Swelling Mechanism of Polyoxazoline-Based Gel Polymer Electrolytes for Lithium-Ion Batteries

Gel polymer electrolytes for lithium-ion batteries are typically prepared by swelling a preprepared cross-linked polymer network with a solution of lithium salt. This step of the electrolyte preparation is assumed to be straightforward, and very little attention has been paid to the investigation of the swelling mechanism. We questioned the common assumption that the feed solution swells the polymer network at the same concentration and fed a polyoxazoline network with several concentrations of lithium bis(oxalato)borate and lithium bis(trifluoromethane)sulfonimide solutions in propylene carbonate and diglyme. We thoroughly analyzed the obtained gels using NMR and Raman spectroscopy and evaluated the content of the solvent and salt in the polymer matrix, their mutual interactions, and mobilities. We found that this assumption was invalid and created a model of swelling of a polyoxazoline network with a salt solution. Our model implies the importance of often neglected solvent–polymer interactions or lack thereof. In the context of the function of the polyoxazoline system, our results provide insights into the efficiency of polymer matrices for gel polymer electrolytes, which will help avoid inefficient polymer matrices in the future.