Fabrication of dual-hollow heterostructure of Ni₂CoS₄ sphere and nanotubes as advanced electrode for high-performance flexible all-solid-state supercapacitors

High-energy-density and flexible supercapacitors have shown numerous application potential in modern portable electronics. However, the relatively low specific capacity, poor rate retentions, and limited durability have hindered their implement. Herein, a novel hierarchical dual-hollow electrode, composed of a hollow Ni₂CoS₄ sphere and outer hollow Ni₂CoS₄ nanotubes (Ni₂CoS₄HS-HTs), is elaborately constructed. The Ni₂CoS₄HS-HT-5 exhibits a high specific capacity of 817.5 C g⁻¹ at a current density of 1 A g⁻¹ with remarkable rate retention of 75.3% at 50 A g⁻¹. In an all-solid-state asymmetric supercapacitor of Ni₂CoS₄HS-HT-5//CAC, a high capacitance of 1511.5 mF cm⁻² at 5 mA cm⁻² is obtained with an exceptional energy density of 13.6 mWh cm⁻³ at a power density of 92.6 mW cm⁻³. In addition, the capacity retention reaches 96% over 2000 cycles at 20 mA cm⁻³, implying the outstanding durability. The flexibility and mechanical stability are demonstrated by the intact electrochemical performances under different bending angles. As a proof-of-concept, two Ni₂CoS₄HS-HT-5//CACs in series could successfully illuminate 31 LED indicators for more than 8 mins. These fascinating electrochemical performances benefit from the novel electrode structure and depict great potential for modern energy storage applications.