Highly reversible zinc metal anode enabled by zinc fluoroborate salt-based hydrous organic electrolyte

Abstract

Although Zn(BF4)2 contains favorable ingredients of fluorine for the formation of ZnF2 rich-Zn2+ ion conducting solid electrolyte interface (SEI), the aqueous electrolyte based on inorganic Zn salt of Zn(BF4)2⋅xH2O shows high Hammett acidity with pH value <1, which gives rise to severe corrosion of metallic Zn electrode and thermodynamically spontaneous hydrogen evolution reaction (HER). Meanwhile, an uneven and arbitrarily aggregated SEI will result in uneven distribution of Zn2+ ion flux and electric field, leading to rampant dendrite growth. Here, we employ a hydrophilic organic solvent of vinylene carbonate (VC) and hydrate Zn(BF4)2⋅4H2O salt for a hydrous organic electrolyte, denoted as ZnBF-VC. With the ZnBF-VC electrolyte used, the VC molecules preferably adsorb on the Zn surface to block H2O molecules and Zn metals. Meanwhile, the unique Zn2+-solvation sheath of Zn(VC)2.89(H2O)1.28(BF4)1.83 is formed, which forms an organic/inorganic hybrid SEI in-situ with ZnF2 and ZnCO3 as inorganic species. The distinct SEI enables favorable Zn2+ ion transport and can effectively protect metallic Zn electrode from corrosion, side reactions and dendrite formation. Consequently, the ZnǀǀZn cells cycled over 2200 h at 0.5 mA cm−2 and the ZnǀǀCu asymmetric cells maintained an excellent Coulombic efficiency (CE) of ∼99.7% over 550 cycles at 1 mA cm−2. Whereas, the ZnǀǀZn cells broke after only 74 cycles in aqueous electrolyte. Additionally, the full cell we assembled with manganese hexacyanoferrate (MnHCF) in ZnBF-VC electrolyte demonstrates excellent cycling stability, achieving a high specific capacity of 146.2 mAh g − 1, and a high retention rate of 85.3% over 1300 cycles at 0.4 A g − 1, while the cell using the referred ZnBF-H2O electrolyte survived only ∼6 cycles. This work proposes a feasible direction for the study of Zn(BF4)2-based organic electrolyte for Zn batteries.