Unveiling the Origins of High Ionic Conductivity in Lithium Phosphorus Oxynitride Amorphous Electrolytes

Lithium phosphorus oxynitride, having an amorphous structure, has enabled all-solid-state thin film batteries with lithium metal anodes and high-voltage cathodes since the nineties. Nevertheless, the origins of its outstanding ionic conductivity compared to its crystalline counterparts, as well as the interplay between structure and ionic transport in this electrolyte, have remained elusive. Herein, we have applied a compelling methodology based on impedance spectroscopy analyses to isolate the distinct energetic contributions for the ionic conduction process, namely, the enthalpies for defect formation and migration. The variations of these enthalpies with the nitrogen content are correlated with structural aspects unveiled by solid-state nuclear magnetic resonance (NMR) and depth profiling X-ray photoelectron spectroscopies. The main findings indicate that the amorphous structure, inherent to radiofrequency magnetron sputtering synthesis, is the root of a striking decrease of the enthalpy related to defect formation, while the nitrogen incorporation plays a crucial role in Li+ ion mobility by forming bridging species, which tend to lower the enthalpy of migration.