Дисертації з теми "Sulfur cathode"

京都大学
新制・課程博士
博士(人間・環境学)
甲第23286号
人博第1001号
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 内本 喜晴, 教授 田部 勢津久, 教授 高木 紀明
学位規則第4条第1項該当
Doctor of Human and Environmental Studies
Kyoto University
DFAM

This study was undertaken to understand the effect of applied pressure on the performance of the lithium sulfur cathode. Compressible carbon based cathodes and novel nickel based cathodes were fabricated. For each cathode, pore volume and void volume were quantified and void fraction was calculated, compression under 0 to 2MPa was measured, and lithium-sulfur cells were assembled and cycled at pressures between 0 and 1MPa. The cathodes studied had void fractions in the range of 0.45 to 0.90. Specific discharge capacities between 200 and 1100 mAh/g under 1MPa were observed in carbon-based cathodes. Nickel-based cathodes showed increased specific discharge capacity of up to 1300 mAh/g, with no degradation of performance under pressure. The high correlation of specific discharge capacity and void fraction, in conjunction with previous work, strongly suggest that the performance of lithium-sulfur cathodes is highly dependent on properties that influence ionic mass transport in the cathode.

Lithium–sulfur batteries are highly attractive energy storage systems, but suffer from structural anode and cathode degradation, capacity fade and fast cell failure (dry out). To address these issues, a carbide-derived carbon (DUT-107) featuring a high surface area (2088 m² g⁻¹), high total pore volume (3.17 cm³ g⁻¹) and hierarchical micro-, meso- and macropore structure is applied as a rigid scaffold for sulfur infiltration. The DUT-107/S cathodes combine excellent mechanical stability and high initial capacities (1098–1208 mA h gs ⁻¹) with high sulfur content (69.7 wt% per total electrode) and loading (2.3–2.9 mgs cm⁻²). Derived from the effect of the electrolyte-to-sulfur ratio on capacity retention and cyclability, conducting salt is substituted by polysulfide additive for reduced polysulfide leakage and capacity stabilization. Moreover, in a full cell model system using a prelithiated hard carbon anode, the performance of DUT-107/S cathodes is demonstrated over 4100 cycles (final capacity of 422 mA h gs ⁻¹), with a very low capacity decay of 0.0118% per cycle. Application of PS additive further boosts the performance (final capacity of 554 mA h gs ⁻¹), although a slightly higher decay of 0.0125% per cycle is observed.

Developing next generation secondary batteries has attracted much attention in recent years due to the increasing demand of high energy and high power density energy storage for portable electronics, electric vehicles and renewable sources of energy. This dissertation investigates sulfur based advanced electrode materials in Lithium/Sodium batteries. The electrochemical performances of the electrode materials have been enhanced due to their unique nano structures as well as the formation of novel composites. First, a nitrogen-doped graphene nanosheets/sulfur (NGNSs/S) composite was synthesized via a facile chemical reaction deposition. In this composite, NGNSs were employed as a conductive host to entrap S/polysulfides in the cathode part. The NGNSs/S composite delivered an initial discharge capacity of 856.7 mAh g-1 and a reversible capacity of 319.3 mAh g-1 at 0.1C with good recoverable rate capability. Second, NGNS/S nanocomposites, synthesized using chemical reaction-deposition method and low temperature heat treatment, were further studied as active cathode materials for room temperature Na-S batteries. Both high loading composite with 86% gamma-S8 and low loading composite with 25% gamma-S8 have been electrochemically evaluated and compared with both NGNS and S control electrodes. It was found that low loading NGNS/S composite exhibited better electrochemical performance with specific capacity of 110 and 48 mAh g-1 at 0.1C at the 1st and 300th cycle, respectively. The Coulombic efficiency of 100% was obtained at the 300th cycle. Third, high purity rock-salt (RS), zinc-blende (ZB) and wurtzite (WZ) MnS nanocrystals with different morphologies were successfully synthesized via a facile solvothermal method. RS-, ZB- and WZ-MnS electrodes showed the capacities of 232.5 mAh g-1, 287.9 mAh g-1 and 79.8 mAh g-1 at the 600th cycle, respectively. ZB-MnS displayed the best performance in terms of specific capacity and cyclability. Interestingly, MnS electrodes exhibited an unusual phenomenon of capacity increase upon cycling which was ascribed to the decreased cell resistance and enhanced interfacial charge storage. In summary, this dissertation provides investigation of sulfur based electrode materials with sulfur/N-doped graphene composites and MnS nanocrystals. Their electrochemical performances have been evaluated and discussed. The understanding of their reaction mechanisms and electrochemical enhancement could make progress on development of secondary batteries.

This PhD project aims to address the low energy storage issues of electrode materials for supercapacitors through morphological and defect engineering. The key scientific contribution in this thesis includes: revealing the superior intrinsic electrochemical properties of NiCo-sulfide to hydroxide/oxides, demonstrating a facial defect engineering to enhance electrochemical properties of CoxNi1-xS2 by low temperature plasma, developing a new method for synthesis of high-performance carbon material derived by biomass.

