In order to further increase the energy density of lithium ion batteries (LIBs), it is of utmost importance to develop advanced electrode materials in combination with suitable electrolytes, which deliver either higher capacities and/or can be operated at high cell voltage with sufficient cycling stability. Here, we introduce (1H-imidazol-1-yl)(morpholino) methanone (MUI) as a cathode electrolyte interphase (CEI) forming electrolyte additive for LiNi1/3Co1/3Mn1/3O2 (NMC111) parallel to graphite cells operated at high cell voltage of up to 4.6 V. The addition of MUI to the carbonate-based reference electrolyte leads to a superior cycling stability in comparison to those with the pure reference electrolyte. The working mechanism of MUI is comprehensively elucidated with various ex situ analytical techniques. A reduction of MUI at the graphite anode is observed starting at a potential of approximate to 0.9 V vs. Li/Li+, resulting in a slightly higher kinetic impairment of lithium ion intercalation/deintercalation into/from graphite. To prevent the unfavorable reduction of MUI at the graphite anode, Li4Ti5O12 (LTO)-based composite anodes are also used to analyze the function of the additive at the cathode surface. We can clearly confirm that a protective film at the cathode surface is formed, helping to suppress parasitic side reactions at the cathode/electrolyte interface during long-term cycling. (C) 2019 The Electrochemical Society.