Thermal Study of Reaction Intermediate Stability during Electrochemical Sulfur Reduction in a Lithium-Sulfur Cell

One of the most promising cell chemistries in the quest to surpass Lithium-ion cells in capacity is the Lithium-sulfur cell. The use of Sulfur, due to its high theoretical specific capacity (1675 mAh/g) and natural abundance, is the basis for this future potential. A step towards future development of this cell is the understanding of the complex reactions that occur at the sulfur cathode. The overall mechanism is the electrochemical reduction of octa-sulfur (S₈) to Li₂S with several intermediate lithium-sulfur composites. Prior research has involved the study of the plateaus and characteristics of the voltage profiles of these cells to try and elucidate the reaction mechanism.  Isothermal micro-calorimetry is a versatile tool to study reactions and understand their thermal behavior. Thermodynamic study of these reactions had been reported by the authors elsewhere. Herein we examine the stability of each intermediate by calculating the entropy change of their reaction step.  

The entropy change has negative value in the first and second electrochemical reaction stages which generate soluble lithium sulfur (Li₂S₈ and Li₂S₄). It changes to positive value when the electrochemical reaction produces insoluble lithium sulfur (Li₂S₂ and Li₂S). The entropy change drops when DOD is 70%. This entropy change pattern exactly corresponds with the voltage profile with various potential plateaus which represents the electrochemical reaction stages. The entropy change values provide a fundamental understanding of the drivers of the electrochemical reaction, can be used to understand the stability of the various intermediates and also predict any new intermediates or reaction steps.