Application of a thiocyclic phosphoester structure compound as a high-voltage additive for electrolytes

Abstract

The present invention discloses the use of a compound with a thiocyclic phosphoester structure as a high-voltage additive for an electrolyte, a composite additive, and a high-voltage electrolyte for a lithium battery. The present invention applies the high-voltage additive to the electrolyte of a lithium-ion battery, thereby forming a stable and dense inorganic CEI film at the positive electrode, stabilizing transition metal ions on the surface of the positive electrode material, inhibiting oxygen precipitation of the positive electrode material, and reducing oxidative decomposition of the electrolyte; the high-voltage additive can also be formed together with the auxiliary additive at the negative electrode to form a composite SEI film, thereby improving the stability of the negative electrode SEI film, enhancing the performance of the lithium battery under high voltage conditions, and extending the long high-temperature cycle life of the lithium battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery technology, and in particular to the application of a compound with a thiocyclic phosphoester structure as a high-voltage additive for an electrolyte. Background Art

[0002] Since their commercialization in the 1990s, lithium batteries have been widely used due to their high energy density, high charge and discharge efficiency, low self-discharge, long service life, and environmental friendliness. They are currently used in consumer electronics, aerospace, military, power tools, and electric vehicles. With technological advancements, the energy density of lithium-ion batteries is becoming increasingly demanding, both in the consumer and power battery sectors. Increasing the operating voltage of lithium batteries has become a key technology for improving their energy density. A variety of high-voltage cathode materials have been developed, including high-voltage lithium cobalt oxide (LCO), high-voltage nickel-manganese oxide (NiMnO), and olivine-structured LiMPO4.

[0003] However, when researching high-voltage lithium batteries, researchers discovered that as the voltage of lithium batteries increases, conventional electrolytes not only undergo oxidative decomposition themselves but also chemically react with the cathode material, leading to deterioration in battery performance and severely shortening the battery life. Therefore, developing an electrolyte that matches high-voltage lithium batteries becomes crucial. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a compound with a thiocyclic phosphoester structure as a high-voltage additive for electrolyte, which can improve the stability of the CEI film on the surface of the battery positive electrode material and improve the performance of the lithium battery.

[0005] The technical solution adopted to achieve the purpose of the present invention is:

[0006] In one aspect, the present invention provides a use of a compound having a thiocyclic phosphoester structure as a high-voltage additive for an electrolyte.

[0007] Specifically, the compound structural formula of the thiocyclic phosphoester structure is:

[0008]

[0009] Among them, R1, R2, R3, and R4 can be selected from hydrogen, an alkyl group, an alkenyl group, an alkynyl group having 1 to 6 carbon atoms, or a cycloalkyl group, a cycloalkenyl group, an aryl group having 6 to 12 carbon atoms, and their halogenated derivatives. The halogenated derivatives of the alkyl group, the alkenyl group, the alkynyl group, and the aryl group are partially substituted or fully substituted, and the halogen in the halogenated part is one or more of fluorine, chlorine, and bromine.

[0010] Furthermore, R1 and R2 are preferably hydrogen, an alkyl group having 1 to 2 carbon atoms, an alkenyl group, an alkynyl group or a fluorinated alkyl group, and R3 and R4 are preferably hydrogen, an alkyl group having 1 to 2 carbon atoms or an alkenyl group.

[0011] Furthermore, the R1 and R2 are preferably one of hydrogen, methyl, ethane, vinyl, ethynyl or trifluoromethyl, the R3 is preferably one of hydrogen, methyl or vinyl, and the R4 is preferably hydrogen or methyl.

[0012] Specifically, the amount of the compound having a thiocyclic phosphoester structure added is 0.05 wt% to 5 wt% of the total mass of the electrolyte.

[0013] On the other hand, the present invention also provides a composite additive, which is a combination of the above-mentioned compound with a sulfur cyclophosphite structure and a compound additive, wherein the compound additive is an auxiliary additive A and / or an auxiliary additive B, wherein the auxiliary additive A is one or more of lithium tetrafluoroborate, lithium bis(oxalatoborate), and lithium difluorooxalatoborate, preferably lithium difluorooxalatoborate (LiDFOB); the auxiliary additive B is one or more of vinylene carbonate, vinyl vinyl carbonate, ethyl vinyl acetate, vinyl sulfite, propylene sulfite, vinyl sulfate, 1,3-propane sultone, propenyl-1,3-propane sultone, 1,4-butane sultone, methylene disulfonate, hexamethyldisilazane, magnesium trifluoromethanesulfonimide, tris(pentafluorophenyl)boron, tris(trimethylsilane)phosphate, and tris(trimethylsilane)phosphite, preferably vinylene carbonate (VC), and the amount of the composite additive added is 0.5wt%-5wt% of the total mass of the electrolyte.

