Superionic Conduction in Co-Vacant P2-Na_xCoO₂ Created by Hydrogen Reductive Elimination

The layered P2-Na_xMO₂ (M: transition metal) system has been widely recognized as electronic or mixed conductor. Here, we demonstrate that Co vacancies in P2-Na_xCoO₂ 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 Na_0.61(H₃O)_0.18Co_0.93O₂. 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 Na_x(H₃O)_yCoO₂ does not exhibit any significant ionic conductivity proves that Co vacancies are essential for the occurrence of superionic conductivity. Migration routes: Na_x(H₃O)_yCo_1-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.