

[0052] In some embodiments, in the non-aqueous electrolyte, the imidazole compound has the following structure:

[0053] R1 and R2 in formula (I) are independently selected from halogen, C1-C20 Alkoxy, C1~C 20 Alkyl or halogen substituted C1~C 20 of alkyl.

[0054] In some embodiments, in the non-aqueous electrolyte, R1 and R2 in formula (I) are each independently selected from halogen, C1-C6 alkoxy, C1-C6 alkyl or halogen-substituted C1-C6 alkyl; and X is selected from O, S or NH.

[0055] In some embodiments, in the non-aqueous electrolyte, the cyclotriphosphazene compound has the following structure:

[0056] R5~R in formula (II) 10 Each independently selected from halogen, C1-C 20 Alkoxy, C1~C 20 Alkyl or halogen substituted C1~C 20 of alkyl.

[0057] In some embodiments, in the non-aqueous electrolyte, R5 to R 10 Each is independently selected from halogen, C1-C6 alkoxy, C1-C6 alkyl or halogen-substituted C1-C6 alkyl.

[0058] In some embodiments, in the non-aqueous electrolyte, R5 to R 10 All are halogen, and further all are -F.

[0059] In some embodiments, in the non-aqueous electrolyte, the weight ratio of the imidazole compound to the cyclotriphosphazene compound is (0.1-10):1, further can be (0.2-10):1, further can be (0.3-10):1, and any of the following weight ratios or any two weight ratios can also be selected: 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc. A more appropriate weight ratio of imidazole compounds to cyclotriphosphazene compounds can further improve the wettability of the electrolyte to the separator, reduce byproducts on the surface of the SEI film, inhibit the structural collapse of the electrode active material, and reduce the decomposition and gas production of the electrolyte, thereby improving the chemical stability of the electrolyte components during battery cycling at high voltages. If the weight ratio of the imidazole compound to the cyclotriphosphazene compound is too high, it may lead to poor interfacial compatibility and poor cycling performance. If the weight ratio of the imidazole compound to the cyclotriphosphazene compound is too low, it may result in insufficient interface protection and excessive HF content, which may still damage the positive and negative electrode interface films.

[0060] In some embodiments, the concentration of the lithium salt in the non-aqueous electrolyte is 0.5 mol / L to 1.5 mol / L (also 0.5 M to 1.5 M), further 0.8 mol / L to 1.3 mol / L, and can also be selected from any one of the following concentrations or any two of the following concentrations: 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, etc. A more appropriate concentration of lithium salt in the non-aqueous electrolyte is conducive to further improving the long cycle life of the battery. If the concentration of lithium salt in the non-aqueous electrolyte is too high, it may cause the viscosity of the electrolyte to be too large, which also affects the rate of the entire battery system; if the concentration of lithium salt in the non-aqueous electrolyte is too low, it may cause the conductivity of the electrolyte to be low, thereby adversely affecting the rate and cycle performance of the entire battery system.

[0061] It can be understood that the volume molar concentration (ie, the amount of substance concentration) represents mol / L and can be abbreviated as "M".

[0062] In some embodiments, in the non-aqueous electrolyte, the lithium salt is selected from at least one of an organic lithium salt and an inorganic lithium salt.

[0063] In some embodiments, in the non-aqueous electrolyte, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium trifluorosulfonyl, lithium difluoro(trifluoromethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methyl and lithium bis(trifluoroimidesulfonate).

[0064] In some embodiments, in the non-aqueous electrolyte, the organic solvent is selected from one or more of carbonate compounds and furan compounds.

[0065] In some embodiments, in the non-aqueous electrolyte, the additive further comprises one or more of a sulfonate compound, a fluorocarbonate, and a nitrile compound, wherein the nitrile compound contains a cyano group in its structure.

[0066] In some embodiments, the non-aqueous electrolyte comprises, by weight, 80 to 120 parts of carbonate compounds, 2 to 5 parts of sulfonate compounds, 2 to 5 parts of nitrile compounds, 6 to 9 parts of fluorocarbonate compounds, 0.1 to 10 parts of imidazole compounds, and 0.8 to 1.2 parts of cyclotriphosphazene compounds.

