Structure of Li3YCl6 and Its Effect on Lithium Ionic Conductivity

Halide-based Li+ solid-state electrolytes have garnered renewed interest in recent years, mainly because of their significantly improved Li+ ionic conductivity and excellent stability against high-voltage cathodes. Numerous studies have reported that synthetic conditions and protocols can profoundly influence cation defect chemistry, thereby affecting Li+ mobility in these materials. Despite extensive investigations, the structures and fundamental mechanisms governing Li+ transport in these compounds remain highly debated, posing challenges for further performance optimization. In this report, we present a comprehensive structural analysis of Li3YCl6 synthesized under various conditions. Our findings reveal that the structural differences among the various forms of Li3YCl6 originate from variations in the type and length scales of Y-vacancy ordering within the hexagonal close-packed (hcp) anion framework. Our results show that samples treated at lower temperatures tend to exhibit a more uniform distribution of Y3+ and vacancies across the two different cation layers. In contrast, high-temperature treatment leads to the preferential occupation of one layer by Y3+, while vacancies dominate the other layer. This temperature-driven redistribution underlies the multiple-phase transitions observed upon heat treatment. Kinetic Monte Carlo simulations on the simulated and experimental structures suggest that the Y-vacancy uniformly distributed structure exhibits more balanced in-plane Li+ diffusion behavior; in contrast to the Y-vacancy segregated phases, where Li+ diffusion is facile in the Y-poor layer but is severely hindered in the adjacent Y-rich layer. This study resolves the longstanding controversies surrounding the structure of Li3YCl6 and highlights the importance of manipulating the cation-vacancy distribution for optimizing Li+ conductivity in hcp-based layered halide electrolytes.