Lithium ion battery electrolyte and lithium ion battery

Abstract

The application provides a lithium ion battery electrolyte, which comprises an organic solvent, a lithium salt and an electrolyte additive, wherein the electrolyte additive comprises a carbodiimide and an isocyanate. The electrolyte additive can react with other substances in the electrolyte to form a SEI film with moderate thickness and compactness, and can also reduce the moisture in the pole core, thereby improving the capacity, cycle performance and storage performance of the lithium ion battery. When the lithium ion battery electrolyte is applied to a lithium ion battery, a lithium ion battery with long cycle performance and low impedance performance can be obtained. The application also provides a power consumption device comprising the lithium ion battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a lithium-ion battery electrolyte and a lithium-ion battery. Background Technology

[0002] The copper foil surface of the negative electrode material in lithium-ion batteries contains a small amount of cuprous oxide. After being immersed in the electrolyte for a period of time, trace amounts of water in the electrode core will slowly be released into the electrolyte, and the reaction will continuously generate Cu. + And water. On the one hand, Cu + On the negative electrode, water is preferentially reduced to metallic copper. The resulting copper dendrites penetrate the separator, increasing the battery's self-discharge. Furthermore, the continuous release of water from the electrode may amplify internal side reactions, leading to increased self-discharge and reduced storage performance and quality. On the other hand, small amounts of water can also participate in the formation of the solid electrolyte interphase (SEI) film, increasing its thickness and battery impedance. Simultaneously, it consumes active lithium released from the positive electrode, resulting in reduced battery capacity and deteriorated cycle and storage performance.

[0003] In addition, the SEI film formed by existing conventional SEI film-forming additives is relatively loose, and when the amount added is small, the SEI film formed is thin, resulting in poor long-term room temperature cycle stability and high temperature performance of the battery; when the amount added is large, the SEI film formed is thick, the battery impedance is large, resulting in poor high-rate power performance and low-temperature discharge performance of the battery. Summary of the Invention

[0004] In view of this, this application provides a lithium-ion battery electrolyte comprising an organic solvent, a lithium salt, and electrolyte additives, wherein the electrolyte additives include carbodiimide and isocyanate. These electrolyte additives can react with other substances in the electrolyte to form a moderately thick and dense SEI film, while also reducing moisture in the electrode core, thereby improving the capacity, cycle life, and storage performance of the lithium-ion battery. Applying this lithium-ion battery electrolyte to a lithium-ion battery can yield a lithium-ion battery that balances long cycle life and low impedance performance. This application also provides an electrical device incorporating this lithium-ion battery.

[0005] The first aspect of this application provides a lithium-ion battery electrolyte, which includes an organic solvent, a lithium salt, and an electrolyte additive. The electrolyte additive includes a carbodiimide as shown in formula (I) and an isocyanate as shown in formula (II), wherein the mass ratio of the carbodiimide to the isocyanate is 1:0.25-4.

[0006]

[0007]

[0008] R1 and R2 are selected from any one of isopropyl, tert-butyl, cyclohexyl and phenyl, respectively;

[0009] R3 is selected from monovalent, divalent, or trivalent substituted or unsubstituted hydrocarbon or sulfone groups;

[0010] n is 1, 2, or 3.

[0011] In this embodiment of the application, the carbodiimide in the lithium-ion battery electrolyte includes one or more of N,N'-diisopropylcarbodiimide, N,N'-ditert-butylcarbodiimide, N,N'-dicyclohexylcarbodiimide, and N,N'-diphenylcarbodiimide.

[0012] In this embodiment of the application, the isocyanate in the lithium-ion battery electrolyte includes monoisocyanate, diisocyanate and triisocyanate; the monoisocyanate includes p-toluenesulfonyl isocyanate, the diisocyanate includes isophorone diisocyanate, diphenylmethane diisocyanate and lysine diisocyanate, and the triisocyanate includes 4,4',4” triphenylmethane triisocyanate.

[0013] In this embodiment of the application, the sum of the mass percentages of the carbodiimide and the isocyanate in the lithium-ion battery electrolyte is 0.5%-3%.

[0014] In this embodiment of the application, the lithium-ion battery electrolyte further includes an SEI film-forming additive, which includes one or more of vinylene carbonate, fluoroethylene carbonate, propylene sulfite, and methylene disulfonate.

