鋰離子電池碳基負極材料表面改質製備與研究

Study on Surface Modification of Carbon Anode for Lithium-ion Batteries

楊乃璇(Nai-Hsuan Yang)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201610220

摘要

本論文利用不同之表面改質技術對碳基負極材料進行處理，進而達到降低首次充放電不可逆電容量、提高循環壽命、改善快速充放電容量下降，以及提升電容量等目標。
首先，無定型碳材料具有極佳的循環壽命穩定性，然而其在首次充放電之不可逆電容量較石墨負極高很多，且電容量也較石墨低，因此本研究使用表面改質的方式對無定型碳材料首次充放電之不可逆電容量較石墨負極高與電容量也較石墨低的問題加以改善。使用不同高分子對無定型碳材料進行表面改質，由結果顯示無定型碳材料之首次不可逆值會受到比表面積之影響。同時，使用不同之瀝青包覆製程，可再次映證表面包覆層之覆蓋均勻度，對無定型碳之表面改質與首次充放電不可逆值有明顯的影響。研究中顯示出使用適量之高分子，對無定型碳材料進行表面包覆，不僅能使其不可逆值降低到20%以內，同時仍能維持材料本身極佳循環壽命之特性。最後，本研究在進行表面包覆時，添加少量之高容量矽奈米顆粒，期望能對無定型碳材料較低之電容量做進一步之改善。結果顯示所添加之矽奈米顆粒能夠均勻的分散在無定型碳材料表面，同時，此材料在1C的速率下進行循環壽命測試，於70圈後仍然有約75% 之可逆電容量保存率。
本研究同時也利用微波輔助，配合液態含矽前驅物，成功合成出具高速充放電穩定性之矽氧化物/石墨複合材料。傳統矽(矽氧化物)/碳複合材料，常使化學氣相沉積法，並配合具毒性之氣態含矽前驅物(矽烷或四氯化矽)製備。此一製程不僅危險、製程時間長且耗能﹔相較於傳統製程，微波製程具有操作時間短、液態前驅物安全、以及具選擇性等優點。在此部分，分別使用了奈米碳管、石墨片與石墨球進行包覆，由掃描式電子顯微鏡可以觀察到，由於微波加熱具有選擇性，碳及石墨基材能在短時間內被加熱到極高的溫度，並使得其周圍之液體裂解，並沉積在基材表面，形成一包覆均勻且完整之矽氧化物鍍膜層。由於此鍍膜層非熱的良導體也非微波的受體，因此鍍膜層達到一定厚度之後便不會再繼續增厚(通常所形成的鍍膜層厚度僅約幾奈米)。所得之矽氧化物/石墨複合材料進行電性測試，發現不僅在電容量上有所提升，改變放電速率時表現出極佳的穩定性，在循環壽命上也有傑出的表現，在快速充放電的條件下100圈後庫倫效率可達99.9%以上，經500次高速充放電後，仍然保有95%左右之可逆電容量的保存率。
本研究使用不同的表面改質技術，皆能夠有效的提升碳基負極材料之可逆電容量，並表現出不錯的循環壽命，同時，本研究所採用之改質技術較傳統製程簡單、安全與快速，提供日後鋰離子電池負極材料材料合成與設計新的概念。

關鍵字

鋰離子電池 ； 負極 ； 石墨 ； 無定型碳材料 ； 矽 ； 微波合成 ； 表面改質

並列摘要

In this work, the reversible capacity, cycle life stability and rate performance can be improved by using different surface modification process.
As we know, disordered carbon reveal good cycle life stability. However, despite of the favorable features, the large initial irreversible capacity and low specific capacity still are big problems and needed to be solved. Surface modification method was used to solve the weak points. The results showed that the first-cycle irreversible capacity of the disordered carbon anode and its specific surface area has a strong positive correlation. And the result was also proved by using different kinds of pitch coating process. From electrochemical test, the initial irreversibiluty can be lower than 20% by using polymer coating and the electrodes still remain good cycle stability. Small amount of Si nanoparticles was used for further improvement of the charge capacity. The Si nanoparticles can be well distribute on the disordered carbon surface by combining polymer and pitch coating. The charge capacity retention is around 75% when cycle at 1C rate after 70 cycles.
Secondly, a Si oxide-coated graphite composite anode for Li-ion batteries (LIBs) was synthesized using a microwave-assisted coating method. In this synthesis, a solution comprising liquid polysiloxanes is used as the Si-containing precursor. For conventional synthesis process, people usually use chemical vapor deposition and toxic Si-containing vapor precursors, such as SinHm or SiHxCly. On the contrary, microwave-assisted coating method is safe, saving energy, and selective when doing the process. Here, carbon nano tube, graphite flake and graphite sphere were used as the microwave absorbent. Heating the graphite or carbon with microwave induces the deposition of a Si-containing conformal layer on the particle surfaces. Because the deposited layer is not microwave absorbent, it is also not good heat conductor. When the coating grew to certain thickness the reaction will stop (the thickness of the coating is around nano-scale). The resulting sample was subsequently calcined to produce SiOx/graphite composite. When tested as a LIB anode, the resulting composites exhibited an improvement in reversible specific capacity, good rate performance and excellent cyclability. When cycle at high C rate, all the composite electrodes showed the average coulombic efficiency higher than 99.9% after 100 cycles and the charge capacity retention were around 95% after 500 cycles. In this work, several polymer were chose to do surface modification.
The reversible capacity, cycle life stability and rate performance of carbon anode can be improved by using different surface modification process. The results suggest new strategies for both designing and synthesizing high-performance anode materials for LIB applications.

並列關鍵字

Li-ion battery ； anode ； graphite ； disordered carbon ； silicon ； microwave synthesis ； surface modification

參考文獻

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