Lithium ion battery secondary liquid injection electrolyte and lithium ion battery

Abstract

The application discloses a lithium ion battery secondary injection electrolyte and a lithium ion battery, which comprises a first injection electrolyte and a second injection electrolyte; the first injection electrolyte at least comprises a first lithium salt, a first solvent and vinylene carbonate; the second injection electrolyte at least comprises a second lithium salt, a second solvent, vinylene carbonate and a lithium salt additive, the mass fraction of the lithium salt additive is 0.2%-1.5%, the lithium salt additive is selected from at least one of lithium difluoro(oxalato)borate, lithium difluorobis(oxalato)phosphate or lithium tetrafluoro(oxalato)phosphate; the content of the vinylene carbonate in the first injection electrolyte is less than that in the second injection electrolyte. The first injection electrolyte focuses on reducing the negative electrode SEI to ensure excellent low-temperature and fast-charging performance; the first injection electrolyte focuses on avoiding the negative electrode film-forming consumption problem at low voltage and improving the additive use efficiency to ensure the cycle and high-temperature performance.

Description

Technical Field

[0001] This invention belongs to the field of electrolyte technology, specifically relating to a secondary electrolyte for lithium-ion batteries and a lithium-ion battery. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Although lithium iron phosphate batteries have advantages such as high safety, long service life and low cost, and their share in the power battery market is gradually increasing, there are still issues that need to be improved in terms of fast charging capability, low temperature performance and high temperature performance in order to meet increasingly higher usage requirements.

[0004] Lithium bis(fluorosulfonyl)imide (LiFSI) exhibits high lithium-ion conductivity and participates in electrolyte exchange film (SEI film), significantly improving battery performance at low temperatures and during fast charging. However, excessive addition can lead to corrosion of the aluminum foil in the electrode material, affecting battery life and performance. Lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate, and lithium tetrafluorooxalate phosphate, as electrolyte additives, can inhibit aluminum foil corrosion, participate in SEI film formation, reduce SEI impedance, and suppress iron ion dissolution, thus improving the high-temperature cycling performance of lithium batteries.

[0005] However, lithium-ion batteries are typically assembled after the electrolyte is fully injected, followed by formation to activate the lithium battery. At this stage, if lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate, and / or lithium tetrafluorooxalate phosphate are added as additives to the electrolyte, these additives tend to preferentially form a film on the negative electrode. If the addition amount is large (e.g., reaching 0.5%), although it can improve the high-temperature cycle performance of the lithium battery, the increased SEI thickness will severely degrade the battery's fast-charging capability and low-temperature performance. Conversely, if the addition amount is too small (e.g., around 0.1%), the consumption of the negative electrode film will significantly reduce the effective amount participating in the positive electrode film formation, making it difficult to achieve the expected goals of reducing the positive electrode CEI impedance and inhibiting aluminum foil corrosion and iron ion dissolution. For these reasons, the application of such lithium salt additives in lithium iron phosphate batteries is relatively limited.

[0006] Furthermore, ethyl acetate, due to its low viscosity and melting point, as well as its high dielectric constant, can significantly improve the low-temperature performance and fast-charging capability of batteries. However, because of its low boiling point, when ethyl acetate is used as the main solvent, it results in poor high-temperature cycle performance of the battery. Therefore, ethyl acetate is generally only suitable as a solvent for low-temperature lithium-ion battery electrolytes and cannot be used in lithium-ion batteries with high high-temperature cycle performance requirements.

[0007] As the most room-temperature electrolyte additive for lithium iron phosphate batteries, VC can effectively improve the high-temperature cycle performance of batteries. However, if the amount added is too large, it can easily cause a significant increase in the proportion of organic components in the SEI, which will seriously deteriorate the fast-charging performance. If the amount added is too small, it will not achieve the expected effect. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a secondary electrolyte for lithium-ion batteries and a lithium-ion battery.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0010] In a first aspect, the present invention provides a secondary electrolyte for lithium-ion batteries, comprising a first electrolyte and a second electrolyte;

[0011] The second electrolyte solution contains lithium salt additives, which are selected from at least one of lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate, or lithium tetrafluorooxalate phosphate.

[0012] During the assembly of lithium-ion batteries, the first electrolyte is injected and formed, and then the second electrolyte is injected.

