**1. Introduction**

As new markets using energy storage devices increase, various types of energy storage devices are striving to enter the market [1, 2]. In the case of electric vehicles (EVs), Li ion batteries (LIBs), fuel cells (FCs), and supercapacitors (SCs) are in a competitive relationship [3]. LIBs are emerging as the most promising candidates because of their high energy density and technical maturity. However, there is a problem of safety. The unstable supply and price of lithium and cobalt are also serious problems. In addition, FCs still have difficulties in

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commercialization due to the disadvantage of obtaining hydrogen from petrochemistry and the reluctance of establishing a hydrogen station. As an alternative to this situation, SCs can be proposed. Up to now, SCs have been applied to an electric bus using regenerative break and electric train powered by pulse power, since the large power density of SCs is suitable for intermittent power supply [4]. However, SCs are facing with critical challenge of low energy density. Therefore, LIBs and SCs are applied together for the complementary purpose. The operating voltage of LIBs is from 3.5 to 4.0 V and that of SCs is approximately under 2.8 V. If SCs achieve the 3.5–4.0 V rated voltage with high energy densities or LIBs achieve high power densities, then the power supply unit can be minimized and integrated [5]. In other words, if SCs archive high energy density without sacrificing power density, SCs can be applied as an alternative energy storage system replacing LIBs and FCs.

operating voltage of SCs. Next alternative organic solvent for high operating voltage is linear sulfones, alkylated cyclic carbonate, and adiponitrile [10–12]. These alternative electrolytes archive 3–3.2 V of operating voltage. Meanwhile they show relatively high viscosity and low ionic conductivity, these characteristic occur decrement of power density. Another type of alternative electrolyte is the LIBs electrolyte system. These electrolytes contain common solvents (cyclic carbonate for high dielectric constant and linear carbonate for low viscosity) and

voltage over 3 V. However, they have some issues of moisture sensitivity (formation of HF

interphase (SEI). These problems are directly connected to electrical performance and safety

To solve these issues, ionic liquids (ILs) are investigated. ILs are thermally stable because they have negligible vapor pressure and they are nonflammable and relatively high ionic conductivity. More importantly, ILs are very effective for widening the operating voltage due to their wide electrochemical stability window over 3 V. Additionally, ILs are composed of only ions, which means that ILs play a role in both salts and solvents. Therefore, there are no additional salts or solvents. For these reasons, ILs have been widely investigated as an

In 1807, Humphry Davy pioneered the study for the electrolysis of molten salts which is referred as ILs later and presented the electrochemical theory of the molten electrolyte, despite the research was concentrated on reactive metal preparation [13]. After that, synthetic method of aluminum with electrolysis of aluminum oxide dissolved in cryolite was suggested by Hall [14]. This method is meaningful that eutectic molten salts were formed at low temperature

ILs have been studied for electrochemical devices such as SCs, FCs, rechargeable batteries, photovoltaic cell, and actuator etc. due to the mobility and flexibility of ions [5, 14–20]. ILs are composed of large and asymmetrical organic cation and charge delocalized inorganic/ organic anion by weak interaction [21]. Despite that ILs exist in the cation-anion state, these structures lower the tendency to crystallize, so they provide a fluid phase with reasonable ion conductivity, and they show no decomposition or significant vapor pressure [21, 22]. As mentioned, ILs are composed of organic ions and can be combined to various structures with easy preparation. Thus, various kinds of ILs can be used for the given application with

Wide tunability of ILs can be combined to satisfy the desired characteristics of SCs such as working voltage, operating temperature range, and internal resistances [23]. For these reasons, ILs have been widely investigated as electrolyte material. As mentioned above, quaternary

with electrolysis, and this method is still applied in aluminum industrial production.

of SCs. Also, organic solvent is highly flammable and thermally unstable.

