Electrolyte and lithium ion battery

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte and a lithium ion battery, wherein the electrolyte comprises a compound A containing a vinyl carbonate structure and an additive B; the mass percentage content of the compound A in the electrolyte is x%, the mass percentage content of the additive B in the electrolyte is y%, and 1 < x / (5y) < 18; the LUMO value of the compound A and the additive B is relatively low, so that the compound A and the additive B preferentially form a film on a negative electrode to the other compounds in the electrolyte, the lithium ion content in the SEI of the negative electrode is effectively improved, and the SEI stability is improved without deteriorating charging; the compound A containing the vinyl carbonate structure and the additive B are combined to effectively inhibit the thickness growth of the battery in the cycle process, improve the thermal box effect of the battery, and thus effectively improve the thermal stability and the charging cycle performance of the lithium ion battery.

Description

Technical Field

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

[0002] To improve the energy density of lithium-ion batteries, the voltage of the positive electrode active material can be increased, or the specific capacity of the negative electrode can be improved. Introducing a material with a higher specific capacity as the negative electrode active material can effectively improve the specific capacity of the negative electrode. Silicon has a theoretical specific capacity of up to 4200 mAh / g, which can effectively improve the specific capacity of the negative electrode, thereby improving the energy density of lithium-ion batteries.

[0003] However, introducing silicon into the anode also brings a series of negative effects. This is because the solid electrolyte interface (SEI) film on the surface of silicon particles has low stability and is more prone to decomposition. Moreover, silicon anode materials undergo huge volume changes during charge and discharge, further increasing the probability of SEI rupture on the anode surface. This accelerates the consumption rate of active lithium ions during cycling. Coupled with the influence of factors such as more side reactions, this leads to a rapid increase in battery thickness during cycling, resulting in reduced battery thermal stability and degraded battery cycle performance. Therefore, there is an urgent need to develop an electrolyte suitable for high-voltage silicon anode systems to suppress the increase in battery thickness during cycling, thereby improving battery performance and extending battery cycle life. Summary of the Invention

[0004] In view of the above-mentioned problems existing in the prior art, the purpose of the present invention is to provide an electrolyte and a lithium-ion battery.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides an electrolyte comprising compound A and additive B, wherein the structure of additive B is shown in Formula 1:

[0007] R1-R2-CN

[0008] Formula 1;

[0009] Where R1 is Any one of them; R2 is an alkyl, alkenyl, or alkynyl group with 2 to 5 carbon atoms;

[0010] Compound A includes an additive and a solvent containing a ethylene carbonate structure;

[0011] The mass percentage of compound A in the electrolyte is x%, and the mass percentage of additive B in the electrolyte is y%, and 1 <x / (5y)<18。

[0012] In some embodiments, the aromatic heterocyclic group in R1 is provided with a side chain group, wherein the side chain group is an alkyl, alkoxy, or carbonyl group having 1 to 5 carbon atoms.

[0013] In some embodiments, the additive B is at least one of the following formulas 2 to 4:

[0014]

[0015] In some embodiments, the additive B has a mass percentage content of 0.3% to 4% in the electrolyte.

[0016] In some embodiments, the additive containing a ethylene carbonate structure includes difluoroethylene carbonate and / or fluoroethylene carbonate.

[0017] In some embodiments, the solvent containing the ethylene carbonate structure includes ethylene carbonate.

[0018] In some embodiments, the difluoroethylene carbonate has a mass percentage content of 0.2% to 10% in the electrolyte; and / or, the fluoroethylene carbonate has a mass percentage content of 1% to 20% in the electrolyte.

[0019] In some embodiments, the solvent of the electrolyte further includes at least one of propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propionate, ethyl propionate, propyl propionate, and methyl acetate.

[0020] In some embodiments, the electrolyte comprises at least one lithium salt selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorodioxarate phosphate, lithium tetrafluorooxarate phosphate, lithium dioxarate borate, lithium difluorooxarate borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(fluorosulfonyl)imide; the lithium salt comprises 8% to 25% by mass in the electrolyte.

