Two-step hydrothermal synthesis of a fireworks-like amorphous Co3S4 for asymmetric supercapacitors with superior cycling stability

Abstract

Transition metal sulfides (TMSs) as pseudocapacitive materials have high specific capacitance. But the main concern of the volume variation as a result of insertion/desertion of electrolyte ions into/from TMSs during the electrochemical processes leads to the terrible cycling stability of supercapacitor devices. To solve this concern, this work reports a fireworks-like amorphous Co3S4 prepared via a facile two-step hydrothermal method containing the synthesis of precursor Co2(CO3)(OH)2 in the first step, which is converted into Co3S4 in the second step via an anion-exchange reaction with S2−. The influence of anion-exchange temperature on the structure and electrochemical performances of as-prepared materials is discussed. It is observed that the structure of materials changes from disorder to order with the increment in anion-exchange temperature, while the fireworks-like morphology is kept almost unchanged. When the anion-exchange temperature is 100 °C, the resultant Co3S4–100 is in an amorphous structure. A remarkable specific capacitance (2520.2 F g−1@1 A g−1) is delivered by Co3S4-100, surpassing that of crystalline Co3S4-220 (1675.4 F g−1), which are attributable to the former's larger specific surface area together with the amorphous structure nature. Apart from eminent energy density (52.4 Wh kg−1@1026.3 W kg−1 power density), the asymmetric supercapacitor (Co3S4–100 positive electrode and activated carbon negative electrode) shows exceptionally good cycling stability (retaining 98.4% of the initial capacitance after 10,000 cycles).