近年來，由於電動車與電子產品的崛起，市場對於具有高能量密度與低成本的充電電池需求快速增加，相較於傳統電池容量較低的鋰離子電池，具有高理論容量且活性材料成本低廉的鋰硫電池，將主宰為下一世代的鋰離子電池系統。
本研究提出一個簡易的雙層結構電極的設計來減緩多硫化物在電解液中遷移的效應。有別於一般文獻所提出的將具有極性的添加劑混入正極材料的方式，這篇論文將鈮摻雜二氧化鈦塗布在富含硫的正極上，形成一層添加層。此添加層得以利用化學吸附方式，抓住溶於電解液中的多硫化物，避免其穿梭至對電極，提高整體電容量與電池壽命。利用此電極設計，電池第一圈可提供高達1880 mA h g-1 的電容量，與未改質的鋰硫電池相比，有顯著的電性提升。此外，本研究利用XPS進一步驗證鈮摻雜二氧化鈦對於多硫化物的化學吸附反應，進一步證明此添加層對於整體電池之循環壽命、電容量與高電流充放電性能的改善。

This study presents a method to suppress the migration of lithium polysulfides in lithium–sulfur batteries by introducing a dual-layer electrode structure. Herein, unlike conventional methods of mixing the polar additives with sulfur/carbon composites, melted sulfur mixed with mesocarbon microbeads are used as the electrode and covered with an additive layer of Nb-doped TiO2/graphite composite via two-step blade coating. By doping TiO2 with Nb, electrical and lithium ion conductivity of TiO2 can be increased, thereby enhancing the redox reaction kinetics. Most importantly, chemisorption of lithium polysulfides to Nb–TiO2 can effectively mitigate the shuttle effect, resulting in higher capacity and longer cycle life. The electrode with the Nb–TiO2 additive layer results in a 1st and 100th cycle specific capacity of 1883 mAh g-1 and 894 mAh g-1, respectively, at 0.1 C (1 C = 1675 mAh), indicating enhanced electrochemical performance as compared with that of bare lithium-sulfur batteries. X-ray photoelectron spectroscopy (XPS) study was conducted to investigate the interaction between polysulfides and Nb–TiO2. The results indicate that the Nb–TiO2–layered electrode efficiently traps polysulfides on the cathode and improves the rate capability, cycle performance, and specific capacity.

Chapter 1 Introduction 2
1.1. Background of lithium-ion battery 2
1.2. Introduction to lithium–sulfur battery 2
1.3. Motivations and objectives in this study 3
Chapter 2 Literature Review 9
2.1. Introduction of Lithium–Sulfur Battery 9
2.1.1. The reaction mechanism of Li–S batteries 9
2.1.2. Critical issues of Li–S batteries 16
2.2. Strategies undertaken for Li–S system improvements 21
2.2.1. Basic Concept of sulfur cathode material 21
2.2.2. Carbon host materials 23
2.2.3. Metal oxide host materials 35
Chapter 3 Experimental Details 45
3.1. Material Preparation 45
3.1.1. Nb0.15Ti0.85O2 synthesis 45
3.1.2. MCMB/sulfur composites 45
3.1.3. Preparation of the cathodes 46
3.1.4. Preparation of the XPS samples 46
3.2. Characterization and Analysis 47
3.2.1. Phase Identification 47
3.2.2. Compositional Evaluation 47
3.2.3. Morphological Observation 47
3.2.4. Thermogravimetric analysis (TGA) 47
3.3. Electrochemical Analysis 48
3.3.1. Battery Assembly 48
3.3.2. Cyclability and Rate Capability Measurement 48
3.3.3. Cyclic Voltammetry (CV) 48
3.3.4. Electrochemical Impedance Spectroscopy (EIS) 49
Chapter 4 Results and Discussions 51
4.1. Cathode characterization 51
4.2. The interaction between NTO15s and lithium polysulfides 56
4.3. The electrochemical behavior of MCMB/S/NTO15 cathodes 61
Appendix
A.1 The morphology of cathodes after cycling 74
A.2 The electrochemical performance of MCMB-S-NTO15 76
A.3 The electrochemical performance of bare NTO15s 79

[1] M. Broussely, P. Biensan, B. Simon, Lithium insertion into host materials: the key to success for Li ion batteries, Electrochimica Acta, 45 (1999) 3-22.
[2] C.M. Hayner, X. Zhao, H.H. Kung, Materials for rechargeable lithium-ion batteries, Annual review of chemical and biomolecular engineering, 3 (2012) 445-471.