[0067] In some embodiments, the non-aqueous electrolyte includes 80 to 120 parts of carbonate compounds by weight, further 100 to 120 parts, further 100 to 110 parts, and can also be selected from any one or two of the following weight parts: 80 parts, 90 parts, 100 parts, 110 parts, 120 parts, etc.

[0068] In some embodiments, the non-aqueous electrolyte comprises 2 to 5 parts by weight of a sulfonate compound, further comprising 2 to 4 parts, and even further comprising 3 to 4 parts. The amount may also be selected from any one or a combination of the following: 2 parts, 3 parts, 4 parts, 5 parts, etc. A suitable amount of sulfonate in the non-aqueous electrolyte further improves the stability of the negative electrode SEI film, thereby enhancing the battery's high-temperature storage performance and long-term cycling performance. If the amount of sulfonate in the non-aqueous electrolyte is too high, it may result in insufficient electrolyte dynamics and poor cycling performance. If the amount of sulfonate in the non-aqueous electrolyte is too low, it may result in insufficient protection of the negative electrode interface and fail to achieve the desired protective effect.

[0069] In some embodiments, the non-aqueous electrolyte comprises 2 to 5 parts of a nitrile compound by weight, further comprising 2 to 4 parts, further comprising 3 to 4 parts, or selected from any one of the following weight parts or any two weight parts: 2 parts, 3 parts, 4 parts, 5 parts, etc. A more appropriate weight ratio of the nitrile compound in the non-aqueous electrolyte is beneficial for further improving the positive electrode interface stability and enhancing high-temperature cycle performance. If the weight ratio of the nitrile compound in the non-aqueous electrolyte is too high, it may lead to poor compatibility with the negative electrode, increased cyclic polarization, and rapid capacity decay; if the weight ratio of the nitrile compound in the non-aqueous electrolyte is too low, it may result in insufficient protection of the positive electrode material and significant capacity decay.

[0070] In some embodiments, the non-aqueous electrolyte includes 6 to 9 parts of fluorocarbonate compounds by weight, and can further be 7 to 8 parts, or can be selected from any one or two parts by weight of the following ranges: 6 parts, 7 parts, 8 parts, and 9 parts.

[0071] In some embodiments, the non-aqueous electrolyte comprises, by weight, 0.1 to 10 parts of an imidazole compound, further 0.2 to 10 parts, further 0.3 to 10 parts, or any one or two parts selected from the following weight ranges: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. A more appropriate weight ratio of the imidazole compound in the non-aqueous electrolyte is beneficial for further improving the cycle characteristics of the resulting battery at higher temperatures. If the weight fraction of imidazole compounds in the non-aqueous electrolyte is too high, it may lead to poor compatibility with the positive and negative electrode interfaces, and thus lead to cycle degradation; if the weight fraction of imidazole compounds in the non-aqueous electrolyte is too low, it may cause the electrolyte to have a low degree of wettability on the diaphragm, reduce the deposition of by-products on the SEI surface during the cycle, increase the amount of other metal elements other than lithium ions in the electrode active material dissolved in the electrolyte, and there is a lot of residual hydrogen fluoride in the electrolyte, the battery produces serious gas, and the capacity retention rate of the prepared battery drops rapidly when cycled at high temperature.

[0072] In some embodiments, the non-aqueous electrolyte comprises 0.8 to 1.2 parts by weight of a cyclotriphosphazene compound, further 0.9 to 1.2 parts, and even further 1 to 1.2 parts, or any one or two parts by weight selected from the following ranges: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, etc. A more appropriate amount of cyclotriphosphazene compound in the non-aqueous electrolyte is beneficial for further improving the cycling characteristics of the resulting battery at higher temperatures. If the weight fraction of cyclotriphosphazene compounds in the non-aqueous electrolyte is too high, it may lead to poor compatibility with the negative electrode side interface, and thus poor cycle stability; if the weight fraction of cyclotriphosphazene compounds in the non-aqueous electrolyte is too low, it may cause the content of the component in the electrolyte that can capture Lewis acid PF5 to decrease, the organic solvent in the electrolyte (such as fluoroethylene carbonate) to decompose to a high degree, and the gas production rate of the electrolyte to be faster, causing excessive expansion of the battery.

