To conclude, sodium storage in hard carbon occurs in 3 different stages, with the last stage having two processes occurring simultaneously: adsorption on reactive surface sites, insertion and adsorption at intralayer defect sites and on pore walls, and intercalation between layers and nanopore filling occurring simultaneously. Based on these findings, we suggest the following design strategies to maximize the sodium storage capacity of HC. The presence and speciation of defects within graphitic domains are crucial for initiating sodium insertion and pore filling. Moreover, the sodium insertion capacity in the slope region depends on the defect concentration and defect type.