Super-long cycle life battery electrolyte applied to lithium iron phosphate system

Abstract

The invention discloses an ultra-long cycle life battery electrolyte applied to a lithium iron phosphate system. The battery electrolyte is prepared from the following components in parts by weight: 70-85 parts of an organic solvent, 10-20 parts of an electrolyte and 0.4-10 parts of a film-forming additive, the film-forming additive is selected from at least three of vinylene carbonate, fluoroethylene carbonate, 1, 4-butane sultone and ethylene sulfate. By compounding the organic solvent and the film-forming additive, the SEI film structure of the battery can be effectively inhibited and improved, the decomposition and consumption of electrolyte are inhibited, the impedance of the battery is reduced, the high-temperature performance of the battery is improved, the film-forming stability of the SEI is improved, and the cycle life of the SEI is prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery electrolytes, and in particular relates to a battery electrolyte with an ultra-long cycle life applied to a lithium iron phosphate system. Background Art

[0002] Renewable energy generation has the problems of instability, randomness, and intermittency, which puts higher requirements on grid frequency control. Therefore, in the new power system, energy storage will become an important part. Driven by new technologies and industrial policies, the home energy storage battery system market will usher in development opportunities in the future, and it is expected that the home energy storage battery system market will grow rapidly by 2029. Therefore, with the development of the future energy storage market, it is still necessary to develop a battery with an ultra-long cycle life. The market development in the field of home energy storage has put forward higher requirements on the energy density and cycle life of the next generation of lithium-ion batteries.

[0003] The life of lithium iron phosphate batteries is one of the important indicators of energy storage system products. Currently, lithium batteries must meet the 20-year warranty requirement. Studies have shown that the life of lithium iron phosphate batteries is affected by many factors. The stability of the electrolyte in lithium-ion batteries plays a vital role in the cycle stability of the battery. During the charge and discharge cycle, as the SEI film is continuously repaired and reorganized, the lithium ions and electrolytes in the electrolyte will continue to be consumed, resulting in a significant reduction in the coulombic efficiency and cycle life of the lithium battery.

[0004] At present, the cycle life of lithium iron phosphate batteries in the energy storage field is generally 3000-4000 cycles, and they are used in general energy storage base stations and other fields. The current market demand for higher battery life is 8000-10000 cycles. Therefore, it is necessary to develop a corresponding lithium-ion battery electrolyte. Summary of the invention

[0005] The object of the present invention is to provide a battery electrolyte with ultra-long cycle life applied to a lithium iron phosphate system, so as to solve the problem of poor battery cycle life in the prior art.

[0006] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is a battery electrolyte for ultra-long cycle life of lithium iron phosphate system, wherein the battery electrolyte is composed of the following components in parts by weight: 70-85 parts of organic solvent, 10-20 parts of electrolyte, and 0.4-10 parts of film-forming additive; The organic solvent is selected from two or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate, ethyl methyl carbonate, propylene carbonate, propyl propionate, ethyl propionate, propyl acetate and ethyl acetate; The film-forming additive is selected from at least three of vinylene carbonate, fluoroethylene carbonate, 1,4-butane sultone, and vinyl sulfate.

[0007] Preferably, the electrolyte is any one or more of lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium bis(fluorosulfonylimide), and lithium difluorooxalatoborate.

[0008] Preferably, the weight portion of the film-forming additive is 1.5-5 parts.

[0009] Preferably, the organic solvent is composed of dimethyl carbonate, diethyl carbonate, ethylene carbonate, ethyl methyl carbonate and propylene carbonate.

