Solid-State Ionics in Halide Superionic Conductors

Abstract

Halides have emerged as a new class of solid electrolytes that can deliver superionic conductivity comparable to that of state-of-the-art sulfide electrolytes as well as electrochemical stability suitable for high-voltage (>4 V) operations. Although these merits have led to an extensive exploration of various halide-based materials in recent years, our understanding of the superionic conduction mechanism and its structure dependency remains in its infancy, impeding a rational design of new halide electrolytes. Herein, we investigate lithium-ion conduction mechanisms in halides by focusing on two representative structural classes: the trigonal phase, which features an hcp-stacking anion framework, and the monoclinic phase, characterized by a ccp-stacking anion framework. For trigonal halides such as Li 3 MCl 6 (M=Y, Er), we find that superionic conduction is primarily controlled by the in-plane lithium percolation pathways and the interlayer spacing, both of which are sensitively altered by the distinct composition/ordering of cations (M) arising from the structural flexibility of Li 3 MCl 6 . Moreover, it is demonstrated that these two factors are inversely correlated with each other by the partial occupancy of M, which turns out to be a diffusion inhibitor and, simultaneously, a pillar that provides a large interlayer space for facile lithium diffusion. These findings infer that there exists a critical range/ordering of M in trigonal halides that can allow high ionic conductivity while elucidating previous reports of the considerable discrepancies in lithium diffusivities observed for Li 3 MCl 6 (M=Y, Er). Accordingly, we propose a general design criterion for superionic trigonal halide solid electrolytes and showcase that even a simple change in the M ratio (per Cl or Li) based on this strategy can result in a remarkable increase of the ionic conductivity, leading to the highest recorded value in the trigonal family (1.19 mS cm − 1 ). Furthermore, we study the fundamental diffusion mechanisms in ccp-stacked monoclinic halides, establishing general design rules for next-generation halide superionic conductors.