Na2Mn3+0.3Mn4+2.7O6.85: A cathode with simultaneous cationic and anionic redox in Na-ion battery

Here we report a high-capacity cathode, Na2Mn3+0.3Mn4+2.7O6.85, based on simultaneous cationic (Mn4+/Mn3+) and anionic (O2-/(O2)n-) redox process in Na-Mn-O system for Na-ion batteries (NIBs). An obvious voltage plateau at ~ 4.2 V vs. Na+/Na is reversibly presented during Na+ deinsertion/insertion demonstrated by XPS analysis of the participation of oxygen redox. This cathode delivers ~ 213 mA h g-1 in the voltage range of 1.5-4.5 V composed of two voltage plateaus from both cationic and anionic redox reaction. The two voltage plateaus exhibit the reversible phase transition upon Na+ insertion/deinsertion corresponding to a solid-solution region (high plateau) and a two-phase region (low plateau), respectively. The results indicate that anionic redox activity is available to develop high-capacity cathodes for NIBs.