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.