Systematic electrochemical impedance spectroscopic (EIS) combining with cyclic voltammograms (CV) and charge/discharge test have been used to investigate the reaction and capacity fade mechanism of LiNi0.5Co0.2Mn0.3O2 electrode with high cutoff voltage 3.0-4.5 V and high working temperature 55 degrees C. It was found that the phase transformation and the Li-Ni site exchange in lattice at high voltage (>4.3 V) acted on charge transfer process were the main reasons of capacity fading with the cutoff voltage of 3.5-4.5 V. The degradation of LiNi0.5Co0.2Mn0.3O2 cathode with high-temperature was mainly associated with the strong catalytic activity of Ni4+ which can cause the side reaction between electrode and electrolyte to form unstable SEI film as well as the acceleration of phase transformation on the surface of electrode with high electrode polarization potential. Furthermore, it was found that improving the performance of SEI film should be one of the most important methods for improving the cycling performance of LiNi0.5Co0.2Mn0.3O2 cathode under high-temperature and we have proved it by systematic discussing the EIS results of the LiNi0.5Co0.2Mn0.3O2 cathode in electrolyte with fluoro-ether as an additive. (C) 2015 The Electrochemical Society. All rights reserved.