Electrolyte and lithium ion battery

Abstract

The application relates to the technical field of secondary batteries, in particular to a high-pressure-resistant lithium ion battery electrolyte. The electrolyte comprises a solvent, a lithium salt, a non-polar diluent and a sulfur-based additive represented by formula (1), (2) or (3). The solvent comprises a carbonic acid ester solvent; the lithium salt is dissolved in the solvent; the mass fraction of the non-polar diluent in the electrolyte is 21%-60%; and the mass ratio of the sulfur-based additive to the non-polar diluent is 1:200-1:10. The electrolyte can improve the cycle capacity of the lithium ion battery and ensure that other performances of the lithium ion battery remain stable.

Description

Technical Field

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

[0002] Currently, lithium-ion batteries are rapidly developing in the fields of electric vehicles and large-scale energy storage, capturing a significant market share. To achieve faster charging speeds and longer driving ranges for electric vehicles, lithium-ion batteries are required to have higher fast-charging capabilities and higher energy density.

[0003] In existing technologies, some cathode materials, such as high-nickel ternary cathodes, can significantly improve the energy density of lithium-ion batteries. However, due to the instability of highly reactive metal ions such as trivalent nickel ions, they react with air to generate surface alkali. This surface alkali has strong catalytic activity on the electrolyte, causing continuous oxidative decomposition of the electrolyte on the cathode surface. This leads to rapid capacity loss and gas generation in the lithium-ion battery, resulting in a decline in battery cycle life. When the lithium-ion battery is in a high state of charge (SOC) state, the oxidative decomposition or other side reactions of the electrolyte on the cathode surface become even more severe, further exacerbating the decline in cycle life. Summary of the Invention

[0004] To address the above problems, this invention provides an electrolyte that, when a lithium-ion battery is in a high SOC state, exhibits a weaker degree of side reactions on the positive electrode surface, thereby significantly improving the cycle life of the lithium-ion battery.

[0005] One aspect of the present invention provides an electrolyte comprising a solvent, a lithium salt, a nonpolar diluent, and a sulfide additive. The solvent comprises a carbonate solvent; the lithium salt is dissolved in the solvent; the nonpolar diluent has a mass fraction of 21%-60% in the electrolyte; the mass ratio of the sulfide additive to the nonpolar diluent is 1:200-1:10, and the sulfide additive is a compound represented by formula (1), formula (2), or formula (3).

[0006]

[0007] Optionally, the non-polar diluent is one or more of trifluorotoluene, difluorobenzene, trifluorobenzene, toluene, ethylbenzene, decafluoropentane, perfluorohexanone, perfluorobutyl ethyl ether, and perfluorobutyl methyl ether.

[0008] Optionally, the non-polar diluent has a mass fraction of 25%-50% in the electrolyte.

[0009] Optionally, the mass ratio of sulfur-based additives to non-polar diluents is 1:100-1:25.

[0010] Optionally, the carbonate solvent is one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0011] A second aspect of the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and the electrolyte described above.

[0012] Optionally, the positive electrode includes a positive current collector and a positive active material, wherein the positive active material includes any one or a combination of at least two of lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0013] Optionally, the negative electrode sheet includes a negative current collector and a negative active material, wherein the negative active material includes any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbide, or lithium titanate.

[0014] In some examples of the present invention, the electrolyte provided achieves stable compatibility with cathode materials such as high-nickel ternary cathodes by using a non-polar diluent.

[0015] Specifically, on the one hand, non-polar diluents possess excellent oxidation stability and are not prone to oxidative decomposition. The addition of non-polar diluents to the electrolyte reduces the contact between carbonate solvents and the positive electrode surface, thereby reducing the probability of carbonate solvent decomposition. On the other hand, non-polar diluents do not dissociate lithium salts; therefore, their addition compresses the solvation structure of lithium ions in the electrolyte, forming an anion-containing solvation structure, thus improving the overall oxidation stability of the electrolyte. Furthermore, the addition of diluents can also reduce the activity of carbonate solvents, further enhancing their oxidation stability.

