Aluminium (Al) batteries are a promising, next-generation technology and current research efforts are aimed at positioning this technology to compete with existing lithium-ion batteries (LIB). The development of non-aqueous electrolyte chemistries for Al battery systems has received renewed attention to address some of the shortcomings associated with LIB. Of particular importance in this development is the liquid electrolyte as its rheology governs the battery chemistry. The goals are to generate a liquid that is compatible with the other (solid) battery components, stable long-term with repeated use, and to optimise the rheology (i.e. target high conductivity and low viscosity). Chloroaluminate room temperature ionic liquid (RTIL) electrolytes made by mixing Lewis acidic aluminium chloride (AlCl₃) salt with a (often chloride-containing) Lewis basic salt e.g. 1-ethyl-3-methylimidazolium chloride (EMIM-Cl) has been extensively studied. This tuneable electrolyte provides good thermal stability, good ionic conductivity, and a wide polarizable potential window. While these traits are advantageous these types of Lewis basic salt precursors are generally expensive, difficult to synthesize and in some instances can be toxic. Recently, ionic liquid analogues (ILA) that are made from abundant, inexpensive and often non-toxic materials have begun to be explored. To date the two most common Lewis basic salts examined have been urea and acetamide but their rheological and electrochemical properties need to be improved in order to complete with RTIL-based electrolytes.

Our group have recently revealed that amidine-based chloroaluminate ILA electrolytes show promise over urea-, acetamide-, and pyrrolidinium-based electrolytes.[1] Specifically, guanidinium chloride (Guan-Cl) and acetamidinium chloride (Acet-Cl) based salts display reversible electrochemical plating/stripping of Al, good ionic conductivities (e.g. 10 mS cm^-1), and moderate viscosities (e.g. 50 cP). Also, in this work we initially proposed a mathematical model to extract the conductivity from these electrolytes by fitting the voltammetric i-E curve (from Al deposition/dissolution) to a linear, modified Butler-Volmer formalism. The characteristic, anodic i-E trace shows a striking linearity often over very large potential ranges (e.g. >2 V), and this response is used to extract ionic conductivity. This represents a novel, electroanalytical method to obtain this important rheological metric.

As such, in this contribution we will highlight our latest efforts to expand, benchmark (to common methods for measuring conductivity), and determine the limits of our i-E curve fitting method to measure ionic conductivity.[2] Specifically, we have examined the AlCl₃:Acet-Cl electrolyte in depth by looking at the potential scanning rate, potential range probed, and the compositional (mole ratio Lewis acid : Lewis base) effect on the conductivity extracted from our fitting method. We have also studied a range of other common chloroaluminate electrolytes to compare with published literature. All of these values are then benchmarked to conductivity data measured both from a traditional impedance-based method and that from a commercial conductivity probe. Lastly, we also have examined temperature-dependent conductivities from all three of these methods. Overall, we find good agreement between values measured from our in situ i-E curve fitting method to those from more traditional conductivity measurement methods. This electroanalytical work serves to deepen our understanding of the conductivity of chloroaluminate ILAs for Al battery applications.

References:

[1] A.J. Lucio, I. Efimov, O.N. Efimov, C.J. Zaleski, S. Viles, B.B. Ignatiuk, A.P. Abbott, A.R. Hillman, K.S. Ryder. Chem. Commun., 2021, 57, 9834-9837.

[2] A.J. Lucio, E. Bulmer, I. Sumarlan, A.R. Hillman, K.S. Ryder. 2022 in preparation.