Synthesis of the Metastable Cubic Phase of Li₂OHCl by a Mechanochemical Method

The oxyhalide-based solid electrolyte Li₂OHCl usually forms the thermodynamically stable orthorhombic phase at room temperature and shows poor lithium ionic conductivity. Above 35 °C, a structural phase transition into the cubic phase occurs and ionic conductivity is enhanced. In this work, mechanochemical synthesis of Li₂OHCl is reported. The as-prepared Li₂OHCl formed a cubic Pm3̅m structure and showed an ionic conductivity of 2.6 × 10^-6 S cm^-1 at 25 °C. Once the cubic phase was treated at 200 °C, the orthorhombic Pmc2₁ structure appeared at 25 °C and the ionic conductivity decreased down to 1.4 × 10^-7 S cm^-1. Formation of the metastable cubic phase could be explained in terms of low crystallinity of Li₂OHCl derived from mechanochemical synthesis.