The deterioration behavior of the Li-ion batteries consisted of LiMn1.7Al0.3O4 and hard carbon were investigated at the storage temperature from 50 to 75°C. The deterioration phenomena were analyzed by using XPS, FT-IR, SEM and TEM for the negative electrode surface. As a result, the negative electrode surface of deteriorated battery showed the dissociation of manganese from the positive active material, and also the decomposition of electrolyte and LiPF6 components was observed. When the storage temperature was over 65°C, the ratio of PO4 3− component in the films particularly increased, suggesting that the transformation was thought to be caused by the decomposition of LiPF6. Furthermore, we developed the solid electrolyte interface (SEI) observation technology. The film thickness of the deteriorated negative electrode was found to be varied from 40 to 200nm.