**4. Conclusions**

PEO-LiTFSI Li+-conducting membranes, containing the PYR14TFSI ionic liquid, were prepared and studied to be addressed as electrolyte separators for all-solid-state lithium polymer batteries operating at medium-high temperatures. A solvent-free procedure was designed to prepare the PEO-LiTFSI-PYR14TFSI electrolytes. These ternary systems have shown remarkably improved thermal, ion transport and interfacial properties with respect to the ionic liquid-free electrolytes. Wide electrochemical stability was observed even at medium-high operating temperatures. In particular, the ionic liquid-based PEO electrolytes are able to sustain high current rates without any appreciable lithium anode degradation, which is not allowed in binary ionic liquid-free, PEO-LiTFSI systems, thus enabling their use in battery systems operating at 80 ◦C or above and high current rates. Battery tests carried out at 80 ◦C in Li/LiFePO4 polymeric systems have shown excellent cycling behavior and capability retention at high current rates, e.g., more than 93.6% of the theoretical capacity (i.e., 99.5% of the initial value) is still delivered after 100 cycles run at 1C with a coulombic efficiency close 100%. This performance largely exceeds that of analogous, ionic liquid-free, polymer lithium batteries at the same operating conditions, nominating the PEO-LiTFSI-PYR14TFSI ternary system as an electrolyte separator for medium-high temperature lithium polymer batteries. It is worth highlighting the absence of dendrite growth on the Li anode during prolonged cycling tests even at high current rates, which is very often not observed in lithium metal polymer batteries.

**Author Contributions:** G.B.A. and S.P. conceived and designed the experiments; G.-T.K. and M.C. performed the experiments; G.-T.K. and G.B.A. analyzed the data; G.B.A. wrote the paper.

**Funding:** This research received no external funding.

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