In this paper, we report positive effect of Tris(2,2,2-trifluoroethyl) phosphite (TTFP) as additive during initial activation and cycling of Li-rich-NMC xLi2MnO3–(1 − x)LiMO2 (x >> 1; M = Ni, Co, Mn) cathode in EC/DMC + 1 M LiPF6 electrolyte. Firstly conductivity and viscosity of electrolyte with x wt.% TTFP; 0 wt.% < x < 20 wt.% were studied. From the electrochemical characterization using cyclic voltammetry, galvanostatic charge/discharge and EIS we show that TTFP passives aluminum current collector and the acts as oxygen scavenger during the extended activation plateau up 4.7 V vs. Li+/Li (up to 301 mAh g−1 in charge). The beneficial effect of TTFP improves the long-term cycling stability of the cathode with more than 95% of initial capacity in Gr//Li-Rich -NMC full cell is maintained after 50 cycles at C rate. Finally, XPS analysis of Li-rich -NMC electrodes show presence of TTFP on the surface during cycling and confirm the presence of Mn3+ at the end of discharge. The convergence of all characterizations indicates that TTFP should act as a catalyst to several surface reactions which are beneficial to long cycling cell performances.