An electrolyte for improving the low-temperature fast charging and cycle performance of lithium-ion batteries

Abstract

This invention discloses an electrolyte for improving the low-temperature fast charging and cycle performance of lithium-ion batteries, and a lithium-ion battery containing the electrolyte. The electrolyte comprises a main lithium salt, an organic solvent, and a first additive, wherein the first additive is a lithium sulfonate salt compound with the structure shown in formula (I); and a second additive, wherein the second additive is lithium difluorophosphate-based fluoroborate or lithium difluorophosphate-based fluorophosphate with the structure shown in formula (II). The definitions of each substituent in formulas (I) and (II) are detailed in the specification. The first additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte; the second additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte. This invention has the advantages of inhibiting aluminum foil corrosion in lithium sulfonate salt electrolyte system batteries, while improving the fast charging performance, low-temperature performance, and cycle performance of lithium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery electrolytes, and particularly to an electrolyte and lithium-ion battery thereof that improves the low-temperature, fast-charging and cycle performance of lithium-ion batteries. Background Technology

[0002] Currently, energy anxiety during long-distance charging of new energy vehicles has become a core concern for car owners. Therefore, the low-temperature fast-charging performance of batteries is receiving increasing attention. To address issues such as power performance degradation caused by charging at -20℃, 4C fast charging, and 2C fast charging cycles, electrolyte modification can be used. Modified electrolytes can reduce the initial DCIR impedance of the battery and suppress impedance growth during low-temperature and fast-charging processes.

[0003] The performance of high-voltage lithium-ion batteries in fast charging and low-temperature electrolytes is mainly limited by two factors. First, the high charge migration resistance of the electrode interface film (SEI) increases electrode polarization during rate charging or low-temperature charging and discharging. Second, under fast charging or low-temperature conditions, lithium plating is prone to occur at the negative electrode during the charging process, leading to a deterioration of the SEI film and a decrease in battery cycle performance. Currently, an important method to improve the fast charging and low-temperature performance of lithium-ion batteries is to use low-resistance additives. By adding low-resistance additives that are more effective than ECs or commonly used organic film-forming additives, the charge transfer resistance at the electrode interface can be reduced. Alternatively, lithium salt deposition improvers can be added to prevent lithium dendrite growth under fast charging or low temperatures, thereby improving battery cycle stability.

[0004] Patent CN106882820A discloses a lithium fluorosulfonate compound, which, when used as an additive in lithium-ion battery electrolytes, can significantly improve the battery's high input-output characteristics, especially at low temperatures (i.e., the battery has low internal resistance), thereby enhancing its fast-charging performance and low-temperature performance, making it better suited for applications such as electric vehicle power supplies and stationary large power supplies. However, the literature "Study on the Interfacial Electrochemical Behavior of Difluorosulfonylimide Lithium Electrolyte and Aluminum Positive Current Collector" (Diss. Huazhong University of Science and Technology, 2017) points out that the lithium fluorosulfonate compound mentioned in this patent can cause corrosion of the battery's positive current collector. Even at a dosage as low as 50 ppm, it can cause corrosion of the aluminum foil. As a result, during battery use, Al ions dissolve in the electrolyte and may even be further reduced on the negative electrode surface, damaging the negative electrode SEI film, increasing side reactions, and leading to a decline in battery cycle performance.

[0005] To address the aluminum foil corrosion problem caused by lithium fluorosulfonate compounds, Zhang et al. (Journal of The Electrochemical Society 153.9(2006):B365.) and Yan et al. (Journal of Solid State Electrochemistry 20.2(2016):507-516.) proposed lithium dioxaborate (LiBOB) and lithium difluorooxaborate (LiDFOB), respectively, which form a stable protective film on the surface of the positive electrode current collector, thereby inhibiting the reaction between lithium fluorosulfonate compounds and the positive electrode current collector and improving the cycle stability of the battery.

