*2.3. Thermal Analysis*

DSC measurements were run using a differential scanning calorimeter (TA Instruments, model Q100, New Castle, DE, USA). The samples, upon housing (within the dry room) in sealed Al pans, were cooled (10 ◦C·min−1) from room temperature down to −140 ◦C and then heated (10 ◦C·min−1) up to 150 ◦C.

The thermal stability was verified in a nitrogen atmosphere through TG analysis carried out by a SDT 2960 equipment, simultaneous TG-DTA (TA Instruments, New Castle, DE, USA) with Thermal Solution Software (version 1.4, Thermal Solutions Inc, Ann Arbor, MI, USA). During the experiments, the atmosphere above the samples was fixed by flowing high purity nitrogen atmosphere at a flow rate of 100 mL·min−1. The experiments were performed on 5–10 mg samples (handled in the dry room), which were housed in platinum crucibles. The thermal stability was initially investigated by running a heating scan from room temperature up to 500 ◦C at a scan rate of 10 ◦C·min−1.

## *2.4. Cell Assembly*

The electrochemical measurements on the polymer electrolyte samples were carried out on two-electrode cells fabricated in the dry room. Two different cell types (active area equal from 2 to 3 cm2) were assembled by sandwiching a polymer electrolyte separator between (i) two Li foil electrodes (50 μm thick, supported onto Cu grids as the current collectors) for determining, respectively, the resistance at the interface with the lithium anode and the limiting diffusion current density; (ii) a nickel foil (working electrode, 100 μm thick, used also as the current collector) and a lithium foil (counter electrode, 50 μm thick, supported onto a Cu grid as the current collector) for the linear sweep voltammetry tests. In the latter kind of cell, a tiny lithium strip (50 μm thick, supported onto a Ni grid as the current collector) was used as the reference electrode.

The electronic conductivity of the ionic liquid-containing LiFePO4 composite cathode was investigated as a function of the carbon content by carrying out impedance measurements on symmetrical Al/cathode/Al cells. The composite cathode tape was interlayered between two Al foils (20 μm thick), which were also used as the current collectors.

The solid-state Li/LiFePO4 batteries (cathode limited) were fabricated (inside the dry room) by laminating a lithium foil (50 μm thick), a P(EO)10(LiTFSI)0.1(PYR14TFSI)0.1 polymer electrolyte separator and a LiFePO4-based composite cathode tape (plated onto a 20 μm thick Al foil). Aluminum and copper grids were used as the cathodic and anodic current collector, respectively. The electrochemically active area of the Li/LiFePO4 cells was 1.13 cm2.

All assembled cells were housed in soft envelopes, evacuated for at least 1 h (10−<sup>2</sup> mbar) and then vacuum-sealed. Finally, the cells were laminated twice by hot-rolling at 100 ◦C to improve the electrolyte/electrode interfacial contact.

## *2.5. Electrochemical Tests*

Impedance measurements were performed on symmetrical Li/polymer electrolyte/Li (frequency range: 65 kHz–10 mHz; temperature range: 20–80 ◦C) and Al/composite cathode/Al (10 kHz–1 Hz, 20 ◦C) cells by a Frequency Response Analyzer, F.R.A. (Schlumberger Solartron, mod. 1260, Leicester, UK). The analysis of the AC responses was carried out by an equivalent circuit model taking into account all possible contributes to the impedance of the cell under test [38]. The validity of the selected circuit was confirmed by fitting the AC responses using a non-linear least-square (NLLSQ) software developed by Boukamp [39,40] (only fits characterized by a χ2 factor lower than 10−<sup>4</sup> were considerable acceptable [39,40]).

The electrochemical stability window (ESW) of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 polymer electrolyte was evaluated by linear sweep voltammetries (LSVs) run at 0.5 mV·s<sup>−</sup><sup>1</sup> in the 20–80 ◦C temperature range. The measurements were performed by scanning the cell potential from the open circuit value (OCV) towards more negative or positive potentials to determine the cathodic and anodic electrochemical stability limits, respectively. The LSVs were performed at least twice on each electrolyte to confirm the results obtained, using fresh samples and clean electrodes for each test. The measurements were performed at 20 ◦C using an Electrochemical Interface (Schlumberger Solartron, mod. 1287, Leicester, UK).

