Critical Role of Anion Donicity in LiS Deposition and Sulfur Utilization in Li-S Batteries.

Lithium-sulfur batteries offer a high theoretical gravimetric energy density and low cost, but the full utilization of the sulfur electrode has been limited by the premature passivation of insulating lithium sulfide (LiS). Anion has been one of the major parameters to improve Li-S batteries in addition to solvent, additives, and electrode structures. Here, we reveal the role of anion donicity on the passivation of Li-S battery and its underlying working mechanism. We show that anions with high donicity effectively reduce the charge-transfer resistance during the cycling of Li-S cells and alleviate the LiS passivation by transforming the dense film LiS to porous three-dimensional flake LiS. UV-vis spectroscopy revealed that anions with higher donicity exhibit higher LiS solubility, which is consistent with their stronger bonding to Li, as revealed by nuclear magnetic resonance and density functional theory calculations. Our study reveals the role of anion donicity in LiS passivation and its underlying mechanism, offering rational design consideration for electrolyte salts in achieving high sulfur utilization and high energy efficiency for Li-S batteries.