Structural Basis for Emergence of Superionic Conductivity by an Ion Exchange

Abstract

Various superionic conductors have been examined in terms of the application to electrolytes for solid fuel cells [1]. Recently we demonstrated by impedance measurements that a simple two-step chemical reaction transformed an electronic conductor NaxCoO2into a superionic one. In the present study, we performed in situ synchrotron X-ray diffraction experiments to investigate a structural mechanism for the superionic conductivity driven by the chemical treatment of the layered oxide NaxCoO2. We developed a temperature- and humidity-controllable capillary cell under hydrogen and helium gas flow to install in the Debye-Scherrer camera at BL44B2 of SPring-8. This cell allows us to explore a structural transformation process by reduction and humidification treatments. Structural identifications and refinements with in situ diffraction data proved that Co vacancies formed by a CoO separation suppressed the electronic conductivity. Meanwhile it turned out from charge estimation in the Na layers that the superionic conductor transition originated from an ion exchange of H3O+for Na+, which was confirmed by Raman spectroscopy measurements. In addition, charge densities clearly visualized the H3O+ions disordering around the Na original sites, suggesting that the H3O+behave as a carrier source. Finally it was found from electrostatic potentials that the disordering H3O+sites were coupled through shallow potential barriers to trace a honeycomb-like ion pathway. In the presentation, I will discuss what a carrier is for the superionic-conductive phase from different viewpoints such as activation energies, concentration cell tests, and molecular dynamics simulations using the experimental structure information.