[0015] In this embodiment of the application, the organic solvent in the lithium-ion battery electrolyte includes one or more of the following: γ-butyrolactone, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, dipropyl carbonate, N-methylformamide, N-methylacetamide, acetonitrile, N,N-dimethylformamide, sulfolane, dimethyl sulfoxide, dimethyl sulfite, and other cyclic or chain organic esters containing fluorine, sulfur, or unsaturated bonds.

[0016] In this embodiment of the application, the lithium salt in the lithium-ion battery electrolyte includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium difluorophosphate, and lithium difluorosulfonylimide.

[0017] A second aspect of this application provides a lithium-ion battery, comprising a battery casing and a positive electrode, a negative electrode, a separator, and an electrolyte housed within the battery casing. The electrolyte includes the lithium-ion battery electrolyte provided in the first aspect of this application.

[0018] In this embodiment of the application, the DC internal resistance of the lithium-ion battery at room temperature is 0.4mΩ-0.6mΩ; the cycle capacity retention rate after 1000 cycles at 60℃ is 85%-90%; and the storage capacity retention rate after 28 days of storage at 60℃ is 95%-97%.

[0019] The third aspect of this application provides an electrical device, which includes the lithium-ion battery provided in the second aspect of this application. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of a lithium-ion battery provided in an embodiment of this application;

[0021] Figure 2 This is a schematic diagram of the structure of an electrical device provided in an embodiment of this application.

[0022] Explanation of icon numbers

[0023] 100 - Lithium-ion battery; 101 - Lithium-ion battery electrolyte; 102 - Separator; 103 - Positive electrode; 104 - Negative electrode; 105 - Battery casing; 200 - Electric vehicle; 201 - Lithium-ion battery. Detailed Implementation

[0024] The present application will be further described in detail below with reference to preferred embodiments, but the scope of protection of the present application is not limited to the following specific embodiments.

[0025] In this application, all technical terms have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this application.

[0026] The copper foil surface of the negative electrode material in lithium-ion batteries contains a small amount of cuprous oxide. After being immersed in the electrolyte for a period of time, trace amounts of water in the electrode core will slowly be released into the electrolyte, and the reaction will continuously generate Cu. + And water. On the one hand, Cu +On the negative electrode, water is preferentially reduced to metallic copper. The resulting copper dendrites penetrate the separator, increasing the battery's self-discharge. Furthermore, the continuous release of water from the electrodes may amplify internal side reactions, leading to increased self-discharge and reduced storage performance and quality. On the other hand, small amounts of water can also participate in the formation of the SEI film, increasing its thickness and battery impedance. Simultaneously, it consumes active lithium released from the positive electrode, resulting in reduced battery capacity and deteriorated cycle and storage performance.

[0027] There are two main methods for removing moisture from the battery core. One is to alter the baking process conditions by increasing the baking temperature, extending the baking time, and reducing the oven pressure. However, these methods significantly increase battery energy loss, thus increasing manufacturing costs. Furthermore, higher temperatures and longer baking times can cause the separator to wrinkle, affecting battery performance. The second method is to replace the PE separator with a PP separator to slow the impact of wetting rate on copper deposition. However, this also carries the risk of separator wrinkling, and the improvement effect in practical applications is not significant.

[0028] In addition, the SEI film formed by existing conventional SEI film-forming additives is relatively loose, and when the amount added is small, the SEI film formed is thin, resulting in poor long-term room temperature cycle stability and high temperature performance of the battery; when the amount added is large, the SEI film formed is thick, the battery impedance is large, resulting in poor high-rate power performance and low-temperature discharge performance of the battery.

[0029] To address the aforementioned problems, this application provides a lithium-ion battery electrolyte comprising an organic solvent, a lithium salt, and an electrolyte additive. The electrolyte additive reacts with other substances in the electrolyte to form a moderately thick and dense SEI film, while also reducing moisture in the electrode core, thereby improving the capacity, cycle life, and storage performance of the lithium-ion battery. Applying the electrolyte with this additive to a lithium-ion battery results in a lithium-ion battery that balances long cycle life and low impedance performance.

