Measuring and Enhancing the Ionic Conductivity of Chloroaluminate Electrolytes for Al-Ion Batteries

Abstract

At the core of the aluminum (Al) ion battery is the liquid electrolyte, which governs the underlying chemistry. Optimizing the rheological properties of the electrolyte is critical to advance the state of the art. In the present work, the chloroaluminate electrolyte is made by reacting AlCl₃ with a recently reported acetamidinium chloride (Acet-Cl) salt in an effort to make a more performant liquid electrolyte. Using AlCl₃:Acet-Cl as a model electrolyte, we build on our previous work, which established a new method for extracting the ionic conductivity from fitting voltammetric data, and in this contribution, we validate the method across a range of measurement parameters in addition to highlighting the model electrolytes' conductivity relative to current chloroaluminate liquids. Specifically, our method allows the extraction of both the ionic conductivity and voltammetric data from a single, simple, and routine measurement. To bring these results in the context of current methods, we compare our results to two independent standard conductivity measurement techniques. Several different measurement parameters (potential scan rate, potential excursion, temperature, and composition) are examined. We find that our novel method can resolve similar trends in conductivity to conventional methods, but typically, the values are a factor of two higher. The values from our method, on the other hand, agree closely with literature values reported elsewhere. Importantly, having now established the approach for our new method, we discuss the conductivity of AlCl₃:Acet-Cl-based formulations. These electrolytes provide a significant improvement (5-10× higher) over electrolytes made from similar Lewis base salts (e.g., urea or acetamide). The Lewis base salt precursors have a low economic cost compared to state-of-the-art imidazolium-based salts and are non-toxic, which is advantageous for scale-up. Overall, this is a noteworthy step at designing cost-effective and performant liquid electrolytes for Al-ion battery applications.

Figure 1

(a) Exemplar experimental cyclic voltammogram (solid and dashed black traces represent cathodic and anodic scans, respectively) demonstrating the i–E curve-fitting (solid red line) method with the AlCl₃:Acet-Cl 2:1 electrolyte at a scan rate of 100 mV s^–1. (b) Corresponding QCM plot at 100 mV s^–1 showing the mass change as a function of potential. The orange shaded region visually represents deposition potentials, whereas the blue shaded region represents dissolution potentials. A solid gray line is included at the zero mass mark for clarity.

Figure 2

Experimental impedance modulus plot (filled black squares) and corresponding fit (solid red line) to an R-CPE electrochemical equivalent circuit (see the inset) for the AlCl₃:Acet-Cl 2:1 formulation. The dashed gray lines are overlaid as a visual aid for estimating the capacitance and resistance. Collected at 0 V with a perturbation amplitude of 10 mV.

Figure 3

(a) Overlay of cyclic voltammograms at five potential scan rates for the AlCl₃:Acet-Cl 2:1 formulation and (b) plot of conductivity (from i–E curve fitting) as a function of scan rate. Error bars represent the standard deviation of 3 replicate measurements.

Figure 4

Overlay of cyclic voltammograms for the AlCl₃:Acet-Cl 2:1 formulation at 1.1, 2.3, and 3.5 V potential widths collected at (a) 100 and (b) 20 mV s^–1.

Figure 5

(a) Overlay of CV cycle 1 (solid black line) and cycle 30 (dashed red line) for the AlCl₃:Acet-Cl 2:1 formulation at a scan rate of 100 mV s^–1. (b) Corresponding conductivity data estimated from i–E curve fitting as a function of CV cycle number.

Figure 6

Overlay of cyclic voltammograms with uncompensated (solid black trace) and iR-compensated (dashed red trace) data for the AlCl₃:Acet-Cl 2:1 liquid at a scan rate of 100 mV s^–1.

Figure 7

(a) Overlay of cyclic voltammograms measured from 25 to 70 °C at 100 mV s^–1. The dashed black arrows point toward increasing temperatures. (b) Arrhenius plots of temperature-dependent conductivity data from EIS data (empty black circles) and CV data (filled black squares). The best fit line is shown for the two data sets along with the line equation and R² coefficient.

Figure 8

(a) Cyclic voltammograms at a scan rate of 100 mV s^–1 for different mole ratios of LA:LB for the AlCl₃:Acet-Cl system. (b) Overlays of the conductivity as a function of mole ratio for three different methods: 100 mV s^–1i–E fitting (black squares), 20 mV s^–1i–E fitting (red circles), and EIS fitting data (blue triangles). Error bars represent the standard deviation of 3 replicate measurements.