Redox Promotion by Prelithiation Modification of the Separator in Lithium–Sulfur Batteries

The sluggish "liquid–solid" kinetics between lithium polysulfide (Li2Sx (x ≤ 4)) and lithium sulfide (Li2S) impedes the electrochemical performance of lithium–sulfur batteries (LSBs). The prelithiation modification of the separator has been proven to promote the reduction of Li2Sx (x ≤ 4), but its mechanism and effect on Li2S-oxidation are still unclear. In this work, three kinds of lithiated metformin [LixMF (x = 0.8, 1.0, 2.0)] were prepared via the lithiation reaction using MF as the precursor. Combining experiments and theoretical calculations, we find that the bidentate coordination bond of Li1(Li1–N1&Li1–N2) in Li1.0MF can be reinforced at charging potentials, which triggers the weakening of the Li–S bond in Li2S, thereby promoting Li2S-oxidation. In contrast, both the Li1′ bidentate coordination bond (Li1′–N1&Li1′–N2) and Li2–N3 bond in Li2.0MF shift the electron cloud between the two Li–S bonds in Li2S but fail to weaken the Li–S bond or facilitate Li2S-oxidation. After LixMF (x = 0.8, 1.0, 2.0) was introduced into the cathode side of the polypropylene (PP) separator, Li1.0MF delivers the excellent cycling stability with the high sulfur loading and lean electrolyte. In other words, the modification of the PP separator needs to be moderately prelithiated, rather than maximumly prelithiated.