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 [1]. 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. Its stable electrochemical performance is restricted by the upper voltage limit of 4.0 V (vs. Na/Na+), which allows for reversibly removing 0.5-0.55 Na+ per formula unit, corresponding to the capacity of 120-130 mAh·g-1. Further reduction of sodium content inevitably accelerates capacity degradation, and this issue calls for a detailed study of the redox reactions that accompany the electrochemical (de)intercalation of a large amount of sodium.