**1. Introduction**

Intense research efforts are still directed to improve the characteristics of lithium battery devices for energy storage. In particular, attention is directed to those devices that possess the requirements of safety and performance for powering electric vehicles [1] or for stationary use in smart-grids [2,3]. In this scenario, grea<sup>t</sup> advances in terms of reliability can be achieved by moving from liquid to polymer electrolytes. Among the different types of polymer electrolytes for lithium batteries, those based on polyethylene oxide (PEO) membranes, have received significant attention [4].

PEO membranes combine high chemical stability, solid-like diffusivity, and good ionic conductivity [5,6]. The main problem that has limited the use of PEO is its poor ionic conductivity at room temperature (10−<sup>8</sup> S/cm) due to the presence of crystalline phases. Conductivity values of interest for applications in lithium batteries (>10−<sup>4</sup> S/cm) are reached only above 65 ◦C, i.e., beyond the melting temperature of the polymer crystalline phase. The reason for this behavior is to be found in the peculiar PEO conduction mechanism as the lithium ion moves from a coordination center (the PEO

oxygen atoms) to the next. This movement is made possible only if the chain undergoes subsequent rearrangemen<sup>t</sup> steps. The dynamics of this process is greatly facilitated by a high mobility of the chains. Consequently, high values of ionic conductivity are reached above the melting temperature in the amorphous phase where long-range chain active motions are possible [7].

In this paper we address the problem of PEO low room temperature conductivity using three different strategies [8]. The first one consists of incorporating inside the fibers of electrospun PEO membranes inert ceramic particles, which act as fillers. The presence of these particles inhibits the PEO chains tendency to crystallize, leading to membranes with a higher amorphous fraction. In addition, as is well documented in the literature, it is known that the fillers stabilize the lithium electrolyte interface and increase the Li+ transference number due to Lewis acid–base type interactions between the ceramic surface groups and the polymer chains coordination sites. As an additional benefit, the presence of this inorganic dispersion improves the mechanical properties of the composite polymer membranes. In general, the best overall performances have been obtained with nanometric particles having acidic surface groups and in a 5–20 wt.% ratio with respect to the polymer [5,8–11]. The second strategy involves the use of lithium salts with low lattice energy, which favors the lithium salt dissociation. To achieve this goal, lithium salts with large and flexible anions, that disperse effectively the charge and are able to increase the free-volume between the polymer chains and thus to promote the ionic mobility, have been used [8,12,13]. The third approach has involved the addition of an aprotic ionic liquid (IL) that increases the total ionic conductivity, competing with lithium-ions toward the binding sites of coordination and creating additional free-volume between the chains [8]. The disadvantages of ionic liquids are their high viscosity, their tendency to form ionic clusters, and their high cost [14]. To mitigate such drawbacks, we have added to the electrolyte formulations alkyl carbonates, which reduce the viscosity of the solution and form a protective passivating film (i.e., a solid–electrolyte interface, SEI) on the lithium anode [8].

In this perspective, we designed and prepared innovative electrolytic systems with the aim of providing a compositional study and elucidation on the role of different components. Emphasis is given to the analysis of the conducting properties of the proposed systems as a figure of merit in view of applications in electrochemical devices. Such electrolytes consist of nanocomposite polymer membranes of PEO/silica produced through the electrospinning technique and then gelled using two liquid solutions. The two solutions, based on the aprotic ionic liquid PYR14TFSI, contain a mixture of two carbonate solvents, i.e., ethylene carbonate and dimethyl carbonate (EC and DMC), and the bis(trifluoromethane)sulfonyl imide lithium salt (LiTFSI). In one of the two solutions, the polysulfide Li2S8 was also added. The addition of this last component is justified by the fact that these systems have been thought to provide possible future applications not only in conventional lithium batteries but, specifically, in lithium-sulfur batteries [15–17]. The dissolution of the sulfur cathode in the form of polysulfides (and the consequent shuttle phenomenon) strongly limits the development of this technology. For this reason, we intend to reduce such dissolution playing on the solubility equilibrium, by adding a buffer polysulfide to the electrolyte [18–20]. Also, some ILs (such as *N*-methyl-*N*-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide, PYR14TFSI) have already demonstrated the ability to suppress the dissolution of polysulfides increasing the performance of sulfur cathodes [16]. The choice of LiTFSI as lithium source is due to its stability, large dimension, and flexible imide structure. These latter properties, together with the strong electron withdrawing behavior of the (trifluoromethane)sulfonyl group, enhance the negative charge delocalization and, in turn, guarantee a high salt dissociation level. Moreover, the TFSI anion is the same contained in the ionic liquid, which should avoid the Li+-ion transference number reduction, possibly occurring when additional ionic species are added to the solution [21].
