Study of Porous Carbon Nanofibers As Interlayer and As Standalone Binder-Free Sulfur Hosts for Li-S Batteries

Abstract

In the first part of the talk, I will present our work on study of free-standing porous carbon nanofibers with tunable surface area and pore structure as an interlayer between the sulfur cathode and the separator to inhibit the shuttling of the intermediate polysulfides in lithium–sulfur (Li–S) batteries. Specifically, the effects of thickness, surface area, and pore size distribution of carbon nanofiber (CNF) interlayers on the performance of Li–S batteries have been studied. The carbon nanofiber interlayer not only reduces the electrochemical resistance but also localizes the migrating polysulfides and traps them, thereby improving the discharge capacity as well as cyclability. A high initial discharge capacity of up to 1549 mA h g−1 at C/5 rate, which is 92% of the theoretical capacity of sulfur, with 98% average coulombic efficiency and 83% capacity retention after 100 cycles was obtained. In the second half of the talk, I will discuss our work wherein we directly incorporate sulfur into the carbon nanofibers as a host using an ultra-rapid (5-second) deposition technique. This procedure melts sulfur into carbon nanofiber mats, which play a significant role as a built-in current collector to provide uninterrupted electron transport pathways throughout the electrode without hindrance of insulating binding agents, thereby eliminating the need for any dead weight from the binders, interlayer or current collector. The large inter-fiber spacing facilitates electrolyte diffusion and provides sufficient space for the volume expansion during lithium-sulfur redox. The cathodes exhibit highly stable discharge capacities at 0.5C with 100% capacity retention over 150 cycles (~550 mAh g-1 after conditioning). This technique completely eliminates the need for slurry-based processing including the insulating binders, toxic solvent, or heavy current collector, producing an effective capacity of ~991 mAh g-1 at 0.5C over 150 cycles. Moreover, conventional sulfur melt deposition techniques require high temperatures (155-300 °C) and are very time consuming (~8-10h) while this ultra-rapid technique requires only 140 °C, 5 seconds, and a small application of pressure (~200 psi). The ultra-rapid method offers a practical approach to sulfur cathode fabrication into simple fiber electrodes compared to conventional techniques that require time-consuming deposition to integrate sulfur into highly complex nanostructures. Furthermore, this free-standing nature of the nanofiber-based design demonstrates a feasible route for evolving technologies like smart fabrics and wearable batteries. Relevant Reference: Singhal, R.; Chung, S-H.; Manthiram, A.; Kalra, V. Free-standing Carbon Nanofiber Interlayer for High Performance Lithium-Sulfur Batteries. Journal of Materials Chemistry A, 2015, 3, 4530.