[0006] However, through NCM / graphite full-cell testing, the inventors discovered that additives such as LiBOB and LiDFOB, which inhibit corrosion of the aluminum foil in the positive electrode current collector, often also have a negative electrode film-forming effect. Furthermore, due to their high negative electrode film-forming potential (i.e., low film-forming potential in the full cell), they easily lead to an increase in the overall internal resistance of the battery, resulting in a decline in the battery's fast-charging and low-temperature performance. Therefore, while the combined use of lithium fluorosulfonate compounds and additives such as LiBOB and LiDFOB can inhibit aluminum foil corrosion and solve the cycle stability problem, it can no longer meet the requirements for fast-charging and low-temperature performance, and thus fails to address the initial goal of alleviating the anxiety associated with long-distance charging in new energy vehicles.

[0007] Therefore, it is both necessary and urgent to propose a method to effectively suppress corrosion of aluminum current collectors while improving the low-temperature performance, fast-charging performance and cycle stability of batteries. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention proposes an electrolyte that simultaneously improves the fast-charging performance, low-temperature performance, and cycle stability of lithium-ion batteries.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] An electrolyte for improving the low-temperature fast charging and cycle performance of lithium-ion batteries comprises a main lithium salt and an organic solvent, and the electrolyte further comprises:

[0011] The first additive is a lithium sulfonate salt compound with the structure shown in formula (I):

[0012]

[0013] In the formula, R1 is selected from fluorine, C1-C3 alkyl or C1-C6 fluoroalkyl;

[0014] The second additive is lithium difluorophosphate fluoroborate or lithium difluorophosphate fluorophosphate with the structure shown in formula (II):

[0015]

[0016] In the formula, M is boron or phosphorus; when M is boron, x is selected from 1, 2, 3 or 4, y is selected from 0, 1, 2 or 3, and x + y = 4; when M is phosphorus, x is selected from 1, 2, 3, 4, 5 or 6, y is selected from 0, 1, 2, 3, 4 or 5, and x + y = 6;

[0017] The first additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte;

[0018] The second additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte.

[0019] Preferably, in formula (I), R1 is selected from fluorine, methyl, ethyl, trifluoromethyl, difluoromethyl, fluoromethyl, 2-fluoroethyl, pentafluoroethyl, heptafluoropropyl or nonafluorobutyl.

[0020] More preferably, the first additive is selected from at least one of the following formulas:

[0021]

[0022] The second additive is selected from at least one of the following structures:

[0023]

[0024]

[0025] Further, the first additive accounts for 0.5 to 2.0 wt% of the total mass of the electrolyte; the second additive accounts for 0.5 to 2.0 wt% of the total mass of the electrolyte.

[0026] Indeed, the first additive of this invention, when added to the electrolyte at a concentration of 50 ppm, can cause aluminum foil corrosion, and the corrosion becomes more severe with increasing dosage, leading to rapid degradation of battery performance. At this point, the addition of lithium difluorophosphate-based fluoroborate or lithium difluorophosphate-based fluorophosphate of this invention can effectively inhibit aluminum foil corrosion. Although the mechanism by which compound II inhibits aluminum foil corrosion in the lithium-ion battery of this invention is not entirely clear, the inventors speculate that: lithium difluorophosphate-based fluoroborate can form AlBO3 passivation on the aluminum current collector surface, thereby inhibiting corrosion; or lithium difluorophosphate-based fluorophosphate can react with Al2O3 on the current collector surface to generate AlF3 passivation on the aluminum current collector, thereby inhibiting corrosion.

[0027] The main lithium salt used in this invention can be any commonly used lithium salt in electrolytes. Preferably, the main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide, accounting for 5-30% of the total mass of the electrolyte. More preferably, the main lithium salt is selected from lithium hexafluorophosphate and / or lithium bis(fluorosulfonyl)imide, accounting for 8-20% of the total mass of the electrolyte. Most preferably, the main lithium salt is selected from a mixture of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, and the amount of lithium bis(fluorosulfonyl)imide added is 2-10%.

[0028] The organic solvent used in this invention can be selected from commonly used solvents in electrolytes. Preferably, the organic solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylic acid ester compounds, sulfone compounds, or ether compounds.

[0029] Furthermore, the C3-C6 carbonate compound is selected from at least one of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, fluoroethylene carbonate, and difluoroethylene carbonate.

[0030] C3-C8 carboxylic acid esters are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate, propyl propionate, ethyl 2,2-difluoroethyl acetate, and ethyl 2,2,2-trifluoroethyl acetate.

