In 2020, 23 countries and states have announced bans on fossil fuel vehicles within the next couple of decades.[1] Current lithium-ion technologies present both gravimetric and volumetric energy density limitations.[2] These limitations can be overcome by the use of solid electrolytes and metallic anodes in all solid-state batteries.[2] Inorganic solid electrolytes can be classified into 4 main categories, oxide, sulfide, halide, and anti-perovskites, each with their own pros and cons. The first anti-perovskites date back to 1915 and are the combination of alkali halides and their hydroxides.[3] Furthermore in the late 1900s, Jansen et al. reported the first ionic conductivities for Na-ion anti-perovskites on the order of 10-5 S/cm at 480 K.[4,5] Throughout the last decade, Na-ion anti-perovskites have been studied with reported conductivities between 10-2 to 10-9 S/cm at room temperature.[6] Through tailoring of the anti-perovskite crystal structure and composition, it is believed that the migration barrier for Na-ion hopping can be reduced, facilitating Na-ion motion through the structure.[7] Because the perovskite and hence the anti-perovskite structure is so versatile, there are many different material compositions that can be studied. The anti-perovskites with most interest have been those with the simple X3BA composition. Recently, interest has been drawn to anti-perovskites found in nature, ex: Kogarkoite Na3(SO4)F.[8] In this work, we have studied various new and existing compositions of anti-perovskites and measured their ionic conductivities for use in solid state batteries. Some of the compositions studied include the new sulfate hydride Na3SO4H, Na2(Ca/Mg)PO4F isostructural to existing Na2MPO4F cathode materials, and Na3(S/SeO4)(F/Cl). Recently, a new composition has been synthesized with conductivities around 10-3 S/cm at RT, a promising value for use as solid ionic conductors.

Synthesis and characterization of sodium anti-perovskites ion conductors for solid state batteries

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