Aluminum Ion Species Transport in Pure and Additive Modulated Deep Eutectic Solvents (DES) Electrolytes

Abstract

The need for safe, low cost, high energy and density storage devices is ubiquitous the world over. Satisfying this need requires various energy storage and conversion applications, one of which is the aluminum ion batteries. One problem with expansion in their development and commercialization is the availability of optimized electrolytes. Recently, deep eutectic solvents (DES) have gained attention as electrolytes for energy storage devices. These solvents are similar to ionic liquids, but are generally cheaper and easier to prepare. Additionally, they are known to dissolve metal oxides which plague AIBs operation, and their solubility is dependent on the hydrogen bond donor. Because of their inherent tunability, we studied the effect of AlCl3 concentration (1:1 – 1.7:1 molar ratio), amide type (acetamide (AA), butyramide (BA), propionamide (PA)), viscosity reducing additive type and concentration, on the aluminum ion species and transport in AlCl3:amide DES electrolytes using multi-Nuclear (1H, 27Al) Magnetic Resonance (NMR) and AC Impedance Spectroscopy techniques, as a function of temperature. Below are a few noteworthy observations. Figure 1. VFT plots of the ionic conductivity for AlCl3:AA (left) and AlCl3:PA (right) DES electrolytes. Thirdly, the variable temperature 1H and 27Al spectra and T1 data for the pure and additives included DES electrolytes were very dependent on concentration, additive type and concentration, and species type. For example, in the 1:1 molar ratio electrolyte, the assigned AlCl2(amide)+ species had the longest T1, while the AlCl2(amide)2 + had the shortest. Additionally, the inclusion of additives such as fluoroethylene carbonate (FEC) and propylene carbonate (PC) caused species specific increasing local ion dynamics, possibly through the reduction of bulk viscosity effects and Coulombic interactions. These results and more will be expounded upon in our presentation.Firstly, for both AlCl3:PA and AlCl3:BA DES electrolytes, a maximum in ionic conductivity was observed at the 1.3:1 molar ratio. For AlCl3:AA DES the maximum occurred at the 1.5:1 molar ratio. Secondly, for the three amides conductivity values fell in the range of ~1 – 11 mS/cm over the temperature range of 288 – 363K. Additionally, the temperature dependence displayed curve-like behavior and were fitted using the VFT equation to reveal high levels of fragility and dynamic behaviors similarities to some pure ILs where the effective inter-conversion between the trans and cis conformations of the anion facilitated faster ion dynamics. Figure 1