[0016] However, nonpolar diluents suffer from poor reduction stability, undergoing continuous reduction decomposition on anode surfaces made of materials such as graphite or silicon. The inventors discovered that adding sulfide additives represented by formulas (1), (2), or (3) to an electrolyte containing a nonpolar diluent can further solve this problem, achieving good compatibility between the nonpolar diluent and the anode without affecting other performance characteristics of the lithium-ion battery. This is because the solid electrolyte interface (SEI) product generated by these sulfide additives at the anode has high polarity, and the solubility of this SEI product in nonpolar diluents is extremely low, thus enabling the formation of a complete and dense SEI film. Simultaneously, the unsaturation and sulfur content of these sulfide additives are higher than those of commonly used sulfide additives (such as vinyl sulfate DTD and 1,3-propanesulfonate PS), resulting in SEI film components with higher cross-linking and higher sulfur content, further improving the density of the SEI film and reducing the solubility of the SEI film components in nonpolar diluents. The sulfur-based additives used in this invention can not only prevent the reduction and decomposition of non-polar diluents on the negative electrode surface, but also construct a high-quality SEI film with very little addition. Detailed Implementation

[0017] 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.

[0018] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

[0019] Solid Electrolyte Interface (SEI): During the charging and discharging process of a lithium-ion battery, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material.

[0020] Battery State of Charge (SOC): SOC is the percentage of remaining battery capacity relative to the battery's rated capacity. It reflects the battery's remaining capacity and indicates its ability to continue operating. SOC is 0 when the battery is fully discharged and 1 when it is fully charged. It is typically expressed as 0-100%.

[0021] In this document, when referring to numerical ranges, unless otherwise specified, the distribution of selectable values ​​within a numerical range is considered continuous, including the two endpoints of the range (i.e., the minimum and maximum values), and every value between these two endpoints. When multiple numerical ranges are provided to describe a feature or property, these numerical ranges can be combined.

[0022] A first aspect of the present invention provides an electrolyte comprising a solvent, a lithium salt, a nonpolar diluent, and a sulfide additive. The solvent comprises a carbonate solvent; the lithium salt is dissolved in the solvent; the nonpolar diluent has a mass fraction of 21%-60% in the electrolyte; the mass ratio of the sulfide additive to the nonpolar diluent is 1:200-1:10, and the sulfide additive is a compound represented by formula (1), (2), or (3).

[0023]

[0024] That is, the sulfur-based additives are one or more of [4,4'-bis(1,3,2-dioxothiacyclopentane)]2,2,2',2'-tetraoxide, glyoxal disulfate, and 1,3,6,8-tetraoxa-2,7-dithiaspiro[4.4]nonane 2,2,7,7-tetraoxide.

[0025] In lithium-ion battery electrolytes, carbonate solvents exhibit good dissociation of lithium salts and good compatibility with graphite anodes. In embodiments of this invention, the solvent in the electrolyte includes carbonate solvents, specifically one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0026] However, carbonate solvents have poor oxidative stability. With the increasing nickel content in ternary cathode materials, conventional carbonate-based electrolytes can no longer meet the requirements for electrochemical stability. Currently, the common approach is to add cathode film-forming additives to the electrolyte to solve the problem of poor electrolyte oxidative stability. However, this method usually leads to the deterioration of other electrical properties, such as power performance and low-temperature performance.