[0073] In some embodiments, in the non-aqueous electrolyte, the carbonate compound is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate and methyl butyrate.

[0074] In some embodiments, in the non-aqueous electrolyte, the furan compound is selected from one or more of tetrahydrofuran, furancarboxylic acid, nitrofurazone and furazolidone.

[0075] In some embodiments, in the non-aqueous electrolyte, the fluorinated carbonate compound is selected from one or more of fluorinated ethylene carbonate, fluorinated ethyl methyl carbonate, and fluorinated propylene carbonate.

[0076] In some embodiments, in the non-aqueous electrolyte, the sulfonate compound is selected from one or more of 1,3-propane sultone, 1,3-propylene sultone, and 3-fluoro-1,3-propane sultone.

[0077] In some embodiments, in the non-aqueous electrolyte, the nitrile compound is selected from one or more of succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile and glyceroltrinitrile.

[0078] The present application also provides a method for preparing the non-aqueous electrolyte according to the first aspect, comprising the following steps:

[0079] The lithium salt, the organic solvent and the additive are mixed to obtain the non-aqueous electrolyte.

[0080] The preparation method of the non-aqueous electrolyte provided in the present application has simple steps, and the obtained non-aqueous electrolyte can be applied to batteries containing high-voltage positive electrode materials.

[0081] In a third aspect of the present application, a secondary battery is provided, comprising the non-aqueous electrolyte described in the first aspect or the non-aqueous electrolyte prepared by the preparation method described in the second aspect.

[0082] The secondary battery provided in the present application has a high capacity retention rate and a low thickness expansion rate when cycled at high voltage and high temperature, and the capacity characteristics of the secondary battery are still good after being stored under high temperature conditions.

[0083] In some embodiments, the secondary battery comprises a positive electrode sheet, a negative electrode sheet, a lithium battery separator and an electrolyte, wherein the electrolyte is the non-aqueous electrolyte described in aspect 1. It is understood that the secondary battery here refers to a lithium-ion secondary battery.

[0084] In a lithium-ion secondary battery, the positive electrode sheet includes a positive electrode current collector and a positive electrode active slurry layer located on the positive electrode current collector, wherein the positive electrode active slurry layer includes a positive electrode active material. The negative electrode sheet includes a negative electrode current collector and a negative electrode active slurry layer located on the negative electrode current collector, wherein the negative electrode active slurry layer includes a negative electrode active material. The specific types of the positive electrode active material, positive electrode binder, and negative electrode active material are not subject to specific restrictions and can be selected according to needs.

[0085] Preferably, the positive electrode active material is one or more selected from lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary material, lithium ferrous phosphate (LiFePO4) and lithium manganese oxide (LiMn2O4).

[0086] Preferably, the negative electrode active material is graphite or graphite doped with silicon. If graphite is selected, it can be natural graphite, artificial graphite, mesophase microcarbon beads (abbreviated as MCMB), hard carbon, and soft carbon. When graphite doped with silicon is selected, it can be silicon oxide, silicon-carbon composite. In addition, the negative electrode materials include: Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O 12 , Li-Al alloys can be used as negative electrode active materials.

[0087] The following are specific examples.

[0088] In order to make it easier to understand and implement the present invention, the following easier-to-implement, more specific and detailed embodiments and comparative examples are provided as reference.

[0089] Unless otherwise specified, the raw materials used in the following experiments can be purchased from the market.

[0090] Example 1

[0091] (1) Preparation of electrolyte

[0092] EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate), and PP (propyl propionate) were mixed in a mass ratio of 1:1:2:6 to form an organic solvent. Additives including PS (1,3-propane sultone), FEC (fluoroethylene carbonate), and the nitrile compounds SN (succinonitrile), AND (adiponitrile), and HTCN (1,3,6-hexanetricarbonitrile) were added to the organic solvent and mixed thoroughly. Lithium hexafluorophosphate was then added to obtain a mixed solution with a lithium hexafluorophosphate concentration of 1.1 mol / L. N-heptafluorobutyrylimidazole (Compound A) and hexafluorocyclotriphosphazene (Compound B) were then added to the mixed solution to obtain an electrolyte (see Table 1).

