Origin of shuttle-free sulfurized polyacrylonitrile in lithium-sulfur batteries

Abstract

Sulfurized polyacrylonitrile (S-cPAN) shows an intrinsic shuttle-free capability during cycling with high reversible capacity, making it a promising material for lithium-sulfur (Li–S) battery. However, the lithiation/delithiation mechanism of S-cPAN is still debatable and unclear. In this work, the fundamental reaction mechanism of S-cPAN cathode material is unveiled by in-situ Raman and in-situ X-ray absorption (XAS) spectroscopies. Together with density functional theory calculation, the formation of -N-Sx-N- (x < 4) bridges besides C–S- and –S-S- bonds during the synthesis process is proposed. These sulfur-nitrogen bonds and their strong interactions in the S-cPAN compounds are first observed to account for the proposed solid-solid transformation during the lithiation/delithiation of S-cPAN. Surprisingly, the cPAN backbone is also found to be involved in the charge compensation while the ordered Li2S along the nitrogen edge on the PAN matrix is suggested to form when S-cPAN is fully lithiated. The proposed modified mechanism deciphers the outstanding electrochemical performance of S-cPAN, providing a new pathway for designing high capacity, shuttle-free cathode materials for next-generation Li–S batteries, and a new perspective of sulfur chemistry.