LiNi0.8Co0.1+xMn0.1-xO2 cathodes with x = 0, 0.03, and 0.06 were prepared by firing a mixture of stoichiometric amounts of LiOH center dot H2O and coprecipitated Ni0.8Co0.1+xMn0.1-x(OH)(2) powders at 800 degrees C for 15 h. Using these powders, their storage characteristics upon exposure to air and electrolytes at 90 degrees C were compared before charging and after charging to 4.3 V with a variation of the storage time. The discharge capacities of the LiNi0.8Co0.1+xMn0.1-xO2 cathodes with x = 0, 0.03, and 0.06 were 192, 186, and 184 mAh/g, respectively, while their irreversible capacity ratios were 13, 12, and 8%, respectively. As the Co content (x) increased in the cathode, both the Ni2+ content in the lithium 3a sites, and the contents of the LIOH and Li2CO3 impurity phases decreased. In particular, changes in the oxidation state of the Ni and Mn ions after 4.3 V charging upon storage at 90 degrees C were monitored using X-ray absorption near-edge spectra, and Ni4+ was found to reduce to Ni3+ while the oxidation state of the predominant Mn4+ did not change. However, residual Mn3+ ions in the cathodes dissolved into the electrolytes. Moreover, the cathodes stored at 90 degrees C for 7 days were transformed into a spinel phase (Fd3m), regardless of the Co content. In an effort to resolve this dissolution problem, Al2O3 and Co-3(PO4)(2) nanoparticles were coated onto the cathode (LiNi0.8Co0.1Mn0.1O2) with the highest amounts of metal dissolution at 90 degrees C. The results showed that the Co-3(PO4)(2)-coated cathode exhibited a greatly decreased metal dissolution and decreased its irreversible capacity by 5%, compared with a bare and Al2O3-coated cathode. (C) 2008 The Electrochemical Society.