*2.1. Reagents*

For the preparation of the solid electrolytes, the following materials were used: PEO: Mn 5 × 10<sup>6</sup> <sup>g</sup>·mol−<sup>1</sup> for the TPE, Mn 6 × 10<sup>5</sup> <sup>g</sup>·mol−<sup>1</sup> for the reference electrolyte, and Mn 4 × 10<sup>5</sup> <sup>g</sup>·mol−<sup>1</sup>

for the positive electrode preparation, all purchased from Sigma-Aldrich (St. Louis, MO, USA). D-α-tocopherol polyethylene glycol 1000 succinate (TPGS), used to prepare the modified sepiolite (TPGS-S), was purchased from Sigma-Aldrich and used as received. Details on the preparation of the TPGS-S have appeared elsewhere [30]. Battery grade LiTFSI and PYR14TFSI with 99.9% of purity were purchased from Solvionic (Toulouse, France). Dry acetonitrile with 99.8% of purity was purchased from Scharlab (Barcelona, Spain). All the reagents were stored in dry room with dew point below −50 ◦C; they were used without further purification.

#### *2.2. Synthesis and Preparation of Materials*

Reference solid polymer electrolyte (PEO-LiTFSI) was prepared as follows: LiTFSI was dissolved in acetonitrile and stirred with a mechanical stirrer for 30 min. PEO, Mn 6 × 10<sup>5</sup> <sup>g</sup>·mol−1, was slowly added and the mixture was stirred for 5 h to guarantee the complete solubilization of all reagents. The molar ratio of EO/Li was chosen to be 20. The amount of solid in the acetonitrile solution was set to 12 wt %. Self-standing membranes of reference PEO-LiTFSI electrolyte were obtained by solvent casting over Teflon sheets. The casted solution was dried for 2 h at 35 ◦C and then for 17 h at 60 ◦C under reduced pressure. PEO-LiTFSI electrolyte formulation is given in Table 1.

TPE was prepared in accordance with method reported earlier [26]. Briefly, all components were physically premixed and then melt compounded in a Haake MiniLab extruder (Haake Minilab, Thermo Fisher Scientific, Waltham, MA, USA). Processing was carried out at a shear rate of 80 rpm during 20 min and at 160 ◦C. Afterwards, TPE extrudate was processed by hot pressing at 75 ◦C. TPE electrolyte formulation is given in Table 1.

**Table 1.** Main features of the investigated solid electrolytes. PYR14TFSI—1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide; LiTFSI—lithium bis(trifluoromethanesulfonyl) imide; PEO—poly (ethylene oxide); TPGS-S—surface modified sepiolite; DSC—differential scanning calorimetry; TPE—thermoplastic polymer electrolyte.


## *2.3. Physicochemical Characterization*

Characterization of electrolytes was done on films of controlled thickness processed by compression molding at 75 ◦C during 3 min.

Scanning electron microscopy (SEM) was performed with a Hitachi SU-8000 (Hitachi Ltd., Tokyo, Japan). Samples were fractured after immersion in liquid nitrogen and the sections were observed unmetalized.

Differential scanning calorimetry (DSC) studies were performed in a TA Instruments Q100 (TA Instruments, New Castle, DE, USA). The heat flow was recorded as follows: two cooling-heating cycles at 10 ◦C·min−<sup>1</sup> from 120 ◦C to −80 ◦C, followed by a second cooling-heating cycle from 120 ◦C to −80 ◦C at 20 ◦C·min−1. DSC data included in Table 1 were obtained from the second DSC heating trace at 10 ◦C·min−1. The crystallinity percentage ( χc) was determined considering 100% crystalline PEO heat of melting as Δ *H* m = 197 J·g<sup>−</sup><sup>1</sup> [31]. The % χc in Table 1 is referred to the weight of the electrolyte and not to the weight fraction of PEO.

Thermogravimetric analysis (TGA) was performed in a TA Q-500 in nitrogen atmosphere at 10 ◦C·min−<sup>1</sup> up to 800 ◦C.

Determination of diffusion coefficients ( *D*) was done by 7Li and 19F pulsed field gradient-NMR (PFG-NMR) in a Bruker AvanceTM 400 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) as reported before [26]. The lithium transference number measured by NMR (*t*NMR Li<sup>+</sup> ) was calculated

using Equation (1). It was not possible to measure *D* of the cation ( *D*Pyr), because of the overlapping with PEO protons, so it was estimated to be about 10% lower than TFSI, according to bibliographic data [32].

$$t\_{\rm Li^{+}}^{\rm NMR} = \frac{D\_{\rm Li^{\rm CLi}}}{D\_{\rm Li^{\rm CLi}} + D\_{\rm TFSI^{\rm TFSI}} + D\_{\rm Pyr}c\_{\rm Pyr}} \tag{1}$$

Creep experiments were done as follows: electrolyte films of about 500 μm were sandwiched between two gold electrodes of 20 mm of diameter, and placed on a heating plate with a 0.5 kg load on top and kept 20 min at 70 ◦C, followed by 20 min at 90 ◦C.

