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

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. © 2023 Published by Elsevier B.V.