Microstructural control and mechanical properties of mullite columnar-based refractory saggars via MgO addition in kaolin-alumina mixtures

With the rapid growth of the electric vehicle industry and secondary battery markets, there is an increasing demand for advanced refractory saggar materials capable of withstanding repeated high-temperature lithiation cycles during the synthesis of cathode active materials. In this study, we systematically investigated the phase evolution, microstructural development, and thermomechanical properties of Al₂O₃–Kaolin–MgO-based ceramic composites with varying MgO content and sintering temperatures, focusing particularly on their corrosion resistance in high -temperature lithium environments. Among the compositions, the M3 sample (Al₂O₃ 44 wt%, Kaolin 44 wt%, MgO 12 wt%) formed a structurally stable and chemically durable multiphase matrix composed of mullite, cordierite, spinel, and sapphirine. This composition exhibited a flexural strength of 76.2 ± 3.81 MPa at 1400 °C and a low thermal expansion coefficient of 4.792 μm/(m·°C). After three cycles of lithium corrosion testing at 900 °C, M3 retained over 66 % of its initial strength and maintained a lithium penetration depth of less than 200 μm. Although its thermal shock resistance was limited by internal stress concentration caused by mismatches in coefficients of thermal expansion among the constituent phases, the M3 composition showed relatively stable strength retention compared to the others. These results demonstrate that a well-balanced multiphase ceramic structure offers a promising strategy for designing high-durability saggar materials suitable for next-generation cathode manufacturing processes under extreme lithium corrosion environments.