Electrolyte and secondary battery

Abstract

The application provides an electrolyte, wherein a long carbon chain ester compound containing X-O (X=S, P, B) is added as a first additive, and an amine compound with a specific structure is added as a second additive, and the two additives are synergistic, can build a stable interface, remove acidic impurities in the electrolyte, and greatly improve the long cycle performance and electrochemical performance of the battery at normal temperature and high temperature.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to electrolytes and secondary batteries. Background Technology

[0002] With the continuous depletion of fossil fuels, resource and environmental issues have received increasing attention. Significantly increasing the proportion of clean and renewable energy generation is an effective way to address current resource and environmental problems. Developing advanced energy storage and conversion devices is crucial for the efficient utilization of intermittent and highly volatile renewable energy power. As a representative of emerging energy storage devices, electrochemical energy storage devices have become a focus of research in various countries due to their advantages such as high energy density, long cycle life, high output voltage, environmental friendliness, and lack of memory effect.

[0003] Electrolytes, often referred to as the "blood" of a battery, play crucial roles in lithium-ion transport and the formation of a solid electrolyte film at the positive and negative electrode interfaces. However, as battery voltage increases, the compatibility between the electrolyte and electrodes becomes increasingly apparent. At high voltages, electrolytes are more prone to oxidative decomposition. If an effective interfacial film cannot be formed, this oxidative decomposition will continue, consuming a large number of active ions and leading to a continuous decline in battery capacity. Simultaneously, acidic substances in the battery system may induce the dissolution of transition metal ions in the positive electrode material, causing irreversible structural changes and also resulting in rapid capacity decay. Currently, to address the incompatibility issue between electrolytes and electrodes, besides changing the solvent system, using additives is a common and effective method. Additives form a film at the electrode-electrolyte interface, removing acidic substances from the electrolyte and significantly improving the battery's cycle stability. Currently, many film-forming additives are patented. Among them, CATL (Contemporary Amperex Technology Co., Limited) disclosed a patent (CN105958120 A) regarding sulfur-containing additives, emphasizing the synergistic effect of film-forming products containing P=O bonds and S=O bonds to suppress SEI impedance growth. Svolt Energy Technology (Ma'anshan) Co., Ltd. disclosed a patent (CN 115763981 B) for an electrolyte containing phosphite or borate ester compounds with specific structures, the function of which is that these compounds can participate in the formation of films at both positive and negative electrodes. Summary of the Invention

[0004] To address the aforementioned technical problems, this application provides an electrolyte and a secondary battery.

[0005] To achieve the above objectives, this application proposes the following technical solution: In a first aspect, an electrolyte is provided, comprising an organic solvent, an additive, and an electrolyte lithium salt; the additive comprises a first additive and a second additive, wherein the first additive is selected from one or more compounds represented by Formulas 1 to 4, and the second additive is selected from one or more combinations of N,N-dimethyltrifluoroacetamide (DMTFA), N,N-dimethylacrylamide (DMAA), dimethylacetamide (DMAC), and 3-fluoroacetanilide (3-FAA). In equations 1 to 4, R 101 R 102 R 103 R 104 R 108 R 109 R 110 R 111 R 112 R 113 Selected independently from hydrogen and C 1~20 Alkyl groups or their halogenated derivatives, C 2~20 alkenyl or its halogenated derivatives, C 3~20 Alicyclic compounds or their halogenated derivatives, C 6~20 One of the aromatic compounds or their halogenated derivatives; and R 101 and R 102 At least one of them, R 103 and R 104 At least one of them, R 108 R 109 and R 110 At least one of them, R 111 R 112 and R 113 At least one of them is independently selected from C 5~20 Alkyl groups or their halogenated derivatives, C 5~20 alkenyl or its halogenated derivatives, C 5~20 Alicyclic compounds or their halogenated derivatives, C 6~20 One of the aromatic compounds or their halogenated derivatives.

[0006] Furthermore, based on the mass of the electrolyte as 100%, the first additive accounts for 0.01% to 10% of the mass of the electrolyte, preferably 0.1% to 3%.

[0007] Furthermore, based on the mass of the electrolyte as 100%, the second additive accounts for 0.01% to 5% of the mass of the electrolyte, preferably 0.1% to 2%.