The limiting diffusion current density of the P(EO)1(LiTFSI)0.1 and P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 polymer electrolytes was determined by potentiodynamic measurements on symmetrical Li/electrolyte/Li cells, i.e., the cell voltage was linearly increased from the OCV value (a few mV) at a scan rate of 0.01 mV·s<sup>−</sup><sup>1</sup> until the current response achieves a steady state. The measurements were performed at temperatures ranging from 40 to 80 ◦C by a potentiostat/galvanostat (MACCOR, mod. 4000, Tulsa, OK, USA).

The cycling performance of the Li/LiFePO4 polymer cells was evaluated under charge/discharge rates ranging from 0.1C (*j* = 0.07–0.08 mA·cm<sup>−</sup>2) to 1C (*j* = 0.7–0.8 mA·cm<sup>−</sup>2) at 80 ◦C. The battery tests were performed using a multiple battery tester (MACCOR, mod. S4000, Tulsa, OK, USA). The voltage cut-offs were fixed at 4.0 V (charge step) and 2.0 V (discharge step), respectively. During the experiments, the cells were held in a climatic chamber (Binder GmbH, mod. MK53, Tuttlingen, Germany) with a temperature control of ±0.1 ◦C.

#### **3. Results and Discussion**

#### *3.1. Ionic Liquid-Based Polymer Electrolytes*

The solvent-free procedure allowed homogeneous, freestanding, polymer electrolyte membranes with good mechanical properties to be obtained. In addition, the ionic liquid-containing P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 sample looks rather sticky, thus resulting (even if not easily handled) in improved contact at the interface with electrodes.

The results of the DSC investigation are illustrated in Figure 1a. The P(EO)1(LiTFSI)0.1 electrolyte shows a broad endothermic melting peak centered around 60 ◦C [21,41] and a weak glass transition (Tg) feature located at −39 ◦C. The pure PYR14TFSI ionic liquid, reported for comparison purposes, exhibits only a melting peak around −7 ◦C [42]; i.e., the absence of glass transition and exothermal "cold crystallization" features sugges<sup>t</sup> that the IL sample was fully crystallized prior to running the DSC measurements [43]. The incorporation of PYR14TFSI into the P(EO)1(LiTFSI)0.1 electrolyte results in almost complete disappearance of the melting peak in the DSC trace, which displays only the

Tg feature around −55 ◦C, clearly indicating that the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 electrolyte is amorphous even at room temperature.

The thermal stability is a mandatory requirement for electrolytes to be addressed to battery systems for medium-high temperature applications. Figure 1b compares the TGA trace (in nitrogen atmosphere) of the P(EO)1(LiTFSI)0.1 and P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 electrolyte membranes. The IL-free sample exhibits a weight loss above 180 ◦C, whereas the addition of the ionic liquid component results in thermal stability increase up to 220 ◦C. It should be noted that PYR14TFSI is seen to be thermally stable up 290 ◦C. Therefore, we can reasonably hypothesize that the ionic liquid, properly incorporated within the polymer host, is able to protect the PEO chains by thermal degradation. Something similar was previously observed in other PEO electrolytes [41], in which the IL agent, suitably dispersed through the polymeric matrix, was seen to prevent the oxidation of the polymer host above 4 V (vs. Li+/Li◦).

**Figure 1.** DSC (panel **a**) and TGA (panel **b**) traces of P(EO)10(LiTFSI)1 and P(EO)10(LiTFSI)1(PYR14TFSI)1 polymer electrolyte samples. Scan rate: 10 ◦C·min−1. The PYR14TFSI ionic liquid is reported for comparison purposes.

