Multifunctional Boehmite Grafting on Al₂O₃-Coated Polypropylene Separators for Fire-Retardant and Fast-Rechargeable Lithium-Ion Batteries

To improve the fire safety in lithium-ion batteries, extensive research has investigated reinforcing the thermal and mechanical properties of the separators. Applying double-sided ceramic coatings on separators has emerged as an irreplaceable strategy because of their electrical insulation and cost-effectiveness. Aluminum oxide (Al₂O₃) has been highlighted as a reasonable ceramic material that satisfies the characteristics. However, when the temperature rises above ~150°C, the Al₂O₃ coating layer undergoes irreversible bond cleavage. This causes the separator to shrink when thermal runaway begins, limiting its heat dissipation ability and leading to short-circuiting in lithium-ion batteries. In this study, we show that introducing a small amount of boehmite (AlOOH) on top of the Al₂O₃ coating layer imparts a highly cohesive binding network, which restrains the bond cleavage motion even at the onset temperature for thermal runaway (shrinkage ratio of 7.9% at 150°C). This strong binding network is achieved by cross-linking the coating layer composed of Al₂O₃ and polyvinylidene fluoride (PVDF)with AlOOH via hydrogen bond formation. Moreover, owing to the presence of abundant polar functional groups in AlOOH, the AlOOH-grafted Al₂O₃-coated separator (AlOOH–Al₂O₃ SP) shows strong permeability to lithium ions, thereby alleviating ion diffusion resistance on the anode side. This ion diffusion behavior can be maintained even when the cells assembled with AlOOH–Al₂O₃ SP are operated under harsh charge–discharge conditions (261.1 mAh/g after 100 cycles at 1000 mA/g and 80°C). Therefore, we believe that this multifunctionality of AlOOH–Al₂O₃ SP can serve as a new approach to simultaneously improve the fire retardant and fast-recharge abilities in lithium-ion batteries.