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.