Local Structure in Sodium Antiperovskite Ionic Conductors
Authors: Kim Junghwa , Kim Dennis S. , Zhang Duhan , Li Yiliang , Chiang Yet-Ming , LeBeau James 
Published: September 2023

Antiperovskites, which can serve as solid state electrolytes, have recently received great attention due to their high ionic conductivity[1]. Compared to typical solid-state ionic conductors, Na3OCl antiperovskite exhibits a unique ionic conductivity behavior that deviates from a linear Arrhenius behavior, in which the ionic conductivity increases rapidly with increasing temperature[2,3]. Many different structural origins affecting the ion conduction mechanism in antiperovskite have been proposed using calculation, such as vacancy/dumbbell migration[4]. The lack of direct experimental local structural investigations makes it challenging to fully understand the structural origins for high ionic conductivity, even resulting in limitations in the development of material designs for good ionic conductors. Complicating their investigation, these materials are also extremely sensitive to air, preventing many structural studies.
Here, local structural investigations of Na3OCl are performed using aberration corrected scanning transmission electron microscopy (STEM) to elucidate the atomistic mechanisms underlying its high ionic conduction. To avoid degradation in air, we use an inert gas transfer system that has several advantages. First, transfer in an inert gas environment without air/moisture exposure enables the sample to retain the on-stoichiometry. Second, both room temperature and cryo experiments are possible, which allows pristine structure investigation at application relevant temperatures. Beyond the environment, Na3OCl is also very electron beam sensitive. We will show that low-dose imaging methods, such as parallel beam electron diffraction and electron ptychography, can provide new insights and allow for direct imaging of the local structure. For example, diffuse electron scattering (scattering between the Bragg reflections) is used to investigate subtle differences resulting from local structure, atomic defects, and phonon dispersion relations through combination with electron scattering simulations and Molecular Dynamics (MD) using machine learned potentials. We will then demonstrate how electron ptychography can achieve deep sub-angstrom spatial resolution with low dose conditions and provide direct insights into the migration of Na-ions. Overall, the inert gas transfer system and low-dose diffraction and STEM ptychography enable direct imaging of local structures even for reactive and electron beam sensitive materials, such as antiperovskite electrolytes. The understanding of local structure related to the high ionic conductivity will be crucial for designing high-performance solid electrolytes.

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[2] Wang, F. et al. Dynamics of hydroxyl anions promotes lithium ion conduction in antiperovskite Li2OHCl. Chemistry of Materials 32, 8481–8491 (2020)
[3] Li, Y. et al. Tuning Ionic Conductivity in Sodium Anti-Perovskite Ionic Conductors in ECS Meeting Abstracts 945 (2020)
[4] Kim, K. & Siegel, D. J. Correlating lattice distortions, ion migration barriers, and stability in solid electrolytes. Journal of Materials Chemistry A 7, 3216–3227 (2019)