Role of FTFSI Anion Asymmetry on Physical Properties of AFTFSI (A=Li, Na and K) Based Electrolytes and Consequences on Supercapacitor Application

Abstract

This study compares the physicochemical properties of six electrolytes comprising of three salts: LiFTFSI, NaFTFSI and KFTFSI in two solvent mixtures, the binary (3EC/7EMC) and the ternary (EC/PC/3DMC). The transport properties (conductivity, viscosity) as a function of temperature and concentration were modeled using the extended Jones-Dole-Kaminsky equation, the Arrhenius model, and the Eyring theory of transition state for activated complexes. Results are discussed in terms of ionicity, solvation shell, and cross-interactions between electrolyte components. The application of the six formulated electro-lytes in symmetrical activated carbon (AC)//AC supercapacitors (SCs) was characterized by cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), electrochemical impedance spectroscopy (EIS) and accelerated aging. Results revealed that the geometrical flexibility of the FTFSI anion allows it to access and diffuse easily in AC whereas its counter ions (Li + , Na + or K + ) can remain trapped in porosity. However, this drawback was partially resolved by mixing LiFTFSI and KFTFSI salts in the electrolyte.

Non‐Flammable Sodium Asymmetric Imide Salt‐Based Deep Eutectic Solvent for Supercapacitor Applications

“…The activation energies and are extracted from the slopes of Figure 2d and tabulated accordingly. The values reveal that the energy barrier of the viscous flow is 41 % higher than the ones containing conventional ternary or binary solvent mixtures (e. g., EC/PC/3DMC or 3EC/7EMC), [20,30] signifying stronger intermolecular interactions. This may be because only EC ensures the solvation of Na + and the complexation of imide anions.…”

“…The changes in electrolyte density ( ρ ) between 10 and 55 °C are shown in Figure 2a. The eutectic mixtures are moderately denser (1.45<ρ<1.5 g cm −3 ) than standard carbonate‐based electrolytes such as EC/EMC (3/7 wt) and 1 mol L −1 NaFTFSI (or 1 mol L −1 NaTFSICN) whose density lies between 1.1 and 1.3 g cm −3 at 25 °C [19,20] . The compactness of the eutectic mixtures leaves little free volume, making them denser.…”

“…The eutectic mixtures are moderately denser (1.45 < 1 < 1.5 g cm À 3 ) than standard carbonate-based electrolytes such as EC/EMC (3/7 wt) and 1 mol L À 1 NaFTFSI (or 1 mol L À 1 NaTFSICN) whose density lies between 1.1 and 1.3 g cm À 3 at 25 °C. [19,20] The compactness of the eutectic mixtures leaves little free volume, making them denser. With the aid of empirical relationships (Equations 4 and 5), together with Glasser's theory [24] (Equations 6 and 7), we model the density variation with temperature.…”

“…A standard carbonate-based electrolyte containing the same salts is used for comparison purposes. [20] Electrolyte E h a � 0:05[kJ mol À 1 ]…”

“…Discharge Capacity [F g Furthermore, accelerated aging floating tests [20,35] were initiated to evaluate the stability of the SCs during long-term cycling. The tests were conducted within a voltage window of 0 to 2.4 V for the NaFTFSI-based electrolyte (Figure 5g) and 0 to 2.0 V for the NaTFSICN-based electrolyte.…”