Quantifying the ion coordination strength in polymer electrolytes

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Chemistry

Background:

• Solid polymer electrolytes (SPEs) are key components in advanced battery technologies.
• Ion transport properties in SPEs are influenced by ion coordination strength, a parameter requiring further investigation.
• Current methods for assessing ion coordination in SPEs are limited.

Purpose of the Study:

• To present and compare qualitative and quantitative methods for measuring ion coordination strength in SPEs.
• To investigate the coordination behavior of Li+, Na+, and Mg2+ cations with different polymer electrolytes.
• To establish the relationship between ion coordination strength and ion transport properties in SPEs.

Main Methods:

• Utilized two qualitative methods probing cation coordination equilibria with polymer ligands or solvent molecules.
• Employed one quantitative method analyzing ion dissociation equilibria in solvent-free polymer systems.
• Studied TFSI-based salts of Li+, Na+, and Mg2+ in polyethylene oxide (PEO), poly(ε-caprolactone) (PCL), and poly(trimethylene carbonate) (PTMC).

Main Results:

• All methods consistently showed polyethylene oxide (PEO) exhibits the strongest ion coordination, while poly(trimethylene carbonate) (PTMC) shows the weakest.
• Magnesium (Mg2+) cations displayed the weakest coordination across all polymers, indicating strong ion-ion interactions in Mg(TFSI)2.
• Lithium (Li+) and sodium (Na+) coordination strength varied based on cation properties and polymer structure, aligning with known transference numbers.

Conclusions:

• Developed and validated methods for assessing ion coordination strength in SPEs.
• Demonstrated that polymer structure and cation type significantly influence ion coordination.
• Confirmed the critical role of ion coordination strength in governing ion transport within solid polymer electrolytes for battery applications.