Understanding Oxygen Redox in Cu-Doped P2-Na_0.67Mn_0.8Fe_0.1Co_0.1O₂ Cathode Materials for Na-Ion Batteries

Oxygen redox chemistry has attracted much attention as an extra charge compensation mechanism to attain the high specific capacity for Na-ion battery cathodes. But how to modulate the crystal and electronic structure to achieve a simultaneous enhancement of anionic and cationic redox is still a big challenge. Herein, the positive effect of Cu doping on both the anionic and cationic redox in P2-Na 0.67 Mn 0.8 Fe 0.1 Co 0.1 O 2 has been reported for the first time, and the roles of Cu have been unveiled. Cu doping increases the lattice parameters a and c as well as enlarges the interslab spacing and Na-O bond length, which reduces the electrostatic attraction between Na and O, and subsequently enhances Na + diffusion coefficient and rate capability. Moreover, Cu doping decreases Mn 3+ /Mn 4+ ratio and alleviates Jahn-Teller distortion, which benefits to the layered structure stability. More importantly, Cu doping reduces the slab thickness of TMO 2 and TM-O/O-O bond length as well as mitigates the lattice volume change and the phase transition, which improves the structure stability and the cycling stability. The strengthened TM-O/O-O bonding energy also suppresses O 2 release and improves the reversibility of oxygen redox, which has been further confirmed by differential scanning calorimeter and ex-situ X-ray photoelectron spectroscopy.