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.