Non-aqueous electrolyte and lithium secondary battery

Abstract

The present invention is used in the field of lithium-ion battery technology, and discloses a non-aqueous electrolyte comprising the following substances by weight: 5-20% electrolyte lithium salt, 70-90% organic solvent, and 2%-9% functional additives, wherein the functional additives include fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene. Compared with the classic ethoxy pentafluorocyclotriphosphazene, fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene has more sulfonyl and ester groups. The corresponding positive electrode electrolyte interface layer (CEI) rich in inorganic lithium salts such as lithium sulfate and lithium fluoride can stabilize the interface between the positive electrode and the electrolyte at high voltage, suppress the rise in impedance, and make the stable circulation of 5-volt spinel a reality. The introduction of ester groups can increase the solubility of lithium salts in the electrolyte, reduce the viscosity of the solution, improve the wetting ability, and have a good flame retardant effect. Batteries containing fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene additive electrolytes show better electrochemical performance than the control group.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, in particular to a non-aqueous electrolyte and a lithium secondary battery. Background Art

[0002] The non-aqueous electrolyte in lithium-ion batteries plays a very important role. Lithium ions are conducted within the battery through the electrolyte, which is used between the positive and negative electrodes. Lithium salts can also affect the battery voltage by adjusting the ion concentration in the electrolyte, making it easier to maintain the battery voltage within the appropriate range. In lithium-ion batteries, the presence of additives in the electrolyte helps inhibit electrochemical side reactions. Electrochemical side reactions refer to physical or chemical changes that occur during the battery's charge and discharge process. If the reaction is too strong, it will lead to the loss of active materials and shorten the battery life, thus affecting battery performance. Lithium salts can form a stable solid electrolyte membrane (SEI and CEI), thereby inhibiting the electrochemical reaction process within the battery and ensuring the long-term reliable use of the battery.

[0003] Due to the high content of conventional chemical substances in the electrolyte and the constraints on the two extreme requirements of oxidation and reduction and solubility, the types of solvents and lithium salts available in the electrolyte are limited, while the amount of additives used is small, which allows them to be consumed and has a larger selection space, and the cost-effectiveness is high.

[0004] Additives often exhibit synergistic effects (mutually promoting) or counter-synergistic effects (mutually hindering and worsening the effect) in performance development, and research into these two effects is extremely valuable. Currently, additives that combine two or more distinct functions are called "multifunctional additives," but this category is relatively rare. Many additives, such as VC and PS, achieve various improvements in electrical performance by improving the SEI, but are still essentially single-function additives. Typical multifunctional additives include FEC (film formation, wetting, and conductivity enhancement) and FB (overcharge and viscosity dilution).

[0005] Currently, bifunctionality can be achieved through structural design. For example, certain polymerizable monomers can be modified to incorporate groups with battery activity, such as cyano, isocyanate, sulfonic acid, phosphate, and boron- and silicon-containing groups. One promising example of an additive is cyclotriphosphazene, primarily ethoxypentafluorocyclotriphosphazene.

[0006] Cyclotriphosphazene was first invented by Mitsubishi Chemical. It is basically the only additive that has flame retardant properties and does not cause too many side effects to the battery system. However, this compound also has several disadvantages: high cost, high system viscosity, and instability when used alone in a high-voltage system.

[0007] There are two main synthetic routes for ethoxycyclotriphosphazene. The first is a two-step method, which uses hexachlorocyclotriphosphazene (P3N3Cl6) as raw material, sodium fluoride as fluorinating agent, and DMI (1,3-dimethyl-2-imidazolidinone) as solvent for fluorination reaction, followed by distillation to obtain hexafluorocyclotriphosphazene (>98%). Then, hexafluorocyclotriphosphazene (P3N3F 6) The crude product is distilled to obtain ethoxypentafluorocyclotriphosphazene with a purity of >99.8%. Another method is to change the fluorination agent from potassium fluoride to pyridine hydrofluoric acid. Summary of the Invention

[0008] The object of the present invention is to provide a non-aqueous electrolyte and a lithium secondary battery to solve the problems raised in the above background art, such as high cost of additives in the current non-aqueous electrolyte, high system viscosity, and instability when used alone in a high voltage system.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a non-aqueous electrolyte comprising the following substances by weight: 5-20% electrolyte lithium salt, 70-90% organic solvent, and 2%-9% functional additives, wherein the functional additives include fluorosulfonyl difluoroacetate-based pentafluorocyclotriphosphazene.

