Synthesis and electrochemical mechanisms of yolk-shell ZnCo2S4 for high-performance supercapacitors

Ternary metal sulfides have excellent electrochemical energy-storage and charge-transfer capabilities, outperforming binary metal sulfides and traditional metal oxides; thus, they have great potential for application in electrochemical supercapacitors. In this study, ZnCo2S4 nanoparticles with a unique yolk-shell microstructure are synthesized through a two-stage hydrothermal method based on controlling the second sulfidation treatment period. The resulting yolk-shell nanoparticle possesses adjustable interspaces between the ZnCo2S4 core and yolk, resulting in high specific surface area and mesoporosity. The material characteristics enlarge the reaction sites and facilitate the charge-transfer pathways, and the material could accommodate volume expansion during ion and electron transport. Moreover, the ZnCo2S4 has a high Co3+ content, which benefits Faradaic redox reactions for intensifying electron transport and charge-storage capability. The desirable material physical/chemical properties are supported with rationally designed ZnCo2S4 nanoparticles, which affect the capacitor electrochemical performance, varied with the sulfidation duration. The 18 h-sulfided yolk-shell ZnCo2S4 exhibits the optimal properties, demonstrating its potential application as a supercapacitor electrode. The resulting asymmetric supercapacitor with the yolk-shell ZnCo2S4 electrode achieves excellent energy densities of 92 Wh kg−1 and 66 Wh kg−1 at power density values of 2400 W kg−1 and 18,000 W kg−1, respectively, with good electrochemical stability.