Publication date: 10th April 2024
In all-solid-state batteries, key issues are ion transport at grain boundaries and the interface between the active material and electrolyte. It is necessary to identify the dominant ion transport mechanism that inhibits ion conduction in batteries. In this study, we visualize Li-ion diffusion at grain boundaries via secondary ion mass spectrometry (SIMS) at −110 °C using an isotope exchange technique for perovskite-type Li0.29La0.57TiO3 (LLTO) as a model solid electrolyte.[1] This technique enables to evaluate the grain boundary diffusion coefficient.
LLTO was prepared via isotope exchange at 22 °C for 59 h and introduced into the SIMS system immediately after 6Li exchange. The SIMS image clearly reveals that the relative 6Li fraction C changes rapidly at the grain boundaries via comparison with the laser microscope image taken from the same position. This result means that the grain boundary resistance is the main factor increasing the total resistance of LLTO solid electrolytes. Considering the continuity of the diffusion flux across the interface between the bulk and the grain boundary, we can calculate the grain boundary diffusion coefficient Dgb from 6Li concentration gradient in the bulk nearby the boundary dC/dy|bulk, the difference in the 6Li concentration at the grain boundary dCgb and the bulk diffusion coefficient Dbulk. Based on the line profile from SIMS image, the derivative coefficient dC/dy|bulk is determined as 1.1 cm−1 using a quadratic function, and dCgb is 0.02. If the Dbulk of 2.6 × 10−8 cm2 s−1 measured by PFG-NMR [2] is used, we obtain Dgb/l = 1.5 × 10−6 cm s−1. Assuming the typical grain boundary thickness l = 0.5 nm [3], then Dgb = 7.6 × 10−14 cm2 s−1, and the calculated Dgb is five orders of magnitude lower than Dbulk.
This study was supported by the Japan Science and Technology Agency ALCA-Specially Promoted Research for Innovative Next Generation Batteries project (grant number JPMJAL1301). This study was also supported by Japan Society for the Promotion of Science KAKENHI Grant Numbers JP19H05814 (Grant-in-Aid for Scientific Research on Innovative Areas “Interface IONICS”) and JP21H02033 (Grant-in-Aid for Scientific Research (B)). We thank Dr. Atsuko Nagataki, Mr. Kyosuke Matsushita, and Ms. Makiko Oshida for their assistance with SIMS and SEM at the NIMS Battery Research Platform (Tsukuba, Japan).
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