Role of Dynamically Frustrated Bond Disorder in a Li+ Superionic Solid Electrolyte

Inorganic lithium solid electrolytes are critical components in next-generation solid-state batteries, yet the fundamental nature of the cation–anion interactions and their relevance for ionic conductivity in these materials remain enigmatic. Here, we employ first-principles molecular dynamics simulations to explore the interplay among chemistry, structure, and functionality of a highly conductive Li+ solid electrolyte, Li3InBr6. Using local-orbital projections to dynamically track the evolution of the electronic charge density, the simulations reveal rapid, correlated fluctuations between cation–anion interactions with different degrees of directional covalent character. These chemical bond dynamics are shown to correlate with Li+ mobility and are enabled thermally by intrinsic frustration between the preferred geometries of chemical bonding and lattice symmetry. We suggest that the fluctuating chemical environment from the polarizable anions functions like a solvent, contributing to the superionic behavior of Li3InBr6 by temporarily stabilizing configurations favorable for migrating Li+. The generality of these conclusions for understanding solid electrolytes and key factors governing the superionic phase transition is discussed.