Superionic Conduction in Co-Vacant P2-Nax CoO2 Created by Hydrogen Reductive Elimination

Area of Science:

• Materials Science
• Electrochemistry
• Solid-state Chemistry

Background:

• Layered P2-NaxMO2 (M: transition metal) compounds are typically electronic or mixed conductors.
• Understanding ionic transport mechanisms in these materials is crucial for energy storage applications.

Purpose of the Study:

• To investigate the role of cobalt vacancies in P2-NaxCoO2 on ionic conductivity.
• To identify the charge carrier responsible for ionic conduction in vacancy-ordered phases.

Main Methods:

• Synthesis of P2-NaxCoO2 with controlled cobalt vacancies via hydrogen reductive elimination.
• In situ synchrotron X-ray powder diffraction and Raman spectroscopy for phase composition analysis.
• Electromotive force measurements and molecular dynamics simulations to determine ion transport species.

Main Results:

• Cobalt vacancies introduced into P2-NaxCoO2 result in significant ionic conductivity (0.045 S/cm at 25°C).
• The superionic phase was identified as Na0.61(H3O)0.18Co0.93O2.
• Protons, not hydroxide ions, were confirmed as the primary charge carriers through experimental and simulation data.
• Co-stoichiometric compounds showed negligible ionic conductivity, highlighting the necessity of vacancies.

Conclusions:

• Cobalt vacancies are essential for achieving superionic conductivity in the P2-NaxCoO2 system.
• Proton transport is the dominant mechanism in these vacancy-ordered materials.
• This research offers insights into designing novel ionic conductors for electrochemical devices.