Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium-Based Compounds

A high-rate of oxygen redox assisted by cobalt in layered sodium-based compounds is achieved. The rationally designed Na-0.6[Mg0.2Mn0.6Co0.2]O-2 exhibits outstanding electrode performance, delivering a discharge capacity of 214 mAh g(-1) (26 mA g(-1)) with capacity retention of 87% after 100 cycles. High rate performance is also achieved at 7C (1.82 A g(-1)) with a capacity of 107 mAh g(-1). Surprisingly, the Na-0.6[Mg0.2Mn0.6Co0.2]O-2 compound is able to deliver capacity for 1000 cycles at 5C (at 1.3 A g(-1)), retaining 72% of its initial capacity of 108 mAh g(-1). X-ray absorption spectroscopy analysis of the O K-edge indicates the oxygen-redox species (O2-/1-) is active during cycling. First-principles calculations show that the addition of Co reduces the bandgap energy from approximate to 2.65 to approximate to 0.61 eV and that overlapping of the Co 3d and O 2p orbitals facilitates facile electron transfer, enabling the long-term reversibility of the oxygen redox, even at high rates. To the best of the authors' knowledge, this is the first report on high-rate oxygen redox in sodium-based cathode materials, and it is believed that the findings will open a new pathway for the use of oxygen-redox-based materials for sodium-ion batteries.