Nitrogen-Doped Hard Carbon from Glucose and Sewage-Sludge Hydrolysate via Cohydrothermal Carbonization for Sodium-Ion Battery Anodes

Abstract Municipal sewage-sludge hydrolysate contains soluble nitrogen-bearing organic matter but is compositionally variable and does not, by itself, provide a controlled carbon framework for hard-carbon synthesis. Here, glucose was used as a reproducible carbon-framework precursor, while sewage-sludge hydrolysate was introduced as a nitrogen-containing functional cofeedstock and structural regulator. A series of precursors containing 5 g glucose and 0–70 mL hydrolysate within a fixed total liquid volume of 70 mL were hydrothermally treated at 240 °C and subsequently carbonized at 1300 °C. The sample prepared with 70 mL hydrolysate (10AQGL) exhibited an expanded interlayer spacing of 0.386 nm, moderate defect density, 0.8 wt % surface nitrogen, and a low specific surface area of 8.91 m2·g–1. These coupled structural and chemical characteristics limited excessive interfacial side reactions and supported an initial Coulombic efficiency of 84.2%. The 10AQGL electrode delivered a reversible capacity of 307.7 mAh·g–1 and retained approximately 240 mAh·g–1 after 600 cycles at 1.0 A·g–1. The improvement cannot be assigned to nitrogen doping alone because nitrogen content, interlayer spacing, defect density, and pore structure changed concurrently. This study therefore demonstrates the use of sewage-sludge hydrolysate as a functional cofeedstock in glucose-derived hard carbon, while industrial scalability and net environmental benefits remain to be established through continuous processing, techno-economic analysis, and life-cycle assessment.