Critical Role of Anion Donicity in Li2S 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 (Li2S). 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 Li2S passivation by transforming the dense film Li2S to porous three-dimensional flake Li2S. UV-vis spectroscopy revealed that anions with higher donicity exhibit higher Li2S4 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 Li2S 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.