It’s hard to be sure of what happens inside a battery while it is in operation. During discharging, lithium diffuses to the cathode and becomes incorporated into polysulfide salts. The chemical species formed by the reduction and oxidation of sulfur with alkali metals in the cells are involved in a complex equilibrium. The performance of Li–S batteries depends on which sorts of polysulfides are formed and how they are transported around the cell in a process known as the “polysulfide shuttle”. However, the lack of techniques available to analyze Li–S cells in situ makes it difficult to understand this shuttling process and fully optimize the cell configuration.

Robert Dominko, National Institute of Chemistry, Ljubljana, Slovenia, and colleagues have developed a special cell configuration that allowed them to use UV/Vis spectroscopy to qualitatively and quantitatively detect soluble polysulfides in the cell separator while the battery was in operation.

Robert Dominko and Manu U. M. Patel now used this technique to compare Li–S batteries with two different types of electrolytes, to quantitatively determine differences in the mechanisms of polysulfide shuttling within the electrolyte. The researchers performed UV/Vis measurements of chemically synthesized stoichiometric mixtures of polysulfides to obtain calibration curves. They then used these calibration curves to recalculate polysulfide concentrations in Li–S batteries that had been analyzed in operando.
The team found a correlation between higher polysulfide concentration and faster capacity fading. Long-chain polysulfides undergo an irreversible reaction on the surface of lithium leading to a reduction in battery capacity.

This new technique should lead to further insights into the factors leading to capacity fading in lithium–sulfur batteries.

^{Image: © Andy Armstrong; www.flickr.com/photos/andyarmstrong/ CC BY-SA 2.0}

• Li-S Battery Analyzed by UV/Vis in Operando Mode,
  Manu U. M. Patel, Rezan Demir-Cakan, Mathieu Morcrette, Jean-Marie Tarascon, Miran Gaberscek, Robert Dominko,
  ChemSusChem 2013.
  DOI: 10.1002/cssc.201300142
• Application of In Operando UV/Vis Spectroscopy in Lithium–Sulfur Batteries,
  Manu U. M. Patel, Robert Dominko,
  ChemSusChem 2014.
  DOI: 10.1002/cssc.201402215