Dielectric Increment of Electrolytes Mediated by Ion Association for Lithium–Sulfur Batteries

Electrolyte engineering for long-lifespan alkali-based batteries focuses on modulating the solvation structure to build the electrode/electrolyte interface and dictate interfacial reactions. Previous strategies have relied on increasing the salt concentration to introduce the anion-derived solid electrolyte interphase (SEI) for considerable interfacial stability, but these strategies are restricted by the poor solubility of film-forming salts in weak solvation electrolytes. Herein, a dielectric increment of weak solvation electrolytes based on the ion dissociation and association chemistry is proposed to realize the high salt solubility. Differing from the dielectric decrement with the addition of salts in strong solvation electrolytes owing to reduced free solvents, the dielectric increment in weak solvation electrolytes is a result of the surplus dielectric polarization of contact ion pairs (CIPs). As a demonstration in salt-concentration-sensitive lithium–sulfur (Li–S) batteries, CIPs facilitate the high solubility of lithium polysulfides (LiPSs) and promote the Li₂S₂/Li₂S nucleation. The CIP-induced dielectric increment of electrolytes yields 96% capacity retention after 175 cycles at 0.2 C in the Li–S cell. This underexplored strategy provides effective guidelines for the design of dielectric-constant-mediated electrolytes for alkali-based battery applications.