Layered Li1+x(Ni0.425Mn0.425Co0.15)(1- x)O-2 materials (0 <= x <= 0.12) were prepared at 1000 degrees C for 12 h in air by a coprecipitation method. As x increased in Li1+x(Ni0.425Mn0.425Co0.15)(1- x)O-2, the substitution of x Li+ ions for x transition metal ions induced for charge compensation an increase in the average transition metal oxidation state. X-ray photoelectron spectroscopy analyses showed that cobalt and manganese were present in these materials in the trivalent and tetravalent states, respectively, and that increasing overlithiation led to the oxidation of Ni2+ ions into Ni3+ ions. The refinement of the crystal structure of these materials in the R (3) over barm space group and magnetic measurements showed a decrease in the Ni occupancy in the Li layers with increasing overlithiation. From an electrochemical point of view, the reversible capacity in the 2 -4.3 V range decreased with overlithiation. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2138573] All rights reserved.