Impact of Structural Coherence and Disorder on the Ionic Transport and Lattice Dynamics in Li + -conducting Argyrodites

Abstract

Solid-state batteries offer improved safety and higher energy density compared to conventional lithium-ion systems. Among candidate solid electrolytes, lithium argyrodites stand out for their exceptional ionic conductivity and compositional flexibility. Recent studies have revealed strongly anharmonic, liquid-like ion and lattice dynamics in these materials, including the collapse of soft phonons driven by Li⁺ diffusion, which impacts both local vibrations and thermal transport. Yet, the connection between local structure, phonon dynamics, and macroscopic heat transport remains unresolved. In this work, we employ post-synthesis processing to tune microstructural parameters-such as crystallite size, strain, and coherence length-in two model systems: Li 5.5 PS 4.5 Cl 1.5 and Li 6 PS 5 Br,. We systematically examine how mechanical treatments influence structural coherence, ion and lattice dynamics, and thermal transport. To further probe the role of structural disorder, we investigate bromide substitution in Li 6 PS 5 I. Across all compounds, thermal transport above 100 K is dominated by diffusons. At lower temperatures, however, structural disorder is significantly more effective than reduced coherence length at suppressing phonongas-type transport, underscoring the crucial role of local structure. Together with a detailed analysis of lithium-ion dynamics, these results provide new insights into how structural coherence and disorder govern both transport and vibrational properties in fast ionic conductors.