**2. Ionic liquid as liquid electrolyte for supercapacitors**

**2.1. Neat ionic liquid as liquid electrolyte for electric double layer**

), slow intercalation reaction of Li cation, and the presence of a solid electrolyte

. These electrolytes exhibit high ionic conductivity and high operating

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 25

lithium salts like LiPF<sup>6</sup>

electrolyte material.

desired properties (**Figure 1**).

from LiPF6

There are two ways to increase the energy density of SCs. One is to enhance capacitance, and the other is to increase the operating voltages, since the energy density is proportional to capacitance and operating voltage. At the initial stage, many researchers focused on widening the electrode area for enhancing electric double layer (EDL) capacitance. Various types of activated carbon (AC) are commercialized as a result of these efforts. Carbon nanomaterials like carbon nanotubes (CNTs) and graphene are also highlighted as large capacitance materials [6]. Additionally, redox active electrode materials like metal oxide and conductive polymer are proposed to enlarge capacitance by pseudocapacitance. However, those materials struggle with process abilities, limiting of electrolyte adoption, charging/discharging properties, minimizing high cost, and checking suitability for commercial production lines. Another approach of redox active materials is electrolyte that contains redox active couples. Representative redox couples are halides, vanadium complexes, copper salts, hydroquinone, methylene blue, indigo carmine, p-phenylenediamine, m-phenylenediamine, lignosulfonates, and sulfonated polyaniline [7]. Most of electrolyte adopting redox couple has a problem of solvent selection because such redox couples are effective in aqueous medium. This means that the operating voltage of aqueous electrolyte containing redox active couple is limited under 1.23 V, which is the electrolysis voltage of water. Such boundaries with electrode materials motivate the development of advanced electrolyte with high operating voltages for high energy densities because the operating voltages for SCs depend on the electrochemical stability window of the electrolyte.

At early stages, classical salts and systems are suggested. The solvent is organic solvents such as acetonitrile (AN) and propylene carbonate (PC) [8]. Quaternary ammonium salts and AN are representative electrolyte of SCs, and their operating voltage is about 2.8 V. The electrolytes adopting AN exhibit large specific capacitance due to high ionic conductivity derived from low viscosity of AN. However, SCs applied AN-based electrolyte have to be operated under 80°C due to the boiling point of AN. Otherwise, PC is less toxic and has higher thermal stability than AN. Thus, the electrolyte adopting PC is generally considered a safe electrolyte. Using PC-based electrolytes can achieve slightly higher operating voltages for SCs than those using AN. However, formation of carbonate and evolution of H<sup>2</sup> and CO<sup>2</sup> caused by reaction of carbon electrode and PC have been identified as the main causes of performance degradation at high voltages [9]. To overcome these drawbacks, many electrolytes have been proposed over the past several years as an alternative electrolyte to increase the operating voltage of SCs. Next alternative organic solvent for high operating voltage is linear sulfones, alkylated cyclic carbonate, and adiponitrile [10–12]. These alternative electrolytes archive 3–3.2 V of operating voltage. Meanwhile they show relatively high viscosity and low ionic conductivity, these characteristic occur decrement of power density. Another type of alternative electrolyte is the LIBs electrolyte system. These electrolytes contain common solvents (cyclic carbonate for high dielectric constant and linear carbonate for low viscosity) and lithium salts like LiPF<sup>6</sup> . These electrolytes exhibit high ionic conductivity and high operating voltage over 3 V. However, they have some issues of moisture sensitivity (formation of HF from LiPF6 ), slow intercalation reaction of Li cation, and the presence of a solid electrolyte interphase (SEI). These problems are directly connected to electrical performance and safety of SCs. Also, organic solvent is highly flammable and thermally unstable.

To solve these issues, ionic liquids (ILs) are investigated. ILs are thermally stable because they have negligible vapor pressure and they are nonflammable and relatively high ionic conductivity. More importantly, ILs are very effective for widening the operating voltage due to their wide electrochemical stability window over 3 V. Additionally, ILs are composed of only ions, which means that ILs play a role in both salts and solvents. Therefore, there are no additional salts or solvents. For these reasons, ILs have been widely investigated as an electrolyte material.
