Unveiling the role of intrinsic defects in N/S Co-Doped hard carbon for superior sodium-ion batteries

Abstract

Hard carbon materials have emerged as a crucial anode choice for commercial sodium-ion batteries (SIBs), owing to their inherent abundance in porosity and the adaptability in adjusting interlayer spacing. However, the low capacity below 1 V and sluggish transportation kinetics respectively hinders the output voltage and rate performance. In this work, a one-step polymerization technique has been proposed to synthesize interconnected three-dimensional N/S-rich molecules via a methylene (–CH₂-) bridge. During the subsequent carbonization process, the in-situ elimination of –CH₂- bridge and partial heteroatoms facilitated the formation of intrinsic defects within the carbon layers, yielding an N/S co-doping hard carbon with intrinsic defect structures. This innovative approach provides a remarkable reversible capacity of 238mAh g⁻¹ at voltages below 1 V, a high-rate capability of 150mAh g⁻¹ at 5 A/g, along with exceptional cyclic stability of nearly 100 % capacity extension after 2000 cycles. This obviously enhancement in low-voltage sodium storage capacity and rate performance is attributed to the enhanced effect through N/S co-doping with intrinsic defect structures. This work highlights the critical role of defect engineering in carbon materials for efficient low-voltage sodium ions storage, offering a promising anode material with superior rate and cyclic stability.