ИСТИНА

The type of the intercalating cation plays crucial role in the phase transformation regime of the cathode materials. Layered Na2FePO4F demonstrates two two-phase transitions in the Na-ion cell, which has confirmations in the literature [1]. In contrast, desodiation of Na2FePO4F in the Li-ion cell demonstrates sloping charge profile suggesting a solid-solution mechanism with no charge ordered intermediate phase formation. This difference in desodiation behaviors is ascribed to a different activity of the A1 and A2 alkali positions which have in their coordination spheres different number of the “dangling” oxygen atoms (bonded only to the alkali and P cations, but not included into the FeO4F2 octahedra), which was confirmed by operando x-ray powder diffraction, ex-situ synchrotron x-ray powder diffraction and 57Fe Mössbauer spectroscopy studies, and were corroborated with density functional theory calculations. This comparative study identifies that the bonding to the “dangling” oxygen atoms is an important factor which must be taken into account while discussing electrochemical activity of alkali cations in the polyanion structures. As the second part of the work, for the first time structural transformations in Na2CoPO4F under cycling in a sodium-ion half-cell have been compared to these of its ferrous analog, and also was obtained and characterized orthorhombic layered Na2Mg0.6Mn0.4PO4F as a sodium-ion cathode containing only abundant elements and theoretically able to undergo two-electron transition of manganese cation. References [1] B. L. Ellis, W. R. M. Makahnouk, W. N. Rowan-Weetaluktuk, D. H. Ryan, and L. F. Nazar, Chem. Mater., 22, 3 (2010), 1059