[0021] The present invention further provides a lithium-ion battery comprising the electrolyte described above.

[0022] In some embodiments, the negative electrode active material of the lithium-ion battery includes graphite and a silicon-containing material, wherein the mass ratio of graphite to the silicon-containing material is z; and z satisfies: 3 <x / (10y)+2z / y<47。

[0023] In some embodiments, the silicon-containing material includes at least one of silicon carbide and silicon oxide.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention adds compound A containing an ethylene carbonate structure and additive B to the electrolyte system. Compound A and additive B, which have partially ethylene carbonate structures, have low LUMO (lowest unoccupied molecular orbital) values, and will preferentially form a film on the negative electrode compared to other compounds in the electrolyte that may serve as SEI components. This effectively increases the lithium-ion content in the negative electrode SEI and improves SEI stability without deteriorating charging performance. The combined use of compound A containing an ethylene carbonate structure and additive B can effectively suppress battery thickness growth during cycling and improve the battery's thermal performance, thereby effectively improving the thermal stability and cycle performance of the lithium-ion battery without deteriorating charging performance. Detailed Implementation

[0026] To make the technical solution and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0027] This invention provides an electrolyte comprising compound A and additive B, wherein the structure of additive B is shown in Formula 1:

[0028] R1-R2-CN

[0029] Formula 1;

[0030] Where R1 is Any one of them; R2 is an alkyl, alkenyl, or alkynyl group with 2 to 5 carbon atoms;

[0031] Compound A includes an additive and a solvent containing a ethylene carbonate structure;

[0032] The mass percentage of compound A in the electrolyte is x%, and the mass percentage of additive B in the electrolyte is y%, and 1 <x / (5y)<18。

[0033] This invention, based on the premise that the electrolyte contains compound A with a ethylene carbonate structure, adds additive B to the electrolyte. Additive B is a multifunctional compound containing aromatic heterocyclic groups. Additive B has a high HOMO (highest occupied orbital) value and a low LUMO (lowest unoccupied orbital) value, similar to compound A containing a ethylene carbonate structure, and can preferentially form a film at the negative electrode to form a highly stable SEI. In addition, additive B contains aromatic heterocyclic groups, all of which are electron-rich structures. Under the electron-deficient state of the high-voltage positive electrode, the electron-rich characteristics of additive B enable it to be efficiently adsorbed on the surface of the positive electrode active particles, facilitating the polymerization of additive B at the positive electrode to form a film and a highly stable SEI. At the same time, the cyano group (-CN) in additive B can also complex with metal ions dissolved in the electrolyte, inhibiting side reactions that may occur after the metal ions dissolve.

[0034] The combined use of compound A, containing an ethylene carbonate structure, and additive B can simultaneously form films on both the positive and negative electrodes, effectively improving the stability of the SEI (Sediment Insulation) layer, enhancing battery cycle performance and thermal stability, and mitigating the adverse effects of introducing silicon into the negative electrode. Compound A, containing an ethylene carbonate structure, helps increase the LiF content in the SEI film; however, LiF has extremely low ionic and electronic conductivity and high mechanical strength. Additive B, under specific conditions, produces a film with higher electronic conductivity than compound A, but carries the risk of electron leakage. Therefore, the combined use of additive B and compound A can improve the strength of the SEI, reducing the risk of breakage, and also enhance its insulation properties. When used together, additive B exhibits superior performance. Simultaneous use of compound A and additive B can effectively suppress battery thickness growth during cycling, improve battery cycle performance and reduce the thermal box effect, thus avoiding safety hazards.

[0035] Furthermore, the aromatic heterocyclic group in R1 is provided with a side chain group, wherein the side chain group is an alkyl, alkoxy, or carbonyl group having 1 to 5 carbon atoms.

