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

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_xA_yM_1-yO₂ superstructure. Here, in situ X-ray diffraction and ex situ ⁷Li nuclear magnetic resonance of P2 type Na_0.6Li_0.2Mn_0.8O₂ 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_0.67Li_0.2Mn_0.73Fe_0.07O₂ 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_0.6Li_0.2Mn_0.8O₂ 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.