Gas adsorption analysis for quantifying the edge sites of graphite

The edge plane exposed on the surface of carbon materials is the active site that determines their properties. When graphite is used as the negative electrode in lithium-ion batteries, the edge plane acts as an access point for lithium insertion/desorption. However, because of the low specific surface area and amorphous carbon coating of graphite particles, accurately estimating the number of edge planes is challenging. Therefore, we propose a technique for determining graphite edge planes based on the stepwise adsorption of Kr onto the hexagonal carbon network surface. Graphite particles with highly modified surface structures were prepared by controlling the particle size, heat treatment temperature, and amorphous carbon coating. Electrochemical evaluation revealed significant differences in charge-transfer resistance, an indicator of edge planes. The edge plane determination results obtained using conventional methods such as X-ray diffractometry and Raman spectroscopy did not correlate well with charge-transfer resistance. Meanwhile, Kr adsorption measurements revealed that the stepwise adsorption behavior, because of the interaction of Kr with the energetically homogenous surface, varied significantly depending on the surface properties of graphite particles. The number of edge planes estimated by gas adsorption strongly correlated with charge-transfer resistance. Consequently, we determined that the graphite edge plane tied to the electrochemical index can be derived using gas adsorption analysis. This edge plane determination technique would be applicable not only to graphite but also to highly crystalline carbon materials with an exposed hexagonal carbon network surface.