An electrolyte and a lithium-ion battery

Abstract

This application specifically discloses an electrolyte and a lithium-ion battery. The electrolyte includes a first additive and a second additive. The first additive includes 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive includes a fluorosulfonate additive. This application has the advantages of promoting the formation of a stable SEI film on the surface of silicon-containing anode materials, improving the volume stability of silicon-containing anodes during lithium battery cycling, and thus improving the rate performance, cycle performance, and safety performance of lithium batteries.

Description

Technical Field

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

[0002] The main components of a lithium-ion battery include the positive electrode, negative electrode, separator, and electrolyte. Among these, the electrolyte is a crucial component. During the initial charge and discharge cycle, the electrolyte reacts with the silicon-based material at the solid-liquid interface, forming a passivation layer (SEI film) at the negative electrode interface. Lithium ions can freely intercalate and deintercalate through the SEI film, enabling the battery to charge and discharge. While traditional lithium-ion batteries are widely used in portable electronic devices and electric vehicles, their limited energy density cannot meet the demands of more demanding applications. Therefore, the research and development of high-energy-density lithium batteries has become a vital research topic.

[0003] High-capacity anode materials are a crucial breakthrough for improving the energy density of lithium-ion batteries. Silicon-based materials are considered the most likely to replace graphite as anode materials due to their high theoretical specific capacity, suitable discharge potential, and relatively abundant reserves in nature. However, the commercial application of silicon-based materials in lithium-ion batteries faces numerous challenges. Silicon-based materials undergo significant volume expansion during cycling, which prevents the formation of a stable SEI film on the anode surface. The repeated rupture and formation of the SEI film consumes a large amount of lithium ions. The repeated growth and rupture of the SEI film at the silicon-containing anode interface significantly affects the rate performance and cycle stability of lithium batteries. In severe cases, a broken SEI film may lead to more violent reactions between the electrolyte and electrode materials, generating heat and gas, increasing the internal pressure of the battery, and potentially causing safety accidents.

[0004] Therefore, it is of great significance to develop an electrolyte suitable for lithium batteries with silicon-containing anode systems to suppress the SEI film damage of silicon-based materials during lithium battery charge-discharge cycles. Summary of the Invention

[0005] To address the problems existing in the prior art, this application provides an electrolyte and a lithium-ion battery.

[0006] In a first aspect, this application provides an electrolyte, which adopts the following technical solution:

[0007] An electrolyte comprising a first additive and a second additive, the first additive comprising 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive comprising a fluorosulfonate additive.

[0008] This application utilizes a combination of a first additive and a second additive. The electron-deficient boron atom in the first additive can capture anions, thus acting as an anion acceptor to coordinate with anions in the electrolyte, promoting lithium-ion dissociation and increasing lithium-ion migration rate. Increased free lithium content reduces SEI film dissolution. Furthermore, the dioxoboronyl group can absorb HF in the electrolyte, all contributing to enhanced SEI film stability and improved rate and cycle performance of the lithium battery. The presence of fluorine in the second additive interacts with free lithium ions in the electrolyte, promoting the formation of LiF and Li2S interfacial films at the negative electrode, thereby improving the stability of the negative electrode SEI film. The high ionic conductivity of LiF and Li2S further enhances the rate performance of the lithium-ion battery. Moreover, the boron in the first additive coordinates with the sulfonic acid groups in the second additive to improve film stability on the electrode surface, while the combination of fluorine and sulfonic acid groups improves the flame-retardant effect of the electrolyte, further enhancing the fast-charging safety of the lithium battery.

[0009] Preferably, the fluorinated sulfonate additive includes at least one of propyl 2,2,3,3,3-pentafluorotrifluoromethanesulfonate, 2,2,3,3-tetrafluoropropyltrifluoromethanesulfonate, 2,2-difluoroethyltrifluoromethanesulfonate, and hexafluoroisopropyltrifluoromethanesulfonate.