[0008] Further, the organic solvent includes one or more of ethylene carbonate, propylene carbonate, butene carbonate, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, tetrahydrofuran, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0009] Furthermore, the electrolyte lithium salt is selected from one or more combinations of lithium hexafluorophosphate, sodium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, sodium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate)borate, lithium di(oxalate)borate, lithium bis(fluorosulfonyl)imide, sodium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium perchlorate, and sodium trifluoromethanesulfonate.

[0010] Further, the concentration of the electrolyte lithium salt in the electrolyte is 0.5M~2.0M; preferably, the concentration of the electrolyte lithium salt is 0.8M~1.5M.

[0011] Secondly, a secondary battery can also be provided, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the aforementioned electrolyte.

[0012] Furthermore, the positive electrode active material in the positive electrode sheet is selected from one or more combinations of lithium-containing metal oxide positive electrode materials, sodium metal oxide positive electrode materials, polyanionic positive electrode materials, and Prussian blue positive electrode materials. Preferably, the positive electrode active material in the positive electrode sheet comprises at least one of high-voltage lithium cobalt oxide and medium-nickel high-voltage materials.

[0013] Furthermore, the negative electrode active material in the negative electrode sheet is selected from lithium metal, sodium metal, natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, silicon-carbon composites, alloys and alloy-like oxide materials, spinel-structured lithiated TiO2 or Li4Ti5O 12 One or more of Li-Al alloys.

[0014] Furthermore, the separator is selected from any one of polyurethane, polyethylene, polypropylene, polytetrafluoroethylene, polyimide, and separators including those coated with ceramic components.

[0015] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: By adding long-chain ester compounds containing XO (X=S, P) as the first additive and amine compounds with specific structures as the second additive to the electrolyte, the two work synergistically to build a stable interface, remove acidic impurities in the electrolyte, and greatly improve the long-cycle performance and electrochemical performance of the battery at room temperature and high temperature. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 These are time-of-flight secondary ion mass spectra of the positive electrode surfaces disassembled after formation of the lithium-ion batteries assembled in Example 1 and Comparative Example 1, where (a) represents C. - Content comparison, (b) is CoO2 - Content comparison, (c) is S - Content comparison.

[0018] Figure 2 These are time-of-flight secondary ion mass spectra of the negative electrode surfaces after formation of the lithium-ion batteries assembled in Example 1 and Comparative Example 1, where (a) represents C. - Content comparison, (b) is S - Content comparison. Detailed Implementation

[0019] Based on the issues of stability and incompatibility between the electrolyte and electrode interface, the applicant has discovered through extensive research that by adding a long-chain ester compound containing XO (X=S, P) as a first additive and an amine compound with a specific structure as a second additive to the electrolyte, the two work synergistically to build a stable interface, remove acidic impurities in the electrolyte, and greatly improve the long-cycle performance and electrochemical performance of the battery at room temperature and high temperature. Based on this, the present invention was completed.

[0020] This invention provides an electrolyte comprising an organic solvent, an additive, and an electrolyte lithium salt; the additive comprises a first additive and a second additive, wherein the first additive is selected from one or more compounds represented by formulas 1 to 4, and the second additive is selected from one or more combinations of N,N-dimethyltrifluoroacetamide (DMTFA), N,N-dimethylacrylamide (DMAA), dimethylacetamide (DMAC), and 3-fluoroacetanilide (3-FAA). In equations 1 to 4, R 101 R 102 R 103 R 104 R 108 R 109 R 110 R 111 R 112 R 113 Selected independently from hydrogen and C 1~20 Alkyl groups or their halogenated derivatives, C 2~20 alkenyl or its halogenated derivatives, C 3~20 Alicyclic compounds or their halogenated derivatives, C 6~20 One of the aromatic compounds or their halogenated derivatives; and R 101 and R 102 At least one of them, R 103 and R 104 At least one of them, R 108 R 109 and R 110 At least one of them, R 111 R 112 and R 113 At least one of them is independently selected from C 5~20 Alkyl groups or their halogenated derivatives, C 5~20 alkenyl or its halogenated derivatives, C 5~20 Alicyclic compounds or their halogenated derivatives, C 6~20 One of the aromatic compounds or their halogenated derivatives.

