Defect evolution in graphene upon electrochemical lithiation.

Despite rapidly growing interest in the application of graphene in lithium ion batteries, the interaction of the graphene with lithium ions and electrolyte species during electrochemical cycling is not fully understood. In this work, we use Raman spectroscopy in a model system of monolayer graphene transferred on a Si(111) substrate and density functional theory (DFT) to investigate defect formation as a function of lithiation. This model system enables the early stages of defect formation to be probed in a manner previously not possible with commonly used reduced graphene oxide or multilayer graphene substrates. Using ex situ and Ar-atmosphere Raman spectroscopy, we detected a rapid increase in graphene defect level for small increments in the number of lithiation/delithiation cycles until the I(D)/I(G) ratio reaches ∼1.5-2.0 and the 2D peak intensity drops by ∼50%, after which the Raman spectra show minimal changes upon further cycling. Using DFT, the interplay between graphene topological defects and chemical functionalization is explored, thus providing insight into the experimental results. In particular, the DFT results show that defects can act as active sites for species that are present in the electrochemical environment such as Li, O, and F. Furthermore, chemical functionalization with these species lowers subsequent defect formation energies, thus accelerating graphene degradation upon cycling. This positive feedback loop continues until the defect concentration reaches a level where lithium diffusion through the graphene can occur in a relatively unimpeded manner, with minimal further degradation upon extended cycling. Overall, this study provides mechanistic insight into graphene defect formation during lithiation, thus informing ongoing efforts to employ graphene in lithium ion battery technology.