Composite electrolyte, method for injecting the same and lithium ion battery

Abstract

This invention discloses a composite electrolyte, its injection method, and a lithium-ion battery, relating to the field of battery electrolyte technology. The composite electrolyte comprises a first electrolyte and a second electrolyte. The second film-forming additive in the second electrolyte includes compound I, whose chemical structural formula is [insert chemical formula here]. The composite electrolyte, comprising the first and second electrolytes, introduces the second film-forming additive into the second electrolyte. By combining the repeated effects of the film-forming additives in the two electrolytes, and utilizing different mechanisms for film formation, the resulting SEI film is more stable, covering the larger specific surface area required for porous carbon with its high specific surface area. This provides better containment of the volume expansion of active materials in the later stages of cycling, thereby improving its electrochemical performance.

Description

Technical Field

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

[0002] Porous carbon materials have gained widespread attention in lithium-ion batteries due to their high specific surface area, pore volume, low density, good chemical stability, and especially their hierarchical pore size. When used as an anode in lithium-ion batteries, the high specific surface area of ​​porous carbon allows it to bind more lithium ions, providing high capacity for the battery. Therefore, more SEI (Sediment Insulation) film protection is required.

[0003] After the first charge, a solid-phase electrolyte interphase (SEI) film forms on the surface of the positive and negative electrode active materials in a lithium-ion battery. This film has a multilayer structure, with a porous side near the electrolyte and a dense side near the electrode. The formation of the SEI film has a crucial impact on the performance of the electrode materials. On the one hand, the formation of the SEI film consumes some lithium ions, increasing the irreversible capacity during the first charge-discharge cycle and reducing the charge-discharge efficiency of the electrode materials. On the other hand, the SEI film is insoluble in solvents and can exist stably in organic electrolyte solutions. Furthermore, this passivation film only allows lithium ions to pass through, effectively preventing the co-intercalation of solvent molecules and avoiding damage to the electrode materials caused by solvent molecule co-intercalation. This significantly improves the cycle performance and lifespan of the electrode. However, the instability of the SEI film at high temperatures affects its high-temperature performance; therefore, improving the performance of the SEI film is an urgent problem to be solved. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a composite electrolyte, its injection method, and a lithium-ion battery.

[0005] The present invention discloses a composite electrolyte comprising a first electrolyte and a second electrolyte. The first electrolyte comprises a first solvent, a first lithium salt and a first film-forming additive, and the second electrolyte comprises a second solvent, a second lithium salt and a second film-forming additive.

[0006] The second film-forming additive includes at least one of lithium difluorophosphate, lithium tetrafluoroborate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinylene sulfate (DTD), lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate (LiODFP), and 1,3-propane sulpholactone; the second film-forming additive includes compound I, whose chemical structural formula is... In the formula, R1 is selected from one of the following: halogen atom, fluorinated C1-C10 alkyl, fluorinated C1-C10 alkoxy, fluorinated C2-C10 alkenyl, fluorinated C2-C10 alkenoxy, fluorinated C2-C10 alkynyl, and fluorinated C2-C10 alkynoxy; and R2 is selected from one of the following: fluorinated or unsubstituted C1-C10 alkyl or alkoxy, fluorinated or unsubstituted C2-C10 alkenyl or alkenoxy, fluorinated or unsubstituted C2-C10 alkynyl or alkynoxy, and halogen atom.

[0007] According to one embodiment of the present invention, the weight X of the first electrolyte and the weight Y of the second electrolyte satisfy the following relationship: 5Y≥X≥3.5Y.

[0008] According to one embodiment of the present invention, the first solvent includes carbonate, carboxylic acid ester and ether, and the weight ratio of carbonate, carboxylic acid ester and ether satisfies the following relationship: A1:B1:C1=(40~65):(25~45):(0~20), A1≥B1 and B1≥C1; wherein, A1 is the weight ratio of carbonate in the first solvent, B1 is the weight ratio of carboxylic acid ester in the first solvent, and C1 is the weight ratio of ether in the first solvent.

