2.2.4. Preparation of Triethylmethylammonium Tetrafluoroborate (TEMABF4)

For the synthesis of TEMABF4, a conventional procedure was followed, exploiting the different solubilities of the desired product and the ammonium chloride.

Here, 22.7 g (0.150 mol) of triethylmethylammonium chloride (TEMACl) were added in a 250 mL round-bottom flask and dissolved in 150 mL of dry acetonitrile. After complete solubilization, 18.9 g (0.180 mol) of NH4BF4 was added. The resulting suspension was stirred overnight at room temperature. The suspension was filtered to remove the solid, and the mother liquor was concentrated at a reduced pressure and then crystallized with diethyl ether. Purified product was filtered and dried under vacuum at 50 ◦C overnight, obtaining an almost quantitative yield. 1H NMR (400 MHz, CD3CN) δ 3.23 (q, J = 7.3 Hz, 6H), 2.84 (s, 3H), 1.47 (d, J = 6.8, 3H), 1.24 (tt, J = 7.3 Hz, J14N = 2.0 Hz, 9H). 13C NMR (101 MHz, CDCl3) δ 56.91 (t, J14N = 3 Hz), 47.48 (t, J14N = 4 Hz) 8.19. 19F NMR (376 MHz, CD3CN) <sup>δ</sup> −151.29 (11B), −151.34 (10B).

#### *2.3. Electrolyte Characterization*

Before preparing the electrolytes, the solvents were stored on molecular sieves (3A) until the water content was reduced to 30–40 ppm, as measured by Karl–Fischer titration. The electrolyte conductivity was measured at 20 ◦C using platinized Pt electrodes (Crison 254). The conductivity meter was previously calibrated with a 0.1 M KCl standard solution (conductivity 12.89 mScm−<sup>1</sup> at 25 ◦C, Hanna Instrument).

The electrochemical stability window (ESW) was evaluated using a Bob's Cell™ electrochemical cell equipped with three electrodes: Au disc electrode (ø 3 mm, embedded in PEEK) as working electrode, Pt wire as counter electrode and an Ag/Ag<sup>+</sup> quasi-reference electrode in a solution of PC (TEMABF4 0.1 M and AgNO3 3 mM). The reference electrode was equipped with a bridge tube filled with supporting electrolyte (PC TEMABF4 1M) and connected to the cell with glass frits (Vycor®). Before each measurement, 5 mL of electrolyte was introduced into the cell and purged with nitrogen under magnetic stirring for 10 min. The magnetic stirring was stopped, and the bubbler was moved from purging to vent position to avoid moisture contamination. Linear Sweep Voltammetry (LSV) measurements were then performed from open circuit potential (OCP) towards both positive and negative potentials with a scan rate of 10 mVs−1.(Figure S5), to evaluate respectively the anodic (Eox) and cathodic limits (Ered). The potential limits were explored separately, and each

measurement was made with fresh electrodes and electrolytes. These limits represent the maximum applicable potential in a classic 3-electrode set-up experiment, for which a faradic current density is not negligible due to the electrochemical decomposition of the electrolyte.

## *2.4. Symmetrical EDLC Assembly*

Each coin cell (CR2016) was prepared cutting two circular AC-based electrodes (ø 12 mm). A cellulosic separator (ø 18 mm Celgard® battery separator) was used as the dielectric separator. Before being cut, the activated carbon electrodes were placed in a vacuum oven at 80 ◦C for 10 h, cooled, and stored in a nitrogen atmosphere.

Before being used, all of the coin cell's components (case, gasket, cap, plate, and spring) were washed and sonicated with detergents, rinsed with ultra-pure water, and dried in a vacuum oven at 80 ◦C. The cells were assembled in a dry room by placing into the case the first electrode, the dielectric separator, the appropriate electrolytic solution, the second electrode, the plate, the spring and finally the cap. The cells thus prepared were sealed with MSK110 manual hydraulic crimping machine (MTI KJ group™) and tested as symmetrical EDLCs.

#### *2.5. EDLC Characterizations*

EDLCs were characterized by cyclic voltammetry (CV), galvanostatic charge-discharge cycles (GCs) and potentiostatic electrochemical impedance spectroscopy (EIS).

