A Rechargeable Lithium Battery with Li2O2 Cathode in Closed Systems

poster abstractLi-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

Organosulfur materials for rechargeable metal-sulfur batteries

Lithium-ion batteries have become the powerhouse of modern human life. The ubiquity of personal electronic devices, the push to electrify the transportation sector, and the need to implement energy storage to make the electric grid resilient while also encouraging the deployment of renewable energy sources have made batteries more important than ever. To support this growth, a worldwide concerted effort is underway to develop batteries that are energy-dense, efficient, cost-effective, safe, recyclable, and most importantly sustainable for use by future generations. In this regard, metal-sulfur batteries check all the boxes. The low molecular mass of sulfur ensures an extremely high theoretical energy density, the redox chemistry is highly reversible and efficient, and finally, the high natural abundance of sulfur presents a low-cost and environmentally benign alternative to the current Li-ion battery materials. But these impressive characteristics are not without challenges. Sulfur chemistry suffers from the “shuttle effect”, poor electronic and ionic conductivities, large volume changes during battery cycling, and limited potential to tinker with the inherent chemistry. The rich and bountiful nature of organic chemistry may possess the key to solving some of these challenges through the use of organosulfur materials. Organopolysulfide materials can 6 minimize the “shuttle effect” owing to their bulkiness, improve both electronic and ionic conductivity, suffer lesser volume changes, and allow for the tuning of redox potential and other properties. In this dissertation, we examine how uniquely tuned organic functional groups affect the performance of batteries. The structure-property relationships were probed using different electrochemical and materials characterization methodologies. We learn that the organic groups have a deep impact on the redox chemistry of sulfur, so organopolysulfide materials can significantly enhance the performance of metal-sulfur batteries. The commercial viability of organopolysulfide materials was determined by testing batteries with practically necessary cell parameters and by fabricating prototype pouch cells wherever possible. This material class shows significant promise in overcoming the limitations of metal-sulfur batteries. We hope that these studies will add to the body of knowledge that will inspire researchers to develop battery materials that ensure the energy security of our civilization.Materials Science and Engineerin

A Graphite-Polysulfide Full Cell with DME-Based Electrolyte

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

The unique chemistry of thiuram polysulfides enables energy dense lithium batteries

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

Bis(aryl) Tetrasulfides as Cathode Materials for Rechargeable Lithium Batteries

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

Development of novel cathodes for high energy density lithium batteries

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 polysulfide 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

Scalable Metal Phosphides as a Dual-Function Catalyst and Lithium–Metal Stabilizer for Lithium–Sulfur Batteries

Although lithium–sulfur batteries are a promising approach for achieving a high energy density, their commercial viability is limited by poor sulfur redox kinetics, polysulfide shuttling, and lithium metal instability. To resolve these issues concurrently, we utilize titanium phosphides (TiPx) as dual-function materials to catalyze the sulfur redox kinetics at the cathode and stabilize the lithium metal at the anode. Importantly, these phosphides are synthesized via a facile, highly scalable, one-step mechanochemical ball-milling process that does not generate toxic phosphine gas as a byproduct, making it more viable compared with traditional synthesis routes. At the cathode, we find that higher phosphorus contents in the phosphide as in TiP2 leads to superior redox kinetics and reduced polysulfide shuttling due to improved polysulfide adsorption ability. Meanwhile, at the anode, TiP2 acts as a lithiophilic seed in a three-dimensional carbonaceous host that can be lithiated to form a Li-TiP2/C composite. This composite alleviates the volume changes of lithium metal by utilizing TiP2 to form a more favorable solid–electrolyte interface. Full coin cells employing TiP2 in the cathode and anode were assembled with a sulfur loading of 4 mg cm–2 and a negative to positive capacity (N/P) ratio of 6. After 50 cycles, the TiP2 full cells retain a higher capacity of 643 mA h g–1 compared to 422 mA h g–1 for the conventional cell, despite the higher N/P ratio of 12 for the conventional cell. Overall, we showcase the viability of employing metal phosphides as catalysts in practical lithium–sulfur batteries

A Class of Organopolysulfides As Liquid Cathode Materials for High-Energy-Density Lithium Batteries

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 C₆H₅S_<i>x</i>C₆H₅ (4 ≤ <i>x</i> ≤ 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 1<i>C</i> rate along with superlative performance up to 10<i>C</i>. 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

Bis(aryl) Tetrasulfides as Cathode Materials for Rechargeable Lithium Batteries

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

Highly Reversible Diphenyl Trisulfide Catholyte for Rechargeable Lithium Batteries

Organotrisulfide (RSSSR) is a new class of high-capacity cathode materials for rechargeable lithium batteries. The organic R group can tune the structure and property of organotrisulfide as well as electrochemical performance in batteries. Herein, a nominal diphenyl trisulfide (DPTS, C₆H₅SSSC₆H₅) catholyte is reported for rechargeable lithium batteries. Three sulfur atoms allow 4e^– storage per molecule, affording DPTS with a theoretical capacity of 428 mAh g^–1. The DPTS catholyte is synthesized from a coupling reaction of diphenyl disulfide (DPDS) and elemental sulfur in liquid electrolyte at 70 °C. It is found that the DPTS catholyte is a mixture of DPTS, DPDS, and elemental sulfur in the electrolyte. The lithium cell with the DPTS catholyte delivers an initial specific capacity of 330 mAh g^–1_DPTS and retains 79% of the initial capacity over 100 cycles at the C/2 rate. The cell delivers an initial discharge specific energy of 751 Wh kg^–1_DPTS with a high energy efficiency of over 95% at the C/5 rate. The achievable energy density of the DPTS catholyte (1.0 M) is 158 Wh L^–1. This study shows that DPTS is a promising high-capacity cathode material for highly reversible lithium batteries