A Multifunctional Sacrificial Plasticizer in PEO-Based Electrolytes for Improved Ionic Transport Properties and a Robust Solid-Electrolyte Interphase

Abstract Poly(ethylene oxide) (PEO)-based electrolytes have attracted extensive attention owing to their excellent lithium-salt-solvating capability and favorable mechanical flexibility. However, the practical application of PEO-based electrolytes is severely hindered by poor ionic transport properties and unstable interfaces between the electrolytes and electrodes. To address these challenges, a flexible organic molecule, 2-(2-(butylthio)–5-(methoxymethyl)–3-methylcyclohexa-2,5-dien-1-yl)–2,2-difluoro-1-phenylethanone (DFAPCS), was rationally designed and synthesized for preparing the PEO/LiTFSI/DFAPCS (PLD) composite electrolyte with a highly homogeneous structure. Such a DFAPCS additive, which contained methoxy (–OCH3) groups and thioether (–S–) linkages, imposes steric hindrance, polar interactions, and conformational disturbances on PEO chains. Such effects disrupted PEO ordered packing and reduced crystallinity to boost ionic conductivity. In addition, the DFAPCS additive possessed a low lowest unoccupied molecular orbital (LUMO) energy level, enabling it to preferentially undergo electrochemical decomposition for the formation of a robust solid-electrolyte interphase (SEI) enriched with LiF, Li2O, and LixSx components. As a result, the PLD electrolyte exhibited a significantly enhanced ionic conductivity of 8.06 × 10–4 S cm–1 at 60 °C, a higher lithium-ion transference number of 0.58, and an expanded electrochemical stability window of 4.6 V. The Li//Li symmetric cell demonstrated excellent long-term stability, achieving an ultra-long cycling lifespan of 4000 h at a current density of 0.1 mA cm–2. The LiFePO4-based full cell delivered outstanding rate capability and cycling performance, maintaining stable operation for 1000 cycles at 1 C with a capacity retention of 66.9% and an average Coulombic efficiency as high as 99.88% after prolonged cycling.