[0036] Furthermore, additive B is at least one of the following formulas 2 to 4:

[0037]

[0038] In this invention, additive B is preferably any one of the three compounds in Formulas 2 to 4; wherein, the structure in Formula 2 is denoted as B1, the structure in Formula 3 is denoted as B2, and the structure in Formula 4 is denoted as B3; it can be seen that R2 in B1 is an alkyl group with 2 carbon atoms, and a side chain group is attached to R1, which is an alkyl group with 1 carbon atom; R2 in B2 is an alkyl group with 2 carbon atoms; R2 in B3 is an alkyl group with 2 carbon atoms, and a side chain group is attached to R1, which is an alkoxy group with 1 carbon atom.

[0039] Furthermore, the mass percentage of additive B in the electrolyte is 0.3% to 4%.

[0040] Furthermore, additives containing a ethylene carbonate structure include difluoroethylene carbonate (DFEC) and / or fluoroethylene carbonate (FEC);

[0041] And / or, solvents containing the ethylene carbonate structure include ethylene carbonate (EC).

[0042] Furthermore, the mass percentage of difluoroethylene carbonate in the electrolyte is 0.2% to 10%; and / or, the mass percentage of fluoroethylene carbonate in the electrolyte is 1% to 20%.

[0043] Furthermore, the solvent of the electrolyte also includes at least one of propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propionate (MA), ethyl propionate (EP), n-propyl propionate (PP), and methyl acetate (MA).

[0044] Furthermore, the electrolyte includes at least one lithium salt selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorodioxarate phosphate, lithium tetrafluorooxarate phosphate, lithium dioxarate borate, lithium difluorooxarate borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(fluorosulfonyl)imide; the lithium salt has a mass percentage content of 8% to 25% in the electrolyte.

[0045] In this invention, compound A includes DFEC, FEC, and solvent EC. Here, the additive containing the ethylene carbonate structure is simply referred to as additive A. Since DFEC and additive B have lower LUMO energies, they preferentially form a film on the negative electrode compared to EC and FEC. However, ultimately, DFEC and additive B will both act together with EC and FEC to form a film; that is, the main components of the film formation include additive A and additive B. Additive A and additive B work together on the SEI, improving its stability and thus enhancing the thermal stability of the lithium-ion battery. They also increase the LiF content in the negative electrode SEI, improving the Li... + The solvation-desolvation process optimizes Li + The solvation structure further improves fast charging performance.

[0046] Furthermore, the electrolyte of the present invention may also contain film-forming additives, including at least one of 1,3-propanesulfonate lactone, vinyl sulfate, tris(trimethylsilane)borate, and tris(trimethylsilane)phosphate; the mass percentage of the film-forming additives in the electrolyte is 0.1% to 20%.

[0047] The present invention also provides a lithium-ion battery comprising the electrolyte described above.

[0048] Furthermore, the negative electrode active material of the lithium-ion battery includes graphite and silicon-containing materials, with a mass ratio of graphite to silicon-containing materials of z; and z satisfies: 3 <x / (10y)+2z / y<47。

[0049] Furthermore, the silicon-containing material includes at least one of silicon carbide and silicon oxide.

[0050] When the value of the relation x / (10y)+2z / y falls within the above range, Li + The number of EP in the outer first solvated sheath layer is reduced to a minimum, and EP basically does not participate in Li. + The solvation of Li, which is mainly responsible for solvation. + The transport carrier has low viscosity, which helps to ensure the charging performance of the battery and is basically not consumed during cycling. The main components of the film include EC, FEC, DFEC and additive B. At this time, the content ratio of LiF to Li2CO3 in SEI reaches the optimal level, and the stability, insulation and ionic conductivity of SEI reach the best level, thereby making the cycle performance and fast charging performance of lithium-ion battery optimal.

[0051] Example 1

[0052] This embodiment provides an electrolyte and a lithium-ion battery including the electrolyte;

[0053] The electrolyte preparation steps are as follows:

[0054] In a glove box filled with argon gas, EC, PC, DEC, and EP were mixed in a mass ratio of EC:PC:DEC:EP = 15:15:40:30 to obtain a mixed solution.

