Investigation On Preparation And Lithium Storage Properties Of Graphene Nanoribbons/SnO₂/TiO₂ Hybrids

Rechargeable lithium ion batteries?LIBs? have long been considered as the most effective energy-storage system for mobile electronics, electric vehicles and renewable energy systems, due to their high energy density, high operation voltage, low self-discharge and environment friendly.Electrode materials play a vital role in their electrochemical performance. However, current commercial graphite anodes with a low theoretical capacity of 372 mAh g-1, cannot fulfil the increasing demands for LIBs. Therefore, it is highly urgent to develop new electrode materials with high reversible capacity, long-term cyclic stability, sound rate performance and high security.In this thesis, a hierarchical hybrid of SnO₂ nanoparticles encased in graphene nanoribbons?GNRs? are synthesized. The effect of different ratio of Sn2+/GNRs in microstructure and electrochemical performance is also studied systematically. The discharge capacity of SnO₂/GNRs-8 can reach 547 and 514 mAh g-1 at current density of 1000 and 2000 mA g-1, respectively. More importantly, after 300 cycles, the reversible capacity of SnO₂/GNRs-8 can remains at 452 mAh g-1 with coulombic efficiency of 100%, showing excellent rate capability and cycling stability.GNRs matrix can significantly enhance the electronic conductivity of the electrode, avoid the aggregation of SnO₂, alleviate the serious volume variation and prevent the pulverization of SnO₂ nanoparticles during the lithiation-delithiation cycles.A porous spherical hybrid of TiO₂ nanoribbons entwined by graphene nanoribbons?GNRs? is synthesized by hydrothermal reaction. Both anatase and TiO₂?B? phases are observed in TiO₂/GNRs composite. The close entwining of GNRs prevents TiO₂ nanoribbons from self-rolling to nanotubes and maintains layered structure. The addition of GNRs can not only improve the electronic conductivity, but also improve the structure stability in cycles, resulting in superior electrochemical performance. The TiO₂/GNRs nanocomposite exhibits high rate capability and cycling performance with a capacity of 131 mAh g-1 accompanying coulombic efficiency of 99.2 % at a current rate of 10 C?1 C=335 mA g-1? over 1400 cycles.A hybrid of SnO₂@TiO₂ core-shell nanospheres wrapped by graphene nanoribbons is synthesized by hydrothermal reaction, followed by sol-gel method and a simple solution diffusion method. TiO₂ in a thickness of 50 nm are uniformly coated on hollowSnO₂ nanospheres, then graphene nanoribbons wrapped and entwined the SnO₂@TiO₂ under electrostatic force. The SnO₂@TiO₂/GNRs electrode delivers remarkable capacities of 843.2, 645.9 and 502.3 mAh g-1 at current densities of 1.0, 2.0 A g-1 and 3.0 A g-1. Moreover, a reversible capacity of 748 mAh g-1 can be obtained at a current density of 1.0 A g-1 even after 300 cycles. The appealing electrochemical performance can be attributed to the synergistic effect of hollow SnO₂ nanospheres, highly crystallized TiO₂ and flexible GNRs.