Li-air batteries (LAB) have been attracting much attention because of their theoretical energy density of 3500 Wh kg⁻¹, which is five times as high as that of Li-ion batteries. However, many problems remain to be challenged in every component of the battery: air electrodes, electrolytic solutions, Li electrodes, etc. As for the Li electrode, we have reported some results on the reversibility of the Li dissolution/deposition reaction by using Li | Li symmetric cells with tetraglyme (G4) -based electrolytic solution [1, 2]. In particular, it has been confirmed that in the 1.0 M LiNO₃/G4 electrolyte under an O₂ atmosphere, a Li₂O protective layer is efficiently formed on the Li electrode at a current density of 0.4 mA cm^-2, and Li dendrite formation is suppressed. In this study, we expanded test conditions in current densities up to 2.0 mA cm^-2 and temperatures at 10 °C to 50 °C to reveal whole picture of dissolution/deposition behavior of the Li electrode.

A Li | Li symmetric cell was assembled using a pair of Li foils inserted by a separator (Celgard 2400) and 1.0 M LiNO₃/G4 or 1.0 M LiTFSI / G4 as the electrolytic solutions in an Ar-filled glove box. The Li dissolution/deposition cycle tests by the cells were carried out at 10, 30 and 50℃ at a constant current density between 0.2 to 2.0 mA cm^-2 regulated by a constant capacity of 0.5 mAh cm⁻². After the tests, the Li electrodes were taken out of the cells followed by rinsing in G4 in the glove box, the surfaces of which were observed by SEM and analyzed by XPS.

Figure 1 showed Li | Li cell test results at 0.4 mA cm⁻² and 30 ℃ in different electrolytic solutions. Figure 1a for that in 1.0 M LiTFSI/G4 showed gradual decrease in overvoltages and unstable polarization curves. These two features suggested that the Li electrode protection by the Li₂O layer from O₂ gas could not withstand the current density and the exposed Li metal reacted with the solvent and electrolyte, resulting in growing Li dendrite deposition. On the other hand, Figure 1B for that in the 1.0 M LiNO₃ / G4 overvoltages, suggesting suppressed Li-electrolytic solution showed very flat polarization plateaus and slightly growing dendrite formation. The difference between Figure 1a and 1b demonstrated the effect of Li₂O protective layer of Li surface formed by the reaction between Li and NO₃⁻.

Figure 2 showed the SEM image of the Li electrode after the dissolution/deposition test for 15 cycles and the surface analysis by C1s XPS. Figure 2a for LiTFSI/G4 electrolytic solution showed bulky surface layer as thick as 30 µm accompanied by a large dent. Figure 2b suggested various kind of C-containing deposits on the Li surface, which was presumably decomposed products of electrolytic solution. Therefore, Figure 2a and 2b were well consistent with Figure 1a which suggested successive surface reactions. Figure 2c and 2d for LiNO₃/G4 electrolytic solution, on the contrary, showed thin surface layer and poor amount of surface products, even though the current density, 0.6 mA cm⁻², was higher than that in Figure 2a, 0.4 mA cm⁻². These results suggested steady surface protected by Li₂O layer and were also consistent with Figure 1b.

Other results presenting the effect of current densities and temperatures will be reported in our presentation to clarify the whole picture of Li dissolution/deposition reaction mechanism.

References

[1] M. Saito et al. J. Electrochem. Soc., 164, 2872–2880 (2017).

[2] M. Saito et al. J. Electrochem. Soc., 168, 010520 (2021).

Acknowledgement

This study was supported by JST Project ALCA-SPRING (JPMLAL1301) and NIMS Joint Research Hub Program, Japan