[0027] Therefore, in some embodiments of the present invention, the electrolyte contains a non-polar diluent component to improve the oxidation stability of the electrolyte and achieve stable compatibility between the electrolyte and cathode materials such as high-nickel ternary cathodes. On the one hand, non-polar diluents themselves have good oxidation stability and are not prone to oxidative decomposition. The addition of non-polar diluents can reduce the contact between carbonate solvents and the cathode surface, thereby reducing the probability of oxidative decomposition of carbonate solvents. On the other hand, non-polar diluents do not have a dissociation effect on lithium salts. Therefore, the addition of non-polar diluents can compress the solvation structure of lithium ions in the electrolyte, causing it to form a solvation structure containing anions, thereby improving the overall oxidation stability of the electrolyte. Furthermore, the addition of non-polar diluents can also reduce the activity of carbonate solvents, which can also improve the oxidation stability of carbonate solvents. In some embodiments of the present invention, the nonpolar diluent is preferably one or more selected from trifluorotoluene, difluorobenzene, trifluorobenzene, toluene, ethylbenzene, decafluoropentane, perfluorohexanone, perfluorobutyl ethyl ether, and perfluorobutyl methyl ether; more preferably, the nonpolar diluent is trifluorotoluene. To effectively improve the overall oxidative stability of the electrolyte and adjust the solvation structure, while maintaining stable conductivity of the electrolyte, the mass fraction of the nonpolar diluent in the electrolyte is preferably 21%-60%, more preferably 25%-50%.

[0028] Through analysis of the test results of lithium-ion battery cycle capability of embodiments of the present invention, the inventors discovered that due to the poor reduction stability of non-polar diluents, they undergo continuous reduction decomposition on the surface of graphite or silicon anodes in lithium-ion batteries. To address this problem, the inventors introduced sulfide additives represented by formulas (1), (2), or (3) into the electrolyte of some embodiments of the present invention, achieving good compatibility between the non-polar diluent and the anode while maintaining good stability of other performance characteristics of the lithium-ion battery. The structural formulas of the aforementioned sulfide additives are as follows:

[0029]

[0030] The reason why the sulfur-based additives with the above-described structure can achieve good compatibility between the non-polar diluent and the negative electrode in the embodiments of the present invention is twofold. Firstly, the SEI film products generated by these sulfur-based additives at the negative electrode have high polarity, and these SEI film products have extremely low solubility in non-polar diluents, thus enabling the formation of a complete and dense SEI film. Secondly, the unsaturation degree and sulfur content of these sulfur-based additives are higher than those of commonly used sulfur-based additives (such as DTD and PS), therefore, the SEI film components generated by their decomposition have higher cross-linking degree and higher sulfur content, thereby further improving the density of the SEI film and reducing the solubility of the SEI film components in non-polar diluents. These sulfur-based additives not only prevent the reductive decomposition of the diluent but also enable the construction of high-quality SEI films with very small addition amounts.

[0031] In some embodiments of the present invention, the sulfur-based additive is preferably [4,4'-bis(1,3,2-dioxothiacyclopentane)]2,2,2',2'-tetraoxide, i.e., formula (1). To achieve good compatibility between the nonpolar diluent and the negative electrode and to maintain the stability of other performance characteristics of the lithium-ion battery, the mass ratio of the sulfur-based additive to the nonpolar diluent, as indicated by formula (1), (2), or (3), is preferably 1:200-1:10, and more preferably, the mass ratio is 1:100-1:25.

[0032] In other embodiments of the present invention, the electrolyte further includes fluorinated additives and lithium salt additives. The fluorinated additives can participate in the formation of the SEI film together with the sulfide additives represented by formulas (1), (2), or (3) introduced in the embodiments of the present invention, forming an SEI film composed of inorganic components such as lithium fluoride and lithium sulfide. This uniform and dense SEI film can ensure the stability of the cycle and high-temperature storage performance of the lithium-ion battery. The lithium salt additives can introduce different anionic groups into the solvation structure of lithium ions. These anionic groups can participate in the formation of the positive electrode CEI / negative electrode SEI, improving the film quality and preventing continuous side reactions of the electrolyte on the electrode surface that could lead to the loss of active lithium. The fluorinated additive is preferably one or more of fluoroethylene carbonate, tris(2,2,2-trifluoroethyl) phosphate, 2-fluoropyridine, and difluorocarbonate ethylene ester, and the mass fraction of the fluorinated additive in the electrolyte is preferably 1%-12%; the lithium salt additive is preferably one or more of lithium difluorooxalate borate (LiDFOB), lithium difluorodioxalate phosphate (LiODFP), lithium difluorophosphate (LiPO2F2), lithium difluorosulfonylimide (LiFSI), and lithium dioxalate borate (LiBOB), and the mass fraction of the lithium salt additive in the electrolyte is preferably 0.1%-5%.

