Synthesis and Characterization of Sodium Anti-perovskite Ion Conductors for Solid State Batteries

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.<br/>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]<br/>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]<br/>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^[9], 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. <br/><br/> <br/>References:<br/>1. Yiu, Nicholas. et al “State of Batteries Report 2020.” BatteryBits, 16 January 2021<br/>2. J. Janek and W. G. Zeier, Nat. Energy, vol. 1, no. 9, 2016, p. 16141.<br/>3. Scarpa G, Atti R. Accad. Naz. Lincci, Sez. II. 1915;24:476-482<br/>4. W. Muller and M. Jansen, Z. Anorg. Allg. Chem., 1990, 591, 41–46<br/>5. M. Jansen, C. Feldmann and W. Muller, Z. Anorg. Allg. Chem., 1992, 611,7–10<br/>6. J.A. Dawson, T. Famprikis, K.E. Johnston. J. Mater. Chem. A, 2021,9, 18746-18772<br/>7. K. Kim, D.J. Spiegel. J. Mater. Chem. A, 2019,7, 3216-3227<br/>8. S. V. Krivovichev, Z. Kristallogr. Cryst. Mater., 2008, 223, 109–113<br/>9. A. Mutschke, G.M. Bernard, M. Bertmer, et al. Angew Chem Int Ed. 2021;60(11):5683-5687.