Water-in-salt electrolytes towards sustainable and cost-effective alternatives: Example for zinc-ion batteries

Since 2015, incredible enthusiasm has erupted around a brand-new form of electrolyte: Water-in-salt electrolytes (WISEs). These liquid electrolytes contain small amounts of water in contrast to very high salt contents. One perceived advantage of these electrolytes is an enlarged electrochemical stability window. Despite the scientific and technological significance of WISEs, these electrolytes run the risk of being restricted to academic usage. This limitation can be associated with the heavy reliance on LiTFSI, which is an expensive and complex salt to produce. As such, it is arguable that any viable WISE should be both economically practical and sustainable. Considering those factors, we believe that Zn-ion batteries with WISEs are the most promising candidates. Additionally, WISEs overcome certain limitations associated to Zn-ion technologies: the electrochemical stability window and cycling stability. In this current opinion, we will first detail the synthesis pathways for LiTFSI and demonstrate its problematic production process. Next, alternative substitutes for highly concentrated electrolytes utilized in aqueous zinc-ion batteries will be discussed. • LiTFSI WISEs require unsustainable raw materials, challenging their viability. • Simple and cheap salts (ZnCl 2 or ZnSO 4 ) can be used to produce WISEs. • Studies using AZIBs containing WISEs have started to appear, these are reviewed. • AZIBs with WISEs are promising for more sustainable and safer energy storage devices. • WISEs can solve the low ESW and low cycling stability of AZIBs.