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Carbon-coated mesoporous Co9S8 nanoparticles supported on reduced graphene oxide (rGO) are successfully synthesized by a simple process.This composite makes full use of the protection of the carbon layer on the surface,the good conductivity and three-dimensional (3D) structure of rGO,the mesoporous structure and nanoscale size of Co9S8,thereby presenting the excellent electrochemical performances in potassium-ion batteries,407.9 mAh·g-1 after 100 cycles at 0.2 A·g-1 and 215.1 mAh·g-1 at 5 A·g-1 in rate performances.After 1,200 cycles at 1.0 A·g-1,this composite still remains a capacity of 210.8 mAh·g-1.The redox reactions for potassium storage are revealed by ex-situ transmission electron microscope (TEM)/high-resolution TEM (HRTEM) images,selected area electron diffraction (SAED) patts and X-ray photoelectron spectroscopy (XPS) spectra.The application of this composite as the host of sulfur for Li-S batteries is also explored.It sustains a capacity of 431.8 mAh·g-1 after 800 cycles at 3 C,leading to a degradation of 0.052％ per cycle.These results confirm the wide applications of this composite for electrochemical energy storage.