Surface Functionalization of Two-Dimensional MXene Nanosheets to Tailor Sulfur-Host Architecture for Metal-Sulfur Batteries

Abstract

Practicality of lithium-sulfur batteries is severely hindered by the notorious polysulfide-shuttle phenomenon, leading to rapid capacity fade. This issue is aggravated with increase in sulfur loading, causing low-coulombic efficiency and cycle life. Herein, we present a facile strategy to combine hydrophobic sulfur and hydrophilic, conductive Ti3C2Tz-MXene via one-step surface functionalization using di(hydrogenated tallow) benzylmethyl ammonium chloride (DHT). The latter renders the Ti3C2Tz surface hydrophobic, making it readily dispersible in sulfur dissolved in a carbon disulfide (CS2) solvent. By evaporating the solvent, we conformally coat the DHT-Ti3C2Tz (DMX) with sulfur. The developed composite, with higher available active area, enables effective trapping of lithium polysulfides (LiPs) on the electroactive sites within the cathode, leading to improvement in electrochemical performance at higher sulfur loadings. The DMX/S cathodes function with high sulfur loading of ∼10.7 mg·cm−2 and deliver a stable areal capacity of ∼7 mAh·cm−2 for 150 cycles in the standard ether electrolyte. Moreover, a DMX/S cathode in a pouch-cell configuration retains ∼770 mAh·g−1 after ∼200 cycles at 0.2C (85.5% retention). Postmortem spectroscopic studies elucidate the nature of the LiPs-MXene interaction and the effect of surface functionalization towards improved performance. To further demonstrate the applicability of such uniquely functionalized MXene sheets, we study them as a host to confine sulfur (S8) triggering a quasi-solid state redox mechanism enabling the use of commercialization-friendly carbonate electrolyte in metal-sulfur batteries. We demonstrate that the multilayer MXene structure provides tunable spacing for S8 confinement and its unique interlayer spacing prevents adverse polysulfide-carbonate reactions resulting in stable battery cycling.