[0093] (2) Battery production

[0094] Production of positive electrode:

[0095] The positive electrode active material LCO, the conductive agent CNT, and the binder polyvinylidene fluoride were thoroughly stirred and mixed in an N-methylpyrrolidone solvent at a weight ratio of 97:1.5:1.5 to form a uniform positive electrode slurry. This slurry was coated on the positive electrode current collector Al foil, dried, and cold pressed to obtain a positive electrode sheet.

[0096] Production of negative electrode:

[0097] Graphite (the negative electrode active material), acetylene black (the conductive agent), styrene-butadiene rubber (the binder), and sodium carboxymethyl cellulose (the thickener) are thoroughly mixed in a suitable amount of deionized water at a mass ratio of 95:2:2:1 to form a uniform negative electrode slurry. This slurry is then applied to the negative electrode current collector, Cu foil, dried, and cold-pressed to form a negative electrode sheet.

[0098] Production of lithium-ion batteries:

[0099] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation. The sheets are then wound into bare cells. The bare cells are placed in outer packaging bags, and the aforementioned electrolytes are injected into the dried cells. After vacuum packaging, resting, formation, and shaping, a lithium-ion battery is obtained.

[0100] High temperature cycle testing of batteries

[0101] Test method: Place the battery in a 45±2℃ environment, follow the standard charge and discharge cycle, cycle rate 1C, charge voltage 3.0~4.5V, and calculate the capacity retention rate of the battery after the cycle. The calculation formula is as follows:

[0102] nth cycle capacity retention rate (%) = (nth cycle discharge capacity) / (first cycle discharge capacity) × 100%

[0103] High temperature storage test of batteries:

[0104] Test method: Charge the divided battery cells to 4.5V at room temperature at a current of 0.5C. Place the fully charged battery in an 85°C environment for 6 hours, measure the thickness expansion rate, and after returning to room temperature, discharge the battery cells at a current of 0.5C to 3.0V. Record the discharge capacity. The test results are shown in Table 2.

[0105] Examples 2 to 6

[0106] The methods for preparing the electrolytes in Examples 2-6 were essentially the same as in Example 1, except that different amounts of Compound A and Compound B were added. The same battery preparation and performance testing methods as in Example 1 were employed, and the test results can be found in Table 2.

[0107] Table 1. Electrolyte compositions in Examples 1 to 6

[0108] Table 2. Battery test results in Examples 1 to 6

[0109] Comparative Examples 1 to 5

[0110] The methods for preparing the electrolytes in Comparative Examples 1 to 5 were essentially the same as those in Example 1, except that the amounts of Compound A and Compound B added were different (see Table 3). The same battery preparation and performance testing methods as in Example 1 were employed, and the test results are shown in Table 4.

[0111] Table 3. Electrolyte compositions in Comparative Examples 1 to 5

[0112] Table 4. Battery test results in Comparative Examples 1 to 5

[0113] It can be seen that the batteries in Examples 1 to 6 have better long-term cycle performance at a higher temperature of 45°C. After 800 cycles of testing at a higher voltage of 3.0 to 4.5V and a rate of 1C, the capacity retention rate is still high and the thickness expansion is small. Moreover, after storage at a high temperature of 85°C for 6 hours, the capacity retention rate is still high and the thickness expansion rate is small.

[0114] The battery in Comparative Example 1, when cycled at a higher voltage of 3.0-4.5V at a rate of 1C at a temperature of 45°C, or stored at a temperature of 85°C for 6 hours, had a significantly lower capacity retention rate than the batteries in Examples 1-6, and a significantly increased thickness expansion rate. This may be due to the addition of only a cyclotriphosphazene compound without an imidazole compound. Although the cyclotriphosphazene compound alone can reduce HF damage to the positive and negative electrode interfaces to a certain extent, the absence of an imidazole compound prevents a synergistic effect, resulting in insufficient protection of the positive electrode interface and poor high-temperature resistance of the battery.

