Three-dimensional graphene encapsulated hollow CoSe₂-SnSe₂nanoboxes for high performance asymmetric supercapacitors

Construction of metal selenides with a large specific surface area and a hollow structure is one of the effective methods to improve the electrochemical performance of supercapacitors. However, the nano-material easily agglomerates due to the lack of support, resulting in the loss of electrochemical performance. Herein, we successfully design a three-dimensional graphene (3DG) encapsulation-protected hollow nanoboxes (CoSe₂-SnSe₂) composite aerogel (3DG/CoSe₂-SnSe₂) via a co-precipitation method coupled with self-assembly route, followed by a high temperature selenidation strategy. The obtained aerogel possesses porous 3DG conductive network, large specific surface area and plenty of reactive active sites. It could be used as a flexible and binder-free electrode after a facile mechanical compression process, which provided a high specific capacitance of 460 F g^-1at 0.5 A g^-1, good rate capability of 212.7 F g^-1at 10 A g^-1The capacitance retention rate is 80% at 2 A g^-1after 5000 cycles due to the fast electron/ion transfer and electrolyte diffusion. With the as-prepared 3DG/CoSe₂-SnSe₂as positive electrodes and the AC (activated carbon) as negative electrodes, an asymmetric supercapacitor (3DG/CoSe₂-SnSe₂//AC) was fabricated, which delivered a high specific capacity of 38 F g^-1at 1 A g^-1and an energy density of 11.89 Wh kg^-1at 749.9 W kg^-1, as well as excellent cycle stability. This work provides a new method for preparing electrode material.