Separator Materials Influence on Supercapacitors Performance in Viscous Electrolytes

It is well known that a high internal resistance limits the power capability of the supercapacitor (SC) and ultimately its application. In SCs a number of sources contribute to the internal resistance and are collectively measured and referred to as the equivalent series resistance. These are resistance of the electrode material, the interfacial resistance between the electrode and the current-collector, the ionic resistance of ions moving in small pores, the ionic resistance of ions moving through the separator and the electrolyte resistance. Separator has an effect on the ionic charge compensation rate between the positively and negatively charged microporous carbon electrodes soaked into the electrolyte solution. Therefore the molar conductivity of ions in the porous separator matrix is extremely important.

The aim of this paper was to study the influence of the separator material characteristics (chemical composition, morphology, thickness, porosity) on the performance of SCs at lower temperatures and viscous electrolytes. For that two different electrolytes were used: 1 M triethylmethylammonium tetrafluoroborate solution in dimethyl carbonate, ethylene carbonate and propylene carbonate mixture (TEMABF₄ + DMC:EC:PC) with 1:1:1 volume ratio and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid. Temperature dependence study was performed with TEMABF₄ + DMC:EC:PC electrolyte at the temperature range from +24 to -30 °C. To study the influence of the separator material characteristics different separator materials were used: commercially available separators prepared from cellulose (TF4425) and polypropylene (Celgard2400), and self-made separators, prepared by electrospinning method from poly(vinylidene fluoride) with different surface morphologies. Carbide-derived carbon electrodes, prepared from TiC at chlorination temperature T=950°C, were used. Our SCs were tested by cyclic voltammetry, constant power discharge and electrochemical impedance spectroscopy methods.

In given paper it will be discussed how high-frequency series resistance, phase angle, characteristic relaxation time constant and specific power values of SC depends on the separator parameters used. Also it will be shown that for the high power pulse generation systems, the properties of the separator material have to be carefully optimized and separator materials effect is highly important for the viscous electrolyte solutions based SCs and for systems working at lower temperatures.