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Na
 0.67
 Mn
 
 (1‐
 x
 )
 
 Fe
 
 x
 
 O
 2
 Compounds as High‐Capacity Cathode Materials for Rechargeable Sodium‐Ion Batteries

Herein we report on electrochemical and structural properties of low‐cost Na0.67Mn(1‐x)FexO2 (NMFx) cathode materials synthesized through substitution of Mn with small amounts of Fe. The materials were fabricated using a simple solid‐state synthesis method. Electrochemical performance of the cathodes was significantly improved when initial electrochemical activation had been performed. Activated Na0.67Mn0.9Fe0.1O2 material displayed a high capacity of 132 mAh/g after 100 cycles and delivered an exceptionally high rate capability of 116 mAh/g at 500 mA/g. Energy‐dispersive X‐ray spectroscopy (EDX), cyclic voltammetry (CV) and ex situ X‐ray photoelectron‐spectroscopy (XPS) analyses suggested that Fe had played a structural as well as an electrochemical role in cathode performance. Ex situ and in situ X‐ray diffraction (XRD) studies were used to analyse the associated structural variations during the capacity evolution.