Figure 1: Schematic illustration of sodium storage in graphite-based materials.

(a) Na⁺ cannot be electrochemically intercalated into graphite because of the small interlayer spacing. (b) Electrochemical intercalation of Na⁺ into GO is enabled by the enlarged interlayer distance because of oxidation. However, the intercalation is limited by steric hindering from large amounts of oxygen-containing groups. (c) A significant amount of Na⁺ can be electrochemically intercalated into EG owing to suitable interlayer distance and reduced oxygen-containing groups in the interlayers.