Equal amounts of Fe- and Ni-substituted Li2MnO3 (chemical formula: Li1+x[(Fe1/2Ni1/2)(y)Mn1-y](1-x)O2, 0 < x < 1/3, 0.2 <= y <= 0.8) were synthesized using coprecipitation-hydrothermal-calcination. Although the samples with y less than 0.5 are only monoclinic Li2MnO3-type structure (C2/m), samples with y larger than 0.6 show a two-phase nature consisting of the monoclinic phase and cubic LiFeO2 phase (Fm (3) over barm). Electrochemical characterization as a positive electrode shows that the Li2O extraction region disappears above y = 0.6 on initial charging and that the energy density is decreased drastically above the composition on initial discharging. The optimized transition metal ratios are y = 0.4 and 0.5 because the initial average discharge voltage increases with y and the maximum initial cycle efficiency is attained. In the optimized composition, the Fe- and Ni-substituted Li2MnO3 is a 3.5 V class positive electrode, with similar charge and discharge profiles to those of the most attractive active material, NMC positive electrode (chemical formula: Li1+x[(Fe1/2Ni1/2)(y)Mn1-y](1-x)O-2, 0 < x < 1/3, 0.2 < y < 0.8). Consequently, Fe can be used as an activator in combination with Ni for constructing "Co-free" Li2MnO3-based positive electrodes. The calcination-condition-dependence of electrochemical properties at the optimized composition is also examined. The effects of the Fe valence state on initial charge-discharge curves are discussed. (C) 2010 Elsevier B.V. All rights reserved.