Synthesis and Electrode Performance of O3-Type NaFeO₂-NaNi1/2Mn1/2O₂Solid Solution for Rechargeable Sodium Batteries

NaFex(Ni1/2Mn1/2)1-xO2 layered oxides are synthesized by a solid-state method, and their electrode properties as positive electrodes for rechargeable sodium batteries are examined. Crystallographic analysis on a series of samples reveals that NaFex(Ni1/2Mn1/2)1-xO2 samples crystallize into a solid solution between two end-members of O3-type Na(Ni1/2Mn1/2)O2 and NaFeO2. A Na/NaFe0.4(Ni1/2Mn1/2)0.6O2 cell delivers 130 mAh g−1 of reversible capacity in a voltage range of 2.0 – 3.8 V. Energy density available based on metallic sodium is estimated to be approximately 400 mWh g−1, which is considerably larger than that of NaFeO2 (∼300 mWh g−1). Moreover, the Na/NaFe0.4(Ni1/2Mn1/2)0.6O2 cell shows relatively good cyclability and rate-capability. From these results, a potential application of the NaFex(Ni1/2Mn1/2)1-xO2 solid solution for rechargeable Na-ion batteries is discussed.