Lithium insertion and extraction for high-capacity disordered carbons with large hysteresis

Disordered carbons heat-treated from 550 to 1000 degrees C containing hydrogen atoms showed high specific capacities with large hysteresis in the potential when used as anodes in lithium-ion cells. The lithium storage mechanism in the disordered carbons has been investigated by the charge-discharge test, X-ray diffraction (XRD) and solid-state Li-7 NMR measurements. Variation of the layer spacing of the disordered carbon heat-treated at 900 degrees C with insertion and extraction indicated that lithium was inserted into the unorganized carbon site (U-site) near O V us Li/Li+ after insertion into the layer structure site (L-site) and removed from the U-site near 1 V after the extraction from the L-site. Li-7 NMR spectra of the lithiated disordered carbons heat-treated at 550 degrees C showed two bands with a relatively small shift (< 10 ppm) from 0 ppm vs LiCl, indicating that stored lithium had an ionic character. The results of Li-7 NMR analysis revealed the existence of the ionic lithium stored in the reversible storage sites and lithium trapped in the irreversible storage site. The high capacity with large hysteresis was attributed to the ionic lithium stored on the condensed aromatic ring in the U-site. (C) 1997 Elsevier Science Ltd.