| Summary: | 碩士 === 國立中正大學 === 化學工程研究所 === 106 === Si materials can offer high theoretical capacities in lithium ion battery anode materials. Unfortunately, they suffer from dramatic volume expansion, resulting in the pulverization of electrode material structure during lithiation/delithiation cycles. In this work, the porous Si powders were made from rice husks by reduction with LiH. Then, the process parameters were optimized by XRD and XPS analysis. The amounts of by-products (Li4SiO4, Li2SiO3) were determined based on the XRD results. We also proposed the reaction equation of RH-SiO2 and LiH for fabrication of porous Si. The stoichiometric coefficients of the reaction equation were balanced according to the experimental data. The pure porous Si particles were characterized by BET, revealing that the average pore size was 207nm.
Fluoroethylene carbonate (FEC) was added to the electrolyte to form a dense and less resistant SEI film on porous Si particles in the initial cycle of electrochemical SEI formation, which can improve the cycle performance. As a result, the porous Si anodes exhibited a high reversible capacity of 1734mAh/g at 0.5C charge rate and long cycle life (with capacity retention of 59.4% after 250 cycles). In addition, EIS analysis of the porous Si electrode was conducted. The total impedence (Rtotal) mainly consisted of the resistance of charge transfer (RCT) and the resistance of SEI film (RSEI). In the first 10 cycles, all of the impedance values decreased as the cycle number increased. However, the impedance values were slightly increased during the following 40 cycles. The porous Si/graphite composite was prepared by ball milling. The porous Si/graphite anodes exhibited a high electrochemical performance. The capacity was 669mAh/g at 0.5C with 84.5% capacity retention after 450 cycles.
In the second part, the binder-free electrode material was investigated. The electrode materials contained porous Si, graphite and polyacrylonitrile (PAN). The binder and carbon black commonly used in conventional electrode was replaced with PAN. The compositions of PAN polymer after heat treatment at 550oC and 700oC was examined using XPS, Raman spectroscopy. The TGA results indicated that the weight ratio of porous silicon to nitrogen-doped carbon was 81:19 at 550oC. The electrochemical measurements showed that the porous Si/carbonized-PAN composite anodes exhibited a specific capacity of 821.5mAh/g at 0.5A/g and with 55.5% capacity retention after 100 cycles. Furthermore, the porous Si/graphite/carbonlized-PAN composite anodes exhibited a reversible capacity of 396.5mAh/g after 100 cycles at current density of 0.5A/g, and with 49.5% capacity retention.
Key words：Lithium-ion battery, Porous Silicon, Rice husks, Polyacrylonitrile, Binder-free electrode
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