Enhancing performance of sandwich-like cobalt sulfide and carbon for quasi-solid-state hybrid electrochemical capacitors

Metal sulfides (MSs) should be feasible candidates for hybrid electrochemical capacitors (HECs) due to their high theoretical specific capacitances. However, their performances are largely hampered by sluggish ion/electron transport kinetics and fast capacitance fading. Here, we provide a new approach to fabricate high-performance dual-structural MSs for long-life electrochemical energy storage devices. With robust, graphitic and nitrogen-doped porous carbon shells and highly conductive reduced graphene oxide (RGO) substrates, our cobalt sulfide-based composite shows 99.7% capacitance retention after 4000 cycles. We also present a sandwich-like carbon electrode with ultrahigh specific capacitance and excellent cycling stability. As a result, a quasi-solid-state HEC comprising the above-mentioned electrodes and a PVA-PAA membrane is fabricated. Its electrochemical performance is superior to those formerly reported for MSs, and our results for the first time provide a solid base for the application of MSs in HECs.