在過去20幾年間，可充式鋰離子電池由於具有較高的工作電壓、理論電容量(≈ 3800 mAh g-1)以及良好的循環穩定性，因此被廣泛運用在各種可攜式電子產品，例如手機、筆記型電腦等。大多數商業化的鋰離子電池通常是使用鈷酸鋰(LiCoO2)或磷酸鋰鐵(LiFePO4)作為陰極材料，然而這種嵌入及嵌出型(insertion-extraction type)的材料所能儲存的鋰離子較為有限，導致無法滿足對電池能量密度的要求。因此，人們迫切需要找到一個具有更高的能量密度的先進電池裝置。
鋰硫電池(Lithium-Sulfur Batteries, LSBs)由於擁有非常高的理論能量密度(2500 Wh kg-1)，再加上其成本低廉及對環境無汙染等優點，因此被視為是下一世代具有前景的新型儲能系統。然而，鋰硫電池仍有許多致命的缺點阻礙其商業化發展，包括易溶解於電解液內的多硫化鋰所造成之穿梭效應(shuttling effect)、硫在充放電過程中的體積變化、硫的絕緣性質和多硫化鋰間緩慢的轉換反應，這些缺點分別會導致嚴重的電容量損失和較低的活性物質硫使用率。針對這些缺失，多數的研究者選擇加入基底材料(sulfur host)來解決問題。在許多已開發的基底材料當中，又以極性材料較為常見，例如 : 金屬氧化物、金屬硫化物、摻雜碳材等，由於多硫化鋰同樣是具有極性的，因此這些極性基底材料可以透過化學吸附的方式以有效捕捉可溶的多硫化鋰以減緩穿梭效應。此外，金屬硫化物更是被許多文獻指出對多硫化鋰間轉換反應有催化的效果，所以被認為是傑出的候選材料。
本研究首先選用具有多孔性結構的有機金屬骨架ZIF-67作為前驅物，透過原位合成(in-situ growth)的方式將ZIF-67生長於表面改質過的多壁奈米碳管，接著經過進一步的鍛燒與硫化，得到由ZIF-67衍生出的二硫化鈷/氮摻雜碳多面體，並由多壁奈米碳管互相串連而成的複合材料CoS2-NC/MWCNT。本研究期望藉由結合ZIF-67的多孔性質、多壁奈米碳管良好的導電性、氮摻雜碳材和金屬硫化物對多硫化鋰強烈的化學吸附性，以及金屬硫化物催化多硫化鋰轉換反應的能力，以改善上述鋰硫電池的缺點。最後利用滲硫法(melt-diffusion method)將CoS2-NCMWCNT與硫複合(S@CoS2-NC/MWCNT)以作為鋰硫電池的陰極材料。發現在充放電速率0.1 C下可以產生1133 mAh g-1的比電容，且當充放電速率增加至2.0 C時仍能保有607 mAh g-1的電容量。同時，其亦具有良好的循環穩定性，在1.0 C下經過300次充放電循環仍有77 %的初始放電容量，平均每次循環的電容量衰退率為0.078 %。
此外，為了進一步增進鋰硫電池之循環穩定性，本研究在鍛燒時額外加入本身含氮比例高的雙氰胺作為氮源，藉此提升材料之氮摻雜程度以加強基底材料對多硫化鋰的化學吸附作用，或是將具有片狀交聯結構的還原氧化石墨烯作為物理屏障與CoS2-NC/MWCNT複合以增加多硫化鋰擴散的阻力，防止其自陰極擴散。透過以上兩種方法，本研究成功地將比電容量的衰退速率由0.078 %分別減少至0.066 %以及0.064 %。

