Covalent Si–S bonding engineering in sulfurized polyacrylonitrile (SPAN): toward enhanced electrochemical stability and kinetics in lithium–sulfur batteries

Abstract

Sulfurized polyacrylonitrile (SPAN) is a promising cathode material for lithium–sulfur batteries due to its superior sulfur retention and long-term stability. However, the practical application of SPAN is hindered by insufficient sulfur anchoring sites and sluggish redox kinetics. Herein, we present an atomic-level bonding strategy to overcome these limitations by incorporating silicon quantum dots (SiQDs) into SPAN under high-temperature sulfurization (450 °C), forming robust covalent Si–S bonds with a high dissociation energy. These covalent Si–S bonds act as stable sulfur anchors, enabling a 19.77% higher sulfur loading with minimal free sulfur, while significantly improving electron/ion conductivity and redox reaction kinetics. The optimized Si_0.05-SPAN cathode exhibits outstanding electrochemical performance, including a high initial discharge capacity of 1432.7 mAh g⁻¹ and a retained capacity of ∼773 mAh g⁻¹ after 500 cycles at 1.5 C, corresponding to an ultralow capacity decay of only 0.023% per cycle with nearly 100% coulombic efficiency. Comprehensive experimental characterization combined with density functional theory (DFT) calculations reveal that Si–S bonding redistributes electron density around sulfur atoms, effectively suppressing sulfur loss and accelerating charge transfer. This covalent bond engineering approach offers a new strategy for developing high-energy-density lithium–sulfur batteries with enhanced stability and kinetics.