Characterization of Surface Free Energy of Cathode Materials for Lithium-ion Batteries

Abstract

While the lithium-ion batteries (LIBs) are expanding their application area from mobile IT devices to the large scale power sources for electric vehicles and renewable energy storages, the limited cycle life of the present LIBs has been pointed out as the main obstacle. The degradation of LIBs employing LiMn2O4 and LiCoO2 cathode materials is known to be initiated by the metal dissolution from the cathode materials. Owing to the slow kinetics of the metal dissolution behavior, the quantitative analysis of the trace amounts of dissolved metal ions has been relied on the spectroscopic techniques such as the inductive coupled plasma (ICP) or the atomic adsorption spectroscopy (AAS). These conventional methods take at least several days and fail to provide any information on the cathode surface properties. We herein report that the SFE of cathode materials is closely related to their metal dissolution behavior. The SFEs of various types of LiCoO2 and LiMn2O4 are obtained by the contact angle measurement: the liquid adsorption method for powder samples and the sessile drop method for electrode samples. We confirm that the metal dissolution is determined dominantly by the polar component of the SFE of the cathode surface. We also found that the Al2O3 coating suppresses the metal dissolution, which is ascribed to the deceased polarity of cathode surface. The basic polarity of cathode electrodes is smaller than that of powders, which suggests a possibility that the metal dissolution can be suppressed through the optimization of the composite electrode components (binders and conductive carbons). We also confirmed the correlation of the SFEs with the types of crystal structures, which was examined by using SEM and XRD analysis. ⓒ 2013 DGIST