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標題: | 鋰離子電池矽鎳複合負極材料合成與分析 Synthesis and Characterization of Si-Ni Composite Anode Materials for Lithium-ion Batteries |
作者: | Hung-Yu Lin 林宏侑 |
指導教授: | 吳乃立(Nae-Lih Wu) |
關鍵字: | 鋰離子電池,矽,矽鎳複合材料負極,電解液添加劑,氟代碳酸乙烯酯, Li-ion battery,Silicon,Si-Ni composite anode material,Electrolyte additive,Fluoroethylene carbonate (FEC), |
出版年 : | 2014 |
學位: | 碩士 |
摘要: | 本論文之主要目的為開發以矽為主體的鋰離子二次電池負極材料,欲透過簡易製備方法得到具高極板密度之矽基負極材料。矽(Si)因擁有絕佳的理論電容量(~3600 mAh/g)、蘊藏量豐富、價格低廉與安全無毒性等特性而受到重視,矽是目前極可能取代石墨(372 mAh/g)成為新時代高容量鋰離子電池的負極材料。然而,由於矽在充放電過程中伴隨著劇烈的體積膨脹與收縮及本身低導電特性,以及固態電解液界質(SEI)的負面影響,導致極板結構的不穩定和電化學特性的不可逆,使得矽負極在鋰電池上的應用受到限制。
首先,以克服極板結構的問題為考量,將矽源與氧化鎳混合後藉由高能球磨與還原燒結的簡易製程,成功地製備出矽鎳複合材料。初步實驗證明不同粒徑大小的矽源(微米矽與奈米矽)會影響其製備後的矽鎳複合物型態與晶相,含有NiSi合金可減緩在充放電過程時所造成的體積變化,進而影響其電化學循環穩定性。 其次,將奈米矽與氧化鎳(Si:NiO)依不同比例(1:2, 1:1, 2:1)混合經由高能球磨與還原燒結製程,得到各別的矽鎳複合物。結果發現,矽含量多雖提高了電容量,但會犧牲循環穩定性,在電容量與循環穩定性的權衡中,1:1具有較佳結果。經過50圈(0.1A/g)循環壽命測試後,其可逆電容量仍保有694 mAh/g。 由於奈米矽的高成本,將矽源換成來自太陽能板的廢棄物,矽-碳化矽(Si-SiC)。相同地,將矽-碳化矽與氧化鎳(Si-SiC:NiO)依適合比例(1.5:1)混合經由高能球磨與還原燒結製程,得到矽-碳化矽-鎳複合物。結果顯示,此矽-碳化矽-鎳複合物在50圈(0.1A/g)循環壽命測試後仍保有443 mAh/g的可逆電容量,優於純矽-碳化矽(<10圈)。 然而,在保有更長的循環壽命考量下,除了改善矽負極的極板結構的崩解問題,持續性SEI膜生成影響極板結構穩定性是個關鍵性問題以及待進一步改善,而SEI膜的穩定性主要由活物層與電解液所影響。因此,分別對活物層與電解液進行改善。針對活物層方面,將矽-碳化矽-鎳複合物作為標準品,將其極板上各別再多塗附一膠著劑層或石墨層,試圖減少矽與電解液接觸而減少SEI膜的生成,結果顯示其改善效果有限。針對電解液方面,利用FEC作為電解液添加劑,由SEM觀察經過50圈循環壽命的極板,結果顯示有5wt% FEC添加劑的極板具有較薄的SEI層,體積膨脹程度為150%;而無FEC添加的極板膨脹程度為215%。經過100圈循環測試後,有5wt% FEC添加劑的極板仍具有581 mAh/g的可逆電容量是高於無FEC添加劑的極板的331 mAh/g。此結果顯示SEI膜持續生成大幅地影響極板穩定性與循環壽命。最後,透過交流阻抗分析探討不同FEC添加劑濃度(5wt%, 10wt%, 50wt%)對於極板電化學特性的影響,結果顯示電化學特性會受到極板主體導電度與持續生成的SEI之權衡影響。 The main purpose of this research is to explore new anode materials based on silicon for lithium ion batteries. It is attempted to obtain the Si-based anode materials with high electrode density through a facile fabrication process. Compared to graphite (372 mAh/g), silicon has attracted much attention because of the unique properties, such as its highest specific capacity (~3600 mAh/g), the second abundant element on earth, low cost and environmental safety. However, Si undergoes a dramatic volume change during cycling and its intrinsically poor conductivity, resulting in the mechanical instability and poor cyclability. Moreover, the irreversibility cased by solid electrolyte interphase (SEI) formation also fastens the capacity fading rate. These problems retard the commercial application of Si. Firstly, from the view point of stabilizing electrode structure and increasing the conductivity, the Si-Ni composites were successfully synthesized with different ratio of Si to NiO by the simple processes of high energy ball milling (HEBM) and reductive calcination (RC). It was found that the different particle sizes (3μm and 100 nm) of Si sources would affect the reactivity of Si with NiO and the morphology of the Si-Ni composites, and further influence the electrochemical performance. Besides, the NiSi alloy is helpful to accommodate the volume change during cycling. Secondly, the nmSi-Ni composites were prepared with three weight ratios (1:2, 1:1, 2:1) of nmSi to NiO by HEBM and RC processes. The results of electrochemical performance show that the specific capacity increases with the amount of Si, but the cycling stability is sacrificed. Among these, the nmSi-Ni composite from the weight ratio of 1:1 demonstrates the optimized trade-off between the specific capacity and cycling stability, and it exhibits the reversible capacity of 694 mAh/g with adequate cycling stability after 50 cycles. Due to the consideration of the high cost of nmSi, the silicon source is replaced by Si-SiC which is the waste powder from solar panels. Likewise, the Si-SiC-Ni composite has been synthesized with the adequate weight ratio of 1.5:1 of Si to NiO via HEBM and RC processes. From the results of electrochemical performance, the Si-SiC-Ni composite performs the reversible capacity of 443 mAh/g after 50 cycles, which is much better than the one of pristine Si-SiC. However, from the view point of maintaining the long-term cycling performance, the negative effect of continuous SEI formation on the stability of the electrode structure is a critical problem and it needs to be further alleviated. The Si-SiC-Ni composite is regarded as the control sample. There are two ideas to deal with this problem, including the aspects of active material and electrolyte. From the aspect of active material, coating the binder or graphite layer on the control sample electrode is attempted to protect the electrode structure from forming the continuous SEI, however, the results exhibit that the effect on reducing continuous SEI formation is limited. From the aspect of electrolyte, fluoroethylene carbonate (FEC) is utilized as an electrolyte additive. From the results of SEM images, the SEI layer of the cycled electrode in 5wt% FEC electrolyte is thinner than the one in FEC-free electrolyte after 50 cycles. Furthermore, the expansion ratio of the cycled electrode in 5 wt% FEC electrolyte is 150%, which is smaller than that of 215% in FEC-free electrolyte. After 100 cycles, the electrode in 5wt% FEC electrolyte shows the improved reversible capacity of 581 mAh/g, while the one in FEC-free electrolyte shows the reversible capacity of 331 mAh/g. The remarkable improvement has been proved that the continuous SEI formation substantially influences the stability of the electrode structure and cycle life. Finally, from the results of rate performance, cycling performance and EIS analyses of the control sample electrode in 5wt%, 10wt% and 50wt% FEC electrolyte, it is found that there's a trade-off between the bulk conductivity and the stability of SEI that influence the electrochemical properties. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/16642 |
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