Waste-to-battery: Valorizing WWTP sewage sludge as an efficient ultra-high sulfur content cathode

Abstract

The growing accumulation of sewage sludge (SS) due to the increased implementation of wastewater treatment plants (WWTPs) gives rise to a need to propose new sustainable management strategies. This study explores an efficient method of transforming SS into functional activated carbon (SS-AC) through a simple, low-cost, and scalable one-step calcination process. The resulting SS-AC features a highly disordered N-doped carbon structure, high carbon content, and outstanding textural properties (SBET: 1011 m2·g−1, VT: 1.025 cc·g−1), making it an ideal conductive matrix for energy storage. A composite material (SS-AC@S) with ultra-high sulfur content (90 wt% S) was evaluated as a cathode for lithium sulfur (Li--S) batteries. Electrochemical characterization revealed excellent performance, with a specific capacity exceeding 1200 mAh·g−1 at 1C over prolonged cycling. The hierarchical porous structure and N-doping enhanced the electron/ion transport and enabled strong physical and chemical sulfur confinement, thus improving the stability of sulfur electrochemistry. In addition, a novel dynamic Li2S6 adsorption experiment confirmed the high efficiency of SS-AC in regard to trapping lithium polysulfides, further supporting its role in mitigating shuttle effects. This study demonstrates that SS, an abundant waste material, can be upcycled into a high-performance cathode for Li--S batteries, offering a highly abundant raw material and sustainable solution for next-generation energy storage technologies.