An electrolyte, its preparation method, and a lithium-ion battery

Abstract

This invention provides an electrolyte, its preparation method, and a lithium-ion battery. The electrolyte includes an organic solvent, additives, and a lithium salt, wherein the additives include additive A. By adding additive A to the electrolyte, the imidazole group in additive A enables it to participate in the formation of both the positive and negative electrode films, improving the stability of the solid electrolyte interface film. Furthermore, additive A can act as a Lewis base to capture H2O and corrosive substances such as phosphorus pentafluoride and hydrofluoric acid in the battery, reducing side reactions between water, gases, and corrosive substances and other components in the battery. Additionally, the cyano, cyanoethyl, or dimethylene cyano groups in additive A can act as a Lewis base to adsorb hydrofluoric acid. Additive A can improve the stability of the solid electrolyte interface film between the positive and negative electrodes, reduce side reactions between water, gases, and corrosive substances and other components in the battery, thereby improving the battery's high-temperature storage performance and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to an electrolyte, its preparation method, and a lithium-ion battery. Background Technology

[0002] In recent years, ternary cathode materials have been used in the power batteries of new energy vehicles. However, under high voltage conditions (>4.3V), the solvent in the electrolyte is prone to oxidative decomposition on the surface of the cathode material. The hydrogen ions produced by the oxidation and decomposition of the solvent are prone to react with hexafluorophosphate ions in the electrolyte to form hydrofluoric acid. Hydrofluoric acid corrodes the cathode, causing the transition metal of the cathode to dissolve, and water and gas are generated inside the battery, which seriously limits the performance and use of the cathode material.

[0003] In related technologies, the electrolyte of ternary high-voltage battery systems (>4.3V) generally contains additives such as hexamethyldisilazane (HMDS), dicyclohexylcarbodiimide (DCC), and trimethylsilyl phosphate (TMSP). Although these additives have the effect of removing acid, their use as electrolyte additives often leads to the deterioration of battery cycle performance.

[0004] Therefore, it is necessary to develop an electrolyte suitable for ternary high-voltage battery systems to improve the cycle performance and storage performance of the batteries. Summary of the Invention

[0005] The embodiments of the present invention provide an electrolyte, a method for preparing the same, and a lithium-ion battery, which can improve the technical problems of poor electrolyte cycle performance and storage performance.

[0006] In a first aspect, embodiments of the present invention provide an electrolyte comprising an organic solvent, an additive, and a lithium salt, wherein the additive comprises additive A, and the structure of additive A is shown in Formula I:

[0007]

[0008] Where n is selected from 0, 1, 2, 3, 4, 5 or 6;

[0009] Substituents R1, R2, and R3 are individually or independently selected from hydrogen atoms, halogens, cyano groups, isocyanate groups, C1-C10 straight-chain or branched alkyl groups, C1-C10 straight-chain or branched alkoxy groups, C1-C10 halogenated straight-chain or branched alkoxy groups, C3-C10 straight-chain or branched carboxyl ester groups, C3-C10 straight-chain or branched alkenyl groups, C3-C10 straight-chain or branched alkynyl groups, C6-C26 unsubstituted aryl groups, C6-C26 aryl groups substituted with alkyl, hydroxyl, and / or halogens, C7-C27 unsubstituted benzyl groups, C7-C27 benzyl groups substituted with alkyl, hydroxyl, and / or halogens, or combinations of these groups.

[0010] In one embodiment, n is selected from 0, 1, or 2; and / or

[0011] The substituents R1, R2 and R3 are each or independently selected from hydrogen atoms and / or C1-C2 straight-chain alkyl groups.

[0012] In one embodiment, the additive A in the electrolyte is 0.2%-2.0% by mass; and / or

[0013] Additive A is selected from at least one of imidazole-1-ylacetonitrile, 2-(2-methyl-1H-imidazole-1-yl)acetonitrile, 2-(2-ethyl-1H-imidazole-1-yl)acetonitrile, 1H-imidazole-1-carboxynitrile, N-(2-cyanoethyl)imidazole, and 1-cyanoethyl-2-methylimidazole.