[0030] This application provides a lithium-ion battery electrolyte comprising an organic solvent, a lithium salt, and an electrolyte additive, wherein the electrolyte additive comprises a carbodiimide as shown in formula (I) and an isocyanate as shown in formula (II), wherein the mass ratio of the carbodiimide to the isocyanate is 1:0.25-4.

[0031]

[0032] R1 and R2 are selected from any one of isopropyl, tert-butyl, cyclohexyl and phenyl, respectively;

[0033] R3 is selected from monovalent, divalent, or trivalent substituted or unsubstituted hydrocarbon or sulfone groups;

[0034] n is 1, 2, or 3.

[0035] In this application, carbodiimide and isocyanate in the lithium-ion battery electrolyte are special SEI film-forming additives. Unlike commonly used SEI film-forming additives containing carbon-carbon double bonds, carbon-oxygen double bonds, and sulfur-oxygen double bonds, the electrolyte additive in the lithium-ion battery electrolyte provided in this application includes both carbodiimide and isocyanate. Lithium-ion battery formation refers to the process of the first charging of a lithium-ion battery, the purpose of which is to form a solid electrolyte interface film, i.e., an SEI film, on the surface of the negative electrode. In this application, during the initial stage of lithium-ion battery formation, carbodiimide and isocyanate can react with solvent molecules and lithium salts in the electrolyte to generate a cyclic polymer with a dense structure, resulting in a moderately thick and dense SEI film. Specifically, both carbodiimide and isocyanate contain unsaturated double bonds with nitrogen atoms. Under low current (i.e., during battery formation), these unsaturated double bonds undergo ring-opening reactions and interconnect to form new cyclic polymers. These cyclic polymers are six-membered rings, exhibiting greater structural stability, and also contain nitrogen atoms, which facilitate coordination with lithium ions, forming a stable and dense SEI film. This dense SEI film effectively prevents the solvent from reacting with the negative electrode active material, allowing lithium ions to intercalate and deintercalate, thereby improving the electrochemical performance of the lithium-ion battery. Furthermore, the moderate thickness of this dense SEI avoids the problems of poor long-term room-temperature cycle performance and high-temperature performance caused by an excessively thin SEI film, while also avoiding the problems of high battery impedance and poor rate performance caused by an excessively thick SEI film. This application's embodiments, by selecting a two-component electrolyte additive of carbodiimide and isocyanate, have obtained a lithium-ion battery electrolyte that balances long-cycle performance and low impedance performance.

[0036] In this embodiment, the carbodiimide includes one or more of N,N'-diisopropylcarbodiimide, N,N'-di-tert-butylcarbodiimide, N,N'-dicyclohexylcarbodiimide, and N,N'-diphenylcarbodiimide. Preferably, the carbodiimide includes at least one of N,N'-diisopropylcarbodiimide and N,N'-di-tert-butylcarbodiimide. A small amount of cuprous oxide exists on the negative electrode material of a lithium-ion battery. After the electrolyte has been immersed for a period of time, trace amounts of water in the electrode core will slowly be released into the electrolyte, and the reaction continuously generates Cu. + And water. On the one hand, Cu +On the negative electrode, water is preferentially reduced to metallic copper. The resulting copper dendrites penetrate the separator, increasing the battery's self-discharge. Furthermore, the continuous release of water from the electrode may amplify internal side reactions, leading to increased self-discharge and reduced battery storage performance and quality. On the other hand, small amounts of water also participate in the formation of the SEI film, increasing its thickness and battery impedance. Simultaneously, it consumes active lithium released from the positive electrode, resulting in reduced battery capacity and poorer cycle and storage performance. The carbodiimide in the lithium-ion battery electrolyte of this application can react with trace amounts of water in the lithium-ion battery electrode and electrolyte to generate urea, thereby removing trace amounts of water from the electrode core. In some embodiments, the carbodiimide in the electrolyte additive is N,N'-diisopropylcarbodiimide, which reacts with trace amounts of water to generate 1,3-diisopropylurea. On the one hand, N,N'-diisopropylcarbodiimide is a liquid at room temperature and is easily and uniformly dispersed after being added to the electrolyte. On the other hand, the generated 1,3-diisopropylurea is soluble in the organic solvent of the electrolyte and does not decompose at 100°C, thus ensuring the stability of the battery's electrochemical performance. In other embodiments, the carbodiimide in the electrolyte additive is N,N'-di-tert-butylcarbodiimide. N,N'-di-tert-butylcarbodiimide reacts with trace amounts of water to generate 1,3-di-tert-butylurea, which is also soluble in the organic solvent of the electrolyte. While removing trace amounts of water from the electrode core, the reaction products do not negatively affect the battery's electrocyclic properties.

