(439b) Continuous Synthesis of Solid Electrolyte Particles for All-Solid-State Lithium-Ion Batteries By Spray-Drying Method

All-solid-state lithium-ion batteries (ASS-LiBs) are attracting attention as next-generation batteries for electric vehicles because of their high capacity and safety. High safety comes from the solid electrolytes, materials unique to ASS-LiBs. However, the productivity of solid electrolytes tends to be low because they are primally produced by a batch-type wet synthesis. For the practical application of ASS-LiBs, it is necessary to establish a continuous synthesis process of solid electrolytes. In addition, high capacity of ASS-LiBs can be expected by mixing and filling solid electrolyte particles with different sizes. Therefore, the objective of this study was the size control of solid electrolyte particles by continuous synthesis using spray-drying method. In spray-drying method, dry particles ca be produced by drying a suspension or solution of a raw material sprayed into a hot airflow. Li4SnS4 (LSS), which can specifically be dissolved in water, was selected as a model solid electrolyte. This study attempted to continuously synthesize LSS particles by spray-drying using LSS aqueous solution. In addition, the effects of spray-drying conditions on the size of the LSS particles were investigated.

LSS was synthesized following the procedure reported by Kanazawa et al.[1] Li₂S and SnS₂ were weighed to chemical amphoteric ratio of 2:1 in a glovebox under an Ar atmosphere. After homogeneous mixing of 30 min using a mortar, the mixed powder was heated to 580°C for 2 h using an electric furnace. The synthesized LSS was dissolved in water, followed by the spray-drying of LSS solutions to obtain LSS precursor particles. Spray-drying using the two-fluid nozzle was performed under the inlet temperatures of 200 and 220°C, the spray pressure of 0.20 MPa, liquid feed rate of 1 g/min, and solution concentration of 6.3 wt%. The LSS precursor particles were heated to 260°C for 2 h under an Ar atmosphere. For comparison with spray-drying, LSS particles were also obtained from the LSS aqueous solution by evaporation to dryness. Evaporation to dryness was performed at 80°C and 400 rpm using a hot stirrer. The LSS precursor particles were also heated to 260°C for 2 h under an Ar atmosphere. The properties of the LSS powders were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). Particle size distributions were obtained by SEM image analysis. The electrochemical property was analyzed by ionic conductivity.

The morphologies of particles obtained by evaporation to dryness and spray-drying were compared through SEM images. The particles obtained by evaporation to dryness were clearly larger than those by spray-drying. In addition, the particles obtained by evaporation to dryness had an indefinite shape, while the particles obtained by spray-drying were micron-order with spherical-shaped. This would be because the particle size and shape obtained by spray-drying strongly depended on the diameter of the spherical droplets. Sauter mean diameter of droplets by spray-drying in this study was approximately 7 µm under the spray pressure of 0.20 MPa. Under the droplet diameter of approximately 7 μm and the solution concentration of 6.3 wt%, particle size obtained by spray-drying would be 1–3 μm. Due to the better filling properties of spherical particle than non-spherical one, the solid electrolyte particles obtained by spray-drying were expected to improve the filling properties of ASS-LiBs. Also, the XRD measurements compared the crystalline of the powder obtained by spray-drying with the orthorhombic LSS structure reported by Kaib et al.[2] The XRD pattern of the powder obtained by spray-drying was similar to that of the orthorhombic LSS, indicating the formation of orthorhombic LSS powder by spray-drying. The ionic conductivity of the LSS powder obtained by spray-drying was 1.65 × 10⁻⁵ S/cm at room temperature. The ionic conductivities of orthorhombic LSS at room temperature have been reported to be on the order of 10⁻⁵ S/cm by Matsuda et al.[3] The LSS particles were successfully produced by spray-drying process. Furthermore, the effect of the operating conditions of inlet temperatures during spray-drying on the particle size distributions of the LSS particles was investigated. The medium diameter became small at high inlet temperature. The high droplet evaporation at high inlet temperature rate would prevent the unification of nuclear particles within a droplet.

In conclusion, the spray-drying of LSS aqueous solution was performed to obtain LSS powder. The physical properties of the LSS powder were analyzed. It was demonstrated the production of solid electrolyte particles by continuous process using spray-drying. In addition, it is indicated that the size of solid electrolyte particles can be controlled by spray-drying conditions.

References

[1] K. Kanazawa, S. Yubuchi, C. Hotehama, M. Otoyama, S. Shimono, H. Ishibashi, Y. Kubota, A. Sakuda, and A. Hayashi, Inorg. Chem., 2018, 57, 9925–9930.

[2] T. Kaib S. Haddadpour, M. Kapitein, P. Bron, C. Schröder, H. Eckert, B. Roling, and S. Dehnen, Chem. Mater., 2012, 24, 2211–2219.

[3] R. Matsuda, T. Kokubo, N. H. H. Phuc, H. Muto, and A. Matsuda, Solid State Ionics., 2020, 345, 115190.