Replacing flammable nonaqueous electrolytes with non-flammable aqueous electrolytes is critical to achieve safe, low-cost and eco-friendly Li-ion batteries. However, the voltage window of an aqueous Li-ion battery is limited by the thermodynamic stability window of water (1.23 V), which limits the energy density of the aqueous Li-ion batteries. Recently developed LiTFSI-based "water-in-salt" electrolytes with highly-concentrated salts (>21 m) have expanded the electrochemical stability window of water to over 3.0 V by decreasing the amount of free water molecules and creating artificial solid-electrode interfaces, however, raise concerns of cost and toxicity.
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