Multi-functional amino acid enabling global ion regulation for high-performance aqueous Zn–I2 batteries

Aqueous Zn–I2 batteries (AZIBs) possess great potential in future energy storage systems while facing severe restriction by the unstable Zn anode and uncontrollable shuttling of polyiodides during cycling. Herein, we propose a multi-functional amino acid, proline (Pro), in the traditional aqueous electrolyte to regulate the overall ion behaviors in both bulk electrolyte and electrode interfaces, thereby enhancing the comprehensive performance of the AZIBs. This amino acid with strong polarity can disrupt the hydrogen bonds among water networks and impair the electrostatic coupling of Zn2+–SO42−. Besides, it can simultaneously regulate the interfacial ion behaviors on both the Zn anode and I2 cathode, thus inhibiting various side reactions and the polyiodide shuttle effect, respectively. As a result, the stability and lifetimes of ZnǁZn symmetric batteries and ZnǁCu asymmetric batteries are significantly improved. Meanwhile, in practical ZnǁI2 full cells, the introduction of Pro can extend the lifetime from 68 to 150 cycles at a high I2 loading of 13 mg cm−2 and improve the capacity retention from 38.6% to 64.0% after 6800 cycles at a common I2 loading of 1.6 mg cm−2. This demonstrates the significant potential of this multi-functional additive in the future development of practical AZIBs.