[0115] The battery in Comparative Example 2, when cycled at a higher voltage of 3.0-4.5V at a rate of 1C at a temperature of 45°C or stored at a temperature of 85°C for 6 hours, has a capacity retention rate significantly lower than that of the batteries in Examples 1-6, and a significantly increased thickness expansion rate. This may be due to the addition of only an imidazole compound without the addition of a cyclotriphosphazene compound. Although the imidazole compound alone can improve the wetting of the electrolyte into the diaphragm to a certain extent, the absence of the cyclotriphosphazene compound does not provide sufficient protection for the positive electrode interface and fails to achieve a synergistic effect, resulting in poor high-temperature resistance of the battery.

[0116] The battery in Comparative Example 3 has a capacity retention rate significantly lower than that of the batteries in Examples 1 to 6 when cycled at a higher voltage of 3.0 to 4.5 V and a rate of 1C at a temperature of 45°C, or stored at a temperature of 85°C for 6 hours. It is also lower than that of the batteries in Comparative Examples 1 to 2. The possible reason is that no cyclotriphosphazene compounds and imidazole compounds are added, and the carbonate compound organic solvent and sulfonate compound and nitrile compound additives in the electrolyte have a weak inhibitory effect on the battery during high temperature cycling or high temperature storage, resulting in poor high temperature resistance of the battery.

[0117] The battery in Comparative Example 4, at a temperature of 45°C, was cycled at a higher voltage of 3.0-4.5V and a 1C rate, or stored at a temperature of 85°C for 6H. Its capacity retention rate was significantly lower than that of the batteries in Examples 1-6. At the same time, the capacity retention rate of the battery decreased significantly faster in the early stage than that of the batteries in Comparative Examples 1-2, and the thickness expansion rate increased faster in the early stage than that of the batteries in Comparative Examples 1-2. The possible reason is that the sum of the added amounts of cyclotriphosphazene and imidazole compounds is high, but the ratio of the two is low, and they fail to play a good role in synergistic protection of the positive electrode. Specifically, in Comparative Example 4, only a small amount of imidazole compounds was added, while the amount of cyclotriphosphazene added was significantly higher, and the latter was added in an amount much higher than the former, resulting in an insignificant improvement in the wettability and interface protection of the diaphragm, and significantly poor interface compatibility on the negative electrode side, thereby further deteriorating the cycle stability of the battery. During the cycle, the electrolyte decomposed and produced gas faster, and the thickness expansion rate was higher than that of Comparative Example 1.

[0118] The battery in Comparative Example 5, when cycled at a higher voltage of 3.0-4.5V at a rate of 1C at a temperature of 45°C or stored at a temperature of 85°C for 6 hours, has a capacity retention rate significantly lower than that of the batteries in Examples 1-6. At the same time, the rate of decrease in the capacity retention rate of the battery in the early stage is significantly faster than that of the batteries in Comparative Examples 1-2. The possible reason is that the sum of the added amounts of the cyclotriphosphazene compound and the imidazole compound is high, but the ratio of the two is high, which fails to play a good role in synergistic protection of the positive electrode; specifically, in Comparative Example 5, a large amount of imidazole compound is added, while the amount of cyclotriphosphazene compound added is extremely low, and the latter is added at a relatively high rate. The amount added is much higher than the former; although the large amount of imidazole compounds added improves the wettability of the electrolyte to the diaphragm, reduces the by-products on the surface of the SEI film, and reduces the decomposition rate of the electrolyte, thereby significantly reducing the early expansion of the battery, the amount of cyclotriphosphazene compounds added to the electrolyte of the battery is too low, the electrolyte has fewer components that can capture Lewis acid PF5, and the organic solvent in the electrolyte (such as fluoroethylene carbonate) has a high degree of decomposition. Finally, the thickness expansion rate of the battery is still high, and too many imidazole compounds may lead to poor interface compatibility with the positive and negative electrodes, ultimately causing the battery to deteriorate in cycle and significantly reduce the capacity retention rate.

