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 at the temperature range of 900℃ to 1200℃. The lithium storage mechanism in these disordered carbons has been investigated by the charge-discharge tester, cyclic voltammeter, XRD(x-ray diffraction), solid-state $^7Li$-NMR(nuclear magnetic resonance) and HRTEM(high-resolution transmission electron microscopy).
As the synthetic temperature decreases, the reversible capacity of the disordered carbons increases and exceeds that of graphite (372mAh/g) in the case of them synthesized below 1100℃ Large hysteresis in the charge-discharge potential profiles is observed and it disappears with the increase of synthetic temperature. Cyclic voltammetric curves show that charging current peak near 0V vs. $Li/Li^+$ and discharging current peak at ca 1.1V vs. $Li/Li^+$ increase gradually with the decrease of synthetic temperature and they correspond to the Plateaus observed in the charge-discharge potential profiles. Micropores are observed in the disordered carbon synthesized below 1000℃ by HRTEM. The size of micropores increases from 0.5nm to 1nm as the synthetic temperature decreases. XRD patterns and NHR spectra suggest that high capacity and large hysteresis of these disordered carbons are due to the storage of lithium in the micropores.
The electrochemical properties of highly ordered graphite coated with Pyrolytic disordered carbons by a tumbling CVD(chemical vapor deposition) method have been studied. The effects of coating layer on the reversible capacity, irreversible capacity, cycle life, and rate capability of graphite electrodes have been investigated by the charge-discharge tester, EIS(electrochernical impedance spectroscopy), SEM(scanning electron microscopy), and HRTEM.
Through the coating of pyrolytic carbon on the surface of graphite, new carbonaceous materials having core-shell structure were synthesized. Core part is a ordered carbon (graphite) and shell part is a disordered carbon (pyrolytic carbon). In the graphite coated with the pyrolytic carbon reduction of irreversible capacity and improvement of cyclability and rate capability are observed without the change of reversible capacity. Initial coulombic efficiency of graphite electrodes increase from 87.21% to 93.32%. Nyquist plots obtained from EIS show that the contact resistance of electrodes distinctly decrease by coating of the pyrolytic carbon. It is suggested that the coating layers suppress the formation of SEI(surface electrolyte interface) layers below 0.8V vs. $Li/Li^+$ by preventing cointercalation of solvents with lithium ions between graphene layers.