In the past two decades, rechargeable lithium-ion batteries (LIBs) have been extensively applied in a wide range of portable electronic devices, such as mobile phones and notebooks, because of their high working voltages, high theoretical capacities (≈ 3800 mAh g-1), and impressive cycling stability. Most of the commercial LIBs use LiCoO2 or LiFePO4 as the cathode materials. These insertion-extraction type of materials, however, can only store relatively limited amounts of lithium ions, resulting in unsatisfactory energy densities of the battery. Consequently, there is an urgent need to develop advanced battery devices capable of delivering higher energy densities.
Lithium-sulfur batteries (LSBs), possessing a high theoretical energy density of 2500 Wh kg-1 and being cost-effective and eco-friendly, are regarded as a prospective next-generation energy storage device. Nevertheless, there are several fatal drawbacks that impede the commercialization of LSBs, including the notorious shuttling effect of soluble lithium polysulfides (LiPS), volume variation of sulfur during charge-discharge cycles, low conductivities of sulfur, and sluggish conversion reaction kinetics of LiPS, leading to severe capacity decay and low utilization of sulfur. For these shortcomings, most researchers choose to add sulfur hosts to address the issues. Among the many developed host materials, polar materials, such as metal oxides, metal sulfides, and heteroatom-doped carbon materials, are commonly utilized because of their abilities to effectively capture soluble LiPS via forming strong chemical bonding with LiPS during charge-discharge cycles to alleviate the shuttling effect. Moreover, transition metal sulfides are considered to be an excellent candidate since they have been widely reported to be able to accelerate the conversion reaction involving LiPS.
Herein, metal-organic frameworks (MOF) derived cobalt disulfide/nitrogen-doped carbon polyhedrons linked with multi-walled carbon nanotubes, termed CoS2-NC/MWCNT, were successfully fabricated through in-situ growth of ZIF-67 on surface treated carbon nanotubes, followed by carbonization and subsequent sulfurization. We attempt to improve the above-mentioned disadvantages of LSBs by combining the porosity of ZIF-67, good conductivity of MWCNTs, strong chemical adsorption between metal sulfides, nitrogen-doped carbon, and LiPS, as well as the ability of metal sulfides to enhance the redox kinetics of LiPS conversion. CoS2-NC/MWCNT was then composited with sulfur with a simple melt-diffusion method to obtain S@CoS2-NC/MWCNT to serve as the cathode of the LSBs. It exhibited a high discharge capacity of 1133 mAh g-1 at 0.1 C, and maintained a decent capacity of 607 mAh g-1 at 2.0 C. In addition, it also showed outstanding cycling stability, with a 77 % capacity retention and a low capacity decay of 0.078 % per cycle over 300 cycles at 1.0 C.
On top of that, in order to further enhance the cycling stability of LSBs, we introduced dicyandiamide (DCD) of a high nitrogen content into the carbonization process to increase the level of nitrogen-doping, which is advantageous to enhance the chemical binding between host materials and soluble LiPS. We also combined flake-shaped and cross-linked reduced graphene oxides (rGO), acting as a physical barrier, with CoS2-NC/MWCNT to lessen shuttling of soluble LiPS. With means of the above two practices, the capacity decay rate was further reduced to 0.066 and 0.064 % per cycle through heavy N-doping and addition of rGO, respectively.

摘要 i
致謝 v
目錄 vi
圖目錄 viii
表目錄 xii
第1章 緒論 1
1-1 前言 1
1-2 電化學反應系統簡介 2
1-3 鋰離子電池 3
1-4 鋰硫電池 5
1-4-1 工作原理 5
1-4-2 電解液系統 8
第2章 文獻回顧 11
2-1 概述 11
2-2 以鎳(II)為開放金屬中心之有機金屬骨架 11
2-3 有機金屬骨架複合物(MOF composites) 13
2-4 中空碗狀摻氮碳材 16
2-5 中空奈米碳管形貌過渡金屬硫化物 18
第3章 實驗方法與儀器 22
3-1 研究動機 22
3-2 實驗藥品 24
3-3 實驗儀器 26
3-4 分析儀器 27
3-5 實驗步驟 30
3-5-1 二硫化鈷/氮摻雜碳複合多壁奈米碳管(CoS2-NC/MWCNT-X)材料製備 30
3-5-2 高氮摻雜—二硫化鈷/氮摻雜碳複合多壁奈米碳管(D-CoS2-NC/MWCNT-X)材料製備 31
3-5-3 CoS2-NC/MWCNT複合還原氧化石墨烯(rGO-CoS2-NC/MWCNT)材料製備 32
3-5-4 陰極複合材料製備 32
3-5-5 多硫化鋰吸附實驗 33
3-5-6 鋰硫電池電極片製作與全電池組裝 33
3-5-7 Li2S6對稱電池電極片製作與電池組裝 33
3-5-8 電化學測試與分析 34
第4章 結果與討論 36
4-1 有機金屬骨架ZIF-67及其複合物和衍生物材料鑑定與分析 36
4-2 二硫化鈷/氮摻雜碳複合多壁奈米碳管(CoS2-NC/MWCNT)材料鑑定與分析 40
4-2-1 表面形貌、內部晶格繞射與元素組成分析 40
4-2-2 不同溫度下鍛燒比較 43
4-2-3 XPS分析 45
4-2-4 比表面積與孔徑分布分析 46
4-3 二硫化鈷/氮摻雜碳複合多壁奈米碳管與硫之複合材料分析 49
4-4 二硫化鈷/氮摻雜碳複合多壁奈米碳管與硫之複合材料應用於鋰硫電池陰極電化學測試與分析 51
4-4-1 不同溫度下鍛燒比較 51
4-4-2 二硫化鈷/氮摻雜碳複合多壁奈米碳管與其前驅物電化學性能測試比較 53
4-4-3 長效循環測試 58
4-4-4 多硫化鋰吸附測試 60
4-4-5 高活性物質硫負載充放電及循環穩定性測試 63
4-5 提升氮摻雜程度—加強化學吸附多硫化鋰能力 64
4-6 額外與可作為物理屏障之材料複合—阻擋多硫化鋰擴散 71
4-7 與其它鋰硫電池文獻性能比較 76
第5章 結論 78
參考文獻 80

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