Guanidine-based chloroaluminate electrolyte: A candidate for a rechargeable aluminum battery

Rechargeable aluminum-ion batteries remain an important technological target as an alternative to lithium. Here, acidic room-temperature ionic liquid analogue electrolytes (ILAs) were synthesized from guanidine hydrochloride salt (GuanHCl) and aluminum chloride, with varying metal-to-salt (AlCl3/GuanHCl) ratios, and were characterized for their potential application in aluminum-ion batteries. The rheological properties of these electrolytes, including viscosity and electrical conductivity, were determined. The viscosity followed an increasing trend with AlCl3 content, while the conductivities followed the inverse trend consistent with Walden’s rule. Both parameters showed an Arrhenius-type behavior with respect to temperature, although, interestingly, the activation energies were all very similar, around 23 kJ mol–1. This is comparable to other chloroaluminate liquids and suggests that the mobile charge-carrying species in all the compositions are similar. The speciation of the liquids was investigated by FT-IR and NMR spectroscopies, showing significant trends that indicate interaction between the guanidinium cation and the chloroaluminate center. Electrochemical activities were correlated with rheology, and the 2.0:1.0 formulation exhibited a gravimetric response closest to the Faradaic model. Coin cell testing of the 2.0:1.0 formulation showed interesting trends in cell specific capacity and efficiency as a function of charge/discharge rate that suggests this electrolyte could be a strong candidate for a rechargeable Al battery. The electrolytes are relatively low-cost, making them suitable for potential industrial-scale applications. Clearly, further detailed studies and life-cycle testing of the cells are required in order to realize this technological potential.