Hydrophilic Cation-MediatedInterfacial Chemistryfor 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.