Sandwich-like solid composite electrolytes employed as bifunctional separators for safe lithium metal batteries with excellent cycling performance

Conventional lithium metal batteries (LMBs) containing bare porous polyolefin separators and a high content of liquid organic solvent always suffer from the risk of fire and powerlessness for ultralong cycle lifetime. In this paper, the sandwich-like solid composite electrolytes (SCEs) fabricated by wrapping a commercialized polypropylene (PP) separator with polyvinylidene fluoride (PVDF)–decabromodiphenyl ether (DBDPO) layers on both sides, are customized as the bifunctional separators for long life-span LMBs with high safety. The combustion resistance of the designed separator comes from the flame retardancy of DBDPO fillers, based on a free radical elimination mechanism. However the uniform lithium deposition for dendrite-free lithium anodes in LMBs can be ascribed to the integrated ion-conducting PVDF–DBDPO network equivalent to numerous ion transport channels, which are able to redistribute Li+ enriched at the pore outlets of the PP substrate. The obtained PVDF–DBDPO/PP separator requires only 10 μL cm−2 of liquid electrolyte to realize stable operation at room temperature. Specifically, the assembled lithium iron phosphate/lithium cells can exhibit an excellent cycling performance with a capacity retention of up to 92.3% at 0.5C and even 100% at 5C after 1000 cycles. The bifunctionality of such a separator in terms of high safety and unexpected cycling stability endows it with powerful competitiveness in comparison with the present separators.