Investigation on the first-cycle charge loss of graphite anodes by coating of the pyrolytic carbon using tumbling CVD

Abstract

The first-cycle charge loss of graphite coated with pyrolytic carbon using a tumbling chemical vapor deposition (CVD) process has been studied for the active material of anodes in lithium-ion secondary batteries. Through the coating of pyrolytic carbons on the surface of graphite particles, carbon materials with a core (graphite)-shell (pyrolytic carbon) structure can be obtained. The irreversible capacity of these carbons at first charge/discharge cycle has been examined using a charge-discharge cycler. The coating of the pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity, which leads to improvement of first cycle coulombic efficiency from 87.21 to 93.32%. The irreversible capacity formed above 0.8 V vs. Li/Li+ at the first charging cycle is proportional to the surface area of the carbon, that is, the coating time of pyrolytic carbon. The plateau of the first-cycle charging curve disappears distinctly with the coating of pyrolytic carbon. The reduction of the initial irreversible capacity in graphite coated with pyrolytic carbon results from this disappearance of the plateau. This means that the coating layer of pyrolytic carbon is more compatible with the electrolyte than the surface of bare graphite. From electrochemical impedance spectroscopy spectra, the increase of contact resistance in bare graphite electrodes occurs at 0.8-0.2 V vs. Li/Li+ in the first charging and the coating of pyrolytic carbon on the surface of graphite can suppress the increment of contact resistance. (C) 2004 The Electrochemical Society.