Enhanced Ionic Conductivity of Protonated Antiperovskite via Tuning on Paddlewheel Effect

The thermodynamically more stable lithium halide hydroxide, Li2OHCl, is experimentally easier to synthesize than Li3OCl. However, the protonated antiperovskite has the low ionic conductivity at room temperature due to the limited reorientation of OH anions. Here, density functional theory calculations were performed to determine the stability, elastic properties and Li diffusions of brominated Li2OHCl (Li2(OH)0.9Br0.1Cl and Li2OHCl0.9Br0.1). The Br substitution weakens the local bonding interactions and promotes the reorientation of OH anions, then increasing the ionic conductivities. Based on this, the reorientation of OH anions is further accelerated by substituting Cl by BH4 anions, and this novel protonated antiperovskite, Li2OHBH4, exhibits the high ionic conductivity of 2.8 mS/cm at room temperature. Our work highlights combining multi-factors, the overall soft lattice and the soft rotation mode of anion groups correlated to Li migrations, can effectively optimize the Li ionic conductivity, which might be a universal descriptor to further screen or design other classes of solid state electrolytes.