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Koji Kandori1 Hisao Yamashige2 Noritoshi Furuta3 Takamasa Nonaka4 Yuki Orikasa1

1, Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, Japan
2, Toyota Motor Corporation, Toyota, Aichi, Japan
3, SOKEN, Inc., Nisshin, Aichi, Japan
4, Toyota Central R&D Labs., Inc., Nagakute, Aichi, Japan

All-solid-state rechargeable batteries are expected to be used widely as post lithium-ion batteries. Recently, the ionic conductivity of the solid electrolyte has been dramatically improved to the similar order as organic electrolytes. However, to improve the performance of all-solid-state rechargeable batteries at the cell level, it is important to not only enhance the ionic conductivity of the solid electrolyte but also to understand the diffusion behavior of carrier ions in composite electrodes. Ion diffusion in composite electrodes is complicated during charging and discharging in all-solid-state rechargeable batteries. In the case of a liquid electrolyte using an organic solvent, the transport number is lower than 1.0[1], and it is known that ion concentration distribution is caused in the electrolyte[2, 3]. On the other hand, in principle, no ion concentration distribution occurs in the solid electrolyte since in the solid electrolyte, the transport number of carrier ions is approximately one. Therefore, it is considered that ion diffusion phenomena in solid electrolytes are different from liquid systems. Although ion diffusion in composite electrodes of bulk-type all-solid-state batteries proceeds through active materials and solid electrolytes, its diffusion path is very complicated, and its analysis is a challenge. Therefore, few observation examples of the dynamic behavior of carrier ions during the operation of the batteries have been reported. Another factor is that lithium ion, which is a general carrier ion, is a light element and it is difficult to detect the ions directly. In this research, as a model case of the all-solid-state rechargeable battery, the diffusion behavior in the composite electrode was directly observed by high energy synchrotron X-ray using the all solid secondary battery cell which uses silver ion as a carrier. Silver ion conductors have the advantage that solid electrolytes exhibiting high conductivity at room temperature have already been reported[4]. Also, silver ions are sensitive to X-ray. By using X-ray transmission imaging method with synchrotron X-ray with high transmittance, spatial resolution, and temporal resolution, the silver ion concentration distribution in the electrode during charging and discharging in the model battery was directly observed[5]. From the obtained results, the apparent diffusion coefficient in the composite electrode was calculated.
A silver-ion conductor, Ag6I4WO4, was prepared as a solid electrolyte for all-solid-state silver battery. To observe the ion concentration distribution in the bulk-type all-solid-state rechargeable battery, an Ag | Ag6I4WO4 | TiTe2 cell was fabricated. Synchrotron X-ray radiography measurements were performed in SPring-8 (Hyogo, Japan). X-rays were irradiated during discharge with a constant potential at 72 mV, and transmission intensity was measured with a two-dimensional detector to obtain an image.
The apparent diffusion coefficient in the composite electrode was estimated from this imaging measurements. This result suggests that ion diffusion in the composite electrode governs the performance. Thus, it has been suggested that ion diffusion in the composite electrode is important at a scale of hundreds of microns of the actual electrode thickness.

References
[1] A. Nyman, M. Behm, and G. Lindbergh, Electrochim. Acta, 53, 6356 (2008).
[2] H. Yamashige, N. Furuta, M. Mukouyama, K. Kawamura, H. Ota, T. Nonaka and H. Kawaura, ECS Meeting Abstracts, MA2014-02, 461 (2014).
[3] S. A. Krachkovskiy, J. D. Bazak, P. Werhun, B. J. Balcom, I. C. Halalay and G. R. Goward, J. Am. Chem. Soc., 138, 7992 (2016).
[4] L. Y. Y. Chan and S. Geller, J. Solid State Chem., 21, 331 (1977).
[5] K. Kandori, H. Yamashige, N. Fruta, T. Nonaka, and Y. Orikasa, Electrochemistry, 87(3), 182 (2019).

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