Applying additives to perovskite precursor solutions is an effective way to passivate perovskite films for the construction of perovskite solar cells (PSCs) with increased power-conversion efficiencies (PCEs) and enhanced stabilities. Ionic liquids (ILs) with halide anions have been applied as additives. Here, we report the application of imidazolium-based ILs, containing the nitrile functionality in the cation and chloride anion, as additives for triple-cation (Cs+, formamidinium [FA(+)], methylammonium [MA(+)]) PSCs. We found that a Cl-I exchange takes place between the I- anion of the triple-cation precursors and the Cl- anion of the IL. The fabricated PSCs containing perovskite films with the ILs show high PCEs (in the best case 22.86%) and high stabilities, with the unencapsulated devices retaining about 95% of their original efficiencies after 1,000 h. The mechanistic basis for the passivation effect of the IL additives in the perovskite films is described.