A Urea Modified Ternary Aqueous Electrolyte with Tuned Intermolecular Interactions and Confined Water Activity for Highly Stable Zinc-Ion Batteries

Abstract

Aqueous zinc-ion batteries (ZIBs) have gained attention as potential energy storage devices due to their high safety.1 However, the narrow electrochemical window and unfavorable side reactions (i.e., hydrogen and oxygen evolution reactions) induced by water decomposition restrict their development.2,3 Thus, constraining water activity is necessary to enhance stability and enlarge electrochemical window. In this study, we prepare a 2.9 m (mol kgsolvent −1) Zn(ClO4)2−CO(NH2)2−H2O ternary aqueous electrolyte with restricted water activity at room temperature. Raman, Fourier transform infrared (FTIR), and nuclear magnetic resonance (NMR) characterization techniques suggest strong intermolecular interactions between CO(NH2)2 and H2O, which decrease the free H2O molecules and reduce their associated activity to suppress the parasitic reactions at the electrodes. Density functional theory (DFT) calculations substantiate the strong hydrogen bonds between urea and water. Compared with eutectic electrolyte and water-in-salt electrolyte, this aqueous electrolyte exhibits a high ionic conductivity (6.83 mS cm−1) as conventional aqueous electrolytes without sacrificing the voltage window (2.5 V). Furthermore, DFT computations reveal the preferential adsorption of urea molecules onto the zinc surface, mediating zinc deposition process. The reduction of urea generates a robust organic solid electrolyte interphase (SEI), promoting homogeneous zinc deposition on the anode and hindering parasitic reactions. Consequently, this ternary aqueous electrolyte not only imparts Zn/Zn symmetric cells with prolonged cycling stability exceeding 1000 h at various current densities but also endows zinc−vanadium batteries a capacity of 250 mAh g−1 for 400 cycles at 1 A g−1. This finding highlights a promising approach for the development of stable aqueous zinc-ion batteries with an expanded electrochemical window.Reference A. Konarov et al. ACS Energy Letters 2018, 3 (10), 2620–2640.C. Li et al. Joule 2022, 6 (8), 1733–1738.J. Zhou et al. Advanced Materials 2021, 33 (33), 2101649. Figure 1