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Aqueous zinc‐iodine (Zn─I2) battery is a promising energy storage system due to its inherent safety, high theoretical capacity, sustainability, and cost‐effectiveness. However, the shuttle effect of polyiodide severely affects the stable loading of active iodine and even accelerates the corrosion of the Zn anode, thus impeding its further advancement. Herein, a unique trimethylsulfonium cation (TMS+) with strong adsorption is proposed to stabilize both the iodine cathode and Zn anode. Benefiting from the robust interaction between TMS+ and polyiodide, the electrolyte can effectively immobilize large‐capacity iodine in the form of oily precipitate, thus avoiding the shuttle effect of polyiodide and the Zn corrosion. Additionally, TMS+ can be preferentially adsorbed on various Zn facets, inducing an electrostatic shielding effect to inhibit Zn dendrite growth. Consequently, Zn anode can be stably cycled over 3400 h at 5 mA cm−2/5 mAh cm−2, and a large areal capacity of 2.71 mAh cm−2 as well as long‐life stability over 6400 cycles is achieved for Zn─I2 battery. Furthermore, cation adsorption engineering is practically utilized in pouch cells, realizing superior fast‐charging stability over 790 cycles. This electrolyte modification with dual stabilizations is anticipated to be applied to other metal‐iodine batteries as a cost‐effective, facile, and safe strategy. [ABSTRACT FROM AUTHOR]