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, Al ₂O₃, 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, 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 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 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.