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The crystal structure of the NASICONNa3V2(PO4)3phase (NVP)has been investigated asa function of temperature. Combining laboratory and synchrotron X-raypowder diffraction with single crystal X-ray diffraction, we demonstratethe presence of four polymorphs of NVP from 30 to 225 °C. Whilethe high temperature γ-NVP crystallizes in the classical rhombohedralcell (S.G. R3̅c, a= 8.73382(4) Å, c= 21.91438(17) Å),the low temperature α-NVP undergoes a monoclinic distortion(S.G. C2/c, a=15.1244(6) Å, b= 8.7287(3) Å, c= 21.6143(8) Å, β = 90.163(2)°) togetherwith an ordering of the Na atoms. In the middle temperature range,two incommensurate modulated structures (β- and β′-NVP)are also reported for the first time. [ABSTRACT FROM AUTHOR]

Inthe search for better batteries, the quest for new positiveelectrode materials, which are derived from earth-abundant elementsand synthesized through low energy demanding processes, has becomean area of interest to the scientific community. Many Li–ironbased polyanionic compounds have already been considered for Li ionbatteries, but there still remain a large variety of Fe-based polyanioniccarboxylate compounds (carbonates, oxalates, malonates, etc.) to beexplored, among which Fe2(C2O4)3·nH2O is commercially available,but for which structure and electrochemical reactivity toward Li arelacking. By means of combined X-ray and neutron powder diffraction,we unravel for the first time the structure of Fe2(C2O4)3·4H2O. It adoptsa triclinic unit cell with iron atoms being octahedrally coordinatedby one water molecule and three oxalate groups, two of them beingtetradentate. This arrangement translates into zigzag chains linkedone to another through the third oxalate molecule to lead to an openedlayered structure into which two additional water molecules are located.Moreover, we demonstrate that it reacts with lithium at an averagepotential of 3.35 V vs.Li leading to a sustainablecapacity of 98 mAh/g. The insertion–extraction of the lithiumprocess follows a biphasic process and involves the Fe3+/Fe2+redox couple as deduced from in situX-ray powder diffraction and operandoMössbauerspectroscopy measurements. [ABSTRACT FROM AUTHOR]