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-NaxVO2 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 VO2 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-Na1/2VO2 phase shows that the Na+ ions are perfectly ordered to minimize Na+/Na+ electrostatic repulsions. Within the VO2 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.
Figures at a glance
Figure 1: X-ray diffraction pattern of the P2-Na0.71VO2 composition obtained by solid-state synthesis. 
It shows that the sample is actually a mixture of two P2-NaxVO2 phases, with small amounts of V2O3 and Na3VO4 impurities (the main Bragg peaks for the V2O3 and Na3VO4 impurities are marked by open squares and open rhombs, respectively). The basic P2-AxMO2 structure type with hexagonal P63/mmc symmetry is given in the inset. The Bragg peak positions are shown by the vertical lines for the major (red) and minor (blue) phases, whose cell parameters are (ahex. = 2.865(1) Å and chex. = 11.260(3) Å) and (ahex. = 2.912(1) Å and chex. = 11.174(3) Å), respectively. The additional weak Bragg peaks are not indexed using the basic hexagonal cell, indicating that a supercell is nedeed for at least one of the two phases to index these peaks.
Figure 2: Evolution of the electrochemical behaviour of the P2-NaxVO2 system. 
a, Evolution of cell voltage as a function of sodium content in NaxVO2 over the 0.5 ≤ x ≤ 0.92 range. The limits of the biphasic domains and the solid solutions are shown by dashed lines. The three single phases for x = 1/2, 5/8 and 2/3 are emphasized by thick blue lines. b, In situ X-ray diffraction data recorded during the galvanostatic intermittent titration technique experiments. Considering the basic structural model with hexagonal P63/mmc symmetry, the main peaks correspond to (004)hex., (100)hex. and (102)hex. reflections. The Bragg peak centred on 33° corresponds to the most intense reflection of a V2O3 impurity (marked by an asterisk). Additional weak Bragg peaks are visible within the 2θ-range 23.4–23.8° (0.61 < x < 0.70) and within the 2θ-ranges 28.9–29.8° and 33.8–34.7° (0.53 < x < 0.57).
Figure 3: Synchrotron diffraction pattern of P2-Na1/2VO2 and Rietveld refinement of its structure. 
The experimental (filled circles) and the calculated (red line) X-ray diffraction patterns are in good agreement with the smooth difference line (blue line). An enlargement in the 2θ-range is shown in the inset (top right): Bragg peaks are indexed considering the orthorhombic supercell with Pnma symmetry (Bragg peak positions are marked by green lines). Characteristic peaks of the superstructure are marked by stars. The distortion of the triangular lattice is illustrated by the splitting of the (100)hex. reflection into (220)orth. and (400)orth. reflections (see inset top left). The V2O3 impurity (most intense reflections marked with an open square) was present in the initial P2-Na0.71VO2 powder that was used in the sodium batteries, whereas the graphite (marked with an open circle) was added to this initial powder when the positive electrode was prepared.
Figure 4: Three-dimensional overview of the structure of P2-Na1/2VO2. 
The trigonal prisms Nae(1)O6 and Naf(2)O6 are shown in orange and yellow, respectively. The octahedra V(1)O6, V(2)O6 and V(3)O6 are shown in grey, light blue and cyan, respectively.
Figure 5: Projection of the structure of P2-Na1/2VO2 along the c axis.
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