Boosting the electrocatalytic activity of 2D ultrathin BiOX/rGO (X = F, Cl, Br, and I) nanosheets as sulfur hosts: insight into the electronegativity effect of halogenated elements on the electrochemical performances of lithium–sulfur batteries

Abstract

Sluggish redox kinetics and the polysulfide shuttle effect hinder the commercialization of lithium–sulfur batteries. Herein, we report an in situ combination of BiOX (X = F, Cl, Br, and I) and rGO that forms an ultrathin layered structure BiOX/rGO as a sulfur host. Polar BiOX, with alternating X⁻ and [Bi₂O₂²⁻] layers, strongly adsorbs lithium polysulfides (LiPSs) owing to the high electronegativity of X and O atoms, which reduces the electron density of Bi. This results in the empty orbitals of Bi providing more Lewis acid sites for accepting external electrons from LiPSs, thus accelerating the redox kinetics of sulfur. The BiOF/rGO-S cathode delivers a high capacity of 997 mA h g⁻¹ over 400 cycles at 0.5 C. Even with a high sulfur loading of 9.1 mg cm⁻² and an E/S ratio of 7.7 μL mg⁻¹, it retains a capacity of 628.8 mA h g⁻¹ after 90 cycles at 0.1 C. In situ studies and DFT calculations reveal that BiOF/rGO promotes the SRR process and inhibits the polysulfide shuttle effect. This work elucidates the correlation between the electron cloud density of Bi in BiOX and the electrocatalytic performances of lithium–sulfur batteries and the underlying mechanisms, providing novel insights for designing high-performance sulfur host materials.