Regulation of the Ionic Transport and Electrochemical Stability in Poly(ethylene oxide) Solid Electrolyte via Terminal Polyfluorinated Modification

Polyoxyethylene (PEO) electrolyte is the most widely used polymer solid electrolyte because of its flexibility and good solubility for lithium salts. However, its application is still hampered by the low ionic conductivity, narrow electrochemical stability window, low Li⁺ transfer number, and poor electrolyte/electrode interfaces. This study reports the modification of a traditional PEO electrolyte with terminal hydroxyl groups (PEO-OH) to two PEO electrolytes with terminal phenyl groups (PEO-P) and perfluorophenyl groups (PEO-FP), respectively. The received PEO-P and PEO-FP electrolytes both exhibit higher ionic conductivity and transfer number due to the terminal phenyl groups that could disturb the PEO's crystal structure and thereby increase the movement of PEO's chain segments. In particular, the electrochemical stability window of the PEO-P and PEO-FP electrolytes is significantly increased to around 5.2 V from 3.8 V of PEO with reactive OH groups. In addition, the PEO-FP electrolyte with the polyfluorinated phenyl group demonstrates superior ability to form stable electrolyte/electrode interfaces, resulting in excellent cycling performance both in Li//Li symmetric cells and LiFeO₄//Li full battery. Furthermore, the much wider electrochemical stability window of the PEO-FP electrolyte also leads to the stable operation of the full cells with a high-voltage NCM523 cathode, and its pouch cell also confirms the excellent flexibility and safety of the PEO-FP electrolyte for practical applications.