Electrolyte and secondary battery

Abstract

According to the electrolyte provided by the invention, the long-carbon-chain ester compound containing X-O (X = S, P or B) is added into the electrolyte as the first additive, the amine compound with a specific structure is added as the second additive, and the first additive and the amine compound cooperate with each other, so that a stable interface can be constructed, acidic impurities in the electrolyte are removed, and the service life of the electrolyte is prolonged. And the long cycle performance and the electrochemical performance of the battery at normal temperature and high temperature are greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to an electrolyte and a secondary battery. Background Art

[0002] With the continuous consumption of fossil energy, resource and environmental issues have received more and more attention. Substantially increasing the proportion of clean, renewable new energy power generation is an effective way to solve the current resource and environmental problems. In order to efficiently utilize intermittent and highly volatile new energy electricity, the development of advanced energy storage and conversion devices is the key. As a representative of emerging energy storage devices, electrochemical energy storage devices have become the focus of research in various countries due to their high energy density, long cycle life, high output voltage, green environmental protection, and no memory effect.

[0003] The electrolyte is known as the "blood" of the battery. It carries important functions such as lithium ion transmission in the battery and forming a solid electrolyte film at the interface of the positive and negative electrodes. However, as the battery voltage increases, the compatibility of the electrolyte and the electrode gradually becomes prominent. Under high voltage, the electrolyte is more likely to undergo oxidative decomposition. If an effective interface film cannot be formed, this oxidative decomposition will continue to occur. This process consumes a large amount of active ions, resulting in a continuous decay of the battery capacity. At the same time, the acidic substances present in the battery system may induce the dissolution of transition metal ions in the positive electrode material, causing the structure of the positive electrode material to undergo irreversible changes, which will also lead to a rapid decay of its capacity. At present, in order to solve the problem of incompatibility between the electrolyte and the electrode, in addition to replacing the solvent system, the use of additives is a common and effective means. Additives can greatly improve the cycle stability of the battery by forming a film at the interface between the electrode and the electrolyte and removing the acidic substances in the electrolyte. At present, a large number of film-forming additives have applied for patent protection. Among them, Contemporary Amperex Technology Co., Ltd. disclosed a patent on sulfur-containing additives (CN105958120 A), emphasizing that the film-forming products containing P=O bond compounds and S=O bond compounds coordinate with each other to inhibit the growth of SEI impedance. Honeycomb Energy Technology (Ma'anshan) Co., Ltd. disclosed a patent for an electrolyte containing phosphite or borate compounds with a specific structure (CN 115763981 B), the role of which is that such compounds can participate in the film formation of positive and negative electrodes. Summary of the invention

[0004] In response to the above technical problems, the present application provides an electrolyte and a secondary battery.

[0005] To achieve the above objectives, this application proposes the following technical solutions: 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 of the compounds represented by Formula 1 to Formula 5, and the second additive is selected from one or more of the compounds represented by Formula 6 to Formula 8, In formula 1 to formula 5, R 101 , R 102 , R 103 , R 104 , R 105 , R 106 , R 107 , R 108 , R 109 , R 110 , R 111 , R 112 , R 113 are independently selected from hydrogen, C 1~20 Alkyl or its halide, C 2~20 Alkenyl or its halide, C 3~20 Alicyclic compounds or their halogenated derivatives, C 6~20 An aromatic compound or a halogenated compound thereof; and R 101 and R 102 At least one of R 103 and R 104 At least one of R 105 , R 106 and R 107 At least one of R 108 , R 109 and R 110 At least one of R 111 , R 112 and R 113 At least one of which is independently selected from C 5~20 Alkyl or its halide, C 5~20 Alkenyl or its halide, C 5~20 Alicyclic compounds or their halogenated derivatives, C 6~20 An aromatic compound or one of its halides; In formula 6, R 201 , R 202 , R 203 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 201 , R 202and R 203 Any two of the are connected to form a 3- to 10-membered ring; In formula 7, R 204 , R 205 , R 206 , R 207 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 204 , R 205 , R 206 and R 207 Any two of the are connected to form a 3- to 10-membered ring; In formula 8, R 208 , R 209 , R 210 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 208 , R 209 and R 210 Any two of the following are connected to form a 3- to 10-membered ring.

[0006] Furthermore, based on the mass of the electrolyte being 100%, the mass percentage of the first additive in the electrolyte is 0.01% to 10%, preferably 0.1% to 3%.

