Comprehensive insights into sodium storage in pitch‐derived porous hard carbon

Abstract The controversies about the mechanism of sodium storage in hard carbon (HC) hinder its rational structural design. A series of porous HC materials using coal tar pitch show a reversible capacity of 377 mAh g−1 and an initial Coulombic efficiency (ICE) of 87% as well as excellent cycling performance. More attention is paid to exploration of the relationships between the sodium status on various storage sites at different sodiation states and the ICE by solid‐state 23Na nuclear magnetic resonance spectroscopy. The adsorbed Na ions contribute the most to the irreversible capacity. The de‐solvated Na ions entering the closed pores are reduced to Na atoms and aggregated to Na clusters. Also, this process contributes the most to the reversible capacity and is characteristic of a long plateau in the voltage profile. Intercalation is partially reversible; it is the main source of capacity for slope‐type HCs but plays a minor role in the reversible capacity of plateau‐type HCs. Therefore, increasing the content of the closed pores can improve the reversible plateau capacity and reducing the open mesopores of HC increases the ICE. These findings provide insights into the structural design and cost‐efficient preparation of high‐performance HC anode materials for advanced sodium‐ion batteries.