Indiana University-Purdue University Indianapolis (IUPUI) / Recent innovations in portable electronics, electric vehicles and power generation by wind and solar have expanded the need for effcient battery storage. Lithium-ion batteries have been the frontline contender of battery storage yet are not able to match current demands. Alternatively, lithium-sulfur batteries are a promising technology to match the consumer demands. Elemental sulfur cathodes incur a variety of problems during cycling including the dissolution of intermediate lithium polysul- fides, an undesirable volume change (~ 80%) when completely reduced and a high dependence on liquid electrolyte, which quickly degrades the cell's available energy density. Due to these problems, the high theoretical capacity and energy density of lithium sulfur cells are unattainable. In this work, A new class of phenyl polysul- fides, C6H5SxC6H5(4 < x <6), are developed as liquid sulfur containing cathode materials. This technology was taken a step further to fulfill and emerging need for exible electronics in technology. Phenyl tetrasulfide (C6H5S4C6H5) was polymerized to form a high energy density battery with acute mobility. Lithium half-cell testing shows that phenyl hexasulfide (C6H5S6C6H5) can provide a specific capacity of 650mAh/g and capacity retention of 80% through 500 cycles at 1C rate along with superlative performance up to 10C. Furthermore, 1, 302W h/ kg and 1, 720W h/L are achievable at a low electrolyte/active material ratio. Electrochemical testing of polymer phenyl tetrasulfide reveals high specific capacities of 634mAh /g at 1C, while reaching 600mAh /g upon mechanical strain testing. This work introduces novel cathode materials for lithium-sulfur batteries and provides a new direction for the development of alternative high-capacity flexible cathode materials.

In this thesis, electrode binders as vital components in the fabrication of composite electrodes for lithium-ion (LIB) and lithium-sulfur batteries (LiSB) have been investigated. Poly(vinylidene difluoride) (PVdF) was studied as binder for sulfur-carbon positive electrodes by a combination of galvanostatic cycling and nitrogen absorption. Poor binder swelling in the electrolyte and pore blocking in the porous carbon were identified as origins of low discharge capacity, rendering PVdF-based binders an unsuitable choice for LiSBs. More promising candidates are blends of poly(ethylene oxide) (PEO) and poly(N-vinylpyrrolidone) (PVP). It was found that these polymers interact with soluble lithium polysulfide intermediates generated during the cell reaction. They can increase the discharge capacity, while simultaneously improving the capacity retention and reducing the self-discharge of the LiSB. In conclusion, these binders improve the local electrolyte environment at the electrode interface. Graphite electrodes for LIBs are rendered considerably more stable in ‘aggressive’ electrolytes (a propylene carbonate rich formulation and an ether-based electrolyte) with the poorly swellable binders poly(sodium acrylate) (PAA-Na) and carboxymethyl cellulose sodium salt (CMC-Na). The higher interfacial impedance seen for the conventional PVdF binder suggests a protective polymer layer on the particles. By reducing the binder content, it was found that PAA-Na has a stronger affinity towards electrode components with high surface areas, which is attributed to a flexible polymer backbone and a higher density of functional groups. Lastly, a graphite electrode was combined with a sulfur electrode to yield a balanced graphite-sulfur cell. Due to a more stable electrode-electrolyte interface the self-discharge of this cell could be reduced and the cycle life was extended significantly. This example demonstrates the possible benefits of replacing the lithium metal negative electrode with an alternative electrode material.

