Current Collector for the Sulfur Electrode

The main criteria for the selection of the current collector material in a central sulfur cell or for the cell case material in a central sodium cell are corrosion resistance to sulfur and sodium polysulfides, good electrical conductivity, and low costs. This cost argument has led to coated materials which have been compared with nickel—chromium alloys (Inconel 600). 

The high resistivity of Inconel 600 (11 OjtI 0 8 Dm) demanded the application of this material as a composite with a central aluminum core. The aluminum was totally enclosed in Inconel 600 so that the Inconel was only exposed to sulfur and polysulfides. In a test over more than three years, cells with a composite current collector of this kind suffered from a high capacity decline. Post-test analysis showed that Inconel sustained polysulfide attack with the formation of a duplex nickel and chromium sulfide layer on the current collector surface. 

Aluminum itself cannot be used as a current collector or cell case material due 

Aluminum itself cannot be used as a current collector or cell case material due to the formation of a high-resistivity aluminum sulfide layer. Therefore a surface protection has to be applied which necessitates a corrosion-resistant but electrically conductive coating. Good results have been obtained with aluminum coated with a layer of nichrome and covered by a secondary protective coating of carbon, which formed a tightly adherent, corrosion-resistant, outer layer. Post-mortem examination after four years of operation showed that such a current collector had been largely unaffected by the very aggressive environment. 

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The theoretical energy density of a lithium-sulfur electrochemical system is 2500 Wh/kg or 2800 Wh/1, which makes it immensely attractive for the development of a chemical power source. This attractiveness is also enhanced by the ready availability and cheapness of sulfur and the absence of environmentally harmful components. And, indeed, attempts of developing a battery using this electrochemical system were made yet in the end of the 1960s of the previous century, at the rise of the studies of electrochemical lithium systems. It was suggested in the beginning to use the negative electrode made of metallic lithium and the positive one of elementary sulfur supported directly on the current collector. The characteristics of these first layouts were clearly unsatisfactory, partly, because sulfur is an insulator. Later, the positive electrode came to be made of a mixture of sulfur and a carbon material (carbon black). 

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