Anion Doping for Layered Oxides with a Solid-Solution Reaction for Potassium-Ion Battery Cathodes

The development of potassium-ion batteries (PIBs) is challenged by the shortage of stable cathode materials capable of reversibly hosting the large-sized K⁺ (1.38 Å), which is prone to cause severe structural degradation and complex phase evolution during the potassiation/depotassiation process. Here, we identified that anionic doping of the layered oxides for PIBs is effective to combat their capacity fading at high voltage (>4.0 V). Taking P2-type K_2/3Mn_7/9Ni_1/9Ti_1/9O_17/9F_1/9 (KMNTOF) as an example, we showed that the partial substitution of O^2- by F^- enlarged the interlayer distance of the K_2/3Mn_7/9Ni_1/9Ti_1/9O₂ (KMNTO), which becomes more favorable for fast K⁺ transition without violent structural destruction. Meanwhile, based on the experimental data and theoretical results, we identified that the introduction of F^- anions effectively increased the redox-active Mn cationic concentration by lowering the average valence of the Mn element, accordingly providing more reversible capacity derived from the Mn^3+/4+ redox couple, rather than oxygen redox. This anionic doping strategy enables the KMNTOF cathode to deliver a high reversible capacity of 132.5 mAh g^-1 with 0.53 K⁺ reversible (de)intercalation in the structure. We expect that the discovery provides new insights into structural engineering for pursuing stable cathodes to facilitate the future applications of high-performance PIBs.