In Situ-Formed Li_2S in Lithiated Graphite Electrodes for Lithium-Sulfur Batteries

摘要

Rechargeable lithium-sulfur (Li-S) batteries have the potential to meet the high-energy demands of the next generation of batteries. However, the lack of lithium in the sulfur cathode requires the use of lithium metal anode, posing safety hazards. Use of Li_2S as the cathode can eliminate this problem, but it is hampered by intrinsic challenges (e.g., high electrical resistivity and reactivity in air). We report here the use of a lithiated graphite electrode to chemically reduce in situ the polysulfide Li_2S_6 in liquid electrolyte to insoluble Li_2S. The chemical reduction slowly draws lithium out of graphite, resulting in a reduction of the d_(002) spacing of graphite from 3.56 to 3.37 A and an increase in the open-circuit voltage of cells from 60 mV to 2.10 V after stabilizing over 6 days. X-ray photoelectron spectroscopic analysis shows 48.4% of sulfur in the polysulfide was converted to Li_2S. The formed amorphous Li_2S shows good cyclability with low charge overpotential. The results demonstrate that lithiated graphite can serve as a lithium donor in lithium-deficient cathodes, which could enable lithium metal-free Li-S, Li-air, or Li-organic batteries.