Effect of lattice flexibility and ion-ion interactions on transport in superionic conductors

Superionic conductors (SICs) play an important role in the development of solid-state batteries and thermoelectrics. The atomic mechanisms for rapid ion transport in such materials, however, remain poorly understood. Here, we use ab-initio and classical molecular dynamics (MD) simulations to study the diffusion of Ag⁺ ions in AgCrSe₂, a common SIC. Our ab-initio simulations reveal the dominant diffusion pathway taken by ions as they hop across the a and b lattice sites. To study macroscopic diffusion across length and time scales inaccessible by the ab-initio simulations, we develop a molecular-mechanics force field to describe the interactions of the mobile ions and the lattice framework. Classical MD simulations based on this force field demonstrate the role of lattice flexibility in modulating the energy barrier and vibrational frequency of Ag⁺ ions, and the role of ion-ion interactions in facilitating the concerted hopping of neighboring mobile ions on the lattice. Such physical insights into the transport mechanisms of ion transport in SICs could help optimize their performance.