Fingerprinting electrode-electrolyte interfaces and intercalant kinematics in flake graphite
Graphite intercalation compounds are critically important in modern technology and today’s economy. For instance, they serve as the anode in the ubiquitous lithium-ion battery. However, despite considerable efforts to understand intercalation compounds, their behavior during charging and discharging at the macromolecular level is still not well understood, even in idealized circumstances. Recent developments in nano-fabrication have brought forth a platform to study this dynamical process with the high spatial resolution of an electron microscope. We expanded on and developed new fabrication techniques to form ultra-thin, in situ fluid cells. With this platform we imaged the graphite intercalation process with an optical and a transmission electron microscope (TEM). Optical videos show, vividly the graphite flake charging and discharging and give insight to the lithium-graphite structure over multiple cycles. Our TEM images provide evidence that defects dominate the intercalation dynamics. We used electron energy loss spectroscopy (EELS) to map, in situ, lithium ion kinematics within a graphite flake by tracking subtle shifts in the graphite plasmon as it intercalates. Using EELS we were able to image and identify two different dendrite types, lithium metal and lithium hydride, that grew on the graphite’s edge during extreme charging conditions. In situ EELS also shows a lithium oxide signal build up at the edge of the graphite and could suggest that it plays an important role in the composition and stability of the solid-electrolyte interface (SEI). Dendrite formation and the SEI are critical in lithium-ion battery safety and overall function. Our results connect bulk and nano-scale experiments of graphite intercalation providing a visual bridge — one that is sorely needed — between the idealized systems normally used in simulations and actual electrochemical batteries.