Low-temperature long-circulation electrolyte, battery and electric equipment

Abstract

The invention relates to the technical field of new energy, in particular to a low-temperature long-circulation electrolyte, a battery and electric equipment. The low-temperature long-circulation electrolyte provided by the invention comprises lithium salt, an organic solvent and an additive, and the lithium salt comprises lithium bis (fluorosulfonyl) imide and lithium difluoro (oxalato) borate; the organic solvent is prepared from methyl 3, 3, 3-trifluoropropionate; the additive comprises fluoroethylene carbonate and a sulfur-containing compound. Therefore, lithium bis (fluorosulfonyl) imide and lithium difluoro (oxalato) borate are used as lithium salts, 3, 3, 3-methyl trifluoropropionate is used as an organic solvent, fluoroethylene carbonate and a sulfur-containing compound are used as additives, and the five raw materials cooperate with one another, so that the conductivity of the electrolyte at low temperature is improved, the low-temperature direct-current resistance of the battery can be reduced, the cycle capacity retention rate is increased, and the service life of the battery is prolonged. And particularly, the cycle retention rate of the battery at low temperature can be improved.

Description

Technical Field

[0001] The present invention relates to the field of new energy technologies, and in particular to a low-temperature long-circulation electrolyte, a battery and an electrical device. Background Art

[0002] With the rapid development of electric vehicles, commercial vehicles, aerospace, and energy storage systems in cold regions, the performance optimization of lithium-ion batteries in low-temperature environments (<0°C) faces severe challenges. Under low-temperature conditions, the electrolyte system has three major technical bottlenecks: (1) ionic conductivity is significantly reduced, resulting in obstructed charge transfer; (2) the electrode interface deintercalation kinetics deteriorate, causing severe polarization; and (3) traditional electrolyte components are prone to side reactions such as solvent decomposition and lithium salt degradation during long-term cycling, resulting in unstable reconstruction of the solid electrolyte interface (SEI) film. These problems collectively lead to key performance degradations such as a sudden drop in battery capacity and a shortened cycle life.

[0003] Current industry research and development of low-temperature electrolytes focuses on two main areas: solvent system optimization and the application of conductivity enhancers. Regarding solvent systems, the following optimization strategies are primarily employed: first, constructing a composite system of low-viscosity carbonates (EMC / DMC) and high-dielectric constant solvents (EC) to synergistically improve ionic conductivity and SEI film stability; second, introducing carboxylate solvents with ultra-low viscosity and high dielectric constants to significantly improve the rheological properties of the electrolyte under low-temperature conditions. However, existing carbonate-based electrolytes have significant performance limitations. First, the inherently high freezing point of carbonate solvents (approximately 36°C) leads to easy crystallization at low temperatures, causing a sharp increase in electrolyte viscosity. Second, linear carbonates such as EMC / DMC lack antioxidant stability, which exacerbates side reactions under high-temperature cycling conditions, severely impacting the battery's long-cycle performance. Carboxylate solvents are prone to react with lithium metal or highly active anodes, necessitating the addition of additives to optimize electrolyte performance. However, the use of additives can interfere with SEI film formation and exacerbate lithium graft growth, thereby impacting battery cycling performance. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defects of the electrolyte in the prior art such as low conductivity and poor cycle performance at low temperatures, thereby providing a low-temperature long-cycle electrolyte, battery and electrical equipment.

[0005] To this end, the present invention provides a low-temperature, long-cycle electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate; the organic solvent comprises methyl 3,3,3-trifluoropropionate; and the additive comprises fluoroethylene carbonate and a sulfur-containing compound.

[0006] Among them, the CAS number of lithium bis(fluorosulfonyl)imide is 171611-11-3, the CAS number of lithium difluorophosphate is 24389-25-1, the CAS number of methyl 3,3,3-trifluoropropionate is 18830-44-9, and the CAS number of fluoroethylene carbonate is 114435-02-8.

[0007] In some embodiments, based on the total weight of the electrolyte, the content of the lithium salt is 10-20 wt%, the content of the organic solvent is 30-88 wt%, and the content of the additive is 0.5-10 wt%. Preferably, the content of the organic solvent is 70-88 wt%.

