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Two Competing Reactions of Sulfurized Polyacrylonitrile Produce High-Performance Lithium-Sulfur Batteries.

Sulfurized polyacrylonitrile (SPAN) is an attractive cathode candidate for the advanced lithium-sulfur (Li-S) batteries owing to its outstanding cyclic stability. Nevertheless, SPAN suffers from inadequate initial Coulombic efficiency (CE) induced by the sluggish reaction kinetics, which is primarily ascribed to the low Li-ion diffusivity and high electronic resistivity of the Li2S product. In this work, an optimal trace amount of soluble lithium polysulfide of Li2S8 is introduced as a redox mediator for a freestanding fibrous SPAN cathode to enhance the reversible oxidation efficiency of Li2S. During the delithiation process, the chemical interactions between Li2S and Li2S8 additive facilitate the electrochemical oxidation of Li2S, resulting in the transformation of not only C-S/S-S bonds in SPAN but also elemental sulfur. Benefiting from the synergistic effect of the two competing reactions, a high initial CE of 82.9% could be achieved at a current density of 200 mA g-1. Moreover, a superior capacity retention along with a high capacity of 1170 mAh g-1 up to the 400th cycle is available at 1000 mA g-1. The study offers a feasible approach for Li-S batteries toward the practical applications of SPAN.