Fundamental Understanding of Li-S Battery Electrochemistry
Authors: Torres-Castro Loraine , Jones Simon C. , Pasalic Jasmina , Bugga Ratnakumar V 
Published: February 2020

In recent years, commercial realization of rechargeable batteries has revolutionized portable electronic industry affecting mostly microelectronics. However, issues such as safety, high/low temperature performance, cost, and environmental concerns are still being addressed, particularly for high power and high energy applications. Li-ion batteries have been successfully employed in several Mars surface missions, e.g., Mars Exploration Rovers (Spirit and Opportunity), Mars Phoenix Lander, and more recently in the Mars Science Laboratory rover (Curiosity) [1]. The diverse benefits in gravimetric and volumetric densities combined with good low temperature performance of Li-ion batteries, contribute to reduced launch costs, increased payload and science capabilities in the space missions. However, NASA’s upcoming missions need energy storage systems with enhanced performance capability, especially higher specific energies and energy densities. A direct application of the technology involves the astronaut Extra Vehicular Activity (EVA), wherein the astronaut’s Portable Life Support System (PLSS) is expected to support 8 hours of EVA. The state of art batteries, with ~200 Wh/kg at the cell level, can support only four hours of EVA.
Among the battery systems, the Li-S is the most attractive due to the high theoretical capacity of sulfur, i.e., 1672mAh/g (~10 times higher than LiCoO2), abundance and low toxicity [2-5]. However, some issues such as (i) mechanical stress on the cathode, (ii) dissolution of sulfur products with the electrolyte, forming redox shuttles have to be understood and mitigated. In recent years, extensive efforts have been made to extend the cycle life of Li-S cells using a variety of approaches. These include new cathode designs with porous carbon structures to contain sulfur and its reduced products, anode coatings to protect against dissolved polysulfides and suitable electrolyte solutions, all of which contributed to improved cycle life in laboratory cells. In the present study, we focused on developing a high energy rechargeable Li-S battery with moderate cycle life that can offer considerable performance enhancements beyond the conventional and advanced Li-ion batteries. To achieve our goal, various advanced cell components for Li-S cell are being developed: (i) a protected Li anode with an intimate thin polymer electrolyte coating stable towards Li organic solvents, (ii) new electrolytes with reduced solubility for polysulfides, (iii) new electrolyte additives to reduce polysulfides shuttle effects and/or to function as redox catalysts for enhancing the sulfide reversibility, and (iv) durable sulfur cathode configurations with a nanoporous and nanostructured substrate (typically carbon or metal) and/ or cathodes blended with electro-active mediators to contain/constrain sulfur and its reduced products. These approaches were studied and analyzed to understand how the enhanced components affects the structure of the cathode, the dissolution of polysulfides in the electrolyte, the kinetics and the electrochemical performance. The results will be presented in detail.
1. B. V. Ratnakumar, W. C. West, P. DeGrosse Jr., M. C. Smart, L. Jones and R.C. Ewell, NASA Battery Workshop, Huntsville, AL, November 6, (2012)
2. Brian L. Ellis, Kyu Tae Lee, and Linda F. Nazar, Positive Electrode Materials for Li-Ion and Li-Batteries, Chem. Mater. 22 (2010) 691–714.
3. Ho Suk Ryu, Zaiping Guo, Hyo Jun Ahn, Gyu Bong Cho, Huakun Liu, Investigation of discharge reaction mechanism of lithium/liquid electrolyte/sulfur battery, J. Power Sources 189 (2009) 1179–1183.
4. A. Manthiram, Y. Z. Fu and Y. S. Su, Challenges and Prospects of Lithium-Sulfur Batteries, Acc. Chem. Res 46 (2013) 1125–1134.
5. Y. X. Yin, S. Xin, Y. G. Guo and L. J. Wan, Angew, Lithium–Sulfur Batteries: Electrochemistry, Materials and Prospects, Chem., Int. Ed., 52 (2013) 13186–13200.