Covalent Confinement of Sulfur Copolymers onto Graphene Sheets Affords Ultrastable Lithium–Sulfur Batteries with Fast Cathode Kinetics

Abstract

Lithium–sulfur (Li–S) batteries have received significant attention due to the high theoretical specific capacity of sulfur (1675 mA h g–1). However, the practical applications are often handicapped by sluggish electrochemical kinetics and the “shuttle effect” of electrochemical intermediate polysulfides. Herein, we propose an in-situ copolymerization strategy for covalently confining a sulfur-containing copolymer onto reduced graphene oxide (RGO) to overcome the aforementioned challenges. The copolymerization was performed by heating elemental sulfur and isopropenylphenyl-functionalized RGO to afford a sulfur-containing copolymer, that is, RGO-g-poly­(S-r-IDBI), which is featured by a high sulfur content and uniform distribution of the poly­(S-r-IDBI) on RGO sheets. The covalent confinement of poly­(S-r-IDBI) onto RGO sheets not only enhances the Li+ diffusion coefficients by nearly 1 order of magnitude, but also improves the mechanical properties of the cathodes and suppresses the shuttle effect of polysulfides. As a result, the RGO-g-poly­(S-r-IDBI) cathode exhibits an enhanced sulfur utilization rate (10% higher than that of an elemental sulfur cathode at 0.1C), an improved rate capacity (688 mA h g–1 for the RGO-g-poly­(S-r-IDBI) cathode vs 400 mA h g–1 for an elemental sulfur cathode at 1C), and a high cycling stability (a capacity decay of 0.021% per cycle, less than one-tenth of that measured for an elemental sulfur cathode).

Trapping of Polysulfides with Sulfur‐Rich Poly Ionic Liquid Cathode Materials for Ultralong‐Life Lithium–Sulfur Batteries

“…As displayed in Figure c, sulfur possesses a sharp exothermic peak at 48.9 °C and an endothermic peak at 118.3 °C, corresponding to the crystallization and melting of sulfur, respectively. Compared with sulfur, poly(S‐ co ‐DVIMBr) has no crystalline and melting peaks in the DSC curves (Figure d), suggesting the amorphous character of poly(S‐ co ‐DVIMBr) . It also indicates that there is no remaining elemental sulfur in the final polymers, in good agreement with the XRD results.…”

“…It is clearly seen from Figure a that there are two distinct discharge platforms at 2.3 and 2.1 V. The high‐voltage platform at 2.3 V is attributed to poly(S‐ co ‐DVIMBr) reduced to high Li 2 S x ( x ≥8) and Li 2 S 8 . Another plateau at 2.1 V can be assigned to the formation of Li 2 S x (4≤ x ≤6) . Upon further discharge, these Li 2 S x (4≤ x ≤6) are finally reduced to fully discharged Li 2 S 2 /Li 2 S .…”

“…In contrast, poly(S‐ co ‐BVIMBr) and poly(S‐ co ‐VIM) have similar XRD patterns to sulfur except for some shifts of prominent peaks at 2 θ =23, 26, and 28°, indicating that there is some crystalline sulfur in poly(S‐ co ‐BVIMBr) and poly(S‐ co ‐VIM). After inverse vulcanization, the formed linear sulfur bridges in poly(S‐ co ‐BVIMBr) and poly(S‐ co ‐VIM) may partly remain crystalline …”

“…Compared with elemental sulfur, the melting of the sulfur in poly(S‐ co ‐BVIMBr) was found to occur over a relatively wide temperature range that centered at 109.2 °C. According to the literature, this may be owing to the random length of polysulfide chains in the copolymer …”

Carbon Foam Fibers with a Concentric Tube‐Core/Three‐Dimensional Nanosheet‐Sheath Structure for High‐Performance Lithium‐Sulfur Batteries

“…It is clear that HPCF‐900 and HPCF‐800 can adsorb Li 2 S 6 , and the color of Li 2 S 6 solution turns from brown‐yellow to light yellow. However, the brown‐yellow color of Li 2 S 6 solution changes into clear for HPCF‐1000 [32] . It means that as‐constructed unique structure of HPCF‐1000 possesses strong ability to trap LiPSs.…”