Porous Carbon Hollow Nanospheres for Confining Polysulfide in Lithium Sulfur Batteries

Lithium–sulfur batteries (Li–S), as one of the most promising candidates, have attracted increasing attention owing to its high theoretical capacity (1675 mAh g^-1), low safety concerns/environmental footprint, high natural abundance and competitive cost. Despite these considerable advantages, practical application of  Li–S batteries is still hindered currently by several challenges, including low electrical conductivity of sulfur/polysulfides, dissolution of lithium polysulfides and the resulting shuttling effect.

To better confine the sulfur/polysulfides in the electrode of Lithium–Sulfur (Li/S) batteries and improve the cycling stability, we developed a porous carbon shell to confine polysulfide. We systematic investigate the effect of pore size on the confining effect of porous C, and found out small pore size down to 2.8 nm gave the best confining effect.  We further developed a core–shell structure with polymer coated porous carbon as shell and S as core. It was first prepared through impregnation of sulfur into hollow carbon spheres under heat treatment, followed by a coating polymerization to give a double layers core–shell structure. From the study of scanning transmission electron microscopy (STEM) images, we demonstrated that the sulfur not only successfully penetrated through the porous carbon shell but also aggregated along the inner wall of the carbon shell, which, for the first time, provided visible and convincing evidence that sulfur preferred diffusing into the hollow carbon rather than aggregating in/on the porous wall of the carbon. Taking advantage of this structure, stable capacity of 900 mAh g-1 at 0.2 C after 150 cycles and 630 mAh g-1 at 0.6 C after 600 cycles could be obtained in Li/S batteries. We also demonstrated the feasibility of full cells using the sulfur electrodes to couple with the silicon film electrodes, which exhibited significantly improved cycling stability and efficiency. The remarkable electrochemical performance could be attributed to the desirable confinement of sulfur through the unique double layers core–shell architectures. [1]

1. W. Zhou, X. Xiao, M. Cai, Y. Li, Nano Letters, 2014, 14 (9), 5250–5256