[0014] Furthermore, the ratio of the compound having a thiocyclic phosphoester structure to the compound additive is 1:1-5, preferably 1:1-3, and more preferably 1:(2±0.5).

[0015] The electrolyte also includes a lithium salt and an organic solvent; further, the lithium salt can be selected from one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium difluorophosphate, lithium bistrifluoromethylsulfonyl imide, and lithium bisfluorosulfonyl imide, preferably lithium hexafluorophosphate; further, the organic solvent can be selected from any one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, ethyl butyrate and their halogenated derivatives, preferably a mixture of ethylene carbonate, fluoroethylene carbonate, diethyl carbonate, and propyl propionate.

[0016] Furthermore, the auxiliary additive can be selected from one or more of vinylene carbonate, vinyl ethylene carbonate, ethyl vinyl acetate, vinyl sulfite, propylene sulfite, vinyl sulfate, 1,3-propane sultone, propenyl-1,3-propane sultone, 1,4-butane sultone, methylene disulfonate, hexamethyldisilazane, magnesium trifluoromethanesulfonimide, tris(pentafluorophenyl)boron, tris(trimethylsilane) phosphate, and tris(trimethylsilane) phosphite, preferably one or more of vinylene carbonate, vinyl ethylene carbonate, ethyl vinyl acetate, vinyl sulfite, propylene sulfite, and vinyl sulfate, more preferably vinylene carbonate, the amount of the lithium salt added is 0.5wt%-20wt% of the total mass of the electrolyte, and the amount of the organic solvent added is 70wt%-90wt% of the total mass of the electrolyte.

[0017] On the other hand, the present invention also provides a high-voltage electrolyte for a lithium battery, comprising the above-mentioned lithium salt, an organic solvent, a high-voltage additive and a compounded additive, wherein the high-voltage additive is the compound with the thiocyclic phosphoester structure described above, and the amount of the high-voltage additive added is 0.05wt% to 5wt% of the total mass of the electrolyte.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention uses a thiocyclic phosphoester structure compound as a high-voltage additive in the electrolyte, which can not only form a stable and dense inorganic CEI film at the positive electrode, stabilize the transition metal ions on the surface of the positive electrode material, inhibit oxygen precipitation of the positive electrode material, and reduce the oxidative decomposition of the electrolyte; it can also form a film together with the auxiliary additive at the negative electrode to form a composite SEI film, improve the stability of the negative electrode SEI film, enhance the performance of the lithium battery under high voltage conditions, and extend the long high-temperature cycle life of the lithium battery. DETAILED DESCRIPTION

[0020] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0021] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0022] Example 1

[0023] The present invention will be described in detail below with reference to the embodiments.

[0024] A high voltage additive, the structural formula of which is as follows:

[0025]

[0026]

[0027]

[0028] Example 1: Preparation of lithium battery

[0029] (1) Lithium cobalt oxide suitable for high voltage was selected as the positive electrode material. The positive electrode materials LiCoO2, CNTs, and PVDF were mixed evenly in a ratio of 97.5:1:1.5, coated on an aluminum foil current collector, dried in an oven, and rolled on a roller press. The compaction density was 4.0 g / cm 3 , and the required positive electrode sheet is obtained.

[0030] (2) Artificial graphite is selected as the negative electrode material, and graphite, CMC, conductive agent, and binder are mixed according to the ratio of 95:1.2:1.8:

[0031] 2 were mixed evenly to obtain a negative electrode sheet with a compaction density of 1.6 g / cm 3 .

[0032] (3) A ceramic-coated PE film (9+3+3) um was selected as the isolation film, and the electrode was made into a 1.5 Ah small soft-pack battery by lamination for testing high-voltage electrolyte.

[0033] Example 2: Lithium Battery Performance Test

[0034] The charge and discharge voltage window of the lithium battery is 3.0-4.45V; the battery cycle test is performed at room temperature of 25°C and high temperature of 45°C, and the charge and discharge current of the cycle is 0.5C.

