High-Voltage Oxygen-Redox-Based Cathode for Rechargeable Sodium-Ion Batteries

Recently, anionic-redox-based materials have shown promising electrochemical performance as cathode materials for sodium-ion batteries. However, one of the limiting factors in the development of oxygen-redox-based electrodes is their low operating voltage. In this study, the operating voltage of oxygen-redox-based electrodes is raised by incorporating nickel into P2-type Na_2/3[Zn_0.3Mn_0.7]O₂ in such a way that the zinc is partially substituted by nickel. As designed, the resulting P2-type Na_2/3[(Ni_0.5Zn_0.5)_0.3Mn_0.7]O₂ electrode exhibits an average operating voltage of 3.5 V and retains 95% of its initial capacity after 200 cycles in the voltage range of 2.3–4.6 V at 0.1C (26 mA g⁻¹). Operando X-ray diffraction analysis reveals the reversible phase transition: P2 to OP4 phase on charge and recovery to the P2 phase on discharge. Moreover, ex situ X-ray absorption near edge structure and X-ray photoelectron spectroscopy studies reveal that the capacity is generated by the combination of Ni²⁺/Ni⁴⁺ and O²⁻/O¹⁻ redox pairs, which is supported by first-principles calculations. It is thought that this kind of high voltage redox species combined with oxygen redox could be an interesting approach to further increase energy density of cathode materials for not only sodium-based rechargeable batteries, but other alkali-ion battery systems.