“Tri-Solvent-in-Salt” Electrolytes for High-Performance Supercapacitors
Authors: Lu Xuejun , Gutiérrez María C , Ferrer M. Luisa , Lu Xuejun , Liu Jian 
Published: July 2022

Electrolytes chemistry for high-performance supercapacitors (SCs) has been addressed recently, where solvents included in electrolyte composition dissolving or mixing the electrochemically active salts or ILs have been typically seen as a mere medium.[1] Specifically, attention regarding the achievement of high-performacne SCs has also been paid to, e.g., water-in-salt (WIS), solvent-in-salt (SIS), and bi-solvent-in-salt (BSIS) electrolytes, demonstrating that solvent molecules may indeed play a more active role.[2
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4] In this presentation, we will talk about the design of a tri-solvent-in-salt (TSIS) electrolyte where every solvent contributed (with an IL, i.e., 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, the eutectic mixture of H2O and dimethylsulfoxide (DMSO) in a 2 to 1 molar ratio primary HB complex structures with mixed EMIMBF4 offers a low melting point and low flammability, then add acetonitrile (CH3CN) in different molar ratios providing an improvement of the rate capability to the resulting electrolyte. As compared to other electrolytes, the TSIS electrolyte composed in a molality of 5.8 m (TSIS-5.8) showed the cost efficiency and exhibited a low self-extinction rate. 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, respectively, power densities of 10,000 and 17,000 W kg-1, the capacitance retention of ca. 82% after 15,000 cycles at 4 A g-1 and a self-discharge as low as 22%. The use of ternary solvent mixtures combining different solvents in the proper molar ratios opens up an easy and low-cost path to design many new electrolytes in terms of non-flammability, non-toxicity, high electrical conductivity, and wide electrochemical stability window (ESW). Forthcoming research could use the knowledge provided by this work in terms of ions solvation and transport in TSIS electrolytes and explore the interfacial interactions between electrolyte and electrode material to determine their respective relevance in the performance of SCs.

Keywords: tri-solvent-in-salt (TSIS), hydrogen bond, eutectic mixtures, supercapacitors

Reference
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[4] X. Lu, et al. Aqueous Co-Solvent in Zwitterionic-based Protic Ionic Liquids as Electrolytes in 2.0 V Supercapacitors. ChemSusChem 2020, 13, 5983.

[5] X. Lu, et al. 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. Energy Storage Mater., 40, (2021), 368-385.