Defect engineering induced heterostructure of Zn-birnessite@spinel ZnMn2O4 nanocrystal for flexible asymmetric supercapacitor

Abstract Defect engineering holds great promise to boost surface charge redox chemistry of pseudocapacitive materials. However, their innovative development on the heterogeneous structure is still lacking. Herein, defect-rich heterogeneous Zn-birnessite nanosheet@spinel ZnMn2O4 nanocrystal composites are designed via an in situ chemical reduction route at a low temperature. We explore the formation mechanism that the generated oxygen vacancy (Vo) in the Zn-birnessite triggers Mn cation migration, leading to birnessite-to-spinel phase transition. The defect-rich heterostructure supplies rich Mn2+/3+/4+ redox couples, multiple electrochemically active sites, and shortened ion-transport pathways. Moreover, the bandgap of the heterostructure is reduced from 1.54 eV to 1.06 eV after introducing Vo, which promotes electron transport and thus bolsters fast redox reaction kinetics. Accordingly, the heterostructure delivers a large areal capacitance of 1903 mF cm−2 at 3 mA cm−2 at a wide potential window of 1.2 V, high rate performance, and long cycle life (93.7% capacitance retention over 16,000 cycles). An asymmetric supercapacitor employing the heterostructure as a cathode and vanadium oxide as an anode exhibits a high voltage of 2.4 V, and possesses a maximum energy density of 6.24 mWh cm−3. This research offers a promising avenue to tailor the electrochemical reactivity of heterostructures through defect engineering.