IONIC-CONDUCTIVITY AND STRUCTURE OF (LICL)2-AL2O3-SIO2 XEROGELS

(LiCl)2-Al2O3-SiO2 xerogels were prepared by a sol-gel process. The structure of the heat-treated xerogels was studied by powder X-ray diffraction, Fourier transform infrared spectroscopy, and Li-7, Si-29, and Al-27 nuclear magnetic resonance techniques. Li-7 NMR studies indicate two kinds of lithium ions in the xerogels: (a) polycrystalline LiCl dispersed in the pores of the xerogel and (b) lithium ions associated with nonbridging oxygens on the silica tetrahedra or associated with AlO4- units. For the samples having the same lithium content (20 mol % (LiCl)2) the ionic conductivity increases with increasing Al2O3/SiO2 ratio. This is attributed to the increasing concentration of lithium ions associated with tetrahedral AlO4 or nonbridging oxygens in the xerogel. A constant activation energy is observed when the (LiCl)2 content increases from 10 to 20 mol % for samples having the sample A12O3/SiO2 ratio of 1/2. This constant activation energy is analyzed in terms of the competition between the decreasing jump distance tending to decrease activation energy and increasing Coulombic interaction tending to increase activation energy.