Engineering ionic pathways by phonon-structure connection in perovskite solid electrolyte Li0.29La0.57TiO3

Lithium lanthanum titanium oxide (LLTO) is a promising solid electrolyte (SE) for solid-state batteries, owing to its excellent chemical and thermal stability, wide electrochemical window, and high ionic conductivity even at room temperature. However, the influence of lattice dynamics and local structural features on lithium transport remains poorly understood. In this study, we employ large-scale machine-learned molecular dynamics (MLMD) simulations alongside inelastic neutron scattering (INS) measurements to reveal the role of strongly anharmonic soft phonons in Li-ion diffusion. Unlike many other materials where soft phonons facilitate ion transport, we find that in LLTO, these phonons suppress Li diffusion by inducing tilt or rotational distortions in the lattice. These distortions constrict the bottlenecks that significantly hinder ionic conductivity. Our results show that stabilising these soft phonon modes and thereby restricting the oxygen dynamics, combined with the inherent anharmonicity of Li-related vibrations, can substantially enhance Li diffusion. Thus, in perovskite-based SEs, soft phonon modes and oxygen dynamics emerge as critical descriptors for designing advanced electrolytes. Additionally, our investigation of lattice thermal conductivity using the Green-Kubo formalism reveals glass-like thermal transport behaviour, arising from the disordered structure and extremely anharmonic soft modes. Lithium lanthanum titanium oxide is a stable, high-conductivity solid electrolyte for solid-state batteries, but the roles of lattice dynamics and local structure in lithium transport remain unclear. Here, the authors use large-scale machine-learned molecular dynamics simulations and inelastic neutron scattering measurements to clarify the role of anharmonic soft phonons in Li-ion diffusion.