Full Dissolution of the Whole Lithium Sulfide Family (Li2S8 to Li2S) 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 Li2S2/Li2S 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 (Li2S8–Li2S). 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. Electrolyte my fire: A new safe eutectic solvent of ϵ‐caprolactam/acetamide can act as an electrolyte for lithium–sulfur batteries. It shows strong resistance to fire, and can dissolve the whole lithium sulfide family (Li2S8–Li2S). It is low‐cost and environmentally friendly. By using this eutectic solvent, capacitance retentions of 68 % over 200 cycles and 95 % over 40 cycles were achieved with and without electrode modification, respectively.