(Invited) High Surface Area N-Doped Carbon Fibers with Accessible Reaction Sites for All Solid State Lithium Sulfur Batteries

Abstract

All solid-state lithium-sulfur batteries (ASSLSBs) are believed to deliver extremely higher energy density and remarkable safety than conventional Lithium-Sulfur batteries using liquid electrolytes. However, the low utilization of active material (sulfur) caused by sluggish reaction kinetics greatly hindered the development of ASSLSBs. It is significant to fabricate an advanced electrode where sulfur has efficient electron/ion accessibility. Porous carbon host, which has been widely used in liquid cells, is proposed to address the challenges. However, the conventional porous carbon host used in liquid cells can not be directly used in ASSLSBs, because the nonmobile solid electrolytes can not reach the sulfur confined in the pores buried deeply inside the carbon. An ideal porous carbon host should own a large specific surface area to provide sufficient sites for sulfur, but it is very critical that the pores should only locate at the out surface of the carbon. Till now, though many works reported the application of porous carbon in ASSLSBs, but none of them discussed the structure requirement of porous carbon. Here, for the first time, we discussed the ideal structure of the porous carbon and developed a polyacrylonitrile-derived porous carbon fibers (named PPCF) with a unique core-shell structure where a layer of micropores cover on dense core contributing to a high specific surface area.Porous carbon fibers were successfully prepared through electrospinning and following carbonization and activation processes. This PPCF owns a unique core-shell structure where a layer of micropores is covered on the dense cores. A high specific surface area of 1519 m2 g-1 was achieved, and the micropores could effectively confine the sulfur avoiding the formation of bulky sulfur. More importantly, the pores mainly distributed on the outer surface which rendered remarkable ion access to sulfur, contributing to high sulfur utilization. The fibrous morphology enabled efficient electron conduction paths through effective percolation. Compared with the vapor grown carbon fibers (VGCF), a common carbon fiber with no surface pores, the cathode using PPCF exhibited improved reaction kinetics with an overpotential reduction by 149 mV. The reduced overpotential explains the faster kinetic and higher utilization of sulfur in S-PPCF-SE. In the dQ/dV measurement, S-PPCF-SE showed strong anodic/cathodic peaks with high intensity of 3156/4367 mAh g-1 V-1 which are significantly higher than 1010/1268 mAh g-1 V-1 in S-VGCF-SE, demonstrating a greatly improved reaction kinetics. The ASSLSBs using PPCF exhibited three times enhanced initial capacity of 1166 mAh g-1 at C/20. After 50 cycles, the capacity gradually decreased to 710 mAh g-1 and was maintained stable after 220 cycles at C/10. A remarkable capacity of 889 mAh g-1 at C/2 was achieved. In comparison, the ASSLSBs using VGCF as carbon additive delivered a low rate capacity of 100 mAh g-1 at C/2.