In Situ Synchrotron Radiation XRD and XAFS Measurements of Multilayer All-Solid-State Lithium-Ion Batteries Using Li2CoP2O7 as a Positive Electrode Material

All-solid-state lithium-ion batteries are expected as next-generation batteries due to their safety and high volumetric energy density. We reported Li 2 CoP 2 O 7 (LCPO) as a positive electrode material with charge/discharge reactions over 5 V. [1–3] In this study, the structural and valence change of LCPO in multilayer all-solid-state lithium-ion batteries during charge/discharge was investigated using in situ synchrotron radiation X-ray diffraction (XRD) and X-ray Absorption Fine Structure (XAFS) measurements. In multilayer all-solid-state lithium-ion batteries, LCPO was the positive electrode active material, Ti-based oxide was the negative electrode active material, and NASICON-type solid electrolyte was used as the solid electrolyte. Each material for the positive and negative electrodes and the solid electrolyte was made into a paste. They were laminated using a screen printing method and integrally sintered to produce multilayer all-solid-state batteries. Synchrotron radiation XRD and XAFS measurements were performed at the large synchrotron radiation facility SPring-8. In situ XRD was measured in a transmission mode using 30 keV X-rays for a multilayer all-solid-state battery. The X-ray beam had to pass through the center of the multilayer all-solid-state battery during the first charge/discharge reactions. In situ XAFS experiments were measured using a fluorescence method. XAFS scans covering the Co K edge peak were taken during the first charge/discharge reactions. Figure 1 shows the XRD patterns of the multilayer all-solid-state battery during the first cycle in the charge/discharge process. The main diffraction peaks of LCPO are located at around 13.0°, 14.4°, and 16.7°. The diffraction peak position at 16.7° gradually shifts to the lower degree side from its uncharged state and the peak intensity decreases during the charging process. In addition, the peak shift returns to the same position at the uncharged state during the discharging process. However, that is not completely. The diffraction peak intensity at 14.4° decreases. In the XAFS measurement results, the Co K edge spectrum shifts to the higher energy side compared to the uncharged state during the charging process. The Co K edge spectrum returns to the same energy position in the uncharged state during the discharging process. These results show that the valence of Co changes from 2+ to 3+ upon charging and returns from 3+ to 2+ upon discharging. We observed how the Co of the LCPO operates while the multilayer all-solid-state battery was charging and discharging. [1] A. Kato, The 395 th Committee Of Battery Technology, (2019) [2] M. Kobayashi, et al., The 60 th Battery Symposium, 2F19, (2019) [3] C. Yamamoto, et al., The 87 th ECSJ Spring Meeting, (2020) Figure 1