Size-dependent ionic conductivity observed for ordered mesoporous alumina-LiI composite

Ordered mesoporous Al2O3 was synthesized by a solution sol-gel method using neutral copolymer surfactants as templates. The pore size was controlled over the range of 2.8–12.5 nm by using different surfactant copolymers under different synthetic conditions. By utilizing cyclohexane as a cosolvent, mesoporous Al2O3 having relatively monodispersed particle size was obtained. Composites composed of the synthesized mesoporous Al2O3 and the lithium ion conductor (LiI) was prepared by heating the composite in a sealed Pyrex tube at 773 K. Composition dependence of the dc conductivity was determined from a single arc in the observed impedance spectra. The maximum dc electrical conductivity, 2.6×10−4 S cm−1 at 298 K, was observed for 50 LiI·50 Al2O3 composite with 4.2 nm averaged mesoporosity, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which the conductivity increased with decreasing the pore size.