Correlating lattice distortions, ion migration barriers, and stability in solid electrolytes

The emergence of solid electrolytes with ionic conductivities comparable to that of liquids has improved the prospects for realizing solid-state batteries. Although a small number of ionically-conducting solids are now known, most exhibit shortcomings such as limited interfacial stability and susceptibility to dendrite penetration. Consequently, the discovery of alternative solid electrolytes remains an important goal. This search has been slowed, however, by incomplete understanding of the elementary chemical and structural features that give rise to high ionic mobility. Here we characterize the atomic-scale connections between mobility, thermodynamic stability, and lattice distortions. The degree of lattice distortion, described by the tolerance factor, t, was systematically varied via isovalent composition variation across a series of model anti-perovskite ion conductors. Larger distortions are observed to correlate strongly with lower energy barriers for percolating ion migration: distortions suppress corrugations of the potential energy surface by widening a subset of migration channels and by destabilizing equilibrium configurations. As larger distortions also correlate with reduced stability, realizing high ionic mobility in this class of conductors requires balancing a mobility/ stability tradeoff. Na3SI is identified as one such balanced material.