Copolymerization of Polythiophene and Sulfur To Improve the Electrochemical Performance in Lithium–Sulfur Batteries

Abstract

We first report on the copolymerization of sulfur and allyl-terminated poly­(3-hexylthiophene-2,5-diyl) (P3HT) derived by Grignard metathesis polymerization. This copolymerization is enabled by the conversion of sulfur radicals formed by thermolytic cleavage of S₈ rings with allyl end-group. The formation of a C–S bond in the copolymer is characterized by a variety of methods, including NMR spectroscopy, size exclusion chromatography, and near-edge X-ray absorption fine spectroscopy. The <b>S-P3HT</b> copolymer is applied as an additive to sulfur as cathode material in lithium–sulfur batteries and compared to the use of a simple mixture of sulfur and P3HT, in which sulfur and P3HT were not covalently linked. While P3HT is incompatible with elementary sulfur, the new <b>S-P3HT</b> copolymer can be well dispersed in sulfur, at least on the sub-micrometer level. Sulfur batteries containing the <b>S-P3HT</b> copolymer exhibit an enhanced battery performance with respect to the cycling performance at 0.5C (799 mAh g^–1 after 100 cycles for <b>S-P3HT</b> copolymer versus only 544 mAh g^–1 for the simple mixture) and the C-rate performance. This is attributed to the attractive interaction between polysulfides and P3HT hindering the dissolution of polysulfides and the charge transfer (proven by electrochemical impedance spectroscopy) due to the homogeneous incorporation of P3HT into sulfur by covalently linking sulfur and P3HT