The Na_xMoO₂ Phase Diagram (¹/₂ ≤ x < 1): An Electrochemical Devil’s Staircase

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 Na_xMoO₂ system has been studied using electrochemistry combined with in situ synchrotron X-ray diffraction experiments. The many steps observed in the electrochemical curve of Na_2/3MoO₂ during cycling in a sodium battery suggest numerous reversible structural transitions during sodium (de)­intercalation between Na_0.5MoO₂ 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 Na_xMoO₂ specific phases that are believed to be characterized by sodium/vacancy ordering as well as Mo–Mo bonds and subsequent Mo–O distances patterning in the structures. Moreover, a room temperature triclinic distortion was evidenced in the composition range 0.58 ≤ x < 0.75, for the first time in a sodium layered oxide system. Monoclinic and triclinic subcell parameters were refined for every Na_xMoO₂ phase identified. Reversible [MoO₂] slab glidings occur during the sodium (de)­intercalation. This level of structural detail provides unprecedented insight on the phases present and their evolution, which may allow each phase to be isolated and examined in more detail.