*3.2. Composite Electrodes*

The LiFePO4 electrode formulation was optimized in terms of carbon content in order to reach a good compromise between electronic conductor content and cathode performance. Therefore, electrode samples containing different carbon contents were prepared and investigated in terms of their electronic conductivity by impedance spectroscopy. The results are reported in Figure 5 as AC responses (panel a) and electronic conductivity vs. carbon content dependence (panel b). The impedance plot of the carbon-free sample (Figure 5a) is constituted by a semicircle (not starting from the axis origin) which does not display any capacitive contribution, indicating charge transfer at the interfaces with the Al◦ collectors [38]. This behavior—i.e., supporting electron conduction through the composite electrode—suggests the establishment of a three-dimensional network (percolation) formed by LiFePO4 particles and, therefore, electronic continuous pathways through the composite cathode [37]. It should be noted that the as-received active material is provided as superficially carbon-coated; this supports the not-very-low electronic resistance (given by the AC plot intercept with the real axis at low frequencies [38]) of the composite cathode (i.e., pure LiFePO4 material exhibits very low electronic conductivity [47]). The addition of KJB carbon around 3–4 wt. % results in a remarkable reduction of the semicircle diameter and a shifting of the low frequency intercept with the real axis towards smaller impedance values, highlighting a decrease of the electronic resistance of the cathode. At a KJB content equal to 6 wt. %, the semicircle practically reduces to a quasi-single point on the real axis, indicating that the electronic conductivity is largely overcome with respect to the ionic one (the electron and ion conductions through the polymer electrolyte are in parallel) of the polymer electrolyte incorporated within the electrode. In such a condition, the electronic resistance of the composite cathode is given by the distance of the "spot" response intercept with the real axis from the origin of the axes [38].

Figure 5b illustrates the electronic conductivity of the composite LiFePO4 cathode as a function of the carbon content. As evinced in Figure 5a, the electron conduction raises up to 7 wt. % of KJB with a gain of about 1.5 orders of magnitude. The further addition of carbon does not lead to any improvement of the electron transport properties, whereas it depletes the active material content and, therefore, the energy density of the composite cathode. Therefore, the KJB content in the LiFePO4 electrode was fixed to 7 wt. %.

**Figure 5.** Panel (**a**): impedance plots of Al/LiFePO4 composite cathode/Al symmetrical cells at different carbon contents. Frequency range: 10 kHz–1 Hz. Temperature: 20 ◦C. Panel (**b**): electronic conductivity of LiFePO4 composite cathode as a function of the carbon content. Temperature: 20 ◦C.

#### *3.3. Battery Tests at 80* ◦*C*

Upon investigation of the electrochemical performance, the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 ionic liquid-based, polymer electrolyte was subjected to tests in Li/LiFePO4 cells at 80 ◦C. Figure 6a compares the voltage vs. capacity profile referring to the 1st charge–discharge cycle run at different current rates. A flat plateau, typical of the Li+ insertion/de-insertion process into the LiFePO4 active material [24,33,34], is observed (in the 3.3–3.6 V range) even at higher rates, with a coulombic efficiency close to 99%. This highlights that IL-incorporating Li/LiFePO4 cells are capable of maintaining the same voltage during almost the entire charge/discharge step. Only a 100 mV increase in ohmic drop is observed on passing from 0.1C through 1C. An initial capacity corresponding to the theoretical value (170 mA·h·g<sup>−</sup>1) is delivered up to the medium rate (0.33C) with just a moderate decrease at high current rates, i.e., more than 160 mA·h·g<sup>−</sup><sup>1</sup> (>94.1% of the theoretical capacity) are discharged at 1C. Figure 6b,c compares the voltage profiles of the selected charge/discharge cycles at 0.1C and 1C, respectively. It is worth noting that the excellent reproducibility of the battery performance, i.e., the profile feature and the delivered capacity, are practically unchanged after 100 consecutive cycles run (at 100% of deep of discharge, DOD) even at high current rates, which is not often reported for lab-scale, lithium metal polymer cells [24]. These results clearly show the very good reversibility of the Li+ intercalation process even under hard operating conditions in combination with an excellent compatibility at the electrolyte/electrode interface and negligible degradation phenomena occurring within the cell components. Such a performance score, however, can be achieved only through good

manufacturing of the electrolyte/electrode components, i.e., high purity levels and careful optimization of the formulation, and of the full cells.

**Figure 6.** Panel (**a**): voltage vs. charge/discharge capacity profile of the 1st cycle of Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/LiFePO4 polymeric cells at 80 ◦C and different current rates. Selected voltage vs. charge/discharge capacity profiles, obtained at 80 ◦C, of Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/LiFePO4 cells at 0.1C (panel **b**) and 1C (panel **c**), respectively.