[0055] Based on the total weight of the electrolyte as 100%, 14% lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution; then 12% FEC, 3% DFEC, and 1.5% B1 were added, and the solution was stirred evenly to obtain the electrolyte.

[0056] The structure of B1 is shown in the following formula:

[0057]

[0058] The manufacturing steps of lithium-ion batteries are as follows:

[0059] The positive electrode active material is lithium cobalt oxide (LiCoO2), the conductive agent is conductive carbon black (Super P, SP), and the binder is polyvinylidene difluoride (PVDF). LiCoO2, SP and PVDF are mixed in a weight ratio of LiCoO2:SP:PVDF = 98.5:0.5:1, and then added to N-methylpyrrolidone and mixed evenly to obtain a lithium-ion battery positive electrode slurry.

[0060] The prepared positive electrode slurry was coated on the current collector aluminum foil, dried at 85°C, and then subjected to cold pressing, edge trimming and slitting processes in sequence. After drying under vacuum at 85°C for 4 hours, the electrode tabs were welded on to obtain the positive electrode sheet, wherein the specific capacity of the positive electrode is 175mAh / g.

[0061] The negative electrode active material is graphite and silicon carbide (SiC), the conductive agent is carbon nanotubes (CNTs), the thickener is sodium carboxymethyl cellulose (CMC), and the binder is polyacrylic acid (PAA). A mixture of graphite and SiC in a weight ratio of graphite:SiC = 85:15 is used as the negative electrode active material. After mixing the various materials in a weight ratio of negative electrode active material:CNT:CMC:PAA = 97:0.8:1.2:1, deionized water is added and mixed evenly to prepare the negative electrode slurry.

[0062] The prepared negative electrode slurry is coated on the current collector copper foil, dried at 85°C, and then subjected to cold pressing, edge trimming and slitting processes in sequence. After drying under vacuum at 85°C for 12 hours, the electrode tabs are welded on to obtain the negative electrode sheet.

[0063] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes, and then wound to obtain a bare cell. The bare cell has a designed capacity of 6.0 Ah and a voltage range of 3.0V to 4.55V. The bare cell is then placed in an aluminum-plastic film outer packaging for sealing, and then baked in an 85℃ vacuum oven for 48 hours. Electrolyte is injected into the dried battery with an injection coefficient of 1.5 g / Ah. The battery after injection is then sealed, allowed to stand, formed, shaped, and capacity tested, and then sealed a second time to control the electrolyte retention coefficient at 1.2 g / Ah, resulting in a lithium-ion soft-pack battery.

[0064] Example 2-11

[0065] Repeat the steps of Example 1 using the parameters specified in Examples 2 to 11 in Table 1;

[0066] Example 12

[0067] The only difference between Example 12 and Example 1 is that a mixture of graphite and SiC in a weight ratio of 80:20 is used as the negative electrode active material.

[0068] Comparative Examples 1-6

[0069] Repeat the steps of Example 1 using the parameters specified in Comparative Examples 1 to 6 in Table 1;

[0070] In Comparative Examples 1-6, the missing content due to the absence of additive B, DFEC, or FEC was made up by the solvent, and the addition ratio of each component in the solvent remained unchanged.

