The continued demand for long-lasting batteries essentially drives us to contribute to a development of high-performance cathode materials, because a cathode material which is one of essential components in Li-ion batteries (LIBs) critically determines an overall energy density of a battery. High-capacity Ni-rich NCM (x > 80%, LiNi_xCo_yMn_zO₂, x+y+z=1) cathode materials have been regarded as one of the most promising candidates to realize high-energy density Li-ion batteries. However, the complete application has been limited due to some critical problems, especially the unstable interfacial stability resulting from the massive formation of residual lithium compounds and the severe electrolyte decomposition. In this study, we investigate the roles of fullerene-based electrolyte additive in Ni-rich NCM-based LIB system and reveal that the fullerene-based additives functionalized with malonic acid moieties, MA-C₆₀, effectively stabilize the unstable interface of Ni-rich cathodes, which are demonstrated by combined experimental and computational studies. According to differential electrochemical mass spectroscopy (DEMS), nuclear magnetic resonance (NMR) analysis and the first principles calculation, the MA-C₆₀ additives deactivate reactive radical species (O₂^•-, LiOCO₃^•, and Li(CO₃)₂^•) induced by electrochemical oxidation of residual lithium compound, Li₂CO₃ on the LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode surface thanks to a highly favorable affinity of the malonic acid moieties with the reactive radical species. Furthermore, the additive mitigates the parasitic reaction of residual lithium compounds with LiPF₆ through the formation of a stable cathode-electrolyte interphase. These roles of the new additive effectively prevent an aging in the cell system by suppressing attacking the interface of Ni-rich cathodes against the harmful reactive oxygen species and drastically enhance the cyclic performance over 400 cycles (nearly ~50 % improvement of capacity retention). Our work offers the potential of new-type of fullerene-based additive for the stable cathode-electrolyte interphase and would help to realize a commercialization of high-energy density LIB cathodes.