[0021] In the aforementioned electrolyte, the applicant discovered through research that adding a long-chain ester compound containing XO (X=S, P) as the first additive and adding an amine compound with a specific structure as the second additive can improve the electrolyte quality. The first additive participates in the formation of the positive and negative electrode interface film during the redox process, constructing a long carbon chain interface film in situ, effectively improving the toughness and stability of the electrode / electrolyte interface film. However, it also generates acidic byproducts, which can adversely affect interface stability and the battery. The addition of amine compounds with specific structures can effectively remove acidic substances in the electrolyte, reduce the decomposition of lithium salts, solvents, etc., and react with the acidic byproducts generated by the first additive during film formation, blocking the subsequent reactions of the acidic byproducts. This is beneficial to improving the interface stability between the positive and negative electrodes and the electrolyte, constructing a stable passivation layer between the electrode / electrolyte interface, suppressing battery gas generation and its impact on electrode materials and electrolyte, and compensating for the shortcomings of long carbon chain ester compounds. Furthermore, the simultaneous addition of the second and first additives can produce an effective synergistic effect, constructing a stable interface layer between the electrode / electrolyte interface, and greatly improving the battery's long-cycle performance, storage performance, and electrochemical performance at both room temperature and high temperature.

[0022] In the electrolyte described in some embodiments of the present invention, specifically, the first additive is selected from one or more combinations of dipentyl sulfite, diheptyl sulfite, dipentyl sulfate, diheptyl sulfate, tripentyl borate, tri-n-hexyl borate, trioctyl borate, tri-n-decyl borate, tri-o-tolyl borate, tri-neopentyl phosphite, tri-n-hexyl phosphite, triphenyl phosphite, trioctyl phosphate, triphenyl phosphate, and diphenyl toluene phosphate; the second additive is selected from one or more combinations of N,N-dimethyltrifluoroacetamide (DMTFA), N,N-dimethylacrylamide (DMAA), dimethylacetamide (DMAC), and 3-fluoroacetanilide (3-FAA).

[0023] In some preferred embodiments, based on the mass of the electrolyte (100%), the first additive accounts for 0.01% to 10% of the mass of the electrolyte, preferably 0.1% to 3%, for example 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, etc.

[0024] In some preferred embodiments, based on the mass of the electrolyte (100%), the second additive accounts for 0.01% to 5% of the mass of the electrolyte, preferably 0.1% to 2%, for example 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, etc.

[0025] In some embodiments, the organic solvent includes one or more of ethylene carbonate, propylene carbonate, butene carbonate, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, tetrahydrofuran, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0026] In some embodiments, the solvent comprises 5% to 40% ethylene carbonate (EC), 5% to 30% propylene carbonate (PC), 10% to 40% diethyl carbonate (DEC), and 10% to 50% propyl propionate (PP) by weight of 100%.

[0027] In some embodiments, the electrolyte lithium salt is selected from one or more combinations of lithium hexafluorophosphate, sodium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, sodium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate)borate, lithium di(oxalate)borate, lithium bis(fluorosulfonyl)imide, sodium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium perchlorate, and sodium trifluoromethanesulfonate.

[0028] In some preferred embodiments, the concentration of the electrolyte lithium salt in the electrolyte is 0.5M to 2.0M; preferably, the concentration of the electrolyte lithium salt is 0.8M to 1.5M, such as 0.8M, 0.9M, 1.0M, 1.1M, 1.2M, 1.3M, 1.4M, 1.5M, etc.

[0029] One embodiment provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the aforementioned electrolyte.

[0030] The positive electrode active material in the positive electrode sheet can be selected from one or more combinations of lithium metal oxide positive electrode materials (such as layered lithium nickel oxide, layered lithium cobalt oxide, layered lithium manganese oxide, layered ternary or multi-element positive electrode materials, spinel-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, etc.), sodium metal oxide positive electrode materials, polyanionic positive electrode materials, and Prussian blue positive electrode materials. Preferably, the positive electrode active material in the positive electrode sheet is a high-voltage positive electrode material, including high-voltage lithium cobalt oxide, medium-nickel high-voltage materials, and other high-voltage materials.

[0031] The negative electrode active material in the negative electrode sheet can be selected from lithium metal, sodium metal, natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, silicon-carbon composites, alloys and alloy oxide materials, spinel-structured lithiated TiO2 or Li4Ti5O 12 One or more of Li-Al alloys.