[0013] Through experiments, the inventors discovered that when a first-fill electrolyte (containing none of the three lithium salt additives mentioned above) is injected into a lithium-ion battery and formed to a certain voltage, a second-fill electrolyte containing lithium salt additives is then injected for further formation before proceeding with the normal process. This solution effectively avoids the problem of lithium salt additives being consumed in the negative electrode film formation, which is present in conventional single-fill solutions. It increases the proportion of these additives participating in the positive electrode film formation, and with a smaller amount added, achieves the beneficial effects of inhibiting aluminum foil corrosion and reducing CEI impedance. This results in a balance between fast-charging performance and high-temperature cycling performance.

[0014] In some embodiments, the mass fraction of lithium salt additives in the second electrolyte injection is 0.2%-1.5%.

[0015] In some embodiments, the first electrolyte injection includes at least a first lithium salt, a first solvent, and vinylene carbonate; the second electrolyte injection also includes the first lithium salt, the first solvent, and vinylene carbonate, wherein the vinylene carbonate content in the first electrolyte injection is less than the vinylene carbonate content in the second electrolyte injection.

[0016] Vinylene carbonate (VC) is a key additive in lithium iron phosphate battery electrolytes. It forms a stable SEI film on the negative electrode surface, improving the battery's high-temperature and cycle performance, especially when ethyl acetate is used as a solvent. Within a certain range, this performance is positively correlated with the amount of VC added. However, excessively high VC content increases the thickness of the SEI film, leading to a higher DC internal resistance (DCR) and consequently, a decrease in fast-charging and low-temperature performance. Therefore, limiting the VC content in the initial electrolyte filling to a low level ensures excellent low-temperature and fast-charging performance of the lithium battery.

[0017] Experiments have shown that when the electrolyte is split into a first-fill electrolyte and a second-fill electrolyte, and vinylene carbonate is added to the basic components (solvent, lithium salt, etc.) of the first-fill electrolyte, and vinylene carbonate and lithium salt additives (the above three) are added to the basic components (solvent, lithium salt, etc.) of the second-fill electrolyte, compared to merging the first-fill electrolyte and the second-fill electrolyte into a single filling, the fast-charging capability, low-temperature cycle performance, and high-temperature cycle performance of lithium batteries can be significantly improved.

[0018] In some embodiments, the mass percentage of vinylene carbonate in the first and second electrolyte injections is z = x*m + y*(1-m) = 0.025 + ε2(N-1000) / 80000, where,

[0019] 2% ≤ z ≤ 6%;

[0020] m represents the percentage of the total electrolyte mass in the first injection, ranging from 78% to 92%.

[0021] x is the mass fraction of vinylene carbonate in the electrolyte during the first injection, x=(2ε1 / λ+1) / 100;

[0022] ε1 and ε2 are both fluctuation coefficients, with values ​​ranging from 0.8 to 1.2. The value of ε1 is related to the negative electrode surface density, the type of negative electrode material (such as particle size, OI value, coating, etc.), the negative electrode formulation and the electrolyte conductivity, etc., while the value of ε2 is related to the selection of positive and negative electrode materials (type, specific surface area, etc.) and the liquid injection coefficient.

[0023] N is the design number of cycles at 45°C when the battery capacity decays to 80% of its initial capacity; its value can be selected as 1000, 1200, 1500, 2000, etc.

[0024] λ refers to the design charging rate factor for the battery's 30%-50% SOC. λC refers to the charging capacity; when λ=1, it means the battery is fully charged in 1 hour; when λ=2, it means the battery is fully charged in 0.5 hours. The larger the value of λ, the better the battery's charging capacity. λ can be an integer or a decimal.

[0025] Preferably, m is 85%-90%.

[0026] Adjusting the VC content in electrolyte A (first-fill electrolyte) and electrolyte B (second-fill electrolyte) through a two-stage electrolyte injection method has achieved good results in balancing low film impedance, high temperature performance, and long cycle life in lithium iron phosphate batteries. However, excessively high VC content will increase the thickness of the SEI film, resulting in a higher DC internal resistance (DCR) and thus a decrease in fast charging and low-temperature performance. Conversely, too low a VC content will not achieve the desired effect. Therefore, the required VC content in the electrolyte varies depending on the battery's performance requirements. Obtaining the required VC content requires extensive experimentation, which consumes a significant amount of time and money.

