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

This article introduces the catalytic polymerization of phenothiazines through the crosslinking of aromatic rings with methylene (–CH2-) groups. By sacrificing methylene at a low carbonization temperature of 700°C, a large number of intrinsic defects are generated in situ within the carbon layers. The coupling effect between N/S co-doping and the intrinsic defect structure not only enhances the sodium storage capacity at low voltages (< 1 V), but also improves the sodium ions transfer capability. • Intrinsic defects were obtained via bridge-linked polymerization and in-situ elimination. • The role of intrinsic defects in N/S doped hard carbon for sodium storage was investigated. • Synergistic effect of co-doping and intrinsic defects enhances low-voltage sodium storage capacity. 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 (–CH 2 -) bridge. During the subsequent carbonization process, the in-situ elimination of –CH 2 - 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.