Study On The Preparation And Electrochemical Performance Of Mesophase-derived Carbon Material

Carbonaceous mesophase has been recognized as exceptional precursors for preparation of carbon materials, because of their rich source, low price and high carbon content, and its structure determines the structure of thereof carbon materials. The materials and the preparation conditions are the important factors that affect the final structure of carbonaceous mesophase. The present study was therefore undertaken to prepare carbonaceous mesophase via pyrocondensation of coal tar pitch or petroleum pitch. On the base of researching the structure of primary quinoline insoluble and mesocarbon microbeads, the mesophase formation mechanism was further completed and verificated. Then some work was carried out to study the effects of the materials, additives and post-treatment conditions, on the structure of carbon materials. Furthermore, the as-obtained carbon materials were used as an anode material in lithium-ion batteries (LIB). In addition, the lithium storage mechanism of different carbon structure was analysed, and then giving a principle for preparing excellent mesophase-derived carbon material.A coal tar pitch with primary quinoline insoluble (PQI) was used to prepare mesocarbon microbeads (MCMB). The microstructural characteristics of PQI and MCMB were discussed by various analysis techniques to evaluate the influence of PQI on MCMB formation process. The results revealed that PQI had a reasonably high degree of structural uniformity, which revealed that PQI could overcome the high energy barrier of a new mesophase formation and then decrease the activation energy of MCMB formation. Thus, it was concluded that PQI could act as nuclei of MCMB formation.The present study was undertaken to prepare nano-iron/MCMB via liquid co-carbonization of a coal tar pitch with ferrocene. The catalytic effects of ferrocene addition on the growth of MCMB and structure of carbonized/graphitized MCMB were discussed. It was found that nano-iron particles accelerated the growth of MCMB and consequently enhanced the structural ordering of spheres during the process of carbonization and graphitization. Electrochemical tests demonstrated that the carbonized/graphitized MCMB catalyzed by iron displayed higher capacity and better cycle performance in comparison with the pure carbonized/graphitized MCMB.Mesophase pitch (MP)/exfoliated graphite nanoplatelets (GNPs) nanocomposite had been prepared by an efficient method with an initiation of graphite intercalation compounds (GIC). It was observed that the GIC had exfoliated completely into GNPs during the formation of MP/GNPs nanocomposite and the GNPs were distributed uniformly in MP matrix, preventing the coalescence of the mesophase domains during heat treatment and allowing the formation of numerous contacts between different GNPs particles. The resulting samples continued to be carbonized under protection of nitrogen and were named as CMP and CMP/GNPs nanocomposite. To demonstrate the potential application of the present CMP/GNPs nanocomposite with enhanced electronic conductivity, we carried out a preliminary investigation into its electrochemical performance toward the lithium insertion/extraction compared with that of CMP. Electrochemical tests demonstrated that the initial charge capacity of CMP/GNPs nanocomposite anode was 381.2 mAh/g, and the initial Coulombic efficiency of CMP/GNPs nanocomposite was 57.3%, which was much larger than that of CMP (48.2%). Such an improvement can be attributed to the uniform distribution of GNPs particles and the formation of a 3D network of GNPs within CMP matrix. Thus the 3D network of GNPs can be considered as a 3D current collector network, which provided negligible times of the electronic carriers to the Li⁺ contact (electrolyte) and thus to minimize the electrode polarization.Exfoliated mesocarbon microbeads (EMCMB750) were prepared from graphitized mesocarbon microbeads (GMCMB) by intercalation reaction and rapid heating exfoliated processes. Scanning electron microscopy and X-ray diffraction analysis techniques were used to characterize the morphologies and structural characters of the samples. The SEM images of GMCMB and EMCMB750 revealed that after exfoliation, some microcracks appeared on the surface of EMCMB750. The XRD results clearly demonstrated that the exfoliated treatment destroyed the ordered structure of GMCMB to some extent. The electrochemical properties of GMCMB and EMCMB750 as anode materials for LIB have been preliminary investigated. The discharge profiles of EMCMB750 displayed dual features of disordered carbon and graphite. At the beginning of discharge, the voltage drops rapidly to 1.5 V and a sloping voltage range of 1.5-0.2 V are observed, which is similar to the typical voltage profiles of disordered carbon. When the voltage decreased to 0.2 V and formed a plateau, corresponding to the insertion of lithium ions into graphite layers. Although the initial discharge capacity of EMCMB750 anode is 600.7 mAh/g, the initial Coulombic efficiency is only 76.3%, which is mainly related to the high lithium storage capacity of non-graphite structure, and the capacity above 0.2 V is 34.8% of the total discharge capacity. EMCMB750 exhibited better discharge/charge performances at higher current density, which is attributed to the diffusion coefficient of EMCMB750 (5.55×10^-9 cm²/s) is higher than GMCMB (1.23×10^-10 cm²/s), then decreasing the electrode concentration polarization.