Structural and Compositional Factors That Control the Li-Ion Conductivity in LiPON Electrolytes

Amorphous Li-ion conductors are important solid-state electrolytes. However, Li transport in these systems is much less understood than for crystalline materials. We investigate amorphous LiPON electrolytes via <em>ab initio</em> molecular dynamics, providing atomistic-level insight into the mechanisms underlying the Li<sup>+</sup> mobility. We find that the latter is strongly influenced by the chemistry and connectivity of phosphate polyanions near Li<sup>+</sup>. Amorphization generates edge-sharing polyhedral connections between Li(O,N)<sub>4</sub> and P(O,N)<sub>4</sub>, and creates under- and overcoordinated Li sites, which destabilizes the Li<sup>+</sup> and enhances their mobility. N substitution for O favors conductivity in two ways: (1) excess Li accompanying 1(N):1(O) substitutions introduces extra carriers; (2) energetically favored N-bridging substitutions condense phosphate units and densify the structure, which, counterintuitively, corresponds to higher Li<sup>+</sup> mobility. Finally, bridging N is not only less electronegative than O but also engaged in strong covalent bonds with P. This weakens interactions with neighboring Li<sup>+</sup> smoothing the way for their migration. When condensation of PO<sub>4</sub> polyhedra leads to the formation of isolated O anions, the Li<sup>+</sup> mobility is reduced, highlighting the importance of oxygen partial pressure control during synthesis. Furthermore, this detailed understanding of the structural mechanisms affecting Li<sup>+</sup> mobility is the key for optimizing the conductivity of LiPON and other amorphous Li-ion conductors.