Microstructure and electrochemical performance of recycled graphite coated with high coking value coal asphalt

Abstract

Graphite-based anode material is easily destroyed by the embedding and decomposing of electrolyte in the graphite layers, resulting in the reducing of battery cycle efficiency and stability. The graphite anode coated with asphalt to form a “core-shell” structure with amorphous carbon layer can alleviate the expansion and exfoliation of graphite layer, and thus improve the electrochemical performance of the graphite anode material. In this study, the coated asphalt with high coking value and high softening point were prepared by air-blowing oxidation and cross-linking polymerization of medium temperature coal-tar pitch. Moreover, recycled graphite anode was coated by as-prepared asphalt using liquid phase coating method, and the coating properties were systematically investigated. The coating morphology and interfacial microstructure of recycled graphite were analyzed by X-ray diffraction (XRD), Raman spectroscopy (Raman), nitrogen adsorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the electrochemical performance of the anode materials were tested. The results show that asphalt coating can effectively improve the microstructure of recycled graphite. With the increase of asphalt coating amount, the surface defects of graphite decrease, and the specific surface area and pore volume reduce as well. After carbonization, the amorphous carbon layer is formed at the coating interface, and the disorder degree of the coating interface increases with the rise of asphalt coating amount. The formation of amorphous carbon on the coating interface significantly improves the performance of graphite anode. The specific capacity of recycled graphite coated with 12% asphalt reaches 450 mA·h/g, which is 140 mA·h/g higher than that of uncoated graphite. And, the first coulomb efficiency is increased 11.5%, and the cycle stability is improved apparently. Likewise, the specific capacity under 5A·h is enhanced to be 285 mA·h/g, that is 242.8 mA·h/g higher than the uncoated recycled graphite. The coating process using as-prepared coal tar pitch achieved an outstanding rate capability of graphite anode.

Keywords

• coating pitch
• “core-shell” structure
• oxidation cross-linking
• liquid phase coating
• recycled graphite