Surface-Modified Mxene Nanosheets to Enable Complete Utilization and Immobilization of Polysulfides in Lithium-Sulfur Batteries

Abstract

Lithium-Sulfur batteries are of great interest owing to sulfur's high theoretical capacity, low-cost and environmental benignity. However, the practicality of these batteries has been impeded due to the infamous “polysulfide-shuttle” effect. Although several research articles in the past decade have focused to mitigate this effect, capacity fade and low coulombic efficiency still prevail. 2D MXenes are early transition metal carbides known for their high metallic conductivity with tunable surface functionalities. MXenes with formula Mn+1XnTx are usually derived from the parental MAX phases by selectively etching the A layer (where M represents a transition metal, A represents the group 13 or 14 elements in the periodic table and X represents carbon or nitrogen, Tx stands for the surface terminations). Thus, the obtained MXenes possess a unique layered structure. After etching, the surface of MXenes is terminated with various functional groups such as hydroxyl (-OH), oxygen (-O), chlorine (-Cl), and fluorine (-F).Rich surface polar sites in Ti3C2Tx enhance electrochemical performance via suppression of polysulfide shuttle. However, past synthesis procedures result in the development of hydrophilic MXene nanosheets unable to be coupled with hydrophobic sulfur. Herein, we propose a new design based on the functionalization of MXene nanosheets with an organic molecule for the effective deposition of sulfur nanoparticles on the surface. This strategy enables the dispersion of MXene sheets in sulfur dissolved solvents enabling the conformal deposition of sulfur on MXene sheets with negligible water in the composite. The metallic sheet-like conductivity (~103 S.cm-1) also aids in faster reaction kinetics during the conversion of polysulfides to Li2S, mitigating the shuttle effect. Consequently, the developed composite enables the efficient utilization of active surface sites to bind polysulfides and alleviate the shuttle effect. This novel cathode delivers a high initial reversible capacity of 1220 mAh·g-1 at 0.1 C and at 0.5C, these cathodes stabilize at 880mAh·g-1 after the first 5 cycles, 85% of which is retained post 500 cycles. Additionally, we developed cathodes with high sulfur loading of ~10.7 mg·cm-2 that delivers a stable areal capacity of ~7 mAh·cm-2 for 150 cycles. Moreover, these cathodes in a pouch cell configuration retained ~770 mAh·g-1 after ~200 cycles at 0.2C (85.5% retention). Ex-situ studies elucidated the nature of the LiPs–MXene interaction and the effect of surface functionalization towards improved performance +RP and VN have equal contribution