Tuning Anion Composition and Mobility to Balance Ionic Conductivity and Cation Selectivity in Solid Polymer...

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Science

Background:

• Solid polymer electrolytes (SPEs) are crucial for safe electrochemical energy storage but face challenges in balancing ionic conductivity and selective cation transport.
• Ion transport in SPEs is often coupled with polymer segmental dynamics, limiting performance.
• Tethered anions in single-ion conductors can improve cation transport but may reduce overall conductivity.

Purpose of the Study:

• To investigate the impact of blending a tethered-anion polymer electrolyte with a flexible polymer and untethered small-molecule lithium salts.
• To explore how salt anion volume and the ratio of tethered-to-untethered anions influence ion conduction and thermal properties.
• To enhance ionic conductivity and cation transference in SPEs for high-temperature battery applications.

Main Methods:

• Synthesized poly-[lithium sulfonyl-(trifluoromethane sulfonyl)-imide methacrylate] (PLiMTFSI), a glassy single-ion-conducting polymer with tethered anions.
• Blended PLiMTFSI with poly-(oligo-oxyethylene methyl ether methacrylate) (POEM) and various small-molecule lithium salts (LiTFSI, LiFSI, LiTf, LiClO4).
• Investigated the ion conduction behavior and thermal properties of the resulting ternary blend electrolytes.

Main Results:

• Achieved a significant enhancement in Li+ conductivity in a POEM-based ternary blend containing bulky TFSI- anions and an equimolar mixture of PLiMTFSI and LiTFSI.
• Observed a Li+ conductivity of 4.8 × 10^-4 S/cm at 100 °C, an order of magnitude higher than a comparable POEM/LiTFSI system (6.3 × 10^-5 S/cm).
• Attributed the conductivity enhancement to a more than 9-fold increase in the lithium transference number (0.66 vs. 0.07), indicating improved selective cation transport.

Conclusions:

• Tuning anion composition and mobility in polymer blends is an effective strategy to enhance ionic conductivity and selective cation transport in SPEs.
• The ternary blend approach offers a viable pathway to develop SPEs for high-performance batteries that operate reliably at elevated temperatures.
• This research provides insights into designing advanced electrolytes for safer and more efficient energy storage solutions.