Spherical natural graphite coated by a thick layer of carbonaceous mesophase for use as an anode material in lithium ion batteries

Abstract

A method for coating a thick layer of carbonaceous mesophase was developed to treat spherical natural graphite (SNG) for use as anodes in lithium ion batteries. The carbonaceous mesophase layer was fabricated by heat treatment of a mixture of SNG and coal tar pitch. The thickness of the carbonaceous mesophase on the surface of the SNG was approximately 2.5 μm, which is effective for enhancing the strength of the carbonaceous mesophase shell and for allowing the shell to maintain good integrity at a high anode density (1.6 g cm−3). The mesophase layer increased the initial columbic efficiency from approximately 90% to 95%, dramatically improved the capacity retention and reduced the irreversible capacity by greatly decreasing the SNG surface area. The initial efficiency, cycle life and rate capability for the SNG anode covered by a thick mesophase layer gave comparable results as the mesocarbon microbeads (MCMB) anode, while the SNG anode reversible capacity of 341 mAh g−1 was higher than that of MCMB, 319 mAh g−1. Electrochemical measurements showed that SNG particles coated by a thick carbonaceous mesophase layer are strong candidates for use as possible anode materials in high energy density lithium ion batteries.