Investgation On Preparation, Decoration And Lithium Storage Properties Of Graphene/TiO₂Hybrids

Lithium-ion batteries (LIBs) are widespreadly used in portable electronic devices,electrical vehicles, energy storage grid and aerospace for their high energy density, highoperation voltage and long life span. The development and storage of clean energybecome a hotspot of materials science, and the study of lithium-ion batteries drawsextensive attention from researchers, with the energy crisis and environmentaldegradation. Nowadays the capacity and safety of LIBs need to be further improved,however, as one of the key components of LIBs, electrode materials play a vital role intheir electrochemical performance. Therefore, it is highly significant to investigateelectrode materials of LIBs.In this thesis, graphene nanoscrolls encapsulated TiO2(B) hybrids have beensynthesized via a hydrothermal reaction, followed by ion-exchange and heat treatment.The excellent electrical conductivity of graphene nanoscrolls favors charge transfer,while the nanopores in them provide abundant sites for lithium ions insertion. Theirunique scrolled structures stabilize the architectures during lithium insertion/extractionand prevent the active materials from peeling, as well as restrict the side reactionsbetween the hydroxyls on the surface of TiO2(B) and the electrolytes. The hybridsexhibit initial reversible capacity of232mAh g-1at0.1C rate with coulombic efficiencyof85%. And they perform typical pseudocapacitive behavior of lithium storage, aftergalvanostatically charged/discharged for300cycles at10C rate, the capacity retains153mAh g-1with capacity retention of94%, showing outstanding lithium storageproperties.Graphene-TiO2(B) hybrids are decorated with hierarchical MnO2nanoplatelets bya water-bath reaction. The high specific surface area of the composites enlarge thecontact area with the electrolytes, thus favors electrochemical reactions. The ultrathinMnO2nanoplatelets possess sufficient pores, which are beneficial for the penetration ofthe solvented lithium ions, hence ensuring high electrochemical reactivity. Comparingwith the hybrids without surficial decoration by MnO2nanoplatelets, the capacity of thecomposites is dramatically improved, exhibiting initial reversible capacity of291mAhg-1, which is obviously higher than that of the graphene-TiO2(B) hybrids (200mAh g-1),and retains243mAh g-1after150cycles at the current density of250mA g-1, withcapacity retention of83.5%. In-situ growth of ultrasmall SnO2nanoparticles on the substrate of graphene-TiO2(B) hybrids are prepared by a hydrothermal reaction, accordingly surfacecarbon-coating under heat treatment. The substrate of graphene-TiO2(B) hybrids caneffectively buffer the volume expansion and shrinkage of SnO2nanoparticles duringlithiation/delithiation, thus keep structural stability of the composites duringcharge/discharge process. The surficially coated carbon layers enhance the electricalconduction of the composites, as well as immobilize the ultrasmall SnO2nanoparticlesto avoid peeling, leading to better cycling performance. Galvanostatic charge/dischargetests show that the capacity of the composites reaches476mAh g-1at the currentdensity of1000mA g-1. After200cycles at the current density of500mA g-1, thereversible capacity retains524mAh g-1, exhibiting excellent electrochemical properties.