[0031] The sulfone compound is selected from at least one of sulfolane, dimethyl sulfoxide, dimethyl sulfone, and diethyl sulfone;

[0032] The ether compound is selected from at least one of triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0033] To further improve the basic performance of the electrolyte, the electrolyte also includes a basic additive, which is selected from at least one of sulfonate compounds, sulfate compounds, fluorocarbonate compounds, unsaturated carbonate compounds or fluorinated lithium salt compounds, and is used in an amount of 0.1 to 5.0 wt% of the total mass of the electrolyte.

[0034] The sulfonate compound is selected from at least one of 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, and methanedisulfonate methane; the sulfate compound is selected from at least one of vinyl sulfate, 4-methyl vinyl sulfate, 4-fluorovinyl sulfate, and 4,4'-divinyl sulfate; the fluorinated carbonate compound is selected from at least one of fluorovinyl carbonate, difluorovinyl carbonate, and trifluoromethylpropylene carbonate; the unsaturated carbonate compound is selected from at least one of vinylene carbonate and vinyl vinyl carbonate; the fluorinated lithium salt compound is selected from at least one of lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorosulfonylimide, lithium ditrifluoromethylsulfonylimide, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate) phosphate, lithium difluorooxalate borate, and lithium tri(oxalate) phosphate.

[0035] In one specific embodiment, the electrolyte comprises 0.1–2.0 wt% of a first additive and 0.1–2.0 wt% of a second additive. The electrolyte can inhibit aluminum foil corrosion caused by the first additive and, compared to using the first additive alone or the second additive alone, can also improve the battery's low-temperature performance, fast-charging performance, and cycle performance.

[0036] In a preferred embodiment, the electrolyte comprises 0.5–1.5 wt% of a first additive, 0.5–1.5 wt% of a second additive, 0.1–2.0 wt% of methylene disulfonate, and 0.1–2.0 wt% of vinyl fluorosulfate. The electrolyte can inhibit aluminum foil corrosion caused by the first additive, and compared to not adding the above two types of basic additives, it can further improve battery cycle performance.

[0037] In another preferred embodiment, the electrolyte comprises 1.0–2.0 wt% of a first additive, 0.1–1.0 wt% of a second additive, 0.2–1.0 wt% of fluoroethylene carbonate, 0.5–2.0 wt% of ethylene sulfate, and 0.1–1.0 wt% of a fluorinated lithium salt compound. This electrolyte can inhibit aluminum foil corrosion caused by the first additive, and compared to not adding the above three basic additives, it can further improve the battery's low-temperature performance and cycle performance.

[0038] In another preferred embodiment, the electrolyte comprises 0.5–2.0 wt% of a first additive, 0.5–2.0 wt% of a second additive, 0.5–2.0 wt% of vinyl sulfate, 0.1–1.0 wt% of 1,3-propanesulfonate lactone, 1.0–2.0 wt% of fluoroethylene carbonate, 0.1–1.0 wt% of vinylene carbonate, 0.1–1.0 wt% of lithium difluorophosphate, and 0.05–0.5 wt% of lithium tetrafluoroborate. This electrolyte can inhibit aluminum foil corrosion caused by the first additive, and compared to not adding the above five basic additives, it can further improve the battery's low-temperature performance, fast-charging performance, and cycle performance.

[0039] The present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and any of the electrolytes described above.

[0040] The active material of the positive electrode is selected from at least one of nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium cobalt oxide, lithium nickel-manganese oxide, spinel manganese oxide, lithium cobalt phosphate, lithium iron-manganese phosphate, and other positive electrode materials that have high oxidizing properties in the fully charged state and use aluminum foil as the current collector.

[0041] The active material of the negative electrode is selected from at least one of commonly used negative electrode materials such as graphite, silicon oxide, silicon carbon, lithium, and lithium titanate.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] This invention suppresses aluminum foil corrosion caused by lithium sulfonate salt compounds by adding lithium difluorophosphate or lithium difluorophosphate. The combined use of the two not only solves the aluminum foil corrosion problem, but also improves the low-temperature performance, fast-charging performance and cycle stability of lithium-ion batteries. Detailed Implementation

[0044] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternatives, improvements, and equivalents that may be included within the scope of the claims.

