CoNi-MOF-S with double optimization of crystal phase and electronic structure for quasi-solid-state supercapacitors

This study modifies the structure of bimetallic cobalt‑nickel metal-organic framework (CoNi-MOF) via a sulfur doping strategy to form a composite material (CoNi-MOF-S) and investigates the mechanism for enhancing its electrochemical performance as a supercapacitor electrode. Compared with undoped CoNi-MOF, the introduction of sulfur plays a dual role: on one hand, sulfur replaces partial elements in organic ligands to form metal‑sulfur covalent bonds, which enhance electron transport efficiency; on the other hand, it successfully induces the formation of cobalt sulfate and nickel sulfate crystalline phases. These newly generated crystal phases construct a stable framework within the nanostructure, effectively alleviating the volume expansion problem during charge-discharge processes. The synergistic effect between the two endows the material with excellent charge-discharge performance and cycling stability. Experimental results show that CoNi-MOF-S2 exhibits superior specific capacitance (701C/g at 0.5 A g −1 ) and cycling stability (capacity retention >85 % after 5000 cycles at 5 A g −1 ) in 6 M KOH electrolyte. Furthermore, a flexible quasi-solid-state supercapacitor with a sandwich structure is constructed using CoNi-MOF-S2 as the positive electrode, activated carbon (AC) as the negative electrode, and a gel electrolyte. This device achieves an energy density of 53.4 Wh kg −1 at a power density of 379.4 W kg −1 , maintains 76.9 % capacity after 5000 cycles at 5 A g −1 , and can operate normally during bending. Demonstrating the synergistic mechanism of crystalline phase reconstruction and electronic structure optimization, this study offers new insights to enhance the performance of CoNi-MOF for electrode materials.