[0008] In some embodiments, the sulfur-containing compound includes at least one of 1,3-propane sultone (CAS No. 1120-71-4), propenyl-1,3-sultone (CAS No. 21806-61-1), 1,4-butane sultone (CAS No. 1633-83-6), vinyl sulfate (CAS No. 1072-53-3), and methylene disulfonate (CAS No. 99591-74-9).

[0009] Preferably, the sulfur-containing compound includes at least one of 1,3-propane sultone, vinyl sulfate, propenyl-1,3-sultone and methylene methanedisulfonate.

[0010] In some embodiments, the lithium salt further comprises at least one of lithium hexafluorophosphate (CAS No. 21324-40-3), lithium difluorophosphate (CAS No. 24389-25-1), lithium perchlorate, lithium trifluoromethanesulfonate (CAS No. 33454-82-9), lithium bis(oxalatoborate) (CAS No. 244761-29-3), lithium tetrafluoroborate (CAS No. 14283-07-9), lithium bis(trifluoromethanesulfonyl)imide (CAS No. 90076-65-6), and lithium bis(pentafluoroethylsulfonyl)imide (CAS No. 132843-44-8).

[0011] In some embodiments, the organic solvent further comprises at least one of ethylene carbonate (CAS No. 96-49-1), diethyl carbonate (CAS No. 105-58-8), ethyl methyl carbonate (CAS No. 623-53-0), ethyl propionate (CAS No. 105-37-3), propyl propionate (CAS No. 106-36-5), ethyl acetate (CAS No. 141-78-6), ethyl butyrate (CAS No. 105-54-4), γ-butyrolactone, difluoroethyl acetate (CAS No. 1550-44-3) and ethyl 2,2,2-trifluoroacetate (CAS No. 406-95-1).

[0012] In some embodiments, the additive includes at least one of vinylene carbonate (CAS No. 872-36-6), tris-(trimethylsilyl) phosphite (CAS No. 1795-31-9), tris-(trimethylsilyl) borate (CAS No. 13399-93-4), ethylene glycol bis(propionitrile) ether (CAS No. 3386-87-6), biphenyl (CAS No. 92-52-4), vinyl ethylene carbonate (CAS No. 4427-96-7), trimethyl phosphate (CAS No. 512-56-1), triphenyl phosphate (CAS No. 115-86-6), tributyl phosphate (CAS No. 126-73-8), tris(2,2,2-trifluoroethyl) phosphate (CAS No. 358-63-4) and fluorinated ether.

[0013] Preferably, the additive includes at least one of tributyl phosphate, ethylene carbonate, tris-(trimethylsilyl) phosphite, and tris-(trimethylsilyl) borate.

[0014] In some embodiments, the content of the lithium difluorooxalatoborate is less than or equal to 0.5 wt %.

[0015] In some embodiments, the content of methyl 3,3,3-trifluoropropionate is 10-30 wt %.

[0016] In some embodiments, the content of the sulfur-containing compound is 0.5-2 wt %.

[0017] In some embodiments, the ratio of the lithium bis(fluorosulfonyl)imide to the lithium difluorooxalatoborate is 1-85. Preferably, the ratio of the lithium bis(fluorosulfonyl)imide to the lithium difluorooxalatoborate is 1-45.

[0018] In some embodiments, the ratio of the content of methyl 3,3,3-trifluoropropionate to the content of fluoroethylene carbonate is 3-30. Preferably, the ratio of the content of methyl 3,3,3-trifluoropropionate to the content of fluoroethylene carbonate is 5-20.

[0019] In some embodiments, the ratio of the lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 0.25 to 17. Preferably, the ratio of the lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 2 to 8.5.

[0020] In addition, the present invention also provides a method for preparing a low-temperature, long-cycle electrolyte, in which a lithium salt, an organic solvent and an additive are mixed in a certain ratio in an inert atmosphere to obtain an electrolyte.

[0021] On the other hand, the present invention also provides a battery comprising the above-mentioned low-temperature long-cycle electrolyte.

[0022] At the same time, the present invention also provides an electrical device including the above-mentioned battery.