[0045] I. Electrolyte Preparation

[0046] Example 1

[0047] Ethylene carbonate and methyl ethyl carbonate were mixed at a mass ratio of 3:7. Then, 6% lithium hexafluorophosphate (LiPF6) and 6% lithium bisfluorosulfonyl imide (LiFSI) were added based on the total mass of the electrolyte. In addition, 0.5% of compound I-3 and 0.5% of compound II-3 were added based on the total mass of the electrolyte.

[0048] Example 2

[0049] Ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed in a mass ratio of 3:2:5. Then, 7% LiPF6 and 7% LiFSI were added based on the total mass of the electrolyte. Next, 0.5% methylene disulfonate (MMDS) and 0.5% 4-fluoroethylene sulfate (FDTD) were added based on the total mass of the electrolyte. Finally, 1.0% of compound I-1 and 1.0% of compound II-2 were added based on the total mass of the electrolyte.

[0050] Example 3

[0051] Ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate were mixed in a mass ratio of 3:2:5. Then, 5% LiPF6 and 9% LiFSI were added based on the total mass of the electrolyte. Next, 0.5% fluoroethylene carbonate (FEC), 2.0% ethylene sulfate (DTD), and 0.5% lithium difluorobis(oxalato) phosphate (LiDFOP) were added based on the total mass of the electrolyte. Finally, 2.0% of compound I-2 and 0.5% of compound II-4 were added based on the total mass of the electrolyte.

[0052] Example 4

[0053] Ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate were mixed in a mass ratio of 3:2:5. Then, 9% LiPF6 and 6% LiFSI (based on the total mass of the electrolyte) were added. Next, 1.0% DTD, 0.5% PS, 2.0% FEC, 0.2% vinylene carbonate (VC), 0.8% lithium difluorophosphate (LDF), and 0.1% lithium tetrafluoroborate (LiBF4) (based on the total mass of the electrolyte) were added. Finally, 0.5% of compound I-1 and 2.0% of compound II-1 (based on the total mass of the electrolyte) were added.

[0054] Example 5

[0055] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-1 and 0.5% of compound II-2.

[0056] Example 6

[0057] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-2 and 0.5% of compound II-2.

[0058] Example 7

[0059] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 0.5% of compound I-3 and 0.5% of compound II-3.

[0060] Example 8

[0061] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 0.1% of compound I-1 and 0.1% of compound II-1.

[0062] Example 9

[0063] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 3.0% of compound I-1 and 3.0% of compound II-1.

[0064] Example 10

[0065] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-4 and 0.5% of compound II-6.

[0066] Example 11

[0067] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-5 and 0.5% of compound II-5.

[0068] Example 12

[0069] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-6 and 0.5% of compound II-5.

[0070] Example 13

[0071] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-7 and 0.5% of compound II-4.

[0072] Example 14

[0073] The operation of this embodiment is the same as that of embodiment 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-8 and 0.5% of compound II-4.

[0074] Example 15

[0075] Ethylene carbonate and methyl ethyl carbonate were mixed at a mass ratio of 3:7, and then 14% LiPF6 based on the total mass of the electrolyte was added. In addition, 2.0% of compound I-1 and 2.0% of compound II-1 based on the total mass of the electrolyte were added.

[0076] Example 16

[0077] Ethylene carbonate and ethyl methyl carbonate were mixed at a mass ratio of 3:7. Then, 6% LiPF6 and 2% LiFSI were added based on the total mass of the electrolyte. In addition, 2.0% of compound I-1 and 2.0% of compound II-2 were added based on the total mass of the electrolyte.

[0078] Comparative Example 1

[0079] The operation of this comparative example is the same as that of Example 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-1 and 0.5% of LiDFOB.

[0080] Comparative Example 2

[0081] The operation of this comparative example is the same as that of Example 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-1 and 0.5% of LiBOB.

[0082] Comparative Example 3

[0083] The operation of this comparative example is the same as that of Example 4, except that 0.5% of compound I-1 and 0.5% of compound II-1 are replaced with 2.0% of compound I-1.

[0084] Comparative Example 4

[0085] The operation of this comparative example is the same as that of Example 1, except that 0.5% of compound II-3 is replaced with 0.5% of LiDFOB.

[0086] Comparative Example 5

[0087] The operation of this comparative example is the same as that of Example 2, except that 1.0% of compound II-2 is replaced with 1.0% of LiDFOB.

