Influence of Sample History on the Morphology and Transport Properties of PEO−Lithium Salt Complexes

We provide clear evidence of the effect of the solvent used for sample preparation on the morphology and transport properties of PEO−lithium salt (LiTFSI, LiBF4, or LiClO4) complexes. The surface structure, thermal behavior, and conductivity of the films along with the measured diffusion coefficients and transport numbers of the ionic species are clearly different whether acetonitrile (good solvent of PEO) or methyl formate (poor solvent) was used. Interestingly, the samples with the lowest conductivity have the highest diffusion coefficients, indicative of a difference in carriers concentration. We infer that the polymer chains keep memory of the solvent, and their generated spatial organization is directly implied in conductivity through the use of specific pathways for ionic mobility. Since the variations observed here are of the same amplitude or greater than those observed for some deliberate modification of the polymer electrolytes (i.e., nanoparticles addition, mechanical strain, etc.), great care must now be exercised in giving precise sample history.