ИСТИНА

Na-ion batteries are considered as future alternative to Li-ion due to low cost and wide abundance of sodium. In turn, O3-type NaNi1-x-yFexMnyO2 cathode materials are an excellent alternative to Li-containing layered oxides such as LiCoO2 and its derivatives. These compounds are isostructural to α-NaFeO2 (s.g. R3̅m) with Ni, Fe, and Mn in NaNi1/3Fe1/3Mn1/3O2 being in the 2+, 3+, and 4+ oxidation states, respectively. The possibility of using Fe3+/Fe4+ redox pair in cathode materials for SIBs is one of the main features of the Na-ion system in contrast to Li-ion. However, the peculiar crystal chemistry of iron cations introduces certain problems in the (de)sodiation process of layered sodium oxides.Here, we present operando and ex situ studies of O3-NaNi1/3Fe1/3Mn1/3O2 using powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS) combined with 57Fe Mössbauer spectroscopy (MS). Our approach revealed the sequence of the redox transitions and allowed us to observe ≈12% of the Fe4+ cations that occupied tetrahedral positions. To the best of our knowledge, this is the first instance of Mossbauer spectroscopy detecting the presence of iron cations in the sodium layer in NaMO2 oxides, as well as the first indication of its oxidation state.