[0014] In one embodiment, the additives further include carbonate additives, sulfur-containing additives, and lithium salt additives.

[0015] In one embodiment, the carbonate additive in the electrolyte is 0.2%-1.0% by mass; and / or

[0016] The sulfur-containing additive in the electrolyte comprises 0.2%-2.0% by mass; and / or

[0017] The lithium salt additive in the electrolyte comprises 0.5%-2.0% by mass; and / or

[0018] The carbonate additives include at least one of vinylene carbonate, fluoroethylene carbonate, and vinyl ethylene carbonate; and / or

[0019] The sulfur-containing additive includes at least one of 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, vinyl sulfate, and methylene disulfonate; and / or

[0020] The lithium salt additive includes at least one of lithium difluorobis(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium bis(oxalato)borate, lithium fluorosulfonylimide, lithium difluorophosphate, and lithium tetrafluoroborate.

[0021] In one embodiment, the lithium salt in the electrolyte comprises 10%-16% by mass; and / or

[0022] The lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonyl)imide, and lithium tetrafluoroborate.

[0023] In one embodiment, the organic solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluorinated ethylene carbonate.

[0024] In one embodiment, in the organic solvent, the volume fraction of ethylene carbonate is 0-30%, the volume fraction of propylene carbonate is 0-20%, the volume fraction of dimethyl carbonate is 0-40%, the volume fraction of diethyl carbonate is 0-20%, the volume fraction of ethyl methyl carbonate is 0-50%, and the volume fraction of fluorinated ethylene carbonate is 0-20%.

[0025] Second, an embodiment of the present invention provides a method for preparing the electrolyte as described above, and the method includes the following steps:

[0026] Under a protective atmosphere, mix the formulated amounts of the organic solvent, lithium salt, and additive evenly to obtain the electrolyte;

[0027] Among them, the additive includes additive A.

[0028] In one embodiment, the order of adding the mixed raw materials is to add the additive to the organic solvent, and then add the lithium salt; and / or

[0029] The protective atmosphere includes a nitrogen atmosphere and / or an argon atmosphere; and / or

[0030] The additive further includes a carbonate additive, a sulfur-containing additive, and a lithium salt additive.

[0031] Third, an embodiment of the present invention provides a lithium-ion battery, and the lithium-ion battery includes the electrolyte as described above.

[0032] In one embodiment, the lithium-ion battery further includes a positive electrode, a negative electrode, and a separator;

[0033] The active material of the positive electrode is a nickel-cobalt-manganese ternary material; and / or

[0034] The active material of the negative electrode is graphite or silicon-carbon.

[0035] In one embodiment, the chemical formula of the nickel-cobalt-manganese ternary material is: Li(Ni x Co y Mn z )O2, where 0.5 ≤ x < 0.9, 0 < y ≤ 0.3, 0 < z ≤ 0.3, and x + y + z = 1.

[0036] The beneficial effects of the embodiments of the present invention:

[0037] In embodiments of the present invention, by adding additive A to the electrolyte, the imidazole group in additive A enables additive A to participate in the formation of both the positive and negative electrode films, and the solid electrolyte interface film composition includes lithium nitride, which can improve the stability of the solid electrolyte interface film. In addition, the carbon-carbon double bonds and carbon-nitrogen double bonds in the imidazole group of additive A are bonded in an sp2 hybridization manner, which gives additive A a planar conjugation effect, allowing it to be delocalized and act as a Lewis base to capture H2O and corrosive substances such as phosphorus pentafluoride and hydrofluoric acid in the battery, reducing side reactions between water, gases and corrosive substances and other components in the battery. Furthermore, the cyano, cyanoethyl or dimethylene cyano groups directly bonded to the nitrogen atom in the imidazole group of additive A can not only participate in the formation of the positive electrode film, but also coordinate with transition metal ions in the positive electrode material to reduce the dissolution of transition metal ions. At the same time, the cyano, cyanoethyl or dimethylene cyano groups can act as a Lewis base to adsorb hydrofluoric acid. Additive A can improve the stability of the solid electrolyte interface film between the positive and negative electrodes, reduce side reactions between water, gas and corrosive substances and other components in the battery, reduce the dissolution of transition metal ions, and thus improve the high-temperature storage performance and cycle performance of the battery. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the present invention. In the present invention, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in actual use or operation; while "inner" and "outer" refer to the outline of the device.

