Elemental sulfur is one of the promising cathode active materials for high-energy rechargeable lithium batteries because of its high theoretical capacity (ca. 1670 mAh-g-1) and relatively low cost [1]. However, sulfur cathode has some disadvantages, such as low electrical conductivity and dissolution as polysulfides into electrolyte during electrochemical cycling, resulting in shuttling in Li-S batteries. Several attempts have been performed to solve these problems, and the major improvements have been made by forming composites with carbon matrix, whereby sulfur was physically confined in some fine structures [2]. Despite the improved electrical transfer and the suppressed shuttling of polysulfides, these S-C composites cells still showed gradual capacity degradation. This is originated from the low binding energy between sulfur and carbon matrix [3].
Another type of S-C composites has been developed, based on the concept of embedding sulfur into a conductive polymer matrix, such as sulfurized poly(acrylonitrile) (S-PAN) [4-6]. In these organosulfur materials, sulfur is thought to be incorporated in cavity of molecular scale, which significantly prevents the dissolution as polysulfides, leading to the excellent cycle stability of the cells. Although many kinds of polymer framework have been investigated, there still remain challenges for exploring the organosulfur materials with higher sulfur contents, using inexpensive and non-toxic reagents [3].
In the present work, we have tried to prepare new type of organosulfur cathode materials using primary alcohol. The structure and the electrochemical properties of the obtained sulfurized alcohol composites (SAC) were examined.
Experiments
SAC was prepared by refluxing primary alcohol 1-CnH2n+1OH (n = 3 – 10) (1g) and elemental sulfur (5g) in a glass tube equipped in an electric furnace, which was heated at 400oC. After cooling, the resulting powder was ground and then heated again at 300oC under N2 flow in order to eliminate residual elemental sulfur to yield the SAC. We also prepared S-PAN after the method reported previously [7]. The obtained SAC was characterized by XRD, Raman spectroscopy, TEM observation, and elemental analysis, as well as S and C K-edge XAFS measurements (SR Center, Ritsumeikan University). Electrochemical lithium insertion / extraction reactions were carried out at 30oC using lithium coin-type cells with 1M LiPF6 / (EC + DMC) electrolyte at a current density of 30 mA-g-1 between 1.0 and 3.0 V initially with discharging. The electrochemical performances were also examined by assembling all-solid-state cells using the Li2S-P2S5 solid electrolyte and indium anode in a similar manner as described previously [8] at a current density of 30 mA-g-1 between 0.4 and 3.5 V.
Results and Discussion
The obtained SAC samples were black in color, and the XRD patterns showed no significant peaks, irrespective of the primary alcohol (n-value). High-resolution TEM observations showed no obvious crystalline domains, indicating an amorphous phase. The EDX mapping showed relatively homogeneous distribution of sulfur and carbon in the SAC samples. The elemental analyses indicated that the sulfur content was more than 60 wt%, which was higher than that reported previously for S-PAN (ca. 30 – 53 wt%) [4,5]. The Raman spectra showed some peaks at 480, 1250, 1440cm-1, suggesting the S-S, C-S, and C-C bonds, respectively. Analyses of the D- and G-bands indicated that the C-C bonds mainly consisted of sp3-type configuration, which makes a clear contrast to the S-PAN where the C-C bonds consisted of mainly sp2-configuration.
The electrochemical tests for the SAC sample cells showed that the initial discharge capacity was ca. 800 - 1000 mAh-g-1, which was higher than that of S-PAN, due possibly to higher sulfur contents. Also, the all-solid-state cells with the SAC samples showed the discharge capacity of ca. 600 – 800 mAh-g-1. The charge/discharge mechanism was examined using S and C K-edge XAFS measurements, and the results will be presented in the conference.
Acknowledgment
This work was financially supported partly by “Next-generation storage battery material evaluation technology development” project of NEDO and METI, Japan.
References
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[6] Y. Liang et al., Adv. Energy Mater., 2, 742 (2012).
[7] J. E. Trevey et al., J. Electrochem. Soc., 159, A1019 (2012).
[8] T. Takeuchi et al., J. Electrochem. Soc., 157, A1196 (2010).