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 Na3OCl0.5(BH4)(0.5), Na3OBr0.5(BH4)(0.5)Na3OI0.5(BH4)(0.5) and Na3OCl0.33Br0.33(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.