Roles of the Polymer\nBackbone for Sulfurized Polyacrylonitrile\nCathodes in Rechargeable Lithium Batteries

Sulfurized polyacrylonitrile (SPAN) has emerged as a\nhighly promising\ncathode material for next-generation lithium–sulfur (Li–S)\nbatteries primarily due to its non-polysulfide dissolution and excellent\ncycle stability. Nevertheless, the specific roles and impacts of the\npyrolyzed polyacrylonitrile, which constitutes the polymer backbone\nof SPAN, remain inadequately understood. In this study, comprehensive\ninvestigations from multiple aspects, including electrochemistry,\nspectroscopy, electron microscopy, and theoretical calculations, were\nconducted on a series of SPAN materials with various sulfur contents.\nThe results reveal that the polymer backbone serves at least four\ncritical functions. First, during the synthesis of SPAN, the polymer\nbackbone provides reactive sites for the incorporation of sulfur through\nchemical bonding. Second, it establishes an extensive π-conjugated\nnetwork via a dehydrogenation reaction by sulfur and serves as a conductive\nframework for SPAN. The chemically bonded sulfur atoms dope the polymer\nbackbone, which narrows the highest occupied molecular orbitals–lowest\nunoccupied molecular orbitals (HOMO–LUMO) energy gap. Third,\nthe polymer backbone plays an essential role in determining the first\nCoulombic efficiency, and the irreversibly inserted Li atoms further\ndope the polymer backbone and reduce the HOMO–LUMO energy gap.\nLastly, the pyridine nitrogen within the polymer backbone exhibits\nan adsorption effect on lithium sulfide (Li<sub>2</sub>S) species,\nwhich stabilizes the cycling performances of the SPAN cathode. It\nis worth noting that the polymer backbone hardly contributes reversible\ncapacity for the SPAN cathode in carbonate electrolytes within the\n1 to 3 V operating voltage range. This study enhances our understanding\nof SPAN and may provide valuable guidance for the development of better\nSPAN materials and rechargeable batteries.