[0033] A second aspect of the present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte described in the above embodiments. During the charging and discharging process, lithium ions repeatedly insert and extract between the positive and negative electrode. The separator is disposed between the positive and negative electrode to provide isolation, and the electrolyte conducts ions between the positive and negative electrode.

[0034] The positive electrode of the lithium-ion battery provided in this invention includes a positive current collector and a positive active material. The positive current collector is typically made of a material with good conductivity and mechanical strength, and is not limited thereto; however, aluminum foil is preferred as the positive current collector in this invention. The positive active material may include any one or a combination of at least two of lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. In the lithium-ion battery provided in this invention, the positive active material is preferably lithium nickel cobalt manganese oxide with the chemical formula LiNi. x Mn y Co z O2(x+y+z=1). Preferably, 0.6≤x<1, more preferably, the positive electrode active material is LiNi. 0.9 Mn 0.05 Co 0.05 O2.

[0035] The negative electrode of a lithium-ion battery includes a negative current collector and a negative active material. The negative current collector can be made of a material with good conductivity and mechanical strength, and there are no limitations on this. Copper foil is preferred as the negative current collector in this invention. The negative active material may include any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxides, silicon carbide compounds, and lithium titanate. Those skilled in the art can select according to actual needs. In the embodiments of this invention, graphite is preferred as the negative active material of the lithium-ion battery.

[0036] The separator is selected from conventional types in the art. For example, a porous polyethylene (PE) or polypropylene (PP) membrane can be used as the separator. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0037] The technical solution of the present invention will be described in detail below through specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the embodiments are all commercially available.

[0038] The electrolyte composition of the different groups of examples and comparative examples is specifically referred to in Table 1.

[0039] Example 1

[0040] This embodiment provides a lithium-ion battery electrolyte. The solvent in the electrolyte is composed of ethylene carbonate and methyl ethyl carbonate in a mass ratio of 1:9. The lithium salt used is lithium hexafluorophosphate (LiPF6) with a concentration of 1 mol / L. The non-polar diluent used is trifluorotoluene, which has a mass fraction of 35% in the electrolyte. The sulfur-based additive used is [4,4'-bis(1,3,2-dioxothiacyclopentane)]2,2,2',2'-tetraoxide, i.e., formula (1), which has a mass fraction of 0.5% in the electrolyte. In addition to the non-polar diluent, sulfur-based additive, and lithium salt, the remaining part of the electrolyte is the above-mentioned mixed solvent.

[0041] Examples 2-7

[0042] The system was the same as in Example 1, except that the mass fractions of trifluorotoluene in the electrolyte were 15%, 21%, 25%, 50%, 60%, and 70%, respectively, and the rest were the same as in Example 1.

[0043] Examples 8-13

[0044] The system is the same as in Example 1, but the mass fractions of formula (1) in the electrolyte are 0.1%, 0.175%, 0.35%, 1.4%, 3.5% and 7%, respectively, and the rest is the same as in Example 1.

[0045] Example 14

[0046] Similar to the system in Example 1, toluene was used as a non-polar diluent, with a mass fraction of 35% in the electrolyte, and the rest was the same as in Example 1.

[0047] Example 15

[0048] Similar to the system in Example 1, ethylbenzene was used as a non-polar diluent, with a mass fraction of 35% in the electrolyte, and the rest was the same as in Example 1.

[0049] Example 16

[0050] Similar to the system in Example 1, decafluoropentane was used as a nonpolar diluent, with a mass fraction of 35% in the electrolyte, and the rest was the same as in Example 1.

[0051] Example 17

[0052] Similar to the system in Example 1, perfluorohexanone was used as a non-polar diluent, with a mass fraction of 35% in the electrolyte, and the rest was the same as in Example 1.