[0027] Through relevant research and development, the inventors linked the VC content in liquid A and liquid B with the design of battery performance indicators. Based on the battery's design performance indicators, the VC content in liquid A and liquid B can be determined quickly and appropriately, saving a significant amount of research and development time and costs.

[0028] In some embodiments, the mass percentage of vinylene carbonate in the first electrolyte injection is 1%-4%; and the mass percentage of vinylene carbonate in the second electrolyte injection is 5%-25%.

[0029] Preferably, the mass percentage of vinylene carbonate in the first electrolyte injection is 1.2%-2.5%; and the mass percentage of vinylene carbonate in the second electrolyte injection is 7%-18%.

[0030] In some embodiments, the mass fraction of lithium salt additives in the second electrolyte injection is 0.2%-1%, preferably 0.4%-0.8%.

[0031] In some embodiments, the first lithium salt and the second lithium salt are both selected from at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), lithium difluorophosphate (LiPO2F2), or lithium tetrafluoroborate (LiBF4).

[0032] Preferably, the first lithium salt or the second lithium salt is LiPF6 or LiFSI.

[0033] Preferably, the sum of the masses of the first lithium salt and the second lithium salt accounts for 10%-20% of the total mass of the electrolyte.

[0034] The total mass of the electrolyte is the sum of the masses of the electrolyte injected in the first injection and the electrolyte injected in the second injection.

[0035] More preferably, the sum of the masses of the first lithium salt and the second lithium salt accounts for 12.5%-16% of the total mass of the electrolyte.

[0036] In some embodiments, the first solvent and / or the second solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), γ-butyrolactone (GBL), ethyl acetate (EA), methyl acetate (MA), methyl propionate (MP), propyl propionate (PP), or ethyl propionate (EP).

[0037] Preferably, the first solvent and / or the second solvent is ethyl acetate. Furthermore, experiments have shown that if ethyl acetate is the primary solvent of the electrolyte, the addition of lithium salt additives and ethylene carbonate to the second electrolyte injection can effectively improve the high-temperature cycle performance of the lithium battery, providing a guarantee for improving the high-temperature cycle performance, low-temperature cycle performance, and fast-charging capability of lithium batteries using ethyl acetate as the primary solvent.

[0038] Preferably, the first solvent accounts for 30%-60% of the mass percentage of the electrolyte injected in the first injection, and more preferably 45%-55%.

[0039] The second solvent accounts for 30%-70% of the mass of the second electrolyte, preferably 40%-65%.

[0040] In some embodiments, the first electrolyte solution further includes a second additive selected from at least one of vinyl sulfate (DTD), vinyl sulfite (ES), methylene disulfonate (MMDS), 1,3-propanesulfonate lactone (PS), tris(trimethylsilane) phosphate (TMSP), fluorovinyl sulfate (FEC), or fluorobenzene (FB).

[0041] Preferably, the second additive accounts for 0.1%-2% of the mass of the electrolyte injected in the first injection.

[0042] Secondly, the present invention provides a lithium-ion battery that uses the aforementioned secondary electrolyte.

[0043] The secondary injection method for the secondary electrolyte is as follows:

[0044] (1) Determine the total amount of electrolyte required for the battery based on the battery capacity and the electrolyte injection coefficient. Inject the mass of liquid A, which is “m*total amount of electrolyte”, into the battery to be injected. Immerse at 45°C for ≥24 hours.

[0045] (2) Set the formation to the target voltage according to the formation process current, and the aging time is ≥8h;

[0046] The second injection is completed by injecting B solution of mass "(1-m)*total injection volume" into the formed and aged battery.

[0047] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0048] This invention utilizes lithium salt additives in the second electrolyte injection in lithium iron phosphate batteries. After the second electrolyte injection and formation, the addition of the lithium salt additive through the second electrolyte injection can effectively avoid negative electrode consumption and achieve preferential positive electrode film formation. This results in beneficial effects such as inhibiting metal ion dissolution, passivating aluminum foil, and improving CEI impedance.

[0049] Ethylene carbonate (VC) is a major additive in lithium iron phosphate battery electrolytes. It can form a stable SEI film on the negative electrode surface, improving the battery's high-temperature and cycle performance, especially when ethyl acetate is used as the main solvent, thus improving the high-temperature cycle performance of the battery.

