Preparation Of High-quality Graphene And The Application Of Its Hybrids In Lithium-ion Batteries

Graphene has excellent charge transport properties, mechanical properties,thermal conductance, and adsorption properties. In recent years, researchers have discovered the potential application of graphene in energy storage,especially in lithium-ion batteries (LIBs). Compared with the traditional graphite anode materials, the high specific surface area and conjugated structure of graphene are beneficial for higher Li-ions storage ability and larger capacity. Furthermore, when graphene is hybridized with other active materials (such as transition metal oxides, silicon-based materials, etc.), the favorable synergistic effect can endow the hybrid with excellent cycling performance and high reversible capacity. Although various methods for preparing graphene have been reported, it is usually difficult to realize the high-quality production at low cost, which greatly restricts the potential applications of graphene.The main contents of this paper include two aspects: firstly, the preparation of high-quality graphene was studied. Graphene sheets with high lattice regularities are prepared by liquid-phase exfoliation method. Secondly,the synthesis of graphene hybrids materials was investigated. We have developed a novel method to fabricate graphene based hybrids besides the traditional approaches, such as hydrothermal synthesis, template method, and chemical vapor deposition. The resultant graphene hybrids exhibit excellent electrochemical performance. The main works are given as follows:(1) Preparation of FeC13/graphene hybrid and its application in lithium-ion batteries. Stage-1 FeCl3 based graphite intercalation compounds(FeCl3-GICs) were prepared by a non-oxidation method and used as precursors. Subsequently, it was sonicated in organic solvent to prepare graphene sheets and FeCl3 intercalated few-layer graphene (FeCl3-FLG).When FeCl3 inserted into the graphite layers, the interlayer spacing of graphite was significantly enlarged, weakening the constraint of the Van der Waals forces between adjacent sheets, making it easy to exfoliate for graphene sheets while avoiding the lattice defects caused by oxidation. At the same conditions,only 32 % of the graphene sheets have thickness less than 2 nm with natural flake graphite as the precursors, while almost all the thicknesses of graphene sheets prepared from the stage-1 FeCl3-GICs precursors fall in the range of 1.0-2.0 nm. Moreover, by controlling the exfoliation time, the FeCl3-FLG hybrid can be conveniently prepared. As anode materials of LIBs, FeCl3-FLG hybrid shows high specific capacity as evidenced by the high reversible specific capacity of 1002 mA h g-1 at the current density of 100 mA g-1. The graphene matrix restricts the volume expansion of FeCl3 particles during the intercalation and de-intercalation processes, and the enhanced interlayer spacing of FeCl3-FLG hybrid facilitates the diffusion of Li-ions.(2) Preparation of high-quality graphene through high-pressure homogenization approach. Graphene sheets were prepared by exfoliating FeCl3-GICs with a high-pressure homogenization (HPH) approach. The effects of homogenization time, pressure and initial concentration of precursor were explored. The HPH approach is effective, continuous and easy to scale-up,making it a promising method for the mass production of high-quality graphene. Large amounts of single-layer graphene sheets with an average thickness of about 0.72 nm were fabricated under 120 MPa pressure after 50 cycles. The as-prepared graphene sheets exhibit high structural integrity and low defects (ID/IG<0.1), giving a high conductivity of 3.7×104 S m-1 for graphene film.(3) Preparation of Sandwich-Structured Fe2O3/graphene hybrid and its appliacation in lithium-ion batteries. By using the HPH approach, we developed a simple and rapid method of graphene hybrid materials. With FeCl3-GICs as the precursors, FeCl3-FLG hybrid was produced by carefully controlling the homogenization time, and Fe2O3/graphene (α-Fe2O3/FLG)hybrid was prepared after heat treatment at 500℃. The as-prepared a-Fe2O3/FLG hybrid shows a unique sandwich-structure with α-Fe2O3 particles uniformly anchored on both the surface and the interlayer of FLG,which can avoid agglomeration, prevent volume expansion and particles crushing during the charge and discharge process. In addition, highly conductive graphene can form a perfect conducting network, improving the conductivity of α-Fe2O3 particles and reducing the diffusion passway of Li-ion.It is worth noting that graphene and a-Fe2O3 particles are closely contact with each other, which could facilitate charge transport and maintains a good cycling stability.As anode material for LIBs,the α-Fe2O3 /FLG hybrid shows a reversible specific capacity of 1100 mA h g-1 even after 350 cycles at a current density of 200 mA g-1.(4) Ternary CoCl2-FeCl3 based graphite intercalation compounds and their derivatives for lithium-ion batteries applications. Ternary CoCl2-FeCl3 based graphite intercalation compounds (CoCl2-FeCl3-GICs)were used as the anode materials of LIBs. Stage-1 CoCl2-FeCl3-GICs have shown excellent cycling performance, and after 400 cycles at a current density of 200 mA g-1, the reversible capacity of it still maintain about 1000 mA h g-1.Moreover, ternary Co3O4-Fe2O3/graphene (Co3O4-Fe2O3/FLG) hybrid was prepared according to the method adopted in part (3). The reversible specific capacity of Co3O4-Fe2O3/FLG hybrid can reach ～700 mA h g-1 after 300 cycles at current density of 1000 mA g-1.