Designing interphases for highly reversible aqueous zinc batteries

Recent efforts to improve zinc metal anode reversibility in aqueous electrolytes have primarily focused on tailoring Zn 2+ solvation. We propose a complementary approach to directly engineer the anode interphase with the help of two co-cations. The designed organic co-cations offer distinct improvements: a partially fluorinated pyrrolidinium cation effectively suppresses parasitic reactions such as hydrogen evolution (<6 μA cm −2 ), while an ether-functionalized ammonium cation inhibits dendrite formation (almost 10 Ah cm −2 cumulative capacity, >1 year, Zn||Zn). Only 3 wt % of the co-cation combination enables full utilization of a 5-mAh cm −2 reservoir with over 99% Coulombic efficiency and 1,000 cycles with 20% reservoir utilization. We further validate this concept in Zn metal batteries with various cathode chemistries (O 2 , polyaniline, and HNaV 6 O 16 ), and we have achieved significant enhancements in performance. This suggests co-cations are a promising and universal approach to improve metal anode reversibility across emerging battery chemistries.