[0039] Among related technologies, the electrolyte cycle performance and storage performance of lithium-ion batteries are poor and need further improvement.

[0040] To address the aforementioned problems, this application provides an electrolyte comprising an organic solvent, an additive, and a lithium salt. The additive includes additive A, the structure of which is shown in Formula I.

[0041]

[0042] Where n is selected from 0, 1, 2, 3, 4, 5 or 6;

[0043] Substituents R1, R2, and R3 are individually or independently selected from hydrogen atoms, halogens, cyano groups, isocyanate groups, C1-C10 straight-chain or branched alkyl groups, C1-C10 straight-chain or branched alkoxy groups, C1-C10 halogenated straight-chain or branched alkoxy groups, C3-C10 straight-chain or branched carboxyl ester groups, C3-C10 straight-chain or branched alkenyl groups, C3-C10 straight-chain or branched alkynyl groups, C6-C26 unsubstituted aryl groups, C6-C26 aryl groups substituted with alkyl, hydroxyl, and / or halogens, C7-C27 unsubstituted benzyl groups, C7-C27 benzyl groups substituted with alkyl, hydroxyl, and / or halogens, or combinations of these groups.

[0044] In this embodiment, by adding additive A to the electrolyte, the imidazole group in additive A enables additive A to participate in the formation of both the positive and negative electrode films, and the solid electrolyte interface film composition includes lithium nitride, which can improve the stability of the solid electrolyte interface film. In addition, the carbon-carbon double bonds and carbon-nitrogen double bonds in the imidazole group of additive A are bonded in an sp2 hybridization manner, which gives additive A a planar conjugation effect, allowing it to be delocalized and act as a Lewis base to capture H2O and corrosive substances such as phosphorus pentafluoride and hydrofluoric acid in the battery, reducing side reactions between water, gases and corrosive substances and other components in the battery.

[0045] In this embodiment, the structure of additive A also includes cyano, cyanoethyl, or dimethylene cyano groups directly bonded to the nitrogen atom of the imidazole group. Besides participating in the formation of the cathode film, these cyano, cyanoethyl, or dimethylene cyano groups can coordinate with transition metal ions in the cathode material to reduce the dissolution of transition metal ions. Simultaneously, these cyano, cyanoethyl, or dimethylene cyano groups can act as Lewis bases to adsorb hydrofluoric acid.

[0046] In one embodiment, n is selected from 0, 1, or 2; and / or, substituents R1, R2, and R3 are each or independently selected from hydrogen atoms and / or C1-C2 straight-chain alkyl groups.

[0047] Optionally, additive A is selected from at least one of imidazol-1-ylacetonitrile, 2-(2-methyl-1H-imidazol-1-yl)acetonitrile, 2-(2-ethyl-1H-imidazol-1-yl)acetonitrile, 1H-imidazol-1-carboxynitrile, N-(2-cyanoethyl)imidazolium, and 1-cyanoethyl-2-methylimidazolium. On the one hand, these substances can improve the high-temperature storage performance and cycle performance of the battery; on the other hand, these substances are commercially available, with readily available raw materials, which can save the synthesis steps of additive A and simplify the electrolyte production process.

[0048] In one embodiment, the mass percentage of additive A in the electrolyte is 0.2%-2.0%. Optionally, the mass percentage of additive A in the electrolyte is any one or any two of 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, and 2.0%. In this invention, if additive A is too little, its effective adsorption of H2O and reactive substances such as phosphorus pentafluoride and hydrofluoric acid is not ideal; if additive A is too much, it can easily lead to an increase in battery impedance.

