This study investigates the relationship between particle size and flow properties of boehmite powders and their impact on ceramic-coated separators for lithium-ion batteries, specifically exploring how these properties influence the performance of the separators. The separators coated with BH-3 powder, with larger sizes and better flow properties than BH-1 and BH-2, showed superior characteristics with the minimum deviation when measuring physical properties such as thickness and roughness. The BH-3 coated separators, with <10% shrinkage at 150 °C for 1 h, showed the highest ion conductivity (0.69 mS cm−1) and superior electrochemical properties compared to bare PE separators. These results highlight the importance of the excellent flow properties of the boehmite powders, which reduces the slurry viscosity, facilitates the formation of a uniform coating layer, and consequently enhances the performance of the separator.