Analysis of Apparent Diffusion Coefficient in Composite Electrode of Solid State Battery Using Ag-Ion

Abstract

All-solid-state rechargeable batteries are expected to be used widely owing to their safety, long lifecycles, and high energy densities. However, it remains a challenge to exceed the performance of a liquid-type lithium-ion battery, primarily because of poor optimization of the processes of fabrication of the composite electrode structures and cell stacks used in all-solid-state batteries. For the development of all-solid-state batteries at the cell level, it is crucial to not only enhance the ionic conductivity of the solid electrolyte but to also clarify the reaction mechanisms peculiar to the solid electrolyte and composite electrode of an all-solid-state battery. Regarding the unique mechanism of the battery cell, the inhomogeneous concentration distribution of the carrier ion occurs in electrolytes and composite electrodes in conventional lithium-ion batteries that employ a liquid electrolyte. Under charge-discharge in a high-rate condition, resistance distribution in the perpendicular direction becomes remarkable, and a reaction distribution occurs, which leads to a decrease of utilization for active materials. 1) In the case of all-solid-state rechargeable batteries, the situation should differ drastically. Since the transport number of lithium-ion in a lithium-ion solid conductor is almost 1.0, the distribution of carrier ion concentration does not exist in principle because only the lithium-ion can move in a solid electrolyte. However, to our knowledge, there is no report on the direct measurement of carrier ion concentration distribution in solid electrolytes during the charge-discharge of all-solid-state batteries in the literature. This status is because direct observation is difficult owing to the lightness of the lithium ion. To detect the macroscopic diffusion phenomena occurring in an all-solid-state battery, we applied synchrotron X-ray radiography to an all-solid-state battery system with a silver-ion solid electrolyte. Because silver is a considerably heavier element than lithium, its diffusion behaviors in the electrolyte and electrode can be visualized based on changes in the transmittance of X-rays. This study examines whether we detect the concentration distribution of the carrier ion in the solid electrolyte and composite electrode of an all-solid-state rechargeable battery. In addition, the apparent diffusion coefficient is evaluated by using time-dependent concentration change of silver-ion in composite electrode. In the solid electrolyte, no change in the concentration of carrier ions is detected. By contrast, in the composite electrode, a preferential reaction at the electrode/electrolyte interface is confirmed in the initial stages of charge and discharge. 2) Although a change in the concentration of the solid electrolyte would be an advantage, reaction distribution in the composite electrode is one of the critical issues from the viewpoint of practical application of high-energy-density, all-solid-state rechargeable batteries. 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 and Y. Uchimoto, Sci. Rep., 6, 26382 (2016). 2). K. Kandori, H. Yamashige, N. Furuta, T. Nonaka and Y. Orikasa, Electrochemistry, in press.