Revisiting the Mechanochemical Preparation of Li3YCl6 Electrolytes for All-Solid-State Lithium Batteries: Decisive Roles of Water Impurity in YCl3 Reactant

Li 3 YCl 6 (LYC) has emerged as a highly promising halide solid electrolyte for next-generation all-solid-state lithium batteries (ASSLBs) due to its high ionic conductivity and good compatibility with high-voltage cathodes. LYC is commonly prepared via mechanochemical balling of LiCl and YCl 3 reactants; however, reported ionic conductivity values vary widely, and reproducibility remains a challenge. Here, we identify trace water in the YCl 3 as a critical impurity that severely degrades the performance of the resultant LYC. Even trace water (< 500 ppm) in YCl 3 impairs the purity of LYC, whereas higher water content (> 1%) causes a 1-2 order of magnitude decrease in its ionic conductivity. This effect is far more detrimental than moisture in LiCl. By using an ultra-dry YCl 3 (H 2 O content < 10 ppm), high purity LYC was obtained after only 4 hours of ball milling, achieving a high and reproducible room-temperature ionic conductivity of 0.56±0.01 mS cm -1 . We also developed a simple dynamic argon-purge method at 720 °C to remove trace water from commercially available anhydride YCl 3 . The LYC prepared from this treated precursor shows a consistent conductivity of 0.42±0.02 mS cm -1 , an order of magnitude higher than that from untreated YCl 3 . More importantly, the ASSLBs constructed with this LYC deliver comparable initial capacity, Coulombic efficiency, rate capability, and capacity retention to those using LYC from ultra-dry YCl 3 precursors. This study highlights the decisive role of trace water in YCl 3 for the mechanochemical preparation of high-quality LYC electrolytes for ASSLBs.