Fluorine-rich modification of self-extinguishable lithium-ion battery separators using cross-linking networks of chemically functionalized PVDF terpolymers for highly enhanced electrolyte affinity and thermal–mechanical stability

Thermal runaway is of great interest for developing high-performance Li-ion batteries (LIBs) and is accelerated by melting a separator at the elevated temperature during the battery failure. Herein, we report a facile polymeric coating method for polypropylene (PP)-based separators (PPSs) that enhances thermal, mechanical, and electrochemical performances. A conceptually designed double bond-containing polyvinylidene fluoride terpolymer (DPVDF) was coated on PPSs (DPSs) employing a simple dip-coating method, followed by a cross-linking process for DPVDF (DPSX). The modulus of DPSX increased from 49.06 MPa (PPS) to 64.29 MPa. The thermal shrinkage area of DPSX decreased by less than 10% at 140 °C for 30 min, whereas PPSs showed a 31.2% shrinkage. High-temperature open-circuit voltage tests (140 °C) showed that LiFePO4 (LFP) half-cells with PPS and DPS were short-circuited after 10 and 40 min, respectively. However, the LFP cell with DPSX showed no voltage drops even after 2 h. Furthermore, electrolyte wetting was significantly enhanced for DPSs and DPSX compared to PPSs. Moreover, electrolyte uptake was also increased from 65.25% (PPS) to 196.63% (DPSX), which is attributed to the enhanced wettability and pore structure. The long-term cycling and rate capability values of the LFP half-cells assembled with DPS and DPSX were significantly higher than those of the LFP half-cell with PPSs, which is closely related to the reduced cell resistance owing to the interfacial affinity of DPSX. Lastly, it was found that the DPVDF coating granted self-extinguishing functions to the separator. Flame tests demonstrated that DPSs and DPSX were instantly extinguished within a second and retained more than 74.89% of their weight after burning tests, while PPSs lost more than 43.46% (dried) and 59.65% (wetted) of their weight.