F-doped O3-NaNi1/3Fe1/3Mn1/3O2 as high-performance cathode materials for sodium-ion batteries

The F-doped O3-type NaNi1/3Fe1/3Mn1/3O2− x F x (x = 0, 0.005, 0.01, 002 and noted as NFM-F0, NFM-F0.005, NFM-F0.01, NFM-F0.02, respectively, united as NFM-Fs) cathode materials were investigated systematically. The rate performance and capacity retention of the O3-type cathode materials are significantly improved as a function of specific F-doping levels. Optimum performance is achieved in the NFM-F0.01 material having a capacity of ~110 mA h g−1 at a current density of 150 mA g−1 after 70 cycles. The results indicate that the binding energy of oxygen changes as a result of F-doping, and in addition, F-doping results in changes to the stoichiometry ofMn3+/Mn4+, which stabilizes the O3-type layered structure, thus allowing cycling performance to be improved. However, NFM-F0.02, having a higher F-doping level, retains a high capacity retention, although a slight loss is observed. The results suggest there is an optimum F-doping level for the NFM-F system to deliver enhanced cycling performance.