Advances in energy storage (ES) devices have the potential to transform nearly every aspect of society, from transportation to communication to electricity delivery, national defense and domestic security. Among the various prevalent energy storage devices, the most prominent to date are related to electrochemical energy storage (EES) technologies. Several EES technologies are either in existence or have evolved over the years. Among the various systems studied, lithium battery technologies (LBs) have emerged in the forefront as a panacea to the high energy and high power problems facing portable electronics, electric powered vehicle, military applications as well as stand-alone stationary power systems integrated into the electric grid.

Despite advances in the anode arena, lithium metal anodes due to the inherent dendrite formation limitations have never attained commercial status. Overcoming these barriers would be a major breakthrough in the search for high energy density anode systems due to its extremely high theoretical specific capacity (~3860 mAh/g) and the lowest negative electrochemical potential (-3.040 V vs. the standard hydrogen electrode). Similarly, pursuit of high energy density rechargeable lithium metal battery (LMB) cathodes have led to sulfur (for a Li-S battery), air electrode (for a Li-air battery) or intercalation compounds (e.g. NbSe₃, V₂O₅) to realize high voltage LMBs exhibiting high energy densities. Li-S batteries, owing to their high theoretical energy density (2600 Wh kg^-1)¹, is considered as one of the most promising Li metal-based batteries. However, the liquid organic electrolytes, S cathodes and Li metal in current LMBs could not be commercialized globally for high energy applications due to low cycle life and safety issues. The low cycle life, low coulombic efficiency (CE) and safety issue of rechargeable LMBs mainly arises due to the uncontrolled cellular and dendritic growth of Li metal during repeated lithium plating/stripping and formation of unstable solid electrolyte interphase (SEI) and the continuous growth of thick SEI during repeated cycling at the solid liquid interface of organic electrolyte based LMBs. In addition, 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, combined with the shuttle effect of highly soluble polysulfide species in the organic ether-based electrolytes has limited the use of Li-S batteries. To address these challenges facing 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 polymer electrolytes⁸ and electrolytes additives have also been studied to develop a viable Li-S battery system. Thin-film solid-polymer electrolyte batteries offer the potential for improved safety because of the reduced activity of lithium with the solid electrolyte, flexibility in design as the cell can be fabricated in various sizes and shapes, and high energy density.

In this work, we demonstrate the use of a composite polymer electrolyte (CPE) with modified physical and chemical properties in Li-S batteries. These CPEs exhibits superior mechanical properties, excellent room-temperature lithium ion conductivity and low electrolyte: sulfur (E:S) ratio. These CPEs, when used as electrolytes for Li-S batteries, helps prevent both polysulfide dissolution and dendrite formation, in addition to providing very high energy density (~750 Wh/kg). Structural, chemical, physical and electrochemical characterization results validating these properties of the CPEs 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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