Impedance Spectra of Nonhomogeneous, Multilayered Porous Composite Graphite Electrodes for Li-Ion Batteries: Experimental and Theoretical Studies

Abstract

The first lithium-ion insertion and extraction processes of a graphite electrode at different potentials were studied by electrochemical impedance spectroscopy (EIS). The results revealed that, below the potential of 0.65 V during the first lithiation, three semicircles were observed in the Nyquist plots; namely, two semicircles appeared in the middle-frequency region. This new phenomenon has been investigated through the detailed analysis of the change of kinetic parameters obtained from simulating the experimental EIS data for lithiumion insertion and extraction in graphite as a function of potential. It has been found that the two semicircles in the middle-frequency region were strongly potential-dependent, and they were both attributed to the charge-transfer process. Evidence was presented (supported by model calculation) that the two semicircles observed in the middle-frequency region originated from the nonhomogeneous, multilayered porous microstructure of the graphite electrode, which resulted from a parallel combination of impedance response of the thinner parts and thicker parts of the electrode.major State Basic Research Development Program of China[2009CB220102]; Fundamental Research Funds for the Central Universities[2010LKHX03, 2010QNB04, 2010QNB05