Authors: Yuichi Shimizu | Tetsuya Yamada | Kazumichi Yanagisawa | Mongkol Tipplook | Fumitaka Hayashi | Katsuya Teshima
Source: ECS Meeting Abstracts (article, 2024-11-22)
Field: Materials Science — Materials Chemistry

Crystal Morphologic Design of Na-Layered Oxide Crystals by Automated High-Throughput Flux Method Experiments Na x CoO 2 (NCO), a type of sodium layered oxide, has attracted attention as a cathode material for sodium ion batteries (SIB). The P2-type NCO has a layered structure and is generally obtained as plate-like crystals. Crystal growth of high aspect ratio NCO would be desirable to improve output density. The flux method, which is a liquid-phase method using flux (molten salt) as a solvent, is a crystal growth method that can control the crystal plane. However, in the case of NCO crystal growth, the specific effects of the flux species have not been clarified. In addition, NCO has several phases (O3, P2, P'3, etc.), depending on the Na ratio. However, fluxes easily change the Na ratio in the reaction field. For this reason, experimental guidelines for crystal control cannot be established, and trial and error must be used. The experimental search space considered by the flux method is enormous, with at least 14,000 possible experimental search spaces (three levels of raw material species and heating temperatures, nine raw material Na:Co ratios, 18 flux species and even two-component mixtures are allowed), making an exhaustive search unrealistic. To overcome this issue, it would be necessary to develop a crystal phase diagram expressing the crystal polymorphism and aspect ratio. For this purpose, identification of fluxes effective for P2 phase formation and aspect ratio changes was carried out using a data-driven, wide-area experimental search. For the data-driven exploration, flux method experiments was conducted using the high-throughput flux method screening (HTFS) system, which was originally developed in our laboratory. In the experiments, the initial training data was collected using a grid search method, and applied to a Bayesian optimization system. In this time, regression model was constructed using the feature values extracted from X-ray diffraction (XRD) patterns as the objective variables and the experimental conditions as the explanatory variables. About 50 experimental data were collected in a grid search using the HTFS. The formation of P2-type NCO crystals was confirmed in more than half of the samples. However, diffraction lines originating from the impurity Co 3 O 4 were observed in some samples, confirming the strong dependence of the product on the experimental conditions. Data-driven experiments were then undertaken. A total of about 30 experiments were carried out, and in many cases P2-type crystal samples were obtained using Na-based fluxes. In addition, the aspect ratio depended on the flux species and barrel-type crystal morphology was grown in some Na-based fluxes. In this presentation, the mechanism of flux contribution to crystal growth is discussed in terms of solute-solvent interaction and surface energy. Acknowledgement This work was partly supported by the Strategic Innovation Program (SIP) of the Cabinet Office, and joint research with companies. Throughput ; Flux (metallurgy) ; Crystal (programming language) ; Materials science ; Oxide ; Crystallography ; Chemical engineering ; Computer science ; Chemistry ; Engineering ; Metallurgy ; Operating system Solid-state spectroscopy and crystallography