[0009] According to one embodiment of the present invention, the concentration of the first lithium salt in the composite electrolyte is 0.5 to 1.5 mol / L, wherein the first lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethanesulfonate, and tris(trifluoromethanesulfonyl)methyllithium.

[0010] According to one embodiment of the present invention, the weight percentage of the first film-forming additive in the first electrolyte satisfies the following relationship: 3M 第一溶剂 ≥M 第一成膜添加剂 ≥0.2M 第一溶剂 , of which M 第一溶剂 M represents the weight percentage of the first solvent in the first electrolyte. 第一成膜添加剂 The weight percentage of the first film-forming additive in the first electrolyte; wherein the first film-forming additive includes at least one of lithium difluorophosphate, lithium tetrafluoroborate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinyl sulfate (DTD), lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate (LiODFP), and 1,3-propane sulpholactone.

[0011] According to one embodiment of the present invention, the second solvent includes carbonate, carboxylic acid ester and ether, and the weight ratio of carbonate, carboxylic acid ester and ether satisfies the following relationship: A2:B2:C2=(40~65):(20~45):(10~20), A2≥B2 and B2≥C2; wherein, A2 is the weight ratio of carbonate in the second solvent, B2 is the weight ratio of carboxylic acid ester in the second solvent, and C2 is the weight ratio of ether in the second solvent.

[0012] According to one embodiment of the present invention, the concentration of the second lithium salt in the composite electrolyte is 0.5 to 1.5 mol / L, wherein the second lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethanesulfonate, and tris(trifluoromethanesulfonyl)methyllithium.

[0013] According to one embodiment of the present invention, the weight percentage of the second film-forming additive in the second electrolyte satisfies the following relationship: 8M 第二溶剂 ≥M 第二成膜添加剂 ≥3M 第二溶剂 , of which M 第二溶剂 M represents the weight percentage of the second solvent in the second electrolyte. 第二成膜添加剂 The weight percentage of the second film-forming additive in the second electrolyte.

[0014] The present invention discloses a method for injecting the composite electrolyte as described above, comprising the following steps:

[0015] S1. Inject the first electrolyte into the casing of the lithium-ion battery. After soaking and formation, extract the excess first electrolyte from the casing.

[0016] S2. The second electrolyte is injected into the casing of the lithium-ion battery, followed by wetting and formation.

[0017] The present invention discloses a lithium-ion battery containing the composite electrolyte as described above.

[0018] Compared with the prior art, the composite electrolyte, its injection method, and the lithium-ion battery of the present invention have the following advantages:

[0019] The composite electrolyte of the present invention includes a first electrolyte and a second electrolyte. A second film-forming additive is introduced into the second electrolyte. By combining the repeated action of the film-forming additives in the two electrolytes, film is formed using different mechanisms, making the formed SEI film more stable. It covers the larger specific surface area of ​​the SEI film required by the high specific surface area of ​​porous carbon, and has better tolerance for the volume expansion of active materials in the later stage of cycling, thereby improving its electrochemical performance.

[0020] In addition, the composite electrolyte of the present invention introduces a second film-forming additive into the second electrolyte, while reducing the amount of other components in the second electrolyte, thereby improving the solubility of the second film-forming additive.

[0021] Furthermore, the method for injecting the composite electrolyte of the present invention involves first injecting a first electrolyte, then extracting the excess first electrolyte after wetting and formation, and then injecting a second electrolyte. This avoids the remaining first film-forming additive after formation from having a deteriorating effect on the battery cell system. Detailed Implementation

[0022] Several embodiments of the present invention will be disclosed below. For clarity, many practical details will be described in the following description. However, it should be understood that these practical details are not intended to limit the invention. That is, in some embodiments of the invention, these practical details are not essential.

