Enhancing ionic conductivity of LiSiPON thin films electrolytes: Overcoming synthesis challenges related to Li-migration in the precursor target

Currently, amorphous LiPON prepared by magnetron sputtering is the most employed thin film electrolyte due to its ionic conductivity (∼10⁻⁶ S.cm⁻¹), negligible electronic conductivity, absence of grain boundaries and ability to passivate Li metal. Despite the outstanding cycling performance that this combination of properties enables, its moderate conductivity hinders the use microbatteries in Internet of Things applications due to the need for short but high current pulses during communication phases. To better meet this requirement, LiSiPON thin films with ionic conductivities more than ten times greater than that of LiPON have been synthesized, while encountering some challenges in controlling the composition and the reproducibility of the synthesis. Herein, we have synthesized LiSiPON thin films from a set of precursor targets having distinct lithium concentrations. The main results indicate that an increase in the lithium content in the target material significantly enhances its ionic conductivity. Curiously, the most conductive target results in lithium-deficient and poorly conductive thin films that are not particularly reproducible in terms of composition and electrical properties. Our results suggest that lithium migration away from the sputtered area (or racetrack), favored by the high ionic conductivity of the target, is the origin of the resulting Li-deficient films. Finally, we have succeeded in preparing LiSiPO targets with sufficiently low Li-ion conductivity that enable the reproducible deposition of highly conductive LiSiPON solid electrolytes.