[0023] The technical solution of the present invention has the following advantages:

[0024] 1. The present invention provides a low-temperature, long-cycle electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate; the organic solvent comprises methyl 3,3,3-trifluoropropionate; and the additive comprises fluoroethylene carbonate and a sulfur-containing compound. The present invention uses lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate as lithium salts. Since lithium bis(fluorosulfonyl)imide has a large anion radius and low binding energy with lithium ions, it is easier to dissociate lithium ions, thereby improving the conductivity of the electrolyte and accelerating the migration of lithium ions. In particular, in a low-temperature environment, lithium bis(fluorosulfonyl)imide has good compatibility with organic solvents, the anion (FSI-) structure of lithium bis(fluorosulfonyl)imide is loose, and its coordination ability with Li+ is moderate, which reduces the viscosity of the electrolyte, thereby maintaining a high ionic conductivity at low temperatures, greatly solving the problem of lithium ion migration being hindered at low temperatures. In addition, the solid electrolyte interface (SEI) formed by lithium bis(fluorosulfonyl)imide is thinner and rich in inorganic components (such as LiF), and has a higher Li+ at low temperatures. + The diffusion rate is increased, further reducing the interfacial impedance, thereby improving the conductivity of the electrolyte in a low-temperature environment; lithium bis(fluorosulfonyl)imide can form a SEI film with lower impedance and better mechanical stability on the surface of the negative electrode, which is conducive to the diffusion of lithium ions at low temperatures, thereby improving the low-temperature performance of the battery. At the same time, this SEI film can also effectively reduce the loss of active lithium caused by side reactions during the low-temperature cycle of the battery. However, lithium bis(fluorosulfonyl)imide has the effect of corroding aluminum foil, and the introduction of lithium difluorooxalatoborate can effectively passivate the aluminum foil.

[0025] The present invention utilizes two lithium salts, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate. Both lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate decompose before lithium nucleation, forming a membrane composed of boron-containing oligomers and lithium salt anion semi-decomposition products. This facilitates uniform and regular nucleation and growth of lithium. However, its presence is temporary; as deposition continues, it transforms into a mature inorganic phase-rich CEI / SEI membrane containing LiF and Li2S, exhibiting good ionic conductivity. Furthermore, the inorganic-based CEI / SEI membrane not only reduces impedance but also improves cycling stability.

[0026] Furthermore, the present invention uses 3,3,3-trifluoropropionic acid methyl ester as an organic solvent, and fluoroethylene carbonate and a sulfur-containing compound as additives. 3,3,3-trifluoropropionic acid methyl ester and fluoroethylene carbonate can generate a large amount of LiF in the SEI film generated by redox decomposition. LiF is an ideal component of the solid electrolyte interface film. The present invention uses the synergistic effect of 3,3,3-trifluoropropionic acid methyl ester and fluoroethylene carbonate to coat the structure of the electrode material on the basis of stabilizing the SEI film, thereby improving the cycle stability of the material. At the same time, 3,3,3-trifluoropropionic acid methyl ester has low solvation energy, and its unique Li + The solvation structure is also conducive to the formation of LiF, which helps to form a dense and well-passivated interface layer, allowing uniform ion flux and reducing interface resistance, thereby achieving improved conductivity. However, the addition of methyl 3,3,3-trifluoropropionate and fluoroethylene carbonate leads to an excessively high fluorine atom content, which may lead to poor ionic conductivity and reduced ion transport capacity. This is also achieved through the synergistic effect of methyl 3,3,3-trifluoropropionate and fluoroethylene carbonate with lithium bis(fluorosulfonyl)imide to take into account both ionic conductivity and interface stability; the anion of lithium bis(fluorosulfonyl)imide (FSI - ) has strong electron delocalization and low coordination ability, which can weaken the binding energy between lithium ions and solvents, form a loose solvation sheath, and reduce the desolvation energy of lithium ions (especially at low temperatures). The other additive sulfur-containing compound used in the present invention has a highly polar group (S=O, SO), which can further destroy the compactness of the solvation structure through competitive coordination, thereby improving the lithium ion migration rate at low temperatures.

[0027] Therefore, the present invention uses lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate as lithium salts, methyl 3,3,3-trifluoropropionate as an organic solvent, and fluoroethylene carbonate and a sulfur-containing compound as additives. Through the synergistic effect of the five raw materials, the conductivity of the electrolyte at low temperatures is improved, the low-temperature DC resistance of the battery is reduced, and the cycle capacity retention rate is improved, especially the cycle retention rate of the battery at low temperatures.

