Investigation of the surface degradation of LiCoO₂ particles in the cathode materials of Li-ion batteries using FIB-TOF-SIMS

Abstract

  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.