Reaction and Mass Transport Simulations By LBM in Electrolyte of Aqueous Lithium-Air Battery
Authors: Shibata Kazuki , Uemura Suguru , Tsushima Shohji , Hirai Shuichiro 
Published: February 2020

The aqueous Lithium-air battery receives much attention because of its large theoretical energy density; however, clarification of reaction and mass transport of electrolytic solution is necessary toward practical use [1]. In this study, Lattice Boltzmann Method (LBM) was made to realize coupled simulation of transport phenomena of Li+ and O2 under discharge process. The numerical results suggested the importance of supplying sufficient O2 in the electrolyte to achieve high current density discharge.
Structure of aqueous Lithium-air battery and calculation area are shown in Fig. 1. In this study, we focused on mass transport in the electrolyte (1M LiCl aq) and employed two-dimensional calculation area. Through the calculation, constant Li+ flux was set on the end of anode side. O2 concentration at the end of cathode side (air/electrolyte interface) was defined as saturation solubility (8.45×10-6 g/cm). Periodic boundary condition in the y direction of the computational domain was imposed. Shape of the carbon porous layer was simplified and three rectangles were set as electrode. O2 and Li+ are consumed on the electrode surface along with the discharge reaction. Ion transport and electric potential distribution were simulated by LBM with using two-dimensional 9 velocities (2D9Q) model [2] and considering the effect of electric field.
Figure 2 shows the results of concentration distribution of Li+ and O2 in 0.1 mA/cm2 discharge process. It was found that sufficient Li+ is supplied for all of electrode surface during discharge process. On the other hand, the distribution of O2 shows quite low concentration.
Since the O2 concentration in the electrolyte decreases significantly with the discharge, we focus on the relationship between local reaction rate on the electrode and O2 concentration. Figure 3 indicates x-directional distributions of them at y = 30 mm (see Fig. 1).
In case of 0.1 mA/cm2 (see Fig. 3(a)), the O2 concentration near the anode side is reduced to less than 50% of the solubility; however, the O2 is supplied to three of electrodes. As a result, discharge reaction occurs at all electrode surfaces and then local reaction rate shows pulsed shape distribution because reaction occurs on the electrode surface.
When the current density is increased to 0.5 mA/ cm2, O2 concentration decreased remarkably near the air/electrolyte interface as shown in Fig.3 (b). O2 concentration is almost zero in the Anode side (0 < x <570 mm). Supplied amount of O2 is seriously insufficient for discharge reaction in this calculation model because the O2 solubility is quite small in the electrolyte, and distance from air/electrolyte interface to electrode is far. As a result, the most of O2 is consumed at the electrode close to air interface and the reaction rate is significantly high. Thus, all of the electrodes are not used efficiently.
The presented results indicate that O2 supply from the air/electrolyte interface became insufficient and it defined critical current density of the battery. Supplying sufficient O2 in the electrode surface by increasing the air interface is one of the effective solutions. The use of high-pressure O2 is also an important method in achieving the high current density because O2 solubility in the electrolyte can be increased by pressure control.

Acknowledgement

The authors would like to thank Prof. Imanishi from Mie University for his assistance with the experimental study of aqueous Lithium-air battery.

References

[1] T. Zhang, N. Imanishi, Y. Shimonishi, A. Hirano, Y. Takeda, O. Yamamoto, Chem. Commun., 2010, 46, pp 1661-1663.
[2] X.Y. He, N. Li, Comput. Phys. Commun, 2000, 129, pp. 158–166.