Energy Storage Kinetics of Hierarchically Designed Co₉S₈@CoNiO₂ Hollow Cubic Supercapacitors with Improved Stability and Energy Density

The delicate design and rational preparation of core–shell heterostructures are effective in improving the energy conversion and storage characteristics in supercapacitors. Herein, we designed and constructed cobalt metal–organic framework (Co-MOF) hollow core–shell nanocubes decorated with abundant Co₉S₈ and CoNiO₂ nanofeatures. The synergistically composed Co₉S₈@CoNiO₂-120 exhibits high electrical conductivity, high cycling stability, and excellent energy density compared to others with different Ni contents. The improvement of structural stability originated from the pseudocapacitive nature of Co₉S₈. The conversion of CoNi-LDH to CoNiO₂ increases the cycle stability by 8.7 times (specific capacity retention of 587.3 C g^–1 after 10 000 cycles at a high current density of 10 A g^–1) with the specific capacity (652.6 C g^–1 at 1 A g^–1) 3.4-fold higher than that of Co₉S₈. Mechanism analysis reveals that the dissociation process of OH^– is more detrimental to the cycle stability. Furthermore, the assembled asymmetric supercapacitor (ASC) device demonstrates a maximum energy density of 50 Wh kg^–1 (49.4 Wh kg^–1 after consideration of iR loss) at a corresponding power density of 800 W kg^–1 (790 W kg^–1 after consideration of iR loss), with 82% capacity retention over 5000 cycles at 5 A g^–1. Our work provides a novel approach for MOF derivative supercapacitors in practical energy storage applications.