The Selenium-Sulfur/Carbon(Se-S/C) composites containing Se, which has much higher electrical conductivity (10^-3 S cm^-1) than S (5*10^-28 S cm^-1), have been suggested as a new cathode material instead of S/C composites [1-3]. Specifically, SeS₂/C composites delivers competitive volumetric capacity (4026 mAh cm^-3) compared to sulfur (3296 mAh cm^-3) due to larger density of Se (4.82 g cm^-3). Despite of these advantages of the Se-S/C, the material could not be used practically due to a severe capacity decay, which is induced by Shuttle Effect in the Se-S/C composites as well as in the S/C composites. It has been considered that one of the good solutions is that the Se-S are confined effectively within the carbon framework during cycling [1-4]. Zhen et al [2]. introduced CMK/SeS₂@PDA cathode material, which utilized the well-aligned mesoporous carbon and the conductive polymer coating to confine soluble intermediates physically, but it showed only 55 % of capacity retention after 500 cycles at 0.2 A g^-1. Even though it exhibited better long-term cyclic performance in comparison with previously reported Se-S/C works, its capacity retention was still low and the extra coating process was also ineffective way to confine Se-S into pores [2].

In this study, we aimed to demonstrate the effects of the structure and dopants of the carbon framework on the electrochemical performances of the Se-S/C cathode. We prepared the SeS₂/C composites by a facile melt-diffusion and an evaporation method employing 4 different carbon frameworks; (i) mesoporous carbon(OMC), (ii) nitrogen-doped ordered mesoporous carbon(NOMC), (iii) sulfur-doped ordered mesoporous carbon(SOMC), and (iv) ketjen black 600JD (KB600, commercial carbon). When preparing the SeS₂/C composites, we intended that the SeS₂ was partially filled on the carbon frameworks by making an inside hole of SeS₂ inside pores of carbon framework, as shown in Fig. 1-2. We believed that it would be beneficial to suppressing volume expansion of chalcogen materials and utilization of the active materials.

From the half-cell tests of four SeS₂/C cathodes (SeS₂/OMC, SeS₂/NOMC, SeS₂/SOMC, and SeS₂/KB600) (Fig.3), the SeS₂/KB600 showed the highest initial capacity due to its largest surface area that provides sufficient electrochemical reaction sites. However, it exhibited a poor cyclic stability due to the continuous dissolution of lithium polysulfides and polyselenides, originated from relatively large mean pore diameter (8.67 nm >) that is not suitable to trap the Se and S physically. Meanwhile, the SeS₂ in the 3 types of ordered mesoporous carbons (OMC, NOMC, and SOMC), which have a large length to pore diameter ratio (L/D), delivered better rate-capability than the spherical KB600 (Fig. 3a). It implies that a long diffusion path for Li ions and partially filled SeS₂ in the ordered mesoporous carbons are favorable to ion transportation inside pores [4]. Among all the SeS₂/C cathodes, SeS₂/NOMC exhibited the best electrochemical performance; nearly 99.99 % coulombic efficiency (no capacity fading during 500 cycles) presumably due to its N dopant that contribute to chemical adsorption of the soluble intermediates (Fig. 3b, 4).

References

[1] K. Eom et al., Nature communications 8 (2017) 13888

[2] Z. Li et al., Adv. Energy Mater (2017) 170028

[3] Sun et al., ACS Nano 6 (2016) 8289-8298

[4] S.Chen et al., Electrochimica Acta 56 (2011) 9549–9555