Unveiling Zn-induced reinforced transition metal-oxygen coupling in layered oxide cathodes for highly stable sodium-ion batteries

Ion doping is an effective strategy to alleviate the phase transition and irreversible lattice oxygen reactions of layered oxide cathodes for sodium ion batteries, but it still lacks sufficient understanding on the mechanism of elemental action and the interactions between transition metal atoms. In this work, Zn/Fe co-doped P2-Na 0.67 Mn 0.6 Ni 0.3 Zn 0.05 Fe 0.05 O 2 (NMNO-ZF) materials are facile synthesized and indicated that the doped Zn enables to sustain the structural stability of NMNO-ZF to inhibit the phase transition and also cooperates with Fe to trigger the reversible lattice oxygen reaction at high voltages. Density functional theory (DFT) calculations reveal that Zn plays the role of inducing enhanced d-p coupling of Mn/Fe-O in NMNO-ZF. Namely, the Zn-reinforced Mn-O bonds give favorable structural stability to NMNO-ZF, while the Zn-modulated Fe-O bonds greatly improves the reversibility of lattice oxygen redox. Consequently, NMNO-ZF can achieve a reversible capacity of 139.17 mAh g −1 , with almost no capacity degradation after 100 cycles at 120 mAh g −1 , and a capacity retention rate of 89 % even after 2000 cycles at 1.2 A g −1 . Moreover, NNMO-ZF also shows stable cycling capability in the full cell with a capacity retention of 81 % after 400 cycles at 600 mAh g −1 . This study unveils the charge compensation mechanism of Zn/Fe co-doping in layered oxide materials, providing more experimental and theoretical supports for designing highly stable cathodes for SIBs.