Three-dimensional Catenated 1-ethyl-3-methylimidazolium Halotitanate Ionic Liquid Electrolytes for Electrochemical Applications

Abstract

A new family of ionic liquid based electrolytes ([EMImCl/(TiCl4)1.4]/(δ-MgCl2)x) for electrochemical applications is proposed. These materials are obtained by direct reaction of 1-ethyl-3-methylimidazolium chloride (EMImCl) with titanium(IV) chloride (TiCl4), and doped with increasing amounts of δ-MgCl2. Modulated differential scanning calorimetry (MDSC) measurements show that in these electrolytes the glass transition temperature, Tg, occurs at −36°C, and that the crystallization and melting transitions are inhibited by the presence of large and highly amorphous charge delocalized anionic domains. Vibrational studies indicate that in the electrolyte the anionic domains consist of large monomeric and dimeric 3D-catenated halotitanate clusters, in which magnesium chloride units are coordinated to Ti species, and whose equilibrium concentration is modulated by the concentration of δ-MgCl2. Density functional theory (DFT) based electronic structure calculations were undertaken to understand the structural features of these materials and to confirm the vibrational frequency assignments. Broadband electrical spectroscopy (BES) studies reveal that the electric response of these materials is modulated by polarization and dielectric relaxation events. These measurements revealed the correlations existent between conductivity mechanisms and the dielectric relaxations of the host matrix of the electrolytes. At 40°C, the conductivity is found to depend on the δ-MgCl2 concentration and approximately on the order of 10−4Scm−1. Preliminary electrochemical measurements reveal a reversible deposition and stripping process of a Mg-Ti alloy, with an average columbic efficiency of 99.3% and a deposition overvoltage of only 10mV.