[0053] Example 18

[0054] Similar to the system in Example 1, perfluorobutyl ethyl ether was used as a non-polar diluent, with a mass fraction of 35% in the electrolyte. The rest was the same as in Example 1.

[0055] Example 19

[0056] Similar to the system in Example 1, glyoxal disulfate, i.e. formula (2), was used as a sulfur-based additive, with a mass fraction of 0.5% in the electrolyte. The rest was the same as in Example 1.

[0057] Example 20

[0058] Similar to the system in Example 1, 1,3,6,8-tetraoxa-2,7-dithiaspiro[4.4]nonane 2,2,7,7-tetraoxide, i.e. formula (3), was used as a sulfur-based additive, with a mass fraction of 0.5% in the electrolyte, and the rest was the same as in Example 1.

[0059] Example 21

[0060] Similar to the system in Example 1, formulas (1) and (2) were used simultaneously as sulfur-based additives, with the mass fractions of formulas (1) and (2) in the electrolyte being 0.3% and 1%, respectively, and the rest being the same as in Example 1.

[0061] Example 22

[0062] Similar to the system in Example 1, formulas (2) and (3) were used simultaneously as sulfur-based additives. The mass fractions of formulas (2) and (3) in the electrolyte were 0.2% and 0.8%, respectively, and the rest was the same as in Example 1.

[0063] Example 23

[0064] Similar to the system in Example 1, formulas (1) and (3) were used simultaneously as sulfur-based additives. The mass fractions of formulas (1) and (3) in the electrolyte were 0.5% and 0.3%, respectively, and the rest was the same as in Example 1.

[0065] Comparative Example 1

[0066] The electrolyte was prepared in the same manner as in Example 1, except that a non-polar diluent was not used. Otherwise, it was the same as in Example 1.

[0067] Comparative Example 2

[0068] The electrolyte was prepared in the same way as in Example 1, except that no sulfur-based additives were used. Otherwise, it was the same as in Example 1.

[0069] Comparative Example 3

[0070] The electrolyte was prepared in the same manner as in Example 1, except that non-polar diluents and sulfur-based additives were not used.

[0071] Comparative Example 4

[0072] Similar to the system in Example 1, the preparation of the electrolyte did not use the sulfur-based additives represented by formulas (1), (2) or (3), but instead used 1,3-propanesulfonate lactone (PS) as an additive, with a mass fraction of 0.5% in the electrolyte, and the rest was the same as in Example 1.

[0073] Comparative Example 5

[0074] Similar to the system in Example 1, the preparation of the electrolyte did not use the sulfur-based additives represented by formulas (1), (2) or (3), but used vinyl sulfate (DTD) as an additive with a mass fraction of 0.5% in the electrolyte, and the rest was the same as in Example 1.

[0075] Table 1. Electrolyte composition of different groups of examples and comparative examples

[0076]

[0077]

[0078] The electrolytes of Examples 1 to 23 and Comparative Examples 1 to 5 were used in the preparation of lithium-ion batteries.

[0079] In this embodiment of the invention, the electrochemical device is a lithium-ion battery, which can be a primary or secondary lithium-ion battery, comprising: a positive electrode, a negative electrode, a separator located between the positive and negative electrodes, and an electrolyte. The preparation method of the secondary lithium-ion battery in this embodiment of the invention is as follows:

[0080] (1) Preparation of positive electrode sheet

[0081] The positive electrode active material (LiNi) 0.9 Mn 0.05 Co 0.05 O2), conductive agent acetylene black, and binder polyvinylidene fluoride are mixed thoroughly in an N-methylpyrrolidone (NMP) solvent system at a mass ratio of 95:3:2 to obtain a positive electrode slurry. The positive electrode slurry is then uniformly coated onto an aluminum foil, which is subsequently dried and cold-pressed to obtain a positive electrode sheet. The thickness of the single-sided active material of the positive electrode sheet after cold pressing is 50 μm.