This master's thesis deals with a topic of determination of the most suitable ratio of cathode materials for the lithium-sulfur systems. The first two chapters provide a general introduction to the topic of electrochemical energy sources and present the commonly used primary and secondary battery systems with emphasis on their characteristics and applications. The core of the theoretical part is dedicated to lithium-ion and lithium-sulfur batteries, their working principles along with the benefits or drawbacks related to the particular systems, and widely used materials. The experimental part briefly comments on determining the suitable electrode paste preparation method, the subsequent main part is focused on evaluation of electrochemical performance of cells using different ratios of cathode materials. Five samples of cathode materials were prepared, where the sulfur ratio is in range from 64 to 88 wt. %. Finally, the comparison of all prepared ratios in terms of their electrochemical properties is provided.

Binder free vertical aligned (VA) CNT/sulfur composite electrodes with high sulfur loadings up to 70 wt% were synthesized delivering discharge capacities higher than 800 mAh g−1 of the total composite electrode mass
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

Binder free vertical aligned (VA) CNT/sulfur composite electrodes with high sulfur loadings up to 70 wt% were synthesized delivering discharge capacities higher than 800 mAh g−1 of the total composite electrode mass.
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

Graphene-directed two-dimensional (2D) nitrogen-doped porous carbon frameworks (GPF) as the hosts for sulfur were constructed via the ionothermal polymerization of 1,4-dicyanobenzene directed by the polyacrylonitrile functionalized graphene nanosheets. As cathodes for lithium–sulfur (Li–S) batteries, the prepared GPF/sulfur nanocomposites exhibited a high capacity up to 962 mA h g⁻¹ after 120 cycles at 2 A g⁻¹. A high reversible capacity of 591 mA h g⁻¹ was still retained even at an extremely large current density of 20 A g⁻¹. Such impressive electrochemical performance of GPF should benefit from the 2D hierarchical porous architecture with an extremely high specific surface area, which could facilitate the efficient entrapment of sulfur and polysulfides and afford rapid charge transfer, fast electronic conduction as well as intimate contact between active materials and the electrolyte during cycling.

Current electrochemical energy storage systems are not sufficient to meet ever-rising energy storage requirements of emerging technologies. Hence, development of alternative electrode materials is inevitable. This thesis aims to establish novel electrode materials demonstrating both high energy and power density with prolonged cycle life derived from fundamental understandings on electrochemical reactions of chalcogens, such as sulfur (S) and selenium (Se). First, the effects of the pore size distribution, pore volume and specific surface area of porous carbons on the temperature-dependent electrochemical performance of S-infiltrated carbon cathodes in electrolytes having different salt concentrations are investigated. Additionally, the carbide derived carbon (CDC) synthesis temperature, electrolyte composition, and electrochemical S utilization have been correlated. The effects of thin Li-ion permeable but polysulfide non-permeable Al2O3 layer coating on the surface of S infiltrated carbon cathode are also examined. Similar with S studies, Se infiltrated ordered meso- and microporous CDC composites are prepared and the correlations between pore structure designing and electrolyte molarity are explored. Finally, this thesis demonstrates a simple process to form a protective solid electrolyte layer on the Se cathode surface in-situ. This technique adopts fluoroethylene carbonate to convert into a layer that remains permeable to Li ions, but prevents transport of polyselenides. As a whole, the correlations of multiple cell parameters, such as the cathode structure, the electrolyte composition, and operating temperature on the performances of lithium-chalcogen batteries are discussed.

Issues with the dissolution and diffusion of polysulfides in liquid organic electrolytes hinder the advance of lithium–sulfur (Li-S) batteries for next generation energy storage. To trap and re-utilize the polysulfides, brush-like, zinc oxide (ZnO) nanowires based interlayers were prepared ex-situ using a wet chemistry method and were coupled with a sulfur/multi-walled carbon nanotube (S/MWCNT) composite cathode. The cell with this configuration showed a good cycle life at a high current rate ascribed to (a) a strong interaction between the polysulfides and ZnO nanowires grown on conductive substrates; (b) fast electron transfer and (c) an optimized ion diffusion path from a well-organized nanoarchitecture. A praline-like flexible interlayer consisting of titanium oxide (TiO2) nanoparticles and carbon (C) nanofiber was further prepared in-situ using an electrospinning method, which allows the chemical adsorption of polysulfides throughout a robust conductive film. A significant enhancement in cycle stability and rate capability was achieved by incorporating this interlayer with a composite cathode of S/MWCNT. These results herald a new approach to building functional interlayers by integrating metal oxides with conductive frameworks. The derivatives of the TiO2/C interlayer was synthesized by changing the precursor concentration and carbonization temperature. Finally, a dual-interlayer was fabricated by simply coating titanium nitride (TiN) nanoparticles onto an electro-spun carbon nanofiber mat, which was then sandwiched with a sulfur/assembled Ketjen Black (KB) composite cathode with an ultra-high sulfur loading. The conductive polar TiN nanoparticles not only have a strong chemical affinity to polysulfides through a specific sulfur-nitrogen bond but also improve the reaction kinetics of the cell by catalyzing the conversion of the long-chain polysulfides to lithium sulfide. Besides, carbon nanofiber mat ensures mechanical robustness to TiN layer and acts as a physical barrier to block polysulfides diffusion. The incorporation of dual interlayers with sulfur cathodes offers a commercially feasible approach to improving the performance of Li-S batteries.