[0010] Preferably, the electrolyte lithium salt is one or a mixture of lithium hexafluorophosphate, lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonamide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium oxalatoborate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorobisoxalatophosphate (LiDODFP), lithium fluoromalonate borate (LiFMDFB), lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI), and lithium difluorophosphate (LiPO2F2).

[0011] Preferably, the organic solvent is one or a mixture of cyclic carbonate compounds, chain carbonate compounds, sulfone and sulfoxide compounds, and chain carboxylate compounds.

[0012] Preferably, the cyclic carbonates include one or more of ethylene carbonate, propylene carbonate, and γ-butyrolactone; the chain carbonate compounds are one or more of diethyl carbonate, methyl ethyl carbonate, and carbonate derivatives synthesized from linear or branched aliphatic monoalcohols with a carbon number of 3-8 and carbonic acid; the sulfone and sulfoxide solvents are one or more of dimethyl sulfoxide, diphenyl sulfoxide, ethyl methyl sulfone, ethyl methoxyethyl sulfone, cyclopentane (SL), dimethyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone, and methyl isopropyl sulfone.

[0013] Preferably, the functional additive further comprises tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, fluoroethylene carbonate, bisfluoroethylene carbonate, ethylene sulfite, ethylene sulfate, propylene sulfate, methylene disulfonate, vinyl ethylene carbonate, 1,4-butane sultone, 1,3-propane sultone, 1-propylene-1,3-sultone, 1,3-propylene sultone, vinylene carbonate, vinyl ethylene carbonate, ethoxypentafluorocyclotriphosphazene, 1-propylphosphoric acid cyclic anhydride, 1-n-butyl cyclophosphoric acid anhydride, Succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, 1,2,3-tris-(2-cyanoethoxy)propane, ethylene glycol bis(propionitrile) ether, 1,2-bis(cyanoethoxy)ethane, pentaerythritol bicyclic sulfate, methyl trifluoroethyl carbonate, 3,3,3-trifluoropropylene carbonate, 4-trifluoromethyl ethylene carbonate, tris(2,2,2-trifluoroethyl) phosphate, tris(2,2,2-trifluoroethyl) carbonate, 2-fluorobiphenyl, 2,4-difluorobiphenyl, 4-methylethylene sulfate, 4-propylethylene sulfate, tripropargyl phosphate or a mixture of one or more thereof.

[0014] Preferably, the specific steps for preparing the fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene are as follows:

[0015] S1. Functionalization of hexachlorocyclotriphosphazene:

[0016] Using hexachlorocyclotriphosphazene as substrate, sodium fluorosulfonyldifluoroacetate as functional group reagent, and n-hexane or sulfolane as solvent, monosubstituted (fluorosulfonyldifluoroacetate) pentachlorocyclotriphosphazene is obtained;

[0017] S2. Fluorination:

[0018] The fluorination reaction is carried out using potassium fluoride as a fluorination reagent and DMI (1,3-dimethyl-2-imidazolidinone) or sulfolane as a solvent, and fluorosulfonyldifluoroacetate pentafluorocyclotriphosphazene is obtained by distillation.

[0019] The present invention also provides a lithium secondary battery comprising a positive electrode, a negative electrode and the non-aqueous electrolyte described above.

[0020] Compared with the prior art, the beneficial effect of the present invention is that the functional additive contains at least fluorosulfonyldifluoroacetate pentafluorocyclotriphosphazene. Compared with the classic ethoxy pentafluorocyclotriphosphazene, fluorosulfonyldifluoroacetate pentafluorocyclotriphosphazene has more sulfonyl and ester groups. The corresponding positive electrode electrolyte interface layer (CEI) rich in inorganic lithium salts such as lithium sulfate and lithium fluoride can stabilize the interface between the positive electrode and the electrolyte at high voltage, inhibit the rise in impedance, and make the stable cycle of 5-volt spinel a reality. The introduction of ester groups can increase the solubility of lithium salts in the electrolyte, reduce the viscosity of the solution, improve the wetting ability, and at the same time have a good flame retardant effect. Batteries containing fluorosulfonyldifluoroacetate pentafluorocyclotriphosphazene additive electrolytes show better electrochemical performance than the control group. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The figure is a schematic diagram of the process for preparing fluorosulfonyldifluoroacetate pentafluorocyclotriphosphazene according to the present invention. DETAILED DESCRIPTION

[0022] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0023] The present invention provides a technical solution: a non-aqueous electrolyte comprising the following substances by weight: 5-20% of an electrolyte lithium salt, 70-90% of an organic solvent, and 2%-9% of a functional additive, wherein the functional additive comprises fluorosulfonyldifluoroacetate-based pentafluorocyclotriphosphazene.

