Browsing by Author "Bhargav, Amruth"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Bis(aryl) Tetrasulfides as Cathode Materials for Rechargeable Lithium Batteries
    (Wiley, 2017) Guo, Wei; Wawrzyniakowski, Zachary D.; Cerda, Matthew M.; Bhargav, Amruth; Pluth, Michael D.; Ma, Ying; Fu, Yongzhu; Department of Mechanical Engineering, School of Engineering and Technology
    An organotetrasulfide consists of a linear chain of four sulfur atoms that could accept up to 6 e− in reduction reactions, thus providing a promising high-capacity electrode material. Herein, we study three bis(aryl) tetrasulfides as cathode materials in lithium batteries. Each tetrasulfide exhibits two major voltage regions in the discharge. The high voltage slope region is governed by the formation of persulfides and thiolates, and the low voltage plateau region is due to the formation of Li2S2/Li2S. Based on theoretical calculations and spectroscopic analysis, three reduction reaction processes are revealed, and the discharge products are identified. Lithium half cells with tetrasulfide catholytes deliver high specific capacities over 200 cycles. The effects of the functional groups on the electrochemical characteristics of tetrasulfides are investigated, which provides guidance for developing optimum aryl polysulfides as cathode materials for high energy lithium batteries.
  • Thumbnail Image
    Item
    A Class of Organopolysulfides As Liquid Cathode Materials for High-Energy-Density Lithium Batteries
    (ACS, 2018) Bhargav, Amruth; Bell, Michaela Elaine; Karty, Jonathan; Cui, Yi; Fu, Yongzhu; Mechanical Engineering, School of Engineering and Technology
    Sulfur-based cathodes are promising to enable high-energy-density lithium–sulfur batteries; however, elemental sulfur as active material faces several challenges, including undesirable volume change (∼80%) when completely reduced and high dependence on liquid electrolyte wherein an electrolyte/sulfur ratio >10 μL mg–1 is required for high material utilization. These limit the attainable energy densities of these batteries. Herein, we introduce a new class of phenyl polysulfides C6H5SxC6H5 (4 ≤ x ≤ 6) as liquid cathode materials synthesized in a facile and scalable route to mitigate these setbacks. These polysulfides possess sufficiently high theoretical specific capacities, specific energies, and energy densities. Spectroscopic techniques verify their chemical composition and computation shows that the volume change when reduced is about 37%. Lithium half-cell testing shows that phenyl hexasulfide (C6H5S6C6H5) can provide a specific capacity of 650 mAh g–1 and capacity retention of 80% through 500 cycles at 1C rate along with superlative performance up to 10C. Furthermore, 1302 Wh kg–1 and 1720 Wh L–1 are achievable at a low electrolyte/active material ratio, i.e., 3 μL mg–1. This work adds new members to the cathode family for Li–S batteries, reduces the gap between the theoretical and practical energy densities of batteries, and provides a new direction for the development of alternative high-capacity cathode materials.
  • Thumbnail Image
    Item
    Development of Novel Cathodes for High Energy Density Lithium Batteries
    (2016-04) Bhargav, Amruth; Fu, Yongzhu; Zhu, Likun; Zhang, Jing; Anwar, Sohel
    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.
  • Thumbnail Image
    Item
    A Graphite-Polysulfide Full Cell with DME-Based Electrolyte
    (ECS, 2016) Bhargav, Amruth; Wu, Min; Fu, Yongzhu; Department of Mechanical Engineering, School of Engineering and Technology
    Over the last decade, vast improvements have been made in the field of lithium-sulfur batteries bringing it a step closer to reality. In this field of research, deep understanding of the polysulfide shuttle phenomenon and their affinity with carbons, polymers and other hosts have enabled the design of superior cathodes with prolonged life. However, the anode side has undergone comparatively less transformation. In this work, we have developed a new electrolyte based on 1,2-dimethoxyethane (DME) solvent that enables reversible intercalation of lithium ions in graphite. A novel method to introduce solid lithium polysulfide into a carbon current collector as the cathode has been demonstrated and the electrode shows stable cycling with the new electrolyte. A full cell consisting of a lithiated graphitic anode and lithium polysulfide cathode is constructed, which exhibits an initial capacity as high as 1,500 mAh g−1 (based on the sulfur in the cathode) and a reversible capacity of 700 mAh g−1 for 100 cycles. This full cell is capable of delivering over 460 mAh g−1 at rates as high as 2C. The cell degradation over prolonged cycles could be due to the polysulfide shuttle which results in instability of the SEI layer on the graphitic anode.
  • Thumbnail Image
    Item
    A Rechargeable Lithium Battery with Li2O2 Cathode in Closed Systems
    (Office of the Vice Chancellor for Research, 2016-04-08) Bhargav, Amruth; Fu, Yongzhu
    Li-O2 batteries have one of the highest theoretical specific energy of 3,458 Wh/kg when the weight of the primary discharge product, i.e., Li2O2, is considered. Thus, this BIL (Beyond Lithium Ion) battery technology, if made practical, will find extensive usage especially in the successful electrification of vehicles. However, there are many challenges. Current Li-O2 batteries demonstrated in labs have been limited to “open systems”, i.e., batteries that have a porous carbon cathode that “breathes” pure oxygen. The limitations of these systems are the requirement of pure oxygen. In addition, the consensus among researchers on specific capacity (mAh/g) calculations based on active materials is lacking because extra oxygen is continuously supplied to cells upon cycling. One solution to these limitations is the use of closed systems, i.e., storage and reuse of O2 within the cell. Recently, our group has demonstrated a closed and rechargeable lithium battery with Li2O2 cathode for the first time. This platform is unique as it shows, for the first time in literature, capacites and rate capability based on mass of Li2O2. The cell shows a close-to-theoretical capacity over 18 cycles and shows 50 cycles when the charge capacity is limited to 50% of theoretical. It allows other studies on the stability of electrolyte, electrode kinetics, and oxygen storage materials. This system can eleminate the issues of open systems such as impurities oxygen gas and evaporation of electrolyte. Unstable electrolytes are a major bottleneck in Li-O2 batteries. Such a system provides a suitable medium to optimize electrolytes and other cell components.
  • Thumbnail Image
    Item
    The unique chemistry of thiuram polysulfides enables energy dense lithium batteries
    (RSC, 2017) Bhargav, Amruth; Ma, Ying; Shashikala, Kollur; Cui, Yi; Losovyj, Yaroslav; Fu, Yongzhu; Engineering Technology, School of Engineering and Technology
    Organosulfur compounds are cheap and abundant cathode materials that can offer high specific energies. Herein, we explore for the first time, the common vulcanization accelerators viz. thiuram polysulfides embedded in carbon nanotubes as binder-free cathodes in lithium batteries that show 3 highly reversible redox reactions (3 discharge plateaus) and high material utilization (up to 97%). We use electrochemical characterization techniques, first-principles calculations, XPS, XRD, FTIR, and SEM to gain insight into the chemical transformations occurring during battery cycling. We identify that the mesomeric form of lithium pentamethylene dithiocarbamate with a positive nitrogen center, formed in the discharge, can act as polysulfide and sulfide anchors through strong coulombic interactions thus enabling a capacity retention of 87% after 100 cycles at C/5 rate. A high loading cathode with an areal capacity of 5.3 mA h cm−2 tested under a low electrolyte to active material ratio of 3 μL mg−1 yields an active material specific energy of 1156 W h kg−1 thus demonstrating the potential of this class of compounds in high specific energy lithium batteries.