Lithium-Sulfur Batteries: The Effect of High Sulfur Loading on the Electrochemical Performance

Abstract

It is no secret that sulfur as a cathode material is a promising candidate for next generation Li metal-based batteries. However, the intrinsic limitations associated with low conductivity of sulfur and instability of sulfur based composite prevents the commercialization of lithium sulfur batteries. For the past years of Li-S batteries research, many research has been carried out on the cathode design to improve the electrochemical performance. However, it has been mathematically determined that even with high sulfur loading amount and effective sulfur utilization, the true value of energy density cannot outperform the currently available LIBs. That is, other inactive materials must be utilized to address the inherent sulfur issues which would only add more weight. Thus, we propose a novel and facile synthesis of binder-free three-dimensional carbon nanotubes (3D CNTs)/sulfur (S) hybrid composite as an electrode material, where 3D CNTs provide a high conduction path and short diffusion length for Li-ions, while providing ample amount of space for higher sulfur loading. In addition to sulfur loading, the amount of electrolyte also determines the electrochemical performance of Li-S cell in terms of sulfur utilization and cycling stability. However, the statistical analysis shows that more than 90% of reported publications have neglected this E/S ratio. This is important parameter for determining sulfur utilization and energy density of the cell. Here, we report the performance of three-dimensional carbon nanotubes/sulfur hybrid composite with different sulfur loading amounts from 1 to 9 mg/cm2 while tuning the electrolyte/sulfur (E/S) ratio from 12 to 1. The results have demonstrated the E/S ratio of 7 (60 µL electrolyte to 8.5 mg cm-2 sulfur) with reversible specific capacity of ~1068 mAh g-1 at 0.1C rate (~1.4mA cm-2) for up to 150 cycles.