Optimizing the electrochemical performance of imidazolium-based polymeric ionic liquids by varying tethering groups.

摘要

Increasing efforts have been made in the area of sustainable alternative energy devices in the past few decades in order to develop high efficiency, low-cost electrochemical devices with sufficient long-term stability. Due to the drawbacks of conventional organic liquid electrolytes, such as leakage, volatility, flammability, and toxicity, the synthesis of solvent-free electrolyte materials has been studied world-wide.;Among all the alternatives, polymer electrolytes are of great interest and have attracted many research groups. Solid-state polymer electrolytes and in particular, polymer ionic liquids (PILs), considered to be promising candidates, have been under studied widely. Ionic Liquids (ILs), defined as organic/inorganic salts with m.p. lower than 100 °C, offer good chemical stability, low flammability, negligible vapor pressure and high ionic conductivity. PILs, as the polymerized state of ILs, not only present some of the unique properties of ILs, but also benefit from the intrinsic properties of polymers, such as better thermal and chemical stability, enhanced mechanical properties, and tunable solution properties. The constrained structure of PILs may help to overcome fabrication and leakage problems associated with simple liquid electrolytes, but typically also leads to lower ionic conductivity. Once polymerized, the ionic conductivity of PILs drops substantially, usually by several orders of magnitude compared to the corresponding monomers. Therefore, to improve PILs chain mobilitiy is crucial. Previous studies suggest that a flexible tethering group should make the polymer backbone less rigid and increase electrolyte ion mobility.;To investigate how tethering groups affect both electrochemical performance and physical properties of free ILs and PILs, we first report the synthesis and characterization of a novel class of imidazolium (Im) based IL model compounds and their corresponding PILs. Poly(ethylene oxide)s (PEOs), considered to be promising candidates for this purpose, were attached as tethering groups to imidazolium cations in order to optimize the Tg and ionic conductivities. Previous research on oligomer/polymer electrolytes showed that attaching PEO to the imidazolium cation lowered the Tg of ILs and increased their conductivity. PEO is also chemically stable, dissolves metal ions, and when incorporated into ionic liquids, provides a solvent free electrolyte. A series of IL model compounds and PILs were first synthesized with various lengths of PEO attached on the imidazole. The thermophysical and electrochemical properties of ILs and PILs, including density, viscosity, conductivity and thermal properties were characterized in order to investigate the effect of tethering groups.