Supercritical carbon dioxide assisted synthesis of ultra-stable sulfur/carbon composite cathodes for Li– S batteries

To mitigate the shuttle effect and enhance the electrical conductivity in lithium battery cathode, the unique characteristics of supercritical CO2 solvent (SC–CO2) and the distinctive porous and layered microstructure of reduced graphene oxide (rGO) are exploited in the fabrication of a high-performance rGO/sulfur composite cathode. Exploiting SC-CO2 technology can realize highly efficient sulfur transfer and precise microstructure regulation of S/C composite cathodes for Li–S batteries. On exposure, due to the sudden pressure release process, the SC-CO2 expands the interlayers of rGO rendering plenty of storage space for small sulfur allotropes in carbon matrices which increases the active sulfur loading. Being a remarkable hydrophobic solvent, the wetting properties of SC-CO2 are excellent, ensuring sulfur dissolution and penetration deep into the voids and interlayers of rGO. This creates intimate contact of sulfur with rGO interlayers, guaranteeing precise sulfur content, uniform sulfur distribution, and strong interaction between sulfur and carbon leading to enhanced electrical conductivity and sulfur utilization efficiency. Another important feature is that the S/C composites can be prepared at room temperature, unlike other conventional techniques which require a higher temperature. Moreover, the product mixture can be separated simply by de-pressuring SC-CO2. Herein, the rGO/sulfur composite cathode prepared on a lab scale showed an initial discharge capacity of 1024 mAh/g at 0.1C rate with capacity retention of 92.2% and coulombic efficiency of 99% even after 200 charge-discharge cycles. The developed cells showed excellent performance (929 mAh/g at 1 C rate) with an ultralow decay of 0.04% per cycle even after 200 charge-discharge cycles. Through this work, we believe that the synergistic effect of SC-CO2 technology and rGO as sulfur host will open up a promising future for the synthesis of efficient S/C composite cathodes with ultra-high cycling stability