[0023] Furthermore, in this invention, the use of terms such as "first" and "second" is for descriptive purposes only and does not specifically refer to any order or sequence, nor is it intended to limit the invention. They are merely used to distinguish components or operations described using the same technical terms, and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of various embodiments can be combined with each other, but only if they are feasible for those skilled in the art. If a combination of technical solutions is contradictory or impossible to implement, such a combination should be considered nonexistent and not within the scope of protection claimed by this invention.

[0024] Example 1

[0025] This embodiment provides a composite electrolyte for use in lithium-ion batteries. The composite electrolyte includes a first electrolyte and a second electrolyte; wherein the first electrolyte includes a first solvent, a first lithium salt, and a first film-forming additive, and the second electrolyte includes a second solvent, a second lithium salt, and a second film-forming additive.

[0026] In this embodiment, the weight X of the first electrolyte and the weight Y of the second electrolyte satisfy the following relationship: 5Y≥X≥3.5Y.

[0027] In this embodiment, the first solvent includes carbonates, carboxylic esters, and ethers. Specifically, the chemical structural formula of the carbonate is as follows: The structural chemical formula of carboxylic acid esters is The structural chemical formula of ethers is R3 to R10 are each independently selected from one of C1 to C10 alkyl or alkoxy, C2 to C10 alkenyl or alkenyloxy, or C2 to C10 alkynyl or alkynyloxy.

[0028] Furthermore, the weight percentages of carbonates, carboxylic esters, and ethers in the first solvent satisfy the following relationship: A1:B1:C1=(40~65):(25~45):(0~20), A1≥B1 and B1≥C1; where A1 is the weight percentage of carbonates in the first solvent, B1 is the weight percentage of carboxylic esters in the first solvent, and C1 is the weight percentage of ethers in the first solvent.

[0029] In this embodiment, the first lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethanesulfonate, and tris(trifluoromethanesulfonyl)methyllithium.

[0030] Furthermore, the concentration of the first lithium salt in the first electrolyte is 0.5–1.5 mol / L.

[0031] In this embodiment, the first film-forming additive includes at least one of lithium difluorophosphate, lithium tetrafluoroborate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinyl sulfate (DTD), lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate (LiODFP), and 1,3-propane sulpholactone.

[0032] Furthermore, the weight percentage of the first film-forming additive in the first electrolyte satisfies the following relationship: 3M 第一溶剂 ≥M 第一成膜添加剂 ≥0.2M 第一溶剂 , of which M 第一溶剂 M represents the weight percentage of the first solvent in the first electrolyte. 第一成膜添加剂 The weight percentage of the first film-forming additive in the first electrolyte.

[0033] In this embodiment, the second solvent includes carbonates, carboxylic esters, and ethers. Specifically, the chemical structural formula of the carbonate is as follows: The structural chemical formula of carboxylic acid esters is The structural chemical formula of ethers is R3 to R10 are each independently selected from one of C1 to C10 alkyl or alkoxy, C2 to C10 alkenyl or alkenyloxy, or C2 to C10 alkynyl or alkynyloxy.

[0034] Furthermore, the weight percentages of carbonates, carboxylic esters, and ethers in the second solvent satisfy the following relationship: A2:B2:C2=(40~65):(20~45):(10~20), A2≥B2 and B2≥C2; where A2 is the weight percentage of carbonates in the second solvent, B2 is the weight percentage of carboxylic esters in the second solvent, and C2 is the weight percentage of ethers in the second solvent.

[0035] In this embodiment, the second lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethanesulfonate, and tris(trifluoromethanesulfonyl)methyllithium.

[0036] Furthermore, the concentration of the second lithium salt in the second electrolyte is 0.5–1.5 mol / L.

[0037] In this embodiment, the second film-forming additive includes at least one selected from lithium difluorophosphate, lithium tetrafluoroborate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinyl sulfate (DTD), lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate (LiODFP), and 1,3-propane sulpholactone. The second film-forming additive also includes compound I, whose chemical structural formula is […]. In the formula, R1 is selected from one of fluorinated C1-C10 alkyl or fluorinated alkoxy or fluorinated C1-C10 alkenyl or alkenoxy or C2-C10 fluorinated alkynyl, alkynoxy or -F group, and R2 is selected from one of C1-C10 fluorinated or unsubstituted alkyl or alkoxy, fluorinated or unsubstituted C2-C10 alkenyl or alkenoxy, C2-C10 fluorinated or unsubstituted alkynyl or alkynoxy, -F group, and at least one of R1 and R2 contains fluorine.