[0028] 2. The present invention provides a low-temperature, long-cycle electrolyte, wherein the ratio of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate is 1-85. When the ratio of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate is lower than 1, the content of lithium bis(fluorosulfonyl)imide is low, which is not conducive to improving the conductivity of the electrolyte, and in particular limits the migration of lithium ions in a low-temperature environment, thereby resulting in low conductivity of the electrolyte at low temperatures and low cycle capacity retention of lithium-ion batteries. In the electrolyte formed when the ratio of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate exceeds 85, due to the excessively high content of lithium difluorooxalatoborate, the decomposition products of lithium difluorooxalatoborate will aggravate the formation of the SEI film, increase the lithium ion transmission resistance, and affect the conductivity of the electrolyte at low temperatures and the cycle stability of the lithium-ion battery. The electrolyte formed when the ratio of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate is 1-85 can not only improve the conductivity of the electrolyte at low temperatures and reduce the low-temperature DC resistance of the lithium-ion battery, but also improve the cycle capacity retention rate at room temperature and low temperature.

[0029] 3. The present invention provides a low-temperature long-cycle electrolyte, wherein the ratio of 3,3,3-trifluoropropionate to fluoroethylene carbonate is 3-30. When the ratio of 3,3,3-trifluoropropionate to fluoroethylene carbonate is 3-30, and the electrolyte formed is less than 3 or greater than 30, during operation, 3,3,3-trifluoropropionate and fluoroethylene carbonate cannot form a stable SEI film, resulting in an unstable interface layer, which not only affects the low-temperature conductivity of the electrolyte, but also affects the cycle capacity retention rate of the lithium-ion battery. When the ratio of 3,3,3-trifluoropropionate to fluoroethylene carbonate is 3-30, not only can the conductivity of the electrolyte at low temperatures be improved, but the DC resistance of the lithium-ion battery is also reduced, thereby improving the cycle capacity retention rate at room temperature and low temperatures.

[0030] 4. The present invention provides a low-temperature long-cycle electrolyte, wherein the content ratio of lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 0.25-17. The present invention utilizes sulfur-containing compounds to solve the problem of weakening the binding energy between lithium ions and solvents due to the strong electron delocalization and low coordination ability of lithium bis(fluorosulfonyl)imide anions by competitive coordination, thereby improving the migration rate of lithium ions at low temperatures. When the content ratio of lithium bis(fluorosulfonyl)imide to sulfur-containing compounds is less than 0.25, corrosion may occur due to the excessively high content of sulfur-containing compounds, thereby causing a surge in interface layer impedance and increasing lithium ion transmission resistance, which not only shortens the service life but also deteriorates the low-temperature performance. When the content ratio of lithium bis(fluorosulfonyl)imide to sulfur-containing compounds is greater than 17, the content of sulfur-containing compounds is too low to destroy the compact type of the solvation structure, resulting in a low lithium ion migration rate at low temperatures and affecting the electrical conductivity of the electrolyte at low temperatures. The electrolyte formed by the content ratio of lithium bis(fluorosulfonyl)imide to sulfur-containing compounds being 0.25-17 can not only effectively improve the electrical conductivity of the electrolyte at low temperatures, but also reduce the DC resistance of the lithium-ion battery at low temperatures and improve the cycle capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 It is the conductivity of the electrolyte at -20°C in the experimental example of the present invention. DETAILED DESCRIPTION

[0033] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0034] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0035] Example 1

[0036] This embodiment provides a method for preparing an electrolyte, and the specific steps and parameters are as follows:

[0037] In a glove box filled with argon, the moisture content is less than 0.1 ppm, the oxygen content is less than 0.1 ppm, and the mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 2:1:3:4. Ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are mixed, and 10 wt% of methyl 3,3,3-trifluoropropionate based on the total weight of the electrolyte is added thereto to form an organic solvent. 10 wt% of lithium hexafluorophosphate, 0.5 wt% of lithium bisfluorosulfonyl imide, and 0.5 wt% of lithium difluorooxalatoborate based on the total weight of the electrolyte are added to the organic solvent to form a mixture of the organic solvent and the lithium salt. Finally, 1 wt% of fluoroethylene carbonate and 2 wt% of vinyl sulfate based on the total weight of the electrolyte are added, and the mixture is stirred at 500 rpm for 20 min to obtain an electrolyte.

