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

Abstract

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 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.