“First-Cycle Effect” of Trace Li₂S in a High-Performance Sulfur Cathode

Lithium–sulfur battery is one of the most promising choices for next-generation batteries due to its high theoretical energy density and natural abundance. However, the sulfur cathode undergoes a stepwise reduction process and generates multiple soluble polysulfide intermediates; for the further conversion from the dissolved intermediates to the final solid product (Li2S), the surface nucleation barrier limits the speed of the electrochemical precipitation, resulting in serious polysulfide diffusion loss and low sulfur utilization. Herein, the trace Li2S (tLi2S) is modified on the carbon fiber (CF) skeleton as preloaded crystal nuclei to boost the electrokinetics of Li2S deposition in the initial cycle. The trace Li2S decreases the nucleation barrier on the modified electrode (tLi2S@CF), resulting in a high initial capacity of 1423 mAh g–1 for the Li2S6 catholyte (0.2 C), which corresponds to a nearly 100% utilization of Li2S6. Furthermore, the trace Li2S nuclei induce a uniform distribution of the redeposited active materials, and the uniform distribution persists in the following cycles, which benefits the cycle life significantly. The sulfur cathode based on the tLi2S@CF matrix maintains a capacity of 1106 mAh g–1 at 1 C rate after 100 cycles. The strategy can provide a new avenue for the rational design of the sulfur cathode.