Honeycomb-Ordered Na₃Ni_1.5M_0.5BiO₆ (M = Ni, Cu, Mg, Zn) as High-Voltage Layered Cathodes for Sodium-Ion Batteries

Developing high-voltage layered cathodes for sodium-ion batteries (SIBs) has always been a severe challenge. Herein, a new family of honeycomb-layered Na₃Ni_1.5M_0.5BiO₆ (M = Ni, Cu, Mg, Zn) with a monoclinic superstructure has been shown to combine good Na⁺ (de)intercalation activity with a competitive 3.3 V high voltage. By coupling the electrochemical process with ex situ X-ray absorption spectroscopy as well as in situ X-ray diffraction, the charge compensation mechanism and structural evolution of these new cathodes are clearly investigated. Interestingly, both Ni²⁺/Ni³⁺ and Cu²⁺/Cu³⁺ participate in the redox reaction upon cycling, and the succession of single-phase, two-phase, or three-phase regions upon Na⁺ extraction/insertion were identified with rather good accuracy. This research strategy could provide insights into the structure-function-property relationships on a new series of honeycomb-ordered materials with the general formula Na₃Ni_1.5M_0.5BiO₆ and also serve as a bridge to guide future design of high-performance cathodes for SIBs.