[0010] Preferably, the first additive accounts for 1%-3% of the mass of the electrolyte, and / or the second additive accounts for 0.5%-2.5% of the mass of the electrolyte.

[0011] By controlling the proportions of the first and second additives in the electrolyte, on the one hand, the first additive increases the amount of free lithium in the electrolyte, while the addition of the second additive can react with the free lithium to form a LiF film. This avoids the situation where the free lithium content in the electrolyte is too high due to the excessive content of the first additive. Excessive free lithium content will lead to a decrease in the ionic conductivity of the electrolyte, a decrease in the stability of the electrolyte during charge-discharge cycles, and cause the electrolyte to decompose and generate gas, which is not conducive to improving the rate performance, cycle performance and safety performance of lithium batteries.

[0012] Preferably, it also includes an organic solvent, a lithium salt, a lithium salt additive, and a third additive.

[0013] Preferably, the third additive includes at least one of 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), and 1,3-propenesulfonate lactone (PST).

[0014] The third additive accounts for 1%-3% of the mass of the electrolyte.

[0015] By introducing a third additive into the electrolyte and controlling the proportion of the third additive, on the one hand, the third additive can further improve the stability of the electrolyte, thereby working with the first additive to improve the stability of the electrode / electrolyte interface. On the other hand, the third additive can form a sulfur-containing interface film at the negative electrode, which helps to improve the high-temperature cycle performance of lithium batteries.

[0016] Preferably, the lithium salt comprises lithium hexafluorophosphate (LiPF6).

[0017] Preferably, the concentration of the lithium salt is 1-1.5 mol / L.

[0018] Preferably, the lithium salt additive includes at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); the lithium salt additive accounts for 0.1%-1% of the mass of the electrolyte.

[0019] Preferably, the organic solvent includes at least one of linear carbonates and cyclic carbonates.

[0020] Preferably, the linear carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), and methyl propyl carbonate (MPC); the cyclic carbonate includes at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC).

[0021] Preferably, the mass ratio of the linear carbonate to the cyclic carbonate is 60-80:20-40.

[0022] By using linear and cyclic carbonates as solvents, it is helpful to adjust the electrolyte to a suitable viscosity, thereby generating a stable SEI film on the surface of the negative electrode active material. Furthermore, it can improve the ionic conductivity and chemical stability of the electrolyte, thus helping the first and second additives to exert a long-term synergistic effect in the electrolyte, which further helps to improve the rate performance, cycle performance and safety performance of lithium batteries.

[0023] Secondly, this application provides a lithium-ion battery, which adopts the following technical solution:

[0024] A lithium-ion battery includes a positive electrode, a separator, a negative electrode, and an electrolyte as described above. Detailed Implementation

[0025] To better understand and implement this application, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of this application, and not all of them.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0027] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0028] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0029] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where there are other unmentioned elements in addition to the mentioned elements.

[0030] All percentages in this application are weight percentages unless otherwise stated.

[0031] Unless otherwise stated, the terms “a,” “an,” “an,” and “the” as used in this specification are intended to include “at least one” or “one or more.” For example, “a component” refers to one or more components, and therefore more than one component may be considered and may be employed or used in the implementation of the described embodiments.

[0032] The first additive used in this application, 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, has the CAS number 1333222-12-0.

[0033] Example 1

[0034] 1. Preparation of electrolyte

[0035] The electrolyte contains 2% by mass of the first additive and 1.5% by mass of the second additive. The organic solvent contains linear carbonate to cyclic carbonate in a mass ratio of 70:30 and lithium salt concentration of 1.2 mol / L.

[0036] The first additive is 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive is propyl 2,2,3,3,3-pentafluorotrifluoromethanesulfonate.

[0037] The lithium salt is lithium hexafluorophosphate (LiPF6);

[0038] Linear carbonates are dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), and cyclic carbonates are fluoroethylene carbonate (FEC) and propylene carbonate (PC) (DMC:EMC:FEC:PC = 20:50:10:20).

