Li-1.0[Li1/7Mn4/7 Ni-2/7]O-2 cathode material was prepared by a facile, one-pot synthesis method. The structure of the material was determined by Rietveld refinement of structural models using high-resolution synchrotron X-ray and neutron powder diffraction data and was found to consist of two distinct phases. The major phase, with composition Li1.25(3)Ni0.17(1) Mn0.61(1)O2, close to the well-characterized L1.2Ni0.2Mn0.6O2 composition can be described as an intergrowth structure of Li2MnO3 and LiMn0.5Ni0.5O2 domains and the second phase is a lithium-deficient layered structure with refined composition Li0.85(1)Ni0.57(1)Mn0.55(1)O2. The composite cathode has a high initial discharge capacity of 250 mAh/g which drops to 225 mAh/g on the 2nd discharge cycle. This capacity is maintained on subsequent cycles. Time-resolved in-situ synchrotron XRD data was used to study the changes in the lattice parameters and phase evolution of the two phases during Li-insertion and extraction. (C) 2013 The Electrochemical Society. All rights reserved.