Li-S Batteries with Li 2 S Cathodes and Si/C Anodes

Abstract

Lithium-sulfur (Li-S) batteries promise improved capacities over lithium ion batteries. While currently mostly metallic lithium anodes are used, the use of silicon-anodes might offer better safety and durability. However, in a lithium-sulfur-Silicon (Li-S-Si) battery, lithium must be introduced either on the anode or on the cathode in form of Li2S. In this study, we have prepared Li2S cathodes in combination with Si anodes (i.e., Si/Li2S full-cells) to investigate both the processes during initial charging/activation of Li2S cathodes and the effect of Li2S cathode activation on the cycling performance of Si/Li2S full-cells. We observed that the initial activation requires a substantially higher charging potential than for the subsequent cycles. In situ XRD analysis of the cathode during the first cycle clearly indicates the gradual transformation of Li2S to polysulfides and finally to crystalline sulfur, i.e., even large Li2S particles (≈20 μm) can be charged completely. The result is further confirmed by ex-situ SEM/EDS analysis, which revealed the formation of large sheets of sulfur at the cathode/separator interface. Similar cycling performance of Si/Li2S full-cells is observed at both 0.1 C and 1 C rates, a clear advantage over Li/Li2S cells, which suffer from severe dendrite formation at 1 C in the case of high Li2S loadings.