[0039] Linear carbonate and cyclic carbonate are stirred and mixed evenly to obtain a first mixture; lithium salt is added to the first mixture and stirred and mixed evenly to obtain a second mixture; a first additive and a second additive are added to the second mixture and stirred and mixed evenly to obtain an electrolyte.

[0040] 2. Preparation of the positive electrode sheet

[0041] Ternary material NCM (LiNi) 0.9 Co 0.05 Mn 0.05 O2) positive electrode active material, binder PVDF (polyvinylidene fluoride), and conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly at a mass ratio of 96:1.8:2.2 to obtain positive electrode slurry. Then, the positive electrode slurry is coated onto aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0042] 3. Preparation of negative electrode sheet

[0043] Silicon-carbon anode material, conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), and binder PAA (polyacrylic acid) are mixed and stirred evenly in a mass ratio of 83.5:8:0.5:8 to obtain a negative electrode slurry. The solid content is controlled at 30%. The negative electrode slurry is then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, a negative electrode sheet is obtained.

[0044] 4. Selection of separator membrane

[0045] Polyethylene (PE) + ceramic was chosen as the separator for lithium-ion batteries.

[0046] 5. Preparation of lithium-ion batteries

[0047] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation, thus obtaining a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping processes, a soft-pack lithium-ion battery is obtained.

[0048] Example 2

[0049] 1. Preparation of electrolyte

[0050] The electrolyte contains 2% by mass of the first additive, 0.5% by mass of the second additive, and 1% by mass of the lithium salt additive. The organic solvent contains linear carbonate to cyclic carbonate in a mass ratio of 60:40, and the lithium salt concentration is 1 mol / L.

[0051] The first additive is 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive is 2,2,3,3-tetrafluoropropyltrifluoromethanesulfonate.

[0052] The lithium salt is lithium hexafluorophosphate (LiPF6);

[0053] The lithium salt additive is lithium bis(fluorosulfonyl)imide (LiFSI);

[0054] Linear carbonates are dimethyl carbonate (DMC) and diethyl carbonate (DEC), and cyclic carbonates are propylene carbonate (PC) and fluoroethylene carbonate (FEC) (DMC:DEC:PC:FEC = 20:40:20:20).

[0055] Linear carbonate and cyclic carbonate are stirred and mixed evenly to obtain a first mixture; lithium salt is added to the first mixture and stirred and mixed evenly to obtain a second mixture; a first additive and a second additive are added to the second mixture and stirred and mixed evenly to obtain an electrolyte.

[0056] 2. Preparation of the positive electrode sheet

[0057] Ternary material NCM (LiNi) 0.9 Co 0.05 Mn 0.05 O2) positive electrode active material, binder PVDF (polyvinylidene fluoride), and conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly at a mass ratio of 96:1.8:2.2 to obtain positive electrode slurry. Then, the positive electrode slurry is coated onto aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0058] 3. Preparation of negative electrode sheet

[0059] Silicon-carbon anode material, conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), and binder PAA (polyacrylic acid) are mixed and stirred evenly in a mass ratio of 83.5:8:0.5:8 to obtain a negative electrode slurry. The solid content is controlled at 30%. The negative electrode slurry is then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, a negative electrode sheet is obtained.

[0060] 4. Selection of separator membrane

[0061] Polyethylene (PE) + ceramic was chosen as the separator for lithium-ion batteries.

[0062] 5. Preparation of lithium-ion batteries

[0063] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation, thus obtaining a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping processes, a soft-pack lithium-ion battery is obtained.

[0064] Example 3

[0065] 1. Preparation of electrolyte

[0066] The electrolyte contains 3% by mass of the first additive, 2% by mass of the second additive, and 1% by mass of the lithium salt additive. The organic solvent contains linear carbonate to cyclic carbonate in a mass ratio of 80:20, and the lithium salt concentration is 1.5 mol / L.