[0049] In one embodiment, the additives further include carbonate additives, sulfur-containing additives, and lithium salt additives. The carbonate additives enable film formation at the negative electrode; the lithium salt additives contribute to film formation at both the positive and negative electrodes, resulting in a denser solid electrolyte interface film that inhibits the dissolution of metal ions from the positive electrode, reduces the destructive effect of additive A and metal ions on the negative electrode solid electrolyte interface film, thereby reducing the deteriorating effect of additive A on the negative electrode and improving film stability; the sulfur-containing additives assist in film formation at both the positive and negative electrodes and exhibit high thermal stability. This invention, by adding carbonate additives, sulfur-containing additives, lithium salt additives, and additive A, utilizes the synergistic effect between the additives to further improve the high-temperature storage performance and cycle performance of the battery.

[0050] In one embodiment, the carbonate additive includes at least one of vinylene carbonate, fluoroethylene carbonate, and vinyl ethylene carbonate.

[0051] In one embodiment, the sulfur-containing additive includes at least one of 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, vinyl sulfate, and methylene disulfonate.

[0052] In one embodiment, the lithium salt additive includes at least one of lithium difluorobis(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium bis(oxalato)borate, lithium fluorosulfonylimide, lithium difluorophosphate, and lithium tetrafluoroborate.

[0053] In one embodiment, the mass percentage of the carbonate additive in the electrolyte is 0.2%-1.0%. Optionally, the mass percentage of the carbonate additive in the electrolyte can be any one or any two of 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, etc. In this invention, if the content of the carbonate additive is too low, the effect of forming a dense solid electrolyte interface film is not ideal; if the content of the carbonate additive is too high, it can easily cause high impedance and exacerbate high-temperature gas generation.

[0054] In one embodiment, the mass percentage of the sulfur-containing additive in the electrolyte is 0.2%-2.0%. Optionally, the mass percentage of the sulfur-containing additive in the electrolyte can be any one or any two of 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, and 2.0%. In this invention, if the content of the sulfur-containing additive is too low, the effect of forming a dense solid electrolyte interface film is not ideal; if the content of the sulfur-containing additive is too high, it can easily lead to over-film formation and a decrease in battery cycle performance.

[0055] In one embodiment, the mass percentage of the lithium salt additive in the electrolyte is 0.5%-2.0%. Optionally, the mass percentage of the lithium salt additive in the electrolyte can be any one or any two of 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, and 2.0%. In this invention, if the content of the lithium salt additive is too low, the effect of forming a dense solid electrolyte interface film is not ideal; if the content of the lithium salt additive is too high, it can easily lead to gas generation and a decrease in electrical performance.

[0056] In one embodiment, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, and lithium tetrafluoroborate.

[0057] In one embodiment, the lithium salt has a mass percentage of 10%-16% in the electrolyte. Optionally, the mass percentage of the lithium salt in the electrolyte can be any one or any two of 10%, 11%, 12%, 13%, 14%, 15%, 16%, etc.

[0058] In one embodiment, the organic solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and fluoroethylene carbonate.

[0059] In one embodiment, the volume fraction of ethylene carbonate in the organic solvent is 0-30%. Optionally, the volume fraction of ethylene carbonate in the organic solvent can be any one or any two of 0%, 5%, 10%, 15%, 20%, 25%, 30%, etc.

[0060] In one embodiment, the volume fraction of propylene carbonate in the organic solvent is 0-20%. Optionally, the volume fraction of propylene carbonate in the organic solvent can be any one or any two of 0%, 5%, 10%, 15%, 20%, etc.

[0061] In one embodiment, the volume fraction of dimethyl carbonate in the organic solvent is 0-40%. Optionally, the volume fraction of dimethyl carbonate in the organic solvent can be any one or any two of 0, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, etc.

[0062] In one embodiment, the volume fraction of diethyl carbonate in the organic solvent is 0-20%. Optionally, the volume fraction of diethyl carbonate in the organic solvent can be any one or any two of 0%, 5%, 10%, 15%, 20%, etc.

[0063] In one embodiment, the volume fraction of methyl ethyl carbonate in the organic solvent is 0-50%. Optionally, the volume fraction of methyl ethyl carbonate in the organic solvent can be any one or any two of 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc.

