AZ31 Magnesium Alloy Foils as Thin Anodes for Rechargeable Magnesium Batteries

Abstract

In recent decades, rechargeable Mg batteries (RMBs) technologies have attracted much attention because the use of thin Mg foil anodes may enable development of high-energy-density batteries. One of the most critical challenges for RMBs is finding suitable electrolyte solutions that enable efficient and reversible Mg cells operation. Most RMB studies concentrate on the development of novel electrolyte systems, while only few studies have focused on the practical feasibility of using pure metallic Mg as the anode material. Pure Mg metal anodes have been demonstrated to be useful in studying the fundamentals of nonaqueous Mg electrochemistry. However, pure Mg metal may not be suitable for mass production of ultrathin foils (<100 microns) due to its limited ductility. The metals industry overcomes this problem by using ductile Mg alloys. Herein, the feasibility of processing ultrathin Mg anodes in electrochemical cells was demonstrated by using AZ31 Mg alloys (3 % Al; 1 % Zn). Thin-film Mg AZ31 anodes presented reversible Mg dissolution and deposition behavior in complex ethereal Mg electrolytes solutions that was comparable to that of pure Mg foils. Moreover, it was demonstrated that secondary Mg battery prototypes comprising ultrathin AZ31 Mg alloy anodes (≈25 μm thick) and Mg_x Mo₆ S₈ Chevrel-phase cathodes exhibited cycling performance equal to that of similar cells containing thicker pure Mg foil anodes. The possibility of using ultrathin processable Mg metal anodes is an important step in the realization of rechargeable Mg batteries.

Keywords: Mg electrodes; batteries; electrochemistry; energy storage; rechargeable Mg batteries.

Figure 1

SEM images of the surface of pristine (a) Mg metal foil, (b) Mg alloy AZ31 thin film, and (c) cross‐section SEM image of ultrathin AZ31 foil.

Figure 2

Voltage profile of dissolution‐deposition process on AZ31 alloy thin film and Mg metal thin film as anodes at current densities of (a) 0.1 mA cm⁻² for 10 h and (b) 1 mA cm⁻² for 1 h, in 0.25 m APC/THF solution, with Mg metal as counter and reference electrodes. The charges involved in these processes were 1 mA and 0.1 mA per cm², respectively.

Figure 3

SEM images of electrodes after dissolution processes. Images a and b relate to pure Mg foil electrodes after dissolution at 1 mA/cm² for 1 h and 0.1 mA/cm² for 10 h, respectively. Images c and d relate to AZ31 Mg alloy foil electrodes after dissolution at 1 mA/cm² for 1 h and 0.1 mA/cm² for 10 h, respectively. 0.25 M APC/THF solutions.

Figure 4

SEM images of Mg electrodes after deposition processes at different current densities. Images a and b relate to pure Mg anodes 1 mA/cm² and 0.1 mA/cm² respectively. Images c, d relate to AZ31 Mg alloy electrodes, −1 mA/cm² (d) 0.1 mA/cm², in 0.25 M APC/THF. The charge involved in these processes were 1 mA and 0.1 mA per cm² respectively.

Figure 5

Measurements of full Mg cells, with Chevrel phase Mg_xMo₆S₈ (0<×<2) cathodes, (a) rate performance and (b) cycling performance of Mg (100 μm), AZ31 (100 μm) and AZ31 (25 μm), galvanostatic profile of various cycles of (c) Mg (100 μm), (d) AZ31 (100 μm) and (e) AZ31 (25 μm) in 0.25 M APC/THF solutions.