Phosphazene based star-branched polymeric cathode materials via inverse vulcanization of sulfur for lithium–sulfur batteries

Abstract

Novel insoluble star-shaped hexa-branched polymeric materials based on a cyclotriphosphazene core are prepared by the inverse vulcanization of elemental sulfur with hexakis(styreneoxy)cyclotriphosphazene resulting in various wt% sulfur content copolymers, explicitly poly(S-random-phenoxy)cyclotriphosphazene, [poly-(S-r-p)p], to be used as cathode materials for lithium–sulfur (Li–S) batteries. Their structural characterization studies are carried out via Fourier-transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and scanning electron microscopy. Since the resulting poly-(S-r-p)p copolymers are not soluble in common solvents, an alternative soluble model compound, namely poly(S-random-phenoxy)pentachlorocyclotriphosphazene is synthesized via the same route of the inverse vulcanization process. Hence, this soluble model compound is able to be tested by means of ³¹P and ¹H nuclear magnetic resonance spectroscopy allowing better understanding of the vulcanization mechanism between sulfur and vinylic groups based on phosphazenes. Later, the electrochemical performances of poly-(S-r-p)p as a Li–S battery cathode material are evaluated and the advantage of the resulting cross-linked polymer network is highlighted by comparison with the bare sulfur cathode. To gain further insight into the lithium storage mechanism, electrochemical impedance spectroscopy measurements are also performed before and after cycling. This work offers valuable design principles for the fabrication of sulfur-based polymers with their promising applicability in future Li–S batteries.