“Dual-side shielding”: Bromide additives enabling robust four-electron aqueous Zn–I₂ batteries

Aqueous Zn–I₂ batteries based on four-electron conversion chemistry have been extensively explored due to their high voltage and large specific capacity. However, the intrinsic instability of I⁺ generated during the high-voltage conversion process and the severe electrochemical corrosion of Zn anode in aqueous electrolyte primarily hinder the construction of high-performance Zn–I₂ batteries. Herein, trimethylammonium bromide (TMBr) with functional cation and anion has been selected as a dual-side shielding electrolyte additive to concurrently resolve these problems. On the one hand, Br⁻ in TMBr could incorporate into the electrolyte solvation structure and reconstitute the hydrogen-bond (H-bond) network, suppressing the activity of free H₂O molecules and consequently enhancing the Zn anode stability. On the other hand, the Br⁻ could electrochemically activate I⁺, followed by the stabilization of I⁺ via complexation interactions with trimethylammonium cations (TM⁺) and Br⁻, synergistically achieving facile high-voltage conversion chemistry of I₂ cathode. As a result, the TMBr additive enables the four-electron conversion-type Zn–I₂ batteries to demonstrate a remarkable capacity of 250 mAh·g⁻¹ coupled with stable cycling exceeding 15,000 cycles at 3 A·g⁻¹. Additionally, the 160 mAh pouch cell delivers an energy density of 367.4 Wh·kg⁻¹ (based on I₂ mass) with a lifetime of over 300 cycles.