Li-S batteries with Li₂S cathodes and Si/C anodes

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.