Sodium-ion batteries have been extensively pursued as economic alternatives to Lithium-ion batteries. Investigating the polyanion chemistry, alluaudite structured Na 2 Fe II 2 (SO 4 ) 3 has been recently discovered as a 3.8 V positive electrode material (Barpanda et al, Nature Commun., 5:4358, 2014). Registering the highest ever Fe III /Fe II redox potential (vs. Na/Na + ) and formidable energy density, it has opened up a new polyanion family for sodium batteries. Exploring the alluaudite family, here we report isotypical Na 2+2x Mn II 2-x (SO 4 ) 3 (x = 0.22) as a novel high-voltage cathode material for the first time. Following low-temperature (ca. 350°C) solid-state synthesis, the structure of this new alluaudite compound has been solved adopting a monoclinic framework (s.g. C2/c) showing antiferromagnetic ordering at 3.4 K.Synergising experimental and ab-initio DFT investigation, Na 2+2x Mn II 2-x (SO 4 ) 3 has been found to be a potential high-voltage (ca. 4.4 V) cathode material for sodium batteries.

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Under identical hydrothermal condition, remarkably, Aurivillius BiREWO 6 nanoparticles crystallize in an orthorhombic phase for RE = Ce and La, while the RE = Nd− Yb materials crystallize in the monoclinic phase. This kind of distinct phase formation in this series is not observed in solid state synthesis, where for all RE substitution, only the monoclinic phase is formed. Moreover, formation of the orthorhombic phase for BiLaWO 6 and BiCeWO 6 has been observed for the first time. Calcination of as-synthesized BiLaWO 6 and BiCeWO 6 nanomaterials results in the monoclinic phase and this indicates that the formation of the orthorhombic phase is favored only under mild reaction conditions. This could be attributed to the crucial role of the ionic radii of RE 3+ and Bi 3+ ions in solution. Rietveld refinement confirms the BiCeWO 6 and BiGdWO 6 are isostrutural to the low temperature (LT) orthorhombic phase and monclinic high temperature (HT) phase of Bi 2 WO 6 , respectively. Additionally phase dependent distinct morphology and photocatalytic activity is observed. As a representative example, it is observed that needle/ plate-shaped monoclinic BiGdWO 6 nanoparticles show superior visible light driven photocatalytic activity for the Congo-red dye degradation over the spherically agglomerated orthorhombic BiCeWO 6 nanomaterial.

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