[0038] Example 2-22

[0039] Compared with Example 1, the difference lies in the composition of the electrolyte. The differences are detailed in Table 1.

[0040] Example 23

[0041] This embodiment provides a method for preparing an electrolyte, and the specific steps and parameters are as follows:

[0042] In a glove box filled with argon, the moisture content is less than 0.1 ppm, the oxygen content is less than 0.1 ppm, and the mass ratio of ethyl acetate, ethyl 2,2,2-trifluoroacetate, ethyl butyrate, and difluoroethyl acetate is 2:1:3:4. Ethyl acetate, ethyl 2,2,2-trifluoroacetate, ethyl butyrate, and difluoroethyl acetate are mixed, and 10 wt% of methyl 3,3,3-trifluoropropionate based on the total weight of the electrolyte is added thereto to form an organic solvent. To the organic solvent, 15 wt% of lithium trifluoromethanesulfonate, 4.5 wt% of lithium bisfluorosulfonyl imide, and 0.5 wt% of lithium difluorooxalatoborate based on the total weight of the electrolyte are added to form a mixture of an organic solvent and a lithium salt. Finally, 1 wt% of fluoroethylene carbonate and 0.5 wt% of vinyl sulfate based on the total weight of the electrolyte are added, and the mixture is stirred at 500 rpm for 20 min to obtain an electrolyte.

[0043] Example 24

[0044] This embodiment provides a method for preparing an electrolyte, and the specific steps and parameters are as follows:

[0045] In a glove box filled with argon, the moisture content is less than 0.1ppm, the oxygen content is less than 0.1ppm, and the mass ratio of ethylene carbonate, ethyl propionate, ethyl methyl carbonate and ethyl 2,2,2-trifluoroacetate is 2:1:3:4. Ethylene carbonate, ethyl propionate, ethyl methyl carbonate and ethyl 2,2,2-trifluoroacetate are mixed, and 10wt% of methyl 3,3,3-trifluoropropionate and 1wt% of fluoroethylene carbonate based on the total weight of the electrolyte are added thereto to form an organic solvent. The electrolyte was stirred at 500 rpm for 20 min to obtain an electrolyte.

[0046] Comparative Example 1

[0047] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that the electrolyte does not contain lithium bis(fluorosulfonyl)imide, and an equal mass of lithium hexafluorophosphate is used to replace the lithium bis(fluorosulfonyl)imide in Example 10.

[0048] Comparative Example 2

[0049] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that the electrolyte does not contain lithium difluorooxalatoborate, and an equal mass of lithium hexafluorophosphate is used to replace the lithium difluorooxalatoborate in Example 10.

[0050] Comparative Example 3

[0051] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that the electrolyte does not contain methyl 3,3,3-trifluoropropionate.

[0052] Comparative Example 4

[0053] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that the electrolyte does not contain fluoroethylene carbonate, and an equal mass of trimethyl phosphate is used to replace the fluoroethylene carbonate in Example 10.

[0054] Comparative Example 5

[0055] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that the electrolyte does not contain vinyl sulfate, and an equal mass of trimethyl phosphate is used to replace the vinyl sulfate in Example 10.

[0056] Comparative Example 6

[0057] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that an equal mass of lithium bis(trifluoromethanesulfonyl)imide is used instead of lithium bis(fluorosulfonyl)imide.

[0058] Comparative Example 7

[0059] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that an equal mass of lithium bis(trifluoromethanesulfonyl)imide is used instead of lithium difluorooxalatoborate.

[0060] Comparative Example 8

[0061] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that an equal mass of ethyl acetate is used instead of methyl 3,3,3-trifluoropropionate.

[0062] Comparative Example 9

[0063] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that an equal mass of trifluoroethyl phosphate is used instead of fluoroethylene carbonate.

[0064] Comparative Example 10

[0065] This comparative example provides a method for preparing an electrolyte. The specific steps and parameters are the same as those in Example 10, except that an equal mass of trifluoroethyl phosphate is used instead of vinyl sulfate.

