Lithium-sulfur (Li-S) batteries are promising candidates for high-energy storage; however, the high electrolyte uptake of porous S cathodes significantly limits their practical energy density. Although ultralight electrolytes (ULEs) can address this issue, they often suffer from low ionic conductivity, unstable interphases, and sluggish kinetics. This study presents a ULE design based on lithium bis(fluorosulfonyl)imide (LiFSI) salt, which simultaneously achieves a low density (0.89 g cm -3) and high Li + conductivity (7.05 mS cm -1). The LiFSI salt facilitates the formation of a LiF-rich solid electrolyte interphase on the Li metal anode, effectively suppressing polysulfide corrosion and enhancing cycle life. Furthermore, its high donor number improves polysulfide solubility, accelerating conversion kinetics and increasing capacity utilization. As a result, high-loading S cathodes (5 mg cm -2) deliver an initial capacity of 1180 mAh g -1 and retain 70.63% of this capacity after 200 cycles. Pouch cells with the LiFSI-ULE exhibit a 42.5% higher energy density and a 133% longer cycle life compared to those with conventional electrolytes. This study successfully extends the application of LiFSI to Li-S batteries, offering a viable pathway toward long-cycling, high-energy-density energy storage.

Lithium-sulfur (Li-S) battery provides a promising solution toward high-energy and cost-effective energy storage. Unlike dense electrodes of lithium-ion batteries, the highly porous S cathode of Li-S battery uptakes a large amount of electrolyte, which significantly lowers the energy density of the battery. To address this challenge, we report here an ultralight electrolyte (ULE) that simultaneously possesses a low mass density and high Li + ion conductivity. ULE utilizes the lightweight lithium-bis(fluorosulfony)imide (LiFSI) salt and removes the heavy 1,3-dioxolan (DOL) solvent, therefore significantly reducing the density from 1.20 to 0.89 g cm -1. Moreover, LiFSI offers LiF-rich interphase at the Li metal anode to suppress the polysulfide corrosion, and its high donor number also improve the solubility of polysulfides to enhance the capacity of the S cathode. The exclusion of DOL can circumvent the harmful DOL polymerization and S cathode agglomeration. Consequently, pouch cells show 42.5% higher energy density and 133% longer cycle life in ULE than in conventional electrolyte.