[0038] Furthermore, the weight percentage of the second film-forming additive in the second electrolyte satisfies the following relationship: 8M 第二溶剂 ≥M 第二成膜添加剂 ≥3M 第二溶剂 , of which M 第二溶剂 M represents the weight percentage of the second solvent in the second electrolyte. 第二成膜添加剂 The weight percentage of the second film-forming additive in the second electrolyte.

[0039] This composite electrolyte comprises a first electrolyte and a second electrolyte. A second film-forming additive is introduced into the second electrolyte. By combining the repeated effects of the film-forming additives in both electrolytes, film formation is achieved through different mechanisms, resulting in a more stable SEI film. This SEI film covers the larger specific surface area required for the high specific surface area of ​​porous carbon, providing better containment for the volume expansion of active materials in the later stages of cycling, thereby improving its electrochemical performance. Furthermore, the composite electrolyte introduces the second film-forming additive into the second electrolyte while reducing the amount of other components in the second electrolyte, thus improving the solubility of the second film-forming additive.

[0040] This embodiment also provides a method for injecting a composite electrolyte, which is the method for injecting the composite electrolyte described above. The method for injecting the composite electrolyte includes the following steps:

[0041] S1. Inject the first electrolyte into the casing of the lithium-ion battery. After soaking and formation, extract the excess first electrolyte from the casing.

[0042] S2. The second electrolyte is injected into the casing of the lithium-ion battery, followed by wetting and formation.

[0043] The method of injecting the composite electrolyte first injects the first electrolyte, and after wetting and formation, the excess first electrolyte is extracted before injecting the second electrolyte. This avoids the remaining first film-forming additive after formation from having a deteriorating effect on the cell system.

[0044] Example 2

[0045] This embodiment provides a lithium-ion battery in which a composite electrolyte as described in Embodiment 1 is injected into the casing, and the electrolyte injection method described in Embodiment 1 is used for the injection.

[0046] To better illustrate the impact of the composite electrolyte of the present invention on lithium-ion batteries, eleven samples of the composite electrolyte are provided below for detailed description.

[0047] Sample 1

[0048] The composite electrolyte provided in Sample 1 includes a first electrolyte and a second electrolyte. The weight of the first electrolyte is X, the weight of the second electrolyte is Y, and 8≤X+Y≤10.

[0049] The first electrolyte comprises a first solvent, a first lithium salt, and a first film-forming additive. The first solvent comprises carbonates, carboxylic acid esters, and ethers. The carbonates are selected from ethylene carbonate (EC) and diethyl carbonate (DEC), the carboxylic acid ester is selected from ethyl propionate (EP), and the ether is selected from 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE). The weight ratio of ethylene carbonate, diethyl carbonate, ethyl propionate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the first solvent is 10:55:25:10. The first lithium salt is lithium hexafluorophosphate (LiPF6), which accounts for 14.5 wt% of the first electrolyte. The first film-forming additives are vinyl sulfate (DTD), 1,3-propane sulpholol (PS), and lithium difluorooxalate borate (LiODFB). The weight percentages of vinyl sulfate (DTD), 1,3-propane sulpholol, and lithium difluorooxalate borate (LiODFB) in the first electrolyte are 0.5 wt%, 0.2 wt%, and 0.5 wt%, respectively.

