The importance of graphene stacking sequence in the Li-intercalation mechanism

Graphenic materials used as anodes in lithium-ion batteries have the lowest specific capacity at around 2000 °C. Consequently, graphite is predominantly used, and few studies have investigated the intermediate annealing temperature range of 2000 °C. Starting from a graphitizable coke, this study investigates the lithium-intercalation capacity of graphite materials at increasing carbonization temperatures (1700, 2000, 2100, and 2500 °C). The results are correlated with the structural evolution determined via X-ray diffraction (XRD). The variation in Li-intercalation performance is assigned to the evolution of graphene stacking sequences. Intercalation begins during the early stages of graphitization with the appearance of AB pairs (AB stacking sequence), indicating that the mechanism starts even before the formation of the true graphitic structure. The sample carbonized at 2100 °C exhibits the highest concentration of AB pairs of graphenes, approaching 50%. Consequently, electrical characteristics during lithiation/delithiation reveal a marked transition from stage 1/stage 2 to stage 2/stage 2L in turbostratically stacked samples, while more graphitized samples with AB pairs and then graphitic (ABA, etc.) sequences display a stage 2/stage 2L to stage 2L/stage 3 transition. Raman spectroscopy and anode color changes confirm the various stages. This research enhances our understanding of the lithium-intercalation process in graphite materials in relation to the structural composition of the average crystallite.