[0035] Example 3:

[0036] In an argon atmosphere and an environmental index of H2O ≤ 0.5ppm, O2 ≤ 2.0ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1.1M. Then, 1% of the auxiliary additive VC was added, and then 1% of the high voltage additive A was added to prepare electrolyte I.

[0037] Example 4:

[0038] In an argon atmosphere and an environmental index of H2O ≤ 0.5ppm, O2 ≤ 2.0ppm in a glove box, the organic solvents ethylene carbonate (EC), fluoroethylene carbonate (FEC), diethyl carbonate (DEC), and propyl propionate (PP) were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1.1M. Then, 1% of the auxiliary additive VC was added, and then 1% of the high voltage additive B was added to prepare electrolyte II.

[0039] Example 5:

[0040] In an argon atmosphere and an environmental condition of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte having a lithium hexafluorophosphate concentration of 1.1 M. 1% of the auxiliary additive VC was then added, followed by 1% of the high-voltage additive C to prepare electrolyte III.

[0041] Example 6:

[0042] In an argon atmosphere and an environmental condition of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte having a lithium hexafluorophosphate concentration of 1.1 M. 1% of the auxiliary additive VC was then added, followed by 1% of the high-voltage additive D to prepare electrolyte IV.

[0043] Example 7:

[0044] In an argon atmosphere and a glove box with environmental indicators of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1.1 M. Subsequently, 1% of the auxiliary additive VC was added, and then 1% of the high-voltage additive E was added to prepare electrolyte V.

[0045] Example 8:

[0046] In an argon atmosphere and a glove box with environmental indicators of H2O≤0.5ppm and O2≤2.0ppm, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP=25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1.1M. Then, 1% of the auxiliary additive VC was added, and then 1% of the high voltage additive F was added to prepare electrolyte VI.

[0047] Example 9:

[0048] In an argon atmosphere and a glove box with environmental indicators of H2O≤0.5ppm and O2≤2.0ppm, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP=25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1.1M. Then, 1% of the auxiliary additive VC was added, and then 0.5% of the high voltage additive E was added to prepare electrolyte VII.

[0049] Example 10:

[0050] In an argon atmosphere and an environmental condition of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte having a lithium hexafluorophosphate concentration of 1.1 M. 1% of the auxiliary additive VC was then added, and then 1% of the high voltage additives E and F were respectively added to prepare electrolyte VIII.

[0051] Example 11:

[0052] In an argon atmosphere and an environmental condition of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte having a lithium hexafluorophosphate concentration of 1.1 M. 1% of the auxiliary additive LiDFOB was then added, followed by 1% of the high voltage additive E to prepare electrolyte IX.

[0053] Example 12:

[0054] In an argon atmosphere and an environmental condition of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte having a lithium hexafluorophosphate concentration of 1.1 M. Subsequently, 2% of the auxiliary additive LiDFOB was added, and then 1% of the high-voltage additive E was added to prepare electrolyte X.

[0055] Example 13:

[0056] In an argon atmosphere and an environmental index of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, DEC, and PP were mixed in a mass ratio of EC / FEC / DEC / PP = 25 / 5 / 50 / 20, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1.1 M. Subsequently, 1% of auxiliary additives VC and LiDFOB were added respectively, and then 1% of a high voltage additive E was added to prepare electrolyte XI.

[0057] Comparative Example 1:

[0058] In an argon atmosphere and a glove box with environmental indicators of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm, the organic solvents EC, FEC, and DEC were mixed in a mass ratio of EC / FEC / DEC = 25 / 5 / 70, and then lithium hexafluorophosphate was added for dissolution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 M. Then, 1% of the auxiliary additive VC was added to prepare a comparative electrolyte 1.

[0059] Comparative Example 2:

[0060] In an argon atmosphere and an environmental condition of H2O ≤ 0.5 ppm and O2 ≤ 2.0 ppm in a glove box, the organic solvents EC, FEC, and DEC were mixed at a mass ratio of EC / FEC / DEC = 25 / 5 / 70, and then lithium hexafluorophosphate was added and dissolved to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 M. Then, 1% of auxiliary additives VC and LiDFOB were added, respectively, to prepare comparative electrolyte 2.

[0061] Table 1 shows the test results of the electrical properties of the electrolytes of Examples 3-13 and Comparative Examples 1 and 2.

