The Na_xMoO₂ 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, 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.

Exploration of the NaxMoO2phase diagram for low sodium contents (x≤ 0.5)

“…11 Recently, we reinvestigated this system in the composition range 0.5 ≤ x < 1 to explain the multiple-step aspect of the electrochemical curve using in situ synchrotron X-ray powder diffraction during the charge and the discharge of a sodium battery made with Na2/3MoO2 as the positive electrode. 13 Thirteen single phase domains were evidenced in the composition range 0.5 ≤ x < 1, 13 whereas the previous study mentioned only five phases in the sodium content range 0.28 ≤ x < 1. 11 Here we report the investigation of the NaxMoO2 phase diagram for sodium contents x ≤ 0.5 using electrochemistry combined with in situ X-ray diffraction.…”

“…The chemical composition was re-evaluated to Na0.63MoO2 according to the initial voltage measured when the material was used as a positive electrode in a sodium ion battery. 13 Electrochemical studies were carried out on Na | NaPF6 + 2% FEC in PC (1M) | NaxMoO2 batteries assembled in airtight 2032 type coin cells. At the positive electrode, the active material was mixed with carbon black to improve its electronic conduction and with PTFE (polytetrafluoroethylene) used as binder in a 88:10:2 weight ratio.…”

“…This was previously reported in our first investigation of the NaxMoO2 system for high sodium contents (x ≥ 0.5) by in situ X-ray diffraction. 13 For x = 0.5 a large voltage step (0.9 V) is observed, indicating the formation of a very stable phase. A detailed study of the material in the vicinity of this composition is presented in the next section.…”

“…The evolution of the position of the 002m reflection is correlated to the distance between two successive (MoO2)n layers and it is a good indicator to differentiate solid solutions from biphasic domains. 13,21 The overall changes in the cell parameters are emphasized by the displacement of the 110m, -201m, -111m and 200m reflections. As sodium is de-intercalated from Na0.5MoO2, X-ray diffraction peaks at high angles broaden until Na0.37MoO2 is formed.…”

Na2Ru1−xMnxO3 as the cathode for sodium-ion batteries

“…There are some peaks locate beyond 3.5 V when x is larger than 0.1, indicating the phase transitions in these materials. [43][44][45] The undesired phase transitions deteriorate the structural stability and aggravate the failure of the capacity eventually. For comparison, there are no peaks above 3.5 V for the sample x = 0 and 0.1, evidencing that there is no phase transition at high potential.…”