LiCo1-2xMgxMnxO2 (0 <= x <= 0.05) materials were prepared from Co1-2xMgxMnx(OH)(2) (0 <= x <= 0.05) co-precipitated precursor materials by mixing precursor materials with stoichiometric amounts of Li2CO3 and heating to 900 degrees C for 10 h. All precursor and lithiated materials were characterized by Scanning Electron Microscopy, X-ray Diffraction (XRD), Inductively Coupled Plasma - Optical Emissions Spectroscopy and electrochemical testing. In situ XRD was performed on LiCo1-2xMgxMnxO2 (x = 0, 0.02, 0.05) electrodes while cycling to study the effects of substitution on phase transitions and unit cell variations. Increasing Mg/Mn substitution in the material was found to slightly increase the 1st charge capacity, decrease the 1st discharge capacity and increase the 1st cycle irreversible capacity (3.6 V-4.7 V). Cells with even 1% Mg/Mn doping were shown to have markedly improved cycling performance, and results suggest that the improvements stem from suppressing the cell impedance growth, not from the suppression of the O3-O6-O1 phase transitions. Differences between LiCoO2 cells and LiCo0.9Mg0.05Mn0.05O2 cells become significant at 4.5 V and above, where the LiCoO2 cells underwent the O3-O6-O1 phase transitions and the LiCo0.9Mg0.05Mn0.05O2 cells appeared to undergo oxygen loss but not this series of phase transitions. (C) The Author(s) 2017. Published by ECS. All rights reserved.