The development of the new cathode and anode materials of Lithium-Ion Batteries (LIBs) with high energy density and outstanding electrochemical performance is of substantial technological importance due to the ever-increasing demand for economic and efficient energy storage system. Because of the abundance of element sulfur and high theoretical energy density, Lithium-Sulfur (Li-S) batteries have become one of the most promising candidates for the next-generation energy storage system. However, the shuttling effect of electrolyte-soluble polysulfides severely impedes the cell performance and commercialization of Li-S batteries, and significant progress have been made to mitigate this shuttle effect in the past two decades. Coordination polymers (CPs) or Metal-organic Frameworks (MOFs) have been attracted much attention by virtue of their controllable porosity, nanometer cavity sizes and high surface areas, which supposed to be an available material in suppressing polysulfide migration. In this thesis, we investigate different mechanisms of mitigating polysulfide diffusion by applying a layer of MOFs (including Y-FTZB, ZIF-7, ZIF-8, and HKUST-1) on a separator. We also fabricate a new free-standing 2D coordination polymer Zn2(Benzimidazolate)2(OH)2 with rich hydroxyl (OH-) groups by using a simple, scalable and low cost method at air/water surface. Our results suggest that the chemical stability, the cluster morphology and the surface function groups of MOFs shows a greater impact on minimizing the shuttling effect in Li-S batteries, other than the internal cavity size in MOFs. Meanwhile, the new design of 2D coordination polymer efficiently mitigate the shuttling effect in Li-S battery resulting in a largely promotion of the battery capacity to 1407 mAh g-1 at 0.1 C and excellent cycling performance (capacity retention of 98% after 200 cycles at 0.25C). Such excellent cell performance is mainly owing to the fancying physical and chemical structure controllability of MOFs or CPs, which has substantial potential for future commercial utilizations.

The shortage of energy sources and the global climate change crisis have become critical issues. Solving these problems with clean and sustainable energy sources (solar, wind, tidal, and so on) is a promising solution. In this regard, energy storage techniques need to be implemented to tackle with the intermittent nature of the sustainable energies. Among the next-generation energy storage systems, lithium sulfur batteries has gained prominence due to the low cost, high theoretical specific-capacity of sulfur. Extensive research has been conducted on this battery system. Nevertheless, several issues including the “shuttle effect” and the growth of lithium dendrites still exist, which could cause rapid capacity loss and safety hazards. Several methods are proposed to tackle the challenges in this dissertation, including cathode engineering, interlayer design, and lithium metal anode protection. An asymmetric cathode structure is first developed by a non-solvent induced phase separation (NIPS) method. The asymmetric cathode comprises a nanoporous matrix and ultrathin and dense top layer. The top-layer is a desired barrier to block polysulfides transport, while the sublayer threaded with cationic networks facilitate Li-ions transport and sulfur conversions. In addition, a conformal and ultrathin microporous membrane is electrodeposited on the whole surface of the cathode by an electropolymerization method. This strategy creates a close system, which greatly blocks the LiPS leakage and improves the sulfur utilization. A polycarbazole-type interlayer is deposited on the polypropylene (PP) separator via an electropolymerization method. This interlayer is ultrathin, continuous, and microporous, which defines the critical properties of an ideal interlayer that is required for advanced Li–S batteries. Meanwhile, a self-assembled 2D MXene based interlayer was prepared to offer abundant porosity, dual absorption sites, and desirable electrical conductivity for Li-ions transport and polysulfides conversions. A new 2D COF-on-MXene heterostructures is prepared as the lithium anode host. The 2D heterostructures has hierarchical porosity, conductive frameworks, and lithiophilic sites. When utilized as a lithium host, the MXene@COF host can efficiently regulate the Li+ diffusion, and reduce the nucleation and deposition overpotential, which results in a dendrite-free and safer Li–S battery.

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 / 新制||人||236(附属図書館) / 2020||人博||1001(吉田南総合図書館) / 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

Indiana University-Purdue University Indianapolis (IUPUI) / Lithium based batteries have become ubiquitous with our everyday life. They have propelled a generation of smart personal electronics and electric transport. Their use is now percolating to various fields as a source of energy to facilitate the operation of devices from nanoscale to mega scale. This need for a portable energy source has led to tremendous scientific interest in this field to develop electrochemical devices like batteries with higher capacities, longer cycle life and increased safety at a low cost. To this end, the research presented in this thesis focuses on two emerging and promising technologies called lithium-oxygen (Li-O₂) and lithium-sulfur (Li-S) batteries. These batteries can offer an order of magnitude higher capacities through cheap, environmentally safe and abundant elements, namely oxygen and sulfur. The first work introduces the concept of closed system lithium-oxygen batteries wherein the cell contains the discharge product of Li-O₂ batteries namely, lithium peroxide (Li₂O₂) as the starting active material. The reversibility of this system is analyzed along with its rate performance. The possible use of such a cathode in a full cell is explored. Also, this concept is used to verify if all the lithium can be extracted from the cathode in the first charge. In the following work, lithium peroxide is chemically synthesized and deposited in a carbon nanofiber matrix. This forms a free-standing cathode that shows high reversibility. It can be cycled up to 20 times, and while using capacity control protocol, a cycle life of 50 is obtained. The cause of cell degradation and failure is also analyzed. In the work on full cell lithium-sulfur system, a novel electrolyte is developed that can support reversible lithium insertion and extraction from a graphite anode. A method to deposit solid lithium polysulde is developed for the cathode. Coupling a lithiated graphite anode with the cathode using the new electrolyte yields a full cell whose performance is characterized and its post-mortem analysis yields information on the cell failure mechanism. Although still in their developmental stages, Li-O₂ and Li-S batteries hold great promise to be the next generation of lithium batteries, and these studies make a fundamental contribution towards novel cathode and cell architecture for these batteries.

