Operando microbeam Raman spectroscopy is used to map the changes in hard carbon during sodiation and desodiation in unprecedented detail, elucidating several important and unresolved aspects of the sodiation mechanism. On sodiation a substantial, reversible decrease in G-peak energy is observed, which corresponds directly to the sloping part of the voltage profile and we argue can only be due to steady intercalation of sodium between the turbostratic layers of the hard carbon. Corresponding reversibility of the D-peak energy change is consistent with intercalation rather than representing a permanent increase in disorder. No change in energy of the graphitic phonons occurs over the low voltage plateau, indicating that intercalation saturates before sodium clusters form in micropores in this region. At the start of the initial sodiation there is no change in G-and D-peak energy as the Solid Electrolyte Interphase (SEI) forms. After SEI formation, the background slope of the spectra increases irreversibly, due to fluorescence. The importance of in situ/operando experiments over ex situ studies is demonstrated; washing the samples or air exposure causes the G-and D-peaks to revert back to their original states owing to SEI removal and sodium de-intercalation and confirming no permanent damage to the carbon structure.

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