Dual Carbon Design Strategy for Anodes of Sodium-Ion Battery: Mesoporous CoS/CoO on Open Framework Carbon-Spheres with rGO Encapsulating.

Transition metal sulfides and oxides with high theoretical capacities have been regarded as promising anode candidates for a sodium-ion battery (SIB); however, they have critical issues including sluggish electrochemical kinetics and poor long-term stability. Herein, a dual carbon design strategy is proposed to integrate with highly active heterojunctions to overcome the above issues. In this new design, CoS/CoO hollow dodecahedron heterojunctions are sandwiched between open framework carbon-spheres (OFCs) and a reduced graphene oxide (rGO) nanomembrane (OFC@CoS/CoO@rGO). The CoS/CoO heterojunctions effectively promote electron transfer on their surface and provide more electrochemical active sites through their hierarchical hollow structures assembled by nanodots. Meanwhile, the dual-carbon framework forms a highly conductive network that enables a better rate capability. More importantly, the dual carbon can greatly buffer volume expansion and stable reaction interfaces of electrode material during the charge/discharge process. Benefitting from their synergistical effects, the OFC@CoS/CoO@rGO electrode achieves a high reversible capacity of 460 mAh g at 0.05 A g and still maintains 205.3 mAh g even when current density is increased by 200 times when used as an anode material for SIBs. Their cycling property is also remarkable with a maintained capacity of 161 mAh g after 3500 charging/discharging cycles at a high current density of 1 A g. The dual-carbon strategy is demonstrated to be effective for enhanced reaction kinetics and long-term cycling property, providing siginificant guidance for preparing other high-performance electrode materials.