High‐Efficiency Lithium Sulfur Cells With Suppressed Polysulfide Solubility Using a Cost‐Effective Ammonium‐Based Ionic Liquid Electrolyte

Lithium–sulfur batteries suffer from polysulfide shuttle (PS) and lithium metal anode instability. We developed a mixed‐anion ionic liquid (IL) electrolyte combining N‐trimethyl‐N‐propylammonium bis(fluorosulfonyl)imide (N 1113 FSI) with LiFSI and LiTFSI, enabling stable lithium metal passivation while suppressing polysulfide dissolution to sub‐mM levels. Electrochemical testing demonstrates a specific capacity of 900 mA h g − 1 with 70% retention after 300 cycles. Electrochemical impedance spectroscopy (EIS) reveals a substantial reduction in charge‐transfer resistance post‐lithiation and stable impedance during extended cycling. Raman spectroscopy confirms effective polysulfide suppression. Notably, cell performance is insensitive to the electrolyte‐to‐sulfur ratio (10–45 µL mg − 1 ), in contrast to conventional ether‐based electrolytes. Depth‐resolved x‐ray photoelectron spectroscopy (XPS) reveals that N 1113 FSI forms abundant LiF through efficient anion reduction, yielding a dense, inorganic‐rich solid‐electrolyte interphase (SEI). In contrast, P 111i4 FSI retains incompletely reduced anions and exhibits diminished LiF, demonstrating cation‐dependent control of SEI chemistry. The N 1113 FSI interphase contains higher levels of LiF, Li‐sulfide, oxidized sulfur, and inorganic oxygen species, which correlate with superior cycling stability. Compared to P 111i4 FSI, N 1113 FSI achieves higher discharge capacity (DC), faster Coulombic efficiency (CE) stabilization, and sustained reversibility. Tailored IL design effectively suppresses polysulfide solubility and engineers efficient SEI chemistry, mitigating shuttle effects and enabling stable Li–S operation across varying electrolyte concentrations.