Preparation of N-doped potassium-intercalated copper sulfide for high-performance potassium-ion and sodium-ion supercapacitors

In this study, a novel method is proposed to synthesize an intercalation compound of N-doped KCu7S4 by directly pre-inserting potassium ions into N-doped Cu7S4 supported on nickel foam (NF). Interestingly, the insertion of potassium ions and N doping increase the interlamellar spacing, facilitating the incorporation of both potassium and sodium ions. At a current density of 9 A g(-1), the fabricated N-doped KCu7S4/NF electrode exhibits specific capacitance values of 2958.0 F g(-1) (1774.8 C g(-1)) and 3223.5 F g(-1) (1934.1 C g(-1)) in KOH (1 M) and NaOH (1 M) electrolytes, respectively. In addition, an aqueous asymmetric supercapacitor is assembled using N-doped KCu7S4/NF as the positive electrode and N-doped KCu7S4/CF (where CF: carbon graphite felt) as the negative electrode in an electrolyte containing 1 M KOH electrolyte. At a current density of 2.5 A g(-1), the N-doped KCu7S4/NF//N-doped KCu7S4/CF supercapacitor demonstrates a specific capacitance of 549.8 F g(-1) (989.6 C g(-1)), an energy density of 80.1 Wh kg(-1) at a power density of 1.8 kW kg(-1), and maintains 114.3 % capacitance retention after 30,000 cycles. Overall, this study presents a novel strategy for the pre-embedding of potassium ions into metal sulfides, which can significantly enhance their energy density.