A LiFSI-based ultralight electrolyte for long-cycle-life and high-energy-density lithium-sulfur batteries

Abstract

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 34.5% 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.