*2.1. Reference Electrode Calibration in Non-Aqueous Solution*

In this work, we used Pt/(I−/I3 −) as the reference electrode. For a facile comparison of electrochemical potentials measured in the different non-aqueous electrolyte solution, the electrode potentials versus I−/I3 <sup>−</sup> were first calibrated using ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal standard [61], as recommended by International Union of Pure and Applied Chemistry. This calibration method based on the measurement of cyclic voltammograms (CV) at the presence of Fc/Fc<sup>+</sup> redox couple in the corresponding catholyte (0.1 M ILs in PC in this work). The obtained CV curves are shown in Figure 1.

**Figure 1.** Cyclic voltammetry measured in [Emim]BF4/PC (**a**), [Bmim]BF4/PC (**b**) [Hmim]BF4/PC (**c**), [Omim]BF4/PC (**d**), [Dmim]BF4/PC (**e**), [Bu4N]BF4/PC (**f**); α: blank, β: 10 mM ferrocene; [Emim]BF4: 1-ethyl-3-methylimidazolium tetrafluoroborate; [Bmim]BF4: 1-butyl-3-methylimidazolium tetrafluoroborate; [Hmim]BF4: 1-hexyl-3-methylimidazolium tetrafluoroborate; [Omim]BF4: 1-octyl-3 methylimidazolium tetrafluoroborate; [Dmim]BF4: 1-decyl-3-methylimidazolium tetrafluoroborate; and [Bu4N]BF4: tetrabutylammonium tetrafluoroborate.

For comparison, the CV curve in the commonly used tetrabutylammonium salt [Bu4N]BF4 catholyte solution is also included. As shown in Figure 1, in all cases, the redox peaks are not observed across the CV curves (the red ones) in the absence of ferrocene. Otherwise, typical redox peaks (the black curves) are observed at the presence of 10 mM ferrocene. The obtained half-wave potentials (E1/<sup>2</sup> = (EPc + EPa)/2) are 0.242 V, 0.231 V, 0.223V, 0.222 V, 0.241 V and 0.242 V in 0.1 M [Emim]BF4/PC, [Bmim]BF4/PC, [Hmim]BF4/PC, [Omim]BF4/PC, [Dmim]BF4/PC and [Bu4N]BF4/PC, respectively. Then, all the potentials in different catholyte solutions are calibrated versus the internal standard if Fc/Fc<sup>+</sup> according to the equation: E (vs. Fc/Fc+) = E (vs. I−/I3 <sup>−</sup>) − E1/<sup>2</sup> [62].
