**1. Introduction**

Highly efficient, light, safe, and long-lasting rechargeable batteries are the goal of all the researchers and producers involved in the energy storage business. So far, lithium ion batteries (LIBs) represent the most promising answer; however, the booming growth of demand spotlighted the drawbacks of such technology. The major intrinsic limitation of LIBs is the low theoretical specific capacity (372 mAh·g<sup>−</sup>1) of the traditional graphite anode, which does not allow the increase of practical LIB energy density to more than 300 Wh·kg−1. Lithium metal represents the best alternative anode material to produce high energy density batteries because it possesses the lowest standard potential (Eo = −3.04 V versus standard hydrogen electrode) and the highest theoretical capacity (3.860 mAh−1) [1]. Unfortunately, this technology is not ideal and presents several issues such as dendrite growth, instability of lithium metal with the most part of classical organic liquid electrolytes, low coulombic efficiency, poor cyclability, and poor safety due to leakage and high flammability of the liquid electrolyte based on a mixture of carbonate solvents [2–4].

Solid polymer electrolytes (SPEs) are without a doubt among the key solutions to overcome such limitations toward high energy density, efficient, and safe solid state batteries (SSB) [4–7]. Indeed, these solid ion conductive membranes can replace microporous separators impregnated by volatile flammable organic electrolytes [5,6], acting as physical barrier against dendrite growth reducing the possibility of short-circuit, thermal runaway, and explosion, significantly improving the safety of the battery [8–10]. However, poor ionic conductivity at room temperature due to low mobility of the lithium cations in the solid polymer matrix, and the loss of mechanical properties in the conductive molten state at higher temperature, limit their spread in the battery market [11]. Several solutions have been proposed to increase ionic conductivity while maintaining good mechanical properties [12,13]. Many of them are based on the addition of low molecular weight compounds with adequate electrochemical properties coupled to the creation of physical or chemical crosslinking sites at the polymer [14]. The employment of inorganic fillers and the introduction of sufficient amount of low molecular weight compounds are listed among the most relevant examples. Adding inorganic fillers proved to favor the performance of the battery by (i) preventing crystallization by hindering the supramolecular arrangemen<sup>t</sup> of the polymer chains; (ii) favoring ionic dissociation, improving the matrix/solid electrolyte interface (SEI) interaction thanks to the contribution of different possible surface groups; and (iii) increasing mechanical resistance and stability [15–17]. The employment of low molecular weight compounds also proved to be an effective measure to enhance the electrochemical performance of a solid state battery [18]. Among them, and despite some drawbacks (high cost and some instability at lithium deposition potential [19]), room temperature ionic liquids (RTILs) are, probably, the most promising materials thanks to their negligible vapor pressure, low flammability, high ionic conductivity in comparison with solid polymer electrolytes, and their ability to form an effective solid electrolyte interphase onto the lithium metal electrode surface [20–22]. Several recent studies demonstrated that the presence of RTILs can enhance significantly the electrochemical properties of the solid state battery, such as, for instance, improving the long-time stability in the stripping/plating from lithium metal electrodes [23,24]. Furthermore, it has recently been demonstrated that free RTILs in a polymeric solid matrix can undergo percolation, creating a highly conductive pathway across the electrolyte and a wet interfacial layer that greatly improves the interfacial compatibility with the electrodes [25].

On the other hand, the always increasing demand of electronic devices goes together with a growing concern about a sustainable future and cost considerations. The combination of these two factors bursts the research toward the development of green processes to obtain high performance materials. In this optic, fast production methods that employ recyclable materials and reduce or eliminate completely the use of harmful organic solvents are the main goal of all the efforts. Thermoplastic polymers represent a valid option to develop solid electrolytes because they can be processed easily by extrusion and shaped by hot-pressing or lamination, none of which require solvent, and can be theoretically recycled and reprocessed, in this way reducing the final cost of solid state batteries, which is crucial for their implementation in the market [26–28].

In this context, this work presents the solvent-free preparation of a thermoplastic polymer electrolyte (TPE) consisting on a polymeric matrix, ad hoc modified inorganic fillers, an ionic liquid, and a lithium salt. More precisely, the TPE is composed by poly(ethylene oxide) (PEO), surface modified sepiolite (TPGS-S), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), prepared by solvent-free extrusion method. This TPE is compared with a well-studied reference electrolyte consisting of PEO and LiTFSI [29]. The extensive physical and electrochemical characterization of the new TPE is presented in this article. The developed solid electrolyte demonstrated high ionic conductivity, good electrochemical stability, excellent compatibility with lithium metal, and promising cycling performance in truly solid state Li-LiFePO4 coin cell prototype.

#### **2. Materials and Methods**
