Tuning the Microstructure and Functional Groups of Coal-Derived Hard Carbons by Urea Additives for 165 Wh kg –1 Sodium Ion Pouch Cells

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 Na 3 V 2 (PO 4 ) 3 cathode shows a capacity retention of 91.2% after 2000 cycles at 5C. The 1.2 Ah pouch cell with a NaNi 1/3 Fe 1/3 Mn 1/3 O 2 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.