[0082] (2) Preparation of negative electrode sheet

[0083] The negative electrode active material graphite, conductive agent acetylene black, binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose were thoroughly mixed in a deionized water solvent system at a mass ratio of 96:2:1:1 to obtain a negative electrode slurry. The negative electrode slurry was then uniformly coated onto copper foil, which was subsequently dried and cold-pressed to obtain a negative electrode sheet.

[0084] (3) Preparation of the diaphragm

[0085] Polyethylene (PE) is used as the diaphragm.

[0086] (4) Preparation of electrolyte

[0087] In a glove box with an actual oxygen content of 0.1 ppm, a moisture content of 0.1 ppm, and a nitrogen content of 99.999%, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 1:9 to form a solvent. The lithium salt used in the electrolyte was lithium hexafluorophosphate (LiPF6), with a concentration of 1 mol / L. The types of non-polar diluents and additives used in the electrolytes of Examples 1-23 and Comparative Examples 1-5, and their mass fractions based on the total weight of the electrolyte, were all as specified in Table 1 above. The electrolyte, excluding the non-polar diluent, additives, and lithium salt, consisted of a solvent. The electrolyte was obtained by mixing the above components. Except for the component ratios specified in the table, the preparation methods of the electrolytes in different examples and comparative examples were the same.

[0088] (5) Preparation of secondary lithium-ion batteries

[0089] The positive electrode, separator, and negative electrode obtained above are stacked sequentially, with the separator positioned between the positive and negative electrode to act as a separator, resulting in a bare cell. The bare cell is then placed in an aluminum-plastic film and baked at 80°C to remove moisture. Subsequently, the corresponding electrolytes from Examples 1-23 and Comparative Examples 1-5 are injected and the cell is sealed. After further processing, including standing, hot and cold pressing, formation, clamping, and capacity testing, the finished soft-pack lithium-ion secondary battery is obtained.

[0090] The performance of lithium-ion batteries assembled using the electrolytes from Examples 1-23 and Comparative Examples 1-5 was tested. The test results are shown in Tables 2, 3, 4, 5, and 6. The test methods are as follows:

[0091] (1) Capacity retention test under ambient temperature cycling at 25℃

[0092] At 25°C, the battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 4.25V until the current was less than 0.05C. After resting for 10 minutes, it was discharged at a constant current of 1C to 2.5V. The discharge capacity of the battery at this point was measured, which is the discharge capacity of the first cycle. The battery was cycled multiple times under the above conditions, and the capacity retention rate after 400 cycles was calculated. The capacity retention rate after cycling was calculated using the following formula:

[0093] Capacity retention rate (%) = (Discharge capacity after 400 cycles / Discharge capacity after the first cycle) × 100%.

[0094] (2) Volume expansion rate test after 30 days of storage at 60℃

[0095] At 25°C, the battery was charged at a constant current of 1C to 4.25V, and then charged at a constant voltage to a current of 0.05C. The battery volume was measured using the water displacement method and recorded as V0. The fully charged battery was then stored in a 60°C oven for 30 days. The volume after storage was measured using the water displacement method and recorded as V1. The volume expansion rate relative to the battery before storage was calculated using the following formula:

[0096] Volume expansion rate (%) = (V1-V0) / V0×100%.

[0097] The test results are shown in Tables 2, 3, 4, 5, and 6.

[0098] Table 2. Performance test results of lithium-ion batteries in Examples 1 and 2-7

[0099]

[0100] Table 3. Performance test results of lithium-ion batteries in Examples 1 and 8-13

[0101]

[0102] Table 4. Performance test results of lithium-ion batteries in Examples 1 and 14-18

[0103]

[0104] Table 5. Performance test results of lithium-ion batteries in Examples 1 and 19-23

[0105]

[0106] Table 6. Performance test results of lithium-ion batteries in Example 1 and Comparative Examples 1-5

[0107]

[0108]

[0109] Analyzing the above data, we can draw the following conclusions:

[0110] Comparing the data in Table 2, it can be seen that when the mass fraction of nonpolar diluent in the electrolyte is 15%-70%, within this range, if the content of nonpolar diluent is too low, it cannot effectively improve the overall oxidation stability of the electrolyte, nor can it adjust the solvation structure, resulting in the continued oxidative decomposition of carbonate solvents. If the content is too high, since the diluent itself cannot dissolve lithium salts, it will cause a decrease in the degree of lithium salt dissociation, leading to a significant decrease in the electrolyte conductivity and preventing the battery from performing as intended. The preferred mass fraction of nonpolar diluent in the electrolyte is 21%-60%, more preferably 25%-50%.