[0050] The second electrolyte comprises a second solvent, a second lithium salt, and a second film-forming additive. The second solvent includes carbonates, carboxylic acid esters, and ethers. The carbonates are selected from ethylene carbonate (EC) and diethyl carbonate (DEC), the carboxylic acid ester is selected from ethyl propionate (EP), and the ether is selected from 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE). The weight ratio of ethylene carbonate, diethyl carbonate, ethyl propionate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the second solvent is 10:55:25:10. The second lithium salt is lithium hexafluorophosphate (LiPF6), which accounts for 14.5 wt% of the second electrolyte. The second film-forming additive consists of 1,3-propanesulfonyl lactone and chemical A, the chemical structure of which is [insert chemical structure here]. 1,3-propanesulfonyl lactone and chemical A account for 0.2 wt% and 3 wt% of the weight of the second electrolyte, respectively.

[0051] Sample 2

[0052] Compared to sample one, the composite electrolyte provided in sample two contains:

[0053] In the first solvent, the weight ratio of ethylene carbonate, diethyl carbonate, ethyl propionate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 10:30:45:15.

[0054] In the second solvent, the weight ratio of ethylene carbonate, diethyl carbonate, ethyl propionate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the first solvent is 10:30:45:15.

[0055] The second film-forming additive is selected from 1,3-propanesulfonyl lactone and chemical compound B. The chemical structural formula of chemical compound B is as follows: 1,3-propanesulfonyl lactone and chemical B account for 0.2 wt% and 3 wt% of the weight of the second electrolyte, respectively.

[0056] The rest is the same as Sample 1, and will not be repeated here.

[0057] Sample 3

[0058] Compared to sample one, the composite electrolyte provided in sample three contains:

[0059] In the first solvent, the weight ratio of ethylene carbonate, diethyl carbonate, ethyl propionate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 5:45:30:20.

[0060] In the second solvent, the weight ratio of ethylene carbonate, diethyl carbonate, ethyl propionate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the first solvent is 5:45:30:20.

[0061] The second film-forming additive is selected from 1,3-propanesulfonyl lactone and chemical C. The chemical structural formula of chemical C is as follows: 1,3-propanesulfonyl lactone and chemical C account for 0.2 wt% and 3 wt% of the weight of the second electrolyte, respectively.

[0062] The rest is the same as Sample 1, and will not be repeated here.

[0063] Sample 4

[0064] Compared to sample one, sample four provides a composite electrolyte in which:

[0065] The first solvent comprises carbonates and carboxylic acid esters, wherein the carbonates are selected from ethylene carbonate (EC) and diethyl carbonate (DEC), and the carboxylic acid ester is selected from ethyl propionate (EP). The weight ratio of ethylene carbonate, diethyl carbonate, and ethyl propionate in the first solvent is 15:45:40.

[0066] In the first lithium salt, lithium hexafluorophosphate accounts for 12.5 wt% of the weight in the first electrolyte.

[0067] The second solvent comprises carbonates and carboxylic acid esters, wherein the carbonates are selected from ethylene carbonate (EC) and diethyl carbonate (DEC), and the carboxylic acid ester is selected from ethyl propionate (EP). The weight ratio of ethylene carbonate, diethyl carbonate, and ethyl propionate in the first solvent is 15:45:40.

[0068] In the second lithium salt, lithium hexafluorophosphate accounts for 12.5 wt% of the weight in the second electrolyte.

[0069] The second film-forming additive is selected from 1,3-propanesulfonyl lactone and chemical D. The chemical structural formula of chemical D is as follows: 1,3-propanesulfonyl lactone and chemical D account for 0.2 wt% and 3 wt% of the weight of the second electrolyte, respectively.

[0070] The rest is the same as Sample 1, and will not be repeated here.

[0071] Sample 5

[0072] Compared to sample one, the composite electrolyte provided in sample five contains:

[0073] In the second film-forming additive, 1,3-propanesulfonyl lactone and chemical A account for 0.2 wt% and 8 wt% of the weight of the second electrolyte, respectively.

[0074] The rest is the same as Sample 1, and will not be repeated here.

[0075] Sample Six

[0076] Compared to Sample 1, Sample 6 provides a composite electrolyte in which:

[0077] In the second film-forming additive, 1,3-propanesulfonyl lactone and chemical A account for 0.2 wt% and 6 wt% of the weight of the second electrolyte, respectively.

