Highly Concentrated Salt Electrolyte for a Highly Stable Aqueous Dual-Ion Zinc Battery

A zinc metal anode for zinc-ion batteries is a promising alternative to solve safety and cost issues in lithium-ion batteries. The Zn metal is characterized by its high theoretical capacity (820 mAh g^-1), low redox potential (0.762 V vs SHE), low toxicity, high abundance on Earth, and high stability in water. Taking advantage of the stability of Zn in water, an aqueous Zn ion battery with low cost, high safety, and easy-to-handle features can be developed. To minimize water-related parasitic reactions, this work utilizes a highly concentrated salt electrolyte (HCE) with dual salts─1 m Zn(OTf)₂ + 20 m LiTFSI. MD simulations prove that Zn²⁺ is preferentially coordinated with O in the TFSI^- anion from HCE instead of O in H₂O. HCE has a broadened electrochemical stability window due to suppressed H₂ and O₂ evolution. Some advanced ex situ and in situ/in operando analysis techniques have been applied to evaluate the morphological structure and the composition of the in situ formed passivation layer. A dual-ion full Zn||LiMn₂O₄ cell employing HCE has an excellent capacity retention of 92% after 300 cycles with an average Coulombic efficiency of 99.62%. Meanwhile, the low concentration electrolyte (LCE) cell degrades rapidly and is short-circuited after 66 cycles with an average Coulombic efficiency of 96.91%. The battery's excellent cycling performance with HCE is attributed to the formation of a stable anion-derived solid-electrolyte interphase (SEI) layer. On the contrary, the high free water activity in LCE leads to a water-derived interfacial layer with unavoidable dendrite growth during cycling.