Optimizing Bromine Complexation and Kinetics: a Bisimidazole Strategy for High-Performance Zn-Br Static Batteries

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• Zinc-bromine (Zn-Br) static batteries offer potential for grid-scale energy storage.
• Practical implementation is limited by the challenge of balancing bromine complexing agent (BCA) properties for strong bromine complexation and fast redox kinetics.

Purpose of the Study:

• To design and synthesize a novel bisimidazolium salt, 1,4-bis(3-methylimidazolium-1-yl) butane dibromide ([bMImB]Br₂), to overcome the limitations of existing BCAs in Zn-Br batteries.
• To demonstrate a molecular design strategy for next-generation BCAs in halogen-based energy storage.

Main Methods:

• Synthesis of a symmetric bisimidazolium salt ([bMImB]Br₂) with tailored molecular structure.
• Electrochemical evaluation of [bMImB]Br₂ in Zn-Br static batteries, including specific energy, coulombic efficiency (CE), and energy efficiency (EE) measurements.
• Testing of flexible pouch cells under mechanical deformation.

Main Results:

• The [bMImB]Br₂ exhibited strong bromine affinity and low steric hindrance, effectively balancing complexation strength and redox kinetics.
• Zn-Br batteries utilizing [bMImB]Br₂ achieved a high specific energy of 116 Wh·kg⁻¹.
• Exceptional performance metrics were recorded: average CE of 99.22% and EE of 89.35% at 10 mAh·cm⁻² for over 150 cycles.
• Flexible pouch cells maintained 110 mAh capacity and 99.7% CE over 100 cycles, even under deformation.

Conclusions:

• The rationally designed [bMImB]Br₂ successfully reconciles the trade-off between bromine complexation and redox kinetics in Zn-Br batteries.
• This bisimidazolium salt enables high energy density and stable cycling performance.
• The study presents a generalizable molecular design approach for developing advanced BCAs for efficient halogen-based energy storage systems.