[0064] In one embodiment, the volume fraction of fluoroethylene carbonate in the organic solvent is 0-20%. Optionally, the volume fraction of fluoroethylene carbonate in the organic solvent can be any one or any two of 0%, 5%, 10%, 15%, 20%, etc.

[0065] This application also provides a method for preparing the electrolyte as described above, comprising the following steps:

[0066] S1. Under a protective atmosphere, the formulated amounts of organic solvent, lithium salt, and additives are mixed evenly to obtain an electrolyte; wherein the additives include additive A. The preparation method provided by this invention is simple to operate, has a short process, and can meet the needs of large-scale industrial production.

[0067] As a preferred technical solution of the present invention, the additives also include carbonate additives, sulfur-containing additives and lithium salt additives in the prescribed amounts.

[0068] In one embodiment, the order of adding the mixed raw materials is to add the additive to the organic solvent, followed by the addition of the lithium salt.

[0069] In one embodiment, the protective atmosphere includes a nitrogen atmosphere and / or an argon atmosphere.

[0070] This application also provides a lithium-ion battery, which includes the electrolyte as described above.

[0071] In one embodiment, the lithium-ion battery further includes a positive electrode, a negative electrode, and a separator.

[0072] Preferably, the active material of the positive electrode is a nickel-cobalt-manganese ternary material.

[0073] Preferably, the active material of the negative electrode is graphite or silicon carbide.

[0074] In one embodiment, the chemical formula of the nickel-cobalt-manganese ternary material is: Li(Ni x Co y Mn z )O2, where 0.5 ≤ x < 0.9, 0 < y ≤ 0.3, 0 < z ≤ 0.3, and x + y + z = 1.

[0075] The above solution will be further described below in conjunction with specific implementation examples. The preferred embodiments of the present application are described in detail as follows:

[0076] Example 1

[0077] This example provides an electrolyte, which consists of an organic solvent, a lithium salt, a carbonate additive, a sulfur-containing additive, a lithium salt additive, and additive A;

[0078] Among them, the lithium salt is lithium hexafluorophosphate, and its mass percentage in the electrolyte is 12.5%;

[0079] The carbonate additive is vinylene carbonate, and its mass percentage in the electrolyte is 0.5%;

[0080] The lithium salt additive is lithium difluorobis(oxalato)borate, and its mass percentage in the electrolyte is 1.0%;

[0081] The sulfur-containing additive is ethylene sulfate, and its mass percentage in the electrolyte is 1.0%;

[0082] Additive A is imidazol-1-ylacetonitrile, and its mass percentage in the electrolyte is 0.5%;

[0083] The organic solvent consists of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and fluorinated ethylene carbonate. Based on the total volume of the organic solvent being 100%, the volume fraction of ethylene carbonate is 15%, the volume fraction of dimethyl carbonate is 30%, the volume fraction of ethyl methyl carbonate is 50%, and the volume fraction of fluorinated ethylene carbonate is 5%.

[0084] This example also provides a preparation method for the above electrolyte. The specific method is as follows:

[0085] Under an argon atmosphere, the formulated amounts of the organic solvent, carbonate additive, sulfur-containing additive, lithium salt additive, and additive A are added to the organic solvent, and then the lithium salt is added. The mixture is stirred at 10°C to obtain the electrolyte.

[0086] Example 2

[0087] The difference between Example 2 and Example 1 is:

[0088] Additive A has a mass percentage of 0.2% in the electrolyte, and the rest is the same as in Example 1.

[0089] Example 3

[0090] The difference between Example 3 and Example 1 is:

[0091] Additive A has a mass percentage of 0.8% in the electrolyte, and the rest is the same as in Example 1.

[0092] Example 4

[0093] The difference between Example 4 and Example 1 is:

[0094] Additive A has a mass percentage of 2.0% in the electrolyte, and the rest is the same as in Example 1.

[0095] Example 5

[0096] The difference between Example 5 and Example 1 is:

[0097] Additive A is 2-(2-ethyl-1H-imidazol-1-yl)acetonitrile, and the rest is the same as in Example 1.

