Applied Physics Seminar: Atomic scale simulations of materials for solid-state energy storage

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. This seminar will describe two recent efforts aimed at closing this knowledge gap.
First, the atomic-scale connections between mobility, thermodynamic stability, and lattice distortions are described. 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. 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.

Second, ab initio molecular dynamics are used to characterize the local structure and migration processes in the prototype Li-ion conducting glass, 75Li2S–25P2S5. A model of the amorphous structure was generated and shown to closely match the measured neutron pair distribution function. Lithium migration is observed to occur via a complex mechanism that combines concerted motion of lithium ions with dynamic coupling to the rotational properties of the PS43- tetrahedra. This latter effect, commonly referred to as the ‘paddlewheel’ mechanism, is generally observed only in crystalline phases that are stable at elevated-temperatures. Surprisingly, in the glass, these calculations indicate that the paddlewheel mechanism contributes to Li-ion mobility at temperatures as low as 300 K. Paddlewheel contributions are confirmed through analyses of spatial, temporal, vibrational, and energetic correlations with Li motion. The possibility for paddlewheel dynamics at low temperature suggests that glasses containing complex anions may be fertile ground in the search for new solid electrolytes.