Deposition and Characterization of Li2O–SiO2–P2O5 Thin Films

Amorphous lithium electrolyte thin films, xLi2O·ySiO2·zP2O5, were deposited by rf magnetron sputtering of pure and mixed-phase lithium silicate, lithium phosphate, SiO2, Li2O, and Li2CO3 targets, and their compositions were determined using proton-induced y-ray emission spectroscopy, energy-dispersive X-ray analysis, Rutherford backscattering spectrometry, and atomic-emission spectroscopy. The deposition conditions were chosen to assure thermalization of the sputtered flux, which proved to be necessary in order to obtain a homogeneous distribution of Si and P in the films. Optical absorption and ac impedance measurements showed that glass-in-glass phase separation occurred in a large SiO2-rich domain of the composition diagram. In contrast to bulk glasses, all of the Li2O–SiO2 films were phase-separated, including those with lithia contents larger than lithium disilicate. High-performance liquid chromatography measurements revealed that, analogous to bulk glasses, the addition of SiO2 to Li2O-P2O5 compositions reduced the number of phosphate anion dimers, trimers, and higher anion polymers in the films through the formation of -Si-O-P-bonds. However, in contrast to bulk glasses, the distribution of phosphate anion polymers followed closely the Flory distribution, with the fraction of anion polymers decreasing monotonically with increasing chain length.