Decoding mixed-ion effects in halide electrolytes: entropy as a unified measure of a soft and disordered lattice

Abstract

Ion mixing has emerged as an effective strategy to modulate the ionic conductivity of halide-based solid-state electrolytes, yet the underlying mechanisms—particularly those involving lattice dynamics—remain elusive. Li lattice softening, manifested as a phonon spectrum redshift or bond length increase, and lattice disordering, reflected in the broadening of phonon density of states (DOS) and radial distribution function, have been explored to be correlated with Li diffusion, yet a quantitative descriptor that unifies the softening and disordering of lattice dynamics is missing. Herein, employing an integrated approach of first-principles calculations and molecular dynamics simulations, we systematically investigate mixed-ion halides to unravel the lattice dynamics origins of both stability and Li-ion diffusion. We reveal that oxidation potential is limited by the anion sublattice softening, which originates from the intrinsic electronegativity of anion species. In contrast, both lattice softening and disordering, induced by ion mixing, are the key to enabling fast Li ion diffusion, bypassing the traditional configurational entropy. Entropy (vibrational entropy and two-body entropy) emerges as the unified quantitative descriptor to capture the combined effects of lattice softening and disordering, revealing that higher entropy correlates with the low activation energy for Li-ion diffusion.