Structure, morphology, and performance of Li1+x(Ni0.33Mn0.33Co0.33)(1-x)O-2 cathode materials with stoichiometric (x=0) and excess Li (x=0.14) were compared on micron-sized, well-formed single crystals. The additional Li in the fresh overlithiated oxide facilitated the formation of a superstructure with root 3a(hex) x root 3a(hex) x c(hex) unit cells, decreased the cell volume change, and improved the initial O3 phase stability upon lithium extraction. Slower kinetics were observed at low Li content where oxide activation dominated, a process that induced significant structure rearrangements and particle damages. For the first time, morphological evaluation during delithiation was well captured on the micron-sized single crystals. An irreversible activation plateau at 4.4 V was only observed during the first charge of the overlithiated crystals. The oxide's higher discharge capacity and better cycling stability between 2.5 and 4.8 V were attributed to its enhanced O3 phase stability upon deep Li extraction. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.038209jes] All rights reserved.