[0088] Comparative Example 6

[0089] The operation of this comparative example is the same as that of Example 3, except that 0.5% of compound II-4 is replaced with 0.5% of LiDFOB.

[0090] Comparative Example 7

[0091] The operation of this comparative example is the same as that of Example 15, except that 2.0% of compound I-1 and 2.0% of compound II-1 are replaced with 2.0% of compound II-1.

[0092] Comparative Example 8

[0093] The operation of this comparative example is the same as that of Example 16, except that 2.0% of compound I-1 and 2.0% of compound II-2 are replaced with 2.0% of compound I-1.

[0094] II. Electrochemical Performance Testing

[0095] The performance of the lithium-ion power batteries (soft-pack cells) prepared in the examples and comparative examples was tested, mainly including:

[0096] (1) Low temperature performance test:

[0097] ① Low-temperature discharge capacity: At room temperature (25℃), the battery was charged at a constant current of 1C to 4.4V, and then charged at a constant voltage until the current dropped to 0.05C. It was then discharged at a constant current of 1C to 2.8V, and the room-temperature discharge capacity C1 was recorded. The above charging steps were repeated to 4.4V, then the ambient temperature was lowered to -20℃, and the battery was left at the ambient temperature for 5 hours to cool it down. Subsequently, it was discharged at a constant current of 0.5C to 2.5V, and the discharge capacity C2 was recorded. The capacity retention rate R1 was also recorded.

[0098] R1 = C2 / C1 * 100%

[0099] ② Low-Temperature DCIR Impedance: Fast Charging Performance Test: At room temperature (25℃), the battery was charged to 50% SOC with a current of 0.2C. Then, the ambient temperature was lowered to -20℃, and the battery was left at the ambient temperature for 5 hours to cool it down. Subsequently, it was discharged at a constant current of 1C I1 for 30 seconds, and the voltage value V1 in the last second of rest and the voltage value V2 in the 30th second of discharge were recorded. The low-temperature DCIR impedance R2 was calculated using the following formula:

[0100] R2=(V1-V2) / I1

[0101] (2) Fast charging performance test:

[0102] At room temperature (25℃), it was charged to 4.40V with a constant current of 4C and discharged to 2.8V with a constant current of 1C. The 4C charging capacity R3 was recorded.

[0103] (3) Cyclic performance test:

[0104] At room temperature (25℃), the battery is charged at a constant current of 2C to 4.40V, then charged at a constant voltage until the current drops to 0.05C, and then discharged at a constant current of 1C to 2.8V. This cycle is repeated, and the discharge capacity of the first week and the discharge capacity of the nth week are recorded. The battery cycle capacity retention rate is calculated using the following formula, and the cycle number change rate R4 is also calculated:

[0105] Capacity retention rate = discharge capacity in week N / discharge capacity in week 1 * 100%.

[0106] When the capacity retention rate is ≤80%, stop the test and record the cycle number n.

[0107] (4) Aluminum foil corrosion problem:

[0108] After the cycle performance test, the battery was disassembled and the negative electrode was dissolved in 40g of 2% dilute nitric acid solution. Then, inductively coupled plasma (ICP) test was performed, and the aluminum ion content in the negative electrode was calculated. If the aluminum ion content was ≥5μg / g, it was considered that there was a corrosion problem of the positive electrode aluminum foil, and it was recorded as Y; otherwise, it was recorded as N.

[0109] The performance test results of the lithium-ion power batteries prepared in each embodiment and comparative example are shown in Table 1 below:

[0110] Table 1 Battery Performance Test Results

[0111]

[0112]

[0113] According to the test results in Table 1 above:

[0114] Comparing Example 1 and Comparative Example 4, or Example 2 and Comparative Example 5, or Example 3 and Comparative Example 6, or Example 4 and Comparative Examples 1 and 2, it was found that when conventional LiDFOB or LiBOB, which inhibit aluminum foil corrosion, were added to the electrolyte containing the first additive, although aluminum foil corrosion could be inhibited, the fast charging performance and low-temperature performance of the battery deteriorated. Only when the second additive of the present invention was used could the fast charging performance and low-temperature performance of the battery be maintained or improved while inhibiting aluminum foil corrosion.

