Oxygen redox activity with small voltage hysteresis in Na_0.67Cu_0.28Mn_0.72O₂ for sodium-ion batteries

Although Li/Na layered oxides that display oxygen redox activity are promising cathodes with attractive energy density, they suffer from large voltage hysteresis and evolution of O₂. In this work, a layered Na_0.67Cu_0.28Mn_0.72O₂ cathode is reported that has reversible lattice oxygen redox activity with small voltage hysteresis. This compound provides a highly reversible capacity of 104 ​mAh g^-1 with a smooth voltage profile originating from both cationic and anionic redox reactions. Density functional theory calculations show that the nonbonding O 2p states along the Cu–O bonds promote the oxygen redox activity. In-situ X-ray powder diffraction patterns and Raman spectra show that the small voltage hysteresis during electrochemical cycling is rooted in the absence of phase transitions and the stable oxygen stacking sequence. These findings may provide new insight into the anionic redox activity and offer a new strategy to design cathodes with high energy density and structural stability.