Probing Ionic Liquid Electrolyte Structure via the Glassy State by Dynamic Nuclear Polarization NMR Spectroscopy

Area of Science:

• Solid-state nuclear magnetic resonance (NMR) spectroscopy
• Materials science
• Electrochemistry

Background:

• Ionic liquids are promising electrolytes for energy storage applications.
• Understanding ion coordination and dynamics in the solid/glassy state is crucial for electrolyte design.
• Solid-state NMR offers atomic-level insights but often suffers from low sensitivity.

Purpose of the Study:

• To investigate the local structure and Li+ ion proximity to the pyrrolidinium cation in a glassy ionic liquid.
• To demonstrate the utility of DNP-enhanced solid-state NMR for studying ion coordination in electrolytes.
• To compare experimental findings with molecular dynamics simulations.

Main Methods:

• Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy was employed.
• A 1.0 m solution of lithium bis(fluorosulfonyl)imide (LiFSI) in N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) was studied in its glassy state at 92 K.
• A 13C-6Li rotational echo double resonance (REDOR) pulse sequence was used with a biradical for signal enhancement to probe Li-C proximities.

Main Results:

• Distributions of Li-C distances were successfully extracted from the DNP-enhanced NMR data.
• A 3D map illustrating the spatial distribution of Li+ ions relative to the C3mpyr cation was generated.
• The experimentally derived 3D map showed excellent agreement with results from liquid-phase molecular dynamics simulations.

Conclusions:

• DNP-enhanced solid-state NMR is a powerful technique for determining ion coordination in glassy ionic liquids.
• The study provides detailed structural information on Li+ interactions within the C3mpyrFSI electrolyte.
• The good agreement validates the use of molecular dynamics simulations for predicting ion behavior in such systems.