Lithium-sulfur (Li-S) batteries are faced with practical drawbacks of poor cycle life and low charge efficiency which hinder their advancements. Those drawbacks are primarily caused by the intrinsic issues of the cathodes (sulfur) and the anodes (Li metal). In attempt to resolve the issues found on the cathodes, this work discusses the method to prepare a binder-free three-dimensional carbon nanotubes-sulfur (3D CNTs-S) composite cathode by a facile and a scalable approach. Here, the 3D structure of CNTs serves as a conducting network to accommodate high loading amounts of active sulfur material. The efficient electron pathway and the short Li ions (Li+) diffusion length provided by the 3D CNTs offset the insulating properties of sulfur. As a result, high areal and specific capacities of 8.8 mAh cm−2 and 1068 mAh g−1, respectively, with the sulfur loading of 8.33 mg cm−2 are demonstrated; furthermore, the cells operated at a current density of 1.4 mA cm−2 (0.1 C) for up to 150 cycles. To address the issues existing on the anode part of Li-S batteries, this work also covers the novel approach to protect a Li metal anode with a thin layer of two-dimensional molybdenum disulfide (MoS2). With the protective layer of MoS2 preventing the growth of Li dendrites, stable Li electrodeposition is realized at the current density of 10 mA cm−2; also, the MoS2 protected anode demonstrates over 300% longer cycle life than the unprotected counterpart. Moreover, the MoS2 layer prevents polysulfides from corroding the anode while facilitating a reversible utilization of active materials without decomposing the electrolyte. Therefore, the MoS2 protected anode enables a stable cycle life of over 500 cycles at 0.5 C with the high sulfur loading amount of ~7 mg cm−2 (~67 wt% S content in cathode) under the low electrolyte/sulfur (E/S) ratio of 6 μL mg−1. This translates to the specific energy and power densities of ~550 Wh kg-1 and ~300 W kg−1, respectively. Additionally, such values far exceed the electrochemical performance of the current Li-ion batteries. Therefore, the synergetic effect of utilizing the 3D CNT-S cathode and the MoS2 protected Li anode will allow the Li-S batteries to become applicable for the transportation and the large-scale energy grid applications.

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Development of alternative cathodes that have high capacity and long cycle life at an affordable cost is critical for next generation rechargeable batteries to meet the ever-increasing requirements of global energy storage market. Lithium-sulfur batteries, employing sulfur cathodes, are increasingly being investigated due to their high theoretical capacity, low cost, and environmental friendliness. However, the practicality of lithium-sulfur technology is hindered by technical obstacles, such as short shelf and cycle life, arising from the shuttling of polysulfide intermediates between the cathode and the anode as well as the poor electronic conductivity of sulfur and the discharge product Li2S. This dissertation focuses on overcoming some of these problems. The sulfur cathode involves an electrochemical conversion reaction compared to the conventional insertion-reaction cathodes. Therefore, modifications in cell-component configurations/structures are needed to realize the full potential of lithium-sulfur cells. This dissertation explores various custom and functionalized cell components that can be adapted with pure sulfur cathodes, e.g., porous current collectors in Chapter 3, interlayers in Chapter 4, sandwiched electrodes in Chapter 5, and surface-coated separators in Chapter 6. Each chapter introduces the new concept and design, followed by necessary modifications and development. The porous current collectors embedded with pure sulfur cathodes are able to contain the active material in their porous space and ensure close contact between the insulating active material and the conductive matrix. Hence, a stable and reversible electrochemical-conversion reaction is facilitated. In addition, the use of highly porous substrates allows the resulting cell to accommodate high sulfur loading. The interlayers inserted between the pure sulfur cathode and the separator effectively intercept the diffusing polysulfides, suppress polysulfide migration, localize the active material within the cathode region, and boost cell cycle stability. The combination of porous current collectors and interlayers offers sandwiched electrode structure for the lithium/dissolved polysulfide cells. By way of integrating the advantages from the porous current collector and the interlayer, the sandwiched electrodes stabilize the dissolved polysulfide catholyte within the cathode region, resulting in a high discharge capacity, long-term cycle stability, and high sulfur loading. The novel surface-coated separators have a polysulfide trap or filter coated onto one side of a commercial polymeric separator. The functional coatings possess physical and/or chemical polysulfide-trapping capabilities to intercept, absorb, and trap the dissolved polysulfides during cell discharge. The functional coatings also have high electrical conductivity and porous channels to facilitate electron, lithium-ion, and electrolyte mobility for reactivating the trapped active material. As a result, effective reutilization of the trapped active material leads to improved long-term cycle stability. The investigation of the key electrochemical and engineering parameters of these novel cell components has allowed us to make progress on (i) understanding the materials chemistry of the applied functionalized cell components and (ii) the electrochemical performance of the resulting lithium-sulfur batteries. / text

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.

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.

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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.