The interface reaction between Al2O3-coated LiLi0.05Ni0.4Co0.15Mn0.4O2 and liquid electrolyte was investigated. The Al2O3-coated LiLi0.05Ni0.4Co0.15Mn0.4O2 showed no large difference in the bulk structure, comparing to bare LiLi0.05Ni0.4Co0.15Mn0.4O2. The coated Al2O3 was found to have an amorphous structure from X-ray diffraction study. A small amount of Al2O3 coating (0.25 wt % in the final composition) showed that a uniform mesoporous Al2O3-coating layer whose thickness is of about 5 nm covers LiLi0.05Ni0.4Co0.15Mn0.4O2 particles, as confirmed by transmission electron microscopy. At higher concentration (2.5 wt % in the final composition), the irregular tens of nanometer-sized Al2O3 powders were observed on the surface of the active material instead of the uniform coating layer. Despite the insulating nature of Al2O3, the thin coating was effective to improve the battery performances, depending on the thickness of the Al2O3-coating layer, and used electrolytic salt. The Al2O3 coating resulted in a higher capacity retention, especially at 60 °C. The alumina layer was significantly protective against HF attack into the electrolyte during cycling so that the decomposition of active material from HF attack would be greatly suppressed. The lower impedance would be ascribed to the positive effects on the electrode/electrolyte interface, the less amount of decomposition of active material by HF and/or scavenging of HF by Al2O3-coating layer into the electrolyte. These effects made it possible to maintain the morphology of active material during extensive cycling. Meanwhile, the bare particles were severely degraded by cycling due to HF.