P2-type Na0.67Fe0.5Mn0.35Co0.15O2 cathode modified by an active coating layer of K2Na(Co(NO2)6) for sodium-ion batteries with improved cycling stability

P2-type Fe-based oxides cathodes (Na0.67Fe0.5Mn0.35Co0.15O2, NFMC) have the disadvantage of a high air sensitivity and are prone to P2-O2 phase transition during charging to high voltage. In this work, an active coating layer of K2Na(Co(NO2)6) (KNC) is successfully coated on the surface of NFMC materials. The ex-situ X-ray diffraction patterns and the microstructure analysis of the KNC@NFMC show that no phase change occurs during charging/discharging in the range of 1.5-4.2 V. The electrochemical tests demonstrate that long-time stability of the active K2Na(Co(NO2)6) coating layer enhances the reversible reaction of Mn3+/Mn4+ and effectively suppresses the increase of charge transfer impedance during cycles. Compared to Na0.67Fe0.5Mn0.35Co0.15O2 oxide, the modified material has superior capacity retention of 89.7% for 200 cycles at 1 C (1 C=180 mAh/g) with a discharge specific capacity of 94.6 mAh/g.