The purpose of this work was to understand the long-term reversibility characteristics of Li-1.1(Ni1/3Mn1/3Co1/3)(0.9)O-2 by investigating the possible phase transitions of this material when subjected to high potential (>4.5 V vs Li+/Li) electrochemical stress. The effect on the crystal structure after first-cycle charge, extended galvanostatic cycling, and potentiostatic stress was examined by in situ X-ray synchrotron powder diffraction and ex situ X-ray powder diffraction. Li-1.1(Ni1/3Mn1/3Co1/3)(0.9)O-2 did not undergo any phase transition when deeply delithiated because of a lithium-nickel exchange degree of about 4% in our sample. The latter property is believed to be the reason for the improved structural stability: Nickel ions present in the interslab space keep the (MO6)(n) slabs in place, thus preventing the O3 phase from converting into the O1 phase.