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

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.