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 Pm3¯ m 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.