Aqueous rechargeable alkali-ion batteries have attracted considerable interest in recent years due to reduced cost, lower environmental impact and, most importantly, and improved safety. Unfortunately, the narrow electrochemical stability window of water (1.23 V) limits the maximum output voltage of aqueous batteries, thus leading to intrinsically low energy density. Water-in-salt electrolytes (WiSE) constituted of, for example, highly concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions, has promoted the development of aqueous LIBs. However, 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.
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