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

Sulfur is one of the promising electrode materials because of a large theoretical capacity (1672 mAh g-1). However, both ionic and electronic conductivities of sulfur are poor and the lithium polysulfide (Li2Sx) dissolves into the electrolyte solution during charge/discharge process [1]. To improve the ionic and electronic conductivities, the composites with carbon, such as nano-carbon architectures [2, 3], graphene [4-6], and carbon nanotubes [7, 8], have been investigated as positive electrode materials. Recently, it was reported that the sulfurized carbon composite prepared by heat treatment of polyacrylonitrile with sulfur exhibits a high capacity and long cyclability [9-11].

In this study, we propose a novel sulfurized carbon composite (here after denoted as SPEG), which is synthesized by heat treatment of polyethylene glycol (PEG: a representative polymer) with sulfur. SPEG showed a second discharge capacity of 488 mAh g(SPEG)-1 and was maintained at 433 mAh g(SPEG)-1 even after the 10thcycle. Although SPEG has high capacity and long life-cycle, crystallographic structure of SPEG itself is not clear. In this paper, we investigate the local structure and chemical states of S in SPEG by X-ray absorption spectroscopy and examined the chemical state of SPEG by Raman spectroscopy.

SPEG was prepared through refluxing 25.0 g of polyethylene glycol (PEG; Kishida Chemical, mean molecular weight: 200 g mol-1) and 51.6 g sulfur (Kanto Chemical) in an alumina tube (Nikkato) in an electric furnace. The mixture of PEG and S was heated up to 673 K, then cooled. The obtained crude product was heated again at 573 K under nitrogen flow (100 ml min-1) in order to remove the residual molecular sulfur. Composite obtained (8.9 g) was pulverized using mortar and pestle. The composite powder is a black powder and contained about 60 wt.% of S in the sample.

The local structure and chemical states of S in SPEG were examined by S K-edge X-ray absorption fine structure (XAFS, BL-10 of the Synchrotron Radiation Center, Ritsumeikan University). The total electron yield method was used and the incident X-ray beam was monochromatized with a Ge(111) crystal (2d = 0.6532 nm) pair. The photon energy was calibrated with the strong resonance of K2SO4 appearing at 2481.7 eV dipole transition. The XANES spectrum of this composite shows three distinct peaks at 2469, 2472 and 2473 eV, which however do not correspond to profiles of pure S and Li2S. Therefore, this suggests that free S is absent in the SPEG.

The chemical states of composites were investigated by Raman spectroscopy (ALMEGA XR, Thermo Fisher SCIENTIFIC) at room temperature. The excitation source is a 532 nm laser. The wavenumber of monochromator was calibrated by a silicon plate at 520 cm-1. The spectrum was recorded by accumulation of 12 scans (each of 5 s) in the range of 100 < ν/cm-1 < 3000. The highest Raman peak of SPEG appears at 1441 cm-1 between G (1600 cm-1) and D (1300 cm-1) bands of graphite. The other peaks appear at 1279, 1066, 846, and 772 cm-1. Those peaks are not assigned to various vibration modes of pure S and carbon. Thus, the new chemical states are formed between S and C in this composite. This result is consistent with result of XAFS. The chemical states in SPEG are related to the electrochemical properties. We will discuss a possible structural model of SPEG and also the correlation between the electrochemical properties and the electronic/local structural changes in SPEG.

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