**4. Conclusions**

An easy way to obtain highly conductive gel polymer electrolytes was proposed in this paper. The swelling ability of high surface area, electrospun PEO membranes was exploited for the absorption of stable IL-based electrolytes. The gelling procedure, in terms of solid-to-liquid weight ratio (i.e., PEO membrane: IL-based electrolyte), was optimized to obtain a reproducible and high swelling degree. The absorption of the liquid component into the PEO membrane strongly reduced or even suppressed the crystallinity of the polymer, giving rise to amorphous, well-plasticized electrolyte systems. The effect of a silica particle additive, dispersed in the polymer matrices during the electrospinning process, was also investigated. A certain degree of crystalline phase, even though very small, was preserved in the membranes after gelling when silica was present, which was attributed to preferential interactions between the inorganic filler and the liquid solution, leaving partially-coordinated PEO chains free to crystallize. This was reflected in the conducting properties of the gel polymer electrolytes, showing lower ion conductivity in the silica-added sample because of a reduced number of available charge carriers.

In view of possible applications in lithium-sulfur batteries, the addition of a polysulfide component (i.e., Li2S8) in the swelling solution was considered. Such a polysulfide affected the ionic conductivity of the silica-free gel polymer electrolytes very little. On the contrary, it had a beneficial effect in the presence of silica. Apparently, the polysulfide opposed the liquid retention of SiO2 particles to the advantage of the transport mechanism.

Overall, selected compositions proposed here showed conductivity values suitable for battery applications in a wide range of temperatures. All these findings address the potentiality of such gelled electrolytes.

**Author Contributions:** M.A.N. and F.C. conceived the work and designed the experiments; L.M. and P.B. synthesized materials and performed the experiments; L.L. and A.T. analyzed the data; M.A.N., S.P. and F.C. revised data analysis and discussion of results; M.A.N. and F.C. wrote the paper.

**Funding:** The results of this work have been obtained with the financial support of the European Community within the Seventh Framework Program LISSEN (Lithium Sulfur Superbattery Exploiting Nanotechnology) Project (project number 314282).

**Conflicts of Interest:** The authors declare no conflict of interest.
