Incorporating Improved Chemical and Electrochemical Reaction Schemes in Electrochemical Engineering Models for Lithium Sulfur Batteries
Authors: Parke Caitlin D. , Subramaniam Akshay , Subramanian Venkat R. , Schwartz Daniel T. 
Published: July 2021

Lithium sulfur batteries have gained popularity due to the promise of high energy density, predicted to be 2-3 times than of lithium ion batteries.1 The challenges that limit commercial success for lithium sulfur batteries stem from the poor cyclability of lithium metal, insulating sulfur products, and the polysulfide shuttle mechanism where soluble sulfur species react with the lithium metal anode, resulting in permanent active material loss.2 The complexity of the challenges facing the adoption of lithium sulfur batteries has led to a focus on electrolyte engineering, additives, and anode coatings. The complex reaction pathway is sensitive to changes in electrolyte and additives, which can alter the shape of the voltage curve and even capacity.3,4 Understanding the effects of different electrolyte systems is important to characterizing the performance. Various techniques like UV-vis and Raman spectroscopy are useful to identifying the speciation. The complex speciation pathway has shown to include dissociation and disproportionation reactions and stabilization of radical species through UV-vis and Raman spectroscopy.5–10

Physics-based modeling can help elucidate underlying phenomena within batteries. Kumaresan and White11 developed a one-dimensional discharge model for lithium sulfur batteries. The model utilized included 5-step reduction scheme with Sn
2- species (n=8,6,4,2,1). The subsequent models that were developed used a similar reaction scheme or a reduced reaction scheme with only 2 or 3 reactions. The chemistry has not been updated to include the new insights available from spectroscopy work. In this work, the following homogeneous dissociation reaction that has been identified in many sulfur electrochemical systems12–17 is added to the reaction scheme

S6
2- ↔ S3
·- (1)

The S6
2- species balance is modified to include a sink for the equilibrium reaction according to reaction (1). The material balance for S3
·- radical species is added with the only source as the equilibrium reaction; the S3
·- species is assumed to be electrochemically inactive. When there is significant S3
·- species, the S6
2- species essentially becomes trapped as the S3
·- radical species, removing it from the reduction scheme. When the reaction is pushed farther to S3
·- radical species, the electrolyte system favors the stabilization of this radical species over the S6
2- species. The forward rate of reaction (1) sets the reaction timescale, and the equilibrium constant determines the ratio of S3
·- and S6
2-. The resulting interplay of these dynamics on the electrochemical behavior of the battery cell will be explored, and the implications on electrolyte engineering will be discussed.

Acknowledgments: The authors are thankful to the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) for funding of this work.

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