Microcubes of ZnSe–SnSe2 Encapsulated within Graphene for High-Performance Asymmetric Supercapacitors

High-capacity SnSe2 cathode materials combine the advantages of conversion and alloying reactions, showing great prospects in supercapacitors. However, they have poor cycling performance and low electronic conductivity. To effectively improve their electrochemical performance, a bimetallic selenide heterostructure was constructed and a three-dimensional graphene (3DG) carbon layer was encapsulated. 3DG increases the specific surface areas of the material, stabilizes the internal structures, and promotes the reaction kinetics. The obvious lattice distortions and mismatches at the ZnSe–SnSe2 heterointerfaces generate a large number of crystal defects and active sites for the adsorption/desorption of OH– ions, which is beneficial for the insertion and extraction of ions during the cycling processes and further enhances the electronic conductivity of the electrode material. Benefiting from these two strategies, 3DG/ZnSe–SnSe2 exhibits a high specific capacity of 1515.2 F g–1 at 1 A g–1 and maintains a capacity retention rate of 90.1% after 3000 cycles at 5 A g–1. Furthermore, the asymmetric supercapacitor 3DG/ZnSe–SnSe2//AC assembled with activated carbon (AC) exhibits excellent electrochemical performance, demonstrating an energy density of 25.3 Wh kg–1 at 750 W kg–1 and a remarkable capacity retention rate of 91.3% after 20,000 cycles at 5 A g–1.