Fiber‐in‐Tube Design of Co9S8‐Carbon/Co9S8: Enabling Efficient Sodium Storage

The sodium‐ion battery is a promising battery technology owing to its low price and high abundance of sodium. However, the sluggish kinetics of sodium ion makes it hard to achieve high‐rate performance, therefore impairing the power density. In this work, a fiber‐in‐tube Co₉S₈‐carbon(C)/Co₉S₈ is designed with fast sodiation kinetics. The experimental and simulation analysis show that the dominating capacitance mechanism for the high Na‐ion storage performance is due to abundant grain boundaries, three exposed layer interfaces, and carbon wiring in the design. As a result, the fiber‐in‐tube hybrid anode shows a high specific capacity of 616 mAh g^?1 after 150 cycles at 0.5 A g^?1. At 1 A g^?1, a capacity of ca. 451 mAh g^?1 can be achieved after 500 cycles. More importantly, a high energy density of 779 Wh kg^?1 and power density of 7793 W kg^?1 can be obtained simultaneously.