Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries because sodium is more abundant and cost-effective when compared to lithium. SIBs require stable negative electrode material as graphite usually does not support sodium intercalation. Hard carbon, a highly disordered non-graphitizable material is a good option, having sodium storage specific capacity in the range of 200–480 mAh g−1 below 1.2 V vs. Na+/Na.1,2 Here, we thoroughly investigate the electrochemical (de)sodiation of hard carbon by means of electrochemical impedance spectroscopy (EIS), the galvanostatic intermittent titration technique, and in situ Raman spectroscopy for fresh and cycled electrodes. The results show G-band reversible shift and a strong change of the charge-transfer resistance above 0.1 V vs. Na+/Na indicating the intercalation of sodium ions into hard carbon, whereas the low-voltage plateau is associated with the pore filling process (Figure 1a). In situ Raman analysis demonstrates that the pore filling includes formation of small sodium clusters in closed pores (Figure 1b). As intercalation mechanism changes to pore filling at ≈ 0.10-0.15 V, apparent diffusivity decreases by an order of magnitude. The gradual growth of the solid-electrolyte interphase layer affects the rise of the interfacial resistance as cycling progresses. In combination with the slower diffusivity, the low￾voltage plateau region strictly impedes fast de/sodiation and eventually causes capacity fade.