Fluorescence correlation spectroscopy based insights into diffusion in electrochemical energy systems.

Area of Science:

• Electrochemistry
• Materials Science
• Physical Chemistry

Background:

• Ion diffusion in battery electrolytes is crucial for capacity and charging rates.
• Electrolyte structuredness and local interactions influence ion transport.
• Lithium-ion battery performance is often limited by electrolyte properties.

Purpose of the Study:

• To characterize local transport properties in Li-ion battery electrolytes using Fluorescence Correlation Spectroscopy (FCS).
• To investigate the effect of lithium-ion concentration on tracer diffusion in EC-DMC solvent mixtures.
• To correlate electrolyte's local behavior with battery performance.

Main Methods:

• Utilized Fluorescence Correlation Spectroscopy (FCS) to measure tracer diffusion of Coumarin 343.
• Studied diffusion in Ethylene Carbonate (EC) and Dimethyl Carbonate (DMC) mixtures with varying LiPF6 concentrations.
• Compared diffusion data with bulk viscosity measurements and Stoke-Einstein relation.

Main Results:

• Tracer diffusion significantly slowed with increasing LiPF6 concentration.
• Discrepancy observed between bulk viscosity and diffusion-inferred local behavior at high salt concentrations.
• Electrolyte homogeneity was found to be limited at molecular scales.

Conclusions:

• Local electrolyte structuredness, not just bulk viscosity, impacts ion transport.
• Slowed tracer diffusion at higher LiPF6 concentrations limits battery performance, especially at fast charging rates.
• FCS provides insights into molecular-scale electrolyte behavior relevant to battery design.