[0066] Table 1 Content of main components of electrolyte

[0067]

[0068]

[0069] Application Example 1-34

[0070] Lithium iron phosphate, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes were dispersed in N-methylpyrrolidone at a ratio of 97.05:2:0.8:0.15 to obtain a uniform positive electrode active material slurry. The slurry was then evenly coated on the surface of the aluminum foil current collector, dried at 100°C for 50 minutes, and compacted with a roller press to obtain a positive electrode sheet with a double-sided density of 420g / cm 2 , compacted density 2.5g / cm 3 ;

[0071] Graphite, sodium carboxymethyl cellulose, conductive carbon black, and styrene-butadiene rubber were mixed and dispersed in deionized water at a ratio of 96.05:1.15:1:1.8 to obtain a negative electrode active material slurry. The prepared slurry was coated on one side of a copper foil, dried, and compacted with a roller press to obtain a negative electrode sheet.

[0072] The electrolytes prepared in Examples 1-24 and Comparative Examples 1-10 of the present invention were used as the electrolytes for each application example;

[0073] Polypropylene diaphragms are used as diaphragms in various applications;

[0074] The prepared positive electrode sheet, separator and negative electrode sheet are stacked in order with the separator in the middle of the positive and negative electrode sheets. The bare battery cells are placed in aluminum-plastic film outer packaging and vacuum dried at 95°C. After the moisture content meets the standard, the electrolyte is injected into the dried battery. The battery is packaged, allowed to stand, hot and cold pressed, formed, and capacity divided to complete the preparation of the lithium-ion battery.

[0075] Experimental example

[0076] The lithium-ion batteries prepared in Examples 1-22 and 25-34 were tested. The test results are shown in Tables 2 and Figure 1 .

[0077] The low-temperature conductivity test method is as follows: take an appropriate amount of electrolyte and place it in a beaker, place it in a -20℃ constant temperature pot for 30 minutes, and then perform a low-temperature conductivity test after the main body temperature drops to -20℃.

[0078] The 25°C cycling performance test method is as follows: Li-ion batteries are placed in a 25°C constant temperature chamber and allowed to stand for 30 minutes to allow the lithium-ion batteries to reach a constant temperature. The lithium-ion batteries that have reached a constant temperature are then charged at a constant current of 1C to 3.65V, then charged at a constant voltage until the current drops to 0.05C, and then discharged at a constant current of 1C to 2.5V. This cycle is repeated, and the discharge capacity of the first and last cycles is recorded.

[0079] The -20°C cycle performance test method is as follows: Place the lithium-ion batteries in a -20°C constant temperature chamber and let them stand for 2 hours to allow the lithium-ion batteries to reach a constant temperature. The lithium-ion batteries that have reached a constant temperature are then charged at a constant current of 0.5C to 3.65V, then charged at a constant voltage until the current drops to 0.05C, and then discharged at a constant current of 0.5C to 2.0V. Repeat this cycle, and record the discharge capacity of the first cycle and the discharge capacity of the last cycle.

[0080] The Xth cycle capacity retention rate (wt%) = (Xth cycle discharge capacity / first cycle discharge capacity) × 100%, where X represents the number of cycles.

[0081] Low-temperature DC internal resistance (DCR) test: Place the lithium-ion battery in a 25°C environment, charge it to 3.65V at a constant current of 1C, then charge it at a constant voltage of 0.05C, let it stand for 30 minutes, then discharge it to 2.5V at a constant current of 1C, let it stand for 30 minutes, repeat this cycle 3 times, and record the average discharge capacity of 3 cycles as C0. Then charge the battery to 3.65V at a constant current of 1C0, then charge it to 0.05C at a constant voltage, let it stand for 30 minutes, then discharge it at 1C0 for 30 minutes, let it stand for 30 minutes. Place the lithium-ion battery in a -20°C environment, let it stand for 2 hours, discharge it at 1C0 for 30 seconds, let it stand for 30 minutes, and then complete the process.

[0082] DCR = (discharge initial voltage - discharge end voltage) / discharge current.

[0083] Table 2 Electrochemical performance of lithium batteries

[0084]

[0085]

[0086] Combined with Table 1-Table 2 and Figure 1 It can be seen that compared with Comparative Examples 1-5, there is a lack of a lithium salt, an organic solvent or an additive. At the same time, compared with Comparative Examples 6-10, the electrolyte formed with lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, ethylene carbonate as the organic solvent and ethylene carbonate as the additive, the electrolyte formed with lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate as the lithium salt, methyl 3,3,3-trifluoropropionate as the organic solvent, fluoroethylene carbonate and sulfur-containing compounds as additives has a higher electrical conductivity at low temperatures. At the same time, the battery formed using the electrolyte has a lower DC resistance at low temperatures and a higher cycle retention rate, especially a good cycle retention rate at low temperatures.