The effect of the incorporation of the PYR14TFSI ionic liquid on the ion transport properties of the polymer electrolyte is summarized in Table 1. A remarkable conductivity increase is observed, especially at ambient temperature and below. For instance, the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 sample shows ion conduction values three and two orders of magnitude higher than that of the IL-free sample at −20 ◦C and 20 ◦C [31,33], respectively. More than 10−<sup>4</sup> S·cm<sup>−</sup><sup>1</sup> are exhibited at 20 ◦C, this is of interest for applications in practical devices and commonly not approached in polymer electrolyte membranes. These results support faster ion conduction through the PEO electrolyte due both to a much larger content of the amorphous phase, in agreemen<sup>t</sup> with the DSC data of Figure 1a, and to the enhanced mobility of the Li+ cations resulting from the presence of PYR14TFSI; i.e., the addition of ionic liquid results in large anion excess with respect to the lithium cations. Therefore, the strength of the Li+··· Anion− interaction reduces the role of the PEO chains in the coordination of the lithium cations, e.g., as a result from the competition with the PEO··· Li+ interactions [24]. At medium-high temperatures, the conductivity of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 electrolyte is seen to approach or exceed 10−<sup>3</sup> <sup>S</sup>·cm<sup>−</sup>1, still displaying a substantial rise with respect to that of the binary IL-free P(EO)1(LiTFSI)0.1 [31,33].



An important requirement for any electrolyte is its capacity to successfully and efficiently allow electrode reactions, at the operating temperature of the device, without appreciable electrochemical degradation (oxidation/reduction) phenomena. Therefore, the electrochemical stability window (ESW) of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 electrolyte system was investigated as a function of the temperature. The results, reported in Figure 2 as linear sweep voltammetry curves, evince only a moderate, even if progressive, reduction of the ESW on passing from 20 to 80 ◦C. In particular, the anodic stability (related to oxidation processes of the electrolyte) detected at 80 ◦C differs by just 200 mV with respect to that recorded at 20 ◦C. Conversely, no practical variation is observed on the cathodic side with the temperature increase, displaying massive electrolyte reduction well below 0 V vs. Li+/Li◦, which allows lithium plating also at 80 ◦C. A very low current flow (<25 <sup>μ</sup>A·cm<sup>−</sup>2) is observed up to the anodic breakdown voltage, thus supporting the high purity of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 sample. On the cathodic verse, three weak ( ≤20 <sup>μ</sup>A·cm<sup>−</sup>2) features, progressively evinced with the temperature increase, are observed around 1.5 V, 0.9 V and 0.5 V vs. Li+/Li◦, respectively. Results previously reported in the literature [44] sugges<sup>t</sup> that the peaks located at 1.5 V and 0.5 V vs. Li+/Li◦ are ascribable to the Li+ cation intercalation process into the native NixO film onto the nickel working electrode surface, whereas the feature at 0.9 V is likely due to impurities, i.e., probably water [45]. To summarize, the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 electrolyte is allowed to successfully operate at medium-high temperatures.

**Figure 2.** Electrochemical stability window of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 polymer electrolyte sample at different operating temperatures. Nickel is used as the working electrode, lithium as counter and reference electrodes. Scan rate: 0.5 mV·s<sup>−</sup>1.

The compatibility with the lithium anode is a key parameter for applications as electrolyte separators in Li metal polymer batteries. Figure 3 compares the impedance plots of Li/P(EO)1(LiTFSI)0.1/Li and Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/Li cells obtained at different temperatures. The AC responses are constituted by a semicircle, taking into account the overall Li/polymer electrolyte interfacial resistance (i.e., charge transfer + passive layer) [38], whereas the high frequency intercept with the real axis is associated with that of the electrolyte bulk [38]. It should be noted that, at 20 ◦C (panel a), the IL-free electrolyte shows a partial semicircle at high-medium frequencies, due to the relatively low conductivity of the sample P(EO)1(LiTFSI)0.1 [31]. Finally, the inclined straight-line, observed at low frequencies, is attributed to diffusive phenomena through the electrolyte (Warburg contribution) [38]. The impedance plots of Figure 3 clearly confirm how the incorporation of ionic liquid results in a significant decrease of the electrolyte resistance, especially from room to medium temperature, in agreemen<sup>t</sup> with the conductivity data reported in Table 1. However, a gain, even if moderate, in interface resistance is also detected. For instance, the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 sample shows, at the interface with Li metal, a resistance of 10–11% lower (i.e., from 830 to 750 cm<sup>2</sup> at 20 ◦C and from 7.0 to 6.3 cm<sup>2</sup> at 80 ◦C) than that of the IL-free electrolyte (Table 1), in the whole investigated temperature range (20–80 ◦C). We can hypothesize that the ionic liquid improves the Li+ cation mobility at the electrolyte/lithium interface.

