Encapsulation of sulfur within well-defined size-tunable ultrasmall carbon nanospheres for superior high-rate and long-stability Li–S batteries

Abstract

A range of ultrasmall microporous carbon nanospheres (ACNS5, ACNS10, ACNS20, and ACNS40) with well-defined tunable diameters ranging from 5 to 40 nm has been designed and employed as sulfur hosts for lithium-sulfur batteries. Sulfur has been encapsulated within their intra-sphere micropores in forms of both chain-like S2-4 (in small micropores with d < 1 nm) and cyclic S8 (in micropores with 1 < d < 2 nm). With their well-defined ultrasmall diameters, these carbon nanospheres have enabled a systematic study on the size effects of microporous carbons on the electrochemical performances of the sulfur composite cathodes. With the decrease of the nanosphere diameter from 40 to 5 nm, it is demonstrated that both the rate performance and cyclic stability of the cathodes are significantly enhanced as a result of improved electrolyte transport within the shortened intra-sphere micropores as well as abundant large inter-sphere meso-/macropores. In combination with a polysulfide-trapping poly (ionic liquid) (PDADMA-T) as the binder, we have further demonstrated that S@ACNS5 electrode exhibits an ultrahigh initial capacity of 1420 mAh g−1 at 0.1C and a superior rate performance with 50% (710 mAh g−1) of capacity retention at 5C. From the cycling test, an excellent capacity retention of 84% has been achieved at 1C even after 1000 cycles with only 0.016% capacity loss per cycle, confirming its outstanding cyclic stability.