Lithium-sulfur batteries: Influence of C-rate, amount of electrolyte and sulfur loading on cycle performance

In the past four years major improvement of the lithium sulfur battery technology has been reported. Novel carbon cathode materials offer high sulfur loading, sulfur utilization and cycle stability. An often neglected aspect is that sulfur loading and amount of electrolyte strongly impact the performance. In this paper, we demonstrate how the amount of electrolyte, sulfur loading, lithium excess and cycling rate influences the cycle stability and sulfur utilization. We chose vertically aligned carbon nanotubes (VA-CNT) as model system with a constant areal loading of carbon. For a high reproducibility, decreased weight of current collector and good mechanical adhesion of the VA-CNTs we present a layer transfer technique that enables a light-weight sulfur cathode. The sulfur loading of the cathode was adjusted from 20 to 80 wt.-%. Keeping the total amount of electrolyte constant and varying the C-rate, we are able to demonstrate that the capacity degradation is reduced for high rates, high amount of electrolyte and low sulfur loading. In addition idle periods in the cycling regiment and lower rates result in an increased degradation. We attribute this to the redox-reaction between reactive lithium and polysulfides that correlates with the cycling time, rather than cycle number.