Superionic Conduction in Co-Vacant P2-NaxCoO2 Created by Hydrogen Reductive Elimination

Abstract

The layered P2-NaxMO2 (M: transition metal) system has been widely recognized as electronic or mixed conductor. Here, we demonstrate that Co vacancies in P2-NaxCoO2 created by hydrogen reductive elimination lead to an ionic conductivity of 0.045 S cm-1 at 25 °C. Using in situ synchrotron X-ray powder diffraction and Raman spectroscopy, the composition of the superionic conduction phase is evaluated to be Na0.61(H3O)0.18Co0.93O2. Electromotive force measurements as well as molecular dynamics simulations indicate that the ion conducting species is proton rather than hydroxide ion. The fact that the Co-stoichiometric compound Nax(H3O)yCoO2 does not exhibit any significant ionic conductivity proves that Co vacancies are essential for the occurrence of superionic conductivity. Migration routes: Nax(H3O)yCo1-δO2 obtained by hydrogen reductive elimination has been found to exhibit superionic conductivity at room temperature. Disordered structures observed at the Na sites 1 and 3 imply the diffusion channels of conducting ion species. In molecular dynamics simulations using the experimental model, H and O delivered from H3O appear to migrate between the sites 2 and 3 and 1 and 2, respectively, forming a double honeycombed sublattice.