[0115] Comparing Example 5 and Comparative Example 3, it was found that adding only the first additive caused corrosion of the battery aluminum foil, which significantly reduced the battery's long-cycle test performance (such as battery cycle performance). Therefore, in the electrolyte system containing the first additive, other substances that can inhibit aluminum foil corrosion (such as the second additive) must be added to improve its long-cycle test performance.

[0116] Comparing Examples 4, 8 and Example 9, it was found that when the amount of the first additive and the second additive is in the range of 0.5% to 2.0%, the battery's low-temperature performance, fast-charging performance and cycle performance are significantly improved.

Claims

1. An electrolyte for improving the low-temperature fast charging and cycle performance of lithium-ion batteries, comprising a main lithium salt and an organic solvent, characterized in that: The electrolyte further includes: The first additive is a lithium sulfonate salt compound with the structure shown in formula (I): In the formula, R1 is selected from fluorine, C1-C3 alkyl or C1-C6 fluoroalkyl; The second additive is lithium difluorophosphate fluoroborate or lithium difluorophosphate fluorophosphate with the structure shown in formula (II): In the formula, M is boron or phosphorus; when M is boron, x is selected from 1, 2, 3 or 4, y is selected from 0, 1, 2 or 3, and x + y = 4; when M is phosphorus, x is selected from 1, 2, 3, 4, 5 or 6, y is selected from 0, 1, 2, 3, 4 or 5, and x + y = 6; The first additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte; The second additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that: In formula (I), R1 is selected from fluorine, methyl, ethyl, trifluoromethyl, difluoromethyl, fluoromethyl, 2-fluoroethyl, pentafluoroethyl, heptafluoropropyl, and nonafluorobutyl.

3. The electrolyte according to claim 2, characterized in that: The first additive is selected from at least one of the following structures: The second additive is selected from at least one of the following structures: 。 4. The electrolyte according to claim 1, characterized in that: The main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide, and accounts for 5 to 30% of the total mass of the electrolyte.

5. The electrolyte according to claim 4, characterized in that: The main lithium salt is selected from lithium hexafluorophosphate and / or lithium bisfluorosulfonyl imide, accounting for 8-20% of the total mass of the electrolyte.

6. The electrolyte according to claim 1, characterized in that: The organic solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylic acid ester compounds, sulfone compounds, or ether compounds.

7. The electrolyte according to claim 6, characterized in that: The C3-C6 carbonate compounds are selected from at least one of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, fluoroethylene carbonate, and difluoroethylene carbonate. The C3-C8 carboxylic acid ester compounds are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate, propyl propionate, ethyl 2,2-difluoroethyl acetate, and ethyl 2,2,2-trifluoroethyl acetate. The sulfone compound is selected from at least one of sulfolane, dimethyl sulfoxide, dimethyl sulfone, and diethyl sulfone; The ether compound is selected from at least one of triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

8. The electrolyte according to claim 1, characterized in that: The first additive accounts for 0.5 to 2.0 wt% of the total mass of the electrolyte; The second additive accounts for 0.5 to 2.0 wt% of the total mass of the electrolyte.

9. The electrolyte according to any one of claims 1-8, characterized in that: The electrolyte also includes a basic additive, which is selected from at least one of sulfonate compounds, sulfate compounds, fluorocarbonate compounds, unsaturated carbonate compounds or fluorinated lithium salt compounds, and is used in an amount of 0.1 to 3.0 wt% of the total mass of the electrolyte. The sulfonate compound is selected from at least one of 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, and methanedisulfonate methane; the sulfate compound is selected from at least one of vinyl sulfate, 4-methyl vinyl sulfate, 4-fluorovinyl sulfate, and 4,4'-divinyl sulfate; the fluorinated carbonate compound is selected from at least one of fluorovinyl carbonate, difluorovinyl carbonate, and trifluoromethylpropylene carbonate; the unsaturated carbonate compound is selected from at least one of vinylene carbonate and vinyl vinyl carbonate; the fluorinated lithium salt compound is selected from at least one of lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorosulfonylimide, lithium ditrifluoromethylsulfonylimide, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate) phosphate, lithium difluorooxalate borate, and lithium tri(oxalate) phosphate.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that: The lithium-ion battery further includes the electrolyte according to any one of claims 1-9.

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