Quasi-2D FCC lithium crystals inside defective bi-layer graphene: insights from first-principles calculations

Quasi-2D crystals inside bilayer graphene have been observed in in-situ TEM experiments [Nature 564 (2018) 234]. It was also revealed that Li crystals have the FCC structure, nucleate at point defects in graphene and contain impurity atoms. Using first-principles calculations, we systematically study the interaction of isolated Li atoms and those assembled in FCC crystals with vacancy-type defects in graphene and show that quasi-2D Li crystals encapsulated between graphene sheets must indeed nucleate at the defects and that the interaction of not only isolated Li atoms but also Li crystals with the defects in graphene is strong. We further demonstrate that a moiré pattern develops at the graphene/Li interface. Finally, we investigate the behavior of impurities most likely to be found in the encapsulated Li crystals, such as O, N, S, and F and show that all impurity atoms take octahedral interstitial positions and strongly interact with atoms in Li crystals, thus impeding the de-lithiation process. Our theoretical work focused on the fundamental aspects of the behavior of Li inside bilayer graphene should help rationalize the results of in-situ TEM experiments and shed light on the role of impurities in the degradation of anode materials during Li-ion battery operation.