Unveiling the Failure Mechanism of Zn Anodes in Zinc Trifluorosulfonate Electrolyte: The Role of Micelle-like Structures

Abstract

Zinc trifluorosulfonate [Zn(OTf)₂] is considered as the most suitable zinc salt for aqueous Zn-ion batteries (AZIBs) but cannot support the long-term cycling of the Zn anode. Here, we reveal the micelle-like structure of the Zn(OTf)₂ electrolyte and reunderstand the failing mechanism of the Zn anode. Since the solvated Zn²⁺ possesses a positive charge, it can spontaneously attract OTf^- with the hydrophilic group of -SO₃ and the hydrophobic group of -CF₃ via electrostatic interaction and form a "micelle-like" structure, which is responsible for the poor desolvation kinetics and dendrite growth. To address these issues, an antimicelle-like structure is designed by using ethylene glycol monomethyl ether (EGME) as a cosolvent for highly reversible AZIBs. The modified electrolyte shows lower dissociation ability to Zn(OTf)₂ and higher coordination tendency with Zn²⁺ compared to the Zn(OTf)₂ electrolyte, resulting in the unique solvation structure of Zn²⁺(H₂O)_1.2(OTf^-)₂(EGME)_2.8, which significantly reduces the charge of micelle, damages the micelle-like structure, and boosts the desolvation kinetics. Moreover, the reduction of EGME and OTf^- can form a robust dual-layered SEI with high Zn²⁺ ion conductivity. Consequently, the Zn/Cu asymmetric coin cell using ZT-EGME can work at a high rate and a capacity of 50 mA cm^-2 and 5 mA h cm^-2 for more than 120 cycles, while its counterparts using ZT can barely work. Moreover, a 505.1 mA h pouch cell with practical parameters including a lean electrolyte supply of 15 mL A h^-1 and an N/P ratio of ∼3.5 can work for 50 cycles.