[0010] The beneficial effects of the present invention are: by using an organic solvent in combination, a higher dielectric constant and ionic conductivity are provided, which is beneficial to the migration of lithium ions in the electrolyte. A film-forming additive composed of at least three substances selected from vinylene carbonate, fluoroethylene carbonate, 1,4-butane sultone, and vinyl sulfate is added to the electrolyte to improve the electrochemical performance of lithium-ion batteries, and can also modify the surface structure of the negative electrode to improve battery performance. It can effectively prevent PC and solvated lithium ions from being co-inserted into graphite, forming a dense and stable SEI film on the surface of the graphite negative electrode, and continuously modifying it as the film structure changes during the cycle, thereby improving the cycle life of the battery and better applying it to fields such as home energy storage and tower energy storage. DETAILED DESCRIPTION

[0011] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0012] The preparation method of the lithium-ion battery electrolyte in the following embodiments is as follows: use a glove box, require the working gas to use 99.999% argon or nitrogen, the moisture content of the glove box environment is less than 10ppm, the oxygen content is less than 10ppm, the purity of the organic solvent is required to be greater than 99.99%, and the moisture content is required to be less than 5ppm. Weigh each material in the glove box according to the mass ratio. When preparing, pay attention to adding the organic solvent and additives (the first film-forming additive and the second film-forming additive) first, and then add the electrolyte. Pay attention to cooling before adding the electrolyte, requiring the temperature to be less than 10°C, and then mix and stir for 1 hour. Example 1

[0013] Weigh 160 g of dimethyl carbonate, 670 g of diethyl carbonate, 400 g of ethylene carbonate, 300 g of ethyl methyl carbonate, 80 g of propylene carbonate, 300 g of lithium hexafluorophosphate, 60 g of fluoroethylene carbonate, 20 g of 1,4-butane sultone, and 10 g of vinyl sulfate, and prepare an electrolyte in a glove box. Example 2

[0014] Weigh 160 g of dimethyl carbonate, 660 g of diethyl carbonate, 400 g of ethylene carbonate, 300 g of ethyl methyl carbonate, 80 g of propylene carbonate, 300 g of lithium hexafluorophosphate, 70 g of vinylene carbonate, 20 g of fluoroethylene carbonate, and 10 g of vinyl sulfate, and prepare an electrolyte in a glove box. Example 3

[0015] Weigh 160 g of dimethyl carbonate, 650 g of diethyl carbonate, 400 g of ethylene carbonate, 300 g of ethyl methyl carbonate, 80 g of propylene carbonate, 300 g of lithium hexafluorophosphate, 60 g of vinylene carbonate, 20 g of fluoroethylene carbonate, 20 g of 1,4-butane sultone, and 10 g of vinyl sulfate, and prepare an electrolyte in a glove box. Example 4

[0016] Weigh 160 g of dimethyl carbonate, 650 g of diethyl carbonate, 400 g of ethylene carbonate, 300 g of ethyl methyl carbonate, 80 g of propylene carbonate, 300 g of lithium hexafluorophosphate, 80 g of vinylene carbonate, 10 g of fluoroethylene carbonate, 10 g of 1,4-butane sultone, and 10 g of vinyl sulfate, and prepare an electrolyte in a glove box.

[0017] Comparative Example 1 160 g of dimethyl carbonate, 760 g of diethyl carbonate, 400 g of ethylene carbonate, 300 g of ethyl methyl carbonate, 80 g of propylene carbonate, and 300 g of lithium hexafluorophosphate were weighed to prepare an electrolyte in a glove box.

[0018] Comparative Example 2 160 g of dimethyl carbonate, 670 g of diethyl carbonate, 400 g of ethylene carbonate, 300 g of ethyl methyl carbonate, 80 g of propylene carbonate, 300 g of lithium hexafluorophosphate, 20 g of 1,3-propane sultone, 60 g of vinylene carbonate, and 10 g of vinyl sulfate were weighed to prepare an electrolyte in a glove box.

[0019] Then, the electrolytes prepared in the above embodiments and comparative examples were injected into batteries in a glove box, and 20AH square soft-pack batteries were selected for electrical performance testing.

