We report herein on the synthesis of "layered-layered" integrated xLi(2)MnO(3)center dot(1 - x)LiMn1/3Ni1/3Co1/3O2 materials (x = 0.3, 0.5, and 0.7) using the self-combustion reaction in solutions containing metal nitrates and sucrose. The nanoparticles of these materials were obtained by further annealing of the as-prepared product in air at 700 degrees C for 1 h and submicrometric particles were obtained by further annealing at 900 degrees C for 22 h. The effect of composition on the electrochemical performance was explored in this work. By a rigorous study with high resolution transmission electron microscopy (HRTEM), it became clear that the syntheses with the above stoichiometries produce two-phase materials comprising nanodomains of both rhombohedral LiNiO2-like and monoclinic Li2MnO3 structures, which are closely integrated and interconnected with one another at the atomic level. Stable reversible capacities similar to 220 mAh/g were obtained with composite electrodes containing submicrometer particles of 0.5Li(2)MnO(3)center dot 0.5LiMn(1/3)Ni(1/3)Co(1/3)O(2). Structural aspects, activation of the monoclinic component, and stabilization mechanisms are thoroughly discussed using Raman spectroscopy, solid-state NMR, HRTEM, and X-ray diffraction (including Rietveld analysis) in conjunction with electrochemical measurements. This work provides a further indication that this family of integrated compounds contains the most promising cathode materials for high energy density Li-ion batteries. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3463782] All rights reserved.