From Computational Screening to Real Devices: The Case of Cyano Esters Based Electrolytes for Electrochemical Double Layer Capacitors

Abstract

In the state of the art electrochemical double layer capacitors (EDLCs), activated carbons (AC) are used as electrode active materials, while solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. These combinations of materials enable the realization of EDLCs with operative voltages of 2.7-2.8 V, extraordinary cycle life (>500.000 cycles), high power (up to 10 kW kg-1) and energy in the order of 5Whkg-1[1]. It has been shown that if the energy of EDLC could be increase up to 10Whkg-1 these devices could be introduced in many new applications, leading to a dramatic increase of their market [1]. For this reason, the development of high energy EDLCs is nowadays considered of crucial importance for the future of this technology. 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 consider of great importance [1]. To avoid time consuming “trial and error” experiments, it is desirable to “rationalize” this search for new electrolyte components. An important step in this direction is the systematic application of computational screening approaches. Via the fast prediction of the properties of a large number of compounds, for instance all reasonable candidates within a given compound class, such approaches should allow to identify of the most promising candidates for subsequent experiments [2]. In this paper we report about the use of computation screening for the identification of novel solvents for EDLCs. As an example application, the known chemical space of almost 70 million compounds is investigated in search of electrochemically more stable solvents. Cyano esters are identified as especially promising new compound class. As a matter of fact, the use of electrolytes containing cyano ester as solvent and Et4NBF4 as conductive salt allows for the realization of EDLCs with operative voltage as high as 3.5 V. These high voltage EDLCs display good performance in term of specific capacitance and capacitance retention. Furthermore, we showed that EDLCs containing this type of solvent are able to retain almost 80% of their initial capacitance after 500 h of float tests carried out at 3.2 V. This extraordinary capacitance retention, which is among the highest so far reported for high voltage EDLC, clearly indicate that cyano ester can be considered as a new and very interesting solvent in view of the realization of high voltage EDLC [3]. Taking these results into account, the use of computational screening appears as a very promising and novel strategy for a rational design of new electrolyte materials for EDLCs. It is important to remark that such approach can be applied not only to the search for new materials, but also to the optimization of mixtures and salt/solvent combinations.