Highly porous poly(m-phenylene isophthalamide)/poly(ether imide) composite separators with a sponge-like structure fabricated using non-solvent induced phase separation and biaxial stretching for lithium-metal batteries

Abstract

Non-solvent induced phase separation (NIPS) is frequently used for rapid phase separation in functional polymers in order to create a porous structure. However, the fabrication of highly porous, uniform structures for advanced separators in lithium metal batteries (LMBs) remains challenging. Herein, a poly(m-phenylene isophthalamide) (PMIA)/poly(ether imide) (PEI) composite separator with a homogeneous, sponge-like porous structure is fabricated by combining NIPS with biaxial stretching. In the NIPS process, delayed demixing is achieved by controlling the coagulation bath parameters. The biaxial stretching process subsequently increases the porosity and decreases the thickness of the separator. The best-performing PMIA/PEI composite separator is obtained by using a biaxial strain of 50 %, and exhibits a bulk porosity of 76.7 ± 1.7 %, along with thermal stability at up to 400 °C, an ion conductivity of 0.95 mS/cm, and an Li+ transference number of 0.59. Enhancements in the electrochemical properties are attributed to improvements in the porosity, electrolyte affinity, and Li+ ion affinity. In particular, PEI contributes to an increase in Li+ ion transport, which promotes the formation of an LiF-rich solid electrolyte interphase layer with dendrite suppression capabilities. The as-fabricated LiFePO4//Li full cell exhibits a high capacity retention of 84.3 % after 300 cycles at a high 3C charge/discharge rate.