Kinetics of Li-ion transfer reaction at LiMn2O4, LiCoO2, and LiFePO4 cathodes

Kinetics of LiFePO4, LiMn2O4, and LiCoO2 cathodes operating in 1 M LIPF6 solution in a mixture of ethylene carbonate and dimethyl carbonate was deduced from impedance spectra taken at different temperatures. The most striking difference of electrochemical impedance spectroscopy (EIS) curves is the impedance magnitude: tens of ohms in the case of LiFePO4, hundreds of ohms for LiMn2O4, and thousands of ohms for LiCoO2. Charge transfer resistances (R (ct)) for lithiation/delitiation processes estimated from the deconvolution procedure were 6.0 Omega (LiFePO4), 55.4 Omega (LiCoO2), and 88.5 Omega (LiMn2O4), respectively. Exchange current density for all the three tested cathodes was found to be comparable (0.55-1 center dot 10(-2) mAcm(-2), T = 298 K). Corresponding activation energies for the charge transfer process, , differed considerably: 66.3, 48.9, and 17.0 kJmol(-1) for LiMn2O4, LiCoO2, and LiFePO4, respectively. Consequently, temperature variation may have a substantial influence on exchange current densities (j (o)) in the case of LiMn2O4 and LiCoO2 cathodes.