Layered Li1+x(Ni0.5Mn0.5)(1-x)O-2 (0 <= x <= 0.15) materials have been synthesized using Li2CO3 and coprecipitated (Ni0.5Mn0.5)(OH)(2). The materials with increased lithium content (x >= 0.10) showed distinct differences in particle morphology, which is attributed to the melting of excess Li2CO3 during calcination. The powder XRD patterns and the initial charging curves to 4.8 V indicated that the excess lithium resides in the transition metal layer to form Li2MnO3-like domains in the electrode structure such that these compounds can be reformulated in a two-component composite notation as 2xLi(2)MnO(3)center dot(1-3x)Li(NiyMn1-y)O-2 {y=(1-x)/[2(1-3x)]} with some cation disorder between the two components. The excess lithium significantly affected the charge/discharge behaviors in the low-voltage range (< 2 V) and the rate capability of the electrode materials, which is attributed to increasing Ni3+ content and decreasing Li/Ni disorder (i.e., Ni content in the lithium layer) in the materials.