Development of Novel Cathodes for High Energy Density Lithium Batteries

Abstract

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.