We investigated the structural characteristics of Li-rich xLi(2)MnO(3) center dot (1-x)Li[MnyNizCow]O-2 cathode material (x around 0.5, y:z:w around 2:2:1) and its electrochemical performance in lithium cells at 30 and 60 degrees C. It was established that nanoparticles of the xLi(2)MnO(3) center dot (1-x)Li[MnyNizCow]O-2 compound are intergrown on the nano-scale and are built of thin plates of 40-50 angstrom. We demonstrated that xLi(2)MnO(3) center dot u (1-x)Li[MnyNizCow]O-2 electrodes exhibited at 60 degrees C high capacities of similar to 270 and similar to 220 mAh/g at 1C and 2C rates, respectively. They can be cycled effectively at 30 and 60 degrees C providing capacity similar to 250 mAh/g in the initial cycles, but it fades upon prolonged cycling due, to some extent, to increasing the electrode impedance (charge-transfer resistance) especially at the elevated temperature. The effective chemical diffusion coefficient of Li+ in these electrodes measured during charge to 4.7 V by potentiostatic intermittent titration technique (PITT) was found to be similar to 10(-10) cm(2)/s. From convergent beam electron diffraction and Raman spectroscopy studies we established, for the first time, that partial structural transition from layered-type to spinel-type ordering in xLi(2)MnO(3) center dot (1-x)Li[MnyNizCow]O-2 electrodes occurred in the initial charge to 4.7 V and even at the early stages of charging at 4.1 V-4.4 V. The thermal behavior of the xLi(2)MnO(3) center dot (1-x)Li[MnyNizCow]O-2 material and electrodes are also discussed. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.070302jes] All rights reserved.