Roles of the Polymer Backbone for Sulfurized Polyacrylonitrile Cathodes in Rechargeable Lithium Batteries

Sulfurized polyacrylonitrile (SPAN) has emerged as a highly promising cathode material for next-generation lithium-sulfur (Li-S) batteries primarily due to its non-polysulfide dissolution and excellent cycle stability. Nevertheless, the specific roles and impacts of the pyrolyzed polyacrylonitrile, which constitutes the polymer backbone of SPAN, remain inadequately understood. In this study, comprehensive investigations from multiple aspects, including electrochemistry, spectroscopy, electron microscopy, and theoretical calculations, were conducted on a series of SPAN materials with various sulfur contents. The results reveal that the polymer backbone serves at least four critical functions. First, during the synthesis of SPAN, the polymer backbone provides reactive sites for the incorporation of sulfur through chemical bonding. Second, it establishes an extensive π-conjugated network via a dehydrogenation reaction by sulfur and serves as a conductive framework for SPAN. The chemically bonded sulfur atoms dope the polymer backbone, which narrows the highest occupied molecular orbitals-lowest unoccupied molecular orbitals (HOMO-LUMO) energy gap. Third, the polymer backbone plays an essential role in determining the first Coulombic efficiency, and the irreversibly inserted Li atoms further dope the polymer backbone and reduce the HOMO-LUMO energy gap. Lastly, the pyridine nitrogen within the polymer backbone exhibits an adsorption effect on lithium sulfide (Li₂S) species, which stabilizes the cycling performances of the SPAN cathode. It is worth noting that the polymer backbone hardly contributes reversible capacity for the SPAN cathode in carbonate electrolytes within the 1 to 3 V operating voltage range. This study enhances our understanding of SPAN and may provide valuable guidance for the development of better SPAN materials and rechargeable batteries.