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Conversion-type anode materials with a high charge storage capability generally suffer from large volume expansion, poor electron conductivity, and slug-gish metal ion transport kinetics. The electrode material described in this paper, namely cobalt sulphide nanopar-ticles encapsulated in carbon cages -(Co9S8@NC), can cir-cumvent these problems. This electrode material exhibited a reversible sodium-ion storage capacity of 705 mAh g-1 at 100 mA g-1 with an extraordinary rate capability and good cycling stability. Mechanistic study using the in situ transmission electron microscope technique revealed that the volumetric expansion of the -Co9S8 nanoparticles is buffered by the carbon cages, enabling a stable electrode–electrolyte interface. In addition, the carbon shell with high-content doped nitrogen significantly enhances the electron conductivity of the -Co9S8@NC electrode material and provides doping-induced active sites to accommodate sodium ions. By integrating the -Co9S8@NC as negative electrode with a cellulose-derived porous hard carbon/graphene oxide composite as positive electrode and 1 M -NaPF6 in diglyme as the electrolyte, the sodium-ion capacitor full cell can achieve energy densities of 101.4 and 45.8 Wh kg-1 at power densities of 200 and 10,000 W kg-1, respectively.