Mesopore size dependence of the ionic diffusivity in alumina based composite lithium ionic conductors

Ordered-mesoporous Al2O3 was synthesized by a sol-gel method using neutral copolymer surfactants as structure-directing agents. The pore size was controlled over the 3-15nm range by the use of various surfactants. Composites composed of the synthesized mesoporous Al2O3 and a lithium ion conductor (LiI) were prepared. The maximum dc electrical conductivity, 2.6 x 10(-4) S cm(-1) at 298K, was observed for 50 LiI.50 Al2O3 composite with 4.2nm average mesopore size, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which conductivity increased with decreasing pore size, except for samples with a pore size of 2.8nm. The lithium ion diffusion coefficient determined by the Li-7 pulsed field gradient nuclear magnetic resonance (PFG-NMR) showed excellent agreement with the measured conductivity calculated by the Nemst-Einstein equation. On the other hand, lithium migration activation energies obtained by quasielastic neutron scattering (QENS) and Li-7 NMR spin-lattice relaxation time (T-1) were considerably smaller than those obtained from electrical conductivity and PFG-NMR. This could be explained by the ion migration mechanism in heterogeneous composites and a possible enhancement of conductivity in mesoscopically confined spaces. (c) 2006 Published by Elsevier B.V.