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

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 Co 3 S 4 prepared via a facile two-step hydrothermal method containing the synthesis of precursor Co 2 (CO 3 )(OH) 2 in the first step, which is converted into Co 3 S 4 in the second step via an anion-exchange reaction with S 2− . 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 Co 3 S 4 –100 is in an amorphous structure. A remarkable specific capacitance (2520.2 F g −1 @1 A g −1 ) is delivered by Co 3 S 4 -100, surpassing that of crystalline Co 3 S 4 -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 (Co 3 S 4 –100 positive electrode and activated carbon negative electrode) shows exceptionally good cycling stability (retaining 98.4% of the initial capacitance after 10,000 cycles).