[0078] The rest is the same as Sample 1, and will not be repeated here.

[0079] Sample 7

[0080] Compared to Sample 1, Sample 7 provides a composite electrolyte in which:

[0081] In the first lithium salt, lithium hexafluorophosphate accounts for 10.5 wt% of the weight in the first electrolyte.

[0082] In the second lithium salt, lithium hexafluorophosphate accounts for 10.5 wt% of the weight in the second electrolyte.

[0083] In the second film-forming additive, 1,3-propanesulfonyl lactone and chemical A account for 0.2 wt% and 5 wt% of the weight of the second electrolyte, respectively.

[0084] The rest is the same as Sample 1, and will not be repeated here.

[0085] Sample 8

[0086] Compared to Sample 1, Sample 8 provides a composite electrolyte in which:

[0087] The second film-forming additives are 1,3-propanesulfonyl lactone and lithium difluorobis(oxalato) phosphate (LiODFP). The weight percentages of 1,3-propanesulfonyl lactone and lithium difluorobis(oxalato) phosphate in the second electrolyte are 0.2 wt% and 3 wt%, respectively.

[0088] The rest is the same as Sample 1, and will not be repeated here.

[0089] Sample Nine

[0090] Compared to Sample 1, Sample 9 provides a composite electrolyte in which:

[0091] In the second film-forming additive, 1,3-propanesulfonyl lactone and chemical A account for 0.2 wt% and 0.5 wt% of the weight of the second electrolyte, respectively.

[0092] The rest is the same as Sample 1, and will not be repeated here.

[0093] Sample 10

[0094] Compared to Sample 1, Sample 10 provides a composite electrolyte in which:

[0095] In the second film-forming additive, 1,3-propanesulfonyl lactone and chemical A account for 0.2 wt% and 10 wt% of the weight of the second electrolyte, respectively.

[0096] The rest is the same as Sample 1, and will not be repeated here.

[0097] Sample 11

[0098] Compared to sample eight, sample eleven provides a composite electrolyte in which:

[0099] In the second film-forming additive, 1,3-propanesulfonyl lactone and lithium difluorobis(oxalato) phosphate account for 0.2 wt% and 0.5 wt% of the weight of the second electrolyte, respectively.

[0100] The rest is the same as sample eight, so I will not repeat it here.

[0101] For ease of explanation, the components of samples one through eleven above are summarized in the following table:

[0102]

[0103]

[0104] Multiple lithium-ion battery semi-finished products were prepared using the same method. The composite electrolytes of samples one through eleven were injected into the casings of the lithium-ion batteries using the same injection method. After encapsulation, settling, formation, shaping, and capacity testing, finished lithium-ion batteries corresponding to samples one through eleven were obtained. The injection method includes the following steps:

[0105] S1. Inject the first electrolyte into the casing of the lithium-ion battery. After soaking and formation, extract the excess first electrolyte from the casing.

[0106] S2. The second electrolyte is injected into the casing of the lithium-ion battery, followed by wetting and formation.

[0107] The lithium-ion batteries from samples one to eleven prepared above were subjected to room temperature cycling performance tests, 14-day high-temperature storage tests at 60°C, DCR (direct current resistance) tests, and thermal shock performance tests. A brief description of each test is provided below:

[0108] Room temperature cycle performance test: At 25℃, the battery after capacity gradation is charged to 4.45V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C. This cycle is repeated for 500 charge-discharge cycles. The capacity retention rate at the 500th cycle is calculated using the following formula:

[0109] 500-week cycle capacity retention (%) = (500-week cycle discharge capacity / initial cycle discharge capacity) × 100%.

