A Research On Wide-temperature And Fire-retardant Electrolytes Of Lithium Ion Batteries

It is difficult for commercial electrolytes of lithium-ion batteries（LIBs）to meet the needs of wide temperature and high safety use,because the commercial electrolytes have a narrow liquid range and the risk of thermal runaway in a certain critical high temperatures or external force impact due to its high volatility.These problems limit the application of lithium ion batteries in resource exploration,aerospace and other fields.In order to solve the above problems,this thesis conducted research on wide-temperature electrolytes and nonflammable electrolytes for LIBs by adjusting the type and concentration of lithium salts and the addition of additives based on Li Fe PO₄/Li cell system.This thesis also proposes the working mechanism of electrolytes at low or high temperature and the design mechanism of wide-temperature electrolytes and non-flammable electrolytes for LIBs.First,wide-temperature electrolyte was designed based on lithium borate difluoroaratate（Li DFOB）and lithium borate dioxalate（Li BOB）with carbonate solvents.Using this electrolyte,a stable and high solvation level was obtained at a high temperature of 100℃,beneficial to the formation of a stable interface passivation film,improved the stability of the electrode-electrolyte interface at high temperatures.Through this way,the operating temperature range of LIBs was widened to-30～100℃.At 25℃,the capacity loss was less than 3%after 1000 cycles.A lifespan of 200 cycles and 160 cycles were achieved at 100℃ and-20℃ with a capacity retention of 90.32%and 84.08%,showing a good prospect for wide-temperature applications.Secondly,further research was carried out to explore the low-temperature electrolytes based on the research results of wide-temperature electrolytes.The low-temperature electrolyte of LIBs was screened by varing the concentration of Li DFOB and lithium bis(trifluoromethanesulfonimide（Li TFSI）in carbonate solvents.With the increase of concentration,the degree of solvation in Li TFSI-based electrolytes gradually increased,while that in Li DFOB-based electrolytes remained low,which was favorable for the low-temperature desolvation process.A high cycle life of 400 cycles at-20℃ was gained using 2 M Li DFOB carbonate solvents electrolyte,maintaining a high capacity retention of 88.87%,showing a good potential for low temperature applications.Finally,triethyl phosphate（TEP）was added to the above wide-temperature electrolytes as a nonflamable additive.The influences of TEP on the high-temperature performance and flame retardancy of LIBs were investigated by testing the electrochemical properties and self-extinguishing time of electrolytes with different TEP contents.An electrolyte with high safety and high-temperature suitablility was obtained,showing a self-extinguishing time of 16 s,which was much lower than 54 s of the commercial electrolyte.