Abstract: Potassium ion capacitors (PICs) are regarded as promising large-scale aqueous energy storage systems. However, due to the poor K⁺ transport kinetics and the structural instability of the cathode materials, the key issues of limited energy density and poor cyclic stability are obstacles to the in-depth growth of PICs. Herein, a novel O-doped perovskite fluoride is demonstrated via an in-situ electrochemical oxidation strategy as the cathode for PICs, introducing additional defects that improve the capacitance and facilitate the reaction kinetics of the electrode. During the electrochemical oxidation process, it is discovered that the perovskite fluoride crystal tends to transform into disordered O-doped KMnF₃ (K_xMnF_yO_z), realizing a structural reconstruction at the electrode material/electrolyte interface. The First-principles calculations based on density functional theory (DFT) are performed to confirm that the improved electrical conductivity and low ionic adsorption energy may be ascribed to the substitution of oxygen for fluorine. The obtained K_1.14MnF_1.17O_1.26 cathode achieves a high specific capacitance of 694 F g^-1 at 1 A g^-1, as well as high capacitance retention of 91.3% after 10,000 charge/discharge cycles in mild K₂SO₄ electrolyte. This study provides an effective strategy to improve the capacitive performance of perovskite fluoride cathode materials in electrochemical energy storage.

Key words: Perovskite fluorides, O-doped, Potassium-ion capacitors, Charge storage mechanism