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 CoNi₂S₄ nanosheets grown directly on nickel foam (PA‐CoNi₂S₄/NF) are successfully synthesized via a facile solvothermal method combined with follow‐up KBH₄ treating. The nanosheets structure optimizes the ion transport path and the amorphous crystals endow PA‐CoNi₂S₄/NF with numerous sulfur vacancies. Therefore, the as‐prepared PA‐CoNi₂S₄/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‐CoNi₂S₄/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.