[0098] Example 6

[0099] The difference between Example 6 and Example 1 is:

[0100] Additive A is 1H-imidazolium-1-carboxynitrile, and the rest is the same as in Example 1.

[0101] Example 7

[0102] The difference between Example 7 and Example 1 is:

[0103] Additive A is N-(2-cyanoethyl)imidazole, and the rest is the same as in Example 1.

[0104] Example 8

[0105] The difference between Example 8 and Example 1 is:

[0106] Additive A is 1-cyanoethyl-2-methylimidazole, and the rest is the same as in Example 1.

[0107] Example 9

[0108] The difference between Example 9 and Example 1 is:

[0109] Additive A is 2-(2-methyl-1H-imidazol-1-yl)acetonitrile, and the rest is the same as in Example 1.

[0110] Example 10

[0111] The difference between Example 10 and Example 1 is:

[0112] The mass percentage of lithium difluorobis(oxalato)borate in the electrolyte is 0, and the rest is the same as in Example 1.

[0113] Example 11

[0114] The difference between Example 11 and Example 1 is:

[0115] The mass percentage of vinylene carbonate in the electrolyte is 0, and the rest is the same as in Example 1.

[0116] Example 12

[0117] The difference between Example 12 and Example 1 is:

[0118] The mass percentage of vinyl sulfate in the electrolyte is 0, and the rest is the same as in Example 1.

[0119] Example 13

[0120] The difference between Example 13 and Example 1 is:

[0121] Additive A has a mass percentage of 3% in the electrolyte, and the rest is the same as in Example 1.

[0122] Comparative Example 1

[0123] The difference between Comparative Example 1 and Example 1 is as follows:

[0124] The electrolyte does not contain additive A, and is otherwise the same as in Example 1.

[0125] Comparative Example 2

[0126] The difference between Comparative Example 2 and Example 1 is as follows:

[0127] Additive A was replaced with imidazole, which had a mass percentage of 0.5% in the electrolyte, and the rest was the same as in Example 1.

[0128] Comparative Example 3

[0129] The difference between Comparative Example 3 and Example 1 is as follows:

[0130] Additive A was replaced with imidazole and acetonitrile, with each imidazole and acetonitrile having a mass percentage of 0.5% in the electrolyte, and the rest was the same as in Example 1.

[0131] Comparative Example 4

[0132] The difference between Comparative Example 4 and Example 1 is as follows:

[0133] Additive A was replaced with 2-propionitrile-1-yl-1H-imidazolium-1-carboxylic acid ester, which had a mass percentage of 0.5% in the electrolyte, and the rest was the same as in Example 1.

[0134] Test methods

[0135] The electrolytes prepared in Examples 1-13 and Comparative Examples 1-4 were applied to lithium-ion batteries, and performance tests were conducted using these lithium-ion batteries. The specific preparation method of the lithium-ion batteries used in the tests included: preparing a slurry from graphite (negative electrode material), acetylene black (conductive agent), carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) in a mass percentage ratio of 94:1:2:3, coating it onto a copper foil current collector, and vacuum drying to obtain the negative electrode sheet; and preparing the positive electrode material NCM613 (LiNi... 0.6 Co 0.1 Mn 0.3 A slurry was prepared by mixing O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) in a mass ratio of 94:3:3. This slurry was coated onto an aluminum foil current collector and vacuum dried to obtain the positive electrode sheet. The positive electrode sheet, negative electrode sheet, separator, and electrolyte prepared in the examples or comparative examples were assembled into a pouch battery. Electrochemical tests were performed using a Xinwei charge-discharge test cabinet, and the hydrofluoric acid (HF) content in the electrolyte was determined using the ice-water titration method.

[0136] (1) Cycle performance test of lithium-ion batteries:

[0137] At 45°C, the lithium-ion battery was charged at a constant current of 1C (nominal capacity) to a voltage of 4.4V, then charged at a constant voltage of 4.4V until the current ≤0.05C. After resting for 30 minutes, it was discharged at a constant current of 1C to a cutoff voltage of 2.8V. This constitutes one charge-discharge cycle. The lithium-ion battery was subjected to 1000 charge-discharge cycles at 45°C under the above conditions.

