Li conduction in sputtered amorphous Ta2O5

Electron and Li ion conducting properties of room temperature sputtered amo rphous tantalum oxide (a-Ta2O5) films were studied in order to evaluate the feasibility of using a-Ta2O5 in electrochromic device applications. The fi lms were investigated using the galvanostatic intermittent titration techni que, impedance spectroscopy, and isothermal transient ionic current measure ments. It was found that the a-Ta2O5 met two out of three requirements pose d on a Li ion conductor in a WO3 based electrochromic device. There was a n egligible intercalation in the potential window used in WO3-based electroch romic devices (above 2.4-2.5 V vs. Li/Li+). Furthermore, in this potential region, the chemical diffusion coefficient for Li was larger than the corre sponding quantity in WO3. However, there was a nonzero electron conductivit y in the a-Ta2O5 films, not observed in the chemical vapor deposition-made beta -Ta2O5 investigated earlier. Still, the ionic conductivity was approxi mately one order of magnitude larger than the electronic one. (C) 2001 The Electrochemical Society.