Synthesis of size-controllable, yolk-shell metal sulfide spheres for hybrid supercapacitors

Developing complex nanostructured materials with amorphous phases remains an important challenge and such materials are potentially valuable for achieving highly efficient electrochemical energy storage. Here, a facile surfactant-free strategy is developed to synthesize size-controllable Ni-Co-Mn sulfide spheres with an amorphous and yolk-shell structure. In the absence of surfactant, monodispersed Ni-Co-Mn glycerate solid spheres with diameters ranging from 650 to 1100 nm are controllably synthesized by regulating the amount of Mn(NO3)2, and subsequently transformed into yolk-shelled amorphous Ni-Co-Mn sulfide spheres via an anion-exchange reaction. Benefitting from compositional and structural advantages, the optimized electrode displays an outstanding specific capacity of 1204 C g−1 at 2 A g−1 with a long cycling lifetime. A hybrid supercapacitor, fabricated with the yolk-shelled amorphous Ni-Co-Mn sulfide spheres as the positive electrode material and activated carbon as the negative one, achieves superior cyclic stability with a high energy density of 71.86 Wh kg−1 at 793.56 W kg−1, demonstrating their potential application for high-performance hybrid supercapacitors.