[0138] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0139] (2) Electrolyte HF content test:

[0140] The electrolyte was stored at 60°C, and the HF content at 0d and 20d was tested by ice-water titration, and recorded as HF-0d and HF-20d respectively.

[0141] (3) High-temperature storage performance test of lithium-ion batteries:

[0142] At 25°C, the lithium-ion battery was charged at a constant current of 0.33C to a voltage of 4.4V, and then charged at a constant voltage of 4.4V to a current of 0.05C. The volume of the lithium-ion battery was measured as V0, and the initial capacity was measured as C0. After that, the lithium-ion battery was placed in a constant temperature chamber at 60°C and stored for 90 days. The volume of the lithium-ion battery was measured and recorded as V1, the capacity was kept as C1, and the capacity was restored to C2.

[0143] The volume expansion rate (%) of a lithium-ion battery after storage at 60°C for 90 days is calculated as (Vn - V0) / V0 × 100%.

[0144] The capacity retention rate (%) of a lithium-ion battery after 90 days of storage at 60°C is (C1 / C0) × 100%, and the capacity recovery rate (%) of a lithium-ion battery after 90 days of storage at 60°C is (C2 / C0) × 100%.

[0145] The test results are shown in Table 1 below:

[0146] Table 1

[0147]

[0148]

[0149] According to the test results in Table 1, the electrolyte provided in the example contains additive A. Additive A can participate in the formation of the positive and negative electrode films, reduce the dissolution of transition metal ions in the positive electrode material, improve the structural stability of the positive and negative electrode materials, reduce the battery cycle impedance, and improve the battery cycle performance. In addition, by adding additive A to the electrolyte, the rise in electrolyte acidity can be effectively suppressed, the high-temperature performance deterioration caused by acid release in the high-voltage system can be improved, and the high-temperature storage performance of the battery can be improved.

[0150] Analysis of Examples 1-4 and 10-12 reveals that adding lithium salt additives to the electrolyte helps form films at both the positive and negative electrodes. The resulting solid electrolyte interfacial film is relatively dense, inhibiting the dissolution of metal ions from the positive electrode and reducing the damage to the solid electrolyte interfacial film at the negative electrode caused by additive A and metal ions. This reduces the deteriorating effect of additive A on the negative electrode and improves film stability. Adding sulfur-containing additives to the electrolyte assists in film formation at both the positive and negative electrodes and exhibits high thermal stability. Adding carbonate additives to the electrolyte also helps in film formation at the negative electrode. This invention, by adding carbonate additives, sulfur-containing additives, lithium salt additives, and additive A, utilizes the synergistic effect between the additives to further improve the high-temperature storage performance and cycle performance of the battery.

[0151] Analysis of Examples 1-4 and Example 13 shows that by controlling the mass percentage of additive A in the electrolyte within the range of 0.2%-2.0%, additive A can effectively adsorb substances such as water, phosphorus pentafluoride, and hydrofluoric acid, while reducing battery impedance, which is more conducive to simultaneously improving the battery's cycle performance and high-temperature storage performance.

[0152] Analysis of Examples 5-9 and Comparative Example 4 shows that the nitrogen atom on the imidazole ring in Additive A is directly connected to groups such as cyano, cyanoethyl, or dimethylene cyano, which is more conducive to Additive A improving the cycle performance and high-temperature storage performance of the battery.

[0153] The embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. An electrolyte, characterized in that, the electrolyte comprises an organic solvent, an additive, and a lithium salt, the additive comprises an additive A, a carbonate additive, a sulfur-containing additive, and a lithium salt additive, the structure of the additive A is shown in formula I: （I） wherein n is selected from 0, 1, 2, 3, 4, 5, or 6; each or independently of the substituents R1, R2, and R3 is selected from a hydrogen atom, a halogen, a cyano group, an isocyanate group, a C1-C10 linear or branched alkyl group, a C1-C10 linear or branched alkoxy group, a C1-C10 halogenated linear or branched alkyl group, a C1-C10 halogenated linear or branched alkoxy group, a C3-C10 linear or branched carboxylate group, a C3-C10 linear or branched alkenyl group, a C3-C10 linear or branched alkynyl group, a C6-C26 unsubstituted aryl group, a C6-C26 aryl group substituted with an alkyl group, a hydroxyl group, and / or a halogen, a C7-C27 unsubstituted benzyl group, a C7-C27 benzyl group substituted with an alkyl group, a hydroxyl group, and / or a halogen, or a combination of these groups.

