Improved Performance of Lithium–Sulfur Batteries Using Nitrogen-Doped Reduced Graphene Oxide-Coated Separators with Optimized Nitrogen Content

Abstract

In this work, a hydrothermally synthesized nitrogen-doped reduced graphene oxide (N-rGO) layer as a barrier to polysulfide diffusion was coated on a commercial polypropylene (PP) separator to enhance charge capacity and cycling stability of Li–S batteries. Coin-cell Li–S batteries were fabricated using sulfur/carbon nanotube/N-rGO cathode with 70% sulfur loading, lithium foil anode, lithium bis(trifluoromethanesulfonyl) imide-based electrolyte, and N-rGO-coated PP separators with varying N contents in the range of 0–20 wt %. Galvanostatic charge–discharge measurements demonstrated that the charge capacity and capacity retention at various current densities improved substantially with increasing N content from 0 to 15 wt % before slightly deteriorating as the N content increases further. The cell with the optimal N-rGO-modified separator delivered high initial discharge capacities of 1563 mA h g–1 at 0.1 C and 1291 mA h g–1 at 1 C and high capacity retentions of 83.7% after 200 cycles at 0.1 C and 79.8% after 500 cycles at 1 C. Chemical analyses of materials verified that the improved energy storage performances by the N-rGO-coated separator could be attributed to the strong affinity of lithium polysulfides around N sites on N-rGO, leading to improved redox kinetics and effective suppression of the polysulfide shuttle effect. Therefore, the hydrothermally synthesized N-rGO-modified separator is highly promising as a means to achieve high-performance and long-life Li–S batteries suitable for mass production and commercialization.