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.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, the 27Al spectra the 1:1 molar ratio DES electrolytes showed three well resolved peaks at about 101, 88 and 74 ppm, assigned to [AlCl2(amide)2]+, AlCl4 -, and [AlCl2(amide)2]+ respectively. An example of this is shown in Figure 1 for the 1.0:1 (left) AlCl3:PA. With increasing AlCl3 concentration, the narrow well resolved peaks gave way to broadened, asymmetric spectra with two to three visible but not separated peaks. This is also shown in Figure 1 (right) for the 1.3:1 (right) AlCl3:PA. A fourth peak also appeared for DES electrolytes which could be due to free AlCl3:amide species. Figure 1. 27Al variable temperature spectra for 1.0:1 (left) and 1.3:1 (right) AlCl3:Propionamide (PA).Thirdly, like the spectra, the 27Al T1 data for the pure DES electrolytes were very dependent on salt concentration and the species type. For the 1:1 molar ratio electrolyte, the AlCl2(amide)+ species had the longest T1, while the AlCl2(amide)2 +had the shortest. Fourthly, with the inclusion of additives such as propylene carbonate (PC) both the 1H and 27Al T1increased, indicating increasing local ion dynamics possibly through the reduction of bulk viscosity effects and Coulombic interactions. In addition to this, PC also caused additional resolution of the [AlCl2(amide)2]+ species - which was not observed in the pure DES. These results and more will be expounded upon in our presentation. Figure 1