Extrinsic Pseudocapacitive CoOOH Achieved by the Suppression of Its Phase Transition Triggered by S^2- Doping

The behavior transformation of battery-type materials to extrinsic pseudocapacitive behavior is accomplished by suppressing their phase transition. The resulting extrinsic pseudocapacitive materials are a valuable supplement to intrinsic pseudocapacitive materials, enriching the connotation of pseudocapacitive materials. Nevertheless, the research on this category of materials remains inactive as of now, probably due to the as yet unclear phase transition suppression mechanism and the associated electrochemical mechanism. In this work, the battery-type Co(OH)₂ is selected as a research object to create an extrinsic pseudocapacitive material by S^2- doping. It is revealed that the K⁺ intercalation/deintercalation is the essential reason for the phase transition of CoOOH that is irreversibly converted from Co(OH)₂, leading to its battery-type behavior. Whereas, the fully protonated CoOOH produced due to the high reaction activity of S^2- doped Co(OH)₂ only allows the intercalation/deintercalation of H⁺. During the process of intercalation/deintercalation of H⁺, CoOOH retains the P3 structure, thereby giving rise to extrinsic pseudocapacitive behavior. The electrochemical measurement results indicate that this extrinsic pseudocapacitive material and the assembled device have electrochemical performance equivalent to that of intrinsic pseudocapacitive materials.