A novel strategy for the synthesis of hard carbon spheres encapsulated with graphene networks as a low-cost and large-scalable anode material for fast sodium storage with an ultralong cycle life

Carbon materials have drawn remarkable attention as promising anode materials for high performance sodium-ion batteries (SIBs). Developing carbon-based anodes is important for the practical utilization of SIBs in energy storage technologies. Herein, hard carbon spheres encapsulated with graphene networks are prepared by a large-scalable and low-cost method, which deliver better electrochemical performance, such as rate capability and long-cycling life with respect to the ever reported hard carbon materials. The electrochemical test indicates that the as-prepared hard carbon anode exhibits a reversible capacity of 140 mA h g(-1) under a current density up to 10 A g(-1). Of special interest is that the capacity of 122 mA h g(-1) is maintained after 4000 cycles, corresponding to the high capacity retention of 87.1%. The robust and flexible structure of the G-HCS material could facilitate a fast electron and sodium ion transport rate, and sustain fast sodium storage at a high current rate owing to the construction of conductive graphene networks with outstanding electrical conductivity, enabling good contact between hard carbon sphere particles and graphene nanosheets. These results provide a rewarding avenue to design and optimize advanced carbon anode materials for next-generation SIBs.