Eutectic Mixtures of Ionic Liquids Electrolytes for Electric Double Layer Capacitors

Abstract

Eutectic mixtures of oxygen-containing spirobipyrrolidiniumbased ionic liquids were investigated as prospective room temperature ionic liquid electrolytes (RTILs) for supercapacitor application. These in-house developed novel ionic liquids (IL) possess wide voltage window (6-7 V) and solubility (3-5.5 M) in conventional solvents, such as acetonitrile and propylene carbonate. The potential window of mixed RTIL was about 5.5 V (at 50 μA/cm 2 threshold). The RTIL mixtures exhibit higher breakdown potential (> 4.6 V) as compared to conventional molecular solvent (acetonitrile)-based cells (~4.0 V) in supercapacitor tests. However, these RTIL mixtures (neat, i.e. w/o any solvent addition) lead to high equivalent series resistance (ESR) initiating around 3.9 V and lower capacitance. Cycling of the negative electrode at 4.6 V led to degradation of the active carbon material, while the positive electrode maintained its integrity.

Understanding transport mechanisms in ionic liquid/carbonate solvent electrolyte blends

“…polymer chains). 21,22,[57][58][59][60] In contrast to proton conduction in aqueous phases, which is often based on vehicular diffusion or Grotthuss-like ''hopping'' of hydrated hydronium ions, 23,26,61 the presence of various solvents and additives in addition to lithium salts and/or ILs renders an explicit description of probable mechanisms and phenomena responsible for an observable charge transport in lithium ion batteries difficult. Nevertheless, the nature of available lithium species and its surrounding ''liquid'' structure is often accessible from MD simulations thus facilitating identification of complex lithium ion transfer mechanisms in liquid electrolytes.…”

“…Another strategy includes the addition of small organic compounds like EC, vinylene carbonate (VC), acetonitrile (AN) or tetrahydrofuran (THF) which based on molecular dynamics (MD) simulations afford lesser coordination of lithium ions with the anion of the ILs and hence better mobility and ionic conductivity. 22,23 While blends of organic solvents are a commodity, 24,25 blends of ILs and organic carbonates were considered by several groups only recently [26][27][28][29][30][31][32][33] particularly focusing on improved safety performance, reduced aluminum current collector dissolution or enhanced electrochemical properties. While selected features of the ion transport in pristine ILs or ILs doped with lithium salts were previously described, 20,[34][35][36][37] the situation is less clear in case of ternary mixtures.…”

Double Layer Impedance in Mixtures of Acetonitrile and Water

“…The strong dependence of the capacitance values on solvents has been applied to expanding the available voltage, improving the reversibility of charge‐discharge processes, selecting materials of cells and electrodes, and suppressing leakage of the charge. As industrial applications, mixed solvents have been used for developing properties of supercapacitors . Mixed solvents have an advantage in providing the opportunity to vary continuously the conditions.…”

The Mass-Balancing between Positive and Negative Electrodes for Optimizing Energy Density of Supercapacitors

“…To date, much effort has been put into enhancing accessible surface area − and modifying the surface chemistry − of electrode materials to boost the specific capacitance and energy density. Meanwhile, research also focuses on the selection and formulation of appropriate electrolytes to widen the operating potential window range − or/and to introduce the pseudocapacitance contribution . In addition, to reasonably construct asymmetric supercapacitors (ASCs) different materials are employed as the positive and negative electrodes can extend working voltage ranges to achieve high energy density of SCs. − Typically, the theoretical voltage of an aqueous-based symmetric system does not exceed 1.2 V, but the operating voltage range of an asymmetric supercapacitor could reach 2.0 V. − …”