# Ionic Liquid Electrolytes for Safer and More Reliable Sodium Battery Systems
## 2 Materials and Methods  
The ionic liquids were synthesized and purified through an eco-friendly route detailly described elsewhere [10,11]. The reagents, namely \(N\)-methyl-imidazole (Sigma-Aldrich, Saint Louis, MO, USA, 99%), \(N\)-methyl-piperidine (Sigma-Aldrich, 99%), bromoethane (Sigma-Aldrich, 98%), 1-bromopropane (Sigma-Aldrich, 99%), 1-bromobutane (Sigma-Aldrich, 99%), sodium bis(fluorosulfonyl)imide (NaFSI, Solvionic, Toulouse, France, >99.9%), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI, 3M, Saint Paul, MN, USA, >99.9%) and (nonafluorobutylsulfonyl)(trifluoromethylsulfonyl)imide acid (HIM\({}_{14}\), 3M, 60 wt.% in water solution) were used as received whereas the trimethylbutylammonium bromide (N\({}_{1114}\)Br, Solvionic, 98%) precursor was purified through sorbents materials [11], i.e., activated carbon (Sigma-Aldrich, Darco-G60) and acidic alumina (Sigma-Aldrich, Brockmann I). The latter were previously treated as reported in ref. [11]. Deionized ultrapure water (Milli-Q, Millipore, Billerica, MA, USA) was used as the only processing solvent.  
The 0.1 NaTFSI-0.9 IL (where 0.1 and 0.9 indicate the mole fractions) [9] electrolytes were prepared by quickly dissolving (at 50 \({}^{\circ}\)C) the proper amount of the NaTFSI (Solvionic, >99.9%, previously vacuum dried overnight at 120 \({}^{\circ}\)C) salt in the ionic liquids.  
The water content was determined (coulombometric titrations) by an automatic [PERSON] titrator (Metrohm, Columbus, OH, USA) within a controlled atmosphere dry room (moisture level < 10 ppm). The content of halide and lithium was checked by X-ray fluorescence (Shimadzu energy dispersion EDX-720 spectrometer, Kyoto, Japan) and atomic absorption analysis (Varian SpetcrAA model 220 atomic absorption spectrometer, SpectraLab, Markham, ON, Canada), respectively.  
The ionic conductivity of the NaTFSI-IL binary electrolytes was determined (through a conductivity meter Amel 160, AMEL, Milan, Italy) by loading (in the dry room) the samples in sealed, glass cells (Amel 192/K1, AMEL, cell constant equal to 1.00 \(\pm\) 0.01 cm) equipped with two porous Pt electrodes. The measurements were carried out in the temperature range from \(-\)40 to 80 \({}^{\circ}\)C (Binder GmbH MK53 climatic test chamber, BINDER, Tuttlingen, Germany). To fully crystallize the IL electrolytes [9], the cells were immersed in liquid nitrogen for about one minute and then immediately transferred in the climatic chamber at \(-\)40 \({}^{\circ}\)C. This route was repeated until the ionic liquid samples kept solid (at \(-\)40 \({}^{\circ}\)C.) After a storage period at \(-\)40 \({}^{\circ}\)C for at least 24 h, the conductivity of the sodium IL electrolytes was measured by running a heating scan at 1 \({}^{\circ}\)C h\({}^{-1}\).  
The thermal stability of the NaTFSI-IL electrolytes was evaluated in argon atmosphere (100 mL min\({}^{-1}\) flow) through isothermal thermo-gravimetrical analysis (TGA) measurements at different temperature steps by using a Pyris Diamond analyzer (Perkin Elmer, Waltham, MA, USA). A very fast ramp (40 \({}^{\circ}\)C min\({}^{-1}\)) was set up for adjusting the temperature step by step, i.e., the time associated to the temperature ramp between two consecutive (temperature) steps, differing in 25 \({}^{\circ}\)C, is extremely short (just a few seconds). This procedure allowed to give more reliable data with respect to those obtained from ramp temperature measurements [9]. The samples (5-10 mg) were placed in sealed, pin-holed, aluminum pans. The TGA measurements have been repeated, on each IL electrolyte sample, at least twice to verify the reproducibility of the results.