The operative voltage (OV) of the investigated electrolytes was defined as the maximum applicable voltage with a Coulombic Efficiency (CE) threshold of 94–95%. CV were recorded with a scan rate of 5 mVs<sup>−</sup>1, starting from 0 V and gradually increasing to the final voltage (Figure S6). The CE was calculated from the ratio between integration of negative (Q−) and positive (Q+) voltammogram areas [44] that represent, respectively, discharge and charge capacitance:

$$\text{CE} = \mid \text{Q}^{-} \mid / \mid \text{Q}^{+} \mid \times 100 \tag{3}$$

The Capacitance Retention (CR) was defined with CV by the ratio of the specific capacitance (SCcv) at different scan rates to that recorded with the scan rate of 5 mVs−<sup>1</sup> (4). The CV scan rate was increased from 5 to 200 mVs−<sup>1</sup> in a potential window from 0 V to OV. In the following characterization, *i*1/2 Vmax is the current density (Acm<sup>−</sup>2) referred to half of the OV, *s* is the applied scan rate (Vs<sup>−</sup>1) and *d* is the active materials loading (gcm<sup>−</sup>2) [45]:

$$\text{CR} = \text{SC}\_{\text{cv}\{\text{x mVs}\}}{^{-1} \text{/} / \text{SC}\_{\text{cv}\ (\text{5 mVs}}\text{}^{-1})} \times 100\tag{4}$$

$$\text{SC}\_{\text{CV}} = i\_{1/2} \,\text{V}\_{\text{max}} / (\text{s} \times d) \tag{5}$$

The GC profiles recorded were elaborated to calculate the specific Capacitance (Csp, Fg−1) and maximum specific Energy (Esp, Whkg−1) and Power (Psp, kWkg−1) according to the following equations [46]:

$$\mathbf{C\_{sp}} = (dt/dV) \times (i/\,\mathrm{m\_{tot}})\tag{6}$$

$$\mathbf{E\_{sp}} = (\mathbf{C} \times \mathbf{O} \mathbf{V}^2) / (2 \times 3600 \times \mathbf{m\_{tot}}) \tag{7}$$

$$\text{P}\_{\text{sp}} = \text{OV}^2 / (4 \times \text{m}\_{\text{tot}} \times \text{ESR}\_{\text{GC}}) \tag{8}$$

where *dt/dV* is the slope of the discharging profile, *i* the applied current, mtot the total mass (in g for Csp and kg for Esp and Psp) of the active materials for the two electrodes, C the capacitance (F), OV the operative voltage (V), 3600 is expressed in second. The ESRGC was calculated according to the following Equation (9), where ΔVohmic is the ohmic voltage drop at the beginning of discharge and *i* was the applied current:

$$\text{ESR}\_{\text{GC}} = \Delta \text{V}\_{\text{ohmic}} / (2 \times i) \tag{9}$$

EIS profiles were recorded in the frequency range between 500 kHz and 10 mHz with 5 mVAC perturbation and 10 points per decade. According to analysis reported by Mei et al. [47], from EIS profile we determined the ESR, the Equivalent Diffusive Resistance (EDR), relative to the ions penetration into the electrode pores, and the bulk resistance (Rbulk), relative to the bulk resistance of the electrolyte in the cell. The different parameters were defined by the segments obtained from the intersection of the Nyquist plot with the x-axis, also exploiting linear fittings in the diffusive-regime (slope ≈ 1) and capacitive-refine (quasi-vertical line). A clarification of the Nyquist plot analysis is shown in Figure S7. The time constant of the investigated materials was evaluated calculating the imaginary part C"(ω) of the complex capacitance according to the work of Taberna et al. [48] and the following equation:

$$\mathbf{C}''(\omega) = -\mathbf{Z}'(\omega) / (\omega \times |\mathbf{Z}(\omega)|^2) \tag{10}$$

where ω is the applied frequency, Z (ω) is the real part of complex impedance related to the Nyquist plot and |Z (ω)|is the impedance modulus related to the Bode plot.