2. The electrolyte according to claim 1, characterized in that, the n is selected from 0, 1, or 2; and / or each or independently of the substituents R1, R2, and R3 is selected from a hydrogen atom and / or a C1-C2 linear alkyl group.

3. The electrolyte according to claim 1 or 2, characterized in that, the mass percentage of the additive A in the electrolyte is 0.2%-2.0%; and / or the additive A is selected from at least one of imidazol-1-ylacetonitrile, 2-(2-methyl-1H-imidazol-1-yl)acetonitrile, 2-(2-ethyl-1H-imidazol-1-yl)acetonitrile, 1H-imidazole-1-carbonitrile, N-(2-cyanoethyl)imidazole, 1-cyanoethyl-2-methylimidazole.

4. The electrolyte according to claim 1, characterized in that, the mass percentage of the carbonate additive in the electrolyte is 0.2%-1.0%; and / or the mass percentage of the sulfur-containing additive in the electrolyte is 0.2%-2.0%; and / or the mass percentage of the lithium salt additive in the electrolyte is 0.5%-2.0%; and / or the carbonate additive comprises at least one of vinylene carbonate, fluoroethylene carbonate, and vinylfluoroethylene carbonate; and / or the sulfur-containing additive comprises at least one of 1,3-propane sultone, 1,3-propene sultone, ethylene sulfate, and methyl methylene disulfonate; and / or the lithium salt additive comprises at least one of lithium difluorobis(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium bis(oxalato)borate, lithium fluorosulfonylimide, lithium difluorophosphate, and lithium tetrafluoroborate.

5. The electrolyte according to claim 1 or 2, characterized in that, the mass percentage of the lithium salt in the electrolyte is 10%-16%; and / or the lithium salt comprises at least one of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethylsulfonyl)imide, and lithium tetrafluoroborate.

6. The electrolyte according to claim 1 or 2, characterized in that, The organic solvent comprises at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate and fluoroethylene carbonate.

7. The electrolyte of claim 6, wherein, In the organic solvent, the volume fraction of the ethylene carbonate is 0-30%, the volume fraction of the propylene carbonate is 0-20%, the volume fraction of the dimethyl carbonate is 0-40%, the volume fraction of the diethyl carbonate is 0-20%, the volume fraction of the methyl ethyl carbonate is 0-50%, and the volume fraction of the fluoroethylene carbonate is 0-20%.

8. A method of preparing an electrolyte as claimed in any one of claims 1 to 7, characterized in that The method comprises the following steps: The formula amount of the organic solvent, the lithium salt and the additive are mixed uniformly under a protective atmosphere to obtain the electrolyte; The additive comprises an additive A.

9. The method of claim 8, wherein, The raw materials are mixed in the following order: the additive is added to the organic solvent, and then the lithium salt is added; and / or The protective atmosphere comprises a nitrogen atmosphere and / or an argon atmosphere; and / or The additive further comprises a carbonate additive, a sulfur-containing additive and a lithium salt additive.

10. A lithium-ion battery, characterized by, The lithium ion battery comprises the electrolyte of any one of claims 1-7.

11. The lithium-ion battery of claim 10, wherein, The lithium ion battery further comprises a positive electrode, a negative electrode and a separator; The active material of the positive electrode is a nickel-cobalt-manganese ternary material; and / or The active material of the negative electrode is graphite or silicon-carbon.

12. The lithium-ion battery of claim 11, wherein, The chemical formula of the nickel-cobalt-manganese ternary material is: Li(Ni x Co y Mn z )O2, wherein 0.5≤x<0.9, 0<y≤0.3, 0<z≤0.3, and x+y+z=1.

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