Hierarchical cobalt-pentlandite (Co, Ni, Zn)9S8 nanostructures: advanced electrodes for flexible solid-state supercapacitors†

Rationally designing electrodes with hierarchical structures, optimized components, and mechanical flexibility offers a viable pathway to boost electrochemical performance and address the growing demands of modern flexible electronic devices. In this study, a unique triple-level hierarchical nanostructure was constructed, where nanoparticles aggregate to form (Co, Ni, Zn)₉S₈ nanoneedles, which in turn assemble into robust hexagonal nanosheets. This hierarchical configuration, achieved via a straightforward two-step hydrothermal strategy, offers a synergistic combination among uniformly distributed nanoparticles, interwoven nanoneedles, and interconnected nanosheets. These features provide an expanded active surface area, improved ion diffusion pathways, and enhanced structural integrity. The optimized distribution of transition metal ions within this structure further contributes to improved electrochemical performance by facilitating efficient redox reactions and ensuring prolonged cycling durability. Density functional theory calculations suggest that Co and Ni significantly enhance OH⁻ adsorption to form abundant redox-active centers, confirming synergistic interaction among Co, Ni, and Zn within (Co, Ni, Zn)₉S₈. Leveraging the benefits of its multiscale structure and well-engineered composition, (Co, Ni, Zn)₉S₈ demonstrates remarkable supercapacitor performance, achieving an impressive specific capacitance of 1664 F g⁻¹ at 1 A g⁻¹, with outstanding rate performance (1391 F g⁻¹ at 20 A g⁻¹) and remarkable cycling stability, retaining 94% of its capacitance after 10 000 cycles. The flexible all-solid-state device utilizing (Co, Ni, Zn)₉S₈ as the positive electrode further underscores its suitability for practical energy storage technologies.