Charge-Discharge Characteristics of Sulfur Positive Electrode with Highly Capacitive Micro-Porous Carbon from N-Doped Carbon Precursor
Authors: Usuki Shinya , Yamagata Masaki , Uchida Satoshi , Hinago Hidenori , Ishikawa Masashi 
Published: February 2020

A lithiumSulfur (Li-S) battery is expected as a next-generation power storage system because of high specific capacity (1,672 mAh g-1) of a S-based positive, which is about 10 times higher than that of a conventional intercalation-based positive in lithium ion batteries. However, lithium polysulfides (Li2Sx (x = 3 — 8)) are formed during charge/discharge and dissolve into an electrolyte. In our previous research, we successfully kept Li2Sx in microporous activated carbon [1], where an electrolyte could not penetrate into a micropore and not react with Li2Sx, though the S content was ca. 30wt%. To prevent diffusion of Li2Sx and also increase the S content, we synthesized microporous activated carbon from N-doped carbon as precursor. As a result, we found a process providing rich micro pores (≤ 2 nm) and we achieved high sulfur loading (≥ 50 wt.%).
N-doped carbon and KOH were mixed as mass ratios of 1 : 1, 1 : 2, and 1 : 4 in deionized water. This mixture was dried at 100°C to prepare the carbon matrix precursor. Then the dried precursor was activated in Ar flow at a rate of 10°C /min and was held for 1 h at 800°C in a silica tube. The synthesized sample was neutralized by dilute hydrochloric acid, and then washed by overly large quantity of deionized water. Next, the washed sample was dried at 100°C under a reduced pressure and we obtained three kinds of activated carbon which were AC/1KOH, AC/2KOH and AC/4KOH.
Sulfur was mixed as 55, 62, and 54wt% with AC/1KOH, AC/2KOH and AC/4KOH, respectively. These three mixtures were heated to 155°C for 5h, and then temperature was increased to 300°C and kept for 2h. We obtained different three AC-S composites which were AC/1KOH@S55, AC/2KOH@S62, and AC/4KOH@S54. The cathodes were prepared from such an AC-S composite, acetyleneblack, CMC, and SBR (91 : 3 : 3 : 3 by weight). Cells were assembled with an AC-S cathode, electrolyte [LiTFSI : tetraglyme : hydrofluoroether (= 10 : 8 : 40 by mol)], separator (polyethylene microporous membrane) and Li foil anode. A typical charge-discharge cycling test was carried out at a current density of 167.2 mA g-1 (0.1 C) for the cells. A different electrolyte [1M LiTFSI in a mixture of 1,3-dioxolane (DOL) / 1,2-dimethoxyethane (DME) (1 : 1 by volume) with 1wt% LiNO3] was used at only AC/2KOH@S62.
We obtained high specific surface area AC from N-doped carbon by alkali activation. We investigated pore distribution of AC/2KOH and AC/4KOH. Probably, micro pore was effectively formed by nitrogen dissipation as well as alkaline activation. However, in case of highly excess KOH, pores tend to expand and mesopore increases. Therefore, AC/2KOH and AC/1KOH have a higher ratio of micropore than AC/4KOH. Each activated carbon material was applied to a cathode of Li-S batteries and its charge-discharge behavior was observed. Discharge curves of three cathodes which are AC/1KOH@S55, AC/2KOH@S62, and AC/4KOH@S54 showed different behavior. AC/2KOH@S62, and AC/4KOH@S54 show a discharge plateau at the first cycle. This is micro-porous carbon peculiar behavior, but their charge curve was two-step reactions. To investigate the origin, we observed characteristics by using a different electrolyte, 1M LiTFSI/DME/DOL with 1wt% LiNO3. The discharge curve showed two-step plateaus. This result means the ether solvent enters into the pores. The sulfur contacts with ether solvent and polysulfides dissolve into electrolyte. While micropore prevented penetration of glyme electrolyte. According to these results, it was found that the pore of less than 2 nm is valid for the glyme electrolyte. The cathode of AC/1KOH@S55 was one-step plateau at both of charge-discharge curves during for 30 cycles. The pore of AC/1KOH has a specific distribution less than 1 nm, so sulfur may exist in the smaller micropore and can not contact with electrolyte. The cycle performances of the sulfur positive electrodes which are AC/1KOH@S55, AC/2KOH@S62, and AC/4KOH@S54. AC/4KOH@S54 showed continuous fading. We obtained AC/4KOH@S54 and AC/1KOH@S55 have different pore size distribution; in the case of AC/4KOH@S54 sulfur preferentially exists in the bigger pores because it tends to be taken in the bigger pores. On the other hand, AC/2KOH@S62 provided higher capacity and the better stability than other electrodes. This is because the dissolution of polysulfide was inhibited and no electrolyte can penetrate into micropore so that it can not react with sulfur.

Acknowledgment

This study was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency.

References

[1] T. Takahashi et al., Prog. Nat. Sci., 23 (2015):available online.