Fundamental Investigations of Factors Affecting Electrical and Transport Properties of Lewis Acidic Chloroaluminate Ionic Liquids for Application in Aluminum Ion Battery
Authors: Ng Kok Long , Lu Zhuole , Wang Yijia , Singh Chandra Veer , Azimi Gisele 
Published: July 2021

High-performance electrochemical energy storage systems are considered key technologies to sustainable energy economy transition. Despite being one of the most mature and leading-edge battery systems, the future of lithium-ion batteries is shadowed by issues related to resource availability, high production cost, and short lifetime. For these reasons, there has been a great impetus towards the development of alternative battery technologies based on abundant and low-cost materials. Among all, aluminum-ion batteries (AIBs) are particularly promising due to high theoretical capacity, low-cost, and ease of handling and storing of metallic aluminum in ambient conditions.

The majority of advanced AIB systems utilizes Lewis acidic chloroaluminate ionic liquid (IL) electrolytes, formed by combining molar excess amount of aluminum chloride (AlCl3) with an organic chloride (RCl; R: organic group). In these systems, the chloroaluminate IL electrolyte behaves as (i) a medium for ion transportation and (ii) an electroactive liquid anode material (also known as anolyte) in AIBs. Among various parameters, the AlCl3/RCl molar ratio (r) and the RCl utilized, are considered the most deterministic factors that affect the performance of AIBs. Several battery and electrolyte performance metrics including achievable specific/volumetric cell-level capacity, electrochemical stability window (ESW), ion transport properties, and conductivity, are known to be dependent on r and the RCl utilized. Despite the importance of chloroaluminate ILs in AIB research field, there is a significant lack of fundamental understandings of the effects of r and RCl on these performance matrices. The elucidation of these effects can provide fundamental insights into approaches for optimizing battery operating conditions as well as revealing important characteristics of high-performance chloroaluminate ILs.

This work is focused on investigating the effects of r on the electrical and transport properties of four AlCl3-RCl ILs, where RCl includes EMIMCl (1-ethyl-3-methylimidazolium chloride), BMIMCl (1-buthyl-3-methylimidazolium chloride), TMAHCl (trimethylamine hydrochloride), and TEAHCl (triethylamine hydrochloride). A combined experimental-computational approach is utilized to obtain fundamental insights into the influence of AlCl3/RCl molar ratio on various properties of chloroaluminate ILs including ESW, ion transference number, and ionic conductivity.

Our results show that the ESW of Lewis acidic chloroaluminate ILs is strongly dependent on r, as r preliminarily governs the concentration of electroactive anionic species (AlCl4
− and Al2Cl7
−) involved in electrochemical redox reactions. For Lewis acidic chloroaluminate ILs, the reduction of Al2Cl7
− to Al (4Al2Cl7
− + 3e− = Al + 7AlCl4
−) sets the cathodic limit while the oxidation of AlCl4
− to evolve Cl2 (2AlCl4
− = Al2Cl7
− + 1/2Cl2 + e−) defines the anodic limit. The cathodic limiting potential shifts positively while the anodic limiting potential shifts negatively, leading to a narrower ESW width with increasing r. Moreover, electrostatic and van der Waals forces significantly affect the cathodic and anodic limiting potentials as these interactions affect the activity of electroactive species involved. The viscosity of chloroaluminate ILs is particularly useful as it reflects the overall degree of interactions in chloroaluminate ILs. Chloroaluminate IL systems that have higher viscosities exhibit a positive shift of anodic and cathodic limiting potentials. The organic cation R+ is determined to contribute substantially to the total ionic conduction in Lewis acidic chloroaluminate ILs. To fundamentally enhance the electrochemical performance of AIBs employing chloroaluminate ILs, strategies focused on the localization of organic cations should be emphasized as they can effectively improve the proportion of ionic conduction by electroactive chloroaluminate anions while hindering the formation of cation-anion aggregates.