A Cation Exchange Strategy to Construct CoNi₂S₄/NiS Nanoframes Composed of Nanosheets with Improved Specific Capacity and Rate Performance for Asymmetric Supercapacitors

Bimetallic sulfide is an outstanding pseudocapacitive material with high theoretical specific capacitance and good electronic conductivity. Herein, nickel-cobalt bimetallic sulfide (CoNi₂S₄/NiS) nanoframes composed of thin sheets are synthesized from Ni-Co Prussian blue analogues (NiCo-PBA) by an ion exchange method. The influence of sodium sulfide solution concentration on the morphology and supercapacitor (SC) performances of sulfides is systematically investigated. Benefiting from the unique nanoframe structure composed of nanosheet morphology and bimetallic active sites, the CoNi₂S₄/NiS-30 electrode displays a high specific capacity of 1243 C g^-1 at 1 A g^-1 and a high capacity retention rate of 75% when increasing the current density by 20-fold. It is worth emphasizing that the rate performance was improved by 68% compared with that of Ni₃S₄/NiS before ion exchange. The assembled asymmetric supercapacitor (ASC) exhibits an energy density of 48.6 Wh kg^-1 at a power density of 963 W kg^-1 and good cycling stability, with a capacity retention rate of 81% after 5000 cycles. Moreover, the all-solid-state supercapacitor (SASC) has an energy density of 46.7 Wh kg^-1 at a power density of 1081 W kg^-1, and two SASCs connected in series can power an LED for exceeding 4 min. This study offers an innovative strategy to develop high-performance bimetallic sulfides for asymmetric supercapacitors.