High Ionic Conductivity, Mechanically Strong Ion Gels for All Solid State Supercapacitor Applications
Authors: Chinnam Parameswara Rao , Simotwo Silas K , Kalra Vibha , Wunder Stephanie 
Published: February 2020

Electrochemical double layer capacitors (EDLCs) have many advantages compared with batteries and fuel cells because of their cycle life and power density. The limitation for use of EDLCs in major applications such as electric vehicles and grid storage is their low energy density. One way to increase energy density of EDLCs is to increase the stability window of the electrolytes. Room temperature ionic liquids (RTILs), which are salts with low temperature melting points, have been investigated as electrolytes in electrochemical devices, since they have useful (i.e. large) electrochemical stability windows (> 4V), are nonvolatile (and as a result often non-flammable), and are therefore safer than conventional aprotic liquid electrolytes.

The utility of a EDLC device increases if it is developed in the form of a thin, solid film since it avoids leakage problems and the use of casings that decrease energy density. One method to make these films is to form iongels, which are ion conducting liquids (such as RTILs) immobilized in a polymer matrix, forming a solid polymer electrolyte (SPE). Since RTILs are typically more viscous (30-200 cP vs ~ 1 cP) than conventional aprotic or aqueous solvents, and therefore less conductive (0.1 to 18 mS/cm), it becomes important to decrease the amount of polymer required to solidify the iongel.

Immobilization of the RTIL can be achieved using gelators that participate in chemical crosslinking reactions or form physical crosslinks. Gelators further increase viscosity and decrease conductivity of RTIL, which decreases the conductivity and increases the equivalent series resistance (ESR) in EDLCs. Therefore, the requirements of iongels in EDLC applications are minimum wt% of gelator, low ESR, good chemical and electrochemical stability with the electrodes and good mechanical properties (to prevent shorting the electrodes under pressure).

In our previous work, we have shown that renewable, environmentally friendly, abundant and inexpensive natural polymers such as methyl cellulose (MC), (as gel former), when combined with an ionic liquid, form tough, temperature stable ion gels1,2. In this work, we developed an iongel with 5% MC as gelator and 95% 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as the ionic liquid, for all solid state supercapacitor applications. The formed iongels have temperature dependent ionic conductivities only 1.5 to 1.8 times lower than the pure RTILs, high thermal stability up to 350oC, solid like behavior over a frequency range of 1 Hz to 100 Hz, at 25oC and a storage modulus of 5 MPa at 25oC, as shown in Figure 1.

The iongel was incorporated into 3-D porous interconnected network of free standing porous carbon nanofibers prepared by an electrospinning technique and further chemically activated, yielding a high surface area of 2282 m2/g as shown in Figure 2.A specific capacitance of 137 F g-1 at 20 mV s-1was obtained for the cell corresponding to a specific energy density of 56 Wh/kg (based on total electrode mass) for a 3.5V window. At the same scan rate 163 F/g was obtained for liquid based cell. In this talk, the performance, stability and postmortem studies of the supercapacitor will be discussed.

In summary we have developed novel iongels that have been solidified with only 5% methylcellulose as gelator, with excellent ionic conductivities (1.5 to 1.8 times lower than the RTIL) and mechanical properties (5MPa at 25oC) as electrolytes for all solid state supercapacitor applications.

Figure 1: Comparison of liquid IL and gel IL; Log σ vs 1000/T (a) 2ndheating DSC thermograms (b) TGA thermograms (c) and storage and loss moduli as a function of frequency for gel IL at 25 °C (d).

Figure 2 top left. SEM micrographs: a-CNFs and right; a-CNFs filled with iongel; bottom left. EIS of liquid- and solid-based cells and right; corresponding CVs

Figure 1