Nano-tube-like surface structure in graphite particles and its formation mechanism: A role in anodes of lithium-ion secondary batteries

Nano-structures on the surface of graphite based carbon particles have been investigated by means of high resolution transmission electron microscopy. The surfaces consist of "closed-edge" structures in a similar manner as carbon nano-tube. That is, they are composed of coaxial carbon tubes consisting of adequate coupling of graphite layer edges. These graphite particles are chemically stable and, therefore, applicable for lithium-ion secondary battery anodes. Molecular dynamics simulations based on the Tersoff potential reveal that the vibrations of the graphite layers at the free edges play an important role in the formation of the closed-edge structures. In lithium-ion secondary batteries, Li ions can intrude into bulk carbon anodes through these closed-edge structures. In order to clarify this intrusion mechanism, we have studied the barrier potentials of Li intrusion through these closed edges using the first-principles cluster calculations. From electrochemical measurements, the carbon anodes composed of these closed-edge structures show actually high battery performance with a large discharge capacity and a small irreversible capacity. This article also implies that we can control these surface structures by choosing some suitable heat treatment conditions and/or pulverization conditions before the final heat treatment process.