Mechanistic Understanding of the Sulfurized-Poly(acrylonitrile) Cathode for Lithium-Sulfur Batteries

Sulfurized−poly(acrylonitrile) compound (SPAN) cathode decipher polysulfide dissolution and shuttling effect and deliver promising electrochemical performance. However, the synthesis reaction mechanism, chemical structure, and lithium storage mechanism of the compound are yet not clearly understood. Herein we report a plausible synthesis reaction mechanism, chemical structure, and lithium storage mechanism of SPAN using high−resolution cross−polarization/magic angle spinning (CP−MAS) solid states nuclear magnetic resonance (ssNMR), Fourier transform infrared spectroscopy (FTIR), X−ray photoelectron spectroscopy (XPS), elemental analysis (EA) techniques, and electrochemical analysis. The analysis reveals that the SPAN structure contains N–S and NC–S bonds in addition to C–S and S–S bonds. Electrochemical cleavage of the covalent bonds results in high initial discharge capacity with high voltage polarization in the first cycle. Once the covalent bonds are cleaved in the first cycle, the electron-donating effect of the Li–C and Li–N bonds increases the electron density of the conjugated heterocyclic structure and benefits the decrease of the charge/discharge voltage hysteresis after the second cycle. As a result, it provides high reversible specific capacity, high coulombic efficiency, and extended cycle life with low voltage hysteresis after the second cycle. The results show that the covalent molecular interactions of SPAN cathode completely overcome the polysulfide dissolution, thereby prevent the loss of active materials. Furthermore, the structural elucidation is further applicable for designing an organic cathode achieving high active materials loading concomitantly with improving the electrochemical performances.