Ion–polymer complexation and ion-pair formation in a polymer electrolyte PEO:LiPF6 containing an ionic liquid having same anion: A Raman study

• Polymer electrolytes based on (PEO + LiPF6) + x wt.% BMIMPF6 for x = 0–20 are reported.
• The dopant salt (LiPF6) and ionic liquid (BMIMPF6) having common anion PF6-.
• Formation of free and contact ion pairs in these electrolytes is also reported.
• These ion pairs are partly responsible for controlling overall ionic conductivity.

Raman spectroscopic studies on a ternary polymer electrolyte (PEO + 10 wt% LiPF6) + x wt% BMIMPF6 are reported in which the dopant salt (LiPF6) and ionic liquid (BMIMPF6) have a common anion PF6−PF6−. Knowledge of the number of mobile “free ions” and their mobility are important for ionic conduction which, in turn, depends on ion–polymer and ion–ion interactions. The present Raman spectroscopic study shows that (a) salt and IL (BMIMPF6) cations (i.e. Li+ and BMIM+) complex weakly with the ether oxygen of the polymer backbone and (b) contact ion pairs (like Li+⋯PF6− & BMIM+⋯PF6−) are present in these polymer electrolytes. The number of free anions and contact ion pairs has been calculated from Raman spectra and their role in ionic transport behavior is pointed out.