[0037] In some embodiments of this application, R3 is selected from substituted or unsubstituted hydrocarbon groups, which can be aliphatic or aromatic hydrocarbon groups. The aliphatic hydrocarbon group can be alkyl, and the alkyl group can be a straight-chain alkyl or a cycloalkyl, wherein the substituted alkyl group can be an arylalkyl. An arylalkyl group is, for example, a phenylalkyl. An aromatic hydrocarbon group can be, for example, aryl or heteroaryl, and the aryl group can be, for example, phenyl, alkylphenyl, or biphenyl. The number of carbon atoms in the substituted or unsubstituted hydrocarbon group is 1-30, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30.

[0038] In some embodiments of this application, R3 is selected from substituted or unsubstituted sulfone groups, wherein the substituted sulfone group may be alkylsulfone, phenylsulfone, alkylphenylsulfone, or phenylalkylsulfone. The number of carbon atoms in the substituted or unsubstituted sulfone group is 1-30, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30.

[0039] Specifically, in some embodiments, when R3 is a monovalent group, the corresponding isocyanate is a monoisocyanate. In some embodiments, R3 can be p-toluenesulfonyl. When R3 is a divalent group, the corresponding isocyanate is a diisocyanate. In some embodiments, R3 can be diphenylmethyl. When R3 is a trivalent group, the corresponding isocyanate is a triisocyanate. In some embodiments, R3 can be triphenylmethyl.

[0040] In this embodiment, the isocyanate can be one or more of monoisocyanates, diisocyanates, and triisocyanates; wherein, the monoisocyanate can be p-toluenesulfonyl isocyanate, the diisocyanate can be one or more of isophorone diisocyanate, diphenylmethane diisocyanate, and lysine diisocyanate, and the triisocyanate includes 4,4',4” triphenylmethane triisocyanate. Isocyanates have high activity and can react with trace amounts of moisture to generate amines and CO2 gas. When the amount of isocyanate added is large and the activity is high, the generated amine can further generate urea. In some embodiments, the isocyanate in the electrolyte additive is p-toluenesulfonyl isocyanate. Cyanate esters are highly efficient dehydrating agents that react with trace amounts of water in the electrolyte to produce p-toluenesulfonamide and carbon dioxide gas. The carbon dioxide gas is removed under negative pressure during subsequent formation of the lithium-ion battery. P-Toluenesulfonamide is soluble in the organic solvent of the electrolyte and remains stable during the charge-discharge process of the lithium-ion battery, without being electrochemically oxidized or reduced to other substances, thus having no significant impact on the battery's electrochemical performance. In other embodiments, isophorone diisocyanate is used as the isocyanate in the electrolyte additive. Isophorone diisocyanate has two isocyanate groups, reacts with water at a moderate rate without a catalyst, and reacts with trace amounts of water to produce isophorone diamine, which is soluble in organic solvents.

[0041] In this embodiment, both carbodiimide and isocyanate in the lithium-ion battery electrolyte can serve as dehydration additives, removing trace amounts of moisture from the electrode core. Carbodiimide reacts with trace moisture to form urea, while isocyanate reacts with trace moisture to form amines or further to form urea. The resulting products are readily soluble in the organic solvent of the electrolyte and maintain excellent stability during battery charging and discharging. Furthermore, the combined addition of carbodiimide and isocyanate can serve as a special SEI film-forming additive. Compared to conventional carbon SEI film-forming additives, carbodiimide and isocyanate can synergistically react with other substances in the electrolyte to form cyclic polymers, creating a dense and moderately thick SEI film. This results in an electrolyte additive that balances the long-cycle performance and low impedance of lithium-ion batteries.

