Influence of Humidity on the Complex Structure of PEO-Lithium Salt Polymer Electrolyte

Solid polymer electrolytes of PEO/LiClO₄ and PEO/LiTFSI solution casting films were prepared with the EO/Li molar ratio of 3: 1, and the effect of relative humidity (RH) on their complex structures were characterized. It is shown that the complex structures were barely changed at RH ≤ 10% while severe differences were shown at RH ≥ 20%. The reason was attributed to the interactions of water with lithium salt, and the formation of PEO–Li⁺–H₂O decreased the interactions between PEO and lithium ions. Furthermore, it was shown that the hydrated samples after heat treatment were still strikingly different in characters from their anhydrous precursors, and the type of lithium salt affected the final structures. It was found that the structure of (PEO)₃LiClO₄ (30% RH) was hardly changed after heating; however, an irreversible compositional transition was discovered in (PEO)₃LiTFSI (30% RH) in which case (PEO)₂LiTFSI was formed.