[0020] 1. 25℃ environmental cycle project: 0.5C constant current constant voltage charge to 3.7V, cut-off current 0.02C, leave for 10min, 0.5C constant current discharge to 2.5V, leave for 10min; cycle 8000 times, calculate the 8000-cycle capacity retention rate, which is required to be ≥80%; 2. 55℃ environmental cycle project: 0.5C constant current constant voltage charge to 3.7V, cut-off current 0.02C, leave for 10min, 0.5C constant current discharge to 2.5V, leave for 10min; cycle for 3500 cycles, and the capacity retention rate for 3500 cycles must be ≥80%; 3. Fully charged and stored at 60℃ for 7 days: charge at 0.5C constant current and constant voltage to 3.7V, cut-off current 0.02C, leave for 10min, discharge at 0.5C constant current to 2.5V, leave for 10min, repeat this cycle twice; then charge at 0.5C constant current and constant voltage to 3.7V, store in a 60±2℃ constant temperature box for 60 days; after 60 days, discharge at 0.5C constant current to 2.5V; then charge at 0.5C constant current and constant voltage to 3.7V, cut-off current 0.02C, leave for 10min, discharge at 0.5C constant current to 2.5V, leave for 10min, repeat this cycle twice, and calculate the capacity retention rate requirement to be ≥98%.

[0021] The test results are shown in the following table .

[0022] From the table we can see that: In the cycle performance charge and discharge test, no additives were added to the electrolyte in Comparative Example 1. When the battery was cycled at 25°C, high temperature cycled at 55°C, and stored at 60°C for 60 days, the battery capacity gradually decreased by 34% with the increase in the number of charge and discharge times, and the decrease was obvious. The 4000-week retention rate was 66.05%, which did not meet the technical requirements.

[0023] The electrolytes with additives, such as Example 1, Example 2, Example 3, and Example 4, have a significantly reduced capacity drop ratio, with the 4000-week drop ratio being 7.1% to 10%. This indicates that the film-forming additives can improve the SEI structure of the battery, and these film-forming additives can reduce the internal resistance of the battery, and can significantly reduce the capacity drop ratio of the battery after charging and discharging.

[0024] Based on all the data, the battery capacity retention rate of Example 3 decreased by only 7.1% after 4000 charge and discharge cycles of the electrolyte, which is at the optimal level; in other Examples 1, 2, and 4, the battery retention rate decreased slightly due to the change in the combination of additives, but all met the requirement of 8000 cycles with a capacity retention rate of >80%; among them, the content of vinyl carbonate additive in Example 4 increased, and the battery capacity retention rate decreased, because the increase in its content would lead to an increase in the internal resistance of the battery, thus affecting the battery cycle life.

[0025] Compared with Examples 1-4, the decrease rate of the battery capacity retention rate after charge and discharge in Comparative Example 2 is larger, indicating that the film-forming additive composed of at least three substances of vinylene carbonate, fluoroethylene carbonate, 1,4-butane sultone, and vinyl sulfate is added to form a film on the negative electrode of the battery, which improves the electrochemical performance of lithium-ion batteries, and can also modify the surface structure of the negative electrode to improve battery performance. However, when the battery is normally cycled to a certain life, the surface modification of the negative electrode disappears. It can effectively prevent PC and solvated lithium ions from being co-inserted into graphite, forming a dense and stable SEI film on the surface of the graphite negative electrode, and continuously modifying it as the film structure changes during the cycle.

Claims

1. A battery electrolyte with ultra-long cycle life applied to lithium iron phosphate system, characterized in that: The battery electrolyte is composed of the following components in parts by weight: 70-85 parts of an organic solvent, 10-20 parts of an electrolyte, and 0.4-10 parts of a film-forming additive; The organic solvent is selected from two or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate, ethyl methyl carbonate, propylene carbonate, propyl propionate, ethyl propionate, propyl acetate and ethyl acetate; The film-forming additive is selected from at least three of vinylene carbonate, fluoroethylene carbonate, 1,4-butane sultone, and vinyl sulfate.

2. The battery electrolyte with ultra-long cycle life applied to lithium iron phosphate system as claimed in claim 1, characterized in that: The electrolyte is any one or more of lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium bis(fluorosulfonylimide), and lithium difluorooxalatoborate.

3. The battery electrolyte for ultra-long cycle life applied to lithium iron phosphate system as claimed in claim 1, characterized in that: The weight portion of the film-forming additive is 1.5-5 parts.

4. The battery electrolyte for ultra-long cycle life applied to lithium iron phosphate system as claimed in claim 1, characterized in that: The organic solvent consists of dimethyl carbonate, diethyl carbonate, ethylene carbonate, ethyl methyl carbonate and propylene carbonate.