Coordination-derived nickel cobalt sulfide solid microspheres with surface defects for high-performance asymmetric supercapacitors

Abstract

Bimetallic sulfides are effective cathode materials for asymmetric supercapacitors because of their excellent electrochemical properties. However, both the low utilization ratio of the electrode material and the low energy storage capacity have restricted further development. In this work, Ni and Co were reacted with terephthalic acid (H2BDC) and isonicotinic acid (Ina) under hydrothermal conditions, followed by hydrolysis in 1 M KOH and vulcanization in thiourea to yield nickel cobalt sulfide (CoNi2S4) solid microspheres (NiCoS) with surface defects consisting of bumps and holes. These structural and morphological features compensate for the shortcomings of bimetallic sulfide electrodes, enhance their energy storage capacity, and improve their capacitance performance. The specific capacitance of the asymmetric supercapacitor assembled with NiCoS and a self-made carbon electrode reached 168.0 and 101.6 F/g at specific currents of 1 and 20 A/g, respectively. An increase in the specific current by a factor of 20 corresponded to an increase in capacitance retention rate to 60.5%, improving the rate performance. When the specific power increased to 800 and 16 000 W/kg, the specific energy reached 59.7 and 36.2 Wh/kg, respectively. Following 5,000 charge/discharge cycles, the capacitance retention rate was 96.78%. Such excellent comprehensive electrochemical performance greatly increases the competitiveness of the NiCoS electrode in the field of electrochemical energy storage.