[0042] In this embodiment, the mass ratio of carbodiimide to isocyanate in the lithium-ion battery electrolyte is 1:0.25-4. In some embodiments, the mass ratio of carbodiimide to isocyanate in the lithium-ion battery electrolyte can be 1:0.25, 1:0.33, 1:0.5, 1:1, 1:2, 1:3, or 1:4. This application adjusts the composition ratio of carbodiimide and isocyanate to achieve a lithium-ion battery electrolyte with both excellent water removal performance and SEI film formation performance. In some embodiments, when the amount of electrolyte additives added is the same, the lithium-ion battery prepared by adding carbodiimide and isocyanate in an appropriate proportion range has lower internal resistance, better cycle performance, and better storage performance than the lithium-ion battery prepared by adding carbodiimide or isocyanate alone. That is, when the electrochemical performance of the prepared lithium-ion batteries is comparable, the total amount of carbodiimide and isocyanate added in an appropriate proportion is less than the amount of carbodiimide or isocyanate added alone.

[0043] In this embodiment, the sum of the mass percentages of carbodiimide and isocyanate in the lithium-ion battery electrolyte is 0.5%-3%. In some embodiments, the sum of the mass percentages of carbodiimide and isocyanate in the lithium-ion battery electrolyte can be 0.5%, 0.75%, 1%, 1.25%, 1.5%, 1.75%, 2%, 2.25%, 2.5%, 2.75%, or 3%. By controlling the sum of the mass percentages of carbodiimide and isocyanate in the lithium-ion battery electrolyte within the range of 0.5%-3%, this application can control the thickness of the SEI film within a suitable range. This avoids the problems of poor long-term room temperature cycle performance and high-temperature performance of lithium-ion batteries caused by an excessively thin SEI film, while also avoiding the problems of high impedance of lithium-ion batteries caused by an excessively thick SEI film, resulting in poor high-rate power performance and low-temperature discharge performance. This approach significantly improves the electrochemical performance of lithium-ion batteries while minimizing costs.

[0044] In this application, the organic solvent in the lithium-ion battery electrolyte includes one or more of the following: γ-butyrolactone, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, dipropyl carbonate, N-methylformamide, N-methylacetamide, acetonitrile, N,N-dimethylformamide, sulfolane, dimethyl sulfoxide, dimethyl sulfite, and other cyclic or chain organic esters containing fluorine, sulfur, or unsaturated bonds. These organic solvents can dissolve the lithium salt while simultaneously dissolving the products of the reaction between carbodiimide and isocyanate with water. Selecting a suitable organic solvent can improve the conductivity and physicochemical stability of the electrolyte. In some embodiments of this application, the solvent is selected from ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, and dimethyl carbonate. In some embodiments, the mass ratio of ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, and dimethyl carbonate is 34:35:13:18.

[0045] In this embodiment, the lithium salt in the lithium-ion battery electrolyte includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium difluorophosphate, and lithium difluorosulfonylimide. The lithium salt, as a provider of lithium ions in the lithium-ion battery electrolyte, is an essential component of the electrolyte. The aforementioned lithium salt and the organic solvent described above can react with the carbodiimide in the electrolyte additive to form a dense SEI film.

[0046] In this embodiment, the lithium-ion battery electrolyte also includes an SEI film-forming additive, which comprises one or more of vinylene carbonate, fluoroethylene carbonate, propylene sulfite, and methane disulfonate. This SEI film-forming additive is a double-bonded substance containing carbon-carbon, carbon-oxygen, or sulfur-oxygen double bonds, etc., with a high reduction potential. In the early stages of battery formation, it preferentially gains electrons and is reduced, combining with lithium ions to form an inorganic component of the SEI film. This SEI film-forming additive can synergistically react with organic solvents, lithium salts, and carbodiimide and isocyanate in the lithium-ion electrolyte to generate a dense SEI film. In some embodiments of this application, the SEI film-forming additive is vinylene carbonate. In some examples, the mass ratio of ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, and vinylene carbonate is 34:35:13:18:3.

[0047] The lithium-ion battery electrolyte provided in this application, by adding carbodiimide and isocyanate as electrolyte additives, reacts with solvent molecules, lithium salts, and SEI film-forming additives in the electrolyte during the initial formation stage of the lithium-ion battery, resulting in a moderately thick and dense SEI film. This dense SEI film effectively prevents the solvent from reacting with the negative electrode active material, allowing lithium ions to intercalate and deintercalate, thereby improving the electrochemical performance of the lithium-ion battery. Furthermore, the moderate thickness of this dense SEI avoids the problems of poor long-term room-temperature cycling performance and high-temperature performance caused by an excessively thin SEI film, while also avoiding the problems of high battery impedance and poor rate performance caused by an excessively thick SEI film. In addition, carbodiimide and isocyanate can also reduce moisture in the electrode core. Applying this lithium-ion battery electrolyte to lithium-ion batteries can yield lithium-ion batteries that balance long-cycle performance and low impedance performance.

