Halogen-Free Polyphosphazene-Based Flame Retardant Cathode Materials for Li-S Batteries

Novel, eco-friendly, and halogen-free polyphosphazene-based flame retardant (FR) cathodes for Li-S batteries are prepared by the inverse vulcanization of elemental sulfur with polybis(2-acrylamidoethoxy)phosphazene (poly(AAE)) resulting in the covalently bonded different wt% sulfur content polymers. Due to the polar amide groups, poly(AAE)-xS (x refer to the wt% of sulfur) composites assist in anchoring intermediate lithium polysulfide species. The structural characterizations of the poly(AAE)-xS polymers are conducted by elemental analysis, Fourier-transform infrared spectroscopy, differential scanning calorimeter, thermogravimetric analysis, and scanning electron microscopy. Their FR properties are elevated by calculation of the limiting oxygen index values and burning tests. Later, lithium-ion storage mechanism of poly(AAE)-55S polymer is evaluated, including cyclic voltammetry, rate performance tests and electrochemical impedance spectroscopy measurements. To compare with the electrochemical performance of poly(AAE)-55S, a noncovalent blend polymer mixture and bare sulfur are also tested. It is found that covalently bonded poly(AAE)-55S obtained by the inverse vulcanization of elemental sulfur and poly(AAE) results much pronounced capacities. Herein, new insights into the design and development of alternative cathode materials for safer Li-S batteries, both providing FR properties and good lithium polysulfides adsorption capabilities, are offered.