In this work, 1, 3, 5-Pentanetricarbonitrile (PTN) is adopted as a novel electrolyte additive to improve the electrochemical characteristics of LiCoO2 cathode operating under high charging cut-off voltage. PTN additive generates an extremely stable CEI film with abundant of CN bond and higher content of LixPOyFz/LixPFy on the LiCoO2 cathode surface, which is beneficial to inhibit the continuous decomposition of electrolyte and decrease the interfacial impedance. With electrolyte containing 0.5 wt.% PTN, the LiCoO2 cathode shows an initial capacity of 180.1 mAh g−1 with a columbic efficiency of 94.0%, and achieves a capacity retention of 90.7% for 300 cycles at 3.0–4.45 V. It makes the mass specific energy density of LCO cathode increases from 564.7 Wh kg−1 to 728.7 Wh kg−1, which leads to nearly 30% of the additional energy.