Controlled synthesis of hierarchical hollow Co _<i>x</i> Ni _{3− <i>x</i>} S ₄ towards enhanced rate capability and excellent cycling stability

The strategic design and advancement of electrode materials are crucial for the effectiveness of energy storage and conversion devices. Herein, we successfully synthesized a series of Co_<i>x</i>Ni_3-<i>x</i>S₄ hollow microspheres through a combined hydrothermal and vapor-phase sulfidation process. These microspheres are composed of interlinked nanosheets featuring uniformly distributed nanoparticles on their surfaces. This distinctive hierarchical structure integrates one-dimensional, two-dimensional and three-dimensional elements, thereby facilitating the exposure of more active sites, enhancing electrolyte-active site interactions, accelerating carrier transport, and mitigating volume expansion during cycling. Specifically, varying the molar ratios results in different nanosheet thicknesses, with higher cobalt content leading to thicker nanosheets and higher nickel content producing thinner ones. The optimized (Co₁Ni₂)S₄ electrode material exhibits an impressive specific capacity of 204.9 mA h g^-1 (1639 F g^-1) at 1 A g^-1, along with remarkable rate capability (holding 80.5% when the current is increased 20 times) and exceptional cycling stability (maintaining 96% of its initial capacity after 15 000 cycles). When paired with lotus pollen-derived activated carbon (AC) to form a supercapacitor device, the (Co₁Ni₂)S₄//AC configuration reaches an energy density of 66.5 W h kg^-1 at 800 W kg^-1. Notably, even after 30 000 cycles, the device retains an outstanding 98% of its initial capacity while sustaining 100% coulombic efficiency. This study presents an encouraging method for the precise fabrication of advanced bimetallic sulfides with enhanced electrochemical characteristics, facilitating their broader application in energy storage technologies.