On the mechanism of ion transport through polyphosphazene solid polymer electrolytes:: NMR, IR, and Raman spectroscopic studies and computational analysis of 15N-labeled polyphosphazenes

Comprehensive investigation of lithium ion complexation with N-15-labeled polyphosphazenes-N-15-poly-[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (IN-MEEP) and N-15-poly-[((2-allylphenoxy)(0.12)(4-methoxyphenoxy)(1.02)(2-(2-methoxyethoxy)ethoxy)(0.86))phosphazene] (N-15-HPP)-was performed by NMR, IR, and Raman spectroscopies. Previous studies characterized the ionic transport through the polymer matrix in terms of jumps between neighboring polymer strands utilizing the electron lone pairs of the etherial oxygen nuclei with the nitrogen nuclei on the polyphosphazene backbone not involved. However, noteworthy changes were observed in the NMR, IR, and Raman spectra with the addition of lithium trifluoromethanesulfonate (LiOTf) to the polyphosphazenes. The data indicate that the preferred association for the lithium ion with the polymer is with the nitrogen nuclei, resulting in the formation of a pocket with the pendant groups folding around the backbone. NMR temperature-dependent spin-lattice relaxation (T-1) studies (C-13, P-31, and N-15) indicate significant lithium ion interaction with the backbone nitrogen nuclei. These studies are in agreement with molecular dynamics simulations investigating lithium ion movement within the polyphosphazene matrix.