Electrical conductivity and dynamics of quasi-solidified lithium-ion conducting ionic liquid at oxide particle surfaces

Lithium ion conducting solid-state composites consisting of lithium ion conducting ionic liquid, lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) dissolved 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide (EMI-TFSA), denoted by [yMLi+][EMI+][TFSA−] in this study, and various oxide particles such as SiO2, Al2O3, TiO2 (anatase and rutile) and 3YSZ are synthesized via a liquid route for the molar concentration of lithium, y, to be 1. The composite consists of SiO2 and the ionic liquid with y = 0.2 was also prepared. The ionic liquid are quasi-solidified at the above oxide particle surfaces when x is below 40 for y = 1 and x is below 30 for y = 0.2, corresponding to the confinable thickness of the ionic liquid at the oxides' surfaces to be approximately 5–10 nm regardless of the oxide compositions. The electrical conductivities of x vol.%[yMLi+][EMI+][TFSA-]–SiO2, Al2O3, TiO2s or 3YSZ composites are evaluated by ac impedance measurements. The quasi-solid-state composites exhibited liquid-like high apparent conductivity, e.g. 10− 3.3–10− 2.0 S cm− 1 in the temperature range of 323–538 K for SiO2–ionic liquid composites with y = 1. The self-diffusion coefficients of the constituent species of x vol.% [yMLi+][EMI+][TFSA−] (x is below 40, y = 0.2 and 1) − SiO2 are evaluated by the Pulse Gradient Spin Echo (PGSE)-NMR technique in the temperature range of 298–348 K. By the quasi-solidification of the ionic liquid at SiO2 particle surfaces, the absolute values of the diffusion coefficients of all constituent species decreased. The SiO2 surfaces work to promote ionization of ion pair, [EMI+][TFSA−], while significant influence on the solvation coordination, [Li(TFSA)n + 1]n−, was not observed. The apparent transport numbers of Li-containing species both in the bulk and the quasi-solidified ionic liquid showed similar values with each other, which was evaluated to be in the range of 0.010–0.017 for y = 0.2 and 0.051–0.093 for y = 1 in the abovementioned temperature range.

► RTIL – Li-salt mixture could be quasi-solidified at oxide particle surfaces.
► Confinable thickness of RTIL – Li-salt at oxide particle surfaces is 5–10 nm.
► The composite had 10− 3.3–10− 2.0 S cm− 1 at 323–538 K.
► Self-diffusion coefficients of constituent species decrease by quasi-solidification.
► SiO2 surface promotes disassociation of ion-pair, [EMI+][TFSA−].