[3] M. Wakihara, O. Yamamoto, Lithium ion batteries: fundamentals and performance, John Wiley & Sons2008.
[4] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Rechargeable lithium–sulfur batteries, Chemical reviews, 114 (2014) 11751-11787.
[5] N. Ding, L. Zhou, C. Zhou, D. Geng, J. Yang, S.W. Chien, Z. Liu, M.-F. Ng, A. Yu, T.A. Hor, Building better lithium-sulfur batteries: from LiNO 3 to solid oxide catalyst, Scientific reports, 6 (2016) 33154.
[6] X. Ji, L.F. Nazar, Advances in Li–S batteries, Journal of Materials Chemistry, 20 (2010) 9821-9826.
[7] K. Patel, Lithium-Sulfur Battery: Chemistry, Challenges, Cost, and Future, The Journal of Undergraduate Research at the University of Illinois at Chicago, 9 (2016).
[8] J.A. Dean, Lange's handbook of chemistry, Material and manufacturing process, 5 (1990) 687-688.
[9] H.B. Wu, S. Wei, L. Zhang, R. Xu, H.H. Hng, X.W.D. Lou, Embedding Sulfur in MOF‐Derived Microporous Carbon Polyhedrons for Lithium–Sulfur Batteries, Chemistry-A European Journal, 19 (2013) 10804-10808.
[10] J. Shim, K.A. Striebel, E.J. Cairns, The lithium/sulfur rechargeable cell effects of electrode composition and solvent on cell performance, Journal of the Electrochemical Society, 149 (2002) A1321-A1325.
[11] J. Guo, Y. Xu, C. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium–sulfur batteries, Nano letters, 11 (2011) 4288-4294.
[12] J. Schuster, G. He, B. Mandlmeier, T. Yim, K.T. Lee, T. Bein, L.F. Nazar, Spherical ordered mesoporous carbon nanoparticles with high porosity for lithium–sulfur batteries, Angewandte Chemie, 124 (2012) 3651-3655.
[13] H. Wang, Y. Yang, Y. Liang, J.T. Robinson, Y. Li, A. Jackson, Y. Cui, H. Dai, Graphene-wrapped sulfur particles as a rechargeable lithium–sulfur battery cathode material with high capacity and cycling stability, Nano letters, 11 (2011) 2644-2647.
[14] R. Elazari, G. Salitra, A. Garsuch, A. Panchenko, D. Aurbach, Sulfur‐impregnated activated carbon fiber cloth as a binder‐free cathode for rechargeable Li‐S batteries, Advanced materials, 23 (2011) 5641-5644.
[15] Q. Pang, X. Liang, C. Kwok, L.F. Nazar, The importance of chemical interactions between sulfur host materials and lithium polysulfides for advanced lithium-sulfur batteries, Journal of The Electrochemical Society, 162 (2015) A2567-A2576.
[16] Q. Pang, D. Kundu, M. Cuisinier, L. Nazar, Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries, Nature communications, 5 (2014) 4759.
[17] X. Liang, A. Garsuch, L.F. Nazar, Sulfur Cathodes Based on Conductive Mxene Nanosheets for High‐Performance Lithium–Sulfur Batteries, Angewandte Chemie International Edition, 54 (2015) 3907-3911.
[18] S. Evers, T. Yim, L.F. Nazar, Understanding the nature of absorption/adsorption in nanoporous polysulfide sorbents for the Li–S battery, The Journal of Physical Chemistry C, 116 (2012) 19653-19658.
[19] Z.W. Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.-C. Hsu, Y. Cui, Sulphur–TiO 2 yolk–shell nanoarchitecture with internal void space for long-cycle lithium–sulphur batteries, Nature communications, 4 (2013) 1331.
[20] Y. Tao, Y. Wei, Y. Liu, J. Wang, W. Qiao, L. Ling, D. Long, Kinetically-enhanced polysulfide redox reactions by Nb 2 O 5 nanocrystals for high-rate lithium–sulfur battery, Energy & Environmental Science, 9 (2016) 3230-3239.
[21] H.J. Peng, G. Zhang, X. Chen, Z.W. Zhang, W.T. Xu, J.Q. Huang, Q. Zhang, Enhanced electrochemical kinetics on conductive polar mediators for lithium–sulfur batteries, Angewandte Chemie, 128 (2016) 13184-13189.