[0032] The separator is selected from any one of polyurethane, polyethylene, polypropylene, polytetrafluoroethylene, polyimide, and separators including ceramic component coating; to ensure the heat resistance and mechanical strength of the separator, a separator including ceramic component coating can also be used, which can have a single-layer or multi-layer structure, and preferably the substrate of the ceramic component coated separator is polyethylene.

[0033] In the preparation of positive electrode sheets, negative electrode sheets, separators, etc., the types and amounts of binders, conductive agents, dispersants and stabilizers used can be conventionally selected according to actual needs.

[0034] To facilitate understanding of the present invention, the invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0035] Example 1: (1) Preparation of the positive electrode: The positive electrode active material LiCoO2, the conductive agent carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 96:2:2 in an appropriate amount of N-methylpyrrolidone (NMP) solvent (following conventional process control) to form a uniform positive electrode slurry. This positive electrode slurry is then coated onto the positive electrode current collector Al foil, and after drying and rolling, a positive electrode sheet is obtained.

[0036] (2) Preparation of negative electrode: The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC) are mixed in an appropriate amount of deionized water solvent (conducted according to conventional process control) at a weight ratio of 97:2:1 to form a uniform negative electrode slurry. This negative electrode slurry is then coated onto the negative electrode current collector Cu foil, and after drying and rolling, a negative electrode sheet is obtained.

[0037] (3) Separating membrane: PE porous polymer film is used as the separating membrane.

[0038] (4) Preparation of electrolyte: In a dry argon-atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) were mixed in a mass ratio of EC:PC:DEC:PP = 20:15:30:25. Additives were then added, dissolved, and thoroughly stirred before adding lithium salt LiPF6. The mixture was then thoroughly mixed to obtain the electrolyte. The concentration of LiPF6 was 1.2 mol / L. The additives included the first and second additives described above. The first additive was dipentyl sulfate (DAS), accounting for 1 wt.% of the total electrolyte mass; the second additive was N,N-dimethyltrifluoroacetamide (DMTFA), accounting for 1 wt.% of the total electrolyte mass. It is worth noting that ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) can all be mixed in a mass ratio of EC:PC:DEC:PP of 5-40:5-30:10-40:10-50 to serve as a solvent for the electrolyte. The concentration of LiPF6 in the electrolyte can be 0.5M to 2.0M.

[0039] (5) Preparation of lithium-ion batteries: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. After welding the tabs, the bare cell is placed in an outer aluminum-plastic film. The prepared electrolyte is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and capacity testing, a soft-pack lithium-ion battery is obtained.

[0040] It is worth noting that the preparation steps of the positive electrode, negative electrode, separator, electrolyte, and lithium-ion battery in the subsequent embodiments are the same as in Embodiment 1.

[0041] Example 2: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 2 is different, including 0.5 wt.% DAS and 1 wt.% DMTFA by the total mass of the electrolyte.

[0042] Example 3: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 3 is different, including 2 wt.% DAS and 1 wt.% DMTFA by the total mass of the electrolyte.

[0043] Example 4: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 4 is different, including 1 wt.% DAS and 2 wt.% DMTFA by the total mass of the electrolyte.

[0044] Example 5: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 5 is different, including 1 wt.% DAS and 3 wt.% DMTFA by the total mass of the electrolyte.

[0045] Example 6: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 6 is different, including 0.5 wt.% of tri-n-hexyl phosphite (THPi) and 0.5 wt.% of 3-fluoroacetanilide (3-FAA) by the total mass of the electrolyte.

[0046] Example 7: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 7 is different, including 1 wt.% THPi and 0.5 wt.% 3-FAA by the total mass of the electrolyte.

[0047] Example 8: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 8 is different, including 2 wt.% THPi and 0.5 wt.% 3-FAA by the total mass of the electrolyte.

[0048] Example 9: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 9 is different, including 0.5 wt.% THPi and 1 wt.% 3-FAA by the total mass of the electrolyte.

[0049] Example 10: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Example 10 is different. The composition is different, including 0.5 wt.% THPi and 2 wt.% 3-FAA by the total mass of the electrolyte.

[0050] Comparative Example 1: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Comparative Example 1 is different, including 0 wt.% DAS and 0 wt.% DMTFA by the total mass of the electrolyte.

