The Li-Mn-Ni-O system has received much attention for potential positive electrode materials in lithium ion batteries. In recent work the phase diagram under certain experimental conditions has been mapped out. Using the phase diagram as a guide, it is possible to select compositions near the boundary of the layered region that are layered-layered nano-composites or are single phase layered depending on the synthesis conditions. Some compositions are single phase layered no matter what synthesis conditions are used. Other compositions near LiNi0.5Mn0.5O2, which lie near the boundary of the single phase layered region, can either be single phase layered when quenched or a layered-layered nano-composite if cooled more slowly in an oxygen poor atmosphere or be a three phase material if cooled slowly in an oxygen rich atmosphere. Here, XRD and electrochemical data are reported on samples synthesized under various oxygen partial pressures for both quenched and slow cooled samples. These results are related to the phase diagram in order to better understand the consequences of the structural changes taking place during cooling on the electrochemistry. Single phase samples show excellent charge-discharge capacity and reversibility. However, there is a dramatic decrease in capacity in samples showing the first signs of forming a layered-layered nano-composite, suggesting that layered-layered nano-composites should be avoided. (C) 2014 The Electrochemical Society. All rights reserved.