**4. Conclusions**

The polymer/ionic liquid thermoplastic solid electrolyte based on PEO, modified sepiolite (TPGS-S), LiTFSI, and PYR14TFSI ionic liquid was successfully prepared using a solvent-free extrusion method beneficial for low cost and environment-friendly solid-state battery mass production. The physical-chemical, electrical, and electrochemical properties of the developed solid electrolyte were comprehensively characterized.

The TPE presented a maximum conductivity 0.5 S·cm<sup>−</sup><sup>1</sup> at 60 ◦C. The LSV curve showed that the electrolyte is stable up to 4.2 V versus Li/Li+ and possesses an excellent compatibility with lithium metal electrode during more than 1600 hours cycling under comparatively harsh cycling conditions (1 mA·cm<sup>−</sup>2, 2 mAh·cm<sup>−</sup>2).

Finally, the developed solid electrolyte demonstrated a promising cycling performance in nonoptimized prototype of truly Li-LiFePO4 solid-state coin cell working under relatively higher charge/discharge C-rates (0.2 C/0.5 C).

Thus, the reported solid electrolyte can be considered as a promising candidate for further development of solid-state batteries with charge cut-off voltage about 4.0 V.

**Author Contributions:** Conceptualization, P.T., A.K., Methodology, O.G., E.F., A.K.; Materials design, F.G, N.G., P.T., Investigation, F.G., O.G.; Formal Analysis, O.G., A.K.; Data Curation, O.G., E.F., A.K.; Writing-Original Draft Preparation, P.T., F.G., O.G., E.F., A.K., I.U.; Writing-Review & Editing, P.T., N.G., O.G., E.F., A.K., I.U.

**Funding:** This study was supported by the Comunidad de Madrid (Project Ref. LIQUORGAS-CM, S2013/MAE-2800). Francisco González is grateful to CONACYT-SENER for the scholarship granted (CVU559770/Registro 297710 and S2013/MAE-2800).

**Acknowledgments:** Leoncio Garrido is acknowledged for the determination of diffusion coefficients.

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