Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

WATANABE Masayoshi

doi:10.5796/electrochemistry.84.642

Ionic liquids (ILs) are defined as salts that have melting points lower than 100°C. Most are organic salts, and these may be designed and tailored to have suitable properties. ILs are recognized as a third group of solvents (and electrolytes), after water and organic solvents. They are characterized by their unique properties such as non-volatilities, high thermal stabilities, and high ionic conductivities. In this article, our work on the design and preparation of ILs is briefly reviewed. The concept of ionicity is proposed as a metric to characterize the basic nature (dissociativity) of ILs, which is affected by the Lewis acidity/basicity of cations/anions (i.e., coulombic interactions), the directionality of interactions between cations and anions, and van der Waals interactions between ions. The ionicity of ILs is dominated by a subtle balance between coulombic and van der Waals interactions, which clearly discriminates ILs from conventional high-temperature inorganic molten salts. Here, the design of protic ILs for fuel cell electrolytes, electron-transporting ILs for dye-sensitized solar cell electrolytes, and Li⁺-conducting solvate ILs for lithium battery electrolytes are discussed based on an understanding of the fundamental properties of ILs. Furthermore, the combination of ILs with polymers and colloidal particles affords intriguing quasi-solid materials (ion gels), and the ion transport in these gels is decoupled from the mechanical relaxation time of these materials, yielding new solid electrolytes. The possibility of temperature and photo-sensitive solubility dependence of polymers in ILs allows the creation of stimuli-responsive materials. Finally, protic ILs/protic salts are demonstrated to be good precursors for N-doped carbon materials.

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

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