Nickel Doped P2-type 〖Na〗_(0.67) 〖Fe〗_(0.5) 〖Mn〗_(0.5) O_2 Cathode Materials for Sodium Ion Batteries

Abstract

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.