**Figure 3.** AC response of Li/P(EO)1(LiTFSI)0.1/Li and Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/Li symmetrical cells at 20 ◦C (panel **a**), 50 ◦C (panel **b**) and 80 ◦C (panel **c**).

Applications such as in automotives, smart grids, etc. require high power and for energy to be readily available; this means that this requires the battery system to be feasibly discharged and charged at high current rates without significantly depleting its performance. For instance, the increase of the current rate promotes the diffusive phenomena within the battery, thus lowering the content of the stored/delivered energy. In electrochemical cells, the redox process kinetics are generally much faster than the active species diffusion through the electrolyte separator. By increasing the current value, the matter transferring process becomes more and more predominant with respect to those at the interfaces with the electrodes. When the current flow through the cell achieves a limiting value, JL (diffusion limiting current), the electrochemical processes are fully governed by the ion diffusion from the electrolyte bulk to the electrode surface and vice versa. Therefore, JL is a key parameter for evaluating the feasibility of an electrolyte at high current rates. The limiting current value was determined as reported in Materials and Methods. For instance, linear sweep voltammetry tests were run (at 0.01 mV·s<sup>−</sup>1) on symmetrical Li/P(EO)1(LiTFSI)0.1/Li and Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/Li cells at temperatures ranging from 40 to 80 ◦C. Figure 4 plots the current density values, recorded during the potentiodynamic measurements, as a function of the cell overvoltage. After an initial step increase, in which the electrolyte membrane shows a quasi-ohmic behavior, the current density is seen to progressively level off, likely associated with the establishment of a concentration gradient within the electrolyte membrane [46], around a time-stable value. Such a behavior indicates that the current density through the cell has reached the limiting value (JL), e.g., the ion conduction processes inside the electrolyte membrane are governed by diffusion phenomena (the concentration gradient extends through the overall electrolyte thickness). In Figure 4, it is shown how the JL value remarkably increases with the operating cell temperature but is not affected by the presence of PYR14TFSI, i.e., from 0.13–017 to 1.2–2.0 mA·cm<sup>−</sup><sup>2</sup> (about one order of magnitude) in passing from 40 to 80 ◦C for both the IL-free (panel a) and the IL-containing (panel b) electrolyte. Therefore, the ionic liquid does not seem to reduce the diffusive phenomena through the PEO electrolyte. However, the current density of the P(EO)1(LiTFSI)0.1 sample, upon achieving the limiting value, quickly shows an abrupt feature during the potentiodynamic measurements at 60 ◦C and 80 ◦C (Figure 4a). This behavior, repeatedly confirmed by several (potentiodynamic) tests carried out on different Li/P(EO)1(LiTFSI)0.1/Li cells and never observed in the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 sample, is ascribable to dendrite growth onto the Li electrode at current rates above 1 mA·cm<sup>−</sup>2. The results reported in Figure 4a sugges<sup>t</sup> that the IL-free electrolyte is not able to sustain high current rates. Conversely, the ionic liquid plays a key role in improving the compatibility at the interface with the lithium anode, in particular when the cell is subjected to high current rates instead of in an open circuit condition as plotted in Figure 3. It is a plausible hypothesis that PYR14TFSI behaves as a protective agen<sup>t</sup> towards the Li metal electrode, allowing the running of charge/discharge cycling tests at a high current density without appreciable degradation phenomena of the lithium anode. Once more, this confirms the beneficial effect resulting from ionic liquid incorporation on battery performance.

**Figure 4.** Current density vs. overvoltage curves obtained from potentiodynamic measurements carried out on Li/P(EO)1(LiTFSI)0.1/Li (panel **a**) and Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/Li (panel **b**) cells at different temperatures.