[0050] This invention establishes a data model between key battery performance indicators (fast charging capability and high-temperature cycle performance) and the VC content in liquid A and liquid B in the secondary electrolyte injection scheme. It provides an effective method for quickly determining the composition of the secondary electrolyte injection and provides an important reference for solving the problem of balancing battery fast charging and cycle performance related to VC content or limiting the lower limit of a single performance indicator and strengthening another related performance problem in battery design. Detailed Implementation

[0051] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0052] The present invention will be further described below with reference to the embodiments.

[0053] The specific fabrication process of a lithium-ion battery with an embedded multilayer negative electrode is as follows:

[0054] 1. Preparation of the positive electrode sheet: Polyvinylidene fluoride (PVDF), conductive agent (Super P), and positive electrode material (LFP) are added to N-methylpyrrolidone (NMP) at a mass ratio of 2:1.5:96.5 and mixed evenly to form a slurry. The slurry is then coated onto an aluminum foil current collector. The surface density of the positive electrode on one side is 220±3 g / m². 2 Cold-pressed to 2.5g / m³ 3 After punching, the positive electrode sheet is obtained.

[0055] 2. Preparation of the negative electrode sheet: Sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber latex (SBR), conductive agent (SuperP), and graphite were added to deionized water at a mass ratio of 1.4:2:1:95.6. The mixture was stirred thoroughly and mixed evenly. The slurry was coated onto a copper foil current collector. The negative electrode surface density was determined according to an N / P ratio of 1.15 and cold-pressed to 1.55 g / m³. 3 After punching, the negative electrode sheet is formed.

[0056] 3. Electrolyte preparation: Solution A (electrolyte for the first injection) is prepared according to the mass ratio of LiPF6:LFSI:EC:EMC:DMC:VC:DTD of 11.5:1.5:25.4:33.7:25.4:2:0.5.

[0057] Solution B (second electrolyte) was prepared with a mass ratio of LiPF6:EC:EMC:DMC:VC:LiODFB of 12.5:23.4:31.2:23.4:9:0.5 to obtain the electrolyte of Example 1.

[0058] 4. Preparation of the battery to be injected with electrolyte: The separator, negative electrode, and positive electrode are stacked in a "Z" shape to obtain the bare cell to be injected with electrolyte. The cell is then packaged in aluminum-plastic film and baked to obtain the battery to be injected with electrolyte. The designed capacity of the battery to be injected with electrolyte is 2Ah.

[0059] 5. Implementation of the two-injection scheme: The electrolyte injection coefficient is calculated at 3.5g / Ah, and solution A is calculated as accounting for 90% of the total injection volume, i.e., 6.3g.

[0060] 6. Inject 6.3g of solution A into the cell to be injected, seal it, and then immerse it at 45°C for 36 hours;

[0061] 1) After soaking, the battery is formed at 0.2C (0.4A) until the voltage cutoff is 3.2V, and then aged at 45℃ for 12 hours;

[0062] 2) Inject 0.7g of solution B into the battery after formation and aging, place it at 25°C for 36 hours, and then perform normal charge and discharge to obtain the battery to be tested.

[0063] Examples 1-8 and Comparative Examples 1-6 have adjusted the electrolyte formulations while keeping other aspects unchanged. The specific electrolyte composition is shown in Table 1.

[0064] Table 1. Electrolyte composition for Examples 1-8 and Comparative Examples 1-6

[0065]

[0066]

[0067]

[0068] Testing of lithium-ion batteries:

[0069] Rate charging performance test: The battery was charged at 1C and 3C constant current and constant voltage to 3.65V, with a cutoff current of 0.05C, and then discharged at 1C to 2.5V. The capacity ratio of constant current charging was calculated. A soft-pack three-electrode was fabricated, and the charging time at different rates was tested. By data aggregation, the maximum charging current at different SOCs was determined. The optimal stepped charging scheme (6 stages) was determined for different batteries, and the shortest charging time from 10% to 90% SOC was determined.

[0070] Low temperature performance test: The 1C discharge capacity and energy of the battery were tested at -20℃ and 25℃ respectively, and the ratio of the two was calculated and recorded as the battery discharge capacity retention rate and energy retention rate at -20℃ respectively.

[0071] High-temperature cycle performance test: At 45℃, the capacity retention rate was tested at different cycles of 1C charge and discharge. The cutoff voltage for charge and discharge was 2.5V to 3.65V, and the cutoff current for constant voltage charging was 0.05C.