## *2.4. Electrochemical Characterization*

The ionic conductivity of the TPE and PEO-LiTFSI electrolytes was determined by electrochemical impedance in a NOVOCONTROL GmbH Concept 40 broadband dielectric spectrometer (Novocontrol Technologies GmbH, Montabaur, Germany) in the temperature range of 50 ◦C to 90 ◦C and in the frequency range between 0.1 Hz and 10<sup>7</sup> Hz. Disk films of dimensions of 2 cm diameter and ~500 μm thickness were inserted between two gold-plated flat electrodes, then a frequency sweep was done every 10 ◦C, cooling to −50 ◦C and then heating to 90 ◦C; thereafter, the same measurements were done but cooling from 85 ◦C to 25 ◦C. Ionic conductivity was calculated by using conventional methods based on the Nyquist diagram and the phase angle as a function of the frequency plot. The values that appear in Table 1 correspond to the second heating measurement.

Lithium transference number (*<sup>t</sup>*Li<sup>+</sup> ) of the TPE was obtained at 60 ◦C by combined alternating current (AC) impedance and direct current (DC) polarization measurements using a Solartron Analytical 1400 CellTest System (cell test, City, UK) coupled with frequency response analyzer 1455 (Ametek) of a symmetrical solid state Li/TPE/Li coin cell (2025, Hohsen, City, Japan). Coin cells were prepared using high-purity lithium metal foil (Albermale, Charlotte, NC, USA) with thickness of 50 μm. Before the measurement, the assembled coin cells were kept at 60 ◦C overnight to achieve a good contact and stable interface between the solid electrolyte and lithium metal electrodes. Successively, a DC potential ( Δ *V* = 5 mV) was applied until a steady current was obtained; then, initial (*I*o, after 5 milliseconds) and steady state (*I*ss) currents that flow through the cell were measured. Impedance spectra were recorded (from 1 MHz to 1 Hz) with 10 mV sinusoidal amplitude before and after DC polarization. Subsequently, initial ( *R*o) and final ( *R*ss) bulk resistances of the electrolyte, and initial ( *R*Co) and final ( *R*Css) charge transfer resistances ( Ω) of the interfacial layers Li metal electrode/electrolyte were derived from electrochemical impedance spectra using ZView software 3.5 (Scribner, Southern Pines, NC, USA) Using these measured values, *<sup>t</sup>*Li<sup>+</sup> was calculated by the following Equation (2) [33,34].

$$t\_{\rm Li^{+}} = \frac{I\_{\rm ss} \cdot R\_{\rm ss} \cdot \left(\Delta V - I\_{\rm o} \cdot R\_{\rm Co}\right)}{I\_{\rm o} \cdot R\_{\rm o} \cdot \left(\Delta V - I\_{\rm ss} \cdot R\_{\rm Cgs}\right)}\tag{2}$$

The electrochemical stability window of the TPE was evaluated in three-electrode cells using a Solartron Analytical 1400 CellTest System (Ametek) coupled with a frequency response analyzer 1455 (Ametek). To do so, a solid-state three electrode cell (HS-3E, Hohsen), using stainless steel as a working electrode, a lithium metal (50 μm) disc as a counter electrode, a lithium metal ring as a reference electrode, and a solid electrolyte membrane (80–100 μm) placed between electrodes was fabricated. The cyclic voltammetry (CV) test was carried out at a linear scan rate of 1 mV·s<sup>−</sup><sup>1</sup> to determine the electrochemical performance in cathodic (from OCV to −0.5 V) range. The oxidation stability of the investigated solid electrolyte was determined by linear sweep voltammetry (LSV) from OCV to 6 V at a scan rate of 1 mV·s<sup>−</sup>1. Both CV and LSV experiments were performed at 60 ◦C using different TPE samples.

Galvanostatic stripping-plating studies were carried out at 60 ◦C in a symmetrical Li/TPE/Li coin cell (2025, Hohsen), using two lithium metal discs (Albermale, high-purity foil, 50 μm) and TPE films (80–100 μm) placed in between. The measurements were performed with the help of BaSyTec cell test system (BaSyTec, Asselfingen, Germany) at 60 ◦C. Galvanostatic cycles were run by applying symmetrical 1 mA·cm<sup>−</sup><sup>2</sup> current for 2 h with depth of cycling of 2 mAh·cm<sup>−</sup>2.

Galvanostatic charge-discharge test in solid-state coin cells with lithium metal anode (Albermale, high-purity foil, 50 μm) and composite LiFePO4 (LFP) cathode was performed at 60 ◦C using the BaSyTec cell test system. The cathode consisted of micro-scale carbon coated LFP material (D50: 2–4 μm), PEO-LiTFSI solid electrolyte (EO/Li~20) as ionic conductive binder, and carbon black as a conductive additive. Superficial capacity of the prepared positive electrode was 0.5 mAh·cm<sup>−</sup>2. A carbon coated aluminum current collector was used to enhance interfacial resistance and avoid aluminum corrosion in the presence of TFSI anions. Solid-state coin cells were assembled in a dry room with dew point below −50 ◦C. Once assembled, the cells were kept for 3 h at 60 ◦C and then cycled within the 2.5–3.8 V range at the same temperature using BaSyTec cell test system. It is important to note that cell design, assembly, and formation procedures were not optimized in this study.
