Full Dissolution of the Whole Lithium Sulfide Family (Li ₂ S ₈ to Li ₂ S) in a Safe Eutectic Solvent for Rechargeable Lithium–Sulfur Batteries

The lithium–sulfur battery is an attractive option for next-generation energy storage owing to its much higher theoretical energy density than state-of-the-art lithium-ion batteries. However, the massive volume changes of the sulfur cathode and the uncontrollable deposition of Li ₂ S ₂ /Li ₂ S significantly deteriorate cycling life and increase voltage polarization. To address these challenges, we develop an ϵ-caprolactam/acetamide based eutectic-solvent electrolyte, which can dissolve all lithium polysulfides and lithium sulfide (Li ₂ S ₈ –Li ₂ S). With this new electrolyte, high specific capacity (1360 mAh g ⁻¹ ) and reasonable cycling stability are achieved. Moreover, in contrast to conventional ether electrolyte with a low flash point (ca. 2 °C), such low-cost eutectic-solvent-based electrolyte is difficult to ignite, and thus can dramatically enhance battery safety. This research provides a new approach to improving lithium–sulfur batteries in aspects of both safety and performance.