Effect of hydrogen-gas treatment on the local structure of graphene-like graphite

Abstract Graphene-like graphite (GLG) has been expected as a new candidate for a large-capacity negative electrode material of a lithium-ion battery. Recently, our group has reported that hydrogen-gas treatment of GLG lowered the mean-discharge potential and increased its capacity. In order to clarify the origin of this improvement, scanning tunneling microscopy (STM) was conducted together with simulation using density functional theory calculation. A GLG mono-sheet suitable for STM observation was synthesized from highly oriented pyrolytic graphite (HOPG) as a raw material. A distorted honeycomb region surrounded by triangular one was observed for GLG mono-sheets and it was well simulated using a model consisting of a topmost graphene with pairwisely introduced ether groups underneath. The above honeycomb region disappeared for GLG treated with hydrogen gas (GLG-H), and it was also well simulated using a model in which each oxygen atom introduced in the graphene was substituted by two hydrogen atoms. Since the resulting local structure of GLG-H was similar to that of graphite which shows low discharge potential, this would lead to the increased accessible capacity under 2.0 V of cut-off voltage.