Understanding the structural evolution and Na⁺ kinetics in honeycomb-ordered O′3-Na₃Ni₂SbO₆ cathodes

The development of new sodium ion battery (SIB) cathodes with satisfactory performance requires an in-depth understanding of their structure-function relationships, to rationally design better electrode materials. In this work, highly ordered, honeycomb-layered Na₃Ni₂SbO₆ was prepared to elucidate the structural evolution and Na⁺ kinetics during electrochemical desodiation/sodiation processes. Structural analysis involving in situ synchrotron X-ray diffraction (XRD) experiments, electrochemical performance measurements, and electrochemical characterization (galvanostatic intermittent titration technique, GITT) methods were used to obtain new insights into the reaction mechanism controlling the (de)intercalation of sodium into the host Na_3-xNi₂SbO₆ structure. Two phase transitions occur (initial O′3 phase → intermediate P′3 phase → final O1 phase) upon Na⁺ extraction; the partial irreversible O′3-P′3 phase transition is responsible for the insufficient cycling stability. The fast Na⁺ mobility (average 10^–12 cm²·s^–1) in the interlayer, high equilibrium voltage (3.27 V), and low voltage polarization (50 mV) establish the linkage between kinetic advantage and a good rate performance of the cathode. These new findings provide deep insight into the reaction mechanism operating in the honeycomb cathode; the present approach could be also extended to investigate other materials for SIBs.