SUMMARY
Layered P2 -type ?Na?_0.67 ?Fe?_0.5 ?Mn?_0.5 O_2 is regarded as a promising cathode material for sodium ion batteries. This layered transition metal oxide material is susceptible to structural instability and suffers from poor capacity retention. In this work P2 -Type ?Na?_0.67 ?Fe?_0.5 ?Mn?_0.5 O_2 cathode material was prepared by the citric acid assisted sol-gel method. To ameliorate the stability of the structure and cycling performance, Manganese (Mn) was partially doped with Nickel (Ni) to form P2 -type ?Na?_0.67 ?Fe?_0.5 ?Mn?_(0.5-x) ?Ni?_x O_2. The increased Ni content reduced the reversible capacity but improved cyclability. The improved electrochemical performance of the ?Na?_0.67 ?Fe?_0.5 ?Mn?_(0.5-x) ?Ni?_x O_2 electrode is attributed to the Ni doping, which alleviates the Jahn-Teller distortion of ?Mn?^(3+) thereby corresponding to a lower capacity decay rate. For long term cycling, it is beneficial to cycle within the voltage range of 1.5V-4.0V, to minimize the strain on the P2-type crystal structure. If the cut-off voltage is any higher even though it may show a higher reversible capacity, decay rate will be further exacerbated, resulting in a decreased coulombic efficiency. The initial discharge capacity for ?Na?_0.67 ?Fe?_0.5 ?Mn?_0.5 O_2 is 125mAh/g and is expected to improve once the pure phase P2 -type crystal structure has been formed. The Ni substituted ?Na?_0.67 ?Fe?_0.5 ?Mn?_(0.5-x) ?Ni?_x O_2 potentially serves as a feasible high capacity and stable cathode material for sodium ion batteries.