[22] X. Ji, S. Evers, R. Black, L.F. Nazar, Stabilizing lithium–sulphur cathodes using polysulphide reservoirs, Nature communications, 2 (2011) 325.
[23] Y. Zhang, Y. Zhao, A. Yermukhambetova, Z. Bakenov, P. Chen, Ternary sulfur/polyacrylonitrile/Mg 0.6 Ni 0.4 O composite cathodes for high performance lithium/sulfur batteries, Journal of Materials Chemistry A, 1 (2013) 295-301.
[24] A. Yermukhambetova, Z. Bakenov, Y. Zhang, J.A. Darr, D.J. Brett, P.R. Shearing, Examining the effect of nanosized Mg0. 6Ni0. 4O and Al2O3 additives on S/polyaniline cathodes for lithium–sulphur batteries, Journal of Electroanalytical Chemistry, 780 (2016) 407-415.
[25] S.L. Gojković, B. Babić, V. Radmilović, N. Krstajić, Nb-doped TiO2 as a support of Pt and Pt–Ru anode catalyst for PEMFCs, Journal of Electroanalytical Chemistry, 639 (2010) 161-166.
[26] M. Yang, D. Kim, H. Jha, K. Lee, J. Paul, P. Schmuki, Nb doping of TiO 2 nanotubes for an enhanced efficiency of dye-sensitized solar cells, Chemical Communications, 47 (2011) 2032-2034.
[27] N. Elezović, B. Babić, L. Gajić-Krstajić, V. Radmilović, N. Krstajić, L. Vračar, Synthesis, characterization and electrocatalytical behavior of Nb–TiO2/Pt nanocatalyst for oxygen reduction reaction, Journal of Power Sources, 195 (2010) 3961-3968.
[28] M. Fehse, S. Cavaliere, P. Lippens, I. Savych, A. Iadecola, L. Monconduit, D. Jones, J. Roziere, F. Fischer, C. Tessier, Nb-doped TiO2 nanofibers for lithium ion batteries, The Journal of Physical Chemistry C, 117 (2013) 13827-13835.
[29] E. Dy, R. Hui, J. Zhang, Z.-S. Liu, Z. Shi, Electronic Conductivity and Stability of Doped Titania (Ti1− XMX O2, M= Nb, Ru, and Ta) A Density Functional Theory-Based Comparison, The Journal of Physical Chemistry C, 114 (2010) 13162-13167.
[30] C.-X. Zu, H. Li, Thermodynamic analysis on energy densities of batteries, Energy & Environmental Science, 4 (2011) 2614-2624.
[31] R. Noorden, A better battery: chemists are reinventing rechargeable cells to drive down costs and boost capacity, Nature, 507 (2014) 26-28.
[32] S.C. Jung, Y.-K. Han, Monoclinic sulfur cathode utilizing carbon for high-performance lithium–sulfur batteries, Journal of Power Sources, 325 (2016) 495-500.
[33] Y.-S. Su, A. Manthiram, A new approach to improve cycle performance of rechargeable lithium–sulfur batteries by inserting a free-standing MWCNT interlayer, Chemical communications, 48 (2012) 8817-8819.
[34] C.-N. Lin, W.-C. Chen, Y.-F. Song, C.-C. Wang, L.-D. Tsai, N.-L. Wu, Understanding dynamics of polysulfide dissolution and re-deposition in working lithium–sulfur battery by in-operando transmission X-ray microscopy, Journal of Power Sources, 263 (2014) 98-103.
[35] M. Cuisinier, P.-E. Cabelguen, S. Evers, G. He, M. Kolbeck, A. Garsuch, T. Bolin, M. Balasubramanian, L.F. Nazar, Sulfur speciation in Li–S batteries determined by operando X-ray absorption spectroscopy, The Journal of Physical Chemistry Letters, 4 (2013) 3227-3232.
[36] G. Li, Z. Li, B. Zhang, Z. Lin, Developments of electrolyte systems for lithium–sulfur batteries: a review, Frontiers in Energy Research, 3 (2015) 5.
[37] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li–O 2 and Li–S batteries with high energy storage, Nature materials, 11 (2012) 19.
[38] Y. Diao, K. Xie, S. Xiong, X. Hong, Shuttle phenomenon–the irreversible oxidation mechanism of sulfur active material in Li–S battery, Journal of Power Sources, 235 (2013) 181-186.
[39] A.F. Hofmann, D.N. Fronczek, W.G. Bessler, Mechanistic modeling of polysulfide shuttle and capacity loss in lithium–sulfur batteries, Journal of Power Sources, 259 (2014) 300-310.
