Novel passive thermal protection coating for Li-ion batteries: synergistic thermal runaway mitigation with insulation and gas quenching

COF-hosted PAL@ZIF67 enables nanoscale P–Co–N synergistic flame retardancy. • 100 μm coating delays thermal runaway by 90 s with limited cycling impact. • Char and gas analyses support shielding, charring, and gas-phase inhibition. To improve lithium-ion battery safety, a ternary flame-retardant epoxy coating (EP/PAL@ZIF67@COF) was developed for passive thermal protection. The covalent organic framework (COF) serves as a porous host for integrating phytic acid aluminum (PAL) and ZIF67, enabling complementary condensed-phase charring, physical barrier, and gas-phase flame-inhibition effects. The coating was directly applied onto commercial 103,450 pouch cells at thicknesses of 50, 100, and 150 μm. Thermal-runaway suppression showed a non-monotonic dependence on coating thickness. The 100 μm coating extended the failure time from 1042 to 1132 s and substantially reduced combustion severity. In contrast, the 150 μm coating triggered failure at 1022 s, earlier than the uncoated cell, although visible combustion was completely suppressed. Preliminary sequential cycling and rate tests did not indicate catastrophic electrochemical performance degradation after coating. Cone calorimetry, char-residue characterization, and TG–FTIR analyses support the contributions of catalytic charring, high-temperature barrier formation, and gas-phase flame inhibition at the material level. These results demonstrate the potential of ultrathin flame-retardant coatings for cell-level passive protection and highlight the importance of coating-thickness optimization for lithium-ion battery safety. Transient CFD simulations further showed that increasing coating thickness progressively reduced the simulated thermal response, whereas elevated ambient temperature accelerated cell heating.