Comprehensively Strengthened Metal-Oxygen Bonds for Reversible Anionic Redox Reaction

Introducing anionic redox in layered oxides is an effective approach to breaking the capacity limit of conventional cationic redox. However, the anionic redox reaction generally suffers from excessive oxidation of lattice oxygen to O-2 and O-2 release, resulting in local structural deterioration and rapid capacity/voltage decay. Here, a Na0.71Li0.22Al0.05Mn0.73O2 (NLAM) cathode material is developed by introducing Al3+ into the transition metal (TM) sites. Thanks to the strong Al-O bonding strength and small Al3+ radius, the TMO2 skeleton and the holistic TM-O bonds in NLAM are comprehensively strengthened, which inhibits the excessive lattice oxygen oxidation. The obtained NLAM exhibits a high reversible capacity of 194.4 mAh g(-1) at 20 mA g(-1) and decent cyclability with 98.6% capacity retention over 200 cycles at 200 mA g(-1). In situ characterizations reveal that the NLAM experiences phase transitions with an intermediate OP4 phase during the charge-discharge. Theoretical calculations further confirm that the Al substitution strategy is beneficial for improving the overlap between Mn 3d and O 2p orbitals. This finding sheds light on the design of layered oxide cathodes with highly reversible anionic redox for sodium storage.