The Li-Mn-Ni-O system has received much attention for potential positive electrode materials in lithium ion batteries. This article is an executive summary of a large project that used combinatorial samples synthesized at over 500 compositions to determine the entire Li-Mn-Ni-O and Li-Co-Mn-O pseudo-ternary systems under various synthesis conditions. During slow cooling, the boundaries of the single phase layered region in the Li-Mn-Ni-O system move significantly making the solid-solution area smaller, while the complex co-existence region, made up of two 3-phase regions, transforms dramatically. The impact of these phase transformations on battery performance is presented here for the first time. The electrochemical data for three new materials present in the co-existence region demonstrates why efforts to design a spinel-layered composite electrode have been so difficult. Furthermore, the layered region is considerably larger than previously discussed in the literature implying that a portion of the region remains unexplored. Additionally, matching published lattice parameters with contour plots obtained from the combinatorial studies shows that the common practice of using excess lithium during synthesis may result in single phase compounds with more lithium than the target stoichiometry, which helps explain the large variation in electrochemical performance seen in the literature. This study is therefore a significant contribution toward a complete understanding of how synthesis conditions affect Li-Mn-Ni-O structures and their electrochemistry. (C) 2013 The Electrochemical Society. All rights reserved.