The NaxMoO2 Phase Diagram (1/2 ≤ x < 1): An Electrochemical Devil’s Staircase

Abstract

Layered sodium transition metal oxides represent a complex class of materials that exhibit a variety of properties, for example, superconductivity, and can feature in a range of applications, for example, batteries. Understanding the structure–function relationship is key to developing better materials. In this context, the phase diagram of the NaxMoO2 system has been studied using electrochemistry combined with in situ synchrotron X-ray diffraction experiments. The many steps observed in the electrochemical curve of Na2/3MoO2 during cycling in a sodium battery suggest numerous reversible structural transitions during sodium (de)intercalation between Na0.5MoO2 and Na∼1MoO2. In situ X-ray diffraction confirmed the complexity of the phase diagram within this domain, 13 single phase domains with minute changes in sodium contents. Almost all display superstructure or modulation peaks in their X-ray diffraction patterns suggesting the existence of many NaxMoO2 specific phases that are believed to be characteri...