[0048] like Figure 1As shown, this application also provides a lithium-ion battery 100, which includes a battery casing 105 and a lithium-ion battery electrolyte 101, a separator 102, a positive electrode 103, and a negative electrode 104 housed inside the battery casing 105, with the lithium-ion battery electrolyte 101 and the separator 102 located between the positive electrode 103 and the negative electrode 104. The separator 102 is a polymer film; the positive electrode 103 includes a current collector aluminum foil and a positive electrode active material disposed on the current collector aluminum foil; the negative electrode 104 includes a current collector copper foil and a negative electrode active material disposed on the current collector copper foil; the battery casing 105 includes, but is not limited to, materials such as steel, aluminum, nickel-plated iron, or aluminum-plastic film.

[0049] In this embodiment, the room-temperature DC internal resistance of the lithium-ion battery is 0.4 mΩ-0.6 mΩ; the cycle capacity retention rate after 1000 cycles at 60°C is 85%-90%; and the storage capacity retention rate after 28 days of storage at 60°C is 95%-97%. In some embodiments, the room-temperature DC internal resistance of the lithium-ion battery can be 0.5 mΩ-0.6 mΩ; the cycle capacity retention rate after 1000 cycles at 60°C can be 85%-89%; and the storage capacity retention rate after 28 days of storage at 60°C can be 96%-97%.

[0050] This application also provides an electrical device that includes the lithium-ion battery described above. This electrical device can be, for example, an electric vehicle, a mobile phone, a tablet computer, a laptop computer, a wearable device (watch, bracelet), a digital camera, etc. Figure 2 The electrical device may be an electric vehicle 200. The electric vehicle 200 includes the lithium-ion battery 201 described above in this application.

[0051] The present application will be further described below with reference to several embodiments:

[0052] Example 1

[0053] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, N,N'-diisopropylcarbodiimide, p-toluenesulfonyl isocyanate, and 1 mol / L lithium hexafluorophosphate in a mass ratio of 34:35:13:14.5:3:0.25:0.25, with a total electrolyte volume of 430 g. Lithium iron phosphate, acetylene black, and polyvinylidene fluoride were dissolved in N-methylpyrrolidone in a mass ratio of 90:5:5 to form a battery slurry with a solid content of 58 wt%. The resulting positive electrode slurry, after thorough stirring, was coated onto... A 13μm thick aluminum foil is baked on both sides at 100℃±5℃, then calendered and vacuum dried to form a material layer with a thickness of 130μm±5μm, thus obtaining the positive electrode sheet. A 2% asphalt-coated natural graphite, conductive carbon black, styrene-butadiene rubber, and carboxymethyl cellulose are dispersed in deionized water at a mass ratio of 95:1:1.1:2.3 to form a negative electrode slurry with a solid content of 60wt%. The uniformly stirred negative electrode slurry is coated on both sides of an 8μm thick copper foil and baked at 100℃±5℃ to obtain the negative electrode sheet.

[0054] The prepared positive and negative electrode sheets were stacked with a 9μm thick polyethylene separator to obtain a square lithium-ion cell. The cell was then placed in an aluminum shell measuring 550mm×100mm×19.5mm and the shell cover was welded to form the cell. The cell was then baked in a tunnel furnace at 100℃±5℃ for 10h±0.5h. After baking, the moisture content of the positive and negative electrode assembly was tested using the Karl Fischer method and found to be 112ppm.

[0055] The prepared lithium-ion battery electrolyte is injected into the baked battery cell, and the cap is welded to seal it. The lithium-ion battery is prepared through steps such as formation, charging, high-temperature aging, and capacity testing.