[0062] Table 1: Summary of electrical performance data of different electrolytes

[0063]

[0064] The following conclusions can be drawn from the comparative analysis of the experimental data in Table 1:

[0065] Comparing Examples 3-8 with Comparative Example 1, it can be seen that after the high-voltage additive of the present invention is added to the electrolyte, the electrolyte performance parameters such as the capacity retention rate after 300 cycles at 25°C and the capacity retention rate after 150 cycles at 45°C are greatly improved compared to when no high-voltage additive is added, verifying that the high-voltage additive of the present invention has excellent performance and the prepared high-voltage electrolyte has good electrical performance.

[0066] It can be seen from Examples 3-8 that the high-voltage additives used are A, B, C, D, E, and F used alone, and the auxiliary additives used are all VC. From the electrical properties of the prepared electrolytes, the electrolyte II obtained in Example 4 has the best electrical properties, with an initial efficiency of 90.14%, a capacity retention rate of 86.4% after 300 cycles at 25°C, and a capacity retention rate of 84.8% after 150 cycles at 45°C. This verifies that the electrolyte has good electrical properties when R1 and R2 in the thiocyclic phosphoester structure compound are hydrogen or an alkenyl group with 1 to 2 carbon atoms, and when R3 and R4 are hydrogen. This indicates that the carbon-carbon double bond plays a key role in the electrolysis reaction of the lithium battery, stabilizes the transition metal ions on the surface of the positive electrode material, inhibits the oxygen precipitation of the positive electrode material, and reduces the oxidative decomposition of the electrolyte.

[0067] The ratio of the thiocyclic phosphoester compound (i.e., the high-voltage additive) to the compounded additive can be 1:1-5, preferably 1:1-3, and more preferably 1:(2±0.5). A comparison of Examples 7 and 9 shows that when both the auxiliary additive VC and the high-voltage additive E are used, varying their dosages reveals that a 2:1 ratio of VC to E yields better electrolyte performance. Similarly, Examples 11 and 12 demonstrate that when LiDFOB and E are used simultaneously in a 2:1 ratio, the electrolyte performs better than a 1:1 ratio. This demonstrates that appropriately increasing the LiDFOB or VC content can yield an electrolyte with superior electrical performance.

[0068] From the comparison between Example 7 or Example 8 and Example 10, it can be seen that when the two high-voltage additives E and F are used simultaneously, compared with the electrolyte obtained by using the high-voltage additive E or F alone, it is found that the capacity retention rate after 150 cycles at 45°C using the two high-voltage additives can reach 84.2%, which is a higher retention rate and better electrical performance. This verifies that when the sulfur cyclophosphamide structure compound contains both an olefinic bond and a fluoroalkyl group, the prepared electrolyte has better electrical performance, indicating that the simultaneous presence of carbon-carbon double bonds and fluorine plays a key role in the electrolysis reaction of lithium batteries.

[0069] Comparative Example 2 and Examples 7, 11, and 13 demonstrate that the electrical performance of the electrolyte increases step by step when the high-voltage additive of the present invention, the auxiliary additive LiDFOB alone, the auxiliary additive VC alone, and the auxiliary additives LiDFOB and VC are added simultaneously. This demonstrates that the combination of the high-voltage additive and the compounded additives LiDFOB and / or VC results in superior electrical performance.

[0070] Specifically, Example 13 and Comparative Example 2 demonstrate that the initial efficiency, capacity retention after 300 cycles at 25°C, and capacity retention after 150 cycles at 45°C of the electrolyte obtained using either VC or LiDFOB are superior to those of the electrolyte without the high-voltage additive. Examples 7 and 11 demonstrate that when high-voltage additive E is used simultaneously and VC and LiDFOB are used as auxiliary additives, the electrolyte obtained using LiDFOB alone achieves an initial efficiency of 90.35%, a capacity retention of 90.6% after 300 cycles at 25°C, and a capacity retention of 87.2% after 150 cycles at 45°C, demonstrating superior electrical performance compared to the electrolyte using VC alone. Examples 11 and 13 demonstrate that when high-voltage additive E is used simultaneously, the capacity retention after 150 cycles at 45°C of the electrolyte obtained using both VC and LiDFOB as auxiliary additives is slightly superior to that of LiDFOB alone.

[0071] In summary, compared with electrolytes 1 and 2 without the addition of high-voltage additives, the use of the high-voltage additive of the present invention has a certain improvement in the long high-temperature cycle life of the lithium battery within the appropriate addition range, and the electrical performance is better after the high-voltage additive is combined with the compound additive.

