*3.2. Electrochemical Investigation*

Figure 2 shows ionic conductivity (σ) values of the TPE and PEO-LiTFSI on heating from −50 ◦C to 90 ◦C. Ionic conductivity data of PYR14TFSI [35] shown in Figure 2 demonstrates its higher conductivity comparison with both solid electrolytes. The σ of TPE increases up to values close to 10−<sup>2</sup> <sup>S</sup>·cm<sup>−</sup>1, and likewise decreases on going from 90 ◦C to 25 ◦C, where it attains a value of 5 × 10−<sup>4</sup> S·cm<sup>−</sup>1. As a consequence of the very low fraction of TPE suffering phase transitions in the temperature range studied, the heating and the cooling cycle measurements produce the same σ values, and so also with regards to σ variation with temperature, the TPE can be considered as a liquid. On the contrary, PEO-LiTFSI suffers the melting of the crystalline phase at about 50 ◦C on heating, and on cooling, an abrupt decrease of σ is seen below 50 ◦C, caused by the crystallization of PEO. Hence, the cooling and heating scans do not coincide in the vicinity of the phase transition. As a consequence, under 50 ◦C, the difference in σ between the TPE and PEO-LiTFSI becomes progressively higher.

**Figure 2.** Ionic conductivity of the TPE and PEO-LiTFSI on heating and cooling scans. Ionic conductivity of 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) data reported by Martinelli et al [35] is given for comparison.

The *<sup>t</sup>*Li<sup>+</sup> is a very important characteristic of an electrolyte. A higher *<sup>t</sup>*Li<sup>+</sup> can reduce concentration polarization during charge/discharge steps and, consequently, can increase power density. Moreover, it can hinder Li metal dendrite growth and avoid decomposition and precipitation of the lithium salt. Figure 3a depicts the chronoamperometry of the symmetrical Li-Li coin cell with the investigated TPE. The AC impedance spectra before and after polarization of the cell are exhibited in Figure 3b. The equivalent circuit used for the determination of *R*o, *R*ss, *R*Co and *R*Css values is shown as an inset in Figure 3b.

**Figure 3.** (**a**) Chronoamperometry of the Li/TPE/Li cell; (**b**) the alternative current (AC) impedance spectra before and after polarization. Inset (**b**): the equivalent circuit used for the fitting of the spectra, *R*1 corresponds to *R*o, *R*ss is the sum of *R*2 and *R*3 to *R*Co and *R*Css. Test was performed at 60 ◦C.

Lithium ion transference number of the investigated TPE with EO/Li ratio 12, as obtained from Equation (2), is 0.08 ± 0.01 at 60 ◦C, which is lower than the value obtained for PEO-LiTFSI (EO/Li ~20), which is 0.25 ± 0.01. Such a difference is the direct consequence of the lower molar fraction of Li cations in the TPE (*χ*Li<sup>+</sup> = 0.17) than in PEO-LiTFSI (*χ*Li<sup>+</sup> = 0.5) or, in other words, while in the reference electrolyte, 50% of the ionic species are Li cations, the amount of Li cations in the TPE is only the 17% of the ionic species. A relatively low transport number (*<sup>t</sup>*Li<sup>+</sup> ) of TPE does not mean lower lithium ion conductivity (<sup>σ</sup>Li<sup>+</sup> ) in comparison with PEO-LiTFSI. In particular, as can be seen in Table 2, TPE has 1.9 times higher Li-ion conductivity compared with the reference at 60 ◦C. Moreover, we anticipate that the difference will increase over an order of magnitude at temperature below 60 ◦C, when the reference electrolyte will be partly crystalline because of PEO, while the TPE will remain amorphous.


**Table 2.** Ionic conductivity values of the investigated solid electrolytes.

Note: a *t*+ calculated by combined alternative current (AC) impedance and direct current (DC) polarization measurements reported above.

The electrochemical stability of the electrolyte is a fundamental property that determines the electrochemical behavior of the whole solid state battery. Figure 4 shows the electrochemical stability of the TPE under study towards anodic oxidation and cathodic reduction reactions. From the cathodic profile, reversible lithium plating and stripping processes are well evidenced. On the other hand, anodic LSV scab showed that the investigated electrolyte is anodically stable up to 4.2 V, which is a typical value for PEO based solid electrolytes.

Lithium metal electrode at contact with unappropriated solid electrolyte may show quite poor electrochemical behaviour (low coulombic efficiency, poor reversibility, and even lithium dendrite growth) due to cycling conditions (temperature, current density, depth of cycling) and properties of solid electrolyte layer (SEI) formed at lithium/solid electrolyte interface (nature, thickness, resistance etc.). Therefore, in this work, the compatibility of the TPE with the Li metal anode was evaluated by a galvanostatic stripping-plating test performed in Li-Li symmetrical coin cell with cycling conditions (temperature, current density, and depth of cycling) similar with full solid state cell application. Striping-plating curves for several separated cycles shown in Figure 5 with the aim of highlighting their evolution during the test. The TPE demonstrated quite low polarization and long term cyclability during more than 400 cycles (>1600 h) under relatively harsh cycling conditions. This result demonstrates that this solid electrolyte is well compatible with lithium metal anode that is necessary requirement for its further application in lithium metal solid state batteries.

**Figure 4.** Cyclic voltammetry (CV) (blue) and linear sweep voltammetry (LSV) (red) curves of the TPE measured at 60 ◦C.

**Figure 5.** Cell voltage versus test time of lithium striping-plating in a symmetrical coin cell Li/TPE/Li measured on 1st, 50th, 100th, and 400th cycles. Cycling conditions: 60 ◦C, current density: 1 mA·cm<sup>−</sup>2, duration of each step 2 h.

Finally, the TPE was tested at 60 ◦C in all-solid-state coin cell with lithium metal anode and composite LiFePO4 cathode. Figure 6a presents charge-discharge curves of solid state coin cells with PEO-LiTFSI and TPE solid electrolytes. On the first cycle, polarization of the cell based on TPE is slightly higher in comparison with reference PEO-LiTFSI. However, during following cycles, charge-discharge profiles of both cells became quite similar.

The cycling performance of the solid state cell with TPE and PEO-LiTFSI is shown in Figure 6b. As it can be observed, upon cycling, solid state coin cell with TPE demonstrated more stable and higher coulombic efficiency (Figure 6a,c) and, as a result, much better electrochemical performance compared with the cell based on the reference PEO-LiTFSI compound.

**Figure 6.** Electrochemical performance of solid-state coin cells Li-LiFePO4: (**a**) charge-discharge curves; (**b**) specific discharge capacity; and (**c**) coulombic efficiency versus cycle number. Cycling conditions: 60 ◦C, Constant Current-Constant Voltage (CCCV) charge at 0.2 C (charge current cut off 0.1 C), discharge at 0.5 C, cycling interval 2.5–3.8 V, positive electrode loading 0.5 mAh·cm<sup>−</sup>2.

It should be noted that relatively fast capacity decay of the nonoptimized solid-state coin cell prototype with TPE electrolyte could be related to several reasons, such as solid electrolyte impurities; traces of water in the electrolyte and cathode; and possible restructurization of TPE, which cointans noticeable amount of PYR14TFSI ionic liquid [23]. We believe that further optimization of solid state cell prototype and assembly-formation procedures may significantly improve its electrochemical performance and durability.

Thus, our investigation demonstrated that the developed polymer/ionic liquid thermoplastic solid electrolyte is a promising candidate for further development of all-solid-state batteries with relatively lower environmental impact.
