Graphite derivatives for Li and Na rechargeable batteries

Abstract

The development of a low-cost and high-performance energy storage device is considered as a key issue in moving toward a sustainable society. The electricity production from renewable energy resources such as solar, wind and geothermal energy does not always coincide with consumption time. Therefore, the energy storage devices is necessary to account for the discrepancy. Currently, various energy storage devices (i.e. Li rechargeable batteries, Na rechargeable batteries, and supercapacitors) are intensively studied for emerging large scale applications.

For satisfying the emerging large scale applications, it is of prime importance in developing low-cost and high energy/power energy storage devices. Graphite is earth-abundant, non-toxic, and it can be obtained by relatively low cost process. Furthermore, graphitic materials have high electric conductivity, which is beneficial to deliver high power density. Under this consideration, graphite derivatives are the most promising electrode materials for next generation energy storage applications. This thesis proposes novel approaches for utilizing graphite derivatives as electrodes, including both anode and cathode, for Li and Na rechargeable batteries, while graphite electrode is currently used only for Li rechargeable battery anodes.

Chapter Ⅱ deals with sodium storage behaviors in natural graphite using ether-based electrolytes. This thesis demonstrates that ether-based electrolytes enable sodium intercalation into natural graphite using sodium-solvent co-intercalation behaviors. The natural graphite could provide superior cycle stability over 2000 cycles without noticeable capacity degradation and supply outstanding rate capability.

Chapter Ⅲ introduces functionalized graphene nano-platelets with porous structures for Li and Na rechargeable battery applications. The functional groups anchored with conductive graphene surface can store Li and Na ions with acceptably high voltage for cathode applications through surface Faradaic reactions. Because the functionalized graphene cathode uses surface Faradaic reactions differently from conventional inorganic electrode materials (i.e. LiCoO2, LiMn2O4, and LiFePO4) utilizing solid-state diffusion, much higher rate capability can be obtained. Furthermore, the surface Faradaic reaction does not accompany large volume change, and therefore, the functionalized graphene cathode can provide excellent cycle performance.

A new type energy storage device, all-graphene-battery, will be introduced in Chapter Ⅳ. All-graphene-battery is composed of functionalized graphene cathode and reduced graphene anode. In this device, the graphene with surface functional groups works as a cathode and that without functional groups functions as an anode. Because both electrodes use surface reactions, high power density can be obtained while maintaining high energy density.