Fiber-in-Tube Design of Co9 S8 -Carbon/Co9 S8 : 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 Co9 S8 -carbon(C)/Co9 S8 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.