[0007] Furthermore, based on the mass of the electrolyte being 100%, the mass percentage of the second additive in the electrolyte is 0.01% to 5%, preferably 0.1% to 2%.

[0008] Further, the organic solvent includes one or more of ethylene carbonate, propylene carbonate, butylene carbonate, ethyl methyl 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] Further, 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 bisoxalatoborate, lithium difluorooxalatoborate, lithium bisfluorosulfonyl imide, sodium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, sodium bistrifluoromethanesulfonyl imide, sodium perchlorate and sodium trifluoromethanesulfonate.

[0010] Furthermore, 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] In a second aspect, a secondary battery may be provided, comprising a positive electrode sheet, a negative electrode sheet, 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 a combination of one or more of lithium-containing metal oxide positive electrode materials, sodium metal oxide positive electrode materials, polyanion 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 material.

[0013] Furthermore, the negative electrode active material in the negative electrode sheet is selected from metallic lithium, metallic sodium, 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 , a combination of one or more of Li-Al alloys.

[0014] Furthermore, the diaphragm is selected from any one of polyurethane, polyethylene, polypropylene, polytetrafluoroethylene, polyimide, and diaphragms 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 a long carbon-chain ester compound containing XO (X=S, P, B) as the first additive to the electrolyte and an amine compound with a specific structure as the second additive, the two work together 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 and high temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 The time-of-flight secondary ion mass spectra of the positive electrode surface disassembled after the lithium-ion battery assembled in Example 1 and Comparative Example 1 is shown in FIG. 1 , where (a) is C - Content comparison, (b) is CoO2 - Content comparison, (c) is S - Content content comparison.

[0018] Figure 2 The time-of-flight secondary ion mass spectra of the negative electrode surface disassembled after the lithium-ion battery assembled in Example 1 and Comparative Example 1 is shown in FIG. 1 , where (a) is C - Content comparison, (b) is S - Content comparison. DETAILED DESCRIPTION

[0019] Based on the problems of stability and incompatibility between the electrolyte and the electrode interface, the applicant has discovered through extensive research that by adding a long carbon chain ester compound containing XO (X=S, P, B) as the first additive to the electrolyte and an amine compound of a specific structure as the second additive, the two can work together 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 has been completed.

[0020] The present 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 of the compounds represented by Formula 1 to Formula 5, and the second additive is selected from one or more of the compounds represented by Formula 6 to Formula 8. In formula 1 to formula 5, R 101 , R 102 , R 103 , R 104 , R 105 , R 106 , R 107 , R 108 , R 109 , R 110 , R 111 , R 112, R 113 are independently selected from hydrogen, C 1~20 Alkyl or its halide, C 2~20 Alkenyl or its halide, C 3~20 Alicyclic compounds or their halogenated derivatives, C 6~20 An aromatic compound or a halogenated compound thereof; and R 101 and R 102 At least one of R 103 and R 104 At least one of R 105 , R 106 and R 107 At least one of R 108 , R 109 and R 110 At least one of R 111 , R 112 and R 113 At least one of which is independently selected from C 5~20 Alkyl or its halide, C 5~20 Alkenyl or its halide, C 5~20 Alicyclic compounds or their halogenated derivatives, C 6~20 An aromatic compound or one of its halides; In formula 6, R 201 , R 202 , R 203 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 201 , R 202 and R 203 Any two of the groups are connected to form a 3- to 10-membered ring. If there are any remaining groups among these three groups, the remaining groups are selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or one of its halides; In formula 7, R 204 , R 205 , R 206 , R 207 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 204 , R205 , R 206 and R 207 Any two of the groups are connected to form a 3- to 10-membered ring. If there are any remaining groups among these three groups, the remaining groups are selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or one of its halides; In formula 8, R 208 , R 209 , R 210 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 208 , R 209 and R 210 Any two of the groups are connected to form a 3- to 10-membered ring. If there are any remaining groups among these three groups, the remaining groups are selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or one of its halides.

[0021] In the above electrolyte, the applicant has found through research that a long carbon chain ester compound containing XO (X=S, P, B) is added as the first additive, and an amine compound with a specific structure is added as the second additive. The first additive participates in the formation of the positive and negative electrode interface films during the redox process, constructs a long carbon chain interface film in situ, and effectively improves the toughness and stability of the electrode / electrolyte interface film, but also produces acidic by-products, which have an adverse effect on the 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., react with the acidic by-products produced by the first additive during the film formation process, and block the subsequent reaction of the acidic by-products, which is beneficial to improve the interface stability between the positive and negative electrodes and the electrolyte, and construct a stable passivation layer at the electrode / electrolyte interface, inhibit battery gas production and the impact on electrode materials and electrolytes, and make up for the shortcomings of long carbon chain ester compounds; and the simultaneous addition of the second additive and the first additive can produce an effective synergistic effect, construct a stable interface layer at the electrode / electrolyte interface, and greatly improve the long cycle performance, storage performance and electrochemical performance of the battery at room temperature and high temperature.

