Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond

Research highlights: → The unsaturated phosphonium ionic liquids showed lower melting point than those of the corresponding saturated phosphonium ionic liquids. → The unsaturated phosphonium ionic liquids exhibited high thermal stability when compared to the corresponding saturated phosphonium ionic liquids. → The unsaturated phosphonium ionic liquids were lower viscous and higher conductive than the corresponding saturated phosphonium ionic liquids. → The unsaturated phosphonium ionic liquids exhibited high cathodic stability as much as the corresponding saturated phosphonium ionic liquids. → Iithium redox reaction was observed in the unsaturated phosphonium ionic liquids, which means the possibility to use for lithium battery electrolytes. - Abstract: Physical and electrochemical characterizations of novel two ionic liquids based on quaternary phosphonium cations containing an unsaturated carbon-carbon bond (triethyl(4-pentenyl)phosphonium and allyltributylphosphonium cations) are presented in this report. It was found that both unsaturated phosphonium cations gave low-melting salts in combination with a bis(trifluoromethylsulfonyl)amide anion. The thermogravimetric analysis suggested that the unsaturated phosphonium ionic liquids showed higher thermal stability than those of the corresponding saturated phosphonium ILs. The unsaturated phosphonium ionic liquids also exhibited relatively low viscosity and high conductivities when compared to those of the corresponding saturated phosphonium ionic liquids. These results indicate an improving effect of introducing a carbon-carbon double bond into the phosphonium cations on both the thermal stability and the transport property. The voltammetric measurements suggested that the triethyl(4-pentenyl)phosphonium-based ionic liquid showed a high cathodic stability, enabling the deposition and dissolution of metallic lithium in the phosphonium ionic liquid system.