[0067] The first additive is 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive is 2,2-difluoroethyltrifluoromethanesulfonate.

[0068] The lithium salt is lithium hexafluorophosphate (LiPF6);

[0069] The lithium salt additive is lithium difluorophosphate (LiPO2F2).

[0070] Linear carbonates are diethyl carbonate (DEC) and methyl propyl carbonate (MPC), and cyclic carbonates are ethylene carbonate (EC) and propylene carbonate (PC) (DEC:MPC:EC:PC = 60:20:10:10).

[0071] Linear carbonate and cyclic carbonate are stirred and mixed evenly to obtain a first mixture; lithium salt is added to the first mixture and stirred and mixed evenly to obtain a second mixture; a first additive and a second additive are added to the second mixture and stirred and mixed evenly to obtain an electrolyte.

[0072] 2. Preparation of the positive electrode sheet

[0073] Ternary material NCM (LiNi) 0.9 Co 0.05 Mn 0.05O2) positive electrode active material, binder PVDF (polyvinylidene fluoride), and conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly at a mass ratio of 96:1.8:2.2 to obtain positive electrode slurry. Then, the positive electrode slurry is coated onto aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0074] 3. Preparation of negative electrode sheet

[0075] Silicon-carbon anode material, conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), and binder PAA (polyacrylic acid) are mixed and stirred evenly in a mass ratio of 83.5:8:0.5:8 to obtain a negative electrode slurry. The solid content is controlled at 30%. The negative electrode slurry is then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, a negative electrode sheet is obtained.

[0076] 4. Selection of separator membrane

[0077] Polyethylene (PE) + ceramic was chosen as the separator for lithium-ion batteries.

[0078] 5. Preparation of lithium-ion batteries

[0079] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation, thus obtaining a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping processes, a soft-pack lithium-ion battery is obtained.

[0080] Example 4

[0081] 1. Preparation of electrolyte

[0082] The electrolyte contains 2.5% by mass of the first additive, 2% by mass of the second additive, and 1% by mass of the lithium salt additive. The organic solvent contains linear carbonate to cyclic carbonate in a mass ratio of 80:20, and the lithium salt concentration is 1.3 mol / L.

[0083] The first additive is 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive is hexafluoroisopropyltrifluoromethanesulfonate.

[0084] The lithium salt is lithium hexafluorophosphate (LiPF6);

[0085] The lithium salt additive is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);

[0086] Linear carbonates are diethyl carbonate (DEC) and methyl propyl carbonate (MPC), while cyclic carbonates are fluoroethylene carbonate (FEC) and propylene carbonate (PC) (DEC:MPC:EC:FEC = 50:30:10:10).

[0087] Linear carbonate and cyclic carbonate are stirred and mixed evenly to obtain a first mixture; lithium salt is added to the first mixture and stirred and mixed evenly to obtain a second mixture; a first additive and a second additive are added to the second mixture and stirred and mixed evenly to obtain an electrolyte.

[0088] 2. Preparation of the positive electrode sheet

[0089] Ternary material NCM (LiNi) 0.9 Co 0.05 Mn 0.05 O2) positive electrode active material, binder PVDF (polyvinylidene fluoride), and conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly at a mass ratio of 96:1.8:2.2 to obtain positive electrode slurry. Then, the positive electrode slurry is coated onto aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0090] 3. Preparation of negative electrode sheet

[0091] Silicon-carbon anode material, conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), and binder PAA (polyacrylic acid) are mixed and stirred evenly in a mass ratio of 83.5:8:0.5:8 to obtain a negative electrode slurry. The solid content is controlled at 30%. The negative electrode slurry is then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, a negative electrode sheet is obtained.

[0092] 4. Selection of separator membrane

[0093] Polyethylene (PE) + ceramic was chosen as the separator for lithium-ion batteries.

[0094] 5. Preparation of lithium-ion batteries

[0095] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation, thus obtaining a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping processes, a soft-pack lithium-ion battery is obtained.

