P2-NaxVO2 system as electrodes for batteries and electron-correlated materials

Abstract

Layered oxides are the subject of intense studies either for their properties as electrode materials for high-energy batteries or for their original physical properties due to the strong electronic correlations resulting from their unique structure. Here we present the detailed phase diagram of the layered P2-Na(x)VO(2) system determined from electrochemical intercalation/deintercalation in sodium batteries and in situ X-ray diffraction experiments. It shows that four main single-phase domains exist within the 0.5≤x≤0.9 range. During the sodium deintercalation (intercalation), they differ from one another in the sodium/vacancy ordering between the VO(2) slabs, which leads to commensurable or incommensurable superstructures. The electrochemical curve reveals that three peculiar compositions exhibit special structures for x = 1/2, 5/8 and 2/3. The detailed structural characterization of the P2-Na(1/2)VO(2) phase shows that the Na(+) ions are perfectly ordered to minimize Na(+)/Na(+) electrostatic repulsions. Within the VO(2) layers, the vanadium ions form pseudo-trimers with very short V-V distances (two at 2.581 Å and one at 2.687 Å). This original distribution leads to a peculiar magnetic behaviour with a low magnetic susceptibility and an unexpected low Curie constant. This phase also presents a first-order structural transition above room temperature accompanied by magnetic and electronic transitions. This work opens up a new research domain in the field of strongly electron-correlated materials. From the electrochemical point of view this system may be at the origin of an entire material family optimized by cationic substitutions.

Pure Single‐Crystalline Na1.1V3O7.9 Nanobelts as Superior Cathode Materials for Rechargeable Sodium‐Ion Batteries

“…According to the XPS spectra, vanadium can interchange from V 5+ to V 2+ during the process of intercalation and deintercalation of Na + ion in the NVONBs and most of these changes are occurred between V 5+ and V 3+ . Although some small amount of irreversible low state vanadium (V 2+ ) exist in these complex electrochemical reactions, the reversible vanadium interchange dominates the Na ion insertion/extraction processes, which is consistent with the result of XRD in Figure 4b and previous reports 14. Figure 5d–f shows XPS spectra of O 1s regions before and after electrochemical reaction, respectively.…”

“…These results confirm the complexity of the electrochemical reactions of AVMs and the cycled product might be composed of Na–V–O and V–O matrixes. [[qv: 12a]],14 …”

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

“…The multi-step nature of the corresponding electrochemical curve, presented in Figure 3a, suggest numerous structural transitions in agreement with the observations reported in the literature 24 . In most of the other NaxMO2 systems (M = Co, V, Cr, Ni...) reported in the literature 6,9,10,[33][34][35][36] , the most important voltage drops occur for the Na1/2MO2 and Na2/3MoO2 compositions which are energetically favorable for sodium/vacancy ordering. The initial sodium content in the starting material NaxMoO2 is therefore reevaluated by fitting the 2.5 V and 1.5 V voltage drops at x= 1/2 and 2/3, respectively.…”

“…This family of compounds composed of [MO2] slabs with Na + ions inserted in the interslab space, can also feature interesting physical properties and thus are of signfiicant interest in the search of new electron correlated materials [1][2][3][4][5][6][7][8][9][10][11] . Notably, the interaction between the sodium and sodium vacancy ordering (whether it is commensurate or not) and the possibility of charge ordering in the [MO2] layers lead to peculiar cationic distributions within the [MO2] layers.…”

“…The result of which is a map of the phase diagram of the NaxMO2 systems studied under the electrochemical conditions employed. 6,9,10,13 .…”