Preparation Of Polystyrene Based Carbon Microspheres

Since Sony Company commercialized lithium ion batteries in 1990, lithium ion battery has been paid much attention by researchers and companies due to their advantages in electrochemical properties. Despite the considerable efforts to find other substituents, carbon materials still remain the only commercial anode for lithium ion batteries because of their steady structure and good circular properties. Among carbon materials, sphere-shaped carbon particles (or called carbon microbeads) are a kind of promising anode materials for Li-ion batteries owing to their regular morphologies, controllable crystal structure, smooth surface and high packing density.Mesocarbon microbead prepared from coal tar pitch or petroleum heavy oil is an attractive and competitive anode material for Li-ion batteries. However, there are some disadvantages, e.g., complicated process, wide size distribution and high cost. This thesis focused on the preparation, structure and electrochemical properties of carbon microbeads from polystyrene (PS) microspheres. Choosing polystyrene microspheres as the starting materials is based on their facile synthesis via the dispersion polymerization and the highly graphitizable ability when they are cross-linked and carbonized at high temperature. The key factor for the preparation of polystyrene-based carbon microbeads is how to realize the stabilization when PS microspheres are heat treated in inert gas atmosphere. Two kinds of techniques were tried to crosslink PS into thermosetting PS resin, i.e., AlCl3 and DVB modification. Carbon microbeads were obtained as anode materials for Li-ion batteries by carbonization and graphitization of the cross-linked polystyrene microspheres. In addition, the reaction mechanism for cross-linking of two-types was elucidated. The electrochemical properties of carbon microbeads were analyzed to find the relation between the structure of microbeads and the specific capacities. The above research provided a guide for preparation and wide applications of new kinds of spherical carbon materials.PS microbeads were prepared by dispersion polymerization. The thermosetting PS resin were obtained via the cross linking by CCl₄ under the catalysis of AlCl₃, and two kinds of carbon microbeads (solid or hollow spheres) with the diameter of 3μm were obtained by carbonization. Meanwhile, the carbonization yield of cross-linked PS microbeads was greatly improved from 1% to 30%. The changes of structure and functional groups of samples before and after modification were characterized by XRD and IR, and it was indicated that PS microbeads were modified by the cross-linked agent of CCl₄ under the catalysis of A1C1₃. With the aid of A1C1₃, there was a balance between the dissolution and solidification of cross-linking for PS microbeads and CCl₄, and the cross-linking reaction on the surface of and inner of PS was promoted by proper amount of CC1₄. It is difficult for CC1₄ to penetrate into the center of PS microbeads with high cross-linking degree, under which polymer microbeads with core-shell structure were obtained, and therefore hollow carbon microbeads were prepared by further carbonization. As anode materials for Li-ion batteries, the specific capacity of PS-based carbon sample after graphitization kept a stable value of 360 mA·h/g after 30 circles under the current density of 0.2 mA/cm² and of 280 mA·h/g after 20 circles under the current density of 0.8 mA/cm², suggesting the material possesses the high specific capacity and a high-rate charge-discharge capabilitis.Microbeads from the styrene--divinylbenzene copolymer were prepared by dispersion copolymerization, and the effect of DVB content on the morphology and thermostability of product were investigated. The results of oxidation, carbonization and graphitization of product with DVB content of 0%, 30%, 50%, 70% and 100% show that the spherical shape of copolymer microbeads could not be maintained after direct carbonization, while could be kept after oxidation and then carbonization at 700℃. The mechanism of cross-linking reaction induced by oxidation was proposed, i.e., during heat-treatment the oxidation crosslinking reaction took place between oxygen and pendant double-bonds in copolymers in air to form carboxyl and hydroxyl groups; and polymer with high degree of cross-linking were obtained by the esterification of active groups. The effect of DVB content on the electrochemical properties of carbonized and graphitized products were studied and the results show that the PD50 kept a stable specific capacity of 250 mA·h/g after 30 circles under the current density of 0.2 mA/cm² and of 220 mA·h/g after 20 circles under the current density of 0.8 mA/cm².In addition, the morphology and electrochemical properties of PVC-based carbon microbeads after graphitization were studied and the results show, that the specific capacity of the product kept a stable specific of 235 mA·h/g after 30 circles under small current density and of 215 mA·h/g after 20 circles under large current density.