Electronic Paddle-Wheels Facilitate Transport in Solid-State Ionic Conductors

Solid-state ionic conductors (SSICs) are promising alternatives to liquid electrolytes in energy storage technologies. The rational design of SSICs and ultimately their deployment in battery technologies requires a thorough understanding of their ion conduction mechanisms. In SSICs containing molecular ions, molecular rotations couple to translational diffusion to create a “paddle-wheel” effect that facilitates conduction. This paddle-wheel mechanism explains many important features of molecular SSICs. However, we lack a similarly detailed explanation for anharmonic lattice dynamics and ion conduction in SSICs composed of monatomic ions. I will discuss our recent theoretical work that provides such an explanation. We predict that ion conduction in many SSICs involves “electronic paddle-wheels,” in which localized lone pair electrons rotate, and these rotations couple to and facilitate ion diffusion. After discussing our evidence from simulation results, I will make analogies to molecular SSICs and argue that the electronic paddle-wheel mechanism creates a unifying principle for understanding ion conductivity in both monatomic and molecular materials. We anticipate that a predictive understanding of electron paddle-wheels in ionic conduction can be leveraged to create design principles for engineering solid-state electrolytes from the electronic level on up.