Unexpected Redox-Inert Behaviors and Their Mechanisms of Doped Transition Metals in K 0.5 MnO 2 Cathodes

Layer-structured metal oxides are promising cathode materials for potassium-ion batteries (PIBs) due to their high specific capacity, low cost, and environmental friendliness. However, the redox activity and behaviors of different metal ions in such oxides remain inadequately understood. Herein, the redox behaviors of various transition metal dopants (Fe, Co, Ni, and Cu) in P3-type K 0.5 MnO 2 cathodes for PIBs are systematically investigated. Although these doped elements are typically redox-active in sodium-ion battery cathodes, synchrotron X-ray absorption spectroscopy reveals their minimal valence changes during electrochemical cycling in PIBs, indicating unexpected redox-inert behavior. Density functional theory calculations elucidate the underlying mechanism: doping with Fe or Cu substantially suppresses their 3 d -orbital conduction bands, while Co or Ni doping leads to larger band gaps compared to that of Mn. This electronic structure rationalizes the observed Mn-dominated redox activity. Electrochemical tests further show that Fe- and Co-doped samples deliver higher specific capacities than their Ni- and Cu-doped counterparts, consistent with reduced bandgaps in the Mn density of states. These findings provide fundamental insights into the unexpected redox-inert role of variable-valence dopants in K x MnO 2 cathodes, offering guidance for designing high-performance PIB cathodes.