Non-Arrhenius Ionic Conductivity Behaviour in Antiperovskite Solid Electrolytes for Sodium Metal Batteries

Abstract

Solid-state sodium metal batteries have been proposed as an alternative to lithium-ion batteries (LIBs) for next-generation applications due to the high abundance and wide distribution of sodium resources.1 Sodium antiperovskites (Na3OX, X = Cl, Br, or BH4) have recently attracted great interest as solid electrolyte candidates due to the relatively low processing temperature (<500 oC), reasonable ionic conductivity (up to 4.4 x 10-3 S cm-1 at room temperature), and great chemical stability against sodium metal anodes.2,3 Typical solid-state ionic conductors have temperature-dependent ionic conductivity that follows Arrhenius behaviour over a broad range of temperatures, corresponding to a constant activation energy Ea. However, this is not the case in Na3OBr, and instead a phenomenon resulting in a higher ionic conductivity and lower activation energy is observed to occur at 255 oC. Similar non-Arrhenius ionic conductivity phenomenon has also been reported in the potassium antiperovskite K3OI. It has been proposed that this behaviour originated from anion disordering, but conclusive experimental evidence to support this claim has not been reported.4 An understanding of the origin of this behaviour could enable the synthesis route to be optimised such that higher ionic conductivities could be achieved.In this work, we combined temperature-resolved bulk and local characterisation methods including Pawley and Rietveld refinements of synchrotron x-ray diffraction (XRD), pair distribution function (PDF), solid-state nuclear magnetic resonance (NMR) spectroscopy, and electrochemical impedance spectroscopy (EIS) to gain an insight into the origin of this non-Arrhenius behaviour. We found that this phenomenon does not originate from anion disordering, as previously speculated, because the lattice parameter of Na3OBr changed linearly with temperature and no major intensity changes were observed in the Na-Br and Na-O signals in the PDF patterns. Other supporting findings that lead us to this conclusion will be discussed in the presentation.References Wang, X. et al. Ultra-stable all-solid-state sodium metal batteries enabled by perfluoropolyether-based electrolytes. Nat. Mater. 21, 1057–1066 (2022).Zheng, J., Perry, B. & Wu, Y. Antiperovskite Superionic Conductors: A Critical Review. ACS Mater. Au 1, 92–106 (2021).Sun, Y. et al. Rotational Cluster Anion Enabling Superionic Conductivity in Sodium-Rich Antiperovskite Na3OBH4. J. Am. Chem. Soc. 141, 5640–5644 (2019).Zheng, J. et al. Antiperovskite K3OI for K-Ion Solid State Electrolyte. J. Phys. Chem. Lett. 12, 7120–7126 (2021).