[0111] Comparing the data in Table 3, it can be seen that when the mass ratio of the sulfur-based additive with the structure of formula (1) to the non-polar diluent in the electrolyte is 1:350-1:5, within this mass ratio range, if the content of the sulfur-based additive with the structure of formula (1) is too low, it will not be sufficient to form a complete SEI film at the negative electrode, thus making it impossible to solve the problem of reduction and decomposition of the non-polar diluent at the negative electrode; if its content is too high, it will significantly increase the viscosity of the electrolyte, affecting the transport of lithium ions in the electrolyte, thus leading to a significant increase in battery impedance, making it difficult for the battery to perform its proper electrical performance. The preferred mass ratio is 1:200-1:10, and more preferably 1:100-1:25.

[0112] Comparing the data in Tables 4 and 6, it can be seen that introducing a non-polar diluent into the electrolyte can significantly improve the oxidation stability of the electrolyte and achieve stable compatibility between the electrolyte and the high-nickel ternary cathode. Lithium-ion batteries composed of this electrolyte can exhibit relatively stable and good cycle performance and can delay the decay of the cycle life of lithium-ion batteries. Among them, the non-polar diluent is preferably trifluorotoluene and perfluorobutyl ether, and more preferably, trifluorotoluene is the non-polar diluent.

[0113] Comparing the data in Tables 5 and 6, it can be seen that introducing sulfide additives with one or more combinations of formulas (1), (2), and (3) into the electrolyte can solve the problem of poor reduction stability of non-polar diluents, resulting in continuous reduction decomposition on the surface of negative electrodes such as graphite or silicon. At the same time, the unsaturation degree and sulfur content of sulfide additives with one or more combinations of formulas (1), (2), and (3) are higher than those of commonly used sulfide additives (such as DTD and PS). Therefore, the SEI film components generated by their decomposition have higher cross-linking degree and more sulfur content, thereby further improving the compactness of the SEI film and reducing the solubility of the SEI film components in the non-polar diluent. Ultimately, this achieves good compatibility between the non-polar diluent and the negative electrode and maintains the stability of other performance characteristics of the lithium-ion battery. Among them, the sulfide additive is preferably a compound represented by formula (1), or a combination of compounds represented by formulas (1) and (3). More preferably, the sulfide additive is a compound represented by formula (1).

[0114] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that, include: Solvents, including carbonate solvents; Lithium salt, dissolved in the solvent; A non-polar diluent, wherein the mass fraction of the non-polar diluent in the electrolyte is 21%-60%; the non-polar diluent is one or more selected from trifluorotoluene, difluorobenzene, trifluorobenzene, toluene, ethylbenzene, decafluoropentane, perfluorohexanone, perfluorobutyl ethyl ether, and perfluorobutyl methyl ether. The sulfur-based additive, wherein the mass ratio of the sulfur-based additive to the non-polar diluent is 1:100-1:25, and the sulfur-based additive is a compound represented by formula (1), (2) or (3). 。 2. The electrolyte as described in claim 1, characterized in that, The non-polar diluent has a mass fraction of 25%-50% in the electrolyte.

3. The electrolyte as described in claim 1, characterized in that, The carbonate solvent is one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

4. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1 to 3.

5. The lithium-ion battery as described in claim 4, characterized in that, The positive electrode includes a positive current collector and a positive active material, wherein the positive active material includes any one or a combination of at least two of lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

6. The lithium-ion battery as described in claim 4, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material, wherein the negative active material includes any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbide, or lithium titanate.

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