Lighting the Path—Design Principles for Halide-Ion Solid Electrolytes Through Spectroscopy and Simulation

Lithium-ion energy storage technology has become ubiquitous due to its unmatched energy density and diverse form factors. However, the large-scale production of lithium-ion batteries raises concerns about material criticality. Halide-ion batteries have emerged as a promising alternative, offering higher theoretical energy densities with substantially less criticality concerns as compared to lithium and post-lithium-ion technologies. This growing interest in halide-ion batteries has spurred research into anion migration mechanisms in periodic solids. Rare-earth oxyhalides, capable of crystallizing in diverse structures with well-separated halide-ion slabs, represent potential solid electrolytes for these batteries. By controlling synthesis methods and processing conditions, we prepare compositionally complex oxyhalides stabilized in different polymorphic forms. We will discuss LaOCl and ErOCl crystallized in PbFCl and and YOF/SmSI structures as model systems to explore composition—structure—function correlations. We will specifically discuss the role of site-selective modification in altering halide-ion migration pathways, diffusion energy barriers, and transition states. Using a combination of first-principles simulations, X-ray spectroscopy techniques, X-ray scattering and electrochemical impedance spectroscopy we examine vacancy formation and its implications for halide-ion mobility. Notably, X-ray Excited Optical Luminescence spectroscopy represents a novel probe for characterizing halide ion defects and mobility. First-principles simulations, including nudged elastic band calculations and ab initio molecular dynamics enable mapping of halide ion migration pathways. We will discuss the role of co-intercalation and anion replacement in introducing localized distortions within the halide layers, which provides access to low-energy anion-hopping mechanism. By examining the modulation of ionic conductivity with site-selective modification on the cation and anion sublattice and developing systematic—structure function correlations, we aim to advance understanding of ion transport phenomena within solid-state electrolyte systems and to design components vital to next-generation halide batteries.