[0024] Table 1 Specific electrolyte formulas for the examples and comparative examples in full battery performance tests

[0025]

[0026]

[0027] Among them, the Chinese names of the abbreviations of lithium salts, solvents, additives, etc. in Table 1 are:

[0028] EC: ethylene carbonate; EMC: ethyl methyl carbonate; DMC: dimethyl carbonate; DEC: diethyl carbonate; PC: propylene carbonate; GBL: γ-butyrolactone; EA: ethyl acetate; VC: vinylene carbonate; VEC: vinylene carbonate; FEC: fluoroethylene carbonate; PS: propenyl sulfite; PST: propenyl-1,3-sultone; BSA: N,O-bis(trimethylsilyl)acetamide; TMSB: tris(trimethylsilylphosphate); LiFSI: lithium bis(fluorosulfonyl)imide; MMDS: methylene methanedisulfonate; TTE: tetrafluoroethyltetrafluoropropyl ether; FPN: ethoxypentafluorocyclotriphosphazene.

[0029] Full battery process parameters:

[0030] Laminated soft-pack battery 856006, N / P ratio 1.10, injection volume 3.2g / Ah, design capacity 2.0Ah.

[0031] (1) The positive electrode active material is high voltage spinel 96%, the binder Solvey polyvinylidene fluoride Solif 5130 is 2.0%, the conductive agent carbon black Super P Li (Swiss Temeco) is 2.0%, the electrode compaction is 3.4, and the single-side loading is 22.6 mg / cm 2 (2) The negative electrode graphite (Dongguan Kaijin, AML-400) has a negative electrode loading of 14.1 mg / cm 2 The coating was pressed to a density of 1.55 g / cm 2 The formula is 96% active matter, 2% CMC / SBR binder (Dacel 2200 / Aiyulong 3001), and 2% SuperP Li conductive carbon black (by weight).

[0032] Soft pack battery production: Specifications: 2.0Ah 5V spinel LiNi 0.5 Mn 1.5 856006 battery of O4 / graphite system. The electrolyte dosage is 3.2g / Ah, and the electrolyte formula is shown in Table 1. The battery is pre-sealed at -90kPa pressure using Shenzhen Kejing Battery Sealing Machine (MSK-115A, MTI Corp.) and left to stand for 24 hours. Then, the battery is transferred to a pressure formation cabinet and connected to the Xinwei automatic testing system. It is formed under 2.8-5.0V and C / 10 conditions. After running one lap, the battery is charged to 4.6V and stopped. Then, the battery airbag is cut and sealed twice using a second sealing machine to remove the gas generated during the formation process. Finally, the battery is taken out for cycle testing (Xinwei), with a cycle voltage range of 3.0-5.0V and a charge and discharge rate of 1C / 1C.

[0033] Table 2 Test data of the embodiment and comparative example in the full battery performance test

[0034]

[0035] It can be clearly seen from Tables 1 and 2 that the battery system containing the fluorosulfonyldifluoroacetate-based pentafluorocyclotriphosphazene electrolyte of the present invention generally exhibits better electrochemical performance and safety performance than the battery system containing the conventional ethoxypentafluorocyclotriphosphazene electrode solution, regardless of the different solvent systems and additive systems. This is in addition to the capacity retention and capacity recovery rates after the rigorous 150°C, 30-minute hot box test, conventional room temperature cycling, high-temperature cycling at 45°C, and high-temperature storage at 60°C for 28 days. For example, Example 6 and Comparative Example 1, in which the main solvent systems are almost the same: Example 6 passed the hot box test, and the capacity retention rate and capacity recovery rate after 28 days of high-temperature storage at 60°C were 88.1% and 97.9%, respectively, which were 4.5% and 5.8% higher than the corresponding values ​​of Comparative Example 1 (75.6% and 92.1%); After 500 cycles at room temperature, the capacity retention rate of the battery of Example 6 was 89.1%, which was 15.9% higher than that of Comparative Example 1; After 200 cycles at a high temperature of 45°C, the capacity retention rate of Example 6 was 84.3%, which was nearly 20% higher than that of Comparative Example 1. In addition, Example 6 and Comparative Example 2 with the same lithium salt type and concentration: Example 6 passed the hot box test, and the capacity retention rate and capacity recovery rate after 28 days of high-temperature storage at 60°C were 88.1% and 97.9%, respectively, which were 5.0% and 3.7% higher than the corresponding values ​​of Comparative Example 2 (83.1% and 94.2%), respectively. The battery of Comparative Example 2 has not passed the hot box test; after 500 cycles at room temperature, the capacity retention rate of the battery of Example 6 was 89.1%, which was 25.4% higher than that of Comparative Example 2; after 200 cycles at a high temperature of 45°C, the capacity retention rate of Example 6 was 84.3%, which was 16.2% higher than that of Comparative Example 1.

