Sulfurized Polyethylene Glycol As Electrode Material for Li-S Battery
Authors: Kojima Toshikatu , Ando Hisanori , Takeichi Nobuhiko , Senoh Hiroshi 
Published: February 2020

The battery of safe, low cost, durable and high capacity is strongly demanded for vehicles, mobile phones, personal computers, power backup systems and so on. Lithium-sulfur batteries gather attentions because sulfur as a cathode material has a high capacity of 1672 mAhg-1, is abandon, cheap and benign to environment. The sulfur cathode, however, has disadvantages such as high resistance and dissolution into electrolyte in the form of Li2Sx, which degrade the charge/discharge efficiency and capacity [1].

 In order to mitigate the dissolution of sulfur species, several kinds of nano carbon materials such as graphene [2], carbon nanotube [3], and nano carbon architectures [4] have been investigated as the container for sulfur. Polymer and sulfur composites also have been examined. Among them, a material prepared through heat treatment of polyacrylonitrile and sulfur mixture showed good capacity and cyclability [5-7].

 We found that heat-treated mixture of cheap polyethylene glycol and sulfur, sulfurized polyethylene glycol (SPEG) also showed good capacity and relatively good cycle stability as Li-S battery cathode.

 SPEG was synthesized by heat treatment 25.0 g of polyethylene glycol (PEG) (Kishida Chemical) and 51.6 g sulfur (Kanto Chemical) in an alumina tube (Nikkato), which was set in an electric furnace. The mixture was heated-up to 723 K under N2 flow then cooled. The reaction of SPEG production proceeded above 523 K with refluxing both of sulfur and PEG releasing H2S. This kind of production reaction for sulfur containing material is quite unique because not only sulfur but also PEG exist both liquid and vapor. The obtained crude SPEG was pulverized and heat-treated again at 573 K under N2 flow for more than 2h in order to take rid of residual molecular sulfur. Black powder was obtained by 8.9 g. The production of SPEG was easily scaled-up to 112 g in an experimental batch.

 SPEG was characterized using elemental analysis, XRD, and TG-DTA.

 For electrochemical measurement, cathode was prepared by mixing 50 wt% SPEG, 40 wt% acetylene black (AB), and 10 wt% polytetrafluoroethylene (PTFE), attached to aluminum mesh. The cathode was assembled into coin-type cells (CR2032), with the use of polypropylene sheet as separator, lithium foil as counter electrode, and lithium bis(triﬂuoromethanesulfonyl)amide (LiTFSA) : tetraglyme (1:1 molar ratio) as electrolyte. Galvanostatic charge-discharge cycle tests were performed at a current density of 25 mA g-1 (C/20 rate) in the potential range of 1.0 – 3.0 V vs. Li/Li+.

 From elemental analysis of SPEG, it consist of C: S: H: O = 35.3: 61.4: 0.4: 2.9 wt%. The content ratio of sulfur in SPEG is relatively higher than that of the other sulfur containing cathode materials.

 The XRD profiles of SPEG showed only a halo profile typical for such as low-crystalline or amorphous material without reflection of crystalline sulfur.

 TG-DTA result showed that there was only 17% weight-loss of SPEG until 773 K. This showed SPEG was a heat-resistive material and strongly held sulfur in it.

 The electrochemical properties of SPEG were investigated in a half cell. The capacity was calculated based on the mass of SPEG not sulfur. SPEG cathode showed large first discharge capacity of 910 mAh/g, and first charge one 550 mAh/g, therefore this material had irreversible capacity of 360 mAh/g. The cathode showed second discharge capacity 488 mAh/g, and kept 433 mAh/g at 10th cycle. The average discharge voltage of the cathode was 1.8 V vs. Li/Li+. Because relatively large capacity cathodes like one of Li-S battery changes its volume during charge/discharge, cathode structure is apt to be destroyed. A suitable binder would improve SPEG cathode cyclability.

 As a conclusion, the cheap law materials of PEG and sulfur were changed through new method to novel cathode material SPEG with good productivity up to 112 g. SPEG has high content ratio of sulfur with heat-resistive up to 773 K, and has large capacity with good cycle stability.

References

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