Enhanced electrochemical performances of coal liquefaction residue derived hard carbon coated by graphene as anode materials for sodium-ion batteries

A graphene coated coal liquefaction residue (CLR) derived hard carbon (HC@G) composite anode for sodium ion batteries was prepared by thermal annealing the mixture of the chemically modified hard carbon with graphene oxide. The irregular shaped hard carbon particles were coated uniformly by wrinkled graphene sheets. Both HC and HC@G possess large inter-layer spacing and poor porosity, but a richer pore structure that facilitating ion transport was generated after graphene coating. Because poor conductive hard carbon particles are bridged by graphene sheets, a better conductivity of HC@G is detected by voltammetry test and the EIS analysis. Graphene coating contributes little to the sodium storage capacity. However, HC@G exhibits a much better rate performance and a dramatic improvement of cycling capability with the capacity retention 83% after 2000 cycles at a high current density of 2 A.g(-1). The impressing rate and cycle performances of HC@G anode are attributed to the hierarchical structure with rich ion diffusion channels and extensive electronic conduction path.