Inhibiting Oxygen Activity and Phase Transition in Cu-F Doped Ni-Mn Layered Oxide Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative for large-scale energy storage due to the abundance of sodium resources. Among cathode materials, layered oxides have shown exceptional potential, yet their practical application is hindered by structural instability during electrochemical cycling. In this study, this challenge is addressed by introducing a novel strategy of Cu and F dual doping into the octahedral ligand field of oxygen-activated P2-type Na_0.67Ni_0.33Mn_0.67O₂ layered oxides. Through a comprehensive suite of advanced characterization techniques, unprecedented insights into the modulation of oxygen redox activity are uncovered. Ex situ X-ray photoelectron spectroscopy and Raman spectroscopy reveal enhanced reversibility and stability in chemical bonding, while in situ X-ray diffraction analysis indicates the suppression of detrimental phase transitions, ensuring a stable and unobstructed Na⁺ diffusion pathway. Density functional theory calculations further elucidate that Cu-F co-doping reduces the overlap between Ni t_{2 g} orbitals and O 2p orbitals, thereby inhibiting oxygen redox activity. Remarkably, the co-doped material exhibits significantly improved capacity retention and rate performance. This work not only advances the fundamental understanding of octahedral ligand field engineering but also provides a transformative approach to designing high-performance and stable cathode materials for SIBs, paving the way for their widespread adoption in energy storage systems.