Positive electrode materials which do not react with electrolyte at high potentials (>= 4.6 V vs. Li/Li+) are essential for developing Li-ion batteries with high energy densities and long cycle lives. Reactions with electrolyte can be detected using precise measurements of coulombic efficiency (CE) and charge end point capacity slippage. Three single-phase layered compositions in the Li-Mn-Ni-O system, Li[Li0.16Ni0.12Mn0.65 square(0.07)]O-2, Li[Li0.12Ni0.32Mn0.56]O-2, and Li[Li0.09Ni0.46Mn0.45]O-2 were studied by high precision coulometry at upper potential limits of 4.6 V and 4.8 V. When cycled to 4.6 v, Li[Li0.16Ni0.12Mn0.65 square(0.07)]O-2 had a reversible capacity of 225 mAh g(-1) after 50 cycles, and maintained a substantially higher CE and a lower charge end point capacity slippage per cycle than Li[Li0.12Ni0.32Mn0.56]O-2, Li[Li0.09Ni0.46Mn0.45]O-2, and industry standard Li[Ni1/3Mn1/3Co1/3]O-2 (cycled to only 4.2 V). Overall, these results highlight the inherent "inertness" of Li[Li0.16Ni0.12,Mn-0.065 square(0.007)]O-2 and its suitability as a thin protective shell in a core-shell particle configuration. (C) 2014 The Electrochemical Society. All rights reserved.