“Water-in-deep eutectic solvent” electrolytes enable zinc metal anodes for rechargeable aqueous batteries

Metallic zinc (Zn) is one of the most promising anodes for aqueous batteries, but so far its applicability for rechargeable systems remains elusive, mainly owing to the free water-induced parasitic reactions. Here, we report a new “water-in-deep eutectic solvent (water-in-DES)” electrolyte (~30 mol.% H2O in a eutectic mixture of urea/LiTFSI/Zn(TFSI)2; TFSI, bis(trifluoromethanesulfonyl)imide), in which all water molecules participate in DES's internal interaction (H-bonding and coordinating) network, leading to a suppressed reactivity with Zn anode from both thermodynamic and electrochemical aspects. Inheriting characteristics from aqueous and DES media, this electrolyte enables stable and reversible Zn plating/stripping with over twentyfold enhancement in cycling life compared to routine aqueous electrolytes, even at low rates. With these merits, a desirable rechargeability (>90% capacity retention after 300 cycles at 0.1 C) is achieved for a 1.92 V (average dicharge voltage) Zn/LiMn2O4 battery, together with a practical energy density of 52 Wh/kg (pouch cell, 2 Ah, ~9.8 × excess Zn on anode).