<p>Polymer electrolytes have been actively investigated as safer alternatives to liquid electrolytes; however, their electrochemical performance often remains limited. In this study, carbonate-based composite polymer electrolytes (CPEs) were prepared using a PEC-LiFSI matrix with two types of silica fillers: SiO₂ nanoparticles and electrospun silica nanofibers (SNFs). Filler contents of 1, 5, and 10 wt% were used to examine the morphology and interface-related effects. Compared with the filler-free PEC-LiFSI electrolyte (SPE), both fillers increased thermal stability. FT-IR analysis of the carbonyl stretching region revealed changes with filler addition, indicating changes in the Li⁺ coordination environment. The ionic conductivity exhibited an optimum at 5 wt% loading for both fillers, followed by a decrease at 10 wt%. Linear sweep voltammetry further indicated improved oxidation stability with the addition of silica, with the highest oxidation onset potential observed at 5 wt% SNFs. The mechanical properties were enhanced much more strongly by SNFs than by SiO₂, especially at high loadings. The capacity of the battery increased during the initial cycles and approached approximately 140 mAh g⁻¹. Overall, these results highlight that the surface/interface effects and loading of silica fillers together influence key thermal, mechanical, and electrochemical properties of carbonate-based CPEs and that SNFs are an effective filler for mechanically robust PEC-LiFSI-based CPEs.</p>