Trapping Polysulfides Catholyte in Carbon Nanofiber Sponges for Improving the Performances of Sulfur Batteries

Abstract

Practically viable Li-S batteries necessitate improvement of their poor cycling performances arising from polysulfide shuttle and associated irreversibility as well as reduction of non-active conductive additives. Here we greatly alleviated the problems by utilizing freestanding carbon nanofiber (CNF) sponges as both catholyte reservoir and porous conductive framework in cathode. The pores in the sponge effectively held liquid-type polysulfide catholyte without noticeable aggregation of sulfur products during cycling, resulting in excellent capacity retentions up to ∼90% in 300 cycles at 0.5-C rate. More importantly, polysulfide deposition and dissolution were studied by measuring impedance spectra at seven different stages during the first charge and discharge cycle, revealing the polysulfide redistribution upon cycling and limited kinetics at the fully discharged state due to the deposition of solid Li2S2/Li2S. Detailed investigation of the effects of catholyte concentration and volume was also conducted to optimize the battery performances. Considering that the sponge structure was synthesized by using a facile one-step chemical vapor deposition method, our approach has a great potential in developing practically viable Li-S batteries.