In Situ Raman Spectroscopy Study: Unusual Properties of Sulfur-Polyacrylonitrile As Cathode Material for Lithium Sulfur Battery
Authors: Huang Chen-Jui , Cheng Ju-Hsiang , Lin Ming-Hsien , Jang Ling-Yun , Hwang Bing-Joe 
Published: February 2020

Lithium-sulfur battery is now one of the promising rechargeable battery systems among the next-generation energy storage systems due to its high theoretical capacity of 1675 mAh/g, high theoretical energy density of 2500 Wh/kg comparing to the conventional lithium ion battery, and the abundant resources of sulfur in nature. However, several intrinsic drawbacks of sulfur such as poor electronic conductivity, huge volume change during the cycling, dissolution of intermediates, and safety issue of lithium metals as negative electrode, which hinder its progress to be commercialized. In order to overcome those problems, a combination of sulfur and carbon-based material was studied and found an obvious enhancement in the electrochemical performance. Sulfur-polyacrylonitrile (SPAN) compound [1] as one of the sulfur-carbon-based materials, it shows an outstanding ability to prevent the dissolution of intermediates and largely enhances its long-cycle stability, which has been seen a potential material for the lithium-sulfur battery.
Although many groups reported different synthesis of the SPAN materials with good cycling abilities, the reaction mechanism of SPAN during the cycling is rarely discussed and still unclear. The structure of SPAN has been reported by several groups, which the sulfur is in favor of small size (Sx, x = 1~3) and connects with carbon on the dehydrated PAN structure. Wei et al. proposed several reaction routes [2], including the cleavage of S-S bonds on the thiolate group (RSSR) to form RSLi or the complete break of C-S to form the RSLi and Li2S at the same time. However, the lithiation reaction is still lacking in direct observation. Therefore, in this study, the SPAN is studied by in situRaman spectroscopy to further elucidate the reaction mechanism.
SPAN material was synthesized by grinding sulfur and PAN in a ratio of 4:1. The well-mixed precursor was then transferred into furnace and heated at 350 ℃ for 3 hours under argon atmosphere to obtain black SPAN powder. As comparison, sulfur/conductive carbon black (S/C) composite was prepared in an optimized ratio of 7:3 (sulfur : carbon), and heated at 155 ℃ for 8 hours to insure liquid sulfur was fully absorbed by carbon matrix. The structure of SPAN was characterized by X-ray diffractometer, which all the diffraction peaks of sulfur disappeared and a broad peak has shown at 27 °. This result may relate to the PAN backbone formed a graphite-like structure after the heat treatment. For the Raman spectrum of as-prepared SPAN, peaks found at 171, 301, and 803 cm−1 are assigned as C-S bonds, and peaks found at 481, and 940 cm−1 are related to S-S bonds. This feature is different from S/C composite, which only observed peaks same as elemental sulfur at 150, 216, and 470 cm−1, showing the formation of covalent C-S bonds between sulfur and PAN backbone rather than simple physical absorption between sulfur and carbon composite.
The electrodes of SPAN were prepared by coating the slurry (active material: carboxymethyl cellulose: Super P = 8 : 1 : 1) on the aluminum foil and dried in the 70 ℃ oven overnight. For the cycling test, the reversible capacity for first cycle reaches 1473 mAhg-1 based on sulfur content, which is close to the theoretical capacity of sulfur, and the capacity retains ~90% of initial reversible capacity after 50 cycles. This shows excellent electrochemical properties of SPAN cathode material. Then the assembled cell using SPAN cathode was analyzed by in situ Raman spectroscopy during charging and discharging processes. In the lithiation process, as the lithium content increases, the S-S bonds are observed the decrease of peak intensity at first, and the C-S bonds are observed to decrease after, indicating lithium ions react with S-S bonds prior to the C-S bonds. This observation is supported by the dissociation energy of C-S bond (272 kJmol-1) and S-S bond (251 kJmol-1) as well. In the delithiation process, the peaks of C-S and S-S have slightly increased to the end of charge. However, the intensity is not as intensive as the initial SPAN, which may indicate the formation of nano-sized sulfur in PAN backbone.
The reaction mechanism of SPAN during the lithiation/delithiation process is further discussed and understood in this work, which could contribute to design and enhance better sulfur-polymer based material for lithium-ion battery use.