**3. Results**

## *3.1. Physicochemical Investigation*

Similar TPE reported before [23,25] have shown two properties that make them interesting candidates as electrolytes, their liquid nature at the microscopic scale and their ability to remain as solids at the macroscale up to 90 ◦C for long periods of time [23]. Figure 1 summarizes the physicochemical characterization performed with both the TPE under study and the reference PEO-LiTFSI, which includes a SEM micrograph of the TPE, and the TGA, DSC, and creep experiments of both electrolytes.

**Figure 1.** (**a**) Scanning electron microscopy (SEM) image of the thermoplastic polymer electrolyte (TPE) (cross-section); (**b**) thermogravimetric analysis (TGA) of the poly(ethylene oxide) (PEO)-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and TPE; (**c**) differential scanning calorimetry (DSC) traces of the PEO-LiTFSI and TPE; and (**d**) pictures showing the electrolyte appearance after the creep test (see text for details).

First of all, Figure 1a shows the excellent dispersion of the sepiolite nanofibers in the electrolyte. Figure 1b shows that the thermal stabilities of the TPE and PEO-LiTFSI in nitrogen are very similar and will favor the overall solid-state battery safety. Figure 1c shows how the TPE has two well defined transitions, the PEO glass transition close to −60 ◦C and a melting endotherm slightly under 40 ◦C

caused by the scarce crystalline phase in the TPE. On its turn, PEO-LiTFSI has a Tg at about −38 ◦C and a melting endotherm at 50 ◦C, the latter caused by the crystalline PEO phase, which amounts to 31%. Both the higher Tg and the higher crystalline fraction of PEO-LiTFSI make this electrolyte more rigid than TPE. Figure 1d and 1e show the appearance of the sandwiches (electrode-electrolyte) of PEO-LiTFSI and TPE, respectively, after the creep tests. No creep is seen in either sample after being subjected to the temperature cycles under pressure.

The ionic diffusivity in the TPE has been obtained by PFG-NMR experiments at 25 ◦C, and values are in the range of those obtained for similar TPE [23]: *D*Li = 0.6 × 10−<sup>12</sup> <sup>m</sup>2·s<sup>−</sup><sup>1</sup> and *D*TFSI = 3.9 × 10−<sup>12</sup> <sup>m</sup>2·s<sup>−</sup>1. A transport number *<sup>t</sup>*Li<sup>+</sup> = 0.03 at 25 ◦C can be estimated from these diffusion coefficients using Equation (1).
