Mixed solvent and polymer coordination in PAN and PMMA gel polymer electrolytes studied by ab initio calculations and Raman spectroscopy

The local and mixed solvent and polymer coordination in PAN and PMMA/carbonate (EC/PC) based lithium ion conducting gel polymer electrolytes has been studied by using ab initio calculations and Raman spectroscopy. The energies of the coordination complexes were evaluated by density functional theory with a hybrid functional and by performing simple thermodynamic calculations. Observed frequency shifts for EC and PAN due to lithium ion coordination were confirmed by computing the Hessian matrixes for pure and mixed complexes. The lithium ion has been found to prefer to coordinate to EC solvent molecules. In the PAN based gels there was only a slight preference, while for PMMA gels the preference is much larger. The temperature-dependent coordination is found to be directly related to the differences in lithium ion binding energies in the mixed solvent and polymer ligand complexes. The implications for the formation and stability of gels and for the lithium ion transport in different gel systems are discussed.