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Rechargeable lithium-metal batteries have gained significant attention as potential candidates of energy storage systems; however, severe safety issues including flammable electrolyte and dendritic lithium formation hinder their further practical application. In this work, we develop a novel intrinsic flame-retardant electrolyte, which enables a stable and dendrite-free cycling with lithium plating/stripping Coulombic efficiency of up to 99.1% over 500 cycles. Raman spectra indicate that no free molecular solvent exists, and X-ray photoelectron spectroscopy reveals the LiF-rich interphase on the Li-metal anode. When coupled with sulfurized pyrolyzed poly­(acrylonitrile) cathode, it shows a benign electrochemical reversibility with the areal capacity of up to 3.41 mAh cm–2 after 70 cycles. To further check its compatibility with sulfur cathode, a higher sulfur content (51.6%) is examined with the areal capacity of 3.92 mAh cm–2 and sulfur utilization of 81.7%. This work provides an alternative for safe and high-performance Li–S batteries via a novel electrolyte strategy.

Sulfurized pyrolyzed poly(acrylonitrile) (S@pPAN) demonstrates high sulfur utilization, no polysulfide dissolution, no self-discharge, and extremely stable cycling. Its precursor, PAN, directly determines the performances of cathode materials, including the sulfur content and its utilization for S@pPAN composite materials. Adopting PAN with the molecular weight approaching 550,000 as the precursor, the sulfur content in S@pPAN approaches 55 wt %, and its reversible specific capacity was 901 mAh g −1 (composite) at 50 °C with sulfur utilization over 98%. Moreover, it enabled stable cycling and excellent high rate capability with a specific capacity of 645 mAh g −1 at 5 C. These significantly enhanced electrochemical properties are mainly due to the high molecular weight of the PAN precursor, which provides more space to accommodate amorphous sulfur, along with improved interfacial resistance of S@pPAN.