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Cui et al. Energy Mater 2023;3:300034 https://dx.doi.org/10.20517/energymater.2023.19 Page 3 of 12

Figure 1. Schematic illustrations of solid-state Li-S batteries using (A) traditional solid polymer electrolyte (SPE) and (B) the PO-PU-
LiTFSI electrolyte, (C) fabrication of the PO-PU-LiTFSI electrolyte and sulfur cathode. SEM images from (D) cross-sectional and (E) top
view of the PO-PU-LiTFSI electrolyte, the inset is an optical image of the electrolyte. (F) FTIR spectrum of the PO-PU-LiTFSI electrolyte.

RESULTS AND DISCUSSION
The synthesis of the PO-PU-LiTFSI electrolyte is given in Figure 1C. First, polyethylene glycol (PEG) and
isocyanate were dissolved in acetonitrile. After stirring at 60 °C for several hours, LiTFSI was added to the
above solution, and the resulting mixture was coated on the PO membrane. The obtained membrane was
heated in an oven at 60 °C to remove the acetonitrile. After removing the solvent, the PO-PU-LiTFSI
electrolyte was obtained. The obtained electrolyte has a uniform thickness (about 19 μm) and a smooth
surface [Figure 1D and E].

To identify the composition of the PO-PU-LITFSI electrolyte, Fourier transform infrared (FTIR) spectra
and thermogravimetric analysis (TGA) curves were recorded. As shown in Figure 1F, the existence of -N-H
bonds (3,560 cm ), C-N- bonds (1,348 cm ), and C=O bonds (1,641 cm ) indicates the existence of
-1
-1
-1
urethane/urea groups formed by polyaddition. No peak of isocyanate (-NCO) stretching band is found at