Non-aqueous electrolyte and lithium secondary battery

Abstract

The invention is applied to the technical field of lithium ion batteries, and discloses a non-aqueous electrolyte which comprises the following substances in percentage by weight: 5-20% of electrolyte lithium salt, 70-90% of organic solvent and 2-9% of functional additive, and the functional additive comprises fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene. Compared with classical ethyoxyl pentafluorocyclotriphosphazene, the fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene has the advantages that sulfonyl and ester groups are added, a correspondingly formed positive electrode electrolyte interface layer (CEI) rich in inorganic lithium salts such as lithium sulfate and lithium fluoride can stabilize an interface between a positive electrode and an electrolyte under high voltage, impedance rise is inhibited, and the service life of the positive electrode is prolonged. And stable circulation of 5V spinel is realized. By introducing the ester group, the lithium salt solubility of the electrolyte can be improved, the solution viscosity can be reduced, the wetting ability can be improved, and meanwhile, the electrolyte has a good flame-retardant effect. Compared with a control group, the battery containing the fluorine sulfonyl difluoroacetate pentafluorocyclotriphosphazene additive electrolyte has better electrochemical performance.

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. The electrolyte electrolyte is needed to conduct lithium ions between the positive and negative electrodes in lithium-ion batteries. 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 plays a role in inhibiting the occurrence of electrochemical side reactions. Electrochemical side reactions refer to physical or chemical changes that occur during the charge and discharge process of the battery. If the reaction is too strong, it will lead to the loss of active materials in the battery and shorten the battery life, thereby affecting the performance of the battery. Lithium salts can form a stable solid electrolyte membrane (SEI and CEI), thereby inhibiting the electrochemical reaction process inside 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 leaves a larger selection space, and the cost-effectiveness is high.

[0004] Additives often have synergistic effects (promoting each other) or reverse synergistic effects (mutual restraint is worse) in performance, and research on these two effects of additives is very valuable. At present, additives that have two or more obviously different functions are called "multifunctional additives", and there are actually not many such additives. Many additives such as VC, PS, etc. achieve improvements in various aspects of electrical performance by improving SEI, and they are essentially single-functional additives. Typical multifunctional additives include FEC (film formation, wetting, enhanced conductivity), FB (overcharge, dilution viscosity), etc.

[0005] At present, dual functions can be achieved through structural design, such as introducing groups that are active in batteries into certain polymerizable monomers, such as cyano, isocyanate, sulfonic acid, phosphate, boron-containing and silicon-containing groups. One of the more promising additive examples is cyclotriphosphazene additives, mainly ethoxy pentafluorocyclotriphosphazene.

[0006] Cyclotriphosphazene was first invented by Mitsubishi Chemical. It is basically the only additive that has flame retardant function and does not bring 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] At present, there are two major 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, and distillation to obtain hexafluorocyclotriphosphazene (>98%); then hexafluorocyclotriphosphazene (P3N3F 6) The reaction is carried out with 2-nitro-1-nitro-2-oxo ... 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 mentioned in the above background technology that the additive cost of the current non-aqueous electrolyte is high, the system viscosity is high, and it is unstable when used alone in a high voltage system.

[0009] To achieve the above object, 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 fluoromalonateborate (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 carboxylic acid esters.

[0012] Preferably, the cyclic carbonates include one or a mixture of ethylene carbonate, propylene carbonate, and γ-butyrolactone, the chain carbonate compounds are one or a mixture of diethyl carbonate, methyl ethyl carbonate, and carbonate derivatives synthesized from straight-chain or branched aliphatic monoalcohols with a carbon number of 3-8 and carbonic acid, and the sulfone and sulfoxide solvents are one or a mixture 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, vinyl sulfite, vinyl 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, vinyl carbonate, vinyl ethylene carbonate, ethoxypentafluorocyclotriphosphazene, 1-propyl phosphoric acid cyclic anhydride, 1-n-butyl cyclic phosphoric 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-trifluoromethylethylene 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 any one of them.

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

[0015] S1. Functionalization of hexachlorocyclotriphosphazene:

[0016] Using hexachlorocyclotriphosphazene as a substrate, sodium fluorosulfonyldifluoroacetate as a functional group reagent, and n-hexane or cyclopentane sulfone as a 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 cyclopentane sulfone as a solvent, and fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene is obtained by distillation.

[0019] The present invention also provides a lithium secondary battery, which comprises 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 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 of lithium sulfate and lithium fluoride can stabilize the interface between the positive electrode and the electrolyte at high voltage, inhibit the rise of 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 have a good flame retardant effect. The battery containing the electrolyte with fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene additive shows 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 fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene of the present invention. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work 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 electrolyte lithium salt, 70-90% of organic solvent, and 2%-9% of functional additives, wherein the functional additives contain fluorosulfonyl difluoroacetate-based pentafluorocyclotriphosphazene.

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

[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 disulfonate; 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 high voltage spinel is 96%, the binder Solvey polyvinylidene fluoride Solif 5130 is 2.0%, the conductive agent carbon black Super P Li (Swiss Termega) is 2.0%, the pole piece is compacted 3.4, and the single-side loading is 22.6 mg / cm 2 (2) Negative electrode graphite (Dongguan Kaijin, AML-400) The negative electrode loading is 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 the 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 electrolyte of the fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene of the present invention generally exhibits better electrochemical performance and safety performance than the battery system containing the conventional ethoxy pentafluorocyclotriphosphazene electrode solution, regardless of different solvent systems and additive systems, whether it is a rigorous 150°C, 30-minute hot box test, or a conventional room temperature cycle, 45°C high temperature cycle, and 60°C, 28-day high temperature storage. After the capacity retention rate and capacity recovery rate. For example, in Example 6 and Comparative Example 1, 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 in Example 6 was 89.1%, which was 15.9% higher than that in 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 in 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 purpose, 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 protection scope of the present invention.

Claims

1. A non-aqueous electrolyte, characterized in that: The following substances are included by weight: 5-20% of electrolyte lithium salt, 70-90% of organic solvent, and 2%-9% of functional additives, wherein the functional additives include fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene.

2. A non-aqueous electrolyte according to claim 1, characterized in that: 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 fluoromalonateborate (LiFMDFB), lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI), and lithium difluorophosphate (LiPO2F2).

3. A non-aqueous electrolyte according to claim 1, characterized in that: The organic solvent is one or a mixture of cyclic carbonate compounds, chain carbonate compounds, sulfone and sulfoxide compounds, and chain carboxylic acid esters.

4. A non-aqueous electrolyte according to claim 3, characterized in that: 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 straight-chain 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, cyclopentane (SL), dimethyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone, and methyl isopropyl sulfone.

5. A non-aqueous electrolyte according to claim 1, characterized in that: The functional additives also include tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, fluoroethylene carbonate, bisfluoroethylene carbonate, vinyl sulfite, vinyl 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, vinyl carbonate, vinyl ethylene carbonate, ethoxypentafluorocyclotriphosphazene, 1-propyl phosphoric 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-trifluoromethylethylene 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.

6. A non-aqueous electrolyte according to claim 1, characterized in that: The specific steps for preparing the fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene are as follows: S1. Functionalization of hexachlorocyclotriphosphazene: Using hexachlorocyclotriphosphazene as a substrate, sodium fluorosulfonyldifluoroacetate as a functional group reagent, and n-hexane or cyclopentane sulfone as a 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 cyclopentane sulfone as a solvent, and fluorosulfonyl difluoroacetate pentafluorocyclotriphosphazene is obtained by distillation.

7. 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 6.

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