Polyacrylonitrile/Phosphazene Composite-Based Heat-Resistant and Flame-Retardant Separators for Safe Lithium-Ion Batteries

A safe lithium-ion battery (LIB) is desirable to attain a high output power density, which facilitates the use of LIBs in electric vehicles and grid-scale energy storage systems. In this work, a polyacrylonitrile (PAN)-based porous composite membrane incorporating a phosphorus flame-retardant agent, hexaphenoxycyclotriphosphazene (HPCTP), was fabricated for a heat-resistant and flame-retardant separator, preventing the combustion of LIBs due to short-circuit failures. Electrospinning was used to obtain the nanofibrous membranes, and the content of HPCTP varied from 0 to 20 wt %. To improve their mechanical and thermal properties, heat treatment was applied to the PAN-based membranes, and high tensile strength (>40 MPa) and low areal thermal shrinkage (<5% at 200 °C for 1 h) were achieved. Notably, the composites containing over 10 wt % of HPCTP showed excellent self-extinguishability, which could ensure the high safety of LIBs. Moreover, the ionic conductivity (0.95 mS/cm) and electrolyte uptake (162%) of the composite membrane were higher than those of a commercial polypropylene (PP) separator (Celgard 2400, 0.65 mS/cm and 63%, respectively). This was due to its interconnected pore structure and hydrophilic nature, affording superior discharge capacity and cycle stability. These results indicated that the PAN/HPCTP composite membranes can be used for high-energy density and safe LIBs as heat- and flame-resistant separators.