High capacity and long cycle life are both desirable features for practical secondary cells. In this study, we compare two different synthesis processes for preparing active materials for lithium/sulfur cells using polysulfide and graphene oxide (GO) showing high sulfur utilization and cycling stability. The key factor determining cell performance is the oxygen-containing functional groups on GO and the fine structures. This study shows the possibility of application of GO for enhanced capacity and cycling stability.

A surfactant material plays a significant role in creating a sulfur/carbon composite for lithium/sulfur cells. CTAB (cetyltrimethylammoniumbromide) is a widely used surfactant material in various fields. In this study we identified the functions of CTAB in a sulfur-graphene oxide composite used as the cathode material for lithium/sulfur cells, and the key features for enhancing the capacity of the cells. The CTAB added in the synthesis procedure reacts with polysulfides during the 155 °C heat treatment and produces cetylamine, cetylmethylamine, cetyldimetylamine, and dimethylpolysulfide as products. Furthermore the composite produced by the reactions between cetyldimetylamine and sulfur cover the surface of the sulfur. These work to enhance the performance of the cells. The findings presented here can be applied to various kinds of sulfur/carbon composites to enhance the performance of the sulfur electrode.

Sulfur deposition is an effective method for creating composites of sulfur with various conductive materials. In this study we investigated the details of the deposition process of sulfur onto graphene oxide (GO). It was revealed that just mixing a polysulfide solution and a GO suspension resulted in the deposition of sulfur onto the GO under alkaline conditions. The combination of the alkaline deposition and subsequent acidic deposition at different temperatures yielded materials with various morphologies. The sulfur deposition rate influenced the morphology of the S deposit and the resulting Li/S cell performance. By controlling the process with a moderate time for the alkaline deposition and subsequent acidic deposition at a decreased temperature, a preferred morphology of the sulfur/GO composite with a high sulfur utilization was successfully synthesized. This preferred morphology of SGO provides for a high-performance Li/S cell.

We have investigated the chemical bonding interaction of S in a CTAB (cetyltrimethylammonium bromide, CH3(CH2)15N+(CH3)3Br-)-modified sulfur-graphene oxide (S-GO) nanocomposite used as the cathode material for Li/S cells by S K-edge X-ray absorption spectroscopy (XAS). The results show that the introduction of CTAB to the S-GO nanocomposite and changes in the synthesis recipe including alteration of the S precursor ratios and the sequence of mixing ingredients lead to the formation of different S species. CTAB modifies the cathode materials through bonding with Na2Sx in the precursor solution, which is subsequently converted to C-S bonds during the heat treatment at 155 °C. Moreover, GO bonds with CTAB and acts as the nucleation center for S precipitation. All these interactions among S, CTAB, and GO help to immobilize the sulfur in the cathode and may be responsible for the enhanced cell cycle life of CTAB-S-GO nanocomposite-based Li/S cells.

The X-ray absorption spectroscopy technique has been applied to study different stages of the lithium/sulfur (Li/S) cell life cycle. We have investigated how speciation of S in Li/S cathodes changes upon the introduction of CTAB (cetyltrimethylammonium bromide, CH₃(CH₂)₁₅N⁺(CH₃)₃Br^-) and with charge/discharge cycling. The introduction of CTAB changes the synthesis reaction pathway dramatically due to the interaction of CTAB with the terminal S atoms of the polysulfide ions in the Na₂S_x solution. For the cycled Li/S cell, the loss of electrochemically active sulfur and the accumulation of a compact blocking insulating layer of unexpected sulfur reaction products on the cathode surface during the charge/discharge processes make the capacity decay. A modified coin cell and a vacuum-compatible three-electrode electro-chemical cell have been introduced for further in-situ/in-operando studies.