"First-Cycle Effect" of Trace Li 2 S 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 (Li 2 S), the surface nucleation barrier limits the speed of the electrochemical precipitation, resulting in serious polysulfide diffusion loss and low sulfur utilization. Herein, the trace Li 2 S (tLi 2 S) is modified on the carbon fiber (CF) skeleton as preloaded crystal nuclei to boost the electrokinetics of Li 2 S deposition in the initial cycle. The trace Li 2 S decreases the nucleation barrier on the modified electrode (tLi 2 S@CF), resulting in a high initial capacity of 1423 mAh g -1 for the Li 2 S 6 catholyte (0.2 C), which corresponds to a nearly 100% utilization of Li 2 S 6 . Furthermore, the trace Li 2 S 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 tLi 2 S@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.