Characterization of Different Conductive Salts in Acn-Based Electrolytes for Electrochemical Double Layer Capacitors

Electrochemical double layer capacitors (EDLCs) are considered among the most promising electrochemical storage device for high power systems. EDLCs display extraordinary cycle life (>500.000 cycles), high power (up to 10 kW kg -1 ) and energy (in the range of 5 Wh kg -1 ), what makes them reliable energy sources for many applications. [1] In state-of-the-art EDLCs activated carbons (AC) are used as electrode active materials, while solutions of tetraethylammonium tetrafluroroborate (Et 4 NBF 4 ) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. These combinations of materials leads to EDLCs with operative voltages of 2.7-2.8 V. [1] Considering the formulas describing the energy (E=1/2 CV²) and power (P=V²/4R) of EDLCs, it is obvious that by increasing the operative voltage, energy and power values will be increased as well. For this reason, many efforts have been made towards the introduction of novel electrolytes. [1] Recently we showed that the use of alternative conductive salts, with respect to Et 4 NBF 4 , represents a promising strategy for the realization of high voltage EDLCs. As a matter of fact, using these salts in combination with PC, it is possible to realize EDLCs with operative voltages higher than 3 V. [2] It is well known that also the chemical-physical properties of the solvent have a strong influence on the operative voltage as well as on the performance, especially the power, of EDLCs. [1] Taking into account the lower viscosity of ACN compared to PC, the use of the former solvent in combination with the above mentioned alternative conductive salts appears therefore of great interest. Such combinations could allow the realization of high voltage and high power devices. In this work we report a systematic investigation about the chemical-physical properties of electrolytes containing the salts (tetraethylammonium tetrafluroroborate (Et 4 NBF 4 ) , tetraethylammonium bis(trifluoromethanesulfonyl)imide (Et 4 NTFSI), N-butyl-N-methylpyrrolidinium tetrafluoroborate (Pyr 14 BF 4 ), 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl)imide (Pyr 14 TFSI)) and ACN as solvent. The ionic conductivity, viscosity, density and electrochemical stability windows (ESW) of these electrolytic solutions are considered in detail. Furthermore, the performance of EDLCs containing these advanced electrolytes is investigated and critically analysed. The results of this study indicate that the solvent-salt interaction has a strong influence not only on the chemical-physical properties of the electrolyte, but also on the energy, power and cycle life of the EDLC. Furthermore, they also indicate that the operative voltage of EDLCs containing alternative conductive salts is strongly influenced by the choice of the solvent. [3] References [1] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Advanced Materials, 26 (2014) 2219-2251 [2] S. Pohlmann, C. Ramirez-Castro, A. Balducci, J. Electrochem. Soc., 162 (2015) A5020-A5030. [3] J. Krummacher, C. Schütter, A. Balducci, manuscript in preparation Figure 1