Regeneration of Activated Sludge into SiO2-Decorated Heteroatom-Doped Porous Carbon as Advanced Electrodes for Li–S Batteries

The regeneration of harmful activated sludge into an energy source is an important strategy for municipal sludge treatment and recycling. Herein, SiO₂-modified N,S auto-doped porous carbon (NSC@SiO₂) with high conductivity (70 S m^-1) is successfully obtained through a simple calcination method of the activated sludge from wastewater treatment. Further, P-doped NSC@SiO₂ (NSPC@SiO₂) is designed to achieve a higher surface area (891 m² g^-1 vs 624 m² g^-1), a larger pore volume (0.87 cm³ g^-1 vs 0.08 cm³ g^-1), and more carbon defects. Due to its special structure, NSPC@SiO₂ is used as a sulfur host of lithium-sulfur batteries. The results of polysulfide adsorption experiments, S 2p X-ray photoelectron spectra (XPS), Li₂S nucleation experiments, polysulfide symmetric cells, measurement of the galvanostatic intermittent titration (GITT), polarization voltage difference, lithium-ion diffusion rate, and Tafel slope verified that NSPC@SiO₂ greatly improved the adsorption capacity of polysulfides, lowered the barrier to Li₂S formation and the internal resistances of cells, and accelerated Li⁺ ion diffusion and the reaction kinetics of polysulfide conversion, resulting in the excellent performance of polysulfide capture and superior rate performance and cyclic stability. By comparing NSPC@SiO₂ with NSC@SiO₂, a higher initial capacity (1377 mAh g^-1 vs 1150 mAh g^-1 at 0.1C), better rate capacity (912 mAh g^-1 vs 719 mAh g^-1 at 2C), and low capacity decay (0.094% per cycle within 200 cycles) are obtained. Our work provides direction for the treatment, disposal, and resource utilization of activated sludge.