Formation Mechanism and Molecular Structure of Sulfurized Polyacrylonitrile

Lithium-sulfur (Li-S) batteries have attracted considerable attention due to their high theoretical energy density and abundant sulfur resources. Sulfurized polyacrylonitrile (SPAN) effectively suppresses polysulfide dissolution and demonstrates excellent cycling stability, making it a promising candidate for cathode materials in practical Li-SPAN batteries. This study elucidates the formation mechanism, chemical bonds, and spatial structures of SPAN. The reaction begins with the generation of ·S₂· diradicals, followed by the dehydrogenation and cyclization of polyacrylonitrile (PAN), ultimately resulting in the formation of C─S and N─S bonds. The multilayered structure of SPAN, characterized by C-S and N-S layers twisted at angles of ≈30 to 40 degrees and interconnected by C─S_x─N bonds, has the potential to chemically confine sulfur up to a maximum of 63.5 wt.%, corresponding to a theoretical capacity exceeding 1000 mAh g^-1. These findings provide fundamental insights for the design of SPAN materials for advanced X-SPAN batteries.