# Salt Concentration Control of Polysulfide Dissolution, Diffusion, and Reactions in Lithium–Sulfur Battery Electrolytes

## TL;DR

This paper studies how salt concentration affects polysulfide behavior in lithium-sulfur batteries, improving understanding for better battery performance.

## Contribution

The study reveals how salt concentration alters polysulfide solubility and transport using operando Raman spectroscopy.

## Key findings

- High salt concentration reduces solubility and transport of polysulfides at the lithium anode.
- Trisulfur radicals decrease more rapidly than their parent polysulfides in concentrated electrolytes.
- Salt concentration changes solvation structures, affecting chemical pathways of polysulfides.

## Abstract

Lithium–sulfur
(Li–S) batteries suffer from the dissolution
of sulfur and polysulfide (PS) species in the electrolyte, leading
to capacity loss, instability, and a shortened lifespan. While highly
concentrated electrolytes have been explored to address this issue,
the underlying mechanisms of S/PS dissolution and subsequent diffusion,
particularly concerning the specific behavior of long- and short-chain
PSs under varying states of charge (SOC), remain poorly understood.
We here employ operando Raman spectroscopy to semiquantitatively
monitor PS solubility and migration across a wide range of LiTFSI
concentrations in DME:DOL (1:1, v/v). We find that both PS dianions
(S4–8
2–) and trisulfur radicals
(S3
•–) decrease at the lithium
anode with increasing electrolyte salt concentration (0.3–7.0
m), indicating reduced solubility and slower transport. Notably, the
concentration of S3
•– decreases
more rapidly than that of its parent PS S6
2–, suggesting less favorable radical formation pathways in highly
concentrated electrolytes, potentially due to Li–TFSI–PS
adduct formation. These changes result from shifts in the local solvation
structure at high salt concentration, thereby controlling the solubility,
transport, and chemical pathways of polysulfides in the electrolyte.
By providing the real-time dynamics of long- and short-chain PSs,
this work advances the mechanistic understanding of PSs in order to
provide valuable insight for further improvement of Li–S battery
performance.