MSE/PTFE PH.D. Defense - Jung Tae Lee
School of Materials Science and Engineering
Date: October 29th, 2014
Time: 1 PM
Location: Manufacturing Related Disciplines Complex (MRDC) Room 3515 (Hightower Conference Room)
Committee Members:
Dr. Gleb Yushin (Advisor, MSE)
Dr. Seung Soon Jang (MSE)
Dr. Faisal Alamgir (MSE)
Dr. Thomas Fuller(CHBE)
Dr. Seung Woo Lee(ME)
Title: CHALGOGEN-CARBON NANOCOMPOSITE CATHODES FOR RECHARGEABLE LITHIUM BATTERIES
Summary:
This thesis aims to prepare novel electrode materials, which can demonstrate both high energy and power density with extended life-time based on fundamental understanding on electrochemical reactions of chalcogens, such as sulfur (S) and selenium (Se). Sulfur can provide ~5 times higher theoretical gravimetric capacity compared to the traditional transition metal oxide cathodes. In addition, S is abundant in nature, safe to utilize, and inexpensive. Similar to S, Se can be a promising cathode material because of its high volumetric and gravimetric capacities. Selenium is more advantageous in high power application due to its several orders of magnitude higher electrical conductivity than that of S. The key challenge in electrochemistry of both Li/S and Li/Se is the dissolution of intermediate redox reaction products (polysulfides and polyselenides) in suitable electrolytes and the resulting capacity fading.
All chalcogen cathode materials used in this thesis adopted porous carbon as a hosting material because porous carbon can not only serve as electrically conductive additive but also effectively (although temporarily) suppress the dissolution of intermediate redox reaction products. Studies on structure-property relationships in chalcogen infiltrated porous carbon can provide new insights to develop novel and stable cathode materials. First, this thesis presents the effects of the pore size distribution, pore volume and specific surface area of porous carbons with controlled but randomly shaped bottle neck pores on the temperature-dependent electrochemical performance of S-infiltrated carbon cathodes in electrolytes having different salt concentrations. Additionally, this thesis provides very attractive performance of S infiltrated carbide derived carbon (CDC) cathodes. These CDC comprise both small micropores and small straight mesopores within the individual particles. The effect of CDC synthesis temperature on S utilization was also explored in electrolytes having different salt concentrations. Further improvement in the cycling stability of Li/S battery was made via formation of thin Li-ion permeable but polysulfide non-permeable Al2O3 layer coating on the surface of S infiltrated carbon cathode.
Based on the knowledge obtained from S projects, we have successfully prepared Se infiltrated ordered meso- and microporous silicon carbide derived carbon (CDC) composites and demonstrated high and stable specific capacities of Se-C composites with high molarity electrolytes. Finally, this thesis shows a simple technique to form a protective solid electrolyte layer on the Se cathode surface in-situ. This approach utilizes the favorable properties of fluoroethylene carbonate to convert into a layer that remains permeable to Li ions, but prevents transport of polyselenides, thus enhancing cell cycle stability. As a whole, this dissertation expands our current understanding of correlations with multiple cell parameters, such as the structure of cathode, the composition of electrolyte, and operating temperature on the performance of lithium-chalcogen batteries and demonstrates the possibility of improving performance via optimizing these components.