Non-Arrhenius Li-ion transport and grain-size effects in argyrodite solid electrolytes

TL;DR

This study combines machine learning and multiscale modeling to understand Li$^+$ transport in argyrodite solid electrolytes, revealing how microstructure and grain size influence ionic conductivity and transport mechanisms.

Contribution

It introduces a novel multiscale modeling framework with machine-learning potentials to analyze Li$^+$ transport in argyrodite electrolytes, accounting for grain boundary effects and microstructure.

Findings

Grain boundaries can enhance or suppress Li$^+$ diffusion depending on the bulk phase.

Nanosizing can activate ionic transport in non-superionic electrolytes.

Grain refinement improves intergranular contacts without reducing superionic conductivity.

Abstract

Argyrodite solid electrolytes, such as Li$_6$PS$_5$Cl, exhibit some of the highest known superionic conductivities. Yet, the mechanistic understanding of Li$^+$ transport in realistic argyrodite microstructures -- where atomic-scale mechanisms interplay with continuum-scale dynamics at grain boundaries -- remains limited. Here, we resolve Li$^+$ transport in silico by developing accurate machine-learning potentials via closed-loop active learning and embedding the potentials in a multiscale modeling framework that integrates molecular dynamics with finite element simulations. We show that bulk diffusion barriers scale linearly with anion radius. Grain boundaries have opposite effects depending on the bulk -- enhancing Li$^+$ diffusion in low-diffusivity phases but suppressing it in fast-diffusing ones. Simulations of polycrystalline Li$_6$PS$_5$I reveal non-Arrhenius transport behaviors consistent with experiments. Grain-size-dependent predictions indicate that grain refinement improves intergranular contacts in argyrodites without compromising superionic conductivity, while nanosizing can activate ionic transport in electrolytes lacking intrinsic superionic behavior. Our findings highlight the decisive role of microstructure in developing solid electrolytes.