Aqueous zinc batteries are attractive for large-scale energy storage due to their inherent safety and sustainability. However, their widespread application has been constrained by limited energy density, underscoring a high demand of advanced cathodes with large capacity and high redox potential. Here, we report a reversible high-capacity six-electron-conversion Se cathode undergoing a ZnSe↔Se↔SeCl₄ reaction, with Br⁻/Br_n⁻ redox couple effectively stabilizes the Zn | |Se cell. This Se conversion, initiated in a ZnCl₂-based hydrogel electrolyte, presents rapid capacity decay (from 1937.3 to 394.1 mAh g_Se⁻¹ after only 50 cycles at 0.5 A g_Se⁻¹) primarily due to the dissolution of SeCl₄ and its subsequent migration to the Zn anode, resulting in dead Se passivation. To address this, we incorporate the Br⁻/Br_n⁻ redox couple into the Zn | |Se cell by introducing bromide salt as an electrolyte additive. The generated Br_n⁻ species acts as a dead-Se revitalizer by reacting with Se passivation on the Zn anode and regenerating active Se for the cathode reaction. Consequently, the cycling stability of the Zn | |Se cell is improved, maintaining 1246.8 mAh g_Se⁻¹ after 50 cycles. Moreover, the Zn | |Se cell exhibits a specific capacity of 2077.6 mAh g_Se⁻¹ and specific energy of 404.2 Wh kg⁻¹ based on the overall cell reaction.