Unveiling the charge storage mechanisms of perovskite fluoride KMnF₃ in neutral aqueous electrolyte

Perovskite fluorides (ABF3) have attracted much attention as an emerging and promising electrode material for electrochemical energy storage. However, to reveal the charge storage mechanisms of ABF3 in neutral media and further facilitate their energy storage utilizations remains very challenging. Herein, we have, for the first time, elucidated the charge storage mechanisms of KMnF3 in neutral aqueous electrolyte (1 M Na2SO4) via diverse ex/ in situ physicochemical/electrochemical techniques, showing the dominated conversion reactions along with partial insertion/extraction reactions and adsorption during the charging/discharging processes. Specifically, the KMnF3 nanocrystals transform into amorphous MnOOH/(hydrated)MnO2 nanosheets via adsorption and conversion mechanisms by formation/oxidation of [Mn-OH] intermediates and Mn2+ accompanying with partial H+ extraction of MnOOH to MnO2 in the first charging period, while the partial Na+ and H+ insert into the MnO2 to form Na0.5MnO2 and MnOOH and further (hydrated)MnO2 and MnOOH convert into Mn3O4 and/or Mn2+ in the discharging period. The as-constructed aqueous electrochemical capacitors (ECs) by matching KMnF3 positive electrode with activated carbon (AC) negative electrode exhibit good energy/power density and spectacular cycling behavior. Overall, this work addresses an in-depth understanding of charge storage mechanisms of KMnF3 in neutral media and plays a milestone-like role in developing state-of-the-art ABF3 in aqueous storage systems.