Synergistic effect of sulfur-rich copolymer/S8 and carbon host porosity in Li-S batteries

Abstract

In this work, we investigate the possible role of host material porosity in performance of Li-S batteries, when sulfur-rich copolymer is used as an active ingredient in the cathode. We synthesized a freestanding cathode material using electrospun carbon nanofibers (CNFs) and poly(sulfur-random-1,3-diisopropenylbenzene)/SDIB (with some residual unreacted sulfur) and used it without adding any binders or current collectors. To study the effect of porosity of CNFs, two different porous samples, microporous (miCNF) and mesoporous (meCNF), were designed. When miCNF was used as a host material, the capacity stabilized to 577 mAh/g within 50 cycles and remained stable up to 600 cycles with a small capacity decay rate of 0.046% per cycle along with an excellent coulombic efficiency of >98% throughout the 600 cycles. On the other hand, SDIB-meCNF cathodes delivered a stable capacity of ~600 mAh/g with negligible decay. However, the stability only lasted for 145 cycles. Despite the positive effect of C-S bond formed in SDIB active material, we hypothesize that the porosity of the cathode host still plays a significant role not only due to residual unreacted S8 in the original cathode, but also due to additional loose soluble -Sn- chains that emerge from S-S bond breakage in C-S-Sn-S-C (SDIB) over the course of cycling. Moreover, using the effective capacity calculation, we compared the discharge capacity of our cathode to other works on sulfur rich copolymers at C/2 and C/5 rates. The effective capacity analysis clearly manifests the advantages of using current-collector and binder-free electrospun CNFs as a host matrix for SDIB.