Les batteries lithium-air et Li-S sont des techniques prometteuses pour un stockage efficace d’énergie électrochimique. Les principaux défis sont de développer un électrolyte solide à haute conductivité ionique et des cathodes efficaces. Dans ce travail, des aérogels de carbone conducteurs avec une double porosité ont été synthétisés en utilisant la méthode de sol-gel. Ils ont été utilisés comme cathode dans des batteries lithium-air. Ces cathodes peuvent fournir deux types de canaux pour le stockage de produits de décharge, facilitant la diffusion gaz-liquide et réduisant ainsi le risque de colmatage. Presque 100 cycles été obtenus avec une capacité de 0,4 mAh et une densité de courant de 0,1 mA/cm². Pour le développement d'électrolyte solide stable et conducteur, les sulfures, en particulier Li4SnS4 et son dérivé Li10SnP2S12 ont été particulièrement étudiés. Ces composés ont été synthétisés en utilisant une technique en deux étapes comprenant la mécanosynthèse et un traitement thermique à température relativement basse qui a été optimisé afin d'améliorer la conductivité ionique. La meilleure conductivité obtenue est de 8,27×10-4 S / cm à 25°C et ces électrolytes présentent une grande stabilité électrochimique sur une large gamme de voltage de 0,5 à 7V. Les couches minces ont également été déposées en utilisant la technique de pulvérisation cathodique, avec en général une conductivité ionique améliorée. La performance des batteries Li-S assemblées avec ces électrolytes massifs doit être améliorée, en particulier en améliorant la conductivité ionique de l'électrolyte
Lithium-air and Li-S batteries are promising techniques for high power density storage. The main challenges are to develop solid electrolyte with high ionic conductivity and highly efficient catalyzed cathode. In this work, highly conductive carbon aerogels with dual-pore structure have been synthesized by using sol-gel method, and have been used as air cathode in Lithium-air batteries. This dual- pore structure can provide two types of channels for storing discharge products and for gas-liquid diffusion, thus reducing the risk of clogging. Nearly 100 cycles with a capacity of 0.4mAh at a current density of 0.1 mA cm-2 have been obtained. For developing stable and highly conductive solid electrolyte, sulfides, especially Li4SnS4 and its phosphorous derivative Li10SnP2S12 have been particularly investigated. These compounds have been synthesized by using a two-step technique including ball milling and a relatively low temperature heat treatment. The heat treatment has been carefully optimized in order to enhance the ionic conductivity. The best-obtained conductivity is 8.27×10-4 S/cm at 25°C and the electrolytes show high electrochemical stability over a wide working range of 0.5 – 7V. Thin films have also been deposited by using the sputtering technique, with generally improved ionic conductivity. The performance of the Li-S batteries assembled with these bulk electrolytes is still to be improved, particularly by improving the ionic conductivity of the electrolyte

Polymer-based carbide-derived carbons (CDCs) with combined micro- and mesopores are prepared by an advantageous sacrificial templating approach using poly(methylmethacrylate) (PMMA) spheres as the pore forming material. Resulting CDCs reveal uniform pore size and pore shape with a specific surface area of 2434 m2 g−1 and a total pore volume as high as 2.64 cm3 g−1. The bimodal CDC material is a highly attractive host structure for the active material in lithium–sulfur (Li–S) battery cathodes. It facilitates the utilization of high molarity electrolytes and therefore the cells exhibit good rate performance and stability. The cathodes in the 5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte show the highest discharge capacities (up to 1404 mA h gs−1) and capacity retention (72% after 50 cycles at C/5). The unique network structure of the carbon host enables uniform distribution of sulfur through the conductive media and at the same time it facilitates rapid access for the electrolyte to the active material.

