Comparative Study of Guanidine-, Acetamidine- and Urea-Based Chloroaluminate Electrolytes for an Aluminum Battery

Abstract

Aluminum-based batteries are a promising alternative to lithium-ion as they are considered to be low-cost and more friendly to the environment. In addition, aluminum is abundant and evenly distributed across the globe. Many studies and Al battery prototypes use imidazolium chloroaluminate electrolytes because of their good rheological and electrochemical performance. However, these electrolytes are very expensive, and so cost is a barrier to industrial scale-up. A urea-based electrolyte, AlCl₃:Urea, has been proposed as an alternative, but its performance is relatively poor because of its high viscosity and low conductivity. This type of electrolyte has become known as an ionic liquid analogue (ILA). In this contribution, we proposed two Lewis base salt precursors, namely, guanidine hydrochloride and acetamidine hydrochloride, as alternatives to the urea-based ILA. We present the study of three ILAs, AlCl₃:Guanidine, AlCl₃:Acetamidine, and AlCl₃:Urea, examining their rheology, electrochemistry, NMR spectra, and coin-cell performance. The room temperature viscosities of both AlCl₃:Guanidine (52.9 cP) and AlCl₃:Acetamidine (76.0 cP) were significantly lower than those of the urea-based liquid (240.9 cP), and their conductivities were correspondingly higher. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) showed that all three electrolytes exhibit reversible deposition/dissolution of Al, but LSV indicated that AlCl₃:Guanidine and AlCl₃:Acetamidine ILAs have superior anodic stability compared to the AlCl₃:Urea electrolyte, as evidenced by anodic potential limits of +2.23 V for both AlCl₃:Guanidine and AlCl₃:Acetamidine and +2.12 V for AlCl₃:Urea. Coin-cell tests showed that both AlCl₃:Guanidine and AlCl₃:Acetamidine ILA exhibit a higher Coulombic efficiency (98 and 97%, respectively) than the AlCl₃:Urea electrolyte system, which has an efficiency of 88% after 100 cycles at 60 mA g^-1. Overall, we show that AlCl₃:Guanidine and AlCl₃:Acetamidine have superior performance when compared to AlCl₃:Urea, while maintaining low economic cost. We consider these to be valuable alternative materials for Al-based battery systems, especially for commercial production.

Figure 1

Chemical structure of Lewis bases: urea, guanidine hydrochloride, and acetamidine hydrochloride.

Figure 2

(a) Conductivity of the three ILAs as a function of temperature. (b) Arrhenius plot.

Figure 3

(a) ¹H NMR of the three ILAs. (b) ²⁷Al NMR for the three ILAs.

Figure 4

(a) Cyclic voltammetry (CV) and (b) anodic linear sweep voltammetry (LSV) of the three ILAs; inset shows the expanded region of the potential scale. Both CV and LSV recorded against an Al wire reference electrode at a potential scan rate of 10 mV s^–1.

Figure 5

Symmetrical coin-cell testing using the Al foil anode and cathode and ILA electrolyte at a current density of 0.1 mA cm^–2 with a time limit of 1 h while charging and discharging for up to 120 cycles.

Figure 6

Cyclic voltammetry (CV) of the three ILAs using a PG working electrode at a scan rate of 0.1 mV s^–1 (against an Al^(III)/Al reference electrode). Oxidative intercalation processes are indicated as peaks a, b, and c. Corresponding reductive deintercalation processes are indicated as peaks a′, b′, and c′.

Figure 7

Performance data for coin cells fabricated from an Al foil anode, PG cathode, and ILA electrolyte; (a) charging (▼)/discharging (●) specific capacity as a function of the applied current; (b) Coulombic efficiency (CE) profile as a function of the applied current; (c) Coulombic efficiency for 100 cycles at 60 mA g^–-1; and (d) charging/discharging potential profiles at the 100th cycle (cells from part (c)).