Optimizing the microstructure and functional groups of hard carbon (HC) is critical to enhancing its sodium ion (Na+) storage performance. Herein, the microstructure and functional groups of HC are modified through the carbonization of urea additives in coal. The as-received HC contains rich electronegative functional groups (pyrrolic-N, pyridinic-N,, oxidized-N, and C═O) with Na+ affinity, pseudographitic nanocrystals with short lateral dimension but thick graphitic layers, and closed pores with an average diameter of 2.5 nm that exhibit an “adsorption-intercalation/pore-filling” Na+ storage mechanism. The HC half-cell delivers a reversible capacity of 356.0 mAh g–1 at 25 °C and 458.4 mAh g–1 at 60 °C at 0.1C. The full cell with a Na3V2(PO4)3 cathode shows a capacity retention of 91.2% after 2000 cycles at 5C. The 1.2 Ah pouch cell with a NaNi1/3Fe1/3Mn1/3O2 cathode exhibits an energy density of 165 Wh kg–1. This work contributes to our understanding of sodium storage mechanisms and offers guidance for the design of carbon-based anodes for sodium-ion batteries.
Zhang et al. (Sun,) studied this question.