Materials engineering and nano-manipulation play a key role in the development of advanced Lithium-Sulphur (Li–S) batteries in terms of energy and power density (both gravimetric and volumetric), cycling stability, rate capability, safety and the cost of production. In this thesis, two strategies are used to address the demands, i.e. fabrication of low cost, environmentally benign and conductive carbon-sulphur (C−S) nanostructured cathodes, and the use of interlayers as a novel battery configuration in Li–S battery systems. In the first strategy, inexpensive, scalable, environmentally-friendly and commercial bamboo biochar was activated via a KOH/annealing process to create an abundant microporous structure. This was then used to encapsulate sulphur to prepare a microporous bamboo carbon–sulphur (BC-S) nanocomposite as the cathode for Li–S batteries. The bamboo carbon micropores can encapsulate sulphur and polysulphides to reduce the shuttle phenomenon during cycling while simultaneously maintaining electrical contact between the sulphur and the conductive carbon framework during the charge/discharge process. The treated BC-S (T_BC-S) nanocomposite with 50 wt% sulphur content delivers a high initial capacity of 1295 mA·h·g−1 at a low discharge rate of 160 mA·g−1 and high capacity retention of 550 mA·h·g−1 after 150 cycles at a high discharge rate of 800 mA·g−1 with excellent coulombic efficiency (≥ 95%).
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

Дисертація присвячена вирішенню актуальної науково-прикладної проблеми підвищення рівня екологічної безпеки експлуатації підземних нафтогазопроводів, внаслідок зменшення відмов в їх роботі за рахунок розроблення та впровадження в практику переізоляцїї трубопроводів інноваційних біостійких протикорозійних покриттів на проблемних ділянках, де можливий ризик розвитку мікробіологічної корозії під дією корозійнонебезпечних грунтових мікроорганізмів. Проведено аналіз відмов підземних нафтогазопроводів з врахуванням біологічного фактора та їх вплив на довкілля. Науково обгрунтовано вибір інгібіторів корозії (біоцидів) для модифікації праймерів та мастик на бітумно-полімерній основі. Проведено комплексні дослідження по визначенню корозійної активності грунтів на ділянках МГ «Пасічна-Долина», «Пасічна-Тисмениця» та «Роздільна-Ізмаїл». Встановлено закономірності впливу природи інгібітора й складу електроліта на водонасичення базової та модифікованих мастик в довготривалому експерименті. Проведено комплекс досліджень по впливу ЧАС та похідних діоксодекагідроакридину на ріст і ферментну активність бактерій циклу сірки та швидкість корозії сталевих зразків, встановлено механізм блокування гідрогеназної реакції СВБ і ТБ бактерій. Вперше експериментально встановлена біорезистентність інгібіторів корозії «Г» і «К» та дано теоретичну оцінку біорезистентності похідних діоксодекагідроакридину. Вперше одержано біостійкі протикорозійні покриття на бітумно-полімерній основі (патенти №№ 82775, 84769, 89709), які успішно пройшли випробування в умовах УМГ «Прикарпаттрансгаз». Запропоновано інноваційне рішення зняття локалізовано незруйнованої ізоляції та пошкодженого праймера за екобіотехнологією (патент № 18222/ЗА/12).
Диссертация посвящена решению актуальной научно-технологической проблемы повышения уровня экологической безопасности эксплуатации подземных нефтегазопроводов, вследствие уменьшения отказов в их работе, за счет разработки и внедрения в практику переизоляции трубопроводов инновационных биостойких противокоррозионных покрытий на проблемных участках, где возможен риск развития микробиологической коррозии под действием коррозионнопасных почвенных микроорганизмов. Проведен анализ отказов подземных нефтегазопроводов с учетом биологического фактора и их влияния на окружающую среду. Научно обоснован выбор ингибиторов коррозии (биоцидов) для модификации праймеров и мастик на битумно-полимерной основе. Впервые изучено влияние гетеротрофных бактерий, выделенных из поврежденного битумного покрытия МГ, на устойчивость модифицированных битумно-полимерных мастик. Установлены закономерности влияния природы ингибитора и состава электролита на водонасыщение базовой и модифицированных мастик в длительном эксперименте. Полученные количественные зависимости водонасыщения базовой и модифицированных мастик от природы ингибитора и состава электролита приводят к повышению диэлектрических свойств изоляционных покрытий и предотвращают возникновение экологических катастроф при использовании модифицированных мастик в болотных, заиленных грунтах, из-за повреждения металла подземных нефтегазопроводов вследствие развития биокоррозии. Проведен комплекс исследований по воздействию четвертичных азотсодержащих ингибиторов коррозии и производных диоксодекагидроакридина на рост и ферментативную активность бактерий цикла серы и установлен механизм блокировки гидрогеназной реакции коррозионноактивных сульфатредуцирующих бактерий. Впервые установлена биорезистентность ингибиторов коррозии «Г» и «К» в длительном эксперименте, использование модифицированных противокоррозионных покрытий с их участием будет способствовать замедлению деградации изоляционного покрытия в процессе эксплуатации. Впервые дана теоретическая оценка биорезистентности производных диоксодскагидроакридина, исходя из значений энергии химической связи между углеродными атомами фенольного ядра и атомами модифицирующих элементов, что дает основание рассматривать их перспективными составляющими полифункциональных ингибирующих систем. Проведено исследование воздействия производных диоксодскагидроакридина и четвертичных азотсодержащих соединений (ЧАС) на ацидофобные тионовые бактерии Thiobacillus sp, среди которых максимально эффективными по отношению к этой группе микроорганизмов оказались ингибиторы 1/0 среди производных диоксодскагидроакридина и ингибитор «Ж» среди (ЧАС), которые проявили степень блокировки роста 95,8 и 97,1% соответственно и почти на 90% скорость коррозии стальных образцов, комплексное использование которых позволит обеспечить невозможность микробной коллонизации поверхности подземных нефтегазопроводов и повысит уровень экологической безопасности на протяжении длительного времени их эксплуатации. Впервые получены биостойкие противокоррозионные покрытия на битумно-полимерной основе (патенты №№ 82775, 84769, 89709), которые успешно прошли испытания в условиях УМГ «Прикарпаттрансгаз», внедрение которых в практику переизоляции действующих трубопроводов повысит уровень экологической безопасности их эксплуатации. Получило дальнейшее развитие изучение физиолого-биохимических свойств микроорганизмов разных экологотрофических групп, которые были выделены из поврежденных праймеров и битумного покрытия магистральных газопроводов, проложенных в грунтах различной коррозионной активности. Полученные результаты составляют основу для разработки современной зкобиотехнологии защиты от микробной коррозии, внедрение которой повысит одновременно производительность и качество ремонтных операций, что приведет к уменьшению отказов и обеспечит экологическую безопасность трубопроводных систем Украины (патент № 18222/ЗА/12).
The thesis is devoted to the solution of current scientific and applied problem devoted to the increase of ecological safety level of subsurface oil and gas pipelines exploitation by reducing failures in their work due to the development and introduction of pipeline reisolation by biostable innovative anticorrosive coatings on the problem areas, where there is a possible risk of microbiological corrosion development under the influence of soil microorganisms, which may cause corrosion. The analysis has been conducted of the subsurface oil and gas pipeline failures taking into account the biological factor and their influence on environment. The choice of corrosion inhibitors (biocides) applied for the modification of asphalt-polymer based primers and mastic has been scientifically substantiated. Complex research has been carried out to determine the corrosion activity of soils at the sites of Main Gaspipelines "Pasichna-Dolyna", "Pasichna-Tysmenytsia" and "Rozdilna-Izmail". During long-term experiment there has been determined the influence regularities of inhibitor and electrolyte composition nature on water saturation of base and modified mastics. A set of studies have been carried out devoted to the influence of nitrogen-containing corrosive inhibitors on the growth and enzyme activity of sulfur cycle bacteria and corrosion rate of steel samples. The blocking mechanism of hydrogenase reaction of sulfate-reducing and thionic bacteria has been established. It is the first time that bioresistance of corrosion inhibitors “G” and “K” has been proved. Besides, theoretical estimation of bioresistance of dioxodecahydroacridine derivatives has been given. It is also the first time when biostable anticorrosive coatings on the asphalt-polymer basis have been developed (Patents №№ 82775, 84769, 89709). The latter have been successfully tested within the conditions of the Department of Main Gas Pipelines “Prykarpattransgas”. The innovative solution has been proposed of removing locally situated undamaged isolation and damaged primer with the application of ecobiotechnology (Patent № 18222/3 А/12).

