Significant improvement in cycling and rate ability originating from regulation of bonding and transport channel via Ba2+ decoration in P2 layer oxide cathode

Abstract

Nowadays, commercialization of layered transition metal oxides is hindered by irreversible phase transition during charging and discharging, low-capacity retention rate at high rate and slow Na+ diffusion kinetics. In this work, Ba2+ was introduced into the alkali metal layer to improve the electrochemical performance of layered transition metal oxides. XRD refinement and first principle calculations demonstrate that the introduced Ba2+ can effectively enlarge the distance between Na layers and reduce the potential barrier of sodium ion migration, thus leading to faster sodium ion diffusion kinetics. Simultaneously, charge distribution density calculation indicates that the introduced Ba2+ enhances overlap of electron clouds between transition metal (TM) and O, which result in more obvious covalence of TM-O bond, bigger electrostatic attraction between TM and O, and stronger bond energy of TM-O. These microstructures are beneficial to maintain structural stability and suppress the formation of OP4 phase during charging, which is demonstrated by XRD during charging and discharging process. The optimized Na0.6Ba0.012Mn0.62Ni0.22Fe0.16O2 provides reversible specific capacity of 133.8mAh g−1 at 0.5C (1C = 150mA g−1) and 122.1mAh g−1 at 2C. The strategy of regulating chemical bond environment by introducing Ba2+ achieves excellent the electrochemical performances and promotes the commercial development of sodium ion batteries.