K-Doping Suppresses Oxygen Redox in P2-Na_0.67Ni_0.11Cu_0.22Mn_0.67O₂ Cathode Materials for Sodium-Ion Batteries

In P2-type layered oxide cathodes, Na site-regulation strategies are proposed to modulate the Na⁺ distribution and structural stability. However, their impact on the oxygen redox reactions remains poorly understood. Herein, the incorporation of K⁺ in the Na layer of Na_0.67Ni_0.11Cu_0.22Mn_0.67O₂ is successfully applied. The effects of partial substitution of Na⁺ with K⁺ on electrochemical properties, structural stability, and oxygen redox reactions have been extensively studied. Improved Na⁺ diffusion kinetics of the cathode is observed from galvanostatic intermittent titration technique (GITT) and rate performance. The valence states and local structural environment of the transition metals (TMs) are elucidated via operando synchrotron X-ray absorption spectroscopy (XAS). It is revealed that the TMO₂ slabs tend to be strengthened by K-doping, which efficiently facilitates reversible local structural change. Operando X-ray diffraction (XRD) further confirms more reversible phase changes during the charge/discharge for the cathode after K-doping. Density functional theory (DFT) calculations suggest that oxygen redox reaction in Na_0.62K_0.03Ni_0.11Cu_0.22Mn_0.67O₂ cathode has been remarkably suppressed as the nonbonding O 2p states shift down in the energy. This is further corroborated experimentally by resonant inelastic X-ray scattering (RIXS) spectroscopy, ultimately proving the role of K⁺ incorporated in the Na layer.