Theoretical analysis of oxygen vacancies in layered sodium cobaltate Na_xCoO_{2-\delta}

TL;DR

This paper uses first principles calculations to analyze oxygen vacancies in sodium cobaltate, revealing that low vacancy concentrations are expected under typical application conditions, which impacts its thermoelectric performance.

Contribution

It provides a theoretical analysis of oxygen vacancy formation in Na_xCoO_2, clarifying contradictory experimental data and predicting low vacancy concentrations at relevant conditions.

Findings

Oxygen vacancies are unlikely to form in Na_xCoO_2 at typical temperatures and pressures.

Theoretical results reconcile conflicting experimental reports on oxygen stoichiometry.

Oxygen vacancies have a minimal impact on thermoelectric properties under normal conditions.

Abstract

Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in Na_xCoO_2 with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications.