Ni-composite microencapsulated graphite as the negative electrode in lithium-ion batteries - II. Electrochemical impedance and self-discharge studies

Electrochemical impedance and self-discharge studies were carried out to investigate lithium intercalation into bare and Ni-coated KS10 graphite. Values of the charge-transfer resistances, exchange current densities, surface film resistances, and lithium-ion diffusion coefficients as functions of the state of charge (SOC) all favored the 10 wt % Ni composite KS10 graphite over bare KS10 graphite when these materials were used as the negative electrode in a Li-ion cell with mixed organic electrolyte. The charge-transfer resistances were always lower and gave rise to between 26 and 27% larger exchange current densities, which increased from 137 to 614 mA/g as the SOC increased. The surface film resistances for Ni composite KS10 were between 0.02 and 0.05 Omega g, slightly smaller than those of 0.03 to 0.08 Omega g for bare KS10, and both surface film resistances decreased with increasing SOC. The lithium-ion diffusion coefficients were always slightly larger, ranging between 1.09 x 10(-9) and 6.7 x 10(-9) cm(2)/s. Results from the self-discharge study also favored the 10 wt % Ni composite KS10, which exhibited less capacity loss over a 10 day period compared to bare KS10. (C) 2000 The Electrochemical Society. S0013-4651(99)09-057-6. All rights reserved.