[0096] Example 5

[0097] The difference between this embodiment and Embodiment 1 is that it also includes a third additive, 1,3-propanesulfonate lactone (PS), and the mass percentage of the third additive in the electrolyte is 1%; the remaining steps and parameter settings are consistent with those in Embodiment 1.

[0098] Example 6

[0099] The difference between this embodiment and Embodiment 1 is that it also includes a third additive, vinyl sulfate DTD, and the mass percentage of the third additive in the electrolyte is 3%; the remaining steps and parameter settings are consistent with Embodiment 1.

[0100] Example 7

[0101] The difference between this embodiment and Embodiment 1 is that the mass percentage of the first additive in the electrolyte is 4%, and the mass percentage of the second additive in the electrolyte is 0.1%; the remaining steps and parameter settings are consistent with Embodiment 1.

[0102] Example 8

[0103] The difference between this embodiment and Embodiment 1 is that the mass percentage of the first additive in the electrolyte is 0.5%, and the mass percentage of the second additive in the electrolyte is 3%; the remaining steps and parameter settings are consistent with Embodiment 1.

[0104] Example 9

[0105] The difference between this embodiment and Example 1 is that an equal weight of propyl acetate (EP) is used instead of the linear carbonate in Example 1; the remaining steps and parameter settings are consistent with Example 1.

[0106] Example 10

[0107] The difference between this embodiment and Embodiment 1 is that the mass ratio of linear carbonate to cyclic carbonate is 20:80; the remaining steps and parameter settings are consistent with Embodiment 1.

[0108] Comparative Example 1

[0109] The difference between this comparative example and Example 1 is that the electrolyte does not contain the first additive and the second additive; the remaining steps and parameter settings are consistent with those of Example 1.

[0110] Comparative Example 2

[0111] The difference between this comparative example and Example 1 is that the electrolyte contains only the first additive; the remaining steps and parameter settings are consistent with those of Example 1.

[0112] Comparative Example 3

[0113] The difference between this comparative example and Example 1 is that the electrolyte contains only the second additive; the remaining steps and parameter settings are consistent with those of Example 1.

[0114] Comparative Example 4

[0115] The difference between this comparative example and Example 1 is that an equal weight of 5-(4,4,5,5-tetramethyl-1,3,2-dioxopentano-2-yl)-1H-indazole is used instead of the first additive 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxopentano-2-yl)pyridine in Example 1; the remaining steps and parameter settings are consistent with those in Example 1.

[0116] Test methods

[0117] I. First Coulombic Efficiency Test of Lithium-ion Batteries

[0118] The lithium batteries in the above embodiments and comparative examples were subjected to the first coulombic efficiency test. The specific test steps were as follows: under the condition of 25°C, the lithium-ion battery was charged to 4.2V at a constant current and constant voltage rate of 0.33C, and left to stand for 10 minutes. Then, the lithium-ion battery was discharged to 2.5V at a constant current rate of 0.33C, and left to stand for 10 minutes. The first coulombic efficiency of the lithium-ion battery was calculated.

[0119] Initial coulombic efficiency (%) = Total capacity of lithium-ion battery during initial discharge at 0.33C / Total capacity of lithium-ion battery during initial charge at 0.33C × 100%.

[0120] II. Lithium-ion battery cycle performance test

[0121] 2.1 Lithium-ion battery capacity retention rate during room temperature cycling

[0122] The lithium batteries in the above embodiments and comparative examples were subjected to a room temperature cycle capacity retention test. The specific test steps were as follows: at 25°C, the lithium-ion battery was charged at a constant current and constant voltage of 1C to 4.2V, with a cutoff current of 0.05C, and left to stand for 10 minutes. Then, the lithium-ion battery was discharged at a constant current of 1C to 2.5V and left to stand for 10 minutes. This constituted one charge-discharge cycle. The lithium-ion battery was subjected to 1000 charge-discharge cycles according to the above method. The capacity retention rate of the lithium-ion battery after 1000 charge-discharge cycles at 1C / 1C was calculated.

