Samples of lithium cobalt manganese oxide were prepared by combinatorial and high throughput methods and were studied by X-ray diffraction to determine the pseudo-ternary phase diagram over the entire metal composition range. This report focuses on the nearly triangular single-phase cubic spinel region found to exist between Co3O4, Co2MnO4 and LiMn2O4 when heated at 800 degrees C and quenched. The position and shape of this spinel region shifted dramatically depending on cooling rate. The two cooling rates used in this study were regular-cooling by turning the furnace off and quenching. The spinet region shift can be attributed to an increase in oxygen content taking place during slow cooling. The higher oxygen content in samples that are regular-cooled favors the spinel region over the layered phase. Lattice constant contour plots for the cubic spinel region were also created for both quenched and regular-cooled Li-Co-Mn-oxide systems through fitting of diffraction patterns. (C) 2013 Elsevier B.V. All rights reserved.