Hybridization of Mesoporous Carbon and Iron Oxide for Better Mitigation of Polysulfide Shuttling in Li-S Batteries

Abstract

With a higher theoretical capacity, lithium-sulfur (Li-S) batteries have been considered as promising candidates for next-generation batteries. Due to the non-conducting nature of sulfur, however, lithium-sulfur batteries tend to exhibit poor performance at a high current rate (C-rate).Here we demonstrate that Fe3O4, synthesized from precursor iron (III) acetylacetonate (AAI), and mesoporous carbon material, Ketjen Black (KB), can be synergistically combined to enhance the electrochemical performance of lithium-sulfur batteries substantially. Instead of adding commercial magnetite particles into Li-S cathodes, iron oxides are synthesized and imbedded into KB from a precursor, AAI, through thermal treatments in air and Ar. The sulfur, then, is incorporated into iron oxides/KB hybrids by melting at an elevated temperature. With air-controlled electrospray as the processing method, sulfur embedded iron oxides/KB, poly(acrylic acid), and rGO sheets are mixed and directly deposited onto the carbon-coated aluminum collector, to employ as the Li-S battery cathode. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) studies confirm that Fe3O4 and iron carbide (Fe3C) are synthesized from the precursor, AAI, embedded in mesoporous carbon materials, and reducing the charge transfer resistance of batteries. The rate-capability tests show that systems with KB/Fe3O4 can achieve enhanced performance compared to batteries without iron oxides, especially at high C-rates. With 14 wt % of solid materials as iron oxides and iron carbide, batteries exhibit 750 mAh/g at 2C discharge/charge rates, which is 83% higher compared to systems without iron oxides. However, incorporated via 250 °C for 0.5 hr in air and 850 °C for 2 hr in Ar, the composite of KB/Fe3O4/Fe3C induces irreversible side reactions during the initial charging process, which causes a huge difference in capacity between the first and the second cycle. To achieve an optimal status with improved rate capability and acceptable initial charging time, we modify the thermal treatments and thus increase the proportion of Fe3O4 in the mixture. According to the postmortem analysis, cathodes with iron oxides can interact with soluble polysulfides strongly and alleviate the polysulfide shuttle effect significantly, compared with those without Fe3O4. Such enhanced rate capability by KB/Fe3O4 (synthesized) over KB only or KB/Fe3O4 (commercial) systems suggests that the incorporation of iron oxides can play an important role in improving the electrical conductivity of cathode and mitigating polysulfide shuttle effect. Figure 1