Designing of a Phosphorus, Nitrogen, and Sulfur Three-Flame Retardant Applied in a Gel Poly-m-phenyleneisophthalamide Nanofiber Membrane for Advanced Safety Lithium–Sulfur Batteries

Based on the urgent demand of non-flammable electrospun nanofiber separators and the strong adsorption to polysulfides through chemical doping in separators for Li-S cell, in this study, a phosphorus, nitrogen, and sulfur three-flame retardant (di-(2-(5,5-dimethyl-2-sulfido-1,3,2-dioxaphosphinan-2-yl)hydrazineyl)-P-ethylphosphinic) was synthesized and a high-performance flame-retarding poly-m-phenyleneisophthalamide (PMIA) membrane was successfully prepared through blend electrospinning with the flame retardant, it is regarded as a promising gel nanofiber membrane with advanced safety for the lithium-sulfur (Li-S) cell, and it was systematically explored and analyzed. It was presented that the modified PMIA electrospun membrane with the synthesized flame retardant possessed excellent flame retardation, outstanding thermal stability, and good mechanical strength. Meanwhile, the prepared membrane showed extraordinarily high uptake and preserving retention of the liquid electrolyte and enhanced ionic conductivity. More importantly, the assembled Li-S cells using the obtained membrane exhibited excellent cycling retention and outstanding rate capability because of its fast ion transportation and good interfacial compatibility. The assembled batteries with the novel membrane exhibited a high first-cycle discharge capacity of 1121.50 mA h g^-1, superior discharge capacity retention of 713.41 mA h g^-1, and high Coulombic efficiency of 98.46% after 600 cycles at the 0.5 C rate. In addition, the limiting oxygen index of the obtained nanofiber membrane with flame retardancy was as high as ∼30.0%, which could greatly enhance the safety of the electrospun nanofiber separator. The excellent electrochemical performances and safety for the battery assembled with the prepared gel PMIA nanofiber membrane were attributed to the significantly prevented "shuttle effect" of lithium polysulfides based on the physical capturing of lithium polysulfides through the obtained jelly-like gel state and chemical binding of polysulfide intermediates through the tridoped phosphorus, nitrogen, and sulfur elements in the PMIA and the flame retardant. All of these excellent properties will promote the great development of the Li-S battery with high performance and satisfactory safety.