[0051] Comparative Example 2: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Comparative Example 2 is different, including 1 wt.% DAS and 0 wt.% DMTFA by the total mass of the electrolyte.

[0052] Comparative Example 3: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Comparative Example 3 is different, including 0 wt.% DAS and 1 wt.% DMTFA by the total mass of the electrolyte.

[0053] Comparative Example 4: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Comparative Example 4 is different, including 0.5 wt.% THPi and 0 wt.% 3-FAA by the total mass of the electrolyte.

[0054] Comparative Example 5: The composition of the electrolyte is basically the same as that of Example 1. The difference is that the composition of the additives used in the electrolyte of Comparative Example 5 is different, including 0 wt.% THPi and 0.5 wt.% 3-FAA by the total mass of the electrolyte.

[0055] Table 1. Additive composition in the electrolytes of Comparative Examples 1-3 and Examples 1-5 Performance evaluation: growth rate of HF content in electrolyte during high-temperature storage, growth rate of thickness during high-temperature storage, capacity retention rate and capacity recovery rate, and cycle capacity retention rate.

[0056] (1) High-temperature storage test of lithium-ion battery electrolyte The electrolytes prepared in different comparative examples and embodiments were stored at 45°C in an argon inert atmosphere for 10 days. Before storage, a suitable amount of electrolyte was taken and the HF content was tested and recorded as C0. After high-temperature storage, a suitable amount of electrolyte was taken and the HF content was tested and recorded as C1.

[0057] The growth rate of HF content in the electrolyte = [C1 / C0] × 100%. The test results of each example and comparative example are shown in Table 2.

[0058] Table 2. High-temperature storage test results of electrolytes from Comparative Examples 1-5 and Examples 1-10 Table 2 shows that, comparing the high-temperature storage test results of the electrolytes in Examples 1-3, the amount of HF produced after high-temperature storage is: Example 2 < Example 1 < Example 3. Based on the composition, it can be seen that, with the same amount of the second additive, the HF content growth rate increases with the increase of the amount of the first additive. Comparing the high-temperature storage test results of the electrolytes in Examples 1, 4, and 5, the amount of HF produced after high-temperature storage is: Example 1 > Example 4 > Example 5. Based on the composition, it can be seen that, with the same amount of the first additive, the HF content growth rate decreases with the increase of the amount of the second additive. Comparing the high-temperature storage test results of the electrolytes in Examples 1, 1, 2, and 3, the amount of HF produced after high-temperature storage is: Comparative Example 3 < Example 1 < Comparative Example 1 < Comparative Example 2, indicating that the addition of the first additive DAS increases the HF content growth rate, while the addition of the second additive DMTAF decreases the HF content growth rate. Based on the above speculation, it can be inferred that this may be because DAS may generate acidic substances during high-temperature storage, and DMTFA can reduce the content of acidic substances generated by DAS.

[0059] As shown in Table 2, comparing the high-temperature storage test results of the electrolytes in Examples 6-8, it can be seen that the amount of HF produced after high-temperature storage is Example 8 > Example 7 > Example 6. Based on the composition, it can be seen that, with the same amount of the second additive, the HF content growth rate increases with the increase of the amount of the first additive. Comparing the high-temperature storage test results of the electrolytes in Examples 6, 9, and 10, it can be seen that the amount of HF produced after high-temperature storage is Example 10 < Example 9 < Example 6. Based on the composition, it can be seen that, with the same amount of the first additive, the HF content growth rate decreases with the increase of the amount of the second additive. Comparing the high-temperature storage test results of the electrolytes in Examples 6, 4, 5, and 1, it can be seen that the amount of HF produced after high-temperature storage is Comparative Example 5 < Example 6 < Comparative Example 1 < Comparative Example 4. Based on the composition, it can be seen that the addition of the second additive significantly reduces the HF content growth rate, while the addition of the first additive increases the HF content growth rate.

[0060] Comparison reveals that DMTFA and 3-FAA effectively inhibit the formation of acidic substances, while DAS and THPi may generate acidic substances during high-temperature storage. Furthermore, DMTFA and 3-FAA effectively inhibit the formation of acidic substances from DAS and THPi in the electrolyte. It can be deduced that the second additive, amine compounds, compensates for the shortcomings of the first additive, long-chain ester compounds, in application.