[0110] 14-day high-temperature storage test at 60℃: The battery was placed at room temperature and charged and discharged once at 0.5C (4.45V-3.0V). The discharge capacity C0 before storage was recorded. Then, the battery was charged to a full state of 4.45V using constant current and constant voltage. The thickness d1 of the battery before high-temperature storage was measured using a PPG battery thickness gauge (500g). The battery was then stored in a 60℃ constant temperature chamber for 14 days. After storage, the battery was removed and the thermal thickness d2 after storage was measured. The battery thickness expansion rate after 14 days of storage at 60℃ was calculated. After the battery cooled at room temperature for 24 hours, it was discharged again at 0.5C to 3.0V using constant current, and then charged to 4.45V at 0.5C using constant current and constant voltage. The discharge capacity C1 and charging capacity C2 after storage were recorded. The remaining capacity and recovery rate of the battery after 14 days of storage at 60℃ were calculated using the following formulas:

[0111] Thickness expansion rate after storage at 60℃ for 14 days = (d2-d1) / d1*100%;

[0112] The remaining capacity after storage at 60℃ for 14 days = C1 / C0*100%;

[0113] Capacity recovery rate after 14 days of storage at 60℃ = C2 / C0*100%.

[0114] The results of the above performance tests are shown in Table 3.

[0115] DCR (DC Impedance) test: At room temperature (23℃±3℃), constant current and constant voltage of 0.5C to 4.45V, cutoff current of 0.02C, then discharge at 0.1C for 9h (adjusted to 10% SOC), then discharge at 0.1C for 10s, record the end voltage V1, discharge at 1C for 1s, and record the end voltage V2.

[0116] DCR calculation formula: DCR=(V1-V2) / (1C-0.1C).

[0117] Thermal shock performance: Under 25℃ ambient conditions, discharge to 3.0V with a given current of 0.2C; rest for 5 minutes; charge to 4.45V with a charging current of 0.2C. When the cell voltage reaches 4.45V, switch to constant voltage charging at 4.45V until the charging current is less than or equal to the given cutoff current of 0.05C; after resting for 1 hour, place the cell in an oven. The oven temperature is increased to 135±2℃ at a rate of 5±2℃ / min and maintained for 30 minutes before stopping. The judgment criterion is that the cell does not catch fire or explode.

[0118] The results of the above tests are as follows:

[0119]

[0120] The test results in the table above show that the composite electrolytes of samples one through seven, with the addition of compounds A, B, C, and D respectively, not only improved cycle life but also enhanced the thermal performance of lithium-ion batteries.

[0121] A comparison of samples 1, 8, and 10 shows that the optimal content of compound I in the second electrolyte is controlled between 3 and 8 wt%. Content above or below this range will affect the cell performance. Within the reasonable range of film-forming additive usage, the solubility of the film-forming additive can be improved by reducing the amount of other components in the second electrolyte. Combined with the repeated action of film-forming additives in the two electrolytes, film formation is achieved through different mechanisms, resulting in a more stable SEI film. This SEI film covers the larger specific surface area required for the high specific surface area of ​​porous carbon, and has better tolerance for the volume expansion of active materials in the later stages of cycling, thereby improving its electrochemical performance.

[0122] Compared to sample eight without the addition of chemical I, the electrolyte with the added film-forming additive can improve thermal stability and enhance its thermal performance. Since compound A contains boron atoms, it can form a complex with lithium salt anions during the reaction process. On the one hand, this stabilizes the cathode material and reduces the dissolution of metal ions. On the other hand, it forms a dense and elastic SEI film, which can cover the larger specific surface area SEI film required by the high specific surface area of ​​porous carbon, thereby improving the cycle performance and interface stability of lithium-ion batteries.

