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Title: | 納菲薄膜於鋰硫電池各組成部分之改質成效分析 Assessments on Nafion Modifications at Different Cell Positions of Lithium Sulfur Batteries |
Authors: | Jing Luo 羅婧 |
Advisor: | 吳乃立(Nae-Lih Wu) |
Keyword: | 鋰硫電池,Nafion高分子,隔離膜,粘著劑,鋰金屬負極, Li-S batteries,Nafion modifications,S/C composite coating,separator,binder,Li metal anode, |
Publication Year : | 2016 |
Degree: | 碩士 |
Abstract: | 現今科技高速發展,儲能設備已然成為生活和產業必不可少的一部分。當人們在追求更小更輕便的便攜式電子產品或者是能效更好、動力更強的電動車時,也是在追求能量密度更高的儲能設備。因此,電池產業一直在尋求鋰離子電池技術的突破,以達電容量大、能量密度高、可快速充放電、安全性佳、便宜以及壽命長的需求。基於高理論電容量和便宜原材料的優點,鋰硫電池吸引著研究者的目光。然而,由於電化學中間產物多硫化鋰的溶解嚴重影響著其效能,鋰金屬直接作為負極也存在著安全疑慮,商業化進程掣步不前。這份研究計劃將著重探討鋰硫電池的改質方法,以提高其庫倫效率、快速充放電能力、安全性以及循環壽命。
在這份研究中,我們設計了一個可充放電的玻璃電池,結合數位電子顯微鏡,可用於觀測鋰硫電池在充放電過程中現象,比如說,電極的硫含量以及電解液濃度對於多硫化鋰溶解的影響。這個設備可以方便的辨識各種改質是否能有效地捕捉多硫化鋰。因為玻璃電池腔體設計很薄,景深問題可以忽略不計。因此,此設備還可用於觀測鋰金屬在充放電過程中的形貌變化。 在眾多多硫化鋰捕捉方式中,陽離子選擇性的Nafion高分子薄膜極具潛力有效阻擋多硫化鋰陰離子的通過。在此,我們將系統性地探討Nafion在電池各組成部分(包括硫正極、隔離膜和鋰金屬負極)的改質效用。我們將以75%高硫含量的硫碳奈米複合材料作為正極材料。正極改質方式有兩種,將Nafion預先包覆在硫碳奈米復合材料的粉體上,或者直接將Nafion溶于粘著劑漿料中。隔離膜和鋰金屬也將分別鍍上Nafion薄膜。此外,因為Nafion源於PVDF(一種傳統的正極粘著劑),也因為有燃料電池領域的報告表明Nafion混合PVDF性能更佳,我們也將探討PVDF在鋰硫電池改質上的效用。有效地改質使庫倫效率、快速充放電能力以及循環壽命得以提高。經Nafion改質的隔離膜可使庫倫效率達到接近100%。將Nafion混于粘著劑使用,可使 2 C 的放電電容量達 600 mAh g-1以上、10 C的放電電容量達400 mAh g-1。 為了能夠直接使用鋰金屬作為負極,我們將對改質過後的鋰金屬進一步探討。鋰金屬對鋰金屬的對稱式電池可以排除硫正極的影響,我們將從阻抗、循環極化程度以及表面形貌的角度,廣泛地探討高分子表面改質對於鋰硫電池的適用性。結果表明,Nafion/PVDF混合高分子改質對於阻擋多硫化鋰及抑制鋰金屬枝精沉積有顯著效果。 In the modern high tech society, the demand for high energy density power source in portable electronic devices and electric vehicles is never diminished. The battery community is eager for practical breakthroughs in Li-ion batteries to give higher energy and power densities with fast charging ability, high safety, low cost and long life time. The advantages of high specific energy and low cost of sulfur have recently brought rechargeable lithium-sulfur (Li-S) batteries to the spotlight. Yet shuttle effects, mainly caused by polysulfide migration in the electrolyte, remain major technical obstacles to perfect Columbic efficiency and long-term cycle life. In this study, a special device has been set up to directly observe the in-operando phenomena of Li-S batteries. Effects of cathode sulfur loading and electrolyte salt concentration on the extent of polysulfide (PS) diffusion were compared. Effectiveness of different PS entrapment methods can be easily determined using this technique. The thin glasscell design eliminated the depth of focus issue and is versatile for direct observation of the macroscopic morphology evolution of the Li metal surface upon stripping and redeposition. Among all the polysulfide trapping methods, the cation-selective Nafion film stands out as a promising physical barrier to the anionic polysulfide species. Effective Nafion modifications at different cell positions, including the sulfur cathode, the polypropylene separator (Celgard 2400) and the lithium (Li) metal anode for the first time were systematically investigated. A high sulfur-content sulfur/carbon (S/C) nanocomposite with 75 wt% sulfur in a porous carbon (Pearl 2000) matrix was employed as the cathode material. The sulfur cathode was modified by either coating the S/C nanocomposite powder with Nafion or incorporating Nafion into the binder material. Solution casting method was utilized to modify the porous separator and the metallic Li anode surface. In addition, in light of the swelling control studies of fuel cell applications, the effects of polyvinylidene fluoride (PVDF, also a conventional binder material for the S composite cathodes) incorporation were also discussed. With proper modifications, the Columbic efficiency and high rate capability were substantially enhanced. Columbic efficiency of nearly 100% was achieved with Nafion coated separator, while in-operando digital microscopic analysis provided direct evidence for inhibited polysulfide passage through a Nafion coated separator. Sulfur cathode with Nafion partially incorporated in binder was capable of delivering 600 mAh g-1 at 2 C and above 400 mAh g-1 at 10 C. In order to fulfill direct utilization of Li metal as the anode for Li-S batteries, the effects of modified Li anode was further investigated in the symmetric cell configuration. The simplified system eliminated the influence of sulfur cathode and allowed generalized application to all sulfur-containing cathodes. Along with the electrochemical-impedance-spectra (EIS) analysis and polarization test, SEM images of the cycled Li metal anode revealed the insights of Li protective coatings. And a blend polymer of Nafion and PVDF was proved to be an effective protective coating for Li metal anode in terms of suppressing dendrite formation and prohibiting PS pathway. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/78120 |
DOI: | 10.6342/NTU201602506 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 化學工程學系 |
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ntu-105-R03524095-1.pdf Restricted Access | 7.31 MB | Adobe PDF |
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