碩士
國立臺灣大學
化學工程學研究所
103
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.

Lithium-sulfur battery based on sulfur cathode has the advantages of high specific capacity, high energy density, environmental benignity and natural abundance of sulfur. These advantages over conventional lithium-ion batteries have driven researchers to make a lot of efforts to understand the redox mechanisms and improve the cathode performance. In order to fully realize the potential of lithium-sulfur battery and to approach commercialization, there are still many problems to overcome. Among them are i) low conductivity of sulfur and the discharge product, ii) lithium polysulfide intermediates dissolution and shuttle phenomenon, iii) volumetric expansion upon discharge and iv) lithium metal dendrite formation on anode side. In this thesis, the work is mostly focused on the cathode materials in order to address the first two problems. In the first part of the thesis, reduced titanium oxide is used as both a highly conductive support and a polysulfide adsorbent to prevent the loss of active materials. Mesoporous Magnéli phase with high surface area is synthesized through a sol-gel method. The reduction to suboxide is realized by carbothermal reaction at high temperature and the porous architecture is attributed to the cross-linking of polymer-mediated gel precursor that undergoes decomposition. The strong interaction between oxide and Li2S4 is confirmed by X-ray photoelectron spectroscopy analysis, based on comparison with carbon materials. This is also visually observed from a polysulfide adsorption study where the oxide and carbons were in contact with Li2S4 solutions. With 60 wt% of sulfur introduced onto synthesized Magnéli phase material by melt-diffusion, the cathode experienced very low capacity fading rate of 0.072% per cycle over 500 cycles at a discharge rate of 2C (full discharge in half an hour). The electrode morphology evolution upon cycling reveled by scanning electron microscopy imaging demonstrated more uniform deposition of discharge products for conductive oxide than carbon, which is due to the in-situ adsorption of lithium polysulfides during discharge. In the second part, another class of metallic non-carbonaceous materials - metal boride - was explored as a sulfur host. This polar material is expected to adsorb hydrophilic polysulfide as well. Both bulk and nanosized borides were synthesized through either simple thermal decomposition of metal borohydride and solid state reaction between elements, respectively. In all samples prepared, crystalline metal boride was confirmed to be the dominant phase, with small amount of oxide forming on the surface. Especially, with the addition of carbon nanotubes for solid state reaction, the particle size of as-synthesized boride/carbon composite was effectively reduced to ~100 nm, which is vital for sulfur impregnation. With 60 wt% of sulfur impregnated, the boride/S nanocomposite exhibited a high initial capacity of 985 mA h/g with only 0.1 % of capacity fading per cycle over 200 cycles at C/2.

