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

摘要

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.