Application of 1-butyl-3-menthylimidazolium-hexafluorophosphate as flame retardant in electrolyte of lithium ion battery

Carbonate-based organic electrolytes with lower flash points are widely used in commercial lithium-ion batteries, boosting the development of energy storage system. Safety problems, arising from the combustion and explosion caused by carbonate-based organic electrolytes, have become one of the bottlenecks restricting large-scale application of lithium-ion batteries. In this work, flame-retardant or non-combustible electrolyte systems were investigated. Because of the excellent flame retardancy, low flammability, and high conductivity, thermal, and electrochemical stability, 1-butyl-3-menthylimidazolium-hexafluorophosphate ([Bmim]PF6) was used to generate imidazolium ionic liquids based electrolytes. The flame retardant and electrochemical performances of [Bmim]PF6-based binary electrolytes with different ionic liquid ratios, and the comparison with commercial carbonate electrolytes (CEs) were characterized. Results show that compared with commercial carbonate solvent electrolytes, battery performances, such as thermal safety and electrochemical stability of the proposed [Bmim]PF6 binary electrolyte, are superior to that of CEs when they are used in LiFePO4 electrode batteries. The binary electrolyte with 5% of [Bmim]PF6 ionic liquid possesses the best comprehensive performance. Explicitly speaking, the ionic conductivity and combustibility of the binary electrolyte are higher and significantly lower than that of CEs, respectively, thus the significantly improving the conductivity and thermal stability. Furthermore, since the battery's discharge capacity and capacity retention rate using the binary electrolyte after 100 cycles of charge and discharge at 1 C charge-discharge rate are higher than those using CEs, the [Bmim]PF6 binary electrolyte also shows good electrochemical stability.