Regulating microstructure in agar-derived N-doped hard carbon towards enhanced sodium ion storage

Hard carbon (HC) with larger interlayer spacing, lower operation potential and stable skeleton is a promising kind of anode materials for sodium-ion batteries (SIBs), where heteroatom doping and structural design have been proved to be effective strategies to improve its Na + storage performance. Herein, biomass agar-derived N-doped porous carbon (NPC-Ts, T  = 750, 900 and 1050 °C) were successfully fabricated by modulating carbonization temperature, where interlayer spacing, specifical surface area (SSA), micro/mesopore amount and N-doping levels and configurations (pyridinic N (N-6), pyrrolic N (N-5) and graphite N (N-Q)) were explored in detail. Among the NPC-900 shows the largest interlayer spacing (0.412 nm), SSA (3991 m 2 /g) and sum percentage of N-6 and N-5 (76.7 %). When applied as anode material for SIBs, NPC-900 exhibits the highly reversible capacity (430.9 mAh/g at 100 mA/g after 100 cycles) and excellent rate capability (162 mAh/g at 5000 mA/g) and outstanding cycle life (242.6 mAh/g at 1000 mA/g after 5000 cycles). More importantly, when coupled with Na 3 V 2 (PO 4 ) 3 (NVP) cathode, the Na + full cell displays the highest practical energy density of 267 Wh/kg to date, to the best our knowledge, revealing its potential practical sodium storage applications.