Performance Enhancement of Rechargeable Lithium-Sulfur Battery Utilizing High Mass-Loading Cathodes of Azulmic Carbon Composite
Authors: Matsui Yukiko , Hinago Hidenori , Yamazaki Shigeaki , Ishikawa Masashi 
Published: February 2020

A sulfur (S) cathode has a high theoretical capacity (1,672 mAh g-1). However, a major problem is the behavior of Li polysulfide (Li2Sn) intermediates, which may dissolve into an electrolyte. If dissolution of Li2Sn occurs, this should lead to a rapid capacity decay. We have investigated microporous activated carbon as matrix stabilizing S and realized stable cycling performance.1-2 However, the S content (ca.30 wt.%) and S loading were relatively low, which remains to be improved. To enhance them, in this study we applied azulmic carbon(AZC)3 with a large pore volume and also a porous 3-D Al current collector. Using these materials together with our fluorinated electrolyte, we have successfully realized stable and high capacity of the S-based cathode.
Methods: azulmic carbon (AZC) used in this study was an alkaline-activated type with basically micropores with some meso pores more than 2 nm in diameter. The AZC-S composite was prepared by mixing AZC with S at a weight ratio of AZC: S = 45:55, and then applying heat treatment: 155°C for 5 h, then 300°C for 2 h. The AZC-S composite cathode was prepared by mixing the AZC-S composite, acetylene black, and alginate binder at a respective weight ratio of 90:5:5 and loading the resulting slurry into a 3-D Al current collector, “celmet (Sumitomo Electric Industries)”. The electrolyte was 1.0 mol dm-3 LiTFSI/FEC:HFE(1:1) +VC [9+1] (by vol.). All the cell components were installed into a pouch cell with a Li anode. A typical charge-discharge cycling test was carried out at 167.2 mA g-1 (0.1 C) with cutoff voltages of 3.0 and 1.0 V at 25°C.
Major results and conclusion: we carried out the thermogravimetric analysis of elemental S and the AZC. The weight loss of AZC-S corresponds to the S content: 55 wt.%. The resulting S mass loading into the 3-D collector was 8.2 mg cm-2. We also investigated the discharge capacities in 1.0 mol dm-3 /FEC: HEF+VC (vinylene carbonate) electrolyte and also 2.0 mol dm-3/EC: DMC+VC electrolyte during cycling. The present fluorinated electrolyte contributed to stable discharge capacity showing 505 mAh (g-S)-1 at the 300th cycle. We found that there was a reduction at the surface of the sulfur at the first discharge, and excellent SEI was formed. It protected from direct contact with the electrolyte. The fluorinated electrolyte is, therefore, applicable to AZC even including the meso pores more than 2 nm as pore size. On the other hand, the carbonate electrolyte system showed two plateaus in the discharge curve at the first cycle. The carbonate electrolyte penetrated into the meso pores and the dissolution of Li2Sn occurred. As a result, the discharge capacity showed only 317 mAh (g-S)-1 at the 170th cycle. Thus, using the fluorinated electrolyte enabled the present activated carbon to cycle, as the meso porous carbon containing a large amount of sulfur. We will also report other results in the improvement of discharge capacity by ball-milling and heat treatment for AZC. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING)” from JST.
[1] T. Takahashi et al., Prog. Nat. Sci., 25 (2015) 612.
[2] S. Okabe et al., Electrochemistry, 85 (2017) 671.
[3] S. Usuki et al., Electrochemistry, 85 (2017) 650.

Figure 1