Engineering Electrolytes with Transition Metal Ions for the Rapid Sulfur Redox and In Situ Solidification of Polysulfides in Lithium-Sulfur Batteries.

Abstract

Lithium-sulfur (Li-S) batteries have been pursued due to their high theoretical energy density and superb cost-effectiveness. However, the dissolution-conversion mechanism of sulfur inevitably leads to shuttle effects and interface passivation issues, which impede Li-S battery practical application. Herein, the approach of adopting transition metal salts (CoI₂) to engineering the electrolyte is proposed. Different from anchored transition metal catalysts in the cathode, soluble cobalt ions can chemically reduce and solidify polysulfides, alleviating the dependence of sulfur conversion on the conductive interface while suppressing the shuttle effect. Importantly, all elements in CoI₂ are in the lowest valence state and solid complexes are formed after the redox reaction, which prevents the migration of high valent Co³⁺ to the anode, thus overcoming the poor compatibility between redox mediator and Li anode. Notably, I₃^- has the function of eliminating dead sulfur and dead lithium, which we apply to Li-S batteries. After activating I₃^- at a certain frequency, Li-S batteries indeed achieve a longer and more stable cycle life. By combining the regulatory behavior of anions and cations, the electrolyte is engineered for Li-S batteries with high capacity, long lifespan, and excellent rate performance.