(Invited) In-Situ X-Ray Imaging Analysis for Dynamic Behavior in All-Solid-State Battery
Authors: Orikasa Yuki 
Published: December 2020

Next-generation rechargeable batteries are expected to exhibit safety, long life, high power and high energy density. Rechargeable batteries are composed of a positive electrode, a negative electrode, and electrolyte. Improvement of their characteristics themselves, such as capacity, conductivity, or stability by tuning materials, has been one of the main research topics. However, practical batteries are complex system using other functional materials in order to improve battery performance at the pack level. Therefore, the behavior of carrier ions inside the practical battery is also important research topics for the development of rechargeable batteries.
For charge/discharge of conventional lithium-ion batteries using an organic electrolyte, an inhomogeneous reaction in composite electrodes related to practical charge/discharge performance has been reported1). Electrodes of lithium-ion batteries are composed of active material, conductive carbon, and binder. Electrolyte is poured into void spaces of composite electrodes, which has a role for ion transport. The electric and ionic conductions must be maintained during charge/discharge. However, these conduction paths depend on the mesoscale structure of composite electrodes, which causes the reaction distribution in composite electrodes. The reaction inhomogeneity is quite related to the practical cell performance of batteries such as safety, cycle life, and rate capability. It has been reported that a reaction distribution forms in the electrode depth direction due to the presence of this nonuniform balance between electric and ionic resistances, and the active area for charge/discharge in composite electrode decreases1). In addition, during high-rate charge/discharge in which a large current flow, not only such an inhomogeneous SOC in electrodes but also concentration change of electrolyte occurs. For electrolyte used for lithium-ion batteries, the lithium-ion transport number is small, about 0.4. Therefore, a salt concentration gradient can be formed in the electrolyte during charge/discharge, and the utilization of lithium-ion is greatly restricted. From the results of NMR measurement, a concentration distribution of 0.78 to 1.27 M is reported2). On the other hand, in the case of the all-solid-state rechargeable battery, the transport number of the carrier ions of the solid electrolyte is 1, so in principle, it is considered that a concentration distribution of carrier is not occurred, and therefore, the high energy density and high rate characteristics are expected. However, direct measurement of concentration distribution in solid electrolyte during battery operation has not been reported.
In this study, we introduce examples of the analysis of reaction distribution using synchrotron radiation X-ray in rechargeable batteries. In lithium-ion batteries, nonuniform reaction phenomena of electrode active material were analyzed by two-dimensional imaging XAFS. The results indicate that ion conduction governs the reaction rate inside the composite electrode. In addition, changes in the concentration of the electrolyte solution were investigated by X-ray radiography. For the analysis in all-solid-state battery, the model system was developed using silver ions as probes3), and the characteristics of the all-solid secondary battery were analyzed under charge/discharge conditions.

References:
1) Y. Orikasa, Y. Gogyo, H. Yamashige, M. Katayama, K.Z. Chen, T. Mori, K. Yamamoto, T. Masese, Y. Inada, T. Ohta, Z. Siroma, S. Kato, H. Kinoshita, H. Arai, Z. Ogumi, Y. Uchimoto, Sci. Rep., 6, 26382 (2016).
2) S.A. Krachkovskiy, J.D. Bazak, P. Werhun, B.J. Balcom, I.C. Halalay, G.R. Goward, J. Am. Chem. Soc., 138, 7992-7999 (2016).
3) K. Kandori, H. Yamashige, N. Furuta, T. Nonaka, Y. Orikasa, Electrochemistry, 87, 182-187 (2019).