From Crystalline to Partially Amorphous: A Facile Strategy toward Sulfur Vacancy‐Enriched CoNi2S4 Nanosheets with Improved Supercapacitor Performance

Abstract Supercapacitors (SCs) are considered to be a promising energy storage technology due to their superior electrochemical properties. However, to meet the continuously rising demands of energy density, SCs need to be equipped with high specific capacitance. For this purpose, partially amorphous CoNi 2 S 4 nanosheets grown directly on nickel foam (PA‐CoNi 2 S 4 /NF) are successfully synthesized via a facile solvothermal method combined with follow‐up KBH 4 treating. The nanosheets structure optimizes the ion transport path and the amorphous crystals endow PA‐CoNi 2 S 4 /NF with numerous sulfur vacancies. Therefore, the as‐prepared PA‐CoNi 2 S 4 /NF possess high specific capacitance (3305 F g –1 at 2 A g –1 ) and excellent capacitance retention of 66.4% when the current density increases from 2 to 100 A g –1 . In addition, an asymmetric supercapacitor device composed of PA‐CoNi 2 S 4 /NF as a positive electrode and activated carbon as a negative electrode exhibits a high energy density of 43.4 Wh kg –1 at a power density of 800.4 W kg –1 , as well as excellent long‐term cyclic stability. This work offers a simple approach for the fabrication of transition metal sulfide materials with high electrochemical activities in SCs and other electrochemical energy‐related fields.