X-Ray CT Study on Sodium-Ion Distribution in All-Solid-State Sodium Battery Electrode

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All-solid-state sodium batteries are attracting attention as next-generation batteries because they are safer than lithium-ion batteries that use organic electrolytes and have abundant sodium resources compared to lithium1). However, all-solid-state sodium batteries have difficulty achieving higher performance than lithium-ion batteries. It is presumed that one of the causes of the performance decline is that reaction non-uniformity caused by the supply balance of ions and electrons limits the electrode thickness. Therefore, there is a need to construct a means to directly observe reaction heterogeneity in the operating state of an all-solid-state secondary battery. Using X-ray imaging, it is possible to analyze the progress of reactions inside all-solid-state secondary batteries. However, when lithium ions are carriers, it is difficult to directly detect them using X-rays because of their low absorbance. In the case of sodium ions, which have higher absorbance than lithium, it is expected that they can be directly detected using X-rays. Therefore, in this study, we investigate a method for analyzing the sodium ion distribution inside an all-solid-state sodium battery using X-ray computed tomography (CT).Na2.88Sb0.88W0.12S4 was synthesized by mixing Na2S, Sb2S3, S, and WS2 using a ball mill under an Ar atmosphere, which was used as an electrolyte. A negative electrode mixture was prepared by mixing hard carbon as an active material, Na2.88Sb0.88W0.12S4 as an electrolyte, and boron nitride in a mass ratio of 1:6:1 in a mortar. Boron nitride is a substance that does not react with sodium and has low absorbance, and we expected that this would reduce the absorbance of the electrolyte and clarify the absorption contrast of sodium ions. An all-solid sodium half cell was fabricated by using the prepared negative electrode mixture, sodium metal for the counter electrode in an acrylic cylinder with a diameter of 1 mm. Operando X-ray CT measurements were performed at BL20XU, SPring-8 during constant current charging and discharging at rate of 0.02C. For X-ray CT measurements, micro-absorption CT measurements (pixel resolution: 0.5 μm) and nanophase CT measurements (pixel resolution: 0.04 μm) were performed. The hard carbon in the negative electrode at an initial state and an estimated capacity of about 350 mAh/g after sodium insertion was compared using a cross-sectional view in the thickness direction of an X-ray CT image obtained from micro-absorption CT measurement. It was confirmed that the hard carbon present at the electrolyte interface appeared brighter in the CT cross-sectional view of the negative electrode after sodium insertion compared to the initial state. However, the hard carbon near the current collector showed almost no change between the initial state and after sodium insertion. This suggests that sodiation of hard carbon occurs preferentially at the electrode/electrolyte interface. Acknowledgments：Part of this research was carried out with the support of the New Energy and Industrial Technology Development Organization (NEDO), JPNP20004. References 1) Kubota, M. Dahbi, T. Hosaka, S. Kumakura, S. Komaba, Chem Rec, 18(4), 459-479 (2018). 2) A. Hayashi, N. Masuzawa, S. Yubuchi, F. Tsuji, C. Hotehama, A. Sakuda, M. Tatsumisago, Nat Commun, 10, 1-6 (2019).