Zwitterionic gemini additive as interface engineers for long-life aqueous Zn/TEMPO flow batteries with enhanced areal capacity

Aqueous Zn-based flow batteries often face issues such as poor reversibility and short lifespan due to irregular Zn deposition and detrimental side reactions. To address these challenges, we developed a zwitterionic gemini additive, N,N’-bis(3-propanesulfonic acid)-3,3’-bipyridinium (SPr-Bpy), to enhance Zn plating/stripping behavior and optimize the Zn²⁺ solvation structure. The dual sulfonate groups influence the Zn²⁺ solvation shell and anchor SPr-Bpy to the Zn surface through multi-site interactions. Additionally, the bipyridinium structure forms an electrostatic shielding layer, suppressing excessive Zn²⁺ accumulation, promoting uniform Zn deposition, thus mitigating dendrite formation and hydrogen evolution. Consequently, the Zn||Zn symmetric cells exhibit an impressive lifespan of 250 hours, while the Zn||Cu asymmetric cells achieve a high average Coulombic efficiency of 99.8% over 450 cycles. Moreover, SPr-Bpy significantly improves Zn/TEMPO flow battery performance, achieving a high areal capacity of 24.4 mAh cm⁻² with an exceptional capacity retention of 99.992%/cycle over 500 cycles.