Visualization of Precipitation Behavior in an Aqueous Lithium-Air Battery Electrode By a Soft X-Ray CT

The great advantage of the aqueous lithium-air battery is that the discharge reaction product LiOH is soluble in H₂O; however LiOH･H₂O precipitates in the air-electrode when concentration excesses the solubility by long time discharge. It is considered to cause cell performance degradation, and its mechanism needs to be clarified. In this study, we succeeded to observe emergence of solid phases on carbon fibers after discharge and discharge/charge cycle by using a soft X-ray cone CT.

An aqueous lithium-air battery is composed of an anode, an electrolyte and the carbon porous layer (SIGRACET^®10AA, SGL GROUP) as an air-electrode. The anode consists of a lithium metal, a lithium-ion conducting polymer buffer-layer (PEO) and a water-stable LATP (NASICON-type glass ceramic, Ohara Co., Japan).^[1] In this study, we used a soft X-ray to visualize light elements such as LiOH･H₂O. Since the soft X-ray is seriously absorbed by LATP, we modified the original cell whose structure enabled us to observe only the air-electrode. Moreover, by using a cone beam CT, it was possible to observe the inside of the air-electrode three-dimensionally and non-destructively. Furthermore, 5.2M LiOH (aq) was used for the electrolyte which was close to the solubility (5.3M). It replicated the condition of the electrolyte after discharge for a long time and contributed to shorten the time required for the precipitation of LiOH･H₂O. We observed the inside of the air-electrode before and after discharge (160 min) as experiment 1 and before and after discharge (60 min)/charge (60 min) cycle for three times as experiment 2. Both experiments were performed under the same condition (60°C, 1 atm).

The results of experiment 1 are shown in Fig.1. When the air-electrode was dry, the structure of the carbon fibers -with a strong contrast was observed (as shown in Fig.1 (a)). On the other hand, we couldn’t observe any carbon fibers in the air-electrolyte saturated with the electrolyte because the electrolyte absorbed much more X-ray than the carbon fibers (as shown in Fig.1 (b)). However, the structure of the carbon fibers was clearly observed after discharge (as shown in Fig.1 (c)). The strong contrast between the carbon fibers and the electrolyte suggested that the solid phases (consisted mainly of LiOH･H₂O) had precipitated on the fibers as LiOH･H₂O(s) absorbed much more X-ray than LiOH(aq). Moreover, the solid phases precipitated in the inside of electrolyte, which means the discharge reaction was occurred while using dissolved oxygen in the electrolyte, not at the interface between the air and the electrolyte. Furthermore, the solid phases precipitated on the particular fibers. It suggested that early precipitation on the fibers acted as a nucleus for solid phase growth. As shown in Fig.2, the discharge characteristics suggested that the overvoltage was increasing during discharge. It is considered that the precipitation of the solid phases inhibited the discharge reaction on the particular fibers and the reaction sites were focused on the surface of the fibers in contact with the electrolyte. As a result, the oxygen supply became insufficient locally near the reaction sites and the concentration overvoltage had increased.

The results of experiment 2 are given in Fig.3. While the electrolyte was observed mainly before discharge, the fibers-with a strong contrast were observed after discharge as well as experiment 1 as shown in Fig.3. Theoretically, LiOH･H₂O(s) dissolves in the electrolyte after the charge process because the concentration of Li⁺ and OH^- becomes lower than its solubility. However, the experimental results indicated that the solid phases didn’t disappear immediately after the charge process. It is considered that the dissolution rate of LiOH･H₂O(s) was smaller than the diffusion rate of OH^- and OH^- needed for the charge reaction was provided from the bulk of the electrolyte where the concentration of OH^-was high.

Present results and discussions suggest that the precipitation of the solid phase caused by the discharge process could strongly affect cell performance of an aqueous Lithium-air battery.

Reference

[1] T. Zhang, N. Imanishi, Y. Shimonishi, A. Hirano, Takeda, O. Yamamoto, Chem. Commun, 2010, 46, pp1661-1663.