**1. Introduction**

Large-scale applications such as automotive, stationary, deep-sea drilling devices need batteries to be capable of operating safely at medium-high temperatures with very good performance and cycle life; i.e., without appreciable degradation phenomena. In addition, even devices generally operating around room temperature could be accidentally subjected to prolonged overheating, thus requiring high thermal stability. In this scenario, electrolytes play a key role.

Rechargeable lithium batteries are an excellent choice as advanced electrochemical energy storage systems due to their high energy density and cycle life [1,2]. Recently published manuscripts report that ion conducting polymer membranes, realized through common materials and up-scalable processes, can act as electrolyte separators for rechargeable lithium battery systems. J.R. Nair et al. [3] have prepared methacrylic-based PEs, reinforced with both cellulose hand-sheets and nanosize cellulose fibers, by UV-induced free radical photo-polymerization. Similarly, rigid–flexible composite electrolyte membranes, based on poly(ethyl *α*-cyanoacrylate) and cellulose backbone, have been prepared through an in-situ polymerization process by P. Hu et al. [4]. These cross-linking techniques, also successfully proposed for Na+ conducting PEs [5], have shown short processing times, easy up-scalability and eco-compatibility, and have enabled gel polymer electrolytes (GPEs) with wide electrochemical stability windows and high room temperature ionic conductivity in combination with good mechanical

properties to be obtained. Poly(vinylidene difluoride)-based GPEs were obtained via the phase-inversion method [6]. The use of nano-clay filler and pore-forming agent, i.e., poly(vinylpyrrolidone), was seen to significantly improve the electrolyte uptake and the ion transport properties. H. Li et al. [7] have combined the advantages of GPEs with those of ceramic conductors to prepare sandwiched structure composite electrolytes with enhanced electrochemical performance. Reviews of GPE systems, addressed to Li/S [8] and Li-ion [9] battery systems, were recently published.

However, commercial lithium-ion batteries, even employing GPEs, do not behave well at medium-high temperatures as the organic electrolyte quickly degrades above 50 ◦C, thus irreversibly ageing the electrochemical device [10–12]. In this scenario, the development of solvent-free polymer electrolytes is undoubtedly appealing from safety and engineering points of view and opens new perspectives to applications in electrochemical devices [8,9,13–17]. In addition, polymer electrolytes (PEs) can be easily and cheaply manufactured into low thicknesses (<100 μm) and shapes not allowed for supported liquid electrolytes, offering a new concept of solvent-free, all-solid-state, thin-layer, flexible (both mechanically and in design), robust, lithium polymer batteries (LPBs). Finally, PEs play a second role in composite electrodes as binders and ionic conductors [18].

Nevertheless, the realization of all-solid-state lithium battery systems has been prevented so far by the low ionic conductivity of PEs, especially at ambient temperature. For instance, poly(ethyleneoxide)- lithium salt (PEO-LiX) complexes, considered to be very good candidates as electrolyte separators for LPBs [13–23], approach conduction values of interest for practical applications (>10−<sup>4</sup> <sup>S</sup>·cm<sup>−</sup>1) only above 70 ◦C, i.e., when the polymer is in the amorphous state [13,14,17,20–22]. However, even at medium-high temperatures ( ≥90 ◦C) LPBs exhibit high performance only at low current rates (≤0.1C) [18,22,23], thus preventing applications requiring high power density.

An appealing way to overcome the conductivity drawback is represented by the incorporation of ionic liquids (ILs) into the polymer electrolytes [24]. ILs, i.e., salts which are molten at room temperature consisting of organic cations and inorganic/organic anions [25–27], display several peculiarities such as their extremely low flammability, negligible vapor pressure, high chemical–electrochemical–thermal stability, fast ion transport properties, good power solvency and high specific heat. In the last years, it was successfully demonstrated [24,28–34] how the addition of ILs to PEO-based electrolytes enhances the ionic conductivity above 10−<sup>4</sup> S·cm<sup>−</sup><sup>1</sup> at 20 ◦C—i.e., more than two orders of magnitude higher than that of ionic liquid-free PEs—allowing LPBs to obtain a significant cycling performance at near room temperature (30–40 ◦C) [24,29–34].

In the present work, we show how the incorporation of ionic liquids improves the performance of PEO-based electrolytes even at medium-high temperatures, especially at high current rates, without any evident material degradation and battery cycle life depletion, making the IL-containing PEO membrane an appealing electrolyte separator for LIBs operating at medium-high temperatures. *N*-butyl-*N*-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) was selected as the ionic liquid [24].

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

#### *2.1. Synthesis of the Ionic Liquid*

The PYR14TFSI ionic liquid was synthesized through an eco-friend route, reported in detail elsewhere [35,36].

#### *2.2. Preparation of the Polymer Electrolyte and the Composite Cathode*

The ionic liquid-based polymer electrolyte and composite cathode were prepared through a solvent-free process [33] carried out in a very low relative humidity dry-room (R.H. < 0.1% at 20 ◦C). The material components, i.e., PEO (Dow Chemical, Midland, MI, USA, WSR 301, M W = 4,000,000 a.u.), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 3M, battery grade) and PYR14TFSI, were vacuum dried at 50 ◦C for 48 h (PEO) and at 120 ◦C for 24 h (lithium salt and ionic liquid). PEO and LiTFSI

(EO:Li mole ratio = 1:0.1) were intimately mixed in a mortar, and then PYR14TFSI was added to achieve a (PYR14) +/Li+ mole ratio equal to 1:1. In previous papers [24,33], we have shown that this ratio represents a good compromise between ion transport properties and interfacial stability. The P(EO)1(LiTFSI)0.1(PYR14TFSI)0.1 past-like electrolyte blend was annealed under vacuum at 100 ◦C overnight in order to allow the full diffusion of the lithium salt and ionic liquid through the PEO host, therefore obtaining a homogeneous mixture. Finally, the so-obtained rubber-like material was hot-pressed at 100 ◦C for 2 min to form 70–80 μm thick films. Ionic liquid-free, P(EO)1(LiTFSI)0.1 binary polymer electrolytes were prepared for comparison purposes.

The cathode tape was prepared by intimately blending LiFePO4 active material (Sud Chemie, Munich, Germany) and KJB carbon (electronic conductor, Akzo Nobel, Amsterdam, The Netherlands). LiFePO4 and KJB were previously vacuum dried at 120 ◦C for at least 24 h. Separately, PEO, LiTFSI and PYR14TFSI were roughly mixed (to obtain a paste-like mixture) and then added to the LiFePO4-KJB blend. The resulting cathodic mixture was firstly annealed at 100 ◦C overnight and then hot-pressed to form preliminary films (200–300 μm thick) which were cold-rolled to obtain the final cathode tape (<50 μm) and to remove any porosity within the composite cathode [37]. Finally, 12 mm diameter cathode discs (active area equal to 1.13 cm2) were punched for the battery tests. The active material mass loading ranged from 4 to 5 mg·cm<sup>−</sup>2, corresponding (accounting for a theoretical capacity of LiFePO4 equal to 170 mA·h·g<sup>−</sup>1) to a capacity from 0.7 to 0.8 mA·h·cm<sup>−</sup>2.
