Carbon-Encapsulated Gemini Ionic Liquid as Advanced Bromine Hosts for High-Performance Zn–Br₂ Batteries

Zinc–bromine (Zn–Br₂) batteries represent a promising aqueous energy storage technology, yet their widespread deployment is hindered by the uncontrolled diffusion of molecular bromine from the cathode side, leading to severe self-discharge and capacity fading. Here, we demonstrate an effective strategy to mitigate bromine crossover by employing gemini-type bromine complexing agents (BCAs) impregnated into porous carbon hosts in their oily phase. This approach facilitates the in situ conversion of bromine into a water-immiscible polybromide BCA phase, thereby confining the active species within the electrode structure. The presence of aromatic linkers in the gemini BCA molecules significantly enhance their retention in the carbon network, most likely through π–π stacking interactions with the carbon framework. Moreover, controlled discharge protocols enabled preferential nucleation of polybromide domains within the confined pore environment, further improving the self-discharge stability of the system. As a result, the Zn–Br₂ cells incorporating gemini-type BCAs exhibited suppressed bromine crossover, enhanced coulombic efficiency, and superior cycling durability compared to conventional systems. This work provides mechanistic insights and practical guidelines for the rational design of molecular complexants and electrode architectures for next-generation Zn–Br₂ batteries.