| Lithium ion batteries show high energy density, high voltage, high safety and little environmental pollution, so they are widely applied in portable electronic devices and electric vehicles etc. In recent years, with the growth of the power of high energy’s demand, scientists do large researches on developing lithium ion batteries of high energy density and high power charge and discharge. Generally speaking, the capacity of lithium ion battery is decided by various elements that compose the battery. Anode materials are the main parts to store lithium, which are the key point to improve the capacity, cycle life, charge-discharge performance and other relevant parameters of lithium ion battery.Silicon is regarded as one of the most promising anodes for the next generation of high energy density Li-ion batteries due to its high theoretical specific capacity and low Li-insertion potential(vs. Li/Li+). However, its commercialization is greatly prevented by drastic capacity fade resulting from excessive volume expansion during electrochemical cycling. In contrast, carbon materials possess stable structure as well as good electro-conductivity, and are considered as ideal "buffering matrix" for silicon anode. The Si/C composites with high capacity and good cycling properties have been synthesized by many methods.This project is cooperated with Saint-Gobain Research Shanghai, which aims to develop a kind of anode material of lithium ion battery with high capacity and great stability. The main research is on the preparation of Si-SiOx species in different heat treated conditions, and then we prepared three-dimensional structure grapheme by vacuum filtration method. After ressembled Si-SiOx species with grapheme, we can get different kinds of Si-SiOx-C composites. We use XRD (X-RayDiffraction), SEM (Scanning Electron Microscope),TEM (Transmission Electron Microscopy) and Blue battery test system to characterize the properties of Si-SiOx species, graphene and Si-SiOx-C composites.The results show that vacuum filtration method can successfully prepare three-Dimensional structure graphene. The electrochemical test results show the first cycle charge specific capacity of graphene is311.2mAh/g, which has little difference with the theory specific capacity of pure carbon, and coulombic efficiency is up to84.9%. After100cycles, the rest capacity is134.1mAh/g. Though decreases, the capacity is still high. In the charge-discharge procedure, the stability is also good, and the coulombic efficiency is97%. These results show that three-dimensional graphene has good electrochemical properties.Using the disproportionation reaction of SiO can successfully prepare Si-SiOx species. The species show different properties because of the different heat treat conditions. The electrochemical test results show that with the raise of the heat treat tempreture, the crystallinity of Si also rises, which results in the decrease of the first cycle capacity. In the cycle performance test, the capacity of Si-SiOx species declines sharply. This owes to the volume expansion of Si during electrochemical cycling. We can get the idea that the lower heat treat tempreture is good for preparing Si-SiOx species of high capacities.After Si-SiOx species ressembled with three-dimensional graphene, we get Si-SiOx-C composites. Compared with Si-SiOx species, the first cycle capacity raises a lot. The charge capacity increases from152.24mAh/g to811.3mAh/g, and the irreversible capacity decreases a lot. The coulombic efficiency increases from14%to52%. After100cycles, the charge capacity still raises a lot and shows good stability. The capacity decreases some because of the volume expansion of Si after several cycles, but it keeps a stable level since the fifth cycle, about150mAh/g, and and the irreversible capacity decreases a lot, the coulombic efficiency is up to almost100%. These results show that the three-dimensional graphene can limit the volume expansion of Si and improve the electrochemical properties of Si-SiOx species. |
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