[0022] In the electrolyte described in some embodiments of the present invention, specifically, the first additive is selected from a combination of one or more of diamyl sulfite, diheptyl sulfite, diamyl sulfate, diheptyl sulfate, tripentyl borate, tri-n-hexyl borate, trioctyl borate, (tri-n-decyl) borate, tri-o-tolyl borate, trineopentyl phosphite, tri-n-hexyl phosphite, triphenyl phosphite, trioctyl phosphate, triphenyl phosphate, and diphenyl phosphate; the second additive is selected from a combination of one or more 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 as 100%, the mass percentage of the first additive in the electrolyte is 0.01% to 10%, 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 as 100%, the mass percentage of the second additive in the electrolyte is 0.01% to 5%, 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, butylene carbonate, ethyl methyl 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, based on 100% by mass of the solvent, the solvent comprises 5% to 40% of ethylene carbonate (EC), 5% to 30% of propylene carbonate (PC), 10% to 40% of diethyl carbonate (DEC) and 10% to 50% of propyl propionate (PP).

[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 bisoxalatoborate, lithium difluorooxalatoborate, lithium bisfluorosulfonyl imide, sodium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, sodium bistrifluoromethanesulfonyl imide, sodium perchlorate and sodium trifluoromethanesulfonate.

[0028] In some preferred embodiments, 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, for example, 0.8M, 0.9M, 1.0M, 1.1M, 1.2M, 1.3M, 1.4M, 1.5M, etc.

[0029] Some embodiments provide a secondary battery, including a positive electrode sheet, a negative electrode sheet, 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 a combination of one or more 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 lithium manganese oxide, spinel lithium nickel manganese oxide, etc.), sodium metal oxide positive electrode materials, polyanion 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 metallic lithium, metallic sodium, 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 , a combination of one or more of Li-Al alloys.

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

[0033] In the preparation process 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] In order to facilitate the understanding of the present invention, the present invention will be described in more comprehensive and detailed manner below in conjunction with the accompanying drawings and preferred embodiments of the specification, but the protection scope of the present invention is not limited to the following specific embodiments.

[0035] Embodiment 1: (1) Preparation of positive electrode sheet: The positive electrode active material LiCoO2, the conductive agent carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in a proper amount of N-methylpyrrolidone (NMP) solvent (controlled according to conventional processes) in a weight ratio of 96:2:2 to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector Al foil, and the positive electrode sheet is obtained after drying and rolling.

[0036] (2) Preparation of negative electrode sheet: The negative electrode active material graphite, the binder styrene butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC) are fully stirred and mixed in a proper amount of deionized water solvent (controlled according to conventional processes) at a weight ratio of 97:2:1 to form a uniform negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector Cu foil, and the negative electrode sheet is obtained after drying and rolling.

[0037] (3) Isolation membrane: PE porous polymer film is used as the isolation 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) are mixed in a mass ratio of EC: PC: DEC: PP = 20: 15: 30: 25, and then additives are added, dissolved and stirred thoroughly, and then lithium salt LiPF6 is added, and the electrolyte is obtained after mixing evenly. Among them, the concentration of LiPF6 is 1.2 mol / L. The additives include the first additive and the second additive described above. Among them, the first additive is diamyl sulfate (DAS), accounting for 1 wt.% of the total mass of the electrolyte; the second additive is N, N-dimethyltrifluoroacetamide (DMTFA), accounting for 1 wt.% of the total mass of the electrolyte. It is worth mentioning that ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) mixed in a mass ratio of EC:PC:DEC:PP of 5-40:5-30:10-40:10-50 can all be used as solvents for the electrolyte, and the concentration of LiPF6 in the electrolyte can be 0.5M~2.0M.

[0039] (5) Preparation of lithium-ion batteries: The positive electrode sheet, the isolation film, and the negative electrode sheet are stacked in order, so that the isolation film is placed between the positive and negative electrode sheets to play an isolating role, and then they are wound to obtain a bare battery cell; after welding the pole ears, the bare battery cell is placed in an outer packaging aluminum-plastic film, and the prepared electrolyte is injected into the dried bare battery cell. After vacuum packaging, standing, formation, shaping, capacity testing and other processes, a soft-pack lithium-ion battery is obtained.