[0072] Negative electrode transition metal deposition test: After the battery is cycled at 45℃ for 1000 times, it is discharged to 2.5V, the battery is disassembled, the negative electrode powder is collected, ICP is tested, and the Fe element content is recorded.

[0073] The test data of lithium-ion batteries assembled using the electrolytes prepared in Examples 1-8 and Comparative Examples 1-6 are shown in Table 2.

[0074] Table 2

[0075]

[0076]

[0077] Note: In Examples 1-3, 5, 8 and Comparative Examples 1-5, the total mass of added VC in the electrolyte after conversion was 2.7%. In Examples 4, 6, 7 and Comparative Example 6, the total mass of added VC in the electrolyte after conversion was 3.45%, 3%, and 3.75%, respectively.

[0078] As shown in Table 1, Comparative Example 4 was obtained by disassembling the electrolyte of Comparative Example 1 according to the mass ratio of electrolytes A and B during injection. Table 2 shows that the battery using electrolyte from Comparative Example 4 exhibits significantly improved performance in fast charging, low-temperature cycling, and high-temperature cycling, demonstrating the advantage of using two electrolyte solutions compared to one.

[0079] Compared with Comparative Example 1, Comparative Examples 2-3 show that the addition of LiODFB can improve the dissolution of Fe ions under high-temperature cycling. However, when the addition amount is 0.1%, the inhibition effect is not obvious. This is mainly because LiODFB participates in the formation of the negative electrode film during formation and is consumed, resulting in less participation in the formation of the positive electrode film. When the addition is increased to 0.5%, the Fe element content detected in the negative electrode is significantly reduced, which greatly improves the high-temperature cycling of the battery. However, the fast charging and low-temperature performance of the battery deteriorates significantly.

[0080] Compared to Comparative Example 4, Example 1, by adding 0.5% LiODFB to solution B (equivalent to 0.05% of the total electrolyte), significantly improved Fe ion dissolution and high-temperature cycle performance of the battery, while fast charging and high-temperature performance were not significantly affected. Compared to Comparative Examples 2-3, the effect of LiODFB was greatly enhanced, proving that by using a two-electrode electrolyte method and adding this lithium salt additive after formation, negative electrode consumption can be effectively avoided, and positive electrode film formation can be prioritized.

[0081] The only variable in Examples 1-4 was the VC content in solutions A and B. Table 2 shows that increasing the total VC content significantly improved the battery's high-temperature cycle performance but worsened its fast-charging and low-temperature performance. Conversely, decreasing the total VC content, especially in solution A, significantly improved the battery's fast-charging performance, but worsened its high-temperature cycle performance. Compared to Example 1, Example 8 changed the type of lithium salt additive in solution B, replacing LiODFB with LiDFBP. This improved the battery's fast-charging and low-temperature performance by sacrificing its high-temperature cycle performance.

[0082] Compared to other embodiments and comparative examples, Examples 5-7 and Comparative Example 6 use EA as the main solvent. Due to its low viscosity and melting point, and high dielectric constant, EA significantly improves the battery's low-temperature and fast-charging performance. However, its low boiling point causes deviations in high-temperature cycle performance. As shown in the battery performance of Examples 5-7, increasing the total VC content significantly improves the battery's high-temperature performance, while its low-temperature and fast-charging performance only slightly deteriorates. Compared to Comparative Example 6, Example 7, by adding LiODFB to solution B, significantly improves the battery's high-temperature cycle performance. This not only provides a feasible solution for the application of EA solvent in power batteries but also further demonstrates the universal applicability of the two-electrode electrolyte of this invention.

[0083] The battery test results of the above examples provide a broad reference for the design of secondary electrolytes. The VC content in liquid A and liquid B is quantitatively designed based on two key battery performance indicators: fast charging and high-temperature cycling. For example, BEV batteries require a minimum charging time of less than 40 minutes for 10% to 90% SOC and more than 1000 cycles at 45°C. The VC content in liquid A is in the range of 1.2%-2.5%, and the corresponding VC content in liquid B is estimated to be 7%-18%. Furthermore, the balance between fast charging, low-temperature performance, and high-temperature cycling performance can be achieved by adjusting the types of solvents and lithium salt additives in liquid B.

