Lattice Dynamics of Superionic Conductors

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Solid state batteries are safer, potentially lighter, have a higher energy density, and have longer lifetimes than current batteries. A solid state battery uses a solid state electrolyte that must conduct ions at rates comparable to those of liquid electrolytes. Such materials are called fast ionic conductors. The microscopic origin of such high ionic conduction in solids is still poorly understood, in particular for systems that exhibit correlated, or liquid-like diffusion. In this early stage report, we argue that lattice dynamics, and more precisely anharmonic vibrational modes with large amplitudes and strong directionality towards the diffusion path, must play a role in fast ionic conduction. We highlight how the lattice softening of such vibrational modes provides exactly those conditions. Then, we explore this hypothesis in Li3N, a simple fast ionic conductor that shows correlated diffusion. We found, using first principles lattice dynamics calculations, that there is significant softening of certain vibrational modes pointing in the direction of the diffusion path. This supports our hypothesis that lattice softening, and fast ionic conduction are linked. It could potentially explain correlated diffusion too. We anticipate our findings to open the way for systematic studies of lattice softening in superionic conductors, which could potentially be turned into a descriptor for high-throughput screening. This would provide a highway for more informed design of solid state batteries, and speed up the whole lab-to-product timeline.