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

Abstract

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 SiO 2, Al 2O 3, TiO 2 (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 SiO 2 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 -]–SiO 2, Al 2O 3, TiO 2s 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 SiO 2–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) − SiO 2 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 SiO 2 particle surfaces, the absolute values of the diffusion coefficients of all constituent species decreased. The SiO 2 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.