Electrochemical characterization
The galvanostatic charge/discharge profiles of the PF-HCS electrodes measured at a constant current density of 20 mA g− 1 between 0 and 2.7 V are presented in Fig. 4. The charge/discharge capacities and coulombic efficiencies of the samples are shown inTable 1. The pyrolyzing temperature has an insignificant influence on charge–discharge characteristics of the resin carbon anodes. As presented in Fig. 4, during the first cycle, the voltage profiles of all the PF-HCS electrodes show an irreversible slope from 1.1 to 0.2 V caused by solid electrolyte interphase (SEI) formation and irreversible trapping of Na at the highly defective graphene sites, resulting in irreversible capacities. The subsequent profiles show good reversibility and repeatability, displaying a sloping-voltage region and a nearly flat plateau at potentials lower than 0.2 V, which can be ascribed to intercalation of sodium ions between graphene layers [7], [9] and [22]. There are subtle differences in initial efficiency and some changes in reversible capacities. As carbonization temperature is increased from 800 to 1500 °C, the initial coulombic