Unraveling the process of highly performing sulfur-metal nanocomposites in lithium battery
Herein we originally depict the process of a novel sulfur nanocomposite by combining X-ray computed tomography at micro- and nanoscale and electrochemistry in lithium cell. The electrode is obtained at mild temperature according to an alternative approach including metal nanoparticles of either tin or nickel in a bulk of melted sulfur in the weight ratio of 85:15, respectively. We show that this pathway leads to highly performing electrodes, matching the state-of-the-art results on the best carbonaceous composites. Indeed, lithium-sulfur cells with a working voltage of about 2.2 V ensure a capacity referred to the sulfur mass approaching 1400 mAh g−1 at a C/3 rate and 740 mAh g−1 at a rate as high as 3C (1C = 1675 mAh g−1), with coulombic efficiency close to 100% and stable cycling trend over 100 cycle. High-resolution imaging sheds light on characteristic morphological features of the electrode that allow these remarkable performances, and reveals the beneficial effect of an actual metal nanoparticle incorporation within the sulfur phase. The various investigation techniques, with particular focus on three-dimensional imaging, suggest a sulfur electrodeposition upon charge preferentially next to electron conductive centers within the electrode support as well as on the metal clusters. A massive microstructural reorganization is observed during the first cycle in lithium cell with concomitant remarkable enhancement of the electrode charge transfer and variation of the reaction potentials. This process is accompanied by a substantial electrode amorphization and migration of the active material towards the current collector bulk. The results obtained in this work, and a comprehensive study designed ad hoc for the sulfur electrode, suggest alternative strategies for ultimately achieving an actual Li/S cell improvement.