Nanostructured Li₂S–C composites as cathode material for high-energy lithium/sulfur batteries

With a theoretical capacity of 1166 mA·h·g^–1, lithium sulfide (Li₂S) has received much attention as a promising cathode material for high specific energy lithium/sulfur cells. However, the insulating nature of Li₂S prevents the achievement of high utilization (or high capacity) and good rate capability. Various efforts have been made to ameliorate this problem by improving the contact between Li₂S and electronic conductors. In the literature, however, a relatively high capacity was only obtained with the Li₂S content below 50 wt %; therefore, the estimated cell specific energy values are often below 350 W·h·kg^–1, which is insufficient to meet the ever-increasing requirements of newly emerging technologies. Here, we report a cost-effective way of preparing nanostructured Li₂S-carbon composite cathodes by high-energy dry ball milling of commercially available micrometer-sized Li₂S powder together with carbon additives. A simple but effective electrochemical activation process was used to dramatically improve the utilization and reversibility of the Li₂S–C electrodes, which was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. We further improved the cycling stability of the Li₂S–C electrodes by adding multiwalled carbon nanotubes to the nanocomposites. With a very high specific capacity of 1144 mA·h·g^–1 (98% of the theoretical value) obtained at a high Li₂S content (67.5 wt %), the estimated specific energy of our cell was ∼610 W·h·kg^–1, which is the highest demonstrated so far for the Li/Li₂S cells. The cells also maintained good rate capability and improved cycle life. With further improvement in capacity retention, nanostructured Li₂S–C composite cathodes may offer a significant opportunity for the development of rechargeable cells with a much higher specific energy.