Superstructure Variation and Improved Cycling of Anion Redox Active Sodium Manganese Oxides Due to Doping by Iron

Abstract Anionic redox provides an effective way to overcome the capacity bottleneck of sodium‐ion batteries. A dominant role is played by the arrangement of alkali A and transition metal M in the Na x A y M 1‐y O 2 superstructure. Here, in situ X‐ray diffraction and ex situ 7 Li nuclear magnetic resonance of P2 type Na 0.6 Li 0.2 Mn 0.8 O 2 with ribbon‐ordered superstructure illustrate structural changes and explain the evolution of the electrochemical behavior of electrodes comprising this active mass, during cycling. Upon substitution of a small amount of manganese by iron, Na 0.67 Li 0.2 Mn 0.73 Fe 0.07 O 2 is formed with a honeycomb‐ordered superstructure. Experimental characterizations and theoretical calculations elucidate the effect of iron on oxygen redox activity. The iron‐doped material considerably outperforms the undoped Na 0.6 Li 0.2 Mn 0.8 O 2 as a cathode material for rechargeable Na‐ion batteries. This research reveals the effect of superstructure transformation on the electrochemical properties and offers a new perspective on element substitution in anionic redox active cathode materials.