Mechanochemical synthesis and ion transport properties of Na₃OX (X = Cl, Br, I and BH₄) antiperovskite solid electrolytes

The push towards the development of next-generation solid-state batteries has motivated the search for novel solid electrolyte materials. Sodium antiperovskites represent a structural family of ion conductors that has emerged as a result, with expected advantages in terms of composition tuning, electrochemical stability, mechanical softness and high ionic conductivity. Here, we report the mechanochemical synthesis of several materials in this structural family, including novel mixed-halide compositions such as Na₃OCl_0.5(BH₄)_0.5, Na₃OBr_0.5(BH₄)_0.5 Na₃OI_0.5(BH₄)_0.5 and Na₃OCl_0.33Br_0.33(BH₄)_0.33. We rationalize the effect of halide substitution on the structure and ion transport properties of these materials through diffraction, impedance spectroscopy and molecular dynamics. We conclude with a discussion on Na₃OBH₄, which has recently been reported to be a fast ion conductor, owing to the rotational disorder of the complex superhalide anion BH₄⁻. We are unable to reproduce the reported high ionic conductivity of Na₃OBH₄ neither by experiment nor ab initio simulation.