The structural and transport properties of an organic ionic plastic crystal (OIPC), triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1444FSI) with the addition of lithium bis(fluorosulfonyl)imide (LiFSI) have been investigated with a combined experimental and computational approach. After establishing the effect of thermal processing on structural and transport properties of P1444FSI, a consistent processing procedure was established to understand how P1444FSI properties change with increasing temperature and LiFSI concentration. Elevated rotational motions of ions on the plastic crystalline lattice coincides with and may facilitate solid-solid phase transitions in P1444FSI, while the transport of lithium cations within LiFSI-doped P1444FSI was found to be dominated by increased translational diffusivity with little change in rotational diffusivity. The trade-off between increased ion conductivity in the solid-state and decreased mechanical robustness with increasing LiFSI concentration presents an important challenge for P1444FSI-based solid-state electrolytes moving forward.
Boynton et al. (Wed,) studied this question.