Structural and Electrochemical Properties of Hard Carbon Negative Electrodes for Sodium Secondary Batteries Using the Na[FSA]–[C3C1pyrr][FSA] Ionic Liquid Electrolyte

Two kinds of hard carbon (HC) were investigated as negative electrodes for sodium secondary batteries using Na[FSA]–[C3C1pyrr][FSA] as an ionic liquid electrolyte at 363 K. The structural properties of HCs were studied by X-ray diffraction (XRD), Raman spectroscopy, small-angle X-ray scattering (SAXS), and field-emission scanning electron microscopy (FE-SEM). The interlayer distance between graphene sheets and the pore size were different for these HCs. Potential slope and plateau regions were observed in charge–discharge curves, which is typical for HC negative electrodes. More detailed examinations revealed that the HC with a larger interlayer distance showed higher capacity in the slope region, while that with a larger pore size exhibited a higher capacity in the plateau region. Finally, the rate capabilities and cycling properties of HC negative electrodes were evaluated.