Soft-carbon-tuned hard carbon anode for ultrahigh-rate sodium storage

High-rate hard carbon anode is critical for achieving fast-charging sodium-ion batteries, whereas the limited ion/electron kinetics caused by unexpected surface defects and unsatisfactory conductivity greatly limits rate capability. Herein, a coconut shell-derived soft-carbon-tuned hard carbon (SHC) with low surface area (4.7 m2 g−1) was prepared. With SHCs as bricks, a high conductivity single-walled carbon nanotube (SWNT)-bonded hard carbon film was constructed. The pitch-derived soft carbon formed on SHCs can effectively decrease the surface defects and simultaneously induce optimized disordered graphite domains into carbon matrix, enabling high Na+ reversibility and ionic/electronic conductivity. The crosslinked SWNTs in-between can provide continuous ion/charge transport ‘highways’, thus ensuring rapid ion/electron kinetics. As a result, such a self-supporting carbon anode exhibits remarkable rate performance (330 mAh g−1 at 0.1 C and 272 mAh g−1 at 5 C), superior initial Coulombic efficiency of 95.2% and outstanding cycling stability.