N-Doping Effect for Polysulfide Reservoir within Mesoscale Electrodes for Practical Application of Lithium Sulfur Batteries

Lithium sulfur batteries have become one of the most promising energy storage technologies, potentially meeting the needs for vehicle electrification and stationary applications, because of its lower costs, environmental friendly property and much higher energy density (cathode theoretical energy: 3500 Wh/kg, or 2800 Wh/L) than that of state-of-art Li-ion batteries. However, many challenges exist for the broad deployment of Li/S system. The main challenge is the low Columbic efficiency and fast capacity decay due to the shuttle reactions caused by the soluble intermediate polysulfide

Nitrogen-doped porous carbon (NPC) and multi-wall carbon nanotube (MWCNT) have been frequently studied to immobilize sulfur in lithium-sulfur (Li-S) batteries. However, neither NPC nor MWCNT itself can effectively confine the soluble polysufides if cathode thickness e.g. sulfur loading is increased. In this work, NPC was combined with MWCNT to construct an integrated host structure to immobilize sulfur at a relevant scale. The function of doped nitrogen atoms was revisited and found to effectively attract sulfur radicals generated during the electrochemical process. The addition of MWCNT facilitated the uniform coating of sulfur nanocomposites to a practically usable thickness and homogenized the distribution of sulfur particles in the pristine electrodes, while NPC provided sufficient pore volume to trap dissolved species. More importantly, the wetting issue, the critical challenge for thick sulfur cathodes, is also mitigated after the adoption of MWCNT, leading to a high areal capacity of ca. 2.5 mAh/cm² with capacity retention of 81.6% over 100 cycles. This work provides clues on the fundamental mechanism of N-doping and the strategies to practically develop nano-structured materials for battery applications.

Acknowledgment

This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The EPR, XPS, and SEM analyses were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL).