Beneficial Effects of FEC on an In-Situ Polymerized Deep Eutectic Electrolyte for Solid-State Batteries

Eutectic-based polymer electrolytes have emerged as promising solid electrolytes because of their ionic liquid-like properties, while modifications are essential to further increase their ionic conductivity at room temperature and solve their compatibility with lithium anode. In this work, an in situ polymerized composite electrolyte is modified by the addition of fluoroethylene carbonate (FEC) whose beneficial effect is systematically investigated in different contents. Poly(ethylene glycol) diacrylate (PEGDA), deep eutectic solvent (LiTFSI:N-methylacetamide = 1:3), and alumina fiber work as the monomer, solvent, and three-dimensional skeleton, respectively. In adjusting FEC content, ionic conductivity at room temperature is dramatically raised by three times to 8.93 × 10^-4 S cm^-1, with a 4-fold increase in lithium-ion transference number to 0.405. Meanwhile, the electrochemical window is widened from 3.5 to 4.8 V. The FEC addition also helps in improving the stability with Li anode, which comes from LiF-rich interphases formed at interfaces. The dynamics of LiFePO₄ is significantly enhanced with higher reversibility in full cells, so that fast capacity decay is inhibited with a specific capacity of 124.1 mAh g^-1 obtained after 300 cycles at 1 C. These results provide an effective modification for the deep eutectic electrolyte, which will boost its development in solid-state batteries.