In-Operando FTIR Study on the Redox Behavior of Sulfurized Polyacrylonitrile as Cathode Material for Li-S Batteries

Lithium-sulfur batteries have shown tremendous potential as a post lithium-ion battery chemistry, with a theoretical capacity of up to 1675 mAh/g. However, they have suffered from fundamental material challenges related to the insulating nature of the active material and discharge products as well as the solubility of intermediary products during the electrochemical reaction . One of the many proposed solutions to the latter problem has been to anchor the sulfur chain chemically to an organic molecule to form an organosulfur material. Sulfurized polyacrylonitrile (SPAN) is one such material that has shown tremendous promise. While SPAN has demonstrated long cycle life in carbonate electrolytes, its viability in ether electrolytes is only possible with the inclusion of high concentrations of lithium nitrate. To identify the chemical species present in charge and discharge cycles, elucidate the mechanisms of capacity fade, and understand why capacity is retained in the presence of lithium nitrate, we combined post-mortem analysis by XPS with an in-operando FT-IR study of three spectral regions. We probed bond vibrations we have assigned to the carbon-sulfur bond that anchors sulfur chains to the organosulfur backbone, the sulfur-sulfur bond in lithium polysulfides that evolve in electrolyte, and ring stretches of the heteropolycyclic cyclized-PAN backbone. We present evidence in support of lithiation of the heteropolycyclic backbone. We identify the formation of a robust cathode-electrolyte interphase to be a critical feature of systems with lithium nitrate present. This allows for the retention of the C-S bond and the suppression of polysulfides that otherwise cause shuttling losses.

Automated summary

Lithium-sulfur batteries have potential challenges, but the use of sulfurized polyacrylonitrile (SPAN) and the presence of lithium nitrate can help retain capacity and suppress polysulfide formation by forming a robust cathode-electrolyte interphase.