| The lithium-ion battery, as a new type of green secondary power supply withadvantages such as high energy density, high power density and high operating voltage,has being developed rapidly. Nowadays, it has already become the main power of theportable electronic devices such as mobile phones, digital cameras and notebook PC.Great expectation is placed on it as the power source of electronic vehicles in future, topartially alleviate the dependence on fossil fuel. For fabricating the high-performancelithium-ion battery, it is crucial to increase the specific capacity of the anode materials.More specifically, the anode materials with a capacity more than800mAh/g arerequired to replace the commercial anode material, graphite (theoretical capacity of372mAh/g). Therefore much attention is paid to search for high-capacity anodematerials.Si-O-C materials are one of the promising anode materials because they exhibithigh capacities and good cyclic performances. The thesis employed polysiloxane as theprecursor to prepare Si-O-C materials with different elemental distribution by apyrolytic polymer-to-ceramic conversion. The effect of preparation method andelemental distribution on their electrochemical performances was studied. Then mucheffort was devoted to the mechanism study of lithium storage in Si-O-C materials inorder to guide the design of high-capacity Si-O-C anode materials.The thesis prepared Si-O-C materials with silicon content of30-47.9wt%andcarbon content of4.1-42.1wt%using polysiloxane by changing the pyrolysistemperature and atmosphere, with addition of divinylbenzene as the carbon source. Andthen, the effect of the preparation method on elemental distribution and electrochemicalperformances of the materials was studied. Finally, the relationship between thecapacity and elemental distribution was investigated.Employing polysiloxane as the precursor, Si-O-C materials with carbon content of22.1-23.2wt%or14.1-15.1wt%were prepared under hydrogen or inert atmosphere,respectively. The C/Si-O-C materials prepared under hydrogen atmosphere exhibitlarger capacities and higher coulombic efficiency than those under Ar atmosphere. Thepyrolysis temperature affects the reversible capacity of C/Si-O-C materials, though thereversible capacity does not linearly increase with the increment of the pysolysistemperature.Employing polysiloxane as the silicon source and divinylbenzene as the carbonsource, C/Si-O-C materials with Si content of29.4-40.3wt%and C content of26.3-42.1wt%were prepared at800-1000℃under hydrogen atmosphere. The reversible capacity gradually rises with the increment of carbon content. Among all theseC/Si-O-C materials, the sample with Si:30.4wt%and C:42.1wt%exhibit the largestcapacity of751mAh/g with coulombic efficiency larger than97.4%.Based on all the C/Si-O-C samples prepared with different preparation methods,the relationship between the reversible capacity and elemental content was studied. It isfound that, when O/Si mol ratio is among1-2and C/Si ratio is between0.5and3.5, thereversible capacity gradually increases with the increase of C/Si ratio, which does notlinearly increase with the increment of O/Si ratio.The main electrochemically active site of C/Si-O-C materials was suggested, bycomparing the derivative curves of anode materials composed of Si, O, or C. Thestructure transformation and the lithium reversibility of silicon species in Si-O-C phasewere studied by29Si MAS NMR and XPS. Finally, the contribution of each siliconspecies to the reversible capacity of the whole materials was evaluated.Si-O-C glass phase was proved to be the main source of lithium storage inC/Si-O-C materials, by comparing the derivative curves of anode materials composed ofSi, O, or C reported in the literature. The mechanism of lithium insertion in Si-O-Cphase is totally different from that of silicon, silicon dioxide and silicon monoxidematerials.Of all the bonded silicon species of Si, C, and O in Si-O-C glass phase,[SiO4],[SiO3C],[SiO2C2] are confirmed to be reversible with lithium, with [SiOC3] irreversibleand [SiC4] inactive, by following the structure changes of silicon species during lithiuminsertion/distraction by29Si MAS NMR and XPS.C/Si-O-C materials with [SiO4],[SiO3C], or [SiO2C2] units as the majorcomponent were prepared by a sol-gel method. The C/Si-O-C materials with88wt%of[SiO4] units can deliver a capacity of653mAh/g after20discharge/charge cycles. Thematerials with78wt%of [SiO4] and [SiO3C] units can offer a capacity of898mAh/g;and those with87wt%of [SiO4],[SiO3C] and [SiO2C2] units exhibit a capacity of725mAh/g. Herein, the large reversibility of [SiO4],[SiO3C], or [SiO2C2] is verified.Based on the independence of the reactions between silicon species and lithium, astatistical model was established to express the relationship between the reversiblecapacity and silicon species distribution. According to the model, the order of reversiblecapacity is as follows:[SiO4]>[SiO3C]>[SiO2C2]>[SiOC3]. Furthermore, the capacityof [SiO4] and [SiO3C] units are3times larger than that of graphite.In order to further increase the capacity, Si/Si-O-C materials with excess ofelemental silicon were prepared by simply pyrolyzing poly (methylsilane) or byutilizing a sol-gel method. The electrochemical performances of these Si/Si-O-C materials were studied.Si/Si-O-C materials with excess silicon of <5.7wt%were prepared by pyrolyzingpoly(methylsilane) under hydrogen atmosphere. The materials formed at900℃,1000℃have a capacity of470mAh/g, and400mAh/g, respectively.Si/Si-O-C materials were also synthesized by utilizing a sol-gel method, with3-10wt%of nano silicon in the precursor. The Si/Si-O-C materials prepared with8wt%ofnano silicon in the precursor at1100℃have the best electrochemical performances.They own a first delithiation capacity of1371.6mAh/g, with a first coulombic efficiencyof78%. After30repeated cycles, the materials still keep a capacity of990mAh/g, witha capacity retaining ratio of72%. Furthermore, the materials still bear good C-rateperformances. They can deliver a capacity of715.9mAh/g after30cycles at a3C rate. |
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