ABSTRACT
Supercapacitors are energy storage devices designed to operate at higher power densities than conventional batteries, but their energy density is still too low for many applications. Efforts are made to design new electrolytes with wider electrochemical windows than aqueous or conventional organic electrolytes in order to increase energy density. Ionic liquids (ILs) with wide electrochemical stability windows are excellent candidates to be employed as supercapacitor electrolytes. ILs containing tetracyanoborate anions [B(CN)4] offer wider electrochemical stability than conventional electrolytes and maintain a high ionic conductivity (6.9â mS cm-1). Herein, we report the use of ILs containing the [B(CN)4] anion for such an application. They presented a high maximum operating voltage of 3.7â V, and two-electrode devices demonstrate high specific capacitances even when operating at relatively high rates (ca. 20â F g-1 @ 15â A g-1). This supercapacitor stored more energy and operated at a higher power at all rates studied when compared with cells using a commonly studied ILs.
ABSTRACT
This work presents the physicochemical characterization of two ionic liquids (ILs) with small phosphonium cations, triethylpenthylphosphonium bis(trifluoromethanesulfonyl)imide ([P2225][Tf2N]) and (2-methoxyethyl)trimethylphosphonium bis(trifluoromethanesulfonyl)imide ([P222(201)][Tf2N]), and their mixtures with Li(+). Properties such as the electrochemical window, density, viscosity and ionic conductivity are presented. The diffusion coefficient was obtained using two different techniques, PGSE-NMR and Li electrodeposition with microelectrodes. In addition, the Li(+) transport number was calculated using the PGSE-NMR technique and an electrochemical approach. The use of these three techniques showed that the PGSE-NMR technique underestimates the diffusion coefficient for charged species. The Li(+) transport number was found to be as high as 0.54. Raman spectroscopy and molecular dynamics simulations were used to evaluate the short-range structure of the liquids. These experiments suggested that the interaction between the Li(+) and the Tf2N(-) anion is similar to that seen with other ILs containing the same anion. However, the MD simulations also showed that the Li(+) ions interact differently with the cation containing an alkyl ether chain. The results found in this work suggest that these Li(+) mixtures have promising potential to be applied as electrolytes in batteries.
ABSTRACT
Given their relevant physicochemical properties, ionic liquids (ILs) are attracting great attention as electrolytes for use in different electrochemical devices, such as capacitors, sensors, and lithium ion batteries. In addition to the advantages of using ILs containing lithium cations as electrolytes in lithium ion batteries, the Li(+) transport in ILs containing the most common anion, bis(trifluoromethanesulfonyl) imide anion ([Tf2N]), is reportedly small; therefore, its contribution to the overall conductivity is also low. In this work, we describe the preparation and characterization of two new and one known IL containing the tetracyanoborate anion ([B(CN)4]) as the anionic species. These ILs have high thermal and chemical stabilities, with almost twice the ionic conductivity of the [Tf2N] ILs and, most importantly, provide a greater role for the Li(+) ion throughout the conductivity process. The experimental ionic conductivity and self-diffusion coefficient data show that the [B(CN)4]-based ILs and their Li(+) mixtures have a higher number of charge carriers. Molecular dynamics simulations showed a weaker interaction between Li(+) and [B(CN)4] than that with [Tf2N]. These results may stimulate new applications for ILs that have good Li(+) transport properties.
ABSTRACT
Electrochemical capacitors (ECs) are electrical energy storage devices that have the potential to be very useful in a wide range of applications, especially where there is a large disparity between peak and average power demands. The use of ionic liquids (ILs) as electrolytes in ECs can increase the energy density of devices; however, the viscosity and conductivity of ILs adversely influence the power density of the device. We present experimental results where several ILs containing different cations have been employed as the electrolyte in cells containing mesoporous carbon electrodes. Specifically, the behavior of ILs containing an ether bond in an alkyl side chain are compared with those of a similar structure and size but containing purely alkyl side chains. Using electrochemical impedance spectroscopy and constant current cycling, we show that the presence of the ether bond can dramatically increase the specific capacitance and reduce device resistance. These results have the important implication that such ILs can be used to tailor the physical properties and electrochemical performance of IL-based electrolytes.
