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

Abstract

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, and thus mitigating dendrite formation and hydrogen evolution. Consequently, the Zn||Zn symmetric cells exhibit an impressive lifespan of 250 h, 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.