Low-Temperature Performance of Vinylene Carbonate Additive Containing Electrolyte for Electric Double-Layer Capacitors
Authors: Väli Ronald , Jänes Alar , Lust Enn 
Published: February 2020

The weakest link in modern electric double-layer capacitors (EDLCs) is the solvents’ low thermodynamic stability. In practical applications, acetonitrile enables voltages up to 2.85 V, but is also extremely toxic. On the other hand, safer solvents like propylene carbonate (PC) are much more viscous, which hinders low-temperature performance. Therefore, much work is devoted on studying solvent mixtures [1,2] which are inspired form the fields of electrosynthesis and battery research [3,4], since the applied voltages extend from 4.2 V in the case of lithium-ion batteries and up to 25 V in electrosynthesis [5].
Herein, we have investigated the electrochemical properties of ternary solvent mixture featuring PC, ethyl acetate (EA) in 1:1 volume ratio with a small volume percentage (0.5 – 5)% of vinylene carbonate (VC). 1M NaPF6 was chosen as the salt due to sodium’s high abundance [6], better performance in EDLCs compared to widely used NaClO4 [1] and to provide potential electrolytes for the emerging chemistry of sodium-ion batteries [6]. EA (η = 0.423 mPa s) was chosen to lower the viscosity of rather viscous PC (η = 2.53 mPa s) in the mixture. VC (ε = 126) was added to enhance electrolyte conductivity.
In the first part of the study, we measured electrolyte conductivities for various solvent systems (using 1M NaPF6): PC, PC + 5% VC, PC:EA (1:1), PC:EA (1:1) + 0.25% VC, PC:EA (1:1) + 0.5% VC, PC:EA (1:1) + 1% VC, PC:EA (1:1) + 2% VC, PC:EA (1:1) + 5% VC. In the second part, we carried out room-temperature electrochemical measurements of the test-cells with beforementioned solvent systems using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and constant current charge-discharge (CCCD) methods. 1M NaPF6in PC:EA (1:1) + 0.5% VC electrolyte showed best overall performance and was therefore chosen as the best candidate for wide-temperature (−40 to 60 °C) electrochemical studies. Again CV, EIS and CCCD were used.
Figure 1 shows cyclic voltammograms measured at −30 °C. At low scan rates the system exhibits almost ideal capacitive behaviour, however at 50 mV s-1 the shape of the CVs starts to deviate due to increased viscosity at such low temperatures. Still, its high-rate performance exceeds that of purely PC-based EDLCs.
Results of rigorous equivalent circuit modelling will be shown.

Acknowledgements

The present study was supported by the Estonian Centre of Excellence in Science project 3.2.0101.11-0030, Estonian Energy Technology Program project 3.2.0501.10-0015, Material Technology Program project 3.2.1101.12-0019, Project of European Structure Funds 3.2.0601.11-0001, Estonian target research project IUT20–13, NAMUR project 3.2.0304.12-0397 and projects 3.2.0302.10-0169 and 3.2.0302.10-0165.

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Figure 1