AbstractLithium‐ion batteries (LIBs) adopting layered oxide cathodes with high nickel content (Ni ≥ 0.9) always suffer from extremely poor cycle life, especially at elevated temperatures and higher charging cut‐off voltages. Adding small amounts of functional additives is considered to be one of the most economic and efficacious strategies to resolve this issue. Herein, cyano‐groups are introduced innovatively into a siloxane to delicately synthesize a novel cyano‐siloxane additive, namely 2,2,7,7‐tetramethyl‐3,6‐dioxa‐2,7‐disilaoctane‐4,4,5,5‐tetracarbonitrile (TDSTCN). Encouragingly, 0.5 wt.% TDSTCN additive enables ultrahigh nickel LiNi_0.9Co_0.05Mn_0.05O₂/graphite (NCM90/Gr) full cells with dramatically increased cycle life, especially at an elevated temperature of 50 °C and a high charging cut‐off voltage of 4.5 V. The characterizations reveal that the TDSTCN additive can scavenge HF and promote the formation of robust stable interface layers on NCM90 cathode and Gr anode due to the synergistic effects of cyano‐groups and Si−O bonds. These results reveal the great significance of designing one single additive with several functional groups in enhancing the comprehensive electrochemical performances of high Ni LIBs.