[0071] Table 1

[0072]

[0073]

[0074]

[0075] Test case

[0076] The lithium-ion batteries obtained in Examples 1 to 12 and Comparative Examples 1 to 6 were subjected to the following performance tests:

[0077] (1) Room temperature cycling performance test

[0078] In an environment of 25℃, the capacity-graded lithium-ion battery was charged to 4.55V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C. Then, it was discharged to 3.0V at a constant current of 0.5C. This cycle was repeated for 400 charge-discharge cycles. The capacity retention rate at the 400th cycle was calculated using the following formula:

[0079] Cycle capacity retention rate at 400th week = (Cycle discharge capacity at 400th week / Initial cycle discharge capacity) × 100%;

[0080] The thickness growth rate of the 400th cycle = (the fully charged thickness of the 400th cycle / the fully charged thickness of the first cycle) × 100%;

[0081] (2) High-temperature cycling performance test

[0082] In an environment of 45℃, the capacity-balanced lithium-ion battery was charged to 4.55V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C. Then, it was discharged to 3.0V at a constant current of 0.5C. This cycle was repeated for 300 charge-discharge cycles. The capacity retention rate at the 300th cycle was calculated using the following formula:

[0083] 300-week cycle capacity retention rate = (300-week cycle discharge capacity / initial cycle discharge capacity) × 100%;

[0084] The growth rate of the thickness in the 300th cycle = (the thickness of the fully charged state in the 300th week / the thickness of the fully charged state in the first cycle) × 100%;

[0085] (3) 24-hour high-temperature storage test at 85℃

[0086] The lithium-ion battery was placed at room temperature and charged and discharged once at 0.5C (voltage 3.0V~4.55V), and the discharge capacity C0 before storage was recorded. Then, the lithium-ion battery was charged to a full charge state of 4.55V (100% SOC) using constant current and constant voltage. The thickness d1 of the lithium-ion battery before high-temperature storage was measured using a PPG battery thickness gauge (600g). The lithium-ion battery was placed in an 85℃ constant temperature chamber for 24 hours. After storage, the lithium-ion battery was removed and the thermal thickness d2 of the battery after storage was measured. The thickness expansion rate of the lithium-ion battery after storage at 85℃ for 24 hours was calculated.

[0087] After the lithium-ion battery has cooled to room temperature for 24 hours, it is discharged again at a constant current of 0.5C to 3.0V, and then charged at a constant current and constant voltage of 0.5C to 4.55V. The discharge capacity C1 and charge capacity C2 of the lithium-ion battery after storage are recorded. The remaining capacity and recovery rate of the lithium-ion battery after storage at 85℃ for 24 hours are calculated using the following formulas:

[0088] Thickness expansion rate after storage at 85℃ for 24 hours = ((d2-d1) / d1)*100%;

[0089] The remaining capacity after storage at 85℃ for 24 hours is calculated as (C1 / C0) * 100%.

[0090] Capacity recovery rate after storage at 85℃ for 24 hours = (C2 / C0)*100%;

[0091] (4) Thermal shock performance test

[0092] In an environment of 25℃, the lithium-ion battery is discharged to 3.0V at a given current of 0.2C and left to stand for 5 minutes; then it is charged to 4.55V at a charging current of 0.2C. When the cell voltage reaches 4.55V, it is switched to constant voltage charging at 4.55V until the charging current is ≤ cutoff current 0.05C; after standing for 1 hour, the lithium-ion battery cell is placed in an oven, and the oven temperature is increased to 135±2℃ at a heating rate of 5±2℃ / min and maintained for 60 minutes before stopping. The qualified judgment criterion is that the cell does not catch fire or explode.

[0093] (5) 3C DC charging constant current ratio test

[0094] In an environment of 25℃, the lithium-ion battery cell is discharged to 3.0V at 0.2C; left to rest for 30 minutes; then charged to 4.55V at a charging current of 3C. When the cell voltage reaches 4.55V, constant voltage charging at 4.55V is switched until the charging current is ≤ the cutoff current of 0.05C. The constant current charging capacity during the entire charging process is recorded as C3, and the total capacity during the charging process is recorded as C4. The constant current charging capacity during the entire DC charging process is calculated using the following formula:

[0095] 3C DC charging constant current ratio = (C3 / C4) * 100%;

[0096] The results of the above performance tests are shown in Table 3;

[0097] The values ​​of x, V, z and their relationships in Examples 1-12 and Comparative Examples 1-6 are shown in Table 2.