[0056] Example 2

[0057] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, N,N'-diisopropylcarbodiimide, p-toluenesulfonyl isocyanate, and 1 mol / L lithium hexafluorophosphate in a mass ratio of 34:35:13:14:3:0.5:0.5, with a total electrolyte volume of 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0058] Example 3

[0059] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, N,N'-diisopropylcarbodiimide, and p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:13.5:3:0.5:1 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0060] Example 4

[0061] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, N,N'-diisopropylcarbodiimide, and p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:13.5:3:1:0.5 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0062] Example 5

[0063] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, N,N'-diisopropylcarbodiimide, and p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:12:3:1.5:1.5 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0064] Comparative Example 1

[0065] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and vinylene carbonate in a mass ratio of 34:35:13:15:3 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0066] Comparative Example 2

[0067] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, and N,N'-diisopropylcarbodiimide in a mass ratio of 34:35:13:13.5:3:1.5 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0068] Comparative Example 3

[0069] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, and N,N'-diisopropylcarbodiimide in a mass ratio of 34:35:13:14.5:3:0.5 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0070] Comparative Example 4

[0071] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, and N,N'-diisopropylcarbodiimide in a mass ratio of 34:35:13:14:3:1 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0072] Comparative Example 5

[0073] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, and p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:13.5:3:1.5 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0074] Comparative Example 6

[0075] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, and p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:14.5:3:0.5 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0076] Comparative Example 7

[0077] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, and p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:14:3:1 with 1 mol / L lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0078] Comparative Example 8

[0079] A lithium-ion battery electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, vinylene carbonate, N,N'-diisopropylcarbodiimide, and 1 mol / L of p-toluenesulfonyl isocyanate in a mass ratio of 34:35:13:12.75:3:0.25:2 with lithium hexafluorophosphate. The total electrolyte volume was 430 g. The preparation of the positive electrode material, the negative electrode material, and the assembly of the lithium-ion battery were the same as in Example 1.

[0080] The lithium-ion batteries prepared in Examples 1-5 and Comparative Examples 1-8 were tested as follows:

[0081] (1) Capacity test

[0082] The lithium-ion battery was installed in the BS-9300 battery performance tester and charged at 0.5C with an upper limit voltage of 3.8V at room temperature of 25℃. Then it was discharged at 1 / 3C with a lower limit voltage of 2.0V. The average value of the three discharge capacities was taken as the capacity value of the lithium-ion battery. The results are shown in Table 1.

[0083] (2) DC internal resistance test at room temperature

[0084] At room temperature of 25℃, the lithium-ion battery was adjusted to 25% SOC. Using a DC internal resistance meter, the lithium-ion battery was discharged at 1.5C current for 10 seconds at room temperature of 25℃. The ratio of the voltage difference before and after discharge to the current value was taken as the room temperature DC internal resistance of the lithium-ion battery. The results are shown in Table 1.

[0085] (3) Low-temperature DC internal resistance test

[0086] At room temperature of 25℃, the lithium-ion battery was adjusted to 25% SOC. Using a DC internal resistance meter, the lithium-ion battery was discharged at a current of 1.5C for 10 seconds at a low temperature of -10℃. The ratio of the voltage difference before and after discharge to the current value was taken as the low-temperature DC internal resistance of the lithium-ion battery. The results are shown in Table 1.

[0087] (4) 60℃ Cyclic Capacity Retention Rate Test

[0088] The lithium-ion battery was installed in the BS-9300 battery performance tester and charged and discharged at a current of 0.5C at 60℃, with an upper limit voltage of 3.8V and a lower limit voltage of 2.0V for 1000 cycles. The ratio of the discharge capacity of the 1000 cycles to the discharge capacity of the first cycle is the 60℃ cycle capacity retention rate of the battery. The results are shown in Table 1.

[0089] (5) 60℃ Storage Capacity Remaining Rate Test

[0090] The lithium-ion battery was installed in the BS-9300 battery performance tester and charged to 3.8V at 0.5C under normal temperature of 25℃. The charging capacity was recorded. Then, the battery was stored at 60℃ for 28 days. After that, the battery was removed and discharged to 2.0V at 0.5C under normal temperature of 25℃. The ratio of the discharged capacity to the charged capacity is the remaining capacity rate. The results are shown in Table 1.

