Revealing the Lithium Storage Mechanism of Sulfurized Polyacrylonitrile Cathode by Ex-Situ NMR and Ex-Situ Raman Spectroscopy

Sulfurized polyacrylonitrile (SPAN) is a promising cathode material for high-performance lithium-sulfur batteries.1-3 However, fundamental insights into its molecular structure and lithium storage mechanism are yet not clearly understood. Herein, we reveal the formation of N-S and C=N-S bonds in addition to the C-S and S-S bonds using high−resolution cross−polarization/magic angle spinning (CP−MAS) solid states nitrogen-15 and carbon-13 nuclear magnetic resonance spectroscopy (15N NMR and 13C NMR). Ex-situ 13C NMR, ex-situ 15N NMR, ex-situ 7Li NMR, and ex-situ Raman studies indicate the formation of Li-C-N-Li and Li-C-C-Li bonds. Li–C and Li–N bonds increase the electron density of the conjugated heterocyclic structure and benefit lowering the charge/discharge voltage hysteresis after the second cycle. These results confirm conjugated C=N and C=C double-bonds involved in the lithium-ion storage mechanism leading to higher practical discharge capacity than the theoretical capacity of elemental sulfur. References Weret, M. A. et al. Mechanistic understanding of the Sulfurized-Poly(acrylonitrile) cathode for lithium-sulfur batteries. Energy Storage Materials 26, 483-493 (2020). Weret, M. A., Su, W.-N. & Hwang, B. J. Strategies towards High Performance Lithium-Sulfur Batteries. Batteries & Supercaps 5, e202200059 (2022). Weret, M. A. et al. Fibrous organosulfur cathode materials with high bonded sulfur for high-performance lithium-sulfur batteries. Journal of Power Sources 541, 231693 (2022).