[0119] The inventors of this application also discovered that butyryl-containing imidazoles are more advantageous for achieving higher capacity retention and lower thickness expansion when the carbon chain length is longer (e.g., hexanoyl), the side chain is shorter (e.g., acetyl), and the substituent of the imidazole nitrogen atom is non-linear. Furthermore, perfluorinated butyrylimidazoles are more advantageous than non-perfluorinated butyrylimidazoles in improving the wettability of the electrolyte to the separator, reducing byproducts on the SEI film surface, inhibiting the collapse of the electrode active material structure, and reducing electrolyte decomposition and gas production.

[0120] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0121] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims, and the specification shall be used to interpret the scope of the claims.

Claims

1. A non-aqueous electrolyte for a secondary battery, characterized in that: The non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive, wherein the additive comprises an imidazole compound and a cyclotriphosphazene compound; The ratio of the weight of the imidazole compound to the sum of the weight of the organic solvent and the additive in the non-aqueous electrolyte is 0.2% to 10%; The ratio of the weight of the cyclotriphosphazene compound to the sum of the weight of the organic solvent and the additive in the non-aqueous electrolyte is 0.5% to 2%.

2. The non-aqueous electrolyte according to claim 1, characterized in that The imidazole compound has the following structure: R1 to R4 in formula (I) are independently selected from halogen, C1 to C 20 Alkoxy, C1~C 20 Alkyl or halogen substituted C1~C 20 An alkyl group; X is selected from O, S or NH; The cyclotriphosphazene compound has the following structure: R5~R in formula (II) 10 Each independently selected from halogen, C1-C 20 Alkoxy, C1~C 20 Alkyl or halogen substituted C1~C 20 of alkyl.

3. The non-aqueous electrolyte according to claim 1 or 2, characterized in that The weight ratio of the imidazole compound to the cyclotriphosphazene compound is (0.1-10):

1.

4. The non-aqueous electrolyte according to any one of claims 1 to 3, characterized in that The concentration of the lithium salt in the non-aqueous electrolyte is 0.5 mol / L to 1.5 mol / L.

5. The non-aqueous electrolyte according to claim 4, characterized in that The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, trifluorosulfonyl lithium, difluoro(trifluoromethylsulfonyl)imide lithium, tris(trifluoromethylsulfonyl)methyl lithium and bis(trifluoroimide)sulfonate lithium.

6. The non-aqueous electrolyte according to any one of claims 1 to 5, characterized in that The organic solvent is selected from one or more of carbonate compounds and furan compounds.

7. The non-aqueous electrolyte according to claim 6, characterized in that The additive further comprises one or more of sulfonate compounds, fluorocarbonate compounds and nitrile compounds, wherein the nitrile compounds contain cyano groups in their structures.

8. The non-aqueous electrolyte according to any one of claims 1 to 7, characterized in that in, In parts by weight, the invention comprises 80 to 120 parts of carbonate compounds, 2 to 5 parts of sulfonate compounds, 2 to 5 parts of nitrile compounds, 6 to 9 parts of fluorocarbonate compounds, 0.1 to 10 parts of imidazole compounds and 0.8 to 1.2 parts of cyclotriphosphazene compounds.

9. The non-aqueous electrolyte according to claim 8, characterized in that Satisfy at least one of the following characteristics: (1) The carbonate compound is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate and methyl butyrate; (2) the furan compound is selected from one or more of tetrahydrofuran, furancarboxylic acid, furazolidone and furanpropylamine; (3) the fluorocarbonate compound is selected from one or more of fluoroethylene carbonate, fluoroethyl methyl carbonate and fluoropropylene carbonate; (4) The sulfonate compound is selected from one or more of 1,3-propane sultone, 1,3-propylene sultone and 3-fluoro-1,3-propane sultone; (5) The nitrile compound is selected from one or more of succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile and glyceroltrinitrile.

10. A secondary battery, characterized in that: The non-aqueous electrolyte comprises the non-aqueous electrolyte according to any one of claims 1 to 9.

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