Effect of Li-Mg Alloy Anode on Li Dendrite Suppressionfor Li-Air Secondary Batteries
Authors: Iwasaki Daiki , Fukuchi Kohei , Azuma Shota , Nomura Akihiro , Ozawa Fumisato , Saito Morihiro 
Published: December 2024

Non aqueous Li-air battery (LAB) has been attracting much attention as one of the next-generation batteries because it has a theoretical capacity about ten times that of the conventional Li-ion battery (3505 Wh kg-1). However, there is a risk of short-circuit due to Li dendrite formation on the surface of Li metal anode by charge/discharge cycling. Recently, a small amount of Mg addition into the Li metal anode was reported to work as a good scaffolding and suppress the Li dendrites in Li-sulfur batteries (LSBs) [1]. In addition, the SEI film derived from the Li-Mg alloy anode reduced an electrolyte decomposition and improved the cycle performance of LSBs. Moreover, the Li-Mg alloy anode has a higher Li diffusion coefficient than pure Li one, and the interfacial stability between the electrolyte and electrode was also improved [2]. However, there are almost no report on its application to LABs. In this study, Li dissolution/deposition behavior under O2 gas atmosphere was investigated using a Li-Mg | Li-Mg symmetric cell to reveal the effect of Mg addition into the Li metal anode.
The Li | Li and Li-Mg | Li-Mg symmetric cells was assembled using 1.0 M LiTFSI/G4 electrolyte solution, a pair of 0.5 mm thick pure Li foil, Li-10 wt%Mg (Li-0.1Mg) and Li-20 wt%Mg (Li-0.2Mg) alloy foil, and PE separator in a dry box filled with Ar gas. The Li dissolution/deposition test was repeated upto 200 cycles at a constant current density of 0.50 mA cm-2 in the voltage range of -2.0 V to 2.0 V under the O2 atmosphere, and the magnitude of overvoltage and flatness of the polarization curves were evaluated. Each resistance value was also analyzed by AC impedance measurement for each charge /discharge cycle. Furthermore, the electrode surfaces before and after the cycle tests were evaluated by a scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) analysis to investigate the differences in the Li and Mg deposition morphology and the chemical composition of SEI films.

Fig. 1 shows the polarization curves for the Li | Li and Li-Mg | Li-Mg symmetric cells. In the case of pure Li metal, the overpotential increased from 120th cycle. This suggests that Li dendrite gradually formed and caused to increase in the surface area of Li metal and electrolyte decomposition, following to generate a relatively thick SEI film. On the other hand, Li-0.1Mg alloy electrode shows smaller overpotential through the 200 cycles. This indicates that the Li-0.1Mg alloy electrode suppressed the Li dendrite by stabilizing the interphase between the electrode and electrolyte solution. In contrast, the Li-20 wt%Mg alloy electrode rather increased the overpotential during initial cycle, but gradually reduced it and was stabilized as same as Li-10 wt%Mg alloy one. These results were in good agreement with the morphologies of electrode surface after the cycling tests by SEM observation.
The results of SEM-EDS and XPS analyses in more detail, their correlation, and the mechanism of the effect of Mg addition into the Li metal electrode will be presented in the meeting.
References
[1] L.-L. Kong et al., Adv. Funct. Mater., 29, 1808756 (2019).
[2] Z. Shi et al., J. Power Sources., 92, 70 (2001).

Figure 1