Hydrophilic Cation-Mediated Interfacial Chemistry for Advanced Multi-Electron Zn–Halogen Batteries

Aqueous Zn-iodine batteries with four-electron I-/I0/I+ conversion (4eZIBs) offer doubled theoretical capacity compared with conventional two-electron systems, yet their application is hindered by polyiodides shuttling, I+ hydrolysis, and sluggish interhalogen conversion. Here, we propose a cation-mediated electrolyte strategy by screening quaternary ammonium (QA) cations with varying hydrophilicity. Among the screened candidates, bis(2-hydroxyethyl)dimethylammonium (BHDA) with higher hydrophilicity mitigates the interfacial aggregation of QA-Ix species by forming oil-like BHDA-Ix species, thereby changing the conventional I2 ↔ ICl2- process into I2 ↔ I5- ↔ ICl2- and facilitating the four-electron I-/I0/I+ conversion. On the Zn anode, the interfacial enrichment of BHDA reconstructs the interfacial hydrogen-bond network, as evidenced by in situ synchrotron-based Fourier-transform infrared spectroscopy, suppressing dendrite growth and parasitic reactions. Consequently, 4eZIB coin cells deliver 375.1 mAh g-1 at 1 C and retain 80.1% of the capacity over 20,000 cycles at 10 C. Furthermore, 700 mAh and 1.2 Ah pouch cells achieve stable cycling with low negative/positive ratios of 3.33 and 1.53, respectively. These results highlight the influence of cation hydrophilicity on high-valence iodine chemistry and provide guidance for electrolyte design in aqueous Zn-halogen batteries.