[0036] The above specific embodiments further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and do not limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A non-aqueous electrolyte, characterized in that: The composition comprises the following substances by weight: 5-20% of an electrolyte lithium salt, 70-90% of an organic solvent, and 2%-9% of a functional additive, wherein the functional additive comprises fluorosulfonyldifluoroacetate-based pentafluorocyclotriphosphazene; The electrolyte lithium salt is one or a mixture of lithium hexafluorophosphate, lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonamide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium oxalatoborate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorodioxalatophosphate (LiDODFP), lithium fluoromalonate borate (LiFMDFB), lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI), and lithium difluorophosphate (LiPO2F2); The organic solvent is one or a mixture of cyclic carbonate compounds, chain carbonate compounds, sulfone and sulfoxide compounds, and chain carboxylic acid esters.

2. The non-aqueous electrolyte according to claim 1, wherein: The cyclic carbonates include one or a mixture of ethylene carbonate, propylene carbonate, and γ-butyrolactone; the chain carbonate compounds include one or a mixture of diethyl carbonate, methyl ethyl carbonate, and carbonate derivatives synthesized from linear or branched fatty monoalcohols with a carbon number of 3-8 and carbonic acid; the sulfone and sulfoxide solvents include one or a mixture of dimethyl sulfoxide, diphenyl sulfoxide, ethyl methyl sulfone, ethyl methoxyethyl sulfone, sulfolane (SL), dimethyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone, and methyl isopropyl sulfone.

3. The non-aqueous electrolyte according to claim 1, wherein: The functional additives further include tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, fluoroethylene carbonate, bisfluoroethylene carbonate, ethylene sulfite, ethylene sulfate, propylene sulfate, methylene disulfonate, vinyl ethylene carbonate, 1,4-butane sultone, 1,3-propane sultone, 1-propylene-1,3-sultone, 1,3-propylene sultone, vinylene carbonate, vinyl ethylene carbonate, ethoxypentafluorocyclotriphosphazene, 1-propylphosphoric acid cyclic anhydride, 1-n-butyl cyclophosphoric acid anhydride, butanediol. Nitrile, adiponitrile, 1,3,6-hexanetrinitrile, 1,2,3-tris-(2-cyanoethoxy)propane, ethylene glycol bis(propionitrile) ether, 1,2-bis(cyanoethoxy)ethane, pentaerythritol bicyclic sulfate, methyl trifluoroethyl carbonate, 3,3,3-trifluoropropylene carbonate, 4-trifluoromethyl ethylene carbonate, tris(2,2,2-trifluoroethyl) phosphate, tris(2,2,2-trifluoroethyl) carbonate, 2-fluorobiphenyl, 2,4-difluorobiphenyl, 4-methylethylene sulfate, 4-propylethylene sulfate, tripropargyl phosphate or a mixture of one or more thereof.

4. The non-aqueous electrolyte according to claim 1, wherein: The specific steps for preparing the fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene are as follows: S1. Functionalization of hexachlorocyclotriphosphazene: Using hexachlorocyclotriphosphazene as substrate, sodium fluorosulfonyldifluoroacetate as functional group reagent, and n-hexane or sulfolane as solvent, monosubstituted (fluorosulfonyldifluoroacetate) pentachlorocyclotriphosphazene is obtained; S2. Fluorination: The fluorination reaction is carried out using potassium fluoride as a fluorination reagent and DMI (1,3-dimethyl-2-imidazolidinone) or sulfolane as a solvent, and fluorosulfonyldifluoroacetate pentafluorocyclotriphosphazene is obtained by distillation.

5. A lithium secondary battery, characterized in that: The invention comprises a positive electrode, a negative electrode and the non-aqueous electrolyte according to any one of claims 1 to 4.

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