Temperature-Dependent Na-Ion Conduction and Its Pathways in the Crystal Structure of the Layered Battery Material Na2Ni2TeO6

Abstract

Na-ion conduction and correlations between Na-ion conduction pathways and the crystal structure have been investigated as a function of temperature in the layered battery material Na2Ni2TeO6 by impedance spectroscopy and neutron diffraction, respectively. The impedance data reveal an ionic conductivity of σ ≈ 2 × 10–4 S/m at 323 K, which strongly enhances with increasing temperature and attains a high value of ∼0.03 S/m at 423 K. The temperature-dependent conductivity data show an Arrhenius-type behavior with an average activation energy (Ea) of ∼0.58(3) eV for T ≥ 383 K. By employing soft bond valence sum analysis of the neutron diffraction patterns, we experimentally demonstrate the site-specific Na-ion conductions through visualization of microscopic sodium-ion conduction pathways and verify the recent theoretical results of molecular dynamics simulation. Our results reveal two-dimensional Na-ion conduction pathways that are confined within the ab planes of Na layers. Crystal structural study indicates that the layered structure involving Na-ion layers is responsible for high ionic conductivity, and the local crystallographic environment of Na-ion sites is responsible for site-specific conductivity. Our study further reveals that, for up to 500 K, the ionic conduction is governed by the Na ions located at the Na1 and Na2 sites, whereas all the Na ions located at the three Na sites contribute to the conduction process above 500 K. Our neutron diffraction study also establishes that the crystal structure of Na2Ni2TeO6 is stable for at least up to 725 K (the highest measured temperature), however, with an anisotropic thermal expansion (αc/αa ∼ 3).