Lithium phosphorus oxynitride, having an amorphous structure, has been enabling all-solid-state thin film batteries with lithium metal anode 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 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$^+$ ions mobility by forming bridging species, which tend to lower the enthalpy of migration.