[0123] 2.2 High-Temperature Cycling Capacity Retention Rate of Lithium Batteries

[0124] The lithium batteries in the above embodiments and comparative examples were subjected to a room temperature cycle capacity retention test. The specific test steps were as follows: at 45°C, the lithium-ion battery was charged at a constant current and constant voltage of 1C to 4.2V, with a cutoff current of 0.05C, and left to stand for 10 minutes. Then, the lithium-ion battery was discharged at a constant current of 1C to 2.5V and left to stand for 10 minutes. This constituted one charge-discharge cycle. The lithium-ion battery was subjected to 1000 charge-discharge cycles according to the above method. The capacity retention rate of the lithium-ion battery after 1000 charge-discharge cycles at 1C / 1C was calculated.

[0125] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0126] III. Ratio Performance Test

[0127] The lithium batteries of the above embodiments and comparative examples were subjected to rate performance testing. The specific test steps were as follows: under 25°C conditions, the lithium-ion batteries were discharged at a constant current rate of 1C to 2.5V, and left to stand for 10 minutes. Then, the lithium-ion batteries were charged at a constant current and constant voltage rate of 6C to 4.2V, with a cutoff current of 0.05C. After standing for 10 minutes, the constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion batteries were recorded. The constant current charge ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charge ratio = constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2 × 100%.

[0128] IV. Safety Performance Testing

[0129] Thermal runaway tests were conducted on the lithium batteries of the above embodiments and comparative examples. The specific test steps were as follows: the test sample was heated from room temperature to 45±2℃ in the chamber and left for 90 minutes. The change in the battery temperature rise rate was detected. If the temperature rise exceeded 0.2℃ within 10 minutes (i.e., SHR>0.02℃ / min), it was considered that a self-exothermic reaction had occurred inside the battery. The adiabatic environment was maintained until the battery experienced thermal runaway. If the temperature rise did not exceed 0.2℃ within 10 minutes (i.e., SHR≤0.02℃ / min), the next temperature rise step test was continued. Each temperature step was 5℃. The steps were repeated on each step. The ARC test temperature range was 45-300℃. The self-heating initiation temperature was T1 (temperature rise rate SHR>0.02℃ / min), and the thermal runaway initiation temperature was T2 (temperature rise rate SHR>1℃ / min).

[0130] Table 1

[0131]

[0132] Based on Example 1, Comparative Examples 1-4, and Table 1, it can be seen that by using 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine in combination with fluorosulfonate additives, this application can not only appropriately increase the free lithium content in the electrolyte and improve the ionic conductivity of the electrolyte, but also promote the formation of stable LiF and Li2S interface films at the recharge interface. This helps to improve the rate performance and first-cycle efficiency of lithium batteries. Furthermore, boron can combine with sulfonic acid groups to improve the film stability on the electrode surface and improve the cycle capacity retention of lithium batteries. The combination of fluorine and sulfonic acid groups can improve the flame retardant effect of the electrolyte, help to increase the self-heating onset temperature of lithium batteries, and further improve the fast-charging safety of lithium batteries.

[0133] Based on Examples 1-4 and Table 1, it can be seen that by introducing lithium salt additives into the electrolyte in this application, the quality of the electrode surface interface film can be improved, the film structure can be refined, and the electrode surface interface film can be made more uniform and dense, which helps to improve the cycle capacity retention rate and rate performance of lithium batteries.

[0134] Combined with Examples 1, 5-6 and Table 1, it can be seen that by adding a third additive to the electrolyte in combination with 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine and fluorosulfonate additives, this application can further improve the stability of the electrode / electrolyte interface, improve the cycle capacity retention rate of lithium batteries, improve the high-temperature cycle performance of lithium batteries, and increase the self-heating onset temperature of lithium batteries.

