Li ion transport properties of amorphous/crystalline Li-La-Zr-Nb-O solid electrolyte thick films prepared by suspension plasma spraying

Abstract

Amorphous Li-La-Zr-O films are considered very attractive as a solid-state electrolyte in future batteries due to their low processing temperatures and resistance to Li-dendrite formation. The knowledge of amorphous Li-La-Zr-O phases is limited and thick films and bulk samples of amorphous Li-La-Zr-O have yet to be explored. Only thin film forms (~0.5 μm) of these amorphous Li-La-Zr-O phases have been prepared. In this work, we investigated the Li-ion transport properties of amorphous/crystalline mixtures of Li-La-Zr-Nb-O thick films (30 to 55 μm) prepared by the suspension plasma spray process. The process was manipulated to alter the amorphous Li-La-Zr-Nb-O content in the as-deposited films. Properties of the films that contained mostly amorphous Li-La-Zr-Nb-O (~76 wt%) were compared vs. mostly cubic Li garnet-type oxide (~55 wt%). The latter showed Li-ion transport properties that were similar to previously reported properties of cubic Li-garnet type oxides whereas the amorphous Li-La-Zr-Nb-O films exhibited significantly different properties. Their AC conductivity spectra show a Jonscher power law (JPL) behavior with the dimensionless exponent of ~0.92 that is followed by superlinear power law (SLPL) behavior with the dimensionless exponent of ~1.65 in the high frequency regime. These behaviors have not been reported earlier for the amorphous Li-La-Zr-Nb-O phases. Both of the JPL and SLPL behaviors have thermally activated characteristic with the activation energies of 0.76 eV and 0.64 eV, respectively. A lower value of ~0.45 eV was evaluated for the activation energy by solid-state 7Li NMR spectroscopy. The effect of temperature on the Li-ion dynamic process was studied by scaling the conductivity spectra and it was found to be temperature independent.