## 2 Materials and Methods
### Methods  
High resolution mass spectrometry (HRMS) experiments were performed with a Synapt G2 HDMS quadrupole/time-of-flight (Manchester, UK). The samples were introduced at 10 \(\upmu\)m\({}^{-1}\) flow rate (capillary voltage +2.8 kV, sampling cone voltage varied between +20 V and +60 V) under a desolvation gas (N\({}_{2}\)) flow of 100 L h\({}^{-1}\) heated at 35 \({}^{\circ}\)C. Accurate mass experiments were performed using reference ions from poly(propylene glycol) (PPG), poly(ethylene glycol) (PEG) or CH\({}_{3}\)COONa internal standards. All the samples were dissolved in methanol doped with 3 mM ammonium acetate prior to analysis. Data analyses were conducted using MassLynx 4.1 programs provided by Waters. Fourier-transformation infrared (FTIR) spectra were recorded using a Perkin Elmer Spectrum Two FTIR Spectrometer with an attenuated total reflectance (ATR) accessory. Nuclear magnetic resonance (NMR) spectra of different nuclei were recorded by using Bruker AC 400 MHz. For the \({}^{7}\)Li NMR measurements of electrolytes, an internal standard LiCl in D\({}_{2}\)O (500 \(\upmu\)L; 0.05 mol L\({}^{-1}\)) was used and the signal related to Li peak was set at 0 ppm. The electrolytes were separated from the LiCl standard by a sealed insert in a 5 mm tube. All NMR experiments were carried out at room temperature. Differential scanning calorimetry (DSC) experiments were carried out on a TA DSC Q20 using a heat/cool/heat cycle from \(-80\)\({}^{\circ}\)C to \(80\)\({}^{\circ}\)C for the electrolyte mixtures and from \(-80\)\({}^{\circ}\)C to \(220\)\({}^{\circ}\)C for zwitterion. The heating and cooling rates were fixed at 10 \({}^{\circ}\)C min\({}^{-1}\) and the measurements were carried out under nitrogen atmosphere. Glass transition temperature (T\({}_{g}\)) and melting point (T\({}_{m}\)) were determined from the second heating cycle of the DSC thermograms. Thermogravimetric analysis (TGA) was performed with a TA Instruments Q500 employing an aluminum pan with a small amount of sample (3-5 mg). The experiments were recorded in dynamic conditions under nitrogen atmosphere heating at a rate of 10 \({}^{\circ}\)C min\({}^{-1}\) from room temperature to 600 \({}^{\circ}\)C. Before measurements, all compounds were dried at 60 \({}^{\circ}\)C for 18 h and stored in a dry room with dew point below \(-50\)\({}^{\circ}\)C.  
The ionic conductivity of the glass fiber separator impregnated with the zwitterion-based electrolytes was measured by electrochemical impedance spectroscopy (EIS) technique in symmetrical coin cells (2025, Hohsen) with blocking stainless steel electrodes (SS) and a glass fiber separator (Albet GF52203254) impregnated with 75 \(\upmu\)L of the investigated electrolyte, using a 1470E potentiostat coupled with 1455 FRA interface (both from Solatron Analytical, UK). A 20 mV sinusoidal amplitude was applied in the 1 MHz-1 Hz frequency range at different temperatures from 80 \({}^{\circ}\)C to 20 \({}^{\circ}\)C every 10 \({}^{\circ}\)C. The tests were performed using a dynamic climatic chamber Binder MK53 (Germany) with a temperature control accuracy of \(\pm 0.5\)\({}^{\circ}\)C. The cells were allowed to stabilize for 1 h at each temperature before measurement. The so obtained EIS spectra were fitted using ZView software 3.5 (Scribner). Three parallel cells of each electrolyte were assembled and measured to guarantee a good reproducibility. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) methods were employed to evaluate the oxidation (anodic) and reduction (cathodic) stability of the investigated electrolytes, respectively. To do this, a three-electrode cell HS-3E (Hohsen) with a stainless steel disc (SS), as the working electrode (area of 1.76 cm\({}^{2}\)), and lithium foil (50 \(\upmu\)m, Albermarle), as the counter and reference electrodes, was assembled in a dry room (dew point \(-50\)\({}^{\circ}\)C). The glass fiber separator impregnated with 150 \(\upmu\)L of the investigated liquid electrolyte was placed in between the counter and the working electrodes.