A new strategy for preparing oligomeric ionic liquid gel polymer electrolytes for high-performance and nonflammable lithium ion batteries

• The nonflammable gel electrolyte's limiting oxygen index can be as high as 29.
• High cell capacity under different C-rates and excellent cycle stability.
• New method to synthesize an oligomeric ionic liquid from conventional epoxy resin.

In the present work, a new strategy is used to economically synthesize an oligomeric ionic liquid from conventional phenolic epoxy resin. This oligomeric ionic liquid is further blended with PVdF-co-HFP and organic liquid electrolyte to prepare a high performance, nonflammable gel polymer membrane. Although the liquid electrolyte uptake is low (< 50%) for this novel gel polymer electrolyte, it possesses high ionic conductivities of 2.0 mS cm−1 at 30 °C and 6.6 mS cm−1 at 80 °C, respectively. The AC impedance results show that the interfacial compatibility between this gel polymer electrolyte and the electrodes is good. These two factors result in high cell capacity under different charge/discharge rates. Further, excellent cell-cycle stability after being charged and discharged 100 cycles is also demonstrated with the columbic efficiency to be up to 99. Due to the existence of the oligomeric ionic liquid, this novel gel polymer electrolyte exhibits superior dimensional stability; that is, at high temperature (150 °C) the dimensional change is less than 1%. Notably, the electrolyte’s limiting oxygen index can be as high as 29, meaning that it achieves the flame-retardant requirement under a normal atmosphere, which is essential to the safety of lithium ion batteries. These features allow this novel gel polymer electrolyte to function as a high performance and high safety lithium ionic conductor as well as a separator for lithium-ion batteries.