Investigation of Electrochemical Sodium-Ion Intercalation Behavior into Graphite-Based Electrodes

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Sodium-ion batteries cannot employ graphite which is a typical negative electrode material for lithium-ion batteries. This is principally because sodium-ion cannot intercalate deeply into graphite, which has been a mystery for many years. Here, the mechanism of electrochemical sodium-ion intercalation into graphitic materials was investigated by using Raman spectroscopy and X-ray diffraction measurement to solve the question. Low stage sodium graphite intercalation compound (Na-GIC) was formed electrochemically only near the surface of graphite by potential holding above the sodium metal deposition potential. On the other hand, the high stage Na-GIC was formed electrochemically in the bulk at the sodium metal deposition potential. In addition, the apparent diffusion distance and the apparent diffusion coefficient of sodium-ion inside graphite were calculated using chronopotentiograms and potentiostatic intermittent titration technique. As a result, the sodium-ion diffusion inside spherical graphite was not slow enough to explain the limited reactivity. Hence, the limitation of sodium-ion intercalation into graphite might be originated from not the kinetic limitation inside graphite but the thermodynamic limitation.