Electrochemical Reaction and System for Conversion of Sulfur Hexafluoride

Technology 

This invention establishes candidate electrolytes in which sulfur hexafluoride may be dissolved to permit electrochemical reactivity and battery development. Solubility of sulfur hexafluoride in tetraethylene glycol dimethyl ether (TEGDME) is comparable to the solubility of 1-10 mM oxygen gas in various nonaqueous electrolytes for lithium-oxygen batteries, thereby facilitating electrochemical reduction of sulfur hexafluoride. Lithium-sulfur-hexafluoride cells are constructed using a pre-stabilized lithium metal anode, a Vulcan carbon cathode, and non-aqueous battery electrolyte such as lithium perchlorate (LiClO4) in TEGDME. The resulting lithium-sulfur-hexafluoride cell exhibits significantly higher discharge capacities of up to ~2500 mAh/gcarbon at 5mA/gcarbon  compared to Li cells discharged under argon (<30 mAh/gcarbon).  

Additionally, the reduction of sulfur hexafluoride on select metal electrode surfaces at room temperature is demonstrated. These electrochemical reactions occur with a carbon electrode and without the need of metal catalysts, generating benign carbon/metal-fluorides within the electrode. The nonaqueous environment of the system therefore hinders the formation of toxic products like hydrogen sulfide (H2S) and hydrogen fluoride (HF). 

Problem Addressed

Maturation of commercialized primary lithium-based batteries like lithium-thionyl chloride and lithium-carbon monofluoride has ultimately led to the tapering off of energy density gains in recent years. This invention utilizes the reaction and destruction of sulfur hexafluoride, a non-reactive greenhouse gas which has been difficult to activate with current chemistry methods, to increase the discharge capacities of lithium batteries. As a result, it addresses two challenges, (1) the capture and destruction of a greenhouse gas, and (2) the increase of lithium battery energy-density. 

Advantages

• Electrochemically reducing conditions, rather than high temperatures, drive molecular activation and reaction of sulfur hexafluoride (SF6) at room temperature with high yields
• Nonaqueous system hinders formation of toxic products like hydrogen sulfide (H2S) and hydrogen fluoride (HF)
• Sulfur hexafluoride mitigation technology developed extracts energy from the molecule which can be applied to increasing the capacity and energy density of batteries

Publications

Li, Y., Khurram, A., & Gallant, B. M. (2018). A High-Capacity Lithium–Gas Battery Based on Sulfur Fluoride Conversion. The Journal of Physical Chemistry C, 122(13), 7128-7138. doi: 10.1021/acs.jpcc.8b00569.

Gao, H., Li, Y., Guo, R., Gallant, B. M. (2019). Controlling Fluoride-Forming Reactions for Improved Rate Capability in Lithium-Perfluorinated Gas Conversion Batteries. Adv. Energy Mater., 9, 1900393. doi: 10.1002/aenm.201900393.