Synergistic Nitrogen-Doping and Defect Engineering in Hard Carbon: Unlocking Ultrahigh Rate Capability and Long-Cycling Stability for Sodium-Ion Battery Anodes.

Hard carbon (HC) anodes for sodium-ion batteries (SIBs) face challenges such as sluggish Na⁺ diffusion kinetics and structural instability. Herein, we propose a synergistic nitrogen-doping and defect-engineering strategy to unlock ultrahigh-rate capability and long-term cyclability in biomass-derived hard carbon. A scalable synthesis route is developed via hydrothermal carbonization of corn stalk, followed by controlled pyrolysis with urea, achieving uniform nitrogen incorporation into the carbon matrix. Comprehensive characterization reveals that nitrogen doping introduces tailored defects, expands interlayer spacing, and optimizes surface pseudocapacitance. The resultant N-doped hard carbon (NC-2) delivers a remarkable reversible capacity of 259 mAh g at 0.1 A g with 91% retention after 100 cycles. And analysis demonstrates a dual Na⁺ storage mechanism combining surface-driven pseudocapacitive adsorption (89% contribution at 1.0 mV s) and diffusion-controlled intercalation facilitated by reduced charge transfer resistance (56.9 Ω) and enhanced ionic pathways. Notably, NC-2 exhibits exceptional rate performance (124.0 mAh g at 1.0 A g) and sustains 95% capacity retention over 500 cycles at 1.0 A g. This work establishes a universal defect-engineering paradigm for carbonaceous materials, offering fundamental insights into structure-property correlations and paving the way for sustainable, high-performance SIB anodes.