Ionic liquids (ILs) are good candidates as electrolytes for electrical energy storage (EES) devices due to their high safety and superior physicochemical and electrochemical properties. Although Li-ion batteries (LIBs) are good candidates for EES and many types of IL electrolytes for LIBs were already reported [1-3], the limited amount of lithium and cobalt resources may lead to the unstable supply of LIBs in the future. Thus, several groups including us developed IL electrolytes for Na-ion batteries (NIBs) utilizing low-cost and ubiquitous resources of sodium and iron [4-8]. However, the standard potential of Na⁺/Na redox couple is lower than that of Li⁺/Li redox couple, which reduces the energy densities of batteries [9].

Under these circumstances, we have focused on K-ion batteries (KIBs) and developed new IL electrolytes containing alkyl-pyrrolidinium and -imidazolium cations. For example, in the K[FSA]-[C₃C₁pyrr][FSA] (C₃C₁pyrr⁺ = N-methyl-N-propylpyrrolidinium) binary system [10], a mixed melt at x(K[FSA]) = 0.20 (molar fraction) shows a reasonably high ionic conductivity of 4.8 mS cm⁻¹ and a wide electrochemical window over 5 V at 298 K. The electrochemical windows of M[FSA]-[C₃C₁pyrr][FSA] (M = K, Na, Li) are compared by cyclic voltammetry. As the result, the K-ion system shows a wider electrochemical window than Na-ion and Li-ion systems due to negative potential of the K⁺/K redox couple, which implies the feasibility of KIBs possessing higher energy densities.

Acknowledgments

This study was partly supported by MEXT program "Elements Strategy Initiative to Form Core Research Center" (since 2012; MEXT: Ministry of Education Culture, Sports, Science and Technology, Japan), and a research grant from Kansai Research Foundation for Technology Promotion, Japan.

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