A Raman study of ion solvation and association in polymer electrolytes

Ion–polymer and ion–ion interactions in polymer electrolytes have been investigated at different temperatures and pressures, and for different polymer molecular weights. Salt–polymer complexes of various triflate salts, M(CF 3 SO 3 ) x (M = Li + , Na + , Ca 2+ , Cu 2+ and Nd 3+ ), in low molecular weight polyethers (PEO and PPO) have been studied using Raman and Brillouin scattering. It is found that anions coordinated to the OH end groups of the polyethers are more stable than cations forming crosslinks between oxygens of adjacent chains, which in turn are more stable than cations coordinated to single chains. We have observed that the number of ion pairs increases as the polymer molecular weight increases. Furthermore, the ion–ion interaction is strongly temperature‐dependent and shows widely different behavior in different systems. In the case of Li + ‐, Na + ‐ and Ca 2+ ‐containing polyether complexes the ion association increases with increasing T. It is constant in Cu 2+ –triflate complexes, while it decreases for complexes containing trivalent Nd 3+ cations. Pressure observations in LiCF 3 SO 3 –PPO complexes reveal increasing solubility and fewer ion pairs as pressure increases. It is shown that theories which consider volume changes in the salt dissociation process can qualitatively explain the effects of molecular weight, temperature and pressure using entropy considerations. The entropy effect includes contributions due to free‐volume dissimilarities between the solvated ions and the macromolecules, structural ordering induced via cationic crosslinking of adjacent chains and electrostriction. Comparing the spectroscopic data with conductivity data, it is found that differences in the “free” solvated ion concentration can account for differences in conductivity.