Synergistic thermal stabilization of ceramic/co-polyimide coated polypropylene separators for lithium-ion batteries

Abstract To improve the safety of lithium-ion batteries (LIBs), co-polyimide (PI) P84 was introduced as a polymeric binder for Al₂O₃/polymer composite surface coatings on polypropylene (PP) separators. By monitoring the dimensional shrinkage of the PP separators at high temperatures, we verified a synergistic thermal stabilization effect between the Al₂O₃ ceramic and the PI polymeric binder. Although PI was thermally stable up to 300°C, a coating consisting solely of PI did not impede the PP separator dimensional changes (-22% at 150°C). On the other hand, the Al₂O₃/PI-coated PP separators efficiently impeded the thermal shrinkage (-10% at 150°C). In contrast, an Al₂O₃/poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) combination lowered the thermal stability of the PP separators (-33% at 150°C). As a result, the Al₂O₃/PI-coated PP separators remarkably suppressed the internal short-circuit of the unit half-cells associated with separator thermal shrinkage (100 min at 160°C), whereas the PVdF-HFP retained only 40 min under identical conditions. The Al₂O₃/PI-coated PP separators achieved rate capabilities and cell performances similar to those of the bare PP separators.