X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), and chemical lithiation experiments were used to evaluate the phases associated with the electrochemistry of the mixed-metal layered LiMn0.5M0.5O2 oxide electrode. These results, along with coin-cell cycling data from the substituted layered (1 - x)Li2TiO3.(x)LiMn0.5Ni0.5O2 composite oxide electrode are reported. The cycling behavior of Li/0.05Li(2)TiO(3).0.95LiMn(0.5)Ni(0.5)O(2) (x = 0.95) cells over an extended voltage window (4.3 or 4.6-1.25 V) under moderate current rate have yielded rechargeable capacities above 250 mAh/g. These large capacities and structural data suggest that both the composite (1 - x)Li2TiO3.(x)LiMn0.5Ni0.5O2 and LiMn0.5Ni0.5O2 (standard) layered electrodes operate predominantly off two-electron redox couples, Ni4+/Ni2+ and Mn4+/Mn2+ approximately between 4.6 and 2.0 V, and between 2.0 and 1.0 V versus metallic Li, respectively. The LiMn0.5Ni0.5O2 layered oxide is shown to reversibly react chemically or electrochemically with Li to form a stable, but air-sensitive dilithium compound, Li2Mn0.5Ni0.5O2 (Li2MO2; M = metal ion) that can be indexed to the space group P-3m1. (C) 2003 Elsevier Science B.V. All rights reserved.