Lithium-ion batteries will be increasingly important in next-generation energy-storage devices. However, the charge-discharge mechanism of the electrode in these batteries has not yet been revealed. It has been reported that the surface crystal structure changes when these batteries degrade, but the phase transition that occurs on the surface of the cathode material has not yet been clarified because observation of the surface of the grains is insufficient. On the other hand, it has been shown that using Ga⁺ primary ion time-of-flight secondary ion mass spectrometry (TOF-SIMS), a regularity can be observed in the fragment patterns of metal compounds that can provide information on their oxidation states and electron affinities. Therefore, in this study the spectra of various cobalt components, including LiCoO₂, CoO, and Co₃O₄ were compared, and depth profile analyses of Li-ion battery’s cathode materials were conducted using a laboratory-made TOF-SIMS apparatus. Variation in the secondary ion mass spectra was observed from the surface to the interior. Although a phase transition was not directly observed via cross-sectional analysis, it was concluded that the surface of the cathode materials comprised of CoO and/or Co₃O₄, while the interior of the cathode consisted of LiCoO₂. Moreover, it was determined that the thickness of the CoO and/or Co₃O₄ layers on the cathode materials was less than 200 nm.

  A ZAF method has been established as a useful quantitative analysis technique for bulk specimens by EPMA. Accurate quantification can be performed when analyzed particle sizes are greater than an x-ray generation region. In measurements of light elements such as oxygen, fluorine, carbon, boron and nitrogen, it is, however, hard to obtain satisfactory results, comparing with heavy elements analyses. In this study, we have compared quantitative accuracies of the results by ZAF methods with that by Monte Carlo method. The general formula of the ionization distribution function derived by the MC method was compared with the several ZAF methods.

  We found that the x-ray intensities of O, S, Ga, Co and Ni calculated from the proposing general formula are almost equivalent to those obtained by MC method. It is also found that the MC method gives more accurate results than the ZAF methods in the accelerating voltage range of 5 - 25 kV. But, for fluorine the quantitative accuracy obtained by our method is lower than that by MC method.