Lithium-ion batteries are leading the path for the power sources for various portable applications, such as laptops and cellular phones, which is due to their relatively high energy density, stable and long cycle life. However, the cost, safety and toxicity issues are restricting the wider application of early generations of lithium-ion batteries. Recently, cheaper and less toxic cathode materials, such as LiFePO₄ and a wide range of derivatives of LiMn₂O₄, have been successfully developed and commercialized. Furthermore, cathode material candidates, such as LiCoPO₄, which present a high redox potential at approximately 4.8 V versus Li⁺/Li, have received attention and are being investigated. However, the theoretical capacity of all of these materials is below 170 mAh g⁻¹, which cannot fully satisfy the requirements of large scale applications, such as hybrid electric vehicles and electric vehicles. Therefore, alternative high energy density and inexpensive cathode materials are needed to make lithium batteries more practical and economically feasible. Elemental sulfur has a theoretical specific capacity of 1672 mAh g⁻¹, which is higher than that of any other known cathode materials for lithium batteries. Sulfur is of abundance in nature (e.g., sulfur is produced as a by-product of oil extraction, and hundreds of millions of tons have been accumulated at the oil extraction sites) and low cost, and this makes it very promising for the next generation of cathode materials for rechargeable batteries. Despite the mentioned advantages, there are several challenges to make the sulfur cathode suitable for battery use, and the following are the main: (i) sulfur has low conductivity, which leads to low sulfur utilization and poor rate capability in the cathode; (ii) multistep electrochemical reduction processes generate various forms of soluble intermediate lithium polysulfides, which dissolve in the electrolyte, induce the so-called shuttle effect, and cause irreversible loss of sulfur active material over repeat cycles; (iii) volume change of sulfur upon cycling leads to its mechanical rupture and, consequently, rapid degradation of the electrochemical performance. A variety of strategies have been developed to improve the discharge capacity, cyclability, and Coulombic efficiency of the sulfur cathode in Li/S batteries. Among those techniques, preparation of sulfur/carbon and sulfur/conductive polymer composites has received considerable attention. Conductive carbon and polymer additives enhance the electrochemical connectivity between active material particles, thereby enhancing the utilization of sulfur and the reversibility of the system, i.e., improving the cell capacity and cyclability. Incorporation of conductive polymers into the sulfur composites provides a barrier to the diffusion of polysulfides, thus providing noticeable improvement in cyclability and hence electrochemical performance. Among the possible conductive polymers, polypyrrole (PPy) is one of the most promising candidates to prepare electrochemically active sulfur composites because PPy has a high electrical conductivity and a wide electrochemical stability window (0-5 V vs Li/Li⁺). In the first part of this thesis, the preparation of a novel nanostructured S/PPy based composites and investigation of their physical and electrochemical properties as a cathode for lithium secondary batteries are reported. An S/PPy composite with highly developed branched structure was obtained by a one-step ball-milling process without heat-treatment. The material exhibited a high initial discharge capacity of 1320 mAh g⁻¹ at a current density of 100 mA g⁻¹ (0.06 C) and retained about 500 mAh g⁻¹ after 40 cycles. Alternatively, in situ polymerization of the pyrrole monomer on the surface of nano-sulfur particles afforded a core-shell structure composite in which sulfur is a core and PPy is a shell. The composite showed an initial discharge capacity of 1199 mAh g⁻¹ at 0.2 C with capacity retention of 913 mAh g⁻¹ after 50 cycles, and of 437 mAh g⁻¹ at 2.5 C. Further improvement of the electrochemical performance was achieved by introducing multi-walled carbon nanotubes (MWNT), which provide a much more effective path for the electron transport, into the S/PPy composite. A novel S/PPy/MWNT ternary composite with a core-shell nano-tubular structure was developed using a two-step preparation method (in situ polymerization of pyrrole on the MWNT surface followed by mixing of the binary composite with nano-sulfur particles). This ternary composite cathode sustained 961 mAh g⁻¹ of reversible specific discharge capacity after 40 cycles at 0.1 C, and 523 mAh g⁻¹ after 40 cycles at 0.5 C. Yet another structure was prepared exploring the large surface area, superior electronic conductivity, and high mechanical flexibility graphene nanosheet (GNS). By taking advantage of both capillary force driven self-assembly of polypyrrole on graphene nanosheets and adhesion ability of polypyrrole to sulfur, an S/PPy/GNS composite with a dual-layered structure was developed. A very high initial discharge capacity of 1416 mAh g⁻¹ and retained a 642 mAh g⁻¹ reversible capacity after 40 cycles at 0.1 C rate. The electrochemical properties of the graphene loaded composite cathode represent a significant improvement in comparison to that exhibited by both the binary S/PPy and the MWCNT containing ternary composites. In the second part of this thesis, polyacrylonitrile (PAN) was investigated as a candidate to composite with sulfur to prepare high performance cathodes for Li/S battery. Unlike polypyrrole, which, in addition of being a conductive matrix, works as physical barrier for blocking polysulfides, PAN could react with sulfur to form inter- and/or intra-chain disulfide bonds, chemically confining sulfur and polysulfides. In the preliminary tests, PAN was ballmilled with an excess of elemental sulfur and the resulting mixture was heated at temperatures varying from 300°C to 350°C. During this step some H₂S gas was released as a result of the formation of rings with a conjugated π-system between sulfur and polymer backbone. These cyclic structures could ‘trap’ some of the soluble reaction products, improving the utilization of sulfur, as it was observed experimentally: the resulting S/PAN composite demonstrated a high sulfur utilization, large initial capacity, and high Coulombic efficiency. However, the poor electronic conductivity of the S/PAN binary composite compromises the rate capability and sulfur utilization at high C-rates. These issues were addressed by doping the composite with small amounts of components that positively affected the conductivity and reactivity of the cathode. Mg₀.₆Ni₀.₄O prepared by self-propagating high temperature synthesis was used as an additive in the S/PAN composite cathode and considerably improved its morphology stability, chemical uniformity, and electrochemical performance. The nanostructured composite containing Mg₀.₆Ni₀.₄O exhibited less sulfur agglomeration upon cycling, enhanced cathode utilization, improved rate capability, and superior reversibility, with a second cycle discharge capacity of over 1200 mAh g⁻¹, which was retained for over 100 cycles. Alternatively, graphene was used as conductive additive to form an S/PAN/Graphene composite with a well-connected conductive network structure. This ternary composite was prepared by ballmilling followed by low temperature heat treatment. The resulting material exhibited significantly improved rate capability and cycling performance delivering a discharge capacity of 1293 mAh g⁻¹ in the second cycle at 0.1 C. Even at up to 4 C, the cell still achieved a high discharge capacity of 762 mAh g⁻¹. Different approaches for the optimization of sulfur-based composite cathodes are described in this thesis. Experimental results indicate that the proposed methods constitute an important contribution in the development of the high capacity cathode for rechargeable Li/S battery technology. Furthermore, the innovative concept of sulfur/conductive polymer/conductive carbon ternary composites developed in this work could be used to prepare many other analogous composites, such as sulfur/polyaniline/carbon nanotube or sulfur/polythiophene/graphene, which could also lead to the development of new sulfur-based composites for high energy density applications. In particular, exploration of alternative polymeric matrices with high sulfur absorption ability is of importance for the attainment of composites that possess higher loading of sulfur, to increase the specific energy density of the cathode. Note that the material preparation techniques described here have the advantage of being reproducible, simple and inexpensive, compared with most procedures reported in the literature.