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

[0041] Embodiment 2: The composition of the electrolyte is substantially the same as that of Example 1, except that the composition of the additives used in the electrolyte of Example 2 is different, including 0.5 wt. % DAS and 1 wt. % DMTFA based on the total mass of the electrolyte.

[0042] Embodiment 3: The composition of the electrolyte is substantially the same as that of Example 1, except that the composition of the additives used in the electrolyte of Example 3 is different, including 2 wt. % DAS and 1 wt. % DMTFA based on the total mass of the electrolyte.

[0043] Embodiment 4: The composition of the electrolyte is substantially the same as that of Example 1, except that the composition of the additives used in the electrolyte of Example 4 is different, including 1 wt. % DAS and 2 wt. % DMTFA based on the total mass of the electrolyte.

[0044] Embodiment 5: The composition of the electrolyte is substantially the same as that of Example 1, except that the composition of the additives used in the electrolyte of Example 5 is different, including 1 wt.% DAS and 3 wt.% DMTFA based on the total mass of the electrolyte.

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

[0046] Embodiment 7: The composition of the electrolyte is substantially the same as that of Example 1, except 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 based on the total mass of the electrolyte.

[0047] Embodiment 8: The composition of the electrolyte is basically the same as that of Example 1, except 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 based on the total mass of the electrolyte.

[0048] Embodiment 9: The composition of the electrolyte is substantially the same as that of Example 1, except 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 based on the total mass of the electrolyte.

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

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

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

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

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

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

[0055] Table 1 Composition of additives in the electrolytes of Comparative Examples 1 to 3 and Examples 1 to 5 Performance evaluation: electrolyte high-temperature storage HF content growth rate, high-temperature storage thickness growth rate, capacity retention rate and capacity recovery rate, 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, an appropriate amount of electrolyte was taken and the HF content therein was tested, which was recorded as C0. After high-temperature storage, an appropriate amount of electrolyte was taken and the HF content therein was tested, which was recorded as C1.

[0057] Increase rate of HF content in electrolyte = [C1 / C0] × 100%. The test results of various embodiments and comparative examples are shown in Table 2.

[0058] Table 2 High temperature storage test results of the electrolytes of Comparative Examples 1 to 5 and Examples 1 to 10 As can be seen from Table 2, by comparing the test results of the electrolytes stored at high temperatures of Examples 1 to 3, it can be seen that the amount of HF generated after the electrolytes are stored at high temperatures is Example 2 < Example 1 < Example 3. Combined with its components, it can be seen that when the amount of the second additive is the same, as the amount of the first additive increases, the HF content growth rate increases; by comparing the test results of the electrolytes stored at high temperatures of Examples 1, 4 and 5, it can be seen that the amount of HF generated after the electrolytes are stored at high temperatures is Example 1 > Example 4 > Example 5. Combined with the components, it can be seen that when the amount of the first additive is the same, as the amount of the second additive increases, the HF content growth rate decreases. Comparing the test results of the electrolytes stored at high temperatures of Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3, it can be seen that the amount of HF generated after the electrolytes are stored at high temperatures is Comparative Example 3 < Example 1 < Comparative Example 1 < Comparative Example 2, indicating that the addition of the first additive DAS will increase the HF content growth rate, while the addition of the second additive DMTAF will reduce the HF content growth rate. Based on the above speculation, this may be because DAS may generate acidic substances during high-temperature storage, and DMTFA can just reduce the content of acidic substances produced by DAS.

[0059] As can be seen from Table 2, by comparing the test results of the electrolytes of Examples 6 to 8 stored at high temperatures, it can be seen that the amount of HF generated after the electrolyte is stored at high temperatures is Example 8> Example 7> Example 6. Combined with the components, it can be seen that when the amount of the second additive is the same, as the amount of the first additive increases, the HF content growth rate increases; by comparing the test results of the electrolytes of Examples 6, 9 and 10 stored at high temperatures, it can be seen that the amount of HF generated after the electrolyte is stored at high temperatures is Example 10<Example 9<Example 6. Combined with the components, it can be seen that when the amount of the first additive is the same, as the amount of the second additive increases, the HF content growth rate decreases; by comparing the test results of the electrolytes of Example 6, Comparative Example 4, Comparative Example 5 and Comparative Example 1 stored at high temperatures, it can be seen that the amount of HF generated after the electrolyte is stored at high temperatures is Comparative Example 5<Example 6<Comparative Example 1<Comparative Example 4. Combined with the components, it can be seen that the addition of the second additive significantly reduces the HF content growth rate, and the addition of the first additive increases the HF content growth rate.

