Growth of dendrites, the low plating/stripping efficiency of Zn anodes, and the high freezing point of aqueous electrolytes hinder the practical application of aqueous Zn-ion batteries. Here, a zwitterionic osmolyte-based molecular crowding electrolyte is presented, by adding betaine (Bet, a by-product from beet plant) to the aqueous electrolyte, to solve the abovementioned problems. Substantive verification tests, density functional theory calculations, and ab initio molecular dynamics simulations consistently reveal that side reactions and growth of Zn dendrites are restrained because Bet can break Zn2+ solvation and regulate oriented 2D Zn2+ deposition. The Bet/ZnSO4 electrolyte enables superior reversibility in a Zn-Cu half-cell to achieve a high Coulombic efficiency >99.9% for 900 cycles (≈1800 h), and dendrite-free Zn plating/stripping in Zn-Zn cells for 4235 h at 0.5 mA cm-2 and 0.5 mAh cm-2 . Furthermore, a high concentration of Bet lowers the freezing point of the electrolyte to -92 °C via the molecular-crowding effect, which ensures the stable operation of the aqueous batteries at -30 °C. This innovative concept of such a molecular crowding electrolyte will inject new vitality into the development of multifunctional aqueous electrolytes.
Keywords: Zn anodes; betaine; dendrite-free deposition; molecular-crowding electrolytes; wide temperature range.
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