LARGE HYSTERESIS DURING LITHIUM INSERTION INTO AND EXTRACTION FROM HIGH-CAPACITY DISORDERED CARBONS

Perylene-based disordered carbon (PBDC) heat-treated at 550 degrees C for anodes in Li-ion cells showed large hysteresis and a high reversib le capacity of 800 mAh/g. The hysteresis was analyzed by polarization and impedance measurements. The overpotential during lithium extractio n increased markedly in the range of open-circuit potential, 0.5-1 V v s Li/Li+. The impedance spectra of PBDC during lithium insertion were significantly different from those during extraction. The charge-trans fer resistance for lithiated PBDC during extraction above 0.5 V was mu ch larger than that for the PBDC during insertion. The chemical diffus ion coefficient of lithium, D-Li of PBDC during lithium insertion decr eased almost linearly from 5 x 10(-10) to 3 x 10(-12) cm(2) s(-1) with increasing lithium storage capacity. The values of D-Li during lithiu m extraction above 0.5 V were much smaller than those during insertion . The large hysteresis was due to the large charge-transfer resistance and the slow diffusion of lithium during lithium extraction from the fully lithiated PBDC. The large charge-transfer resistance during lith ium extraction has been interpreted as the rectification of lithiated PBDC, which is similar to that of n-type semiconductors under anodic p olarization.