[0087] As a better choice, according to the settings of embodiments 1-22 of the present invention, combined with Figure 1From the results in Table 2, it can be seen that when the ratio of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate is in the range of 1-85, the ratio of methyl 3,3,3-trifluoropropionate to fluoroethylene carbonate is 3-30, and the ratio of the lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 0.25-17, the formed electrolyte has a high conductivity at low temperatures, and the battery formed using the electrolyte has a low DC resistance at low temperatures and a high cycle retention rate, especially good cycle retention rate at low temperatures. Good; further, the ratio of the content of the lithium bis(fluorosulfonyl)imide to the lithium difluorooxalatoborate is 1-45, the ratio of the content of the methyl 3,3,3-trifluoropropionate to the fluoroethylene carbonate is 5-20, and the ratio of the content of the lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 2-8.5. The formed electrolyte has higher conductivity at low temperatures, and the battery formed using the electrolyte has lower DC resistance at low temperatures and higher cycle retention rate, especially better cycle retention rate at low temperatures.

[0088] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A low-temperature long-cycle electrolyte, characterized in that: Including lithium salts, organic solvents and additives, The lithium salts include lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate; The organic solvent includes methyl 3,3,3-trifluoropropionate; The additives include fluoroethylene carbonate and sulfur-containing compounds.

2. The low-temperature long-circulation electrolyte according to claim 1, characterized in that Based on the total weight of the electrolyte, the content of the lithium salt is 10-20 wt%, the content of the organic solvent is 30-88 wt%, and the content of the additive is 0.5-10 wt%; and / or, The sulfur-containing compound includes at least one of 1,3-propane sultone, propenyl-1,3-sultone, 1,4-butane sultone, vinyl sulfate, and methylene methanedisulfonate.

3. The low-temperature long-circulation electrolyte according to claim 1, characterized in that The lithium salt further comprises at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bisoxalatoborate, lithium tetrafluoroborate, lithium bistrifluoromethanesulfonyl imide, and lithium bis(pentafluoroethylsulfonyl)imide; and / or, The organic solvent further comprises at least one of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl butyrate, γ-butyrolactone, difluoroethyl acetate and ethyl 2,2,2-trifluoroacetate; and / or, The additive includes at least one of vinylene carbonate, tris-(trimethylsilyl) phosphite, tris-(trimethylsilyl) borate, ethylene glycol bis(propionitrile) ether, biphenyl, vinyl ethylene carbonate, trimethyl phosphate, triphenyl phosphate, tributyl phosphate, tris(2,2,2-trifluoroethyl) phosphate and fluorine-containing ether.

4. The low-temperature long-circulation electrolyte according to claim 2, characterized in that The content of the lithium difluorooxalatoborate is less than or equal to 0.5 wt %; and / or, The content of methyl 3,3,3-trifluoropropionate is 10-30 wt%; and / or, The content of the sulfur-containing compound is 0.5-2 wt%.

5. The low-temperature long-circulation electrolyte according to claim 4, characterized in that: The ratio of the content of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate is 1-85.

6. The low-temperature long-circulation electrolyte according to claim 4, characterized in that: The ratio of the content of methyl 3,3,3-trifluoropropionate to fluoroethylene carbonate is 3-30.

7. The low-temperature long-circulation electrolyte according to claim 4, characterized in that The content ratio of the lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 0.25-17.

8. The low-temperature long-circulation electrolyte according to claim 5, 6 or 7, characterized in that: The ratio of the content of lithium bis(fluorosulfonyl)imide to lithium difluorooxalatoborate is 1-45; and / or, The ratio of the content of methyl 3,3,3-trifluoropropionate to fluoroethylene carbonate is 5-20; and / or, The content ratio of the lithium bis(fluorosulfonyl)imide to the sulfur-containing compound is 2-8.

5.

9. A battery, characterized in that: Comprising the low-temperature long-cycle electrolyte according to any one of claims 1 to 8.

10. An electrical device, characterized in that: A battery comprising the battery of claim 9.

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