孔洞碳材在鋰硫電池中的應用及反應機制的探討

近年來隨著科技的進步，對於儲電元件的要求越來越高，科學家不斷的想發展具有高能力密度、高電容值與高工作電壓的電池，而鋰硫電池(Lithium sulfur batteries) 就是被視為下一世代的主要電池之一。原因在於相較於目前常見之鋰離子電池 (Lithium ion batteries)，鋰硫電池有較高的電容量 (~1675 mAh/g) 與能量密度 (~2600 W h kg-1)，且陰極端的硫為大自然中相當豐富的原料之一，因此可壓低電池的生產成本。但鋰硫電池同時也存在許多的問題以至於目前還無法大量的生產應用，其中一個問題於陰極端的硫在反應的過程中會生成許多的聚硫化物 (lithium polysulfides)，而這些聚硫化物易溶於電解液中而導致活化物質 (active material) 的損失以及穿梭效應 (shuttle effect) 的產生，會導致電池之電容值以及穩定性隨充放電次數的增加而減少。另一個問題在於陽極端的鋰金屬為活性大的鹼金屬，在電池充放電的過程中容易因為表面的不平整或其他因素而產生鋰金屬樹枝狀結晶 (lithium dendrite)，進一步刺破中間的隔離膜 (separator) 而與陰極端相觸導致短路，嚴重的話可能會引發爆炸，因此在安全上還有所疑慮，種種缺點導致目前鋰硫電池無法被商業化。本論文主要從陰極端的材料改良下手，目的就是為了解決反應過程中聚硫化物易溶於電解液中的缺點。我們利用特殊孔洞結構之碳材來載入硫作為陰極材料，並在充放電過程中利用孔洞來保留住聚硫化物來避免產生穿梭效應。我們從國立成功大學林弘萍教授實驗室取得多種來源為生質原料或工業廢棄物的孔洞碳材，其中對於兩種原料為瀝青的碳材最感興趣，分別是純微孔的XU75以及同時具有微孔、中孔及大孔且孔洞間彼此為相互交錯的XU76，同時在實驗中我們將兩者與市面上常用於鋰硫電池的碳材，也同時具有微孔、中孔與大孔的Ketjen black (KB) 做比較。我們認為具有微孔、中孔及大孔且孔洞間彼此為相互交錯的XU76有利於硫、聚硫化物以及硫化鋰在孔洞間穿梭而能避免聚硫化物溶於電解液當中。透過UV-Vis實驗也證實了以上的觀點，而在電池的表現上也發現XU75與XU76贏過Ketjen black (KB)，其中又以XU76表現結果較佳，且XU76/S的厚度約為1.4 mg/cm2。因此碳材若能同時具備微孔、中孔及大孔的性質，且孔洞結構上能夠相互交織，就可以利用孔洞作為硫、聚硫化物以及硫化鋰的反應槽且吸附能力佳能避免其溶於電解液中，使得電池的電容值與循環穩定性變得更好。本論文的最後也透過in situ的技術來探討不同的孔洞碳材對於鋰硫電池在反應過程中機制上差異，儘管XU75、XU76以及KB三種碳材在in situ Raman spectroscopy以及in situ X-ray absorption spectroscopy的實驗結果對於聚硫化物的生成種類與相對量並沒有太大的不同，但我們還是可以從結果中得知鋰硫電池在反應過程中並非以兩兩電子接受的形式生成聚硫化物，電子在系統中為任意的轉移，又或者會產生歧化/解離反應 (Disproportionation/Dissociation reaction)，因此在Raman數據中才會看到多種聚硫化物的產生。而在X-ray absorption圖譜中則是可以看到硫、聚硫化物以及硫化鋰隨充放電的過程中而有消長的趨勢。

關鍵字

鋰硫電池；聚硫化物；穿梭效應；孔洞碳材；原位拉曼光譜；原位X光吸收光譜

並列摘要

In recent years, with the advance of technology, the requirements of energy storage components are getting higher and higher. Scientists are continually trying to develop the batteries with high energy density, high capacity and high working voltage, while lithium sulfur batteries are considered as one of the next generation batteries. Comparing with the lithium ion batteries, which are used for mobile phones or electric cars now, lithium sulfur batteries have higher capacity (~1675 mAh/g) and energy density (~2600 W h kg-1). Although sulfur-carbon cathode is one of the most abundant natural materials, lithium polysulfides formed during discharge process are soluble in the electrolyte and result in the active material loss and shuttle effect. The battery performance will decrease during cycling. In addition, the lithium metal anode is one of the high activity alkali metals, it will form lithium dendrite during charge and discharge process, mainly due to the surface uneven, resulting in a short circuit and safety issues. In this thesis, I focus on improvement of the materials on the sulfur-carbon cathode to trap soluble polysulfides on the cathode composite during cycling. We report the use of porous carbon materials with different architectures to load sulfur as the active material on the sulfur-carbon cathode and preserve the polysulfides in the cathode composite during cycling. We get various porous carbon materials from National Cheng Kung University, Professor Lin Hong-Ping. XU75 and XU76 carbon materials synthesized by using asphalt which are from industrial wastes. XU75 contains micropores. XU76 contains micropores, mesopores and macropores, and they are interlaced together. For the comparison, the commercial carbon material, Ketjen black (KB) contains micropores, mesopores and macropores. We consider that XU76 has special hole structure, so the sulfur, polysulfides and lithium sulfide can flow between the pores and have the strong absorption to avoid them dissolve into the electrolyte. We use UV-Vis experiment to improve the above idea. Also, compared with the battery performance of XU75, XU76 and KB, XU75 and XU76 are better than KB, and XU76 performs best. The thickness of sulfur-XU76 composite is 1.4 mg/cm2. So we consider that the carbon contains micropores, mesoporoes and macropores, also staggered together to have special hole structure, the pores can be a reservoir and absorb sulfur, polysulfides and lithium sulfide to solve the problem that polysulfides are easily dissolved into the electrolyte and get better performance and stability of batteries. We also use in situ technique to research the mechanism of lithium sulfur batteries using different porous carbon material. Although the results of XU75, XU76 and KB in in situ Raman spectroscopy and in situ X-ray absorption spectroscopy are almost the same, but we still use in situ Raman to know that polysulfides are not produced in the form of getting two electrons, electrons transfer randomly in the system, or it will generate disproportionation/dissociation reaction when charging and discharging, so we can find numerous signals of polysulfides in Raman spectroscopy. In X-ray absorption spectroscopy, we can also see the grown and decline of sulfur, polysulfides and lithium sulfide when going charge and discharge in lithium sulfur batteries.

並列關鍵字

lithium-sulfur batteries ； polysulfides ； porous carbon material ； in situ Raman spectroscopy ； in situ X-ray absorption spectroscopy

參考文獻

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孔洞碳材在鋰硫電池中的應用及反應機制的探討

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