Direct Observation of Silver Dendrite Formation in Glass Electrolyte Using X-Ray Computed Tomography of All-Solid-State Battery

Abstract

Lithium-ion batteries have high capacity and energy density, but there is a problem in terms of safety because an organic electrolyte solution is flammable. Therefore, development of large-scale lithium-ion batteries with high safety is a challenge for practical use. All-solid-state batteries are attracting attention because they use non-flammable inorganic solid electrolytes. In particular, all-solid-state batteries using metal anode have been expected to be next-generation rechargeable batteries because of their high-energy density using metal anode. Nevertheless, they have not yet been used because metal dendrites cause short circuits during charge and discharge processes. Metal dendrites grow along grain boundaries in polycrystalline solid electrolytes.1) However, there is no report on direct observation of dendritic growth in amorphous solid electrolytes without grain boundaries. To elucidate the mechanism of dendrite formation in solid glass electrolytes, we observed direct three-dimensional images during electrochemical reactions of all-solid-state cell by using operando X-ray computed tomography (CT) with synchrotron radiation. Although lithium metal is generally used as anode, such a light element is difficult to be detected using X-ray absorption techniques. In this study, as a model case of dendrite formation in all-solid-state batteries using metal anode, a battery cell using silver-ion solid electrolytes was examined. Silver-ion conductors have the advantage that their solid electrolytes exhibit high conductivity at room temperature.2) 3) Also, the variation of glass composition is enough, which makes easy to design the battery cell for X-ray CT measurement. Study on silver-based glass electrolytes is mainly being proceeded on oxide-based and sulfide-based materials. Among them, AgI-Ag2O-P2O5 glass electrolyte was selected because it has high flexibility in composition and high chemical stability in air.4) AgI, Ag2O, and P2O5 were mixed at mole ratio of 1:1:1. The mixed powder was calcined at 520 °C for 2 hours. After that, glass electrolyte was obtained by quenching AgI-Ag2O-P2O5 melt to room temperature. X-ray diffraction measurements of the synthesized material revealed the amorphous phase. In this study, a vacuum pump and a quartz glass tube with an inner diameter of 0.24 mm were used to mold the glass electrolyte. First, AgI-Ag2O-P2O5 melt was quenched in a quartz glass tube. Second, an electrode material prepared by mixing silver and AgI-Ag2O-P2O5 glass powder at a mass ratio of 3:1 was added from both ends of the glass tube. Finally, a titanium wire with a diameter of 0.20 mm was connected to the electrode. In this way, we prepared Ag |AgI-Ag2O-P2O5| Ag cell in a diameter of 0.24 mm. Micro-X-ray CT measurements were performed at BL20XU in Spring-8. The X-ray energy was 20 keV. The CT scans were performed before and after the chronopotentiometry measurement.Before the electrochemical measurement, the observed CT images showed the micro-cracks in the glass solid electrolyte. After the chronopotentiometry measurement, it was confirmed that silver metal grew from the point of the initially presented crack. Also, the crack expanded based on the presented crack. It means that the silver metal formation induces additional cracks. The CT images obtained before and after the chronopotentiometry measurement indicate that the silver metal dendrite formation causes the stress in the solid electrolyte, resulting in additional crack formation.1) R. Sudo, Y. Nakata, K. Ishiguro, M. Matsui, A. Hirano, Y. Takeda, O. Yamamoto and N. Imanishi, Solid State Ionics, 262, 151 (2014).2) J. C. Burbano, J. E. Diosa, and D. Peña Lara, Molecular Simulation 45, 724-727 (2019).3) T. Takahashi, S. Ikeda, and O.Yamamoto, J. Electrochem. Soc. 120, 647 (1973).4) M. Tatsumisago and A. Hayashi, J. Chem. Soc. 35, 3 (2006).