Understanding the Synergistic Effects and Structural Evolution of Co(OH)2 and Co3O4 toward Boosting Electrochemical Charge Storage

In this study, a novel Co3O4/Co(OH)2 heterostructure is obtained via electrodeposition on nickel (Ni) foam, forming sandwich‐like structure and freestanding electrode. The outer Co(OH)2 with layered structure can provide sufficient absorption sites and enable facile ion intercalation, meanwhile the presence of a conductive and robust interfacial Co3O4 layer between Ni foam and Co(OH)2 is found effectively minimizes the charge transfer resistance and stabilizes the interface, thus improving the electrode's rate and cycling performance with high capacity preserved synergistically. Furthermore, the structural evolution of Co(OH)2 and Co3O4 upon cycling are elucidated systematically using a series of in situ and ex situ techniques. The Co(OH)2 is found irreversibly changed to CoOOH upon first charge, which is then reversibly converted to CoO2 during the subsequent charge–discharge cycles. The Co3O4 exhibits negligible phase changes of the bulk upon cycling, indicating its good structural integrity that contributes to the significantly improved cyclability. In general, this work not only offers an ease and effective approach to optimize the charge storage properties of Co3O4/Co(OH)2 heterostructure via interfacial layer control, but also provides valuable insights in understanding their charge storage mechanisms, which may inspire the development of more heterostructures or extend to other applications. Interfacial layer control of freestanding sandwich‐like Co3O4/Co(OH)2 heterostructure is reported as a generalized and effective approach to optimize the electrode's charge storage properties, leading to improved rate and cycle performance. This work also provides valuable insights in understanding the charge storage mechanisms of Co3O4 and Co(OH)2, which will promote the rational design of more heterostructures with excellent electrochemical performance.