Sodium Nitrate/Formamide Deep Eutectic Solvent as Flame-Retardant and Anticorrosive Electrolyte Enabling 2.6 V Safe Supercapacitors with Long Cyclic Stability

Safe operation of electrochemical capacitors (supercapacitors) is hindered by the flammability of commercial organic electrolytes. Non-flammable Water-in-Salt (WIS) electrolytes are promising alternatives; however, they are plagued by the limited operation voltage window (typically =2.3 V) and inherent corrosion of current collectors. Herein, a novel deep eutectic solvent (DES)-based electrolyte which uses formamide (FMD) as hydrogen-bond donor and sodium nitrate (NaNO3) as hydrogen-bond acceptor is demonstrated. The electrolyte exhibits the wide electrochemical stability window (3.14 V), high electrical conductivity (14.01 mS cm(-1)), good flame-retardance, anticorrosive property, and ultralow cost (7% of the commercial electrolyte and 2% of WIS). Raman spectroscopy and Density Functional Theory calculations reveal that the hydrogen bonds between the FMD molecules and NO3- ions are primarily responsible for the superior stability and conductivity. The developed NaNO3/FMD-based coin cell supercapacitor is among the best-performing state-of-art DES and WIS devices, evidenced by the high voltage window (2.6 V), outstanding energy and power densities (22.77 Wh kg(-1) at 630 W kg(-1) and 17.37 kW kg(-1) at 12.55 Wh kg(-1)), ultralong cyclic stability (86% after 30 000 cycles), and negligible current collector corrosion. The NaNO3/FMD industry adoption potential is demonstrated by fabricating 100 F pouch cell supercapacitors using commercial aluminum current collectors.