Sulfur is a promising cathode material for high-energy cells due to its abundance and high specific capacity (1675 mAh/g). However, its use has challenges. Sulfur undergoes a conversion reaction during charge and discharge, forming soluble polysulfides that can cause side reactions and active material loss. Additionally, sulfur is electrically insulating, requiring a significant amount of conductive additives. Finally, sulfur’s expansion during conversion affects the mechanical stability of Li-S batteries.
An alternative to elemental sulfur is sulfurized polyacrylonitrile (SPAN), where sulfur is covalently-bound to a polyacrylonitrile (PAN) polymer backbone. This structure prevents polysulfides from migrating and allows a solid-solid reaction between sulfur and lithium compounds. The PAN backbone also buffers volumetric expansion and improves conductivity, though it reduces the material’s specific capacity.
SPAN offers advantages in water-based processing, with shear-thinning behavior that makes the slurry stable and easy to coat. This allows for high sulfur content (90 wt.-%, 38 wt.-% S) while maintaining good coating quality. Research is ongoing to increase sulfur content (48 wt.-% sulfur) to reach higher capacities.
In this work, we simplified the process by replacing the CMC/SBR binder-system with polyacrylonitrile, streamlining the process. We also switched from batch to continuous slurry production via extrusion, improving throughput and enhancing electrode properties, such as a 25% increase in discharge capacity at 1 and 2C. The water-based process not only reduces environmental impact but existing lithium-ion electrode production techniques can be applied due to the fact that the coating and drying mechanisms are similar to graphite anodes.
To further improve SPAN cathodes, we coated the slurry onto primed current collectors using roll-to-roll coating. The thin carbon layer on the substrate reduces interface resistivity by up to 77% and leading to an improved electrochemical performance during both charging and discharging.