Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences

Energy storage devices, e.g., supercapacitors (SCs) and zinc-ion batteries (ZIBs), based on aqueous electrolytes, have the advantages of rapid ion diffusion, environmental benignness, high safety and low cost. Generally, SCs provide excellent power density with the capability of fast charge/discharge, while ZIBs offer high energy density by storing more charge per unit weight/volume. Although the charge storage mechanisms are considered different, manganese dioxide (MnO2) has proven to be an appropriate electrode material for both SCs and ZIBs because of its unique characteristics, including polymorphic forms, tunable structures and designable morphologies. Herein, the design of MnO2-based materials for SCs and ZIBs is comprehensively reviewed. In particular, we compare the similarities and differences in utilizing MnO2-based materials as active materials for SCs and ZIBs by highlighting their corresponding charge storage mechanisms. We then introduce a few commonly adopted strategies for tuning the physicochemical properties of MnO2 and their specific merits. Finally, we discuss the future perspectives of MnO2 for SC and ZIB applications regarding the investigation of charge storage mechanisms, materials design and the enhancement of electrochemical performance.