[0123] The above description is merely an 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 principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for injecting a composite electrolyte, characterized in that, The composite electrolyte includes a first electrolyte and a second electrolyte. The first electrolyte includes a first solvent, a first lithium salt, and a first film-forming additive. The second electrolyte includes a second solvent, a second lithium salt, and a second film-forming additive. The second film-forming additive includes at least one selected from lithium difluorophosphate, lithium tetrafluoroborate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinylene sulfate (DTD), lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate (LiODFP), and 1,3-propane sulpholactone; the second film-forming additive also includes compound I, the chemical structural formula of which is... In the formula, R1 is selected from one of halogen atoms, fluorinated C1~C10 alkyl, fluorinated C1~C10 alkoxy, fluorinated C2~C10 alkenyl, fluorinated C2~C10 alkenoxy, fluorinated C2~C10 alkynyl, and fluorinated C2~C10 alkynoxy; R2 is selected from one of fluorinated or unsubstituted C1~C10 alkyl or alkoxy, fluorinated or unsubstituted C2~C10 alkenyl or alkenoxy, fluorinated or unsubstituted C2~C10 alkynyl or alkynoxy, and halogen atoms. The method for injecting the composite electrolyte includes the following steps: S1. The first electrolyte is injected into the casing of the lithium-ion battery. After wetting and formation, the excess first electrolyte in the casing is extracted. S2. The second electrolyte is injected into the casing of the lithium-ion battery, followed by impregnation and formation.

2. The method for injecting the composite electrolyte according to claim 1, characterized in that, The weight X of the first electrolyte and the weight Y of the second electrolyte satisfy the following relationship: 5Y≥X≥3.5Y.

3. The method for injecting the composite electrolyte according to claim 1, characterized in that, The first solvent includes carbonates, carboxylic esters and ethers, and the weight ratio of carbonates, carboxylic esters and ethers satisfies the following relationship: A1:B1:C1=(40~65):(25~45):(0~20), A1≥B1 and B1≥C1; Wherein, A1 is the weight percentage of carbonate in the first solvent, B1 is the weight percentage of carboxylic acid ester in the first solvent, and C1 is the weight percentage of ether in the first solvent.

4. The method for injecting the composite electrolyte according to claim 1, characterized in that, The concentration of the first lithium salt in the composite electrolyte is 0.5~1.5 mol / L, wherein the first lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethanesulfonate, and tris(trifluoromethanesulfonyl)methyllithium.

5. The method for injecting the composite electrolyte according to claim 1, characterized in that, The weight ratio of the first film-forming additive in the first electrolyte satisfies the following relationship: 3M 第一溶剂 ≥M 第一成膜添加剂 ≥0.2 M 第一溶剂 , of which M 第一溶剂 M represents the weight percentage of the first solvent in the first electrolyte. 第一成膜添加剂 The weight percentage of the first film-forming additive in the first electrolyte; wherein the first film-forming additive includes at least one of lithium difluorophosphate, lithium tetrafluoroborate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinyl sulfate (DTD), lithium difluorooxalate borate (LiODFB), lithium difluorobis(oxalate) phosphate (LiODFP), and 1,3-propane sulpholactone.

6. The method for injecting the composite electrolyte according to claim 1, characterized in that, The second solvent includes carbonates, carboxylic esters and ethers, and the weight ratio of carbonates, carboxylic esters and ethers satisfies the following relationship: A2:B2:C2=(40~65):(20~45):(10~20), A2≥B2 and B2≥C2; Wherein, A2 is the weight percentage of carbonate in the second solvent, B2 is the weight percentage of carboxylic acid ester in the second solvent, and C2 is the weight percentage of ether in the second solvent.

7. The method for injecting the composite electrolyte according to claim 1, characterized in that, The concentration of the second lithium salt in the composite electrolyte is 0.5~1.5 mol / L, wherein the second lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethanesulfonate, and tris(trifluoromethanesulfonyl)methyllithium.

8. The method for injecting the composite electrolyte according to claim 1, characterized in that, The weight ratio of the second film-forming additive in the second electrolyte satisfies the following relationship: 8M 第二溶剂 ≥M 第二成膜添加剂 ≥3 M 第二溶剂 , of which M 第二溶剂 M represents the weight percentage of the second solvent in the second electrolyte. 第二成膜添加剂 The weight percentage of the second film-forming additive in the second electrolyte.

9. A lithium-ion battery, characterized in that, The lithium-ion battery contains the composite electrolyte as described in any one of claims 1 to 8.

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