## 1 Introduction  
Ionic liquids (ILs) have a long history [1], but they were the object of a rapid increase in interest in the last 25 years [1], witnessed also from the development of industrial processes involving ILs [2]. In general, they possess peculiar properties, such as a low vapor pressure, high thermal and electrochemical stability, large liquid ranges that can extend below room temperature, high ionic conductivity and good solubility properties; therefore, they were proposed for uncountable applications [3, 4, 5, 6], such as solvents, electrolyte components, lubricants and heat transfer media. Electrochemistry is one of the most studied application for ILs, as they are considered as valid alternatives to the more usual organic alkyl-carbonates, which are the main components of lithium cell electrolytes [6]. In the last years, the application of batteries to the automotive sector has pushed research towards innovative cells, with even higher energy and power densities, that need novel materials to fulfill these requirements. In this framework, ILs have the potentiality to highly surpass the performances of organic solvents because of their higher thermal stability, non-flammability and larger anodic stability [6]. High energy and power density values in general require the use of materials at higher operative voltages (around 5 V). There is a limited number of ILs that can work at such high potentials without the occurrence of decomposition (oxidation), and all of them contain highly fluorinated anions. The most widely used anions are bis(trifluoromethylsulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI). However, other fluorinated anions, such as (trifluoromethyluormethanesulfonyl) (nonafluorobutanesulfonyl)imide (IM14), (difluoroethanesulfonyl)(nonafluorobutanesulfonyl)imide (IM24), (nonafluorobutanesulfonyl)(toluenesulfonyl)imide, (IMT4) and 1,3-hexafluoropropane-disulfonylimide (IM3) were also investigated, providing good electrochemical performances [7]. Moreover, the fluorosulfonyl-(trifluoromethyluormethanesulfonyl)imide (FTSI) asymmetric anion was additionally proposed [8]. The first study on FTSI-based ILs reported a thermal stability above 250 \({}^{\circ}\)C, a strong resistance to crystallization, values of ionic conductivity above 10\({}^{-4}\) S cm\({}^{-1}\) even at -40 \({}^{\circ}\)C and an electrochemical stability exceeding 5 V [8]. The use of FTSI-containing ILs was also studied in dual-ion cells, in comparison with liquids based on the TFSI anion [9; 10]. Some studies were devoted to the investigation of the anodic stability of the aluminum current collectors for high voltage supercapacitors [11], sodium [12] or lithium cells [13] in the presence of FTSI-based ILs. This class of ILs was also used as components of polymeric electrolytes based on polyethylene oxide [14; 15]. Concentrated electrolytes composed of LiFTSI, LiFSI and ether solvents were reported for improving the cycling stability of lithium metal and inhibiting Li-metal dendritic growth [16]. ILs combining alkoxy-functionalized ammonium cations with asymmetric FTSI anions seem to give rise to optimized ILs with improved fluidity [17]. FTSI ILs were used as electrolytes also in combination with organic carbonates, providing some eutectic composition [18; 19]. Certain recent studies were devoted to the investigation of the Solid-Electrolyte Interphase (SEI) developed when FTSI-based ILs are used as electrolytes [20; 21] or components of a polymer electrolyte [15]. The TFSI and FTSI salts are reported to lead to thicker, more highly passivating surfaces than those developed in the presence of FSI salts [20]. Ionic liquids based on the FTSI anion, combined with the trimethyl-isobutyl-ammonium cation (N111 id)\({}^{+}\), have shown interesting physicochemical and electrochemical properties and the ability of keeping in a super-cooled state for more than one year [22]. However, LiFTSI and LiFSI salts are favorable for forming less dendritic, denser and more compact lithium metal deposits [15].