EMIMBF₄ in ternary liquid mixtures of water, dimethyl sulfoxide and acetonitrile as “tri-solvent-in-salt” electrolytes for high-performance supercapacitors operating at -70 °C

For many years, the performance of Li-ion batteries (LIBs) and supercapacitors (SCs) has relied mainly on two factors, (1) the optimization of electrodes composition and/or structure and (2) the selection of salts or ionic liquids (ILs) matching well with the compositional and/or structural features of electrodes. Solvents included in electrolyte composition have been typically seen as a mere medium where the electrochemically active salts or ILs are dissolved or mixed. More recently, attention has also been paid to specific issues, such as flammability, toxicity, electrical conductivity and/or electrochemical stability window (ESW). Recent reports describing water-in-salt (WIS), solvent-in-salt (SIS) and bi-solvent-in-salt (BSIS) electrolytes demonstrated that solvent molecules may indeed play a more active role in the achievement of high-performance LIBs and SCs. This work accomplished the design of a tri-solvent-in-salt (TSIS) electrolyte where every solvent contributed (with an IL such as EMIMBF₄) to the formation of an electrochemically active hydrogen bond (HB) complex structure. Raman and NMR spectroscopies, as well as molecular dynamic (MD) simulations helped elucidate the ratio among all compounds (e.g., solvents and IL) in the HB complex structure that best works as an electrolyte. For instance, one could start from the eutectic mixture of H₂O and dimethylsulfoxide (DMSO) in a 2 to 1 molar ratio and then add acetonitrile (CH₃CN) in different molar ratios. Thus, the 2H₂O:DMSO mixture offers low melting point and low flammability, and CH₃CN provides an improvement of the rate capability to the resulting electrolyte. As compared to other electrolytes, the TSIS electrolyte composed of 1.5EMIMBF₄:2H₂O:DMSO:3.5CH₃CN (5.8 m, TSIS-5.8) was cost efficient and exhibited self-extinction rates as low as 40 s g^-1. Moreover, SCs operating with TSIS-5.8, at -70 °C and up to 2.7 V provided energy densities of ca. 49 and 18 Wh kg^-1 at, respectively, power densities of 10,000 and 17,000 W kg^-1, a capacitance retention of ca. 82% after 15,000 cycles at 4 A g^-1 and a self-discharge as low as 22%.