Sulfur Speciation in Li–S Batteries Determined by Operando X-ray Absorption Spectroscopy

Abstract

Among the most challenging issues in electrochemical energy storage is developing insightful in situ probes of redox processes for a working cell. This is particularly true for cells that operate on the basis of chemical transformations such as Li−S and Li−O 2 , where the factors that govern capacity and cycling stability are difficult to access owing to the amorphous nature of the intermediate species. Here, we investigate cathodes for the Li−S cell comprised of sulfur-imbibed robust spherical carbon shells with tailored porosity that exhibit excellent cycling stability. Their highly regular nanoscale dimensions and thin carbon shells allow highly uniform electrochemical response and further enable direct monitoring of sulfur speciation within the cell over the entire redox range by operando X-ray absorption spectroscopy on the S K-edge. The results reveal the first detailed evidence of the mechanisms of sulfur redox chemistry on cycling, showing how sulfur fraction (under-utilization) and sulfide precipitation impact capacity. Such information is critical for promoting improvements in Li−S batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport

A highly efficient polysulfide mediator for lithium–sulfur batteries

“…5a) 32,39 , with their 1:2 ratio suggesting a chain length approximately equal to Li 2 S 6 owing to the 2:4 ratio of terminal/bridging sulfur in this LiPS. Elemental sulfur was also detected at this and other intermediate states of discharge, consistent with incomplete reaction and/or the disproportionation of metastable polysulfides such as Li 2 S 8 to form Li 2 S 6 and sulfur as previously reported 45 . S2p peaks between 166 and 170 eV correspond to two sulfur environments: one at 167.2 eV (thiosulfate) and another at 168.2 eV, which is assigned to a polythionate complex as discussed above.…”

Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries

“…The interaction of different sulphur host materials (Ti 4 O 7 , VC and graphite) with LiPSs was probed using a combination of XPS and visual discrimination, taking Li 2 S 4 as the representative polysulphide. The latter was prepared using previously reported methods 31 . An equivalent amount of target host material (based on surface area) was first added to a solution of Li 2 S 4 in tetrahydrofuran.…”

“…Surfacemediated reduction of the sulphide species on Ti 4 O 7 was further confirmed by an operando XANES study. This powerful tool has been previously used by us for monitoring sulphur speciation in Li-S cells 31 , and a significant theoretical understanding of the spectroscopic signatures has been recently reported 40 . Sulphur K-edge spectra collected throughout discharge and charge of a Ti 4 O 7 /S electrode were fitted with a linear combination of a-S 8 , Li 2 S 6 , Li 2 S 4 and Li 2 S and compared with a VC/S electrode.…”

“…Sulphur K-edge spectra collected throughout discharge and charge of a Ti 4 O 7 /S electrode were fitted with a linear combination of a-S 8 , Li 2 S 6 , Li 2 S 4 and Li 2 S and compared with a VC/S electrode. In order to necessarily avoid self-absorption in the XANES experiments 31 and also to preferentially probe the sulphur host interface, both working electrodes were prepared with a low sulphur weight fraction (see details in Supplementary Fig. 7).…”

X-ray Absorption Spectroscopic Characterization of the Synthesis Process: Revealing the Interactions in Cetyltrimethylammonium Bromide-Modified Sulfur–Graphene Oxide Nanocomposites

“…[20][21][22][23][24] In this work, we have employed the S K-edge XAS to monitor the evolutions of the S-containing species in each step of the synthesis. S K-edge XAS probes the transition from the S 1s to the 3p orbital.…”