Superior ionic conduction in LiAlO₂thin-film enabled by triply coordinated nitrogen

Developing a highly Li+-conductive thin-film electrolyte remains a challenge for the application of all-solid-state thin-film batteries. This paper reports an N-doped LiAlO2 glassy thin-film with high ionic conductivities prepared by reactive sputtering under an N2 atmosphere after the pattern of the gold standard LiPON thin-film solid electrolyte. The resulting Li+-conductive thin-film is dense without cracks and shows good adhesion to substrates. The doped N partially substitutes O and yields a triple coordination structure in the thin-films, which leads to an enhancement of ionic conductivity up to 3 orders of magnitude compared to pristine LiAlO2. On one hand, the triply coordinated nitrogen facilitates the formation of a cross-linked network and provides continuous pathways for Li+ transport. On the other hand, the triply coordinated nitrogen could provide additional hopping sites for Li+, which have a relatively weak bonding force to Li+ than O. This gives rise to an N-doped LiAlO2 thin-film with an ionic conductivity of 3.99 × 10-6 S/cm at room temperature. To the best of our knowledge, this is one of the highest ionic conductivities ever reported for LiPON-like thin-film solid electrolytes.