Nickel-rich transition metal oxides is widely regarded as promising cathode materials for high energy density lithium-ion batteries for emerging applications in electric vehicles. However, achieving high energy density in Ni-rich cathodes is generally accompanied with substantial obstacles of lifetime and safety. The major issues of Ni-rich cathodes at high working potentials are originated from the unstable cathode-electrolyte interface, while the underlying mechanism of parasitic reactions to material degradation at the surface of cathode materials is not well understood. In this work, we report the parasitic-reaction-driven structural degradation in LiNi0.83Mn0.1Co0.07O2 cathode by altering interfacial chemistry by synthesizing cathode material in different environments. Our home-build high precision leakage current (HpLC) system verifies that Li2CO3 impurity generated in harsh synthesis environment is a nature root of electrolyte decomposition and parasitic reactions on Ni-rich cathodes. Different rates of parasitic reactions are strongly correlated to drastic discrepancy in electrochemical performance of Ni83 cathodes at different synthesis conditions. The post-mortem characterizations reveal that parasitic reactions promote severe rock-salt phase transformation with more Ni reduction and O deficiency at the surface of cathode materials. This study suggests the significance of developing impurity-free and mitigated parasitic reactions in Ni-rich transition metal oxides for enabling high energy density. Acknowledgement Research at the Argonne National Laboratory was supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. Argonne National Laboratory is operated for the DOE Office of Science by UChicago Argonne, LLC, under Contract DE-AC02-06CH11357. Figure 1