A Thin and Homogeneous Solid Electrolyte Interface Enriched with ZnF2 and ZnS for Highly Reversible Zinc Batteries

Abstract

Rechargeable Zn batteries have potential for large-scale energy storage due to their low cost and abundant Zn resources, but their reversibility and cycling life are limited by unstable electrode/electrolyte interface in traditional electrolytes, such as: surface corrosion, hydrogen evolution reaction (HER), and dendritic Zn growth. Solid electrolyte interface (SEI) plays a crucial role in stabilizing the interface and its composition and morphology significantly influence the electrochemical performance of Zn batteries. Herein, we develop a thin and homogeneous SEI enriched with ZnF₂ and ZnS for stable Zn batteries. Unlike previous reports, the ZnF₂ and ZnS are uniformly distributed with depth in this SEI, with a thickness of only ∼20 nm, which not only inhibits the interfacial side reaction and the formation of dendritic Zn, but also optimizes the diffusion behavior of Zn²⁺ at the interface, leading to stable Zn²⁺ transfer and the deposition of Zn along the Zn (002) direction. Such a SEI film is achieved by designing a dilute Zn fluoroborate salt-based eutectic electrolyte by coupling Zn(BF₄)₂ salt with dimethyl sulfoxide (DMSO), where a unique and water-scarce Zn²⁺ solvent sheath of Zn(DMSO)_3.72(H₂O)_1.23(BF₄)_1.06 can be formed, thereby generating this thin and homogeneous SEI film. Meanwhile, the DMSO molecules can preferably adsorb on the Zn surface, blocking direct contact between H₂O molecules and Zn metal. Thus, the Zn ǀǀ Zn symmetric cells can maintain a stable plating/stripping process over 5000 h, and Zn | Cu half cells can cycle stably with a high coulombic efficiency (CE) of ∼99.8% over 1500 cycles at the current density of 5.0 mA cm⁻², manifesting one of the best results in Zn metal batteries. This work provides a feasible route for the development of eutectic electrolytes.