| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 101 === The main purpose of this research is to develop a high tap-density anode material based on silicon for lithium ion batteries. Silicon, in addition to its abundance on earth and its environmentally-friendly property, it also possesses a high theoretical capacity ( > 3600 mAh/g) compared to graphite (372 mAh/g). However, the dramatic volumetric variations during cycling and intrinsic low conductivity result in structural instability and poor cyclability. Moreover, the irreversibility caused by solid electrolyte interphase (SEI) formation accelerates the capacity fading as well. In order to solve those problems, we use sol-gel process to make a porous zirconia-silicon composite material and use different carbon coating process to improve the electronic insulation property of zirconia.
To synthesize Si-ZrO2 composite, nano-sized Si is dispersed in iso-propanol and at the same time, the zirconia gel forms by sol-gel method with zirconium propoxide as precursor. After deriving Si-ZrO2 gel, high temperature treatment is conducted to have porous Si-ZrO2 powder. The pore volume and strong mechanical property of zirconia are utilized to buffer the irreversible expansion of Si during cycling.
The preliminary work is to make a porous zirconia which can provide sufficient pore volume for buffering expansion of Si. This research has developed a rather-simple process (without supercritical drying, which is typically regarded as indispensable drying method for deriving aerogel) by controlling several important factors in sol-gel process, such as concentration of precursor, calcination temperature, water content, and gel state to derive porous zirconia. Besides, high-temperature treatment under vacuum environment can preserve more pore volume than that under 3% H2/N2 environment ( T > 500℃ ). After all the factors are well studied, the silicon is mixed with zirconia aerogel and then calcined to have porous Si-ZrO2 composite powder.
Besides, carbon coating methods consist of fructose carbon coating and pitch carbon coating. The former one is directly soaking stable Si-ZrO2 gel in fructose solution and the fructose solution can permeate into the porous gel and form a fructose layer on Si surface. The fructose layer decomposes into carbon layer after high temperature treatment. The experimental result indicates that the impedance of Si-ZrO2 electrode decreases a lot with increasing conductivity of Si-ZrO2-C. Different from fructose carbon coating, pitch carbon coating adopted two-step calcinations. Si-ZrO2 proceeds the 1st calcination under low temperature (400℃) to have Si-ZrO2 powder and then the powder is mixed with pitch in acetone solution. After drying process, the collected powder proceeds the 2nd calcination to have Si-ZrO2-C. The experimental result shows that the dissociated carbon indeed fills the pore volume and reduces the surface area of porous Si-ZrO2 structure to improve the irreversible capacity from SEI formation.
Last, two kinds of nano silicon with different sizes and qualities are used to form Si-ZrO2-C composite. The experimental result shows that Si-ZrO2-C electrode can retain 70% of 1st cycle charge capacity after running 50 cycles.
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