The Influence of Ion-Solvent Interactions on the Performance of Electrochemical Double Layer Capacitors Containing Non-Conventional Solvents

Abstract

In the state of the art electrochemical double layer capacitors (EDLCs), solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. Using these electrolytes in combination with activated carbons (AC) based electrodes is possible to realize EDLCs with operative voltages of 2.7-2.8 V and energy in the order of 5Whkg-1. The power of these devices is up 10 kW kg-1 and their cycle life higher than 500.000 cycles [1]. Presently the realization of high energy EDLCs is seen as one of the most important goal for an increase of the market of these devices [1]. The most straightforward strategy to increase the EDLC´s energy is to increase their operative voltage. Since such desirable increase is not possible when the state-of-the-art electrolytes are used, the introduction of new electrolytic solution is presently considered of great importance [1] In order to realize advanced electrolytes for EDLC, the introduction of new conducting salts and/or new solvents appears a viable strategy [1,2]. In the past years we showed that using non-conventional salts is possible to realize PC-based EDLCs with operative voltages higher than 3 V [1,2]. More recently, taking advantaged of computational screening [3], we identified cyano esters as new solvent for the realization of high voltage and high energy EDLCs [4,5]. We showed that cyano ester-based EDLCs display operative voltage as high as 3.5 V and good performance in term of specific capacitance and capacitance retention. After 500 h of float tests carried out at 3.2 V EDLCs containing this type of solvent are able to retain almost 80% of their initial capacitance [4]. It is well known that the ion-solvent interaction might have a dramatic effect on the behaviour of EDLCs [1,2]. While these interactions have been considered for conventional salt-solvent combinations, only little information is available on the case of non-conventional electrolyte components [2]. Herein, we report a systematic investigation about the chemical-physical properties of electrolytes containing the salts tetraethylammonium tetrafluroroborate (Et4NBF4), tetraethylammonium bis(trifluoromethanesulfonyl) imide (Et4NTFSI) and N-butyl-1-methylpyrrolidinium tetrafluoroborate (Pyr14BF4), in the state of the art solvent ACN, and in the alternative solvents Adiponitrile (ADN), 2-methylglutaronitril (MGN) and 3-cyanopropionic acid methyl ester (CPAME). The ionic conductivity, viscosity, density and electrochemical stability windows (ESW) of these electrolytic solutions are considered. Furthermore, the electrochemical performance of EDLCs containing these electrolytes are investigate in details. Finally, also the anodic dissolution of the aluminium current collector in these electrolytes is analysed and discussed [6].