Beyond the anion-rich solvation paradigm: anion-mediated ion-pair aggregation enables robust SEI for highly reversible zinc batteries

Rechargeable zinc batteries are promising candidates for large-scale electrochemical energy storage, owing to their low cost and the abundance of zinc resources. However, their practical application is limited by an unstable electrode/electrolyte interface that leads to issues such as surface corrosion, the hydrogen evolution reaction, and dendritic Zn growth. Herein, we report a new mechanism for interfacial stabilization via anion-mediated ion-pair aggregation, challenging the prevailing anion-rich solvation paradigm. When highly concentrated TFSI – anion is introduced into a conventional 2.0 M Zn(OTF) 2 aqueous electrolyte, it does not convert the Zn 2+ solvation sheath from [Zn(H 2 O) 6 ] 2+ to an anion-rich solvation structure; instead, it promotes the formation of aggregate ion pairs (AGG), which adsorb preferentially onto the Zn surface, effectively suppressing H 2 O-induced side reactions and maintaining high ionic conductivity. At the same time, synergistic Li + cations can assemble into loosely bound solvation clusters surrounding the Zn 2+ solvation sheath, immobilizing active H 2 O molecules and weakening the Zn 2+ –dipole interaction; this not only facilitates the Zn 2+ transport kinetics and desolvation but also preferentially reduces and forms a robust LiF-modified SEI and F-rich SEI layer (only ∼15 nm thick) on the Zn electrode surface, thereby improving the interfacial stability. Thus, Zn||Zn symmetric cells can achieve exceptional cycling stability over 4000 h, and Zn|Cu half-cells can run stably with a high Coulombic efficiency of 99.92% for over 3000 cycles at a current density of 5.0 mA cm –2 , among the best performances reported for zinc batteries. This work provides new insights into electrolyte engineering by leveraging ion-pair aggregation.