(Invited) Electrochemical Properties of Ionic Liquids Containing Aprotic Heterocyclic Anions (AHA ILs) and Their Mixtures with Lithium Salts

Abstract

Ionic liquids containing aprotic heterocyclic anions, such as pyrrolides, pyrazolides, imidazolides, triazolides and tetrazolides, were originally developed for CO2 capture applications since these anions can chemically react with CO2.1 Subsequently, we have been investigating the potential use of this class of “AHA” ILs for a variety of electrochemical applications. We have synthesized a series of room temperature ionic liquids (RTILs) based on 1-ethyl-3-methylimidazolium ([emim]+) and tetra-alkylphosphonium ([P222n]+) cations with different aprotic heterocyclic anions (AHAs) and characterized them as potential electrolyte candidates.2,3 The density and transport properties of these ILs were measured over the temperature range between 283.15 K and 343.15 K at ambient pressure. The temperature dependence of the transport properties (viscosity, ionic conductivity, self-diffusion coefficient and molar conductivity) was fit well by the Vogel-Fulcher-Tamman (VFT) equation. The ionicity of these ILs was quantified by the ratio of the molar conductivity obtained from the ionic conductivity and molar concentration to that calculated from the self-diffusion coefficients using the Nernst-Einstein equation. Exceptionally high values of this molar conductivity ratio, even greater than 0.9, were reported. The results showed that many of the AHA ILs exhibit very good conductivity for their viscosities, and provided insight into the design of ILs with enhanced dynamics that may be suitable for electrolyte applications. Here we will explore how the addition of lithium AHA salts affects the transport properties of the IL/salt mixtures in order to evaluate the use of AHA ILs for lithium ion battery applications. In particular, we investigate the ionic conductivity and viscosity, as well as anion, cation and lithium diffusivity, as measured by Pulse-Gradient-Spin-Echo NMR spectroscopy. The focus of this investigation is imidazolium cations paired with pyrrolide and pyrazolide based anions. The pure lithium AHA salts, which exhibit some interesting properties themselves, are characterized by a variety of analytical (e.g., TGA, DSC) and spectroscopic (e.g., NMR) techniques. Finally, we explore the influence of additives on viscosity and ionic conductivity, and on anion, cation and lithium diffusivity. If ionic liquids could be used as battery solvents, they could greatly reduce the fire hazard presented by the currently used organic carbonate solvents. B. Gurkan, B. F. Goodrich, E. M. Mindrup, L. E. Ficke, M. Massel, S. Seo, T. P. Senftle, H. Wu, T. W. Rosch, M. F. Glaser, J. K. Shah, E. J. Maginn, J. F. Brennecke, and W. F. Schneider, “Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture,” J. Phys. Chem. Let., 1(24), 2010, 3494-3499.L. Sun, O. Morales-Collazo, H. Xia and J. F. Brennecke, “Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity and Self-diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) RoomTemperature Ionic Liquids. 1. Variation of Anionic Species,” J. Phys. Chem. B, 2015, 119, 15030-15039.L. Sun, O. Morales-Collazo, H. Xia and J. F. Brennecke, “Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity and Self-diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in the Phosphonium Cation,” submitted to J. Phys. Chem. B, 2016.