Rapid Sulfur-Melt Integration into Electrospun Carbon Nanofibers As Free-Standing, Binder-Free Cathodes in Lithium Sulfur Batteries

Abstract

Lithium sulfur (Li-S) has stimulated intensive research efforts due to its relatively high specific energy density of about five times that of Li-ion (2600 vs. 500 Wh kg-1, respectively), making it a conceivable alternative for use in electric vehicles, portable electronics, etc. However, the electronically insulating nature of sulfur and diffusion of reaction intermediates (shuttling effect), has steered efforts to utilize conductive polymers, graphene-oxides, and carbon scaffolds1to encapsulate sulfur and reduce shuttling, yet still combine these materials with binders and additional conductive additives to fabricate the final cathode upon a current collector. Herein we report a rapid, single-step sulfur-melt technique to impregnate electrospun carbon nanofibers (CNF), producing entirely free-standing cathodes, without the use of any binder/additives or additional current collector. Free-standing CNFs are produced by electrospinning polyacrylonitrile solution and subsequently heating the electrospun fibers for carbonization of the polymer chains to form stable CNFs. Commercial sulfur is integrated into the CNF mat by rapidly heating sulfur above its melting temperature (~120°C) combined with applied pressure. This quick melting-technique produces the final electrode, free of binding agents and completely device-ready; no further vacuum filtration or processing is required. This novel cathode fabrication utilizes the facile technique of electrospinning for structurally stable fiber mats in conjunction with a straight-forward method of infusing active material (sulfur). These cathodes were directly used as electrodes in CR2032 coin cells for electrochemical evaluation. Cathodes with a sulfur loading of 1.0 mg cm-2 exhibited high initial discharge capacities were 1382, 1310, and 1140 mAh g-1 at C/10, C/5, and C/2 rates, respectively (1C=1675 mAh g-1). The high initial discharge capacities indicate the CNF structure facilitates high sulfur utilization. One cathode with 0.6 mg cm-2 (~25wt%) sulfur loading tested at C/10 rate exhibited a very high initial discharge capacity of 1625 mAh g-1 (97% of the theoretical maximum, 1675 mAh g-1) and 1000 mAh g-1 at 100 cycles. The initial discharge capacity of this cell relative to the weight of the entire electrode (active sulfur + inactive materials) was 406 mAh g-electrode-1. Cathodes with higher sulfur loading (~1 mg cm-2 & ~50wt%), tested at faster rates (C/5) also show initial discharge capacities greater than 400 mAh g-electrode-1. These capacities are well above that of typical Li-ion. For comparison, the maximum achievable capacity of LiCoO­2 cathode is ~140 mAh g-1, which on a per gram-electrode basis, reduces to approximately 98 mAh g-electrode-1 when the current collector/binder weight (~30-50% of electrode weight) are factored into performance.2 The cathode design described in this work reduces processing steps, eliminates the dead-weight of binders and current collectors, and shows promising, practicalelectrochemical performance for the future of energy storage devices beyond lithium ion. 1. X. Ji, K. T. Lee and L. F. Nazar, Nat Mater, 2009, 8, 500-506. 2. N. Li, Z. Chen, W. Ren, F. Li and H.-M. Cheng, Proceedings of the National Academy of Sciences of the United States of America, 2012, 109, 17360-17365. Figure Caption: (a) SEM micrograph of pristine electrospun carbon nanofibers (CNF); (b) Discharge capacity of a sulfur-CNF cathode relative to weight of active material (sulfur); (c) Discharge capacities of a S-CNF cathode and the achievable capacity of Li-ion (LiCoO2) on a per-electrode-weight basis. Figure 1