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

ABSTRACT

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.