[0135] Combining Examples 1, 7-8, and Table 1, it can be seen that when the content of 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine is too high and the content of fluorosulfonate additives is too low, the free lithium content in the electrolyte may be too high and cannot be consumed to form a LiF interface film, resulting in a decrease in electrolyte stability, especially the cycle capacity retention rate and self-heating onset temperature of the lithium battery, which show a significant downward trend compared to the lithium battery in Example 1; when 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine is too high, the free lithium content in the electrolyte may be too high and cannot be consumed to form a LiF interface film, resulting in a decrease in electrolyte stability, especially the cycle capacity retention rate and self-heating onset temperature of the lithium battery, which show a significant downward trend compared to the lithium battery in Example 1; When the content of 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine is too low and the content of fluorosulfonate additives is too high, the absorption effect of the dioxoboronyl group on HF in the electrolyte is not significant, and it cannot promote the generation of free lithium in the electrolyte, resulting in a decrease in the stability of the interface film at the negative electrode, and a significant decrease in the rate performance and cycle capacity retention of the lithium battery. This application, by controlling the mass ratio of 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine and fluorosulfonate additives in the electrolyte to meet the above range, can help improve the rate performance, cycle performance and safety performance of the lithium battery.

[0136] Based on Examples 1, 9-10 and Table 1, it can be seen that by using linear carbonates and cyclic carbonates with the above-mentioned ratios as organic solvents, this application helps to improve the stability of the electrolyte system and the stability of the first and second additives in the electrolyte, allowing them to work together effectively. On the other hand, it can adjust the appropriate electrolyte viscosity and generate an SEI film on the surface of the negative electrode active material, which helps to improve the rate performance and cycle performance of the lithium battery.

[0137] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application, but such modifications or substitutions are all within the scope of protection of this application.

Claims

1. An electrolyte, characterized in that: The electrolyte includes a first additive and a second additive. The first additive includes 2-(difluoromethoxy)-5-(4,4,5,5-tetramethyl-1,3,2-dioxoboron-2-yl)pyridine, and the second additive includes a fluorosulfonate additive. The first additive accounts for 1%-3% of the mass of the electrolyte, and the second additive accounts for 0.5%-2.5% of the mass of the electrolyte.

2. The electrolyte according to claim 1, wherein the fluorinated sulfonate additive comprises at least one selected from propyl 2,2,3,3,3-pentafluorotrifluoromethanesulfonate, 2,2,3,3-tetrafluoropropyltrifluoromethanesulfonate, 2,2-difluoroethyltrifluoromethanesulfonate, and hexafluoroisopropyltrifluoromethanesulfonate.

3. The electrolyte according to claim 1 or 2, characterized in that: It also includes organic solvents, lithium salts, lithium salt additives, and third-party additives.

4. The electrolyte according to claim 3, characterized in that: The third additive includes at least one of 1,3-propanesulfonate lactone, vinyl sulfate, and 1,3-propenesulfonate lactone. The third additive accounts for 1%-3% of the mass of the electrolyte.

5. The electrolyte according to claim 3, characterized in that: The lithium salt additive includes at least one of lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium difluorophosphate, and lithium bis(trifluoromethanesulfonyl)imide. The lithium salt additive accounts for 0.1%-1% of the mass of the electrolyte.

6. The electrolyte according to claim 3, characterized in that: The organic solvent includes at least one of linear carbonates and cyclic carbonates.

7. The electrolyte according to claim 6, characterized in that: The linear carbonate includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate; the cyclic carbonate includes at least one of ethylene carbonate, fluoroethylene carbonate, and propylene carbonate.

8. The electrolyte according to claim 6, characterized in that: The mass ratio of the linear carbonate to the cyclic carbonate is 60-80:20-40.

9. A lithium-ion battery, characterized in that: It includes a positive electrode, a separator, a negative electrode, and an electrolyte as described in any one of claims 1-8.

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