X-ray spectroscopy as a probe for lithium polysulfide radicals

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• The discharge mechanism of lithium-sulfur batteries remains poorly understood.
• Polysulfide radicals are hypothesized to play a significant role in battery discharge.
• Accurate quantification of these species is crucial for elucidating electrochemical pathways.

Purpose of the Study:

• To simulate and analyze X-ray absorption spectroscopy (XAS) spectra of polysulfides.
• To differentiate spectral features of neutral, radical, and dianionic polysulfide species.
• To assess the feasibility of detecting radical species using S K-edge XAS.

Main Methods:

• Density Functional Theory (DFT) based simulations.
• Calculation of X-ray Absorption Spectroscopy (XAS) spectra at the sulfur K-edge.
• Analysis of spectral features for neutral, radical (-1), and dianionic (-2) polysulfides.

Main Results:

• The low-energy pre-edge feature in S K-edge XAS is primarily due to terminal atoms in linear polysulfides, not exclusively radicals.
• A unique spectral feature for radicals, arising from 1s → π* transitions, was identified 0.5-1 eV below the main pre-edge.
• Existing XAS measurements show no evidence of this radical-specific feature.

Conclusions:

• The observed pre-edge feature in S K-edge XAS is not solely indicative of radical species.
• A distinct spectral signature for linear polysulfide radicals exists but is weak.
• Detection of linear radicals using XAS is feasible only at high mole fractions (>20%).