High-performance rechargeable batteries are in great demand for portable electronic devices, electrical vehicles and grid-scale energy storage systems. Lithium sulfur (Li-S) batteries is considered as one of the most promising Li metal-based batteries due to their high theoretical energy density (2600 Wh kg^-1)¹, low cost and natural abundance². However, the large volume expansion (~80%)³ of sulfur accompanying the electrochemical cycling, the low utilization of sulfur resulting from poor room – temperature electronic conductivity of sulfur (~10^-15 S/cm)⁴, shuttle effect of highly soluble polysulfides in the organic ether-based electrolytes and Li dendrite growth results in fast capacity fading leading to safety concerns. As a result, commercialization of Li-S batteries thus far has been largely limited.

To address these challenges facing the sulfur cathodes, significant efforts have been made to demonstrate advanced composite cathodes using various carbon materials⁵, polymers⁶ and metal-organic framework (MOF) materials⁷. In addition, solid electrolytes⁸ and electrolytes additives have also been identified to engineer a viable Li-S battery system.

Despite these efforts to improve the performance of sulfur cathodes, suppression of dendrites growth on the lithium anode is the most crucial factor for practical application of Li-S batteries. Li metal, due to its high reactivity interacts instantly with most organic electrolyte solvents and Li salts to form a solid electrolyte interphase (SEI) layer that prevents further consumption of Li and electrolyte⁹. However, the SEI layer cannot withstand the volume change from repeated Li plating/stripping process due to its low mechanical strength. Hence, Li ions would aggregate toward the fresh Li exposed to the electrolyte due to the “tip effect”¹⁰ leading to uncontrolled Li dendrite growth. The dendrites protruding from the anode surface may pierce through the separator and cause internal short circuits and safety concerns. Due to the low utilization of Li metal and rapid consumption of electrolyte resulting from the Li dendrite growth, practical application of lithium metal as anode is highly restricted.

In this work, we demonstrate the use of a composite polymer electrolyte (CPE) with fine-tuned physical and chemical properties to act as a separator-electrolyte complex in Li-S batteries. The superior mechanical properties of the CPEs coupled with excellent electrochemical conductivity and low electrolyte: sulfur (E:S) ratio will significantly facilitate prevention of both polysulfide dissolution and dendrite formation. Results of these studies will be presented and discussed.

Acknowledgements: The authors acknowledge the financial support of DOE grant DE-EE 0006825 and DE-EE-0008199, Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM).

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