[0072] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A use of a compound having a thiocyclic phosphoester structure as a high-voltage additive for an electrolyte, characterized in that: The compound of the thiocyclic phosphoester structure is generally formulated as follows: Among them, R1, R2, R3, and R4 are hydrogen, an alkyl group, an alkenyl group, an alkynyl group having 1 to 6 carbon atoms, or a cycloalkyl group, a cycloalkenyl group, an aryl group having 6 to 12 carbon atoms, and their halogenated derivatives, the halogenation in the halogenated derivatives of the alkyl group, alkenyl group, alkynyl group, and aryl group is partial or full substitution, and the halogenated halogen is one or more of fluorine, chlorine, and bromine.

2. The use according to claim 1, characterized in that The R1 and R2 are hydrogen, an alkyl group with 1 to 2 carbon atoms, an alkenyl group, an alkynyl group or a fluorinated alkyl group, and the R3 and R4 are hydrogen, an alkyl group with 1 to 2 carbon atoms or an alkenyl group.

3. The use according to claim 1, characterized in that The R1 and R2 are one of hydrogen, methyl, ethane, vinyl, ethynyl or trifluoromethyl, the R3 is one of hydrogen, methyl or vinyl, and the R4 is hydrogen or methyl.

4. The use according to claim 1, characterized in that The added amount of the compound is 0.05 wt% to 5 wt% of the total mass of the electrolyte.

5. A composite additive, characterized in that: The composite additive is a combination of a compound with a thiocyclic phosphoester structure and a compound additive. The general formula of the compound with a thiocyclic phosphoester structure is wherein R1, R2, R3, and R4 are hydrogen, an alkyl group having 1 to 6 carbon atoms, an alkenyl group, an alkynyl group, or a cycloalkyl group having 6 to 12 carbon atoms, a cycloalkenyl group, an aryl group, and halogenated derivatives thereof, wherein the halogenated derivatives of the alkyl group, alkenyl group, alkynyl group, and aryl group are partially or fully substituted, and the halogenated halogen is one or more of fluorine, chlorine, and bromine; The compound additive is an auxiliary additive A and / or an auxiliary additive B, wherein the auxiliary additive A is one or more of lithium tetrafluoroborate, lithium bis(oxalatoborate), and lithium difluorooxalatoborate, and the auxiliary additive B is one or more of vinylene carbonate, vinyl ethylene carbonate, ethyl vinyl acetate, vinyl sulfite, propylene sulfite, vinyl sulfate, 1,3-propane sultone, propenyl-1,3-propane sultone, 1,4-butane sultone, methylene disulfonate, hexamethyldisilazane, magnesium imide trifluoromethanesulfonate, tris(pentafluorophenyl)boron, tris(trimethylsilane)phosphate, and tris(trimethylsilane)phosphite.

6. The composite additive according to claim 5, characterized in that The auxiliary additive A is lithium difluorooxalatoborate, and the auxiliary additive B is vinylene carbonate.

7. The composite additive according to claim 5, characterized in that The ratio of the compound with the thiocyclic phosphoester structure to the compound additive is 1:1-5.

8. The composite additive according to claim 5, characterized in that The ratio of the compound with the thiocyclic phosphoester structure to the compound additive is 1:1-3.

9. The composite additive according to claim 5, characterized in that The ratio of the compound with the thiocyclic phosphoester structure to the compound additive is 1:(2±0.5).

10. A high-voltage electrolyte for lithium batteries, characterized in that: The composite additive according to any one of claims 5 to 9 further comprises a lithium salt and an organic solvent.

11. The high-voltage electrolyte for lithium batteries according to claim 10, characterized in that: The lithium salt is one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium difluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(fluorosulfonyl)imide, and the amount of the lithium salt added is 0.5wt%-20wt% of the total mass of the electrolyte; The organic solvent is any one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, ethyl butyrate and their halogenated derivatives, and the amount of the organic solvent added is 70wt%-90wt% of the total mass of the electrolyte.

12. The high-voltage electrolyte for lithium batteries according to claim 10, characterized in that: The lithium salt is lithium hexafluorophosphate.

13. The high-voltage electrolyte for lithium batteries according to claim 10, characterized in that: The organic solvent is a mixture of ethylene carbonate, fluoroethylene carbonate, diethyl carbonate and propyl propionate.

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