Structural and electrochemical characterisation of nitrogen enriched carbons produced by the co-pyrolysis of coal-tar pitch with polyacrylonitrile

Carbonaceous materials ranging from soft carbons of moderate nitrogen content (up to 2.5 wt.%) to typical hard carbons with an excess of nitrogen (up to 6 wt.%) were produced by carbonisation at 1050 degreesC of coal-tar pitch (CTP)-polyacrylonitrile blends with various ratio of the components. The resultant carbons were characterised by elemental analysis, optical and transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy and sorption of nitrogen and carbon dioxide. The electrochemical lithium insertion has been investigated in these carbons by a galvanostatic technique, using carbon/lithium two-electrode cells. Independently of the nitrogen content, the electrochemical performance of the soft carbons is comparable. The nitrogen enriched hard carbons demonstrate a relatively low value of reversible capacity, due to the absence of available nanopores. On the other hand, the irreversible capacity increases with the proportion of nitrogen, especially in the form of pyridinic groups. The lone pair of electrons contribute to the trapping of solvated lithium cations on these groups which act as active sites for the electrolyte decomposition during the first reduction, leading to an enhanced irreversible capacity. (C) 2003 Elsevier Ltd. All rights reserved.