Data-Driven Synthesis of Multi-Element Substituted Fluoride Crystals for Improved Ionic Conductivity

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Barium fluoride (BaF2), with its fluorite-type structure, is anticipated to be a solid electrolyte material for high voltage fluoride-ion batteries. Previous studies have demonstrated that partial substitution of barium by lanthanum in BaF2 can increase the ionic conductivity by four to five orders of magnitude [1]. This finding implies that elemental substitutions could significantly improve the ionic conductivity of BaF2. However, a single elemental substitution limits the elemental exploration space, thereby reducing the chances of discovering compositions with high conductivity. Expanding the search space through multi-element substitution is expected to be effective; however, it is impractical to exhaustively explore over 3000 potential compositions in multi-elemental substitution. Therefore, we introduced a "Material-Conductivity Search Space" that offers a comprehensive overview of both the compositions of solid solutions and their conductivities, facilitating an efficient search. To create this system, we aimed to elucidate the relationship between the substitution elements and solid solution formation and conductivity properties using a data-driven approach. Based on the D-optimal design (D-opt), twenty candidate compositions for multi-element substitution were synthesized. Subsequently, additional experiments were performed using the data-driven design of experiment (DOE) approach. In this case, a regression model was constructed by using elemental species and amounts as the explanatory variables, and "number of peaks", which was extracted from the X-ray diffraction (XRD) pattern as the objective variable. Samples prepared using the data-driven DOE approach showed a significant reduction in the average 'Number of peaks' in XRD patterns, from 41.83 to 19.65, compared to those synthesized under D-opt guidance. This result indicates that data-driven approach can effectively search solid solution composition. Next, the electrical conductivity properties of the synthesized multi-elemental substituted BaF2 were examined at room temperature and revealed that their conductivities ranged from 10^-7 to 10^-3 S/cm. Notably, samples including potassium fluoride (KF) as a substitution element exhibited a significant enhancement in conductivity. We suggest that this improvement is due to the formation of additional ionic conduction pathways resulting from charge compensation, and/or activation energy change by multi-elemental substitution. We are now improving the material-conductivity search space by combining these experimental results as learning data. In the presentation, we will also report on the effective search for high conductivity materials using this system.Acknowledgments: This research was partially supported by the Aichi Knowledge Hub Aichi Priority Research Project and the Strategic Innovation Program (SIP) of the Cabinet Office.[1] Kazuhiro Mori, Atsushi Mineshige, Takashi Saito, et al., ACS Appl. Energy Mater., 2020, 3, 2873-2880.