[0084] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A secondary electrolyte for lithium-ion batteries, characterized in that: This includes the first and second injections of electrolyte; The second electrolyte solution contains lithium salt additives, which are selected from at least one of lithium difluorooxalate borate, lithium difluorobis(oxalate) phosphate, or lithium tetrafluorooxalate phosphate. The first-fill electrolyte contains at least a first lithium salt, a first solvent, and vinylene carbonate; the second-fill electrolyte also contains the first lithium salt, the first solvent, and vinylene carbonate, with the vinylene carbonate content in the first-fill electrolyte being less than that in the second-fill electrolyte; wherein, in the second-fill electrolyte, the mass fraction of lithium salt additives is 0.2%-1.5%; the mass percentage of vinylene carbonate in the first and second-fill electrolytes relative to the total mass of the electrolytes is [missing information]. z=x m+y (1-m)=0.025+ε2(N-1000) / 80000, Among them, 2%≤z≤6%; m represents the mass percentage of the electrolyte injected in the first injection, which is 78%-92% of the total electrolyte. x represents the mass fraction of vinylene carbonate in the electrolyte during the first injection. x = (2ε1 / λ+1) / 100; ε1 and ε2 are both fluctuation coefficients, with values ​​ranging from 0.8 to 1.2; N is the number of cycles designed to reduce the battery capacity to 80% of its initial capacity when cycling at 45°C. λ refers to the battery's design charging rate factor; m is 85%-90%.

2. The lithium-ion battery secondary electrolyte according to claim 1, characterized in that: In the first electrolyte injection, the mass percentage of vinylene carbonate is 1%-4%; in the second electrolyte injection, the mass percentage of vinylene carbonate is 5%-25%.

3. The lithium-ion battery secondary electrolyte according to claim 2, characterized in that: In the first electrolyte injection, the mass percentage of vinylene carbonate is 1.2%-2.5%; in the second electrolyte injection, the mass percentage of vinylene carbonate is 7%-18%.

4. The lithium-ion battery secondary electrolyte according to claim 2, characterized in that: In the second electrolyte injection, the mass fraction of lithium salt additives is 0.2%-1%.

5. The lithium-ion battery secondary electrolyte according to claim 4, characterized in that: In the second electrolyte injection, the mass fraction of lithium salt additives is 0.4%-0.8%.

6. The lithium-ion battery secondary electrolyte according to claim 1, characterized in that: Both the first lithium salt and the second lithium salt are selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorophosphate, or lithium tetrafluoroborate.

7. The lithium-ion battery secondary electrolyte according to claim 6, characterized in that: The first or second lithium salt is LiPF6 or LiFSI.

8. The lithium-ion battery secondary electrolyte according to claim 6, characterized in that: The combined mass of the first lithium salt and the second lithium salt accounts for 10%-20% of the total mass of the electrolyte.

9. The lithium-ion battery secondary electrolyte according to claim 8, characterized in that: The combined mass of the first and second lithium salts accounts for 12.5%-16% of the total mass of the electrolyte.

10. The lithium-ion battery secondary electrolyte according to claim 1, characterized in that: The first solvent and / or the second solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, γ-butyrolactone, ethyl acetate, methyl acetate, methyl propionate, propyl propionate, or ethyl propionate.

11. The lithium-ion battery secondary electrolyte according to claim 10, characterized in that: The first solvent and / or the second solvent is ethyl acetate.

12. The lithium-ion battery secondary electrolyte according to claim 10, characterized in that: The first solvent accounts for 30%-60% of the mass percentage of the electrolyte injected in the first batch. The second solvent accounts for 30%-70% of the mass of the electrolyte injected in the second injection.

13. The lithium-ion battery secondary electrolyte according to claim 12, characterized in that: The first solvent accounts for 45%-55% of the mass of the electrolyte injected in the first batch; The second solvent accounts for 40%-65% of the mass of the electrolyte injected in the second injection.

14. The lithium-ion battery secondary electrolyte according to claim 1, characterized in that: The first electrolyte also includes a second additive, which is selected from at least one of vinyl sulfate, vinyl sulfite, methylene disulfonate, 1,3-propanesulfonate lactone, tris(trimethylsilane) phosphate, fluorovinyl sulfate, or fluorobenzene.

15. The lithium-ion battery secondary electrolyte according to claim 14, characterized in that: The second additive accounts for 0.1%-2% of the mass of the electrolyte injected in the first injection.

16. A lithium-ion battery, characterized in that: It uses the secondary electrolyte as described in any one of claims 1-15.

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