Indiana University-Purdue University Indianapolis (IUPUI) / The studies of rechargeable lithium-sulfur (Li-S) batteries are included in this thesis. In the first part of this thesis, a linear sweep voltammetry method to study polysulfide transport through separators is presented. Shuttle of polysulfide from the sulfur cathode to lithium metal anode in rechargeable Li-S batteries is a critical issue hindering cycling efficiency and life. Several approaches have been developed to minimize it including polysulfide-blocking separators; there is a need for measuring polysulfide transport through separators. We have developed a linear sweep voltammetry method to measure the anodic (oxidization) current of polysulfides crossed separators, which can be used as a quantitative measurement of the polysulfide transport through separators. The electrochemical oxidation of polysulfide is diffusion-controlled. The electrical charge in Coulombs produced by the oxidation of polysulfide is linearly related to the concentration of polysulfide within a certain range (≤ 0.5 M). Separators with a high porosity (large pore size) show high anodic currents, resulting in fast capacity degradation and low Coulombic efficiencies in Li-S cells. These results demonstrate this method can be used to correlate the polysulfide transport through separators with the separator structure and battery performance, therefore provide guidance for developing new separators for Li-S batteries. The second part includes a study on improving cycling performance of Li/polysulfide batteries by applying a functional polymer on carbon current collector. Significant capacity decay over cycling in Li-S batteries is a major impediment for their practical applications. Polysulfides Li2Sx (3 < x ≤ 8) formed in the cycling are soluble in liquid electrolyte, which is the main reason for capacity loss and cycling instability. Functional polymers can tune the structure and property of sulfur electrodes, hold polysulfides, and improve cycle life. We have examined a polyvinylpyrrolidone-modified carbon paper (CP-PVP) current collector in Li/polysulfide cells. PVP is soluble in the electrolyte solvent, but shows strong affinity with lithium polysulfides. The retention of polysulfides in the CP-PVP current collector is improved by ~50%, which is measured by a linear sweep voltammetry method. Without LiNO3 additive in the electrolyte, the CP-PVP current collector with 50 ug of PVP can significantly improve cycling stability with a capacity retention of > 90% over 50 cycles at C/10 rate. With LiNO3 additive in the electrolyte, the cell shows a reversible capacity of > 1000 mAh g ⁻¹ and a capacity retention of > 80% over 100 cycles at C/5 rate. The third part of this thesis is about a study on a binder-free sulfur/carbon composite electrode prepared by a sulfur sublimation method for Li-S batteries. Sulfur nanoparticles fill large pores in a carbon paper substrate and primarily has a monoclinic crystal structure. The composite electrode shows a long cycle life of over 200 cycles with a good rate performance in Li-S batteries.

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A systematic development of 2D alloy catalyst with synergistic performance of high lithium polysulfide (LiPS) binding energy and efficient Li+ ion/electron conduction is presented. The first section of work found that Li+ ions can flow through the percolated ion transport pathway in polycrystalline MoS2, while Na+ and K+ ions can easily flow through the percolated 1D ion channel near the grain boundaries. An unusually high ionic conductivity of extrinsic Li+, Na+, and K+ ions in 2D MoS2 film exceeding 1 S/cm was measured that is more than two orders of magnitude higher than those of conventional solid ionic materials, including 2D ionic materials. The second section of this dissertation focus on catalyzing the transformation of LiPSs to prevent the shuttle effect during the battery cycling by synthesizing 2H (semiconducting) – 1T (metallic) mixed phase 2D Mo0.5W0.5S2 alloy on CNF paper, using two step sputtering and sulfurization method. The lithium sulfur (Li-S) battery cell assembled with the 2D Mo0.5W0.5S2/CNF/S cathode shows a high specific capacity of 1228 mAh g-1 at 0.1C and much higher cyclic stability over 4 times as compared to the pristine cathodes. The high LiPSs binding energy of catalyst efficiently prevents the shuttling effect and corrosion of Li anode after long term stability test for over 400 cycles. The defect engineered MoWS catalyst on CNF showed significantly enhanced polysulfide transformation resulting in specific capacity of 1586 mAh g-1 at 0.05C for the full cell Li-S battery and much higher cyclic stability over 1000 cycles. Stacked layers of D-MoWS-CNF-S cathodes can result in an increased sulfur loading up to 10 mg cm-2 with highest achievable areal capacity of 13.5 mAh/cm2. The efficient sulfur utilization and reduced negative-to-positive capacity (N/P) ratio by D-MoWS catalyst significantly increased the gravimetric energy density to the highest reported value of 1090 Wh kg-1 w.r.t the total weight of anode and cathode.