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

摘要

Anionic redox provides an effective way to overcome the capacity bottleneckof sodium-ion batteries. A dominant role is played by the arrangement ofalkali A and transition metal M in the Na_xA_yM_(1-y)O_2 superstructure. Here, insitu X-ray diffraction and ex situ 7Li nuclear magnetic resonance of P2 typeNa_(0.6)Li_(0.2)Mn_(0.8)O_2 with ribbon-ordered superstructure illustrate structuralchanges and explain the evolution of the electrochemical behavior of electrodescomprising this active mass, during cycling. Upon substitution of asmall amount of manganese by iron, Na_(0.67)Li_(0.2)Mn_(0.73)Fe_(0.07)O_2 is formed with ahoneycomb-ordered superstructure. Experimental characterizations and theoreticalcalculations elucidate the effect of iron on oxygen redox activity. Theiron-doped material considerably outperforms the undoped Na_(0.6)Li_(0.2)Mn_(0.8)O_2as a cathode material for rechargeable Na-ion batteries. This research revealsthe effect of superstructure transformation on the electrochemical propertiesand offers a new perspective on element substitution in anionic redox activecathode materials.