Coupled Dynamics of Mobile Ions and Anharmonic Phonons in Solid Electrolytes

The design of new solid-state electrolytes (SSEs) hinges on identifying and tuning relevant descriptors. While static structural descriptors have been correlated with fast diffusion, the host framework vibrations and their complex coupling with mobile ions remain less studied. Further, anharmonic effects in SSEs are drastic, leading to a breakdown of textbook models of harmonic phonons, necessitating more advanced considerations of mode spectral functions. We investigate phonons in a series of fast ion conductors (Li+, Na+, Cu+, Ag+) using a combination of neutron scattering measurements, ab-initio molecular dynamics (AIMD), and machine-learning molecular dynamics (MLMD) simulations. Using inelastic neutron scattering (INS) and MD simulations, we find that soft anharmonic phonon modes play an important role in facilitating ionic hops, reflecting the softness in the potential energy surface. The quasi-elastic neutron scattering (QENS) measurements, supplemented with large-scale MD simulations, probe the diffusivity and the diffusion characteristics, for instance whether hops occur in a cooperative fashion. Further, we find a clear crossover from crystal-like vibrations to a more liquid-like dynamics upon warming in several instances of SSEs [1-4]. These results offer detailed microscopic insights into the dynamic mechanism of fast ion diffusion and provide an avenue to search for further solid electrolytes. Identifying and controlling the pertinent phonon modes coupled most strongly with ionic conductivity, and assessing the role of anharmonicity, could pave the way for discovering and designing new SSEs.

[1] J. Ding, et al. Nature Physics 21, 118–125 (2025)

[2] M K. Gupta et al. Advanced Energy Materials 12, 2200596 (2022)

[3] M K. Gupta et al. Energy and Environmental Science 14, 6554 - 6563 (2021)

[4] J. L. Niedziela et al. Nature Physics, 15, 73–78 (2019)