Three-Dimensional Flower-Shaped Activated Porous Carbon /Sulfur Composites As Cathode Materials for Lithium-Sulfur Batteries

Abstract

Lithium sulfur battery, which use sulfur as a cathode and Li as an anode, have taken more and more attention for its high theoretical specific capacity of 1675 mA h g−1 and high theoretical specific energy of 2600 Wh kg−1. Despite these particular advantages, several troublesome issues associated with the sulfur cathode severely limits the practical use of sulfur in an electrode, such as, the highly electrical insulating nature of sulfur, the redox shuttle of dissolved polysulfide ions and the volume expansion of sulfur cathode materials. In our work, three-dimensional flower-shaped activated porous carbon/sulfur composites (FA-PC/S) are fabricated for the first time via a simple method utilizing flower-shaped ZnO as a template and pitch as the carbon precursor, followed by carbonization-activation and thermal treatment. A typical flower-shaped FA-PC spherical structure is composed of many nanopetals intersecting with one another. Because of the unique three-dimensional flower-shaped porous spherical structure, FA-PC has high reactivity, high electrical conductivity and a short transport length for the Li ion. Furthermore, the rich micropores of the FA-PC offer enough space to accommodate the volume expansion that occurs during the discharge process of the encapsulated sulfur as well as confine the electrochemical reaction products of the sulfur cathode within the micropores. The composites are also characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis, Raman spectroscopy, X-ray powder diffraction (XRD), and thermogravimetric (TG) measurements. The results show that sulfur is well dispersed and encapsulated homogeneously in the micropores of the flower-shaped activated porous carbon (FA-PC) with excellent electrical conductivity, high surface area and large pore volume. The electrochemical tests show that the FA-PC/S composites with 60 wt. % S have a high initial discharge capacity of 1388 mA h g-1 at 100 mA g-1, a good cycling stability (reversible discharging capacity of approximately 600 mA h g-1 at 1600 mA g-1) and excellent rate capability.