Design and theoretical study of novel deep eutectic solvents: The effects of bromine and chloride anions on solvation structure and supercapacitor performance

Abstract

Designing high efficiency electrolytes with wide voltage window and operating temperature range is crucial for achieving high capacity and electrochemical stability of activated carbon-based supercapacitors. Here, novel deep eutectic solvent (DES) electrolytes utilizing tetraethylammonium bromide (TEAB) or tetraethylammonium chloride (TEAC) as hydrogen bond acceptor (HBA) and ethylene glycol (EG) as hydrogen bond donor (HBD) are proposed to modulate the temperature dependent ion transport and desolvation within activated carbon (AC) electrodes. Compared with conventional aqueous electrolytes, the present DES electrolytes have broad electrochemical stable window (up to 3.03 V), wide operating temperature range (−40 to 115 °C), considerable ionic conductivity (8.70 mS cm−1 at 25 °C) and low viscosity (18.13 mPa s at 25 °C). Significantly, theoretical simulations reveal that the highest occupied molecular orbitals (HOMO) energies of DES complexes decrease obviously, indicating that the electron transitions become more difficult and the DES electrolytes are more stable after the formation of hydrogen bond interactions. The AC||AC supercapacitors using DES electrolytes exhibit expanded cell voltage (2.4 and 1.8 V at −40 and 25 °C, respectively), increased energy density of 32.2 Wh kg−1 and impressive cycling stability (92.8% capacity retention at 1 A g−1 after 10000 cycles).