[0060] By comparison, it can be found that DMTFA and 3-FAA can effectively inhibit the generation of acidic substances, while DAS and THPi may generate acidic substances when stored at high temperatures. Moreover, DMTFA and 3-FAA can effectively inhibit DAS and THPi from generating acidic substances in the electrolyte. It can be inferred that the second additive amine compound makes up for the shortcoming of the first additive long carbon chain ester compound in the application process.

[0061] (2) Lithium-ion battery high temperature storage thickness test: At 25°C, the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C. At this time, the thickness of the lithium-ion battery is tested and recorded as h0; then the lithium-ion battery is placed in a 60°C constant temperature box, and after 30 days of storage, the thickness of the lithium-ion battery is tested and recorded as h1. 15 lithium-ion batteries are tested in each group, and the average value is taken.

[0062] The thickness expansion ratio (%) of the lithium-ion battery after storage at 60°C for 30 days = [(h1-h0) / h0]×100%.

[0063] (3) High temperature storage capacity retention test of lithium-ion batteries At 25°C, the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and then discharged to 3.0V at 0.5C constant current. The discharge capacity of the lithium-ion battery at this time is recorded as D0; then, the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and then the lithium-ion battery is stored at 60°C for 30 days. After the storage is completed, it is left at 25°C for 4 hours and discharged to 3.0V at 0.5C constant current. The discharge capacity of the lithium-ion battery at this time is recorded as D1. 15 lithium-ion batteries are tested in each group and the average value is taken.

[0064] The capacity retention rate (%) of the lithium-ion battery after storage at 60° C. for 30 days = [D1 / D0]×100%.

[0065] (4) High temperature storage capacity recovery rate test of lithium-ion batteries At 25°C, the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and then discharged to 3.0V at 0.5C constant current, and the discharge capacity of the lithium-ion battery at this time is recorded as D0; then, the lithium-ion battery is continued to be charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and then the lithium-ion battery is stored at 60°C for 30 days. After the storage is completed, it is left at 25°C for 4 hours and discharged to 3.0V at 0.5C constant current, and then the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and discharged to 3.0V at 0.5C constant current, and the cycle is repeated for 3 times. At this time, the discharge capacity of the lithium-ion battery is tested and recorded as D2. 15 lithium-ion batteries are tested in each group, and the average value is taken.

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

[0067] (5) Cycle life test of lithium-ion batteries At 25°C, the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and then discharged to 3.0V at 0.5C constant current. At this time, the discharge capacity of the lithium-ion battery is tested and recorded as D0; then the lithium-ion battery is charged to 4.5V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and discharged to 3.0V at 0.5C constant current, and the cycle is repeated for 299 times. The discharge capacity at this time is recorded as D3. 15 lithium-ion batteries are tested in each group, and the average value is taken.

[0068] The remaining capacity retention rate of the lithium-ion battery = [D3 / D0] × 100%.

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

[0070] Table 3 Performance test results of lithium-ion batteries assembled with electrolytes of Comparative Examples 1 to 5 and Examples 1 to 10 From the performance test results in Table 3, it can be seen that the use of the first additive or the second additive alone improves the high temperature performance of the battery cell, but the improvement is limited; when the first additive and the second additive are used at the same time, the high temperature performance and cycle life of the battery cell are significantly improved. Therefore, it can be concluded that the first additive and the second additive play a synergistic role in improving battery performance.

[0071] (6) Lithium-ion battery positive and negative electrode interface detection The lithium-ion batteries of Comparative Example 1 and Example 1 were formed, specifically: at 25° C., the lithium-ion batteries were charged to 4.5V at a constant current and constant voltage of 0.1C, with a cut-off current of 0.05C, and then discharged to 3.0V at a constant current of 0.1C, for 3 cycles. The lithium-ion batteries were then disassembled, and the positive and negative electrode sheets were taken out respectively, and the interface components were detected by negative ion mode of time-of-flight secondary ion mass spectrometry.