The cycling performance of the Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/LiFePO4 solid-state cells, tested at 80 ◦C and different current rates, is depicted in Figure 7a. An excellent capacity retention (as also evinced in Figure 6b,c) with a coulombic efficiency quickly leveling above 99.5% (100% at 0.1C) is recorded even at higher rates, i.e., more than 99.5% and 94% of theoretical capacity are initially delivered at 0.33C and 1C, respectively, with a very modest decay (>98% and 93.6%, respectively) after 100 consecutive cycles. This corresponds to a capacity fading around 0.005% per cycle and, in conjunction with the very good charge/discharge efficiency, once more highlights a highly reversible lithiation process in combination with the high purity level and high compatibility of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 polymer electrolyte towards electrodes, in particular with the lithium metal anode, even at high current rates. Also, it should be noted that very clean lithium metal tapes were used for the cell manufacturing in order to obtain an optimal Li/electrolyte interface. Especially, we would like to point out the absence of dendrite growth on the Li electrode during prolonged cycling tests run also at 1C, i.e., very rarely encountered in lithium metal polymer batteries operating at medium-high temperatures under high rates [24]. These experimental data, in rather good agreemen<sup>t</sup> with the results derived from potentiodynamic measurements depicted in Figure 4, once more demonstrate that the incorporation of ionic liquids such as PYR14TFSI significantly improves the PEO electrolyte interface with the lithium anode, allowing high current rates to be sustained for prolonged cycling tests without appreciably depleting the cell performance.

**Figure 7.** Capacity and coulombic efficiency vs. cycle number evolution at different current rates (panel **a**); and theoretical capacity vs. current density dependence (panel **b**) of Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/ LiFePO4 polymeric cells at 80 ◦C. The corresponding current rates are also reported.

The capacity vs. current density dependence (80 ◦C) is plotted in Figure 7b, which evinces a very good rate capability. Above 94% of the theoretical value is still obtained at 1C, supporting an excellent rate capability up to 1C, i.e., corresponding to about 0.7 mA·cm<sup>−</sup>2, which represents a very interesting current value for an all-solid-state polymer electrolyte. A further increase of the current rate up to 2C, i.e., around 1.4 mA·cm<sup>−</sup>2, leads to a reduction of the delivered capacity which levels off at 57% of the theoretical value. This behavior, ascribable to diffusive phenomena within the electrolyte separator, is in good agreemen<sup>t</sup> with the results obtained by the potentiodynamic measurements (Figure 3b), which indicates that above a current density of about 1.2 mA·cm<sup>−</sup><sup>2</sup> (determined as JL value), the electrochemical processes through the cell are controlled by the diffusive phenomena occurring within the polymer electrolyte. However, despite the capacity decay due to the operating current density exceeding the limiting value, the Li/P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1/LiFePO4 cells are still able to deliver about 100 mA·h·g<sup>−</sup><sup>1</sup> at a rate as high as 2C (about 1.4 mA·cm<sup>−</sup>2), i.e., representing a remarkable capacity value for an all-solid-state polymer electrolyte.

The battery performance of the P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 electrolyte, detected at 80 ◦C in Li/LiFePO4 cells, is compared with that of other lithium-conducting, ionic liquid-free, PEO membranes, recorded in Li/LiFePO4 and Li/V2O5 systems at temperatures from 90 ◦C to 100 ◦C [18,22,23]. The data, reported in Table 2, show how appreciable capacities, i.e., from 70 to 96% of the cell theoretical value, are delivered only at low-medium rates (0.2C–0.33C). However, a capacity decay down to 45–60% of the theoretical value is observed after 100 consecutive charge/discharge cycles, with a fading corresponding to 0.26–0.36% per cycle. Conversely, very modest capacities, i.e., from 8 to 14% of the theoretical value, are obtained when the current rate is increased up to 0.8C–1C. From the data illustrated in Figures 6 and 7 and Table 2, it is evident how, at medium-high temperatures, the PYR14TFSI-incorporating lithium polymer batteries behave much better in terms of their delivered capacity and cycling performance than the IL-free ones. For instance, the addition of suitable ionic liquid is able to largely improve the performance of the LPBs not only at ambient or near ambient conditions, as previously reported in the literature [18,20,22,23], but even at medium-high temperatures. Therefore, the PEO-LiTFSI-PYR14TFSI Li+-conducting membranes are very promising candidates as electrolyte separator systems for all-solid-state lithium polymer batteries operating around 100 ◦C.

**Table 2.** Summary of the battery performance of the P(EO)1(LiTFSI)0.1(PYR14TFSI)1 polymer electrolyte at 80 ◦C compared with that of lithium-conducting, ionic liquid-free, PEO membranes at medium-high temperatures. (a) From reference [22]; (b) from reference [23]; (c) from reference [18]; (d) this work.

