Lithium-ion power batteries offer the advantage of high energy density but present a risk of thermal runaway. Under the conditions such as mechanical impact or overcharging, thermal runaway can initiate an explosive combustion, rapidly propagating throughout the entire battery pack and severely threatening life and property safety. Passive thermal protection systems represent one of the most economical and effective strategies for battery thermal protection. However, the surface temperature of ternary lithium batteries during thermal runaway exceeds 700 ℃, imposing severe challenges on the high-temperature stability of thermal insulation materials. Conventional materials such as mica and ceramic fiber wool are unable to meet these requirements. In contrast, fiber-reinforced aerogel composites emerge as an ideal choice due to their non-combustibility, excellent thermal insulation properties, and compressibility. Their core component like silica aerogel possesses a nanoporous structure (with the porosity of 99.8%) and employs multiple heat-blocking mechanisms. These characteristics render it one of the solid materials with the lowest thermal conductivity (approximately 0.014 W/(m·K)). There exist some critical challenges, i.e., organic fibers (e.g., aramid) exhibit an insufficient temperature resistance, high-performance ceramic fibers (e.g., mullite, silicon carbide) incur high costs and involve complex manufacturing processes, and lower-cost glass fiber-reinforced aerogel felts demonstrate an inadequate high-temperature performance. Basalt fiber becomes a preferred reinforcement material due to its high strength, high-temperature resistance, low cost, and environmentally friendly properties. Previous studies demonstrated that aerogel composites prepared via supercritical drying with basalt fibers could exhibit a superior thermal insulation performance, compared to glass fiber systems. Furthermore, industrialization requires overcoming limitations in the drying process. The predominant supercritical drying method utilizes expensive equipment and has operational hazards. Ambient pressure drying presents greater application prospects due to its advantages of low cost, safety, and operational simplicity. This study employed low-cost basalt fiber combined with ambient pressure drying to develop aerogel composites that integrated excellent thermal insulation, robust mechanical properties, and economic viability. This approach was to meet the thermal protection requirements for power batteries in new energy vehicles.

Basalt fiber needle-punched felt was employed as a reinforcement phase. Composites were prepared via regulating the concentrations of tetraethyl orthosilicate (TEOS) (0.67-1.17 mol/L) as well as by sol-gel process, hydrophobic modification using trimethylchlorosilane (TMCS), and ambient pressure drying. The surface morphology and structure of the composites were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, and Fourier transform infrared spectroscopy (FTIR). The thermal properties were characterized by HotDisk thermal conductivity analysis, thermogravimetric analysis-differential scanning calorimetry (TG-DSC), and a hot plate temperature rise test at 600 ℃, respectively. The compression testing was performed by a universal testing machine. The flame resistance was analyzed via exposing the composites into a butane torch flame at 1090 ℃ for 20 min.

As the TEOS concentration increases from 0.67 mol/L to 1.17 mol/L, the aerogel matrix becomes progressively denser and the cracks disappear. The sample with 1.00 mol/L TEOS has the lowest shrinkage, a density of 0.159 g/cm³, a specific surface area of 465.7 m²/g, and an average mesopore diameter of 12 nm. The enhanced structural integrity resulting from this density variation leads to a decrease in room-temperature thermal conductivity from 0.0274 W/(m·K) to 0.025 W/(m·K). After 10-min exposure to the hot surface at 600 ℃, the cold-side temperature is only 106 ℃, demonstrating the supervior thermal insulation performance. The sample with 1.17 mol/L TEOS displays the optimal compression performance, with a compressive modulus of 625 kPa and a compressive strength of 179 kPa. The fiber can effectively distribute the load, achieving a compression ratio of exceeding 60%, indicating the excellent cushioning and energy absorption capability. After annealing at 300 ℃, the removal of organic components increases the specific surface area to 581.5 m²/g and results in more ordered pore structures, while the compressive strength increases to 400 kPa. At 500 ℃, the thermal conductivity is 0.049 W/(m·K)), which is still lower than that of comparable ceramic fiber-reinforced aerogels. Beyond 700 ℃, the aerogel fragments, and the basalt fibers oxidize (turning red), while their skeleton dimensions remain stable. The total weight loss at 1000 ℃ is only 10%. Upon exposure to a butane flame at 1090 ℃ for 20 min, sintering occurs to a depth of only 2 mm in the cross-section, while the internal fibers remain intact. The mass loss is 3.8%, indicating the outstanding high-temperature service performance. These properties meet the requirements for passive thermal protection under extreme operating conditions in power batteries.

The TEOS concentration significantly affected the structural integrity of the material. At the TEOS concentration of 1.00 mol/L, the aerogel matrix achieved a density of 0.159 g/cm³ with minimal shrinkage, forming a continuous mesoporous structure (i.e., the specific surface area of 465.7 m²/g). This structure imparted the excellent thermal insulation performance (i.e., the thermal conductivity of 0.025 W/(m·K) at room temperature, with a cold-side temperature of only 106 ℃ under a hot surface at 600 ℃) and mechanical properties. The material maintained a functional stability under high-temperature conditions. After heat treatment at 500 ℃, its thermal conductivity remained as low as 0.049 W/(m·K). Moreover, the compressive strength increased due to the homogenization of the pore structure. Although particle formation occurred in the aerogel at 700 ℃, the basalt fiber framework remained structurally intact, and the weight loss was only 10% at 1000 ℃. The results of Butane flame exposure tests for 20 min demonstrated the outstanding flame resistance of the material. A sintered protective layer of aerogel and fiber could form on the surface, effectively inhibiting heat penetration. The sample showed no burn-through, with sintering confined to a shallow depth, while the internal structure remained intact.