[0061] (2) Thickness test of lithium-ion battery at high temperature: At 25℃, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C. The thickness of the lithium-ion battery at this point was measured and recorded as h0. The lithium-ion battery was then placed in a constant temperature chamber at 60℃ and stored for 30 days. After storage, the thickness of the lithium-ion battery at this point was measured and recorded as h1. Fifteen lithium-ion batteries were tested in each group, and the average value was taken.

[0062] Thickness expansion rate (%) of lithium-ion battery after storage at 60℃ for 30 days = [(h1-h0) / h0]×100%.

[0063] (3) High-temperature storage capacity retention test of lithium-ion batteries At 25℃, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C. It was then discharged to 3.0V at a constant current of 0.5C. The discharge capacity of the lithium-ion battery at this point was recorded as D0. Subsequently, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C. The lithium-ion battery was then stored at 60℃ for 30 days. After storage, it was left to stand at 25℃ for 4 hours and then discharged to 3.0V at a constant current of 0.5C. The discharge capacity of the lithium-ion battery at this point was recorded as D1. Fifteen lithium-ion batteries were tested in each group, and the average value was taken.

[0064] Capacity retention rate (%) of lithium-ion batteries stored at 60℃ for 30 days = [D1 / D0] × 100%.

[0065] (4) High-temperature storage capacity recovery rate test of lithium-ion batteries At 25℃, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C. It was then discharged to 3.0V at a constant current of 0.5C. The discharge capacity of the lithium-ion battery at this point was recorded as D0. Subsequently, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C. The lithium-ion battery was then stored at 60℃ for 30 days. After storage, it was left to stand at 25℃ for 4 hours and discharged to 3.0V at a constant current of 0.5C. This cycle was repeated 3 times. The discharge capacity of the lithium-ion battery at this point was measured and recorded as D2. Fifteen lithium-ion batteries were tested in each group, and the average value was taken.

[0066] The capacity recovery rate (%) of a lithium-ion battery stored at 60°C for 30 days is calculated as [D2 / D0] × 100%.

[0067] (5) Cycle life test of lithium-ion batteries At 25℃, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C. It was then discharged to 3.0V at a constant current of 0.5C. The discharge capacity of the lithium-ion battery at this point was measured and recorded as D0. Subsequently, the lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C, and discharged to 3.0V at a constant current of 0.5C. This cycle was repeated 299 times, and the discharge capacity at this point was recorded as D3. Fifteen lithium-ion batteries were tested in each group, and the average value was taken.

[0068] Lithium-ion battery remaining capacity retention rate = [D3 / D0] × 100%.

[0069] The performance test results of the lithium-ion batteries assembled in each embodiment and comparative example are shown in Table 3.

[0070] Table 3 Performance test results of lithium-ion batteries assembled with electrolytes from Comparative Examples 1-5 and Examples 1-10 As shown in Table 3, the performance test results indicate that using the first additive or the second additive alone improves the high-temperature performance of the battery cell, but the improvement is limited. However, when both the first and second additives are used simultaneously, the high-temperature performance and cycle life of the battery cell are significantly improved. Therefore, it can be concluded that the first and second additives have a synergistic effect on improving battery performance.

[0071] (6) Detection of positive and negative electrode interfaces of lithium-ion batteries The lithium-ion batteries of Comparative Example 1 and Example 1 were formed as follows: at 25°C, the lithium-ion batteries were charged to 4.5V at a constant current and constant voltage of 0.1C, with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 0.1C, for 3 cycles. Subsequently, the lithium-ion batteries were disassembled, and their positive and negative electrode plates were taken separately. The interface composition was detected by time-of-flight secondary ion mass spectrometry in negative ion mode.

[0072] Figure 1 C on the cathode surface after formation in Comparative Example 1 and Example 1 was detected by time-of-flight secondary ion mass spectrometry. - CoO2 - S - The content comparison chart shows that the CoO2 content on the positive electrode surface of Comparative Example 1 and Example 1 is significantly higher. - The content of these components is relatively equal, indicating that the ratio of the interfacial film to the cathode material is essentially the same. In Example 1, S was detectable on the cathode surface after formation. - and higher C content - Analysis suggests that this may be due to the long-chain ester compound DAS participating in interfacial film formation, and the long-chain compound did indeed enter the interfacial film, altering the composition of the interface.

