Studying Na criticality in NaxMn2−xO2 (x = 1.05–1.3) type Na-rich disordered rocksalt cathode for higher capacity

Abstract

Disordered rocksalt (DRX) can achieve far better capacity than layered oxide cathodes, with the potential to use as cathodes for next-generation batteries, but are limited by their poor ion diffusion kinetics. Initial assessments on Li-rich disordered cathode suggest that Li percolation is heavily influenced by transition metal (TM) polyhedral surrounding Li diffusion path by a repulsive force. However, the kinetics and influence of TM polyhedral structures are not investigated for sodium rich DRX cathodes. Herein, we study the influence of Na stoichiometry in Na-rich DRX cathode—NaxMn2-xO2 (x = 1.05–1.3) and its influence on Na percolation. Increasing the Na stoichiometry systematically gives better insight into Na percolation network, TM cluster formation, and its effect on sodium diffusivity on Na-DRX cathodes. The cathode material is analyzed by transmission electron microscopy and X-ray absorption spectroscopy analysis, giving better understanding on influence of Na stoichiometry on its neighboring polyhedral structures. The Na-rich samples were initially assessed by electrochemical studies, which showed about 110 mAhg−1 of capacity for 0.5 Ag−1 current density. The study provides an insight into crystal structure of Na-rich disordered cathode and ways to improve its capacity and rate performance.