Polymer Gel Membranes for Aluminum Deposition and Rechargeable Aluminum Ion Batteries

Aluminum and its many versatile alloys are routinely used as surface coatings for the corrosion protection of many metals, offering both barrier and sacrificial protection. Among various methods for aluminum coating, electrodeposition is the most attractive one because it leads to thin, economical coatings that usually are adherent and do not affect the structural and mechanical properties of the substrate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Room-temperature ionic liquids (low-temperature molten salts) possess a unique set of physical properties such as nonflammability, negligible vapor pressure, high ionic conductivity, and high thermal, chemical and electrochemical stability. Therefore, they open the door for the electroplating of reactive elements, which is impossible using aqueous or other organic solvents. Thus far, most research about the use of ionic liquids for electrodeposition of aluminum and its alloys has focused on chloroaluminate anions, which were obtained by mixing anhydrous AlCl₃ with organic chloride salt such as 1-ethyl-3-methyl imidazolium chloride (EMImCl) and 1-(1-butyl)pyridinium chloride (N-BPCl) etc. Unfortunately, because of the hygroscopic nature of AlCl₃ and the chloroaluminate, the electroplating has to be performed in an inert-gas atmosphere. ¹

One way to reduce the moisture sensitivity of the chloroaluminate based ionic liquids for practical application is to form polymer gel electrolytes, either by impregnating liquid electrolytes into the preformed membranes or co-cast polymer and liquid electrolytes, or by copolymerization of monomers in the presence of plasticizers. In this paper, polymer gel membranes were synthesized using the latter approach, that is, copolymerization of monomers in the presence of plasticizers. The gel membranes are transparent with good mechanical properties, which can be used for practical applications. Fig. 1 shows the ionic conductivities of the gel membranes containing different amount of eutectic mixture of AlCl₃ and 1-ethyl-3-methylimidazolium chloride (EMIm.Cl) in a molar ratio of 1.5:1. At 20^oC the ionic conductivities of the gel membranes containing 50, 60, 70 and 80 wt% of AlCl₃-EMIm.Cl are 5.3 x 10^-5, 2.0 x 10^-4, 8.7 x 10^-4 and 1.7 x 10^-3 S cm^-1, respectively. Fig. 2 shows a typical cyclic voltammetry of the membrane containing 60 wt% of AlCl₃-EMIC (1.5:1) with copper plate as the working electrode and aluminum plate as the counter electrode. Reversible aluminum deposition and stripping is clearly observed. It is noticed that the current densities increase with increasing the scan cycles, indicating an activation process, probably due to the residual surface oxide on the Al plate.

For rechargeable aluminum ion battery application, reversible aluminum deposition/ stripping with high coulombic efficiency is a necessary step to realize stable cycling performance. Therefore, this gel membrane can also be used for rechargeable aluminum ion batteries. ²More details about the gel membrane will be presented at the meeting.

Acknowledgements:This work was funded by the Strategic Environmental research and Development Program (SERDP) (WP2316).

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