Synthesis and Electrode Performance of O3-Type NaFeO₂-NaNi_1/2Mn_1/2O₂Solid Solution for Rechargeable Sodium Batteries

NaFe x (Ni 1/2 Mn 1/2 ) 1- x O 2 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 NaFe x (Ni 1/2 Mn 1/2 ) 1- x O 2 samples crystallize into a solid solution between two end-members of O3-type Na(Ni 1/2 Mn 1/2 )O 2 and NaFeO 2 . A Na/NaFe 0.4 (Ni 1/2 Mn 1/2 ) 0.6 O 2 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 NaFeO 2 (∼300 mWh g −1 ). Moreover, the Na/NaFe 0.4 (Ni 1/2 Mn 1/2 ) 0.6 O 2 cell shows relatively good cyclability and rate-capability. From these results, a potential application of the NaFe x (Ni 1/2 Mn 1/2 ) 1- x O 2 solid solution for rechargeable Na-ion batteries is discussed.