Using a novel synthesis technique similar to the Columbite method, we have prepared materials from the series Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2, 0 <= x <= 1/2, with high density and average particle size greatly in excess of 1 mu m, without the aid of a sintering agent. The structure of NixMn((1-x))(OH)(2) precursors, as a function of temperature and composition, was studied in detail. These materials phase-separate into mixed oxides at approximately 500 degrees C for most compositions, but compositions with x >= 3/14 convert to single-phase spinel upon further heating. This allows for the production of a single-phase precursor with a homogeneous cation distribution and large particle size and density. By reacting these mixed-metal oxide spinel precursors with LiOH center dot H2O, we have prepared dense Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 materials without heating over 900 degrees C. These dense materials will allow production of electrodes for advanced Li-ion cells with greater volumetric energy density. (c) 2005 The Electrochemical Society.