Enabling high-temperature processing of thin film Li-ion batteries using a LISICON based solid-state electrolyte

Lithium-ion batteries employing solid-state electrolytes (SSEs) are emerging as a safer and more compact alternative to conventional batteries using liquid electrolytes, especially for miniaturized energy storage systems. However, the industry-standard SSE, LiPON, imposes limitations due to its incompatibility with high-temperature processing. In this study, we investigate Li_4-<i>x</i> Ge_1-<i>x</i> P _<i>x</i> O₄ (LGPO), a LISICON-type oxide, as a promising alternative thin-film SSE. LGPO thin films are fabricated using pulsed laser deposition under four distinct deposition conditions, with <i>in situ</i> impedance spectroscopy enabling precise conductivity measurements without ambient exposure. We systematically correlate deposition temperature, background pressure, chemical composition, crystallinity, and morphology with ionic transport properties. Polycrystalline LGPO films grown at high temperature (535 °C) and low oxygen pressure (0.01 mbar) exhibited the highest room-temperature ionic conductivity (∼10^-5 S cm^-1), exceeding that of LiPON by an order of magnitude, with an activation energy of 0.47 eV. In contrast, amorphous films show significantly lower conductivity (∼5.2 × 10^-8 S cm^-1) and higher activation energy (0.72 eV). The results reveal that crystallinity, chemical composition, and grain boundary density critically affect ion transport, highlighting the importance of microstructural control. This work establishes LGPO as a viable, high-performance oxide SSE compatible with high-temperature processing for next-generation microbattery architectures.