Two-channel sodium-ion conducting polymer electrolytes based on perfluorinated linear hetero-oligomers prepared by enzymatic synthesis and NaTFSI

Abstract

Perfluorinated polyethyleneoxides and polymethyleneoxides are very promising host materials for Li- and Na-ion conducting polymer electrolytes due to their chemical stability and fire-retardant properties. In this study, we have carried out the enzymatic synthesis of a linear copolymer based on perfluoropolyether (PFPE) and polyethylene glycol methyl diester (PEG) units, which cannot be achieved by conventional organic chemistry methods. The numerical molecular weight of the resulting hetero-oligomer, named PFPE–PEG–PFPE, was measured to be ca. 3881 g mol⁻¹. The hetero-oligomer is suitable to dissolve the salt NaTFSI up to 20.0% by mass, yielding a series of eight samples with formula PFPE–PEG–PFPE/(NaTFSI)_x (0.000 ≤ x ≤ 2.160), without the need of a plasticizer. The polymer electrolytes reveal glass transition temperatures which increase in the range from −77 to −49 °C with salt concentration. The copolymer and the PEs were thermally stable up to at least 320 °C and 305 °C, respectively. A vibrational spectroscopy study allowed us to clarify the role of CO and CF₂ moieties, OH groups, and ester groups in promoting salt dissolution and assisting ionic mobility. The ionic conductivity measured at 25 °C is as high as 6.6 × 10⁻⁵ S cm⁻¹ and 3.6 × 10⁻⁵ S cm⁻¹ in separate PEG and PFPE/PEG salt-doped phases, respectively. The corresponding cationic transference numbers were as high as 0.80 and 0.94, respectively. This novel biphasic ion-conductor exhibits two distinct conductivity channels and marks a significant advancement towards solvent-free Na–polymer batteries.