Energy storage devices with high energy density and low cost are highly demanding since the existing state-of-the-art Li-ion battery systems have intrinsic limitations in energy and price. Thus, lithium-sulfur (Li-S) batteries have attracted a great attention because sulfur is abundant and it has high theoretical capacity (1672 mAh g^-1) and energy density (2600 Wh kg^-1) when coupled with metallic lithium anode [1]. However, it is still challenging to device Li-S batteries offering a long-term cycling stability at a high level of sulfur loading due to the dissolution of polysulfides (Li₂S_x, 4<x<8) into the electrolyte and their shuttle effect causing poor capacity retention and low coulombic efficiency [2]. Recently, many important technological breakthroughs have been reported to suppress the polysulfides shuttle effects, which potentially leads to long cycle life of Li-S batteries [3]. Nonetheless, the loading level of sulfur has to be much increased for the Li-S batteries to go beyond current LIBs in energy density. In this aspect, a rational design of sulfur cathode is still regarded as one of the most important measures in mitigating the polysulfides shuttle problems.

In this study, we employed mesoporous carbons (MCs) and N-doped mesoporous carbon (N-MC) as the sulfur host materials. The MCs were prepared by a sol-gel process in an acidic aqueous mixture of commercial colloidal silica (20nm in diameter) as the template and sucrose as the carbon source [4]. The ratio of colloidal silica/sucrose was varied to obtain MCs with different textual properties. The N-MC was prepared by aniline polymerization in MC dispersion with ammonium persulfate as the initiator followed by carbonization of resulting composite product. The sulfur loading level of S/MCs and S/N-MC was varied in the range of 35-80 wt%, which corresponds to the sulfur loading density of 0.6~4 mg/cm². We also studied the effects of sulfur loading methods (melt-impregnation in a flow or a closed system) and the use of a conductive separator (CS) on the electrochemical performances of Li-S batteries. The CS was prepared by direct coating of a flake-type graphite on a polypropylene (PP) membrane (Cellgard 2400). The electrochemical tests were conducted using CR203 coin-type cells with Li foil as the counter electrode. The working electrode was prepared by casting a paste that consisted of the active material, conductive additive and PVdF binder with a mass ratio of 7:1:2 or 8:1:1 onto a Al foil current collector. 1.0 M LiTFSI in 1,2-dioxolane(DOL)/1,2-dimethoxyethane(DME) containing 0.1 or 0.4M LiNO₃ was used as the electrolyte. The cells were cycled in a cut-off voltage range of 1.5-3.0V Li⁺/Li.

The MC-1(S_BET = 1213 m²/g, V_pore = 2.75 cm³/g) is highly porous with pore size distribution in the range of 10-30 nm as shown in the TEM image in Fig. 1a. As compared in Fig. 1b and 1c, flake-type graphite are evenly coated onto the surface of porous PP membrane. The reversible capacities of various Li-S batteries at 0.5C are compared in Fig. 1d as a function of sulfur content. In overall, the reversible capacity was decreased with sulfur content. However, the reversible capacity was improved with strategic approaches depicted in Fig. 1d; (1) S/MC-1 showed much higher capacity than S/MC-2 (S_BET = 887 m²/g, V_pore = 1.55 cm³/g ), (2) S/N-MC-1 showed much enhanced capacity compared to S/MC-1 at high sulfur content, and (3) the use of CS exhibited further improvement in the reversible capacity. As shown in Fig. 1e, an electrode of S/MC-1 (S = 65.7wt%) assembled with CS exhibited an excellent cycling stability at 0.5C up to 300 cycles.

[1] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater., 11, 19–29 (2012).

[2] Y. Diao, K. Xie, S. Xiong, X. Hong, J. Power Sources, 235, 181 –186. (2013).

[3] A. Rosenman, E. Markevich, G. Salitra, D. Aurbach, A. Garsuch, and F. F. Chesneau, Adv. Energy Mater., DOI: 10.1002/aenm.201500212 (2015).

[4] H. I. Lee, G. D. Stucky, J. H. Kim, C. Pak, H. Chang, and J. M. Kim, Adv. Mater., 23, 2357-2361 (2011).

Fig.1. (a) TEM image of MC-1 (Inset shows pore size distribution of MCs), SEM images of (b) PP membrane and (c) conductive separator, (d) reversible capacities (after 50 cycles at 0.5C) of various samples as a function of sulfur content, and (e) cycling performance of S/MC-1 assembled with CS.