Preparation And Characterization Of Graphene-Based Composite Materials And Their Electrochemical Performance

The energy density of lithium-ion batteries mainly depend on the electrode potentials and reversible specific capacities of both cathode and anode materials. Graphite is the widely commercial anode material for lithium-ion batteries because of its excellent cycle performance. However, the theoretical specific capacity of graphite is only 372 mAh g^-1, so that the energy density of lithium-ion batteries cannot satisfy the needs of the people. To improve the energy density of lithium-ion batteries, many new anode materials with high specific capacities, such as metal oxides, silicon, and alloy, have been explored. However, their application in practical lithium-ion batteries is still hindered by the poor cycling performance due to the huge volume change during Li insertion/extraction process.Composite materials technologies can play the respective advantage of the composed materials and conquer the disadvantage of single material, which can be used to explore novel anode materials for lithium-ion batteries. Among the explored composites, carbon based composites have been widely concerned the most promising anode materials for lithium-ion batteries due to their excellent electrochemical performances. Graphene is a novel 2D nano-sized cabon with high electrical conductivity, which has good application prospect in energy storage field. To improve the reversible specific capacities of anode materials for lithium-ion batteries, we prepared graphene and graphene-based materials Fe₃O₄-graphene, SnO₂-graphene, Fe₃O₄-SnO₂-graphene. The techniques of XRD, SEM, and TEM were adopted to characterize the microstructure and morphology of the as-prepared materials. The electrochemical methods including galvanostatically discharged and charged test, cyclic voltammetry, and impedance measurements were used to systematically investigate their electrochemical performance.Firstly, graphene sheets with fewer layers were prepared through thermal exfoliation, and their microstructure, morphology and electrochemical performance were investigated. It is found that the graphene sheets possess a curled morphology consisting of a thin wrinkled paper-like structure, fewer layers （less than 2.1 nm, ⁴ layers） and large specific surface area (492.5 m²g^-1). The first reversible specific capacity of the prepared graphene sheets was as high as 1264 mAh g^-1 at a current density of 100 mA g^-1. Even at a high current density of 1000 mA g^-1, the reversible specific capacity remained at 445 mAh g^-1. After 40 cycles, the reversible capacity was still kept at 848 mAh g^-1 at the current density of 100 mA g^-1. These results indicate that the prepared graphene sheets with fewer layers possess excellent electrochemical performances for lithium storage. Secondly, to resolve the problem that the poor dispersion of graphene sheets in water and aggregating easily, a gas/liquid interface reaction method, without the solvent of water, was developed to prepare metal oxides-graphene nanocomposites. Ethylene glycol was used as both the dispersant for graphene sheets and the solvent for metal salt in the method, the metal oxides-graphene nanocomposites were formed by the reaction between NH3 and metal ions at the gas/liquid interface. The reaction was limited at the interface, so that the particle growth is localized, which is important for synthesizing monodispersed nanoparticles with a narrow size distribution. In addition, ethylene glycol is a good dispersant for graphene sheets, which is necessary for forming homogeneous metal oxides-graphene nanocomposites.Thirdly, Fe₃O₄-graphene nanocomposite was prepared by the gas/liquid interface reaction, the reaction mechanism was investigated by the characterization methods of X-ray diffraction, SEM and TEM, and the electrochemical performance was investigated through galvanostatically discharged and charged test. TEM observation reveals that the average particle size of Fe₃O₄ nanoparticles in the Fe₃O₄-graphene nanocomposite is about 12.5 nm. Electrochemical tests show that the initial reversible specific capacity of the as-prepared Fe₃O₄–graphene nanocomposite is as high as 1060 mAh g^-1, after 90 cycles, the remaining capacity is 99% of the initial reversible specific capacity. The enhanced cycling performance can be attributed to the facts that the graphene sheets distributed between the Fe₃O₄ nanoparticles can prevent the aggregation of the Fe₃O₄ nanoparticles, and the Fe₃O₄-graphene nanocomposite can provide buffering spaces against the volume changes of Fe₃O₄ nanoparticles during electrochemical cycling.Fourthly, SnO₂-graphene nanocomposite was firstly prepared via the facile gas/liquid interfacial synthesis approach , and the prepared SnO₂-graphene nanocomposite were investigated by employing the techniques of XRD, SEM, and TEM. SEM and TEM observation reveals that the SnO₂ nanoparticles with the particle size 2-6 nm were deposited onto the graphene sheets during the gas/liquid interface reaction process. The as-prepared SnO₂-graphene nanocomposite exhibits an unprecedented high reversible specific capacity of 1304 mAh g^-1 at a current density of 100 mA g^-1, which is much higher than its theoretical specific capacity (858 mAh g^-1) calculated according to the theoretical specific capacity of SnO₂ (782 mAh g^-1 based on the conventional alloying mechanism) and graphene sheets (1116 mAh g^-1). To understand the phenomenon, both of the fresh SnO₂-graphene electrode sheet and fully charged （3.0 V） one after 150 cycles were characterized by X-ray diffraction. The bare SnO₂ nanoparticles electrode sheets were also characterized for comparison. It is found that the conversion reaction, SnO₂ + 4Li⁺+4e^-â†'2Li₂O + Sn, which is usually reported to be irreversible, can become reversible in the SnO₂-graphene nanocomposite due to the very small Sn nanoparticles. However, the above conversion reaction is not reversible in the bare SnO₂ nanoparticles. These results indicate that the graphene sheets in SnO₂–graphene nanocomposite can not only improve the cycling performance of SnO₂, but also affect its reaction mechanism.Finally, to improve the conversion reaction kinetics of Fe₃O₄ and reduce the initial irreversible capacity of SnO₂, a novel Fe₃O₄-SnO₂-graphene ternary nanocomposite was designed. It was also prepared by using the gas–liquid interfacial synthesis approach. The as-prepared Fe₃O₄-SnO₂-graphene nanocomposite was characterized by XRD, SEM and TEM. Its electrochemical performance was also investigated. The as-prepared Fe₃O₄-SnO₂-graphene nanocomposite as an anode material for lithium-ion batteries exhibits high reversible specific capacity, outstanding cyclability and good rate capability due to the synergistic effect existing not only between the graphene and metal oxides but also between the Fe₃O₄ and SnO₂. The reversible specific capacity is as high as 1198 mAh g^-1 at a current density of 100 mA g^-1, even at a high specific current of 2000 mA g^-1, the reversible capacity still remains 521 mAh g^-1.