[0073] Figure 2 C on the negative electrode surface after formation in Comparative Example 1 and Example 1 was detected by time-of-flight secondary ion mass spectrometry. - S - A comparison chart of their contents. Similarly, S was detected on the surface of the positive electrode after formation in Example 1. - and higher C content - This further demonstrates that DAS participates in interfacial film formation, and that long carbon chains do indeed enter the interfacial film, altering the composition of the interface.

[0074] Combination Figure 1 and Figure 2This fully demonstrates that the first additive plays a role in the formation of the positive and negative electrode interface film in the secondary battery, thus, the first additive is beneficial to the formation of the interface film. Combined with the aforementioned performance tests and analysis, it can be inferred that the addition of the second additive helps reduce the acidic substances generated by the first additive, thereby improving the stability of the positive and negative electrode interface films. Therefore, the second additive can synergistically enhance the performance of the lithium-ion battery with the first additive.

[0075] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that, The additives include organic solvents, additives, and electrolyte lithium salts; the additives include a first additive and a second additive, wherein the first additive is selected from one or more combinations of dipentyl sulfite, diheptyl sulfite, dipentyl sulfate, diheptyl sulfate, tripentyl phosphite, tri-n-hexyl phosphite, triphenyl phosphite, trioctyl phosphate, triphenyl phosphate, and toluene diphenyl phosphate, and the second additive is selected from one or more combinations of N,N-dimethyltrifluoroacetamide (DMTFA), N,N-dimethylacrylamide (DMAA), dimethylacetamide (DMAC), and 3-fluoroacetanilide (3-FAA).

2. The electrolyte according to claim 1, characterized in that, Based on the mass of the electrolyte (100%), the first additive accounts for 0.01% to 10% of the mass of the electrolyte.

3. The electrolyte as described in claim 2, characterized in that, Based on the mass of the electrolyte (100%), the first additive accounts for 0.1% to 3% of the mass of the electrolyte.

4. The electrolyte according to claim 1, characterized in that, Based on the mass of the electrolyte (100%), the second additive accounts for 0.01% to 5% of the mass of the electrolyte.

5. The electrolyte according to claim 4, characterized in that, Based on the mass of the electrolyte (100%), the second additive accounts for 0.1% to 2% of the mass of the electrolyte.

6. The electrolyte according to claim 1, characterized in that, The organic solvent includes one or more of the following: ethylene carbonate, propylene carbonate, butene carbonate, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, tetrahydrofuran, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

7. The electrolyte according to claim 1, characterized in that, The electrolyte lithium salt is selected from one or more combinations of lithium hexafluorophosphate, sodium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, sodium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate)borate, lithium di(oxalate)borate, lithium bis(fluorosulfonyl)imide, sodium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium perchlorate, and sodium trifluoromethanesulfonate.

8. The electrolyte according to any one of claims 1 to 7, characterized in that, The concentration of the electrolyte lithium salt in the electrolyte is 0.5M~2.0M.

9. The electrolyte as described in claim 8, characterized in that, The concentration of the electrolyte lithium salt in the electrolyte is 0.8M~1.5M.

10. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is any one of claims 1 to 9.

11. The secondary battery as described in claim 10, characterized in that, The positive electrode active material in the positive electrode sheet is selected from one or more combinations of lithium metal oxide positive electrode materials, sodium metal oxide positive electrode materials, polyanionic positive electrode materials, and Prussian blue positive electrode materials.

12. The secondary battery as described in claim 11, characterized in that, The positive electrode active material in the positive electrode sheet includes at least one of high-voltage lithium cobalt oxide and medium-nickel high-voltage material.

13. The secondary battery as described in claim 10, characterized in that, The negative electrode active material in the negative electrode sheet is selected from lithium metal, sodium metal, natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, silicon-carbon composites, alloys and alloy oxide materials, spinel-structured lithiated TiO2 or Li4Ti5O 12 One or more of Li-Al alloys.

14. The secondary battery according to claim 10, characterized in that, The separator is selected from any one of polyurethane, polyethylene, polypropylene, polytetrafluoroethylene, polyimide, and separators coated with ceramic components.

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