We conducted a long-term cycling test of a commercial 18650-type lithium-ion battery with a capacity of 3100 mA • h at room temperature and investigated the degradation mechanism of the battery's LiNi 0.82 Co 0.15 Al 0.03 O 2 positive electrode. Sphericalaberration-corrected scanning transmission electron microscopy (Cs-STEM) revealed the presence of a thick degraded surface layer on the surface of the positive active material after the cycling test, and electron energy loss spectroscopy (EELS) revealed that the degraded surface layer continuously evolved from a LiNiO 2 layered structure to a NiO structure, in the direction from the bulk toward the surface. Hard X-ray photoemission spectroscopy (HX-PES) indicated that the majority of low-valence Ni existed on the surface of the positive active material, which was charged after the cycling test, and that the degraded surface layer was inactive against charge reaction. The results suggest that the degraded surface layer was responsible for battery degradation during the cycling test. X-ray photoemission spectroscopy (XPS) indicated that Li 2 CO 3 increased on the surface of the positive electrode after the cycling test. The phenomena would contribute to the formation of the degraded surface layer on the surface of the positive active material.

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