In situ-formed Li2S in lithiated graphite electrodes for lithium-sulfur batteries

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 Li2S 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 Li2S6 in liquid electrolyte to insoluble Li2S. The chemical reduction slowly draws lithium out of graphite, resulting in a reduction of the d002 spacing of graphite from 3.56 to 3.37 Å 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 Li2S. The formed amorphous Li2S 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.