博士
國立臺灣科技大學
材料科學與工程系
105
In the field of lithium-sulfur (Li-S) battery, intense research has been made in the past decades to find high capacity, long cycle life, improved safety, high rate capability, and high sulfur loaded cathode material for Li-S batteries to be applicable for commercialization. Although significant achievements have been established, problems that hindered real applications of Li-S batteries have not been fully resolved. Therefore, this dissertation focused on the design of sulfur nanocomposite cathode material for high performance Li-S batteries and characterizing their property through various spectroscopic and microscopic techniques followed by measuring their performance to be suitable for real application after coin cell fabrication. In the first part of this study, hybrid nanostructured microporous carbon-mesoporous carbon doped titanium dioxide/sulfur composite (MC-Meso C-doped TiO2/S) was we designed as a cathode material for Li-S batteries. The hybrid MC-Meso C-doped TiO2 host material was produced by a low-cost hydrothermal and annealing process. It found that the resulting conductive material showed dual microporous and mesoporous behavior, which enhanced the effective trapping of sulfur and polysulfides. The hybrid MC-Meso C-doped TiO2/S composite material possessed rutile TiO2 nanotube structure with successful carbon doping, while sulfur was uniformly distributed in the hybrid MC-Meso C-doped TiO2 composite materials after the melt-infusion process. Electrochemical measurements of the hybrid cathode material also showed improved cycling stability and rate performance with high sulfur loading (61.04 wt %). Moreover, the material delivered an initial discharge capacity of 802 mAh g-1 and maintained at 578 mAh g-1 with the coulombic efficiency greater than 97.1% after 140 cycles at 0.1 C iv rates. This improvement was thought to be attributed to the unique hybrid nanostructure of the MC-Meso C-doped TiO2 host and the good dispersion of sulfur in the narrow pores of the spherical microporous carbon (MC) and the Meso C-doped TiO2 nanotube support. Secondly, the novel nanocomposite cathode materials consisting of sulfur (80 wt%) embedded within nitrogen doped three-dimensional reduced graphene oxide (N-3D-rGO) was designed by a controllable sulfur impregnation method. Nitrogen doping helped to increase the surface area by ten times and pore volume by seven times from pristine graphene. These structural features allowed the cathode to accommodate more sulfur. Moreover, the cathode adsorbed polysulfides and prevented their detachment from host materials, thereby achieving stable cycle performance. The solution drop sulfur impregnation method provided uniform distribution of nanosized sulfur in a controlled manner. Furthermore, the cathode delivered high initial discharge capacities of 1042 mAhg-1 and 916 mAh g-1 at 0.2 C and 0.5 C with excellent capacity retention of 94.8% and 81.9% after 100 cycles respectively, with a low decay rate of 0.062% per cycle after 500 cycles. Thus, the combination of solution drop sulfur impregnation and nitrogen doping opens a new chapter for resolving capacity fading, as well as long cycling problems, and creates a new strategy to increase sulfur loading in controlled mechanism. In the final part of this dissertation, a novel design of dual-confined sulfur cathode with core-shell architecture, where sulfur (72.5 wt%) was first encapsulated in MC cores and embedded by graphene (G) shells was reported. Larger, soluble polysulfide intermediates (Li2Sx, 4≤x≤8) were trapped by G shells, which prevented the dissolution of soluble polysulfide intermediates into the organic electrolyte, so that a stable cycling v performance could be achieved. Moreover, the G shell created the hollow space in-between, which helped ensure the integrity of the hybrid cathode against the volume expansion upon cycling. On the other hand, MC confined smaller sulfur (S2-4) molecules within its small pores and suppressed the formation of soluble polysulfides. The resulting electrode delivered a high initial discharge capacity of 982 mAh g-1 with enhanced capacity retention of 85.4% after 100 cycles at 0.2 C rates. More importantly, the cathode exhibited a high discharge capacity of 886 mAh g-1, and maintained at 601 mAh g-1 after 500 cycles at 0.5 C with the coulombic efficiency of nearly 100%, which is the best performance among reported cycle stabilities.

