Structural and electrochemical properties of Li(Ni1-2xCoxMnx)O-2 (x = 0.1 - 0.3) as positive electrode materials for lithium batteries were investigated. The materials were prepared via a coprecipatation process. From X-ray diffraction studies, it was found that the synthesized materials were crystallized into an alpha-NaFeO2 layered structure (R $(3) over bar $m). With increasing Co and Mn content in Li ) Ni1-2xCoxMnx ( O2, the corresponding lattices were slightly contracted. X-ray absorption near edge spectra of Li[Ni1-2xCoxMnx]O-2 [x = 0.1 - 0.3] also showed that the oxidation state of Ni is changed from 3+ to 2+ when tetravalent Mn is present in Li[Ni1-2xCoxMnx]O-2 oxide(x = 0.2 - 0.3). From Rietveld refinements of powder X-ray diffraction data, it was found that further increase in Co and Mn content also resulted in reduced cation mixing between Li+ and Ni2+ in Li layers. Electrochemical properties and thermal stability were also affected by the physical properties. When tetravalent Mn appeared in the structure, the corresponding cyclability was greatly improved compared to that of Li[Ni0.8Co0.1Mn0.1]O-2. Furthermore, thermal stability at highly delithiated state was also significantly enhanced by the existence of electrochemically inactive tetravalent Mn in the oxide matrix. (c) 2007 The Electrochemical Society.