[0091] Table 1. Electrochemical performance test data of lithium-ion batteries in Examples 1-5 and Comparative Examples 1-8

[0092]

[0093] The data from Examples 1-5 and Comparative Examples 1-8 show that, compared to Comparative Example 1 (which does not contain carbodiimide and isocyanate) and Examples 2-7 (which only contain carbodiimide or isocyanate), the electrolytes in Examples 1-5, by simultaneously adding both carbodiimide and isocyanate, produce batteries with roughly the same capacity but significantly reduced DC resistance at both room temperature and low temperature, with a more pronounced reduction at low temperature. Furthermore, compared to Comparative Example 8, Examples 1-5 of this application, by controlling the mass ratio of carbodiimide and isocyanate within a suitable range, can further reduce battery resistance and improve cycle performance. In addition, the 60°C cycle capacity retention rate is improved by more than 5%, and the 60°C storage capacity remaining rate is also improved to some extent. Therefore, lithium-ion batteries prepared using the lithium-ion battery electrolyte containing carbodiimide and isocyanate provided in this application exhibit superior electrochemical performance compared to lithium-ion batteries obtained by adding only conventional additives or only single-component additives.

[0094] The preferred embodiments have been described in detail above, but the present invention is not limited to the specific implementation methods described above. Those skilled in the art can make various specific modifications under the guidance of this application without departing from the scope of protection of this application, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A lithium-ion battery electrolyte, characterized in that, The lithium-ion battery electrolyte includes an organic solvent, a lithium salt, and an electrolyte additive. The electrolyte additive is composed of a carbodiimide as shown in formula (I) and an isocyanate as shown in formula (II), wherein the mass ratio of the carbodiimide to the isocyanate is 1:0.25-4. （Ⅰ） （Ⅱ） R1 and R2 are selected from any one of isopropyl, tert-butyl, cyclohexyl and phenyl, respectively; R3 is selected from monovalent, divalent, or trivalent substituted or unsubstituted hydrocarbon or sulfone groups; n is 1, 2, or 3.

2. The lithium-ion battery electrolyte as described in claim 1, characterized in that, The carbodiimide includes one or more of N,N'-diisopropylcarbodiimide, N,N'-di-tert-butylcarbodiimide, N,N'-dicyclohexylcarbodiimide, and N,N'-diphenylcarbodiimide.

3. The lithium-ion battery electrolyte as described in claim 1 or 2, characterized in that, The isocyanate includes one or more of monoisocyanate, diisocyanate and triisocyanate; the monoisocyanate includes p-toluenesulfonyl isocyanate; the diisocyanate includes one or more of isophorone diisocyanate, diphenylmethane diisocyanate and lysine diisocyanate; the triisocyanate includes 4,4',4'' triphenylmethane triisocyanate.

4. The lithium-ion battery electrolyte according to any one of claims 1-3, characterized in that, The sum of the mass percentages of the carbodiimide and the isocyanate in the lithium-ion battery electrolyte is 0.5%-3%.

5. The lithium-ion battery electrolyte according to any one of claims 1-4, characterized in that, The lithium-ion battery electrolyte also includes an SEI film-forming additive, which includes one or more of vinylene carbonate, fluoroethylene carbonate, propylene sulfite, and methylene disulfonate.

6. The lithium-ion battery electrolyte according to any one of claims 1-5, characterized in that, The organic solvent includes one or more of the following: γ-butyrolactone, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, dipropyl carbonate, N-methylformamide, N-methylacetamide, acetonitrile, N,N-dimethylformamide, sulfolane, dimethyl sulfoxide, dimethyl sulfite, and other cyclic or chain organic esters containing fluorine, sulfur, or unsaturated bonds.

7. The lithium-ion battery electrolyte according to any one of claims 1-6, characterized in that, The lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium difluorophosphate, and lithium difluorosulfonylimide.

8. A lithium-ion battery, characterized in that, The lithium-ion battery includes a battery casing and a positive electrode, a negative electrode, a separator, and an electrolyte housed inside the battery casing, wherein the electrolyte includes the lithium-ion battery electrolyte according to any one of claims 1-7.

9. The lithium-ion battery as described in claim 8, characterized in that, The lithium-ion battery has a DC internal resistance of 0.4 mΩ-0.6 mΩ at room temperature; a cycle capacity retention rate of 85%-90% after 1000 cycles at 60℃; and a storage capacity retention rate of 95%-97% after 28 days of storage at 60℃.

10. An electrical appliance, characterized in that, The electrical equipment includes a lithium-ion battery as described in claim 8 or 9.

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