Nitriles as efficient electrolyte additives are widely used in high-voltage lithium-ion batteries. However, their working mechanisms are still mysterious, especially in practical high-voltage LiCoO2 pouch lithium-ion batteries. Herein, we adopt a tridentate ligand-containing 1,3,6-hexanetricarbonitrile (HTCN) as an effective electrolyte additive to shed light on the mechanism of stabilizing high-voltage LiCoO2 cathode (4.5 V) through nitriles. The LiCoO2/graphite pouch cells with the HTCN additive electrolyte possess superior cycling performance, 90% retention of the initial capacity after 800 cycles at 25 °C, and 72% retention after 500 cycles at 45 °C, which is feasible for practical application. Such an excellent cycling performance can be attributed to the stable interface: The HTCN molecules with strong electron-donating ability participate in the construction of cathode-electrolyte interphase (CEI) through coordinating with Co ions, which suppresses the decomposition of electrolyte and improves the structural stability of LiCoO2 during cycling. In summary, the work recognizes a coordinating-based interphase-forming mechanism as an effective strategy to optimize the performance of high voltage LiCoO2 cathode with appropriate electrolyte additives for practical pouch batteries.