Unlocking the Low-Temperature Potential of Biomass-Derived Hard Carbon Anodes via Synergistic N/S Co-Doping for High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are a viable alternative to lithium-ion systems, yet their commercialization is hindered by the lack of high-performance, cost-effective anode materials. Hard carbon is promising but typically requires energy-intensive high-temperature carbonization (>1200 °C) to achieve satisfactory capacity. Herein, we demonstrate a synergistic nitrogen and sulfur co-doping strategy that compensates for the capacity and rate deficiencies of hard carbon synthesized at significantly reduced temperatures (750–950 °C). Using waste banana peels as a renewable precursor and a simple dilute sulfuric acid pretreatment, we achieve in situ N/S co-doping within a porous carbon matrix. The N/S synergy introduces abundant active sites and enhances electronic conductivity, thereby boosting the slope-region capacity (Na+ adsorption) to offset the reduced plateau capacity at lower carbonization temperatures. The optimal sample, carbonized at 850 °C (HC-850), exhibits outstanding long-term cycling stability (76.0% capacity retention after 1900 cycles at 1 A g–1) and a high initial discharge capacity of 278 mAh g–1 at 1 A g–1. This work presents a low-energy, scalable, and fundamentally insightful strategy for designing high-performance hard carbon anodes from biomass waste, challenging the necessity of ultrahigh-temperature processing.