Interfacial impedance study of Li-ion composite cathodes during aging at elevated temperatures

The interfacial impedance behavior of carbon black, LiNi0.8Co0.15Al0.05O2, and LiFePO4 composite electrodes during storage in 1.2 M LiPF6, ethylene carbonate/ethyl methyl carbonate 3:7 wt % electrolyte at 45 degrees C was investigated. The effect of thermal pretreatment (120 degrees C under vacuum) prior to aging on the electrochemical properties of the electrodes was studied in detail. Electrochemical impedance measurements, which were carried out periodically during storage, showed that (i) the electrodes' interfacial impedance decreased substantially with the heat-treatment time, (ii) the rate of impedance increase upon storage in electrolyte at elevated temperatures was predetermined by the heat-treatment parameters, and (iii) physicochemical properties of carbon (carbon black) were primarily responsible for the observed interfacial impedance behavior of the electrodes during aging. Scanning electron microscopy, energy-dispersive X-ray, and Fourier transform infrared analysis of aged electrodes revealed a thin surface film of electrolyte decomposition products. Adsorbed water and oxygen containing surface groups on carbon reacted with the electrolyte to produce inorganic and organic surface deposits. The thin-film formation on carbon black particles during aging, and consequently, a gradual loss of electronic conductivity within the electrode is most likely responsible for degradation of electrochemical performance.