Reaction mechanism studies towards effective fabrication of lithium-rich anti-perovskites Li3OX (X = Cl, Br)

• Used reducing reagents (Li metal and LiH) as starting materials, trying to eliminate OH impure phases in the final product.
• Used Li2O and LiBr as starting materials, avoiding OH phases at the beginning; used ball milling to activate the reaction.
• Explored the reaction mechanism for solid-state synthesis of Li3OX (X = Cl, Br) by IR, TG-DSC, theoretical calculation, EIS, etc.

Lithium-rich anti-perovskites (LiRAPs), with general formula Li3OX (X = Cl, Br), recently reported as superionic conductors with 3-dimensional Li+ migrating channels, are emerging as promising candidates for solid electrolytes in all-solid-state lithium-ion batteries (LIBs). However, great challenges remain in the fabrication of pure LiRAPs due to difficulties such as low yield, impurity phases, thermodynamic instabilities, and moisture-sensitivity. In this work, we thoroughly studied the formation mechanism of Li3OCl and Li3OBr using various solid-state reaction routes. Different experimental strategies were developed to improve the syntheses, namely, for the purposes of phase stability, phase purity, and large-scale production. One feasible method is to use the strong reducing agents Li metal or LiH to eliminate the OH species. The results show that LiH is more effective than Li metal, mainly due to negatively charged H− and reaction uniformity. The other successful method employs a solid diffusion approach using Li2O and LiX as the starting reagents, thereby avoiding OH entirely; ball milling of reagents under Ar atmosphere was utilized to decrease initial grain size and increase the reaction rate. Fourier transform infrared spectroscopy (FTIR), thermal analyses, and first-principles calculations were performed to give indications on the reaction pathway.