Elasticity and Cooperative Ion Motion in a Polymeric Ionic Liquid Loaded with Li Salt

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Chemistry

Background:

• Solid polymer electrolytes (SPEs) offer enhanced safety and stability for next-generation batteries but face challenges with slow ion transport and poor mechanical strength.
• Polymeric ionic liquids (PILs) show promise for high salt dissolution and ion transport, yet balancing segmental motion for conductivity with mechanical integrity remains difficult.

Purpose of the Study:

• To develop and characterize a novel polymeric ionic liquid (PIL) with a flexible siloxane backbone for improved solid polymer electrolyte (SPE) performance.
• To investigate the relationship between polymer backbone flexibility, salt loading, and the resulting ionic conductivity and mechanical properties of the PIL-based SPE.

Main Methods:

• Synthesized and characterized a PIL with a siloxane backbone (PMS-ImTFSI).
• Evaluated ionic conductivity using electrochemical impedance spectroscopy.
• Assessed mechanical properties and viscoelastic behavior via shear rheology.
• Investigated ion transport mechanisms and microstructure using NMR diffusometry, inverse Haven ratios, and X-ray scattering.

Main Results:

• PMS-ImTFSI demonstrated enhanced Li+ conductivity (up to 2 × 10-5 S/cm at 90 °C) and a stable rubbery plateau up to 20 wt% salt loading, indicating improved elasticity.
• A transition from salt-in-polymer to polymer-in-salt regimes was observed across multiple techniques, correlating with optimal properties at low salt concentrations.
• High salt loadings led to microstructure loss and diminished performance.

Conclusions:

• PILs with flexible, nonpolar backbones can form ion-rich domains at low salt concentrations, simultaneously achieving high Li+ conductivity and robust mechanical properties.
• The findings provide a design strategy for advanced SPEs by optimizing the interplay between polymer structure, salt concentration, and resulting microstructural evolution.
• This research paves the way for safer and higher-performance batteries utilizing novel solid polymer electrolytes.