[0098] Table 2

[0099]

[0100]

[0101] Table 3

[0102]

[0103] As can be seen from the test results in Table 3, compared with Example 1, Example 2 and Example 3 both used additive B with different structures, while in Examples 7-9 only the amount of additive B was adjusted. The overall lithium-ion battery performance test results obtained in Examples 1 and Examples 7-9 are better than those in Examples 2 and 3, which proves that using additive B1 can achieve better results than B2 and B3.

[0104] Compared to Example 1, the amount of DFEC added in Examples 4-6 was adjusted, and the resulting lithium-ion batteries showed better overall performance in the test results. This proves that keeping the amount of DFEC added in the range of 0.2% to 10% can improve the thermal stability and charging cycle performance of lithium-ion batteries to a certain extent.

[0105] Compared to Example 1, the amount of FEC added in Examples 10 and 11 was adjusted, and the resulting lithium-ion batteries showed better overall performance in the test results. This proves that keeping the amount of FEC added in the range of 1% to 10% can improve the thermal stability and charging cycle performance of lithium-ion batteries to a certain extent.

[0106] Compared with Example 1, in Example 12, the weight ratio of graphite to SiC, i.e., the z value, was adjusted, further proving that when the relational expression x / (10y)+2z / y in this solution remains within the range of 3 < x / (10y)+2z / y < 47, it can have a certain improvement effect on the thermal stability performance and charge-discharge cycle performance of lithium-ion batteries.

[0107] Compared with Example 1, in Comparative Example 1, only DFEC was used, and additives B and FEC were not used; in Comparative Example 2, only additive B was used, and DFEC and FEC were not used; in Comparative Example 3, only FEC was used, and DFEC and additive B were not used; Comparative Example 6 was used as a blank control group, and DFEC, FEC, and additive B were not used. It can be seen from Table 2 that the overall performance test results of the lithium-ion batteries obtained in Comparative Examples 1-3 are better than those in Comparative Example 6. This is because in Comparative Examples 1-3, whether any one of DFEC, FEC, or additive B is added to the electrolyte, they can form a film on the negative electrode to improve the cycle performance and the thermal stability of SEI to a certain extent; in addition, Comparative Example 2 using additive B shows better performance in the test of thermal shock pass rate than Comparative Examples 1 and 3, while Comparative Examples 1 and 3 show better performance in the test of thickness expansion rate than Comparative Example 2; it can be seen that the use of additive A containing ethylene carbonate structure can improve the problem of volume change during the charge-discharge process of lithium-ion batteries, effectively inhibit the thickness growth of the battery during the cycle, and improve the cycle efficiency of the battery, but additive A has no improvement effect on the phenomenon of battery thermal runaway; while the use of additive B can effectively inhibit the heat accumulation inside the battery and improve the battery thermal box effect, but it has no improvement effect on the problem of battery thickness growth; in the case of the combined use of additive B and additive A containing ethylene carbonate structure, such as in Example 1, the improvement of the volume expansion rate, charge-discharge cycle, and thermal box effect of the battery can be achieved simultaneously, and the improvement degree is better than the case of using the first additive or the second additive alone, such as in Comparative Examples 1-3.

