The incorporation of highly polarized inorganic compounds in functional separators is expected to alleviate the high temperature safety- and performance-related issues for promising lithium-sulfur batteries. In this work, a unique Co3O4 polyhedral coating on thermal-stable polyimide (PI) separators was developed by a simple one-step low-temperature calcination method utilizing metal-organic framework (MOF) of Co-based zeolitic-imidazolate frameworks (ZIF-Co) precursors. The unique Co3O4 polyhedral structures possess several structural merits including small primary particle size, large pore size, rich grain boundary, and high ionic conductivity, which endow the ability to adequately adsorb dissolved polysulfides. The flexible-rigid lithium-lanthanum-zirconium oxide-poly(ethylene oxide) (LLZO-PEO) coating has been designed on another side of the polyimide non-woven membranes to inhibit the growth of lithium dendrites. As a result, the as-fabricated Co3O4/polyimide/LLZO-PEO (Co3O4/PI/LLZO) composite separators displayed fair dimensional stability, good mechanical strength, flame retardant properties, and excellent ionic conductivity. More encouragingly, the separator coating of Co3O4 polyhedrons endows Li-S cells with unprecedented high temperature properties (tested at 80 °C), including rate performance 620 mAh g-1 at 4.0 C and cycling stability of 800 mAh g-1 after 200 cycles-much better than the state-of-the-art results. This work will encourage more research on the separator engineering for high temperature operation.
Keywords: energy storage systems; functional separators; high temperature operation; lithium sulfur batteries; polyimide.