Lower reorganization energy raises Marcus electron transfer rate and enables fast reaction kinetics in lithium-organosulfur batteries

Abstract

Organosulfur polymer is an emerging cathode material for lithium-sulfur batteries due to its solid–solid reaction and shuttle effect-free kinetics behavior. Here, The organosulfur polymers polypropyl trisulfide (P3S) and polypropyl tetrasulfide (P4S) containing short-chain sulfur by interfacial polycondensation reaction are synthesized. Then selenium (Se) atoms are introduced into the short-chain sulfur structure and form a P4SSe. Based on Marcus-Gerischer theory calculation, selenium-doping decreases the electron reorganization energy λ of discharge intermediates (CSSe⁻∙) of P4SSe, thereby accelerating the electron transfer rate K_ET and reaction kinetics. DFT calculation demonstrates that selenium atoms can availably improve the frontier molecular orbital energy matching degree between the LUMO level of electrophile (Li⁺) and HOMO level of nucleophile (CSSe⁻∙), thus speeding up the formation of bonding orbital σ_Li-Se. Reduced Gibbs free energy of the discharging reaction of organosulfur polymers and decreased LUMO-HOMO energy gap contribute to accelerating the redox kinetics of organosulfur polymer. The electrochemical performance results reveal that the organosulfur polymer/CNT-composited cathode has good rate stability and capacity reversibility. The P4SSe/CNT cathode exhibits excellent cycling performance with an initial specific capacity of 749.9 mAh g⁻¹ at 1 A g⁻¹, accounting for 70.65 % of the theoretical specific capacity, suggesting that a small amount of selenium doping could improve the Marcus electron transfer rate and cycling specific capacity for the organosulfur polymer/CNT-composited cathode.