Room temperature ionic liquid-lithium salt mixtures: optical Kerr effect dynamical measurements

Area of Science:

• Physical Chemistry
• Materials Science
• Electrochemistry

Background:

• Ionic liquids (ILs) are promising electrolytes for lithium-ion batteries due to their unique properties.
• However, adding lithium salts to ILs increases viscosity and decreases ionic mobility, limiting their practical application.
• Understanding the dynamics of these mixtures is crucial for optimizing battery performance.

Purpose of the Study:

• To investigate the effects of lithium bis(trifluoromethylsulfonyl)imide addition on the orientational relaxation dynamics of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.
• To analyze the changes in dynamics across various time scales (1 ps–16 ns) and lithium salt concentrations.
• To evaluate the applicability of mode coupling theory (MCT) in describing the dynamics of IL-salt mixtures.

Main Methods:

• Optically heterodyne-detected optical Kerr effect spectroscopy was employed to probe ultrafast dynamics.
• Measurements were conducted on the pure ionic liquid and mixtures with varying lithium salt concentrations.
• Data analysis involved fitting to power laws and a schematic mode coupling theory model.

Main Results:

• Two temperature-independent power laws (intermediate and von Schweidler) and a final exponential relaxation were observed for the pure ionic liquid.
• Lithium salt concentration significantly affected both power laws, with a notable discontinuity near a mole fraction of 0.2.
• Mode coupling theory successfully described the dynamics of both the pure ionic liquid and the salt mixtures.

Conclusions:

• Lithium salt addition alters the orientational relaxation dynamics of ionic liquids in a concentration-dependent manner.
• The observed dynamics are well-explained by mode coupling theory, providing insights into ion transport mechanisms.
• These findings are critical for the development of advanced electrolytes for lithium-ion batteries.