Breaking Quasi‐Chelation in Thioether Electrolytes for Synergistic Interfacial Stabilization in Lithium‐Sulfur Batteries

Lithium‐sulfur batteries (LSBs) have attracted extensive attention owing to their high energy density. However, their development remains severely hindered by sluggish interfacial reaction kinetics, unstable electrode–electrolyte interphases, lithium dendrite growth, and polysulfide shuttling. In this work, we identify a hidden quasi‐chelation effect in ether‐based nitrile solvents, which undermines the effectiveness of weakly solvating electrolyte design, and demonstrate that this limitation can be effectively overcome through a thioether‐substitution strategy. Specifically, replacing the ether oxygen in methoxyacetonitrile with sulfur to form (methylthio)acetonitrile disrupts the quasi‐chelating coordination motif and weakens solvent‐Li + interactions, and induces anion‐rich solvation structures. Spectroscopic characterizations combined with molecular dynamics simulations reveal that breaking this quasi‐chelation effect lowers the Li + desolvation barrier and accelerates ion‐transport kinetics. Meanwhile, the preferential decomposition of the thioether solvent promotes the formation of sulfur‐rich, inorganic‐dominated interphases, thereby enabling simultaneous stabilization of both the lithium metal anode and the SPAN cathode. As a result, lithium dendrite growth and polysulfide shuttling are effectively suppressed, and Li||SPAN full cells retain 86% of their initial capacity after 600 cycles under high‐loading conditions. This work demonstrates that quasi‐chelation breaking can serve as an effective molecular design principle for constructing weakly solvating electrolytes toward practical lithium‐sulfur batteries.