碩士
國立高雄科技大學
化學工程與材料工程系
107
In this study, the hollow Ni(OH)2 microspheres (about 5~6 μm in diameter) composed of nanosheets were synthesized by hydrothermal synthesis method. Hollow nickel oxide microspheres were obtained by heating the Ni(OH)2 microspheres at different temperatures under air atmosphere. The hollow microspheres were filled with sulfur and then coated on the carbon paper as the composite cathode for lithium-sulfur battery. The advantage of this cathode was to use the hollow metal oxide microspheres that could offer high sulfur loading and reduce the shuttle reaction of lithium polysulfides, as compared with a pure sulfur cathode. The surface morphology, structure, pore size distribution, and electrochemical performance of the hollow microspheres were investigated. Results indicated that when the calcination temperature increased, the nanosheets became smaller. Transmission electron microscopy and isothermal adsorption and desorption curves revealed that the pore size distribution of hollow microspheres was significantly changed by the calcination process. Hollow NiO microspheres obtained at 400℃ exhibited bimodal pore size distribution including the mesopores and macropores. This unique structure could serve as a hosting matrix to facilitate the sulfur diffusion and capture the lithium polysulfide, leading to the suppression of shuttle effect and capacity degradation. The initial discharge capacity of the hollow microspheres/sulfur electrode using NiO (calcined at 400℃) could reach 1365 mAh g-1 at a current density of 0.2 C and 260 mAh g-1 at 5.0 C. After 75 charge/discharge cycles, the capacity retention of electrode was 96 %, and the Coulombic efficiency of the electrode remained high (about 98 %). As a whole, the hollow NiO microspheres with bimodal pore size distribution are promising cathode materials for application in lithium-sulfur batteries.

Rechargeable lithium sulfur (Li-S) batteries are potentially safe, environmentally friendly and economical alternative energy storage systems that can potentially be combined with renewable sources including wind solar and wave energy. Sulfur has a high theoretical specific capacity of ~1680 mAh/g, attainable through the reversible redox reaction denoted as S8+16Li ↔8Li¬2S, which yields an average cell voltage of ~2.2 V. However, two detrimental factors prevent the achievement of the full potential of the Li-S batteries. First, the poor electrical/ionic conductivity of elemental sulfur and Li2S severely hampers the utilization of active material. Second, dissolution of intermediate long-chain polysulfides (Li2Sn, 2