Lithium nickel-manganese-cobalt oxide (Li(Ni_xMn_yCo_z)O₂ with x+y+z=1, written as NMC), when used as a cathode in Li-ion batteries, has an achievable capacity of approximately half of its theoretical capacity. The inability to utilize the entire Li content in NMC and achieve the full theoretical capacity is due, in part, to instabilities at high voltage. A higher charge cut-off potential, while increasing achievable capacities, leads to poor long-term cycling performance and a diminished useful battery lifetime. To explore the mechanism of the undesirable decomposition in the overcharged, high voltage limit, we studied the role of Ni content and charge cut-off potential on the efficiency of Li (de)intercalation in NMC oxides by using a combination of in situ differential electrochemical mass spectrometry (DEMS), isotopic labeling, and complementary surface characterization. The results show that gaseous decomposition products are evolved in several distinct regimes, in which the phenomena of Li plating at the Li anode, electrolyte oxidation, and cathode decomposition can be distinguished. Our findings highlight that impurity surface species on the cathode play a significant role in first cycle inefficiency.