[0108] In Comparative Example 4 and Comparative Example 5, although the addition amounts of Additive B, DFEC, and FEC, as well as the weight ratio of graphite to SiC, are all controlled within the ranges given in this solution, the performance of the obtained lithium-ion batteries is still not ideal. This is because they do not satisfy the relationships 1 < x / (5y) < 18 and 3 < x / (10y) + 2z / y < 47. At this time, there may be two situations inside the battery: one is that the content of Li2CO3 in the SEI is relatively high. Although it brings higher ionic conductivity and enables the lithium-ion battery to have better fast charging performance, it also makes the stability and insulation of the SEI worse, and the SEI cannot maintain stability, making it easier for electrons to pass through the SEI and react with the electrolyte in a side reaction; the other is that the content of LiF in the SEI is too high. Although the SEI has high stability, the ionic conductivity of LiF is low, resulting in poor fast charging performance of the lithium-ion battery. Only when the relationships 1 < x / (5y) < 18 and 3 < x / (10y) + 2z / y < 47 are satisfied, Li + The number of EPs in the outermost first solvation sheath is reduced to the lowest, and EPs basically do not participate in the solvation of Li + but mainly serve as a transport carrier for solvated Li + and are basically not consumed during the cycling process; therefore, the main components of the formed film include Additive B, DFEC, FEC, and EC. At this time, the content ratio of LiF to Li2CO3 in the SEI reaches the optimal value, and the stability, insulation, and ionic conductivity of the SEI reach the best, thereby making the cycling performance and fast charging performance of the lithium-ion battery reach the optimal level. In summary, when the relationship values are not within this range, the performance of the lithium-ion battery will deteriorate.

[0109] By introducing Additive A and Additive B containing ethylene carbonate structures into the electrolyte system, the present invention enables Additive A and Additive B containing ethylene carbonate structures to form a film on the negative electrode, effectively improving the cycling performance and thermal stability performance of the SEI, improving the cycling performance and thermal shock resistance performance of the battery through the electrolyte, inhibiting the growth of the battery thickness during cycling, and avoiding potential safety hazards; at the same time, controlling the values of the relationships 1 < x / (5y) < 18 and 3 < x / (10y) + 2z / y < 47 can make the SEI film reach the best state, thereby making the various performances of the lithium-ion battery reach the optimal level.

[0110] The above are only partial or preferred embodiments of the present invention, and thus cannot limit the scope of protection of the present invention. Any equivalent structural transformation made using the description of the present invention under the overall concept of the present invention, or any direct / indirect application in other related technical fields, is included within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that, It includes compound A and additive B, the structure of which is shown in Formula 1: R1-R2-CN Formula 1; Where R1 is Any one of them; R2 is an alkyl, alkenyl, or alkynyl group with 2 to 5 carbon atoms; Compound A includes an additive and a solvent containing a ethylene carbonate structure; The mass percentage of compound A in the electrolyte is x%, and the mass percentage of additive B in the electrolyte is y%, where 1 < x / (5y) < 18; the value of y% ranges from 0.3% to 4%. The additive containing the ethylene carbonate structure includes difluoroethylene carbonate and / or fluoroethylene carbonate; the solvent containing the ethylene carbonate structure includes ethylene carbonate.

2. The electrolyte according to claim 1, characterized in that, The aromatic heterocyclic group in R1 has a side chain group, which is an alkyl, alkoxy, or carbonyl group with 1 to 5 carbon atoms.

3. The electrolyte according to claim 1, characterized in that, The additive B is at least one of the following formulas 2 to 4:

4. The electrolyte according to claim 1, characterized in that, The difluoroethylene carbonate has a mass percentage content of 0.2% to 10% in the electrolyte; and / or, the fluoroethylene carbonate has a mass percentage content of 1% to 20% in the electrolyte.

5. The electrolyte according to claim 4, characterized in that, The solvent of the electrolyte includes at least one of propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, methyl propionate, ethyl propionate, propyl propionate, and methyl acetate.

6. The electrolyte according to claim 1, characterized in that, The electrolyte comprises at least one lithium salt selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorodioxarate phosphate, lithium tetrafluorooxarate phosphate, lithium dioxarate borate, lithium difluorooxarate borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(fluorosulfonyl)imide; the lithium salt comprises 8% to 25% by mass in the electrolyte.

7. A lithium-ion battery, characterized in that, Includes the electrolyte as described in any one of claims 1 to 6.

8. The lithium-ion battery according to claim 7, characterized in that, The negative electrode active material of the lithium-ion battery includes graphite and silicon-containing material, and the mass ratio of graphite to silicon-containing material is z; and z satisfies: 3 < x / (10y) + 2z / y < 47.

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