In Situ Hierarchical Polymerization Enables Ambient-Pressure Drying of High-Performance Al₂O₃–SiO₂ Aerogel Composites for Battery Thermal Protection

Ambient pressure drying (APD) is an effective method for producing Al₂O₃-SiO₂ aerogels for thermal insulation, with hydrophobic modification crucial for APD success. However, challenges persist in combining thermal insulation with mechanical strength due to the hindrance of doping Al on hydrophobic modification and the inherent brittleness of pure aerogels. We present an in situ hierarchical polymerization strategy, where the subsequently formed Si-O-Si network protects the preferentially polymerized Al-O-Al network, enabling successful hydrophobic modification and APD. Mullite fibers are incorporated to create a load-bearing framework, surrounded by a 3D aerogel matrix formed from nanoparticle aggregates. The nano aerogel serves as the load-bearing unit and creates compartments to suppress thermal diffusion. The resulting mullite fiber-reinforced Al₂O₃-SiO₂ aerogel composites (MF/ASAs) demonstrate high compressive strength (0.41 MPa at 10% strain), excellent fatigue resistance (2.39% plastic deformation after 1000 cycles), and superior thermal properties (thermal conductivity: 0.034 W·m^-1·K^-1 at 25 °C and 0.081 W·m^-1·K^-1 at 1000 °C). A 2 mm-thick MF/ASA effectively blocked thermal runaway in lithium battery modules, with a 526 °C maximum cell-to-cell temperature gap. This approach enables cost-effective, scalable applications of Al₂O₃-SiO₂ aerogels in high-temperature fields.