Hard Carbon with Intensified Graphitization Induced By Graphene Oxide for High Initial Coulombic Efficiency Anode in Sodium Storage

Abstract

Hard carbons hold the most promising practical application among all anode materials for sodium-ion batteries. However, the low initial Coulombic efficiency (ICE) limits their further commercialization. Herein, one kind of hard carbon with Intensified graphitization induced by graphene oxide was prepared by a new coupled carbonization strategy, in which the mixture of phenolic resin (PF) and grapheme oxide (GO) was heat-treated at high temperature directly. The as-prepared hard carbon exhibited outstanding electrochemical behaviors such as superior ICE. We also believe that the strategy may provide a constructive instruction for high performance hard carbon material designing.For synthesis of hard carbon (GHCs), GO aqueous suspension was first prepared via a modified Hummer’s method. Then PF was dissolved in the GO aqueous suspension (mass ratio of PF:GO was 50:1 to 200:1). The slurry was cured and dried at 120 °C, and further carbonized above 1000 °C.With increasing mass ratios of GO to PF, the interlayer distance of carbon decreased from 3.88 Å to 3.70 Å, less than 3.93 Å of pure PF, suggesting that the addition of graphene oxide may enhance the graphitization of PF derived carbon. La and Lc of GHCs also increased accordingly with the increasing of GO proportion, indicating the development of graphitic microcrystallite. C1s X-ray photoelectron spectra showed that, the sp2 C content of GHCs increased from 48.4% to 54.5%, which was several time larger than the pure contribution from physical addition of graphene theoretically (from 0.5% to 2%, corresponding to the mass ratio of PF:GO from 200:1 to 50:1 ). From HRTEM and SAED images, a significant improvement of short-range ordering degree can be clearly detected with the increasing of GO content. Those results were so consistent, indicating that GO gives an effcient induced local graphitization in hard carbon.The sodium storage performances of GHCs were characterized in a half cells with metal Na as the counter electrode, using 1.0 M NaPF6 in ethylene carbonate /dimethyl carbonate (1/1 in volume) as the electrolyte. As-prepared hard carbon delivered a superior ICE over 90%, high reversible capacity of 343 mAh/g and excellent cycling performance beyond 150 cycles with a capacity retention over 90%. This may be due to the characteristics of GHCs with less defective and lower surface area , especially with the Intensified local graphitization induced by graphene oxide. Figure 1