鋰硫電池高硫含量正極材料之製備與分析

Synthesis and Characterization of High Sulfur Content Cathode For Lithium Sulfur Batteries

馬心惠(Xin-Hui Ma)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.01252

摘要

現今，鋰離子電池已經被廣泛用在眾多便攜式設備中，例如智慧型手機和筆記型電腦等。然而，目前的鋰離子電池在基於傳統的嵌入機制上，無法滿足高科技產業和電動車的需求。因此，去尋求一個新的鋰電池系統，可以顯著降低成本、增加容量和提高能量密度，為現在首要之務。在各種被廣泛研究的新型電池中，鋰硫電池憑藉著其高理論電容量，對於世界各地的研究者來說十分地具有吸引力。在這個研究計畫中我們將認入地探討鋰硫電池：活物中硫含量之提高、電極材料之修飾以及高分子膜對電池電化學性能之影響。
首先，為了提高活物中硫的含量，合成硫碳奈米複合材料將採用兩種不同的方法。其一是真空加熱法，其二是反溶劑加熱法。真空加熱法具有可以把殘留在多孔碳外部的硫燒掉的優勢。但是於此同時，由於燒結方法上的問題而局限了硫碳奈米複合材料中硫的含量。因此，利用反溶劑加熱法來合成高硫含量的硫碳納米複合材料。反溶劑法是基於兩種溶劑不互溶的特性來製備材料，目前已被廣泛應用於工業生產中。此外，為了改善鋰硫電池在高電流密度和長時間充放電下的循環穩定性和電化學性能，我們將導電石墨塗於鋁箔表面進行改質後作為電流收集板。從電化學性能測試來看，尤其是在高電流密度下的充放電表現上，是否使用表面改質後的鋁箔作為電流收集板對電池的起始可逆電容量和極化現象有著明顯的影響。由結果可以得知石墨不但是一種具有高度發展潛能的材料而且能有效地提高鋰硫電池在高電流密度下的電性。因此，在高電流密度和長期充放電循環下，將反溶劑加熱法和表面改質鋁箔之結合並應用於高硫含量之鋰硫電池是可行且有效的。
基於之前實驗室學長之研究，Nafion高分子披覆的隔離膜可以有效抑制多硫化鋰。因而，在此利用浸漬法將Nafion高分子均勻塗於隔離膜的表面，將此改質後的隔離膜用於高硫含量鋰硫電池中。在0.2 C充放電速率下，初始電容量具有1060 mAh g-1，而經過100圈循環測試後仍保有650 mAh g-1之可逆電容量。即使是在1 C 較高充放電速率和200圈循環測試下，仍保有590 mAh g-1之可逆電容量，其表現優於同等條件下沒有使用Nafion高分子披覆的隔離膜之鋰硫電池，且將Nafion披覆於隔離膜表面也改善了高硫含量鋰硫電池的庫倫效率。由實驗結果可以發現到，經Nafion高分子修飾的隔離膜所具有的均勻且小空結構不但允許鋰離子之傳輸且可以抑制多硫化鋰在電解液中之傳遞。從電性結果可知，Nafion高分子層能有效減少多硫化鋰在正負極間的傳遞且提升了鋰硫電池之穩定性和充放電可逆性。

關鍵字

鋰硫電池 ； 反熔劑加熱法 ； 高硫含量 ； Nafion 高分子

並列摘要

Nowadays, lithium-ion batteries (LIBs) are extensively applied in numerous portable devices such as smart-phones and laptops. However, current LIBs based on the conventional intercalation mechanism cannot meet the requirements of the electronics industry and electric vehicles. Therefore, it is extremely urgent to seek for systems with a significant reduction in cost and increase in capacity and energy density. Among various promising candidates, lithium–sulfur (Li–S) batteries with a high theoretical capacity are very attractive. This study aims at significantly raising the sulfur content of active material, decorating the electrode material and influence of polymer film in Li-S battery.
Firstly, in order to increase the sulfur content of the active material, the synthesis of the sulfur carbon nanocomposite material was introduced by two different methods which were vacuum heating and anti-solvent heating methods. Except for the advantages that less sulfur particles left outside the pores of carbon, vacuum heating method encountered a limitation of sulfur content in the S-C nanocomposite material, due to the calcination temperature and carbon pore volume. Therefore, high sulfur content nanocomposite material could be synthesized by another method, the anti-solvent heating method. Anti-solvent processes are largely used in the industry, which were based on the polarity of two solvents immiscible to each other. Furthermore, to enhance the cycling stability and rate capability, the surface modified Al foil was applied as the current collector, especially for the long-term cycling at high current density. From the electrochemical performance, particularly the c-rate performance, the obvious differences of the initial reversible capacity and polarization between using the graphite coated Al foil and without coating could be observed. The favorable performance obtained by using the conductive material coated on Al foil demonstrated that graphite was a promising material for enhancing the electrochemical performance at higher current density. Hence, the combination of anti-solvent heating method and graphite coated Al foil was a feasible approach to test the higher sulfur content Li-S batteries at high current density for a long-term cycles.
Based on the previous work, the use of a Nafion-ionomer film in Li-S battery could efficiently confine the polysulfides. Therefore, a novel separator coating with a Nafion polymer film was prepared by dipping that was used in high sulfur content (75 wt.%) Li-S batteries. The S-C nanocomposite of 75wt% sulfur content featuring a Nafion coated separator exhibited an initial capacity of 1060 mAh g-1 at 0.2 C, and the discharge capacity declined slowly, to 650 mAh g-1, after 100 cycles. Even at high c-rate of 1 C, the cell with Nafion-coated separator presented a reversible capacity of 590 mAh g-1 after 200 cycles which was superior than that without Nafion-coated separator. The Nafion-coated separator also improved Coulombic efficiency of high sulfur content Li-S cells at various current densities. The Nafion polymer coated separator displayed a structure of few small and uniform pores that allowed penetration of lithium-ions transmission, meanwhile, it could effectively prevented polysulfide anions transporting in the electrolyte, as well. It is demonstrated from the electrochemical performance that the Nafion-coated separator was quite effective in reducing shuttle effect, enhancing the stability and the reversibility of the electrode.

並列關鍵字

Li-S batteries ； anti-solvent heating method ； high sulfur content ； Nafion polymer

參考文獻

1. J. B. Goodenough and Y. Kim, "Challenges for Rechargeable Li Batteries", Chemistry of Materials, 22, 587-603 (2010)

2. M.-K. Song, S. Park, F. M. Alamgir, J. Cho, and M. Liu, "Nanostructured electrodes for lithium-ion and lithium-air batteries: the latest developments, challenges, and perspectives", Materials Science and Engineering: R: Reports, 72, 203-252 (2011)

3. J.-S. Lee, S. Tai Kim, R. Cao, N.-S. Choi, M. Liu, K. T. Lee, and J. Cho, "Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air", Advanced Energy Materials, 1, 34-50 (2011)

4. L. Miao, W. Wang, K. Yuan, Y. Yang, and A. Wang, "A lithium-sulfur cathode with high sulfur loading and high capacity per area: a binder-free carbon fiber cloth-sulfur material", Chemical Communications, 50, 13231-13234 (2014)

5. J. Nelson, S. Misra, Y. Yang, A. Jackson, Y. Liu, H. Wang, H. Dai, J. C. Andrews, Y. Cui, and M. F. Toney, "In Operando X-ray Diffraction and Transmission X-ray Microscopy of Lithium Sulfur Batteries", Journal of the American Chemical Society, 134, 6337-6343 (2012)