Electrochemistry (Oct 2024)
Functionalization and Applications of Molten Salts and Ionic Liquids
Abstract
The author and co-workers developed 1-ethyl-3-methylimidazolium fluorohydrogenate (fluorohydrogenate anion: (FH)nF−), which had the ionic conductivity of 100 mS cm−1, the highest among those of ionic liquids reported to date, and clarified its physical properties and structures. We also applied an ionic liquid consisting of cesium fluorohydrogenate to an electrolytic bath for the production of fluorine gas, F2, by electrolysis of metal fluorides dissolved in it. We proposed a fuel cell using a fluorohydrogenate ionic liquid. In this system, the fluorohydrogenate anion acts through a unique mechanism transporting hydrogen and charge, regarded as a completely new type of fuel cell that can operate even at medium temperatures (>80 °C) under non-humidified conditions. We investigated the properties of binary and ternary mixed salt systems of alkali metal salts systematically using bis(trifluoromethylsulfonyl)amide (TFSA) and bis(fluoromethylsulfonyl)amide (FSA) as anions, which had high electrochemical stability and low melting points, and succeeded in developing a series of ionic liquids that enabled deposition of metallic lithium and sodium. We also fabricated a sodium secondary battery by combining a metallic sodium anode and a NaCrO2 cathode to demonstrate excellent charge/discharge and cycle performance. We also succeeded in developing an ionic liquid with a low-melting-point (61 °C), NaFSA-KFSA binary system, and reported high-performance operation of Na-Sn alloy-based anodes and NaCrO2 cathodes. We created an FSA-based inorganic-organic hybrid ionic liquids with wide temperature ranges and high lithium and sodium ionic conductivities. Taking advantage of the heat resistance of the ionic liquid, we achieved ultra-fast charge/discharge by operating it at 90 °C. Furthermore, we produced a 27 Ah prismatic battery using a hard carbon negative electrode and a NaCrO2 positive electrode, and achieved weight and volume energy densities of 75 Wh kg−1 and 125 Wh L−1, respectively. The power density of this battery was 225 W kg−1 and capacity retention rate at 500 cycles of charge/discharge was 87 %.
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