A Triple‐Shield Supramolecular Film: Simultaneously Enabling High‐Voltage Insulation, Thermal Conduction, and Flame Retardancy for Battery Safety

ABSTRACT The escalating risk of thermal runaway in lithium‐ion batteries (LIBs) demands advanced materials that integrate efficient thermal management, superior electrical insulation, and exceptional flame retardancy. Inspired by supramolecular assembly strategies, we designed a novel multifunctional composite by incorporating boron nitride nanosheets (BNNS) into a hydrogen‐bonded network formed between polyimide (PI) and a specially synthesized phosphorous‐nitrogen flame retardant (FR). The hydrogen bonding networks between PI and FR effectively drive reorganization and densification of the molecular chains, leading to an expanded bandgap, deep charge traps, and significantly enhanced breakdown strength. Simultaneously, the well‐dispersed BNNS construct efficient thermal pathways, boosting the thermal conductivity, while synergizing with FR to form a robust char barrier that suppresses heat and mass transfer during combustion. Consequently, the optimized PI/FR@BNNS composite exhibits an exceptional combination of a high breakdown strength (750 MV/m), enhanced thermal conductivity (1.85 W/mK), and outstanding flame retardancy. When applied as a protective layer on LIBs, this composite simultaneously provides effective heat dissipation, reliable electrical insulation, and active fire suppression, demonstrating great potential as an integrated material for ensuring the safety of high‐energy‐density battery systems.