Although the non-aqueous lithium-ion batteries (LIBs) are achieving a huge market share, their cost and safety issues caused by flammable and volatile organic electrolytes hinder their wide application in large-scale stationary energy storage area, in which cost and safety are the determining factors instead of energy density. In this context, rechargeable aqueous batteries, which use non-flammable and less-volatile aqueous electrolytes become an appealing alternative..
The first aqueous LIBs based on LiMn2O4//VO2 was proposed by Dahn’s group in 1994. The limited electrochemical window of aqueous electrolytes was extended in 2015 by using a “water-in-salt” electrolyte consisting of 21 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as designed by Wang’s group, demonstrating a LiMn2O4//Mo6S8 full aqueous LIBs with a high output potential of 2.3 V. Afterwards, multiple fluorine-containing salts, such as sodium trifluoromethane sulfonate (NaOTF) and potassium trifluoromethane sulfonate (KCF3SO3), have also been reported to enable high-voltage rechargeable aqueous sodium-ion batteries (ASIBs) and aqueous potassium-ion batteries (APIBs), respectively. However, in spite of the remarkable achievements, these highly concentrated electrolytes based on fluorinated salts spoil the two major advantages of aqueous batteries, which are the low cost and reduced environmental impact.
In this work, concentrated halide-free electrolytes are developed for aqueous batteries beyond lithium. Specifically, a variety of concentrated electrolytes consisting of acetate or sulfate salts are studied for ASIBs, APIBs, aqueous zinc-ion batteries (AZIBs) and aqueous ammonium-ion batteries (AAIBs). Classical Molecular Dynamics (MD) simulations are employed to understand molecular interactions as function of salt concentration. Additionally, the structural evolution of active materials is investigated by in situ X-ray diffraction (XRD) to verify the electrode reaction mechanism. The electrode surface chemistry is also investigated by X-ray photoelectron spectroscopy (XPS) to unveil the nature of the electrode/electrolyte interphase, and differential electrochemical mass spectrometry (DEMS) measurements are used to monitor gas evolution.
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