Study Of Graphene Hybrid Materials For Anode And Cathode Of Li/Na-ion Batteries

Graphene, a one-atom thick layer of graphite, is an atomic-scale honeycomb lattice made of sp2-bonded carbon atoms. It possesses unique2-D structure and excellent electrical, mechamical and thermal properties, considered as a promising candidate for future electrode materials of Li/Na-ion batteries. In this study, graphened-based anode and cathode hybrid materials were prepared by hydrothermal/solvothermal methods. Their growth mechanism and improved Li/Na-storage properties have been studied in detail.This study used graphene (G) as matrix to prepare specific electrode materials of unique morphologies in order to improve their electrochemical properties. Ultra-thin NiS/G nanohybrid has been synthesized by one-pot hydrothermal route. The hybrid exhibits unique sheet-on-sheet structure, where ultrathin NiS sheets (below5nm) are anchored on few-layer (below8layers) graphene sheets, while bare NiS shows flower-like structure with aggregated nanorods. Experiment results indicate that the recrystallization of NiS from nanorods to nanosheets experienced an exfoliation process, in which new chemical bonding occurs between residual oxygen-containing groups on graphene sheets and dangling or unsaturated bonds related to-S or=S on NiS nanorods.In this work, the Li-storage and Na-storage properties of NiS/G hybrid have been studied. Results indicate that NiS/G hybrid exhibites remarkably improved electrochemical properties. As the anode materials for Li-ion batteries, after100cycles at current density of50mA/g, NiS/G hybrid retains a reversible capacity of481mAh/g,96.2%of its theoretical capacities (of about500mAh/g); while the capacity of bare NiS rapidly decays to140mAh/g after mere10cycles. As the anode materials for Na-ion batteries, NiS/G hybrid yeilds a charge capacity over150mAh/g at current density of400mA/g, while that of bare NiS is less than80mAh/g. Graphene acts not only as mechniacally stable buffer to accomodate the volume effect, but also conductive network to enhance the electronic conductivity of hybrid materials. Its immobilizing effect also prevents the aggregation during cycling.In this study, LiFePO4/G hybrid has been synthesized by solvethermal method. LiFePO4particles with diameter of less than100nm are anchored on graphene sheets. At current density of5C, the LiFePo4/G hybrid yields a reversible capacity of105mAh/g, while that of bare LiFePO4is65mAh/g. A flexible and continuous conductive network formed by graphene in the cathode materials can effectively enhance electron transportation and better the wetting of electrolyte, therefore contributes to improved rate capability.