Layered sodium transition metal oxides represent a complex class of materials that exhibit a variety of properties, e.g. superconductivity, and can feature in a range of applications, e.g. 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 suggests 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, thirteen 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 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 NaxMoO2 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.

Neutron diffraction is a powerful technique to localize and quantify lithium in battery electrode materials. However, obtaining high-quality operando neutron diffraction data is challenging because it requires achieving good electrochemical performance while cycling a large amount of active material to ensure optimal signal-to-noise ratio. We have developed a cylindrical cell specifically suited for operando neutron diffraction studies, and used it to investigate the structural changes in the Ni-rich LiNi 0.6 Co 0.2 Mn 0.2 O 2 cathode material during cycling between 2.5 and 4.3 V vs. Li + /Li. The cell demonstrates reliable electrochemical performance, even after long-term cycling, and the important crystal structure parameters of the active material, including Li occupancy, could be successfully refined with the Rietveld method using neutron diffraction data collected in operando after appropriate background subtraction.

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