Activating Oxygen Redox in Layered K_xTMO₂ to Construct High‐Capacity and Enable Phase‐Transition‐Free Potassium Ion Cathode

Abstract Mn‐based layered oxides are one of the most appealing cathodes for potassium‐ion batteries (PIBs) due to its cost‐effective potassium and manganese resources, and high theoretical specific capacity. However, severe phase transitions and Jahn‐Teller distortion of Mn 3+ have already hindered its practical application. To circumvent these issues, a layered P2‐type K 0.67 Mn 0.75 Ni 0.23 Nb 0.02 O 2 (P2‐KMNNb) cathode is proposed where the nickel and niobium substitution enables increase non‐bonding oxygen states and structural stability. Via electrochemical test and physicochemical characterizations, it is demonstrated that the substitution induces oxygen redox activity in potassium ion battery system, favoring the highly reversible K + (de) intercalation capability. The P2‐KMNNb exhibits a high reversible capacity of 134.8 mAh g −1 at a current density of 10 mA g −1 , with a capacity retention of 85.2% after 150 cycles. The full cell, composed of the P2‐KMNNb cathode and hard carbon anode, also shows excellent electrochemical performance, achieving a high specific capacity of 63.6 mAh g −1 at a large current density of 200 mA g −1 , with an initial coulombic efficiency of 95.2%. Meanwhile, in situ X‐ray powder diffraction patterns and Raman spectra show that the absence of phase transitions and the stable oxygen stacking sequence. These findings provide new strategies for modulating the redox activity and reversibility of structure.