[0072] Figure 1 The C on the positive electrode surface after formation of Comparative Example 1 and Example 1 detected by time-of-flight secondary ion mass spectrometry - 、 CoO2 - , S - It can be found that the CoO2 on the positive electrode surface of Comparative Example 1 and Example 1 - The content of S is comparable, indicating that the ratio of the film to the positive electrode material on the interface is basically comparable. Example 1 After formation, S can be detected on the positive electrode surface - And higher content of C - After analysis and speculation, this may be because the long carbon chain ester compound DAS participated in the interfacial film formation, and the long carbon chain did enter the interfacial film and changed the composition of the interface.

[0073] Figure 2 The C on the negative electrode surface of Comparative Example 1 and Example 1 after formation detected by time-of-flight secondary ion mass spectrometry - , S - Similarly, S was detected on the positive electrode surface after the formation of Example 1. - And higher content of C - , further indicating that DAS is involved in interfacial film formation and that long carbon chains do enter the interfacial film, changing the composition of the interface.

[0074] Combination Figure 1 and Figure 2, which fully demonstrates that the first additive plays a role in the formation of the interface film between the positive and negative electrodes of the secondary battery, and therefore, the first additive is conducive to the formation of the interface film. Combined with the aforementioned performance test and analysis, it can be inferred that the addition of the second additive is conducive to reducing the acidic substances produced by the first additive, and thus is conducive to improving the stability of the positive electrode interface and the negative electrode interface film, so the second additive can cooperate with the first additive to improve the performance of the lithium-ion battery.

[0075] 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 principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that The invention comprises 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 of the compounds represented by formula 1 to formula 5, and the second additive is selected from one or more of the compounds represented by formula 6 to formula 8, In formula 1 to formula 5, R 101 , R 102 , R 103 , R 104 , R 105 , R 106 , R 107 , R 108 , R 109 , R 110 , R 111 , R 112 , R 113 are independently selected from hydrogen, C 1~20 Alkyl or its halide, C 2~20 Alkenyl or its halide, C 3~20 Alicyclic compounds or their halogenated derivatives, C 6~20 An aromatic compound or a halogenated compound thereof; and R 101 and R 102 At least one of R 103 and R 104 At least one of R 105 , R 106 and R 107 At least one of R 108 , R 109 and R 110 At least one of R 111 , R 112 and R 113 At least one of which is independently selected from C 5~20 Alkyl or its halide, C 5~20 Alkenyl or its halide, C 5~20 Alicyclic compounds or their halogenated derivatives, C 6~20 An aromatic compound or one of its halides; In formula 6, R 201 , R 202 , R 203 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 201 , R 202 and R 203 Any two of the are connected to form a 3- to 10-membered ring; In formula 7, R 204 , R 205 , R 206 , R 207 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 204 , R 205 , R 206 and R 207 Any two of the are connected to form a 3- to 10-membered ring; In formula 8, R 208 , R 209 , R 210 are independently selected from hydrogen, C 1~12 Alkyl or its halide, C 2~12 Alkenyl or its halide, C 3~12 Alicyclic compounds or their halogenated derivatives, C 6~12 An aromatic compound or a halogenated compound thereof; or R 208 , R 209 and R 210 Any two of the following are connected to form a 3- to 10-membered ring.

2. The electrolyte according to claim 1, characterized in that Taking the mass of the electrolyte as 100%, the mass percentage of the first additive in the electrolyte is 0.01% to 10%, preferably 0.1% to 3%.

3. The electrolyte according to claim 1, characterized in that Taking the mass of the electrolyte as 100%, the mass percentage of the second additive in the electrolyte is 0.01% to 5%, preferably 0.1% to 2%.

4. The electrolyte according to claim 1, characterized in that The organic solvent includes one or more of ethylene carbonate, propylene carbonate, butylene carbonate, ethyl methyl 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.

5. 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 bisoxalatoborate, lithium difluorooxalatoborate, lithium bisfluorosulfonyl imide, sodium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, sodium bistrifluoromethanesulfonyl imide, sodium perchlorate and sodium trifluoromethanesulfonate.

6. The electrolyte according to any one of claims 1 to 5, characterized in that 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.

7. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, characterized in that 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, polyanion 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 material.

9. The secondary battery according to claim 7, characterized in that: The negative electrode active material in the negative electrode sheet is selected from metal lithium, metal sodium, 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 , a combination of one or more of Li-Al alloys.

10. The secondary battery according to claim 7, characterized in that: The diaphragm is selected from any one of polyurethane, polyethylene, polypropylene, polytetrafluoroethylene, polyimide, and diaphragms coated with ceramic components.

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