Improved Performance of Lithium Sulfur Battery with Fluorinated Electrolyte

Lithium-ion secondary battery is very promising for energy storage in powering electric vehicles (EV's) due to its high energy density and high durability against many charges and discharge cycles. However, currently these batteries cannot offer a suitably long driving range (i.e., >300 km) for pure electric vehicles. To compete in the market with gasoline-based vehicles, new batteries are required for the next-generation EV’s to provide much higher energy density, and reduce cost factors. ¹

Lithium-sulfur battery is a promising energy storage system due to its superior specific capacity (1675 mAh per gram of sulfur), its wide range of operating temperature, intrinsic overcharge protection and more importantly low cost. However, this battery technology has not matured to date due to several technological barriers such as rapid capacity fading and low coulombic efficiency, which are believed to be mainly associated with the loss of sulfur active material during repeated charge and discharge process through the dissolution of lithium polysulfides into the electrolyte and the side reactions of dissolved polysulfide species with the electrolyte solvent and the lithium anode. ^[2-7]

To mitigate the severe shuttling effect of polysulfides in the conventional electrolyte, a new electrolyte approach was adapted by incorporating a fluorinated solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) in the electrolyte formulation. Based on the density functional theory, the fluorine substitution in ethyl propyl ether lowers both LUMO and HOMO energy levels, resulting in simultaneously high reduction potential and oxidation stability of the fluorinated ether. The theoretical calculation indicates that the fluorinated electrolyte solvents are thermodynamically more favorable for electrochemical reduction reaction forming a passivation layer on both electrodes than their non-fluorinated counterparts under certain voltage conditions. Our experimental results demonstrated that a binary solvent electrolyte comprising of TTE and 1, 3-dioxolane (DOL) displayed a superior cycling performance in Li-S cell employing a carbon/sulfur nanocomposite electrode with 75% of sulfur content. The new fluorinated electrolyte suppressed the deleterious shuttling effect and therefore improved the capacity retention and coulombic efficiency in cell tests. SEM/EDS analysis confirmed the improved performance was due to the detainment of polysulfides inside the electrode after the formation of a passivation layer.  Also, this electrolyte has an amazing effect on the self-discharge of the Li-S cell and to the best of our knowledge; this is the first electrolyte which has shown excellent shelf life at the fully charged stage.

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