The commercialization of lithium-sulfur (Li-S) batteries faces persistent challenges, including the polysulfide shuttle effect, poor electrical conductivity of sulfur cathodes, and severe volumetric expansion during cycling. To address these limitations while aligning with carbon neutrality objectives, this study innovatively utilizes waste-activated sludge (a byproduct of wastewater treatment) as a sustainable precursor to synthesize hierarchical porous, heteroatom-doped biochar (designated SC) through a multi-step process involving chemical etching, K₂CO₃ activation, and controlled pyrolysis. Structural characterization demonstrates that the optimized SC material exhibits a three-dimensional interconnected hierarchical pore architecture, achieving a remarkable specific surface area of 1643.3 m²•g⁻¹ which is higher compared to unactivated sludge-derived carbon (USC). Crucially, HF etching prior to activation effectively removes inert SiO₂ components, while K₂CO₃ activation at 800 ℃ under N₂ atmosphere induces micropore formation and graphitization, as evidenced by X-ray diffraction (XRD). Chemical composition analysis reveals that SC possesses dual heteroatom doping, featuring abundant polar functional groups including C=O, C—O, graphitic-N, pyrrolic-N and pyridinic-N, which synergistically enhance polysulfide chemisorption. Ultraviolet-visible spectroscopy confirms SC's exceptional adsorption capacity for Li₂S₆, with complete elimination of characteristic absorption peak at 350 nm, outperforming USC and raw sludge-derived carbon (designated DS). When employed as a sulfur host, the SC-based cathode delivers a high initial discharge capacity of 1165.5 mAh•g⁻¹ at 0.1 C, maintaining 92.3% capacity retention after 50 cycles and achieving an ultralow capacity decay rate of 0.05% per cycle over 1000 cycles. Even under high sulfur loading (8 mg•cm⁻²), the cathode retains 55.0% capacity after 500 cycles at 0.5 C, demonstrating practical viability. Mechanistic studies attribute this performance to the synergistic effects of physical confinement (hierarchical pores), chemical adsorption (polar functional groups), and enhanced reaction kinetics (low charge-transfer resistance, R_ct=177.9 Ω; fast Li⁺ diffusion coefficient). Furthermore, sludge-derived SC reduces material costs lower compared to commercial carbon while aligning with carbon neutrality goals via waste-to-resource conversion. This work provides a green and cost-effective paradigm for designing high-energy-density Li-S batteries and valorizing biowaste in energy storage systems.

Key words: lithium-sulfur battery, shuttle effect, sewage sludge, biochar