[40] M.R. Busche, P. Adelhelm, H. Sommer, H. Schneider, K. Leitner, J. Janek, Systematical electrochemical study on the parasitic shuttle-effect in lithium-sulfur-cells at different temperatures and different rates, Journal of Power Sources, 259 (2014) 289-299.
[41] C. Barchasz, J.-C. Leprêtre, F. Alloin, S. Patoux, New insights into the limiting parameters of the Li/S rechargeable cell, Journal of Power Sources, 199 (2012) 322-330.
[42] S.-E. Cheon, S.-S. Choi, J.-S. Han, Y.-S. Choi, B.-H. Jung, H.S. Lim, Capacity fading mechanisms on cycling a high-capacity secondary sulfur cathode, Journal of The Electrochemical Society, 151 (2004) A2067-A2073.
[43] H. Zhang, X. Li, H. Zhang, Li-S and Li-O2 Batteries with High Specific Energy, Springer2017.
[44] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon–sulphur cathode for lithium–sulphur batteries, Nature materials, 8 (2009) 500.
[45] B. Zhang, X. Qin, G. Li, X. Gao, Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres, Energy & Environmental Science, 3 (2010) 1531-1537.
[46] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow carbon nanofiber-encapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries, Nano letters, 11 (2011) 4462-4467.
[47] Z. Xiao, Z. Yang, L. Wang, H. Nie, M.e. Zhong, Q. Lai, X. Xu, L. Zhang, S. Huang, A lightweight TiO2/graphene interlayer, applied as a highly effective polysulfide absorbent for fast, long‐life lithium–sulfur batteries, Advanced materials, 27 (2015) 2891-2898.
[48] R.F. Bartholomew, D. Frankl, Electrical properties of some titanium oxides, Physical review, 187 (1969) 828.
[49] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W. Barsoum, Two‐dimensional nanocrystals produced by exfoliation of Ti3AlC2, Advanced Materials, 23 (2011) 4248-4253.
[50] H. Yang, C.-K. Lan, J.-G. Duh, The power of Nb-substituted TiO2 in Li-ion batteries: Morphology transformation induced by high concentration substitution, Journal of Power Sources, 288 (2015) 401-408.
[51] X. Tang, D. Li, Sulfur-doped highly ordered TiO2 nanotubular arrays with visible light response, The Journal of Physical Chemistry C, 112 (2008) 5405-5409.
[52] X. Tao, J. Wang, Z. Ying, Q. Cai, G. Zheng, Y. Gan, H. Huang, Y. Xia, C. Liang, W. Zhang, Strong Sulfur Binding with Conducting Magnéli-Phase Ti n O2 n–1 Nanomaterials for Improving Lithium–Sulfur Batteries, Nano letters, 14 (2014) 5288-5294.
[53] H. Yamin, A. Gorenshtein, J. Penciner, Y. Sternberg, E. Peled, Lithium sulfur battery oxidation/reduction mechanisms of polysulfides in THF solutions, Journal of the Electrochemical Society, 135 (1988) 1045-1048.
[54] Y. Sun, G. Li, Y. Lai, D. Zeng, H. Cheng, High rate lithium-sulfur battery enabled by sandwiched single ion conducting polymer electrolyte, Scientific reports, 6 (2016) 22048.
[55] Z. Deng, Z. Zhang, Y. Lai, J. Liu, J. Li, Y. Liu, Electrochemical impedance spectroscopy study of a lithium/sulfur battery: modeling and analysis of capacity fading, Journal of The Electrochemical Society, 160 (2013) A553-A558.
[56] Y. Zhao, L. Peng, B. Liu, G. Yu, Single-crystalline LiFePO4 nanosheets for high-rate Li-ion batteries, Nano letters, 14 (2014) 2849-2853.
[57] D. Moy, A. Manivannan, S. Narayanan, Direct measurement of polysulfide shuttle current: A window into understanding the performance of lithium-sulfur cells, Journal of the electrochemical society, 162 (2015) A1-A7.
[58] Y.-S. Su, A. Manthiram, Lithium–sulphur batteries with a microporous carbon paper as a bifunctional interlayer, Nature communications, 3 (2012) 1166.
[59] J. Ming, M. Li, P. Kumar, L.-J. Li, Multilayer approach for advanced hybrid lithium battery, ACS nano, 10 (2016) 6037-6044.