Preparation, Structures And Properties Of Pitch-based Graphene Nanosheets

The development of preparation and application of graphene nanosheets (GNS) and carbon anode materials for lithium ion battery were reviewed in detail. On the basis of that, carbonization products from mesophase pitch at low temperatures were used to prepare the GNS via vacuum pyrolysis at high temperatures. The relation among preparation process, structures and properties of the GNS were studied. In addition, pyrolysis behavior of the mesophase pitch was investigated in this paper.The pyrolysis behavior of the mesophase pitch was studied by XRD, FT-IR and TGA. Results show that the interlayer spacing of the mesophase pitch decreases and then increases with increasing carbonization temperature, and meanwhile the average crystallite height of the mesophase pitch gradually decreases after 500℃. When the carbonization temperature arrives at 800℃, the graphite-like layered structure can be obtained. In addition, the functional groups and heteroatoms of the mesophase pitch prolapse and its carbon skeleton is rearranged, while vibration absorption peaks of the benzene ring become weak. Furthermore, the initial decomposition temperature and the final decomposition temperature of the mesophase pitch increase, but its carbonization yields decrease. A kinetics model is proposed according to TGA results, which reveals the pyrolysis behavior of the mesophase pitch.The influence of preparation process on the structures of the GNS was investigated by XRD, FESEM, TEM, N2 adsorption-desorption and Raman spectroscopy. Results show that the GNS is formed from graphite flakes with a network structure, which then change into transparent nanosheets with a stacked lamellar and wrinkled structure at high temperature. The GNS is made up of 10～15 graphene layers that still have high crystalline structure, and their BET specific surface areas range from 14.24m2/g to 33.53m2/g. The optimum condition for preparing the GNS is carbonization temperature 1800℃, vacuum degree 700Pa, heating rate 20℃/min and ultrasonic power 400W.The electrochemical performances of the GNS as anode material for lithium-ion battery were investigated. Results show that the discharge capacity of the GNS increase with increasing carbonization temperature, but the initial coulomb efficiency decreases. In addition, the initial irreversible capacity of the GNS increases and the initial coulomb efficiency decreases with increasing ultrasonic power. The graphitization degree and the interlayer spacing of the GNS increases, but the layer number of the GNS decreases after the high-temperature carbonization. The GNS possess good cycling performance with over 93% of coulomb efficiency after 20 cycles, due to the internal lamellar structure of the GNS.