Effect of Reversible Lithium Ion Intercalation on the Size-Dependent Optical Properties of Graphene Quantum Dots

Abstract

Three different sizes of graphene quantum dots have been synthesized in order to explore the size - dependent effects on the efficiency of energy storage and optical behavior while functioning as an active material in rechargeable lithium ion batteries (LIBs). Subtle changes in the dimensional as well as optical properties of GQDs upon Li-ion intercalation and de-intercalation have inexorably been investigated as a function of state of charge (SOC). Systematic experiments performed at three different SOC's (∼05%, ∼50% and ∼95%) show interesting, reversible changes in the UV-VIS absorption spectra which could be explained by the mechanism of charging-discharging involving the influx/out flux of Li-ions. We have also explored the possibility of designing a new smart battery electrode with intrinsic state-of-health indicator based on the light emission properties of GQD as an active material. More importantly, the preferred armchair edges or sites for a reversible lithium intercalation in GQDs in contrast to the zigzag sites are evident from Raman analysis. Interestingly, excitation dependence in the PL spectra has been observed after Li intercalation whereas the pristine GQDs reveal excitation independent emission which is explained due to the involvement of surface states / traps. Some of these results could be useful for understanding the energetic of Li-ion intercalation and deintercalation in multilayer graphene and related composites.