Unveiling the charge storage mechanisms of Co-based perovskite fluoride in a mild aqueous electrolyte

This study is an in-depth exploration of the charge storage mechanisms of KCoF₃ in 1 M Na₂SO₄ mild aqueous electrolytes via an array of ex situ/in situ physicochemical/electrochemical methods, especially the electrochemical quartz crystal microbalance (EQCM) technique, showing a combination of conversion, insertion/extraction and adsorption mechanisms. Specifically, during the first charge phase, Co(OH)₂ is formed/oxidized into amorphous CoOOH and Co₃O₄, and then CoOOH undergoes partial proton extraction to yield CoO₂, which is simultaneously accompanied by the transformation of Co₃O₄ into CoOOH and (hydrated) CoO₂. During the first discharge process, the partial insertion of H⁺ into (hydrated) CoO₂ leads to the formation of CoOOH and Co₃O₄, with the conversion of Co₃O₄ into CoOOH and both Co₃O₄ and CoOOH undergoing further transformations into (hydrated) Co(OH)₂via the insertion of H⁺. This work offers valuable references for the development of aqueous energy storage.