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[Mg_0.2Mn_0.6Co_0.2]O₂ 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[Mg_0.2Mn_0.6Co_0.2]O₂ 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 (O^2−/1−) is active during cycling. First-principles calculations show that the addition of Co reduces the bandgap energy from ≈2.65 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.