Effects of synthesis temperature on the electrochemical characteristics of pyrolytic carbon for anodes of lithium-ion secondary batteries

The electrochemical properties of new disordered carbon materials obtained by a gas-phase reaction of LPG (liquid propane gas) have been studied. Pyrolysis of LPG was performed in the temperature range 900 to 1200 degrees C. The lithium storage mechanism in these disordered carbons has been investigated by the charge-discharge tester, cyclic voltammeter, X-ray diffraction (XRD), solid-state Li-7 nuclear magnetic resonance (NMR), and high-resolution transmission electron microscopy (HRTEM). As the synthesis temperature decreases, the reversible capacity of the disordered carbons increases and exceeds that of graphite (372 mAh/g) in the case of those synthesized below 1100 degrees C. A large hysteresis in the charge-discharge potential profiles is observed, but it disappears with an increase of the synthesis temperature. Cyclic voltammetric curves show that the charging current peak near 0 V vs. Li/Li+ and the discharging current peak at ca. 1.1 V vs. Li/Li+ increase gradually with a decrease of the synthesis temperature, these peaks correspond to the plateaus observed in the charge-discharge potential profiles. Micropores are observed in the disordered carbon synthesized below 1000 degrees C by HRTEM. The size of the micropores increases from 0.5 to 1 nm as the synthesis temperature decreases. XRD patterns and NMR spectra suggest that the high capacity and large hysteresis of these disordered carbons are due to the storage of lithium in the micropores. (C) 1999 The Electrochemical Society. S0013-4651(99)03-114-6. All rights reserved.