Metal oxide coating on positive active materials in lithium-ion batteries is an effective way to improve its cycleability. However, coating state of the metal oxide layer and the mechanism of the improvement have not been fully understood. In this paper, the coated state of ZrO2 on a LiCoO2 thin-film electrode was investigated using a ferrocene redox reaction, and the role of the ZrO2-coating on the degradation phenomena of the LiCoO2 was investigated. The redox behavior of ferrocene revealed that the ZrO2 layer did not cover the entire surface area of LiCoO2, that is, the ZrO2 layer was not compact and contained cracks. In addition, a lithium-ion deficit phase (Li1-x CoO2) was irreversibly formed on the uncovered LiCoO2 areas. In spite of the imperfect ZrO2 layer, the formation of the lithium-ion deficit phase inside the bulk was suppressed. It is clarified that a partial ZrO2 coating on the LiCoO2 is sufficient to suppress the growth of the lithium-ion deficit phase inside the bulk of LiCoO2.