Effects of O2 Concentration and Precipitates on the Lithium Air Battery
Authors: Uemura Suguru , Hirai Shuichiro , Sasabe Takashi , Imanishi Nobuyuki , Torikai Kosuke , Furuyama Tomoaki 
Published: February 2020

Lithium-air battery attracts great attention because of its high energy density. In this study, we focus on the aqueous lithium–air secondary battery. The aqueous electrolyte realizes cost reduction and ensures the safe use of a battery. However, to realize a more powerful lithium–air battery, it is necessary to clarify and improve the reaction and mass transport phenomena in the cathode [1]. Low solubility of oxygen in aqueous electrolyte limits oxygen transport to the reaction site. Discharge reaction products in cathode electrode cause cell performance degradation. In this study, effect of dissolved oxygen concentration in the electrolyte on cathode performance is investigated. Low energy X-ray CT was employed to observe discharge reaction products in a porous cathode electrode.
Figure 1 shows experimental apparatus, which is composed of two main parts: a beaker cell and control section for oxygen concentration in the electrolyte, respectively. The cell is consisted from composite anode, cathode, and reference electrode. The composite anode was water-stable and had a multilayer structure for the aqueous lithium air battery [2]. LATP [Li1+x+yAlxTi2-xSiyP3-y] (Ohara Co., Kanagawa, Japan), which is a water-stable lithium-ion-conducting glass ceramic, was used, along with PEO18LiTFSI [poly (ethylene oxide) with lithium bis (trifluoromethanesulfonyl) imide] as the organic electrolyte. Because the cell resistance was drastically decreased [3], beaker cell was heated in the constant temperature bath to keep the cell temperature at 60°C. For the cathode, SIGRACET® 10AA (SGL Group, USA) was used as the carbon porous layer with no catalyst. Reaction area of anode/cathode was f4 mm. In order to control the dissolved oxygen concentration in electrolyte (1M LiCl aq.), a micro-bubble generator (Nagoya　Oshima　machinery Co.,Ltd.) was employed. Acceleration of dissolution rate and the overconcentration condition were achieved by generating oxygen micro-bubbles. Normal dissolved oxygen concentration was approximately 6.5 mg/L under 60 °C heating condition. On the other hand, the generator realized high concentration up to 15 mg/L.
Figure 2 shows cathodic polarization curves. The cathode performance was improved by using high oxygen concentration electrolyte. However, performance degradation was still caused under high current density condition. When the cathode potential excesses approximately -1.0 V, plot trend of all results changes and shows same plateau. It means that the discharge reaction on the cathode was shifted from oxygen reduction to hydrogen evolution reaction under high current density condition [4].
In order to observe precipitate of discharge products in the cathode, low energy X-ray CT and visualization cell as shown in Figure 3. By using closed type cell and aqueous solution (saturated LiOH with 10 M LiCl), experimental condition mimicked less oxygen condition involving precipitation of discharge products. As shown in Figure 4, emergence of bubble and solid precipitate were clearly visualized by low energy X-ray CT. It is considered that emergence of bubble and solid precipitate depended on oxygen concentration. Solid precipitate (Li2O2
[5]) was emerged at the bottom part of cathode where oxygen was able to reach by diffusion. On the other hand, oxygen was not transported efficiently to the top part of cathode, and discharge reaction was shifted. As result, electrolysis of water (H2bubble generation) has occurred.
In order to realize higher performance under high current density condition, electrolyte was flowed through the porous cathode. As shown Figure 5, cathode performance was significantly improved. It is considered that oxygen concentration overpotential was reduced by supplying oxygen rich electrolyte.

Acknowledgement
This work was supported by JSPS KAKENHI Grant Number 15K13881, 15H02347.

Reference
[1] K. Shibata, S. Uemura, S. Tsushima, S. Hirai, ECS Trans. 2015, 64(19), pp 39-46.
[2] T. Zhang, N. Imanishi, S. Hasegawa, A. Hirano, J. Xie, Y. Takeda, O. Yamamoto, N. Sammes, J. Electrochem. Soc., 2008, 155, A965.
[3] T. Zhang, N. Imanishi, A. Hirano, Y. Takeda, and O. Yamamoto, Electrochem. Solid-State Lett., 2011, 14, A45.
[4] H. He, W. Niu, N. M. Asl, J. Salim, R. Chen, Y. Kim, Electrochimica Acta, 2012, 67, 87.
[5] M.Matsui, A. Wada, Y. Maeda, H. Ohkuma, O. Yamamoto, N. Imanihsi, 224th ECS meeting abstract, 2013, MA2013-02, 411.

Figure 1